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A quantitative palynofacies study was performed on upper Tithonian-Berriasian, open-marine, fine-grained carbonates outcropping at Berrias (Berriasian stage stratotype), Broyon, and Angles in the Vocontian basin of southeast France. Organic constituents are subdivided into a relatively autochthonous marine fraction (mainly dinocysts and secondarily foraminiferal linings) and an allochthonous land-derived fraction (higher plant debris and sporomorphs). Fluorescent amorphous organic matter is absent throughout these organic-lean carbonates.
Within a precise biostratigraphic framework based on ammonites and calpionellids, the three studied sections are interpreted in terms of sequence stratigraphy, using field observations, micro- and macrofacies analysis, and palynofacies data. Within this sequence stratigraphy framework, trends in the distribution of organic matter bear the imprint of sea level fluctuations. Moreover, a model of an idealized palynofacies sequence in open marine carbonates is proposed for each systems tract:
-- The lowstand systems tract (LST) is dominated by large-scale, angular, and/or degraded terrigenous fragments;
-- The transgressive systems tract (TST) is marked by the upward decrease in abundance, size, and angularity of terrigenous fragments, and by the upward increase in blade-shaped black humic fragments (approx. inertinite) and in dinocysts diversity and abundance, which peak at the maximum flooding surface (mfs);
-- The highstand systems tract (HST) shows reversed trends, both dinocysts and blade-shaped black humic fragments decreasing, and terrigenous fragments increasing.
The outlined methodology has a definite application in petroleum geology. It can be used in different depositional environments, notably in more organic-rich sediments, where the potential presence of fluorescent amorphous organic matter, particularly abundant in transgressive episodes, provides an extra palynofacies parameter.
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