Inside baleen: Exceptional microstructure preservation in a late Miocene whale skeleton from Peru

Exceptionally preserved delicate baleen microstructures have been found in association with the skeleton of a late Miocene balaenopteroid whale in a dolomite concretion of the Pisco Formation, Peru. Microanalytical data (scanning electron microscopy, electron probe microanalysis, X-ray diffraction) on fossil baleen are provided and the results are discussed in terms of their taphonomic and paleoecological implications. Baleen fossilization modes at this site include molding of plates and tubules, and phosphatization. A rapid formation of the concretion was fundamental for fossilization. We suggest that the whale foundered in a soft sediment chemically favorable to rapid dolomite precipitation, allowing the preservation of delicate structures. Morphometric considerations on the baleen plates and bristles coupled with the reconstructed calcification of the latter permit speculation on the trophic preferences of this balaenopteroid whale: the densely spaced plates and the fine and calcified bristles provide evidence for feeding on small-sized plankton, as does the modern sei whale

Looking for the key to preservation of fossil marine vertebrates in the Pisco Formation of Peru: new insights from a small dolphin skeleton

The upper Neogene Pisco Formation of Peru is known worldwide as one of the most significant Cenozoic marine vertebrate Konservatt-Lagerstätten, even featuring cetacean specimens that retain remains of soft tissues or stomach contents. Previous works showed that biomediated precipitation of dolomite concretions around large-sized decaying carcasses was one of the most relevant processes responsible for exceptional fossil preservation. In turn, little is known about the modes of fossilization of well-preserved small-sized vertebrates, which are rather common in the Pisco Formation, but mostly do not exhibit dolomite concretions. We report on a cetacean specimen, identified as belonging to the extinct short-snouted, small dolphin species Brachydelphis mazeasi (Pontoporiidae), preserved within a late Miocene sandy deposit at the site of Pampa Corre Viento. This specimen consists of a moderately disarticulated partial skeleton exhibiting well-mineralized bones; it is not enclosed within a dolomite concretion, being however delimited by an evident dark boundary in the host sediment. Scanning electron microscopy and microanalytical investigations identify Mn-oxides and apatite as early diagenetic minerals around the skeleton. We argue that a rapid burial of the specimen was pivotal for the preservation of the bones, and allowed the early establishment of anoxic processes for degradation of organic matter. Coupled with availability of P in porewater, the reducing conditions and the lowered pH allowed precipitation of Ca-phosphate while increasing Mn solubility close to the pontoporiid carcass. Mn-oxides precipitated at the redox boundary, the latter defining the outer edge of the volume of sediment affected by altered chemical conditions due to the decaying processes. The permeability of the sediment and the small size of the carcass were possible factors unfavorable to extensive sulfate reduction, thus preventing the formation of a dolomite concretion and allowing bone phosphatization. This record emphasizes the role of conditions favorable to bone mineralization in absence of an isolating carbonate concretion, in cases of high quality preservation of small-sized vertebrates observed in the Pisco Formation. The observation of patterns in the distribution of diagenetic minerals in the sediment enclosing vertebrate remains without, or with limited carbonate concretions provides insights into early taphonomic processes

Stratigraphic framework of the late Miocene to Pliocene Pisco Formation at Cerro Colorado (Ica Desert, Peru)

This paper describes a 200 m-thick section of the Pisco Formation exposed at Cerro Colorado, an important fossiliferous site in the Ica desert. In order to properly place the fauna in its correct relative position, this study establishes the stratigraphic framework within which the different fossil-bearing intervals of this site can be compared and may prove invaluable in future high-resolution studies on the faunal change. Most of the Pisco Formation deposits exposed at Cerro Colorado consist of gently dipping fine-grained sandstones, diatomaceous siltstones and diatomites with minor ash layers and dolomites deposited within nearshore and offshore settings. To facilitate detailed stratigraphic correlations within the Pisco strata for a 30 km2 area, eight marker beds have been defined and large-scale (1:10,000 scale) geological mapping conducted to determine fault positions, styles and offsets. The geological map shows that there are two important angular unconformities in the study area. The first one is the interformational basal unconformity of the Pisco Formation against folded, faulted, and planated Oligo-Miocene rocks of the Chilcatay Formation. The second is a low-angle intraformational erosional discontinuity of up to 48 angular discordance that allows the subdivision of the Pisco stratigraphy exposed in the study area into two informal allomembers. Dating of the exposed succession by diatom biostratigraphy suggests that the age of the lower allomember is late Miocene, whereas the upper allomember is late Miocene or younger

