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The Jurassic-Cretaceous boundary interval in the Jura Mountains and the Vocontian Basin: Sedimentological aspects
Andreas Strasser

Building: Muséum d'Histoire Naturelle de Genève
Room: Amphithéâtre
Date: 2018-12-06 02:00 PM – 02:30 PM
Last modified: 2018-11-23

Abstract


During the latest Jurassic and earliest Cretaceous, the palaeogeographic realm of the Jura was a topographic high between the Tethys Ocean and the Paris Basin, on which a shallow carbonate platform developed. Hiatuses are abundant and biostratigraphically relevant fossils are rare. In the Vocontian Basin, lowstand sediments predominated, precluding an undisturbed stratigraphic record.

In the Swiss Jura, the Tithonian strata are represented by the Twannbach Formation where dolomitization and features implying subaerial emergence are common. It is overlain by the early Berriasian Goldberg Formation, which corresponds to the “Purbeck facies”. Periods of subaerial exposure are indicated by black pebbles and calcretes.

Overlying the Goldberg Formation, the Pierre-Châtel Formation developed in fully marine, shallow-lagoonal conditions. The climate was semi-arid. Its base, dated to the Subalpina subzone of the middle Berriasian Occitanica zone (Clavel et al., 1986), represents a first transgressive surface that can be correlated over the entire Jura platform. The overlying Vions Formation records a more humid climate, indicated by terrigenous input of clays and quartz, iron-staining, root traces, and coal layers. The Chambotte Formation displays bioclastic and oolitic limestones, again implying fully marine conditions in a more arid climate. This is the result of a second transgressive pulse, dated to the base of the Otopeta subzone of the late Berriasian Boissieri zone (Charollais et al., 2008). A tectonic tilt of the Jura platform resulted in non-deposition and/or erosion of parts of the Vions and Chambotte formations.

The early Valanginian is represented by the reddish limestones (“calcaires roux”) of the Vuache Formation (Strasser et al., 2016). Locally, the base of the formation contains a marly interval (“Marnes d’Arzier”). The limestone beds display hummocky and swaley cross-stratification implying storm activity. The facies are bioclastic and oolitic packstones to grainstones rich in echinoderm and bryozoan fragments. The depositional environment was that of an open shelf. Ammonites and dinocysts assign the main body of the Vuache Formation to the Pertransiens and Neocomiensiformis ammonite zones, while the Inostranzewi and Verrucosum zones are condensed at its top (Monteil, 1993; Charollais et al., 2008).

In the Vocontian Basin, the Tithonian commonly displays thickly bedded hemipelagic limestones (“barre tithonique”). Facies and sedimentary structures imply deposition mainly as mudflows, grainflows, debris flows, and occasional turbidites. The early Berriasian is dominated by slumps and debris flows. Starting within the Privasensis subzone (Occitanica zone), cyclic hemipelagic sedimentation resulted in a more expanded stratigraphic record. The sections are rich in ammonites, which allows a relatively precise biostratigraphic dating (e.g., Le Hégarat, 1971; Busnardo et al., 1979; Atrops & Reboulet, 1993).

Some of the prominent sequence-stratigraphic elements of Hardenbol et al. (1998) can be correlated between platform and basin (Pasquier & Strasser, 1997). For example, the transgressive surface above sequence boundary Be4 corresponds to the base of the Pierre-Châtel Formation on the platform. Tresch & Strasser (2010) have shown that the transgression occurred step-wise, following the platform morphology, and that it occurred within a time span of about 200 kyr. While the platform started to be flooded, lowstand conditons continued in the basin. The transgressive surface seen in the basin (in the Dalmasi subzone) corresponds to an important increase in accommodation on the platform (Pasquier & Strasser, 1997).

Both the Tithonian-Berriasian and Berriasian-Valanginian boundaries are badly defined on the Jura platform. The transgressive surface at the base of the Pierre-Châtel Formation may be relatively well dated and correlatable over a wide area but, being within the Berriasian stage, is not a good candidate for a system boundary. In the Vocontian Basin, the Tithonian-Berriasian boundary is commonly characterized by gravity deposits. Geochemistry is not helpful as there were no significant oceanic events occurring in the Jurassic-Cretaceous boundary interval. Therefore, other palaeogeographic areas with continuous, undisturbed stratigraphic records have to be evaluated.