Candeleros Formation Research Articles

They all floated in the cretaceous: new rebbachisaurid (Sauropoda, Diplodocoidea) with a highly pneumatized skeleton from the Upper Cretaceous (lower Cenomanian) of Patagonia, Argentina

ABSTRACT Rebbachisaurids are a group of basal diplodocoid sauropods that diversified primarily in Gondwana between the early Lower Cretaceous and the early Upper Cretaceous. Their phylogenetic relationships are still under debate given the fragmentary and incomplete nature of its remains. Here, we provide a detailed description of the postcranial remains of Campananeyen fragilissimus gen. et sp. nov. from the Candeleros Formation (lower Cenomanian) of Neuquén Province, Argentina. Campananeyen presents notable features that allow it to be recognised as a new species by the presence of an underdeveloped crista prootica, paraoccipital process internally hollow and a fuse dorsal alar arm of the ilium. The phylogenetic analysis recovered this taxon as a basal rebbachisaurid closely related to Zapalasaurus, Sidersaura and the specimen from La Amarga Formation MACN-Pv-N 35. The new rebbachisaurid is recovered as the sister taxon of Sidersaura due the presence of procoelous posterior caudal vertebrae and dorsoventrally flattened posterior caudal centra. C. fragilissimus presents an extreme pneumatization of the sacral region with an autapomorphic dorsal alar arm of the sacral transverse processes. Finally, the phylogenetic analysis supports a South American origin for Rebbachisauridae and an early diversification to Africa and Europe by the Barremian-Aptian as previously suggested.

Pterosaur tracks from the Upper Cretaceous (Cenomanian) Candeleros Formation of northwestern Patagonia, Argentina: Ichnotaxonomic and palaeoecological perspectives from Gondwana

New pterosaur tracks from the Cenomanian Candeleros Formation (Neuquén Basin) have been discovered in the Aguada de Tuco locality, northwestern Patagonia, Argentina. Preserved in medium-grained sandstones originating from alluvial deposits, these tracks include both isolated manus and pes impressions. The manus tracks are tridactyl and display a marked asymmetry, while pes tracks exhibit three-digit impressions and a subrectangular to subtriangular shape. The absence of a clearly defined trackway, coupled with the limited preservation quality of the documented pes tracks and distinctions from known pterosaur ichnotaxa, prompted the provisional assignment of these tracks to aff. Pteraichnus isp. They also exhibit a different morphology from previously recorded tracks within the same unit, thus contributing to a greater morphological variability of pterosaur tracks for the Candeleros Formation. Additionally, these tracks are found in association with narrow furrows interpreted as traces left by pterosaur manus and pes claws. Considering the shape and preservation of these tracks and the fragmentary record of pterosaurs from the Neuquén Basin, it is possible to attribute the producers to pterodactyloid pterosaurs. Moreover, the observed variability in manus track size, all sharing a similar morphology, suggests having been produced by individuals of different ages rather than different species of pterodactyloid pterosaurs. This study significantly contributes to our understanding of the pterosaur track record from Gondwana and the Cenomanian worldwide.

The last of the oldies: a basal rebbachisaurid (Sauropoda, Diplodocoidea) from the early Late Cretaceous (Cenomanian–Turonian) of Patagonia, Argentina

ABSTRACT Rebbachisauridae is mainly recorded during the early Late Cretaceous in Gondwana, and in South America in particular. Sidersaura marae gen. et sp. nov. was found in rocks of the Huincul Formation (upper Cenomanian – Turonian) of Neuquén Province, Argentina. Sidersaura has notable characteristics that allow recognising it as a new species and defining its phylogenetic relationships. First, it has a frontoparietal foramen, as in dicraeosauridae. It bears fourteen caudal vertebrae with neural arches displaced closer to the anterior margin of the centrum and a ventral longitudinal hollow on middle of the centra, both characters previously considered as titanosaurian synapomorphies, and here recognised in these diplodocoid sauropods. Sidersaura shows a peculiar tarsal condition with an unusual calcaneum morphology which resembles that of basal sauropods. The haemal arches have a stellate morphology with two sets of projections comparable to the specimen MMCh-PV 47 from the Candeleros Formation (Cenomanian), previously described as a titanosaurian. The phylogenetic analysis retrieves Sidersaura as a basal Rebbachisauridae more closely related to Zapalasaurus than to Limaysaurinae. The presence of a basal taxon at Cenomanian – Turonian times, so close to the extinction of the group, implies that the evolutionary history of rebbachisauridae was more complex than previously thought.

