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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.

The moderately rich past diversity of the superfamily Inioidea (Cetacea, Odontoceti) in both the Atlantic and Pacific oceans contrasts with the present survival of a single genus (Inia, Amazon river dolphin, family Iniidae) in freshwater deposits of South America and of a single species (Pontoporia blainvillei, franciscana, family Pontoporiidae) along the eastern coast of that continent. However, part of the late Miocene – Pliocene inioid fossil record consists of relatively fragmentarily known species, for which systematic affinities remain poorly understood. Based on a sample of six cranial specimens from lower upper Miocene (Tortonian, 9.5–8.6 Ma) marine deposits of the Pisco Formation exposed at four localities of the East Pisco Basin (southern coast of Peru), we describe a new genus and species of inioid, Samaydelphis chacaltanae. This mesorostrine, small-sized species is characterized by an upper tooth count of c. 30 teeth per row, a moderately elevated vertex of the cranium displaying a long anteromedial projection of the frontals and interparietal, and the plesiomorphic retention of a premaxilla–nasal contact. Recovered as a member of the family Pontoporiidae in our phylogenetic analysis, S. chacaltanae falls as sister group to Meherrinia isoni, from the upper Miocene of North Carolina (USA), which has previously been tentatively referred to the Iniidae or regarded as a stem Inioidea. Originating from the P1 allomember of the Pisco Formation, the mesorostrine S. chacaltanae was contemporaneous and sympatric with two other inioids, the brevirostrine pontoporiid Brachydelphis mazeasi and the longirostrine iniid Brujadelphis ankylorostris.

Along the Peruvian coast, the sedimentary succession of the East Pisco Basin is exposed in the Ica Desert. At Cerro Los Quesos, laterally continuous dolomite layers characterise the diatomaceous sediments of the P2 sequence of the Miocene Pisco Formation, where a large number of marine vertebrates are exceptionally preserved, many enclosed in dolomite nodules. In this work, cemented layers from this sequence were described and sampled for petrographic, chemical, microscopic and isotopic analyses. Dolomite occurs in continuous 10–50 cm thick well cemented layers, formed by sediment of different nature: biogenic, terrigenous, volcanoclastic, and phosphatic. The underlying sediments exhibit a yellow layer with sparse dolomite crystals, a black layer with abundant Mn-oxides, and a reddish layer rich in Fe-oxides, indicating redox-related fronts. Two generations of dolomite can be recognised: an early diagenetic microcrystalline one, and a sparry one, filling the large cavities. As observed in both thin sections and on broken surfaces, microcrystalline dolomite also fills the inner spaces of the diatom areolae replicating their finest ultrastructure, such as foramina and cribra and replace calcite shells of foraminifera. δ18O and δ13C values from the microcrystalline dolomite of two layers, selected based on the absence of other carbonate phases (e.g. calcite) and the lack of sparry cement, are in agreement with those reported for the Peru margin and fall in the fields of either sulphate-reduction or methanogenesis. All the data point to dolomite precipitation associated with low-temperature early diagenesis that typically occurs in upwelling settings, where high surface water productivity is responsible for high rates of organic carbon flux to the sea bottom and for the cyclical oxygen depletion at the bottom. Such conditions also promote high abundances of marine vertebrates and the exceptional preservation of their skeletons in the sediments.

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 measuring of seven sections (totaling a height of almost 1000 m) 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, 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).

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.

The Eocene sediment successions of the East Pisco Basin (southern Peru) host an exceptionally rich and well-preserved 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 of ocean currents at the Eocene-Oligocene transition. However, the taxonomic composition of the diatom assemblage suggests seasonal rather than persistent upwelling conditions.

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.

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.

Tephra layers are a unique tool for stratigraphy. Their geologically instantaneous deposition together with their dispersion on wide areas make them a powerful instrument for dating and correlating simultaneous events at different localities. In this paper, we present and discuss the application of tephra fingerprinting in the upper Miocene portion of the Pisco Formation. The Pisco Formation is an important marine vertebrate Fossil-Lagerstätte that crops out in the Ica Desert, on the Peruvian Coast. The paleontological relevance of this formation has entailed the necessity of reconstructing a chronostratigraphic framework, using 40Ar/39Ar dating and correlations on volcanic ashes. Tephra layers from the volcanic activity of the Peruvian Andes are very frequent in the Pisco Formation and many represent a primary air-fall deposition on the sea floor. Therefore, many samples from tephra layers within measured stratigraphic sections at different sites in the study area were fingerprinted with a combined approach, using petrographic and chemical analyses on phenocrysts and volcanic glasses, as well as glass shard morphology and granulometric analyses, performed on 53 selected samples, collected from different localities. Based on the results obtained, we correlate some tephra layers in two localities at several km distance, where the correlation between the measured sections was previously only hypothetical. Major element chemistry of biotite proved a valuable tool to discriminate different tephra in the upper Miocene portion of the Pisco Formation, due to the presence of peraluminous as well as metaluminous compositions. This work shows that it is possible to apply tephra fingerprinting and tephrostratigraphy in a complex situation such as that represented by the Pisco Formation, where a high number of apparently similar volcanic ash layers were deposited in a shallow marine environment. The application of this correlation method allows us to increase the chronostratigraphic detail in the studied interval of the Pisco Formation, providing an accurate framework in which fossil marine vertebrates are stratigraphically located. The obtained high-resolution chronostratigraphy will greatly clarify the evolutionary history of Cetaceans.

