AL-TAWIL, AUS A. and J. FRED READ, Department of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061-025O.

ABSTRACT: Regional Characteristics of 5-10 M Scale Carbonate Cycles of Late Mississippian Greenbrier Depositional Sequences, West Virginia

The 0-500m thick Late Mississippian Greenbrier Group (Big Lime) is a major hydrocarbon reservoir in the Appalachians. It consists of ramp carbonates that contain several depositional sequences (tens of meters thick) of transgressive shaly/muddy carbonates, skeletal/oolitic grainstones and red beds. Sequence boundaries updip have erosional unconformities, caliches, breccias and channelling, whereas downdip they are relatively conformable and overlain by lowstand red beds. The sequences contain high frequency, 5-10 m carbonate cycles, possibly of 100 to 400 KY duration. Updip, few cycles are developed. These consist of grainstone-dominated facies with sharp/erosional tops, locally developed beach facies, eolianites and caliches. Tidal flat facies are rare and thin. In the thicker downdi sections, there are more carbonate cycles. These are dominated by skeletal packstone grading up into skeletal/oolitic grainstone, and some thick (up to 8 m) tidal flat laminated dolomites. The oolites likely are sheets to tidal bar sands and channelled bodies. Oolites are rare in the furthest downdip sections on the ramp. Downdip cycle boundaries rarely have caliches, but erosional and microkarstic surfaces are common. Comparison of the observed sections with computer generated synthetic cross sections suggests that the cycles were developed under moderate amplitude (tens of meters) high frequency Milankovitch eustatic sea level fluctuations dominated by 100-400 KY periods, which mark the onset of the Late Carboniferous Gondwana glaciation. The modelling closely simulates limited updip ycle development, and abundant cycle development downdip, along with locally well developed lowstand tidal flats. Even though high frequency sea level changes exposed the platform many times, the semi-arid climate limited diagenesis to minor, near surface dolomitization and little subaerial cementation. Consequently, the oolite porosity is dominantly interparticle, supplemented by microporosity.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.