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Abstract: Sequence Stratigraphy and Tectonics of the Pre-Uralian Foredeep of the Southern Ural Mountains A Preliminary Assessment

SNYDER, WALTER S., TAMRA A. SCHIAPPA, CLAUDE SPINOSA, VLADIMIR I. DAVYDOV, (Permian Research Institute, Department of Geosciences, Boise State University, Boise, ID 83725); DORA M. GALLEGOS, (Albertson College of Idaho); and SCOTT M. RITTER, (Brigham Young University)

A preliminary sequence stratigraphic framework has been established for the southern Pre-Uralian Foredeep, Russia and Kazakhstan. The Late Carboniferous through Lower Permian strata of the Pre-Uralian Foredeep includes fluvial, deltaic, reef, marginal marine, shelf, and basinal facies and reflects a complex interaction between eustatic sea level changes, paleotopography, and tectonism. Detailed biostratigraphic data have been critical for delineating the complex stratigraphy, structure and paleogeography.

The Pre-Uralian Foredeep is at the juncture of two major oil-producing regions, the Volga-Ural Basin and the new fields within and adjacent to the Caspian Basin The Southern Urals near Aqtibe (formerly Aktyubinsk), and Aidaralash Creek in particular, expose the southern-most Paleozoic strata of the region; to the south and west, these strata are buried beneath Cretaceous and younger units. Precise stratigraphic and paleogeographic relationships between the southern Pre-Uralian Foredeep and northeastern Pre-Caspian Basin (Fig. 1) have yet to be established. Nevertheless, these Upper Paleozoic exposures may be the surface analogues of proven and potentially new source rocks and reservoir units of the Pre-Caspian Basin. In addition, the Cisuralian (Lower Permian) stratotype sections will be established in the Southern Urals. Our studies at Aidaralash Creek, Kazakhstan formed the basis for the Global Stratotype Section and Point (GSSP) for the base of the Permian at this location. The proposal was formally accepted by the lUGS in August, 1996.

Third order sequence boundaries within the Upper Carboniferous-Lower Permian sections along Aidaralash Creek do not consistently match published Permian sea level curves, and this could be ascribed to: (1) poor biostratigraphic data, (2) autogenic fluctuations in sediment supply (not real sequence boundaries), (3) tectonism (tectonic sequence boundaries), or, (4) incorrect sea level curves. Biostratigraphic data suffice to date the third-order sequences. Autogenic changes in sediment supply can be inferred from the facies patterns and may account for some boundaries, but those marked by conglomerate-sandstone horizons are clearly lowstand deposits. The role of tectonism versus eustasy must continue to be evaluated by additional studies of the regional stratigraphy and expanded detailed biostratigraphic investigations.

The Late Carboniferous through Lower Permian Pre-Uralian Foredeep has been modeled as a flexural foreland basin, however, several lines of evidence suggest that this may be an overly simplistic scenario. The overall paleogeography of the Pre-Uralian Foredeep does not correspond to one expected of a simple flexural-loaded foreland basin. Therefore, although the foredeep was a structural basin that received a large volume of orogenic detritus, the Uralian orogeny may not have spawned a classic, Sevier-type foreland basin. Furthermore, the eastern edge of the foredeep is truncated by a major fault, suggesting post-Permian movement along this fault may have included a large component (mostly?) of strike-slip displacement; its relation to the Main Uralian Fault is problematic. This stratigraphic and tectonic scenario raises questions about the nature of the orogeny; what were the space/time patterns of terrane and continent collisions that produced the Pre-Uralian Foredeep, and to what extent did the accreted terranes and microcontinents over-ride the Previous HittaperedTop edge of the East European continent, and where the terranes subsequently translated along the margin ?

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah