ASTEROID IMPACT EJECTA UNITS OVERLAIN BY IRON-RICH SEDIMENTS IN 3.5-2.4 GA TERRAINS, PILBARA AND KAAPVAAL CRATONS: ACCIDENTAL OR CAUSE-EFFECT RELATIONSHIPS?

Abstract

The significance of temporal and spatial associations between asteroid/comet impact ejecta units and overlying iron-rich sediments, including banded iron-formation (BIF), jaspilite and ferruginous shale, observed in the Pilbara Craton, Western Australia and the Barberton Greenstone Belt (BGB), Kaapvaal Craton, South Africa, is considered. Such associations include (1) 3470.1 ± 1.9 Ma impact spherule units and tsunami-type breccia overlain by jaspilite in the Antarctic Chert Member, central Pilbara Craton; (2) 3258 ± 3 Ma impact spherule unit (S2) in the BGB overlain by BIF, jaspilite and ferruginous shale; (3) 3243 ± 4 Ma impact unit (S3) in the BGB overlain by iron-rich sediments (Ulundi Formation); (4) two formations of BIF and ferruginous shale at the base (Nimmingarra Iron Formation) and lower part (Paddy Market Formation) of the Gorge Creek Group, central Pilbara Craton, which overlie 3235 ± 3 Ma felsic volcanics along a boundary correlated with the BGB-S3 unit; (5) 2629 ± 5 Ma impact spherule unit and tsunami-type deposit overlain by BIF and ferruginous shale (Marra Mamba Iron Formation), central Pilbara Craton; (5) 2481 ± 4 Ma spherule unit intercalated at the lower part of the Dales Gorge Iron Member of the Brockman Iron Formation, Hamersley Basin, and possible equivalents in the Kuruman Formation, western Transvaal. No significant thicknesses of iron-rich sediments are known to overlie carbonate-hosted impact ejecta units, including the Bee Gorge Member (BGM) of the Wittenoom Formation (2561 ± 8 Ma), Carawine Dolomite (eastern Hamersley Basin, Pilbara Craton), Monteville Formation and Reivilo Formation (Griqualand West Basin, Transvaal Group), or the Graensco-Vallen spherule occurrence (southwest Greenland). The juxtaposition between impact ejecta units and overlying BIF and jaspilite may be accidental or purely related to preservation of these units in below-wave-base environments. Alternatively, this association may hint at enrichment of seawater in soluble iron under low-oxygen atmosphere/hydrosphere conditions due to enhanced denudation of mafic volcanic terrains uplifted/exposed through impact-induced tectonic movements and/or impact-triggered mafic volcanic and hydrothermal activity. The general absence of iron-rich sediments above carbonate-hosted impact ejecta units may be attributed to lack of Fe enrichment or to pH and Eh conditions unsuitable for the precipitation of iron oxides. The near-absence of BIF in high-energy shallow water environments overlying impact ejecta units may reflect retardation of colloidal/chemical sedimentation in such environments. A systematic association between impact ejecta and iron-rich sediments, if confirmed, may yield useful stratigraphic tracers in the search for impact signatures in early Precambrian terrains. © 2006 Elsevier B.V. All rights reserved.