The Plio-Pleistocene Calvello-Anzi Basin (CAB) represents a synorogenic shallow marine to alluvial succession deposited during the late phase of the Apennine Orogenic event. While the stratigraphy and sedimentary cycles are well known, the precise role and timing of the tectonic phases affecting this area are unclear. In this paper we report the major findings from a study aimed at unraveling the signatures of the Plio-Quaternary tectonic events affecting the CAB and this portion of the Southern Apennines by using a field integrated approach characterized by facies analysis, structural surveying, and basin architecture. The CAB is located in the axial portion of the Southern Apennine chain and has a roughly E-W orientation which is at a high angle with respect the main NW-SE striking regional tectonic structures controlling the foreland-directed migration of the compression. In the study area, an approximately 1,200-meter-thick package of Zanclean (early-middle Pliocene) to Santernian (middle Pleistocene) shelf, shallow marine, and alluvial deposits accumulated during the late stages of the Apennine orogenic evolution. Within the CAB deposits we recognize two main depositional motifs which roughly correspond to previously established regional cycles that are genetically linked to two corresponding tectonic phases of the Southern Apennine chain. Our results reveal how these two main tectonic phases played a primary role in controlling the depositional architecture, facies, depocenter distribution, and the basin shape throughout the CAB's lifespan. In the proposed model, during the first depositional stage (Zanclean), the CAB formed as a wedge-top basin, controlled by the subsurface propagation of NW-SE-trending thrusts related to the main NE-SW directed shortening of the Apennines. This stage is recorded in the basal units of the CAB infill by progressive unconformities, the development of syn-tectonic shallow marine sedimentary wedges along a high gradient tectonically controlled depositional surface, and a marked SE migration of the main depocenter. During the second depositional stage (upper Piacenzian to Santernian), the CAB was controlled by a series of roughly E-W-trending normal faults which crosscut the preexisting compressional structural fabric. Syndepositional activity along these faults played a direct role in creating an E-W oriented tectonic depression and controlling the accommodation space for the upper portion of the CAB infill. This second stage is marked by two points of evidence. The first point is revealed by a southwestward migration of the CAB depocenter driven by ~90-degree rotation from a roughly NW-SE elongated pattern to roughly elongated W-E transverse to the Pliocene regional structural fabric. This depocenter drift also coincides with an abrupt change in depositional regime from shallow marine to continental sedimentation. The second and most important point of evidence is the presence of several basin-bounding extensional fault zones which cut the lower Pliocene basin-fill units. We relate these features to the middle upper Pleistocene transition to along-strike extension, widely reported in the Southern Apennines, which is related to the overall extensional regime that is acting on the chain also at present. Data from this study provide additional insight into the evolution of the Southern Apennine chain, the formation of the Plio-Pleistocene CAB, and the debated role of Quaternary tectonics on the basin's development during the post-collision orogenic phases. Given the proximity of the study area with several oil fields and considering that hydrocarbon traps in Southern Apennines are Plio-Quaternary in age, the constraints provided on the timing and the style of the regional deformation may improve the definitions of hydrocarbon traps in the buried Apulian platform.

Bedforms and resulting sedimentary structures of interpreted upper-flow-regime origin are fairly common in various ancient and modern depositional settings, yet outcrop examples of shallow-water strata dominated by sedimentary structures arising from such flow conditions are rarely documented. This outcrop-based study presents the first detailed analysis of mid-Pleistocene beachface-shoreface strata containing sedimentary structures interpreted to have been formed by storm-generated backwash flows during the transition in flow regime from supercritical to subcritical. Here we propose that moving down the steep beachface, backwash flows rapidly accelerated and became thinner, experienced abrupt deceleration and flow thickening passing through an erosional hydraulic jump in the trough at the toe of the beachface, and then waned to thicker and subcritical conditions just downflow of the hydraulic jump. In this frame, landward-dipping backset laminae deposited on the downflow side of the trough evolved into thinner, upper-stage plane-parallel laminae further downstream. The transition from the proximal to the distal upper shoreface records a significant increase of preserved wave ripples and burrowed intervals, in concert with a progressive decrease in thickness and grain-size of individual sets of plane-parallel laminae and increase of gravel-filled gutter casts.

