Gosses Bluff - a latest Jurassic impact structure, central Australia. Part 1: geological structure, stratigraphy, and origin

Gosses Bluff consists of a prominent circular ridge, 4.5 km in diameter, surrounded by a less well-exposed deformed outer ring, 24 km in diameter, which incorporates annular breccia troughs. The circular ridge, which forms part of an eroded central uplift, is composed of fractured and brecciated Ordovician to Devonian sandstone and shale, capped in places by overturned megabreccia. The structure was formed by the impact of an asteroid or comet. Evidence for an extraterrestrial impact origin includes: (1) the circular symmetry of the disturbed zone, which comprises outcrops of vertical to overturned strata whose original stratigraphic position would be at depths of <3-4 km; (2) the presence of shatter cones and rhomboidal fracture patterns diagnostic of intense shock; (3) shatter-cone axes that define a structurally central focus at shallow depth beneath the palaeosurface when reconstructed to their pre-impact orientation; (4) outward ejection of large blocks; (5) melting of sandstone and siltstone to form melt breccia; (6) a gradation with increasing depth from shock-melted breccia into recrystallised and unheated breccia, suggesting a high central heat source; (7) a depth limit of the structural disturbance defined by continuous seismic reflectors below about 3500 m; (8) the absence of gravity anomalies which would provide evidence for deep-seated mass excess or deficiency. Mineralogical and microstructural features diagnostic of instantaneously applied shock pressures abound. Quartz in both breccia and bed-rock shows shock-induced fractures and planar deformation features. The melt breccia at Mount Pyroclast records higher shock levels: quartz has been transformed to glass, partly recrystallised into tridymite, and subsequently converted to solid-state diaplectic quartz. The fusion of shale resulted in potassium-enriched hot solutions circulating below the crater floor, and recrystallisation into pumiceous aggregates of sanidine accompanied by zeolites and hematite. Ar-Ar plateau ages of this sanidine-rich material suggest recrystallisation at 142.5 ± 0.8 Ma, which - along with the orientation and reverse nature of the geomagnetic field at the time of the event - points to a latest Jurassic age. Calculations indicate that a crater the diameter of the Gosses Bluff structure reflects the release of energy in the order of 105_106 Mt, which could have been generated by an asteroid or comet estimated to have been about 2 km in diameter.