Emplacement of lobate sheet flows with hyaloclastites onto soft sediment: The Erquy Neoproterozoic lava pile, Armorican Massif (France)

Abstract

The Neoproterozoic volcano-sedimentary succession of the Heussaye Headland (Erquy) belongs to the Cadomian block located in the north-east of the Armorican Massif (France). The volcanism, dated at approximately 608 ± 7 Ma (zircon U/Pb data), occurred in a back-arc basin. This site exhibits a noteworthy verticalized pile of moderately metamorphosed (greenschist facies) basaltic andesite flows of arc/back-arc tholeiite affinity, alternating with metasedimentary layers (sandstones and siltstones). A large range of volcanic facies, characteristic of an underwater soft-substrate environment, can be continuously studied for over than 200 m parallel to the flow edges. They include pillow lavas, lava lobes, fluidal peperites and sediment-matrix hyaloclastites. The presence of parallel lava lobes in a sector where one lava flow is capped by a variably thick hyaloclastite carapace, together with the occurrence of asymmetric turbiditic folds in interbedded metasedimentary layers, suggests the existence of an easterly-facing gentle paleoslope. The sudden thinning of this hyaloclastite carapace in two spots located approximately 60 m from each other, is attributed to the synvolcanic activity of two conjugate normal paleofaults. This hypothesis is supported by the observation of a hyaloclastite accumulation in one of the two spots close to a strike-slip fault, which may have been extensional at the time of the volcanism. Water entered the lava flow along the faults, resulting in an interstitial network of hyaloclastites by quench fragmentation in the upper part of the hanging wall block. All these observations are consistent with a model of lava emplacement along a subaqueous slope resulting from synmagmatic extensional faulting in a marginal basin context.