Dinosaur tracks with skin impressions in the La Buitrera Paleontological Area (Candeleros Formation, Cenomanian), Río Negro Province, Argentina

The La Buitrera Paleontological Area, in the north of the Río Negro Province, Argentina, preserves the uppermost section of the Cenomanian Candeleros Formation, as well as a rich vertebrate fauna that inhabited the Kokorkom Desert paleo-erg. Candia Halupczok et al. (2017) reported tracks found on wet and dry interdune. New evidence from 2020 to 2022 fieldtrips enabled the recognition of tracks in cross-section in non-channelized ephemeral fluvial dominated facies within the main aeolian setting located in the vicinity of the wettest recorded area of the La Buitrera Locality: the Cañadón de las Tortugas. The cross-section tracks commonly identified in these facies are from 30 to 75 cm long and 20–30 cm deep. Some of them preserve thin and shallow structures in the filling of the original track wall interpreted here as traces of the trackmaker integument. Specimens preserve two kinds of skin impressions. The first ones are parallel, mostly oblique striations, which sometimes cross each other at a high angle, produced by the pes during penetration and withdrawal from the substrate. The second ones are subrounded to polygonal, here interpreted as scale traces of the sole foot. Two clear, subtriangular to curved, and elongated, claw impressions are recorded in one track that allow us to relate it with a sauropod trackmaker.

A new chelid turtle, Prochelidella palomoi sp. nov., from campanian Anacleto Formation, Neuquén Basin, North-Western Patagonia, Argentina.

The new turtle remains were found in the Anacleto Formation (Campanian) exposed in the Aguada Grande site. This area is located 10 km from the south of the city of Rincón de Los Sauces, in the north of the province of Neuquén, Patagonia, Argentina. The specimen (MAU-Pv-AG-452) consists of a partially preserved skull, carapace remains, an almost complete plastron, and a left ilium. The skull shows some features (a developed retropterygoid foramen, and a big muscle attachment area over the basisphenoid) that allow us to refer this specimen to the genus Prochelidella. The visceral view of the first costal bone exhibits deeper axillary buttresses that extend 2/3 of the costal width. This condition is also observed in the costal bone of Prochelidella buitreraensis from the Candeleros Formation (Cenomanian). The scute pattern observed in the anterior lobe of the plastron shows a large gular that extends along the entoplastra length, and small extragulars over the epiplastra. In contrast with the scute scheme observed in Prochelidella cerrobarcinae and Prochelidella portezuelae where the gular does not extend beyond the first 1/3 of the entoplastron. The phylogenetic analyses performed recovered this new species as a member of Prochelidella. The material recovered from the Anacleto Formation represents the first undoubtedly finding of Prochelidella in the late Cretaceous (Campanian) and the second skull recognized for the genus.

Earliest Tuatara Relative (Lepidosauria: Sphenodontinae) from Southern Continents

The New Zealand tuatara (Sphenodon) is the sole surviving rhynchocephalian of a once thriving group across Pangea during early Mesozoic times. Outside New Zealand, close relatives of the tuatara (sphenodontines) are known from a few Jurassic records in North America and Europe and from end-Cretaceous incomplete remains in Patagonia. Still, the evolutionary relationships of most of them remain elusive. Here we describe a new sphenodontine, Tika giacchinoi gen. et sp. nov., based on well-preserved cranial and postcranial remains from upper levels of the Candeleros Formation (Cenomanian) at the Konservat-Lagerstätte of ‘La Buitrera Paleontological Area’ in northern Patagonia, Argentina. Our phylogenetic analysis recovered Tika as a close relative of the tuatara, together with Laurasian and Patagonian taxa. The new finding represents the oldest certain sphenodontine from the gondwanan continents and reinforces the hypothesis that particular terrestrial ectothermic tetrapods attained a circumantarctic Cretaceous-Tertiary distribution. Tika is inferred to have fed upon a variety of prey items including small vertebrates, similar to the extant tuatara, but ecologically different from the large herbivorous sphenodontians already known from La Buitrera. Therefore the new taxon expands the known diversity of sphenodontians during the Late Cretaceous in Patagonia and indicates that Rhynchocephalia, although declining or extinct in Laurasia, were still taxonomic and ecologically diverse in southwestern Gondwana.

