Sohm Abyssal Plain: Evaluating Proximal Sediment Provenance DANIEL JEAN STANLEY, PATRICK T. TAYLOR, HARRISON SHENG, and ROBERT STUCKENRATH CONTRIBUTIONS TO THE MARINE SCIENCES SERIES PUBLICATIONS OF THE SMITHSONIAN INSTITUTION Emphasis upon publication as a means of "diffusing knowledge" was expressed by the first Secretary of the Smithsonian. In his formal plan for the Institution, Joseph Henry outlined a program that included the following statement: "It is proposed to publish a series of reports, giving an account of the new discoveries in science, and of the changes made from year to year in all branches of knowledge." This theme of basic research has been adhered to through the years by thousands of titles issued in series publications under the Smithsonian imprint, commencing with Smithsonian Contributions to Knowledge in 1848 and continuing with the following active series: Smithsonian Contributions to Anthropology Smithsonian Contributions to Astrophysics Smithsonian Contributions to Botany Smithsonian Contributions to the Earth Sciences Smithsonian Contributions to the Marine Sciences Smithsonian Contributions to Paleobiology Smithsonian Contributions to Zoology Smithsonian Studies in Air and Space Smithsonian Studies in History and Technology In these series, the Institution publishes small papers and full-scale monographs that report the research and collections of its various museums and bureaux or of professional colleagues in the world of science and scholarship. The publications are distributed by mailing lists to libraries, universities, and similar institutions throughout the world. Papers or monographs submitted for series publication are received by the Smithsonian Institution Press, subject to its own review for format and style, only through departments of the various Smithsonian museums or bureaux, where the manuscripts are given substantive review. Press requirements for manuscript and art preparation are outlined on the inside back cover. S. Dillon Ripley Secretary Smithsonian Institution SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES • NUMBER 11 Sohm Abyssal Plain: Evaluating Proximal Sediment Provenance Daniel Jean Stanley, Patrick T. Taylor, Harrison Sheng, and Robert Stuckenrath SMITHSONIAN INSTITUTION PRESS City of Washington 1981 ABSTRACT Stanley, Daniel Jean, Patrick T. Taylor, Harrison Sheng, and Robert Stuck- enrath. Sohm Abyssal Plain: Evaluating Proximal Sediment Provenance. Smithsonian Contributions to the Marine Sciences, number 11, 48 pages, 23 figures, 5 tables, 1981.—The southernmost part of the Sohm Abyssal Plain in the Northwest Atlantic Basin is geographically distal with respect to the major source of Quaternary terrigenous material transported from the Canadian Maritime Provinces. An assessment of the proportion of more locally intro- duced sediment relative to that derived from distal sources is based largely on size and compositional analyses of Quaternary piston core samples. These data are supplemented by radiocarbon dating of selected core samples, bottom photographs, conductivity-temperature-depth profiles, and seismic records. The premises of the study are that (a) locally derived sediment should be most abundant near high-relief bathymetric features such as seamounts and abyssal hills, and (b) such material should contain enhanced proportions of reworked volcanic debris and alteration products. Core analyses reveal that the amounts of these are directly related to proximity of volcanic ocean- bottom features, and that a significant, although not total, amount of such volcanic materials recovered from cores are derived from submarine weath- ering of basalt. Associated with this assemblage are nannofossils, dating from the Quaternary to the Upper Cretaceous, reworked from older strata. This increased proportion of volcanic and related products and reworked faunas near seamounts and basement rises strongly implies that such topographic features continue to serve as major source terrains. Locally derived volcanic materials, however, are usually disseminated and masked on the Sohm Abyssal Plain, particularly in sectors receiving large amounts of terrigenous turbidites and biogenic suspensates, and/or undergoing reworking by bottom currents. We propose that the volcanic fraction can serve as a useful index, or "yardstick," to interpret the role of locally derived material in abyssal plain sedimentation. A sedimentation model is developed to illustrate the premise that as access to land-derived sources diminishes, the proportion of terrigenous components is reduced while pelagic and volcanic fractions are enhanced. Thus, sediment accumulating in abyssal plains almost totally isolated from terrigenous sources would comprise significant amounts of pelagic (including wind-blown) and volcanic components. Our model illustrates that even in an abyssal plain, such as the Sohm, which has had an important and direct access to abundant distally derived terrigenous sources, particularly during the Pliocene and Quaternary, the locally supplied reworked volcanic products account for a significant fraction of the total abyssal plain sediment fill. OFFICIAL PUBLICATION DATE is handstamped in a limited number of initial copies and is recorded in the Institution's annual report, Smithsonian Year. SERIES COVER DESIGN: Seascape along the Atlantic coast of Eastern North America. Library of Congress Cataloging in Publication Data Sohm Abyssal Plain (Smithsonian contributions to the marine sciences ; 11) Bibliography: p. I. Marine sediments—Sohm Plain (Atlantic Ocean) I. Stanley, Daniel J. II. Series: Smith- sonian Institution. Smithsonian contributions to the marine sciences ; no. 11. GC383.S64 551.46'4 81-607056 AACR2 Contents Page Introduction 1 Acknowledgments 4 Methods 4 Physiography of Study Area 7 Sediment Acoustic Facies in Southern Plain 7 General Distribution 7 Perched Sediment Basin 14 Other Areas near Congress and Lynch Seamounts 15 Core Lithologies: General Description 16 Rates of Sediment Accumulation 17 Textural and Compositional Characteristics 20 Grain Size 20 Coarse Fraction Composition 20 Fine Silt and Clay Fraction Composition 21 Water-Mass Movement 26 Discussion and Conclusions 27 Appendix: Tables 37 Literature Cited 46 Sohm Abyssal Plain: Evaluating Proximal Sediment Provenance Daniel Jean Stanley, Patrick T. Taylor, Harrison Sheng, and Robert Stuckenrath Introduction Following the classic work of Heezen and Ew- ing (1952), studies of North Atlantic abyssal plains during the past three decades have empha- sized turbidity currents as a major transporting agent in the displacement of sediment to remote regions. This process has been proposed largely as a result of piston core investigations which show that surficial Atlantic sequences commonly contain a sand fraction comprising clastic, shal- low-marine or land-derived components (Ericson et al., 1961; Heezen, 1963; Horn et al., 1971). The acoustic patterns on seismic reflection pro- files of the Northwest Atlantic Basin show mul- tiple returns (termed acoustically layered) that are generally interpreted as stacked sequences of turbidites (Ewing et al., 1973). Seismic surveys in this region also indicate the presence of transpar- ent acoustic series, primarily on topographic highs, which are believed to have a different origin (Ewing et al., 1973; Bowles, 1980). Inter- pretations of acoustically transparent layers are based on JOIDES deep-sea drilling results, bot- tom photography (Heezen and Hollister, 1971), nephelometry (Biscaye and Eittreim, 1977), and Daniel Jean Stanley and Harrison Sheng, Division of Sedimentology, Smithsonian Institution, Washington, D.C. 20560; Patrick T. Taylor, Earth Survey Applications Division, NASA Goddard Space Flight Center, Greenbelt, Md. 20771; and Robert Stuckenrath, Radiation Biology Laboratory, Smithsonian Institution, Washington, D.C. 20560. studies of water-mass motion (Laine, 1978). Var- ious transport models, for the most part involving resuspension by bottom currents, have evolved from these investigations. Recent reassessment of the origin of sediments in the Northwest Atlantic Basin, including the large, deep (>5000 m), T-shaped Sohm Abyssal Plain south of the Canadian Maritime Provinces (Heezen and Tharp, 1968), indicates an even more complex depositional origin (Figure 1). Bis- caye and Eittreim (1977) summarize the impor- tance of concentrated suspended material in the water masses on and above this Plain, and attri- bute the almost ubiquitous bottom-hugging nepheloid layer to erosion and resuspension of the seafloor by bottom-water flow. Amos and Gerard (1979), however, suggest that a nepheloid layer in the northern sector of the Sohm Plain is due to material carried southward by a transport process related to turbidity currents. The discovery of a large clockwise-circulating gyre associated with the Gulf Stream System (Worthington, 1976) has led to a modification of the classic turbidite and nepheloid layer emplacement models for this area. According to Laine and Hollister (1981), flow associated with this gyre resuspends seafloor sediment and also entrains turbidity current flows periodically introduced into the Sohm Abyssal Plain from the Canadian Maritime Margin. All the above sedimentation schemes, and com- SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES ■",, GB ■& 80° 50° NT l ' 45° 25'" A-i '^%:A: l: ... ■•••• ...-.A^Ai :^f 40° w FIGURE 1.—Chart of Northwest Atlantic Basin (abbreviations for features cited in text: GrB = Grand Banks, NSS = Nova Scotian Shelf, GB = Georges Bank, CH = Cape Hatteras, HAP = Hatteras Abyssal Plain, SAP = Sohm Abyssal Plain, KSC = Kelvin Seamount Chain, MAR = Mid-Atlantic Ridge, CR = Corner Rise, B = Bermuda; C = Congress Seamount, L = Lynch Seamount; core locations: Ly = Lynch 710-78 cores, A = Atlantis 153 core, V = Vema 22 and 26 cores). binations thereof, are generally applicable on a basin-wide scale and tend to emphasize remote provenance and long-distance dispersal of terri- genous and biogenic material by near-bottom current activity. There has been a tendency, how- ever, to overlook the contribution of locally de- rived material in regional depositional interpre- tations. In order to focus more precisely on this potentially important aspect, we selected for study the southernmost part of the Sohm Abyssal Plain. This area represents a geographically distal sector with respect to the major source of the Quaternary terrigenous material transported onto the plain, i.e., the Canadian Maritime Mar- gin (Horn et al., 1971). Thus, our study will attempt to assess the proportion of more locally introduced sediment relative to that derived from distal sources. It could be assumed that the amount of locally derived sediment should be higher in cores recovered from this remote region than those collected in more proximal areas. NUMBER The southern sector of the Sohm Abyssal Plain is bordered on three sides by abyssal hills of low to moderate relief (Heezen et al., 1959; Horn et al., 1971). Samples and seismic data were col- lected near the center of this region, specifically around Congress and Lynch seamounts, isolated bathymetric highs in this region (Figures 1 and 2). These data supplement more widely spaced geological and geophysical measurements previ- ously collected by others—principally from the Lamont-Doherty Geological Observatory (L- DGO)—in the plain (Ewing et al., 1974:138,229). Our working premises for this study are (a) that locally derived sediment should be most abun- dant near high-relief bathymetric features (e.g., seamounts, ridges, and large abyssal hills), and (b) that such material should contain enhanced proportions of volcanic debris. To test this idea, LYNCH SEAMOUNT ( Depth in fathoms ) 33°30'N 33°15'N 33°N 32°45N 32°30'N 55°W 54°30'w 54°W FIGURE 2.