A Bigger Splat: Rock fall impact processes in wet sediment - an example from Clachtoll, NW Scotland

Abstract

At Clachtoll, in NW Scotland, an enigmatic outcrop of Lewisian gneiss is observed. This zone, ~60 m by 100 m in aerial view, contains abundant fractures filled with clastic material from the immediately adjacent and stratigraphically overlying Stoer Group. This zone also displays foliation that is misoriented by 90 degrees, relative to that in the surrounding basement. Fractures on top of this outcrop contain sedimentary structures, such as laminations, indicative of passive infilling, whilst fractures at the base and on the lateral flanks show characteristics more indicative of forceful injection.

This study proposes that the aforementioned zone represents a hitherto unrecognised fallen block, termed the Clachtoll Megablock, lying on the regional scale nonconformity between the Lewisian gneiss and the Mesoproterozoic sedimentary rocks of the Stoer Group (ca. 1.2 Ga). It is proposed that vertical emplacement of a falling block onto unconsolidated and fluid laden sediment, could cause rapid over-pressuring and liquefaction of said sediment, leading to hydrofracture and sediment slurry injection into the impacting block. Fractures on the top of the block would then be passively infilled under gravity. Simple numerical modelling yields a fall height of 6 m as being sufficient for rapid liquid over-pressuring to overcome the tensile strength of the gneiss.

Several models of emplacement were considered in an attempt to reconcile all of the features observed in the block. Features, including WNW-ESE oriented tensile fractures in the back of the block, and a small thrust associated with brittle folding in the front of the block, indicate an incomplete syn-emplacement disaggregation of the Clachtoll Megablock, consistent with gravity-driven transport from the NNE. Emplacement must have involved an element of sliding, and the misorientation of foliation inside vs. outside of the block can only be explained by rotation of the block.

Evidence of soft sediment deformation in the basal part of the Stoer Group, along with knowledge of the active rift setting of the event suggest that emplacement of the Clachtoll Megablock was triggered by an earthquake ca. 1.2 Ga. Weathering processes, such as freeze-thaw, in an arid environment likely contributed to this.