Eolian Deposits of the Younghusband Carbonate Barrier, South Australia: Analog for Ancient Eolian Petroleum Reservoirs
Steven G. Fryberger, Brett Walker, Rob Rutherford, 2001. "Eolian Deposits of the Younghusband Carbonate Barrier, South Australia: Analog for Ancient Eolian Petroleum Reservoirs", Modern and Ancient Carbonate Eolianites: Sedimentology, Sequence Stratigraphy, and Diagenesis, F. E. (Rick) Abegg, David B. Loope, Paul M. (Mitch) Harris
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The Holocene eolian barrier of the Younghusband peninsula, South Australia, is an accessible and well-studied modern analog for ancient eolian carbonate and mixed quartz-carbonate barriers, as well as the associated suites of sedimentary structures that might be expected in such settings. The barrier has formed from the shoreward drift of Lacepede Shelf biogenic carbonate fragments as well as the longshore drift of quartz sand from the Murray River.
The Younghusband barrier carbonate content increases from roughly 12 percent at the mouth of the Murray River southeastward to nearly 75 percent at Lacepede Bay, near Kingston. The barrier consists mainly of eolian dune and sand-sheet deposits. Dune bedforms consist of parabolic, blowout, coppice, and various subtypes of transverse dunes including barchan, barchanoid ridge, and transverse ridge dunes. There is a low foredune along the shore, landward of which lies a narrow sabkha mat separates this foredune from the main barrier. Where vegetation has been destroyed or thinned, deflation forms large blowouts, parabolic dunes, and transverse dunes, whose advance is causing the barrier to shift northeastward into the Coorong Lagoon.
The abundant carbonate material within the barrier includes transported bryozoans and coralline algae that originated on the cool-water Lacepede Shelf. These materials, along with broken shells of lagoonal and nearshore fauna such as gastropods and mollusks, have been incorporated into the cross stratification. This process has produced such coarse, shelly textures in the eolian barrier sediments, particularly the sand-sheet deposits, that it would be difficult to identify these sediments as eolian in the ancient record. This is not merely because of the presence of shell debris but also because of unusual sedimentary structures. Nevertheless, common eolian sedimentary features are present and identifiable, and provide a key to proper interpretation of this group of sediments in ancient rocks. The Holocene Younghusband barrier and the landward Pleistocene carbonate barriers are viable analogues for subsurface petroleum reservoirs. The Younghusband barrier, if buried and preserved, would be a very narrow but elongate sand lens about a kilometer wide at maximum, and up to 30 meters thick, with enormous reservoir volume because of its great length. It would consist mainly of clean eolian sands, composed dominantly of either quartz or carbonate depending upon position along the barrier. These sediments would be interbedded with nearshore and lagoonal facies (potential source rocks).
Recognition of the eolian nature of such a barrier-originated petroleum reservoir would allow correct paleogeographic placement of the sediments, and would facilitate both development drilling and further exploration for new barriers or extensions of the current field. Further, the knowledge that the modern barriers exist in trends controlled by sea-level stands would suggest that petroleum reservoirs in such ancient systems might occur in multiple fields parallel to bathymetric contours of an ancient marine shelf. The current preservation of eolian parts of older barriers in the South Australia region onshore but below sea level (observed in shallow-drilling programs) and offshore as subsea “ranges” proves that such preservation of the eolian component of barriers is not only possible but should be expected in the ancient record.
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Modern and Ancient Carbonate Eolianites: Sedimentology, Sequence Stratigraphy, and Diagenesis
Carbonate eolianites had long been considered to be limited to the Quaternary, but a number of Mesozoic and Paleozoic examples have been documented in the past 15 years. Thus, an increased awareness of carbonate eolianites is required to properly interpret the rock record and to assess their spatial and temporal distribution. The papers of this volume will help geologists to: (1) recognize carbonate eolianites and understand their preservation potential—recognitional criteria for most carbonate environments are common knowledge, but this is less true for carbonate eolianites; (2) understand their sedimentologic and diagenetic variability—diagenesis of carbonate eolianites has important economic considerations. Whereas Quaternary eolian limestones are commonly porous, Paleozoic and Mesozoic examples are typically tight owing to compaction; (3) understand the Psilionichnus (marginal marine) and Scoyenia (nonmarine) Ichnofacies—carbonate eolianites are not devoid of trace fossils; (4) interpret them in a sequence stratigraphic framework—interpretations of relative sea level during eolian deposition can be difficult, as differences between transgressive, regressive, and deflationsourced eolianites are subtle. Thus, the placement of sequence boundaries within interbedded eolian and subtidal carbonate successions is not entirely straightforward.