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Characteristics of Permian Tanqua Karoo Deepwater Mudstones

By
Bruce M. Samuel
Bruce M. Samuel
1
C & C Technologies, 730 E. Kaliste Saloom Road, Lafayette, LA 70508
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Arnold H. Bouma
Arnold H. Bouma
2
Department of Geology & Geophysics, Louisiana State University, Baton Rouge, LA 70803
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Published:
January 01, 2003

Abstract

Turbidite-related mudstones are an integral part of any submarine fan complex. The Permian Tanqua Karoo in southwestern South Africa is an excellent example. Thick-bedded mudstones, 10–50 m thick, subdivide the individual sand-rich fans within a fan complex. The 30–50 cm thick medium-bedded mudstones and 1–30 cm thick thin-bedded mudstones occur between the sandstone layers of a fan system and may act as baffles to fluid flow.

Characteristics of thick-bedded shales are that the middle fan areas (leveed channels) show predominantly alternating fining-upward sequences of parallel laminated mudstones and mud-shales (∼50–60% clay) and clay-stones and clay-shales (∼80–95% clay). A sharp contact typifies the location of the contact between the mud and the overlying clay.

Medium-bedded shales are typically more siliceous. They are comprised of cyclic fining-upward sequences of quartz-rich siltstones, mudstones and sometimes a clay-shale lamination. However, clay-shale preservation is rare. A thin-bedded siltstone (∼10–20% clay), up to 0.5 cm thick, predominates at the base of each sequence and is capped by laminations of mud-shale (∼35–60% clay), which are generally less than 5 mm thick.

Thin-bedded shales are characterized by fining-upward sequences comprised of siltstone (∼20–30% clay), mud-shale (∼35–55% clay), and clay-shale lamina. The sequence is typically mud-rich and the mud-shale often expresses an amalgamated contact with the clay-shale material above.

The limited thickness of thin-bedded shales and the siliceous nature of medium-bedded shales suggests that fluid and gas flow may be possible. Even minor tectonics result in fractures. The combination of the clays with some micas and organics suggest that many of these mudstones can be source rocks and gas reservoir at the same time.

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SEPM Miscellaneous Publication

Siltstones, Mudstones and Shales: Depositional Processes and Characteristics

Erik D. Scott
Erik D. Scott
Shell International Exploration & Production, 200 North Dairy Ashford, Houston, Texas 77079
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Arnold H. Bouma
Arnold H. Bouma
Dept. of Geology & Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803
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William R. Bryant
William R. Bryant
Dept. of Oceanography, Texas A&M University, College Station, Texas 77843
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SEPM Society for Sedimentary Geology
Volume
1
ISBN electronic:
9781565760943
Publication date:
January 01, 2003

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