Australia: The Land Where Time Began

A biography of the Australian continent 

Mt Jacob Region

The Tapley Hill  Formation records deposition immediately after the Sturtian glaciation in the Adelaide Geosyncline (Thompson et al., 1964). This unit is basin wide in area, and lies conformably and disconformably above glacial diamictites of Sturtian age, in the Merinjina Tillite, in the northern part of the Adelaide Geosyncline (Preiss, 1987). to the east of Paralana Fault at Mt Jacob, where there are 5 distinct stratigraphic units that can be identified, it attains its maximum thickness of 550 m, the 5 units being referred to here as the Tindelpina Shale Member (Thompson et al., 1964), or more informally as the siltstone unit, cyclic unit, quartz-sandstone unit, and transitional shale unit.

The base of the Tapley Hill Formation is defined by the Tindelpina Shale Member, that is laterally persistent, has an average thickness of about 50 m in the sections used by the authors1.There is a thin cap carbonate at the base of the Tindelpina Shale Member, less than 10 m thick, that mainly consists of peloidal dolomite situated above the glaciogenic Merinjina Tillite with sharp conformable contact. The Tindelpina Shale Member is characterised by dark grey, pyritic dolomite shale that has regular dolomite interbeds beds about 5 cm thick, and the cap carbonates grades into the upper part of it. These dolomite interbeds towards the top of the Tindelpina Shale Member disappear, the unit grading into the overlying siltstone unit.

Calcareous siltstone comprises the siltstone unit often forms a prominent ridgeline about 50-70 m from the base of the Tapley Hill Formation. A change in the dominant carbonate mineralogy, from dolomite to calcite, and an increase in the abundance of detrital quartz, coincides with the siltstone unit in the Mt Jacob section. The cyclic unit, that overlies the siltstone unit is characterised by calcareous shale that is finely laminated with interbeds of carbonate-rich (30-60 %) carbonate, that is very regularly spaced about 10-20 cm thick. In the stratigraphic section the spacing of the carbonate-rich horizons is consistently lass the 4 m. A stratigraphic interval in the middle of the Tapley Hill Formation, overlying the cyclic unit, is characterised by calcareous shale that is interbedded with siltstone, fine sandstone, and sometimes pebbly sandstone (quartz-sandstone-unit). A prominent feature through this interval are partial Bouma sequences (commonly AB and BCD divisions). There are also minor interbeds (2-5 cm thick) of limestone-edgewise-conglomerate near the base of the quartz-sandstone unit. They have tabular, thin limestone intraclasts, that are typically less than 3 mm thick, that are commonly curved, resembling closely the stromatolitic limestones present at the base of the overlying Balcanoona Formation, indicating that the deposition and erosion of the Balcanoona Formation carbonates, that were of equivalent shallow-water origin, was underway before the deposition of the quartz-sandstone unit.

The upper regressive part, the transitional shale unit, of the Tapley Hill Formation lies above the quartz-sandstone unit, the contact between them being abrupt. Near Mt Jacob the unit, dominated by finely laminated grey, silty shale with thin interbeds of carbonate-rich siltstone and limestone-edgewise conglomerate that is identical to those present at the base of the quartz-sandstone unit, reaches a thickness of  up to 30 cm. Through the conformable and gradational contact with the overlying Balcanoona Formation the number of edgewise beds increase in number and thickness, reaching thicknesses up to 1 m. The Balcanoona Formation was first defined by Coats & Blissett (1971) and subsequently re-defined by Preiss et al., (1998). It is comprised of a base of stromatolitic limestone, lower than about 20 m, with interbeds of edgewise-conglomerate from 5 cm to 3 m thick, and minor calcareous shale. Purely stromatolitic dolomite overlies these, the stromatolites being dominantly of broad domes that range from about 30 cm to about 2 m wide.

See Interglacial Carbonates, Umberatana Group, Flinders Ranges, South Australia

Sources & Further reading

  1. Giddings, J.A., Wallace, M.W. and Woon, E.M.S., 2009, Interglacial carbonates of the Cryogenian Umberatana Group, Flinders Ranges, South Australia, Australian Journal of Earth Science (2009) 56 (907-925).

 

 

 

Author: M. H. Monroe
Email:  admin@austhrutime.com
Last Updated 07/05/2012
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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading