Earth History Research and News

The Coconino Sandstone (Permian) of Grand Canyon, Arizona, is conventionally thought to have been deposited in a desert environment. However, studies of fossil trackways preserved in the Coconino have caused some geologists to question the traditional aeolian interpretation. A new study of sand-filled cracks penetrating the Hermit Shale at the base of the Coconino also suggests that alternative depositional models should be considered. These cracks have usually been thought to be ‘mudcracks’ caused by desiccation in a terrestrial environment, but there are several features of the cracks that render this interpretation suspect. For more information, here’s the abstract of the new study. A full-text PDF of the article can be downloaded by Creation Research Society members.

Whitmore JH, ‘Origin and significance of sand-filled cracks and other features near the base of the Coconino Sandstone, Grand Canyon, Arizona, USA’, Creation Research Society Quarterly 2005;42(3):163-180.
http://www.creationresearch.org/crsq/abstracts/Abstracts42-3.htm

Abstract: Conventional geology proposes that the Coconino Sandstone formed when wind-blown desert sand migrated over the mud-cracked floodplains of the Hermit Formation. The contact between these two Permian formations was studied along ten trails in the Grand Canyon. Special attention was given to sand-filled cracks that occur at the base of the Coconino penetrating the Hermit, features usually interpreted as mud cracks. The most notable cracks are widest (up to 25 cm) and deepest (up to 10 m) along the Bright Angel Fault on the South Rim. Cracks are always present near major faults, but become narrower, shallower and are sometimes absent altogether as horizontal distance from faults increases and vertical displacement along faults decreases. Vertical laminations within the cracks, U-shaped cracks, cracks that dissipate upwards, slickensides not caused by faulting and other features make the mud crack theory suspect. They might be better explained as clastic dikes (or sand intrusions) which originated by injection during tectonic activity after the deposition of the Coconino Sandstone. Evidence near the base of the Coconino such as load casts, burrows and vertebrate trackways, suggests the Coconino was rapidly deposited in an aqueous environment. Cross-cutting relationships indicate the Bright Angel Fault was active during the Precambrian, then quiescent until the Cenozoic (Miocene to Pliocene). If the clastic dikes were caused by tectonic activity, either the Coconino was unlithified or only partially lithified in excess of 200 million years (unlikely, in a conventional scenario) or that only a short amount of time passed between deposition and faulting, greatly reducing the supposed duration of geologic time.

The accompanying photograph shows the abrupt contact between the Coconino Sandstone (above) and the Hermit Shale (below) exposed along the Bright Angel Trail in Grand Canyon, Arizona.