International audience; wo-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.

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 report on bite marks incising fossil mammal bones collected from upper Miocene deposits of the Pisco Formation exposed at Aguada de Lomas (southern Peru) and attributed to the giant megatooth shark Carcharocles megalodon. The bitten material includes skull remains referred to small-sized baleen whales as well as fragmentary cetacean and pinniped postcrania. These occurrences, the first in their kind from the Southern Hemisphere, significantly expand the still scarce record of bite marks for C. megalodon; moreover, for the first time a prey (or scavenging item) of C. megalodon is identified at the species level (as Piscobalaena nana, a diminutive member of the extinct mysticete family Cetotheriidae). Due to the fragmentary nature of the studied material, the exact origin of the detected marks (i.e., by scavenging or by active predation) cannot be ascertained. Nevertheless, relying on actualistic observations and size-based considerations, we propose that diminutive mysticetes (e.g., cetotheriids) were some of the target prey of adult C. megalodon, at least along the coast of present-day Peru. C. megalodon is thus here interpreted as an apex predator whose trophic spectrum was focused on relatively small-sized prey. Lastly, we propose a link between the recent collapse of various lineages of diminutive mysticetes (observed around 3 Ma) and the extinction of C. megalodon (occurring around the end of the Pliocene).

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.

Large discordant breccia bodies (LDBBs) are important record keepers of the tectonic and gravitational evolution in platform-to-basinal settings, and have important implications for fluid-flow migration and compartmentalization of tight carbonate reservoirs. In the Gargano Promontory of southern Italy, LDBBs occur within a Cretaceous slope and basinal carbonate succession. We use field mapping and Unmanned Aerial Vehicle (UAV) -based Structure from Motion (SfM) Photogrammetry to document otherwise inaccessible cliff-side exposures of seismic-to subseismic-scale vertical discordant breccias. LDBBs are up to 50 m in width, more than 80 m in height and display internal chaotic or aligned clast fabrics. The formation which generally contains the LDBBs is characterized by beds of cherty pelagic limestone intercalated with calcarenites, calciturbidites and horizons of mass transport deposits. The mass-transport deposits can be correlated across the chaotic breccia bodies, indicating only slight or no vertical displacement across the adjacent walls. The bases of the breccia bodies are always hidden below current sea level, while the rarely exposed tops are capped by bedded intervals of the host rock formation. Timing and origin of the studied breccias were determined using several lines of evidence, such as stratigraphic provenance of clasts in breccias, mutual relationships of structural and sedimentologic features, and previous studies which establish that the age of dolomitizing cements in the LDBBs formed at different times and by different processes (fault shearing and solution collapse). This work investigates the size, shape and geometry of these breccia bodies whilst also providing cm-scale detail of the textural features in otherwise inaccessible outcrops. We suggest that breccias formed as a result of solution exploiting a pre-existing fracture network characterized by large-scale vertical strike-slip or oblique-slip faults. Initial displacement along these faults created a wide fault damage zone, where fluid migration was later focused to create a zone highly susceptible to solution and subsequent periodic sidewall collapse.

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-Lagerstatte 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.

Lepidocyclinids are one of the most common group of Cenozoic large benthic foraminifera and, thanks to their clear evolutionary patterns, they are extensively used in biostratigraphy. They originated during the Eocene in the American region and then spread eastward, reaching a worldwide distribution in the Oligocene. This paper investigates the southernmost population of lepidocyclinids ever found along the Pacific coast of South America, located in the lower part of the Paracas Formation (Los Choros Member) of the East Pisco Basin of Peru. The examined Los Choros strata are composed of mixed siliciclastic-bioclastic coarse-grained deposits. The skeletal assemblage is dominated by large benthic foraminifera and mollusks with subordinate echinoids and barnacles, suggesting a deposition in a shallow, tropical, shelf environment with a moderate nutrient supply. The large benthic foraminiferal assemblage is largely dominated by Lepidocyclina rdouvillei. This species is extremely primitive, exhibiting ancestral characters such as a straight wall separating the protoconch and deuteroconch, very large principal auxiliary chambers and a poorly organized equatorial plane. The presence of common and primitive L. rdouvillei, supported by the associated occurrence of Polylepidina, suggests an age comprised between 43.6 Ma and 40.5 Ma for the Los Choros Member in the study area. This range is confirmed by nannofossil assemblages that indicate an age between 42.37 Ma and 40.34 Ma for the base of the overlying Yumaque Member of the Paracas Formation. These results highlight the potential of lepidocyclinids for biostratigraphy and paleoenvironmental reconstructions in South America.

