STRATIGRAPHY AND DEPOSITIONAL ENVIRONMENT OF THE HORTON GROUP IN THE ST-MARYS BASIN, CENTRAL MAINLAND NOVA-SCOTIA

The St. Marys Basin, central mainland Nova Scotia, contains an Upper D evonian(?)-Lower Carboniferous intracontinental alluvial fan-fluviatil e-lacustrine basin-fill sequence that occupies the current boundary be tween the Meguma and Avalon terranes of the Canadian Appalachian oroge n. The basin rocks belong to the Horton Group, and are here divided in to six partially laterally equivalent formations. The stratigraphicall y lowest rocks are predominantly exposed in the central part of the ba sin in a series of en echelon anticlinal closures. These rocks were de posited in a longitudinal drainage system and represent elastic strata deposited in lacustrine (Little Stewiacke River Formation) and braide d fluvial (Barrens Hills and Lochiel formations) environments. Toward the southern flank of the basin the sequence is essentially a southeas t-facing monocline in which the beds coarsen and thicken upward. Along this flank, elastic rocks were predominantly deposited in alluvial fa ns (Cross Brook and West River St. Marys formations) derived from the Meguma terrane to the south. These rocks unconformably overly the Megu ma Group, reflecting a decrease in accommodation space and implying th at the St. Marys Basin is underlain, at least in part, by Meguma basem ent. In the northwestern part of the basin, elastic sedimentary rocks have mixed Avalonian-Meguma provenance (Graham Hill Formation) reflect ing influence of the Avalonian margin to the north and confirming the status of the Horton Group as a post-Acadian overstep sequence. The de position of coarse conglomerate along the southern flank of the basin suggests a strong tectonic influence on sedimentation where subsidence along this basin margin occurred along northerly dipping listric norm al faults. In contrast, the character of the sediments does not vary w ith proximity to the northern margin of the basin (Chedabucto Fault), suggesting that the fault does not constitute the original basin margi n, and that an unknown portion of the basin and its Meguma basement ha ve been tectonically removed.