The Vocontian clastic dykes and sills: a geometric model

Abstract

Abstract Distal Aptian-Albian deep water channelled massive sands of the Vocontian Basin (SE France) are often associated with sand injections. The Bevons and Rosans areas in the Vocontian domain present probably the most spectacular outcrops showing complex networks of clastic sills and dykes injected into a thick marly/limy succession. Most injections are found in the channel banks, fed laterally from sandy channels. The sills are up to 10 metres thick in the vicinity of the connection with the channel feeder; they thin out and die into marls 2 or 3 kilometres away from it. Most dykes are injected from the sills rather from the channel itself: a few small dykes can be found under the channel fill. They are most abundant within a few hundred metres of the channel. Today, injections extending downwards from sills have up to 275 metres vertical extent, whereas injections extending upwards from sills never reach the contemporaneous palaeo-sea floor. Ptygmatic folding of the dykes by mechanical compaction indicates the amount of local post-injection compaction of shale and clearly shows that sand injection occurred prior to burial. Outcrop mapping shows that channel bank fracturing is contemporaneous with channel infilling. This is evidence of early syn-depositional injection of the sandy material. Vocontian clastic injections provide good geometrical analogues to deep offshore clastic injectite networks and the opportunity to better understand genetic processes.