Illite crystallinity patterns in the Anti-Atlas of Morocco

The low-grade metamorphism of the sedimentary cover of the Moroccan Anti-Atlas is investigated using Illite crystallinity (IC) method. More than 200 samples from three key areas (southwestern, central and eastern Anti-Atlas) have been taken from a maximum of different stratigraphic levels and have been analysed. The metamorphism is of low to very low degree throughout the southern flank of the Anti-Atlas. It increases from northeast to southwest. Whereas in the eastern Anti-Atlas diagenetic and anchizonal IC-values are predominant, in the western and central Anti-Atlas also epizonal IC-values are found. In every respective area the IC improves with stratigraphic age. At the scale of the entire Palaeozoic Anti-Atlas basin the IC correlates best with estimated paleo-overburden. However, burial metamorphism cannot be the cause even though considering missing sedimentary pile of Late Carboniferous age. The ‘abnormal' paleo-geothermal gradient of 43-35 °C/km we evidenced for the Carboniferous is a true one, and has to be related to a basement sequence enriched in heat producing elements such as series of the West African Crato

Characterization of metalliferous sediment from a low-temperature hydrothermal environment on the Eastern Flank of the East Pacific Rise

Metalliferous deposits are described from the eastern flank of the East Pacific Rise (EPR) offshore Costa Rica, close to a basaltic seamount called “Dorado high”. Based on heat-flow data and porewater profiles, the site is an area of active low-temperature hydrothermal discharge. We focus on the mineralogical and chemical analysis from a 124 cm long gravity core (GC50), located on the northwestern slope of the 100 m high Dorado. In this core, the sediments consist of detrital clay minerals as well as authigenic minerals such as zeolites, apatites, and Fe/Mn-rich oxyhydroxides. In contrast, the reference sediments from adjacent areas without hydrothermal activity are olive gray hemipelagic muds composed of volcanic glass particles, clay minerals, siliceous microfossils, and some detrital quartz and feldspar. Bulk sediment chemistry and chemical enrichment factors calculated with respect to the reference sediment indicate that the most important chemical changes occurred at the base of the core from 100 to 124 cm bsf, with strong enrichments in MnO, CaO, P2O5, and Fe2O3. These enrichments are correlated with the occurrence of authigenic Fe-oxyhydroxide (goethite) and Mn oxide (todorokite and vernadite, at 100 cm bsf), and hydrothermal apatite (110–124 cm bsf). In the upper section of the core from 12 to 70 cm, the sediment is composed of abundant smectite and authigenic phillipsite, and only minor chemical changes can be observed with respect to the reference sediments. The ubiquitous presence of phillipsite suggests that the entire sedimentary column of core GC50 was first affected by diagenesis. However, below 70 cm bsf, these phillipsites are partially dissolved and Fe oxides occur from 110 to 124 cm, followed upward by Mn oxides at 100 cm. This transition from Fe to Mn-rich sediments can be interpreted in terms of an upward increasing redox potential. PAAS-normalized REY patterns of GC50 sediments present clearly negative Ce and positive Y anomalies inherited from seawater at the base of core GC50. These anomalies decrease upward, which we interpret together with the transition from Fe to Mn-rich sediments by an upward migrating low-temperature hydrothermal fluid. Thus, after a first stage of diagenesis, the discharge of a low-temperature hydrothermal fluid occurred through the sedimentary column, leading to the precipitation of hydrothermal compounds that are lacking towards the surface

Reconciling strontium-isotope and K-Ar ages with biostratigraphy: the case of the Urgonian platform, Early Cretaceous of the Jura Mountains, Western Switzerland

During the late Early Cretaceous, the shallow-water domains of the western Tethys are characterized by the widespread deposition of Urgonian-type carbonates rich in rudists, corals and other oligotrophic, shallow-marine organisms. In the Helvetic Alps, the Urgonian occurrences have been dated by ammonite biostratigraphy as Late Barremian and Early Aptian. For the more proximal occurrences in the western Swiss Jura, a recent age model based on bio-, chemo- and sequence stratigraphy has been proposed, which allows for an improved correlation with the Helvetic counterparts. In order to corroborate the recently proposed age model for the Jura, a set of well-preserved rhynchonellids collected from five different lithostratigraphical formations and members ("Marnes bleues d'Hauterive”, "Marnes d'Uttins”, basal marly layers within the "Urgonien Jaune”, "Marnes de la Russille”, "Urgonien Blanc”) has been analysed for its strontium-isotope ratios (87Sr/86Sr). In addition, K-Ar dating was performed on well-preserved glauconite grains from two different levels ("Marnes d'Uttins” and a basal layer within the "Urgonien Jaune”). The correlation of the Sr-isotope data set with a belemnite-based, ammonite-calibrated reference curve provides an age model which is coherent with recently published ages based on calcareous nannofossil biostratigraphy and the correlation of trends in chemo- and sequence stratigraphy. K-Ar dating on well-preserved glauconite grains from the "Marnes d'Uttins” and lowermost part of the "Urgonien Jaune” delivered ages of 127.5±2.3 and 130.7±2.6Ma, respectively. Whereas the age of the glauconitic level near the base of the "Urgonien Jaune” is chronostratigraphically meaningful, the K-Ar age of the "Marnes d'Uttins” appears too young relative to the presently used time scale. This may be related to rejuvenation of the K-Ar chronometer due to post-depositional Ar loss, most likely during hardground formation. The ages obtained here confirm the Late Barremian age for the onset of the Urgonian platform, an age which is conform with ages obtained in the Helvetic Alps and elsewhere along the northern Tethyan margi

