Claudio Riccomini

Soft-sediment deformation at the base of the Neoproterozoic Puga cap carbonate (southwestern Amazon craton, Brazil): Confirmation of rapid icehouse to greenhouse transition in snowball Earth

Stratigraphic and isotopic data identify the lower 45 m of the Araras Group, on the southwest margin of the Amazon craton, as a Neoproterozoic platform cap carbonate deposited below wave base upon Varanger glacial diamictites of the Puga Formation. The basal beds consist of moderately deep water pinkish dolomudstone with stratiform to wavy fenestral microbialites locally cut by tube-like structures and fenestral nonmicrobial planar laminites with tepee-like features. Above the basal carbonates are deep-water bituminous lime mudstones with alternating thin calcite crusts and lime mudstone laminae commonly disrupted by calcite crystal fans (pseudomorphs after aragonite). The basal contact of the Puga cap exhibits soft-sediment deformational structures (principally load casts) that are here attributed to rebound-induced seismicity acting upon both recently deposited carbonate sediments and underlying unconsolidated diamictite. These features constitute the first clearly recognized sedimentological evidence for the rapid change from icehouse to greenhouse conditions as postulated in the snowball Earth model of Neoproterozoic glaciation.

Multiscale Analysis of Topographic Surface Roughness in the Midland Valley, Scotland

Surface roughness is an important geomorphological variable which has been used in the Earth and planetary sciences to infer material properties, current/past processes, and the time elapsed since formation. No single definition exists; however, within the context of geomorphometry, we use surface roughness as an expression of the variability of a topographic surface at a given scale, where the scale of analysis is determined by the size of the landforms or geomorphic features of interest. Six techniques for the calculation of surface roughness were selected for an assessment of the parameters behavior at different spatial scales and data-set resolutions. Area ratio operated independently of scale, providing consistent results across spatial resolutions. Vector dispersion produced results with increasing roughness and homogenization of terrain at coarser resolutions and larger window sizes. Standard deviation of residual topography highlighted local features and did not detect regional relief. Standard deviation of elevation correctly identified breaks of slope and was good at detecting regional relief. Standard deviation of slope (SDslope) also correctly identified smooth sloping areas and breaks of slope, providing the best results for geomorphological analysis. Standard deviation of profile curvature identified the breaks of slope, although not as strongly as SDslope, and it is sensitive to noise and spurious data. In general, SDslope offered good performance at a variety of scales, while the simplicity of calculation is perhaps its single greatest benefit.

A Complete Skull of an Early Cretaceous Sauropod and the Evolution of Advanced Titanosaurians

Advanced titanosaurian sauropods, such as nemegtosaurids and saltasaurids, were diverse and one of the most important groups of herbivores in the terrestrial biotas of the Late Cretaceous. However, little is known about their rise and diversification prior to the Late Cretaceous. Furthermore, the evolution of their highly-modified skull anatomy has been largely hindered by the scarcity of well-preserved cranial remains. A new sauropod dinosaur from the Early Cretaceous of Brazil represents the earliest advanced titanosaurian known to date, demonstrating that the initial diversification of advanced titanosaurians was well under way at least 30 million years before their known radiation in the latest Cretaceous. The new taxon also preserves the most complete skull among titanosaurians, further revealing that their low and elongated diplodocid-like skull morphology appeared much earlier than previously thought.

Neotectonic activity in the Serra do Mar rift system (southeastern Brazil)

Abstract The Serra do Mar rift system is the most significant Cenozoic tectonic feature of the onshore continental area of southeastern Brazil. This system exhibits a two-fold tectonic evolution, marked by a distensive synsedimentary stage in Paleocene-Oligocene times and a strike-slip stage of post-Oligocene age. Recent investigations and re-examination of previously described outcrops in the Resende and Taubate Basins, depressions of the rift system, have revealed late Pleistocene sedimentary deposits also affected by strike-slip faulting of the second tectonic stage or even of a third one. In the Taubate Basin, some colluvium and stone-lines are faulted, with development of positive and negative flower structures. On the other hand, positive flower structures of the Resende Transpressional Zone have tilted alluvial terraces of the Paraiba do Sul valley. Both situatins are related to major E-W transcurrence. No absolute age dates are available for Taubate and Resende deposits. However, comparisons with the adjacent Paraiba do Sul River stratigraphy suggest that colluvium and stone-lines, as well as alluvial terraces, are of late Pleistocene age or younger. Fault plane solutions of two swarms of reservoir-induced events, close to the area concerned, had strike-slip and overthrust focal mechanisms related to an average E-W compressive regional stress direction. The similarity of the stress fields is thus strongly indicative of a neotectonic E-W compressive event that appears to have been active, at least as residual tectonism, up to the present day.

