SANDSTONE PROVENANCE OF THE MARINE JURASSIC OF WESTERN CANADA AND CENTRAL MONTANA

In the Jurassic (Oxfordian to Bajocian), a retroarc foreland basin covered part of western North America, forming the Sundance Seaway. The Ellis Group of Montana and the coeval Fernie Formation of western Canada represent the marine fill of this basin. The location of the entrance to the seaway is uncertain; one hypothesis places it as far north as the border between the Yukon Territory and British Columbia, and another puts it 1,700 kilometers south near the U.S./Canada border. Sandstone provenance analysis of the Jurassic is used here to identify potential sediment sources and may offer insight into the location of the entrance to the seaway. If the northern hypothesis is correct, sediment sources would come from the craton as well as the fold and thrust belt, with sandstones containing quartz as well as quartz and lithics. If the southern hypothesis is correct, a single cratonic provenance dominated by quartz would be expected.

Thin sections of twelve sandstones were prepared, doubly stained for plagioclase, and point counted to 200 sand-sized grains and then plotted on ternary diagrams to determine provenance.

Sandstones from the Ellis Group have a greater proportion of lithics, mainly glauconite, shale, sandstone, and volcanic clasts than those of the Fernie. Ellis Group sandstones also contain more fossils, mainly bivalves and oysters, whereas the Canadian samples lack any fossils. Sandstones from both regions contain abundant quartz and chert clasts with minimal feldspars.

Sandstone grain size and composition in Canada changes across an uppermost Fernie unconformity. Sandstones below this have a grain size of lower to upper fine sand, and chert and hematite are uncommon. Above the unconformity, grain size is lower to upper medium with abundant chert and hematite.

The abundance of quartz and chert in both regions supports a recycled orogen provenance. The increased amounts of lithics in the Ellis Group could reflect a larger input from the fold and thrust belt to the west. The change in provenance in Canada across the unconformity in the uppermost Fernie may indicate the initiation of the western sediment source in the latest Jurassic.