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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 48 (1964)

Issue: 4. (April)

First Page: 538

Last Page: 539

Title: Aspects of Reef Developments as Illustrated by the Devonian Ancient Wall Reef Complex, Alberta: ABSTRACT

Author(s): E. W. Mountjoy

Article Type: Meeting abstract


The Ancient Wall reef complex occurs in thrust sheets of the western Front Ranges of the Alberta Rocky Mountains north of Jasper. The reef complex is exposed

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in two thrust sheets for a distance of 26 miles. The reef complex consists of a lower dolomitic stromatoporoidal limestone (Cairn formation) overlain by well bedded aphanitic limestone (Southesk formation) which contain abundant non-skeletal grains locally mixed with fossil fragments. This reef is comparable to adjacent reefs in the mountains and to some of the subsurface reefs.

The depositional history of the reef has been reconstructed from the stratigraphy, reef geometry, and a detailed study of the well exposed south margin. Thin argillaceous and stromatoporoidal carbonates were deposited over the entire area (Flume formation). Thicker and more fossiliferous portions of the Flume occur beneath the margin of the overlying Cairn reef and appear to represent shoals on and around which organic growth flourished. Differential subsidence combined with a rising sea level drowned most of the stromatoporoidal platform. In the relatively shallow waters above the more positive portions of the basin, stromatoporoidal growth continued. Between 400 and 500 feet of thick, massive stromatoporoidal carbonates and detritus accumulated above these more positive areas, formi g the Cairn biostromes of the Ancient Wall and adjacent reefs. The surrounding basin was partly filled with black, predominantly euxinic shales (Perdrix formation). A lowering of sea-level throughout the basin caused shallowing above the biostromal area. Thus the environment was changed to one of very shallow-water bank conditions in which fine carbonate sands and muds with a pelletoid texture (Southesk formation) were deposited.

Gradual subsidence permitted the accumulation of between 500 and 600 feet of carbonate sands. The Southesk bank is flanked by brachiopod limestones and shales which contain local, small, coral biostromes. These fossiliferous carbonates grade into thin-bedded calcareous shales and limestones (Mount Hawk formation) of the surrounding basin. Basin relief was gradually decreased by influx of terrigenous muds and fine carbonate muds and detritus derived from the bank. Further shallowing and basin-filling permitted lateral extension of the bank environment and associated carbonate sands. During the Sassenach transgression the bank was emergent and the margins were eroded slightly.

The abrupt variations in thickness and character at the reef margins indicate differential subsidence during and after accumulation of the reef complex. Thus both differential subsidence and changes in sea-level have exerted control on reef development.

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