—Sampling stations around the Congress and Lynch seamounts in the Southern Sohm Abyssal Plain; note the perched basin on western flank of the Congress seamount (bathymetry from Lynch cruise 710-78 in 1978; solid circles = core stations, squares = camera stations; triangles = CTD stations, solid bars (a, b, c) = 3.5 kHz profiles, shown also, in part, in Figure 10; depths shallower than 2910 fathoms given in hundreds of fathoms). SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES six cores recovered on or within 220 km (120 nm) of Congress Seamount (Figure 1) were examined: two each near the Congress, on the southern Abyssal Plain proper, and on the somewhat higher relief volcanic topography peripheral to the plain. ACKNOWLEDGMENTS.—We thank the U.S. Navy for supporting cruise 710-78 of the USNS Lynch and, particularly, Capt. John T. Lawrence, the officers, crew, and scientific party who ably as- sisted in the work at sea. Dr. David Greenewalt, Naval Research Laboratory, Washington, D.C, provided us with preliminary interpretations of the conductivity-temperature-depth data col- lected on this cruise. We have benefited greatly from access to the core, bottom photograph, and seismic data li- braries of the Lamont-Doherty Geological Ob- servatory of Columbia University, Palisades, New York, and are grateful to Drs. F. W. McCoy and W. Ludwig, and to Ms. B. Batchelder and Mr. L. Sullivan for their help in providing us with data used in this study. Appreciation is also expressed to Drs. S. Street- er, Lamont-Doherty, and C. C. Smith, U.S. Geo- logical Survey, Washington, D.C, for identifica- tion and assistance in the interpretation of the benthic foraminifera and nannofossils, respec- tively. The paper has been reviewed by L. A. Barnard, Texas A & M University, T.-C. Huang, Univer- sity of Rhode Island, and G. H. Keller, Oregon State University. Methods The Congress and Lynch seamounts were sur- veyed by the USNS Lynch (cruise 710-78) from 30 September to 6 October 1978. Bathymetric data were obtained in fathoms with the Lynch's hull- mounted 12 kHz echo-sounding system and con- toured in these units; the uncorrected contours shown in Figures 2 and 3 incorporate previous hydrographic ship crossings of this feature (data obtained from the Defense Mapping Agency, Hydrographic Topographic Center Data Library, Brookmount, Maryland). Throughout the text of this paper, depths and other units are given in metric units; in most instances, equivalent Eng- lish units are presented for convenience. Seismic reflection profiler data (36 kilojoule sparker sys- tem) collected during the Lynch cruise were sup- plemented by airgun (25 cu in, or 0.4 m ) records obtained during Vema cruises 22 and 26 (8 June 1966 and 8 August 1968, Ewing et al., 1974:138, 229). The more than 1000 line-kilometers of Lynch reflection data include 554 km in an east-west direction and 482 in north-south and diagonal directions (Figure 3). Line spacing around the Congress ranges from 3 to 10 km. Lynch subbot- tom 3.5 kHz profiles also were obtained adjacent to and on the mount proper, in the vicinity of the two conductivity-temperature-depth (CTD) pro- files shown in Figure 2. At these CTD stations a Neil Brown instrument (Brown, 1974) was used. The first station (33°09'N, 54°58'W) was on the western flank of the northern Congress peak some three kilometers east of Lynch core 710-78-1; the second CTD (32°56'N, 54°57'W) was on the western flank of the southern Congress peak (Fig- ure 2). Seafloor photographs were taken at three sta- tions on and near the Congress (Figure 2). Two stations were occupied during the Lynch cruise and one station on Vema cruise 26. These are situated as follows: Lynch camera station 1, 33°09'N, 54°50'W, 3110-3290 m (1700-1800 fm); Vema cruise 26, camera station 7(13 August 1968), on the southern Congress peak, 32°55.2'N, 54°55.6'W, 2850-2815 m (1560-1540 fm); Lynch camera station 2 on the Sohm Abyssal Plain west of the seamount, 32°56'N, 55°12.5'W, about 5367 m (2935 fm). Two piston cores were obtained during the Lynch cruise from the immediate vicinity of the Congress (Figure 2): core Ly 710-78-1, taken near the base of the seamount in a perched basin (or pond) near the point of thickest sediment accu- mulation about 20 km west of the northern Con- gress peak, is 558 cm long (33°09.3'N, 55°00.2,W, 5207 m, or 2847 fm, depth); core Ly 710-78-2, from the adjacent plain, returned only a few NUMBER V-26 ( Depth in fathoms ) 32"45N 33°30'N 32°30'N 55°W 54°30 W 54 W FIGURE 3.—Chart showing location of continuous seismic sparker profiles made on Lynch cruise 710-78 (selected records, bounded by points A through X, shown in Figures 5-9; dotted line denotes airgun profile from Vema cruise 26, shown in Figures 4A and 13; depths shallower than 2910 fathoms given in hundreds of fathoms). centimeters of surficial sediment due to pretrip- ping of the piston in the core barrel (32°57.3'N, 55°06.6'W, 5359 m, or 2930 fm, depth). These materials were supplemented by four L-DGO cores (for locations see Figure 1 and also Horn et al., 1971, their fig. 5). Two are located on the plain proper, northeast of the seamount: R/V Atlantis core 153-141 (33°26.5'N, 53°48'W; 5350 m, or 2925 fm, depth; 1025 cm core length; also described by Ericson et al., 1961), and R/V Vema core 22-231 (33°29'N, 54°10'W; 5535 m, or 3026 fm, depth; 1507 cm core length). A third is lo- cated on the surrounding abyssal hills to the east of the southern plain: R/V Vema core 26-6 (32°42'N, 52°36'W; 5207 m, or 2847 fm, depth; 587 cm core length). The fourth was recovered on the southernmost fringe of the plain southwest of the Congress: R/V Vema core 26-9 (31°43'N, 56°12'W; 5546 m, 3032 fm, depth; 609 cm core length). The lithofacies of the cores are based on visual observations of split cores; only Lynch core 710- SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES 78-1 was x-radiographed. Core analyses also in- cluded textural measurements (percentages by weight of sand, silt, and clay fractions); these data are presented in Table 1. Compositional descrip- tions were made on a total of 75 samples taken from the 5 cores and presented in Tables 2 to 5. Mineralogical studies included the following: (1) estimated proportions of components of sand and coarse silt (>15 /mi) sizes (Table 1) using petro- graphic (transmitted and reflected light) and bin- ocular microscopes; (2) x-ray diffraction of the clay and silt fractions; (3) x-ray fluorescence W VEMA 26 4 5 6 Congress j | Seamount P /v. \\(D i 2,° hours , i 1 ,i ! I: i- I 090° J 8 August 1968 J/§ « y§ OOf 01 0.2 I 03 , 04,, \ "ft , K 7 1 —7 '■,#"»!' T -NORTH L X SW NEvA/NORTH SOUTH w FIGURE 8.—Seismic profiles showing the acoustically layered reflection pattern of the Sohm Abyssal Plain sediments surrounding the Congress and Lynch seamounts; this facies abuts directly against topographically opaque hills (profiles M-N, O-P) or the acoustically transpar- ent pattern (profiles G-H, I-J), or both; the layered facies is commonly disrupted by a reflection-free, diapir-like pattern of unknown origin (for other explanations see legend for Figure 5). 12 SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES WEST EAST NW SE WEST EAST H FIGURE 9.—Seismic profiles showing acoustically transparent sediment north of the northern Congress peak; the internal reflectors (IR) are areally more limited than in the perched basin, compare with profiles in Figure 7 (for other explanations see legend for Figure 5). This layered series directly abuts the base of the seamounts, in sectors where the acoustically trans- parent layer is absent (Figure 7, profile J-K, and Figure 5, profile E-F). The Sohm Abyssal Plain almost always dis- plays the acoustically layered facies overlying acoustically transparent or opaque seismic reflec- tion patterns; we presume the latter to be base- ment. Sediment thicknesses range from about 0.15 to 0.5 seconds (one-way travel time) between areas of relief on the basis of Vema (Figure 4) and Lynch profiles. The contact between the layered and the opaque basal seamount flank facies is well illustrated in the 3.5 kHz records obtained while underway to and from CTD Station 2 on the southern Congress peak (Figure 10, profile c). NUMBER 11 13 2l60fm (3950m)£§ 2200fm^ (4023m)«f I400fm (2560m) I A I500fm (2743 m) FIGURE 10.—Selected 3.5 kHz subbottom profiles obtained near (a, b) and west (c) of the southern Congress peak (location of profiles shown in Figure 2); compare the subdued layering of sediment in profiles a and b with the distinctly layered Sohm Abyssal Plain sediments abutting the base of the mount c (horizontal scale applies to profiles b and c; profile a made while ship was drifting). 55°W tV/vSECTION ABOVE * \ ' INTERNAL REFLECTOR , IR rV/iiSECTION WITHOUTS OR INTERNAL REFLECTOR 14 33°15'N 33°N SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES 54°30'W 54°W 32°45'N 32°30'N FIGURE 11.—Isopach map of acoustically transparent facies, showing two sedimentary config- urations: sections above internal reflectors (IR) and sections without an internal reflecting horizon (dense stippling pattern = sediment thicknesses >0.20 seconds of two-way travel time; lighter stippling = 0-0.20 seconds). PERCHED SEDIMENT BASIN.