The Backpit seam, Sydney Mines Formation, Nova Scotia: A record of peat accumulation and drowning in a Westphalian coastal mire
Abstract Extensive Westphalian D to Stephanian coal seams divide the Sydney Mines Formation of Cape Breton Island into large-scale repetitive sedimentary packages. The Backpit seam, one of the most continuous seams within this formation, was studied in detail to establish compositional trends and relate these to paleomire development within an overall transgressive setting. The seam is of high-volatile B to A bituminous rank and ranges in thickness from 0.6 to 1.5 m onshore. Agglutinated foraminifera occur in strata directly below and above the seam, indicating a coastal setting for the mire. Ash and sulphur contents average 15.3 ± 6 and 5.2 ± 2%, respectively. Sulphur, predominantly in the form of pyrite, increases near the roof of the seam, consistent with a brackish influence in the roof strata. The planar Backpit mire was subjected to widespread, periodic flooding, marked by thin dull to coaly shale lithotype intervals. Some intervals can be correlated across the onshore portion of the basin for more than 45 km and this distribution suggests regional controls on their formation. Seam lithology changes frequently in vertical section and banded lithotypes predominate. Vitrinite macerals and vitrinite-rich microlithotypes are abundant and thin discrete fusain bands, the remains of ancient fires, are also common. Coal facies patterns record a series of wetting upward pulses in the upper portion of the seam that culminated in the drowning and termination of the mire. A broad, relatively shallow embayment subsequently formed that supported a fresh- to brackish water fauna.