Stratigraphic Analysis and Possible Tidal Influence in the Thorburn Member and Coal Brook Member, Stellarton Formation, Nova Scotia.
MetadataShow full item record
The Late Carboniferous Stellarton Basin is located in central Nova Scotia. It is bounded by the Cobequid Fault to the north and the Hollow Fault to the south. Dextral movement on these faults during the Carboniferous formed the 6 km by 18 km pull-apart Stellarton Basin. The basin-fill is composed of approximately 2600 m of coal- and oil shale-bearing strata of the Stellarton Formation. The intervals studied are within the Thorburn and Coal Brook Members of the Stellarton Formation, respectively. They provide a range of lithofacies from proximal to distal with respect to the sediment source. The fissile and torbanite oil shales were deposited under quiet, anoxic, sediment starved conditions. Massive and laminated grey mudstones formed in a sediment deprived environment with limited current action indicated by small scale cross-stratification. An increase in biological activity including burrowing implies a move to oxygenated bottom waters. A lithofacies consisting of interlayered sandstone and mudstone shows an increased proximal component with unsteady current flow exhibited by sandy cross-beds overlain by mudstone drapes. Weakly stratified sandstone were deposited during periods of high sediment load and energetic water movement. A coal facies as well as a paleosol facies is encountered locally in the Thorburn interval. These indicate sub-aerial or near-sub-aerial exposure and form the proximal end-member of the facies range. Horizontal and vertical lithofacies variations are found in the two studied intervals. The Thorburn interval shows an overall coarsening-upward grain size trend in cycles approximately 10 to 20 m thick, from oil shale to medium-grained sandstone. The coarsening-upward cycle is overlain by a minor fining-upward cycle that includes paleosols. The Coal Brook interval ranges from 11 to 20 m thick and shows an overall coarsening-upward grain-size trend. The interval also reveals lateral thickening of units and an increase in coarse-grained units towards the inferred sediment source in the southeastern portion of the study area. The overall coarsening-upward trend in the intervals implies deposition in standing bodies of water with prograding coarse-grained deposits. Paired mudstone drapes, considered characteristic of tidal influence, were identified in the interlayered sandstone and mudstone of the Thorburn interval. The mudstone drapes cap sandy climbing and truncated cross-sets of approximately 3 cm thickness. Although not all mudstone drapes in the facies are paired, individual sets show as much as 50% pairing. Mud drape deposition is inferred to occur during the slackwater period of tidal reversal. The pairing is associated with thickness differences in the sandstone cross-sets implying differences in current strength and direction between a dominant and a subordinate tide. The low-sulphur coals and lack of strictly marine fossils indicate that the tidal influence was not accompanied by increased salinity. Supporting marine influence has been documented in other Upper Carboniferous areas of Atlantic Canada including Sydney, Joggins, and Port Hood. This is the first documentation of marine influence in the Upper Carboniferous strata of the Stellarton Basin. Pages: 94 Supervisor: Martin Gibling