A seaway may have occupied a geosynclinal trough in Washington and Oregon during Early Devonian time. Its existence is postulated from the distribution and sedimentary environment of isolated deposits of Early Devonian and probably Early Devonian age in the northern Rocky Mountains. Regional evidence suggests that the Beartooth Butte formation of Early Devonian age was laid down along the eastern margin of a sea and on the landmass that bordered it in southern and central Montana and northern Wyoming. The Williston basin area in North Dakota was apparently a part of the landmass. The Water Canyon, Maywood, and Ghost River formations which may be in part correlative with the Beartooth Butte were laid down in a nearshore, shallow-water marine environment in northern Utah an southern Idaho, western Montana, and west-central Alberta, respectively. These lie west of the marginal marine deposits of the Beartooth Butte but several hundred miles east of the postulated north-south axis of the seaway.

Discontinuous deposits of the Beartooth Butte formation, which formerly was considered to be a local channel filling at a few localities in northern Wyoming, have now been widely recognized. The formation consists of grayish red and yellowish gray silty dolomite and dolomitic siltstone, sandstone, conglomerate, and breccia. It is generally less than 10 feet but locally as much as 170 feet thick. The continental beds of the Beartooth Butte were laid down on a land surface of generally low relief with karst topography in places. Redbeds filling channels and sinkholes were derived mostly from red soils that had developed on carbonate rocks possibly in a humid, tropical, or subtropical climate. Marginal marine beds of the formation were probably deposited in estuaries, bays, and lagoons a ong a drowned coast characterized by long, narrow marine embayments.

The Water Canyon formation of Early Devonian age in northern Utah is about 400 feet thick. It is composed of intraformational breccia, silty dolomite, cherty dolomite, and dolomitic sandstone and sandy dolomite containing

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fish remains. The lower part of the Maywood formation in western Montana is 200 feet thick and may be of Early Devonian age. It is composed of thin-bedded silty dolomite and dolomitic siltstone with imbedded crystals of dolomite pseudomorphous after halite and, like the Water Canyon formation, dolomitic sandstone containing fish remains. The upper part of the Ghost River formation in west-central Alberta is lithologically similar to the lower part of the Maywood and may also be of Early Devonian age. Discontinuous shallow-water, near-shore marine deposits of these three formations were probably laid down at the mouths of bays.

The probable subsurface occurrence of the Beartooth Butte formation and its tentative correlatives provides a hitherto unrecorded and untested stratigraphic trap that might be considered in planning petroleum exploration.

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