Oxfordian reef architecture of the La Manga Formation, Neuquén Basin,

PALMA, R.M.; LÓPEZ GÓMEZ, J.,; PIETHÉ. R.D.; KIETZMANN, D.A.; ADAMONIS, S.

Hammamet

Simposio; IGCP 506 Fifth Symposiums; 2008

Faculty of Sciences of Bizerte (FSB), Faculty of Sciences of Tunis (FST)

Upper Jurassic reefs are widely developed in many areas of the world, but inArgentina are poorly represented. The stratigraphical and paleontological studyof the outcrops of the La Manga Formation (60 m), Argentina, located nearBardas Blancas region, Mendoza province (35ºS and 69ºO) allow thereconstruction of the sedimentary environments of an Oxfordian carbonateramp, where outer ramp, middle ramp, inner ramp (oolitic shoal), inner rampmargin (patch reef) lagoon and paleokarst were differentiated. The reefs consistof in-situ framework of coral boundstones (18m) that was formed at the top ofshallowing-upward succession, and back reef facies.Coral reefs were analyzed by defining coral colonies shapes, paleontologicalcontent, coral diversity and taphonomy studies. In some studied sectionsabundant fragments of gripheids, incrustant bryozoans and isolated spongesprovided a suitable substrate for coral colonization, however, other sectionsshow an increase in the proportions of ooids, peloidal and coral intraclasts.The core-reef facies is composed of white-gray unstratified low diversityscleractinian corals limestone dominated by robust and thinly branching coralswith phaceloid–meandriform growths, and domal coral colonies with cerioidphocoidgrowths.The assemblage is characterized by Actinastraea sp. and Australoseris sp.Internal facies organization and different types of coral colonies allow torecognize the develop of varying framework as well as intercolonies areas. Thereef fabric is superstratal, which vertical coral growth allows more effectivebiological and physical processes.On the base of coral growth fabric (domal and branching types) the reef of LaManga Formation is considered as a typical mixstones. The intercolonies areasconsist of biomicrites and biomicrorudites conta due to ining abundant coralfragments, parautochthonous grifeids and another bivalves (Ctenostreon sp.)gastropods (Harpagodes sp., Natica sp.), echinoderms (Plegiocidaris sp.) andechinoids spines, miliolids, Cayeuxia sp., Acicularia sp., Salpingoporella sp.,and intraclasts, ooids, peloids and coated grains.Due to growth forms, the domal ones forms are probably more protectedagainst biological and physical destruction, meanwhile delicate branching formswith very open and fragile framework were more affected and fragmented dueto wave action and bioerosion.The reef fabric shows different intervals of truncation as consequence oferosion resulting from coral destruction by storm waves or currents. Themaximum flooding surface separates oolitic shoal facies below from theaggradational and progradational coralline limestones facies above. An episodeof sea-level fall and karstification (148 Ma) affected reef and oolitic facies.