Insights into the diagenetic environment of fossil marine vertebrates of the Pisco Formation (late Miocene, Peru) from mineralogical and Sr-isotope data

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 Cacarbonate dissolution. The diagenetic chemical environment was, nevertheless, favorable to bone preservation

Fossil diatom (Bacillariophyceae) associations from the Mio-Pliocene laminated sediments of the Pisco Formation (East Pisco, Peru): relations with the preservations of marine vertebrates in the fossil record and investigations on the influence of volcanic activity on primary production. Associazioni a diatomee (Bacillariophyceae) fossili delle laminiti della Formazione mio-pliocenica di Pisco (bacino orientale di Pisco, Peru): relazione con la conservazione nel record fossile di vertebrati marini e indagini sull'influenza dell'attività vulcanica sulla produttività primaria.

Objectives - The Mio-Pliocene diatomaceous Pisco Formation, Peru, was deposited under upwelling conditions in the Pisco Basin, one of the many forearc basins scattered along the Peruvian coast. This formation is particularly popular among vertebrate palaeontologists due to its fossil content. Indeed, because of the high spatial concentration and continuous temporal distribution of perfectly preserved fossil marine vertebrate specimens, the Pisco Fm. is classified as a Konservat-Lagerstätte; this term identifies a place or a quarry characterised by exceptional preservation of fossils. Focusing on the sites of Cerro Colorado and Cerro Los Quesos, Ica Desert, Peru, this Thesis aims to understand which mechanisms brought to the formation of the Pisco Lagerstätte and whether the deposition of diatoms contributed to it in any way. Taking into consideration the geology of the area, three hypotheses on the formation of the Pisco Lagerstätte are advanced and investigated in this Thesis: 1) high sedimentation rates: rapid burial is listed in all the paleontology books as the main process that lead to the preservation of fossils. This hypothesis has been already taken into consideration by previous authors as the cause of the formation of the Pisco Lagerstätte. In particular, sedimentation rates of diatom frustules were advocated as responsible for the burial of whale carcasses by these authors. In this Thesis also the hypothesis that volcanic ashes could increase sedimentation rates by enhancing primary production is advanced. 2) anoxic/disossic environment: along the modern eastern Pacific continental margin, high amounts of organic matter produced during upwelling lead to a great consumption of oxygen. In the intermediate waters dissolved oxygen is < 1 ml/L. This same condition may have persisted during the deposition of the Pisco Fm., therefore inhibiting the aerobic decomposition of carcasses on the sea floor. 3) Harmful algal blooms (HABs): HABs are phenomena known to kill large amounts of marine species. Those blooms are related to different planktonic groups and are known to kill marine organisms either by: a) the production of a large amount of toxins; b) the lowering of dissolved O2 in the water column (anoxia of the water column). To verify the high sedimentation rates hypothesis, a detailed stratigraphic framework of the sites of Cerro Colorado and Cerro Los Quesos has been carried out, both by means of diatom biostratigraphy and by means of 39Ar/40Ar ages. Sedimentation rates have been subsequently calculated and discussed. Moreover, because diatomites are often laminated, a lamina-scale study has been carried out on the sediments underlying fossil whales to unravel the seasonality of diatom species deposition and consequently of the water column. This study also provided additional information on yearly sedimentation rates of the Pisco Fm. During the development of both biostratigraphic and lamina-scale studies, particular attention was paid to the identification of potential HABs. Field observations highlighted that most of the fossils at Cerro Colorado and Cerro Los Quesos are wrapped up in carbonatic concretions; analyses revealed that all of them are dolomite nodules. Therefore, a specific investigation on dolomite nodules was undertaken in order to understand whether they played a role in the formation of the Lagerstätte. Beside the study on the formation of the Lagerstätte, this Thesis aimed to study the role of volcanic ashes deposition on the fertilisation of the ocean surface. Indeed, scientists have recently become aware of the fertilizer role played by volcanic ash: following major volcanic eruptions, airborne volcanic ash may fall onto the ocean surface, where it releases nutrients (NH+4, NO-3, NO-2, PO3 -4, Si) and Fe.The Pisco Fm. is characterized by a great number of ash layers, making the Pisco Fm. a natural laboratory to study this phenomenon in the Neogene. This Thesis aimed to contribute to the development of an investigation method relevant to fossil contexts. Therefore, observations were made both on the field and in the laboratories. The importance of volcanic ashes in increasing primary production takes much more relevance in the general context of this Thesis if this capability is interpreted as the role of ashes in increasing sedimentation rates (therefore further favouring rapid burial of carcasses). Results - Results obtained by diatom stratigraphy and tephrochronology converged in stating that the sites of Cerro Colorado and Cerro Los Quesos are late Miocene in age. Results highlight that sedimentation rates in the Pisco Fm. spanned between 19 cm/ka ± 1 to ca. 33 cm/ka (0.2 mm/a to 0.3 mm/a). These sedimentation rates are calculated for compacted sediment and can be transformed into ‘real sedimentation rates’ (sedimentation rates for non compacted sediment) applying a compaction factor equal to 60%; applying this factor, sedimentation rates equal to 32 cm/ka and 55 cm/ka, respectively, are obtained. Taking into account only the yearly sequence of diatom laminae, sedimentation rates reach a peak of 150 cm/ka (1.5 mm/a) for compacted sediments, equal to a ‘real’ sedimentation rate of 250 cm/ka (2.50 mm/a). All the listed sedimentation rates are in line with those calculated for Quaternary sediments in high-productivity