Lateral and vertical sedimentologic and paleoecologic analyses of rocks and fossils of the lower part of the Dugger Formation (Pennsylvanian, Desmoinesian) in Indiana has delineated the following deltaic sub-environments: (1) distributary sands, (2) upper-, middle-, and lower-interdistributary estuaries, (3) delta-plain lakes, and (4) swamps. In this part of the Illinois basin, terrigenous influx was from the northeast across an extremely low depositional gradient. Because of high surface area to depth ratio, current and wave energies of this shallow epicontinental sea were diminished. Thus, differential compaction rather than marine reworking dominated the deltaic destructive phase in this area. Differential compaction resulted in accumulation of localized anomalously th ck sequences. The thickened units occur directly adjacent to penecontemporaneous distributary sandstones because of the greater degree of syndepositional subsidence in these areas. The dominance by differential compaction produced an unusual situation wherein a slow marine transgressive or deltaic abandonment stage was followed by a rapid marine regressive or deltaic progradational stage. Vertical and lateral changes in body- and trace-fossil communities reflect the encroachment of marine conditions and the relatively sudden onset of freshwater deposition. Maximum marine inundation of the area coincided with carbonate deposition. Maximum water depth in the area is estimated to have been less than 80 ft (25m) based on lateral relationships of interdistributary lithofacies with distributar sandstone and delta-plain sediments. The marine-influenced lithology deposited in shallowest water was black shale, which grades downward into coal and upward into calcareous gray shale and limestone. Lateral facies gradation indicates that these divergent lithotypes were produced penecontemporaneously at the delta margin.

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