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Phosphorite deposits form through phosphogenesis, a mechanism of diffusion and concentration of interstitial phosphate in sediments leading to Ca-phosphate mineral precipitation in a favorable sedimentary regime at the seafloor. Stretching along the Peruvian coast, the East Pisco Basin is known to host phosphorite beds of Miocene age. In this study, we investigate the petrographic, mineralogical and chemical composition of the Miocene phosphorite layers of the East Pisco Basin, and discuss the phosphogenetic mechanism(s) and sedimentary dynamics that led to the genesis and accumulation of P-rich deposits. Phosphorite samples were collected from layers overlaying stratigraphic unconformities within the Pisco Formation and subsequently analyzed through multiple methods: optical microscopy, XRD, XRF, SEM-EDS, EPMA, ICP-MS and LA-ICP-MS. Our results indicate that these layers consist of phosphatic clasts and nodules along with dolomite clasts, basement boulders, shark teeth, mollusk molds and vertebrate bone fragments. The P-rich clasts are mainly composed of phosphatic intraclasts and small phosphatic nodules. They occur associated with minor amounts of dolomite clasts coated by Fe-oxyhydroxides and laminites. Clasts and nodules are cemented by both early and late diagenetic cements, such as dolomite, calcite, silica, gypsum/anhydrite and baryte. Our observations suggest that the Pisco phosphorite deposits are transgressive lags laying on ravinement surfaces. They formed just below the sediment-water boundary during the early transgression phases, when the porewaters were enriched in phosphorus due to the abundance of organic-rich sediments and the microbial activity, as typical of upwelling settings. These deposits formed in shallow marine conditions, characterized by high biological productivity and low net sedimentation rates, with suboxic conditions at and beneath the sediment-water interface. The concentration of phosphate-coated clasts was then favored by a sedim

The fossil record of cetaceans documents how terrestrial animals acquired extreme adaptations and transitioned to a fully aquatic lifestyle 1,2. In whales, this is associated with a substantial increase in maximum body size. Although an elongate body was acquired early in cetacean evolution 3, the maximum body mass of baleen whales reflects a recent diversification that culminated in the blue whale 4. More generally, hitherto known gigantism among aquatic tetrapods evolved within pelagic, active swimmers. Here we describe Perucetus colossus—a basilosaurid whale from the middle Eocene epoch of Peru. It displays, to our knowledge, the highest degree of bone mass increase known to date, an adaptation associated with shallow diving 5. The estimated skeletal mass of P. colossus exceeds that of any known mammal or aquatic vertebrate. We show that the bone structure specializations of aquatic mammals are reflected in the scaling of skeletal fraction (skeletal mass versus whole-body mass) across the entire disparity of amniotes. We use the skeletal fraction to estimate the body mass of P. colossus, which proves to be a contender for the title of heaviest animal on record. Cetacean peak body mass had already been reached around 30 million years before previously assumed, in a coastal context in which primary productivity was particularly high.

The Cenozoic succession of the East Pisco Basin preserves the sedimentary record of several episodes of deformation of the forearc crust along the Peruvian margin. The 1:50,000 scale geological map presented here encompasses an area of about 1,000 km2 lying astride the Ica River, and contributes to our understanding of the timing and mode of basin filling and deformation. Our novel two-fold megasequence framework provides a sound basis for establishing a first-order tectono-stratigraphic setting of the mid-Eocene–upper Miocene succession exposed in the study area. We interpret that the mid-Eocene to lower Oligocene succession studied in this work (megasequence P) was deposited in a single forearc basin, which was dissected into the present-day West and East Pisco basins by a fault-bounded basement high during the late Oligocene, and subsequently overlain by the Miocene fill of the East Pisco basin (megasequence N).

