Luís V. Duarte

Organic-Rich facies in the Sinemurian and Pliensbachian of the Lusitanian Basin, Portugal: Total organic carbon distribution and relation to transgressive-regressive facies cycles

The upper Sinemurian to Pliensbachian series of the Lusitanian Basin (Portugal) correspond to marly limestone sediments rich in benthic and nektonic macrofauna. This sedimentary record includes several intervals of organicrich facies, which are particularly well developed in the western sectors of the basin. They correspond to grey and dark marls locally showing strong lamination (black shale type) and are recognized as one of the most important potential oil source rocks. This study shows the vertical and lateral distribution of these organic-rich intervals, supported by over 550 total organic carbon (TOC) determinations. The results presented reveal two important intervals, with several black shale occurrences, in the Oxynotum(?)–Raricostatum (Polvoeira Member of Agua de Madeiros Formation) and at the top of the Ibex-upper part of Margaritatus zones (top of the Vale das Fontes Formation), showing in the distal (western) sectors up to 22% and 15% TOC, respectively. TOC values decrease progressively towards the proximal sectors, the youngest organic-rich interval being the most expressive at the basin scale. This lateral TOC distribution, the facies stacking patterns and the decrease observed in benthic macrofauna confirm that these intervals are related to 2nd-order transgressive phases. 2nd-order regressive phases, developed during the uppermost Raricostatum and Spinatum zones respectively, show lower TOC values. TOC distribution combined with other stratigraphic and sedimentological parameters enabled seven facies maps to be created for the time interval studied. At the regional scale, this study shows for the first time the good similarity between the upper Sinemurian-Pliensbachian sedimentary successions of the Lusitanian and Basque- Cantabrian basins.

Lower to Middle Toarcian from the Coimbra region (Lusitanian Basin, Portugal): sequence stratigraphy, calcareous nannofossils and stable-isotope evolution

This multidisciplinary study (facies analysis, sequence stratigraphy, calcareous nannofossils and carbon stable isotopes) focuses on the Lower to Middle Toarcian succession cropping out in the Coimbra region (Northern sector of the Lusitanian Basin), Portugal. Deposited on homoclinal carbonate ramp, the sampled hemipelagic series, consisting of marl/limestone alternations can be subdivided into three third-order depositional sequences (ST1, ST2 and ST3) characterised by different vertical facies arrangements and palaeontological contents. The sequence boundaries lie within the Polymorphum Zone, around the Polymorphum/Levisoni Zone boundary and in the Bifrons Subzone (Bifrons Zone). The calcareous nannofossils provide a continuos succession of age-significant assemblages, and a useful set of nannobiohorizons that include the LO of Calcivascularis jansae , the LOs of Biscutum grande and Biscutum finchii , and the FOs of Carinolithus cantaluppii , Carinolithus superbus , Discorhabdus striatus and Discorhabdus ignotus . The evolution of the ð13C agrees with the sequential developments of the series because the positive excursions roughly coincide with transgressive depositional phases, whereas a negative trend is observed during regressive phases.

THE GEOLOGICAL HERITAGE OF THE LOWER JURASSIC OF CENTRAL PORTUGAL: SELECTED SITES, INVENTORY AND MAIN SCIENTIFIC ARGUMENTS

The Lower Jurassic is well represented in west-central Portugal (Lusitanian Basin), where several outcrops with exceptional exposures exist. Among these, three sites have a major interest for Geological Heritage: S. Pedro de Moel, Rabacal and Peniche. These localities display important sections for the study of the Liassic carbonate successions, whose scientific and educational value, for the domains of Palaeontology, Stratigraphy, Sedimentology and Geomorphology is widely known. The aim of this work is to enumerate the main scientific arguments for considering these Portuguese Liassic localities as national and international relevance and as an important heritage value (Geosite).

Tautomers and UV-Induced Photoisomerization of a Strongly Intramolecularly H-Bonded Aromatic Azo-Dye: 1-(Cyclopropyl)diazo-2-naphthol

Aromatic azo compounds have a wide range of industrial applications as dyes in optical and color-changing materials and can also be exploited in the design of new photodynamic molecular systems. The azo derivative 1-(cyclopropyl)diazo-2-naphthol was isolated in low-temperature cryogenic matrices, and its molecular structure, tautomeric equilibrium, and photochemical transformations were characterized by infrared spectroscopy and theoretical calculations. Only azo-enol forms having the OH group involved in a strong intramolecular hydrogen bond, forming a six-membered ring with the azo group, were found experimentally. Irradiation with a narrowband source in the near-UV range generates different rotameric and tautomeric azo-enol and keto-hydrazone forms that can be interconverted at different irradiation wavelengths.

Photoisomerization of Saccharin

Most known applications of saccharin and saccharyl derivatives and their potential for new uses rely on the thermal and photochemical stability of the saccharyl system. Here, we show that saccharin undergoes structural rearrangement when subjected to a narrow-band ultraviolet irradiation. Monomeric saccharin was isolated in low-temperature argon matrices and its photochemistry was characterized by means of infrared spectroscopy and DFT calculations. Among several DFT methods used, the O3LYP/6-311++G(3df,3pd) level gave the best match with the experimental spectra. Irradiation of matrix-isolated saccharin, with a narrow-band source (290 nm), generates a so far unknown isomer that we call iso-saccharin. The structures of the conjugate bases of saccharin and iso-saccharin were also computed theoretically. Their free energies and dipole moments suggest that both anions may be relevant in systems where saccharin participates, as is the case of the recently proposed saccharin-based ionic liquids.

