Dating and characterising a newly discovered sedimentary basin in the east Tennant region

Abstract

An unknown sedimentary sequence was discovered in the newly defined Brunette Downs Rift Corridor. The sequence is interpreted to lie within a half graben structure, beneath the Helen Springs Volcanics (297m) and extend to basement (724m). This study analyses these sediments to establish when they were deposited, under what conditions, and from what source. The unknown sedimentary rocks were defined as Units A through F, with a notable unconformity splitting units A and B. Maximum depositional ages were established above and below the unconformity using U–Pb detrital zircon dating. These yielded ages of 902 ± 34 Ma and 1649 ± 37 Ma respectively, based on the youngest concordant grain. In-situ laser ablation inductively coupled plasma reaction cell mass spectrometry (LA-ICP-MS/MS) Rb–Sr dating established a minimum depositional age of 1547 ± 13 Ma for shales in the sequence below the unconformity. Initial 87Sr/86Sr values and rare earth element compositions of these shales suggest that this age dates diagenetic phases that grew in equilibrium with Mesoproterozoic sea water. Therefore, this age may effectively date deposition of this sedimentary package. NDIBK10 sediments were compared to a database of surrounding sedimentary and igneous provinces. The age distribution of detrital zircon grains show that the unknown sediments are most similar to the lower McArthur Group (Glyde Package), or Bullita Group (Favenc Package). The shale Rb–Sr age of 1547 ± 13 Ma suggests that the Favenc Package is the most likely of these possibilities. The sandstones are likely sourced from the Aileron Province, possibly due to uplift related to the Chewings Event. La/Sm ratios from shales indicate that the source was primarily felsic, comparable to average upper continental crust (AUCC). Th/U ratios and Ce anomalies indicate that ocean waters were oxic/sub-oxic becoming more oxic during deposition.