An Upper Miocene volcaniclastic interval, up to about 2.5 m thick, is exposed at several locations along the deep-water foreland basin system of central Apennines. Geochronological dating converges to indicate that these volcanic sediments were deposited at about 5.5 Ma (e.g., Odin et al., 1997), whereas mineralogical analyses indicate that the component strata comprise 80% of blocky and vesicular glassy shards and that the mineralogical phases represent the remaining 20% (Potere et al., 2022). These features, and the observed silica and alkali content, suggest that the volcaniclastic layers can be considered as the product of a calc-alkaline rhyolitic magma (Guerrera et al., 1986; Potere et al., 2022; Trua et al., 2010). In order to determine transport directions and depositional processes, in this study the volcaniclastic strata exposed on a recently made road cut near the town of Amandola (FM) have been investigated. At this new outcrop, the volcaniclastic horizon is 1.90 m thick and it comprises twenty-three beds, each recording a discrete density flow depositional event. Individual beds are characterized by sedimentological features typical of the product of turbidity currents, such as: i) flame structures along the contacts; ii) overall normal grain-size grading formed during temporal flow deceleration; and iii) the occurrence of a massive (i.e., ‘‘structureless’’) Ta division overlain by planeparallel and cross-laminated Tb and Tc divisions, with the massive division indicating deposition at rates of suspended load fallout high enough to suppress tractional transport and the laminated divisions indicating final flow stages, in which flow density and velocity are sufficiently low for bedform development to occur. The absence of laminated Tb and Tc divisions in some of the beds can be explained by the narrow grain-size distribution (i.e. the presence of well-sorted sediment) or by the post-depositional erosion of these divisions by succeeding sediment gravity f

We report on an aturiid (Cephalopoda: Nautiloidea) shell from the Pisco Formation, a Neogene marine sedimentary unit of the East Pisco Basin (southern Peru) that is widely known for its rich and exquisitely preserved marine vertebrate fossil content, including an outstanding cetacean assemblage. The studied specimen was collected from Middle Miocene strata exposed in the vicinity of Cerro Submarino. It consists of an internal mould of a phragmocone and is tentatively identified herein as belonging to the widespread, long- ranging species Aturia cubaensis. This fossil represents the first occurrence of Aturia in the Middle Miocene of the Pacific margin of South America; as such, it fills a gap in the chronostratigraphic distribution of the Southeastern Pacific finds of this genus, helping to bridge the Lower and Upper Miocene segments of its regional fossil record. The rarity of Aturia in the shelfal Cenozoic deposits of the East Pisco Basin may reflect the palaeoenvironmental habits of this extinct cephalopod genus, which may have lived in the upper bathyal zone, at about 250-350 m water depth. Despite recent suggestions that some extinct and extant marine mammal ecomorphotypes (including some odontocetes) were likely predators of nautiloids, there is no indication that any member of the diverse and abundant toothed whale faunas of the Pisco Formation exploited these shelled cephalopods as a relevant food source.

The detailed study of diatom laminations conducted by means of backscattered electron imaging serves as tool to unravel details of past ocean dynamics. In this paper we apply this method to the analysis of the diatomites of Cerro Los Quesos, Upper Miocene Pisco Fm, Peru. Numerous studies have been conducted on the Pisco Fm; however, a focus on its paleoceanographic significance is still lacking. In this work, we provide information on the oceanographic setting in the area at the time of diatomites deposition. The high abundance of deep-living Coscinodiscus laminae, proceeded by either a mixed lamina or a terrigenous one, let us hypothesize a deep position of the thermocline during the deposition of the Pisco diatomites; together with the scarcity of Chaetoceros Hyalochaete spp. resting spores, this evidence confutes the belief that equals high biogenic silica content in marine sediments with enhanced upwelling. Conversely, the depositional setting of the Pisco Fm diatomites is more similar to what is known as “permanent El Niño” (or “El Padre”) state, meaning a constant weakened upwelling (or upwelling of nutrients-poor waters). Climate modeling warns that an increase in atmospheric CO2 may lead to this mean state in the near future. Thanks to this study we also obtained refined information on the diatomites sedimentations rates. The comparison of the Pisco diatomites sedimentation rates with those of Quaternary diatomites gave strength to the hypothesis that the formation of the vertebrate Lagerstätte may have been enhanced, among others, by the so-called “impact-burial” mechanism.

