The stratigraphy within the Wittenoom Formation's Paraburdoo Member of the Hamersley Basin in Western Australia has lacked a significant and identifiable marker bed until the recent discovery of the Paraburdoo Spherule Layer (PSL). This 1.5-2 cm thick spherule layer, which was produced by a large bolide impact in the Neoarchean, records more depositional events than presented thus far within the literature. Furthermore, large bolide impacts in the Phanerozoic produced global change identifiable in the post-impact sediments. Aside from a few isolated examples, however, post-impact change associated with Precambrian impacts is sparse. In these studies, we (1) correlate this stratigraphy residing at the three known exposed locations of the PSL within the Hamersley Basin and use these correlations in conjunction with detailed observations and analyses to assess the type of sedimentation recorded by these alternating 'sand' and 'mud' beds. (2) We characterize in detail these same several meters of stratigraphy using the PSL as a distinctive marker bed and add to the existing depositional model of the Hamersley Basin. (3) We use the PSL as a case study to search for impact-induced change in the sediments above the spherule layer. (4) Finally, we investigate microbreccia features surrounding the PSL and outline a new depositional history for the PSL and impact-related strata. We find that the strata surrounding the PSL were not deposited by sediment gravity flows, but rather long-acting deep-marine bottom current(s), the rhythmicity recorded as alternating 'sand' and 'mud' beds is the result of increasing and decreasing current velocities, grain size decreases while mud content increases to the east within the basin, bed thinning occurs to the west within the basin, microbial activity may be recorded in laminations in the finer-grained mud beds present within these strata, and the diagenetic history changes across the Hamersley Basin. We also find possible minor sedimentary changes across the PSL that may be due either to a disturbance by bottom currents or changing diagenetic conditions. Contrary to the trends found with several post-Great Oxidation Event large bolide impacts, we find no evidence of shifts in tectonic regime, sediment deposition, paleoenvironment, or weathering induced by the PSL impact, for which the impactor has been estimated to be approximately three times larger than that of the K-Pg boundary layer. This lack of weathering may be due to one or both of the following hypotheses: either the PSL's deposition in several-hundred-meter-deep water within the Hamersley Basin of Western Australia caused the water to act as a buffer against such changes, or the Neoarchean's high-CO2 atmospheric composition acted as a threshold below which introducing more impact-produced gases would not have produced the expected climatic and weathering changes. We also report higher iron and arsenic concentrations in the sediments immediately above the PSL as well as distinctive layers of hematite nodules bracketing the spherule layer. These geochemical changes may record ocean overturn of the Neoarchean stratified water column, which brought slightly oxygenated waters to depth, and/or heating of the global oceans by tens to hundreds of degrees Celsius in the wake of the PSL impact. Either or both of these mechanisms in addition to impact-induced shallow-water ocean evaporation may have caused a massive die-off of microbes, also producing a post-impact increase in iron and arsenic concentrations. Finally, we find that the PSL is bracketed by thin layers of microbreccia. The microbreccia layer underlying the spherules contains K-phyllosilicate mud clasts, detrital quartz grains, iron oxide grains and nodules, mud aggregates, and muddy areas within carbonate interstitial material. This underlying layer formed via debris flow or possible bottom-current flow before the PSL spherules fell through the water column. Afterwards, meteoritic dust from the impact along with sediment suspended in the impact-mixed water column settled out on top of the spherules. The pressure of the spherules and overlying mud on top of the underlying microbreccia layer caused the microbreccia to pipe upwards, pooling out on top of the spherules, forming an overlying microbreccia layer. Additional impact-related microbreccia features occur at only one of the three PSL deposition sites, including microbreccia-filled dikes in the strata below the PSL and further water escape features upward through the strata above the PSL, suggesting differential diagenesis or local variability between the three exposed PSL sections. Carbonate, iron oxide, and silica precipitation within the microbreccia layers likely occurred during diagenesis. / Geoscience

The late Archean Mount McRae Shale of the Hamersley Basin in Western Australia may record the presence of oxygen in the atmosphere before the Great Oxidation Event (2.4-2.3 Ga). Several prior studies (Anbar et al., 2007; Blum and Anbar, 2010; Duan et al., 2010; Kakegawa et al., 1998; McManus et al., 2006) have used isotopic systems to analyze the Mount McRae Shale and conclude that there was a presence of oxygen before the Great Oxidation Event. The purpose of this study is to determine if the U-Pb system can be used to see through later events to the initial conditions. The uranium-lead values of the Mt McRae Shale provide evidence of the mobilization of U and Pb gain. The geochemical disturbances have been linked to the tectonic activity (460 Ma) in the neighboring Canning basin, which could have possibly opened the geochemical system. In terms of the depositional environment the U-Pb data gathered here do not point to oxygenation of the atmosphere.

