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Chert deposits preserve a record of secular change in the oceanic silica cycle. The evolutionary radiation of silica-secreting organisms resulted in a transition from abiological silica deposition, characteristic of the Archean and Proterozoic eons, to the predominantly biologically controlled silica deposition of the Phanerozoic. Comparative petrography of Phanerozoic and Precambrian cherts indicates that an earlier change in chert deposition occurred toward the end of the Paleoproterozoic era (ca. 1.8 Ga). In Neoproterozoic and Mesoproterozoic strata, early diagenetic chertification is largely restricted to peritidal environments. These early diagenetic cherts typically occur as nodules or discontinuous beds within carbonate deposits that have similar depositional textures. The cherts formed primarily by carbonate replacement with subsidiary direct silica precipitation. Some of the Paleoproterozoic cherts associated with iron formations, however, are distinctly different from younger cherts and appear to have formed largely by direct silica precipitation at or just below the seabed. These primary cherts lack ghosts or inclusions of carbonate precursors, have fine-scale grain fracturing (possibly from syneresis), exhibit low grain-packing densities, and are not associated with unsilicified carbonate deposits of similar depositional composition. Cherts in some Paleoproterozoic iron formations (e.g., the Gunflint Formation, northwestern Lake Superior region) are composed of silica types similar to those in Phanerozoic sinters (e.g., the Devonian Rhynie and Windyfield cherts, Scotland). Such cherts may provide evidence that basinal, and perhaps global, oceanic silica concentrations were higher during the Paleoproterozoic era than at later times. Keywords: Precambrian, chert, diagenesis, silica cycle.

A Late Archean layer rich in sand-sized spherules of former silicate melt in the Monteville Formation (Transvaal Supergroup, South Africa) has Ir concentrations as high as 6.4 ppb and is clearly enriched in Ir relative to associated tuffs, carbonates, and shales. The Monteville spherule layer is also enriched in other siderophile elements, including the platinum group elements (PGEs). The PGEs in the spherule layer produce a flat (meteorite like) pattern when they are normalized to chondritic abundances. The abundances of Ir and other siderophile elements are similar to broadly contemporaneous spherule layers in the Hamersley basin of Western Australia. That the mineral compositions, textures, and sedimentary structures of the spherule layers in the Transvaal Supergroup and Hamersley basin are also very similar suggests that they were all formed by the same processes. We think that the best way to explain the high Ir concentrations and other characteristics of the Monteville spherule layer is that it represents distal impact ejecta. There are, however, significant differences between the Monteville spherule layer and Early Archean spherule layers in the Barberton greenstone belt, including much higher average and maximum concentrations of Ir in the latter. The data presented here clearly show that each Precambrian spherule layer is unique and needs to be characterized individually, as is true for the impact spherule layers of the Phanerozoic. Keywords: impacts, spherules, Archean, iridium, platinum group elements.

Samples from a ca. 2.54 Ga layer rich in microkrystites (sand-size spherules of former silicate melt) in the Hamersley Group (Western Australia) are enriched in Ir up to 50 fold over associated carbonates and shales, but few of the other siderophile elements display significant anomalies or chondritic interelement ratios. However, similar concentrations and interelement ratios are observed in ejecta from the ca. 590 Ma Acraman impact structure and have been attributed to diagenetic redistribution. The microkrystite layer also contains sand-size quartz grains with internal textures typical of regionally metamorphosed rocks rather than evidence of shock metamorphism. We suggest that the microkrystites were created by an impact that took place in a deep ocean basin rather than on a continent, and that the associated quartz is epiclastic detritus brought in by unusually high energy waves and/or currents, not as ballistic ejecta.

Detailed geologic field work in the Hamersley basin of Western Australia has identified a single horizon that contains predominantly sand-sized spherules similar to those found in impact ejecta such as at the Cretaceous-Tertiary boundary. Evidence suggests that these spherules represent a reworked distal strewn field formed by a large bolide impact in late Archean time. This so-called 'spherule marker bed' occurs throughout the main body of the Hamersley basin and in stratigraphically equivalent, but shallower-water lithologies in the northeastern corner. Carbonate constitutes both a matrix component of the spherule marker bed and minor interbeds in the local stratigraphic section. We have utilized carbonate Pb-Pb dating methods to provide, for the first time, an age estimate (2541 +18/-15 Ma) for this important marker bed and therefore of the bolide impact event. Samples from both facies define, within error, an identical isochron age that is also consistent with the few zircon-based age estimates for the Hamersley succession. These findings highlight the great potential of carbonate Pb-Pb geochronology in the dating of Archean and Proterozoic sedimentary rocks.

Predominantly sand-sized spherules of K-feldspar with spherulitic, vesicular, and other crystalline textures help to define a unique marker bed in the 2.6 Ga Wittenoom Formation which persists for more than 300 km across the Hamersley Basin of Western Australia. These spherules usually occur as a minor constituent of a single turbidite, but locally they are restricted to centimeter-thick lenses of pure spherules along a single horizon in the argillite close beneath it. In the shallower northeastern part of the Basin, similar spherules (as well as larger, more heterogeneous particles) likewise occur at only one horizon, but here they are a minor constituent of a thick dolomitic debris-flow deposit. as the Wittenoom Formation and the Carawine Dolomite are thought to be stratigraphically equivalent, this debris-flow deposit is believed to be a proximal equivalent of the spherule-bearing turbidite. On the basis of their similarity to microtektites and microkrystites in shape, size, and mode of occurrence, the spherules are interpreted as droplets of silicate melt that were generated and dispersed across the Hamersley Basin by a major bolide impact. Moreover, the mass of the spherules preserved in the Hamersley Basin is of the same order of magnitude as the estimated masses of microtektite glass in major Cenozoic strewn fields, despite the fact that the spherules cover an area which is 2 to 3 orders of magnitude smaller. The layers which host the spherules are interpreted as the deposits of a major sediment gravity flow which exhumed and redeposited most of the spherules after shallow burial. The flow, however, is not believed to have been a direct result of the proposed impact. If the spherule-bearing layers constitute a chronostratigraphic marker, as suggested here, there are important implications for the deposition of carbonates and banded iron formations (BIF) within the Hamersley Basin. Similar spherules also occur at a single horizon [is congruent to] 250 m higher stratigraphically in the Brockman Iron Formation, suggesting that two impacts occurred near the Hamersley Basin in relatively rapid succession. Correlative strata may be present in the Transvaal Basin of South Africa.

The large extraterrestrial object striking Earth at cosmic velocity melts and vaporizes silicate materials, which can condense into highly spheroidal, sand size particles that get deposited hundreds to thousands of kilometers form the point of impact. The spherules and spherule layers are and the processes that create them, then speculate about what might be learned through wider identification of and more extensive study of impact spherule layers is reviewed.