New evidence of ammonites and its sedimentary context within Upper part of Chokolate Formation (Yura, Arequipa)

Abstract

In District of Yura, Province and Department of Arequipa (southern Peru), there are several studies in petrology and stratigraphy (Jenks, 1958; Benavides, 1962; Vargas, 1970; Vicente, 1981; León, 1981; Vicente qt al., 1982). In this study, we show an update on knowledge of sedimentological and paleontological elements in Chocolate Formation, in cerro Yanacoto; reporting for this area fluvial and marine sedimentary sequences, both containing fauna of ammonites in marine facies. This study relates paleontological features on the sedimentological context in which the ammonites were found. At cerro Yanacoto, up to 170 m. in the stratigraphic column (at the top of the Sequence 1) at Chocolate Formation, there are layers containing carbonated silty sediments, that contain two fossiliferous levels, separated each other by 50 cm. in thickness by fine grained sandstone, collected at total about ten internal molds of ammonites found in normal position and regular state conserved. These are ammonites, which paleontological features consists of small size, umbilical diameter about 2 to 5 cm., first whorl height range between 1.5 to 2 cm., evolute to slightly involute, some samples, can be observed at least 4 whorls, venter is slightly rounded and very narrow, and the ornamentation consists of simple and straight ribs on the flanks; and on its ventro-lateral shoulders, bending are observed toward growth direction of the shell. These characteristics correspond to Arietitidos. Regarding Uptonia sp., they are compressed, with coarse ribs and coarse to slightly regular folds relatively next to ventral zone, they are not tuberculated.