ABSTRACT

The overall patterns of facies and thickness variations that occur in the Purcell rocks of Clark Range, in the eastern Rocky Mountains, can be outlined in terms of the relationships among six composite stratigraphic sections. Extrapolations of trends in facies and thickness variations from Clark Range confirm regional correlations that are based on the apparent equivalence of distinctive rock types and thin 'marker' units. The resulting interpretation of regional patterns of facies and thickness variations provides a basis for deducing the provenance and environment of deposition of the Purcell sediments.

All current data concerning the character and regional relationships of the Purcell rocks are consistent with the hypothesis that the Purcell sediments were deposited on and adjacent to the flood plain of a large subsiding delta along the western margin of the craton, under conditions analogous to those in the Gulf Coast geosyncline. This implies that the large volume of fine terrigeneous clastic sediment that constitutes the bulk of the Purcell succession was derived from the older Precambrian rocks which occur in the interior of the continent, far from the site of deposition; and that none of it need have originated in some supposed western landmass.

The Waterton Formation in the eastern Rocky Mountains and the lower part of the Fort Steele Formation in the western Rocky Mountains are shallow water, shelf-type deposits that mark the base of the exposed Purcell succession and appear to be of about the same age. They are overlain, and perhaps replaced westward, by an eastward-tapering wedge of fine terrigenous clastic sediment that forms the 10,000 to 15,000 foot thick Aldridge (Prichard) Formation in the Purcell Mountains, and a 2,000 foot thick upper unit of the Altyn Formation in southwestern Clark Range. This large volume of relatively uniform fine clastic sediment appears to have been deposited along the advancing front of a delta, in relatively deep water on the continental slope and outer part of the continental shelf, during a stage of aggradation. It is replaced to the east by a condensed sequence of near shore, shallow water, carbonate muds, about 500 feet thick, that forms the Altyn Formation of eastern and northern Clark Range. The younger formations appear to be flood plain, tidal-flat, and shallow water carbonate-bank deposits that accumulated on the extensive, slowly subsiding deltaic plain built during the stage of aggradation.

The shallow water muds, silts and sands of the Appekunny and Grinnell Formations thicken southeastward and westward and are equivalent to the Creston Formation (Ravalli Group) of the Purcell Mountains. The lower member of the Siyeh Formation represents a transition to the shallow water carbonate mud facies of the middle member. The latter occurs in a condensed, near shore facies in northwestern Clark Range, thickens southeastward into Montana, and is transitional westward into a thicker shallow water succession, the Kitchener Formation (middle unit of the Piegan Group), that contains a higher proportion of terrigenous sediment. The upper member of the Siyeh Formation consists of varicoloured shallow water muds and silts. It becomes thicker and coaser southeastward and westward. It is represented by the Snowslip Formation at Marias Pass, Montana, and the upper unit of the Piegan Group and the lower part of an unnamed unit that was formerly called the "Siyeh Formation" in the Purcell Mountains of Montana and British Columbia respectively. The Purcell Lava forms the only reliable time-stratigraphic marker between the eastern edge of the Rocky Mountains and the Purcell Mountains. The distinctive shallow water carbonate rocks and sandstones of the Sheppard Formation can be recognized at the base of the Gateway Formation and the latter should be restricted to exclude them. The Sheppard Formation unconformably overlaps the Purcell Lava and the upper member of the Siyeh Formation in the western Rocky Mountains near the International Boundary, and may also be unconformable over a younger, unnamed sequence of interbedded lavas and sediments (formerly the upper part of the "Siyeh Formation") in the Purcell Mountains nearby. The

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Sheppard Formation thickens southeastward into Montana. It may be equivalent to the lower part of the Dutch Creek Formation in the western Purcell Mountains. The Gateway Formation thickens southeastward and is represented by the Shields Formation at Marias Pass, Montana, where the red siltstones and argillites of the lower member have replaced most of the green dolomitic argillites and sandstones of the upper member. The colour of the lower member changes from red in the eastern Rocky Mountains to greenish grey in the western Rocky Mountains and eastern Purcell Mountains, but other characteristic features remain unchanged. The Phillips Formation persists across the Rocky Mountains into the eastern Purcell Mountains as a distinctive marker unit. It is equivalent to the Red Plume Formation at Marias Pass, Montana. The Roosville Formation thins toward the northwest in the southern Rocky Mountains and southeastern Purcell Mountains as a result of southeastward epeirogenic tilting followed by pre-Middle Cambrian erosion. The Mount Nelson Formation of the western Purcell Mountains may be partly equivalent to the Roosville Formation or younger than it. It is overlain unconformably by a thick succession of Precambrian Windermere rocks that pass upward into rocks of probable Lower Cambrian age. Windermere, Lower Cambrian, Middle or Upper Cambrian, and Devonian rocks are unconformable on the Purcell succession along a structural high that extends into the southeastern Purcell Mountains from Lizard Range and the southern part of Hughes Range in the western Rocky Mountains.