1,721,141 research outputs found
Birth and closure of the Kallipetra Basin: Late Cretaceous reworking of the Jurassic Pelagonian-Axios/Vardar contact (northern Greece)
Full text linkSome 20 Myr after the Late Jurassic to Early Cretaceous obduction and collision at the eastern margin of Adria, the eroded Pelagonia (Adria) Axios/Vardar (oceanic complex) contact collapsed, forming the Kallipetra Basin, described around the Aliakmon River near Veroia (northern Greece). Clastic and carbonate marine sediments deposited from the early Cenomanian to the end of the Turonian, with abundant olistoliths and slope failures at the base due to active normal faults. The middle part of the series is characterized by red and green pelagic limestones, with a minimal contribution of terrigenous debris. Rudist mounds in the upper part of the basin started forming on the southwestern slope, and their growth competed with a flux of ophiolitic debris, documenting the new fault scarps affecting the Vardar oceanic complex (VOC). Eventually, the basin was closed by overthrusting of the VOC towards the northeast and was buried and heated up to ~ 180 °C. A strong reverse geothermal gradient with temperatures increasing up-section to near 300 °C is recorded beneath the VOC by illite crystallinity and by the crystallization of chlorite during deformation. This syntectonic heat partially reset the zircon fission track ages bracketing the timing of closure just after the deposition of the ophiolitic debris in the Turonian. This study documents the reworking of the Pelagonian Axios/Vardar contact, with Cenomanian extension and basin widening followed by Turonian compression and basin inversion. Thrusting occurred earlier than previously reported in the literature for the eastern Adria and shows a vergence toward the northeast, at odds with the regional southwest vergence of the whole margin but in accordance to some reports about 50 km north
Carbonate platform evidence of ocean acidification at the onset of the early Toarcian oceanic anoxic event
No full textThe early Toarcian oceanic anoxic event (Early Jurassic; ~183 Myr ago) is associated with one of the largest negative carbon isotope excursion (CIE) in the whole Phanerozoic (3–7‰). Estimates of the magnitude and rate of CO2 injection in the ocean-atmosphere system are compatible with a scenario of ocean acidification. Many carbonate platforms drowned in the Pliensbachian, well before the early Toarcian event. In this paper we test the hypothesis of surface water ocean acidification by presenting data from a resilient carbonate platform: the Apennine Carbonate Platform of southern Italy.
The studied sections document a dramatic shift of the carbonate factory from massive biocalcification to chemical precipitation. Lithiotis bivalves and calcareous algae (Palaeodasycladus mediterraneus), which were the most prolific carbonate producers of Pliensbachian carbonate platforms, disappear during the first phase of the early Toarcian CIE, before the most depleted values are reached. We discuss the local versus supraregional significance of this shift and propose a scenario involving abrupt decline of carbonate saturation, forced by CO2 release at the beginning of the early Toarcian CIE, followed by a calcification overshoot, driven by the recovery of ocean alkalinity. Attribution of the demise of carbonate platform hypercalcifiers to ocean acidification is supported by palaeophysiology and reinforced by experimental data on the detrimental effects of ocean acidification on recent shellfishes and calcareous algae
Application of organic matter and clay minerals studies to the tectonic history of the Molise area, Central Apennines
No full textThe Abruzzo-Molise-Sannio region is part of the Central Apennines foreland fold and thrust belt, a structure that formed during the Neogene as a result of the continental collision between the European and Adria plates. Certain areas of this crust experienced anomalously high temperatures due to deep burial, while others did not. This burial has been investigated using optical indicators of organic matter (OM) maturity and clay mineralogy. The maximum depths of burial of Tertiary surface sequences have been established based on the vitrinite reflectance (Ro) and thermal alteration index (TAI) of OM dispersed in synorogenic sediments and from clay mineralogy of pre-thrusting clayey deposits. The synorogenic deposits show low levels of organic maturity having Ro values less than 0.6% indicating early mature to mid-mature hydrocarbon generation. The pre-thrusting deposits are shales characterised by a very high percentage of clay minerals with lesser amounts of quartz, K-feldspar, plagioclase and calcite. The dominant minerals are highly smectitic interstratified illite/smectite (RIS). Both Ro and I/S ratios were used to calibrate burial and thermal evolution of this chain sector by numerical modelling. The results indicate that the structural units which outcrop in the study area (particularly the Molise Unit) were never overthrust by significant thicknesses of rocks during mountain building, at least before Pliocene times; the hypothesis that more internal units overthrust the presently exposed units also seems to be unlikely
Integrated mineralogical and rock magnetic study of Deccan red boles
No full textWe conducted a detailed rock magnetic and mineralogical study of bole beds from the Deccan magmatic province, India. Magnetic susceptibility of 15 bole beds showed two contrasting patterns, with susceptibility values either increasing or decreasing up the profile. We then focused on two representatives red boles located in the Western Ghats, the RBB and RBAN profiles, to unravel the nature and origin of these contrasting magnetic susceptibility patterns. The presence of smectite argues against significant secondary thermal alterations. Major-elemental compositions obtained by X-ray fluorescence spectrometry of RBB and RBAN red boles are comparable to the parent basal
Sedimentary mercury enrichments as evidence of volcanism in the Tethyan carbonate platforms during Oceanic Anoxic Event-2
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Paleoecology of the Cretaceous-Tertiary mass extinction in planktonic foraminifera
No full textPaleobiogeographic patterns of the Cretaceous-Tertiary (K-T) mass extinction in planktonic foraminifera in Tunisia, spanning environments from open marine upper bathyal, to shelf and shallow marginal settings, indicate a surprisingly selective and environmentally mediated mass extinction. This selectivity is apparent in all of the environmental proxies used to evaluate the mass extinction, including species richness, ecological generalists, ecological specialists, surface and subsurface dwellers, whether based on the number of species or the relative percent abundances of species. The following conclusions can be reached for shallow to deep environments: about three quarters of the species disappeared at or near the K T boundary and only ecological generalists able to tolerate wide variations in temperature, nutrients, salinity and oxygen survived. Among the ecological generalists (heterohelicids, guembelitrids, hedbergellids and globigerinellids), only surface dwellers survived. Ecological generalists which largely consisted of two morphogroups of opportunistic biserial and triserial species also suffered selectively. Biserials thrived during the latest Maastrichtian in well stratified open marine settings and dramatically declined in relative abundances in the early Danian. Triserials thrived only in shallow marginal marine environments, or similarly stressed ecosystems, during the latest Maastrichtian, but dominated both open marine and restricted marginal settings in the early Danian. This highly selective mass extinction pattern reflects dramatic changes in temperature, salinity, oxygen and nutrients across the K-T boundary in the low latitude Tethys ocean which appear to be the result of both long-term environmental changes (e.g., climate, sea level, volcanism) and short-term effects (bolide impact). (C) 2002 Elsevier Science B.V. All rights reserved
Detecting eustatic and tectonic signals with carbon isotopes in deep-marine strata, Eocene Ainsa Basin, Spanish Pyrenees
No full textGoing Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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