basins, confirming that sedimentation rates during the deposition of the Pisco Fm. were high but not exceptional. Investigations conducted on laminae were also used to interpret the seasonal stratification that characterised the water column during the deposition of the Pisco Fm. The sequence of terrigenous lamina – Coscinodiscus ooze – mixed lamina (Chaetoceros RS/Thalassionema plus terrigenous material) has been interpreted as the yearly turnover of diatom species in an upwelling environment. The terrigenous lamina represents the austral summer sediment input. Coscinodiscus oozes are the product of the summer flourishing of this genus at the thermo/nutricline and of its subsequent late summer/early autumn sedimentation (the so-called ‘fall dump’) which follows the breakage of the summer water column stratification. The mixed lamina represents the spring bloom. Many clues suggest that sediments of the Pisco were not completely anoxic: 1) the presence of mollusc; 2) the pervasive bioturbations recognised both in the diatom laminae and at the top of tephra; 3) the coupling of fossil vertebrates with dolomite nodules surrounding the fossils themselves. Indeed, if the sediments of the Pisco had been anoxic, due to the high amounts of organic material (diatom cells) exported to the sea floor, dolomite would have precipitated also far from the vertebrate carcasses. On the contrary, dolomite nodules are always coupled to fossils. This means that only in correspondence of the carcasses, oxygen levels were reduced to a point that lead to sulphate-reduction and methanogenesis with consequent precipitation of dolomite. Dolomite nodules have had a pivotal role in preserving the marine vertebrates in the fossils record: rapid dolomitisation prevented the skeletons from being disarticulated and protected bones from chemical and mechanical erosive agents. Therefore fast dolomitisation processes played a pivotal role in the formation of the Pisco Lagerstätte. However, although not exceptional, also high sedimentation rates favoured the complete burial of carcasses under the sea bottom surface, where sulphate reduction, methanogenesis and, consequently complete dolomitisation, were favoured. Sediment porosity, may have favoured rapid burial and dolomitisation of carcasses as well. Indeed, sediment structures underlying some carcasses suggest that carcasses ‘sank’ into the sediment as they arrived at the sea bottom. Finally, I cannot exclude that the high concentration of carcasses in the Pisco Fm. is due to high density of the marine mammal populations in the Pisco Basin during the late Miocene; to answer this question a dedicated study with a more biological approach should be developed. For what the HABs hypothesis is concerned, no evidence harmful diatoms has been found during the development of this Thesis; similarly, the study concerning the enhancing of primary production due to volcanic ashes needs to be further developed and no conclusions can be drawn on at the moment for this hypothesis. As a final statement, I can affirm that the formation of the Pisco Lagerstätte was caused by the concurrence of many diverse ecological factors; however, the geochemical conditions of the sea bottom and high sedimentation rates were certainly two of the most important ones. Dolomite nodules frequently cemented siliciclastic sediments, not only diatomaceous ones, confirming that sedimentation of diatom frustules to the sea floor is to be considered just one of the many factors concurring in the formation of the Pisco Lagersätte, not its principal cause. Obiettivi - La Formazione diatomica Mio-Pliocenica di Pisco, in Perù, si è depositata nel bacino del Pisco, uno dei tanti bacini forearc che caratterizzano la costa peruviana, in condizioni di upwelling. Questa formazione è particolarmente popolare tra i paleontologi dei vertebrati grazie al suo contenuto in fossili. Infatti, a causa dell’elevata concentrazione spaziale di esemplari fossili perfettamente conservati e del loro record temporale continuo, la Pisco Fm. è classificata come un Konservat-Lagerstätte; questo termine identifica un luogo o una formazione caratterizzati da un’eccezionale conservazione dei fossili. Ponendo il fuoco sui siti di Cerro Colorado e Cerro Los Quesos (deserto di Ica, Perù), questa Tesi si propone di capire quali meccanismi hanno portato alla formazione del Pisco Lagerstätte e se la deposizione delle diatomee ne abbia in qualche modo favorito la formazione. Prendendo in considerazione la geologia della zona, sono state avanzate e studiate tre ipotesi: 1) alti tassi di sedimentazione: il rapido seppellimento è elencato in tutti i libri di paleontologia, come il processo principale che porta alla conservazione dei fossili. Questa ipotesi è già stata presa in considerazione dai precedenti autori come la causa della formazione del Pisco Lagerstätte. In particolare, è stato ipotizzato che siano stati i tassi di sedimentazione dei frustoli delle diatomee a favorire il rapido seppellimento delle carcasse. In questa Tesi viene avanzata anche l'ipotesi che le ceneri vulcaniche possano aver aumentato la produzione biosilicea incrementando, di conseguenza i tassi di sedimentazione delle diatomee stesse. 2) ambiente anossico / disossico: lungo il moderno margine continentale del Pacifico orientale, elevate quantità di materia organica prodotte a causa dell’upwelling portano ad un grande consumo di ossigeno. Nelle acque intermedi ossigeno disciolto è <1 ml / L. Questa stessa condizione può essere persistita durante la deposizione della Pisco Fm., inibendo quindi la decomposizione aerobica delle carcasse sul fondo del mare. 3) fioriture algali velenose (Harmful algal blooms, HABs): gli HABs sono fenomeni noti per uccidere grandi quantità di organismi marini. Tali fioriture sono tipiche di diversi gruppi planctonici e sono note per uccidere organismi marini attraverso: a) la produzione di una grande quantità di tossine; b) l'abbassamento di O2 disciolto nella colonna d'acqua (anossia della colonna d'acqua). Per verificare l'ipotesi relativi ai tassi di sedimentazione è stato realizzato un dettagliato quadro stratigrafico dei siti di Cerro Colorado e Cerro Los Quesos, sia attraverso