--> --> Abstract: Origin and Stratigraphic Expression of Pliocene Climate Cycles in the Caspian Basin, by Dag Nummedal; #90082 (2008)

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Origin and Stratigraphic Expression of Pliocene Climate Cycles in the Caspian Basin

Dag Nummedal
Colorado Energy Research Institute, Colorado School of Mines, Golden, CO

Climate fluctuations did exert a dominant control on the style of sedimentation in the South Caspian Basin, through their direct impact both on lake levels and sediment supply. Spectral analyses of gamma logs reveal systematic patterns in sand/mud ratios. These patterns also reveal a set of nested cycles that correspond to the relative durations of Milankovitch precession cycles (~20 ky), short eccentricy cycles (~100 ky) and long eccentricity cycles (~400 ky). The Kirmaky Suite reveals a strong signal of sandstone-shale successions ranging in thickness between 12 and 22 meters, attributed to the ~20 ky precession cycles. Search for longer period cycles was limited to one subsurface gamma log of the Balakhany Suite. The associated spectrum revealed peaks in the 55 to 80 m range and 210 to 300 m range, attributed to short (~ 100 ky) and long (~ 400 ky) eccentricity cycles.

The observed sediments were deposited in fluvial, lake-margin playa, shoreline and open lake depositional systems. The sequence boundary is an exposure surface within mudstones. The palynology of mudstones documents an associated dry climate. Above the sequence boundary lays a forestepping succession of terminal splay sandstones and mudstones suggesting slowly rising lake level; the palynomorphs indicate corresponding increase in humidity. Above this there is a stack of braided stream deposits that generally represent the dominant sandstone interval of the entire sequence. This is interpreted the lowstand systems tract, which is abruptly truncated by a lacustrine flooding surface, which in turn is capped by a backstepping succession of more terminal splay deposits. The palynomorphs indicate that the climate remained humid during deposition of this transgressive systems tract.

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