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Sharp-based, flood-dominated mouth bar sands from the Burdekin River delta of northeastern Australia; extending the spectrum of mouth-bar facies, geometry, and stacking patterns

Christopher R. Fielding, Jonathon D. Trueman and Jan Alexander
Sharp-based, flood-dominated mouth bar sands from the Burdekin River delta of northeastern Australia; extending the spectrum of mouth-bar facies, geometry, and stacking patterns
Journal of Sedimentary Research (January 2005) 75 (1): 55-66

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.


ISSN: 1527-1404
EISSN: 1938-3681
Serial Title: Journal of Sedimentary Research
Serial Volume: 75
Serial Issue: 1
Title: Sharp-based, flood-dominated mouth bar sands from the Burdekin River delta of northeastern Australia; extending the spectrum of mouth-bar facies, geometry, and stacking patterns
Affiliation: University of Nebraska, Department of Geosciences, Lincoln, NE, United States
Pages: 55-66
Published: 200501
Text Language: English
Publisher: Society of Economic Paleontologists and Mineralogists, Tulsa, OK, United States
References: 46
Accession Number: 2005-036029
Categories: Sedimentary petrologyQuaternary geology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. sects., sketch maps
S20°00'00" - S20°00'00", E148°00'00" - E148°00'00"
Secondary Affiliation: University of Queensland, AUS, AustraliaUniversity of East Anglia, GBR, United Kingdom
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute. Reference includes data supplied by SEPM (Society for Sedimentary Geology), Tulsa, OK, United States
Update Code: 200513
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