C.R. ALEXANDER, JR et al. ..... spectrometry to determine the depth of 137Cs penetration (ALEXANDER, 1985; ...... Hemphill Publishing, Austin, Texas, 57 pp.
('ontinental Shelf Research, Vol. 6. No. I/2. pp. 337 to 357, 1986. Printed in Great Britain.
0278M.343/86$3.0t)+ [).l~l F'ergamonJournalsI.td.
High-resolution seismic stratigraphy and its sedimentological interpretation on the Amazon continental shelf CLARK R . ALEXANDER,
JR,* CHARLES
A . NITFROUER* a n d DAVID J. DEMASTER*
(Received for publication 13 December 1985)
Abstract--Seismic profiles (3.5 kHz) from over 6000 km of shiptrack and piston cores from 26 stations were examined to evaluate the seismic-stratigraphic framework of the Amazon subaqueous delta. Acoustic reflectors in the seismic profiles were related to sedimentological and acoustic properties in piston-core sediment. Grain size in cores from the Amazon shelf correlates well with seismic velocity and saturated bulk density, and thus with changes in acoustic impedance. Acoustic reflectors on the Amazon shelf generally correspond to coarser layers in the seabed. The observed acoustic reflectors can be attributed to both depositional and erosional processes. Depositional reflectors, which correspond to stratal surfaces, reveal typical stratification of the subaqueous delta. In a seaward direction, topset strata (70 m water depth) dip gently (2 m thickness) and coarse (5.1 phi, mean grain size) as compared to dcpositional reflectors. This erosional reflector overlies two seismic sequences of top-truncated foreset reflectors. North of the river mouth, an acoustically transparent layer, covering 1.5 xl04km 2 and representing 100-200 years of sediment accumulation, overlies truncated relict mud
1.56
÷J- + +
0
~
1.54 ~.sz
1.50 1.48 1.46
I
I
5
6
I
7 Mean
Grain
I
I
I
e
9
I0
Size
(~)
Fig. 7. Plot of grain size and seismic velocity for all piston core samples; linear correlation coefficient R = -0.85. The relationship between grain size and seismic velocity results because grain size affects the porosity, and thus the compressibility of the medium.
349
High-resolution seismic stratigraphy
205-
+
-Jf-
2.00-
o
,9o-
o,
185-
~
L80-
o
1,75-
-"z,
,7o-
v
+
+
+;+
+
++
1,65-
~ 2 •
÷÷,
++*
1.601.55I. 5 0 1.45
-ti 5
i 6
, 7
Mean
i 8
Groin
, 9
, I0
Size ( 4 )
Fig. 8. Plot of grain size and saturated bulk density for all piston core samples; linear correlation coefficient R = -0.89. The relationship between grain size and saturated bulk density results because grain size affects porosity, which is related to saturated bulk density.
depth of penetration in large-diameter gravity cores from the same station. This comparison is possible because the large diameter gravity corer consistently retrieves the sediment-water interface (KUEHL et al., 1985). Interference effects can obstruct the accurate correlation of sedimentological and acoustic properties with reflectors in the seabed (see Background). Significant interference effects are possible in seismic profiles of topset strata from the Proximal-Mud region, where coarser layers are closely spaced (< 1 m separation). Where layer spacing is greater than the wavelength of the seismic-source pulse (i.e. over most of the shelf) interference effects probably are insignificant. Synthetic seismograms for comparison with seismic profiles could not be generated because the characteristics of the waveform of the 3.5-kHz seismic-source pulse were not adequately recorded at the time of profiling. However, the conclusion from the preceding discussion is that acoustic reflectors observed in 3.5-kHz profiles from the Amazon shelf generally corresponded to coarser layers.
