Gifford H. Miller John W. Magee Marilyn L. Fogel Matthew J. Wooller Paul P. Hesse Nigel A. Spooner Beverly J. Johnson Lynley Wallis
A bolide that impacted NW Australia during the Late Quaternary left a circular depression more than 100 m deep and nearly a kilometer in diameter, with a crater rim ∼30 m above the regional terrain. T..
A bolide that impacted NW Australia during the Late Quaternary left a circular depression more than 100 m deep and nearly a kilometer in diameter, with a crater rim ∼30 m above the regional terrain. The resultant crater is a window into the regional water table. The surface of the contemporary central pan is 25 m below the adjacent terrain, coincident with the late Holocene regional water table modified by local evaporative processes. Shielded from aeolian deflation by the crater rim, the central depression has slowly filled with dust, sand, and chemical precipitates, estimated to be 20–100 m thick based on geophysical surveys, one of the few continuous depocenters in the Australian Arid Zone. The nature of the crater's sediment fill is controlled by interactions between the water table, primarily in response to changes in summer monsoon rain, changes in the delivery of sand and dust to the crater by the prevailing easterly winds, and the level of the sedimentary fill surface. Optically Stimulated Luminescence (OSL) and 14C dates constrain an age model indicating the upper 10 m of sediment fill recovered from the central pan span the past ∼60 ka. The lowest 3 m consist of clayey sand deposited in perennial water during Marine Isotope Stage (MIS) 3. The water table subsequently dropped rapidly ∼35 ka and remained more than 7 m below the late Holocene level through most of MIS 2, during which 2 m of sandy clay was deposited on a dry crater floor, confirming a dry and dusty Last Glacial Maximum (LGM) climate. By 14 ka a rising water table intersected the crater surface, modifying the upper 50 cm of LGM sediment, and syndepositionally modifying another 60 cm of subsequent sandy clay deposition. Aeolian sediment delivery effectively ceased ∼13 ka, and the upper 4.8 m is a gypsum-dominated precipitate, which initially accumulated rapidly, before equilibrating with the late Holocene water table shortly after 6 ka. Lacustrine carbonate encrustations on rocks at the base of the crater wall and ∼4 m above the central pan with 14C ages >40 ka document a time when regional groundwater maintained a water body in the crater 3.5–4.5 m above the modern groundwater level. The crater wall deflected the prevailing easterly winds, creating a horseshoe-dune extending westerly on both sides of the crater, with an extension rate of 35 m ka−1. An augered hole through the northern dune revealed 10 m of sediment overlying ferricrete. The lowest meter is a mixture of broken ferricrete and sand that we interpret to be debris from the bolide impact. Three OSL dates through the dune project an age for the debris-dune contact of 120 ± 10 ka. Changes in physical properties and bulk sediment δ13C through the 9 m of aeolian sediment indicate the lowest 1.8 m was deposited during MIS 5 (120–85 ka), under a uniformly wetter climate than present. The overlying 4.3 m of sediment was deposited between 85 and 14 ka (MIS 4, 3, 2) and exhibits transitional characteristics between the lower unit and the upper 3.8 of sand, which was deposited primarily during the Holocene. Large changes in the regional water table occurred over the past 60 ka, including an LGM water table persistently ≥7 m lower than late Holocene levels, and 3.5–4.5 m higher prior to 40 ka, plausibly in MIS 5, indicative of a stronger Australian Summer Monsoon than at any time subsequently. Age models and sediment properties from the two sedimentary records indicate the crater was formed >60 ka and most likely ∼120 ka, more recently than previous estimates.