Correlated Biostratigraphy and Palaeoecology of Microplankton from the Bearpaw Formation (Campanian-Maastrichtian) of Alberta, Canada

by Dalzell, Matthew Thomas

Marine palynomorph assemblages from the Campanian-Maastrichtian Bearpaw Formation, consisting of 34 genera and 69 species of dinoflagellate cysts and 8 forms of acritarchs, were recovered from a composite section in the Cypress Hills and the Research Council of Alberta Castor borehole in southern Alberta, Canada. Correlation of the sections with magnetostratigraphy and radiometric dates of bentonites within the Bearpaw Formation provided a chronostratigraphic control that enabled correlation between the two sections and the Campanian-Maastrichtian Boundary Global Stratotype Section and Point at Tercis les Bains, France. Correlation of first and last occurrence data of key taxa in the boundary section at Tercis with the same taxa in the studied sections support the placement of the Campanian-Maastrichtian Boundary at the transition between magnetochrons 32n.1n and 31r within the Baculites baculus ammonoid range zone of the Western Interior of North America.

Semi-quantitative analyses of the assemblages record the transgressive-regressive episodes of the Bearpaw cyclothem. Increases in the relative abundances of gonyaulacacean cysts, numbers of dinocysts compared to terrestrial palynomorphs and assemblage diversity correlate with transgressive episodes, with the richest assemblages occurring during periods of open, offshore to neritic conditions correlated with the cyclothems transgressive peak.

Two new species of dinoflagellate cyst are described. The first, Dinoflagellate sp. 1, is a novel taxon, while the second, Downiesphaeridium sp. A, is a chorate cyst similar to Mesozoic forms previously ascribed to Cleistosphaerdium diversispinosum Davey et al. emend. Eaton et al.