Abstract

The Calcareous Member of the Blairmore Group in Alberta is a key stratigraphic unit within the Lower Cretaceous strata of the Western Canada Sedimentary Basin because of its regional extent and the unique limestone-bearing lithology. In this study, the type section of the Member (the combined Mill Creek-Gladstone Creek section) has been remeasured and described in greater detail using a sequence stratigraphic framework supported by palynostratigraphic data. Two more measured outcrop sections, one of which can be used as a reference section (on Hwy. 541) have also been described and correlated with the type section.

In the type section, the Calcareous Member has 16 individual beds which can be grouped into seven coarsening-upward parasequences. A typical parasequence, two to five metres thick, consists of a basal black shale with millimetre-thick graded beds of shell hash (distal tempestites) sanding upward to a very fine-grained, burrowed calcarenite with wave-ripple or combined flow-ripple lamination (proximal tempestites). A good recovery of palynomorphs including stenohaline dinoflagellates from the section resulted in the subdivision of the Calcareous Member into four palynostratigraphic units, some of their boundaries coinciding with those of the parasequences.

The stacking pattern of parasequences in the type section has been interpreted in terms of a sequence stratigraphic model. The four lower parasequences with a progradational stacking pattern, and the upper three with a retrogradational pattern, constitute the Transgressive Systems Tract (TST) of a second order depositional sequence (Blairmore Group). A transgressive surface marked by a major palynological break belonging to a higher order sequence separates the two. The top of the Calcareous Member is defined in one example by a second order maximum flooding surface, and in the other by a sequence boundary.