Redescription of the Cranial Morphology of Mariliasuchus Amarali, and Its Phylogenetic Affinities (crocodyliformes, Notosuchia)

Abstract The cranial morphology of Mariliasuchus amarali, a poorly known notosuchian from the Late Cretaceous of Southeastern Brazil, is redescribed based on new material. Its phylogenetic affinities within Crocodylomorpha are evaluated through a parsimony analysis involving 46 taxa and 198 characters. Mariliasuchus is nested well inside the clade Notosuchia, as the sister group of Comahuesuchus, a derived notosuchian from the Late Cretaceous of Argentina. Both taxa share the following unambiguous synapomorphies: ventral half of the lacrimal tapering posteroventrally, not contacting or only slightly contacting the jugal; presence of a large foramen on the lateral surface of the anterior part of the jugal; presence of procumbent premaxillary and anterior dentary alveoli; and ectopterygoids that do not participate of the palatine bar. The presence of procumbent premaxillary teeth, specialized tooth crown morphology, and fore–aft jaw movements suggests that this group presented complex jaw movements related to specialized feeding habits.

Closing the Clymene ocean and bending a Brasiliano belt: Evidence for the Cambrian formation of Gondwana, southeast Amazon craton

We report new paleomagnetic and geochronological data from Ediacaran rift-drift carbonates in the Paraguai belt at the southern end of the suture zone between the Amazon craton and the Sao Francisco and Rio de Plata cratons, South America. Early thrusting resulted in remagnetization ca. 528 ± 36 Ma or later; the mean age is established by 40 Ar/ 39 Ar encapsulation dating of mixed authigenic and detrital illite from remagnetized carbonates from the unmetamorphosed fold-thrust belt. This remagnetization overlaps with a 525 Ma Gondwana reference pole. Metamorphic illite from the slate belt yields 40 Ar/ 39 Ar ages of 496–484 Ma, the timing of peak regional metamorphism. Oroclinal bending of the Paraguai belt was caused by a 90° clockwise rotation of the east-west limb after ca. 528 Ma, probably refl ecting the irregular margin of the southeast Amazon craton. The age of the Paraguai belt overlaps with that of the Pampean orogeny farther south along the western margin of the Rio de Plata craton, suggesting a coeval closure for the Clymene ocean separating the Amazon craton from the Sao Francisco and Rio de Plata cratons.

A New Notosuchian from the Late Cretaceous of Brazil and the Phylogeny of Advanced Notosuchians

A new notosuchian crocodyliform from the Late Cretaceous Bauru Group found in the southeastern State of São Paulo (Brazil) is described here. The new taxon, Caipirasuchus stenognathus, is referred as a new species of the recently erected genus Caipirasuchus within the clade Sphagesauridae based on a phylogenetic analysis of basal mesoeucrocodylians. Caipirasuchus stenognathus is represented by an almost complete skull and lower jaw that has autapomorphic characters that distinguish it from other species of Sphagesauridae. These autapomorphies include: maxilla forming part of the orbital margin (absence of lacrimal-jugal contact), nasal with smooth depressions on the posterior region close to the contact with the maxilla and lacrimal, postorbital with posterior palpebral facet that extends posteriorly underneath the ear-flap groove, and a distinct anterior process of the medial flange of the retroarticular process. Additionally, the new taxon lacks autapomorphic features described in other sphagesaurids. The phylogenetic analysis results in a monophyletic genus Caipirasuchus, that is the sister group of a clade fomed by Sphagesaurus huenei, Caryonosuchus pricei, and Armadillosuchus arrudai. Sphagesaurids also include a basal clade formed by Adamantinasuchus navae and Yacarerani boliviensis. Other notosuchian taxa, such as Mariliasuchus amarali, Labidiosuchus amicum, Notosuchus terrestris, and Morrinhosuchus luziae are successive sister taxa of Sphagesauridae, forming a clade of advanced notosuchians that are restricted to the Late Cretaceous of South America. These results contrast with most previous phylogenetic hypotheses of the group that depicted some members of Sphagesauridae as more closely related to baurusuchids, or found Asian (e.g., Chimaerasuchus) or African (Malawisuchus, Pakasuchus) forms nested within advanced notosuchians that are, according to our analysis, endemic of the Late Cretaceous of South America.

