MICROSTRATIGRAPHY OF A UNIQUE BONEBED NEAR THE BASE OF THE OWL ROCK MEMBER OF THE CHINLE FORMATION, PETRIFIED FOREST NATIONAL PARK

In 2008, Yale Peabody Museum field crew discovered a unique, dense association of bones in the Chinle Formation from the Painted Desert portion of Petrified Forest National Park. Subsequent excavation has so far yielded a nearly complete, semi-articulated skeleton of the aetosaur Typothorax, numerous Typothorax plates, isolated rauisuchian teeth, and a partially complete, disarticulated large phytosaur. This bonebed is unique among finds in the park to date in that the bones are extremely well preserved, yet very delicate, articulated, highly concentrated, and representing multiple species. This locality marks the highest occurrence of Typothorax in the park and the first good material collected from the Owl Rock Member.

The largest bone accumulation lies at the bottom of a lens of alternating beds of sandstone and conglomerate with an areal extent of around a hundred square meters. The deposit is bounded top and bottom by pedogenic purple shales.

The lens has a maximum thickness of nearly two meters and is comprised mostly of red sandstones, interbeded with thinner layers of red conglomerate. Sandstone grain size ranges from very fine to very coarse sand, with the conglomerates mostly of small pebbles. Gray nodules, presumably from local reduction, are common throughout all layers and range in size from barely visible to three or four centimeters. Phytosaur teeth are frequently present in the larger nodules. Bone appears to lie predominately at the top of the conglomerate layer near the sandstone contacts. The individual layers of sandstone and conglomerate increase in thickness from roughly ten centimeters at the top, to over 20 cm at the bottom of the section. The thickest layer is 81 cm of sandstone containing ripple marks just above the main bone layer. At the top of the section the bone fragments are heavily weathered and rounded from transport, but the material improves greatly in the lower conglomerate layers. Based on the stratigraphy, we infer that the material was collected and preserved during a flood event, perhaps in a chute bar or transverse bar.