Introduction

Re-exploring classic localities and concepts can be vital in reshaping our present views or verifying the classic concepts. I recently was given the opportunity visit and collect one such locality—the Grand Canyon. The research trip was organized by Karl Karlstrom from the University of New Mexico and involved 14 days of a rafting trip down the Grand Canyon. One of the goals of the trip was to reinvestigate the Cambrian of the Grand Canyon, which included collecting fossil localities in the Tonto Group (we had National Park Service collecting permits). This was the first time in over 70 years that Cambrian fossils have been collected, since Edwin McKee.

Why is this important? Besides the reinvestigation of Charles Resser’s trilobite locations and taxonomy, the Grand Canyon has been a classic model of a transgressive package used in several geology/earth science textbooks. The model suggests that the Tapeats Sandstone is a diachronous transgressive sandstone (beach), migrating from west to east, followed by the Bright Angel Shale (off shore, marine), and then the Mauv Limestone (carbonate bank) progressing to eventually cover this portion of the craton.

Of course the original model proposed by McKee and Resser in 1945 was more complex with a transition zone between the Tapeats and Bright Angel and extensive intertongueing of the Bright Angel and Mauv. Work by Eben Rose has also shown that the stratigraphic relationships are more complicated. Being able to collect the old and new trilobite localities and the dating of detrital zircons allows us to test these models. One of the preliminary findings is the base of the Bright Angel is diachronous, with Glossopleura faunas found in the eastern Grand Canyon and Olenellus faunas found in the west. However, the radiometric dating of detrital zircons, which provides a maximum depositional age (work done by Mark Schmitz and his student Michael Mohr at Boise State University) provides some breakthroughs: 1) the Sixtymile Formation below the Tapeats Sandstone is Cambrian, not Precambrian as previously assumed; 2) The transgression of the Cambrian environments did not occur in 40-60 million years, but only in about 10 million years; and 3) the extinction of Olenellus occurred after 506.5 million years ago, long after the appearance of Paradoxides in Europe.

Sincerely trilobitic,

Fred Sundberg

Show Low

Arizona

freddeb85@cableone.net


Links

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The Trilobite Papers 20 are part of the PaleoNet system. Sponsorship of The Palaeontological Association is gratefully acknowledged.