—The largest areal distribution and thickest section of unconsoli- dated sediment above the acoustic basement of the Congress Seamount is situated to the west of its northern peak (Figures 11, 12). At this location, sediment has accumulated in a saddle between the two major peaks in a depression bounded to the west by a nearly continuous topographic ridge (Figure 7, profiles J-K and N-O). Sediment in this perched basin comprises both distinct and weak acoustically layered material as illustrated by Figures 7 (profile J-K) and 13. Maximum thickness of this material ranges to about 520 m, assuming a velocity of 1.90 km/sec (cf. Houtz, 1974). Three distinct and continuous reflectors forming the top 75 m thick section in Lynch profiles are an acoustic artifact, i.e., multiple reflections from the sediment-water interface caused by several bubble pulses produced by the seismic sparker. The high-resolution 3.5 kHz rec- ords along the same traverse show marked lateral variation rather than continuity of the upper sediment layers (Figure 10). Within this large sediment basin, another series of distinct multiple reflectors (hereafter referred to as internal reflectors, IR) occur at about 200 m below the sediment-water interface (Figure 7, arrow on profile J-K; Figure 13). This IR, unlike the reflecting horizon at the sediment-water in- terface, is probably composed of several horizons of distinct acoustic impedence, and it divides this thick sequence of weakly layered facies (Figure NUMBER 15 CONTOURED SEDIMENT THICKNESS BELOW INTERNAL REFLECTOR ( IR ) 33°15'N 33°N 55°W 54°30'W - 32°45'N 54°W 32°30'N FIGURE 12.—Isopach map of acoustically transparent facies beneath internal reflectors (IR); note that IR presence is restricted to regions north and west of Congress peak (dense hatching = thicknesses beneath IR >0.20 seconds of two-way travel time; light hatching = 0.10 to 0.20 seconds). 13). At the thickest point 285 m of sediments lie above (Figure 11), and 235 m lie below (Figure 12), this IR. OTHER AREAS NEAR CONGRESS AND LYNCH SEA- MOUNTS.—A depositional configuration compa- rable to the perched basin cited above (two layers of acoustically transparent sediment separated by strong IR) also occurs in the sector north of the northern peak of Congress (Figures 3, 9, 11, and 12). Elsewhere in the region surveyed, including the Lynch Seamount, smaller ponds of acousti- cally transparent sediment occur without the IR (Figures 6 and 11). The thick (380 m) accumulation of acoustically transparent material situated to the north of Con- gress thickens westwardly, and abuts the base of another large ridge (Figure 9, profile H-I). A short north-south seismic traverse west of the northern peak (Figure 8, profile I-J) shows thick- ening of the acoustically transparent facies in a southerly direction; the southern termination of this facies is abruptly truncated by the acousti- cally layered facies of the Sohm Plain. The great- est thickness of transparent series (about 140 m) on the Lynch Seamount lies northwest of the peak (Figures 6, 11), while thinner accumulations are found north and south of the peak. High- resolution 3.5 kHz profiles also show the indis- tinct, more poorly defined layering that charac- terizes the thin accumulation of acoustically transparent facies on the upper Congress flank (Figure 10, profiles a and b). 16 SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES ife^fi W ^7 mm It'.. ■ ! ' ? WPPHB Ij'.Hl 11|| CONGRESS | ,:ffiijfi SEAMOUNT i P mm* mm HUB »' |'Afl HW'fflJ aits ! si A? - ■ I < m o ?Flfer:fflS mi -26 JO n FIGURE 13.— Vema 26 airgun profiles of one sector of seafloor west of the Congress Seamount (for location see Figure 3, also cf. 4A), showing perched basin (p) and internal reflectors (IR); left profile is a high-pass filtered record that reveals layering in the "acoustically transparent" section above IR; right profile is a low-pass filtered record, in which layering above the internal reflectors IR, is absent; note that the reflector at the sediment-water interface at p is layered in both sections suggesting that this is an acoustic artifact (both profiles were made on 8 August 1968 between 1100 and 1300 hours; reproduced from Ewing et al., 1974, with permission from the Lamont-Doherty Geological Observatory). Core Lithologies: General Description In this and the following sections we describe the petrologic characteristics of five cores in the following sequence (Figure 1): southern Sohm Abyssal Plain proper {Atlantis 153-141, Vema 22- 231, and Lynch 710-78-2); on, and adjacent to, the surrounding flanking hills (Vema 26-6 and 26-9); and perched basin on the lower flank of the Congress Seamount {Lynch 710-78-1). The Atlantis core is by far the coarsest of the five and includes numerous, distinct, coarse sand layers, particularly in the lower two-thirds of the section (Ericson et al., 1961, fig. 11). Contacts between the layers of sand, muddy sand, and mud are generally sharp, as revealed by distinct changes in grain size and color. Sand layers, some in excess of 10 cm, may consist of largely biogenic components (commonly foraminifera), or mixed biogenic and terrigenous fractions. In the lower part of the core, however, terrigenous material predominates. The sand layers, some graded, are NUMBER 17 probably turbidites. Colors, recorded only after the core had dried, are yellowish brown and olive grey. Vema core 22-231, while only 30 km to the west and lying about 185 m deeper than the Atlantis core, displays a significantly different lithology. It is composed almost exclusively of silty clay and clayey silt, with only rare thin silt and sand layers. Coarse sand layers are generally graded and mi- caceous and are interpreted to be turbidites. Some finer-grained sections display bioturbation, while others comprise varve-like silt laminations that occur irregularly and could represent fine-grained turbidites. The effect of bottom-current activity, however, cannot be discounted. From the Lamont core log description, the section consists of alter- nating moderate- to reddish-brown (5YR4/4), pale-brown (10YR6/2), and light-olive-grey (5YR5/2) units. Some darkly streaked zones ap- parent in split cores consist of iron sulfide-rich layers often related to burrowing structures. In contrast to the Vema 22 core, Vema 26-6 and 26-9 are fine textured, more homogeneous, and do not display distinct sand or silt laminae. They do, however, show localized patches of light, al- most cream-colored sections 1 to 5 cm thick. These patches have been interpreted as weath- ered volcanic products, possibly palagonite (un- published L-DGO core descriptions, 1969). Both cores recovered largely foraminiferal silty clay that includes only a minor fraction (I percent or less) of terrigenous or nonforaminiferal biogenic components. The cores display moderate-yellow- ish-brown (10YR5/4), moderate-brown (5YR3/ 4) and light-brown (5YR6/4) sections. Some units show intensely burrowed structures. Core Lynch 710-78-1, from the perched basin, most closely resembles Vema core 22-231 in terms of vertically varying grain size (clayey silt and silty clay) and a cyclic pattern of sedimentation (Figure 14). The down-core variation in the Lynch core is characterized by fine-grained ooze (30% pelagic organisms) of different colors: pale-brown (10YR6/2) mud, alternating with yellowish- brown (10YR4/2) and moderate- to reddish- brown (5YR4/4) mud layers. Most of these layers range from about 20 to 50 cm thick, although two of them exceed 1 m. Textural analyses (Table 1) show that the yellowish-brown (and in some cases grey) layers are clayey-silt ooze dominated by silt, with minor proportions of sand. The moderate- to reddish-brown ooze layers tend to be thinner and somewhat finer grained (silty clay) than the yellowish-brown muds (as shown in Figure 14). Two features of this core are a distinct angular truncation at about 435 cm, and highly bioturbated zones with large vertical and/or hor- izontal (transverse) burrows located at about 210, 330, 500 and 540 cm (sections at 210, 330, and 500 cm are noted on x-radiographs, Figure 15). A distinct vertically graded sand (largely foramini- fera) to mud layer in the pale-brown zone is clearly observed between 303 and 290 cm; an x- radiograph (Figure 15) shows that this unit ac- tually fines upward to about 275 cm. It is inter- preted as a turbidite. Rates of Sediment Accumulation Five radiocarbon dates were obtained on sam- ples collected from core Lynch 710-78-1 (Figure 14). Results, listed below, are in years before present, and centimeters refer to sample depth below core top: 11,070±85 at 12-20 cm; ll,160±500at 20-30 cm; 22,390±470 at 170-185 cm; 33,760±800 at 260-274 cm, and >43,000 at 310-325 cm. Rates of accumulation in centime- ters per 1000 years or cm/ka, between these dated horizons are approximately <2, 13, 13.5, 8, and <4.5; a time integration of the overall sedimen- tation rate is approximately 7 cm/ka. It is note- worthy that the highest rates apply to the yellow- ish-brown mud immediately below the signifi- cantly more slowly accumulating Holocene core top (Figure 14). This yellowish-brown layer ac- cumulated between 40 and 165 cm from approx- imately 20,000 to 12,000 B.P., about the time of the most recent eustatic low sea level stand, fol- lowed by a rapid sea level rise. This was a cold period during and following major glaciation at the end of the Pleistocene (Laine, 1978). We also obtained two dates from Vema 22-231, on the plain proper, nearest the Lynch core: 31,700±1400 SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES ► 10 YR 4/2 H C l4 11070 ± 85 (12-20cm) i!_3ll5.£!T'.bi_ J C14 III60±500 (20-30cm) \ (34-36cm)-< MODERATE TO REDDISH BROWN (80-82cm)- • YELLOWISH BROWN r14 22390±470 (I70-I85cm)- 5 YR 4/4 5 YR 4/4 •Vaguely laminated - Transverse burrows -Vertical burrows (I95-I97cm)- (225-227cm)- Highly burrowed '— Bioturbation ► 10 YR 6/2 CU 33760 ±800 (260-274cm) J Blo^rbation Cl4> 43000 (310-325, (297-299cm). (299-30lcm:^300 (30l-303cm',-J (304-306cm)' . ' \ "Graded Foram sand !\ M» -Transverse burrows V 5 YR 4/4 (345- 347cm )- (373-375cm)- 10 YR 6/2 — Highly burrowed HI SM*M>-' ooooooo FIGURE 14.—Detailed log of Lynch core 710-78-1 collected in the perched ba- sin on west flank of the Congress Sea- mount (see Figure 2), showing princi- pal sediment types, color, and physical and biogenic structures, and the posi- tion and age (in years before present) of radiocarbon-dated samples; (5YR4/4 = moderate to reddish brown; 10YR4/2 = yellowish brown; 10YR6/2 = pale brown; dark vertical bars at right = position of x-radio- graphed core sections illustrated in Figure 15; see Table 1 for grain-size analyses of selected segments identi- fied at left of log). (434-435cm)- (443-445cm)" (542-544cm)-< 550 (557-559cm)-<559 ► 5 YR 4/4 .Sharp, angular facies break > 10 YR 6/2 Highly mottled 5 YR 4/4 S ( Large mottles . highly bioturbated NUMBER 11 19 -^■felgife 190 FIGURE 15.—X-radiographic prints of selected sections of Lynch core 710-78-1 collected in the perched basin on the west flank of the Congress Seamount (position of sections in centimeters below core top as shown in core log in Figure 14), revealing various degrees of bioturbation including some horizontal and vertical burrows, and completely reworked mottled sections, and a graded turbidite consisting of bioclastic sand to clayey silt (303 cm to 280 cm). 