Sequence response of a distal-to-proximal foreland ramp to glacio-eustasy and tectonics; Mississippian, Appalachian Basin, West Virginia-Virginia, U.S.A
A.T.wil Aus; Wynn Thomas, C.; Read, J.F.ed
Special Publication - Society for Sedimentary Geology 78: 11-34
This paper evaluates the limits of using well-cuttings data and wireline logs in conjunction with limited core and outcrop data to generate a regional, high-resolution sequence stratigraphy for upper Mississippian (Chesterian) Greenbrier carbonates, West Virginia, U.S.A. These data are then used to document the stratigraphic response of the distal to proximal foreland basin to tectonics and Carboniferous glacio-eustasy during the transition into ice-house times. The major mappable sequences are fourth-order sequences, a few meters to over a hundred meters thick. They consist of updip red beds and eolianites, lagoonal muddy carbonates, ooid grainstone and skeletal grainstone-packstone shoal complexes, open-ramp skeletal wackestone, and slope-basinal laminated argillaceous carbonates. The sequences are bounded downdip by lowstand sandstones and calcareous siltstones, and locally on the ramp by basal transgressive shales; only a few sequence boundaries are calichified, compared with updip sections in Kentucky, where caliche and breccias are common. Transgressive systems tracts range from thin units to others that constitute the lower half of the sequence. The highstand systems tracts contain significant grainstone units. Maximum flooding surfaces on the ramp slope occur at the base of slope or basinal facies that rest on lowstand to transgressive complexes, whereas on the ramp they occur beneath widespread grainstones that overlie nearshore shale or lime mudstone. In the Greenbrier succession, fourth-order sequences are arranged into weak third-order composite sequences bounded updip by red beds, and by lowstand sands and oolite along the ramp slope. The composite sequences contain three to four fourth-order sequences. Correlation of the foreland basin units with third-order global sea-level curves, and with high-frequency sequences within the intracratonic Illinois Basin, shows that in spite of differential subsidence rates ranging from 2 to 30 cm/ky across the foreland, global third- and fourth-order sea-level changes whose amplitude increased with time were a strong influence on sequence development. Thrust-load-induced differential subsidence of fault blocks of the foreland basement controlled the rapid basinward thickening of the depositional wedge, and modified the eustatic effects on the accumulating succession. Moderate-amplitude eustatic sea-level change and semiarid climate were the dominant causes of the widespread reservoirs of ooid grainstone and lowstand sands. The overall stratigraphy suggests upward increase in amplitudes of sea-level change, and cooling, which likely records the initiation of Gondwana ice-sheet growth.