* Astronomy

Spider is an impact structure (or astrobleme), the deeply eroded remnant of a former impact crater, situated in the Kimberley Region of northern Western Australia, 18 km east of the Mount Barnett Roadhouse on the Gibb River Road. Due to very rugged terrain the site is effectively inaccessible. The name is derived from the visually striking spider-like radiating ridges of quartzite prominently visible from the air or on satellite images.    
The age of the Spider impact event is not accurately constrained, but it has been argued it occurred after gentle folding of the Palaeoproterozoic quartzite, but before a regional episode of glacial erosion; if correct it occurred between 900-600 Ma i.e. during the Neoproterozoic.

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Longitude:  16°44'34.92"S,  Latitude: 126° 5'21.89"E


Title: Spider impact structure, Kimberley Plateau, Western Australia: interpretations of formation mechanism and age based on integrated map-scale data
Author:  Abels, A.

The centre of the 13 x 11 km Spider impact structure, Western Australia, displays an unusual system of eroded folds and imbricated thrusts surrounding a sandstone dome. As inferred from GIS-integrated remote sensing, geological and digital elevation data, the structural setting of the original crater was influenced by, and hence post-dates, the formation of the Mt Barnett Syncline, the east-west-oriented axis of which runs through the Spider structure. The syncline formed during the regional Yampi Orogeny ( ca 900 Ma), thus constraining the maximum age of the impact event. The sandstone dome in the centre of Spider formed prior to the imbrication, as interpreted from the present setting that indicates a deflection of the southward moving material during the crater collapse. Two modes of formation are discussed in order to explain the south-directed shortening in the Spider impact structure: (i) impact into the bottom of a syncline-controlled palaeovalley leading to uplift of the central crater floor followed by gravity-driven asymmetric sliding preferentially from the northern crater wall and valley slope, respectively; and (ii) moderately oblique (~10 - 30°) impact from the north onto the axis of the syncline, producing a central uplift under the influence of downrange residual momentum and, thus, asymmetric deformation inside the uplift and farther downrange. Neither model alone explains all the observations, and only a combination of both may provide a satisfactory solution.

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