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Tar
Springs Sandstone, Illinois Basin
The 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