SANDSTONE PETROGRAPHY OF THE PENNSYLVANIAN CUMBERLAND GROUP, CUMBERLAND BASIN, NOVA SCOTIA

Coal-bearing strata of the Pennsylvanian Cumberland Group make up the upper 4300 meters of the Joggins Fossil Cliffs UNESCO World Heritage Site. These strata include (from oldest to youngest): the Boss Point, Little River, Joggins, Springhill Mines, and Ragged Reef Formations. Thin sections from 27 fluvial sandstones were described and point counted to provide new information about the paleoenvironmental conditions, composition, provenance, and reservoir potential of these units.

Sandstones in the lower units are generally fine to medium grained; intraformational lags are present at the base of some channel bodies. Channel bodies within the Little River and Ragged Reef Formations are medium to course grained; pebbly sandstones and conglomerates are common in the Ragged Reef Formation.

Channel body sandstones in the Cumberland Group are lithic arenites with variable rounding and sorting. Sandstones within the type section of the Ragged Reef, Springhill Mines, Joggins, and Boss Point Formations have 79% quartz, 19% lithic, and 2% feldspar grains (average, QFL normalized values). Sandstones in the Little River Formation and the Spicer’s Cove member of the Ragged Reef Formation have 69% quartz, 30% lithic, and 1% feldspar grains (average, QFL normalized values). Lithic grains are dominantly metamorphic (phyllite and schist), with a subordinate amount of igneous and sedimentary (chert) fragments. Accessory minerals include micas (muscovite), pyroxenes, and sphene. Calcite cement is common in the Springhill Mines, Joggins, and Little River Formations; silica cement is common in the Ragged Reef and Boss Point Formations. Pore space increases from 5% in the older units to 15% in the younger, less deeply buried, units.

The abundance of lithic fragments and the paucity of clayey matrix indicate that much of the sediment was derived locally and deposited in energetic streams. Feldspar is largely absent, suggesting tropical weathering and/or largely metamorphic source areas. The coarse grain size and abundance of lithic fragments in the Little River Formation corresponds to a westerly paleoflow anomaly, which may record tectonism or a reorganization of the basin drainages. A similar lithic anomaly is present in the Spicer’s Cove member of the Ragged Reef Formation, probably because of its proximity to the basin margin.