ABSTRACT: Origin and Stratigraphic Significance of Several Anomalously Thick Sandstone Trends in the Middle Triassic Doig Formation of West-Central Alberta, Canada

WITTENBERG, JOERG, and THOMAS F. MOSLOW, University of Alberta, Edmonton, Alberta, Canada

The Middle Triassic Doig Formation of west-central Alberta is composed of several westward-prograding parasequences. In the established stratigraphic nomenclature, the Doig Formation is subdivided from the overlying and laterally correlative Halfway Formation. The Doig Formation is considered to be the deeper water depositional equivalent of westward-prograding shoreface systems established in the Halfway Formation. This subdivision is confusing, since it is based on lithofacies and interpreted depositional environments and is therefore avoided in this study. The stratigraphic model presented emphasizes the recognition of several regionally correlatable flooding surfaces in cored sequences and on well logs. The westward-thickening of the entire Middle Triassic section within the study area, and the superposition of shallow marine shoreface sandstones on middle to outer shelf sediments, suggest stable coastal depositional systems that were not influenced by large eustatic or relative sea level fluctuations.

Anomalously thick sandstone bodies (up to 60 m thick and less than 2.0 km in width) in the Doig Formation have been previously interpreted to be of a fluviatile or incised estuarine valley-fill origin. However, in this study it has been recognized that the thick sandstone bodies never truncate the marine flooding surfaces (radioactive, phosphatic shale markers) that bound each progradational parasequence. Second, the presence of marine macrofossils and ichnofossil assemblages in the thick sandstones precludes their interpretation as having a fluviatile or estuarine origin. Third, the distribution of facies within the north-south-trending thick sandstone bodies is suggestive of an east to west sediment transport direction (coincident with the direction of progradation of the parasequen es) as opposed to a north-south transport direction, as would be the scenario for incised channels. It is therefore interpreted that the anomalously thick sandstones were deposited penecontemporaneously with the shoreface sandstones, typifying the Halfway Formation, established to the east of the thick sandstone trends in each parasequence. Based on detailed sedimentological descriptions of cored sequences, the anomalously thick Doig sandstones are interpreted as being the products of shoreface sourced mass wasting events occurring in an outer shelf to shelf margin setting.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)