ABSTRACT: Preservation of Daily Tidal Cycles and Stacked Alluvial Swamp Deposits: Depositional Response to Early Compaction of Buried Peat Bodies

Timothy M. Demko, Robert A. Gastaldo

The character of the clastic depositional environments represented in the lower Mary Lee coal zone of the Pennsylvanian Pottsville Formation in the Warrior basin, Alabama (tidally influenced mud flats and alluvial swamps) was controlled by the compaction of buried peat bodies.

The lowest mineable coal in the Mary Lee coal zone, the Jagger, is overlain by laminated shale and sandstone exhibiting pronounced cyclic bedding. This bedding records daily tidal cyclicity in the form of sandmud couplets. These correspond to flood-current deposition of the coarser fraction, followed by fallout of the finer grained fraction during ensuing slack-water periods. These couplets are cyclically bundled--sandier bundles corresponding to spring tides and muddier bundles to neap tides (lamination counts suggest a 24-30-day cycle).

The clastic sequence above the overlying Blue Creek coal is characterized by a series of stacked alluvial swamp horizons. These can be identified by autochthonous fossil plants and pedological features indicative of gleyed paleosols. Catastrophic flooding buried and preserved these horizons.

The rapid, early compaction of the buried Jagger and Blue Creek peat bodies created accommodation space that allowed both the preservation of tidalites in the Jagger coal to Blue Creek coal interval and the stacking of alluvial swamp paleosols above the Blue Creek seam. Carboniferous peats were comprised of highly compressible plant parts and, hence, were sensitive to sediment loading. Once the peat bodies had compressed to a certain extent, stability of the overlying sediment surface created conditions amenable to resumption of peat accumulation.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990