ABSTRACT

Eleven successive microfaunal assemblages are present in the Alberta Group (Cretaceous) of the Rocky Mountain Foothills. In ascending order these are: (1) Miliammina manitobensis-Verneuilina canadensis, (2) Verneuilinoides perplexus gleddiei, (3) lower pelagic, (4) Pseudoclavulina-Gaudryina-Haplophragmoides, (5) Trochammina, (6) Anomalina, (7) Brachycythere-Bullopora, (8) Planulina, (9) upper pelagic, (10) Trochammina ribstonensis, and (11) Robulus assemblages. All of these assemblages are useful in local identification of stratigraphic level, and many are of significance in regional to interregional correlation.

Concordant relationships of the microfaunal assemblages with the megafaunal zones and rock-stratigraphic units are shown for the Blackstone Formation. Stratigraphic discrepancies between the occurrences of microfaunas and megafaunas in the Wapiabi Formation seem largely the result of ecological conditions, with the distribution of the megafossils being more subject to environmental control. Selection of the base of the upper white-speckled shale "zone" and its associated pelagic foraminiferal fauna as a datum plane for correlation purposes shows the Thistle Member of the Wapiabi Formation to be markedly diachronic, becoming younger in a northerly direction.

The composition of the microfaunas indicates a shallow, probably turbid, nearshore environment of deposition for most of the Alberta Group. The microfaunal sequence provides supporting evidence for the theory that the base of the Blackstone is time transgressive, becoming progressively younger in a southerly direction. Microfaunal evidence indicates that the final recession of the Late Cretaceous sea began earlier in the southern Foothills than in the northern areas. The flooding in the central and northern Foothills represented by the Robulus fauna in the Nomad Member of the Wapiabi seems contemporaneous with that represented by a similar fauna in the upper part of the Lea Park Formation and lower marine tongues of the Belly River Formation in eastern Alberta.