During Turonian time, the western North American Cretaceous seaway covered a shallow shelf upon which sediments were deposited predominantly from western sources. Pulsating rates of sediment supply resulted in local regressions and transgressions along the seaway margins. The last three of these regressive-transgressive sequences affecting the Powder River basin are recorded by two sandstone complexes with western sources: the Torchlight and First Frontier sandstones, and the Turner sandstone, a shale-sandstone complex which prograded westward from the craton.

The Torchlight and First Frontier sandstones represent deltaic depocenters which consisted of distributary-channel and delta-front sand complexes, capped by well-sorted transgressive, reworked sands. The Turner sandstone represents isolated individual bars, channels, and splays within a mud-rich deltaic complex.

Stratigraphic analyses show that the Torchlight sandstone is slightly older than the First Frontier, and that the Turner complex is somewhat younger than the First Frontier. An isopach map of the lower, calcareous part of the overlying Niobrara Formation shows that calcareous shales are thickest over the western deltaic complexes, decreasing to zero along the seaward edges of the Turner lobes. Interpretation of the calcareous shales as representing periods of slow siliciclastic sediment influx indicates that the western complexes were inactive as depocenters for a significant period of time whereas the Turner distributaries were still supplying significant siliciclastic sediments from the craton to the eastern parts of the Powder River basin.

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