20 SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES at 44-45 cm, and 37,500±1500 at 89-100 cm. Rates of accumulation between these dated ho- rizons are about 1.5 and 7.5, and the averaged rate is about 2.5 cm/ka, lower by a factor of three than the Lynch perched basin section. In both cores, the presence of sharp contacts between layers indicate probable hiatuses. The tops of these two cores may not have been recovered and, therefore, computed rates should be used with caution. In order to compare our values with the rates of sedimentation for areas bounding the Sohm Plain we examined data from two Deep Sea Drilling Project and one Giant Piston Core sites. These data reveal extreme areal variability in the accumulation of sediment. The rates in the south- ern Sohm Plain are comparable to averaged long- term accumulation values (about 12 cm/ka) for the Quaternary and Upper Pliocene sections forming the abyssal plain at the base of the Nashville Seamount (JOIDES drill site 382) about 150 km northwest of the Congress (Tuch- olke and Vogt, 1979). In contrast, extreme vari- ations occur in the hills bordering the southern sector of the plain. On the lower flank of the Mid- Atlantic Ridge to the east, near Vema 26-6, JOIDES drill site 10 revealed a considerably reduced Quaternary section. Very low sedimen- tation (<1 cm/ka) perhaps affected by sediment compaction, is recorded there (Peterson, Edgar, et al., 1970). To the west of the plain, however, radiocarbon dates from a Giant Piston Core (GPC site 5) reveal rates two orders of magnitude higher than were measured near the Congress Seamount (Silva et al., 1976). In brief, these highly variable rates can be explained in large part by regional removal and subsequent redep- osition of sediment resulting from large-scale wa- ter-mass circulation (see earlier sections on sedi- ment acoustic facies distribution and Figures 4 to 12). Textural and Compositional Characteristics GRAIN SIZE.—Size analysis included the sand (>63 jtim), silt, and clay (<2 /z,m) fractions. These results are presented in Table 1, and graphically depicted by bars in Tables 2 to 5. The upper third of A tlantis core 153-141 consists of low proportions of sand; samples below 400 cm, however, contain abundant (locally to >65%) sand. The silt/clay ratio in the upper third of the core is highly variable, recording alternating amounts of silty clay and clayey silt. Below 400 cm the core is generally characterized by greater proportions of silt than clay (Table 2). In contrast, Vema core 22-231 is largely mud (silt and clay mixtures, Table 1) and contains a few, thin sand layers (Table 3). The silt/clay ratio is usually greater than 0.5, and the core includes larger proportions of silty clay than does the Atlantis core. Vema cores 26-6 and 26-9 are even finer grained than the above, and contain only traces of sand and a range of low to very low (0.57-0.14) silt/clay ratios (Table 4). This analy- sis highlights the prevailing silty clay texture and generally homogeneous nature of these two abys- sal hill cores. Lynch core 710-78-1 recovered in the perched basin on the Congress Seamount flank is most similar to, but somewhat siltier than, Sohm Plain Vema core 22-231 (Table 5). One distinct turbidite (sand to clayey silt) occurs at about 300 cm (Figures 14, 15). Overall, this core is characterized by highly variable silt/clay ratios ranging from about 0.5 to 2.7 (Table 1). COARSE FRACTION COMPOSITION.—Size analyses show that, with the exception of Atlantis core 153- 141, most samples contain a low proportion (<1% or trace) of sand. A semiquantitative estimate of the proportion of the 16 most common compo- nents of the coarse fraction (>15 jum)—coarse silt and sand—of selected samples is depicted in Ta- bles 2 to 5. The coarse fraction of the upper part of core Atlantis 153-141 is dominated by sand of biogenic origin (largely planktonic foraminifera) and car- bonate fragments, with smaller amounts of other bioclastic, terrigenous, and apparently volcanic components (Table 2). In the lower two-thirds of the core, quartz, heavy minerals, and mica of sand size prevail. Volcanic products (optically NUMBER 11 21 identified palagonite, sideromelane, and chloro- phaeite) are found locally, and occur as present- to-few throughout (Table 2). The coarse-fraction composition of Vema 22- 231 is much more variable than for the Atlantis core: some samples are dominated by biogenic components, others by terrigenous, and some in- clude both (Table 3). Moreover, the proportion of volcanic products is also highly variable, with the highest concentration near the core top (prob- ably due to winnowing). The biogenic fraction is largely carbonate fragments (predominantly bro- ken foraminifera) and coccoliths rather than un- broken foraminiferal tests. Manganese micro- nodules, siliceous aggregates (probable volcanic alteration products), and plant fibers are irregu- larly distributed in this core (Table 3). Cores Vema 26-6 and 26-9 are distinct from the above cores primarily by their uniformly high proportion of volcanic and related products, in- cluding manganese micronodules (L. A. Barnard, pers. comm.; see also Figures 16, 17), siliceous aggregates (Figure 16), devitrified glass, palagon- ite, zeolites and altered feldspars (Figure 17), and pyroxenes. This assemblage argues against a dom- inantly diagenetic origin. Moreover, evidence for reworking is provided by the presence of quartz along with high proportions of carbonate frag- ments and coccoliths in the upper parts of the cores (Table 4). Core Lynch 710-78-1 is most similar to core Vema 22-231 in that the coarse component assem- blages vary vertically, and are dominated by carbonate fragments (broken foraminifera), whole foraminiferal tests, and coccoliths; siliceous aggregates and brown mica are also present, with smaller proportions of weathered glass, spicules, radiolaria, quartz, and other light and heavy minerals (Table 5). The difference between Sohm Plain core Vema 22-231 and Lynch core 710-78-1, which was obtained at more than 325 m above the plain on the flank of Congress, is that the latter contains substantially lower amounts of quartz, no plant matter and a somewhat higher proportion of volcanic products (Table 5), includ- ing manganese micronodules. Petrographic mi- croscope and Scanning Electron Microscope (SEM) analyses of the Lynch coarse silt fraction show variable proportions of mica, plagioclase feldspars, siliceous microfossils (Figure 18A,B), cal- careous nannofossils (Figure 18C,D), foraminiferal fragments (Figure 18C,D), dolomite rhombs (Fig- ure 18E), and angular volcanic shards (Figure 18F). Palagonite, sideromelane and/or chloro- phaeite are found in every sample examined throughout the core. The abundance of these volcanic products, usually of coarse silt-size, varies considerably from sample-to-sample (Table 5). Many of the carbonate microfossils in the Lynch core are poorly preserved (Figure 18), possibly from dissolution and recrystallization. The mod- erate- to reddish-brown layers contain a much higher proportion of siliceous microfossils and a lower proportion of coccoliths than the yellowish- brown layers (Figure 18A). In many samples of the latter sediment facies, as well as in the graded bed (at 300 cm), mixed faunas comprising Late Quaternary (Figure 19A), Tertiary (Figure 19B,C), and Upper Cretaceous (Figure 19D) are found (C. C. Smith, pers. comm., 1980). The benthic fora- minifera in the graded bed at about 300 cm provide evidence of downslope displacement of at least 1000 m; their original habitat is normally about 3000 to 4000 m (S. Streeter, in litt., 1979). Other layers in the cores also indicate displace- ment of benthic foraminifera. Further insight on modification of faunal con- tent related to dissolution associated with depth is provided by Lynch core 710-78-2, essentially a surficial Sohm Abyssal Plain seafloor sample. This small core is a clayey silt with a trace of sand (>63 jitm) that consists largely of foraminifera and mica grains; it is texturally and mineralogically analogous to the upper samples of Lynch core 710- 78-1. The foraminifera, however, tend to be some- what more poorly preserved than in Lynch core 1; thin tests that break easily during preparation may have been affected by deposition below the carbonate compensation depth (CCD). FINE SILT AND CLAY FRACTION COMPOSITION.— X-ray diffraction analyses of the less than 2 /xm size fraction from all cores examined confirm the 22 SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES FIGURE 16.—Photomicrographs of selected samples from core Vema 26-9: A and c, sand fraction from 435-437 cm and 503-505 cm, respectively, showing manganese micronodules (m), feldspars (f), quartz (q) and siliceous aggregates (a); B and D, sectioned micronodules from 453- 457 cm and 503-505 cm, respectively, mounted in Lakeside© cement (matrix) and viewed in reflected light; the manganese-rich grains show no obvious internal structure (scale in /xm). patterns of clay-mineral distribution found in this general region by Biscaye (1965). Core Atlantis 153-141 (Table 2) illustrates that the clay-mineral suite throughout is dominated by the mica group (illite, muscovite). Kaolinite and chlorite together account for about a third of this size fraction, while feldspars and calcite ac- count for the remainder. Montmorillonite (used in this paper synonymously with smectite) occurs only in trace amounts. The silt-fraction suite is dominated by plagio- clase, dolomite, and calcite. In addition, mica, kaolinite and chlorite, quartz, K-feldspar, and amphibole are generally present in variable pro- portions. The clay-size fraction from core Vema 22-231 (Table 3) is generally comparable to the Atlantis core. The composition of silt is also similar, but shows a slightly higher amount of dolomite. The clay-mineral fraction of cores Vema 26-6 and 26-9 (Table 4) is markedly different from the previously discussed cores in that the proportion NUMBER 23 FIGURE 17.—Scanning electron micrographs of selected grains from Vema 26 cores: A, manganese micronodule ( Vema 26-6, 474-476 cm); B, palagonite glass ( Vema 26-9, 503-505 cm); c, zeolite (phillipsite, note twinning) in Vema 26-6, 474-476 cm; D, plagioclase feldspar, weathered and iron-coated ( Vema 26-6, 45-47 cm) (scale in jttm). 24 SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES FICURE 18.