Facies Architecture of a Devonian Soft-Sediment-Deformed Alluvial Sequence, Broken River Province, Northeastern Australia
Simon C. Lang, Christopher R. Fielding, 1991. "Facies Architecture of a Devonian Soft-Sediment-Deformed Alluvial Sequence, Broken River Province, Northeastern Australia", The Three-Dimensional Facies Architecture of Terrigenous Clastic Sediments and its Implications for Hydrocarbon Discovery and Recovery, Andrew D. Miall, Noel Tyler
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The Late Devonian Rockfields Member of the Bulgeri Formation in northeast Queensland, Australia, was deposited in a tectonically active alluvial basin. The source area was an uplifted igneous and metamorphic terrain to the south of a major oblique-slip fault zone, forming the basin margin. The member is characterized by relatively uniform, very fine- to medium-grained, soft-sediment-deformed sandstones, interbedded predominantly with slightly reddened siltstones. Detailed mapping of sediment body geometries and internal structures has resulted in the recognition of eight architectural elements. Channel-fill elements include laminated sand sheets, dune complexes, scour fills, laterally-and downstream-accreting macroforms, and linguoid barforms. Floodplain elements include levees, crevasse splays, and overbank fines.
The abundant soft-sediment deformation structures (mainly convolute lamination) were produced by liquefaction of sandy bedforms, during or immediately after rapid deposition. Larger scale structures, however, deform multiple erosion surfaces. These may be due to liquefaction of saturated unconsolidated sands during earthquakes of magnitude >5, with inferred epicenters along the fault zone 20 km to the south.
The proposed depositional model comprises a broad, sandy braidplain with marked discharge variation, resulting in widespread seasonal flooding. Channel-avulsion events, combined with a relatively high subsidence rate associated with tectonic activity, resulted in the accumulation of laterally extensive, sheet-like, channel sandbodies and fine-grained floodplain deposits. A modem analog is the seasonally inundated low-sinuosity channel system of the alluvial plains and fans surrounding the Gulf of Carpentaria, north Queensland. This model bears some resemblance to published models for low-sinuosity streams, but sufficient differences warrant recognition as a distinct fluvial style.
The unit represents an example of a strongly layered sequence with laterally extensive (>1,500 m) channel sandstone bodies 3-10 m thick, separated by floodplain deposits 0.3-2 m thick of comparable lateral extent (1,500 to >4,000 m). Hydrocarbon reservoirs with similar architecture, uniform grain size, and a general lack of internal fine partings could be considered to behave isotropically with respect to fluid flow within individual bodies.