The boundary of the Chukchi Sea (Chukot in transliterated Russian) with the Beaufort Sea is drawn along the 156 degree meridian, and that with the East Siberian Sea along the 180 degree line.
The South Chukchi downwarp extends along the coast of Siberia eastward from Wrangel Island for some 800 km; it is more than 200 km wide. Structures within it from west to east are the Long depression, Shmidt depression, Onman arch, Hope depression, Kotsebu arch, and Selavik downwarp.
South Chukchi downwarp has a basement of Mesozoids of the Novo-Sibirsk-Chukot system. Its fill consists of volcanics and clastics of Late Mesozoic-Cenozoic age more than 4 km thick. Five seismo-acoustic units are recognized within this fill.
The first unit has formation velocities of 1.7-1,9 kmps and appears to be present over the entire downwarp. It is Neogene- Quaternary in age and is composed of clastics up to 750 m thick.
The second unit has a formation velocity of 2.2 kmps and is interpreted as being coal-bearing sediments of late Paleogene age.
The third unit with a formation velocity of 3.1-3.6 kmps in the eastern part of the Hope depression rests on folded basement of late Kimmerian consolidation, where formation velocities are 4.5-5.2 kmps. In the western part of the Hope depression and in the Shmidt depression it is underlain by sediments that have a velocity of 2.5 kmps. This unit is interpreted as Late Cretaceous Turonian and Campanian in age and composed of continental coal-bearing deposits. Thickness in the Hope depression is 800-1000 m and in the Shmidt depression more than 700 m.
The fourth unit has a velocity of 2.5 kmps and is found only in the western part of the Hope depression and in the Shmidt depression. It appears to be a weathered zone 150-400 m thick of post-Cenomanian but pre-Turonian age.
The fifth unit has avelocity of 4.5 kmps and is present only in the western part of the Hope depression and in the Shmidt depression. It rests on intensively deformed basement of Kimmerian consolidation, where velocity is 6.1 kmps. This unit is interpreted as upper molasse ofAlbian-Cenomanian age. Aeromagnetic data indicate that volcanics, probably andesites, are at the top of the unit. Thickness of this fifth unit is about 1 km in the Hope depression and more than 1.5 km in the Shmidt. (Continued in next News Letter)
Taken from Ya. I. Polkin,1986, one trctonic map and three
cross section; digested in Petroleum Geology, Vol. 23, No. 5/6.
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