Sedimentology of the Marinoan Type Section, Marino Rocks to Hallett Cove area, South Australia

Abstract

Sections measured in detail through the older part of the Late Precambrian Marinoan sequence in the Type Area of Marino and Hallett Cove, south of Adelaide, confirmed a mainly clastic shelfal sequence, which shows the influence of strongly fluctuating sea level. The sequence is dominated by silt deposition, with only occasional, often thin, sandstone intervals. In the relatively small area studied the palaeoshoreline can be interpreted as running north-north-west and south-south-east. Tidal currents at a high angle to the shoreline influenced sedimentation. Alongshore currents and wave action also have played a role in deposition. Storm wave action may have reworked some of the units. The lower part of the Marinoan Type Section represents a transgressive sequence from the supratidal mudflat environment of Unit 1 to the below storm wave base silts and shales of Unit 3. The sequence is then dominated by deep water silts and shales, with minor shallowing episodes recorded in the sandstones of Units 4 and 6. The Reynella Siltstone Member contains massive diamictites, sandstone channels, rhythmites, granule trains and two possibly ice-wedged palaeosoil horizons. These features indicate that glaciogenic processes played a role in the deposition of this Unit. The Seacliff Sandstone Member consists of massive sandstone beds, with dewatering structures, and lenses of dolomite and silt. The massive sands were deposited from proximal liquefied flows. The Nuccaleena dolomite equivalent and Unit 13 of the Wilpena Group represent a transgression near the end of the Marinoan sequence in the Type Area. The interbedded silts and sands of Unit 13 were deposited by distal turbidity currents. The upper part of the Marinoan Type Section records regression, as the turbidite sequence shallows upwards to the storm-deposited ABC Range Quartzite. The fluctuating sea level may reflect eustatic rises and falls in sea level produced by associated glacial, or tectonic activity. The Marinoan glaciation is directly reflected in the Reynella Siltstone Member but the indirect effects of a more prolonged glacial period may have influenced deposition over much of the lower part of the Marinoan-Type Section.