Silurian Shelf Sequences, Southern Wabash Platform, Indiana, U.S.A

Spengler, Alison¹, Fred Read¹ (1) Virginia Tech, Blacksburg, VA

Twelve cores (30 to 200m) were logged from the Wabash Platform, southern Indiana, bordering the subsiding proto-Illinois Basin. The shelf contains lagoonal silty dolomitic mudstone, minor shoal water, sheet-like crinoidal packstone, deeper platform skeletal wackestone and a margin of clinoformed, stromatactis wackestone- mudstone and dolomitized pinnacle build-ups, with crinoidal flank beds. At least five, basinward thickening (30 to 200m), third-order sequences occur, the upper two offlapping basinward. Sequences contain three to five, fourth parasequences each. LSTs of sequences consist of silty, dolomitic mudstone (Osgood, Waldron and lower and upper Mississinewa units). TST's are skeletal wackestone with local crinoidal packstone updip. HST's are upward shallowing successions of skeletal mudstone grading up into skeletal wackestone and local packstone. The upper sequences thicken drastically (to over 100 meters) into local buildups over the Terre Haute Bank. The Wabash Platform in southern Indiana lacks peritidal facies and grainstones are rare. This reflects low productivity platformward of the margin, due to the epicontinental sea setting and predominantly deeper subtidal deposition, resulting in much unfilled accommodation produced by eustacy and subsidence. Even on local buildups, up building generally lagged relative sea level rise, resulting in overall catch up stacking, once accommodation started to decrease. Third-order unconformities may have resulted from Early Silurian glacio-eustasy. Lower amplitude glacio-eustasy may also have formed the later Silurian sequences but must have been less than 40 meters increasing into the end Silurian. Sea level changes involved in the fourth order parasequences must have been smaller, indicating only moderate to minor ice.