MONTEVILLE SPHERULE LAYER (NEOARCHEAN, TRANSVAAL SUPERGROUP, SOUTH AFRICA) LOOKS FAMILIAR IN THREE NEW CORES THAT TRIPLE ITS AREAL EXTENT

Layers rich in sand-size spherules of former silicate melt constitute the only known record of terrestrial impacts by asteroids and/or comets from the Archean and earliest Proterozoic. One example, the Monteville spherule layer (MSL), was deposited in deep shelf paleoenvironments on the Kaapvaal margin amidst carbonates and shales ca. 2.63 Ga (billion years ago). Sedimentological and geochemical studies suggest the MSL was produced by a large extraterrestrial impact dispersing spherules for thousands of kilometers which were then reworked by impact-generated waves and currents. The latter included bottom return flow (backwash) from the northeast that transported quartzose sands anomalously far out on the shelf. Three new cores have intersected the MSL: Agouron cores GKF-1 and GKP-1 drilled ca. 30 km west of the southernmost outcrop of the MSL and a third core drilled near Trompsburg ca. 270 km ESE of the Griqualand West basin. The MSL contains similar medium to coarse sand-size spherules in all 3 cores, but differs in character core to core. The MSL in GKF-1 is 90 cm thick and contains large rip-up clasts of basinal carbonate and early diagenetic pyrite. The MSL in GKP-1 is only 1.5 cm thick, consists largely of fine carbonate sand, and contains reworked pyrite including disks up to ~1 cm long. The MSL In the Trompsburg core is sandy throughout, consisting of a mixture of spherules; carbonate peloids and ooids; reworked pyrite (making up 50% or more of the basal ~ 42 cm); and quartz-rich medium to coarse sand. The upper ~ 35 cm of the MSL in this core is finer and probably of backwash origin. The degree to which the MSL varies in the new cores does not exceed the range of variability seen in one continuous exposure in a single ~ 45 m-long cliff at its type section. However, coarse quartzose sand had heretofore only been observed in the MSL at the northern edge of the Griqualand West basin. The presence of coarse quartzose sand in the MSL in the Trompsburg core indicates the Kaapvaal paleo-shoreline angled strongly northwest-southeast at the time of the MSL impact. The new cores also provide new support for the oceanic impact model for the formation of the MSL; demonstrate its usefulness as a unique regional (if not intercontinental) time-stratigraphic marker; and expand its minimum preserved area from ca. 15,000 to 46,000 km².