Fluvial shoal water deltas: pre‐vegetation sedimentation through the fluvial–marine transition, Lower Cambrian, English Channel region

Abstract

AbstractLower Palaeozoic fluvial systems tend to be more sand‐prone than those of later eras and the nature of coastal environments less certain. Field studies are presented that characterize the fluvial to marine transition over a distance of 80 km, in the Lower Cambrian of the Cotentin Peninsula, northern France. The sedimentary rocks are divided into six facies associations which represent deposition in proximal fluvial, distal fluvial, delta plain, delta front, pro‐delta and offshore carbonate bank environments. The basin fill is sandstone‐dominated and subdivided into three stratigraphic intervals. A 200 to 300 m thick basal interval contains very coarse‐grained fluvial sandstones deposited during a relative sea level lowstand. An overlying interval, 250 to 1500 m thick, is a facies mosaic. Fluvial strata in the north‐west pass laterally south‐east into deltaic and shallow marine pro‐delta sediments. The delta front deposits show repetitively stacked, upward‐coarsening parasequences, 8 to 10 m thick, which reflect the repeated progradation of lobate, fluvially‐dominated deltas onto a shallow marine shelf. The deltas formed following marine transgression and accumulated during a period of gradually rising relative sea level. An upper unit, 130 m thick, containing offshore stromatolitic and oolitic limestones, caps the study interval and represents deposition during a relative sea level highstand. The fluvial and delta distributary channel sandstones of the middle unit contain <1% mudstone. The cohesionless substrate determined that deltaic distributaries were predominantly braided in character and subject to common bifurcations which resulted in an ordered diminution of channel size and competence in a seaward direction. Terminal distributary channels show evidence of migratory levées and mouth‐bars and consistently delivered fine to medium‐grained sand to the delta front. The study highlights an example of pre‐vegetation deltaic sedimentation that was hydraulically organized and predictable, despite being fed by braided fluvial systems with high levels of peak discharge.