--> Abstract: Facies Mosaic Snap-shot of a Fluvio-paralic Depositional System: The Late Permian Moranbah Coal Measures, Bowen Basin, Australia, by P. Michaelsen, R. A. Henderson, and P. J. Crosdale; #90937 (1998).

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Abstract: Facies Mosaic Snap-shot of a Fluvio-paralic Depositional System: The Late Permian Moranbah Coal Measures, Bowen Basin, Australia.

MICHAELSEN, PER, ROBERT A. HENDERSON, PETER J. CROSDALE, James Cook University.

The Late Permian Moranbah Coal Measures represents part of the basinal fill in the Bowen Basin, eastern Queensland, Australia. Deposition occurred in a major, north-south trending, rapidly subsiding retroarc foreland basin. Compressive deformation and contemporaneous volcanism to the east facilitated the supply of abundant volcanoclastic sediments, resulting in rapid basinal aggradation. Sedimentation was dominated by cold climate alluvial environments. To the south, progradation of marine sequences, characterised by deltaic to inner shelf marine sediments occurred, resulting in lateral interfingering relationships with the German Creek- and MacMillan Formation.

The lithofacies architecture of the Moranbah Coal Measures has been investigated in extensive opencut coal mine exposures, in drillcore from intensively drilled areas down dip from the open cut and spaced regional drillcore has revealed eight reoccurring lithofacies associations: (i) erosively based, vertically stacked, sandstone ribbons (up to 50m thick by 2km wide); (ii) restricted lobes of interbedded sandstone and siltstone proximal to channels; (iii) laterally extensive lobes to sheets of interbedded/laminated sandstone and siltstone, and laminated carbonaceous claystone; (iv) laterally extensive sheets of interlaminated claystone and coal stringers; (v) laterally extensive (up to 130km) sheets of bituminous coal; (vi) thin, laterally extensive sheets of laminated, varve-like, claystone (up to 1m thick and 15km across); (vii) erosively based, bioturbated sandstone and sheets of strongly bioturbated, interbedded/laminated sandstone, siltstone and carbonaceous claystone, characterised by abundant ichno fossils; (viii) sheets of bioturbated sandstone with shellbeds. This facies assemblage is interpreted to correspond to eight fluvio-paralic-shallow marine depositional systems; (i) major, low sinuosity, distributary channels; (ii) levee and proximal crevasse splay complex; (iii) distal crevasse splays/overbank; (iv) marsh; (v) peat mires; (vi) floodbasin lakes; (vii) interdistributary bay and intertidal flats and (viii) marine inner shelf, respectively.

The sedimentary record indicates that fluvial deposition in the northern part of the basin developed as overlapping lobes, generating significant aggradational relief. Sequential compaction of thick tracts of peat mire deposits by elastic sediments appears to have controlled the facies stacking pattern on a local scale. Exceptionally well developed, composite, crevasse splay deposits (up to 25m thick by >3km wide) with feeder channels (up to 30m thick by 0.5km wide) indicate that crevassing distributed significant amounts of sediment across the floodplain. Crevassing is considered to be coupled to a flashy fluvial style.

Channel tracts were relatively stationary in position over enduring periods, and developed stacked sediment accumulations. Extensive lateral bodies of crevasse splay deposits developed as lateral accumulations to these tracts. Abandoned clastic systems became the sites of peat mire development, expressed in the stratigraphy as coal rider seams.

The regionally developed P-Tuff Bed (c. 1m thick and traceable >300km along strike) provides a detailed regional snap-shot of the fluvio-paralic facies mosaic which existed immediately before and immediately after the volcanic eruption. Prior to the emplacement of the P-Tuff, the peat precursor of seam coal was part of a lateral facies mosaic, representing mire accumulation within a plexus of fluvial channels, crevasse micro-deltas and floodbasin lakes. The extensively developed Pleiades Seam, underlying the P-Tuff, accumulated over abandoned fluvial tracts on a broad coastal plain close to the palaeoshoreline.

A similar peatland environment developed following deposition of the P-Tuff. The P-Tuff is capped by the patchily developed P-Tuff Seam. This seam is in turn overlain by paralic deposits, which in the southern part of the basin, are characterised by abundant bioturbation (e.g. Rosselia isp.). Strata reflecting this marine incursion represent another important chronostratigraphic marker horizon within the basinal fill. A marine influence on coal development, in terms of coastal ponding during transgression, is indicated by the sediment package between the P-Tuff and the base of the paralic interval.