—Scanning electron micrographs of selected samples from Lynch core 710-78-1: A, B, dominantly :iliceous fraction (silicoflagellates and diatom fragments) from the moderate to reddish-browi. and pale brown facies, 345 and 374 cm, respectively, from the top of the core; c, D, carbonate fraction dominated by coccoliths and recrystallized foraminifera from the yellowish brown ~acies, 14 and 302 cm, respectively; E, dolomite rhomb, 14 cm; F, volcanic shard, 226 cm (scale given by thin horizontal bar in jum). NUMBER 11 25 FIGURE 19.—Scanning electron micrographs of selected microfossils in Lynch core 710-78-1: A, Gephyrocapsa oceanica, a typical Quaternary coccolith species (at 543 cm from the top of the core) commonly found throughout this core and mixed with the reworked species illustrated in the other components of this illustration; B, Discoaster variabilis (arrow) of Middle Miocene to Pliocene age (at 301 cm); c, Discoaster (arrow 1) of probable Pliocene age and Prediscosphaera cretacea of Cretaceous age (arrow 2) at 543 cm; D, Watznaueria barnesae (arrow) of Cretaceous age at 543 cm (scale in fim). of montmorillonite is generally over 25%, and ranges up to about 50%. Moreover, the relative percentages of kaolinite plus chlorite are some- what higher than those of the mica group. With regard to the silt fraction, these two Vema cores generally contain higher mica, kaolinite plus chlo- rite, and plagioclase, with notably lower calcite than the Atlantis and Vema 22 cores. The Lynch core 710-78-1 (Table 5) clay-size samples contain montmorillonite in amounts greater than in Atlantis or Vema 22-231 cores, but lower than the two Vema 26 cores. The mica group, moreover, is more abundant than the kaolinite and chlorite components, and the pro- portion of quartz is about the same as in the Vema 26 cores. In the silt fraction of the Lynch core, the 26 SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES dolomite is usually present, calcite amounts are extremely variable, and the proportions of mica, kaolinite, and chlorite are more similar to the Vema 26 cores. Amphibole is not detected, but samples from the four other cores yield traces to 5%. X-ray diffraction mineralogy reveals a gen- erally lower calcite content in the silt- and clay- size fractions, and X-ray fluorescence shows a substantially higher iron and, in some instances, manganese content in the moderate- to reddish- brown layers of the Lynch core. X-ray fluorescence analyses of manganese and iron in the clay-size fractions from the five cores reveal rather consistent concentrations of iron throughout these samples. The relative amount of manganese, however, is extremely variable (values expressed as counts per 0.1 mg per 200 seconds; see Tables 2-5): in the Atlantis core, values range from 3.2-7.8; Vema 22, 6.2-10; Vema 26-9, 8.3-32.5; Vema 26-6, 7.1-60.9; and Lynch-l, from 1890 to 4620. We interpret the thousand- fold increase from the Atlantis to the Lynch core to be a result of the latter's proximity to the volcanic Congress Seamount. In contrast, the lowest values in the Atlantis core result from its greater distance to volcanic sources and dilution by terrigenous and biogenic components. It may be noted that energy dispersive x-ray analysis of a typical mi- cronodule (Figure 16) of the sand-size fraction from the Lynch-l core (Table 5) highlights the values of the two most abundant elements: man- ganese, 27,555, and iron, 13,365 (lesser amounts of copper, nickel, zinc, chromium, and cobalt also are measured). These results are comparable to electron microprobe analyses from Vema 26 cores that indicate some micronodules of coarse silt and fine sand size (Figure 1 7A) are comprised largely of manganese, iron, and nickel. We note that these manganese micronodules are associated with palagonitized glass (Figure 17B), zeolites (probably phillipsite, Figure 17c), smectites (montmorillonite, Table 4), and plagioclase feld- spar (Figure 17D). This assemblage occurring in Vema 26 cores 6 and 9 is generally diagnostic of altered volcanic materials (cf. Peacock and Fuller, 1928; Arrhenius, 1963; Honnorez, 1978; Stone- cipher, 1978; Scott et al., 1979). Some of these products, such as smectite, may also have been derived from land sources. Water-Mass Movement The general circulation in the Atlantic, sum- marized by Wiist (1949; and Heezen and Hollis- ter, 1971), shows the dominance of the Arctic and Antarctic water masses on abyssal circulation. A recent hypothesis concerning the circulation in the Northwest Atlantic proposed by Worthington (1976, fig. 11) shows a major NNE-SSW elon- gated gyre related to the Gulf Stream system. This large clockwise-rotating gyre is located east of Cape Hatteras, extends to about 40° W longi- tude, and is positioned over the Bermuda Rise and most of the Sohm Abyssal Plain. It is believed to influence the circulation pattern of the deeper waters, including those above the southern Sohm Abyssal Plain (Schmitz, 1977; Laine, 1978). Two CTD vertical profiles through the water column were made on the western flanks of the Congress Seamount to ascertain the nature of the intermediate and deep waters in the study area. CTD Station 1 was situated on the perched basin about 3 km east of core Lynch 710-78-1, and CTD Station 2 was obtained on the southern peak (Figure 2). Three distinct water masses are rec- ognized: North Atlantic Central Water, extend- ing from the surface to a depth of about 930 m (510 fm); a second layer, about 170 m thick, composed in part of Mediterranean Water, rang- ing from 930 m (510 fm) to about 1100 m (600 fm); and below this, North Atlantic Deep Water, extending almost to the bottom of the profiles on the flanks of the seamount. The presence of Ant- arctic Bottom Water (-0.5°C, 34.65%o) is sug- gested by the lowermost readings of the temper- ature-salinity curve. The profiles indicate the pos- sibility of slight mixing of Antarctic Bottom Wa- ter with North Atlantic Deep Water at, and just above, the sediment-water interface on the lower flanks of the seamount and on the abyssal plain (D. Greenewalt, in litt., 1979). While these data are admittedly limited, they tend to support the NUMBER 11 27 oceanographic model of Wiist (1949), Heezen and Hollister (1971, fig. 9.45), and others that show the northward extension of the Antarctic Bottom Water to the region encompassing the souther- most part of the Sohm Abyssal Plain. Since current-meter data are lacking, an indi- cation of water flow and direction is obtained by noting the nature of bedforms recorded by com- pass-oriented bottom photographs. Three camera stations located around the Congress Seamount (Figure 2), reveal predominant transport from SSE to NNW. This flow direction at the three localities was determined from ripple marks (Fig- ures 20, 21), craig-and-tail features (Figure 22), coarse lag accumulated against obstacles on the bottom (Figure 20A), and asymmetrically de- formed mud structures (Figure 22). Although this bottom transport pattern may probably represent only recent and short-term dispersal, it is note- worthy that (a) the isopach map indicates local- ized thickening of sediment south, north, and west of both Lynch and the northern peak of Congress (Figure 11), and (b) selected seismic profiles show accumulation of transparent acous- tic series east of obstacles (Figure 9, profile H-I). These latter observations could be interpreted as the result of long-term sediment dispersal trends toward the north and west quadrants. These patterns tend to support the oceanographic model of a westerly-directed return flow of the southern leg of the Gulf Stream Gyre above the Sohm Plain study area (Worthington, 1976; Laine, 1978). Discussion and Conclusions A review of sedimentation models in the abys- sal plains of the Atlantic and other world oceans reveals an emphasis on gravitative processes, largely turbiditic, and long distance dispersal of terrigenous material, primarily sands (e.g., Hee- zen and Laughton, 1963). There is, in fact, ample documentation for the Northwest Atlantic Basin that the processes of turbidity currents and re- lated gravity flows of sediment account for a significant proportion of abyssal plain deposits, especially the sands (Horn et al., 1971; Pilkey et al., 1980). In addition to this particular land- derived component, most workers also call atten- tion to the contribution from clastic particulate matter, of both terrigenous (fine silt and clay) and biogenic (largely planktonic tests) origin, which settle from suspension. The deposits from these two differing modes of sedimentation have been interpreted from seismic profiler data and core analyses. Sediments lacking internal acoustic reflectors ("acoustically transparent" layers) are generally inferred to be pelagic or resuspended deposits which may be composed of well-bedded clays (Fox et al., 1967), foraminiferal ooze with clays (Taylor and Hekinian, 1971), or a complex mixture of clay, sand, and carbonate-rich layers (Silva et al., 1976). The relative proportion of carbonate tests within these pelagic sediments, largely foraminifera and coccoliths, tends to be highest in areas shallower than the carbonate compensation depth (CCD). In contrast, distinct acoustic reflectors revealed by seismic profiler data and coarse silt- to sand-sized sediment re- covered in cores are generally thought to have formed as turbidites. Recent studies show that clay- to sand-size sediments emplaced by either turbidity currents or pelagic suspensate deposition can be, and prob- ably in most instances are, modified by the action of bottom currents (Heezen and Hollister, 1971; Laine, 1978). Evidence for this bottom-current erosion in the southern Sohm Abyssal Plain study area includes lineated bedform features revealed by photographs (Figure 22), textural properties of the coarser-grained fractions (Hubert, 1964), and the age measurement (in excess of 30,000 yrs B.P.) near the top of core Vema 22-231, which reveals a truncated upper sediment layer. All of the above depositional mechanisms em- phasize distal transport. We note in abyssal plain studies, however, that the role of proximal or more local-intrabasinal sediment source and dis- persal (in particular, the displacement of volcan- ically derived sediments, cf. Heath and Dymond, 1977) is for the most part neglected. This is indeed surprising if one views a physiographic chart of 28 SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES FIGURE 20.—Lynch 710-78 camera station 1, oblique view bottom photographs on the northern Congress peak (between 3110-3290 m, location shown in Figure 2): A, subdued asymmetric ripples of sand and mud transported toward NNW (arrow), note coarse lag against small outcrop in background; B, mixed biogenetic-volcanigenic sediment veneer of bedrock, presum- ably volcanic, note distinctly rippled sediment in background (compass diameter = 6.3 cm). NUMBER 11 29 FIGURE 21.