The Miocene Gorgoglione Flysch Formation records the stratigraphic product of protracted sediment transfer and deposition through a long‐lived submarine channel system developed in a narrow and elongate thrust‐top basin of the Southern Apennines (Italy). Channel‐fill deposits are exposed in an outcrop belt approximately 500 m thick and 15 km long, oriented oblique to the palaeoflow, which was roughly south‐eastward. These exceptional exposures of channel‐fill strata allow the stacking architectures and the evolution of the channel system to be analyzed at multiple scales, enabling the effects of syn‐sedimentary thrust tectonics and basin confinement on the depositional system development to be deciphered. Two end‐member types of elementary channel architecture have been identified: high‐aspect‐ratio, weakly‐confined channels, and low‐aspect‐ratio, incisional channels. Their systematic stacking results in a complex pattern of seismic‐scale depositional architectures that determines the stratigraphic framework of the deep‐water system. From the base of the succession, two prominent channel‐complex sets have been recognized, namely CS1 and CS2, consisting of amalgamated incisional channel elements and weakly‐confined channel elements. These channelized units are overlain by isolated incisional channels, erosional into mud‐prone slope deposits. The juxtaposition of different channel architectures is interpreted to have been governed by regional thrust‐tectonics, in combination with a high subsidence rate that promoted significant aggradation. In this scenario, the alternating ‘in sequence’ and ‘out of sequence’ tectonic pulses of the basin‐bounding thrusts controlled the activation of coarse‐clastic inputs in the basin and the resulting stacking architectures of channelized units. The tectonically‐driven confinement of the depositional system limited the lateral offset in channel stacking, preventing large‐scale avulsions. This study represents an excellent opportunity to analyze the stratigraphic evolution of a submarine channel system in tectonically‐active settings from an outcrop perspective. It should find wide applicability in analogous depositional systems, whose stratigraphic architecture has been influenced by tectonically‐controlled lateral confinement and associated lateral tilting.

A newly discovered elasmobranch assemblage from the fossil-bearing area of Zamaca (Chilcatay Formation, southern Peru) is described herein, providing a first comprehensive view on the early Miocene shark and ray paleocommunities of the East Pisco Basin, whose sedimentary infill represents one of the most important Cenozoic Fossil-Lagerstatten worldwide. The studied assemblage includes at least twenty-two species attributed to twelve families and five orders. Thirteen taxa are recorded for the first time from the Chilcatay Formation, and four of them are recorded for the first time from the Pacific coast of South America. The reconstructed paleoenvironmental scenario is consistent with a shallow-marine coastal area, representative of a sheltered shelfal setting, influenced by both brackish and open-ocean waters. This paleoenvironment was inhabited by a community of small mesopredator elasmobranchs that exploited the Zamaca area as a nursery ground and recruitment area. The structure of the Zamaca assemblage is mainly explained by three key-features: 1) a taxonomic composition dominated by two shark lineages, Lamniformes and Carcharhiniformes, the former being predominant; 2) the leading role played by two species, Carcharhinus brachyurus and †Cosmopolitodus hastalis, accounting for about 60% of the analyzed specimens; 3) the distinctly juvenile imprint of the entire assemblage. Striking similarities emerge between the elasmobranch assemblage from Zamaca and the late Miocene one from Cerro Colorado (Pisco Formation, East Pisco Basin), thus suggesting the persistence of a peculiar “biological enclave” driven by the concurrence of the ecological, environmental, and oceanographic factors that characterized the coast of present-day Peru during the Neogene.

International audience; Cetaceans originated in south Asia more than 50 million years ago (mya), from a small quadrupedal artiodactyl ancestor [1, 2, 3]. Amphibious whales gradually dispersed westward along North Africa and arrived in North America before 41.2 mya [4]. However, fossil evidence on when, through which pathway, and under which locomotion abilities these early whales reached the New World is fragmentary and contentious [5, 6, 7]. Peregocetus pacificus gen. et sp. nov. is a new protocetid cetacean discovered in middle Eocene (42.6 mya) marine deposits of coastal Peru, which constitutes the first indisputable quadrupedal whale record from the Pacific Ocean and the Southern Hemisphere. Preserving the mandibles and most of the postcranial skeleton, this unique four-limbed whale bore caudal vertebrae with bifurcated and anteroposteriorly expanded transverse processes, like those of beavers and otters, suggesting a significant contribution of the tail during swimming. The fore- and hind-limb proportions roughly similar to geologically older quadrupedal whales from India and Pakistan, the pelvis being firmly attached to the sacrum, an insertion fossa for the round ligament on the femur, and the retention of small hooves with a flat anteroventral tip at fingers and toes indicate that Peregocetus was still capable of standing and even walking on land. This new record from the southeastern Pacific demonstrates that early quadrupedal whales crossed the South Atlantic and nearly attained a circum-equatorial distribution with a combination of terrestrial and aquatic locomotion abilities less than 10 million years after their origin and probably before a northward dispersal toward higher North American latitudes

