| La Boca Formation | |
| Type: | Geological formation |
| Period: | Lower Jurassic |
| Age: | Lower Pliensbachian-Latest Aalenian, [1] |
| Prilithology: | Red sandstones, mudstones, and siltstones |
| Otherlithology: | Pyroclastic volcanic rocks |
| Region: | Tamaulipas |
| Unitof: | Huizachal Group |
| Subunits: |
|
| Underlies: | La Joya Formation |
| Overlies: | Huizachal Formation |
| Thickness: | <10 m |
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.[2] 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.[3]
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.[4] Deposits of Late Triassic Age referred to this unit have been reclassified in a new formation, El Alamar Formation.[5] 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.[6]
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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.[7] 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.[7]
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.[7] 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.[7] 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.[7]
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.[8]
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.[8]
| Genus | Species | Location | Material | Type | Made by | Notes | Images |
|---|---|---|---|---|---|---|---|
| Mermia |
|
| Locomotion trace | Domichnia & Fodinichnia |
| Densely looped grooves or ridges. Taxon linked with the major Early Jurassic flooding of the Huizachal Valley, developed locally mostly on a gravel-bed braided fluvial style with paleocurrents oriented west-northwest | |
| Scoyenia |
|
| Burrows | Domichnia & Fodinichnia |
| Burrow fossils in lacustrine or fluvial environments, probably made by arthropods | |
| Genus | Species | Location | Stratigraphic position | Abundance | Notes | Images |
|---|---|---|---|---|---|---|
| Bocaconodon[9] | B. tamaulipensis | Jim's Joy, Huizachal Canyon | Lower Part | Teeth | A basal Mammaliaform | |
| Bocatherium[10] [11] | B. mexicanum | Huizachal Canyon | Lower Part | IGM 3492, Skull | A Tritylodontid | |
| Huasteconodon | H. wiblei | Huizachal Canyon | Lower Part | Teeth | A Gobiconodont | |
| Victoriaconodon | V. inaequalis | Rene's Roost, Huizachal Canyon | Lower Part | Teeth | A Triconodontid | |
| Unnamed Mammaliaforms (IGM 6622,IGM 6855, and IGM 6856) | Indeterminate |
| Lower Part | IGM 6855, partial right dentary; IGM 6856, left dentary; IGM 6622, partial right dentary | ||
| Genus | Species | Location | Stratigraphic position | Abundance | Notes | Images |
|---|---|---|---|---|---|---|
| Clevosaurus | aff. C. sp. |
| Lower Part |
| A Sphenodontidae Rhynchocephalian of the family Clevosauridae. | |
| Cynosphenodon[14] [15] | C. huizachalensis | Huizachal Canyon | Lower Part |
| A sphenodontine rhynchocephalian closely related to the living tuatara.[16] | |
| Opisthias | aff. O. sp. |
| Lower Part |
| A Sphenodontidae Rhynchocephalian of the family Opisthodontia. | |
| Sphenovipera[17] | S. jimmysjoyi | Jim's Joy, Huizachal Canyon | Lower Part | IGM 6076, an almost complete right lower jaw with teeth | A possible venomous Sphenodont | |
| Zapatadon[18] | Z. ejidoensis | Tierra Buena, western part of the Huizachal Canyon | Lower Part | IGM 3497, crushed skull, missing part of the skull table and roofing bones | A dwarf Sphenodont | |
| Genus | Species | Location | Stratigraphic position | Abundance | Notes | Images |
|---|---|---|---|---|---|---|
| Metasuchia | Indeterminate | Huizachal Canyon | Lower Part | IGM 3498 & additional specimens. Partial skulls and postcranial skeletons | Preliminary results suggest it may be a stem metasuchian. | |
| Protosuchia[21] [22] | Indeterminate | Huizachal Canyon | Lower Part | Skull fragment | A possible member of Protosuchia. Found to be sister taxon of Platyognathus hsui from the Lower Jurassic Lufeng Formation of Yunnan. | |
