Santa Juana Formation a

Late Triassic The Late Triassic is the third and final epoch (geology), epoch of the Triassic geologic time scale, Period in the geologic time scale, spanning the time between annum, Ma and Ma (million years ago). It is preceded by the Middle Triassic Epoch a ...

(

Carnian The Carnian (less commonly, Karnian) is the lowermost stage (stratigraphy), stage of the Upper Triassic series (stratigraphy), Series (or earliest age (geology), age of the Late Triassic Epoch (reference date), Epoch). It lasted from 237 to 227.3 ...

stage) sedimentary rock formation near Santa Juana in the lower course of the

Biobío River The Biobío River (also known as Bío Bío or Bio-Bio) is the second largest river in Chile. It originates at Icalma and Galletué lakes in the Andes and flows to the Gulf of Arauco (in Spanish) on the Pacific Ocean. The major tributaries of ...

in south-central Chile.Santa Juana Formation
at

Fossilworks Fossilworks was a portal which provides query, download, and analysis tools to facilitate access to the Paleobiology Database, a large relational database assembled by hundreds of paleontologists from around the world. History Fossilworks was cr ...

.org Lithologies range from conglomerate

sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...

arkosic Arkose () or arkosic sandstone is a detrital sedimentary rock, specifically a type of sandstone containing at least 25% feldspar. Arkosic sand is sand that is similarly rich in feldspar, and thus the potential precursor of arkose. Components ...

sandstone,

siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility. Although its permeabil ...

and

mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from ''shale'' by its lack of fissility.Blatt, H., and R.J. Tracy, 1996, ''Petrology.'' New York, New York, ...

.Nielsen, 2005

Description

The sediments that consolidate into the rocks of the formation deposited in

alluvial Alluvium (, ) is loose clay, silt, sand, or gravel that has been deposited by running water in a stream bed, on a floodplain, in an alluvial fan or beach, or in similar settings. Alluvium is also sometimes called alluvial deposit. Alluvium is ...

, fluvial, lacustrine and

playa lake A dry lake bed, also known as a playa (), is a basin or Depression (geology), depression that formerly contained a standing surface water body, which disappears when evaporation processes exceed recharge. If the floor of a dry lake is covered b ...

environment. Overall, the formation is rich in

plant fossil Paleobotany or palaeobotany, also known as paleophytology, is the branch of botany dealing with the recovery and identification of plant fossils from geological contexts, and their use for the biological reconstruction of past environments (pale ...

s. The formation was deposited over the rocks of the

Coastal Batholith of central Chile The Coastal Batholith of central Chile is a group of plutons in the Chilean Coast Range of Central Chile appearing contiguously from 33° S to 38° S. At a latitude of 40° S an outlying group of plutons of the batholith appear in a more eastward ...

in a sedimentary basin that formed along the Gastre Fault. It has been posited that the basin developed as a

rift In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics. Typical rift features are a central linear downfaulted depression, called a graben, or more commonly a half-graben ...

during the early break-up of Gondwana in the Triassic.