| Roca Formation | |
|---|---|
| Stratigraphic range:Maastrichtian-late Danian (pre-Tiupampan) ~68–63 Ma | |
 Roca Formation inBarda Norte,General Roca, Río Negro | |
| Type | Geological formation |
| Unit of | Malargüe Group |
| Sub-units | 3 members |
| Underlies | El Carrizo &Pircala Formations |
| Overlies | Jagüel Formation |
| Thickness | Up to 58 m (190 ft) |
| Lithology | |
| Primary | Limestone,claystone,siltstone |
| Other | Gypsum |
| Location | |
| Coordinates | 38°54′S67°36′W / 38.9°S 67.6°W /-38.9; -67.6 |
| Approximate paleocoordinates | 41°30′S56°00′W / 41.5°S 56.0°W /-41.5; -56.0 |
| Region | Río Negro,Neuquén,La Pampa &Mendoza Provinces |
| Country | Argentina |
| Extent | Neuquén Basin |
| Type section | |
| Named for | General Roca |
   Roca Formation, Argentina (Argentina) | |
TheRoca Formation is aCretaceous toPaleogenelithostratigraphic unit, located in theNeuquén Basin.[1] It crops out in the Argentinian provinces ofRío Negro,Neuquén,La Pampa, andMendoza.[2] Its deposition isdiachronous, beginning during theMaastrichtian in the north of its distribution, and later moving to the south, where its strata reached the LateDanian. It lies transitionally above theJagüel Formation, and the top of the formation is marked by a regionalunconformity due to anEocene andOligoceneorogenic pulse. These two units belong to theMalargüe Group. The marine sediments of the Jagüel and Roca Formations were deposited during atransgression from theAtlantic Ocean, beginning in the Maastrichtian and ending in the Danian.[3][4]
Thestratotype of the Roca Formation is located 12 kilometres (7.5 mi) north ofGeneral Roca, Río Negro (39º40´S, 67º32´W). The fossiliferous beds of the Roca Formation were discovered by G. Rohde Windhausen (1914), who was also the first author to describe these sediments. Schiller (1922) took samples of one section along the Zanjón Roca, from the northern part of General Roca toHorno de Cal (lime kiln). This author proposed to name the lime kiln as the "classic area", and the westward cliffs from the lime kiln as the "model area" of these beds. The lithological composition of thistype locality contains gray-yellowish and highly fossiliferous limestones, with greenish claystones andmarls, and abundant gypsum at the top. The basal and middle sections are approximately 26 metres (85 ft) thick (Weber, 1972).[5]
In its type locality, the Roca Formation is divided into three sections. The first section, that is, the base, is approximately 20 metres (66 ft). It consists of intercalated bioclasticlimestones and green claystones.[6] The calcareous materials are highly fossiliferous with textures likewackestone andpackstone without an obvious orientation of the shells. Theclaystones are composed ofmontmorillonite withcalcite andquartz.[7] The second section (middle) is not over 8 metres (26 ft) and consists mainly of limestones. Such section has very thinstrata of varied yellow limestones and siltstones, interbedded with yellow porous and coarse-grained limestones. The limestones of the middle section are almost devoid of fossil invertebrates. The third section (upper) is 25 to 30 metres (82 to 98 ft). It is mainly composed of whitegypsum in large crystals. It has lenticular intercalations of greenish siltstones withwave-formed ripples and fragments of gypsum.[7]
The faunal character on the fossils of marine invertebrates in the Roca Formation suggests a transgression from the Atlantic Ocean. By contrast, the marine invasions inArgentina andChile during theJurassic andEarly Cretaceous were caused by an inflow of seawater from thePacific Ocean.[8][3]
The first section of the Roca Formation has abundant fossiliferous content, includingbivalves,gastropods,bryozoans,echinoderms, crustaceans,ostracods,foraminifera, and calcareousnanoplankton, as well as remains of fish.[5][7][8]