Stratigraphy and Paleogeography of Mabou Group and Related Middle Carboniferous Facies, Nova Scotia, Canada

Abstract

Marine and nonmarine facies in Nova Scotia, Canada, of Visean through Westphalian A (middle Carboniferous) age are closely related to a tectonic framework that consists of a narrow region of highly mobile blocks surrounded by a broad region of stable blocks. Defining and mapping new lithogenetic units and a more precise method of time control (fossil microspores) supports this hypothesis. Middle Carboniferous units of previous workers, the Windsor, Canso, and Riversdale “groups,” are time-stratigraphic and hence inadequate for defining facies. New lithogenetic units of group rank were defined in a previous report (Belt, 1964) to include strata represented by those time-stratigraphic units. The new units are termed the Windsor Group (restricted Windsor “group”), the Mabou Group (previously a formation), and the Coarse Fluvial Facies (an informal term). Existing formations have, wherever possible, been regrouped into these new units. A new formation (Emery Brook Formation) and a tongue (Glengarry tongue of the Pomquet Formation) are herein proposed. Windsor, Canso, and Riversdale time-stratigraphic units are redefined as Windsorian, Cansoan and Riversdalian Stages. The term Mabou Group is applied to fine-grained “red” and “gray” fluvial and lacustrine strata that succeed the widespread marine and evaporite deposits of the Windsor Group. The Mabou facies represent basin-fill sediments that underlie and also are laterally equivalent to coarse marginal deposits of the Coarse Fluvial Facies. The major zone of mobility during middle Carboniferous time, the Fundy Basin, was irregular in shape and not entirely a region of subsidence. The Caledonia and Cobequid arches, narrow upthrown blocks within the basin, supplied much of the sediment. The New Brunswick and Meguma platforms, bordering the Fundy Basin on the northwest and south, respectively, do not represent a major source of sediment, although much sediment moved across them from outside the Maritime region. The platforms are distinguished from the basins by a thin veneer of relatively undeformed strata; they alternated between gentle uplift and gentle subsidence. In contrast, the subsiding portions of the Fundy Basin received up to 20,000 feet of middle Carboniferous sediment that was subsequently highly folded and faulted and locally mildly metamorphosed. The margin of the Fundy Basin is interpreted as a region of high-angle faults because the thickness of Carboniferous sediment decreases abruptly from the basin onto the platform, Mabou facies grade laterally from “gray” (lacustrine) to “red” (fluvial) facies toward the margin of the basin, and coarse alluvial fans were deposited on the basin side of long, sinuous faults. The margin is believed to parallel the area of long faults that today separate the area of pre-Carboniferous platform, with its thin, slightly deformed Carboniferous cover, from the area of the basin, with its thick, highly deformed Carboniferous strata.