Abstract

The Tar Springs Sandstone (Upper Mississippian) crops out in a thin belt around the southern margin of the Illinois Basin. The formation is composed of fine-grained, well-sorted quartz arenites and sublitharenites. Four facies recognized by their lithologies, sedimentary structures, geometries, and geographical locations, comprise the Tar Springs Sandstone. These facies and their dominant characteristics are: 1) Facies A, cross-bedded sandstone; 2) Facies B, horizontally bedded sandstone; 3) Facies C, flaser- and lenticular-bedded sandstone and shale; and 4) Facies D, interbedded sandstones and shales. The Tar Springs Sandstone represents deposition by prograding fluvial-deltaic and paralic sedimentary environments. Specific depositional environments are fluvial and distributary channels (Facies A), shoreface and foreshore (Facies B), tidal flats (Facies C), and pro-delta, interdistributary bay, floodplain, and destructional bars (lower, middle, and upper Facies D). The distribution of the deposits reflects a longshore change in marine processes from wave-dominated in the southeast to tide-dominated in the northwest. This suggests that there was either a change in bathymetry or coastal configuration that resulted in diminished wave energy and an enhanced tidal regime in a longshore direction away from the major sediment sources. Interpretations of the depositional history of the Tar Springs Sandstone suggest the following sequence of events. Fluvial and distributary channels traversed a broad coastal plain and built high-constructive lobate deltas which prograded into a shallow eperic sea. Marine currents reworked the deltaic sand bodies, and northwest flowing longshore currents provided sands to strike-fed mainland beaches. To the northwest of the deltas, where fluvial sediment contributions were minor, thin marine sheet sands were deposited. In these distal areas, sedimentation was dominated by tidal processes, and extensive tidal-flats developed. With the cessation of terrigenous influx, deltaic deposits were reworked into destructional bars. Finally, conditions once again became favorable for "clear-water" carbonate deposition on the epeiric sea floor.

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