Large-impact triggered deposits in the deep sea are usually massflows and turbidites. These deposits can be distinguished from normal turbidites by the incorporation of impact ejecta, such as microtektite-like spherules, but otherwise are thought to have no special diagnostic features. Numerous K/T boundary outcrops in eastern Mexico contain a sandstone layer whose origin is clearly related to the Chicxulub impact because it contains impact ejecta from Chicxulub such as tektite-like droplets and spherules. However, often these clastic layers do not display the standard Bouma Ta-Te sequence, but have repeated graded intervals, reversed current directions and channel-like geometry. Stinnesbeck et al. (2001) describe in the same sections up to four separate spherule layers supposedly separated by normal Cretaceous pelagic intervals. We found these ‘normal’ layers to consist of large, soft, rip-up clay blebs. The interpretation that each layer represents a separate impact is unlikely, because the tektite-like and the limestone ejecta grains are identical in all three layers. The probability of three impacts producing exactly the same ejecta seems extremely small. The Late Archean Wittenoom (spherule) layer in the Hamersley basin (Western Australia) often occurs in the form of a single turbidite, with impact spherules at the base. In places such as the Wittenoom Gorge, however, the layer is complex, shows repeated grading and reversed current directions, and is quite similar to the K/T sandstone layer. We interpret such complex sandstone layers as tsunami wave-induced deposits and/or turbidites modified by tsunami-induced currents and countercurrents.
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Dawn Y. Sumner
Department of Geology
University of California
Davis, CA 95616
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