158 Paleontology: Taphonomy, Paleoecology, and Diversity Dynamics (Posters)

Tuesday, 6 November 2012
9:00 AM - 6:00 PM
Hall B

MICROSTRATIGRAPHIC ANALYSIS OF BURROW-REWORKED DINOSAUR TRACK BED AT JOANNA'S TRACK SITE, CRETACEOUS GLEN ROSE FORMATION, GLEN ROSE, TEXAS

Paper number: 158-17
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

Although dinosaur trackways are common in the Cretaceous Glen Rose Formation of Texas, the recently discovered Joanna track site illustrates a unique ichnological relationship where dinosaur tracks were disrupted by invertebrate burrows made long after burial. In an effort to document the precise sequence of events, we described the interval from 0.3 m below the track layer through 2.7 m above it in a vertical outcrop adjacent to the track site, focusing on the 70-cm of strata immediately above the track horizon. An 8-meter N-S cross-section of this 70-cm interval was power-washed, examined for trace fossils, body fossils, and lithology in the field, and the interval sampled as a hand-cut 2-piece 20 cm square core. To prevent disintegration, the rock was coated in epoxy as it was removed. The lower piece was dry cut into a few 3-4 cm thick vertical slabs; the upper part will be prepared similarly. These are to be dry sanded and polished to reveal microstratigraphic details. A continuous series of thin sections will also be made through the thickness of the sample.

The track layer contains burrows of at least two ichnogenera, Arenicolites and Thalassinoides. The former is restricted to the track layer, whereas Thalassinoides originates in two generations, one before formation of the Arenicolites and another from the top of a highly bioturbated 30 cm bed of ostracod and foraminifera-rich mud overlying the track surface. This layer contains unidentified crustaceans, a turtle shell fragment, shark teeth, and small vertebrae. Capping this is a thin bed of pyritic mud; 20 cm of burrowed mud with bivalves in life position and Planolites; Thalassinoides (near top, filled with grainier sediment); and Teichichnus (spreiten showing retrusive movement). A 20 cm grainstone bed tops the interval. Beneath the tracks, the track layer wackestone grades into 20 cm of ostracod packstone with mud-filled burrows.

An upward increase in mud and deeper water fauna suggests a transgressive succession, and the burrowed mud bed overlaying the tracks suggests episodic burial events. Relationships between tracks and traces suggest that Thalassinoides and Arenicolites tracemakers colonized the track layer prior to the creation of tracks, then the second generation of Thalassinoides formed after burial of the dinosaur tracks by at least 30 cm of mud.