| Genus | Species | Location | Stratigraphic position | Abundance | Notes | Images |
|---|---|---|---|---|---|---|
| Ceratosauria | Indeterminate | Casa de Fidencio, Huizachal Canyon | Lower Part | IGM 6625, craneal fragmentary elements | A possible basal ceratosaur related with the African Berberosaurus. | |
| Heterodontosauridae[23] | cf.H. sp. | Huizachal Canyon | Lower Part | Teeth | An Ornithischian of the family Heterodontosauridae. | |
| "Megapnosaurus"[24] | "M." mexicanum [25] | Casa de Fidencio, Huizachal Canyon | Lower Part | (IGM 6624) partial twelfth dorsal vertebra, partial thirteenth dorsal vertebra, partial synsacrum, incomplete fused pelvis | An indeterminate Coelophysoidean. | |
| Neotheropoda | Indeterminate | Huizachal Canyon | Lower Part | Isolated teeth | Several morphotypes, maybe related with Coelophysoidea, Dilophosauridae or Tetanurae. | |
| ?Sauropodomorpha | Indeterminate | Rene's Roost, Huizachal Canyon | Lower Part | Large bone fragments | A possible Basal Sauropodomorph. | |
| Genus | Species | Location | Stratigraphic position | Material | Notes | Images |
|---|---|---|---|---|---|---|
| Araucariacites |
|
|
| Pollen | A Pollen Grain, affinites with the family Araucariaceae inside Pinales. Conifer pollen from medium to large arboreal plants. | |
| Exesipollenites |
|
|
| Pollen | A Pollen Grain, affinities with the Hirmeriellaceae in the Pinopsida. | |
| Dapcodinium[26] |
|
|
| Cysts | A Dinoflajellate of the family Rhaetogonyaulacaceae inside Gonyaulacales | |
| Eucommiidites |
|
|
| Pollen | A Pollen Grain, afinnities with Erdtmanithecales inside Spermatophytes. | |
| Exesipollenites |
|
|
| Pollen | A Pollen Grain, affinities with the family Cupressaceae in the Pinopsida. Pollen that resembles that of extant genera such as the genus Actinostrobus and Austrocedrus, probably derived from dry environments. | |
| Dictyophillidites |
|
|
| Spores | ||
| Krausellisporites |
|
|
| Spores | A Miospore, affinities with Selaginellaceae or Lycopodiaceae inside Lycopsida. | |
| Nannoceratopsis |
|
|
| Cysts | A Dinoflajellate of the family Nannoceratopsiaceae inside Nannoceratopsiales | |
| Ovalipollis |
|
|
| Pollen | A Pollen Grain, afinnities with Caytoniales inside Gymnospermopsida. | |
| Pareodinia |
|
|
| Cysts | A Dinoflajellate of the family Pareodinioideae inside Gonyaulacales | |
| Quadraeculina |
|
|
| Pollen | A Pollen Grain, affinities with Podocarpaceae and Pinaceae inside Coniferophyta. | |
| Rhaetogonyaulax |
|
|
| Cysts | A Dinoflajellate of the family Peridiniphycidae inside Dinophyceae | |
| Spheripollenites |
|
|
| Pollen | A Pollen Grain, affinities with the Hirmeriellaceae in the Pinopsida. | |
| Vitreisporites |
|
|
| Pollen | A Pollen Grain, afinnities with Caytoniales inside Gymnospermopsida. | |
| Genus | Species | Location | Stratigraphic position | Material | Notes | Images |
|---|---|---|---|---|---|---|
| Agathoxylon[27] |
|
|
|
| Affinities with Cheirolepidiaceae or Araucariaceae inside Pinales. Includes petrified wood logs up to 3.5 m in size | |
| Cheirolepidium |
|
|
|
| Affinities with Cheirolepidiaceae inside Pinales. | |
| Cephalotapsis |
|
|
|
| Affinities with Cupressaceae inside Pinales. | |
| Cycadolepis |
|
|
|
| Affinities with Cycadeoidaceae inside Bennettitales. | |
| Ctenophyllum |
|
|
|
| Affinities with Williamsoniaceae inside Bennettitales. | |
| Laurozamites[28] |
|
|
|
| Affinities with Williamsoniaceae inside Bennettitales. Representative of large arboreal to low arbustive Bennetittes. The dominant foliar remain recovered on the formation, with up to 50 specimens | |
| Otozamites |
|
|
|
| Affinities with Williamsoniaceae inside Bennettitales. | |
| Piazopteris[29] [30] |
|
|
|
| Affinities with Matoniaceae inside Gleicheniales. | |
| Podozamites |
|
|
|
| Affinities with Krassiloviaceae inside Voltziales. | |