About Ga Impact Clusters and Associated Tsunami

The Jeerinah Impact layer (JIL) (Simonson et al., 2000; Glikson, 2004) consists of a sequence of siltstone, chert and mafic volcanics, capped by a ^40-cm-thick microkrystite spherule-bearing rip-up breccia, a lenticular spherule unit up to about 60 cm thick, with an overlying ~70-cm-thick boulder-size debris-flow conglomerate. The sequence represents initial current and/or seismic disturbance of the seabed, settling of microkrystite spherules, subsequent slumps and debris flow (Fig. 8.5.A-D). The age of JIL is constrained by U-Pb zircon date of 2629 ± 5 Ma of overlying volcanic tuff (Nelson et al., 1999) and U-Pb ages on zircon from sediments from the base of the Jeerinah Formation (2684 ± 6 Ma, 2690 ± 6 Ma; Arndt et al., 1991). Pending further isotopic age studies, it is possible that the JIL impact occurred close to the late Archaean (^2.7-2.68 Ga) volcanic peaks that dominate greenstone belts of the Yilgarn, Superior, Brazil, and part of the Kaapvaal cratons. Significantly, by analogy to banded iron formations which overlie the ^3.26-3.225 impact boundary in the Pilbara and Kaapvaal, the JIL fallout units is located immediately below the banded iron formation of the Marra Mamba Iron Formation.

Fig. 8.5. More than 2.63-Ga Jeerinah Impact Layer. (A) Polished section of rip-up ferruginous siltstones (FS) with intervening microtektites (T) and microkrystite spherules (MKS). (B) Boulder breccia-conglomerate overlying the main spherule layer. knife - 8 cm. (C) JIL microkrystite spherule showing inward radiating K-feldspar fans and an offset central vesicle - both hallmarks of condensate spherules. (D). JIL microtektite showing crude flow bands and quartz-filled vesicles.

Fig. 8.5. More than 2.63-Ga Jeerinah Impact Layer. (A) Polished section of rip-up ferruginous siltstones (FS) with intervening microtektites (T) and microkrystite spherules (MKS). (B) Boulder breccia-conglomerate overlying the main spherule layer. knife - 8 cm. (C) JIL microkrystite spherule showing inward radiating K-feldspar fans and an offset central vesicle - both hallmarks of condensate spherules. (D). JIL microtektite showing crude flow bands and quartz-filled vesicles.

A microtektite and microkrystite-bearing impact spherule-bearing megab-reccia unit (SBMB), possibly correlated with the 2.56-Ga SMB impact fallout unit (Fig. 8.6), consisted of 20-30-m-thick brecciated chert and dolomite fragment-rich carbonate breccia occurs in the lower part of the ~2.54-2.56-Ga Carawine Dolomite over 100-km NW-SE in the eastern outlier of the Hamers-ley Basin (Simonson, 1992; Hassler et al., 2001; Glikson, 2004). The megabrec-cia is either excavated into, or conformably overlies, layered carbonate, and consists of chert and dolomite fragments and blocks derived from the underlying carbonates (Fig. 8.7.A-E). The SBMB constitutes a unique time-event marker located below wave base in subbasins between, as well as below, the stromatolite reef. Where stratigraphic relations are established, the stromatolites appear mainly later than the impacts. No impact debris was found to date in the possibly contemporaneous stromatolite reefs, such as in the Gregory Range - a lack attributed to high-energy currents over the shallow reef environment. Preservation of intact spherules within the veins is attributed to dilation of the injected veins under extreme hydraulic pressures caused by

Fig. 8.6. About 2.56-Ga impacts. (A) Classic outcrops of SMB-1, Munjina Gorge, showing a basal microkrystite layer (MKR) (under Swiss knife), Bouma-cycle turbidites (BC), the base of the cross-rippled tsunami zone (TZ) with climbing ripples is indicated by an arrow. Knife is 8 cm. (B) Pocket of chert pebble-conglomerate of SMB-2 overlying silicified siltstone of the "Quiet Zone" (QZ) which, in turn, overlies eddie-structured turbidites of SMB-1 (ED). Bee Gorge; (C) Climbing ripples of SMB-1 overlain by siltstones of the "Quiet Zone" (QZ), overlain by spherule-bearing siltstone of SMB-1. Bee Gorge area; (D) Turbulence flame structures (FS) overlying silicified siltstone (S) which in places contains basal spherule lenses. The FS zone is overlain by siltstone that contains turbulent eddie structures; SMB-1, Crossing Pool, Wittenoom Gorge. Arrows from base to top point to (1) base of SMB-1 turbidite; (2) base of flame structured zone; (3) base of siltstone-hosted eddie zone; (4) turbulence eddie; base of "Quiet Zone"; (E). Turbulence eddies in SMB-1, Crossing Pool, Wittenoom Gorge. Arrows from base to top point to: (1) base of cross-layered zone; (2) top of cross-rippled zone; (3) Siltstone-hosted eddie zone; (4) turbulence eddie; (5) base of Bouma cycle zone; (6) top of Bouma cycle zone; (F) Convoluted turbulence eddie structures, SMB-1, Bee Gorge; (G) Microkrystite spherules from SMB-1, Munjina Gorge, displaying radial inward radiating devitrification features (d) of K-feldspar fans and randomly oriented K-feldspar microlites. Note the offset vesicle (V) in the right-hand spherule; (H) Mi-crotektite (T) consisting of microlitic K-feldspar and located near a microkrystite spherules of similar mineralogy but showing inward-radiating textures and offset vesicles. These textural differences are interpreted, respectively, in terms of derivation from microtektite melt droplets and vapor condensation melt droplets.