New remains from the Cenomanian Candeleros Formation, Neuquén Basin (Patagonia, Argentina) provide insights into the formation of the sacro-urostylic complex in early pipimorph frogs (Amphibia, Anura)

An incomplete anuran skeleton collected from outcrops of the Cenomanian Candeleros Formation on the western shore of the Ramos Mexia Reservoir, Neuquén Province, Patagonia, is described herein. It consists of several partially articulated postcranial bones that are exposed in ventral view and belong to a post-metamorphic frog that is probably an adult. The axial column consists of eight opisthocoelous presacral vertebrae; fusion of the first two anterior vertebrae cannot be determined. Most of the sacrum is formed by the expanded diapophyses of Vertebra IX. However, Vertebra X also has distally expanded diapophyses that fuse to those of Vertebra IX. Traces of a suture between the urostyle and sacrum are evident. The morphology of the sacro-urostylic region casts doubts on the previous interpretation of the fused condition of sacrum and urostyle in the holotype of Avitabatrachus uliana from the same stratigraphic unit. The material described here contributes new information about the formation of the sacrum and suggests that different ontogenetic pathways are involved in formation of different sacral morphologies. The sacrum formed only by Vertebra IX probably is plesiomorphic in pipimorphs. It is hypothesized that the more complex structure described here evolved from it and incorporates postsacral vertebrae as well as synostotic fusion with the urostyle. Complete synostotic fusion of sacrum and urostyle, when present, seems to have been an event that occurred relatively late in the ontogeny of early pipimorphs. This fusion might have shifted to earlier developmental stages relative to the ancestor independently on different branches of the pipimorph tree.

Controls of pedogenesis in a fluvial-eolian succession of Cenomanian age in northern Patagonia

Although interactions between eolian and fluvial systems are well documented in both modern and ancient environments, there are few studies of such deposits that also address the paleosols that developed within them. The Cenomanian Candeleros and Huincul formations of the Neuquén Basin, Northern Patagonia, Argentina, provides an excellent opportunity to do so. These formations preserve mixed fluvial-eolian and fluvial systems, respectively, with associated pedogenesis. We identify five lithofacies associations (fluvial channel deposits [FA1], deposits of unconfined fluvial flows [FA2], floodplain deposits [FA3], eolian-dune deposits [FA4], and wet interdune deposits [FA5]), and five pedotypes (calcic Protosols [P1], Vertisols [P2], vertic and gleyed Protosols [P3a and P3b, respectively], vertic Calcisols [P4], and argillic Protosols [P5]). The lower Candeleros Formation is characterized by eolian dune deposits with calcic Protosols and wet interdune deposits with Vertisols and vertic and gleyed Protosols. The middle part of the formation is dominated by deposits of unconfined fluvial flows with vertic Protosols and, in the upper part of the unit, there are channel deposits with vertic Calcisols. The Huincul Formation is characterized by fluvial deposits with argillic Protosols.Changes in paleosol development have been recorded throughout the two formations. Small-scale changes, recognized in the lower Candeleros Formation, comprise a chrono-toposequence, in which calcic Protosols, Vertisols, vertic and gleyed Protosols are genetically associated with FA4 and FA5. Calcic Protosols developed on the dunes and gleyed Protosols in the inner part of the interdunes subjected to temporal waterlogging. When the dunes were vegetated and pedogenesis promoted dune fixation, the rate of dune mobility was slow and the ancient soils developed into Vertisols. In turn, the absence of vegetation was linked to a relatively high rate of dune migration, the time of soil development was limited, and, as a result, vertic Protosols were preserved. The analysis of soil-forming factors as large-scale controlling factors showed that throughout the succession, variations on the type of paleosol and the sedimentary environment over time were controlled by changes in ancient climate. Mid-Cretaceous global climate reconstructions suggest that the paleosols developed in northern Patagonia were located near the limit between the Southern Mid-latitude Warm Humid Belt and the Southern Hot Arid Belt. Seasonally arid and semi-arid conditions prevailed in this area, although there was a temporal trend towards a temperate subhumid climate.

