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The AAPG/Datapages Combined Publications Database
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Abstract
2004. The American Association of Petroleum Geologists. All rights reserved.
Sequence Sets, High-accommodation Events, and the Coal Window in the Carboniferous Sydney Coalfield, Atlantic Canada
Martin R. Gibling,1 K. I. Saunders,2 N. E. Tibert,3 J. A. White1
1Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia, Canada
2Petro-Canada, Calgary, Alberta, Canada
3Department of Environmental Science and Geology, Mary Washington College, Fredericksburg, Virginia, U.S.A.
ACKNOWLEDGMENTS
We are grateful to John Calder, Don MacNeil, and Rob Naylor for discussion, Allen Archer and Tim Demko for their helpful reviews of the manuscript, and Jack Pashin and Bob Gastaldo for editorial assistance. Andy Henry and Dalhousie Graphics are thanked for their drafting skills. Financial assistance through a Natural Sciences and Engineering Research Council of Canada (NSERC) Research Grant to M. R. Gibling, an NSERC Scholarship to K. Saunders, and a Texaco Scholarship to J. White are gratefully acknowledged.
ABSTRACT
Economic coals of the Sydney Basin lie in high-frequency sequences mostly bounded by calcretes, indicative of base-level lowering on the coastal plain. Most coals represent blanket coastal peats that accumulated just prior to maximum transgression, which is commonly marked by dark limestone and shale with a restricted marine fauna. Thin coals are also present in highstand deposits. Coal and sequence thickness show a general correlation, confirming a link between potential accommodation and peat accumulation. Composite sequences commence with sustained coastal progradation and base-level lowering, and coals are especially prominent in a transgressive sequence set with a slightly retrogradational to aggradational style. Sequence architecture was controlled by high-accommodation events of relative sea level rise followed by relative falls, the expression of glacioeustatic events in a cratonic basin with moderate subsidence rate. Although thin (average 19 m [62 ft]) and thick (average 55 m [180 ft]) sequences show apparently similar architecture, thick sequences contain thick alluvium with cryptic transgressive units, and their detailed architecture probably reflects channel switching, climatic, and/or tectonic effects during prolonged, low-accommodation periods. Thick sequences represent the most landward transgressions and probably pass basinward into composite sequences.
The coal-bearing interval or coal window
in the basin fill is about 1.5 km (0.9 mi) thick and reflects long-term accommodation driven by subsidence, as well as climatic control. Economic coals formed while the outcrop belt lay within range of high-accommodation, relative sea level events, and they show little petrographic change through the coal window. Upward loss of coals reflects regional progradation of the alluvial plain, coupled with climatic change as Pangea drifted northward.
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