TEXTURAL VARIANCE OF SOUTH AFRICAN IMPACT SPHERULES DUE TO WEATHERING AND LATERAL POSITION

Most extraterrestrial impacts are inferred from structures on the continental portion of the earth's surface, but evidence of this sort is unavailable for oceanic impacts older than the Mesozoic due to tectonic recycling. Therefore, the only record of these impacts we can hope to find are layers of impact ejecta generated by such collisions. At least 8 thin layers of sand-size spherules of former silicate melt have been found in several Late Archean to Paleoproterozoic successions in South Africa and Western Australia and interpreted as ejecta from oceanic impacts. The Monteville Formation in the Griqualand West Basin (South Africa), discovered in 1995, contains one of the best preserved and most extensive of these layers. This basin provided an excellent site for preservation of the layer with its low energy environment and lack of bioturbation, and the strata are generally flat-lying and undeformed despite their age of ca. 2.6 Ga. The layer has been sampled from both surface exposures and drill cores. Although originally glassy, devitrification and replacement by other minerals (mostly K4eldspar) has left many of the spherules with radiating fans, random laths, and with other distinct textures. We petrographically analyzed these spherules with 3 goals in mind: 1) to shed light on their original composition and cooling histories, 2) to see if there was any systematic lateral variation that could be used for paleogeographic analysis and 3) to see how much textural information was lost by alteration through surface weathering. Preliminary work to date on these samples suggests the following: 1) the internal textures of the spherules change along the 160 km long transect studied and 2) textures of the spherules in the surface samples are less diverse than those in the drill cores.