The Río Mayer Formation is sequence of

sedimentary rock Sedimentary rocks are types of rock (geology), rock formed by the cementation (geology), cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or de ...

s of

Early Cretaceous The Early Cretaceous (geochronology, geochronological name) or the Lower Cretaceous (chronostratigraphy, chronostratigraphic name) is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 143.1 ...

age that form part of the northern

Magallanes Basin The Magallanes Basin or Austral Basin is a major sedimentary basin in southern Patagonia. The basin covers a surface of about and has a NNW-SSE oriented shape. The basin is bounded to the west by the Andes mountains and is separated from the Mal ...

(Austral Basin) in Santa Cruz Province, Argentina. It is a lateral equivalent of the

Zapata Formation Zapata Formation () is a sedimentary formation of Lower Cretaceous age in the Magallanes or Austral Basin of Argentina and Chile. Much of the formation is folded and faulted as consequence of the Andean orogeny.Fildani et al., 2008 In outcrops o ...

, as described in Chile further south within the same basin.

Tectonic setting

The Río Mayer Formation and it lateral stratigraphic equivalents form part of the post-rift sequence following the major back-arc

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 ...

ing event in the Jurassic, which was associated with the formation of a significant

back-arc basin A back-arc basin is a type of geologic Structural basin, basin, found at some convergent boundary, convergent plate boundaries. Presently all back-arc basins are submarine features associated with island arcs and subduction zones, with many found ...

, greater than 100 km wide, the

Rocas Verdes Basin The Rocas Verdes ophiolites () are a series of greenschists and other rocks constituting ophiolites in Magallanes Region, southernmost Chile. The Rocas Verdes ophiolites represent the continental-oceanic crust that existed in a back-arc basin in th ...

. The rifting was accompanied by eruption of a thick sequence of silica-rich volcanic rocks, forming the

Chon Aike CHNOPS and CHON are mnemonic acronyms for the most common elements in living organisms. "CHON" stands for carbon, hydrogen, oxygen, and nitrogen, which together make up more than 95 percent of the mass of biological systems. "CHNOPS" adds ...

Large Igneous Province A large igneous province (LIP) is an extremely large accumulation of igneous rocks, including intrusive ( sills, dikes) and extrusive (lava flows, tephra deposits), arising when magma travels through the crust towards the surface. The format ...

. The back-arc rifting is interpreted to have propagated northwards, leading to a northwards-narrowing rift. During the late Cretaceous, the basin closed up, locally with

obduction Obduction is a geological process whereby denser oceanic crust (and even upper mantle) is scraped off a descending ocean plate at a convergent plate boundary and thrust on top of an adjacent plate. When oceanic and continental plates converge, ...

of back-arc basin

oceanic crust Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramaf ...

, forming

ophiolite An ophiolite is a section of Earth's oceanic crust and the underlying upper mantle (Earth), upper mantle that has been uplifted and exposed, and often emplaced onto continental crustal rocks. The Greek word ὄφις, ''ophis'' (''snake'') is ...

s, and the development of a major

fold and thrust belt A fold and thrust belt is a series of mountainous foothills adjacent to an orogenic belt, which forms due to contractional tectonics. Fold and thrust belts commonly form in the forelands adjacent to major orogens as deformation propagates outwards ...

, accommodating 100s of kms of shortening. After deposition of the Río Mayer Formation, the Magallanes Basin shows a transition to a

foreland basin A foreland basin is a structural basin that develops adjacent and parallel to a mountain belt. Foreland basins form because the immense mass created by crustal thickening associated with the evolution of a mountain belt causes the lithospher ...

, related to this shortening.

Stratigraphy

The Río Mayer Formation lies conformably above the Springhill Formation, which marks the ending of the rift episode, infilling the rift topography. The top of this formation varies from uppermost Jurassic to Lower Cretaceous (uppermost

Valanginian In the geologic timescale, the Valanginian is an age or stage of the Early or Lower Cretaceous. It spans between 137.05 ± 0.2 Ma and 132.6 ± 0.2 Ma (million years ago). The Valanginian Stage succeeds the Berriasian Stage of the Lower Cretac ...

) going from south to north within the basin. To the south, in the Largo Argentino and Río Guanaco areas, the Rio Mayer Formation is overlain by the Cerro Toro Formation. Further north, the younger sequence is formed by the Lago Viedma Formation in the

Lago Viedma Viedma Lake (, ) is a Patagonian lake in the province of Santa Cruz Province (Argentina), Santa Cruz, Argentina, situated near its border with Chile. Measuring approximately 50 miles (80 kilometers) in length, it is a major elongated trough lake ...

area and the

Piedra Clavada Formation Piedra is a hair disease caused by a fungus, which causes formation of nodules on the hair shaft.Veasey JV, Avila RB, Miguel BAF, Muramatu LH. White piedra, black piedra, tinea versicolor, and tinea nigra: contribution to the diagnosis of superfic ...

in the

Lago San Martin __NOTOC__ Lago, which means "lake" in several languages, may refer to: Places *Lago, Calabria, a ''comune'' in the Province of Cosenza, Italy *Lago, Mexico, a municipality zone in the State of Mexico *Lago District, a ''distrito'' in Niassa Provin ...

area.

Depositional environment

The sequence is entirely marine in character, following on from the Springhill Formation, which is interpreted to represent deposition in a continental environment changing to a marine environment following a transgression.

Structure

This overall fine-grained sequence is generally highly deformed, with the development of tight asymmetric folds, such as the overturned

syncline In structural geology, a syncline is a fold with younger layers closer to the center of the structure, whereas an anticline is the inverse of a syncline. A synclinorium (plural synclinoriums or synclinoria) is a large syncline with superimposed ...

that outcrops on the Loma del Pliegue Tumbado (literally ''the hill of the recumbent fold'') and the nearby Loma de las Pizarras, west of

El Chaltén El Chaltén is a small mountain village in Santa Cruz Province, Argentina. It is located on the riverside of Rio de las Vueltas, within the Los Glaciares National Park (section ''Reserva Nacional Zona Viedma'') near the base of Cerro Torre and ...

. The boundaries with the older Jurassic

rhyolite Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture (geology), texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained matri ...

s of the El Quemado Complex are all marked with

thrust fault A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks. Thrust geometry and nomenclature Reverse faults A thrust fault is a type of reverse fault that has a dip of 45 degrees or less. I ...

References

{{Reflist Geologic formations of Argentina Cretaceous Argentina