Drill holes CRP-2 and 2A are located 300 m north of seismic line NBP9601-89 (Fig. 1.3, shot point 1998), a W-E line across Roberts ridge. CRP-212A reached a depth of 624.15 mbsf, equivalent to 525 ms twt bsf (Fig. 1.4). At least 15 seismic events can be identified at this depth or above, and can be related to the cored section (Fig. 2.25).
These are summarized in table 2.3.
The seismic stratigraphy of the Victoria Land Basin was establishedby Cooper &Davey (1985), whoidentified a number of major seismic units (Vl-VS) separated by basin-wide reflectors. We sought to sample and calibrate stratigraphically two critical seismic reflectors, one separating V3 from V4 and the other V4 from VS; for theV3lV4 reflector, there were two possible depth interpretations. CRP-212A sampled the sedimentary
sequences from just above the upper alternativeV3N4 boundary, penetrated the lower alternative V31V4 boundary and terminated c. 120 m above the presumed position of the V 4 N 5 boundary, at a depth of about 624 mbsf. W e have taken the logs of sonic velocity and density from the whole-core logging and used these to derive a time-depth relationship. We have then converted the seismic travel times to depth, and thus derived a down-hole reflectivity profile. This profile has then been used to link the seismic data with the lithological logs (Fig. 2.25).
A re-evaluation of the correlation of seismic reflection data from the central Victoria Land Basin to the CRP drill sites (Henrys et al., 1998) suggested that the interpreted correlations are not unequivocal. However, it should be noted that uncertainty in the association of astrongreflector with a particular named interface does not mean that there is any uncertainty in the existence of that reflector as a physical feature. The interpretation here will be limited to an analysis of the major units; detailed linkages are uncertain because of the low resolution of the seismic signal (wavelength
-
30m). However, reflectioncoefficient data and changes in physical properties that extend over-18 Initial Report o n CRP-2/2A
lull 2 3 - Conelation between seismic reflectois, reflectivity and vclouty \ tilucs Jiom nic<is~~iements on thecoie, cind l ~ t h o s t i ~ i t i ~ ~ i l ~ l l i i ~ d l units i i i ('l<l' '/'A
- -
Seismic Lithostratigraphical correlation Comments and inferences Reflector and depth
Twt bsf
(ms) - .. - .
:I+ 50 LSU 4.1 at 48 - 52 mbsf tor at a 4-in-thick diainict: corresponds to minor vclorily ;nid impedance change. Correlates to CRP- 1.
I) :!c 83 Boundary between LSU 6.2 and 6.3 Significant impedance and velocity change: 8-m-thick diamiclite.
at 90 mbsf Upper alternative for V3/V4.
c 1 10 Boundary between LSU 7.1 and 7.2 Major impedance change between thick diamictite (7.1) and an ash-bearin;' unit at 109 mbsf with a 1.2-111-thick ash bed. Reflection not strong.
(1 145 Near the base of LSU 8.1 at 125-130 mbsf Velocity increase within lower part of a diamictite: Oligoccne-Mioccnc hoiii~liiiy placed at 130 in on biostratigraphical evidence. Minor reflector.
1801 Boundary between LSU 9.1 and 9.2 at Sharp increases in velocity in diamictite and sandstone. respectively; appear to 190 183 mbsf or between LSU 9.2 and 9.3 at concspond to weak seismic reflectors.
194 mbsf
F" 215 Boundary within LSU 9.4 at 220 mbsf Reflector à § f does not corresponds to an identifiable velocity change.
Lower alternative for V3/V4.
Sea-floor multiple if~tersects CRP-2A at 220 ins bsf. Below this. interpretation of reflectors is more difficult.
g 241 Possibly boundary between LSU 9.5 and 9.6 at c. 240 rnbsf
l) 265 Middle of LSU 9.8 at c. 276 mbsf Velocity fall and increased reflection coefficient.
i 290 LSU 10.1 at 296 - 306 mbsf Sharp velocity increase at top of unit, associated with impedance changes:
an equally sharp decrease velocity at the base of LSU 10.1 corresponds to ;I inajoi impedance change and angular unconformity.
.:. .:.
.,. .,. 3 l 5 Base of LSU 11.2 at c. 328 mbsf Slight velocity increase.
, 350 Boundary within LSU 12.1 at c . 365 or Velocity increase and significant change in reflectivity; marks the beginning of :I
boundary between LSU 12.1 and 12.2 at zone of highly variable physical properties down to 420 mbsf.
378 mbsf
k 380 LSU 12.3 at 420 mbsf LU 12 contains several thick diamictites of varying impedance and velocity, but only that in the base of LSU 12.3 corresponds to a step of significant d ~ ~ r a l i o i l . I* 405 LSU 12.4 to 13.1 at 437 - 443 mbsf Top of 6-m-thick diamictite corresponds to major velocity change and to a
reflector extrapolated through the sea-floor multiple.
m* 440 Boundary between LSU 13.2 and 13.3 at 495 Corresponds to significant velocity change and a reflector.
mbsf
Second sea-floor multiple iiztersects CRP-2A at 440 m s bsf. Below this, interpretation of reflectors is much more d~fficult.
11 4601 Boundary between LSU13.3 and 14.1 at Corresponds to a significant velocity change and a faint reflector.
475 516 mbsf
4901 Base of LSU 15.2 at 570 mbsf Lowest significant velocity change in CRP-2A, within well-cemented sandstone:
520 corresponds to a major reflector traced to a 100-m wide bench on the sea floor at
c. 570 mbsf.
Note: *strongest and most persistent reflectors. **not seen on large scale near-trace plot.
about 20 m have been used to improve correlations. We also note that diamictites do not always correspond to a significant velocity or impedancechange (e.g. at 230 mbsf), and in some cases strong and continuous reflectors have no associatedchange in velocity (see reflector f at 220 msec TWT bsf in Fig. 2.25).
A major lithological change (diamictite to mudstone), sampled at about 142 mbsf in CRP- 1, is correlated with the
base of a 4-m thick Early Miocene diamictite forming Lithostratigraphical Sub-Unit (LSU) 4.1 in CRP-2 at a depthof about 52 mbsf; this is consistentwith thecorrelation from CRP-1 to CRP-2 using seismic reflection data.
Table 2.3 summarizes the correlation between seismic reflectors from line NBP9601-89 and lithostratigraphical units in CRP-212A. Down-hole logging data and synthetic seismograms will be included in the Scientific Report.