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Local tectonic control on parasequence architecture: Second Frontier sandstone, Powder River Basin, Wyoming

¹ Australian School of Petroleum, Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC), University of Adelaide, Adelaide, SA 5005, Australia
² Geosciences Department, University of Houston, Houston, Texas 77204

Boyan K. Vakarelov is a lecturer at the Australian School of Petroleum, University of Adelaide, Australia, in sedimentology and sequence stratigraphy. His research interests are in shallow-marine clastic sedimentology, high-resolution sequence stratigraphy, and CO₂ sequestration. He received a B.Sc. (honors) degree from the University of Toronto in 2001 and a Ph.D. from the University of Texas at Dallas in 2006.

Janok P. Bhattacharya is the Robert E. Sheriff Professor of Sequence Stratigraphy at the University of Houston. His research interests include deltaic sedimentology and sequence stratigraphy, the local control of structure on stratigraphy, and reservoir architecture of clastic depositional systems. He received his B.Sc. degree in 1981 from the Memorial University of Newfoundland, Canada, and completed his Ph.D. in 1989 from McMaster University, Hamilton, Ontario, Canada.

The Cenomanian Second Frontier sandstone, one of the major producing units of the Frontier Formation in the Powder River Basin, Wyoming, is a basin-isolated sand body that pinches out to zero thickness in both seaward and basinward direction. The Second Frontier has previously been viewed as a single, relatively homogeneous, wave-dominated sediment succession. Our high-resolution study, integrating detailed facies relationships in outcrop, core, and well logs, shows that the Second Frontier sandstone comprises an offlapping parasequence set, formed from seven regionally mappable wave-dominated parasequences. The parasequences are bounded by minor flooding surfaces that represent previously unrecognized potential flow barriers or baffles. Parasequence boundaries are oriented obliquely to the well-defined regional north-northwest–south-southeast–elongated trend of the unit, and successive parasequences offlap to the south in an along-strike direction. The seaward pinch-out of the Second Frontier sandstone is depositional in nature, as illustrated by well-preserved healing-phase deposits, whereas the basinward pinch-out is caused by marine truncation over a tectonically uplifted area. Parasequence architecture was strongly affected by syndepositional tectonic movement, which determined both (1) the distribution of the entire parasequence set, forming a depositional remnant, and (2) the position and shape of parasequences internal to the remnant. The Second Frontier is thus interpreted as a depositional remnant of a once more extensive wave-dominated deltaic shoreface complex.