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Sohm Abyssal Plain: Evaluating 

Proximal Sediment Provenance 

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DANIEL JEAN STANLEY, PATRICK T. TAYLOR, 

HARRISON SHENG, and ROBERT STUCKENRATH 

"*-* • 

CONTRIBUTIONS TO THE MARINE SCIENCES 
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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 
ABSTRAC T 

Stanley, Daniel Jean, Patrick T. Taylor, Harrison Sheng, and Robert Stuckenrath. 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 introduced 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 oceanbottom features, and that a significant, although not total, amount of such 

volcanic materials recovered from cores are derived from submarine weathering 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: Smithsonian 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 Ewing (1952), studies of North Atlantic abyssal 

plains during the past three decades have emphasized 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, shallow-marine or land-derived components (Ericson 

et al., 1961; Heezen, 1963; Horn et al., 1971). 

The acoustic patterns on seismic reflection profiles of the Northwest Atlantic Basin show multiple 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 transparent acoustic series, primarily on topographic 

highs, which are believed to have a different 

origin (Ewing et al., 1973; Bowles, 1980). Interpretations of acoustically transparent layers are 

based on JOIDES deep-sea drilling results, bottom 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). Various 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). Biscaye and Eittreim (1977) summarize the importance of concentrated suspended material in the 

water masses on and above this Plain, and attribute 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-
80° 

50° NT 

SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES 

40° w 

45° 

l ' 

•", , 

GB 

•& 

25'" A-i '^%:A: l: ... 

••••• ...-.A^Ai :^f 

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 terrigenous and biogenic material by near-bottom 

current activity. There has been a tendency, however, to overlook the contribution of locally derived material in regional depositional interpretations. 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 Margin (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 collected 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 previously collected by others—principally from the 

Lamont-Doherty Geological Observatory (LDGO)—in the plain (Ewing et al., 1974:138,229). 

Our working premises for this study are (a) that 

locally derived sediment should be most abundant 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, 

33°30'N 

33°15'N 

33°N 

32°45N 

32°30'N 

LYNCH SEAMOUNT 

( Depth in fathoms ) 

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 assisted 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 collected on this cruise. 

We have benefited greatly from access to the 

core, bottom photograph, and seismic data libraries of the Lamont-Doherty Geological Observatory 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. Streeter, Lamont-Doherty, and C. C. Smith, U.S. Geological Survey, Washington, D.C, for identification and assistance in the interpretation of the 

benthic foraminifera and nannofossils, respectively. 

The paper has been reviewed by L. A. Barnard, 

Texas A & M University, T.-C. Huang, University of Rhode Island, and G. H. Keller, Oregon 

State University. 

Methods 

The Congress and Lynch seamounts were surveyed by the USNS Lynch (cruise 710-78) from 30 

September to 6 October 1978. Bathymetric data 

were obtained in fathoms with the Lynch's hullmounted 12 kHz echo-sounding system and contoured 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 English units are presented for convenience. Seismic 

reflection profiler data (36 kilojoule sparker system) collected during the Lynch cruise were supplemented 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 subbottom 3.5 kHz profiles also were obtained adjacent 

to and on the mount proper, in the vicinity of the 

two conductivity-temperature-depth (CTD) profiles 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 (Figure 2). 

Seafloor photographs were taken at three stations 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 accumulation about 20 km west of the northern Congress 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 

33°30'N 

32"45N 

32°30'N 

( Depth in fathoms ) 

V-26 

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 pretripping 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 located 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 included textural measurements (percentages by 

weight of sand, silt, and clay fractions); these data 

are presented in Table 1. Compositional descriptions 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 petrographic (transmitted and reflected light) and binocular 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: iI 090° J 

8 August 1968 J/§ « y § 

OOf 01 0.2 I 03 , 04, , <Mlt Qj6,-J 07 08 09 10,,, 14 A ,15 . 16.-. 17 ,18 19 20 21 . 22 hour s 

-i'Jr-r— fr. i T ^--HH'-TTHTr-r i V -U- ^ T r~l' . T -rhrm #r:f-tari it,-;^^r^f{i^^r;rT|rvr-r Ii -1-r-t-7T:^^—rt-rH,-rr |—^—i — H ••,. 

300° 

FIGURE 4.—Selected airgun profiles from Vema cruises 26 and 22 (profiles A and B, respectively) 

across the southern Sohm Abyssal Plain in, and in the vicinity of, our study area (from Ewing 

et al., 1974), showing examples of different acoustic seismic patterns: (1) acoustically layered, 

(2) acoustically transparent, (3) opaque (basement); note the perched basin (p) on the western 

flank of the Congress Seamount (vertical scale denotes seconds of two-way travel time; 

horizontal scale expressed in hours; speed estimated at 9 knots, or 17 km per hour). 
NUMBER 11 

(counts per 0.1 mg of sample per 200 seconds) of 

selected sand, silt, and clay samples; and (4) 

Scanning Electron Microscope (SEM, Cambridge 

MKIIA) and electron probe energy dispersive 

analysis (Tracor Northern Energy Dispersive 

Spectrometer) of selected silt and fine sand grains. 

Radiocarbon dates were determined for Lynch 

core 710-78-1 (5 successful dates), and from Vema 

core 22-231 (2 successful dates). 

Physiography of Study Area 

The Sohm Abyssal Plain, covering an area of 

about 660,000 km2, is a T-shaped province situated south of Georges Bank, Nova Scotian Shelf, 

and Grand Banks (Heezen et al., 1959). The 

southern portion of the vertical bar of the T 

extends about 780 km (420 nm) south of the 

Maritime Continental Margin and lies east of 

Bermuda (Figure 1). Abyssal depths range from 

5120 m (2800 fm) to 5670 m (3100 fm). Bathymetric detail in the study area, between 31° and 

34°N latitude and bisected by the 55°W meridian, remains poorly defined. Two large seamounts, the Congress and the Lynch, are located 

in the approximate center of this sector of the 

plain (Figure 1). Results from widely spaced 

seismic profiles across this region are summarized 

by Ewing et al. (1973). The southern Sohm Plain 

is not a featureless region as has been generally 

depicted. Typical airgun profiles across the area 

show the near-horizontal plain broken by numerous topographic features of highly variable size 

and relief. Flat monotonous stretches of plain are 

generally less than 50 km wide between these 

highs (Figure 4). 

