La Boca Formation

The La Boca Formation is a geological formation in Tamaulipas state, northeast Mexico. It was originally thought to date back to the Early Jurassic, concretely the Pliensbachian stage epoch of 193-184 Ma. Later studies found that while the unit itself was likely deposited during the earliest Pliensbachian, as proven by zircon dating 189.0 ± 0.2 Ma, the local vulcanism (related to the aperture of the Atlantic Ocean and the several Rift Events) continued until the Bajocian.

However, the lower section of the fossil taxa deposited on the rocks above the La Boca Formation is likely of Late Pliensbachian-Lower Toarcian age, and the upper section of Late Toarcian-Late Aalenian age.

Due to successions of Aalenian depositional systems on the upper layers of the Huizachal Canyon, has been delimited the formation to the Toarcian stage, being the regional equivalent of the Moroccan Azilal Formation. Deposits of Late Triassic Age referred to this unit have been reclassified in a new formation, El Alamar Formation. In North America, La Boca Formation was found to be a regional equivalent of the Eagle Mills redbeds of southern United States, the Todos Santos Formation of southern Mexico and the Barracas Group of the Sonora desert region.

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Paleoenvironment

La Boca Formation is genetically related to the Nazas volcanic Arc of the same age (Pliensbachian-Aalenian, ~189.5–171.6 Ma), which was created when Mexico evolved in a convergent plate margin, with the Gulf of Mexico remaining as a restricted basin and a passive margin. The influence of this arc is seen on the continental units such as Todos Santos Formation, which deposited volcanic materials in both nonmarine strata and marginal marine red beds of eastern Mexico.

La Boca Formation left its sediments on a basin formed between the Nazas Volcanic Arc center and the so-called Huizachal-Peregrina Anticlinorium, giving the basin layers whose origin is linked with braided river deposits with different flooding levels, channels fills, and channel belts filling valleys.

In locations such as Aramberri, the development of fluvial channels and the flooding of surfaces was restricted due to the presence of flanking volcanic activity, as well the local Paleozoic basement highs. In this outcrop the fluvial system evolved in several ways. Towards the north, it meanders from braided to ephemeral sandy, with the presence of common laminated sands sheets. The latter are likely a local indicator of unconfined flash floods across floodplains, with some sections recovering periods of desiccation thanks to the presence of mudcracks.

Towards the south, in localities such as El Olmo Canyon, the layers show gravelly braided rivers, oriented east-west, which then evolve into high-sinuosity single-thread meandering rivers. Other southern localities, such as the Caballeros Canyon and Huizachal Canyon, have layers that record gravel-bed braided rivers over a floodplain with high-energy flows: an element recorded on the local stratigraphy as older layers which were highly degraded by the increased force of the younger flows. The rock fragments moved by the currents are bigger in upper layers. In the southernmost outcrop, in Miquihuana the sheet sands show greater flooding events than on any other location.

In the main fossiliferous level of the Huizachal Canyon, in which more than 8000 specimens have been found, the preservation of delicate specimens such as Pterosaurs suggests an environment with little transportation and reworking. Yet the fossils were not buried in situ, as most of the smaller specimens show disarticulation.

All data trends suggest a highly unusual debris-flow environment in which local fluvial alluvial bodies were not big enough to sustain large freshwater biota such as fishes, and most of the preserved specimens were fast-buried near the place of death.

Fossil record

Ichnofossils

Synapsida

Lepidosauromorpha

Sphenodontia

Pterosauria

Crocodylomorpha

Dinosauria

Palynology

Macroflora

See also

*
* List of dinosaur-bearing rock formations
* List of pterosaur-bearing stratigraphic units

References

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Jurassic Mexico
Geography of Tamaulipas
Early Jurassic North America
Middle Jurassic North America
Geologic formations of Mexico
Natural history of Tamaulipas
Paleontology in Mexico
Jurassic System of North America