Looking for the Jurassic-Cretaceous system boundary in the Vocontian Trough (S-E France): Sedimentological problems

Resedimented conglomerates and slump deposits coming from the slopes of the Vocontian Trough are widespread in the corresponding basinal successions ( Ferry & Grosheny , 2013; Ferry , 2017). The maximum stratigraphic extent of a first pulse of redeposited sediments in this basin spans the uppermost Oxfordian to the lower Berriasian. This prevents the identification of a continuous succession spanning the Tithonian-Berriasian transition in most of the basinal sections. Recently, several sections of the upper Drome River valley have been studied by several authors to improve the ammonite biostratigraphy. It shoud be remembered that all these sections are located on the left side of a breccia lobe about 50 km long and 20 km wide, running approximately along the present-day Drome River valley ( Courjault et al. , 2011). The thickest breccia accumulation is within the lower Tithonian. The degree of reworking dimnishes in size and frequency during the deposition of the basinal equivalent of the upper Tithonian-lower Berriasian “Calcaires Blancs“, originally defined on the western border of the basin. It should also be remembered that hiatuses which cannot be identified in the field are frequent, like the one in the middle of a limestone bed which imperceptibly superimposes Berriasian limestone over upper Kimmeridgian limestone ( Ferry et al. , 2015, Fig. 8). This is true even in successions devoid of resedimented bodies like in the Angles section ( Le Hegarat & Ferry , 1990), better known for hosting the Barremian hypostratotypic section. The best basinal area for finding an adequately exposed and continuous stratigraphic succession in this basin is probably in the vicinity of the Veynes village ( Ferry et al. , 2015, Fig. 3). According to Courjault et al. (2011), this location corresponds to the tail of the Drome River lobe, in an area overhanging a deeper submarine terrace where the gully-connected breccia lobe of Ceuse ( Ferry et al. , 2015) is located.