Congress Seamount (33°07'N, 54°50'W), located at the distal southeast end of the Kelvin 

Seamount chain, some 1114 km (600 nm) due 

east of Bermuda (Figure 2), was first charted by 

the United States Hydrographic Survey ship USS 

Congress in 1910. Its nearest large neighbors are 

the Wyoming and Nashville seamounts, about 

150 and 210 km to the northwest (Tucholke and 

Vogt, 1979), and a previously unnamed mount, 

herein referred to as the Lynch Seamount, approximately 90 km to the southeast (Figure 2). 

Congress is a twin-peaked seamount (Figures 2, 

3, and 5, profile E-F). The NE-SW trending 

peaks, separated by 27 km (15 nm), lie at approximately the same depth; the northern summit is 

slightly shallower (2554 m, 1397 fm) than the 

southern peak (2578 m, 1410 fm). They rise 2803 

m (1533 fm) and 2779 m (1520 fm), respectively, 

above the floor of the surrounding Sohm Abyssal 

Plain (5357 m, 2930 fm). The average overall 

slope of the northern peak is approximately 9°; 

the slope increases toward the summit (13°), and 

diminishes near the base (<3°). The average slope 

of the southern peak is about 10°, the near-summit slope is 16°, and the basal slope <4° 

The depth of the Sohm Abyssal Plain around 

the Lynch and Congress seamounts is approximately the same. The relief of the Lynch Seamount (2642 m, 1445 fm), however, is somewhat 

less than that of the Congress inasmuch as the 

Lynch peak is about 179 m (98 fm) deeper. The 

basal area of the single-peaked Lynch mount is 

comparable in size to that of either of the Congress peaks (Figure 6). 

One area of the Congress Seamount selected 

for detailed study is the perched basin between 

the western flank of the northern peak and a 

topographic ridge just to the west (Figure 2 and 

Figure 7, profile J-K). The surface of this depression lies at about 4882 m (2870 fm), or about 475 

m (260 fm) above the Sohm Abyssal Plain. The 

maximum dimensions of this perched, beanshaped depression are 30 km in an E-W direction 

and about 29 km in a N-S direction; its approximate surface area is 450 km2

 (Figure 2). 

Sediment Acoustic Facies in Southern Plain 

GENERAL DISTRIBUTION.—The closely spaced 

grid of Lynch seismic profiles near the Congress 

and Lynch seamounts (Figure 3) provides the 

means to define the distribution of two types of 

unconsolidated sediment in the center of the 

southern Sohm Plain study area. The survey 

reveals the configuration of (a) flat-lying, acoust-
SOUTHWEST 

SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES 

NORTHEAST 

FIGURE 5.—Seismic profiles (30 kj sparker) showing the twin peaks of Congress Seamount 

(profile E-F); note the flat-lying, acoustically layered facies forming the Sohm Abyssal Plain 

and acoustically transparent pattern in the saddle between two peaks (profile C-D) (letters 

refer to Lynch track lines in Figure 3; heavy horizontal bar represents 10 km; vertical scale given 

in seconds of two-way travel time from the surface). 

ically layered sediment, representing the most 

common reflection pattern in the plain surrounding the seamounts (Figure 4, pattern 1; Figure 7, 

profiles D-E and L-M; and Figure 8), and (b) 

the more localized, acoustically transparent pattern (Figure 4, pattern 2; Figure 7, profiles J-K, 

P-Q, and N-O; and Figure 9). An acoustically 

opaque series forms the seamount structure, and 

possible some volcaniclastic apron sediment (Figure 4, pattern 3; and Figures 5 and 6). This 

opaque pattern underlies either the acoustically 

layered or transparent layer, or both (Figure 7, 

profile N-O). The acoustically layered series occurs at depths greater than 5360 m (2930 fm). 
NUMBER 11 

NW 

FIGURE 6.—Seismic profiles showing the single-peaked Lynch Seamount; note crater-like 

depression on peak (profile R-S), acoustically transparent pattern localized on southern flank 

(profile T-U), and acoustically layered abyssal plain facies (profile U-V); sharp peak on profile 

U - V may be a side echo (for other explanations see legend for Figure 5). 
WEST EAST 

WEST" 

E 

EAST 

D 

4-

FIGURE 7.—East-West seismic profiles across the Congress 

Seamount: profiles Q- P and J- K traverse the northern peak; 

profile L-M crosses the southern peak; profile N- O traverses 

saddle between the peaks; profile D-E, Sohm Abyssal Plain 

south of southern peak; the upper three profiles illustrate the 

perched basin containing ponded acoustically transparent 

sediment facies separated by internal reflectors (IR) (for 

other explanations see legend for Figure 5). 
NUMBER 11 11 

WEST ^EASTy-S-^-N-j-WEST 

I H ^ S 

I \' I I' 

EAST. 

SOUTH NORTH SOUTHj —_ i ' >\ "ft 

, K 7 1 — 7 '•,#"»!'

 T 

-NORTH 

L X SW NEv A /NORT H 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 transparent 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 TH E 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 transparent layer is absent (Figure 7, profile J-K, and 

Figure 5, profile E-F). 

The Sohm Abyssal Plain almost always displays the acoustically layered facies overlying 

acoustically transparent or opaque seismic reflection patterns; we presume the latter to be basement. 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). 
14 

55°W 

SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES 

54°30'W 54°W 

33°15'N 

33°N 

32°45'N 

tV/vSECTION ABOVE * \ ' 

INTERNAL REFLECTOR , IR 

rV/iiSECTION WITHOUTS 

OR INTERNAL REFLECTOR 32°30'N 

FIGURE 11.—Isopach map of acoustically transparent facies, showing two sedimentary configurations: 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 unconsolidated 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 records 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 interface, is probably composed of several horizons 

of distinct acoustic impedence, and it divides this 

thick sequence of weakly layered facies (Figure 
NUMBER 15 

55°W 54°30'W 54°W 

33°15'N 

33°N 

CONTOURED 

SEDIMENT THICKNESS 

BELOW INTERNAL 

REFLECTOR ( IR ) 

- 32°45'N 

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 SEAMOUNTS.—A depositional configuration comparable 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 acoustically transparent sediment occur without the IR 

(Figures 6 and 11). 

