Bulletin 027

Bulletin 027

BUL 027

Lithostratigraphy of the Uppermost Cretaceous (Lance) and Paleocene Strata of the Alberta Plains

Author(s) Carrigy, M.A. Date 1971-01-01

In Alberta relatively undisturbed non-marine Cretaceous and Paleocene strata are preserved as a westerly dipping homoclinical wedge of sediment east of the Rocky Mountain Foothills, analogous to the molasse facies bordering the European Alps. This report describes the lithology of the uppermost Cretaceous and Paleocene beds overlying a thick widespread volcanic ashfall (Kneehills Member of the Edmonton Formation) which was deposited 65 million years ago. The post- neehills strata of the Alberta Plains are of considerable interest, for during their deposition thick beds of coal were formed, the Rocky Mountains began to emerge and dinosaurs became extinct.

The nomenclature of the post-Kneehills strata is revised by placing he lower boundary of the Paskapoo Formation at the top of the Kneehills Member of the Edmonton Formation in central Alberta and by lowering the base of the Ravenscrag Formation to the top of the Battle Formation in southeastern Alberta The redefined Paskapoo and Ravenscrag Formations thus become equivalent to the Willow Creek Formation of southwestern Alberta and now include the Lance equivalent beds and the Cretaceous-Tertiary transition. Evidence ispresented to show that the Porcupine Hills Formation is much more extensive than formerly mapped and that it overlies the Paskapoo Formation in the vicinity of the Bow River. This discovery dates the hrust-folded foothills structures as an intra-Paleocene event.

Analysis of the cross stratification in the post-Kneehills strata indicates that a significant change in paleoslope took place between the deposition of the Paskapoo and Porcupine Hills Formations. A shift in the source area from a dominantly volcanic terrain in the interior of British Columbia in late Cretaceous and early Paleocene time to a dominantly sedimentary terrain in northern Montana during late Paleocene time is postulated to account for the change in detrital composition of the sandstones with time.

Most of Upper Cretaceous and Paleocene sandstones are subgreywackes r greywackes. Sandstones of the Paskapoo and Ravenscrag Formations are composed of detrital quartz, chert, volcanic and non-volcanic rock fragments, and minor amounts of clastic carbonates. The heavy mineral assemblage is dominated by first-cycle grains of euhedral iotite, zircon, apatite, epidote and hornblende. Most common among a rich and varied series of intergranular authigenic minerals are clay inerals (montmorillonite, chlorite, kaolinite), zeolites (clinoptilolite) and carbonates (calcite). However, the bulk chemical composition of these sandstones is remarkably uniform, the most notable feature being the high alumina content and the predominance of soda over potash.

The Porcupine Hills Formation sandstones are composed of detrital quartz, chert, nonvolcanic rock fragments and clastic carbonates. The heavy mineral assemblage is a residual suite of small abraded grains of zircon, tourmaline and apatite. In contrast to the Paskapoo and Ravenscrag Formations, only three common intergranular mineral cements are present; quartz kaolinite and calcite. The bulk chemical composition of the Porcupine Hills Formation sandstone is haracterized by a low alumina and high lime contents, and the predominance of potash over soda.

Carrigy, M.A. (1971): Lithostratigraphy of the uppermost Cretaceous (Lance) and Paleocene strata of the Alberta Plains; Research Council of Alberta, RCA/AGS Bulletin 27, 175 p.