Facies and Carbon Isotope Chemostratigraphy of Lower Jurassic Carbonate Deposits, Lusitanian Basin (Portugal): Implications and Limitations to the Application in Sequence Stratigraphic Studies

Abstract The Lower Jurassic carbonate series of the Lusitanian Basin (Portugal), due to their remarkable exposure and high quality outcrops, are currently regarded as a pivotal point to the comprehension of the less-known offshore basins located along the Atlantic Margin. This study presents a detailed sedimentological and biostratigraphical framework and the carbon and oxygen isotopic record of one of the main organic-rich intervals of the Lusitanian Basin, the marly limestones with organic-rich facies member of the Vale das Fontes Formation (Pliensbachian). We report the occurrence of secular δ13C trend of the Lusitanian Basin concordant with the (second-order) Pliensbachian transgressive–regressive facies cycle. An overall increase in carbon isotopic record with transgression, reaching its maximum just above the maximum flooding interval and enhanced organic matter preservation is also observed. When compared with the defined third-order sequences, a transient δ13C increase during the onset of transgression is recognizable. Higher frequency variations in the carbon isotopic record can possibly be traced along the offshore studied sections and are interpreted to be the reflections of local controls which may eventually outpace the regional variation.

Sponge-microbialite buildups from the Toarcian of the Coimbra region (Northern Lusitanian Basin, Portugal): paleoecological and paleoenvironmental significance

Small-scale siliceous sponge-microbialite buildups are recorded in the Middle–Upper Toarcian hemipelagic succession (marl–limestone alternations) of the Lusitanian Basin. A detailed study of these buildups from different outcrops recorded in the eastern area of the basin (Zambujal, Ordem, and Roça Cú) made it possible to characterize the typology of these buildups, identify the different components and facies, and discuss the controlling factors involved. The studied buildups developed in three growth stages: (I) a colonization stage of the muddy bottom by epibenthic macroinvertebrates, resulting in bioclastic patches providing a hard substrate, functioning as benthic islands; (II) a growth stage dominated by dish-shaped siliceous sponges encrusted by thin microbial crusts; (III) a growth stage dominated by thick microbial growth (thrombolites and minority leiolites) on siliceous sponges. This sequence is repeated in more complex buildups with cycles of growth, interruption, and resumption. The competition for available hard substrates among sessile benthic organisms, including microbial communities, reinforced the buildup growth with respect to the surrounding unfavorable soft substrate and controlled the high variability of the accompanying fauna. The installation of these buildups in the outer homoclinal ramp was favored by a low sedimentation rate, water depth, and type and availability of nutrients. Changes in the composition of sponge assemblages indicate decreasing dissolved organic carbon and colloidal substances along with increasing particulate organic matter, probably related with a shallowing-upward trend and a regressive phase.

Charcoal evidence that rising atmospheric oxygen terminated Early Jurassic ocean anoxia

The Toarcian Oceanic Anoxic Event (T-OAE) was characterized by a major disturbance to the global carbon(C)-cycle, and depleted oxygen in Earths oceans resulting in marine mass extinction. Numerical models predict that increased organic carbon burial should drive a rise in atmospheric oxygen (pO2) leading to termination of an OAE after ∼1 Myr. Wildfire is highly responsive to changes in pO2 implying that fire-activity should vary across OAEs. Here we test this hypothesis by tracing variations in the abundance of fossil charcoal across the T-OAE. We report a sustained ∼800 kyr enhancement of fire-activity beginning ∼1 Myr after the onset of the T-OAE and peaking during its termination. This major enhancement of fire occurred across the timescale of predicted pO2 variations, and we argue this was primarily driven by increased pO2. Our study provides the first fossil-based evidence suggesting that fire-feedbacks to rising pO2 may have aided in terminating the T-OAE.

Palaeogeographical Evolution of the Lusitanian Basin (Portugal) During the Jurassic. Part I: The Tectonic Constraints and Sedimentary Response

In this paper, we present the general tectonic framework that conditioned the evolution of the Lusitanian Basin during the Jurassic, when most of the filling of the basin was accumulated. The rifting episodes and the evolution of the basin’s geometry are presented and their close relationship with the main basin-wide unconformities is discussed. The unconformity-bounded sequences are briefly presented as a basis for understanding the evolution of the sedimentary environments. This constitutes the basic geological information to construct the palaeogeographical maps presented in Part II of this paper (this volume).

Advances in the Stratigraphy and Geochemistry of the Organic-Rich Lower Jurassic Series of the Lusitanian Basin (Portugal)

Here we present the main results of a study of the high-resolution stratigraphy of the Lower Jurassic organic-rich marine series in the Lusitanian Basin, Portugal. In this basin, the Lower Jurassic consists mainly of an alternating marl–limestone succession (upper Sinemurian–Toarcian), with some intervals enriched in organic matter (upper Sinemurian and Pliensbachian). These Lower Jurassic carbonate units correspond to the Coimbra, Agua de Madeiros, Vale das Fontes, Lemede, and S. Giao/Cabo Carvoeiro formations. In the last few years, these units have been the subject of a multidisciplinary and integrated stratigraphic analysis, based on the study of a range of biotic and abiotic parameters. In our study, high-resolution analysis of these units in the main reference sections of the basin have allowed seven main themes to be developed, involving sedimentological and sequential analysis, ammonite biostratigraphy, macroinvertebrate palaeontology and palaeoecology, ostracod analysis, chemostratigraphy, organic and petrographic geochemistry, and gamma-ray analysis. Besides the novel results obtained in each scientific domain, the integration of all the collected data has improved our knowledge of not only the stratigraphy and sedimentary knowledge of the Lower Jurassic carbonates but also the hydrocarbon generation potential of the different Sinemurian and Pliensbachian units.