Facies analysis, stratigraphy and marine vertebrate assemblage of the lower Miocene Chilcatay Formation at Ullujaya (Pisco basin, Peru)

Abstract

This paper is the first integrated account of the sedimentology, stratigraphy, and vertebrate paleontology for the marine strata of the Chilcatay Formation exposed at Ullujaya, Pisco basin (southern Peru). An allostratigraphic framework for the investigated strata was established using geological mapping (1:4000 scale) and conventional sedimentary facies analysis and resulted in recognition of two unconformity-bounded allomembers (designated Ct1 and Ct2 in ascending order). The chronostratigraphic framework is well constrained by integration of micropaleontological data and isotope geochronology and indicates deposition during the early Miocene. The marine vertebrate fossil assemblage is largely dominated by cetaceans (odontocetes), whereas isolated teeth and spines indicate a welldiversified elasmobranch assemblage. Our field surveys, conducted to evaluate the paleontological sensitivity of the investigated strata, indicate that vertebrate remains only came from a rather restricted stratigraphic interval of the Ct1 allomember and reveal the high potential for these sediments to yield abundant and scientifically significant fossil assemblages.

Palaeoenvironmental analysis of the Miocene barnacle facies: case studies from Europe and South America

“…These characteristics suggest significant reworking, compatible with a setting above fair-weather wave-base. This hypothesis is in agreement with and Di Celma et al (2018b), which interpreted Ct1b as a shoreface deposit, probably formed at a water depth of less than 15-20 m (assuming a storm-wave base around 15-20 m in accordance with the models of Hernández-Molina et al 2000;Massari & Chiocci 2006). The same authors interpreted Ct1a as an offshore deposit resulting from the downslope transport of shoreface material (below 30-40 m of water depth since the height of the clinoforms is about 15-20 m).…”

“…The overlaying Ct2 deposits include massive and intensely bioturbated sandstones, changing upwards into massive siltstones with dolomitized mudstone layers. A tephra layer near the top of this sequence locates the Chilcatay Formation in the Burdigalian -which agrees with diatom and silicoflagellate biostratigraphy (Di Celma et al 2017, 2018b. These allomembers are well exposed at Ullujaya ( Fig.…”

“…The East Pisco Basin was a Cenozoic semi-enclosed forearc-embayment, protected by an archipelago of islands, located on the southern coast of Peru (DeVries & Jud 2018;Di Celma et al 2018b). It has been mainly investigated for its diverse and exceptionally-preserved Neogene fossil vertebrates (including pinnipeds, sharks, crocodiles, seabirds, turtles and bony fish) that characterize several outcrops west of the Ica River (e.g., Bianucci et al 2015Bianucci et al , 2016, and previous references therein; Lambert et al 2014Lambert et al , 2015Lambert et al , 2017Landini et al 2017Landini et al a, b, 2018Marx et al 2017;Gioncada et al 2018).…”

“…Recent surveys in the Western side of the Ica River Valley have recognized within the Chilcatay Formation distinct depositional sequences, separated by basinwide unconformities (Di Celma et al 2017, 2018a. Based on that, the Chilcatay Formation has been divided into the Ct1 and Ct2 allomembers (Di Celma et al 2018b). Ct1 is further subdivided into two facies associations: Ct1a, composed of sandstones and conglomerates (less common than sandstones) alternating with siltstones, and Ct1b, including clinobedded coarse-grained mixed siliciclastic/bioclastic arenites.…”

“…The studied barnacle-rich skeletal assemblages are located in the well studied successions of the Pietra da Cantoni Basin in Italy, of the Sommières Basin in France, and of the East Pisco Basin in Peru ( Fig. 1; Vannucci et al 1996;Bicchi et al 2006;Reynaud & James 2012;Coletti et al 2015;DeVries & Jud 2018;Di Celma et al 2018b). Previous research provides a firm basis for the interpretation of the barnacle facies of these basins, which has received limited attention until now.…”