Textural characteristics and interpretation of second-cycle, debris-flow-dominated alluvial fans (Devonian of Northern Scotland)

Abstract

Devonian conglomerates in Northern Scotland are interpreted as a product of recycling of pre-existing, variously reworked gravels and sands and deposition in a form of alluvial fans. These second-cycle fanglomerates fringe the western margin of the Golspie Basin (marginal sector of the Orcadian Basin) and are dominated by texturally heterogeneous, structureless debris-flow conglomerates of Facies B and B1. The textural heterogeneity of Facies B, manifested through chaotically arranged bimodal and polymodal textures, highly variable amounts of matrix and unremoulded sandy blocks, indicates that the cohesive and highly competent debris flows originated through remobilisation of previously reworked, sand-enriched gravels. The debris flows of Facies B1 formed as result of resedimentation and remoulding of two, texturally and compositionally distinct, groups of sediment—angular, poorly sorted, locally derived gravels and fine-grained, well sorted, exotic sands. The debris-flow deposits are accompanied by texturally homogeneous, massive to planar-stratified and cross-stratified conglomerates and sandstones of Facies C and C1. The latter deposits represent turbulent sheetfloods that were less competent than the debris flows and that acquired their sediment load through surficial reworking of the alluvial-fan gravels. Texturally homogeneous and structureless talus deposits of Facies A, and massive to planar-stratified sheetflood conglomerates of Facies C2 are locally derived and show first-cycle characteristics.