Isotopic age determinations on precambrian rocks of the carpentaria region, Northern territory, Australia

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Abstract
A sequence up to 40,000 ft thick of unmetamorphosed and only slightly deformed sedimentary and volcanic rocks occurs in the Carpentaria Province of Northern Australia. Metamorphic and granitic rocks form the basement to this sequence, and K‐Ar and Rb‐Sr age measurements show that the basement granites are about 1,800 ± 50 m.y. old. Associated in space and time with the granitic rocks are acid volcanics which form the basal unit in the overlying sequence. Glauconites in sedimentary rocks from this succession yield dates ranging from 1,600 m.y. in the Tawallah Group, the second lowest unit, to about 1,390 m.y. in the Roper Group, the uppermost unit. Plagioclase and pyroxene from dolerites intrusive into the Roper Group give K‐Ar dates ranging from 1,100 to 1,280 m.y.; the older date provides a younger limit to the age of the Roper Group. Following slight folding the Wessel Group was deposited unconformably on the Roper Group; a single glauconite from the topmost formation of the Wessel Group yields concordant Rb‐Sr and K‐Ar dates of 780 ± 20 m.y. The results generally are internally consistent and provide much information, not previously available, as to the age of the Precambrian rocks in this region. Correlation with other Precambrian sequences in Australia now becomes possible as more dates are measured on rocks from other areas. Three alternatives are offered for the subdivision of Precambrian time in Australia; (i) the adoption of an arbitrary time‐scale independent of rock sequences, (ii) the adoption of the Canadian system of nomenclature, and (iii) the definition of standard time‐rock units for use throughout Australia. The third alternative is strongly recommended and such time‐rock units should be bounded by horizons that are amenable to accurate and precise dating by isotopic methods. By the judicious choice of several sequences it should be possible to obtain a satisfactory time scale for all Precambrian rocks in Australia. Part of the sequence developed in the Carpentaria Province is proposed as a time‐rock unit to be known as Carpentarian.