| Author||Jasper Dooge|
|Title||The stratigraphy of an Upper Devonian carbonate-shale transition between the North and South Ram rivers of the Canadian Rocky Mountains|
|Journal||Leidse Geologische Mededelingen|
|Abstract||The Upper Devonian Fairholme Group in the Canadian Foothills and Rocky Mountains is correlated to the reef-bearing subsurface Woodbend-Beaverhill Lake Formations of the Alberta Plains. The group is divisible into the upper Southesk and the basal Cairn Formation, each of which occurs in a carbonate and a clastic facies, commonly referred to as the reef and shale provinces.|
This thesis reports the results of a detailed study of the transition between the carbonate and shale provinces, which is well exposed in the Cripple Creek area of the Front Range of the southern Rocky Mountains. This transition only occurs in the Southesk Formation at this locality and takes place gradually over a distance of eight miles, reflecting depositional patterns in successive positions on a shallowing shelf.
Biostratigraphic zonation allows the delineation of areally segregated carbonate facies, characterized by distinctive lithological and faunal criteria.
A concentration of coral biostromes occurs at the transition between the shallow shelf and deeper water marginal seas. This zone of biostromal dolomite grades seaward through fossiliferous, nodular and argillaceous limestones to calcareous, open marine shales. The transition from shelf to sea areas coincides with transition from complete to incomplete dolomitization. Complete dolomitization is indicated to occur when no argillaceous admixture is present. The zone of biostromes grades shoreward into intertidal and algal deposits towards the central platform area. A progressive increase in water salinity is indicated by a marked decrease in faunal content accompanied by a differentiation in the faunal composition from predominantly coralline to an algal-ostracod-calcisphere assemblage. The dolomite sequences are assigned to the semi-restricted and restricted dolomite facies.
Regional studies indicate the presence of other types of carbonate-shale transitions in the Fairholme of the Rockies. The abrupt juxtaposition of dolomites and shales at Wapiabi Creek is in sharp contrast to the gradational nature of the Cripple Creek transition. The occurrence of detrital carbonates, swept off the platform, indicates the presence of relief between the carbonate and shale provinces. At Cripple Creek, a buffer zone of intervening carbonate types indicates the absence of any appreciable amount of relief. The Cripple and Wapiabi types constitute the end members of a range of carbonate-shale transitions. The areal distribution of these transition types are explained by variation in terms of paleogeographic setting during Southesk time. The gradational Cripple Creek type transition is postulated to occur in areas on the leeward side of the Fairholme shelves, protected from the influences of direct wind and current action. The Wapiabi type occurs in windward areas. This postulation infers the predominance of northeasterly winds during Southesk time.
The study of the spatial arrangement of the Upper Devonian carbonate facies in the Cripple Creek area has resulted in the definition of a depositional carbonate pattern, designated as the ”carbonate model”. This model constitutes a synthesis of all pertinent data, necessary to represent the areal configuration, composition and environmental milieu of the major facies, into a simple and orderly form.
The model can be applied profitably on a world-wide basis to illustrate the facies distribution of carbonate-shale transicions in other geologic systems with only minor changes in the composition of the faunal assemblages.
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