7thISS - Book - Porifera Research.pdf - Porifera Brasil - UFRJ

14Precordillera (Cuyania) Terrane, WesternArgentinaThe Argentine Precordillera is situated along the forefrontof the high Andes at approximately 28º to 37º S, and it isa major geologic province in northwestern Argentina. Itcontains a complete thick sequence of Early Paleozoic rocks.The Precordillera, as part of the Cuyania Terrane, wasformed during the Andean (Tertiary) crustal shortening. Thisdistinctive terrane can be recognized mainly on the basis ofits key stratigraphic composition, involving biostratigraphic,sedimentary and magmatic events; its boundaries withadjacent geologic regions are abrupt (Ramos et al. 1986).In accordance with the Terrane concept, the presentPrecordillera, plus the San Rafael Block and San JorgeLimestones, integrate a unique geologic entity, the so-calledPrecordillera Terrane or Cuyania Terrane.Two hypotheses exist regarding the origin of thePrecordillera: 1) the Precordillera represents a terrane ofLaurentian origin that became attached to Gondwana (westernArgentina) already during Ordovician times (Thomas andAstini 2003). It includes either the classical Precordillera aswell as the San Rafael Block, to the south in the provinceof Mendoza, and the San Jorge Limestones cropping out inthe Province of La Pampa, within the Sierras Pampeanasstructural setting as an allochthonous terrane “Cuyania”,accreted to Gondwana during the lower Paleozoic. 2) thePrecordillera is considered as an autochthonous Gondwananfragment (Baldis et al. 1989, Aceñolaza et al. 1999, 2002)displaced by simple transcurrence mechanics, from ahypothetical intermediate sector between South America,Africa, and Antarctica. Recently, U-Pb geochronologyof detrital zircons indicated a Gondwanan provenancefor Lower Cambrian and Upper Ordovician sandstonesof the Precordillera of western Argentina, supporting theautochthonous Gondwanan nature of the PrecordilleraTerrane (Finney et al. 2003a, 2003b).Cambrian PoriferaThe Cambrian system of the Argentinian Precordillerais represented by a carbonate platform, in the east, and ofa continental slope, in the west. Cambrian platform andslope facies, containing spicules and chancelloriids scleritesare located in the Precordillera of San Juan and northernMendoza provinces, western Argentina (Fig. 1C-D).Cambrian sponges are known mainly from fragmentsof skeletal nets and dissociated spicules from the shallowcarbonate platform sequences of the upper-Lower to MiddleCambrian in the eastern and central belts, and from the slopeolistostromic sequences with allochthonous blocks in thewestern part of the Precordillera of San Juan Province. Thespicules from the Upper Cambrian were collected in TontalRange, San Juan Precordillera (Beresi and Banchig 1997)and in the La Cruz Olistolith, San Isidro area (Beresi andHeredia 1995), Precordillera of Mendoza Province.Sclerites of Chancelloria (Coeloscleritophora) occur inshallow carbonate platforms and allochthonous blocks inthe Precordillera of San Juan and Mendoza. Well preservedsclerites are associated with protospongiid spicules (Fig. 2B-G).Cambrian assemblagesTwo spicule assemblages occur in the Cambrian facies(Beresi and Rigby 1994, Beresi 2003a). The autochthonousassemblage corresponding to material collected from theupper Lower to Middle Cambrian platform sequence of theeastern Precordillera of San Juan. This assemblage consistsof a variety of stauractines and sclerites of Chancelloria eros(Walcott, 1920). The Protospongiidae are represented bytriradiate prodianes, pentactines and hexactines, all belongingto Kiwetinokia utahensis Walcott, 1920, Protospongia andanthaspidellid fragments. This fossil fauna represents theoldest assemblages known of Argentina.The allochthonous assemblage proceeds from the diverseCambrian carbonate olistoliths of slope sequences of thewestern San Juan Precordillera and from the classical area ofSan Isidro, Mendoza Province. The assemblages consist ofthe first precordilleran Protospongia with body preservation,Diagoniella Rauff, 1894, Kiwetinokia Walcott, 1920 andChancelloria and skeletal net with hexactines and monaxons(Beresi and Banchig 1997).Demosponges have a limited record in the Cambrianof the Precordillera. Typically anthaspidellid fragmentswith dendroclones (Fig. 2A) have been reported from thecarbonate platform and slope sequences of the San Juan andMendoza Precordillera (Beresi and Rigby 1994).Ordovician PoriferaDeposits of Ordovician carbonate basins occur in thecentral and eastern Precordillera. The Lower-MiddleOrdovician sediments of the Precordillera represent adrowning carbonate platform with a diverse and relativelycomplete fossil record.Well-preserved and diverse faunas of sponges have beencollected from limestones of the San Juan Formation (UpperTremadoc-Early Llanvirn) in the Precordillera basin of SanJuan province (Fig. 1C-D). This fauna represents the mostsignificant Ordovician sponge fauna known from SouthAmerica and provides the first extensive record of spongesderived from a stable carbonate platform, constituting one ofthe most important Early Ordovician sponge associations ofthe world.Precordilleran sponge faunas are dominated by orchocladlithistid demosponge genera, although hexactinellids areknown from loose spicules and root tufts, and calcareousheteractinid sponges are known from isolated octactinespicules and only one genus. Spicules assemblages werereported from diverse localities of the Precordillera (Beresiand Esteban 2003, Carrera 2003).The San Juan Formation was deposited on an opencarbonate shelf, bounded to the west by continental slopeand oceanic basin deposits. The diverse marine fauna andthe lack of specific structures indicative for shallow water