A complex diagenetic history for the Miocene invertebrates of the East Pisco basin (Peru).

The Miocene marine sediments of the East Pisco Basin (southern Peru) are well-known for their extraordinary concentration of exceptionally well-preserved fossils of marine vertebrates together with invertebrate remains (Di Celma et al., 2017). In the western Ica Valley, the late Oligocene-early Miocene Chilcatay Formation consists of sandstones and siltstones interbedded with layers of cobbles and overlaid by clinoformed coarse-grained biocalcarenites. Fossil invertebrates are common and well-preserved, but the assemblage is characterized by low biodiversity, with only few species of barnacles, echinoids, tube worms and bivalves (mainly pectinids and oysters). Overall, biogenic Cacarbonates are present and well-preserved. In the same region, the late Miocene Pisco Formation consists of three unconformity-bounded depositional sequences (Di Celma et al., 2017), made of sandstone at the base and diatomaceous siltstones at the top. It shows mollusk-rich layers characterized by low biodiversity and high dominance (DeVries, 1988; Di Celma et al., 2017). Mollusks are commonly preserved only as gypsum casts or dolomite/gypsum molds. Pristine mollusk shells are rare and pure Ca-carbonates are only occasionally observed. With the aim of reconstructing the diagenetic histories of both formations, mollusk and barnacle samples have been first analyzed under the optical microscope and SEM-EDS, and then analyzed via Raman spectroscopy (to recognize mineral phases) and cathodoluminescence (to discriminate pristine carbonates and different cement generations). The results of mineralogical and geochemical analyses, integrated with taphonomic observations, enabled us to reconstruct the sequence of the diagenetic processes; for example, Sr-isotope ratios measured on mollusk samples from the Pisco Formation proved that dolomite precipitation occurred during the early diagenesis as recently proposed (Gariboldi et al., 2015). These analytical results will allow to reconstruct the still rather neglected but remarkably complex diagenetic history of the Miocene invertebrates of the East Pisco Basin.