SEDIMENTOLOGICAL INFLUENCES ON VARIATION AND PRESERVATION OF DINOSAUR TRACKS IN THE CARRERAS PAMPA SITE, TORO TORO NATIONAL PARK, BOLIVIA

A single bedding surface at the Carreras Pampa site (CP bed) in the Toro Toro National Park, Central Bolivia hosts thousands of dinosaur tracks arranged in hundreds of trackways, including tail marks and swim tracks that vary in depth, expulsion rims, and track types. Our objective was to assess the degree of sedimentological variability in the tracked bed and determine its impact on track morphology. We collaborated with the Park to conduct fieldwork and collect samples. Our findings will improve understanding of track preservation in sand-sized sediments and be used by local docents when educating park visitors.

The CP bed, encased by claystones, is part of the Maastrichtian-Danian El Molino Formation. Within the study area, CP thickens toward the north and east, ranging from 30 cm to 70 cm. CP is exposed over approximately 7,400 m² and we traced the bed’s edge over 2 km.

Three main components comprise the tracked bed:: nested ostracod grains (NOGs), ooids, and siliciclastics, with proportions varying at the millimeter to centimeter scale with a small percentage of skeletal fragments and intraclasts. Calcitic matrix occurs in some laminae. Cements consist of calcite, primarily acicular syntaxial, with spar and microspar and traces of degraded hydrocarbons. Detrital textures consist of very fine-grained siliciclastics dominated by quartz with minor feldspar, along with fine-grained ooids and ostracods. NOGs are composed of 5–7 valves cemented by acicular cement around a micritic center. CP exhibits three predominant facies: wave-rippled, trough cross-bedded, and horizontally laminated and are composed of similar lithologies. Each facies contain burrows. Paleocurrent indicators trend towards NW-NE.

Ostracods and palynomorphs suggest a lacustrine environment. Siliciclastics were derived from extrabasinal sources, while the carbonate grains were intrabasinal. Vertical changes in the CP bed show a decrease in siliciclastic content up section, with the proportion of NOGs to ooids also increasing. The upper 10 cm of CP was uniformly reworked by waves, and tracks formed only on the upper surface. Track morphology was controlled by the degree of cementation and water saturation of the substrate while tracks were preserved due to early, partial, cementation followed by thick clay deposition as water depth increased.