Microtektite

Fig. 8.7. Outcrops of spherule-bearing megabreccia (SBMB) of the Carawine Dolomite. Swiss knife, 8 cm. Hammer, 30 cm. (A) Base of the SBMB showing breccia excavated in relief into underlying carbonates, cutting through bedding planes, Warrie Warrie Creek. BR, breccia; CD, Carawine Dolomite; (B) A 7-m-long (only part shown) detached carbonate layer segment overlying and underlying megabrec-cia, Ripon Hills; (C) Excavated base of the SBMB showing alternating carbonate blocks of Carawine Dolomite (CD) and layer-parallel injected breccia [B] which contains isolated microkrystite spherules. Warrie Warrie Creek. Swiss knife is 8 cm. (D) Top of the SBMB, showing undisturbed carbonates (CD) capping breccia (BR). Arrows point to contact zone; (E) Top of the SBMB, knife and arrow point to a sharp contact between breccia (BR) and layered carbonate (CD).

Fig. 8.7. Outcrops of spherule-bearing megabreccia (SBMB) of the Carawine Dolomite. Swiss knife, 8 cm. Hammer, 30 cm. (A) Base of the SBMB showing breccia excavated in relief into underlying carbonates, cutting through bedding planes, Warrie Warrie Creek. BR, breccia; CD, Carawine Dolomite; (B) A 7-m-long (only part shown) detached carbonate layer segment overlying and underlying megabrec-cia, Ripon Hills; (C) Excavated base of the SBMB showing alternating carbonate blocks of Carawine Dolomite (CD) and layer-parallel injected breccia [B] which contains isolated microkrystite spherules. Warrie Warrie Creek. Swiss knife is 8 cm. (D) Top of the SBMB, showing undisturbed carbonates (CD) capping breccia (BR). Arrows point to contact zone; (E) Top of the SBMB, knife and arrow point to a sharp contact between breccia (BR) and layered carbonate (CD).

the tsunami. The field relations suggest ejecta fallout preceded arrival of the tsunami waves. The tsunami dispersed the upper soft sedimentary layer of the seabed, including the spherule layer, as a subaqueous mud cloud. Disruption of below-wave base sediments and their underlying substratum to depths in the order of over 100 m require tsunami amplitudes on the scale of hundreds of meters. The tsunami did not necessarily originate from impact craters, and could have emanated from faults and plate margins reactivated by impact-triggered seismic activity.

Stratigraphic correlations and isotopic Pb-Pb carbonate ages tentatively support a correlation between the ~2.54-2.56-Ga SBMB megabreccia and the ~2.56-Ga Spherule Marker Bed (SMB) multiple impact unit, Bee Gorge Member, Wittenoom Formation, central Hamersley Basin (Simonson, 1992; Simonson and Hassler, 1997; Glikson, 2004) (Figs 8.6 A-H). The SMB-1 and SMB-2 impact fallout layers each consists of a spherule layer or lenses overlain by Bouma cycle graded turbidites and/or by current perturbed turbidites containing turbulence eddies or conglomerate. The two impact cycles are separated by a stratigraphically consistent little disturbed silicified siltstone layer. A weakening of tsunami effects from the eastern to the central Hamersley Basin (Simonson, 1992; Glikson, 2004) over a distance of about 200 km, suggests a northeast origin of the tsunami and deepening of the basin in the southwest direction.

A stratigraphically consistent <20-cm-thick unit of microkrystite spherules and microtektite-bearing impact fallout ejecta overlies 2.47-2.50-Ga volcanic tuff (DGS4) of the Dales Gorge Member, Hamersley Basin. Locally the unit incorporates up to meter-scale fragments and boulders of banded chert and stromatolite carbonate, suggesting tsunami transport postdating spherule deposition (Fig. 8.8 A-D). The unit displays anomalous platinum group element (PGE), Ni, Cr, and Co abundances and ratios, indicating meteoritic contamination. Mixing calculations suggest a contribution of 2.5-3% projectile component to the impact-generated volatile cloud. Conservative mass balance estimates derived from the Ir and Pt flux, assuming global extent of a 10-cm-thick spherule unit and chondritic projectile composition, suggest an asteroid diameter on the scale of ^30 km. Similar estimates are obtained from spherule sizes, which in DGS4 reach a mean diameter of ^2.0 mm in aerodynamically elongate spherules. The evidence implies formation of an impact basin on the scale of 400 km in simatic/oceanic regions of the early Proterozoic crust (Glikson et al., 2004).

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