Mid-proterozoic dolomitic varves and microcycles from the McArthur basin, Northern Australia

Abstract

The middle Proterozoic ( −1.75 Ga) Wollogorang Formation of the unmetamorphosed intracratonic McArthur Basin of northern Australia contains a dolomitic black shale facies that has distinctive millimetre and sub-millimetre scale bedding laminations and microcycles of alternating organic-rich mudstone and organic-poor dolostone. The microcycles are between 1 and 20 mm thick and their internal laminations are arranged rhythmically into a lower subdivision of mainly dolostone with subordinate mudstone laminae, and an upper subdivision of mainly mudstone with thin dolostone laminae. The microcycles vary systematically in both thickness and character. The thinnest laminations visible (around 0.2 mm thick) are interpreted as chemical varves; the microcycles represent bundles of varves deposited over longer periods. Over these longer periods each microcycle shows a gradual upward decrease in the amount of dolomite precipitated and/or preserved. This is related in part to original climatic controls in the environment of deposition, and also to a secondary (but pre-lithification) process of dissolution similar to that invoked by Trendall (1972, 1973) for the formation of mesobands in the BIF's of the Hamersley Basin, Western Australia. Simple statistical calculations of varve thickness variations on about 100 microcycles, from a 1.5 m long core, indicate the presence of 7- to 11-year and - 300-year periods. These periods are comparable with those published by Williams (1981) for glaciogenic varves from late Proterozoic ( - 680 Ma) rocks in South Australia; he relates the former to sunspot cycles and the later to longer term climate periods as evident from tree-ring studies. Although nowhere near as conspicuous as the sunspot cycles described by Williams (1981) or the double sunspot cycles of Trendall (1972) from the latest and earliest Proterozoic (respectively), these microcycles from the middle Proterozoic may be due to similar sunspot-related processes and therefore add support to their suggestions for long-term stability of solar processes.