INSTITUTO DE GEOCRONOLOGIA Y GEOLOGIA ISOTOPICA
Unidad Ejecutora - UE
congresos y reuniones científicas
Paleoenvironments of the Cañadón Asfalto Formation (type locality), Cañadón Asfalto basin (Jurassic). Patagonia, Chubut province, Argentina. Stable isotopes, sedimentology and Paleontology
CABALERI, N.G; CAGNONI, MARIANA; ARMELLA, CLAUDIA; GALLEGO, OSCAR; MONFERRAN, MATEO; VOLKHEIMER, W
Congreso; ISC 2014 Sediemntology at the crossroad of new frontiers; 2014
Cañadón Asfalto Formation (Bajocian-Tithonian) represents the lacustrine sedimentation during the development of the rift of the Cañadon Asfalto basin (Patagonia, Argentina). At Cañadón Asfalto locality two members of this unit crop out (Las Chacritas y Puesto Almada). Las Chacritas member represents the lacustrine sedimentation, and the carbonate facies are arranged in expansion and contraction cycles, with a shallowing trend and reduction of the waterbody. In Cañadón Asfalto area, the deepest zone of the paleolake is represented by extended littoral and marginal facies with palustrine facies in the upper- middle part of the section. This lacustrine sedimentation was interrupted by several basaltic flows. The littoral and marginal facies (expansion stage) are represented by mudstone, microbialitic mudstones, stromatolites, wackestones with benthic fauna and paleosols. The invertebrate fauna is represented by conchostraca euestheriodea and eosestheriodea which well developed shells indicate a stable environment during a period of time. On the other hand, mollusk bivalves (Sphaeridae?) are indicative of a lenthic environment with intermediate currents that not affect bottom sedimentation and less than 10 m in depth and cf. Diplodon shells evidence poor or less transport. The lacustrine system received siliciclastic inputs as the result of surface inflows. The palustrine facies (contraction stage) are represented by bioturbated mudstone with benthic fauna and carbon debris, shales, tempestitic levels and evaporites which prevailed within the middle and upper part of the sequence. The long-lasting palustrine environmental conditions are 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. Puesto Almada member represents the restricted stage of the paleolake. The lower section is characterized by shallow water and marginal deposits with pyroclastic material. This sedimentary record suggests a drying trend in the climatic conditions. The conchostraca found in these levels are eosestheriodea, afrograptideos and antronestherideos with small shells. These features could reflect an accommodation to the ephimeral water level with adverse physical and chemical conditions for the development of populations over a long period of time. 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 and mudstone with ripples. The bivalve shells are normally small probably indicating that they were opportunist individuals which development was related with episodes of favorable environmental conditions. Insect larvae (tricopteros) were also found. Carbon and oxygen isotope data of the lacustrine carbonates show moderate positive covariance (r= 0.68 Las Chacritas member) to strong covariance (r=0.89 Puesto Almada member). The two separate covariant trends reflect changes in the basin hydrology between Las Chacritas y Puesto Almada 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?) reflects 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 waterbody, in agreement with the sedimentological and paleontological investigations.