Palaeoecology and sedimentology across a Jurassic fault scarp, NE Scotland

Abstract

A detailed palaeoecological and sedimentological analysis of the Kimmeridgian coastal outcrops around Helmsdale (northeast Scotland) has been related, for the first time, to a detailed biostratigraphical scheme allowing the depositional history of this important area (adjacent to some North Sea oilfields) to be elucidated. A broad, shallow marine, sub-tropical, shelf was disrupted by the onset of normal faulting along the Helmsdale fault in earliest Kimmeridgian times. Initially, water depths remained shallow on the hanging wall, reflected by a normal benthic fauna, but a rapidly steepening southeast-directed palaeoslope produced frequent sediment sliding. Shoreface sands developed immediately to the west of the fault and were transferred to the hanging wall via a fault-transfer zone where they now constitute the Allt na Cuile Formation. Continued rapid subsidence during the remainder of the Kimmeridgian and the Portlandian produced deep, poorly oxygenated to anoxic benthic conditions on the hanging wall in which a combination of rock slide/debris flow/turbidity current deposition alternated with hemipelagite deposition to produce the Helmsdale Boulder Bed Formation. The Eathie locality (Cromarty), previously regarded as a deeper, more distal setting than Helmsdale, contains a slightly more diverse benthic fauna implying shallower water. The deepest waters were immediately adjacent to the fault scarp as predicted by the standard half-graben depositional model.