Late Carboniferous cyclothems and alluvial paleovalleys in the Sydney Basin, Nova Scotia

Abstract

Cyclothems are documented for the first time in alluvial to coastal plain strata (West-phalian D to Stephanian) of the Sydney Basin, Nova Scotia. Well-developed cyclothems commence with an extensive carbonaceous lime-stone or shale with a restricted marine biota that indicates maximal flooding. Succeeding gray detrital bay and lake fills with distributary channel deposits are overlain by stacked channel deposits (paleovalley fills) that mark an abrupt basinward facies shift and a profound change in alluvial style. Channel fills and red vertisols were deposited on an alluvial plain above the filled valleys, and the paleovalley is equated with thick vertisols in adjacent, inter-fluve areas. The cyclothems are capped by gray coastal deposits with economic coals. Agglutinated foraminifera are present in seat earths beneath the coals, which formed as a result of a brief stillstand during transgression. Red mudstones pass laterally into gray detrital bay fills in regions of greater subsidence associated with faulted basement blocks. Poorly developed cyclothems are bounded by extensive but split coals, and autogenic effects partially obscure allogenic patterns. Mean cyclothem duration approximates 200 ka.