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

Authors


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    Higgs Petroleum Consulting Ltd., 743 Oliver Street, Victoria, British Columbia, Canada V8S 4W5.

ABSTRACT

The 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.

Ancillary