LITHOSTRATIGRAPHY
Introduction
Two lithostratigraphic
units are recognized in Hole 901 A. Unit I
was recovered only in a wash core (149-901A-1W) and consists of
nannofossil clays
and clays with silt yielding late Aptian, middle
Miocene, and late
Pliocene ages. As the hole was washed down to 182
m, the depth of
the boundary between Units I and II has been con
strained to lie
only between 0 and 182 mbsf. The wash core barrel was
recovered after
the drilling rate slowed, indicating that the bit had
encountered harder rocks. This suggests that the Unit I rocks cored
probably came from
just above 181 mbsf. Unit II was cored discontinuously over an
interval of about 57 m and consists of olive black
clays and
parallel-laminated calcareous sandstones of early Tithonian
age. The ages,
lithologic composition, colors, facies and depositional
environment, and
cored intervals for Units I and II are summarized in
Table 2.
Seafloor to Core 149-901
A-1W-2, 50 cm
Depth: 0(?) to about 182(?) mbsf
Age: Pleistocene(?) to late Aptian
General Description
This unit was recovered
in a single washed core that contains
pinkish gray (5YR 8/1), yellowish gray (5YR 8/1), and moderate
yellowish brown (5YR 6/2) nannofossil clay and clay with silt. The
massive, structureless appearance of the sediments may result from
extreme core
disturbance and/or bioturbation. A thin (5 mm) layer of
gray clay at
Sample 149-901 A-1W-2, 50 cm, yielded nannofossils
indicating a late
Aptian age; the overlying lighter-colored clays gave
middle Miocene and
late Pliocene ages (for details, see "Biostratigraphy" section, this
chapter). The Pleistocene age for the top of the unit
is not proven
biostratigraphically, but assumed by comparison with
other Leg 149
sites.Depositional Processes
The sediments recovered
from Unit I are broadly comparable to
those in Subunits
IB and IC at Site 900 and are probably pelagic and/or hemipelagic, although a contribution from muddy turbidity flows
cannot be ruled
out. The absence of siliciclastic silts and sands is consistent with a continental-rise setting where bypassing of coarser sediment would have
occurred.
Cores 149-901 A-1W-2, 50
cm, to 149-901A-7R-1, 41 cm
Depth: 182(?) to
247.8 mbsf
Age: early Tithonian
General Description
Core recovery for Unit II
totaled 4.9 m (excluding wash cores) and
ranged from 0% to almost 20%. About 90% of
the total section
recovered consists of olive black (5Y
2/1) clay or clay with silt, which
in places contains thin
laminae of silt and black plant debris. The dark
color of the clays
is probably the result of a high content of organic
matter. Intervals
showing color banding (up to 1 cm thick) in shades
of gray show no
signs of bioturbation.
Greenish gray (5GY 6/1)
and light olive gray (5Y 6/1) parallel
laminated calcareous sandstone forms about 10% of the unit. Many
laminae are formed of black plant debris. The sandstone intervals
range in thickness
between 10 and 20 cm and often are fractured along
laminae. Apparent
dips of up to 10° probably result from tilting of the
core pieces during
coring.
Occasional pieces of thick
(up to 10 cm), very fine-grained,
olive gray (5Y 4/1) dolomite with calcite
occurs within the dark
colored clays.
Depositional Processes
In the absence of any
diagnostic structures or facies sequences, no
precise
interpretation of the depositional environment of Unit II can
be given. The
presence of black, presumably organic-rich claystones,
the common occurrence of plant debris, the apparent lack of bioturbation, and the
abundance of calcareous nannofossils in some intervals
indicate that
deposition took place in a relatively deep marine basin
with anoxic bottom
conditions and that was fringed by well-vegetated
areas. Influxes of
terrigenous sands were frequent, but the processes
of transportation
and deposition are not clear.
The early Tithonian
sedimentary assemblage characteristic of Unit
II is unknown
elsewhere in the North Atlantic. Pre-rift sediments overlying
continental basement drilled on the Galicia margin during ODP
Leg 103 consist of
Tithonian and lowermost Cretaceous neritic lime
stones and
dolomites, essentially devoid of nannofossils (Boillot,
Winterer, Meyer, et al., 1987). In the western North Atlantic, the
equivalent
formation of the same age is the Cat Gap Formation, drilled
in the
Blake-Bahamas Basin at Site 534 (DSDP Leg 76) (Sheridan,
Gradstein, et al.,
1983) is Kimmeridgian to upper Tithonian in age. It
is composed mostly
of reddish gray and dark green to gray calcareous
claystones
interbedded with micritic and bioclastic limestones.
