Abstract

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.

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