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Russian Geology and Geophysics

2004 year, number 3

LITHOLOGICAL AND GEOPHYSICAL CHARACTERISTICS OF CRETACEOUS DEPOSITS IN SOUTHERN TRANSURALIA

B.P. Ryzhyi and E.O. Amon*
Institute of Geophysics, Uralian Branch of the RAS, 100 ul. Amundsena, Ekaterinburg, 620016, Russia
* Institute of Geology and Geochemistry, Uralian Branch of the RAS, 7 Pochtovy per., Ekaterinburg, 620151, Russia
Keywords: Lithology, geophysics, stratigraphy, Cretaceous marine deposits, Transuralia
Pages: 303-311

Abstract

Southern Transuralia lies at the junction of three largest geologic structures: the Uralian Mountains, West Siberian Plate, and Turan Plate. In the Cretaceous, the conditions of sedimentation were drastically different in all structures. In southern Transuralia these conditions interfere with each other to provide a mixed lithology of the formations making up the Cretaceous section, which hinders stratigraphic correlation between sections, tracing of formations, understanding of the specific geologic history of the region, and reconstruction of paleogeography and parameters of paleoenvironment. This paper suggests a system of mutually related lithological and geophysical characteristics permitting these difficulties to be overcome. Steady benchmarks of electrical logging, whose origin is linked with a drastic change of lithological characteristics of formations, allows a strict correlation between sections of boreholes drilled without core sampling. Using them we also can trace the character of distribution of formations within the study area, record the features of their occurrence (depths of sole and roof, thickness) and structure, providing much new information, which is very important in search for and prospecting of ores, in drawing of paleogeographical and other schemes. Correlative geophysical schemes illustrate the behavior of some horizons in space, permitting us to evaluate the change of their composition, and degree of homogeneity, which is especially important for geological survey and mapping of the territory and for prediction of ore potential. Our lithological and geophysical investigations allow core-free drilling of the cover rocks, which makes the field works less labor- and time-consuming.