Invertebrate taphonomy can provide significant information about the post-mortem processes that affected the fossil record. In the East Pisco Basin of southern Peru, a Panopea Ménard de la Groye, 1807 shell bed was found in the upper Miocene strata of the Pisco Formation, hinting at a peculiar biostratinomic and diagenetic history. This bed contains abundant invertebrate fossil molds cemented by dolomite. The specimens of the deep infaunal bivalve, Panopea sp., occur together with bivalves representative of shallow infaunal species (Trachycardium sp. and Dosinia ponderosa [Gray, 1838]) and balanid barnacles, which are sessile encrusters. The Panopea specimens host compound molds evidencing an abundant encrusting fauna, including serpulids, ?foraminifera, bryozoans, and barnacles that colonized the inner surfaces of the valves before their final burial. We hypothesize that short-term, storm-related processes exhumed the living bivalves, resulting in a sedimentological concentration of relatively well-preserved shells. After the death of the exhumed bivalves, the inner surfaces of the articulated Panopea shells, representing hard-substratal, sheltered environments on an otherwise unstable sandy seafloor (i.e., “benthic islands”), were colonized by different encrusting organisms. Following the final burial, dolomite precipitated, cementing the sediment infill of the valves. Lastly, a decrease of pH occurred at the sulfate reduction-methanogenesis boundary, inducing the dissolution of the shell carbonate.

We report 130 vertebrate fossils preserved as bony elements and the co-occurring assemblage of fish teeth and spines from the lower strata of the Pisco Formation exposed along the western side of the lower Ica Valley (East Pisco Basin, Peru). Geological mapping at 1:10,000 scale reveals that all these fossils originate from the Langhian–Serravallian P0 allomember. In the study area, P0 is up to ∼40 m thick and features a sandy lower portion, reflecting shoreface deposition, that fines upwards into a package of offshore silts. Marine vertebrates only occur in the lower sandy layers and include whales, dolphins, reptiles, birds, and bony and cartilaginous fishes. The reconstructed paleoenvironment is consistent with a warm-water, marginal-marine setting with a strong connection to the open ocean. This work helps to elucidate the rich yet still poorly understood middle Miocene portions of the Pisco Formation, and highlights the need to conserve this outstanding Fossil-Lagerstätte.

Fossil bones, together with teeth, are the most common remains of vertebrates that could manage to get preserved over geological times, providing information on the diagenetic and fossilization processes that occurred in the depositional paleoenvironment. Fossil bones from the marine vertebrate Konservat-Lagerstätte in the East Pisco Basin and Sacaco area (Peru) show a high variety of different textural and chemical features, suggestive of different processes variably contributing to the fossilization path. At the macroscopic scale, bone samples can be grouped into six different categories on the basis of the color (red to gray to white) and hardness (which relates to the mineralization degree); a variety of case studies can be found between these categories. Microscopically, the original microstructure of the bone tissue, both compact and cancellous, is well preserved in all the studied samples, with differences in cavity fillings, distribution of microcracks, and presence of Fe oxides in the diverse bone types. The bone composition and mineralogy correspond to fluorapatite. Differences in color, mineralization degree and geochemistry can be interpreted in terms of different fossilization paths, from burial at the seafloor to exposure in the present-day desert environment. The fossilization paths are strongly conditioned by the factors controlling the interplay of the mechanisms of apatite dissolution-recrystallization and dolomite precipitation (formation of carbonate concretions) as well as the fixation of iron in finely disseminated sulfides in the very early stages of fossilization.

Among the most outstanding Cenozoic marine Fossil-Lagerstätten worldwide, the Peruvian Pisco Formation is renowned for its exceptional preservation and abundance of fossil vertebrates, especially cetaceans. We present an updated overview and interpretation of taphonomic data gathered during fifteen field campaigns (2006-2019) on 890 fossil marine vertebrates from the Miocene strata of the Pisco Formation exposed in the Ica Desert. In order to assess the factors that led to the formation of such an exceptional Konzentrat- and Konservat-Lagerstätte, we made observations that range from the taxonomic distribution, articulation, completeness, disposition and orientation of skeletons, to the presence of bite marks, associations with shark teeth and macro-invertebrates, bone and soft tissue (i.e., baleen) preservation, and the formation of attendant carbonate concretions and sedimentary structures. We propose that the exceptional preservation and abundance of the Pisco Formation specimens cannot be ascribed to a single cause, but rather to the interplay of favorable palaeoenvironmental factors and suitable timing of mineralizing processes, such as: i) low concentration of dissolved oxygen at the seafloor; ii) the early onset of mineralization processes; iii) rapid burial of the carcasses; and iv) original biological richness in the southeastern Pacific. Our observations provide a comprehensive overview of the taphonomic characteristics of one of the most significant fossiliferous deposits of South America and lead to the elaboration of a complex scenario for the preservation of its marine vertebrates that might serve as a reference for explaining the formation of other marine vertebrate Fossil-Lagerstätten worldwide.

