Abstract

Distinguishing the deposits of mouth bars from delta distributary channels in the rock record may not be as straightforward as often portrayed, because mouth-bar deposits can be more variable than usually presumed. Mouth bars with triangular plan-view subaerial geometry are well developed at the mouth of the modern Burdekin River of northeastern Australia. This planform is intermediate between the elongate, lozenge-shaped mouth bars typical of river-dominated deltas and the beach-ridge geometries characteristic of wave-dominated deltas. Surface and shallow-subsurface bar deposits are predominantly moderately sorted, coarse-grained sand, similar to that of adjacent, lower-delta-plain-channel floor. Sedimentary textures are modified at seaward sides of mouth bars by waves into a foreshore of well-sorted fine to medium-grained sand, and mouth bar sands pass distally and laterally into more mud-dominated lithologies. The Holocene section beneath the lower delta plain is dominated by 5–8 m thick, sharp-based bodies of coarse-grained sand, texturally indistinguishable from the modern mouth bar with no vertical grain-size trend, a slight upward-fining trend, or in a few cases a coarsening-upward trend. These sand bodies have low-angle seaward-dipping internal bedding surfaces and are bounded by surfaces of similar attitude. The Holocene Burdekin Delta was and is flood-dominated (rather than strongly wave-influenced, as proposed previously) and prograded by rapid deposition of mouth bars during river floods (most of which last for only a few days). Mouth-bar construction takes place over tens of years. The variable vertical grain-size profiles of mouth-bar deposits suggests formation by both aggradation and progradation, and this is supported by the cross-sectional geometry of mouth-bar clinoform sets imaged geophysically. Once a mouth bar has become emergent and is stabilized by vegetation, a new bar is initiated seaward. In this way, delta "lobes" are constructed over 100s to 1000s of years before being abandoned following an avulsion of the trunk river channel to another part of the delta. Caution is needed in the interpretation of ancient shallow-water deltaic successions, where sharp-based, fining-upward mouth bar deposits may be confused with distributary-channel facies.

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