The Bude Formation (Lower Westphalian), SW England: siliciclastic shelf sedimentation in a large equatorial lake

Abstract

ABSTRACTThe 1300‐m‐thick turbiditic Bude Formation was deposited in a lake, Lake Bude, but disagreement persists over whether the environment was a deltaic or deep‐water fan. The tectonic setting of the lake was the northern flank of a northerly advancing Variscan foreland basin, close to the Westphalian palaeo‐equator. Palaeocurrents indicate sediment sourcing from all quadrants except the south.There is a dm‐m scale cyclicity, whereby sandstone bodies comprising amalgamated event beds alternate with mudstone intervals containing non‐amalgamated event beds. The ‘ideal’ cycle is a symmetrical coarsening‐up/fining‐up cycle, consisting of three facies (1, 2 and 3) arranged in 12321 order. Facies 3, in the middle of the cycle, is an amalgamated sandstone body up to 10 m thick which interfingers laterally with thin (cm) mudstone layers. The sandstone body comprises amalgamated beds of very fine sandstone which are largely massive and up to 0.4 m thick. Channels are absent except for scours up to 0.2 m deep which truncate the interfingering mudstone layers. Sandstone bodies are inferred to be tongue‐shaped in three dimensions. Facies 1 and 2, completing the 12321 cycle, are respectively dark‐grey fine and light‐grey coarse, varved(?) mudstone containing thin (< 0.4 m) sandstone event beds. Fossils and burrows indicate that facies 1 and 2 were deposited, respectively, in brackish (rarely marine) and fresh water. Hence, the ideal cycle (12321) reflects an upward decrease then increase in salinity (brackish‐fresh‐brackish); this is attributed to the lake sill being periodically overtopped by the sea, due to glacio‐eustatic sea‐level oscillations. The resulting oscillations in lake depth produced the coarsening‐up/fining‐up (regressive‐transgressive) cyclicity, the central sandstone body representing the regressive maximum.Event beds are interpreted as river‐fed turbidites deposited during catastrophic storm‐floods. Combined‐flow ripples and other wave‐influenced structures occur in event beds throughout the ideal cycle, suggesting deposition of the entire Bude Formation above storm wave base. The proposed environment is a shelf, of continental‐shelf dimensions, but lacustrine instead of marine. Sandstone bodies are interpreted to be river‐connected tongues or lobes. The absence of cycles containing nearshore or emergent facies is attributed to: (i) the lake sill preventing the water level from falling below sill level, thereby insulating the lake floor from eustatically forced emergence; and (ii) relatively distal deposition, beyond the reach of shoreline progradations. The lack of palaeoflow from the south is attributed to a (now eroded?) deep‐water trough lying to the south, in front of the northerly advancing orogen.Some facies 2 laminated mudstone beds grade laterally into massive and/or contorted beds, interpreted as in‐situ seismites (Facies 4), consistent with an active foreland basin setting. Development of seismites was possibly facilitated by gas bubbles and/or weak cohesion in the (fresh water) bottom mud.The late Quaternary Black Sea, with its broad northwestern shelf, is probably a good physiographical analogue of Lake Bude, and was likewise fresh at times.