International audience; The taphonomy and palaeoecology of the early Miocene (Burdigalian) vertebrate assemblage of Ullujaya (East Pisco Basin, Peru) is here described. Vertebrate remains are concentrated in marine facies (Ct1a association) of the exposed Chilcatay Formation (dated 19-18 Ma) deposited within a 30-40 m deep, semi-enclosed, offshore environment. Coupled with ichnological observations, the size distribution of pyrite framboid relics reveals fluctuation of euxinic and oxic-dysoxic conditions at the seafloor. The assemblage is dominated by toothed cetaceans (kentriodontids, squalodelphinids, physeteroids, and the eurhinodelphinid-like Chilcacetus), together with a large dermochelyid turtle, some bony fish, and diverse elasmobranchs, mostly juveniles of Carcharhinus brachyurus and Cosmopolitodus hastalis. The vertebrate assemblage comprises a coastal community, dominated by mesopredators, representative of a warm-temperate, sheltered embayment connected with riverine and open-ocean environments. Vertebrate skeletons are typically disarticulated and incomplete, and some bone elements display shark bite marks. Microborings are observed at the bone surface. Bones exhibit a good degree of apatite mineralisation and bone cavities are locally filled by Ca-Mg carbonates. Our taphonomic observations suggest prolonged flotation of carcasses during which they were subject to biogenic and physical processes of partial destruction (including scavenging by sharks), before final deposition on a soft compact substrate. Preservation was favoured by the oxygen-deficient bottom conditions that inhibited the action of benthic macro-scavengers.

This outcrop-based study reports diffuse joints and veins, normal to strike-slip fault zones and minor folds that developed, from Miocene to Quaternary, within the clastic to siliceous sedimentary record of the forearc Pisco Basin of southern Peru. Patterns, orientations, dimensional parameters and other outcrop-scale characteristics of the various deformation features are illustrated and their genetic mechanisms and timing of development are inferred. These new structural data and interpretations allow a better constraint of the structural style and evolution of the Pisco Basin, and can represent useful guidelines for characterizing the outcrop-scale deformation affecting similar forearc basins along the Peruvian coast. Major results of this study are that the development of the documented deformation features, their patterns, dimensional parameters and kinematics seem influenced by local perturbations of the paleostress field by mechanic processes partly independent of plate tectonics forces. These processes include strain localization on both pre-existing and progressively forming new structural discontinuities, and cyclic switches of the horizontal, principal stress axes σ2 and σ3. In particular, we discuss how different normal fault patterns, from sub-parallel to multidirectional/polygonal, could form in a same deformation phase in response of the local σ2/σ3 magnitude ratio, as an evolution of stratabound, mutually orthogonal vein sets.

A key step in the evolutionary history of Odontoceti (echolocating toothed cetaceans) is the transition from the ancestral heterodont condition–characterized by the presence of double-rooted cheek teeth bearing accessory denticles–to the homodont dentition displayed by most extant odontocete species. During the last few decades, new finds and the reassessment of specimens in collections revealed an increased morphological disparity amongst the Oligo–Miocene heterodont odontocetes. Based on a partly articulated skeleton from late Early Miocene (Burdigalian, 18.8–18.0 Ma) beds of the Chilcatay Formation (Pisco Basin, Peru), we describe a new genus and species of heterodont odontocete, Inticetus vertizi, in the new family Inticetidae. This large dolphin is characterized by, amongst other things, a long and robust rostrum bearing at least 18 teeth per quadrant; the absence of procumbent anterior teeth; many large, broad-based accessory denticles in double-rooted posterior cheek teeth; a reduced ornament of dental crowns; the styliform process of the jugal being markedly robust; a large fovea epitubaria on the periotic, with a correspondingly voluminous accessory ossicle of the tympanic bulla; and a shortened tuberculum of the malleus. Phylogenetic analyses (with and without molecular constraint; with and without down-weighting of homoplastic characters) yielded contrasting results, with Inticetus falling either as a stem Odontoceti or as an early branching member of a large Platanistoidea clade. With its large size, robust rostrum and unusual dental morphology, and the absence of conspicuous tooth wear, Inticetus increases the morphological and ecological disparity of Late Oligocene–Early Miocene heterodont odontocetes. Finally, this new taxon calls for caution when attempting to identify isolated cetacean cheek teeth, even at the suborder level. http://zoobank.org/urn:lsid:zoobank.org:pub:5B306B49-EB1B-42F9-B755-B0B05B4F938F.

Field mapping and sedimentological study of outcrop sections exposed along the Ica River valley permitted the establishment of a regional allostratigraphic framework for the upper Miocene portion of the Pisco Formation. The stratigraphy of the studied interval is illustrated using a new 1:20,000-scale geological map which reveals that this formation is a cyclical sedimentary succession composed of three fining-upward allomembers. The bounding surfaces defining each allomember are transgressively modified subaerial unconformities. They converge and merge landward into a single composite surface representing the time-transgressive lower boundary of the Pisco Formation. Accordingly, the extent of the stratigraphic gap associated with the basal unconformity varies significantly throughout the basin and increases toward the basin margins. The timing of allomember-bounding surfaces coincides with that of major oxygen-isotope maxima in the deep-sea oxygen isotopic record and matches the ages of eustatic sequence boundaries identified elsewhere, indicating glacio-eustatic falls due to the growth of Antarctica ice sheets as a viable mechanism for their development.