The thick (380 m) accumulation of acoustically 

transparent material situated to the north of Congress 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 thickening of the acoustically transparent facies in a 

southerly direction; the southern termination of 

this facies is abruptly truncated by the acoustically layered facies of the Sohm Plain. The greatest 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. Highresolution 3.5 kHz profiles also show the indistinct, more poorly defined layering that characterizes 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 

W 

WPPHB 

Ij'.Hl 

P 

HW'fflJ 

11| | CONGRESS | 

,

:

ffiijfi SEAMOUNT i 

mm mm* 

H UB » ' 

|'Afl 

^7 mm aits ! si 

It'. . • ! ' ? 

A? -

• I 

mi 

-26 JO 

< m 

o 

n 

ife^fi ?Flfer

:fflS 

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 micaceous 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 alternating moderate- to reddish-brown (5YR4/4), 

pale-brown (10YR6/2), and light-olive-grey 

(5YR5/2) units. Some darkly streaked zones apparent 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, almost cream-colored sections 1 to 5 cm thick. 

These patches have been interpreted as weathered volcanic products, possibly palagonite (unpublished 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-yellowish-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 yellowishbrown (10YR4/2) and moderate- to reddishbrown (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 horizontal (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 foraminifera) to mud layer in the pale-brown zone is 

clearly observed between 303 and 290 cm; an xradiograph (Figure 15) shows that this unit actually fines upward to about 275 cm. It is interpreted as a turbidite. 

Rates of Sediment Accumulation 

Five radiocarbon dates were obtained on samples 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 centimeters 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 sedimentation rate is approximately 7 cm/ka. It is noteworthy that the highest rates apply to the yellowish-brown mud immediately below the significantly more slowly accumulating Holocene core 

top (Figure 14). This yellowish-brown layer accumulated between 40 and 165 cm from approximately 20,000 to 12,000 B.P., about the time of 

the most recent eustatic low sea level stand, followed 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 TH E MARINE SCIENCES 

C

 l4

 11070 ± 85 (12-20cm) i!_3ll5.£!T'.bi_ J 

C

14

 III60±500 (20-30cm) \ 

(34-36cm)-< 

MODERATE TO 

REDDISH BROWN (80-82cm)-

• YELLOWISH BROWN 

r

14

 22390±470 (I70-I85cm)-

(I95-I97cm)-

(225-227cm)-

H 

C

U

 33760 ±800 (260-274cm) J 

(297-299cm). 

(299-30lcm:^ 3 0 0 

(30l-303cm',-J 

(304-306cm)' . 

' \ C

l 4 > 43000 (310-325, 

(345- 347cm )-

(373-375cm)-

FIGURE 14.—Detailed log of Lynch core 

710-78-1 collected in the perched basin on west flank of the Congress Seamount (see Figure 2), showing principal sediment types, color, and physical 

and biogenic structures, and the position and age (in years before present) 

of radiocarbon-date d samples; 

(5YR4/4 = moderate to reddish 

brown; 10YR4/2 = yellowish brown; 

10YR6/2 = pale brown; dark vertical 

bars at right = position of x-radiographed core sections illustrated in 

Figure 15; see Table 1 for grain-size 

analyses of selected segments identified at left of log). 

(434-435cm)-

(443-445cm)" 

(542-544cm)-< 

550 

(557-559cm)-<559 

oooooo o SM*M>-' 

• 10 YR 4/2 

5 YR 4/4 

5 YR 4/4 

•Vaguely laminated 

- Transverse burrows 

-Vertical burrows 

Highly burrowed 

'— Bioturbation 

• 10 YR 6/2 

Blo^rbation 

"Graded Foram sand 

!\ M» -Transverse burrows 

V 5 YR 4/4 

10 YR 6/2 

— Highly burrowed 

• 5 YR 4/4 

H I .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 horizons 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 southern Sohm Plain are comparable to averaged longterm 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 (Tucholke and Vogt, 1979). In contrast, extreme variations occur in the hills bordering the southern 

sector of the plain. On the lower flank of the MidAtlantic Ridge to the east, near Vema 26-6, 

JOIDES drill site 10 revealed a considerably 

reduced Quaternary section. Very low sedimentation (<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 redeposition of sediment resulting from large-scale water-mass circulation (see earlier sections on sediment 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 analysis highlights the prevailing silty clay texture and 

generally homogeneous nature of these two abyssal 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 components of the coarse fraction (>15 jum)—coarse silt 

and sand—of selected samples is depicted in Tables 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 carbonate 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 chlorophaeite) are found locally, and occur as presentto-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 include both (Table 3). Moreover, the proportion 

of volcanic products is also highly variable, with 

the highest concentration near the core top (probably due to winnowing). The biogenic fraction is 

largely carbonate fragments (predominantly broken foraminifera) and coccoliths rather than unbroken foraminiferal tests. Manganese micronodules, siliceous aggregates (probable volcanic 

alteration products), and plant fibers are irregularly 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, including manganese micronodules (L. A. Barnard, 

pers. comm.; see also Figures 16, 17), siliceous 

aggregates (Figure 16), devitrified glass, palagonite, zeolites and altered feldspars (Figure 17), and 

pyroxenes. This assemblage argues against a dominantly diagenetic origin. Moreover, evidence for 

reworking is provided by the presence of quartz 

along with high proportions of carbonate fragments 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 assemblages 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), including manganese micronodules. Petrographic microscope and Scanning Electron Microscope 

(SEM) analyses of the Lynch coarse silt fraction 

show variable proportions of mica, plagioclase 

feldspars, siliceous microfossils (Figure 18A,B), calcareous nannofossils (Figure 18C,D), foraminiferal 

fragments (Figure 18C,D), dolomite rhombs (Figure 18E), and angular volcanic shards (Figure 

18F). Palagonite, sideromelane and/or chlorophaeite 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 moderate- to reddish-brown layers contain a much 

higher proportion of siliceous microfossils and a 

lower proportion of coccoliths than the yellowishbrown 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 foraminifera 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 displacement of benthic foraminifera. 

Further insight on modification of faunal content 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 somewhat 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 account for the remainder. Montmorillonite (used 

in this paper synonymously with smectite) occurs 

only in trace amounts. 

The silt-fraction suite is dominated by plagioclase, dolomite, and calcite. In addition, mica, 

kaolinite and chlorite, quartz, K-feldspar, and 

amphibole are generally present in variable proportions. 

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 somewhat higher than those of the mica group. With 

regard to the silt fraction, these two Vema cores 

generally contain higher mica, kaolinite plus chlorite, 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 proportion 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 generally lower calcite content in the silt- and claysize fractions, and X-ray fluorescence shows a 

substantially higher iron and, in some instances, 

manganese content in the moderate- to reddishbrown 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 thousandfold 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 micronodule (Figure 16) of the sand-size fraction 

from the Lynch-l core (Table 5) highlights the 

values of the two most abundant elements: manganese, 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 feldspar (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; Stonecipher, 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, summarized by Wiist (1949; and Heezen and Hollister, 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 elongated gyre related to the Gulf Stream system. 

This large clockwise-rotating gyre is located east 

of Cape Hatteras, extends to about 40° W longitude, 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 recognized: North Atlantic Central Water, extending 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, ranging 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 Antarctic Bottom Water (-0.5°C, 34.65%o) is suggested by the lowermost readings of the temperature-salinity curve. The profiles indicate the possibility of slight mixing of Antarctic Bottom Water 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 southermost part of the Sohm Abyssal Plain. 

Since current-meter data are lacking, an indication of water flow and direction is obtained by 

noting the nature of bedforms recorded by compass-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 (Figures 20, 21), craig-and-tail features (Figure 22), 

coarse lag accumulated against obstacles on the 

bottom (Figure 20A), and asymmetrically deformed mud structures (Figure 22). Although this 

bottom transport pattern may probably represent 

only recent and short-term dispersal, it is noteworthy that (a) the isopach map indicates localized 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 acoustic 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 abyssal 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., Heezen and Laughton, 1963). There is, in fact, ample 

documentation for the Northwest Atlantic Basin 

that the processes of turbidity currents and related 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 landderived component, most workers also call attention 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 recovered 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 probably 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 emphasize distal transport. We note in abyssal plain 

studies, however, that the role of proximal or 

more local-intrabasinal sediment source and dispersal (in particular, the displacement of volcanically 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, presumably 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 dominated by positive topographic relief (ridges, fracture zones, abyssal hills, seamounts, knolls, etc., 

many shown on Figure 1) of predominantly volcanic 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 turbidity-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 nepheloid 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-water circulation (Worthington, 1976; Laine, 1978); 

and (d) it lies near the geographic center of the 
30 SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES 

B 

f

 / '' ' / , ' 

- ','',<••, /"'''' TA v . ' ' 

' "'" 

• 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, Bermuda Rise, and Kelvin and Corner Rise seamounts, 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 consist of variable mixes of the following: (a) land 

and continental margin-derived elastics (including 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 continental margin-derived materials, the southern 

Sohm Plain piston cores reveal a dominant terrigenous 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 examined also show an important pelagic component (e.g., core Vema 22-231) that is related to the 

well-defined distribution pattern of the suspension-rich, benthic nepheloid layer (Biscaye and 

Eittreim, 1977). Nevertheless, we have found that 

all cores have at least some silt- and even sandsize volcanic components (Tables 2-5). Despite 

the significant amounts of land-derived turbidite 

sediments that dominate abyssal plain cores Atlantis 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 relief; 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 micronodules, zeolites, palagonite, chlorophaeite, sideromelane, 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 components were derived from diagenetic remobilization (i.e., manganese micronodules, see Scott et 

al., 1979) and/or from distal land sources (smectite). 

As a test of the true importance of this allochthonous 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 location 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 bottom-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 sparker 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 moderate- to reddish-brown layers (see horizons labeled 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 enhanced fraction of montmorillonite; (2) an abundance of planktonic microfossils including foraminifera (often poorly preserved and recrystallized), 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 intensely 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 biogenic 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 suspended 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. Worthington, 1976). Three bottom photograph stations 

(Figures 20-22) provide evidence for seafloor erosion, 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 horizons labeled 10YR4/2) have (a) somewhat larger 

proportions of better preserved planktonic foraminifera and calcareous nannofossils, and (b) tests 

of the latter, many of which are obviously reworked 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, discontinuous reflectors noted on 3.5 kHz profiles (Figure 

10). Emplacement of this sediment type, particularly the graded beds, is almost certainly by 

gravitative mechanisms, including turbidity currents. 

The ubiquitous, but irregular, abundance in 

this Lynch core of coarse silt- to sand-sized volcanic 

products, including primary (sideromelane, angular 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 microfossils as old as Upper Cretaceous (Figure 19). 

The proximity of the sediment pond to the seamount and other adjacent basement rises suggests 

that these igneous features are major-source terrains. In addition to this igneous supply, material 

reworked from the older depositional cover draping these basement rises also appears to serve as 

a sediment source (cf. Taylor and Hekinian, 

1971). This volcanic-biogenic assemblage is similar to that described by Van Andel and Komar 

(1969), Nayudu (1971), and others. Bottom photographs show that currents on these bathymetric 

highs are capable of eroding and transporting 

weathered volcanic material along with microfossil-rich (pre-Quaternary to Recent) pelagic sediments and the recently deposited suspensates 

(Figure 20). This combined regime of suspensate 

deposition and reworking by bottom-current activity has resulted in accumulation of mixed biogenic-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. Stanley and Taylor, 1977). Specific examples of downslope displacement are provided by the graded 
NUMBER 11 33 

foraminiferal sand bed (Figure 14, base at 303 

cm) and by the displacement of benthic foraminifera 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 processes: suspensates plus older locally derived pelagic deposits subsequently have been reworked 

by bottom currents and displaced downslope by 

gravitative mechanisms. Sediments derived from 

more distal sources supplement these ponded deposits 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 include a volcanic dust (cf. Huang et al., 1975, 

