Evolution of the Middle and Late Jurassic lacustrine systems in the Fossati Depocenter, Gan Gan area, Chubut, Patagonia (Argentina).

CABALERI, NORA; ARMELLA, CLAUDIA; VOLKHEIMER, WOLFGANG; GALLEGO, OSCAR F.; SILVA NIETO, DIEGO; MONFERRAN, MATEO

Mendoza, Argentina

Congreso; 18th International Sedimentological Congress; 2010

International Association of Sedimentologist

The objective of this contribution is the presentation of the facies evolution of the lacustrine Jurassic Fossati Depocenter (Gan Gan area) of the Cañadón Asfalto Basin. The sedimentation in this pull-apart basin begins early in the Middle Jurassic, accompanied by effusions of olivinic basalts. The lower lacustrine, Las Chacritas Member, of the here outcropping Cañadón Asfalto Formation is composed by 28 m of laminated micritic limestones, alternating with tuffs, covered by 36 m of stromatolitic limestones, alternating with subordinated tuffs and some fluvial channels. The upper, Puesto Almada Member, of the Cañadón Asfalto Formation (in this area not in con - tact with the underlying Las Chacritas Member) is composed by an alternance of 128,52 m of tuffites, tuffs and several intercalations of dark shales bearing assemblages of palynomorphs indicating the presence of wetlands characterized by planctonic coccal green algae (Botryococcus), bentonic Zygnematacean algae and the presence of conchostracans, ostracods and insects remains. The prevailing vegetation of the lowlands was characterized by widely distributed conifers (Cheirolepidiaceae), represented by high percentages of the pollen genus Classopollis, indicating well drained soils and warm climatic conditions. At topographically higher positions Araucariaceae and Podocarpaceae were frequent, as demonstrated by the presence of Callialasporites spp., Araucariacites and Podocarpidites. The palynologic assemblages identified in the wetlands allow interesting environmental conclusions, but the species present are of a wide Jurassic and Cretaceous distribution. The invertebrates identified for the Puesto Almada Member consist of "conchostracans" assigned to Congestheriella rauhuti Gallego and Shen, and specimens related to the families ?Euestheriidae and Palaeolimnadiopseidae (Asiolimnadiopseinae,? Eosolimnadiopsis Chen). Ostracods recognized correspond to the Darwinulacea Penthesilenula sarytirmenensis (Sharapova) and Cytheracea Theryosinoecum barrancalensis minor (Musacchio et al.); unidentified remains and fish scales. This association indicates the presence of not very profound benthonic faunas, well oxygenated environment and associated vegetation. The known radiometric age of the upper part of the Puesto Almada Member at the type locality is 147.1±3.3 Ma (Tithonian). In three cases paleosoils developed at the top of the wetland deposits. Levels of gravity flows could be observed in the middle and upper part of the here out - cropping Puesto Almada Member. The thickness of these fining upward lobes is varying from 1.5 to 15.0 m. Evolution of the lacustrine system: The facies evolution within the Las Chacritas Member corresponds exclusively to littoral lacustrine environments. The sub-environments are profound (the lower part), shallow (the middle part) and eulittoral (the upper part of the section). The profound subenvironment is characterized by laminated micritic limestones. The shallow subenvironments are represented by wackestones and packstones, with tempestite levels and fluvial channels. The eulittoral subenvironment is characterized by domal and bulbous stromatolites. The evolution of the body of water during the deposition of the Puesto Almada Member is as follows: 1. Reduction of the water body. 2. Beginning of the filling of the basin with pyroclastic material. 3. Installation of three successive wetland episodes with palustrine environments. 4. Final filling with pyroclastic material and associated gravity flows. This paper was financed with funds of the PIP 5760 (CONICET)