ABSTRACT

The transition from platform/miogeocline to orogenic foreland basin occurs within the Jurassic. It is marked in the west by a change in sediment source from cratonic (east) to orogenic (west) in the Upper Jurassic, and by subsidence of the foredeep. To the east, the Williston Basin is characterized in the Middle Jurassic by pronounced downwarping and eventual inversion in the Upper Jurassic.

Seven stratigraphic packages are regionally significant in the Jurassic databases. From youngest to oldest, these are:

7. Kootenay, Nikanassin, Minnes, Success, "Detrital", Waskada (part) (package includes some lowest Cretaceous strata);

6. Green beds, upper Fernie shale, Swift, Masefield (= upper Vanguard), Waskada (part);

5. Grey beds and equivalents, Rierdon, Roseray (= middle Vanguard), Rush Lake, upper Melita;

4. Highwood, Sawtooth, Shaunavon, upper Gravelbourg, lower Melita;

3. Rock Creek, lower Gravelbourg, Reston, upper Watrous, upper Amaranth;

2. Poker Chip shale;

1. Nordegg, Red Deer, and associated units of the Fernie Formation.

The Lower Jurassic packages (packages 1,2) are thin, starved shelf deposits with many disconformities, deposited only on the western miogeocline and adjacent shelf. The early Middle Jurassic strata (3) are clastic in the west, but in the Williston Basin (where they are the oldest Jurassic strata), they contain evaporites and limestones. Younger Middle Jurassic beds (4,5), somewhat thicker, are shale, with minor sandstone and limestone. The first deposits in the foredeep are the Green beds of the Fernie Formation (6), a starved shelf assemblage overlain by a coarsening- and shallowing-upward foredeep-fill succession. Equivalent strata farther east on the craton are the Swift and Masefield formations. The youngest strata (packages 6,7), continuous into the Early Cretaceous, constitute the first of several Mesozoic foreland-basin clastic sequences.

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