The Cypress Hills Formation (Upper Eocene to Miocene): a semi-arid braidplain deposit resulting from intrusive uplift

Abstract

The Upper Eocene – Miocene Cypress Hills Formation of the Cypress Hills plateau, in southwestern Saskatchewan and southeastern Alberta, is interpreted as a braidplain deposit. The regional paleoslope dipped to the northeast, and the east–west distribution of outcrop exposes facies representing lateral variation across the slope of the braidplain. Overall, the average clast size of the deposits decreases from west to east, with western area sediments dominated by boulder-sized gravels deposited on longitudinal bars. The eastern outcrop area contains deposits of braided channels cut into and interbedded with finer interchannel material including lacustrine marlstones, silcretes, and debris-flow deposits, the latter commonly containing abundant fossils.The gravels of the Cypress Hills Formation are multicyclic; they were originally derived from the western ranges of the Rocky Mountains during Laramide orogenesis and later shed farther into the basin during rebound due to unloading of the Laramide thrusts by erosion. Most recent transport resulted from uplift by intrusive activity of the Sweetgrass Hills, the Bearpaw Mountains, and the Highwood Mountains in northern Montana. Transport from the uplifted source areas was largely restricted to valley-confined rivers with the braidplains beginning beyond the valley termini. The lateral extent of the gravel braidplain was limited by the position of valleys and resulted in the observed variation in facies. Climate, as indicated by the sedimentology, faunal assemblages, silcretes, and palynology, was semi-arid with seasonal rainfall.