Paper No. 246-6
Presentation Time: 2:05 PM
IN SUPPORT OF EARLY PEAT TO COAL COMPACTION; EVIDENCE FROM PENNSYLVANIAN STRATA OF THE APPALACHIAN AND ILLINOIS BASINS
Conceptually, the coalification process involves significant compaction. For Pennsylvanian-age, bituminous coals, peat:coal compaction ratios are commonly interpreted as 10:1 or more. Furthermore, compaction is generally accepted to be a long-term process associated with coalification, lasting perhaps millennia. Nadon (1998) argued for early compaction citing examples of geometrically preserved dinosaur tracks at the top of Cretaceous coals, among other data, as evidence. Several lines of evidence support early compaction in many Carboniferous coal beds: (1) The geometric relationship of clastic partings to coal beds along channel margins, (2) the geometry of seams containing coal balls relative to surrounding strata, (3) differential compaction and offsetting sandstones in successive depositional sequences, which suggest that compaction of underlying peats must have happened prior to deposition of the offset channels to have influenced subsequent paleotopography, (4) in widespread coalbeds, the lack of significant coal thickness change relative to position beneath sandstones (noncompacting) and shales (compacting), (5) relatively concordant bedding a short distance above scour-filling coals, (6) preservation of three-dimensional shape and lack of major discordance in laminations around in situ trees above coal beds, (7) sharp juxtaposition of low-ash coal benches beneath inorganic partings without mixing, and (8) a general lack of extensive soft-sediment deformation through partings in coals and in overlying siltstones, which might be expected to occur from dewatering of large interstitial water volume in the peat during compaction. When these criteria are examined, it appears that substantial compaction of original vegetation took place during and almost immediately after initial burial, with much of the remainder occurring within one depositional sequence of the burial of the peat. There may be a wide variety of compaction ratios for different types of peats in different environments.