The La Voulte Lagerstätte (Callovian): Evidence for a deep water setting from sponge and crinoid communities
Résumé
The siliceous sponges and crinoids from the Chénier Ravine (France, Lower Callovian) are used here as biological markers to characterize the palaeoenvironment of the adjacent and contemporaneous La Voulte Lagerstätte that is remarkable for its unique soft-bodied fauna (e.g. worms, cirrate octopods, vampire squids). Sponges are abundant with dominant hexactinellids (80%) and lithistids (20%). Four lines of fossil evidence, supported by Recent analogues, indicate that this sponge community inhabited deepwater setting under dysphotic or aphotic conditions: (1) the dominance of hexactinellids; (2) the prevalence of cone-shaped and erect morphologies that usually characterize Recent bathyal sponges; (3) the close similarities with Recent hexactinellids from the continental slope and; (4) the lack of encrustation by photophilic organisms. Attachments of sponges on hard substrate (e.g. crystalline basement) and their preservation in soft sediments (e.g. muds) indicate heterogeneous bottom conditions. The Chénier fauna also contains small stalked, asymmetric cyrtocrinid crinoids that are known to live on hard substrates in bathyal environments such as the SW Pacific steep seamounts. Convergent palaeoenvironmental clues obtained from both crinoids and siliceous sponges support the notion that the La Voulte area, including the La Voulte Lagerstätte, was situated in the upper part of the bathyal zone near the slope-basin transition with a water depth most probably exceeding 200 m. Supporting evidence from heterogeneous substrates and complex fault systems indicate a depositional environment along the external part of the slope where steep topographies and blocks usually favour the settlement of cyrtocrinid crinoids and hexactinellid sponges. La Voulte may therefore be one of the rare and extremely precious Mesozoic Lagerstätten to have fossilized a deep marine environment