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Effects of Global Eustatic Sea Level Variations and Tectonism on Stratigraphy of Iraq

Susan L. Gawarecki, Steven Schamel

The stratigraphy of Iraq is marked by complex vertical and lateral facies sequences controlled predominantly by two factors: (1) eustatic sea level variations, and (2) tectonic movements. Ten major depositional cycles from the Late Triassic through the Tertiary correlate well to high sea level stands identified by P. R. Vail et al, and minor unconformities correspond to small-scale regressions. The most important effect of tectonism was to control basin configuration and provide new sediment sources. Prior to the Tethyan orogeny, epeirogenic movements and Triassic passive margin development were the major tectonic influences on Iraq stratigraphy. Tectonic effects on facies variations are greatest in the northeast and are primarily related to the Late Cretaceous closing of the Tethys and late Miocene renewal of this compression. During the Neogene, relatively low eustatic sea level combined with orogenic uplift-restricted marine sedimentary cycles to the highest sea level stands.

Analysis of the sedimentary cycles provides a framework for evaluating the relative economic importance of transgressive versus regressive facies within the Iraq stratigraphic succession. Most reservoir rocks, principally reefal and neritic limestones and to a lesser extent deltaic facies, were deposited during relatively high sea level stands. Source rock depositional environments in Iraq were typically either deep subsiding or shallow restricted intrashelf basins. These environments were not controlled by sea level, but primarily by local tectonics. Applying modern theories of plate tectonics and sea level control of facies to this well-studied petroleum province allows new interpretations of the region's geologic evolution.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.