1979; Sigurdsson et al., 1980), may also be a 

minor distal component. In addition, the measured 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 influencing the transport of material from distal 

sources to the southern Sohm Plain include eustatic 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; Ruddiman, 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 material 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 duration, these mass flows resulted in the deposition 

of distinct volcaniclastic (possibly hyaloclastite) 

layers characterized by a distinct acoustic contrast, the internal reflectors (IR), within the sediment series (Figure 13). The areally restricted 

distribution of the internal reflectors in the depressions 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 smallscale example of an acoustic reflector comparable 

to the one covering a large area in the Northwest 

Atlantic Basin surrounding Bermuda, Horizon 

A

v

 (Tucholke and Mountain, 1979; Bowles, 

1980). The Worzel ash layer (Worzel, 1959) in 

the eastern Pacific, while having a subaerial explosive 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, seamounts, and rifts as illustrated by Heezen and 

Hollister (1971, their chapters 12 and 13). In 

particular, depressions in physiographically complex terrains tend to trap and concentrate locally 

derived sediments, including material from volcanic terrains and products of submarine weathering (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 concentrated in depressions within fracture zones and 

minor bathymetric lows on the Mid-Atlantic 

Ridge (Fox and Heezen, 1965; Siever and Kastner, 1967; Van Andel and Komar, 1969; Latouche and Parra, 1979; Scott et al., 1979). 
34 SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES 

TERRIGENOUS =T 

OBSERVATIONS IN SOHM PLAIN I l0°°/c 

Canadian Maritime Outer Shelves 

Coarse surficial sediments 

Northern Sohm Plain 

Examples: Lamont-Doherty cores. 

in Horn et al. (1971) 

Southern Sohm Plain 

Examples: core ATLANTIS 153-141 

core VEMA 22-231 

cores VEMA 26-6,-9 

Congress Mount Perched Basin 

Example: core LYNCH 710-78-1 

ITHE0RET1CAL CONTROLLING PARAMETERS| 

Proximal and/or direct access 

to terrigenous source(s), especially 

via turbidity currents. 

ESTIMATED COMPOSITION 

70-90% T 

10-30% P 

0-5% V 

More distal and/or less direct access 

to terrigenous source(s). 

ESTIMATED COMPOSITION 

50-75% T 

* • 25-50% P 

0-10% V 

Partially isolated and/or only 

limited access to landderived 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 

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 Seamount (Figure 12), such locally derived volcanic 

and reworked mixed volcaniclastic-biogenic materials are disseminated on the surrounding Sohm 

Abyssal Plain. In these instances, sediment provided from nearby volcanic topography is present, but in minor amounts, i.e., dispersed and 

masked within the reworked terrigeneous turbidite 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 features or markedly diminished (perhaps by distance from source), the proportion of pelagics 

and/or volcanics would be enhanced. To illustrate 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 framework 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 recognized: one is dominated by terrigenous or terrigenous plus pelagic components; the other comprises 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 terrigenous 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 components (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 circulation 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 adjacent to, this study area. These physical oceanographic conditions result in the blurring of the 

distinction between the two major fields as depicted by the closed curve in Figure 23. There are 

obviously many possible ways in which these 

factors would apply. For example, moving bottom 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 volcanic content in some abyssal plains, and illustrations 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 abyssal plain sedimentation, is largely idealized, and 

based on general concepts. We have indicated the 

approximate location of the southern Sohm Abyssal 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 abundant 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 proportion 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 derived 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 

depth(cm) 

ATLANTIS 

7-8 

49-50 

109-110 

172-173 

212-213 

240-241 

265-266 

327-328 

384-385 

400-401 

465-466 

500-501 

575-580 

660- 661 

720-721 

745-746 

807-809 

923-924 

985-986 

VEMA core 

0-2 

11-13 

85-87 

122-123 

168-170 

245-247 

253-255 

401-403 

498-500 

672-673 

681-683 

913-915 

927-929 

1050-1052 

1221-1223 

1236-1237 

1304-1305 

1353-1354 

1460-1462 

1480-1481 

% 

sand 

core 153-

T 

T 

T 

T 

T 

T 

T 

T 

5 

56 

68 

67 

67 

24 

51 

36 

65 

65 

67 

' 22-231 

T 

2 

T 

T 

T 

T 

T 

T 

T 

T 

T 

T 

T 

T 

T 

T 

42 

T 

T 

T 

% 

silt 

-141 

50 

77 

62 

32 

35 

31 

88 

38 

78 

34 

26 

24 

26 

37 

31 

37 

28 

27 

33 

37 

35 

36 

30 

28 

36 

37 

72 

60 

38 

48 

49 

46 

58 

73 

51 

40 

78 

37 

62 

% 

clay 

50 

23 

38 

68 

65 

69 

12 

62 

17 

10 

6 

9 

7 

39 

18 

27 

7 

8 

T 

68 

65 

64 

70 

72 

64 

63 

28 

40 

62 

52 

51 

54 

42 

27 

49 

18 

22 

63 

38 

silt/clay 

ratio 

1.00 

3.34 

1.63 

0.47 

0.53 

0.44 

7.33 

0.61 

4.5 8 

3.40 

4.3 3 

2.66 

3.71 

0.94 

1.72 

1.37 

4.0 0 

3.37 

>33. 0 

0.54 

0.53 

0.56 

0.52 

0.38 

0.56 

0.58 

2.57 

1.50 

0.61 

0.92 

0.96 

0.85 

1.38 

2.70 

1.04 

2.22 

3.54 

0.58 

1.63 

Sample 

depth(cm) 

