Paleoenvironmental framework using microfacies analyses of a nautilid concentration, Agrio Formation, Neuquén Basin, Argentina.

TUNIK, MAISA ANDREA; CICHOWOLSKY, MARCELA; PAZOS, PABLO; AGUIRRE-URRETA, M.B

Mendoza

Congreso; International Sedimentological Congress; 2010

International Association of Sedimentologists

The presence of an exceptional accumulation of nautilid shells in sediments belonging to the Agrio Formation has been previously reported. The objective of this work is to analyze the microfacies of the beds that contain the nautilids and to compare them with the deposits above and below the exceptional nautilid accumulation. The logged section was accomplished at El Salado locality, and was assigned to the middle-upper part of the Pilmatué Member of the Agrio Formation, belonging to the Holcoptychites agrioensis biosubzone (Early Hauterivian). The sediments located below the nautilid bearing beds are heterolithic deposits exhibiting a thickening and coarsening upward trend. They are composed of interstratifications of sandstones, siltstones and claystones.Overlying the heterolithic deposits, up to three amalgamated limestone beds with nautiloids were identified.They are covered by gray bioclastic limestones. The limestones with nautilids were interpreted as amalgamated storm deposits in a ramp environment (Cichowolski 2008). Petrographic studies were performed to the heterolithic deposits, to the beds containing nautilid shells and also to the limestones located above the nautilid levels. The heterolithic and nautilid levels are made up of allochemic sandstones and sandy allochemic limestones. The identified allochems in both types of samples are fragmented oysters and other bivalves, serpulids, echinoderms, and abundant bioclastic debris. Most of the bioclasts are abraded and highly fractured but without signs of important bioerosion. Instead, micritization of ooids, grapestones and intraclasts suggest certain degree of rework of the original facies. Some samples show a linear pattern of the allochems and micas and some show chaotic arrangement of the fragmented bioclasts. The main siliciclastic components are quartz and feldspars with minor volcanic and sedimentary lithics and micas. The proportion of siliciclastic and carbonate material is variablefrom sample to sample. Using the Mount (1984) scheme, the origin of these mixed samples is probably facies mixing for the heterolithic levels and punctuated mixing for the nautilid level. Through this process sediments are transferred between contrasting depositional environments during rare, high intensity sedimentation events, like the storms. In that case we suggest that the benthic communities were disrupted by major storm but they recovered rapidly on fair-weather conditions. Petrographic studies made on samples located few meters above thenautilid beds allowed us to classify them as bioclastic wackestones to packstones, with a wide variety of bioclasts remains and grumose micrite. The identified bioclasts are: bioeroded and fragmented oyster shells, echinoderm plates and spines, gastropod sections, serpulids in longitudinal and transversal section, and a high proportion of undetermined highly fragmented bioclasts. It is remarkable the presence of calciespheres and scarce reworked and micritized oolites. Few monocrystalline quartz, plagioclase and glauconitic grains were identified assiliciclastics material in the samples. The observed microfacies are similar to those described previously for open marine facies for the Agrio Formation and can be classified as RMF7 from Flügel (2004). These microfacies are indicative of deposition on a middle ramp environment. The microfacies study clearly discriminates between the fair weather prograding deposits and storm facies from the normal middle ramp environment.