Testing the persistence of Carcharodontosauridae (Theropoda) in the Upper Cretaceous of Patagonia based on dental evidence

The deposits corresponding to the Upper Cretaceous Neuquén and San Jorge Gulf basins from northern and central Patagonia have provided two of the most complete sequences of terrestrial vertebrate faunas of all Gondwanan landmasses. Among the carnivorous components, the carcharodontosaurid theropods appeared as common elements during the Early Cretaceous and the earliest Late Cretaceous in northern and central Patagonia. Although recorded mostly in the lower Turonian, isolated teeth suggest their presence in younger strata in northern and central Patagonia, reaching the clade in the region as late as the early Maastrichtian. Here, we verify the assignment of such isolated teeth previously identified as belonging to Carcharodontosauridae from the Upper Cretaceous strata of northern and central Patagonia. Using three different methods, namely a cladistic analysis performed on a dentition-based data matrix, and discriminant and cluster analyses conducted on a large dataset of theropod crown measurements, we assign a tooth from Candeleros Formation to carcharodontosaurid theropods and teeth from Cerro Lisandro, Bajo Barreal, Portezuelo, Plottier and Allen formations to abelisaurid theropods. These new reappraisals provide additional evidence about the extinction of Carcharodontosauridae in South America at about the late Turonian–earliest Coniacian as part of a general faunistic turnover event, with the last clear evidence of this lineage in Patagonia coming from the early–middle Turonian.

Morphological variations in dinosaur tridactyl tracks from the Candeleros Formation (Upper Cretaceous) of northwestern Patagonia, Argentina

Several dinosaur tridactyl tracks showing inter- and intra-trackways morphological variations from the Cenomanian Candeleros Formation of the Neuquén Basin, Argentina, have been documented. Footprints occur on three different stratigraphic levels and are impressed on medium-grained sandstone of fluvial origin. Level 1 includes deep tridactyl tracks with metatarsal impressions (average length of 31 cm) preserved as true tracks and undertracks that could have been generated due to simply sinking of the pes into a soft substrate and also probably by a continuous walk in a squatting position. Level 2 shows the largest tridactyl tracks (average length of 58 cm) recognized in this work, situated a few meters close to an already studied sauropod trackway. The different preservation between these bipedal and quadrupedal tracks could be explained due to different substrate properties and dissimilarities of the pedal shape between trackmakers. Level 3 comprises small to large tridactyl tracks (length from 6 to 34 cm) including intra-trackway morphological variation, that is interpreted as a result of changes in the substrate properties, mainly moisture content, along the tracking surface. Taking into account that the footprints studied here are poorly preserved, they were not assigned to any particular ichnotaxon. The large tridactyl tracks have been attributed to carcharodontosaurid theropod trackmakers while medium and small tracks to undetermined bipedal dinosaur. One pair of these large theropod tracks was closely documented on the same tracking surface of the aforementioned studied sauropod trackway, attributed to rebbachisaurids. This suggests that both carcharodontosaurid and rebbachisaurid trackmakers coexisted in the same habitats as the osteological record noted. This tracksite represents an example of the morphological variability of dinosaur tridactyl tracks controlled mainly by the substrate properties, and how an approach aimed at understanding footprint formation and preservation is necessary before beginning ichnotaxonomical studies, among others.

Report of a giant titanosaur sauropod from the Upper Cretaceous of Neuquén Province, Argentina

One of the most fascinating research topics in the field of sauropod dinosaurs is the evolution of gigantism. In the particular case of Titanosauria, the record of multi-ton species (those exceeding 40 tons) comes mainly from Patagonia. The record of super-sized titanosaur sauropods has traditionally been extremely fragmentary, although recent discoveries of more complete taxa have revealed significant anatomical information previously unavailable due to preservation biases. In this contribution we present a giant titanosaur sauropod from the Candeleros Formation (Cenomanian, circa 98 Ma) of Neuquén Province, composed of an articulated sequence of 20 most anterior plus 4 posterior caudal vertebrae and several appendicular bones. This specimen clearly proves the presence of a second taxon from Candeleros Formation, in addition to Andesaurus, and is here considered one of the largest sauropods ever found, probably exceeding Patagotitan in size. While anatomical analysis does not currently allow us to regard it as a new species, the morphological disparity and the lack of equivalent elements with respect to coeval taxa also prevent us from assigning this new material to already known genera. A preliminary phylogenetic analysis places this new specimen at the base of the clade leading to Lognkosauria, in a polytomy with Bonitasaura. The specimen here reported strongly suggests the co-existence of the largest and middle-sized titanosaurs with small-sized rebbachisaurids at the beginning of the Late Cretaceous in Neuquén Province, indicating putative niche partitioning. This set of extremely large taxa from Patagonia has contributed to a better understanding of the phylogenetic relationships of titanosaurs, revealing the existence of a previously unknown lineage and shedding new light on body mass evolution.

