Can biogeochemistry aid in the palaeoenvironmental/early diagenesis reconstruction of the ˜187 Ma (Pliensbachian) organic-rich hemipelagic series of the Lusitanian Basin (Portugal)?

Abstract

Data on lipids, carbohydrates and proteins of the most expressive black shale (s.l.) intervals of the Early-Late Pliensbachian (Early Jurassic,~187 Ma) organic-rich hemipelagic series of the Lusitanian Basin (Portugal) were determined using a method that has been successfully applied over the last two decades in the characterization of biomass and very immature sediments. The goal of this paper is to test the applicability of these techniques to the ancient geological record. To our knowledge, this is the first time that this type of biogeochemical data from sedimentary series older than Oligocene is reported and tentatively used for palaeoenvironmental/diagenetic inferences. Carbohydrates and proteins are present in low concentrations, reaching up to 385.13 and 451.13 μg/g rock, respectively. The main variations are observed in the lipid contents, ranging from 197.67 to 8446.36 μg/g rock. The samples with the highest amounts of lipids seem to correlate with low [O 2 ] time intervals determined by independent data, such as organic petrography, micropalaeontology and sedimentology. This was probably related with selective lipid preservation under oxygen and hydrogen sulfide-rich depleted environments. The good overall match between the determined lipid contents and specific depositional/early diagenetical conditions seem to favor the idea that the easy to perform and inexpensive method applied here has the potential to add useful information to the study of ancient organic-rich carbonate sedimentary series.

3D numerical modelling of marine organic matter distribution: example of the early Jurassic sequences of the Lusitanian Basin (Portugal)

“…This assumption is justified by the Tmax value of around 425–430 °C reported in numerous studies (e.g. Baudin et al ., ; Ferreira et al ., ,b; Duarte et al ., ; Silva et al ., ), which indicates a limited effect of thermal alteration on the organic matter. This leads us to consider that the marine fraction of the measured signal represents the initial TOC value, because of the low maturity values recorded in the Northern Lusitanian Basin.…”

“…The fraction of terrestrial organic matter records a similar impact on the measured signal throughout the basin. This observation is supported by palynofacies analyses, which indicate that organic material is dominated by amorphous organic matter (>60%) with a significant proportion of phytoclasts (5–19%) and palynomorphs (3–18%) (Ferreira et al ., ,b; Duarte et al ., ; Silva et al ., ; Ribeiro et al ., ). The proportion of amorphous organic matter in TOC varies from 60 to 80%.…”

“…The development of carbonate ramp systems in the Northern Lusitanian Basin led to the deposition of marly organic‐rich intervals, as recorded in outcrops along the coast of central Portugal (e.g. Duarte & Soares, ; Duarte et al ., , , ; Oliveira et al ., ; Duarte, ; Silva et al ., , ; Silva & Duarte, ). The organic‐rich facies are similar throughout the Lower Jurassic of the Lusitanian Basin (Duarte et al ., ), corresponding to dark‐grey marl‐limestone alternations.…”

“…The organic‐rich facies are similar throughout the Lower Jurassic of the Lusitanian Basin (Duarte et al ., ), corresponding to dark‐grey marl‐limestone alternations. Black shale intervals can be observed up to 50‐cm thick (Duarte et al ., , , ; Duarte, ; Silva et al ., , ; Silva & Duarte, ). The two main organic‐rich intervals correlate with the Late Sinemurian Oxynotum‐Raricostatum Zone, and the earliest part of the Late Pliensbachian at the top of the Ibex to Margaritatus Zone (e.g.…”

The Lower Jurassic at Peniche (Lusitanian Basin): recent advances in Stratigraphy and Sedimentary Geology

“…2), located about 80 km north of Lisbon, presents the most complete succession of Lower Jurassic sediments in Portugal, framed by an important geological heritage character and a spectacular scenic environment (Duarte, 2004). In recent years, this section has been the focus of intense scientific activity, related mainly to stratigraphy and sedimentary geochemistry (e.g., Rocha, 2007;Hesselbo et al, 2007;Suan et al, 2008aSuan et al, , 2008bSuan et al, , 2010Mattioli et al, 2008Mattioli et al, , 2009Duarte et al, 2010Duarte et al, , 2011Reggiani et al, 2010aReggiani et al, , 2010bSilva et al, 2011bSilva et al, , 2012. In addition to introducing the sedimentary context, describing the stratigraphy, and characterizing the whole succession (Stop 1), on this field-trip we also discuss: a) the Pliensbachian organic-rich marl-limestone deposits, one of the main intervals of hydrocarbon source potential in the West Iberian Margin (Stop 2); b) the Pliensbachian-Toarcian boundary at Ponta do Trovão, which has been ratified as the Toarcian Global Boundary Stratotype Section and Point (GSSP) (Stop 3); and c) the early Toarcian oceanic anoxic event (T-OAE) and its record at Abalo beach (Stop 4).…”