The East Pisco Basin is a Cenozoic forearc basin placed along the southern Peruvian coast. Its youngest sedimentary unit, the Miocene-Pliocene Pisco Formation, is famous for hosting a globally renowned marine vertebrate fossil-Lagerstätte (Gariboldi et al., 2015). Along the western side of the Ica River, the Pisco Formation is comprised of three allomembers representing three transgressive cycles (Di Celma et al., 2017). Southeast of the locality of Cerro Colorado, the basal sandstones of the P1 allomember host an unusual invertebrate-rich bed. An abundant mollusk fauna with a peculiar taphonomic and diagenetic history characterizes a 10 cm-thick layer. Only the internal molds are preserved in a variably cemented sandstone. Specimens of Panopea cf. coquimbensis, an infaunal species living at more than one meter of depth into the seafloor, Trachycardium sp. and Dosinia ponderosa, which are shallow infaunal species, occur together with barnacles and other epifaunal encrusters. Panopea specimens are preserved as internal molds of articulated valves complete with the cast of muscles and pallial line. The molds show serpulids, bryozoans and barnacles encrusting the inner side of the valves, suggesting that the Panopea shells were exhumed, exposed at the seafloor and colonized before the successive burial. Microscopical, XRD and XRF analyses reveal that the molds are constituted by terrigenous clasts cemented by ankerite. Gypsum and anhydrite are also present. We hypothesize that a storm event or strong currents firstly uncovered the Panopea specimens and accumulated them after a short transport, allowing articulation to be preserved. After death of the Panopeaindividuals, the inner part of the shell, representing a hard substrate and a sheltered evironment, was colonized by different organisms. After the final burial, ankerite precipitated cementing the sediment infill of valves. Finally, secondary circulation of fluids caused the dissolution of the calcite shell and precipitation of gypsum/anhydrite (Gioncada et al., 2018), leaving only the internal molds.
Bosio, G., Bracchi, V., Malinverno, E., Basso, D., Collareta, A., Gioncada, A., et al. (2019). Taphonomic and diagenetic history of a Panopea-rich layer from the Pisco Formation, Peru. Intervento presentato a: Congresso SIMP-SGI-SOGEI 2019 - Il tempo del pianeta Terra e il tempo dell'uomo: Le geoscienze fra passato e futuro, Parma.
Taphonomic and diagenetic history of a Panopea-rich layer from the Pisco Formation, Peru
Bosio, G.;Bracchi, V.;Malinverno, E.;Basso, D.;Coletti, G.;
2019
Abstract
The East Pisco Basin is a Cenozoic forearc basin placed along the southern Peruvian coast. Its youngest sedimentary unit, the Miocene-Pliocene Pisco Formation, is famous for hosting a globally renowned marine vertebrate fossil-Lagerstätte (Gariboldi et al., 2015). Along the western side of the Ica River, the Pisco Formation is comprised of three allomembers representing three transgressive cycles (Di Celma et al., 2017). Southeast of the locality of Cerro Colorado, the basal sandstones of the P1 allomember host an unusual invertebrate-rich bed. An abundant mollusk fauna with a peculiar taphonomic and diagenetic history characterizes a 10 cm-thick layer. Only the internal molds are preserved in a variably cemented sandstone. Specimens of Panopea cf. coquimbensis, an infaunal species living at more than one meter of depth into the seafloor, Trachycardium sp. and Dosinia ponderosa, which are shallow infaunal species, occur together with barnacles and other epifaunal encrusters. Panopea specimens are preserved as internal molds of articulated valves complete with the cast of muscles and pallial line. The molds show serpulids, bryozoans and barnacles encrusting the inner side of the valves, suggesting that the Panopea shells were exhumed, exposed at the seafloor and colonized before the successive burial. Microscopical, XRD and XRF analyses reveal that the molds are constituted by terrigenous clasts cemented by ankerite. Gypsum and anhydrite are also present. We hypothesize that a storm event or strong currents firstly uncovered the Panopea specimens and accumulated them after a short transport, allowing articulation to be preserved. After death of the Panopeaindividuals, the inner part of the shell, representing a hard substrate and a sheltered evironment, was colonized by different organisms. After the final burial, ankerite precipitated cementing the sediment infill of valves. Finally, secondary circulation of fluids caused the dissolution of the calcite shell and precipitation of gypsum/anhydrite (Gioncada et al., 2018), leaving only the internal molds.
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