New age estimates obtained via Strontium Isotope (87Sr/86Sr) Stratigraphy and new paleoclimatic data are here presented for the Miocene Chilcatay and Pisco formations exposed in the East Pisco Basin, an Andean forearc basin of southern Peru, which is renowned worldwide for its exceptional content of fossil marine vertebrates. Mollusk and barnacle shells, carbonate nodules, and shark teeth were collected along three stratigraphic sections for applying Strontium Isotope Stratigraphy on both carbonates and phosphates. To avoid diagenetic biases, mollusk and barnacle shells were analyzed in detail by means of optical and scanning electron microscopy, cathodoluminescence, and inductively coupled plasma-optical emission spectrometry, whereas only the enameloid from the best-preserved shark teeth was sampled. The obtained 87Sr/86Sr ages confirm a late early Miocene (Burdigalian) age for the Chilcatay strata, and reveal middle Miocene (Langhian to Serravallian) ages for the lower Pisco unit (i.e., the P0 sequence) – a result that matches the relatively archaic aspect of its cetacean fossil assemblage. New and literature data about the fossil assemblage of the lower Pisco beds highlight the presence of several thermophilic invertebrates and vertebrates, thus suggesting a warm-water, tropical paleoenvironment for this middle Miocene sequence. Such a paleoenvironmental scenario recalls the warm conditions associated with the Chilcatay Formation, rather than the cooler setting inferred for the remainder of the Pisco Formation (i.e., the P1 and P2 sequences). This pattern likely reflects the late Miocene trend of global cooling, as well as a middle to early late Miocene strengthening of the Humboldt Current.

We seek to elucidate the Cenozoic history of the upwelling system off Peru-Chile, which currently forms the basis for one of the most productive fisheries in the world. To pursue this aim, we investigate the outstanding record of Eocene– Pliocene sedimentation and associated marine fossils in the East Pisco Basin of southern Peru, where the lower Miocene Chilcatay Formation is overlain by the Pisco Formation, the latter containing three allomembers, each reflecting a transgressive cycle: P0, P1 and P2, in ascending stratigraphic order. The Chilcatay Formation and the overlying middle Miocene P0 allomember (based on our recent strontium isotope stratigraphy) are comprised of lithologies (sandstones and siltstones) that suggest a limited upwelling. Furthermore, a warm-water palaeoenvironment is indicated by the thermophilic fossil assemblage in these units, including mollusks (architectonicids, cypraeids, and the genus Ficus), the only coral colony known from the basin (a rhizangiid scleractinian from P0), and the extinct snaggletooth shark Hemipristis serra. The overlying P1 and P2 allomembers are late Miocene in age and are characterized by abundant diatomites that suggest high primary productivity conditions. The present-day Peruvian coastal upwelling system results from a combination of the Peru Coastal Current and the offshore Humboldt Current. A precursor of this upwelling system emerged after the K/Pg boundary, and a proto-Humboldt Current originated only in the late Eocene. The Miocene transition in the East Pisco Basin from warm-water conditions to high-productivity and cooler conditions shows that there was a strengthening of the Humboldt Current during the late Miocene.

