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Demise of the northern Tethyan Urgonian carbonate platform and subsequent transition towards pelagic conditions: The sedimentary record of the Col de la Plaine Morte area, central Switzerland
Auteur(s)
Gainon, François
Date de parution
2008
In
Sedimentary Geology, Elsevier, 2008/205/3-4/142-159
Résumé
The sedimentary succession of the Col de la Plaine Morte area (Helvetic Alps, central Switzerland) documents the disappearance of the northern Tethyan Urgonian platform in unprecedented detail and suggests stepwise platform demise, with each drowning phase documented by erosion and phosphogenesis. The first identified drowning phase terminated Urgonian carbonate production in a predominantly photozoan mode. Using a correlation of the whole-rock δ<sup>13</sup>C record with the well-dated record from SE France, its age is inferred to as Middle Early Aptian (near the boundary between the <i>weissi</i> and <i>deshayesi</i> zones). A subsequent drowning phase is dated by ammonites and by a correlation of the whole-rock δ<sup>13</sup>C record as Late Early Aptian (late <i>deshayesi</i> to early <i>furcata</i> zone). A third drowning phase provides an ammonite-based age of Early Late Aptian (<i>subnodosocostatum</i> and <i>melchioris</i> zones) and is part of a widely recognized phase of sediment condensation and phosphogenesis, which is dated as latest Early to Middle Late Aptian (late <i>furcata</i> zone to near the boundary of the <i>melchioris</i> and <i>nolani</i> zones). The fourth and final drowning phase started in the latest Aptian (<i>jacobi</i> zone) as is also indicated by ammonite findings at the Col de la Plaine Morte. The phases of renewed platform-carbonate production intervening between the drowning phases were all in a heterozoan mode. <br> During the ultimate drowning phase, phosphogenesis continued until the Early Middle Albian, whereas condensation processes lasted until the Middle Turonian. Coverage of the external margin of the drowned Urgonian platform by a drape of pelagic carbonates started only in the Late Turonian. During the Santonian, the external part of the drowned platform underwent normal faulting and saw the re-exposure of already lithified Urgonian carbonates at the seafloor. <br> Based on the here-inferred ages, the first drowning phase just precedes oceanic anoxic episode 1a (OAE 1a or “selli event”) in time, and the second drowning phase partly overlaps with OAE 1a. The onset of the third drowning event slightly predates two further periods of increased organic-matter accumulation in the Vocontian Basin (Noir and Fallot levels), and the onset of the fourth and final drowning phase may coincide with two further periods of increased organic-matter accumulation in the Vocontian Basin (Jacob and Kilian levels, part of OAE 1b). These correlations indicate a relationship between the so-called anoxic episodes and the stepwise demise of the Urgonian platform, even if the onset of environmental change is registered earlier on the platform than in basinal sediments.
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Type de publication
journal article
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