Sequence Development on a Sediment-Starved, Low Accommodation Epeiric Carbonate Ramp: Silurian Wabash Platform, USA Mid-continent During Icehouse to Greenhouse Transition
Spengler, Alison Elizabeth
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A study of 12 cores and 2 wells with cuttings through the Silurian (444 to 416 m..y.) succession of the Wabash Platform, Indiana was done to establish the high resolution sequence stratigraphy of a sediment-starved low-latitude epeiric sea platform during the transition from Ordovician icehouse to Devonian greenhouse. The Wabash Platform (approximately 200,000 square km area) is bounded to the north by the Michigan Basin, to the east by the Appalachian Basin, and passed to the southwest into the Vincennes Basin, which was open to the ocean. Facies developed include: crinoidal grainstone-packstone sheets (updip shoals), buildup facies (stromatactis wackestone - lime mudstone, below storm wave-base settings; stromatoporoid skeletal wackestone - floatstone, storm wave-base to fair-weather wave-base; and crinoidal rudstone to packstone; flank facies); non-cherty, skeletal packstone, wackestone, mudstone (sub-fair-weather- to storm wave-base); and cherty, skeletal wackestone - mudstone and variably argillaceous carbonate mudstone (below storm wave-base). Eight thin sequences (1.3 to 4 m.y. duration) occur and range from 2 m to 10 m, with the upper two sequences up to 20 m downdip; most of the sequences can be correlated to global cycles. Except for the lower three disconformity-bounded sequences, most sequences are relatively conformable and lack well defined sequence boundaries or subaerial exposure surfaces. The most easily mapped surfaces are the transgressive surfaces, given that the correlative conformities are cryptic. Lowstand system tracts probably include downdip grainy facies and the deep ramp seaward of updip late highstand deposits. Transgressive systems tracts are upward deepening, upward fining carbonate units, some of which become more argillaceous and silty upward. This contrasts with the usual association of clastic-prone units with lowstand to early transgressive systems tracts. Highstand systems tracts are subtly upward coarsening from carbonate mudstone to skeletal wackestone/packstone and rarely skeletal grainstone. During deposition of the uppermost two sequences, mudmound barrier banks 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 relatively ice-free greenhouse later Silurian except over the buildups which locally shallowed to sea level. Parasequence development in high accommodation settings elsewhere in North America are compatible with the transition from moderate ice-sheets to an ice free world. However, this is poorly expressed on the Wabash Platform due to the dominantly deeper subtidal setting. The Silurian provides a window into climate change from a global cool period to global hothouse, which may have implications for understanding future climate change.
- Masters Theses