Bioapatite of fossil bone and teeth is susceptible to alteration and ion exchange during burial and diagenesis, varying its Sr content through the geological time. Nevertheless, fossil shark teeth are a powerful proxy for both chronostratigraphic and paleoecological reconstructions, thanks to the presence of the enameloid, a hard outer layer consisting of resistant fluorapatite crystallites. Here, we analyze fossil shark teeth from the Miocene sediments of the Chilcatay Formation of the Pisco Basin (southwestern Peru) with the aim of dating poorly constrained strata in this region. (Ultra)structural and compositional analyses of fossil lamniform and carcharhiniform teeth are performed through macroscopical observations, optical microscopy and SEM-EDS for evaluating the preservation state of the collected teeth. Shark teeth display a compact and well preserved outer enameloid layer formed by highly ordered bundles of crystallites that is distinctly separated by a more porous and heterogeneous inner core of dentine featuring diagenetic artefacts and microborings. Compositional mapping highlights differences in the distribution of Ca, P, F, and S in the enameloid and dentine, and chemical results show a Sr content that is consistent with the range reported for extant shark teeth. The best preserved teeth were selected for Strontium Isotope Stratigraphy (SIS), measuring the 87Sr/86Sr values in the enameloid and obtaining numerical (absolute) age estimates. In the Ica River Valley, SIS dates the Chilcatay strata to the Burdigalian (between 19.1 and 18.1 Ma), in agreement with previous radiometric, isotopic and biostratigraphic ages obtained in the same region. At Media Luna, the Chilcatay strata are dated herein for the first time, resulting in a slightly older age of 21.8–20.1 Ma (late Aquitanian–early Burdigalian). These results strengthen the notion that the Sr-ratio of shark teeth can be successfully applied for obtaining reliable age estimates via SIS.

Based on mapping of laterally traceable stratigraphic discontinuities, we propose a high-resolution allostratigraphic scheme for one of the world’s foremost fossil marine vertebrate Lagerstätten: the lower Miocene strata of the Chilcatay Formation exposed along the Ica River near Zamaca, southern Peru. Measured sections combined with 1:10,000 scale mapping of a 24 km2 area provide an overview of the stratal architecture, as well as a general facies framework and interpretation of the various depositional settings. As a whole, the Chilcatay alloformation is bounded by the CE0.1 unconformity at the base and the PE0.0 unconformity at the top. An internal Chilcatay surface, termed CE0.2, splits the alloformation into two distinct allomembers (Ct1 and Ct2). The Ct1 allomember comprises three facies associations recording deposition in shoreface, offshore, and subaqueous delta settings. The Ct2 allomember comprises two facies associations, recording deposition in shoreface and offshore settings. Using these data, we place the rich marine vertebrate assemblage in a precise spatial and stratigraphic framework. The well-diversified vertebrate assemblage is dominated by cetaceans (mostly odontocetes) and sharks (mostly lamniforms and carcharhiniforms); rays, bony fish, and turtles are also present. Taxonomic novelties include the first records of baleen whales, platanistids, and eurhinodelphinids from the Chilcatay Formation.

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 pr

The Miocene Pisco Formation, broadly exposed in the Ica Desert of southern Peru, is among the most outstanding Cenozoic marine Fossil-Lagerstätten worldwide. It is renowned for its exceptional preservation and abundance of vertebrate fossils, including a rich assemblage of whales and dolphins (Cetacea). Here, we integrate taphonomic data on 890 marine vertebrate fossils, gathered through 16 different localities. Our observations range from the taxonomic distribution, articulation, completeness, disposition and orientation of skeletons, to the presence of bite marks, associations with shark teeth and macro-invertebrates, bone and soft tissue preservation, and the formation of attendant carbonate concretions and sedimentary structures. We propose that the exceptional preservation characterising many Pisco vertebrates, as well as their exceptionally high abundance, cannot be ascribed to a single cause like high sedimentation rates (as proposed in the past), but rather to the interplay of several favourable factors including: (i) low levels of dissolved oxygen at the seafloor (with the intervention of seasonal anoxic events); (ii) the early onset of mineralisation processes like apatite dissolution/recrystallisation and carbonate mineral precipitation; (iii) rapid burial of carcasses in a soupy substrate and/or a novel mechanism involving scour-induced self-burial; and (iv) original biological richness. Collectively, our observations provide a comprehensive overview of the taphonomic processes that shaped one of South America’s most important fossil deposits, and suggest a model for the formation of other marine vertebrate Fossil-Lagerstätten.

