Sequence Development from a Sediment-Starved, Low Accommodation Epeiric Carbonate Ramp: Silurian Wabash Platform, U.S.A. Mid-Continent During Icehouse to Greenhouse Transition

Alison Spengler¹ and James F. Read^2
1Chevron, New Orleans, LA
²Geosciences, Virginia Tech, Blacksburg, VA

Cores and cuttings through the Wabash Platform Silurian, Indiana, sample a sediment-starved epeiric sea platform during icehouse to greenhouse transition. Facies include: crinoidal grainstone-packstone sheets (updip shoals); buildup facies; skeletal packstone to mudstone (above storm wave-base); cherty, skeletal wackestone - mudstone and argillaceous carbonate mudstone (below storm wave-base).

Most of the 8 sequences (each 1.3 to 4 m.y. duration; 2 m to 20 m thick), can be correlated to global cycles. The lower three sequences are disconformity-bounded. The remainder are relatively conformable and lack well defined sequence boundaries. The most easily mapped surfaces are the transgressive surfaces, given that the correlative conformities are cryptic. Lowstand system tracts include downdip grainy facies. Transgressive systems tracts are upward deepening, upward fining carbonate units, some of which become more argillaceous upward. Highstand systems tracts are subtly upward coarsening from carbonate mudstone to skeletal wackestone/packstone and rare skeletal grainstone. During deposition of the uppermost two sequences, mudmound barrier banks (Terre Haute Bank) grew upward into shallow water buildups to form a discontinuous raised rim (40 m relief) to the ramp.

Even though subsidence rates were very low (<1 cm/k.y.), the low sedimentation rates (0.3 cm/k.y. to 0.8 cm/k.y.) generally prevented the seafloor from building to sea level except for the basal three sequences in which Early Silurian third order glacio-eustacy generated disconformable boundaries. Thus the ramp remained subtidal through most of the later Silurian. Parasequence development in high accommodation settings elsewhere are compatible with transition from moderate ice-sheets to an ice-free world but this is poorly expressed on the Wabash Platform due to the deeper subtidal setting.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas