Microfacies Study of Rundle Group (Mississippian) of Front Ranges, Central Alberta, Canada

Abstract

The Rundle Group (Mississippian) of Western Alberta consists of a predominantly clastic carbonate sequence exhibiting variations in texture and composition which, when analyzed by petrographic methods, reveal rhythmical changes of environments of deposition and provide means for fine correlation. More than 800 thin sections were studied from three sections in the front ranges of the Rocky Mountains of the Ram River area. The indices of clasticity and frequency were determined for crinoid fragments, pellets, oolites, and quartz grains, and curves for each of the parameters were plotted. The presence of arenaceous foraminifera, bryozoan fragments, algae, sponge spicules, and echinoid spines was recorded. The statistical data, combined with the distribution of the detrital matrix and biochemical cement, reveal five microfacies which represent different environments of deposition, ranging from a shallow lagoonal to a deep open environment. Microfacies 1 is the deepest and consists predominantly of small crinoid fragments and sponge spicules in a fine bioclastic and argillaceous matrix. Microfacies 2 represents a more shallow environment which is transitional between microfacies 1 and 3, the latter consisting of oolites and well rounded, well sorted crinoid fragments and pellets in a clear calcite cement. Microfacies 4 consists almost entirely of interlocking crinoid fragments, whereas microfacies 5 is characterized by calcilutite with scattered crinoid fragments and oolites. Locally thi microfacies may be entirely pelletoidal. The variations of the parameters of the measured components in relation to each other produce a consistent rhythmic pattern of curves when the microfacies are superposed in a succession from 1 to 5 or 5 to 1, indicating that such a succession expresses an ideal cycle, which has been interpreted laterally as a lagoonal environment (microfacies 5) grading into a zone of crinoid colonies and agitated water (microfacies 4) which in turn grades into a succession of environments of gradually deepening water in which mechanical distribution of the particles is predominant (microfacies 3 to 1). The ideal cycle was used for the analysis and construction of a relative bathymetrical curve for each section. General trend curves were drawn to stress the over-all evolution of the sequences, on the basis of which correlations were made, which afford more detail than megascopic field correlations and are in general agreement with them. A comparison of corresponding microfacies of the three sections indicates an alternation of two conditions of the sea floor: either a slope in which the eastern part was consistently more shallow than the western or uniform depth conditions.