Sea level control of carbonate-shale deposition during progradational basin-filling: the upper Devonian Duvernay and Ireton Formations of Alberta, Canada
Creators: name: Stoakes, Franklin Arthur
Publisher: name: University of Calgary
Year: 1979
Subjects: subject: QE 471.15 C3 S86 1979 Fiche
subject: Rocks, Carbonate
subject: Shale - Alberta
subject: Sedimentation and deposition - Alberta - Central region
subject: Geology, Stratigraphic - Devonian

description: Bibliography: p. 262-279.
description: Within basin-filling shales of the Frasnian Duvernay and Ireton Formations a number of inclined electric log markers indicate the presence of significant submarine topography durtng deposition. Log marker surfaces comprise a submarine hardground on the platform and upper foreslope, and may be traced into a thin carbonate-rich layer on the middle foreslope. Calciturbidites containing hardground-derived lithoclasts may directly overlie this layer, establishing the temporal equivalence of these two lithologies. These markers form in response to a relaxation in terrigenous sedimentation against a background of continuous carbonate deposition and may be considered essentially synchronous surfaces. The presence of time-synchronous stratigraphic markers allows precise depth estimates of lithofacies and biofacies ranges to be made. The broad topography of the marker surface defines the three major depositional settings of platform, slope and basin. Cored intervals may be related to their respective log marker and, based on the topographic position they occupy, be assigned a minimum quantitative depositional water depth. Five bathymetrically controlled lithofacies associations have been distinguished: (la) Open marine shale platform 0-35 m (lb) Open marine carbonate platform 0-25 m (2) Upper foreslope nodular limestones 30-55 m (31 Middle foreslope bioturbated shales 55-90 m (4) Toe of slope interbedded micritic limestone and calcareous shale 90-100 m (5) Dark bituminous limestone and shale&gt; 100 m. Biofacies distribution reflects a tripartite layering of the water column probably reflecting levels of oxygenation: An aerobic zone with abundant calcareous fauna 0-45 m; a dysaerobic zone with abundant infauna 45-100 m; and anaerobic zone lacking any indigenous fauna below 100 m. Exact values vary as the basin develops and circulation patterns improve. Average values however differ significantly from those postulated in other studies of shale basins. Depositional slopes average 4-7 m/km (0.23 - 0.4°]. Log markers may be used to divide the Duvernay and Ireton Formations into a total of seven infonnal chronostratigraphic units termed depounits (DUI - CU7}. Depounits are broadly sigmoidal in cross-section and imbricated basinwards, their platform portions forming a stacked sequence of upward shoaling units in the east of the basin. They indicate that deposition tool place during a number of episodic rises in sea level. Growth of surrounding Leduc reefs took place at the same time as shale 5asin - filling. A reciprocal pattern of sedimentation involving repeated sea level rise and still-stand characterizes and explains depositional patterns in this stratigraphic interval. During"high stands" carbonate deposition takes place, on reefs and in suitable water depths on the platform, in a predominantly vertical sense. During stillstands terrigenous sedimentation gradually becomes reesta5lished continuing basin-filling in a lateral sense. Significant carbonate platform deposition first took place with deposition of the Camrose Member which exhibits a shoaling upward sequence from a basal hardground and thins basinwards into shales of the upper foreslope. It is overlain by a thin shale break which thickens basinwards. This carbonate-shale couplet is the fullest lithologic expression of the reciprocal pattern of sedi mentation and may be used to explain depositional patterns in the laterally equivalent Grosmont Formation to the north and overlying Winter5urn Group to the west. Sedimentation patterns in the Western Canadian Frasnian cycle of onlap and offlap may be explained in terms of a reciprocal model of sedimentation involving episodic sea level rise and stillstand. The recognition of widespread submarine hardground surfaces provides the key to understanding the stratigraphic relations within, and evolution of, the East Shale Basin. Recognition of these surfaces in the Upper Devonian of Western Canada should allow a better understanding of stratigraphic relationships within the Frasnian cycle of onlap and offlap.