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An Overview of the Ross Formation, Shannon Basin, Western Ireland

By
Trond Lien
Trond Lien
1
Norsk Hydro Research Center, Bergen, Norway
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Ole J. Martinsen
Ole J. Martinsen
1
Norsk Hydro Research Center, Bergen, Norway
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Roger Walker
Roger Walker
2
Roger Walker Consulting, Inc., Calgary, Alberta, Canada
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Published:
January 01, 2008

Abstract

The Upper Carboniferous deep-water sediments of the Shannon Group were deposited in the extensional Shannon basin of County Clare in western Ireland, and exposures occur along the sea cliffs of the Shannon Estuary (Figure 1). Carboniferous limestone floors the basin, and the basin-fill succession begins with the deep-water Clare Shales. These shales are overlain by various turbidite facies of the Ross Formation (460 m [1509 ft] thick). The type of turbidite system, scale of turbidite sandstone bodies, and the overall character of the stratigraphic succession, make the Ross Formation well suited as an analog for sand-rich turbidite plays in passive-margin basins around the world. The lower 170 m (558 ft) of the Ross Formation contains tabular turbidites with no channels. There is an overall tendency for the Ross to become sandier upwards, although there are no small-scale thickening- or thinning-upward successions. The upper 290 m (950 ft) consist of turbidites, commonly arranged in thickening-upward packages, and amalgamated turbidites. These form channel fills that are each up to 10 m (33 ft) thick. A few of the upper channels have an initial lateral-accretion phase with interbedded sandstone and mudstone deposits, which is followed by a vertical-aggradation phase with thick-bedded, amalgamated deposits. As the channels filled, more turbidites spilled farther overbank. Superb outcrops show that thickening-upward packages developed when channels initially spilled muds and thin-bedded sands up to 1 km (0.6 mi) overbank, followed by thick-bedded, amalgamated turbidites that spilled close to the channel margins. The paleocurrent directions associated with the amalgamated channel fills suggest a low channel sinuosity.

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Contents

AAPG Studies in Geology

Atlas of Deep-Water Outcrops

Tor H. Nilsen
Tor H. Nilsen
Desceased
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Roger D. Shew
Roger D. Shew
University of North Carolina
Wilmington, North Carolina
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Gary S. Steffens
Gary S. Steffens
Shell International E&P
Houston, Texas
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Joseph R. J. Studlick
Joseph R. J. Studlick
Maersk Oil America Inc.
Houston, Texas
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American Association of Petroleum Geologists
Volume
56
ISBN electronic:
9781629810331
Publication date:
January 01, 2008

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