40.
WESTERN NORTH ATLANTIC: SEDIMENTARY EVOLUTION
AND ASPECTS OF TECTONIC HISTORY
1
Brian E. Tucholke, Lamont-Doherty Geological Observatory of Columbia University, Palisades, New York
and
Peter R. Vogt, Naval Research Laboratory, Washington, D.C.
ABSTRACT
Drilling results from DSDP Leg
43
and earlier legs in the western
North Atlantic are synthesized to examine the sedimentary history
and aspects of the tectonic evolution of the basin. In the Late
Jurassic, reddish calcareous claystones and limestones were
deposited in a pelagic environment near and above the CCD. Lime¬
stone deposition persisted throughout the basin until the end of
Neocomian time when a sharp rise of the CCD to less than 2800
meters occurred. This event was coeval with the beginning of "black-
clay' ' deposition and formation of euxinic basin conditions at least
below the CCD. The stagnant deep basin probably was created by
the formation of circum-Atlantic deep-circulation barriers coinci¬
dent in time with tectonic readjustments in the North Atlantic and
the initial opening of the South Atlantic. At the same time, rudist-
coral reefs flourished along the continental margin as far north as
Nova Scotia. Although the CCD remained shallow until the
Maestrichtian, the deep basin became oxygenated in the late Ceno-
manian/Turonian, probably because of the introduction of
deep
and
bottom water from the South Atlantic, and multicolored clays were
deposited at true pelagic rates of 1-3 m/m.y. Little terrigenous
detritus entered the deep basin because of the Cretaceous trans¬
gression and because of (now largely extinct) shelf-edge barrier reefs.
Volcanic detritus from the then active New England Seamounts and
from other background volcanism comprises a locally significant
component of the multicolored clays. A sharp depression of the
CCD to more than 5400 meters occurred in the late Maestrichtian to
earliest Paleocene. This event resulted in basin-wide chalk deposition
and may relate to the end-Cretaceous extinctions. Terrigenous
detritus began to enter the basin in the early Paleocene, and locally
deposited black clays indicate that deep-basin circulation was again
sluggish. Highly siliceous sediments were deposited during the latest
Paleocene to late Oligocene in response to developing abyssal circu¬
lation, upwelling, and enrichment of nutrients in surface water by
volcanism. The latter played an especially significant role in late-
early to early-middle Eocene rapid deposition of biogenic silica and
development of "Horizon A" cherts. Siliceous turbidites flooded the
basin during this interval, but the Bermuda Rise was isolated from
the influx when it was uplifted during the middle to late Eocene.
During the Oligocene to earliest Miocene, the abyssal circulation in¬
tensified because of climatic cooling and tectonically controlled in¬
troduction of bottom water from the Norwegian-Greenland seas;
several hundred meters of sediment were eroded along the existing
continental rise, forming a major unconformity. A sharp change to
depositional conditions occurred in the early to middle Miocene, and
rapid Neogene hemipelagic sedimentation controlled by moderately
strong bottom currents is responsible for formation of the present
continental rise and outer-ridge systems. The CCD has gradually
deepened from about 4 km to more than 5 km during the Tertiary,
but little carbonate is present in sediments other than on the Mid-
Atlantic Ridge because of dilution by current-transported terrige¬
nous debris.
'Contribution Number 2658 of Lamont-Doherty Geological Observatory.
791
B.E. TUCHOLKE, P.R. VOGT
INTRODUCTION
Leg
43
of the Deep Sea Drilling Project was designed
to study three primary problems of the history of the
western North Atlantic. First, drill sites were selected in
areas where we anticipated that we could core previous¬
ly unrecovered sections of the sedimentary record in
order to document more completely the sedimentary
evolution of the basin. These sites also were located
specifically in areas where the basin's major seismic
reflectors could be cored, dated, and correlated with
sediment types. Determining whether these seismic
markers were of the same age and indicated similar
lithofacies throughout the basin was necessary to test
the validity of reflector mapping as a tool for inter¬
preting paleosedimentation patterns.
Secondly, a major objective of Leg 43 was to sample
basalt from the prominent /-Anomaly basement ridge
between the New England Seamounts and the Grand
Banks. A very strong magnetic anomaly, locally greater
than 1000 gammas, is associated With this feature, and
geochemical and magnetic studies of the basalt would
help establish whether unusual basement composition
was responsible for the anomaly. The anomaly does not
result from a geomagnetic field effect because a similar
anomaly is not associated with Pacific crust of compar¬
able age.
Finally, two sites were drilled along the New England
Seamounts to determine the age and nature of volcan-
ism that constructed this linear seamount chain. If vol-
canism had occurred as the North American plate moved
north-westward over a stationary mantle hot-spot,
progressively more youthful volcanism toward the
southeast should be observed along the seamount chain.