The Eocene sediment successions of the East Pisco Basin (southern Peru) host an exceptionally rich and wellpreserved assemblage of vertebrate fossils. However, due to the dearth of geochronological and biostratigraphic controls as well as of stratigraphic correlations, our understanding of these rocks and their fossil content remains elusive. This paper provides a comprehensive calcareous nannofossil, diatom, and silicoflagellate biostratigraphic framework for the Eocene strata exposed at four localities along the Ica River Valley, permitting a robust chronological calibration of the marine vertebrate fauna entombed therein and a better definition of important appearance/ extinction events. The Paracas Formation, deposited directly on top of the Proterozoic and Paleozoic rocks of the crystalline basement, is formed by a siliciclastic-bioclastic sand-sized deposit (Los Choros member) and calcareous-terrigenous siltstone (Yumaque member) that was deposited from the Lutetian (47.8–41.2 Ma) through the Bartonian (41.2–37.7 Ma) to the early Priabonian (37.7–33.9 Ma). The unconformably overlying Otuma Formation consists of a basal sandstone, followed by calcareous siltstone intercalated by diatomite layers towards the top. In the study area, the Otuma Formation is Priabonian in age and is truncated at the top by an unconformity at the base of the overlying Miocene Chilcatay Formation. Due to the angular nature of the unconformity, the upper Otuma strata reach the Oligocene elsewhere. Average sedimentation rates range from 17 to 24 m/My in the Yumaque member of the Paracas Formation and increase to 147–170 m/My in the Otuma Formation. The microfossil assemblages witness a coastal setting with warm-temperate conditions for the Paracas Formation that become slightly cooler (though still temperate) in the upper Otuma Formation. Diatomaceous layers in the upper Otuma Formation indicate an overall increase in nutrient availability, which could reflect the global reorganization

Exquisitely preserved fossils of a new reef-building vermetid species from shallow-marine lower Miocene (Burdigalian) deposits of the Chilcatay Formation and upper Miocene (Tortonian) sediments of the Pisco Formation of Peru are here reported and described in detail for the first time. These finds are assigned to the living genus Thylacodes and recognized as representatives of a new species, Thylacodes devriesi sp. nov. This new taxon is known by long, almost straight tube-like shells that display peculiar ornamentations in form of striated lamellae and are arranged in an organ-pipe fashion. This discovery represents an important addition to the knowledge of the systematics and distribution of Thylacodes in South America in the geological past. Paleoenvironmental and taphonomic inferences drawn by the fossil remains of this reef-forming species are herein discussed for both the Chilcatay and Pisco Formations in the broader framework of the South American fossil record of Vermetidae.

As other marine minerals, phosphates - and in particular bioapatite - incorporate Sr from seawater during formation (Staudigel et al., 1985). Due to long-term variations in the relative abundance of Sr isotopes in the global ocean, 87Sr/86Sr values can be measured in minerals and compared to calibration curves for obtaining age estimates via Strontium Isotope Stratigraphy (SIS) (McArthur et al., 2020). Although hydroxyapatite is susceptible to alteration and Sr concentration in fish teeth can change during burial and diagenesis (Martin & Scher, 2004), fossil shark teeth have been successfully used for Sr-dating, especially when enameloid is analyzed (Schmitz et al., 1997; Harrell et al., 2016). In order to test the feasibility of this method in the fish tooth-rich marine sediments of the East Pisco Basin (Peru), and aiming to date some poorly-constrained strata of this region, we analyzed the ultrastructure and composition of fossil shark teeth from the Chilcatay Formation. This Miocene formation consists of massive sandstones and basement boulders overlain by bioclastic sandstones and diatomaceous and tuffaceous siltstones (Di Celma et al., 2019). It is characterized by an abundant marine vertebrate assemblage, among which elasmobranchs are present (Landini et al., 2019). Teeth of Isurus sp., Cosmopolitodus hastalis, Isurus oxyrinchus, Megalolamna paradoxodon and Physogaleus contortus were collected from Chilcatay beds at the localities of Zamaca, Media Luna and near Cerro Colorado. Teeth were investigated through an optical microscopy and SEM. After taphonomic observations, 11 teeth underwent ICP-OES and 87Sr/86Sr analyses. Shark teeth show a compact and non-porous outer enameloid layer that is distinctly separated from the more porous and heterogeneous inner core of dentine. Ultrastructure analysis shows that the enameloid is formed by highly-ordered bundles of fluoroapatite crystallites, which are often well-preserved, whereas the dentine displays a bone-lik

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.

