FEATURES OF THICK-BEDDED SANDSTONES IN A PROXIMAL FLYSCH SEQUENCE, UPPER CAMBRIAN, SOUTHWEST TASMANIA

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ABSTRACT

The Upper Cambrian Singing Creek Formation, southwest Tasmania, includes a number of thick-bedded, generally non-graded or poorly graded sandstones as well as normal graded sandstones (turbidites), laminated siltstones, conglomerates, and numerous slump sheets. Most of the thick beds occur in composite units up to 30 m or so thick, within which there is interfingering and inter-grading with thinner sandstones, and frequent amalgamation of beds. The composite units are separated by siltstone sequences, and in some cases at least appear to be large lenses. They are interpreted as being mainly channel deposits in a submarine fan complex.

Many of the thick beds are either structureless throughout or have only a thin division of flat lamination (with parting lineation) near the top, followed in some cases by ripple mark. Some beds, however, show unusual wavy lamination or “dish structure” beneath the flat lamination, and this in turn may be underlain by contorted lamination. Narrow vein-like de-watering channels (elutriation columns) occur in the wavy-laminated division of some beds.

The thick beds form a gradational series with the normal graded beds, and apparently represent the over-thickened basal parts of such beds, i.e., they are lateral variants of the normal turbidites. Similar beds called “grain-flow deposits” by Stauffer (1967) also appear to be proximal turbidites. The origin of the wavy lamination and dish structure is problematical but could be related to mass shearing and dewatering during consolidation in the manner observed by Middleton (1967) for experimental high-concentration underflows.

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