U/PB AGE FOR A PIPELINE DINOSAUR SITE: NEW TECHNIQUES AND STRATIGRAPHIC CHALLENGES

In September of 2006 a multi-taxon bone bed in the Upper Jurassic Morrison Fm. was discovered on the McKinsey Ranch south of Laramie, WY, directly in the path of the Kinder Morgan Rockies Express Pipeline (REX). While some fossil bone was impacted by the construction, approximately 20,000 pounds of additional bone and rock matrix were removed intact from the site. The fauna includes adult Camarasaurus, juvenile Apatosaurus, juvenile and adult Allosaurus, an as-yet unidentified theropod, crocodile, turtle, choristodere, and fish. The preparation of these fossils is currently underway in the University of Wyoming Geological Museum as a public exhibit.

During the excavation of the McKinsey-REX site, rock matrix was collected for radiometric dating. The matrix was a moderately smectitic mudstone which is interpreted to be a slightly reworked tuff. The rock was disaggregated using an ultrasonic method, and zircons were concentrated using conventional separation techniques. Zircons were dissolved and analyzed by a chemical abrasion method (CA-TIMS)to reduce the effects of lead loss. Of 5 analyses, 3 yielded concordant data. The date recommended is 152.37±62 Ma and includes variation based on possible source regions for the volcanism as well as U decay errors.

Without the pipeline construction, this locality would never have been found. The locality is in the Laramie Basin, with no outcrops of any kind nearby making stratigraphic placement difficult. A small outcrop of resistant freshwater limestone occurs in the Morrison Fm. a mile from the site along the pipeline trench, and a strike and dip were taken from this outcrop. During the salvage of the bone bed there was no opportunity to take detailed notes of the rock exposed in the trench between this outcrop and the locality, although a preliminary description was recorded. Construction of the Overland Pass Pipeline through this same area during the winter of 2008 will give us the opportunity to collect enough data to place this important locality in stratigraphic context.