SEDIMENTOLOGY OF TRIASSIC DINOSAUR TRACKWAY LOCALITIES OF THE EASTERN ALPS, SWITZERLAND

The Late Triassic period was a time of rapid diversification of the major groups of dinosaurs (Sauropodomorpha and Therapoda). We investigated trackways of these groups in Late Triassic sediments in the Nature Park Ela at seven different stratigraphic levels within the Kössen Formation and the Hauptdolomit (HD), and at Piz S-Chalambert Dadaint (near Scuol) in the HD. Unequivocal sauropod footprints are in the lowermost part of the Kössen Formation (Alplihorn Member) in Parc Ela. They represent the oldest record of advanced sauropod tracks in the world. A refined model of the environment in which the trackways were made is necessary for a better understanding of early dinosaur behavior and to aid in future exploration of dinosaur fossils and trackways.

During the Triassic, central Europe was located at the northern edge of the Tethys Sea, a large embayment in eastern Pangea. The area was covered by warm shallow seas and dominated by large carbonate platforms. During the Late Triassic, 100’s of meters of limestone and dolomite were deposited. Deposition occurred in intertidal, supratidal and lagoonal settings on the landward edge of the platform, which was occasionally subaerially exposed and subjected to input of clastic sediment. At Scuol footprints are in the uppermost HD which consist of buff to grey shallowing-upward bioturbated and oolitic dolomite beds capped by stromatolites and trackways. A meter-thick cross-bedded quartzose sandstone unit lies about a meter beneath the trackways. At Parc Ela the Uglix Plattenkalk Member of the HD and the overlying Kössen Formation consists mostly of amalgamated thin beds of muddy limestones with varying amounts of sand-sized grains (packstone-wackestone), including shell fragments, intraclasts and pellets. Erosive bases and fining upward nature suggest deposition by storm events. The relatively low diversity of fauna suggests a restricted-marine lagoonal environment. Some of the beds are capped by thin layers of stromatolites and rarely by trampled surfaces (trackways), paleokarst surfaces or red paleosols, indicating periods of subaerial exposure. One layer consisted of organic-rich mud with a thin partially coalified interval and amber remains. Exposure surfaces cap shallowing upward sequences and are interpreted as third- and fourth-order sequence boundaries.