Abstract: High-Resolution Cyclostratigraphy of a Mid-Triassic Succession: Marine to Penesaline-Lacustrine Cycles, Palmyrid Trough, Central Syria

D. H. Craig, P. W. Choquette, D. M. Cooper, J. C. Steinmetz

A rhythmic succession of 47+ anhydrite-capped, meter-to-dekameter cycles characterizes the mid-Triassic "C" member of the Kurrachine Formation along the axis of the Mesozoic Palmyrid trough. This succession, 250 m thick on average, has been examined in detail in extensive cores from wells drilled by Marathon Petroleum Syria in a 2500 sq km area.

Of the cycles, 42 are short (4.7 m avg). They consist of a thin basal shale, commonly an intermediate interval of lime mudstone, laminated dolomudstone, and locally thin porous dolograinstone, with a thin cap of bedded and laminated, non-supratidal anhydrite. Five persistent long cycles (12.6 m avg) occur irregularly through the section. These too have a basal shale and capping anhydrite, but with a thicker interval of marine limestone and evaporitic dolomite. Some mud-rich carbonates include persistent thin "event beds" of fine-grained bioclastic debris and/or intraclasts, and oolites.

All cycles occur in all wells, comprising an extensive high-resolution cyclostratigraphy believed to be of previously unreported type and origin. Profiles of clay mineralogy and carbon and oxygen isotopic compositions, and fossil assemblages, suggest repeated interludes of mud-dominated sedimentation which began in low-energy brackish to restricted shallow-marine shelf "basins" under arid/semiarid climate, and ended with sulfate precipitation in shallow, penesaline coastal lakes or lagoons. Cycle thicknesses reflect original accommodation depths of 5 to 50 m (est). The cyclostratigraphy may be controlled by sea-level fluctuations of the sort predicted by Milankovitch theory, with possible influences from local tectonism.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994