Palinostratigraphy and Paleoenvironments in the Gorodishchi Section (Middle Volga River Region, Kimmeridgian–Hauterivian)

—Eight dinocyst-based and three spore- and pollen-based biostratigraphic units are defined in the Kimmeridgian, Volgian, and Hauterivian of the Gorodishchi section, based on a biostratigraphic analysis of the successions of marine and terrestrial palynomorphs. Algological assemblages are described in more detail, and additional criteria for the definition of dinocyst zones established by previous researches are given. A more detailed biostratigraphic subdivision of the middle part of the Volgian is proposed. A local dinocyst zone in the Hauterivian and a biostratigraphic succession of spore–pollen units in the entire section are described for the first time. The research results demonstrate that the boundaries of many palynostratigraphic units exhibit a considerable correlation potential. Based on a biofacies analysis of the microphytoplankton, the dynamics of transgressive–regressive events is studied in relation to the accompanying oxygen and trophic conditions. Possible relationships between marine paleoenvironments and climatic changes reconstructed on the basis of spore–pol­len data are discussed.

Cerro del Hierro, Spain: the largest exposed early Cambrian palaeokarst

We study the largest exposed example of an early Cambrian palaeokarst, associated with laterites and developed during rifting of the Ossa–Morena Zone. The lithostratigraphy, biostratigraphy, facies and the genesis reflect episodes of sea-level fall (Cerro del Hierro Regression) related to tectonic events and palaeoclimate. This palaeokarst can be primarily considered as the result of early Cambrian polyphase karstification in an extensional tectonic regime, later modified by Neogene–Quaternary geomorphological processes. The event may correlate with other regressive events of a similar age in Spain, Italy, United Kingdom, South America and Australia. This episode also has local names (e.g. Cerro del Hierro Regression in the Mediterranean region; Woodlands Regression in the UK). It is sometimes accompanied by additional karst development outside of Spain that is compared and interpreted in a global context.

Lithostratigraphic classification of the Tsodilo Hills Group: a Palaeo- to Mesoproterozoic metasedimentary succession in NW Botswana

The Tsodilo Hills Group strata exposed in the Tsodilo Hills are an association of meta-arenites, meta-conglomerates, quartz-mica schists, sandstone, red siltstone and sedimentary breccia deposited on an open siliciclastic marine shelf between the Late Palaeoproterozic and Late Mesoproterozoic, and outcropping in NW Botswana. The succession is dominated by three micaceous quartzite units interlayered with subordinate lenses and wedges of other rock types. Facies gradients from S to N are expressed by decreasing content of muscovite at all levels of metasediment organisation from thin wedge-shaped units to thick quartzite complexes, as well as a decrease in pebble content and increase in the arenaceous matrix in some metal conglomerate beds, matching regional palaeotransport direction. Well-rounded pebbles of rocks are flat, suggesting redeposition from a beach environment. Lenticular conglomeratic bodies with erosional lower boundaries represent infills of local incisions in the sandy bottom sediments. The abundance of laterally discontinuous lithological units reflects shelf palaeotopography controlled and modified by deposition and migration of large bed forms, ranging from megaripple marks (or submarine dunes) to sand waves. Deposition was influenced by tides and two regressive events. The older regression resulted in a marker unit of tidal mudflat-related red-bed facies: mudstone, siltstone, channel-fill sandstone and sedimentary breccia. The second regression is indicated by a tabular conglomerate marker reflect-ing increased input of coarse terrigenous material.

p75NTR and DR6 regulate distinct phases of axon degeneration demarcated by spheroid rupture

The regressive events associated with trophic deprivation are critical for sculpting a functional nervous system. After nerve growth factor withdrawal, sympathetic axons maintain their structural integrity for roughly 18 hours (latent phase) followed by a rapid and near unison disassembly of axons over the next 3 hours (catastrophic phase). Here we examine the molecular basis by which axons transition from latent to catastrophic phases of degeneration following trophic withdrawal. Prior to catastrophic degeneration, we observed an increase in intra-axonal calcium. This calcium flux is accompanied by p75 neurotrophic factor receptor (NTR)-Rho-actin dependent expansion of calcium rich axonal spheroids that eventually rupture, releasing their contents to the extracellular space. Conditioned media derived from degenerating axons is capable of hastening transition into the catastrophic phase of degeneration. We also found that death receptor 6 (DR6) but not p75NTR is required for transition into the catastrophic phase in response to conditioned media but not for the intra-axonal calcium flux, spheroid formation, or rupture that occurs toward the end of latency. Our results support the existence of an inter-axonal degenerative signal that promotes catastrophic degeneration among trophically deprived axons.

