ABSTRACT: The Role of Evaporites in the Sequence Stratigraphic Framework of Carbonate Platforms.

MOORE, CLYDE H., and WILLIAM J. WADE, Louisiana State University, Baton Rouge, LA

Shallow marine carbonate platform and shelfal sequences commonly exhibit multiple stacked shoaling upward cycles. High-frequency cycle stacking patterns and the sedimentologic architecture of the cycles are useful in determining third order sequence boundaries. In this context, the nature of included evaporite deposits may be particularly critical.

Shoaling upward cycles capped with thin nodular displacive evaporites associated with cryptalgal limestones and dolomites are thought to have been deposited under sabkha conditions. These sequences are commonly associated with highstand systems tracts and suggest limited accommodation space. Sabkha evaporite capped cycles therefore occur more commonly near sequence boundaries.

Thick, dominantly subaqueous evaporite units occurring on a carbonate platform demand a barrier between the site of evaporite deposition and open marine waters, as well as adequate sediment accommodation space. While an appropriate barrier can form under transgressive as well as highstand conditions, adequate accommodation space for thick subaqueous evaporite cycles can only be found during the later stages of the lowstand and during the onset of transgressive systems tracts.

These thick platform evaporite lowstand-transgressive systems tracts, therefore, commonly mark major "turnaround" events and show onlap onto previous sequences. Their presence suggests a period of increasing sediment accommodation space and may often

be a key factor in recognition of sequence and supersequence boundaries.

These relationships emphasize the pressing need for the critical differentiation of evaporites deposited under subaqueous shelf lagoon versus sabkha depositional environments.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)