The Stratigraphy, Sedimentology and Petrography of the Upper Cretaceous Wapiabi and Belly River Formations in Southwestern Alberta

Abstract

<p>The Upper Cretaceous Wapiabi and Belly River Formations are thick clastic units which are well exposed in the foothills along the southeastern margin of the Canadian Cordillera. The Wapiabi Formation constitutes the uppermost unit of a thick marine sequence known as the Alberta Group. The transition to the overlying nonmarine Belly River Formation marks the onset of a long period of molasse type sedimentation in the southern Alberta Basin.</p> <p>The Thistle Member of the Wapiabi Formation is the lowermost unit studied and consists of marine shale with abundant siltstone and thin sandstone laminae which were probably introduced into the basin by turbidity currents. In the southern part of the study area, the overlying Chungo Member is characterized by a lower thick coarsening upward shoreline sandstone body which is overlain by approximately 60 meters of nonmarine coastal plain sediments. The coarsening upward sequence is dominated by turbidites and hummocky and swaley sequence cross stratification. These structures record a significant storm influence in a shallow marine setting. In the central part of the study area, nonmarine Chungo strata are absent. The nonmarine strata appear to have been replaced by thick sections of nearshore to shallow marine sandstones and mudstones which are organized into several coarsening upward sequences. Each sequence is capped by a thin transgressive conglomerate or pebbly mudstone. In the northern part of the study area, the Chungo Member is represented by a 5 meter interval of bioturbated sandstone which overlies an anomalously thick interval of bioturbated sandy mudstones of the Hanson Member. The Chungo interval is interpreted to have been deposited by a wave and storm dominated shoreline complex which prograded from south to north into the Alberta Basin.</p> <p>The Chungo regression was terminated by the Nomad transgression which shifted the Cretaceous shoreline approximately 150 km to the south. This shoreline retreat is essentially nondepositional although the base of the overlying marine sequence is commonly marked by a thin pebbly mudstone or conglomerate. The transition from the marine Nomad Member to the overlying nonmarine Belly River Formation is typically abrupt and occurs across less than 5 meters of section. In the south, this transition is marked by a sharp based trough cross stratified sandstone unit approximately 5 meters in thickness which rests directly on bioturbated mudstone and which is overlain by rooted carbonaceous nonmarine strata. These fining upwards sandstone units are interpreted as low energy tidally influenced shoreline deposits. In the northern half of the study area, the Nomad to Chungo transition is marked by a thin coarsening and thickening upward sandstone-mudstone sequence. This sequence is capped by a trough cross stratified and parallel laminated sandstone which is thought to have been deposited by the progradation of a beach type shoreline.</p> <p>Little paleocurrent data were available from this Nomad-Belly River regressive sequence but previous heavy mineral studies have indicated that the source area lay to the northwest and that major drainage systems flowed to the east and southeast.</p> <p>Petrographic data suggest that the Chungo and Belly River Formation, were derived from different source areas. During Chungo time, much of the sedimentary detritus was derived from the Elkhorn volcanic field of west central Montana. During deposition of the lower Belly River sandstones much of the sediment was derived from erosion of uplifted sedimentary, volcanic, and metamorphic rocks in the rising Cordillera to the west and northwest.</p>