The Arauz Formation is a geological formation located in the Neuquén Basin of west-central Argentina. It is a lithostratigraphic unit of the Neuquén Group, predominantly composed of continental sedimentary rocks dating back to the Early Late Cretaceous period.
Location and Extent
The Arauz Formation is primarily exposed in the Neuquén Province and parts of Río Negro Province in Argentina, within the larger Neuquén Basin. Its outcrops are found along river valleys and eroded areas, offering insights into the regional stratigraphy.Lithology
The formation is characterized by a succession of coarse-grained clastic sedimentary rocks. Its lithology typically includes:- Sandstones: Dominate the sequence, often reddish-brown to reddish-purple, fine to coarse-grained, frequently arkosic or subarkosic, with cross-bedding indicating fluvial deposition.
- Conglomerates: Common, especially in the lower parts, consisting of well-rounded pebbles and cobbles of quartz, igneous, and metamorphic rocks, embedded in a sandy matrix.
- Siltstones and Claystones: Interbedded with sandstones and conglomerates, these finer-grained layers are often reddish and represent floodplain or overbank deposits.
- Calcrete paleosols: Occasional presence of ancient soil horizons, indicating periods of subaerial exposure and pedogenesis.
Depositional Environment
The sedimentary characteristics of the Arauz Formation indicate a [[Fluvial|fluvial]] depositional environment. It represents a system of braided and meandering rivers, with associated floodplains. The presence of large-scale cross-bedding suggests active river channels, while finer-grained sediments point to overbank flooding events and temporary standing water bodies. The overall reddish coloration is indicative of oxidizing conditions during deposition and subsequent diagenesis.Stratigraphy
The Arauz Formation is typically considered part of the [[Neuquén Group]], a thick sequence of Late Cretaceous continental deposits. Its exact stratigraphic position and status can vary slightly in literature, but it is generally understood as:- Lower Neuquén Group: It is often regarded as one of the basal units of the Neuquén Group in certain areas, or a lower member of the [[Candeleros Formation]] (part of the [[Rio Limay Subgroup]]).
- Relationship with Candeleros Formation: In some interpretations, the Arauz Formation is considered to laterally interfinger with or immediately underlie the Candeleros Formation, which is also part of the Rio Limay Subgroup.
- Underlying Units: It typically rests unconformably on older units, such as the [[Rayoso Formation]] (Early Cretaceous) or even Jurassic units, depending on the specific location within the basin.
Age
Based on biostratigraphic correlation and radiometric dating of associated units, the Arauz Formation is assigned an [[Early Late Cretaceous]] age. Specifically, it is correlated with the [[Cenomanian]] epoch (approximately 100 to 93.9 million years ago).Paleontology
While not as prolific in vertebrate fossils as some overlying formations of the Neuquén Group, the Arauz Formation has yielded some important paleontological discoveries, particularly [[Dinosauria|dinosaur]] remains. These include:- Fragments of large [[Sauropoda|sauropod]] dinosaurs.
- Isolated teeth and bone fragments of [[Theropoda|theropod]] dinosaurs.
- Evidence of [[Invertebrate paleontology|invertebrates]] and [[Paleobotany|paleoflora]] from floodplain deposits. These fossils contribute to understanding the terrestrial ecosystems present in South America during the early Late Cretaceous.