The Monte Ascensione succession (c. 2.65–2.1 Ma) is a well-exposed example of an exhumed submarine canyon fill embedded within slope hemipelagic mudstones. This gorge represented a long-lasting pathway for sediment transport and deposition and during the Gelasian delivered Apennine-derived clastic sediment to the adjacent Peri-Adriatic basin. A total of six principal lithofacies types, representing both canyon-confining hemipelagic deposits and canyon-filling turbidity current and mass-transport deposits, can be delineated in the studied sedimentary succession. The canyon-fill deposits display a marked cyclic character and the component lithofacies succeed one another to form at least fifteen fining-upward stratal units, which are interpreted to represent high-frequency, unconformity-bounded depositional sequences. Variability in the vertical repetition of constituent lithofacies allows the identification of three basic styles of sequence architecture that can be interpreted in terms of differing positions along a conceptual down-canyon depositional profile. An integrated chronology, based on biostratigraphic data and on palaeomagnetic polarity measurements, strongly supports a one-to-one correlation between the sequence-bounding surfaces and oxygen isotope stages G2–78, suggesting that the most feasible sequence-engendering mechanism is that of orbitally dictated glacio-eustatic changes in sea level, which regulated timing of sediment storage on the shelf and its redistribution beyond the shelf edge. One of the most significant aspects of this study is the demonstration that processes occurring within upper slope canyons can be expected to be strongly influenced by variations in sea level; that is, the erosional and depositional features evident in these deposits can be strongly controlled by allocyclic processes rather than autocyclic or random processes.

International audience; Although combined molecular and morphological analyses point to a late middle Eocene (38–39 million years ago) origin for the clade Neoceti (Odontoceti, echolocating toothed whales plus Mysticeti, baleen whales, and relatives), the oldest known mysticete fossil dates from the latest Eocene (about 34 million years ago) of Antarctica [1, 2]. Considering that the latter is not the most stemward mysticete in recent phylogenies and that Oligocene toothed mysticetes display a broad morphological disparity most likely corresponding to contrasted ecological niches, the origin of mysticetes from a basilosaurid ancestor and its drivers are currently poorly understood [1, 3, 4, 5, 6, 7, 8]. Based on an articulated cetacean skeleton from the early late Eocene (Priabonian, around 36.4 million years ago) of the Pisco Basin, Peru, we describe a new archaic tooth-bearing mysticete, Mystacodon selenensis gen. et sp. nov. Being the geologically oldest neocete (crown group cetacean) and the earliest mysticete to branch off described so far, the new taxon is interpreted as morphologically intermediate between basilosaurids and later toothed mysticetes, providing thus crucial information about the anatomy of the skull, forelimb, and innominate at these critical initial stages of mysticete evolution. Major changes in the morphology of the oral apparatus (including tooth wear) and flipper compared to basilosaurids suggest that suction and possibly benthic feeding represented key, early ecological traits accompanying the emergence of modern filter-feeding baleen whales’ ancestors.

The Upper Jurassic to Lower Cretaceous platform-slope to basinal carbonate strata cropping out in Gargano Promontory (southern Italy) are partly dolomitized. Fieldwork and laboratory analyses (petrographic, petrophysical and geochemical) allowed the characterization of the dolomite bodies with respect to their distribution within the carbonate succession, their dimensions, geometries, textural variability, chemical stability, age, porosity, genetic mechanisms and relation with tectonics. The dolomite bodies range from metres to kilometres in size, are fault-related and fracture-related, and probably formed during the Early Cretaceous at

The development of emerging digital technologies that allow the collection and analysis of field data represents a significant innovation in field-based geological studies. The integration of these digital techniques with traditional sedimentological field methods facilitates considerable improvements in outcrop characterization. An example of this integrated modern approach for geological data collection is employed for the detailed characterization of a turbidite channel-lobe system of the Gorgoglione Flysch Formation in Southern Italy. The study area, exposed above the village of Castelmezzano, has been measured and described in detailed stratigraphic sections, providing data for both sedimentological analysis and correlation of the stratigraphy. In order to gain a complete perspective on the exposure and stratigraphic elements, analysis of physical outcrop data was enhanced by the use of high-resolution Gigapixel imagery and 3D photogrammetric outcrop reconstructions. The Santa Maria section has been assessed in terms of vertical and lateral facies stacking arrangements and subdivided into two component facies associations separated by a prominent concave-up erosional boundary. The lower facies association, interpreted as a frontal lobe complex, consists of tabular, thick-bedded coarse sandstones interbedded with persistent heterolithic packages of thin-bedded sandstones and mudstones, and minor soft-sediment deformed strata. The upper facies association represents the infill of a channel-form and consists of a basal conglomerate, passing gradually upwards into massive amalgamated sandstones overlain by large-scale cross-laminated sandstones. The excellent exposure of the Santa Maria section records the complete evolution of a channel-lobe system, transitioning from frontal lobe deposition through channel incision and bypass, to progressive backfilling. This study shows how facies characterization, stratigraphic correlations and reconstruction of the depositional architectures have been substantially enhanced by the use of emerging digital techniques for geological data collection.