— Vema 26 camera station 7 vertical-view bottom photographs on the southern Congress peak (between 2815 and 2850 m, location shown in Figure 2): A, B, volcanic pillows thinly covered by sediment mantle; c, very coarse biogenic-volcanigenic sediment and rock fragments; D, asymmetric ripples of muddy sand transported almost due north (arrow) (compass diameter = 7 cm; photos courtesy of Lamont-Doherty Geological Observatory of Columbia University). the oceans (Heezen and Tharp, 1968), which reveals a ubiquitous, highly variable vista domi- nated by positive topographic relief (ridges, frac- ture zones, abyssal hills, seamounts, knolls, etc., many shown on Figure 1) of predominantly vol- canic origin. It is pertinent with respect to the problem of proximal versus distal provenance that our study area is situated in the southermost section of the Sohm Abyssal Plain because (a) it has received a dominant proportion of its sediment fill by tur- bidity-current processes, with materials derived largely from the Canadian Maritime Provinces (Horn et al., 1971); (b) it underlies the region of densest suspended particulate matter, or nephe- loid layer, concentration (>3000 jug/cm2) in the North Atlantic Ocean (Biscaye and Eittreim, 1977, fig. 5); (c) it is influenced by bottom-current activity (bottom photographs and seismic data, this study) resulting from large scale bottom-wa- ter circulation (Worthington, 1976; Laine, 1978); and (d) it lies near the geographic center of the 30 SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES f / '' ' / ,' - ','',<■■, /"'''' TAv . ' ' ■ B B ' "'" U^M Jj FIGURE 22.—Lync/z 710-78 camera station 2, oblique-view bottom photographs on Sohm Abyssal Plain west of the southern Congress peak (about 5365 m, location shown in Figure 2): A and B reveal small craig-and-tail features and asymmetrically deformed mud structures oriented toward the northwest (arrows) (compass diameter = 6.3 cm). NUMBER 11 31 Northwest Atlantic Basin, remote from land sources and almost completely surrounded by major volcanic features (including the lower flanking hills of the Mid-Atlantic Ridge, Ber- muda Rise, and Kelvin and Corner Rise sea- mounts, see Figure 1 and also Ewing et al., 1973). Thus, the Southern Sohm Abyssal Plain is an appropriate area to evaluate the role of more proximal and volcanically derived sediments. On a global scale, abyssal plain sediments con- sist of variable mixes of the following: (a) land and continental margin-derived elastics (includ- ing ice-rafted debris) and/or bioclastic materials; (b) suspensates of clastic (including wind-blown) and biogenic origin; (c) authigenic sediment and diagenetic products; and (d) volcanically derived materials. As in the case of most abyssal plains that have direct access to both land and conti- nental margin-derived materials, the southern Sohm Plain piston cores reveal a dominant terri- genous fraction. This is best illustrated by core Atlantis 153-141. As in many other cores from this large abyssal plain, the significant proportion of terrigenous material was initially emplaced by turbidity current flows (Horn et al., 1971; Pilkey et al., 1980). All four Sohm Plain cores we ex- amined also show an important pelagic compo- nent (e.g., core Vema 22-231) that is related to the well-defined distribution pattern of the suspen- sion-rich, benthic nepheloid layer (Biscaye and Eittreim, 1977). Nevertheless, we have found that all cores have at least some silt- and even sand- size volcanic components (Tables 2-5). Despite the significant amounts of land-derived turbidite sediments that dominate abyssal plain cores At- lantis 153 and Vema 22, significant amounts of volcanic materials, including palagonite and manganese micronodules, are present (Tables 2, 3). The proportion of these volcanically derived products is related to the proximity of the cores to volcanic physiographic features of varying re- lief; see, for example, the large concentration of volcanic products near the top of core Vema 22- 231 (Table 3) and the position of this core relative to volcanic physiographic highs (Figure 4B). Our study clearly demonstrates that cores Vema 26-6 and -9, from an area near the margins of the Sohm Plain bounded by abyssal hills and shielded from clastic land sources, contain a considerably enhanced fraction of volcanically related material of clay as well as silt and sand sizes (Table 4). These components include manganese microno- dules, zeolites, palagonite, chlorophaeite, sidero- melane, and smectite. Some of these materials are almost certainly supplied from the submarine weathering of basalt and its alteration products (Arrhenius, 1963; Nayudu, 1971; Honnorez, 1978; Scott et al., 1979; Furnes, 1980). We do not exclude the possibility that some of these com- ponents were derived from diagenetic remobili- zation (i.e., manganese micronodules, see Scott et al., 1979) and/or from distal land sources (smec- tite). As a test of the true importance of this al- lochthonous volcanically derived component in the Sohm Abyssal Plain setting, we examined core Lynch 710-78-1, which was recovered from a sediment basin perched about 400 m above the abyssal plain on the flank of Congress Seamount (Figures 2, 7). About 500 m of sediment has accumulated in this elevated and isolated basin. The principal significance of the Lynch core loca- tion is that it is shielded from the direct influence of large-scale, bottom-hugging turbidity currents, which tend to follow along deeper contours of the abyssal plain. From the Lynch seismic survey, it is apparent that distribution of the ponded sediment series is largely controlled by local topography and bot- tom-water flow. In this respect, the perched basin serves as a particularly valuable example (Figures 11, 12). Most of the ponded sequence in this basin, while acoustically transparent to the spar- ker seismic frequency (about 100 Hz), is in effect layered (Figure 13) when insonified with airgun frequencies (about 20 Hz). Sections of core Lynch 710-78-1, from this pond, particularly the mod- erate- to reddish-brown layers (see horizons la- beled 5YR4/4, Figure 14), provide evidence of pelagic deposition. The upper reddish-brown layer, deposited in post-Pleistocene time, and sim- 32 SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES ilar deeper horizons (Figure 14) are characterized by the following: (1) clay mineral assemblages of the type occurring throughout this North Atlantic region (Biscaye, 1965) but also including an en- hanced fraction of montmorillonite; (2) an abun- dance of planktonic microfossils including fora- minifera (often poorly preserved and recrystal- lized), radiolaria, and other siliceous microfossils whose proportions vary markedly vertically; (3) a very fine-grained texture (largely silty clay); (4) accumulation rates that for the most part are less than 3 cm/ka; (5) vague laminations and in- tensely bioturbated layers (Figure 15); and (6) the presence of siliceous agregates (Table 5) that may have been produced by benthic activity accompanying slow deposition, or may be of bio- genic opaline derivation (T. C. Huang, pers. comm., 1980). To help explain the above, we recall that the study area lies within the region of highest sus- pended particulate matter concentration in the North Atlantic Ocean (Biscaye and Eittreim, 1977). Moreover, Congress Seamount is located adjacent to a zone of marked bottom-current activity related to the westward-directed return flow of the southern Gulf Stream Gyre (cf. Worth- ington, 1976). Three bottom photograph stations (Figures 20-22) provide evidence for seafloor ero- sion, inferring flowing water on and around the seamount. Thus, the pelagic deposition in the perched basin is complicated by the interaction with moving bottom water in this region. In addition to this pelagic transport regime, the marked cyclic nature of core Lynch 710-78-1 (Figure 14) shows that other modes also were responsible for deposition in this pond during the Quaternary. In comparison to the reddish-brown layers, the thicker, but somewhat less oxidized, yellowish-brown ooze layers (Figure 14, see hori- zons labeled 10YR4/2) have (a) somewhat larger proportions of better preserved planktonic fora- minifera and calcareous nannofossils, and (b) tests of the latter, many of which are obviously re- worked from older strata and dated from Pliocene to Upper Cretaceous (Figure 19); also, they (c) are coarser grained (clayey silt); (d) may display graded bedding (Figure 15, 303-280 cm); and (e) record much higher rates of accumulation (locally 13 cm/ka) in the Upper Pleistocene. Such layers probably account for some subdued, discontin- uous reflectors noted on 3.5 kHz profiles (Figure 10). Emplacement of this sediment type, partic- ularly the graded beds, is almost certainly by gravitative mechanisms, including turbidity cur- rents. The ubiquitous, but irregular, abundance in this Lynch core of coarse silt- to sand-sized volcanic products, including primary (sideromelane, an- gular glass shards) components and secondary alteration derivatives (palagonite, chlorophaeite, zeolites), plus the importance of montmorillonite in the clay fraction indicate a steady, important supply of weathered material to the pond from an igneous source (Table 5). These products are associated, in some instances, with reworked mi- crofossils as old as Upper Cretaceous (Figure 19). The proximity of the sediment pond to the sea- mount and other adjacent basement rises suggests that these igneous features are major-source ter- rains. In addition to this igneous supply, material reworked from the older depositional cover drap- ing these basement rises also appears to serve as a sediment source (cf. Taylor and Hekinian, 1971). This volcanic-biogenic assemblage is simi- lar to that described by Van Andel and Komar (1969), Nayudu (1971), and others. Bottom pho- tographs show that currents on these bathymetric highs are capable of eroding and transporting weathered volcanic material along with microfos- sil-rich (pre-Quaternary to Recent) pelagic sedi- ments and the recently deposited suspensates (Figure 20). This combined regime of suspensate deposition and reworking by bottom-current ac- tivity has resulted in accumulation of mixed bio- genic-volcanigenic deposits on the seamount. These materials subsequently fail on the relatively steep slopes of the mount and adjacent highs and are resedimented to greater depths by a complex mechanism involving bottom current, creep and mass flow, including turbidity-current (cf. Stan- ley and Taylor, 1977). Specific examples of down- slope displacement are provided by the graded NUMBER 11 33 foraminiferal sand bed (Figure 14, base at 303 cm) and by the displacement of benthic forami- nifera to a depth of at least 1000 m below their normal range as recorded in various layers of the Lynch core. The perched sediment fill in the basin thus reveals a complex history of depositional pro- cesses: suspensates plus older locally derived pe- lagic deposits subsequently have been reworked by bottom currents and displaced downslope by gravitative mechanisms. Sediments derived from more distal sources supplement these ponded de- posits in the perched basin. These latter distal materials would likely include resuspended Sohm Abyssal Plain sediments transported by bottom currents (Biscaye and Eittreim, 1977; Laine, 1978). Wind-blown silt, some of which may in- clude a volcanic dust (cf. Huang et al., 1975, 1979; Sigurdsson et al., 1980), may also be a minor distal component. In addition, the mea- sured accelerated rate of sedimentation in the Lynch core during the period from about 20,000 to 12,000 years B.P. (Figure 14) almost certainly records large-scale oceanographic phenomena that affected extensive parts of the North Atlantic during the late Pleistocene. These factors influ- encing the transport of material from distal sources to the southern Sohm Plain include eus- tatic oscillations coupled with important changes in the composition and circulation pattern of deep-water masses associated with the advances and retreats of polar fronts (Schnitker, 1974, 1979; CLIMAP project members, 1976; Ruddi- man, 1977; Ruddiman and Mclntyre, 1976, 1979; Duplessy et al., 1980; and others). The perched sediment basin well illustrates a situation where there has been a concentration of an important contribution of volcanic-rich ma- terial emplaced by bottom-current erosion of the adjacent seamount and by coarse-grained mud flows, turbidity currents, and related submarine gravity flows. During a period of unknown du- ration, these mass flows resulted in the deposition of distinct volcaniclastic (possibly hyaloclastite) layers characterized by a distinct acoustic con- trast, the internal reflectors (IR), within the sed- iment series (Figure 13). The areally restricted distribution of the internal reflectors in the de- pressions west and north of the northern Congress peak (Figure 12) also indicates a local derivation and a probable volcanic origin. While no direct evidence for the origin or nature of these reflectors is available, similar seismic reflection profiles showing large acoustic impedences have been drilled west of the study area (Tucholke and Vogt, 1979; Bowles, 1980). This drilling revealed volcaniclastic layers. The IR acoustically "hard" layer can be traced almost uniformly across the perched basin (Figure 7, profile J-K); elsewhere around the Congress Seamount, the more restricted distribution of the internal reflectors is observed (Figure 9, profile A-B-C). The well-defined IR horizon is a small- scale example of an acoustic reflector comparable to the one covering a large area in the Northwest Atlantic Basin surrounding Bermuda, Horizon Av (Tucholke and Mountain, 1979; Bowles, 1980). The Worzel ash layer (Worzel, 1959) in the eastern Pacific, while having a subaerial ex- plosive volcanic origin, also produces a similar seismic profiler record. In general, the transport processes and resulting deposits that have accumulated in the Congress Seamount perched basin appear analogous to locally derived sediments revealed in and around other igneous regions, such as rises, ridges, sea- mounts, and rifts as illustrated by Heezen and Hollister (1971, their chapters 12 and 13). In particular, depressions in physiographically com- plex terrains tend to trap and concentrate locally derived sediments, including material from vol- canic terrains and products of submarine weath- ering (Honnorez, 1978). An example includes the Nazca plate of the eastern Pacific (Heath and Dymond, 1977). In the northwest Atlantic, near the study area, are occurrences of volcanic-rich and mixed volcaniclastic and biogenic oozes con- centrated in depressions within fracture zones and minor bathymetric lows on the Mid-Atlantic Ridge (Fox and Heezen, 1965; Siever and Kast- ner, 1967; Van Andel and Komar, 1969; La- touche and Parra, 1979; Scott et al., 1979). 34 SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES TERRIGENOUS =T OBSERVATIONS IN SOHM PLAIN I l0°°/c ITHE0RET1CAL CONTROLLING PARAMETERS| Canadian Maritime Outer Shelves Coarse surficial sediments Northern Sohm Plain Examples: Lamont-Doherty cores. in Horn et al. (1971) More distal and/or less direct access to terrigenous source(s). ESTIMATED COMPOSITION 50-75% T *■ 25-50% P 0-10% V Southern Sohm Plain Examples: core ATLANTIS 153-141 core VEMA 22-231 cores VEMA 26-6,-9 Partially isolated and/or only limited access to land- derived terrigenous material via turbidity currents. Also effects of reworking via oceanic circulation. Reduction of carbonates luding pelagic tests) ssolution below CCD Abyssal plain almost totally isolated from terrigenous source(s) via turbidity currents; proximate to volcanic sources and an accumulation of a large pelagic component. ESTIMATED COMPOSITION *► 5-20% T 40-90% P 7-40% V Congress Mount Perched Basin Example: core LYNCH 710-78-1 Proximal and/or direct access to terrigenous source(s), especially via turbidity currents. ESTIMATED COMPOSITION 70-90% T 10-30% P 0-5% V I007« VOLCANIC =V 100% PELAGIC =P (INCLUDING WIND-BLOWN) FICURE 23.—Deposition model depicting different origins of sediment in the southern Sohm Abyssal Plain based on volumetric compositional analyses from this study. This scheme involves three major components: terrigenous (including bioclastic), pelagic (including wind-blown), and volcanic. This model indicates processes and is suggested to apply for abyssal plains in general. Where topographic damming is absent on the eastern and southern flanks of Congress Sea- mount (Figure 12), such locally derived volcanic and reworked mixed volcaniclastic-biogenic ma- terials are disseminated on the surrounding Sohm Abyssal Plain. In these instances, sediment pro- vided from nearby volcanic topography is pres- ent, but in minor amounts, i.e., dispersed and masked within the reworked terrigeneous turbi- dite and biogenic suspensate sequences of the Sohm Plain as illustrated by the Atlantis and Vema 22 cores (Tables 2, 3). On the basis of the above, it is reasonable to expect that where the terrigenous supply is either blocked by positive or negative bathymetric fea- tures or markedly diminished (perhaps by dis- tance from source), the proportion of pelagics and/or volcanics would be enhanced. To illus- trate how volcanics can be used as an index or "yardstick" of modes of abyssal plain sedimenta- NUMBER 35 tion, a simple diagram is presented (Figure 23). This scheme uses a ternary diagram as a frame- work in order to illustrate the temporal or spatial sediment distribution. The three end-members of this triangle represent the major components of sedimentation: terrigenous, pelagic (including wind-blown), and volcanic materials (and their alteration products). Two distinct fields are rec- ognized: one is dominated by terrigenous or ter- rigenous plus pelagic components; the other com- prises largely pelagic or pelagic plus volcanic components. The former field (dot pattern on Figure 23) illustrates the masking of the pelagic and volcanic fraction by the preponderance of land-derived elastics in areas adjacent to land sources or along turbidity current paths. Thus, as distance increases or as access to land-derived sources diminishes, the proportion of the terrigen- ous component is decreased. In contrast, abyssal plain sediment accumulating in a region almost totally isolated from terrigenous sources would include primarily the pelagic and volcanic com- ponents (i.e., the latter field, dashed pattern in Figure 23). This simplified pattern in the Sohm Abyssal Plain is complicated by dynamic factors such as fluctuations of large-scale bottom-water circula- tion and the variability of the depth at which the dissolution of carbonate (CCD) occurs (e.g., Jansa et al., 1979; Thiede et al., 1980; and others). These factors would explain downcore variability in carbonate test content and the significant areal variation in sediment rates observed in, and ad- jacent to, this study area. These physical ocean- ographic conditions result in the blurring of the distinction between the two major fields as de- picted by the closed curve in Figure 23. There are obviously many possible ways in which these factors would apply. For example, moving bot- tom waters can winnow and/or transport locally derived volcanics (cf. Huang, 1980) and older weathered sediments to areas remote from their provenance. In another case, increased rates of carbonate dissolution (due to changes in the CCD) could reduce the numbers of preserved calcareous pelagic tests, thus markedly enhancing the proportion of siliceous pelagic tests, debris, and terrigenous and/or volcanic components. These phenomena might explain the high vol- canic content in some abyssal plains, and illustra- tions of this on a more regional scale have been presented (Heath and Dymond, 1977). It would also help account for vertical lithofacies variations noted in the cores we examined, particularly Lynch-1. This diagram (Figure 23), which models abys- sal plain sedimentation, is largely idealized, and based on general concepts. We have indicated the approximate location of the southern Sohm Abys- sal Plain cores on the theoretical plot. It is noted that even in an abyssal plain such as the Sohm, which has had ample and direct access to abun- dant terrigenous sources, particularly during the Pliocene and Quaternary, the volcanic fraction is significant. Although not dominant, this fraction may constitute as much as 1 percent, and locally up to 5 percent, of the total sediment fill. These values are modest when compared to the propor- tion of volcaniclastic sediments in regions close to volcanic arcs (Sigurdsson et al., 1980). We predict that further detailed petrographic examination of cores collected in the Sohm and other abyssal plains will reveal that locally de- rived reworked material of both sedimentary and volcanic origin may account for a significant fraction of the total sediment fill of abyssal areas. Appendix Tables TABLE 1.