Syn-rift sedimentation in the upper jurassic (helmsdale boulder beds) of the inner moray firth

Abstract

The Helmsdale Boulder Beds of the Inner Moray Firth are genetically related to the Late Jurassic rifting phase in the North Sea. Exposed on the northeastern shore of Scotland, the syn-rift formation accumulated along the Helmsdale Fault, a basin-bounding master fault to the Inner Moray Firth basin. As a result of tectonic activity along the Helmsdale Fault, as well as sea-level and/or climatic variations, three different sedimentary facies were deposited: (1) a conglomerate facies, of rockfall and debris flow origin; (2) a sandstone-mudstone facies, of turbiditic (dilute) to suspension-fallout origin; and (3) a sandstone facies, of turbiditic (concentrated) origin. The three sedimentary facies were found to show different modes of vertical organisation. At the largest scale (850+ m), the succession displays a well defined fining-upward trend. This trend is expressed chiefly by an upward reduction in sand-shale ratios, and is believed to have been mainly the result of progressive fault-controlled subsidence, to which sediment supply could not keep pace. At the scale of tens to hundreds of metres, the succession has been subdivided into seven units: four conglomeratedominated units, and three sandstone-mudstone-dominated units. These units have 10-100 m thick transitional intervals between them, and do not show any obvious grain-size asymmetry. The conglomerate-dominated units are interpreted as the products of repeated earthquake activity, whereas the sandstone-mudstone-dominated units represent periods of relative tectonic quiescence. At the intermediate scale (tens of metres), a number of fining-upward cycles, which predominate over their upward-coarsening counterpart, are present. These fining-upward cycles were probably also controlled by temporal fluctuations in earthquake activity. At the small scale, facies were found to alternate in a random fashion. Individual conglomerate beds or layers are interpreted as the direct result of earthquake tremors along the Helmsdale Fault, whereas the sandstones and mudstones represent background sedimentation in the form of suspension fallout and periodic turbidity flow activity. The hypothesis of an earthquake origin for the conglomerates is supported by a quantitative analysis showing that the observed frequency-clast size (MPS) relationship of the conglomerate layers is closely similar to the known frequency-magnitude relationship of earthquakes. In contrast, the vertical distribution of sandstones and mudstones appears to have been only remotely influenced by earthquakes; it is suggested that sea-level fluctuations and/or climatic variations had a more influential effect.