Sequence development on a sediment-starved, low accommodation epeiric carbonate ramp: Silurian Wabash Platform, USA mid-continent during icehouse to greenhouse transition

Abstract

Abstract Twelve cored wells through the Silurian section of the Wabash Platform, Indiana, U.S.A., and 2 non-cored basinal wells with cuttings, were studied to define the sequence stratigraphy of a sediment-starved ramp within a low-latitude epeiric sea during the latest Ordovician icehouse to Silurian greenhouse transition. The Wabash Platform (approximately 200,000 km2 area) was bounded to the north by the Michigan Basin, to the east by the Appalachian foreland basin, and passed to the southwest into the Vincennes Basin, which was open to the ocean. Facies developed include: platform-fringing buildups (stromatactis wackestone core, stromatoporoid skeletal wackestone caps and crinoidal rudstone/packstone flank facies); and non-buildup crinoid grainstone–packstone sheets (shoals), skeletal wackestone (variably cherty; between fair weather and storm wave base; deep lagoon and deep ramp) and carbonate mudstone (clean to argillaceous; sub-storm wave base, deep lagoon and deep ramp/basin). The eleven sequences, 2 m to 30 m thick and 1 to 4 m.y. durations (average ∼ 2.5 m.y.), generally can be correlated to global sea-level cycles. The lower three sequences are disconformity-bounded (reflecting moderate ice sheets) but the others are relatively conformable. The most easily mapped regional surfaces are the transgressive surfaces, because the correlative conformities are cryptic. The wide well spacing prevented mapping of lowstand system tracts. The transgressive systems tracts are upward-deepening, upward-fining carbonate units, that become more argillaceous and silty upward. The highstand systems tracts are upward-coarsening carbonate mudstone to wackestone–packstone and rare grainstone. Barrier bank complexes and isolated buildups aggraded during deposition of the upper 4 sequences to form a discontinuous raised rim (∼ 40 m relief) to the ramp while the interior of the platform remained sediment starved. Subsidence rates in the basin were very low (0.6 to 1.2 cm/k.y.), but the low platform accumulation rates (0.2 cm/k.y. to 0.8 cm/k.y.) prevented the seafloor from building to sea level in all but the lower three sequences and later over barrier banks (accumulation rates 2 cm/k.y.). Parasequence development in high-accommodation settings elsewhere in the Silurian in North America records transition from moderate ice-sheets to an ice-free world. This is weakly expressed on the Wabash Platform due to the dominantly deeper subtidal setting. The remarkably low sedimentation rates resulted from the epeiric sea being isolated from the open ocean, and they likely are typical of the large epeiric seas of the Paleozoic.