Kim et al.: High Resolution Seismic Studies in the Laptev Sea Shelf
685
Data base for the proposed seismic stratigraphy
Single-channel HRS data were acquired simultaneously with CDP seismic regional profiling
along some of MCS lines in the south-eastern part of the LSS. The acquisition was performed
in analog form aboard RlV "Geolog Dmitry Nalivkin" cruising at a speed 4.5-4.8 knots.
Observation interval was 17 m. A "Sparker" wave source with 10.5 K-joules power capability
was towed at 0.5 m depth, and a standard streamer 48NS301 was applied as receiving device.
Depth resolution of the survey was within 3 m, with total depth penetration not exceeding 200250ms TWT.
For interpretation of obtained HRS data, a seismic stratigraphic concept was implied based on
the assumption that seismic reflectors recognized in the HRS sections represented offshore
continuations of regional disconformities which had either been known from bedrock outcrops
on coastal mainland and in the New Siberian Islands, or penetrated by shallow wells drilled
both on land and from the sea ice (Kim, 1992). The key sections for stratigraphic correlation
were chosen from two sites nearest to the southernmost extent of HRS lines: the Cenozoic
section documented in well # 0-6 immediately south of the western promontory of the Bol 'shoi
Lyakhovsky Island, and another section drilled in well # 15 in the south-eastern coastal rim of
the Laptev Sea (Figure 1).
Interpretation of the uppermost sub-bottom deposits was additionally verified by abundant
gravity coring data. The latter throughout much of the LSS allowed to assess only a minimal
thickness of the Holocene veneer, but several cores reached the top of the Pleistocene, thus
providing the information about total thickness of the Holocene sediments.
The results of detailed paleogeographic analysis performed in VNIIOkeangeologia for the
whole of the Russian East Arctic shelf were also considered. With few exceptions (e.g.
Alekseev et ai., 1992; Kim and Slobodin, 1991; Kim, 1992), these results remain largely
unpublished and are available for analysis only as 1 :5,000,000 maps (authors' drafts) showing
paleoenvironmental reconstructions for several crucial age levels in the Cenozoic history.
Seismic stratigraphy
Four regional reflectors were recognized in HRS profiles (Table 1):
The lowest Reflector I is clearly imaged in HRS records only on profiles 86707( 1) (Figure
2a) and 86712. The horizon has a subdued relief with amplitudes not exceeding 35-40 ms
TWT.
Reflector 2 has been recognized on HRS sections of lines 86707/1/ and 86705 (Figure 2a,
2b), as well as on lines 86703 and 86712. It is marked by a rugged relief most likely indicating
an erosional nature of this seismic boundary.
Reflector 3 on lines 86707/1/ and 86705 (Figure 2a, 2b) appears to form the top of a
sedimentary sequence which was accumulated in depressions of the erosional surface marked
by Reflector 2. Reflector 3 is characterized by a relatively smooth relief and almost permanent
depth at about 80 ms TWT which on lines 86707/1/ and 86705 is equivalent to the level of
highest elevations of Reflector 2. On line 86709 (Figure 2c) identification of Reflector 3 is less
confident due to proximity of the first sea-floor multiple and lack of evidence of underlying
boundaries, and its recognition here is based mostly on analogy with other lines suggesting
persistent presence at this depth of a real seismic horizon.
Reflector 4 is the uppermost regional seismic marker horizon recognized along all profiles at
constant depths close to 60 ms TWT. It is represented by a flat, almost strictly horizontal and
very clearly defined base of an acoustically homogenous sequence bounded at the top by the sea
bottom.
Four seismic sequences separated by reflection horizons discussed above are labeled (in
685
Data base for the proposed seismic stratigraphy
Single-channel HRS data were acquired simultaneously with CDP seismic regional profiling
along some of MCS lines in the south-eastern part of the LSS. The acquisition was performed
in analog form aboard RlV "Geolog Dmitry Nalivkin" cruising at a speed 4.5-4.8 knots.
Observation interval was 17 m. A "Sparker" wave source with 10.5 K-joules power capability
was towed at 0.5 m depth, and a standard streamer 48NS301 was applied as receiving device.
Depth resolution of the survey was within 3 m, with total depth penetration not exceeding 200250ms TWT.
For interpretation of obtained HRS data, a seismic stratigraphic concept was implied based on
the assumption that seismic reflectors recognized in the HRS sections represented offshore
continuations of regional disconformities which had either been known from bedrock outcrops
on coastal mainland and in the New Siberian Islands, or penetrated by shallow wells drilled
both on land and from the sea ice (Kim, 1992). The key sections for stratigraphic correlation
were chosen from two sites nearest to the southernmost extent of HRS lines: the Cenozoic
section documented in well # 0-6 immediately south of the western promontory of the Bol 'shoi
Lyakhovsky Island, and another section drilled in well # 15 in the south-eastern coastal rim of
the Laptev Sea (Figure 1).
Interpretation of the uppermost sub-bottom deposits was additionally verified by abundant
gravity coring data. The latter throughout much of the LSS allowed to assess only a minimal
thickness of the Holocene veneer, but several cores reached the top of the Pleistocene, thus
providing the information about total thickness of the Holocene sediments.
The results of detailed paleogeographic analysis performed in VNIIOkeangeologia for the
whole of the Russian East Arctic shelf were also considered. With few exceptions (e.g.
Alekseev et ai., 1992; Kim and Slobodin, 1991; Kim, 1992), these results remain largely
unpublished and are available for analysis only as 1 :5,000,000 maps (authors' drafts) showing
paleoenvironmental reconstructions for several crucial age levels in the Cenozoic history.
Seismic stratigraphy
Four regional reflectors were recognized in HRS profiles (Table 1):
The lowest Reflector I is clearly imaged in HRS records only on profiles 86707( 1) (Figure
2a) and 86712. The horizon has a subdued relief with amplitudes not exceeding 35-40 ms
TWT.
Reflector 2 has been recognized on HRS sections of lines 86707/1/ and 86705 (Figure 2a,
2b), as well as on lines 86703 and 86712. It is marked by a rugged relief most likely indicating
an erosional nature of this seismic boundary.
Reflector 3 on lines 86707/1/ and 86705 (Figure 2a, 2b) appears to form the top of a
sedimentary sequence which was accumulated in depressions of the erosional surface marked
by Reflector 2. Reflector 3 is characterized by a relatively smooth relief and almost permanent
depth at about 80 ms TWT which on lines 86707/1/ and 86705 is equivalent to the level of
highest elevations of Reflector 2. On line 86709 (Figure 2c) identification of Reflector 3 is less
confident due to proximity of the first sea-floor multiple and lack of evidence of underlying
boundaries, and its recognition here is based mostly on analogy with other lines suggesting
persistent presence at this depth of a real seismic horizon.
Reflector 4 is the uppermost regional seismic marker horizon recognized along all profiles at
constant depths close to 60 ms TWT. It is represented by a flat, almost strictly horizontal and
very clearly defined base of an acoustically homogenous sequence bounded at the top by the sea
bottom.
Four seismic sequences separated by reflection horizons discussed above are labeled (in