Geology Reference
In-Depth Information
16
Hypertidal Facies from the
Pennsylvanian Period:
Eastern and Western Interior
Coal Basins, USA
Allen W. Archer and Stephen F. Greb
Abstract
Siliciclastic tidal facies have been recognized in Pennsylvanian coal measures of
the Eastern Interior (Illinois) and Western Interior (Forest City) basins. In particular,
rhythmic tidal laminations or tidal bundles are recorded in shale-rich, heterolithic
estuarine and coastal paleofacies, as well as within tidal and fluvio-estuarine
channels. The tidal facies are recurring and range from the upper Morrowan (Early
Pennsylvanian) through at least the Desmoinesian (late Middle Pennsylvanian).
Laminae-thickness series within tidal facies in both basins exhibit a variety of
well-developed, tidal cycles that include semidiurnal, diurnal, neap-spring,
apogean-perigean, and seasonal to annual periodicities.
Study of modern analogs, predominantly from hypertidal settings, provides
evidence to suggest the presence of elevated paleotidal ranges in the Pennsylvanian
seaways in both basins. The tidal facies are best developed within transgressive
systems, particularly within incised valley-fill sequences. During sediment accumu-
lation, the extreme tidal dynamics resulted in widespread deposition of rhythmites.
Cyclic rhythmites that contain high-resolution records of daily to yearly periodici-
ties are much more locally restricted. Preservation of tidal rhythmites was likely
aided by (1) rapid, high-magnitude changes in global paleosealevel, (2) strongly
resonant depositional embayments, (3) formation of large tropical Pangean rivers
during lowstand that were converted to estuaries during subsequent periods of
glacial melting and the resultant sea-level rise, and (4) a strongly resonant, extensive
global paleo-ocean. Preservation of cyclic tidal rhythmites that contain high-
resolution records were likely controlled by the generation of local accommodation
space via (1) peat compaction, (2) faulting, and (3) tidal and fluvial channel avulsion.
16.1
Introduction
16.1.1 Geographic and Geologic Setting
A variety of tidally influenced facies occur within
Pennsylvanian strata in the interior coal basins of the
eastern half of the United States. In general, the bedrock
geology of the basins consists of flat-lying, cratonic