la biostratigrafia a diatomee, sia per mezzo delle datazioni 39Ar / 40Ar. Si è quindi potuto calcolare e discutere i tassi di sedimentazione per i due siti. Dal momento che le diatomiti risultano spesso laminate, è stato intrapreso uno studio a scala di singole lamine per studiare la stagionalità della deposizione delle diverse specie di diatomee, conseguentemente, dei movimenti verticali a cui era sottoposta la colonna d’acqua. Questo studio, che ha preso in considerazione i sedimenti sottostanti i fossili di balena, ha anche fornito ulteriori informazioni sui tassi di sedimentazione annuali della Pisco Fm. Oltre alle osservazioni sui tassi di sedimentazione, sulle condizioni di ossigenazione del fondo del mare e sugli HABs, osservazioni sul campo hanno evidenziato che la maggior parte dei fossili a Cerro Colorado e Cerro Los Quesos sono avvolti in concrezioni carbonatiche; le analisi hanno rivelato che tutti i noduli sono noduli dolomitici. Pertanto, è stata svolta anche un'indagine specifica sui tali noduli per verificarne l’eventuale influenza sulla formazione del Lagerstätte. Accanto allo studio sulla formazione del Pisco Lagerstätte, in questa Tesi si è tentato di evidenziare il ruolo della deposizione delle ceneri vulcaniche nel fertilizzare la superficie dell’oceano. Recentemente, alcuni studi hanno infatti evidenziato il ruolo di fertilizzante della cenere vulcanica: dopo grandi eruzioni, le ceneri possono cadere sulla superficie dell'oceano, dove rilasciano sostanze nutritive (NH+ 4, NO-3, NO-2, PO3 - 4, Si) e Fe. La Formazione di Pisco. è caratterizzata da numerosi di livelli di cenere, rendendo la Pisco Fm. un laboratorio naturale per studiare questo fenomeno nel Neogene. A riguardo, l’obiettivo di questa tesi è stato quello di sviluppare un metodo di indagine applicabile ai contesti fossili. Pertanto, sono state effettuate osservazioni sui livelli di cenere sia sul campo che in laboratorio. Nel contesto generale di questa tesi, uno studio sulla capacità di fertilizzazione delle ceneri vulcaniche acquista rilevanza se si considera la potenziale influenza di questo fenomeno sull’incremento della velocità di sedimentazione delle diatomee (favorendo quindi ulteriormente il rapido seppellimento delle carcasse). Risultati - I risultati ottenuti dalla biostratigrafia a diatomee e dalla tefrocronologia convergono nell'affermare che i siti di Cerro Colorado e Cerro Los Quesos sono di età tardo miocenica. I risultati mettono in evidenza che i tassi di sedimentazione della Pisco Fm. oscillano tra i 19 cm / ka ± 1 e ca. 33 cm / ka (0,2 mm / a 0,3 mm / a). Questi tassi di sedimentazione sono calcolati per sedimenti compattati e possono essere trasformati in 'tassi di sedimentazione effettivi' (velocità di sedimentazione per i sedimenti non compattati) applicando un fattore di compattazione pari al 60% ; in questo modo si ottengono rispettivamente tassi di sedimentazione pari a 32 cm / ka e 55 cm / ka. Tenendo conto solo la sequenza annuale di deposizione delle diatomee (studiata attraverso le lamine), i tassi di sedimentazione raggiungono un picco di 150 cm / ka (1,5 mm / a) per sedimenti compattati, pari a un tasso di sedimentazione 'reale' di 250 cm / ka (2,50 mm / a). Tutti i tassi di sedimentazione elencati sono in linea con quelli calcolati per sedimenti quaternari in bacini sedimentari ad alta produttività primaria, confermando che i tassi di sedimentazione durante la deposizione del Pisco Fm. erano alti, ma non eccezionali. Le indagini condotte sulle lamine sono state utilizzate per interpretare la stratificazione stagionale che caratterizzava la colonna d'acqua durante la deposizione della Pisco Fm. La sequenza lamina terrigena - Coscinodiscus ooze - lamina mista (Chaetoceros RS / Thalassionema e materiale terrigeno) è stata interpretata come la sequenza annuale di deposizione del sedimenti in un ambiente di upwelling. La lamina terrigena rappresenta l'input di sedimenti nel bacino durante l’estate australe. Gli oozes a Coscinodiscus sono il prodotto della fioritura estiva di questo genere a livello del nutri/termoclino e della successiva deposizione durante la fine dell’estate / inizio autunno (il cosiddetto fenomeno del 'fall dump') che consegue alla rottura della stratificazione della colonna d'acqua. La lamina mista rappresenta infine la fioritura primaverile. Molti indizi suggeriscono che i sedimenti del Pisco non erano completamente anossici durante la deposizione della formazione stessa. Infatti si è osservato: 1) la presenza di molluschi; 2) diffuse bioturbazioni sia nelle laminazioni a diatomee, sia al top dei livelli di ceneri; 3) l'associazione dei vertebrati fossili ai noduli dolomitici che circondano i fossili stessi. Infatti, se i sedimenti del Pisco fossero stati anossici, l’elevata quantità di materia organica (diatomee) esportata al fondo dell’oceano, avrebbe causato la precipitazione della dolomite anche lontano dalle carcasse dei vertebrati. Al contrario, i noduli dolomitici sono sempre associati ai fossili. Ciò significa che solo in corrispondenza delle carcasse i livelli di ossigeno venivano ridotti al punto tale da attivare i processi che causano la precipitazione della dolomite, vale a dire riduzione dei solfati e metanogenesi. Gli studi condotti sui noduli dolomitici hanno rivelato che questi hanno favorito la preservazione dei vertebrati marini nel record fossile: la rapida dolomitizzazione ha infatti inibito la disarticolazione delle scheletri e li ha protetti dagli agenti erosivi, chimici e meccanici, che agiscono sul fondo dell’oceano. Pertanto, i rapidi processi di dolomitizzazione hanno svolto un ruolo fondamentale nella formazione del Pisco Lagerstätte. A questi hanno contribuito gli alti tassi di sedimentazione che, seppur non eccezionali, hanno favorito la completa sepoltura delle carcasse sotto la superficie di fondo del mare, dove hanno luogo riduzione dei solfati e metanogenesi,favorendo, di conseguenza, una rapida dolomitizzazione. La porosità dei sedimenti può aver a sua volta favorito una rapida sepoltura e dolomitizzazione delle carcasse. Infatti, le strutture sedimentarie sottostanti alcune carcasse suggeriscono che queste ‘affondarono’ nel sedimento una volta arrivate sul fondo

