MICROSTRATIGRAPHY AND FAUNA OF A SINGLE BLOCK FROM THE LATE TRIASSIC WHITAKER (COELOPHYSIS) QUARRY (ROCK POINT FORMATION: CHINLE GROUP), NORTH-CENTRAL NEW MEXICO

Abstract

New Mexico Museum of Natural History’s Apachean age Whitaker Quarry block (C-8-82) has been under preparation since September, 2003. The volume of the 2-m-long, 1.4-m-wide, and 1.1-m-thick block is ~2 m3 . Microstratigraphic study of the block shows two fining upward sequences above a basal sandy siltstone. The upper 5 cm of this bed contains the ostracode, Darwinula , and the conchostracan, Lioestheria , that likely indicate a topographic low containing a (probably ephemeral) pond. This is the first report of calcareous microfossils from the Whitaker Quarry. The basal coarse silt and very fine sand of sequence one fines up to very fine silt with clay lenses. Sequence two has a basal matrix-supported conglomerate that fines up to very fine silt. All beds above the invertebrates contain elongate rip-up mud clasts that are closely aligned and trend WNW-ESE. These flow indicators agree well with the established scheme of a northwest-draining Chinle basin. The block stratigraphy indicates two flood events of increasing energy. To date, the fauna from the block comprises the aforementioned invertebrates; the redfieldiid fish Synorichthyes , the coelacanth fish Chinlea , the archosauromorph Vancleavea , a phytosaur, probably pertaining to Redondasaurus gregorii , and the theropod dinosaur Coelophysis bauri. Additionally, an indeterminate redfieldiid-like fish, several isolated enigmatic scutes, and teeth of a possible sphenosuchian or ornithischian dinosaur are present. In general, the fossil material coarsens up throughout the block. Sequence one contains (ascending order) the invertebrates, fish scales and bones, whole fish, non-dinosaurian tetrapods and