Print this pageThe Mississippian Tar Springs Sandstone, a cratonic fluvial-deltaic sequence, was deposited from north to south as one cycle in a rhythmic carbonate/siliciclastic package that is typical of the Chesterian in the Illinois Basin. Formation and net sand isopach maps and detailed cross-sections indicate a regional architecture consisting of distributary channels which form composite sand bodies up to 125 feet thick, 1 to 2 miles wide and interfluves of shaley fine sand and silt. The channel bases cut downward into older Tar Springs, delta fringe fine sands and shales, and the underlying Glen Dean Limestone. These stacked channel sands are concentrated regionally in sinuous, north-south oriented paleovalley-fills or fluvial channel systems 6 or 8 miles wide and can be correlated north-southward for up to 100 miles. Tidal rhythmites have been observed in fine sands and silts in Indiana. Other tidal indicators have been observed at the southern most extent of the Tar Springs, along sandstone outcrops near Alto Pass, Illinois. Above the stacked sandstones are intertidal to supratidal, interbedded fine sands and silts, and then shales with local coaly stringers, which are capped by the thin, widespread, shallow marine, Vienna Limestone.
The Tar Springs was deposited as a southward prograding deltaic sedimentary package. Tidal processes are believed to have reworked the fluvial channel deposits found near the mouth of the delta. The heterogeneous nature of the formation contributed to the formation of stratigraphic hydrocarbon traps in the Illinois Basin. Knowledge of Tar Springs sand body architecture will thus improve the search for new fields and lead to a better understanding of existing ones.
AAPG Search and Discovery Article #90926©1999 AAPG Eastern Section Meeting, Indianapolis, Indiana