HOW LONG DID DINOSAUR TRACKS ACCUMULATE ON A SINGLE BED? SEQUENCE AND TIMING OF DEPOSITION, BURROWING, TRACK MAKING, AND LITHIFICATION IN THE MAIN TRACKLAYER OF THE PALUXY RIVER, LOWER CRETACEOUS LOWER GLENN ROSE FORMATION, GLEN ROSE, TEXAS

The Cretaceous Glen Rose Formation exposed in the Paluxy River contains horizons of well-preserved dinosaur tracks. The main tracklayer contains a high density of theropod and sauropod trackways. One of the most dense and extensive of the track sites in this layer is known as the Blue Hole Ballroom. This study seeks to find clues to the sequence of events from the initial deposition to the final lithification and burial of track-bearing layers, utilizing a sample collected from the main tracklayer at the Blue Hole Ballroom.

The main tracklayer at this site is a dolomitic marl averaging approximately 20 cm thick. Below is a fine quartz silt, free of macrofossils, and above is the “Steinkern Marl”, a rubbly-bedded unit containing abundant marine fossils including clams, oysters, dasyclad algae, and small serpulid mounds. Swales several meters across are apparent on the surface of the main tracklayer, and variation in depth, outline definition and inferred foot position in different footprints of single trackways suggests that these swales were depositional, and that the consistency of the tracklayer varied spatially. The tracklayer is perforated by vertical burrows ranging 4 to 10 mm in diameter.

Thin sections reveal that silt inside burrows was derived from the underlying silt. Element mapping shows that the bed contains finely disseminated pyrite, which may explain iron staining in dinosaur tracks.

The first sample was a loose piece chosen so as not to disturb any visible tracks. This was sliced both vertically and horizontally. Cutting revealed that burrows are vertical. The vertical sections show that, along the “missing” edge of the sample, these burrows are bent and compressed, presumably by the edge of a dinosaur footprint. This suggests that vertical burrowing preceded dinosaur track making.

Other than bending and compression under the apparent dinosaur footprint, there is no evidence of compression in the tracklayer burrows. This suggests that the layer may have solidified at least somewhat before or soon after burial. It is possible that, along with burrow density, mineralogical and textural clues might help constrain the timing from initial bed deposition to solidification and burial, thereby constraining the temporal span during which the bed was capable of recording tracks.