ABSTRACT

A complete core of the Upper Cretaceous Bearpaw Formation at Castor and surface exposures on the Bow and Red Deer Rivers in south-central Alberta have yielded a microfauna and microflora consisting of foraminifera, ostracodes, echinoid debris, diatoms, megaspores and seeds. Illustrations and descriptions or pertinent remarks are given for 25 species of foraminifera and 5 types of diatoms, none of which is proposed as new.

The 470-foot thick Bearpaw Formation at Castor reveals an alternating succession of five shale and sand members which have been informally designated in ascending order as the lower shale unit, second Castor sandstone, middle shale unit, first Castor sandstone and upper shale unit. Although no truly distinctive foraminiferal zonation is apparent in this section, an Eoeponidella strombodes assemblage has been designated for the lower and middle shale units and a Cassidella tegulata-Marginulina cf. dorsata assemblage for the uppermost beds of the middle shale unit and the upper shale unit. Some of the diatoms seem restricted to the lower part of the formation both at Castor and on the Bow River.

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The foraminiferal fauna seems correlative with North and Caldwell's Anomalinoides sp. fauna from the Snakebite Member of the Bearpaw Formation in the South Saskatchewan River Valley, equated with the Baculites cuneatus - B. jenseni zonal interval of the late Upper Campanian. The microfauna includes a large proportion of species present in the Lower Campaian Lea Park Formation and marine tongues of the Belly River Formation, implying that much of the fauna was reintroduced into the area with the Bearpaw transgression after having been forced to migrate eastward during the deposition of the nonmarine phases of the Belly River Formation. The composition of the Bearpaw foraminiferal fauna shows a closer relationship to that of the Sentinel Hill Member of the Schrader Bluff Formation of northern Alaska than to contemporaneous faunas in either the Gulf or Pacific Coast regions of the United States.

The foraminiferal faunas are generally characterized by agglutinated forms indicating a marginal marine environment for these beds, which accumulated near the northwest margin of the Bearpaw sea -- the sea that caused the last widespread flooding of the western plains region in Late Cretaceous time. Within the succession, five episodes of deepening water are inferred from intervals in which the calcareous foraminiferal element dominates the agglutinated. The earliest of these is recognizable in the basal Bearpaw of both the Castor and Bow River sections, whereas the final episode is expressed in the upper shale unit at Castor, the upper Bearpaw above the marine sandstone on the Bow River, and at the same stratigraphic level above the Dorothy bentonite on the Red Deer River.