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

Duarte, Luís Vitor and Silva, Ricardo and Oliveira, Luiz Carlos Veiga de and Comas Rengifo, María José and Silva, Francisco (2010) Organic-Rich facies in the Sinemurian and Pliensbachian of the Lusitanian Basin, Portugal: Total organic carbon distribution and relation to transgressive-regressive facies cycles. Geologica acta, 8 (3). pp. 325-340. ISSN 1695-6133

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Abstract

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 Água 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.


Item Type:Article
Uncontrolled Keywords:Lower Jurassic, Organic-rich marls, TOC, Sequence stratigraphy, Palaeogeography, Lusitanian Basin.
Subjects:Sciences > Geology > Stratigraphic geology
ID Code:12186
Deposited On:07 Feb 2011 12:30
Last Modified:07 Feb 2011 12:30

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