Paleoenvironments of the Cañadón Asfalto Formation (type locality), Cañadón Asfalto basin (Jurassic). Patagonia, Chubut province, Argentina. Stable Isotopes, Sedimentology and Paleontology

CABALERI, N.; CAGNONI, M.; ARMELLA, C.; GALLEGO, O.F.; MONFERRAN, M.D.; VOLKHEIMER, W.

Ginebra

Congreso; 19th International Sedimentologycal Congress; 2014

Université de Genève

Cañadón Asfalto Fm (CA) with Las Chacritas (LCh) and Puesto Almada (PA) members, represents the lacustrine sedimentation during the rift development of the CA basin. The lower member, LCh, is characterized by expansion/contraction cycles, with a shallowing trend and reduction of the waterbody. The lacustrine sedimentation was interrupted by several basaltic flows. In Cerro Cóndor depocenter (CA creek, type locality), the paleolake is defined by extended littoral and marginal facies with palustrine facies which prevailed within the middle and upper part of the sequence. The littoral and marginal facies (expansion stage) are represented by mudstone, microbialites, stromatolites, wackestones and paleosols. The fauna is represented by euestherids and eosestherids conchostracan which well developed shells indicate a stable environment during a period of time. On the other hand, bivalves (Sphaeridae?) suggest a lenthic environment with moderate currents and less than 10 m in depth while cf. Diplodon shells evidence poor transport. The lacustrine system received siliciclastic inputs from surface inflows. The palustrine facies (contraction stage) are represented by bioturbated mudstone with benthic fauna and carbon debris, shales, storm levels and evaporites. The long-lasting palustrine environment is characterized by long periods of decrease in freshwater discharge, fluctuating wet lake margins and high organic productivity under reduction conditions that slowed organic matter oxidation. PA Mb represents the restricted stage of the paleolake. The lower section is characterized by shallow water and marginal deposits with pyroclastic inputs. This record suggests a drying trend in the climatic conditions. The conchostracan found are eosestherids, afrograptids and anthronestherids with small shells. This feature could reflect an accommodation to the ephemeral water level with adverse physical and chemical conditions for their development. As ostracods, the monospecific associations in abundant populations are indicative of adverse paleoenvironmental conditions. The marginal littoral environment is represented by mudstones interbedded with palustrine shales. In the upper part of the section, tuffs are dominant and associated with mudstone with mudcracks or with ripples. The small bivalve shells suggest that they were opportunist fauna which development was related with episodes of favorable conditions. Insect larvae (trichopterans) were also found. δ13C and δ18O data of the lacustrine carbonates show moderate positive covariance (r= 0.68 LCh Mb) to strong covariance (r=0.89 PA Mb). The two separate covariant trends reflect changes in the basin hydrology between LCh and PA carbonates. The former trend indicates that carbonates precipitated in a waterbody periodically hydrologically closed with sporadic water discharges (δ18O between -12.1? and -17.5?). Maxima in δ13C (2.0?) is associated with productivity (photosynthesis). The latter trend is characteristic for carbonates precipitated under closed lake environmental conditions (r≥0.8). The higher values in δ13C (maximum 5.1?) are associated with atmospheric CO2 exchange due to extended water residence time while the higher values in δ18O (maximum -2.1?) reflect aridity and high evaporation rates in the paleolake. In conclusion, identification of two different covariant trends confirmed the hydrological changes of the basin which have affected the isotopic identity of the water body, in agreement with the sedimentological and paleontological data.