Mathematics – Logic
Scientific paper
Dec 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufm.b32b..04b&link_type=abstract
American Geophysical Union, Fall Meeting 2004, abstract #B32B-04
Mathematics
Logic
0400 Biogeosciences
Scientific paper
In July-August 2004, the NASA Astrobiology Drilling Program sponsored the coring of 3 deep diamond-drill holes in the Pilbara Craton of northwest Australia. The holes targeted the lowest grade and least deformed sedimentary sections of 4 stratigraphic units: the 2.4-2.6 Ga Hamersley Group, the 2.7 Ga Tumbiana Formation of the Fortescue Group, the 3.4 Ga Warrawoona Group, and the 3.5 Ga Coonterunah Group. ABDP 8 cored the unconformity between the Warrawoona and Coonterunah Groups to a depth of 330 metres, intersecting it at 155 metres. Because of syn-depositional erosion, the Strelley Pool Chert was attenuated and the alteration zone beneath the unconformity was scoured and filled to a depth of 10 metres by quartz arenite. As a result, no definitive lithological determination on its status as a potential paleosol could be made. Secondary oxidative alteration was present in Coonterunah cherts to depths of at least 220 metres down-hole. ABDP 9 cored 984 metres of the lower Hamersley Group, from the Dales Gorge Member of the Brockman Iron Formation into the Paraburdoo Member of the Wittenoom Formation. Though the hole was intended to penetrate the uppermost Fortescue Group, drilling was terminated early because of equipment damage by fractured rock, loss of water circulation clogging the hole with cuttings and unanticipated thickening of the Paraburdoo Member by dilational fracturing, expansive brecciation and cavity formation. 79 samples for organic geochemical analysis of biomarker syngenesis were collected under clean conditions immediately the core surfaced. A horizon of impact spherules was intersected in the Bee Gorge Member of the Wittenoom Formation; unlike surface exposures, it was markedly silicified and chloritized in drill-core. ABDP 10 cored 210 metres of the Tumbiana Formation, intersecting the entire Meentheena Carbonate Member, the upper Mingah Tuff Member and terminating just below 4 scoriaceous basalt flows. Large and complex calcareous stromatolites, which were partly silificified in surface exposures, contained no chert in drill-core. Ooid grainstone was a prominent lithology, attesting to a shallow subaqueous depositional environment. Pseudomorphs possibly after diagenetic gypsum were encountered in a fine tuffaceous wacke unit, but no other evaporite minerals were evident. 8 samples for biomarker analysis were collected. The cores will be archived at the Geological Survey of Western Australia core library in Perth. They will be cut using non-magnetic saw blades to allow paleomagnetic studies. Quarter-core splits will be sent to the USA and Japan for detailed examination.
Bonser L. C.
Buick Roger
Dunlop James S.
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