Middle Jurassic Mammalian and dinosaur footprints and petrified forests from the volcaniclastoc La Matilde Formation.

RICARDO NESTOR MELCHOR; SILVINA DE VALAIS,; FERNANDO GENISE, JORGE

First International Congress on Ichnology, Fieldtrip Guidebook

Ichnia 2004

Lugar: Trelew; Año: 2004; p. 47 - 63

The main purpose of the day three of the fieldtrip is to visit the exposures and quarries of the La Matilde Formation at Estancia Laguna Manantiales (ELM). This unit contains a diverse tetrapod track assemblage and some invertebrate trace fossils, in addition to abundant permineralized plant remains. At the same locality we also will examine paleosols with a particular ichnofabric from the overlying Bajo Tigre Formation, which is composed of Early Cretaceous yellowish tuffaceous sediments that form the border of mesas at the estancia. The visit to this area will give an opportunity to contrast paleosols developed in similar substrates of different age (Middle Jurassic and Early Cretaceous) with those visited at the previous stops. In addition, we will also stop at the Monumento Natural Bosques Petrificados (Petrified Forests Natural Monument, PFNM). ªóis national reserve contains huge araucarian petrified logs, which occur in the same Jurassic volcaniclastic succession exposed at Estancia Laguna Manantiales. ªóe area to be visited is located at the southern part of Patagonia and the northeast corner of the Santa Cruz province (Fig. 1, Introduction). Main neighboring cities are at the seaside, mainly Puerto Deseado and Puerto San Julian to the southeast and Caleta Olivia and Comodoro Rivadavia to the northeast. The area is closer to the ocean (about 300 km) than to the Andes (500 km). ªóe relief is low and dominated by basalt-topped mesas, as in most of the extra-Andean Patagonia. Exposures are located in small gulchs and canyons crossed by dry (ephemeral) creeks and rivers. The most important of these courses is the Deseado river, which we will cross on national route 3 on the way to Petrified Forests Natural Monument. Traditionally, the economy of the region is based on sheep raising. However, the recent discovery and development of epithermal gold deposits related to the Jurassic volcanism is an important alternative activity. Petrified wood, cones, and seeds used to be common in the area to be visited but they have become almost ¡°extinct¡± due to the activities of fossil hunters (Andreis et al. 1992). Remains of Indian arrowheads, flakes and stone artifacts are also abundant at the field. ªóey were commonly made using silicified wood. Institutional abreviations used in the text are: MLP, Museo de La Plata and MPEF-IC, Icnolog¨ªa, Museo Paleontol¨®gico ¡°Egidio Feruglio¡±.