No description available.

[Truncated abstract] Banded iron formations of the ~27702405 Ma Hamersley Province of Western Australia were locally upgraded to high-grade hematite ore during the Early Palaeoproterozoic by a combination of hypogene and supergene processes after the initial rise of atmospheric oxygen. Ore genesis was associated with the stratigraphic break between Lower and Upper Wyloo Groups of the Ashburton Province, and has been variously linked to the Ophthalmian orogeny, late-orogenic extensional collapse, and anorogenic continental extension. Small spot PbPb dating of in situ baddeleyite by SHRIMP (sensitive highresolution ion-microprobe) has resolved the ages of two key suites of mafic intrusions constraining for the first time the tectonic evolution of the Ashburton Province and the age and setting of iron-ore formation. Mafic sills dated at 2208 ± 10 Ma were folded during the Ophthalmian orogeny and then cut by the unconformity at the base of the Lower Wyloo Group. A mafic dyke swarm that intrudes the Lower Wyloo Group and has close genetic relationship to iron ore is 2008 ± 16 Ma, slightly younger than a new syneruptive 2031 ± 6 Ma zircon age for the Lower Wyloo Group. These new ages constrain the Ophthalmian orogeny to the period <2210 to >2030 Ma, before Lower Wyloo Group extension, sedimentation, and flood-basalt volcanism. The ~2010 Ma dykes present a new maximum age for iron-ore genesis and deposition of the Upper Wyloo Group, thereby linking ore genesis to a ~21002000 Ma period of continental extension similarly recorded by Palaeoproterozoic terrains worldwide well after the initial oxidation of the atmosphere at ~2320 Ma. The Lower Wyloo Group contains, in ascending order, the fluvial to shallow-marine Beasley River Quartzite, the predominantly subaqueously emplaced Cheela Springs flood basalt and the Wooly Dolomite, a shelf-ramp carbonate succession. Field observations point to high subsidence of the sequence, rather than the mainly subaerial to shallow marine depositional environment-interpretation described by earlier workers. Abundant hydro-volcanic breccias, including hyaloclastite, peperite and fluidal-clast breccia all indicate quench-fragmentation processes caused by interaction of lava with water, and support the mainly subaqueous emplacement of the flood basalt which is also indicated by interlayered BIF-like chert/mudstones and below-wave-base turbiditic mass-flows.

The Pilbara region of Western Australia hosts the Hamersley Province, an area of abundant iron ore resources located in the lower Hamersley Groups, Brockman and Marra Mamba Iron Formations. This study consists of a geotechnical and a geochemical and mineralogical investigation into the Banded Iron Formation (BIF) and shale deposits of the lower Hamersley Group that reside in the pit walls of RTIO mines in Western Australia. Areas throughout Tom Price, Paraburdoo, Marandoo and West Angelas iron ore mines are geotechnically investigated for rock mass conditions through the use of the Slope Mass Rating (SMR) classification system and through point load and slake durability testing. Selected samples from these areas were then geochemically and mineralogically tested by X-ray Fluorescence (XRF), X-ray Diffraction (XRD) and microscopic analysis, to determine the geochemical and mineralogical changes of BIF and shale as they alter and weather through hypogene and supergene alteration and Recent weathering. It was found that the most efficient method for determining the alteration and/or weathering of lower Hamersley Group BIF and shale deposits was by the use of a chemical alteration index, calculated from enriched and depleted major elements in the BIF and shale as they alter and weather. It has been suggested here that this Pilbara Iron alteration index can be calculated efficiently and effectively from geochemical testing in intervals down boreholes throughout future or developing open pit mines to assist in estimating slope stability conditions. It is also suggested that many boreholes should be analysed in section or in 3D space to create cross sections or block models showing the varying extent of alteration and weathering throughout the area being studied. From the geotechnical investigation, it was found that the weakest region, in terms of pit slope stability, were the highly and extremely altered and/or weathered regions with Pilbara Iron alteration indices of between 61 and 80, and 81 and 100, respectively. If these zones are identified, slope stability analysis can be focused on these geotechnically vulnerable areas. Slope stability analysis should be completed by using a suitable technique, such as by the use of SMR, which, along with other risk identification measures, will identify potentially unstable areas and suggest the required course of action. Further hazard and risk analysis should be undertaken in potentially unstable areas and remedial measures undertaken as appropriate. Thereby, the Pilbara Iron alteration index can be used in the Hamersley Province as a predictive tool for pit slope stability.