Chacarilla Formation
Introduction The '''Chacarilla Formation''' () is an Oxfordian to Early Cretaceous geologic formation of the Tarapacá Basin in northern Chile, close to the border with Bolivia. The marine and fluvial formation preserves several dinosaur trackways and has been declared a Natural Sanctuary () in 2004.
Description Stratigraphic column of the formationThe formation comprises a sequence of rhythmically alternating shales and red sandstones with a minimum thickness of . The lower part of the formation was deposited under marine conditions and the upper part in a meandering river floodplain and point bar environment. Paleocurrent analysis demonstrated a flow direction towards the west, northwest and west-northwest.The formation contains ichnofossils of theropods and ornithopods, occurring in the Early Cretaceous upper part of the formation, which is marked by an angular unconformity, overlain by volcanic and clastic rocks of the Late Cretaceous to Early Paleocene Cerro Empexa Formation. The top of the formation is not younger than Aptian.
Fossil content Dinosaur tracksite of the Chacarilla FormationFossil stegosaur, sauropod and theropod tracks and fossil flora have been reported from the formation.The fourteen trackways of the Chacarilla III tracksite consist of 76 individual footprints. Two of the trackways consist of large ornithopod footprints (average footprint length  and average width ). Two other trackways consist of small theropod footprints (less than  long). The other ten trackways were made by large theropods (footprint length more than ). The large theropod tracks are tri− and tetradactyl, mesaxonic, and have lengths and widths between  and , respectively. Nearly all digit impressions possess claw marks, but they lack clear impressions of digital pads. The stride length varies between . The speed of the dinosaurs leaving the tracks is estimated at .Additionally, in the Jurassic part of the formation, fossil flora was reported, containing fossils of ''Posidonomya'', ''Perisphinctes'', ''Baiera sp. '', ''Brachyphyllum sp. '', ''Cladophlebis sp. '', ''Dictyophyllum sp. '', ''Equisetites sp. '', ''Nilsonia sp. '', ''Pterophyllum sp. '', ''Ptilophyllum sp. '', ''Taeniopteris sp. '', and Filicales.
See also * List of dinosaur-bearing rock formations** List of stratigraphic units with ornithischian tracks*** Stegosaur tracks* Arcabuco Formation, contemporaneous ichnofossil-bearing formation in Colombia* La Puerta Formation, contemporaneous ichnofossil-bearing formation in Bolivia* Baños del Flaco Formation, contemporaneous ichnofossil-bearing formation in central Chile
References === Bibliography ===* *  50px Material was copied from this source, which is available under a  Creative Commons Attribution 4.0 International License. * *
Further reading * C. G. Oliver and C.A. Menéndez. 1968.Geología de la Quebrada Juan de Morales, Tarapacá, Chile y su flora Jurásica. Terceras Jornadas Geologicas Argentinas 163-171* R. J. Dingman and C. Galli O. 1965.Geology and ground-water resources of the Pica area, Tarapacá Province, Chile. United States Geological Survey Bulletin 1189:1-113