POPE, MIKE¹, JOHN GROTZINGER², and B. CHARLOTTE SCHREIBER^3
1Mobil Technology Corp., Dallas, TX
²EAPS, Massachusetts Institute of Technology, Cambridge, MA
³Dept. of Geology, Appalachian State University, Boone, NC

Abstract: Evaporitic Subtidal Stromatolites Produced By In Situ Precipitation

The transition between carbonate platforms or isolated carbonate buildups and overlying evaporites commonly is marked by assemblages of stromatolites in shallow water and interlaminated carbonates and evaporites in deeper water settings. The stromatolites commonly have isopachous laminae that are remarkably uniform, varying little in thickness over a given lateral distance. These isopachously-laminated stromatolites apparently developed due to increasing salinity, temperature and anoxia induced by stratification, which created ecologic restriction and an opportunity for stromatolite growth. The isopachously-laminated stromatolites are interpreted to have formed as a result of in situ precipitation of sea-floor encrusting calcite and possibly dolomite. These unique stromatolites and the correlative interlaminated carbonates and evaporites are interpreted to have formed as chemical precipitates.

These unique facies are best developed atop Proterozoic and Paleozoic carbonate platforms underlying major evaporites. Similar Mesozoic and Cenozoic carbonate to evaporite transitions contain are marked by stromatolites with clastic textures and by low-diversity diatomaceous and coccolith mudstones. Accumulation of the mudstones would have imposed two important effects on in situ precipitation. Production of coccoliths would have helped extract calcium carbonate from seawater, thus lowering the growth potential for precipitation of sea floor encrusting stromatolites. Also, settling of both coccoliths and diatoms would have created a sediment flux to the sea floor which would have impeded growth of precipitated stromatolites by smoothering growing crystals and disrupting mineral nucleation.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas