Early calcite cementation and its role in preserving a complex vertebrate track assemblage in the El Molino Formation, Torotoro National Park, Bolivia

Fine-scale timing of early cementation and its role in the preservation of vertebrate tracks in the Cretaceous El Molino Formation in Bolivia explain a composite stratigraphy of track overprinting. A remarkable bedding surface (7400 m2) in Torotoro National Park, Bolivia (Carreras Pampa; CP), exhibits the largest reported assemblage of dinosaur walking and swimming tracks and tail marks, and hosts abundant bird tracks. The CP bed is one of several resistant beds, less than 1 m thick, encased in 2–15 m of mudstone near the base of the middle member of the Maastrichtian–Danian El Molino Formation. The tracked bed prograded northward over mudstone and is sharply overlain by mudstone.

CP bed composition is dominated by nested ostracod grains (40%, 400 μm), fewer ooids and coated grains (550 μm), dispersed quartz (bimodal, 55 and 400 μm), and irregular quartz laminae. The upper centimeters of the CP bed are wave-ripple cross-laminated, and wave-ripple bedforms are preserved in patches on the upper surface.

The deepest tracks at the CP surface, cut wave-ripple bedforms where ripples occur. Medium-depth tracks generally overprint deep tracks and are overprinted by the shallowest tracks. Sediment excavated by invertebrates overlies dinosaur tracks, and both dinosaur tracks and burrowed sediment are overprinted by bird tracks. The youngest traces are swim tracks that cut all other features. A composite stratigraphy developed from overprinting relations shows increasing substrate firmness through time at the bed surface and indicates that the tracks survived initial flooding. This stratigraphy can be explained by early, progressive calcite cementation. This interpretation is supported by relatively open packing preserved by fibrous to micro-bladed isopachous cements, and by later pore-filling fibrous to microspar calcite cements.

Bedforms, overprinting, packing textures, and cement textures indicate preservation by early calcite cementation. Thus, track stratigraphy was a potent tool for unraveling fine-scale temporal events.