ABSTRACT

Quartzose sandstones which overlie Lower Jurassic strata in the Medicine River area can be divided into three different units, each separated by an unconformity. The two lower units (J2 and J3) fill a deep valley cut into older strata; the upper unit (Ellerslie) covers the area.

Uniform, porous, fine, supermature quartzarenite up to 50 m thick comprises the J2, the oldest of the three quartzose sandstones units. The high degree of mineralogic and textural maturity and associated small-scale cross stratification suggest that the sand was transported to a local watershed by eolian processes, then concentrated in the valley by fluvial action.

Sediments of the J3 unit are immature and submature, kaolinitic chertarenites and sublitharenites derived by humid weathering of Paleozoic carbonates. The unit as a whole coarsens upward, indicating valley-fill by progradation into a standing body of water.

Ellerslie sediments are principally mature and supermature quartzarenites and interbedded mudstones whose constituents were derived from the Shield to the east. Extensive bioturbation of sands, the presence of limestone beds and, toward the top of the formation, glauconite pellets, suggest that deposition took place in a brackish-to-marine standing body of water.

The three quartzose sandstone units are not unique to the Medicine River area and can be compared with similar units elsewhere in western Canada. J2 and J3 sandstones are the respective lithologic counterparts of the Roseray and Success Formations of southwestern Saskatchewan. Ellerslie sediments are the marine equivalents of widespread continental lower Mannville deposits in eastern Alberta and Saskatchewan.