Diatom Biostratigraphy at the Miocene Sites of Cerro Colorado and Cerro Los Quesos, Pisco Formation, Peru

The Pisco Formation is a Mio-Pliocene formation cropping out for 300 km along the coast of Peru. This mainly diatomaceous formation is famous worldwide for being a fossil marine vertebrates Lagerstätte. Diatom biostratigraphy was applied to date the Pisco Fm. in few previous works (Macharé et Fourtanier, 1987; Schrader et Ronning, 1988; Koizumi, 1992); those works applied species ranges used in both the Cenozoic middle- to high-latitude North Pacific zonation and Cenozoic low-latitude zonation of Barron, 1985. During recent fieldworks stratigraphic sections at Cerro Colorado and Cerros Los Quesos (Ica desert, Peru), were measured and samples for biostratigraphy were collected ca. every 5 m. Difficulties arose during the biostratigraphic investigations because of the paucity of index species: indeed, coastal and upwelling conditions, which characterized the two sites, lead to the proliferation of species with long stratigraphic ranges such as Odontella aurita, Rhaphoneis spp., Delphineis spp., Coscinodiscus spp., Chaetoceros spp., etc. Results were obtained merging together ranges of species used both in the equatorial Pacific and in the North Tropical Pacific schemes. Cerro Colorado is characterized by an angular discordance that divides it into a lower and an upper allomember (Di Celma et al., 2015). The co-occurrence of the diatom species Denticulopsis hustedtii and Lithodesmium reynoldsii allows the attribution of the lower allomember to the late Miocene, while the only index species present in the upper allomember is Thalassiosira antiqua, which has a stratigraphic range that goes from the late Miocene to the late Pliocene. Thus samples from the upper allomember alone do not permit an accurate dating of the post-discordance sediments. The base of the Cerros Los Quesos sequence is characterized by a younger form of T. antiqua, differring from that of Cerro Colorado by having a more regular central area. Therefore, speculations on the time-relationship between the two sites can be made based on differences in T. antiqua. The presence of many plicated Thalassiosira specimens ascribable to Thalassiosira flexuosa, suggest that Cerros Los Quesos, although younger than Cerro Colorado, is still late Miocene in age