A NEW GEOLOGICAL MODEL BASED ON A SEISMIC-STRATIGRAPHIC ANALYSIS OF THE EASTERN INHAMBÚ OIL FIELD, ONSHORE ESPÍRITO SANTO BASIN, BRAZIL

ABSTRACT. The Espírito Santo onshore basin is an important hydrocarbon province on the eastern Brazilian continental margin that accounts for almost 9% of Brazil’s onshore oil production. This study aims to understand the stratigraphy and geologic evolution of the eastern Inhambú onshore oil field from the post-rift section (Cricaré Formation, Mucuri Member) to the drift section (Rio Doce Formation). We interpret the main stratigraphic surfaces and seismic facies from the basement to the upper pre-Eocene unconformity to build a chronostratigraphic chart and identify the main transgressive and regressive events. Lithological and geophysical logs from new wells allowed the identification of six 3rd -order cycles, including four regressive and two transgressive events. Based on these results, a new conceptual geological model for the eastern Inhambú Oil Field is proposed. Keywords: Chronostratigraphic Chart, Seismic Facies, Canyon Fazenda Cedro, Espírito Santo Basin.RESUMO. A bacia terrestre do Espírito Santo é uma importante província produtora de hidrocarbonetos na margem leste do Brasil, respondendo por quase 9% da produção brasileira de petróleo em terra. O presente trabalho teve como objetivo entender a estratigrafia e a história evolutiva, desde a seção pós-rifte (Formação Cricaré, Membro Mucuri) até a seção drifte (Formação Rio Doce), de uma área a leste ao Campo de Inhambú e assim propor um novo modelo geológico para a área. Neste estudo, foram interpretadas as principais superfícies estratigráficas e identificadas as principais sismofácies desde o embasamento até a discordância do pré-Eoceno Superior, criando-se uma carta cronoestratigráfica de eventos. A interpretação de dados litológicos e geofísicos de poços permitiu o reconhecimento de seis ciclos de 3a ordem, onde foi possível identificar quatro eventos com caráter regressivo, e dois de caráter transgressivo. Como resultado final foi criado um novo modelo geológico conceitual para a área leste do campo petrolífero de Inhambú.Palavras-chave: Carta Cronoestratigráfica de Eventos, Sismofácies, Cânion de Fazenda Cedro, Bacia do Espírito Santo.

Ammonoid multi-extinction crises during the Late Pliensbachian – Toarcian and carbon cycle instabilities

Five crises affecting ammonite evolution occurred during the Late Pliensbachian to Late Toarcian stages (Early Jurassic). The first two (Gibbosus and end-Spinatum zones) occurred during highly cold and regressive conditions which were followed by a global anoxic event generated during a supergreenhouse warm event (Levisoni subzone) concomitant with a worldwide transgressive event. The last two (Late Variabilis and Late Insigne zones) are related to regressive events. We present new carbon isotope data from Southern Peru that demonstrate that most of the major Toarcian crises affecting ammonite evolution can be correlated with the variations affecting the δ13C during that unstable period. The morphogenetic reactions of the ammonites during the five different upper Liassic critical episodes are analysed, showing how this group can be used as stress indicators.

High-precision U–Pb zircon ID–TIMS dating of two regionally extensive bentonites: Cenomanian Stage, Western Canada Foreland BasinThis article is one of a series of papers published in this Special Issue on the theme of Geochronology in honour of Tom Krogh.

The stratigraphy of Cretaceous rocks in the Western Canada Foreland Basin is well constrained by dense borehole data that allow three-dimensional mapping of transgressive–regressive events, paleogeography, and subsidence patterns. However, it is difficult to estimate rates of change or to place events in a precise temporal framework because very few of the many bentonite beds have been dated using modern techniques. In this study, two bentonites, of at least regional extent, were dated using isotope dilution – thermal ionization mass spectrometry (ID–TIMS) U–Pb methods on chemically abraded zircon crystals. The older bentonite correlates with the “X” bentonite of the late middle Cenomanian and yields an age of 95.87 ± 0.10 Ma. The Bighorn River Bentonite, which lies just below the Cenomanian–Turonian boundary, yields an age of 94.29 ± 0.13 Ma and is interpreted to equate with the “B” bentonite and bed 80 at Pueblo, Colorado. Both bentonites dated here are older than the previously reported 40Ar/39Ar ages from correlative United States samples, supporting the observation that 40Ar/39Ar ages may systematically underestimate ages of Cretaceous bentonites by ∼1%, as suggested by other recent studies. The X bentonite immediately precedes a major late middle Cenomanian eustatic transgression that inundated the Dunvegan delta complex. The new ages for the X and Bighorn River bentonites indicate an average minimum subsidence rate of ∼0.27 mm/year in the most proximal part of the Cenomanian foredeep in northeast British Columbia during this time. The new age of the Bighorn River Bentonite, coupled with the orbitally tuned time scale of Sageman et al. (2006) , suggests a Cenomanian–Turonian boundary age of 94.12 ± 0.13 Ma.