MAGNITUDE AND TIMING OF PEAT-TO-COAL COMPACTION

The peat-to-coal transition is commonly assumed to be accompanied by c ompaction that decreases the thickness of the organic deposit to value s of 10% or less of the original peat thickness. Decompaction modeling using such values for coal seams in contact with penecontemporaneous channel sandstones leads to impossible depositional geometries for the sandstones, which have a final thickness that is 90% of their origina l thickness. Similarly, decompaction modeling of the fragments of orga nic material within channel lags using the assumed large peat compacti on value results in the destruction of the associated sedimentary stru ctures, such as trough cross-beds. Dinosaur tracks in the roofs of coa l mines show a shallow depth of penetration and a preservation of foot morphology that is not possible unless the peat the animals walked up on was very firm. These data confirm the macroscopic observations from coal balls that the compaction associated with the peat-to-coal trans formation is minimal. Equally important, the data also show that most of the compaction occurs essentially at the surface and not at the dep ths where coalification takes place.