Cranial and postcranial remains of a new species of Prochelidella (Testudines: Pleurodira: Chelidae) from ‘La Buitrera’ (Cenomanian of Patagonia, Argentina), with comments on the monophyly of this extinct chelid genus from southern Gondwana

The genus Prochelidella is the oldest known panchelid taxon. It is widely distributed in several Cretaceous basins throughout Patagonia, Argentina. It was previously known from three named species. A fourth species presented herein, Prochelidella buitreraensis sp. nov., preserves both cranial and postcranial material from the Candeleros Formation (La Buitrera Area) of Río Negro Province, Argentina. Features observed in the nuchal bone (i.e. wide, trapezoidal cervical vertebrae, shallow nuchal notch) allow us to assign this specimen to the genus Prochelidella. Several specific traits (i.e. a large open retropterygoideum foramen, and a strongly developed muscle attachment site on the basisphenoid) suggest that the form from the Candeleros Formation is a new species of the genus Prochelidella. A phylogenetic analysis that includes the three best-preserved species of Prochelidella recovers this taxon as monophyletic. The Cenomanian age of this new taxon allows us to compare the panchelid turtles from the Patagonian (Neuquen) and Australian (Surat) basins.http://zoobank.org/urn:lsid:zoobank.org:pub:E9D194C7-126D-46B6-BF75-BD1A51A74971

Gregarious behaviour among non-avian theropods inferred from trackways: A case study from the Cretaceous (Cenomanian) Candeleros Formation of Patagonia, Argentina

Gregarious behaviour among dinosaurs has been inferred from several lines of evidence (monospecific bonebeds, skeletal morphology, phylogenetic inferences, comparison with modern ecosystems and parallel trackways with particular characteristics), but is relatively poorly documented for non-avian theropods. Here, we report five parallel theropod trackways of large track size (average length of 28 cm) in the Cretaceous (Cenomanian) Candeleros Formation from northwestern Patagonia, Argentina. The tracks are provisionally assigned to aff. Asianopodus pulvinicalx and considering the autopod morphology of the theropod taxa documented in the Candeleros Formation, abelisaurid theropods are suggested as trackmakers. The trackways possess similar stride, speed estimation, direction and preservational features, and track show uniform depth and do not overlap. Physical barriers (i.e., large fluvial channels or perennial lake) that could influence the direction of the theropod trackmaker gaits were not recognized in the section. Taking into account these considerations, a gregarious behaviour for the abelisaurid theropod trackmakers is proposed. The tracks are preserved as shallow undertracks in a medium-grained sandstone bed deposited in a floodplain setting. The tracking surface is interpreted as the overlaying layer of muddy-siltstone. The occurrence of swelling clays and microbial mats in the track-bearing level could have contributed to the substrate stabilization and their role in the preservation is discussed.

A new narrow-gauge sauropod trackway from the Cenomanian Candeleros Formation, northern Patagonia, Argentina

Dinosaurs are extremely abundant in the Upper Cretaceous Neuquén Group of the Neuquén Basin (Argentina). Nevertheless, while osteological remains are rich the ichnological record is more restricted. A new sauropod dinosaur trackway with five manus-pes imprint sets discovered in the Cenomanian Candeleros Formation is described here. The trackway belongs to the narrow-gauge type that is identified for the first time in the Cenomanian and possibly for the Late Cretaceous. It is preserved as concave epirelief in fine-grained sandstones from floodplain deposits. The tracks, which are large in size (average length of 98 cm), include conspicuous rims with very well-preserved symmetrical ripples on top that are documented for the first time in the track record. Due to their preservation and the absence of clear anatomical details, the trackway was not assigned to any particular ichnotaxon. Taking into account the presence of rebbachisaurid diplodocoid remains in the Candeleros Formation, the classical association of narrow-gauge trackways with diplodocoids and the inferred gleno-acetabular distance, it is suggested that the studied trackway might belong to a large-sized rebbachisaurid. The worldwide record of Cenomanian dinosaur tracks includes only a few records of sauropod tracks. Thus, this new finding contributes to increase the knowledge about the early Late Cretaceous sauropods and their tracks.

