Caliche Crusts: Microbially Induced Sedimentary Structures on Siliceous Substrates

The lower Pennsylvanian strata in Indiana, Illinois, and western Kentucky rests unconformably on Mississippian to Devonian deposits within the midcontinent Illinois Basin. These deposits fill a complex drainage network of sub-Pennsylvanian paleovalleys with as much as 140 m of shale, sandstone, coal, and mudstone, Paleovalley widths are variable ranging from a kilometer to several kilometers wide and trend roughly northeast to southwest. Timing of valley incision is difficult to constrain and may have been diachronous. Filling of the southern reaches or the drainage system commenced during earliest Pennsylvanian while incision may have continued in the more northern (upland) reaches in Indiana. The paleovalleys progressively filled from south to north during an overall transgressive sea-level rise. During sea-level rise, these valleys became the sites for estuarine deposition. Two relatively small paleovalleys (1 lo 1.5 km wide) have been studied in detail in south-central Indiana. Lithofacies can be mapped down the valleys for several kilometers by using subsurface and outcrop data. Within these valleys, conglomerate and conglomeratic sand-dominated inner (upper) estuarine, mud-dominated estuarine central basin, and sand-dominated outer (lower) estuarine deposits can be identified. Central basin sediments overlie fluvial deposits. Direct evidence of tidal influence within the central basin deposits can be recognized by the presence of intertidal rhythmites that exhibit well developed neap-spring cycles. Outer estuarine deposits overlie the central basin deposits and accumulated during the final stages of valley-filling.