Conversely, volcanism which was essentially syn¬
chronous along the chain could indicate widespread ac¬
tivity along a tectonic (fault?) boundary within the
plate.
The first two of these subjects are covered in this
report. A companion paper (Vogt and Tucholke, this
volume) explores the evolution of the New England Sea¬
mount Chain.
Six holes at six sites (382 to 387) were drilled on Leg
43 to meet the objectives outlined (Figures
1
and
2).
Site
45
C
85°
80°
75°
70
I " ' ' 'I
DSDP BOREHOLES
LEGS 1,2,4,11,43,44
40
e
1.
zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Locations of Deep Sea
Drilling
Project sites in the western North Atlantic. Leg 43 sites
are
underlined.
792
SEDIMENTARY EVOLUTION
383,
drilled into the Sohm Abyssal Plain above the
/-Anomaly Ridge, was the only site which failed to meet
our minimum objectives; the site was abandoned after
only 120.3 meters of penetration when caving Pleisto¬
cene sands threatened to anchor the drill string.
In the following pages we summarize major aspects
of the depositional and tectonic history of the North
American Basin, based on Leg
43
results and integrated
with data from DSDP Legs
1,2,4,
and
11
in the western
North Atlantic. The reader will find it useful to refer to
the color foldouts at the back of
this
volume which sum¬
marize Hthology versus age for western North Atlantic
drill sites. Summaries of sediment compositions in this
chapter are based on appropriate results reported in this
volume and in Ewing, Worzel, et al. (1969); Petersen,
Edgar, et al. (1970); and Hollister, Ewing, et al. (1972).
In our discussion we will refer to a summary diagram
showing the history of the calcite compensation depth
(CCD) in the western North Atlantic (Figure 3). Details
on the construction of this figure are given in Appendix
A. The CCD marks the level in the water column at
which carbonate input from surface waters is balanced
by dissolution in the undersaturated deep water. For the
purpose of constructing Figure 3, we have followed the
usage of van Andel (1975), placing the boundary be¬
tween calcareous and non-calcareous pelagic sediments
at 20 per cent CaCO
3
; a boundary of 2 per cent CaCO
3
was used for hemipelagic deposits with major dilution
by terrigenous sediment.
UPPER JURASSIC LIMESTONES
Leg
43
sites did not penetrate Jurassic sediments, but
a brief discussion of earlier results from Leg 11 is in
order here. Figure 3 shows a compilation of lithofacies
variations along sea-floor age-depth curves of western
North Atlantic sites. The Oxfordian/Kimmeridgian red¬
dish limestones generally contain more clay and less car¬
bonate than overlying limestones; in fact the two Oxfor-
dian cores above basalt at Site 105 have 20 per cent or
less carbonate, indicating that the CCD was shallower
than
3
km at this site. At Site 100, however, Oxfordian
greenish gray limestones have higher carbonate content,
and they suggest a deeper CCD near the Bahama Banks.
Alternatively, it also is possible that Site 100 and Holes 4,
5,
and 99A nearby have suffered anomalous subsidence
because of their location near the Bahama Banks. If so,
these sites accumulated carbonate above the CCD level
depicted in Figure 3. Because the biostratigraphic con¬
trol on ages at these sites is still imprecise, it is difficult
to obtain any detail on Late Jurassic fluctuations of the
CCD.
However, in view of the apparently elevated Ox¬
fordian CCD at Site 105, it would not be surprising to
find that some pre-Oxfordian sediments in the North
Atlantic are clay-rich and carbonate-poor. The subse¬
quently deposited, carbonate-rich Tithonian limestones
demonstrate depression of the CCD that continued into
the Early Cretaceous.
LOWER CRETACEOUS (NEOCOMIAN) LIMESTONES
Description
Site 387 was the only Leg 43 drill site that penetrated
Neocomian sediments; however, the Site 387 results are
especially important because these sediments record
Neocomian ridge-flank deposition, in contrast to
previously drilled "basin-margin" holes (4, 5, 99A, 100,
101,
105).
At Site 387 the top of the limestones correlates with
HorizonzyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA ß (Site 387 Report; Tucholke, this volume).
Similar correlations occur at Sites 5,
101,
and 105 where
Horizon ß can be identified unambiguously (Ewing and
Hollister, 1972). Horizon ß in the vicinity of Site 387
tends to drape over the irregular acoustic basement, and
the seismic interval between the horizon and basement
does not exhibit clear acoustic lamination. This is in
marked contrast to the strong acoustic lamination in the
ß to basement interval near Sites 5 and 101, but it is
similar to the pre-/5 acoustic section at Site 105.
The Neocomian limestones cored below Horizon ß
generally have similar compositions and textures in all
the western North Atlantic drillsites. At Site 387 there
are two predominant limestone facies; one is a light col¬
ored, hard limestone which has high carbonate content
(>90%) and is extensively bioturbated. This facies
dominates the central part (Cores 40-47) of the ß to
basement limestone sequence. At the top and bottom of
the sequence (Cores 38-39 and 48-49) a second limestone
facies is more common and consists of softer, darker,
laminated limestones with lower carbonate content
(<
50-90%).
Although these facies tend to be dominant
in the intervals as described, the two facies are inter¬
bedded throughout the section. Overall core recovery in
the limestones was low (<25%) at Site 387, but it gen¬
erally was comparable to that encountered in the lime¬
stones at other sites. The noncarbonate fraction of the
sediment at Site 387 contains mostly clay minerals,
primarily illite and minor montmorillonite (Koch and
Rothe, this volume). Traces of clinoptilolite and 1-10%
quartz (probably authigenic) are present. Compared to
Site 387, Neocomian limestones at other sites have an
average composition with less illite (Holes 99A, 100, 101,
105) and more montmorillonite (Site 105); part of this
difference probably results from different sampling
densities in clayey versus limey interbeds at the various
sites.
The micritic calcite at all sites probably is derived
from nannoplankton which are often well preserved in
the clayey interbeds but are totally recrystallized in the
hard limestones. Radiolarians, often pyritized, occur
locally, and foraminifers are rare or absent. Organic
carbon contents of Site 387 limestones tend to be high
(up to 4.8%) in comparison to other sites (mostly
<1%),
but this may be due to the Site 387 shipboard
sampling being somewhat biased toward darker, marly
interbeds in the limestones. Quartzose chert is present
throughout the ß-to-basement sequence at Site 387, and
it also is common in the limestones at Holes 99A and 100;
793
B.E. TUCHOLKE, P.R. VOGT
SOHM ABYSSAL PLAIN zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
383 -5277 m
J-ANOMALY RIDGE
384 -3910 m
386
BERMUDA RISE
-4783 m 387 -5118 m
EXPLANATION
Hiatus
Nannoplankton
ooze,
Chalk
Nanno-For;
ooze,
Chalk
in contrast, limestones at Sites
101
and
105
rarely contain
chert. Rates of accumulation for the Neocomian lime¬
stones at Site 387 are about
11
m/m.y. (2.4 kg/cm
2
-m.y.
at an average dry bulk density of 2.16 g/cm
3
), which is
slightly higher than the rate for the corresponding litho¬
facies at Holes 5, 99A, and 105 (Table 1).
Depositional Environment
The depositional environment during the Neocomian
probably was much the same throughout the southern
part of the North American Basin. The CCD was deeper
than 4 km (Figure 3), but the carbonate lysocline for
foraminifers probably was shallower, thus accounting
for the paucity of foraminifers in the limestones.
Pelagic deposition on a quiescent, oxygenated sea floor
persisted during the Berriasian, but beginning in the
Valanginian there is evidence for increasingly common
brief episodes of poorly oxygenated bottom water and
consequent deposition of dark> carbon-rich, finely lami-
nated, and burrow-free clayey interbeds in the lime¬
stones. At Site 387, cyclic variations in deposition are
observed, consisting of (from the base): (a) dark, lami¬
nated, marly beds, (b) gray laminated limestones, and
(c) light bioturbated limestones. One possible inter¬
pretation is that each cycle represents a progressive in¬
crease in deep-water oxygenation and surface produc¬
tivity of nannoplankton. The sluggish circulation sug¬
gested for the deep water by the darker, carbonaceous
interbeds may also have prevailed in surface waters,
with reduced upwelling of nutrient-rich water, reduced
productivity, and consequent deposition of marly rather
than limey sediments. Alternatively, cycles of deep-
water sluggish circulation and of surface water produc¬
tivity may have acted independently, with the former
cycle determining the preservation of any calcareous
organisms reaching the sea floor.
There is little doubt that radiolarians were the main
source of chert-forming silica at Holes 99A, 100, and
794
Figure 2.zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Lithologic summary of Leg 43 drill sites.
NASHVILLE SMT.
382 -5527 m
-
-
= ^
Quaternary
1
zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
1
£
U
1
M
,
j
L
■
\
HU
β
■
Silt and clay
turbidites
Green-gray
hemipelagic
clay
Variegated
volcanogenic
clays,
siltstone,
sandstone;
volcaniclastic
breccia
VOGELSMT.
385 -4956 m
_
_
Quat.
Olig.
M
Eoc.
L
Paleoc.
