Basin analysis of the Triassic succession, Bowen Basin, Queensland

The Permo-Triassic Bowen Basin (Queensland) forms part of an extensive retro-arc foreland basin complex continuous with the Gunnedah and Sydney basins of New South Wales. While basin inception and the early development of depocentres was complex, the basins acted as a true foreland basin during the Late Permian to Mid Triassic. The Triassic part of the basin fill has been subdivided into three major stratigraphic units with two subunits of formation rank (Jensen 1975) namely the Rewan Group (Sagittarius Sandstone, Arcadia Formation), Clematis Group (Glenidal Formation, Expedition Sandstone) and Moolayember Formation (lower and upper; Alcock 1970), in ascending stratigraphic order. The basal part of the Rewan Group (lower Sagittarius Sandstone) is of Permian age, but the unit ranges into the late Early Triassic. The depositional environments of all Triassic units were studied at outcrop and in drillcore, with particular focus on the Clematis Group.  Seven genetically related stratigraphic intervals bounded by hiatal surfaces were identified on a basinwide level and utilised to construct time-slices of basin development. Stage 1.    Initially the basin underwent rapid subsidence (Sagittarius Depositional Episode) with the depo-centre located to the north of the depo-centre of the underlying Late Permian deposits, near the eastern basin margin.   Volcanic-lithic detritus was shed into the basin from the orogenic hinterland in the east.   Alluvial fans accumulated thick successions of coarse conglomerates along the eastern basin margin (Cabawin Fans).  A basinwide episode of structural deformation is inferred for the end of Stage 1 from an erosional unconformity at outcrop on the western basin margin and in the south-western subsurface. Confirmation of the basinwide extend of this deformation will require more detailed correlations in this part of the succession. Stage 2.  Sediment accumulation on the basin margins continued during the second (Arcadia) depositional stage in fine grained floodplains and small anastomosing and meandering channels. Detritus was still derived predominantly from the east and resulted in volcanic-lithic petrographic compositions of the sandstones. Mudrocks accumulated in well drained, oxygenated conditions of predominantly distal floodplain facies associations, resulting in thick successions of reddened and variegated mudrocks.  There is evidence to suggest the presence of a saline lake in the central part of the basin at this stage.  Subsidence rates are interpreted to have remained high, resulting in a predominance of fine grained material reaching the central and western parts of the basin.  The drainage systems comprised both axial and transverse streams and most likely formed an internal drainage. The total thickness of sediment accumulated during stages 1 and 2 exceeds 3.5km in the depo­centre.  Stage 3.  The onset of the third depositional stage (Glenidal) is marked by the rapid change to more quartzose sediment composition.  Sediment accumulation initially still occurred in streams confined by cohesive substrate, but towards the top of this stage braided stream depositional environments became dominant.  Locally there is evidence for the presence of small lakes within the floodplain.   Sediment was now mainly derived from the west, which constituted a major reorganisation of dispersal patterns.  Sediment was quartzose, even near the eastern basin margin, in close proximity to the areas which previously provided copious amounts of volcanic lithic detritus to the basin.  In the south-west the Roma Shelf had undergone uplift, either associated with the basinwide structuring event at the end of stage 1 or as a result of more local tectonic movements.  During stage 3 incision into the exposed areas may have begun to form east-west trending valleys on the Roma Shelf.  Stage 4  of the basin development (Expedition Depositional Stage) is characterised by a basinwide change to almost exclusively braided stream environments accompanied by a  coarsening of the sandstones.   The detritus was still derived from the west and both transverse and axial dispersal systems were active.  Incision into the exposed Roma Shelf continued during this time.   Most of the sediment accumulated was deposited by streams entering the basin transversely.   In the central part of the outcrop belt and near the eastern margin deposits containing a higher proportion of large cross-beds were deposited in large south-flowing rivers, presumably forming an axial drainage system.  Westerly palaeo-flow directions are interpreted from cross-beds in the western area, possibly indicating some spill-over from the Bowen into the Galilee Basin.  Subsidence rates during stages 3 and 4 are interpreted to have been lower than during stages 1 and 2. The deposits of the Glenidal and Expedition stages are much thinner ( < 1 km max. in depocentre) and show a less well developed asymmetry of the depo-centre. The cause for this change in subsidence patterns and the sudden influx of quartzose material into the basin can only be speculated.  On the balance of the available evidence tectonic mechanisms are considered more likely than eustatic or climatic variations. These mechanisms could result from variations in the response of the crust underlying the basin to the applied load in the New England Orogen. Alternatively deformation of the orogen could have undergone a period of relative quiescence. The final resolution of this fundamental problem must await further research, despite considerable efforts during the course of this study.  Stage 5  (lower Snake Creek Depositional Stage) comprises a transgressive facies association of fluvial and lacustrine deposits. An overall rise of the base-level within the basin resulted in the establishment of open circulation lacustrine conditions over most of the southern parts.  On the Roma Shelf the basal pans of Stage 4 deposits comprise poorly sorted, coarse fluvial deposits, deriving detritus locally from exposed granitic areas. These streams began to fill the fluvial valleys incised into the Roma Shelf during Stages ?3 and 4, but gave soon way to lake margin deposits of a transgressing lacustrine environment. A single lake with a maximum extend comparable in size to the modern day Lake Erie or lake Victoria flooded the southern parts of  the basin, while poorly drained wetlands prevailed in the northeastern parts of the study area. Where the northern coast line of the lake developed embayments, deposits mimic estuarine facies assemblages with a complex interplay of fluvial and lacustrine deposits. On the eastern basin margin sediment derivation from the east is evident by conglomerates interbedded with finer grained deposits of large wetland areas.  These conglomerates may be part of a large prograding coarse clastic wedge, which dominated the following depositional stage (Stage 6). The development of poorly drained and lacustrine conditions throughout the basin is attributed to a renewed onset of basinwide subsidence resulting in a relative rise of the local base-level.  Stage 6  (lower Moolayember) While the onset of subsidence outstripped sediment supply during Stage 5, resulting in the formation large lakes, Stage 6 is marked by prograding clastic wedges rapidly infilling the basin. On the western basin margin this was in from of lithic sandstones and interbedded grey mudrocks deposited by meandering streams dissecting vegetated floodplains.  On the eastern basin margin large stream-dominated alluvial fans prograded into the basin and gave rise to thick successions of conglomerate with minor interbedded sandstones and mudrocks.   Sediment dispersal occurred again in both transverse and axial fashion.   Wetlands persisted in the central parts of the basin from the previous stage. Stage 7  (upper Moolayember) was not studied here in great detail, but on the western basin margin the change from coarse clastics of Stage 6 to fine grained deposits of Stage 7 is dramatic.  In GSO Taroom 16 conglomerates of Stage 6 are sharply overlain by overbank mudrocks of Stage 7, indicating an intervening period of non-deposition.  A change from prograding to aggrading log patterns can be noted throughout the basin on wireline logs and marks the lower boundary of Stage 7.  Sediment accumulation in the Bowen Basin ended at the end of Stage 7 with a period of uplift and erosion, resulting from the culmination of   compressive tectonism in the New England Orogen and the Bowen Basin.