High cadmium concentrations in Jurassic limestone as the cause for elevated cadmium levels in deriving soils: a case study in Lower Burgundy, France

Cadmium (Cd) is a highly toxic element and its presence in the environment needs to be closely monitored. Recent systematic surveys in French soils have revealed the existence of areas in eastern and central France, which show systematically high cadmium concentrations. It has been suggested that at least part of these anomalous levels are of natural origin. For the Lower Burgundy area in particular, a direct heritage from the Jurassic limestone bedrock is highly suspected. This potential relationship has been studied in several localities around Avallon and this study reports new evidence for a direct link between anomalously elevated cadmium contents of Bajocian and Oxfordian limestone and high cadmium concentrations in deriving soils. Soils in this area show cadmium concentrations generally above the average national population values, with contents frequently higher than the ‘upper whisker' value of 0.8μgg−1 determined by statistical evaluation. In parallel, limestone rocks studied in the same area exhibit cadmium concentrations frequently exceeding the mean value of 0.030-0.065μgg−1 previously given for similar rocks by one order of magnitude, with a maximum of 2.6μgg−1. Mean ratios between the cadmium concentrations of limestone bedrock and deriving soils (Cdsoil/Cdrock), calculated for different areas, range from 4.6 to 5.7. Calculations based on the analyses of both soils from a restricted area and fragments of bedrock sampled in the immediate vicinity of high-concentration soils are around 5.5-5.7. Cdsoil/Cdrock is useful in determining the potential of soils in Lower Burgundy to reflect and exacerbate the high concentrations of cadmium present in parent bedrock

Chicxulub impact spherules in the North Atlantic and Caribbean: age constraints and Cretaceous-Tertiary boundary hiatus

The Chicxulub impact is commonly believed to have caused the Cretaceous-Tertiary boundary mass extinction and a thin impact spherule layer in the North Atlantic and Caribbean is frequently cited as proof. We evaluated this claim in the seven best North Atlantic and Caribbean Cretaceous-Tertiary boundary sequences based on high-resolution biostratigraphy, quantitative faunal analyses and stable isotopes. Results reveal a major Cretaceous-Tertiary boundary unconformity spanning most of Danian subzone P1a(1) and Maastrichtian zones CF1-CF2 (~400 ka) in the NW Atlantic Bass River core, ODP Sites 1049A, 1049C and 1050C. In the Caribbean ODP Sites 999B and 1001B the unconformity spans from the early Danian zone P1a(1) through to zones CF1-CF4 (~3 Ma). Only in the Demerara Rise ODP Site 1259B is erosion relatively minor and restricted to the earliest Danian zone P0 and most of subzone P1a(1) (~150 ka). In all sites examined, Chicxulub impact spherules are reworked into the early Danian subzone P1a(1) about 150-200 ka after the mass extinction. A similar pattern of erosion and redeposition of impact spherules in Danian sediments has previously been documented from Cuba, Haiti, Belize, Guatemala, south and central Mexico. This pattern can be explained by intensified Gulf stream circulation at times of climate cooling and sea level changes. The age of the Chicxulub impact cannot be determined from these reworked impact spherule layers, but can be evaluated based on the stratigraphically oldest spherule layer in NE Mexico and Texas, which indicates that this impact predates the Cretaceous-Tertiary boundary by about 130-150 k

Field guide to Cretaceous-tertiary boundary sections in northeastern Mexico

This guide was prepared for the field trip to the KT elastic sequence of northeastern Mexico, 5-8 February 1994, in conjunction with the Conference on New Developments Regarding the KT Event and Other Catastrophes in Earth History, held in Houston, Texas. The four-day excursion offers an invaluable opportunity to visit three key outcrops: Arroyo El Mimbral, La Lajilla, and El Pinon. These and other outcrops of this sequence have recently been interpreted as tsunami deposits produced by the meteorite impact event that produced the 200 to 300-km Chicxulub basin in Yucatan, and distributed ejecta around the world approximately 65 m.y. ago that today is recorded as a thin clay layer found at the K/T boundary. The impact tsunami interpretation for these rocks has not gone unchallenged, and others examining the outcrops arrive at quite different conclusions: not tsunami deposits but turbidites; not KT at all but 'upper Cretaceous.' Indeed, it is in hopes of resolving this debate through field discussion, outcrop evaluation, and sampling that led the organizers of the conference to sanction this field trip. This field guide provides participants with background information on the KT clastic sequence outcrops and is divided into two sections. The first section provides regional and logistical context for the outcrops and a description of the clastic sequence. The second section presents three representative interpretations of the outcrops by their advocates. There is clearly no way that these models can be reconciled and so two, if not all three, must be fundamentally wrong. Readers of this guide should keep in mind that many basic outcrop observations that these models are based upon remain unresolved. While great measures were taken to ensure that the information in the description section was as objective as possible, many observations are rooted in interpretations and the emphasis placed on certain observations depends to some degree upon the perspective of the author

Maastrichtian-Paleocene deposition in the Paraíba basin, NE Brazil

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