Evidence for Deep-Water Evaporite Deposition in the Miocene Kareem Formation, Gemsa Basin, Eastern Egypt

MAY, JEFFREY A., S. A. STONECIPHER, and J. C. STEINMETZ, Marathon Oil Company, Littleton, CO, and J. N. DYESS, Marathon Petroleum Egypt, Maadi, Egypt

The correct interpretation of intercalated Miocene siliciclastics and evaporites of Gemsa basin is crucial for understanding early rift tectonics of the Gulf of Suez, pinpointing the timing of isolation of the Gulf from the Mediterranean, and developing exploration plays. Evaporites of the Kareem Formation comprise celestites and massive, "chicken-wire," and laminated anhydrites. Although previously interpreted as sabkha deposits, our sedimentologic, petrographic, and paleontologic analyses indicate these evaporites more likely formed in a submarine setting.

Marls that encase the evaporites contain a diverse and abundant assemblage of nannoplankton, planktonic foraminifera, diatoms, pteropods, and fish scales indicative of basinal deposition. Associated turbidites also denote deep-water sedimentation. The paucity of benthic diatoms and foraminifera, plus the presence of unburrowed shales, phosphate nodules, early ferroan carbonate cements, and authigenic pyrite, suggest periodic anoxic, or at least disaerobic, bottom waters.

Directly beneath the evaporites, foram-rich shales grade upward to diatomaceous dolomites, organic-rich dolomites, and varved pocellanite. The dolomites change upward into dolomitic shales containing celestite and then chicken-wire anhydrite. Above the anhydrites, calcareous shales and shaly limestones occur.

These sequences probably represent partial isolation of the Gulf of Suez by middle Miocene, producing periodic basin restriction and evaporative drawdown. Episodes of increasing salinity likely caused the progressive decreases in foram abundance and diversity in marls beneath the anhydrites, culminating in subaqueous evaporite formation. Diverse, indigenous nannoplankton assemblages from shale seams within the anhydrites suggest Gemsa basin was stratified; shallow open-marine conditions coexisted with anhydrite crystallization from deeper hypersaline waters.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)