Paper No. 178-15
Presentation Time: 5:15 PM-5:30 PM
BILODEAU, William L., Dept. of Geology, California Lutheran Univ, 60 W. Olsen Rd. #3700, Thousand Oaks, CA 91360,

Quartzite cobbles in the Upper Triassic Shinarump Conglomerate (basal member of the Chinle Formation) exposed along the Little Colorado River from Winslow to Holbrook, Arizona are covered with unusual pockmarks and fractures. These brittle deformation features include circular to semicircular millimeter- to centimeter-sized pockmarks or shallow depressions underlain by bright halo zones of intense microfracturing. Commonly, several larger radial fractures diverge outward from these depressions or they are surrounded by concave spallation fractures. The cobbles typically are well rounded, polished and range in size from 3-10 cm in diameter. These fabric elements are best developed on cobbles in clast-supported beds and are absent from cobbles in matrix-supported zones or on isolated cobbles in sandstone beds.

In this area, the Shinarump Conglomerate is a braided stream deposit that fills paleovalleys eroded into the Lower Triassic Moenkopi Formation. The outcrops between Winslow and Holbrook contain the coarsest known sections of the Shinarump. At Holbrook, well-bedded, clast-supported cobble conglomerate up to 10 meters thick is interbedded with coarse sandstone beds. To the west, towards Winslow, the thickness of the conglomerate decreases and is restricted to 0.1 - 5 meter thick lenses enclosed in coarse sandstone. Regionally, the Shinarump is coarse sandstone containing thin lenses of pebble to cobble conglomerate with rare clast-supported zones.

Similarly ornamented cobbles in the Triassic Buntsandstein conglomerates in northeastern Spain occur near two meteorite impact sites and the fabric has been attributed to post-depositional clast-to-clast impact deformation due to the propagation of subsurface shock waves from the meteorite impacts. The close proximity of the Shinarump exposures, containing a similar shock deformation fabric, to the Barringer Meteorite Crater (Meteor Crater), suggests that these features may be the result of the impact of the Barringer Meteorite 49,000 years ago. A much older impact at an, as yet, unknown location, is also being considered.

2002 Denver Annual Meeting (October 27-30, 2002)
Session No. 178
Impact Stratigraphy
Colorado Convention Center: A102/104/106
1:30 PM-5:30 PM, Tuesday, October 29, 2002

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