Geomorphic Characterization Of The Gilbert River Distributive Fluvial System (Dfs) And Implications For Cretaceous Coastal Fluvial Successions

Abstract

The Gilbert River distributive fluvial system (DFS) of Queensland, Australia, flanks the Gulf of Carpentaria, an epeiric sea that occupies a slowly subsiding intracratonic basin. Approximately 13 km of progradation has occurred over the last 6 ka, largely due to high sedimentation rates, low regional slope, and a slight sea level fall. This system exhibits the same depositional patterns as purely continental DFSs: 1) a radial channel pattern, 2) a down-DFS decrease in both channel and grain size (the latter inferred), 3) a lack of lateral channel confinement, 4) a broad fan shape, and 5) a down-DFS increase in floodplain/channel area. The coastal plain portion (influenced by sea level changes) is characterized by: a) a contact between DFS and marginal-marine deposits, b) channel incision, confinement and lateral movement, c) increased channel width due to tidal influence, d) sediment redistribution (spits, small-scale deltas), and e) evidence of shoreline progradation (wave-cut platforms and beach ridges). Other coastal DFSs are present in passive-margin settings in Australia, India, and Africa. Few modern examples of DFS spanning the terrestrial to marine realm exist, as: 1) modern coastlines are presently flooded due to high-amplitude Quaternary sea level fluctuations, 2) many rivers are incised into large valleys (Mississippi River) or incised into pre-existing coastal DFS deposits (Canterbury Plains of New Zealand, Texas Gulf coastal plain), and 3) anthropogenic modification conceals surface expressions and hinders natural channel behavior (Godhavari River of India). Geomorphic observations on these systems ultimately lead to sedimentologic and stratigraphic predictions regarding coastal DFS deposits that cross the fluvial-marine interface, such as the Cretaceous Williams Fork Formation of Colorado. A purely progradational succession should be characterized by basal shoreface strata cut into by tidally influenced channels, and exhibit an upsection increase in grain size, sand:mud ratios, and channel amalgamation, with a corresponding decrease in tidal influence and coals. The DFS concept may explain common patterns (e.g. upsection changes in sand:mud, sandbody thickness and architecture) observed in Cretaceous rock record examples and is valuable in reservoir modeling at the basin scale.