| Chacarilla Formation | |
| Type: | Geological formation |
| Period: | Tithonian |
| Age: | Oxfordian-Aptian ~ |
| Prilithology: | Sandstone, shale |
| Namedfor: | Quebrada de Chacarilla |
| Region: | Tarapacá Region |
| Coordinates: | -20.6°N -69.1°W |
| Paleocoordinates: | -21.5°N -36.1°W |
| Underlies: | Cerro Empexa Formation |
| Overlies: | not exposed |
| Thickness: | min. 1100m (3,600feet) |
| Extent: | Tarapacá Basin |
The Chacarilla Formation (Spanish; Castilian: Formación Characilla) 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 (Spanish; Castilian: Santuario de la Naturaleza) in 2004.[1]
The formation comprises a sequence of rhythmically alternating shales and red sandstones with a minimum thickness of 1100m (3,600feet). 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.[2] The top of the formation is not younger than Aptian.[3]
Fossil stegosaur, sauropod and theropod tracks and fossil flora have been reported from the formation.[4]
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 39cm (15inches) and average width 32cm (13inches)). Two other trackways consist of small theropod footprints (less than 25cm (10inches) long).[5] The other ten trackways were made by large theropods (footprint length more than 30cm (10inches)). The large theropod tracks are tri− and tetradactyl, mesaxonic, and have lengths and widths between 31cmto65cmcm (12inchesto26inchescm) and 21cmto46cmcm (08inchesto18inchescm), respectively. Nearly all digit impressions possess claw marks, but they lack clear impressions of digital pads. The stride length varies between 230cmand307cmcm (90inchesand121inchescm). The speed of the dinosaurs leaving the tracks is estimated at 4km/h7km/h.[6]
Additionally, in the Jurassic part of the formation, fossil flora was reported, containing fossils of Posidonomya, Perisphinctes,[7] Baiera sp., Brachyphyllum sp., Cladophlebis sp., Dictyophyllum sp., Equisetites sp., Nilsonia sp., Pterophyllum sp., Ptilophyllum sp., Taeniopteris sp., and Filicales.[8]