CONTROLS ON PEAT ACCUMULATION AND DEPOSITIONAL-ENVIRONMENTS OF A COAL-BEARING COASTAL-PLAIN SUCCESSION OF A PULL-APART BASIN - A PETROGRAPHIC, GEOCHEMICAL AND SEDIMENTOLOGICAL STUDY, LOWER JURASSIC, DENMARK

The paralic, Lower-Middle Jurassic Baga Formation of the Island of Bor nholm, Denmark, was deposited in a fault-bounded, subsiding, pull-apar t basin. The formation is up to 400 m thick and contains more than 50 coal seams. Twelve of these have been investigated petrographically an d geochemically to provide basic information on the composition of the relatively unknown Jurassic coals. The peat-forming environments repr esented by the seams and the associated siliciclastic sediments are in terpreted. The seams represent three types of environments with organi c matter deposition. Feat accumulation occurred in low-lying areas sit uated between river channels in a coastal plain environment undergoing overall transgression. The coals have a relatively uniform, huminite- rich petrographic composition, indicating that the precursor mires wer e dominated by persistent, water-saturated and anoxic conditions. The swamps were probably occupied by a small-statured flora with cellulose -rich tissues. Significant bacterial activity in the peat swamps is su ggested by an abundance of hopanes. Influence from marine water was no t common but occurred occasionally. During peat accumulation, the depo sitional conditions were stable and quiet. The small thicknesses of th e seams (8-57 cm thick) indicate relatively short periods of peat form ation (average c. 2300 yr), due to continued base-level rise, controll ed by subsidence, and an overall eustatic rise, causing repeated chang es in the sedimentary regimes. The coal seams are of low rank and were buried to a depth of 1100-1200 m before uplift, due to Late Cretaceou s-early Tertiary basin inversion and Neogene uplift.