Biostratigraphy, geochronology and sedimentation rates of the upper Miocene Pisco Formation at two important marine vertebrate fossil-bearing sites of southern Peru

The Mio-Pliocene Pisco Formation, Peru, is a world renowned Konservat-Lagerstätte. Beside its potential in preserving important information on the history of the Pacific Ocean during the Miocene, the Pisco Fm. provides palaeontologists with the unique opportunity to study in detail the evolution of marine vertebrate fauna during the Neogene. Because diatomites are one of the main lithologies of the Pisco Fm., previous authors have hypothesised exceptionally high sedimentation rates related to the deposition of diatom frustules as the main cause leading to the formation of the Lagerstätte. However, sedimentation rates were never calculated. With the perspective to build a solid stratigraphic framework for the whole Pisco Fm., we started to investigate two important fossiliferous sites: Cerro Colorado and Cerro Los Quesos. Within these two areas, measurement of multiple stratigraphic sections combined with extensive field mapping of a number of distinct marker beds was carried out, which allowed a high-resolution correlation and creation of a local lithostratigraphic framework. Integrated tephrostratigraphy, 40Ar/39Ar dating and diatom biostratigraphy permitted an accurate chronostratigraphy to be established. Both biostratigraphic and radiometric ages converge to indicate a late Miocene age for the two sites, amending previous attribution of Cerro Colorado and Cerro Los Quesos to the middle Miocene. Age models for the site of Cerro Los Quesos reveal that sedimentation rates are high but in line with those of high-productivity environments, thus invalidating the hypothesis of exceptional sedimentation rates as the main cause for the formation of the Lagerstätte. These results are extremely important also in the perspective of building a complete stratigraphic framework for the Pisco Fm. © 2017 Gebrüder Borntraeger, Stuttgart, Germany