How do you grade peperites?

Abstract

A spectacular, graded peperite of volcanic origin from the Palaeogene Mull Lava Field of NW Scotland, the Carraig Mhór Bed (CMB), consists of a basal facies of pale, sub-horizontally-aligned, fluidal-shaped clasts of hydrothermally-altered basalt set in a brown, complexly-laminated, silt- to clay-grade, host siliciclastic sediment. The basal facies of the CMB grades upwards over an interval of ∼ 10 m into an upper facies dominated by smaller and more angular clasts of identical composition, set in a subordinate proportion of the same sediment matrix. The distinctive textural characteristics of the CMB, together with its remarkably uniform grading motif, may be explained in terms of the mingling of basaltic magma and fluidised siliciclastic sediment, resulting in the ductile fragmentation of magma, and production of the fluidal clasts which dominate the basal facies. Cooling of the system led to limited brittle fragmentation, yielding the finer-grained, sediment-poor upper facies of the CMB. During cooling, the larger fluidal-shaped clasts settled to the base of the unit, through the saturated sediment, producing the vertical (stratigraphic) grading now preserved. Grading occurred, essentially, in situ during peperite formation and cannot be attributed to remobilisation, mass flow or pyroclastic phenomena.