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 intertidal mud flats of the encroaching sea. The Cameron Creek grades upward into marine strata of the Lower and Middle Pennsylvanian Alaska Beach Limestone. Pennsylvanian deltaic deposition ceased in the region as the transgressing sea inundated the terrains that had previously supplied sediments to the Tyler delta.--Modified journal abstract.