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Fluvial Sediments as from Analogue for a Regional Seal: Observations from the Channel Country of Central Australia

Amos, Kathryn J.1; Hasiotis, Stephen T.2; Morris, Jennifer L.3; Wright, Paul 4; Krapf, Carmen B.1
1 Australian School of Petroleum, The University of Adelaide, Adelaide, SA, Australia.
2 Department of Geology, University of Kansas, Lawrence, KS.
3 School of Earth and Ocean Sciences, Cardiff University, Cardiff, United Kingdom.
4 BG Group, Reading, United Kingdom.

Regional expanses of mudstone to very fine sandstone that form cap-seal facies within continental successions are usually interpreted as lacustrine in origin. This need not always be the case, however. The Channel Country rivers of central Australia are mud-dominated, dryland, anabranching rivers with complex channel pattern and floodplain geomorphology. Floodplains reach widths of 70 km over valley lengths of hundreds of kilometres, are comprised of sediments that have been shown to seal effectively when wet, and provide a useful analogue for large-scale regional seals from ancient continental drylands.

A large number of geomorphic elements occur within these floodplains, including anabranching channels, levees, waterholes, splays, and floodplain-surface channels with associated bar-form floodplain highs. Fieldwork, however, has revealed that this abundance of geomorphic features is not likely to be represented in the sedimentary record. Detailed sedimentologic observations are presented from sites on the Diamantina River, one of the principal rivers of the Channel Country and the main contributor of discharge to Lake Eyre. In several locations, channel bed sediments were mud-dominated and pedogenically altered, and indistinct from adjacent levee and floodplain sediments. At other sites, excavation of accretionary bench and bar deposits within anabranching and floodplain-surface channels yielded sedimentary structures comprised almost entirely of mud-aggregate particles with sizes ranging from fine-grained sand to gravel. In some locations, thin sandy and mud-aggregate laminae were interbedded within low-angle planar and cross-bedded sediments.

The Channel Country rivers have been cited as a modern analogue for a number of ancient mud-rich dryland deposits. Here, we explore the preservation potential of the sedimentary structures observed within these muddy deposits and present a detailed comparison between the deposits of the Diamantina River with the mudrock facies of the Late Silurian - Early Devonian lower Old Red Sandstone (ORS), UK. Similar sedimentary structures from the Diamantina River and ORS will be presented and their thicknesses, extent, and composition will be compared. Similarities and differences that may be expected between mud-rich fluvial deposits and lacustrine sediments in the rock record are explored and summarized.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009