Seismic stratigraphy Topset, foreset, and bottomset strata of the subaqueous delta. The topset, foreset, and bottomset strata are defined by the dip of stratal surfaces. The topset strata are defined in this study as those strata lying shoreward of the foreset strata (1:1500. These strata generally occur in water depths between 30 and 70 m, however the gradient and depth range of foreset strata varies significantly. South of 2°N, the foreset strata dip relatively gently (-1:1000) and occur between water depths of 25 and 40 m. Farther north they steepen (to as much as 1:50, seaward of the Sandy-Surface region) and are found in deeper water (between 30 and 75 m, Fig. 10A). Foreset strata in the Distal-Mud region are less steep (-1:350) and occur between water depths of 40 and 75 m (Fig. 10B). Seaward of the Relic~-Mud region, gradients are again steep (-1:150), and foreset strata occur between water depths of 40 and 65 m (Fig. 10C). Foreset strata merge seaward (generally between 65 and 70 m water depth) into the bottomset strata to form a prograding sigmoidal clinoform. The bottomset strata are defined as those strata lying seaward of the foreset strata and having gradients 70 m and downlap onto the transgressive sand layer. Seismic profiles of bottomset strata typically show thin (6.8 x 10 3 km 2 (i.e. almost all of the Distal-Mud region). The upper boundary of this seismic sequence is the seafloor, and the lower boundary is a strong reflector, which truncates underlying stratal surfaces with foreset dips (-1:600) (Fig. 12A). The layer generally thins shoreward and northwestward. Seaward it merges into the foreset beds. The thickest portion of the layer ( - 1 0 m) corresponds with the region of highest accumulation rates ( - 1 0 cm y-~; KUEHL et al., This Volume a). Assuming an accumulation rate of 10 cm y I and a thickness of 10 m, the layer represents about 100 years of accumulation; similar calculations with thickness and accumulation rates applicable to other areas of the layer yield durations of accumulation ranging from 100 to 200 years. Planimetry of sediment thicknesses in the layer (Fig. 13) coupled with porosity data indicates that about 7.6 x 10 m tons of
t~
~
.;
~
,~
,
iI
1
15m
W
E
I
I
o
2 km
B
NW
SE
I
I
0
2km
Fig. 12. Seismic profiles (3.5-kHz) of the acoustically transparent layer. Calculations show the layer to be about 1(10-200 years in age. (A) The layer covers most of the Distal-Mud region, and overlies erosionally truncated foreset reflectors with gradients of about 1:600 (profile H in Fig. 1, water depth 25 m). (B) Toward the southern portion of the Distal-Mud region, reflectors are present within the layer (profile I in Fig. 1, water depth 14 m). The reflectors indicate a transition zone between the acoustically transparent layer and the Sandy-Surface region, and probably are caused by migration or expansion of the processes responsible for the Sandy-Surface region
354
C . R . ALEXANDER, JR et al. s?w I
gN-
sdw ,
,
i
l
l
0m6~k,~, Relict ~/INN'~ ~~~5(//1 "~ I~~ N '
I
I
I
I
I
ISOPACH MAP OF ACOUSTICALLY TRANSPARENT LAYER (Contours In M e t e r s ) .
-
i!!
89 '
,9
%%% "-,
• . :'!~.
' '..:.:. ... -;~,
.".:-.
,,~ ~
~
~
.s
o,
.#
Fig. 13. Isopach map (thickness in meters) of the acoustically transparent layer. The thickest portion of the layer corresponds to the area of highest accumulation rates on the Amazon shelf. Planinietry measurements coupled with porosity data show 7.3 x 10 '° tons of sediment are stored in the layer. This mass of sediment represents 95-190 years of accumulation, if present accumulation rates have been relatively constant over this period.
sediment are stored in the layer. If most of the sediment accumulating on the Amazon shelf, 4-8 × 108 tons y-~ (KUEHLet al., This Volume a), accumulated within the transparent layer (a reasonable assumption considering the distribution of accumulation rates on the shelf), the layer would have required 95-190 years to form. Both sets of calculations above assume that accumulation rates have remained relatively constant for the duration of accumulation. Southeastward, near the Sandy-Surface region, a transition zone between the acoustically transparent layer and the Sandy-Surface region is evidenced by the appearance of strong reflectors in the seabed (Fig. 12B). The presence of reflectors in the layer to the south indicates increased amounts of coarse material in the seabed. Northwestward, the layer thins to
