Beacon Group Sediments

Beacon sediments are restricted to the western half of the valley with the most easterly exposures occurring on peaks to the north of Lake Vida. Sequences of Beacon sandstone show a marked thickening toward the west, for while the most easterly outcrop was only 50 feet of basal sediments, a virtually continuous sequence of 3500 feet was examined west of the Webb Glacier. In one eastern section, onlap of near horizontal sediments onto a planed basement surface dipping at 12° west shows the area of sedimentation at that page 8 time was gradually extending towards the east.

Small variations in a generally uniform quartz sandstone mark off broad divisions of the sedimentary column. The lowest bed is a conglomerate layer which passes upwards into a cross-bedded quatrz sandstone with numerous worm tracks and abundant pyrites as scattered crystals or concretions. Overlying this are 500 feet of pure quartz sandstones with occasional beds showing mud-cracks and ripple-marks. Several coarse conglomerate layers up to 6 feet thick follow, marking the base of a 2500 feet thick coal-measure sequence of cross-bedded sandstones, felspathic grits and green-coloured siltstones. Numerous thin carbonaceous layers occur at intervals throughout this sequence, and occasional impure coal seams reach a maximum thickness of 3 feet.

Samples of fossil leaf and stem impressions from this sequence have not yet been studied, but they might give an age to the sediments which are at present loosely dated as ranging from Devonian to Jurassic. Shale and carbonaceous siltstones were sampled for pollen-analysis also. No results are yet available.