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Pennsylvanian sedimentary rocks in the northern Rocky Mountains and in the northern Great Plains of the United States were deposited primarily on a broad marine shelf between the North American craton and the late Paleozoic continental margin in Idaho and adjacent states. The Lower Pennsylvanian (Morrowan) Tyler Formation comprises detrital sediments and some limestone beds in Montana and North Dakota that were deposited along an eastward-transgressing marine shoreline after regional uplift, warping, and faulting had resulted in an erosional unconformity on top of Mississippian strata. The Lower Pennsylvanian shoreline finally extended onto the cratonic interior in eastern North Dakota.
Initial Tyler sediments were deposited as a deltaic and fluviolacustrine complex succeeded by littoral deposits as the Early Pennsylvanian shoreline transgressed eastward across the shelf. Sediments were derived mostly from Mississippian rocks exposed and eroded in south-central Montana and adjacent parts of Wyoming, and from the cratonic hinterland in eastern North Dakota.
The Tyler Formation is subdivided into the Stonehouse Canyon Member at the base, the Bear Gulch Member, and the Cameron Creek Member at the top. The Stonehouse Canyon Member consists of dominantly carbonaceous mudstone and lenticular sandstone beds that accumulated on a delta shore and plain as offshore bar, beach, channel, levee, lake and swamp, and related deposits. The Bear Gulch Member, which intertongues with the upper part of the Stonehouse Canyon Member, consists mainly of planar-bedded calcareous mudstone that probably formed in a bay or an estuary adjacent to the delta. The Cameron Creek Member, which overlies the two other members, is dominantly red mudstone and represents oxidized subaerial and shoreface deposits on the upper delta platform, the alluvial plain, and the inte tidal mud flats of the encroaching sea. The Cameron Creek grades upward into marine strata of the Lower and Middle Pennsylvanian Alaska Bench Limestone. Pennsylvanian deltaic deposition ceased in the region as the transgressing sea inundated the terrains that had previously supplied sediments to the Tyler delta.
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