--> ABSTRACT: Sequence Stratigraphic Distribution of Coaly Rocks: Fundamental Controls and Paralic Examples, by Kevin M. Bohacs, John R. Suter; #91020 (1995).

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Sequence Stratigraphic Distribution of Coaly Rocks: Fundamental Controls and Paralic Examples

Kevin M. Bohacs, John R. Suter

Significant volumes of terrigenous organic matter can only be preserved to form coals when and where the overall increase in accommodation is approximately equal to the accumulation rate of peat. Accommodation in paralic and non-marine settings is generally a function of changes in base level and subsidence. For mires, however, base level is very specifically the groundwater table. In paralic settings, this is strongly controlled by sea level and the precipitation/evaporation ratio. Peat accumulates over a range of rates, but always with a definite maximum rate set by original organic productivity and space available below depositional base level (groundwater table).

Below a threshold (non-zero) accommodation rate, no continuous peats accumulate due to falling or low groundwater table, sedimentary bypass, and extensive erosion by fluvial channels. This is typical of late-highstand, lowstand-fan, and early lowstand wedge systems tracts. Higher accommodation rates provide relatively stable conditions with rising groundwater tables. Mires initiate and thrive, quickly filling local accommodation vertically and expanding laterally, favoring accumulation of laterally continuous coals--in paralic zones during both mid-lowstand and mid-highstand systems tracts. If the accommodation increase balances or slightly exceeds organic productivity, mires accumulate peat vertically, yielding thicker, more isolated coals--most likely at late-lowstand--early transgr ssive and late-transgressive--early highstand times. At very large accommodation increases, mires are stressed and eventually inundated by clastics or standing water (as in mid transgressive systems tracts).

These relations should be valid for mires in all settings: alluvial and lake-plain as well as paralic. The tie to eustasy in paralic zones is dependant on local subsidence, sediment supply, and groundwater regimes.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995