Absolute Dating of Permian Ash-Fall in the Rio Bonito Formation, Paraná Basin, Brazil

Abstract The Rio Bonito Formation in southern Parana basin contains a set of tonsteins interbedded with coal-seams. These tonsteins are composed mainly of kaolinite with zircon, apatite and beta-quartz paramorphs as accessory minerals, and were interpreted as volcanic ashes deposited by ash falls over pits protected by barrier islands in a barrier-lagoon system. A U-Pb dating of zircons in the tonstein A, which furnished an age of 267.1 ± 3.4 Ma (Early Permian) confirming previous age-dates based on palynology and correlating them with one of the main periods of volcanic activity in the Gondwana. The source of the pyroclastic material was attributed to the early Permian Choiyoi magmatic arc in Argentina, developed during the Sanrafaelic orogeny, and with a main peak of volcanic activity between 260 and 272 Ma.

The puzzle assembled: Ediacaran guide fossil Cloudina reveals an old proto-Gondwana seaway

ABSTRACTDuring the Ediacaran the Clymene Ocean separated the Lau-rentia, Amazonia, and Rio Apa cratons from several landmasses to the west forming the proto-Gondwana supercontinent. However, no clear evidence about the existence of Ediacaran epeiric seas over those landmasses has been found. Here we report and discuss the discovery of the Ediacaran guide fossil Cloudina sp. associated with other metazoan body and trace fossils in the Bambui Group (central eastern Brazil). The Ediacaran age of the Bambui Group and the paleogeographic position of Cloudina -bearing successions in Brazil, Antarctica, Namibia, and Argentina suggest a scenario of ocean con-nectivity among coeval intracratonic basins of South America, Africa, and Antarctica at the end of Neoproterozoic time. The new fi nding epitomizes one of the most important paleontological discoveries ever made in South America, helping to solve an old paleogeographic puz-zle of the Gondwana supercontinent.INTRODUCTION The Bambui Group crops out in central eastern Brazil, and is one of the most studied Neoproterozoic sedimentary successions in South America. It comprises an extensive, fl at-lying cratonic cover in erosional contact with Paleoproterozoic and Archean basement rocks of the Sao Francisco craton. In its eastern and westernmost occurrences, the Bam-bui Group is deformed and metamorphosed by the Brasilia and Aracuai mobile belts, respectively (Fig. 1A). The regional tectonostratigraphy has been interpreted as refl ecting deposition in a foreland basin related to Gondwanan collisions involving the Sao Francisco craton (Pimentel et al., 2011).The Bambui Group (BG) overlies rocks of the Macaubas Group and Jequitai Formation and the Carrancas conglomerate, units supposedly de-posited during Sturtian glaciation. The unit is 700–1000 m thick (Misi et al., 2007) and comprises limestones and dolomites of the Sete Lagoas For-mation at the base that grade upward to shales and siltstones of the Serra de Santa Helena Formation, followed by marls, siltstones, limestones, and sandstones of the Lagoa do Jacare, Serra da Saudade, and Tres Marias Formations (Fig. 1B).The precise age of the BG is not well established, precluding accurate correlation with other supposedly coeval successions such as the Corumba Group (Brazil), Itapucumi (Paraguay), Arroyo del Soldado (Uruguay), Si-erras Bayas (Argentina), and Nama Group (Namibia). The presence of

Morphotectonics of a high plateau on the northwestern flank of the Continental Rift of southeastern Brazil

Abstract Integration of landform and structural analysis allowed the identification of Late Pleistocene–Holocene pulses of tectonic activity in the Campos do Jordao Plateau with ages and regimes similar to the ones from the continental rift. Fault reactivation along Precambrian shear zones give rise to a series of conspicuous morphotectonic features, determine the formation of stream piracy phenomena, and divide the plateau into smaller blocks. Recognition of these tectonic pulses as well as of their effects in landform development—particularly clear on the Campos de Sao Francisco at the highest area of the SE edge of the plateau—show that besides the climate-related Quaternary environmental changes significant neotectonic instability should be considered in the geomorphic evolution of the Campos do Jordao Plateau.