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Four layers, S1-S4, containing sand-sized spherical particles formed as a result of large meteorite impacts, occur in 3.47-3.24 Ga rocks of the Barberton Greenstone Belt, South Africa. Ir levels in S3 and S4 locally equal or exceed chondritic values but in other sections are at or only slightly above background. Most spherules are inferred to have formed by condensation of impact-produced rock vapor clouds, although some may represent ballistically ejected liquid droplets. Extreme Ir abundances and heterogeneity may reflect element fractionation during spherule formation, hydraulic fractionation during deposition, and/or diagenetic and metasomatic processes. Deposition of S1, S2, and S3 was widely influenced by waves and/or currents interpreted to represent impact-generated tsunamis, and S1 and S2 show multiple graded layers indicating the passage of two or more wave trains. These tsunamis may have promoted mixing within a globally stratified ocean, enriching surface waters in nutrients for biological communities. S2 and S3 mark the transition from the 300-million-year-long Onverwacht stage of predominantly basaltic and komatiitic volcanism to the late orogenic stage of greenstone belt evolution, suggesting that regional and possibly global tectonic reorganization resulted from these large impacts. These beds provide the oldest known direct record of terrestrial impacts and an opportunity to explore their influence on early life, crust, ocean, and atmosphere. The apparent presence of impact clusters at 3.26-3.24 Ga and approximately 2.65-2.5 Ga suggests either spikes in impact rates during the Archean or that the entire Archean was characterized by terrestrial impact rates above those currently estimated from the lunar cratering record.

QeEOLIAN DEPOSITS (HOLOCENE)-Well-sorted loose sand deflated from Columbia River floodplain that is now drowned behind dams Qb BOG DEPOSITS (HOLOCENE)-Peat and alluvium. Poorly drained and at least intermittently wet. Grades into unit Qa Qt TALUS DEPOSITS (HOLOCENE)-Nonsorted angular boulder gravel to boulder diamicton. At lower altitudes only arbitrarily distinguished from unit Oa. At higher altitudes in western part of quadrangle includes Holocene moraines, rock glaciers, and protalus ramparts LANDSLIDE DEPOSITS -Consist of: Qlsi Incipient blockslides (Holocene)-Large nonrotated mass of bedrock extensively crevassed as a result of slight movement toward nearby free faces. Crevasse -arrow symbol (not in digital coverage) shows direction of movement Qls Landslides (Holocene and Pleistocene)-Diamicton of angular clasts of bedrock and surficial deposits derived from upslope. Locally includes rotated blockslides. Arrows (not in digital coverage) denote downslope direction Qs ALLUVIUM (HOLOCENE AND PLEISTOCENE)-Moderately sorted cobble gravel along rivers grading to poorly sorted gravelly sand on small-tributary fans; as mapped, includes some material in fans similar to unit Qt Ql LOESS (HOLOCENE AND PLEISTOCENE)-Slightly compacted well-sorted silt. In places below edge of Columbia Plateau grades into unit Qe Qr REGOLITH (HOLOCENE AND PLEISTOCENE)-Slightly compacted to loose silt, diamictite, and soils. Extensive and thick on upland surfaces north and northwest of Columbia River. Includes considerable loess and weathering debris. Mostly mapped on basis of gentle ridgetop topography; contacts approximate GLACIAL DRIFT AND RELATED DEPOSITS Qgd GLACIAL DRIFT (PLEISTOCENE)-Ranges from till in uplands and upvalley areas t o gravelly outwash on broad valley floors. In lower Chelan Valley and in large valleys of Wenatchee drainage east of Lake Wenatchee, outwash gravel is interbedded with till and minor lacustrine sediment; in these areas gravel fraction is cognate with unit Qtg. Mostly last-glacial deposit, but in some places passes upslope into older, weathered drift. Conspicuous, large, sharp-crested moraines distinguished by dot-dash pattern

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The Swaziland Supergroup in the Barberton Greenstone Belt (BGB) consists of a lower, predominantly volcanic sequence, the Onverwacht Group; a middle volcaniclastic and quartz-poor clastic succession, the Fig Tree Group; and an upper quartzose terrigenous unit, the Moodies Group. In classic sections in the Onverwacht anticline, the Onverwacht Group includes 8 to 10 km of komatiitic, basaltic, and dacitic volcanic rocks and thin, silicified sedimentary layers that have been subdivided, from base to top, into the Komati, Hooggenoeg, and Kromberg Formations, and a new unit, the Mendon Formation. The ages and stratigraphic relationships of the highly altered Sandspruit and Theespruit Formations in the anticline are not fully resolved, but the latter includes felsic volcanic components that are in part older than the Komati Formation and in part correlative with dacitic volcanic units at the top of the Hooggenoeg Formation. However, in the Steynsdorp anticline, rocks assigned to the Theespruit Formation lie stratigraphically below the Komati Formation and include the oldest dated stratigraphic units in the Swaziland Supergroup. In the central part of the belt, north of the Granville Grove fault and south of the Inyoka fault, komatiitic volcanic rocks of the Onverwacht Group are younger than those of the Komati Formation and are here assigned to a new unit, the Mendon Formation. Exposed portions of the formation appear to young northward across a series of fault-bounded outcrop belts. North of the Inyoka fault, the Onverwacht Group includes a thick succession of komatiitic and basaltic volcanic rocks and tuffs, layered ultramafic intrusions, and thin cherty units. These rocks are here grouped into a new lithostratigraphic unit, the Weltevreden Formation. Age data suggest that the Weltevreden Formation is equivalent to at least the upper part of the Mendon Formation. The overlying Fig Tree Group consists of interstratified terrigenous clastic units and dacitic to rhyodacitic volcaniclastic and volcanic rocks. South of the Inyoka fault, these strata appear to include two formation-level units: the Mapepe and Auber Villiers Formations. The Mapepe Formation includes as much as 700 m of shale, chertgrit sandstone, and chert-clast conglomerate interstratified with fine-grained felsic pyroclastic and volcaniclastic rocks. Chert, jasper, and barite make up a minor part of most sections. Deposition took place in alluvial, fan-delta, and shallow to perhaps moderately deep subaqueous environments. Dacitic tuffs have yielded single-crystal zircon ages of 3,252 ± 6, 3,243 ± 4, and 3,226 ± 4 Ma. The Auber Villiers Formation includes 1,500 to 2,000 m of dacitic tuff; coarse volLowe, D. R.Downloaded from caniclastic sandstone, conglomerate, and breccia; and terrigenous chert-clast conglomerate, chert-grit sandstone, and shale. This sequence and the locally overlying Moodies strata form the hanging-wall succession above a thrust fault, named the 24-hour Camp fault. The footwall sequence includes rocks of the Mendon and Mapepe Formations...

Beds of sand-sized spherules in the 3400-million-year-old Fig Tree Group, Barberton Greenstone belt, South Africa, formed by the fall of quenched liquid silicate droplets into a range of shallow-to deep-water depositional environments. The regional extent of the layers, their compositional complexity, and lack of included volcanic debris suggest that they are not products of volcanic activity. The layers are greatly enriched in iridium and other platinum group elements in roughly chondritic proportions. Geochemical modeling based on immobile element abundances suggests that the original average spherule composition can be approximated by a mixture of fractionated tholeiitic basalt, komatiite, and CI carbonaceous chondrite. The spherules are thought to be the products of large meteorite impacts on the Archean earth.

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