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CSPG Special Publications

Abstract


Intl. Symposium of the Devonian system: Papers, Volume II, 1967
Pages 387-408
Reefs and Carbonates

Factors governing the Development of the Frasnian, Miette and Ancient Wall reef complexes (banks and biostromes), Alberta

Eric W. Mountjoy

Abstract

The stratigraphy of the Upper Devonian Miette and Ancient Wall carbonate complexes indicates that the two complexes are similar and have undergone parallel development and depositional cycles. The Ancient Wall complex differs from the Miette complex in a lack of bank margin dolomites, presence of more carbonate debris beds, size, and the presence of a thicker Sassenach Formation.

Three major depositional phases are recognized which are primarily related to sea level fluctuations: an initial transgression (Cairn), an intermediate period of stability (lower and middle Southesk), and a regression (uppermost Southesk). A number of cycles are clearly evident within the major phases and reflect varying depths of water and rates of basin sedimentation. The Cairn Formation consists of stromatoporoidal biostromes and associated calcarenites whereas the Southesk Formation represents development of a non-skeletal carbonate sand bank. Wave resistant structures (bioherms?) locally formed during the deposition of uppermost Cairn and lowermost Southesk on the flanks of the biostromes and banks. Three main environments are recognized from bank to basin: banks or biostromes; transitional sequences, including carbonate debris beds, which flank the banks; and the surrounding basin. Carbonate debris beds together with other strata provide useful marker beds for correlating bank margins with basin shales.

The principal factors which controlled development of these biostromes and banks are:

1) fluctuating sea levels, resulting in part from epeirogenic crustal movements, local subsidence, and eustatic sea level changes.

2) the tendency of the Western Alberta arch to remain positive during the Late Devonian, and differential subsidence across this arch possibly related to local tectonic movements or warpings along zones of weakness.

3) shoals, resulting from thicker carbonate accumulations in the upper Flume which formed the substrate for the carbonate. complexes.

The fluctuations of sea level outlined for these two widely spaced carbonate complexes can also be recognized in much of the adjacent Devonian basin and influenced the growth and development of other biostromes and banks.


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