--> ABSTRACT: Climate-controlled Sedimentary Architecture of the Paleo-Volga Delta in the South Caspian Basin, by Dag Nummedal and Edward Clifton; #90906(2001)

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Dag Nummedal1, Edward Clifton2

(1) University of Wyoming, Laramie, WY
(2) U.S. Geological Survey, Los Altos, CA

ABSTRACT: Climate-controlled Sedimentary Architecture of the Paleo-Volga Delta in the South Caspian Basin

The Messinian and early Pliocene succession of the paleo-Volga delta in the South Caspian basin contains a very distinct record of precession- (20 ky) and eccentricity- (100 ky and 400 ky) driven climate cycles. The obliquity signal (40 ky), if present at all, is severely suppressed, consistent with the predictions of the Milankovitch theory for a basin situated at about 35 - 40 degrees north. Eccentricity cycles are expressed through systematic changes in the sand-to-shale ratios of the deposits of successive precession cycles.

The depositional sequence corresponding to each 20-ky cycle is typically about 15 to 20 meters thick and bounded by exposure surfaces (sequence boundaries) characterized by mud cracks, paleosols, and occasional root casts. The exposure surfaces occur near the middle of the shaliest part of the section. Sedimentology, palynology and ostracod assemblages in outcrop and core demonstrate that lake level rise and increased discharge accompanied deposition of fluvial sandstones in the section, a few meters above each exposure surface. The sedimentary structures clearly demonstrate that most sandstones were deposited by flash-floods. Coarsening-upward delta-front successions are rare, as are wave deposits. The fluvial sandstones are capped by a burrowed transgressive surface, overlain in turn by shale containing ostracod evidence for water depths in excess of 50 meters. This shale records lake level highstand and is capped by another exposure surface. The maximum flooding surface and the sequence boundary, therefore, are one and the same. There is no deposition during the intervening phase of lake level fall.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado