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.