VEMA core 

2-3 

29-32 

45-47 

124-126 

227-229 

272-274 

321-323 

344-346 

393-395 

474-476 

544-546 

VEMA core 

1-3 

44-46 

51-53 

159-161 

225-227 

280-282 

407-409 

435-437 

503-505 

593-595 

% % 

sand silt 

26-6 

4 

T 

T 

T 

T 

T 

1 

3 

T 

T 

T 

26-9 

T 

T 

T 

T 

T 

T 

T 

T 

T 

T 

LYNCH core 710-78 

13-15 

34-36 

80-82 

195-197 

225-227 

297-299 

299-301 

301-303 

304-306 

345-347 

372-375 

434-436 

443-445 

542-544 

557-559 

2 

T 

T 

T 

T 

4 

10 

50 

3 

T 

1 

T 

T 

1 

3 

35 

34 

29 

25 

20 

21 

19 

27 

17 

27 

18 

34 

34 

30 

14 

14 

14 

14 

14 

14 

13 

-1 

42 

35 

37 

52 

73 

— 

— 

25 

--

42 

55 

32 

72 

46 

51 

% 

clay 

61 

66 

71 

75 

80 

79 

80 

70 

83 

73 

82 

66 

66 

70 

86 

86 

86 

86 

86 

86 

87 

56 

65 

63 

48 

27 

— 

--

25 

--

58 

44 

68 

28 

55 

46 

silt/clay 

ratio 

0.57 

0.51 

0.40 

0.33 

0.25 

0.26 

0.23 

0.38 

0.20 

0.36 

0.21 

0.51 

0.51 

0.42 

0.16 

0.16 

0.1 6 

0.16 

0.16 

0.16 

0.14 

0.75 

0.53 

0.58 

1.08 

2.70 

--

— 

1.0 

— 

0.72 

1.25 

0.47 

2.57 

0.83 

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 

DEPTH 

(cm from 

core top) 

7-8 

49-50 

109-110 

172-173 

212-213 

240-241 

265-266 

327-328 

384-385 

. 400-401 

465-466 

500-501 

575-580 

660-661 

720-721 

745-746 

807-809 

923-924 

985-986 | 

SIZE 

ANALYS1 

(percentag Sand Silt 

'•:'•:•• : 

::••':: 

#*** — 

^»if»> "T**.' iff 

H S F 

Ji 

" • -'••."• 

•".•.•- ' 

mk 

:•.••:•:•:::• — 

SILT AND SAND FRACTIONS "• = Abundant ^ = Few ^ —— , 

c ^ ^ = Common p - _ ^m 1 

a) MICROSCOPE EXAMINATION y-_ Present ' _lrac e

 | ^—Sil t 

Clay 

fera 

J 

Foramin 

• 

- -

- '/ 

A A 

y 

A 

_ / 

! ' 

Coc eolith 

Carbonate 

fragment 

• 

/y 

// 

_J 

4 

A 

A 

Radioiaria // 

y 

j 

L 

Sponge spicule 

y 

' 

—£ 

Diatom 

A 

y 

A 

/ 

. / 

Opaques Manganese micronodules 

y 

y 

>/ 

'/ 

Siliceous 

aggregate 

V 

y~ 

7 

Heavies A 

y 

y 

? V 

/{ 

Brown mica 

^ 

Quartz 

7 J [ 4 

A • 

y 

v 

•• 

Dolomite 4 

4 

•-

i 

//, 

•

 f 

• //< 

Organic Fiber 

Palagonite 

fA 

4 

y 

A 

4 

A 

. y 

Sideromelane A 

y 

//, 

— 

Chlorophaeite A 

A 

A 

L.,.. 
NUMBER 11 39 

TABLE 2.—Continued 

CLAY-SIZ E FRACTIO N 

X RAY DIFFRACTION 

(percentage) 

Montmorillonite r 

T 

Mica 

Kaolinite plus 

Chlorite 

•42|33 

K3I37 

TH4 6 

r 

1I55 

J 

•40 

\ 

1, 

Amphibole 

1 -

1 

• . 1 

Quartz 

K feldspar 8 

5 

4 

Plagioclase 9 

9 

5 

Calcite 8 

2 

-

T 

Dolomite 

SILT - SIZ E FRACTI O 

X RAY DIFFRACTION 

(percentage) 

Mica 

8 

5 

Kaolinite plus 

Chlorite 

8 

2 

1 18 | 20 

1 5 l 23 

3 

6 

4 

3 

3 

8 

5 

3 

Amphibole 

2 

T 

1 

T 

Quartz 7 

5 

10 

8 

K feldspar 

Plagioclase 

Calcite 

N Dolomite 

7 | 231 20 1 25 

7l 29I24I28 

. 

1 15 | 16 | 15 

4 | 15 | 20 

1 1 | 24 1 29 10 

15 

20 

! 

! 

TI 8 | 10 | 28| 11 1 29 

2 

1 

7 l M l 32 1 11 25 

1 

i 

r 

1 nl i/l 1 12I is Manganese, fluorescence 

(as counts/0.1 mg/200 sec) 

5.9 

7.8 

6.8 

3.2 

4.6 

5.2 

6.0 

5.2 

REMARKS 

Few aggregates are iron cemented, some are dark red; 

inorganic crystalline calcite and feldspar present. 

Inorganic crystalline calcite abundant; silt is 

coarse-grained. 

Terrigenous sand with abundant feldspar and with 

inorganic crystalline calcite; silt is poorly sorted. 

Same as above. 

Same as above. 
40 SMITHSONIAN CONTRIBUTIONS TO TH E MARINE SCIENCES 

TABLE 3.—Summary of results of grain-size and compositional analyses of core Vema 22-231, 

depth 5535 m 

DEPTH 

(cm from 

core top) 

0-2 

11-13 

86-87 

122-123 

168-170 

245-247 

253-255 

. 401-403 

498-500 

672-673 

681-683 

913-915 

927-929 

1050-1052 

1221-1223 

1236-1237 

1304-1305 

1353-1354 

1460-1462 

1480-1481 

SIZE 

ANALYSIS 

(percentage) Sand 

•:•...• 

^ ^ 

j | . 

— 

1 

— 

= 

:^=^ = 

- a 

-

—& 

SILT AND SAND FRACTIONS "• = Abundant f T = Few ^^m~ ] 

^ = Common f/ -_ T j 1 

MICROSCOPE EXAMINATION y = Present ' " ^ 1—^--Sil t Foraminifera 1 

y 

7 " 

/ 

/ 

s 

y 

A 

A 

/ 

/ 

Coccolith 

Carbonate 

fragment 

M 

/, 

A 

-A 

r— 

A

M 

y 

• " 

J 

AL 

Radioiaria A 

7 

Sponge spicule 

A , 

y— 

A 

~y "* 

- ^ ^ 

1 ! 