The life history strategies of sharks often include the use of protected nursery areas by young-of-the-year and juveniles. Nursery areas can be primary (i.e., grounds where the sharks are born and spend the very first part of their lives) or secondary (i.e., grounds inhabited by slightly older but not yet mature individuals). Criteria utilized to recognize these strategic habitats include: high concentration of young sharks, high food availability, and low predation risk. Since the fossil record of sharks consists mainly of isolated teeth, identification of paleonurseries involves a series of problems due to difficult application of actualistic criteria. A rich shark tooth-bearing level (ST-low1) has recently been discovered in the upper Miocene deposits of the Pisco Formation exposed at Cerro Colorado (southern coast of Peru). Most of the teeth collected from this level belong to the extant copper shark Carcharhinus brachyurus. These teeth are small and compatible with those of extant juveniles. This observation, coupled with other paleoenvironmental considerations, indicates that the ST-low1 horizon could have represented a nursery ground for juvenile individuals of C. brachyurus. The absence of very small-sized teeth (i.e., referable to young-of-the-year) suggests a secondary nursery ground inhabited by immature copper sharks. Observations on the tooth size of other Lamniformes, Carcharhiniformes, and Myliobatiformes occurring along with C. brachyurus point to a significantly juvenile structure of this elasmobranch assemblage, thus supporting the hypothesis of a communal use of the Cerro Colorado paleonursery.

New knowledge on patterns of fibrous gypsum veins, their genetic mechanisms, deformation style and weathering is provided by a field- and laboratory-based study carried out on the Neogene to Quaternary Pisco Basin sedimentary strata exposed in the Ica desert, southern Peru. Gypsum veins vary considerably in dimensions, attitudes and timing, and can develop in layered and moderately fractured rocks also in the absence of evaporitic layers. Veins occur both as diffuse features, confined to certain stratigraphic levels, and localized within fault zones. Arrays formed by layer-bounded, mutually orthogonal sets of steeply dipping gypsum veins are reported for the first time. Vein length, height and spacing depend on the thickness of the bed packages in which they are confined. Within fault zones, veins are partly a product of faulting but are also inherited layer-bounded features along which faults are superimposed. Owing to the different petrophysical properties with respect to the parent rocks and their susceptibility to textural and mineralogical modifications, water dissolution and rupture, gypsum veins may have a significant role in geofluid management. Depending on their patterns and grade of physical and chemical alteration, veins may influence geofluid circulation and storage, acting as barriers to flow and possibly also as conduits.

The early Pleistocene clastic succession of the Peri-Adriatic basin, eastern central Italy, records the filling of a series of piggyback sub-basins that formed in response to the development of the eastward-verging Apennine fold-thrust belt. During the Gelasian (2·588 to 1·806 Ma), large volumes of Apennine-derived sediments were routed to these basins through a number of slope turbidite systems. Using a comprehensive outcrop-based dataset, the current study documents the depositional processes, stratigraphic organization, foraminiferal age and palaeodepth, and stratigraphic evolution of one of these systems exposed in the surroundings of the Castignano village. Analysis of foraminiferal assemblages consistently indicates Gelasian deposition in upper bathyal water depths. Sediments exposed in the study area can be broken into seven main lithofacies, reflecting specific gravity-induced depositional elements and slope background deposition: (i) clast-supported conglomerates (conglomerate channel-fill); (ii) amalgamated sandstones (late stage sandstone channel-fill); (iii) medium to thick-bedded tabular sandstones (frontal splay sandstones); (iv) thin to thick-bedded channelized sandstones (sandy channel-fill); (v) medium to very thin-bedded sandstones and mudstones (levee-overbank deposits); (vi) pebbly mudstones and chaotic beds (mudstone-rich mass-transport deposits); and (vii) massive mudstones (hemipelagic deposits). Individual lithofacies combine vertically and laterally to form decametre-scale, disconformably bounded, fining-upward lithofacies successions that, in turn, stack to form slope valley fills bounded by deeply incised erosion surfaces. A hierarchical approach to the physical stratigraphy of the slope system indicates that it has evolved through multiple cycles of waxing then waning flow energy at multiple scales and that its packaging can be described in terms of a six-fold hierarchy of architectural elements and bounding surfaces. In this scheme, the whole system (sixth-order element) is comprised of three distinct fifth-order stratigraphic cycles (valley fills), which define sixth-order initiation, growth and retreat phases of slope deposition, respectively; they are separated by discrete periods of entrenchment that generated erosional valleys interpreted to record fifth-order initiation phases. Backfilling of individual valleys progressed through deposition of two vertically stacked lithofacies successions (fourth-order elements), which record fifth-order growth and retreat phases. Fourth-order initiation phases are represented by erosional surfaces bounding lithofacies successions. The component lithofacies (third-order element) record fourth-order growth and retreat phases. Map trends of erosional valleys and palaeocurrent indicators converge to indicate that the sea floor bathymetric expression of a developing thrust-related anticline markedly influenced the downslope transport direction of gravity currents and was sufficient to cause a major diversion of the turbidite system around the growing structure. This field-based study permits the development of a sedimentological model that predicts the evolutionary style of mixed coarse-grained and fine-grained turbidite slope systems, the internal distribution of reservoir and non-reservoir lithofacies within them, and has the potential to serve as an analogue for seismic or outcrop-based studies of slope valley fills developed in actively deforming structural settings and under severe icehouse regimes.

