New Mexico Geological Society Annual Spring Meeting — Abstracts

One of the key questions surrounding mass accumulations of fossil bones of any age is the mechanism of the bonebed's formation. Various characteristics of the assemblage can be used to interpret how the bones were deposited en masse. Depending upon the site, several explanations can be proposed. The Upper Triassic Snyder Quarry (NMMNH locality L-3845) in the Petrified Forest Formation of the Chinle Group in northern New Mexico has excellent potential to further explore the question of bone bed formation. The main bone-bearing horizon is less than 0.3 m thick and, in initial stratigraphic assays, the northeastern edge of the unit, as well as the next stratigraphically higher conglomeratic unit, slope gently upwards until they are truncated by a horizontal conglomeratic bed. An initial interpretation suggets that the bones are concentrated in an ancient stream channel with a depositional dip down to the southwest. Examination of the lateral extent of the main bone horizon and the condition of the bones themselves will help to resolve further how the assemblage developed. The bonebed itself is exceptionally diverse with both invertebrates and vertebrates preserved. All of the larger bones are disarticulated exceot for some aetosaur scutes, and some of the individual coelophysids are still associated. Also, exceptionally fragile skeletal elements are preserved together with larger and stronger bones.

Bones from the Snyder Quarry are notable for their deep black color and, in some cases, light purple iridescent sheen, preservation that is unique to the Chinle. A fragment of an aetosaur scute from the quarry was analyzed using an energy dispersive spectrometer (EDS) on a JEOL Scanning Electron Microscope (SEM) at the University of New Mexico. To account for the color of the bones, we expected to find a high degree of manganese present in the fragment. However, the SEM spectra recorded very low levels of manganese and unexpectedly high amounts of titanium in the scute fragment. The titanium would explain the Iridescence seen on the surface of numerous bones. A probable source of the titanium is the volcanic detritus found in the Chinle sediments, especially the bentonitic and smectitic muds possibly derived from the volcanics associated with the opening of the Gulf of Mexico. If the source of the titanium is volcanogenic, then it is highly probable that portions of the bone are being replaced by a mineral such as ilmenite. Further analysis of clay samples and multiple bone fragments throughout the quarry and from different species will perhaps yield further information about the elements that were incorporated into the bones as they fossilized.