A 

A 

A 

Diatom 

/ 

Opaques Manganese micronodules 

A 

A 

Siliceous 

aggregate 

y 

y 

'A 

• ^ A 

A 

• 

\ 

A 

f 

y 

y~ 

Heavies 

/ 

A 

. / 

A 

A 

y 

y 

Y 

A 

Brown mica 

/ 

/ 

Quartz 

7^ 

A 

/ 

Dolomite 

_JL 

Organic fiber 

• 

m 

M 

A A 

A 

A A 

y \ 

A 

i 

i 

A 

/A 

^U ' • A 

•~ 

A 

A 

A 

7 

_L4^ 

A 

• 

7 

V 

Palagonite 

'/ 

Sideromelane 

Chlorophaeite 

_A_A ji 

s 

/, 

y 

A 

/ 

A 

/4 

V~ 

AA AA 

/ / 

/ 

A 

/ 

A 

A 

y 

/ 

. / 

A \ /< 

S, 

A 

A 

A 

/ 

/ 

Si 
NUMBER 11 41 

TABLE 3.—Continued 

C L A Y- S 1 Z E FRACTIO N 

X RAY DIFFRACTION 

(percentage) 

Montmorillonite 

Mica 

| 

IM53 

* 

TB49 

- - -

TB41 

. 1 

Kaolinite plus 

Chlorite 

| 3 3 

1, Amphibole 

II 

1 

20 

Quartz 

K feldspar 

T 

T 

6 

Plagioclase 

Calcite 

T | 25 

T 

T 

T 

11 

T 

22 

Dolomite 

SILT-SIZ E FRACTIO N 

X RAY DIFFRACTION 

(percentage) 

Mica 

Kaolinite plus 

Chlorite 

M | 17 

6 4 

211 25 

6 

8 

6 

7 

si 11 

1 5 | 19 

1 

8 | 9 

7 

T | 10 

I 

7 

9 

15 

Amphibole 

Quartz 

cn 

4 4 

1 1 14 

1 

2 

1 

3 

7 

5 

9 

10 

K feldspar 

Plagioclase 

5 | 16 

Calcite 15 

Dolomite 

24 

7 1 ie l 2d | 31 

3 1 12 | 121 12 

111 27| 12 

4 | 20 I 32 

5 | 18p 18 

?\ 17 4 

1 

30 

22 

30 

29 

T | 12| s l 231 15 | 22 

T 

1 

1 

2 

11 

7 

13 

7* 221 20 | 25 

3 | 13^341 23 

7

I

 24i 1 15 

Manganese, fluorescence 

(as counts/O.I mg/200 sec) 

7 . 3 

8.1 

10.0 

6.2 

8.6 

8.5 

7 . 6 

REMARK S 

Most aggregates are iron stained; some 

manganese droplets are oxidized . 

Siliceous aggregates are light brown. 

Siliceous aggregates are gold colored; 

silt grains are unusually large. 

Inorganic crystalline calcite grains are 

chemically etched. 

Aggregates are light brown or white; silt 

is coarse-grained; one grain of garnet. 

Aggregates are white and occasionally light 

brown; inorganic crystalline calcite noted. 

Garnet is present. 

Terrigenous sand, poorly sorted . 

Terrigenous sand, poorly sorted. 

Terrigenous sand, poorly sorted, with garnet; 

aggregates are iron stained. 
42 SMITHSONIAN CONTRIBUTIONS TO TH E 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 

DEPTH 

( cm from 

core top) 

•." 

,*. 

'.'. 

2-3 

-

29-32 

45-47 : 

124-126 = 

227-229 

272-274 

321-323 = 

344-346 ,: 

393-395 = 

474-476 

544-546 E 

VEMA 26-9 |(= 

1-3 :z 

44-46 

51-53 -

159-151 E 

225-227 E 

280-282 = 

407-409 E 

435-437 = 

503-505 = 

593-595 |E 

SIZE 

ANALYSIS 

(percentage) Sand Silt 

:

:

:'.' 

•;.*.• , 

-•".•• 

-

o 

U 

_ 

— 

SILT AND SAND FRACTIONS "• = Abundant ^ = Few ^ 

^ = Common 17"" _T 

MICROSCOPE EXAMINATION p " = Present ' Foraminifera 

/ 

r 

- -

y 

•/ 

y 

y 

— 

Coccolith 

Carbonate 

fragment 

M'A 

~ ^ 

j/ 

A 

/ 

/ 

A 

M

M

4 

_y 

A 

A 

A 

Radioiaria 

Sponge spicule 

V 

y 

Diatom 

Opaques Manganese micronodules 

y 

y 

" 

r 

Siliceous 

aggregate 

A 

y 

A 

y~ m 

y • 

y m 

/ 

V 

y 

V 

1 

A 

y 

y 

• 

A 

y 

: 

r 

r 

Heavies 4 

4 

/ 

A 

/ 

A 

A 

A 

Brown mica 

/ 

Quartz A 

4 

V \ 

A 

A 

4 

V 

y 

V 

Y A 

V 

> sb 

Dolomite A 

A 

S< 

4 

V 

AA ,A; 

r_ 

V~~ 

s 

WA 

A 

y 

„y< 

y 

4 

A 

-

Organic fiber 

7 

'A 

A 

r 

'A 

'A 

'A 

'/ 

y 

y 

y 

y 

LLA— Silt Palagonite 4 

V~ 

Sideromelane 

/A 

Chlorophaeite A 

A A 

Ay 

A 4 yj 

4 

vy x 

A | J J 

\A^ 

''il l 

y^y j ' 

^4 

A 

A 

y , 

\~AA 

y 

AL 

A 

y 

A 

z 

r 

• 4 V w 

rmm A 

_ • . mi. m 

7 
NUMBER 11 43 

TABLE 4.—Continued 

1

 CLAY-SIZ E FRACT I 

X RAY DIFFRACTION 

(percentage) 

Montmorillonite 

Mica 

Kaolinite plus 

Chlorite 

1 18 | 23 

t 

Lo 

I44 

Li 

Li 

[I 

If 

| 2 8 

T 

7 

25 

28 

13 

26 

29 

25 

12 

21 

18 

14 

21 

39 

46 

38 

_26_ 

20 

.2zl 16 

261 31 

30M 23 

2m 19 

461 9 

21 

28 

27 

26 

45 

54 

45 

34 

44 

39 

291 

28 

25 

5? 