Dinosaur tracks in the Kokorkom Desert, Candeleros Formation (Cenomanian, Upper Cretaceous), Patagonia Argentina: Implications for deformation structures in dune fields

The Kokorkom desert extended over an area of 826 km2 in the central-west sector of Neuquén and Río Negro provinces along the area of the backbulge basin within the Andean Foreland basin in the Neuquén Basin. Its deposits constitute the middle-upper section of the Candeleros Formation (Cenomanian) and reach approximately 130 m thick. Tracks are found on wet and dry interdune and within draa slipface deposits. They constitute biogenic deformation structures characterized by folded-up and/or brecciated sandstone levels formed under dry and/or wet substrate conditions with passive filling. The degree of preservation varies, but the identification of digit impression suggests that they were produced by theropods or iguanodontians.

Neuquén Group (Upper Cretaceous): A case of underfilled-overfilled cycles in an Andean foreland basin, Neuquén basin, Argentina

The Nenquén Group was deposited during a period dominated by the Cretaceous Greenhouse and can be divided in three cycles correlated with large-scale changes in the evolution of the Andean foreland basin. The filling of the Neuquén Group is constituted by a complete cycle and two incomplete cycles of underfilled-overfilled, separated by first-order discontinuities assigned to the uplift of the Agrio fold-and-thrust belt during the Chasca/Catequil, Mid Ocean Ridge (CCMOR) collision, coinciding with first-order climatic changes within the Cretaceous greenhouse cycle.The Candeleros Formation in the base of this group was deposited in late underfilled conditions, showing prominent forebulge zones. It is demonstrated that during the Albian, with the cratonward migration of the uplifting forebulge zones, the axis of backbulge zones also migrated cratonwards and a wide uplifted forebulge zone was formed. On top, the Huincul Formation was deposited in an overfilled period without orogenic load, while the Cerro Lisandro Formation was deposited in early underfilled conditions with orogenic load. The Río Neuquén Subgroup started with a late underfilled period (Portezuelo Formation -second-order discontinuity), and after wards the Plottier Formation was deposited in an overfilled period without orogenic load. Finally, the Río Colorado Subgroup was deposited under late and early underfilled conditions (Bajo de la Carpa and Anacleto Formations respectively).

New rebbachisaurid cranial remains (Sauropoda, Diplodocoidea) from the Cretaceous of Patagonia, Argentina, and the first endocranial description for a South American representative of the clade

ABSTRACTHere we report new rebbachisaurid material (MMCh-PV 71) recovered from the Candeleros Formation (Cenomanian) of northwest Patagonia, Argentina. The cranial remains consist of a partial braincase and a right quadrate. Fractures in the braincase exposed the endocranial cavity, allowing the first study of the brain and inner ear morphologies of a South American rebbachisaurid. The braincase and cranial endocast both exhibit traits similar to those observed in the Cretaceous rebbachisaurs Nigersaurus from Africa and Limaysaurus from Argentina, although in terms of osteology, the South American taxa are highly similar. The endocast of MMCh-PV 71 is more similar to that of Nigersaurus than to those of Diplodocus and Camarasaurus, suggesting some probable rebbachisaurid features such as the noteworthy presence of the flocculus. The overall morphology of the quadrate shows similarities with Limaysaurus and Nigersaurus. However, differences such as the broader posterior fossa and the shape and orientation of the head and the pterygoid process indicate that the new specimen could represent a distinct taxon.Citation for this article: Paulina Carabajal, A., J. I. Canale, and A. Haluza. 2016. New rebbachisaurid (Sauropoda, Diplodocoidea) cranial remains from the Cretaceous of Patagonia, Argentina, and the first endocranial description for a South American representative of the clade. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2016.1167067.

Small-sized abelisaurid (Theropoda: Ceratosauria) remains from the Upper Cretaceous of northwest Patagonia, Argentina

A new abelisaurid theropod from the Cenomanian of Candeleros Formation (Neuquén Province, NW Patagonia, Argentina) is described. It includes an isolated frontal, partial pelvis, two fused sacral centra, femoral shaft, rib fragments, partial metatarsal II and shaft of metatarsal III. A paleohistological analysis shows that MMCh-PV 69 was somatically mature at time of death. The prevalence of relatively ordered intrinsic fibers in the primary bone indicates a relatively lower growth rate of MMCh-PV 69 compared with other abelisauroids, such as Aucasaurus garridoi. Phylogenetic analysis found MMCh-PV 69 as a basal abelisaurid, sister group of the node formed by Majungasaurinae and Brachyrostra. We estimated the body mass of MMCh-PV 69 in 240 kg, which makes it one of the smallest abelisaurids recorded. This new form adds to the extensive theropod fauna of the Candeleros Formation, probably the most diverse association of meat-eating dinosaurs recorded in a Cretaceous Formation from South America.