—Grain-size analyses from selected samples of cores studied, showing percentages of sand (>63 jtim), silt, and clay (<2 jtim) and also the silt/clay ratio (T = trace; dashed lines = not processed) Sample % % % silt/clay Sample % % % silt/clay depth(cm) sand silt clay ratio depth(cm) sand silt clay ratio ATLANTIS core 153- -141 VEMA core 26-6 7-8 T 50 50 1.00 2-3 4 35 61 0.57 49-50 T 77 23 3.34 29-32 T 34 66 0.51 109-110 T 62 38 1.63 45-47 T 29 71 0.40 172-173 T 32 68 0.47 124-126 T 25 75 0.33 212-213 T 35 65 0.53 227-229 T 20 80 0.25 240-241 T 31 69 0.44 272-274 T 21 79 0.26 265-266 T 88 12 7.33 321-323 1 19 80 0.23 327-328 T 38 62 0.61 344-346 3 27 70 0.38 384-385 5 78 17 4.58 393-395 T 17 83 0.20 400-401 56 34 10 3.40 474-476 T 27 73 0.36 465-466 68 26 6 4.33 544-546 T 18 82 0.21 500-501 67 24 9 2.66 575-580 67 26 7 3.71 VEMA core 26-9 660- 661 24 37 39 0.94 720-721 51 31 18 1.72 1-3 T 34 66 0.51 745-746 36 37 27 1.37 44-46 T 34 66 0.51 807-809 65 28 7 4.00 51-53 T 30 70 0.42 923-924 65 27 8 3.37 159-161 T 14 86 0.16 985-986 67 33 T >33.0 225-227 T 14 86 0.16 280-282 T 14 86 0.16 VEMA core ' 22-231 407-409 T 14 86 0.16 435-437 T 14 86 0.16 0-2 T 37 68 0.54 503-505 T 14 86 0.16 11-13 2 35 65 0.53 593-595 T 13 87 0.14 85-87 T 36 64 0.56 122-123 T 30 70 0.52 LYNCH core 710-78 -1 168-170 T 28 72 0.38 245-247 T 36 64 0.56 13-15 2 42 56 0.75 253-255 T 37 63 0.58 34-36 T 35 65 0.53 401-403 T 72 28 2.57 80-82 T 37 63 0.58 498-500 T 60 40 1.50 195-197 T 52 48 1.08 672-673 T 38 62 0.61 225-227 T 73 27 2.70 681-683 T 48 52 0.92 297-299 4 — — -- 913-915 T 49 51 0.96 299-301 10 — -- — 927-929 T 46 54 0.85 301-303 50 25 25 1.0 1050-1052 T 58 42 1.38 304-306 3 -- -- — 1221-1223 T 73 27 2.70 345-347 T 42 58 0.72 1236-1237 T 51 49 1.04 372-375 1 55 44 1.25 1304-1305 42 40 18 2.22 434-436 T 32 68 0.47 1353-1354 T 78 22 3.54 443-445 T 72 28 2.57 1460-1462 T 37 63 0.58 542-544 1 46 55 0.83 1480-1481 T 62 38 1.63 557-559 3 51 46 1.10 37 38 SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES TABLE 2.—Summary of results of grain-size and compositional analyses of core Atlantis 153-141, depth 5350 m SIZE ANALYS1 (percentag SILT AND SAND FRACTIONS "■= Abundant ^ = Few ^ ——, c ^^ = Common p- _ ^m 1 a) MICROSCOPE EXAMINATION y-_ Present ' _lrace | ^—Silt DEPTH Sand Silt Clay fera J Coc eolith Carbonate fragment Radioiaria Sponge spicule Diatom Opaques Heavies Brown mica Quartz Dolomite Organic Fiber Palagonite Sideromelane Chlorophaeite (cm from core top) Manganese micronodules Siliceous aggregate '•:'■:■•: Foramin ::••':: ■ ■ // A y V ^ 7 J [ 4 4 fA 4 A 7-8 /y 4 49-50 109-110 172-173 - - 212-213 240-241 - '/ // A y y y A y y~ A A ■ y A A 265-266 A A 327-328 384-385 y A ' y y ■■ 4 4 y A . 400-401 #***— 465-466 ^»if»> "T**.' iff 500-501 575-580 HSF •- i A //, A _J j / >/ 7 y v //, 660-661 Ji 720-721 — 745-746 "• -'■■."■ •".•.•- ' 807-809 mk A 923-924 _ / L —£ . / '/ ? V ■ f . y 985-986 | :•.••:•:•:::• — ! ' /{ ■ //< L.,.. NUMBER 11 39 TABLE 2.—Continued CLAY-SIZE FRACTION X RAY DIFFRACTION (percentage) SILT- SIZE FRACTIO X RAY DIFFRACTION (percentage) N Manganese, fluorescence (as counts/0.1 mg/200 sec) Montmorillonite Mica Kaolinite plus Chlorite Amphibole Quartz K feldspar Plagioclase Calcite Dolomite Mica Kaolinite plus Chlorite Amphibole Quartz K feldspar Plagioclase Calcite Dolomite REMARKS r ■42|33 8 9 8 8 8 2 7 7| 231 201 25 5.9 Few aggregates are iron cemented, some are dark red; inorganic crystalline calcite and feldspar present. 5 2 T 5 7l 29I24I28 7.8 1 18| 20 1 10 . 1 15| 16| 15 T K3I37 6.8 Inorganic crystalline calcite abundant; silt is coarse-grained. 15l 23 8 4| 15| 20 15 3.2 TH46 ■40 5 9 3 3 T 11| 241 29 10 20 Terrigenous sand with abundant feldspar and with inorganic crystalline calcite; silt is poorly sorted. r \ 4.6 ! ! 1I55 1, 4 5 2 6 8 TI 8| 10| 28| 111 29 5.2 Same as above. 1 4 5 2 7l Ml 32 111 25 6.0 1 - 1 - 1 i r J ■ . 1 T 3 3 1 nl i/l 1 12I is 5.2 Same as above. 40 SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES TABLE 3.—Summary of results of grain-size and compositional analyses of core Vema 22-231, depth 5535 m SIZE ANALYSIS (percentage) SILT AND SAND FRACTIONS "•= Abundant fT = Few ^^m~] ^ = Common f/-_T j 1 MICROSCOPE EXAMINATION y = Present ' " ^ 1—^--Silt DEPTH Sand - a Foraminifera 1 Coccolith Carbonate fragment Radioiaria Sponge spicule Diatom Opaques Heavies Brown mica Quartz Dolomite Organic fiber Palagonite Sideromelane Chlorophaeite (cm from core top) Manganese micronodules Siliceous aggregate •:•...• y M A y / 7^ A _JL • '/ 0-2 /, _A_A ji 11-13 ^^ / A y / m s A S, A 86-87 j| . M 122-123 7 " A r— 168-170 — 'A A A ■ /, / A 1 245-247 — 253-255 = -A A ■ ^ A / 7 y A A . 401-403 A 498-500 - 672-673 / AM A . / A A A V A A A 681-683 :^=^= / y 7 ■ A y \ A A , 913-915 927-929 i i s ■ " y— A A A / A /A A / y / 1050-1052 —& 1221-1223 1236-1237 y A J A ^U A A /4 / A AL A f y ' ■ A / A 1304-1305 // \ 1353-1354 / ~y "* A y y~ y Y A ■~ 7 / . / Si -^ ^ _L4^ 1460-1462 / 1 ! V~ 1480-1481 AA AA A \ /< NUMBER 11 41 TABLE 3.—Continued C L A Y-S 1 Z E FRACTION X RAY DIFFRACTION (percentage) SILT-SIZE FRACTION X RAY DIFFRACTION (percentage) Manganese, fluorescence (as counts/O.I mg/200 sec) Montmorillonite Mica Kaolinite plus Chlorite Amphibole Quartz K feldspar Plagioclase Calcite Dolomite Mica Kaolinite plus Chlorite Amphibole Quartz K feldspar Plagioclase Calcite Dolomite REMARKS M| 17 cn 5| 16 15 24 7.3 Most aggregates are iron stained; some manganese droplets are oxidized. | |33 T T| 25 6 4 4 4 71 iel 2d| 31 8.1 Siliceous aggregates are light brown. 211 25 11 14 31 12| 121 12 IM53 1, T T 10.0 Siliceous aggregates are gold colored; silt grains are unusually large. 6 6 1 7 111 27| 12 30 8 7 2 5 4| 20I 32 22 T si 11 1 9 5| 18p 18 30 6.2 Inorganic crystalline calcite grains are chemically etched. * II 15| 19 3 10 ?\ 17 4 29 Aggregates are light brown or white; silt is coarse-grained; one grain of garnet. 1 8.6 Aggregates are white and occasionally light brown; inorganic crystalline calcite noted. TB49 1 T T 1 8| 9 T| 12| sl 231 15| 22 T Garnet is present. - - - 8.5 Terrigenous sand, poorly sorted . 20 6 7 7 1 11 7* 221 20| 25 TB41 11 22 T| 10 9 1 7 3| 13^341 23 Terrigenous sand, poorly sorted. . 1 I 15 2 13 7I 24i 1 15 7.6 Terrigenous sand, poorly sorted, with garnet; aggregates are iron stained. 42 SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES TABLE 4.—Summary of results of grain-size and compositional analyses of cores Vema 26-6, depth 5207 m, and 26-9, depth 5546 m SIZE ANALYSIS (percentage) SILT AND SAND FRACTIONS "■= Abundant ^ = Few ^ ^ = Common 17"" _T MICROSCOPE EXAMINATION p" = Present ' LLA— Silt DEPTH Sand Silt o U Foraminifera Coccolith Carbonate fragment Radioiaria Sponge spicule Diatom Opaques Heavies Brown mica Quartz Dolomite Organic fiber Palagonite Sideromelane Chlorophaeite ( cm from core top) Manganese micronodules Siliceous aggregate :::'.' •;.*.■, •." ,*. -•".•• '.'. / M'A y A A 7 4 /A A 2-3 - 29-32 45-47 : r ~^ y A 4 A 'A V~ A A 124-126 = - - - j/ " y V \ S< A Ay A 4 4 227-229 r A 4 r yj 272-274 y~ m A A 'A vy x 321-323 = _ A y ■ 4 4 A | J J 344-346 ,: V \A^ 393-395 = / y m y 474-476 / V / V 'A ''ill 544-546 E y A Y A 'A y^y j ' VEMA 26-9 |(= 1-3 :z ■/ MM4 / A V > sb 4 ^4 AL A y _y A V y A / V A A A z 44-46 AA ,A; 51-53 - y A y y r_ WA A y , \~AA y 159-151 E V ■ A '/ y r ■ 4 225-227 E A / y V w 280-282 = — — y 1 A A y y rmm A 407-409 E y V~~ „y< y 435-437 = s y 503-505 = : r A 4 - y _ ■ . mi. m 593-595 |E r A 7 NUMBER 11 43 TABLE 4.—Continued 1 CLAY-SIZE FRACTI X RAY DIFFRACTION (percentage) O N S 1 LT-SIZE FRACTION X RAY DIFFRACTION (percentage) Manganese, fluorescence (as counts/O.I mg/200 sec) Montmorillonite Mica Kaolinite plus Chlorite Amphibole Quartz K feldspar Plagioclase Calcite Dolomite Mica Kaolinite plus Chlorite Amphibole Quartz K feldspar Plagioclase Calcite Dolomite REMARKS 1 18| 23 10 |49 ,0 U T 5 3l 1?|47 8 9.7 Large platy iron fragments. 7.1 t 13 21 5 I 33 26.3 Lo 26 28 T 7 16 17 2 13 8 221 '3| 9 31.5 Inorganic crystalline calcite; weathered glass with abundant inclusions. I44 29 27 23 25 3 14 11 24 37.1 Aggregates are iron stained; some weathered glass; rod-shaped silt crystals probably pyroxenes. Li 25 26 4 10 19 20 2 17 12 30 37.4 Few feldspar grains. Li 12 45 6 T T 18 26 15 9 32 54.9 Few feldspar grains; these and other sand grains are rounded to subrounded. [I 21 54 16 36 9 20 19 34.9 Few feldspar grains; silt poorly sorted. If 18 14 45 T T 6 35 2 9 13 30 60.9 Few feldspar grains. 34 5 43 5 19 28 58.9 Few feldspar grains; silt poorly sorted with rod-shaped grains. |28 21 39 46 44 T 14 23 9 34 20 49.5 Few feldspar grains; silt is coarse-grained. T 7 39 1 22 15 15 1 11 6 121 19 21 8.3 Aggregates are occasionally iron stained; inorganic crystalline calcite in silt. 291 4 5 9 18 14 5 11 8 20 24 12.3 Few feldspar grains; silt, poorly sorted, with rod-shaped grains. 25 38 28 4 5 15 15 4 11 8 22 25 12.8 Few feldspar grains; rod-shaped grains in the silt fraction. 28 _26_ 20 25 5? 25 23 13 14 25 20.7 Siliceous aggregates were originally brown glass. T 28 24 13 13 22 17.4 Weathered glass present. .2zl 16 261 31 30M 23 2m 19 461 9 57 43 T 23 26 13 13 20 23.8 Feldspar and weathered glass present. 23 26 13 18 20 29.5 Few aggregates are iron cemented; few feldspar grains. 4.7, 52[ 23 20 11 17 29 25.8 Few feldspar grains; weathered glass present; rod-shaped grains in silt. 21 29 11 16 23 29.1 Aggregates are iron stained; rod-shaped grains in silt fraction. | 45[_ 1 16 .30 20 25| 9 32.5 Aggregates are white or red; few feldspar grains; rod-shaped grains in silt fraction. 44 SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES TABLE 5.—Summary of results of grain-size and compositional analyses of core Lynch 710-78-1, depth 5207 m DEPTH (cm from core top) IS SIZE ANALYS (percentag SILT AND SAND FRACTIONS MICROSCOPE EXAMINATION (| = Abundant p7" = Few r = Common r/"" T __ X = I race W^ = Present Opaques Sand "D- 13-15 34-36 80-82 195-197 225-227 297'-299 299-301 rA V 6-A 2. z y v A 7 7 y V y 7~ ..A A_A 301-303 304-306 345-347 373-375 434-436 443-445 542-544 557-559 'V m : A l A ^ i _^.. V y v—y- Jl A A A 1 A 4 A. A A ,4 A _A A NUMBER 11 45 TABLE 5.—Continued CLAY-SIZE FRACTION X RAY DIFFRACTION (percentage) SILT-SIZE FRACTION X RAY DIFFRACTION (percentage) Manganese, fluorescence (as counts/O.I mg/200 sec) Montmorillonite Mica Kaolinite plus Chlorite Amphibole Quartz K feldspar Plagioclase Calcite Dolomite Mica Kaolinite plus Chlorite Amphibole Quartz K feldspar Plagioclase Calcite Dolomite REMARKS __V7 14 20 28 5 2 4 J 24 1 16 9 3 ll|38 12 3630 Few foraminifera tests are iron stained; ash is devitrified. 47 53 30 2 1 2 4 26 31 10 4 12 T 17 1890 All foraminifera tests and quartz grains are iron stained. 31 ,IP_ 8 18 3 1 4 22 38 14 3 14 9 1890 3310 Abundant devitrified glass. 42 30 2 2 4 8 2 16 15 11 4 IT! 16 21 Feldspar and large quartz grains. 54 33 2 4 14 13 8 3 14 ■48 2800 Foraminifera fragments are ironstained; hornblende. 6| 29| 17 5I42 7 7 7 Tl 12|55| 12 3110 14 30 49 41 5 3 7 21 16 14 6 21 22 2350 Few foraminifera fragments are ironstained. 15 12 13 15 30 1 5 18 19 17 6 23 17 2580 46 35 39 30 2 2 3 5 T 25 22 9 3 10 7 14 3470 36 24 4 2 20 20 ?3 7 is! 12 17 1840 All foraminifera fragments are iron stained. 4 4| 15 15 12 9 3 15I29 4200 91 15 16 2 /ll 1 , . 5 3 13^56 ,0 4620 Literature Cited Amos, A. 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