THE PLIOCENE PALEO-VOLGA DELTA: SEDIMENTARY RESPONSE OF A LARGE LACUSTRINE DELTA TO CYCLIC CLIMATE CHANGES

D. Nummedal¹, Elmira Aliyeva², Dadash Huseinov², Julio Friedmann^3
1 Institute for Energy Research, Dept. of Geology and Geophysics, University of Wyoming, Laramie, WY 82071
² Geological Institute of Azerbaijan, Baku, AZ
³ Dept. of Geology, University of Maryland, and Institute for Energy Research, University of Wyoming

During the latest Miocene and much of the Pliocene (from 5.9 Ma to about 3.6 Ma), a large integrated fluvial system, the paleo-Volga, drained the Russian platform and delivered mature quartz-rich sand to a huge fluvial and delta system on the northern flank of the South Caspian basin, in today’s Azerbaijan. The resulting Productive Series constitute major petroleum reservoirs that have recently been intensively studied.

The Productive Series represents fluvial sheet-flood deposits, braided stream, lacustrine, and subordinate eolian strata, constituting a large, terminal-fan type of delta. Gamma logs reveal a distinct cyclic stratigraphic pattern. Spectral analysis of the logs demonstrates the presence of a fundamental cycle about 15 m thick (varies a bit by formation) nested into larger cycles of about 75 and 300 m thickness. We interpret these cycles as 20-ky Milankovitch precessional cycles modulated by 100-ky and 400-ky eccentricity cycles.

Insolation changes drove dramatic lake level changes in the Caspian Sea as well as climate and sediment yield of the drainage basin. The cycles of the Productive Series show frequent alternation between lacustrine mudstone and subaerial exposure, and the ostracode fauna suggests fairly deep water (100 m+) in the lacustrine facies. We estimate lake level oscillations of > 100 m and shoreline fluctuations of > 100 km on the timescales of the Milankovitch cycles. Alternating herbaceous and arboreal palynofacies document dramatic climatic shifts from dry to humid on the same timescales. The phase relationships between the climate signal and the sedimentary cycles reveal that fluvial sandstones represent phases of increasing humidity, rising fluvial discharge and rising Caspian lake levels. In contrast, falling lake level was associated with little or no sediment influx to the lake.