Controls on Sediment Distribution in the Late Permian Rangal Coal Measures of the Nebo Synclinorium

Abstract

A detailed evaluation of the Late Permian Rangal Coal Measures (RCM) and enveloping strata in the Nebo Synclinorium of the Bowen Basin, Central Queensland, was conducted to assist prediction of interburden character during mining. Wireline logs from ~4700 exploration boreholes were used to correlate coal seams and interburden lithologies from the upper Fort Cooper Coal Measures (FCCM), through the RCM to the basal Rewan Group across an area of approximately 5000 sq. km. High wall mine exposures in key locations were used to examine sedimentary architecture and were supported by downhole image logs and borehole core that yielded depositional environment indicators and paleocurrent determinations. Interpretations were based on correlation of the Girrah and Yarrabee tuff marker horizons and the regionally extensive Vermont and Leichhardt Seams, with sediment stacking patterns revealed by local correlation of several minor seams. Sediment composition and distribution were illustrated using isopach and sandstone percent maps to assist with definition of channel geometry.The results are presented in stratigraphic order from the oldest to youngest strata. Coarse sandstone forms a multi-storey, south-east trending, linear braided channel belt up to 60m thick and 50 km across in the upper FCCM with little equivalent fine-grained packages preserved suggesting significant sediment bypass at this time. In contrast the overlying RCM comprises abundant lake and floodplain deposits punctuated by meandering channel systems and well preserved crevasse-splays. In the RCM, peat mires were extensive and long-lived, with the Vermont and Leichhardt Seams coalescing into one 12m thick ‘superseam’ in the central north of the synclinorium while up to 70m of channel and floodplain sediments intercalated with numerous coal seams accumulated in the south. Above the Leichhardt Seam a southeast trending linear belt of sandstone dominated sediments accumulated along the eastern margin of the synclinorium before being capped by a thin coal seam (the Phillips Seam) which marks the final period of peat accumulation in the RCM. A distinctive ‘marker’ mudstone unit overlies the Phillips Seam and is traceable for ~2000 sq. km across the western half of the synclinorium where it directly overlies the merged Leichhardt-Phillips seam package and defines the upper limit of the RCM. Sediments of the overlying Rewan Group show a distinct change in depositional environment. Repeated sets of alternating sandstone and siltstone forming sheets up to 10m thick and traceable for tens of kilometres create a relatively uniform architecture covering most of the Nebo Synclinorium. The reconstruction of the sedimentary architecture in the Nebo Synclinorium illustrates the balance between tectonic and compaction driven subsidence mechanisms. The upwards fining from basal coarse-grained braided channel fill in the FCCM, through meandering river and floodplain deposits to lacustrine sediments in the RCM reflects accelerating subsidence driven by foreland loading to the east. Differential subsidence rates between the north-west and south-east influenced seam thickness and splitting patterns and resulted in variations in inertinite contents within seams across the Nebo Synclinorium.The distribution of channel belts in the RCM records diversion around paleo-highs created through differential subsidence of the underlying sediments of the upper FCCM. This facilitated the development of thick and extensive peat islands over previously deposited sandstone channel belts. The relatively high compaction potential of the peat influenced the sedimentary architecture of the overlying sediments forming steeply inclined bedding induced by post depositional compaction related rotation. Eastward migration of the major channel systems toward the foreland depocentre resulted in sediment starvation in the west where peat accumulation was unable to match subsidence and a large lake developed. The lake and equivalent peat mire in the east were eventually over-run by the distal expressions of a large alluvial distributary fan system in the Lower Rewan Group reflecting basin overfilling and westward dispersal towards the Galilee Basin.