The early Pleistocene stratigraphic succession of the Peri-Adriatic basin, eastern central Italy, records the filling of an elongate, N–S oriented piggy-back basin located east of the growing Apennine fold-thrust belt. During the Gelasian (2.588–1.806 Ma), large volumes of gravel, sand and mud derived from the emergent Apennines were redistributed into the basin through a number of slope erosional fairways. These sediment conduits are preserved in the rock record as a series of coarse-grained canyon-fill successions that provide an opportunity for assessing, from an outcrop perspective, how this type of deep-water depositional systems evolves and fills. The present study uses measured stratigraphic sections, photopanels, paleocurrent data, careful lithological mapping, and well-log data from a nearby exploration well to constrain the internal organization of one of these canyon fills, referred to herein as the Offida Canyon. A detailed facies analysis suggests that a variety of gravity-driven subaqueous flows were involved in sediment transport and deposition within the submarine canyon, including slumps, cohesive debris flows, and high- and low-density turbidity currents. Five main lithofacies reflecting both canyon-bounding slope deposits and canyon-filling turbidite and debrite depositional elements have been identified within the exposed succession: (i) clast-supported conglomerates (gravel-rich channel complexes); (ii) medium- to thick-bedded sandstones (overbank lobe); (iii) medium- to very thin-bedded sandstones and mudstones (levee-overbank); (iv) pebbly mudstones and chaotic beds (mudstone-rich mass-transport deposits); and (v) massive mudstones (hemipelagic background deposits). These lithofacies are organized in recurring successions and define fining-upward packages that are regarded as the deep-water expression of high-frequency depositional sequences. Each sequence comprises the sedimentary record of major phases of canyon activity and comprises the following surfaces and systems tracts, in ascending stratigraphic order: (i) a pronounced surface of erosion (sequence boundary) generated by efficient turbidity currents during a period of erosion and complete bypass of sediment to more basinward settings; (ii) a lowstand systems tract composed of a diverse assemblage of genetically related lithofacies (channel-complex conglomerates, levee sandy heterolithics, and overbank lobe sheet-like sandstones) laid down by turbidity currents largely bypassing the area; and (iii) a transgressive to forced regressive systems tract comprising mass-transport deposits produced by instability of shelf-edge staging areas and/or failure of canyon walls when coastal sediment sources were far from the shelf edge. Correlation between sequences and oxygen isotope curve suggests that the recurring fluctuations in sedimentary activity of the submarine canyon are related to the switching on and off of coarse clastic sediments to the slope in response to obliquity-driven (41 ky duration) glacio-eustatic sea-level oscillations, which modulated timing of sediment storage on the shelf and its redistribution beyond the shelf edge.

The early Pliocene sediments of the Sua Member (Onzole Formation) are spectacularly exposed along coastal cliffs in the surroundings of Sua (northwestern Ecuador). The shellbed at the base of these strata contains rich mollusk and fish assemblages and provides a rare opportunity to document the virtually unknown Pliocene shallow-water faunas of Ecuador. Stratigraphic context, faunal composition (both mollusks and fishes), biofabric trend, ichnological signatures, and taphonomic features presented here, all indicate that the shellbed is a stratigraphically condensed (hiatal) skeletal concentration generated primarily by low rates of net background sedimentation. Shellbeds of this type, immediately overlying a Glossifungites-demarcated ravinement surface, are regarded as onlap shellbeds and record landward stratal convergence and attenuation during the earliest phases of marine transgression. Although composed of largely autochthonous-parautochthonous specimens, the fossil assemblage contains mollu...