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A probable end-Eocene impact structure under the Timor Sea

Andrew Glikson1 and Dariusz Jablonsky2

1 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
2 Finder Exploration Pty Ltd.

The end-Eocene (c. 35 Ma) constitutes a period of major extraterrestrial impacts, including the Popigai (D=100 km; 35.7 ± 0.2 Ma), Chesapeake Bay (D = 85 km; 35.3±0.1) and a major tektite strewn field in northeastern America. The end-Eocene is also a key period at which the Drake Passage (Chile - Antarctic Peninsula) opened, the Antarctic ice sheet began to form, sharp global cooling occurred accompanied with mass extinction of about 10 percent of species. Here we report the discovery of a new, hitherto unknown, asteroid impact structure of end-Eocene age. New seismic reflection data and petrological studies of drill cuttings from the Mount Ashmore structural dome, west Bonaparte Basin, Timor Sea, suggest the dome represents the central uplift of an extraterrestrial impact of end-Eocene, pre-Oligocene, age. The dome is located below a major unconformity. Isopach maps and reconstruction of structural depths reveal kinematic deformation patterns and structural orientations consistent with centripetal deformation toward the domal axis, as well chaotic megabreccia structures. A study of early Oligocene to early Jurassic drill cuttings from the Ashmore 1B petroleum exploration borehole reveals a dominance of flow-textured comminuted microbreccia with particles ranging to submicron scale, referred to as pseudotachylite. X-ray diffrcation analysis indicates the microbreccia includes about one third of poorly-diffracting particles, likely derived from carbonate and clay-dominated material. The microbreccia contains corroded relic quartz grains which display heavy irregular fracturing and incipient planar fractures. Similar pseudotachylite fragments occur in basal Oligocene sediments, likely representing erosion of the structural dome which protrudes above the pre-Oligocene unconformity. Scanning electron microscopy (SEM) coupled with Energy Dispersive Spectrometry (EDS) indicate the pseudotachylite consists of micron to tens of micron-size particles with low-totals and non-stochiometric heterogeneous compositions, including Si, Al-Si, Si-Ca-Al, Si-Al-Ca, Si-Mg, Fe-Mg-Ca, Fe-Mg-Ca, Fe-Mg and carbonate-dominated particles (see figure). No volcanic material or evaporites are encountered in the core, which militates against interpretations of the structure in terms of volcanic or salt dome origin. The chaotic seismic structure of the domal core, the centripetal sense of intra-domal deformation, the micro-brecciated flow-textured nature of the cuttings, the poorly diffracting nature of the pseudotachylite matrix and the heavily fractured state of quartz grains are consistent with an interpretation of the Mount Ashmore dome in terms of a central rebound uplift of an impact structure.