The late Miocene Pisco Formation of Peru is an outstanding example of richness and high-quality preservation of fossil marine vertebrates. In order to reconstruct the fossilization path, we present new textural, mineralogical and Sr-isotope data of diagenetic minerals formed in correspondence of fossil specimens such as marine vertebrates and mollusks. These fossil specimens were found at Cerro los Quesos, in the Ica Desert, within the diatomaceous strata of the Pisco Formation. Dolomite, gypsum, anhydrite and Mn minerals are the main phases found, while the calcium carbonate originally forming the mollusk valves is replaced by gypsum. An early formation of dolomite and of Mn minerals, triggered by the modifications of the geochemical environment due to organic matter degradation, is suggested by the textural relationships and is confirmed by the Sr isotopic ratio of dolomite, which agrees with that of seawater at the time of sedimentation. Instead, gypsum Sr isotopic ratios indicate a pre-Miocene seawater-derived brine circulating within the sedimentary sequence as a source for Sr. Oxidation of diagenetic sulfide causing a lowering of the pH of porewater is proposed as an explanation for Ca-carbonate dissolution. The diagenetic chemical environment was, nevertheless, favorable to bone preservation

Two-mica—biotite and muscovite—volcanics are particularly rare in the geological record. One of the several dozen volcanic ash layers from Central Andes volcanoes found in the upper Miocene marine succession of the Pisco Formation (Ica Desert, Peru) contains juvenile biotite and muscovite, sillimanite/andalusite, feldspars, and rhyolitic glass. 39Ar–40Ar dating on biotite and muscovite concordantly constrain an age of 7.96 Ma for this two-mica ash layer. A second tephra in the Pisco Formation has a similar biotite composition and an age between 7.45 and 6.93 Ma. The peculiar mineral assemblage and the chemical composition of biotite indicate a strongly peraluminous composition of the erupted magmas and, together with the 39Ar–40Ar ages, suggest to consider a correlation of these ash layers to the eruptions of the Miocene Macusani (Peru) or Morococala (Bolivia) volcanic complexes in the Eastern Cordillera of Central Andes. The major and trace element composition of glass supports the correlation with Macusani. A provenance from Morococala seems less likely given the large distances involved. These results provide new data on the volcanic activity of the Eastern Cordillera revealing ash that dispersed to over 500 km to the west, in the forearc marine basins. This finding highlights that the exhumed forearc East Pisco Basin is highly promising as an archive of distal ash for the reconstruction of the volcanic activity of Central Andes during the Miocene silicic flare-ups.

Strata of Chilcatay and Pisco formations exposed in the Ica Desert (East Pisco Basin, southern Peru) preserve one of the most complete and rich records of Miocene marine vertebrates of the world. Despite its exceptional importance, the chronostratigraphy of these fossil-bearing deposits has been only sporadically studied in the literature until recently. This work presents a detailed reconstruction of the chronostratigraphic framework, achieved by mapping and logging of seven sections of the Miocene Chilcatay and Pisco formations along the western side of the Ica River. The Chilcatay Formation consists of two allomembers, namely Ct1 and Ct2, bounded at the base by unconformities CE0.1 and CE0.2, respectively. Similarly, the immediately overlying Pisco Formation is divided into allomembers P0, P1, and P2, bounded at the base by unconformities PE0.0, PE0.1 and PE0.2, respectively. The new 39Ar–40Ar results presented here, combined with ages of previous work, provide precise constraints on the age of several stratigraphically referenced volcanic ash layers intercalated in the studied fossil-bearing succession, placing its vertebrate fossil fauna within a refined temporal framework and laying the solid ground for its detailed regional and global comparison. The ages of the allomembers, and thus their associated faunas, can be reliably estimated by the combination of 39Ar–40Ar dating on tephra layers with diatom biostratigraphy. In the study area, the two methods are mutually consistent and constrain the deposition of the Chilcatay Formation between 19.2 and 18.0 Ma, that of P1 between 9.5 and 8.6 Ma, and that of P2 between 8.4 and 6.7 Ma. In the absence of direct dating of the P0 allomember, which lacks both preserved tephra suitable for 39Ar–40Ar dating and microfossils, its age can be constrained to the temporal gap between the youngest age available from the underlying Chilcatay strata (18.0 Ma) and the oldest age available from the overlying P1 strata (9.5 Ma).