Sedimentary and structural features of the Bell shale correlate (Early Devonian), Strahan quadrangle, western Tasmania

Abstract

The sequence of interbedded mudstone, siltstone and very fine sandstone in the Strahan Quadrangle, correlated with the Bell Shale is at least 500 m thick. Two broad associations can be defined. The ratio of very fine sand to mud in the lower association is between 3:2 and 2:3 whereas in the upper association only occasional thin sandstone beds occur. Sedimentary structures such as lenticular bedding, symmetrical ripple marks and the lack of deep water sedimentary features suggest that deposition took place in shallow water. This is supported by palaeontological evidence. Some scouring and upward fining beds suggest that deposition from waning currents occurred sporadically within this environment. The Bell Shale correlate has been folded during two phases. The first phase produced folds plunging between 300 and 900 NW and the second phase produced folds plunging shallowly WNW or ESE. Cleavage was developed only locally during each phase. Folds of both phases produced a primary axial surface cleavage and the second phase produced crenulation cleavage in some areas. The amplitude of folds produced during the first phase decreases towards the centre of the basin. The locus of strain during the second phase of deformation was along the Firewood Siding Fault. The existence and timing of these deformation events demonstrates the uniformity in orientation of Tabberabberan structures throughout north and western Tasmania.