Estratigrafia y sedimentologia del abanico terminal de la Formacion Candeleros (Grupo Neuquen), Cretácico Inferior, Cuenca Neuquina, provincias de Neuquen y Mendoza, Argentina.

The Candeleros Formation is represented by a succession of deposits that represent terminal fans starting to fill a foreland basin (back-arc) during the Albian. The different sub-environments of the terminal fan suggest low gradients during the onset of deposition of the Rio Limay Subgroup (Neuquen Group). The terminal fan system is represented by distributary zone (TF-DZ), distributary zone-base zone (TF-DZBZ) and base zone deposits (TF-BZ). Its most notable feature is the presence of bitumen in “pockets” and scattered in the sandstone channel fill and crevasse splays deposit. In this system the feeder zone of the terminal fan system has not been identified, however, the distribution of architectural elements and paleocurrent data allow us to infer that the catchment area of the backbone would have been located to the NNW/NW. Distributary zone deposits are characterized by thick deposits of low sinuosity channel, with a high degree of connectivity and strong vertical aggradation of channel belts. Deposits of distributary zone-base zone are characterized by a higher density of channel deposits/distributary plain, but with belt channel less thick, longer laterally, greater width/thickness ratio and connectivity in the braided deposits. The distal parts of the terminal fan are well represented and vary only in the channelized deposits and mantle/floodplain ratio, which is abruptly reduced towards the flood basin connection. The importance of Candeleros Formation deposits is that they are remnants of accommodation that preserved the original architecture and represent exhumed reservoirs.

Mineral reactions associated with hydrocarbon paleomigration in the Huincul High, Neuquén Basin, Argentina

Bleaching of red beds by the migration of reducing pore fluids is a widespread phenomenon, with most previous work on the topic focused on detailed studies in the southwestern United States, and to a lesser extent Germany. Herein, we report on widespread bleaching of Cretaceous red beds in the Neuquen Basin and the relationship of this bleaching to hydrocarbon migration. In the Cerro Granito area of the Huincul High, Neuquen Basin, the basal deposits of the Neuquen Group (Candeleros and Huincul Formations) are bleached. This alteration was apparently controlled by fluid and host-rock composition as well as by the intensity and span of the fluid-rock interaction. Red, fine-grained sandstones and mudstones of the Candeleros Formation were deposited in fluvial and swamp environments and contain authigenic hematite, Na-rich corrensite, micro- and mesoquartz, calcite, and analcite. Coarse-grained sandstones and mudstones of the Huincul Formation, which overlie the Candeleros Formation, were deposited in a braided fluvial system, under more humid and acidic conditions than the Candeleros Formation. Sandstones are similar in composition and texture to the Candeleros sandstones, except they contain less basic volcanic detritus and lack corrensite. Bleached facies in both formations contain bitumen and clusters of calcite + bitumen concretions, and partially dissolved detrital clasts and most cements, including early Fe-oxides, which resulted in an increase in secondary porosity and bleaching of the red beds. Alteration of the Candeleros Formation is confined to medium- to coarse-grained sandstone in which corrensite is partially dissolved and its interlayer charge is satisfied by Ca instead of Na, as in corrensite from the red sandstones. Extensive dissolution occurred in the Huincul Formation, which favored the precipitation of new authigenic minerals such as smectite, mixed-layer chlorite-smectite, and pyrite. Bleaching and associated alteration most likely resulted from interaction of hydrocarbons with the red beds. Hydrocarbon paleomigration in the Cerro Granito area occurred in higher-permeability portions of the Candeleros and Huincul Formations. The seal provided by the Lisandro Formation was breached during uplift driven by Tertiary tectonism, resulting in the exhumation of the Huincul and Candeleros Formations. This study highlights the importance of a variety of alterations, including complex clay mineral paragenesis, contemporaneous with bleaching of the red beds. Such alterations can potentially be used to provide evidence for the passage of chemically reducing fluids associated with hydrocarbons in other regions, including cases where the original red color has been completely removed.