Kim et at.: High Resolution Seismic Studies in the Laptev Sea Shelf
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demonstrates that the greatest thickness of the Holocene sediments (in excess of 10 m) is
associated with the areas characterized by a minimal distribution of submarine permafrost.
According to paleogeographic reconstruction for the Pleistocene/Holocene boundary, at this
time the whole LSS was occupied by a lacustrine-fluvial plain dissected by numerous
paleoriver valleys controlling accumulation of the Holocene sediments. HRS lines 86707(1)
and 86714 where the thickness of the Holocene deposits was locally observed to exceed 15 m
follow approximately the Yana River paleovalley, whereas another depression in the base of the
Holocene recorded at about intersection of HRS lines 86709 and 86705 seems to correspond to
a major right tributary of the paleo-Lena River. Position of paleovalleys and related thickest
Holocene accumulations is broadly consistent with most prominent bathymetric lows.
Conclusions
I. The stratigraphy of the uppermost part of the LSS sedimentary cover was studied within
penetration depth of HRS system used in the survey; this depth averaged between 100 and 130
m and locally exceeded 250 m. Four units corresponding to Pliocene, EoPleistocene-Middle
Pleistocene, Upper Pleistocene, and Holocene intervals were identified.
2. Reflectors bounding the lowest Unit A are correlated with erosional unconformities of Late
Miocene and Late Pliocene age. Reflector at the top of unit B is more likely to represent
predominantly lithofacial change accompanied by moderate erosion at the Middle/Late
Pleistocene boundary. The uppermost subbottom reflector at the base of the Holocene deposits
(unit D) has a composite nature related to thermal and abrasive activity of advancing Holocene
transgression.
3. Permafrost zonation in the central and eastern LSS is defined by the existence of three
major categories of frozen rocks differing in duration of Pleistocene subaerial exposure and
related freezing history. Available HRS evidence is too limited to allow unambiguous
recognition of permafrost in seismic data. The areas least subjected to latest Pleistocene
subaerial freezing seem to contain the thickest Holocene deposits. It may further be speculated
that in the same areas no significant permafrost was formed in earlier epochs as well. If proved
correct, this assumption may appear very important for selection of NADP drill sites.
4. The thickest Holocene sediments (and paleoriver valleys controlling their distribution)
seem to follow the major structural depressions filled by older (pre-Quaternary) sedimentary
sequences, although they do not necessarily coincide with the deepest parts of earlier grabens.
A preliminary proposal for selection of NADP drill sites shown in Figure I is essentially based
on this structural consideration and related assumption that the proposed sites may provide most
complete late Cenozoic sections.
5. Additional high resolution seismic surveys in the LSS are absolutely critical for better
understanding of regional distribution and paleoenvironmental significance of Late Cenozoic
sequences. In particular, the areas of short-term permafrost formation and its subsequent
degradation during the Holocene must be delineated with greater confidence for final selection
of optimal sites for paleoenvironmental drilling and recovery of continuous sedimentary record
of climate change.
Acknowledgements
Thanks are due to Prof. Karl Hinz and Dr. Sergei Drachev for their constructive review of the
manuscript and well-meant critical comments which helped to improve the paper. We
appreciate the courtesy of Dr. Rinat Murzin, Director General of MAGE, whose permit enabled
us to reproduce in this paper some of MAGE's unpublished data. We are also grateful for
691
demonstrates that the greatest thickness of the Holocene sediments (in excess of 10 m) is
associated with the areas characterized by a minimal distribution of submarine permafrost.
According to paleogeographic reconstruction for the Pleistocene/Holocene boundary, at this
time the whole LSS was occupied by a lacustrine-fluvial plain dissected by numerous
paleoriver valleys controlling accumulation of the Holocene sediments. HRS lines 86707(1)
and 86714 where the thickness of the Holocene deposits was locally observed to exceed 15 m
follow approximately the Yana River paleovalley, whereas another depression in the base of the
Holocene recorded at about intersection of HRS lines 86709 and 86705 seems to correspond to
a major right tributary of the paleo-Lena River. Position of paleovalleys and related thickest
Holocene accumulations is broadly consistent with most prominent bathymetric lows.
Conclusions
I. The stratigraphy of the uppermost part of the LSS sedimentary cover was studied within
penetration depth of HRS system used in the survey; this depth averaged between 100 and 130
m and locally exceeded 250 m. Four units corresponding to Pliocene, EoPleistocene-Middle
Pleistocene, Upper Pleistocene, and Holocene intervals were identified.
2. Reflectors bounding the lowest Unit A are correlated with erosional unconformities of Late
Miocene and Late Pliocene age. Reflector at the top of unit B is more likely to represent
predominantly lithofacial change accompanied by moderate erosion at the Middle/Late
Pleistocene boundary. The uppermost subbottom reflector at the base of the Holocene deposits
(unit D) has a composite nature related to thermal and abrasive activity of advancing Holocene
transgression.
3. Permafrost zonation in the central and eastern LSS is defined by the existence of three
major categories of frozen rocks differing in duration of Pleistocene subaerial exposure and
related freezing history. Available HRS evidence is too limited to allow unambiguous
recognition of permafrost in seismic data. The areas least subjected to latest Pleistocene
subaerial freezing seem to contain the thickest Holocene deposits. It may further be speculated
that in the same areas no significant permafrost was formed in earlier epochs as well. If proved
correct, this assumption may appear very important for selection of NADP drill sites.
4. The thickest Holocene sediments (and paleoriver valleys controlling their distribution)
seem to follow the major structural depressions filled by older (pre-Quaternary) sedimentary
sequences, although they do not necessarily coincide with the deepest parts of earlier grabens.
A preliminary proposal for selection of NADP drill sites shown in Figure I is essentially based
on this structural consideration and related assumption that the proposed sites may provide most
complete late Cenozoic sections.
5. Additional high resolution seismic surveys in the LSS are absolutely critical for better
understanding of regional distribution and paleoenvironmental significance of Late Cenozoic
sequences. In particular, the areas of short-term permafrost formation and its subsequent
degradation during the Holocene must be delineated with greater confidence for final selection
of optimal sites for paleoenvironmental drilling and recovery of continuous sedimentary record
of climate change.
Acknowledgements
Thanks are due to Prof. Karl Hinz and Dr. Sergei Drachev for their constructive review of the
manuscript and well-meant critical comments which helped to improve the paper. We
appreciate the courtesy of Dr. Rinat Murzin, Director General of MAGE, whose permit enabled
us to reproduce in this paper some of MAGE's unpublished data. We are also grateful for