Flood-generated hyperpycnal delta front sands of the Brora Arenaceous Formation (upper Callovian–middle Oxfordian) of the Inner Moray Firth, Scotland, record the onset of rifting

Abstract

Sandstones of the Middle–Upper Jurassic Brora Arenaceous Formation of the Inner Moray Firth, NE Scotland have hitherto been interpreted as representing coastal, tidally-influenced bars. The formation is exposed close to the northern basin-bounding Helmsdale Fault, and the middle member of the formation, the Clynelish Quarry Sandstone, consists of thick, mainly structureless sandstone beds with wavy, commonly amalgamated boundaries. It also includes sandstone bodies with sigmoidal clinothems, erosional surfaces and backset beds. Rich marine faunas dominated by bivalves and ammonites occur at a few levels, whereas trace fossils are rare or absent. The Clynelish Quarry Sandstone is here reinterpreted as reflecting deposition by hyperpycnal sandy density flows in flood-generated marine, subaqueous, delta-scale clinoforms and lobes in front of local mountain streams. The reinterpretation of these sandstones implies the presence of a tectonically controlled, relatively steep basin margin along the line of the Helmsdale Fault. The Brora Arenaceous Formation thus dates the onset of Jurassic rifting in the Inner Moray Firth to the latest Callovian rather than the late Oxfordian as previously interpreted from seismic data.