Maestr.
Upper
Cretaceous
■■
■H
■
—
■■
"―β
M
MH
Mi
US
Clay
Rad.
clay and
ooze,
chert
Zeolitic
silty clay
Marly nanno
Volcanogenic
clay, silt, sand;
basalt,
volcaniclastic
breccia
Figure 2.zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA (Continued).
387,
and that siliceous organisms were more common in
the original sediments at these sites than is now ob¬
served. This observation, considered with the scarcity of
radiolarians nearer the continental margin (Site 105),
suggests that siliceous productivity may have been
restricted to a zone extending from the Bahama Banks
into the central basin of the Atlantic. Such a produc¬
tivity belt could be expected along the path of presumed
circumglobal flow through the Tethys, Atlantic-
Caribbean, and Pacific in the Early Cretaceous, when the
easterlies wind belt extended perhaps 10° farther north
than it does today (Berggren and Hollister, 1974;
Luyendyk et al., 1972).
SEDIMENTARY EVOLUTION
Increasingly frequent episodes of bottom-water stag¬
nation near the end of the Neocomian are indicated by
the abundance of dark marly interbeds near the top of
the HorizonzyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA ßzyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA to basement limestone sequence. This
process culminated during the Barremian in a probably
sharp rise of the CCD (Figure 3) and resultant deposi¬
tion of black clays. Horizon ß marks this change in
lithofacies, the reflector arising from the impedance
contrast at the top of the shallowest high-velocity car¬
bonates underlying the black clays. Possible causes for
the change in depositional regime are discussed in the
following section.
BARREMIAN TO CENOMANIAN BLACK CLAYS
Description
Sites 386 and 387 both penetrated Cretaceous black
clays.
The 240-meter black-clay section at Site 386 is ex¬
ceptionally thick because the site was drilled within a
fracture valley that accumulated pelagic sediment rapid¬
ly shed from adjacent bathymetric highs. The only other
locations with such thick sections are Site 101 (193-296
m) and Site 391 (about 275 m; Benson, Sheridan et al.,
1976) near the continental margin; at Site
101
the top of
the black clays is truncated by an erosional unconformi¬
ty (Horizon A
u
t
see Tucholke, this volume), so the
original thickness must have been even greater. Other
sites (5, 105, and 387) all have a black-clay thickness of
about 100 meters.
Site 386 is also unusual in that the black clays are
deposited directly on lower Albian basaltic basement.
Although this fracture-valley basement was below the
mean ridge-crest elevation during Albian time, it shows
that the black clay depositional field extended to depths
at least as shallow as 3200 meters (Figure 3). Some of
the adjacent peaks which shed calcareous debris into the
valley were above the CCD and probably above the level
of anoxic bottom water. The shallowest of these peaks,
a few kilometers south of Site 386, rises 1800 meters
above the fracture valley basement and had a ridge-crest
paleodepth approximately 1400 meters below sealevel.
TABLE 1
Thickness and Approximate Accumulation Rates
a
of Some Dominant Lithofacies in the North American Basin
Site
Hemipelagic
clay
Eocene
siliceous
turbidites
Maestrichtian
chalks/
limestone
Multicolored
clays
Black
clay
White and
gray limestone
Red clayey
limestone
Homogeneous
green-gray
limestone
101
>203m
(20m/m.y.)
293-340m
(~12m/m.y.)
>92m
(?18m/m.y.)
-
100
-
-
-
>9m
(?)
39-64m
(~8m/m.y.)
41m
(~8m/m.y.)
5
-
-
100-105m
(~3m/m.y.)
>93m
(~7m/m.y.)
-
105
>193m
(19m/m.y.)
49-97m
(??2m/m.y.)
~llOm
(~5m/m.y.)
~139m
(~lOm/m.y.)
(~lOm/m.y.)
99A
-
-
-
158-181m
(~7m/m.y.)
>13m
8 387 7
>177m 41-98m
(~13m/m.y.)
(?2m/m.y.)
>56m >210m >>lOm
(?)
(~20m/m.y.)
25.6m
9.5-19.Im
>18m
(<lm/m.y.)
95-114m
(4-5m/m.y.)
199-2O8m
(10-llm/m.y.)
- - -
6
161-190m
(?4m/m.y.)
>66m
(?15-20m/m.y.)
-
-
-
385
>108
(4-5m/m.y.)
45m
-
-
-
386
148m
(9m/m.y.)
285-314 m
b
(~27m/m.y.)
2.5m
90-109m
(~3m/m.y.)
240m
(~16m/m.y.)
-
-
9
-
>75m
(~3m/m.y.)
-
-
-
In parentheses,
t>lncludes calcareous turbidites.
795