Extending the paleogeographic range and our understanding of the Neoarchean Monteville impact spherule layer (Transvaal Supergroup, South Africa)

Abstract

The Monteville spherule layer (MSL) was deposited in the Griqualand West Basin (GWB) on the Kaapvaal Craton approximately 2.63 Ga. The spherules were generated by a large impact and reworked by impact-generated waves and/or currents. The MSL has been intersected in three previously undescribed cores. Two of the cores, GKF-1 and GKP-1, were drilled ~30 km west of the southernmost outcrop of the MSL. The third core, BH-47, was drilled ~250 km south and east of the GWB. The MSL contains medium to coarse sand-size spherules like those described previously in all three cores but each one was emplaced in a different way. In GKF-1, the MSL is 90 cm thick and contains large rip-up clasts of basinal carbonate and early diagenetic pyrite. In GKP-1, the MSL is only 1.5 cm thick and consists largely of fine carbonate sand, yet it contains pyrite intraclasts up to ~1 cm long. In BH-47, the MSL consists of a lower coarse sandy zone ~37 cm thick rich in spherules, carbonate peloids/ooids, pyrite intraclasts, and quartzose sand and an upper, finer sandy zone ~46 cm thick; neither zone contains any large intraclasts. The new occurrences triple the known extent of the MSL from ~15,000 to ~46,000 km2, support the oceanic impact model for the formation of the MSL, demonstrate that it is a persistent regional time-stratigraphic marker, place new constraints on the Kaapvaal paleoshoreline at the time of impact, and support the existence of Vaalbara.