57 

43 

4.7, 

52[ 

Amphibole 

| 

45[ _ 

Quartz 5 

6 

T 

4 

T 

K feldspar 4 

T 

5 

4 

T 

Plagioclase 

10 

O N Calcite 

| 4 9 

I 33 

T 

10 

T 

T 

T 

7 

1 22 

9 

5 

Dolomite 

1 

S 1 Mica , 0 

16 

23 

19 

18 

16 

6 

5 

14 

15 

18 

15 

25 

28 

23 

23 

23 

21 

16 

LT-SIZ E FRACTIO N 

X RAY DIFFRACTION 

(percentage) 

Kaolinite plus 

Chlorite 

U 

17 

25 

20 

26 

36 

35 

43 

23 

15 

14 

15 

23 

24 

26 

26 

20 

29 

.30 

Amphibole T 

2 

3 

2 

2 

1 

5 

4 

Quartz 5 

13 

14 

17 

15 

9 

9 

5 

9 

11 

11 

11 

13 

13 

13 

13 

11 

11 

20 

K feldspar 

Plagioclase 

Calcite 

3 l 1?|4 7 

8 

11 

12 

9 

20 

13 

19 

34 

6 

8 

8 

14 

13 

13 

18 

17 

16 

Dolomite 8 

2 2 1 '3 | 9 

24 

30 

32 

19 

30 

28 

20 

121 19 

20 

22 

25 

22 

20 

20 

29 

23 

25| 9 

21 

24 

25 

Manganese, fluorescence 

(as counts/O.I mg/200 sec) 

9 .7 

7.1 

26.3 

31.5 

37.1 

37.4 

54. 9 

34.9 

60. 9 

58. 9 

49. 5 

8 .3 

12.3 

12.8 

20.7 

17.4 

23.8 

29.5 

25. 8 

29.1 

32.5 

REMARK S 

Large platy iron fragments. 

Inorganic crystalline calcite; weathered 

glass with abundant inclusions. 

Aggregates are iron stained; some weathered 

glass; rod-shaped silt crystals probably pyroxenes. 

Few feldspar grains. 

Few feldspar grains; these and other sand grains 

are rounded to subrounded. 

Few feldspar grains; silt poorly sorted. 

Few feldspar grains. 

Few feldspar grains; silt poorly sorted wit h 

rod-shaped grains. 

Few feldspar grains; silt is coarse-grained. 

Aggregates are occasionally iron stained; 

inorganic crystalline calcite in silt . 

Few feldspar grains; silt, poorly sorted, 

with rod-shaped grains. 

Few feldspar grains; rod-shaped grains in 

the silt fraction. 

Siliceous aggregates were originall y 

brown glass. 

Weathered glass present. 

Feldspar and weathered glass present. 

Few aggregates are iron cemented; few 

feldspar grains. 

Few feldspar grains; weathered glass present; 

rod-shaped grains in silt . 

Aggregates are iron stained; rod-shaped 

grains in silt fraction . 

Aggregates are white or red; few feldspar 

grains; rod-shaped grains in silt fraction . 
44 SMITHSONIAN CONTRIBUTIONS TO TH E 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) 

13-15 

34-36 

80-82 

195-197 

225-227 

SIZE 

ANALYS 

(percentag 

IS 

SILT AND SAND FRACTIONS 

MICROSCOPE EXAMINATION 

( | = Abundant p7" = Few 

r

= Common r/"" T 

_ _ X = I race 

W^ = Present 

Sand "DrA 

V 

6-A 

2. z 

Opaques 

y 

v 

A 

7 7 

y 

V 

y 

7~ 

..A 

A_A 

297'-299 

299-301 

301-303 

304-306 

345-347 

373-375 

434-436 

443-445 

542-544 

557-559 

'V 

A l 

: 

A 

V 

^ i A 

m _^. . 

v—yy 

Jl A 

A 

A 4 

1 A. 

A 

A 

,4 

A 

_A 

A 
NUMBER 11 45 

TABLE 5.—Continued 

CLAY-SIZ E FRACTIO N 

X RAY DIFFRACTION 

(percentage) 

Montmorillonite 

__V7 

14 

31 

,IP_ 

8 

Mica 

20 

47 

53 

42 

54 

Kaolinite plus 

Chlorite 

28 

30 

18 

30 

33 

6 | 29 | 17 

14 

15 

12 

13 

15 

30 

49 

46 

35 

39 

9 1 15 

41 

30 

30 

36 

24 

16 

Amphibole 

Quartz 5 

2 

3 

2 

2 

5 

2 

4 

K feldspar 2 

1 

1 

2 

3 

1 

2 

4 

2 

Plagioclase 

Calcite 

4 J 24 

2 

4 

4 

4 

4 

8 

5I4 2 

7 

5 

3 

2 

5 

4 | 15 

/ l l 

Dolomite 2 

T 

SILT-SIZ E FRACTIO N 

X RAY DIFFRACTION 

(percentage) 

Mica 

1 

26 

22 

16 

14 

7 

21 

18 

25 

20 

15 

Kaolinite plus 

Chlorite 

16 

31 

38 

15 

13 

7 

16 

19 

22 

20 

12 1 , . Amphibole Quartz 9 

10 

14 

11 

8 

7 

14 

17 

9 

?3 

9 

5 

K feldspar 

3 

4 

3 

4 

3 

Plagioclase 

Calcite 

ll|3 8 

12 

14 

T 

I T ! 16 

14 

Dolomite 12 

17 

9 

21 

• 4 8 

Tl 12|55 | 12 

6 

6 

3 

7 

3 

3 

21 

23 

10 7 

is! 

15I29 

13^56 

22 

17 

14 

12 

17 

,0 

Manganese, fluorescence 

(as counts/O.I mg/200 sec) 

3630 

1890 

1890 

3310 

2800 

3110 

2350 

2580 

3470 

1840 

4200 

4620 

REMARKS 

Few foraminifera tests are iron stained; ash is 

devitrified. 

All foraminifera tests and quartz grains are 

iron stained. 

Abundant devitrified glass. 

Feldspar and large quartz grains. 

Foraminifera fragments are ironstained; 

hornblende. 

Few foraminifera fragments are ironstained. 

All foraminifera fragments are iron stained. 
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