688
Land-Ocean Systems in the Siberian Arctic: Dynamics and History
further to the north, where the role of rifting in Cenozoic history of the LSS has probably been
more pronounced, one can expect even stronger structural control of distribution of Unit A and
its much greater thickness in structural lows, despite anticipated increase in the same direction
of influence of marine sedimentation. The existence of tectonically controlled sharp gradients in
Cenozoic sections is illustrated in Figure 3 by much greater thickness of sediments in well # 15
which is located within a major graben.
Reflector 2 is correlated with the unconformity at the top of the Upper Pliocene section which
is well exposed in coastal and island outcrops and also documented in well sections presented
in Figure 3 where a transgressive nature of this unconformity is confirmed by transition of
predominantly continental Pliocene facies to essentially marine environment during
Eopleistocene-Middle Pleistocene interval. A rugged relief of Reflector 2 suggests that
transgression was accompanied by differentiated erosion of Unit A. In coastal outcrops and
boreholes the change of facies across the PliocenelPleistocene time boundary is mainly recorded
in microfossil data, whereas offshore it is likely to be also reflected in a higher proportion in
Unit B of marine silts and clays as compared to more coarse material in underlying Unit A.
Reflector 3 is correlated with the base of the Upper Pleistocene sequence. A flat relief of this
seismic interface may indicate the absence of significant erosion during its formation. Such
conclusion is confirmed by stratigraphic record in well # 15 demonstrating the absence of a
disconformity associated with this boundary. Therefore reflector 3 may to a large extent mark a
change in depositional environment which has, in fact, been observed at the Middle/Upper
Pleistocene boundary in onshore sections and in both wells shown in Figure 3. Lithology of the
Upper Pleistocene Unit C is presumably dominated by silts, sands and silty sands with
subordinate coarser sediments. It is believed that formation of Unit C was controlled by
multiple sea level oscillations which occurred in a generally regressive environment. The latter
led to gradual retreat of the sea and replacement of marine and shallow marine facies by
lacustrine and alluvial deposits.
Despite distinct morphology of Reflector 4, interpretation of its geological nature is not
simple, and direct correlation of this horizon with erosional unconformity observed in onshore
outcrops and wells at the top of the Upper Pleistocene deposits is, perhaps, not quite as
apparent as it may seem at the first glance.
It is commonly believed that during pre-Holocene regression the sea retreated almost to the
edge of modern continental slope, and the whole shelf became exposed to a relatively shortlived subaerial freezing. During subsequent Holocene transgression, this subaerial frozen
surface remained well preserved on non-flooded land, but over greater part of the shelf it was
considerably modified by advance of the sea and accompanying thermal-abrasive reworking.
As the result, Reflector 4 observed in HRS profiles may correspond not to the original postPleistocene subaerial surface but to a somewhat younger interface of a more complex nature. It
may, perhaps, mark the base of a layer at the top of the uppermost Pleistocene deposits in
which the latter were thawed, reworked and mixed with clastic material supplied to the LSS
during the Holocene.
Evidence of permafrost distribution
Direct evidence of the existence of submarine permafrost was obtained only from numerous
shallow wells (not exceeding 200 m depth) drilled close to islands coastline. Not only the
presence of subbottom cryolithic zone was confirmed, but it also appeared possible to establish
its subcontinuous to discontinuous distribution. as well as to determine the depth to the ,urface
of frozen rocks and their thermal condition in the drilled interval (Soloviev et aI., 1987).
Outside the area studied by drilling, the presence of submarine permafrost may, perhaps, be
exemplified by an apparent break in correlation of Reflector 3 on Line 86709 (see Figure 2c).
Land-Ocean Systems in the Siberian Arctic: Dynamics and History
further to the north, where the role of rifting in Cenozoic history of the LSS has probably been
more pronounced, one can expect even stronger structural control of distribution of Unit A and
its much greater thickness in structural lows, despite anticipated increase in the same direction
of influence of marine sedimentation. The existence of tectonically controlled sharp gradients in
Cenozoic sections is illustrated in Figure 3 by much greater thickness of sediments in well # 15
which is located within a major graben.
Reflector 2 is correlated with the unconformity at the top of the Upper Pliocene section which
is well exposed in coastal and island outcrops and also documented in well sections presented
in Figure 3 where a transgressive nature of this unconformity is confirmed by transition of
predominantly continental Pliocene facies to essentially marine environment during
Eopleistocene-Middle Pleistocene interval. A rugged relief of Reflector 2 suggests that
transgression was accompanied by differentiated erosion of Unit A. In coastal outcrops and
boreholes the change of facies across the PliocenelPleistocene time boundary is mainly recorded
in microfossil data, whereas offshore it is likely to be also reflected in a higher proportion in
Unit B of marine silts and clays as compared to more coarse material in underlying Unit A.
Reflector 3 is correlated with the base of the Upper Pleistocene sequence. A flat relief of this
seismic interface may indicate the absence of significant erosion during its formation. Such
conclusion is confirmed by stratigraphic record in well # 15 demonstrating the absence of a
disconformity associated with this boundary. Therefore reflector 3 may to a large extent mark a
change in depositional environment which has, in fact, been observed at the Middle/Upper
Pleistocene boundary in onshore sections and in both wells shown in Figure 3. Lithology of the
Upper Pleistocene Unit C is presumably dominated by silts, sands and silty sands with
subordinate coarser sediments. It is believed that formation of Unit C was controlled by
multiple sea level oscillations which occurred in a generally regressive environment. The latter
led to gradual retreat of the sea and replacement of marine and shallow marine facies by
lacustrine and alluvial deposits.
Despite distinct morphology of Reflector 4, interpretation of its geological nature is not
simple, and direct correlation of this horizon with erosional unconformity observed in onshore
outcrops and wells at the top of the Upper Pleistocene deposits is, perhaps, not quite as
apparent as it may seem at the first glance.
It is commonly believed that during pre-Holocene regression the sea retreated almost to the
edge of modern continental slope, and the whole shelf became exposed to a relatively shortlived subaerial freezing. During subsequent Holocene transgression, this subaerial frozen
surface remained well preserved on non-flooded land, but over greater part of the shelf it was
considerably modified by advance of the sea and accompanying thermal-abrasive reworking.
As the result, Reflector 4 observed in HRS profiles may correspond not to the original postPleistocene subaerial surface but to a somewhat younger interface of a more complex nature. It
may, perhaps, mark the base of a layer at the top of the uppermost Pleistocene deposits in
which the latter were thawed, reworked and mixed with clastic material supplied to the LSS
during the Holocene.
Evidence of permafrost distribution
Direct evidence of the existence of submarine permafrost was obtained only from numerous
shallow wells (not exceeding 200 m depth) drilled close to islands coastline. Not only the
presence of subbottom cryolithic zone was confirmed, but it also appeared possible to establish
its subcontinuous to discontinuous distribution. as well as to determine the depth to the ,urface
of frozen rocks and their thermal condition in the drilled interval (Soloviev et aI., 1987).
Outside the area studied by drilling, the presence of submarine permafrost may, perhaps, be
exemplified by an apparent break in correlation of Reflector 3 on Line 86709 (see Figure 2c).