Sedimentology and early diagenesis of the Broadford Beds (Lower Jurassic), Skye, north‐west Scotland

Abstract

The Broadford Beds comprise a basal carbonate-dominated unit overlain by dark muddy sandstones and an upper cross-bedded sandstone unit. The limestones include coralliferous and oolitic lithologies and the sandstones include pisolitic berthierine ironstones and abundant phosphate nodules. The Broadford Beds have a maximum thickness of 140 m and can be subdivided into 17 parasequences, each initiated by an approximately 20m rise in relative sea level. Marine flooding surfaces are most readily recognized in the lower parts of the succession, where diagenetic data can be utilized to locate emergence surfaces. There is no diagenetic evidence of early emergence in the upper parts of the sequence, but a variety of sedimentological data can be used to identify parasequence boundaries. Deposition of the Broadford Beds occurred largely around the shores of a number of islands in the northern Hebridean area. These islands partly isolated the Skye-Applecross area from a deeper marine basin to the west, and this may have enhanced the formation of authigenic phosphates and iron silicates. It is possible to use the regional variability of parasequence thicknesses to investigate the role of active local tectonism in governing differential subsidence and sediment supply.

Geology and petroleum prospectivity of the Sea of Hebrides Basin and Minch Basin, offshore NW Scotland

“…environments of deposition and the potential extent of different facies. (Howarth 1956;Amiri-Garroussi 1978;Morton 1989;Ebdon & Jacovides 1990;Bradshaw et al 1992;Morton 1992;Thrasher 1992;Searl 1992;Hesselbo & Coe 2000;Ainsworth & Boomer 2001). (Pentex 1989;BP 1990;Chevron 1991;Thrasher 1992;Butterworth et al 1999;Emeleus & Bell 2005;Fyfe et al 2021;BGS 2020c).…”

“…Permian-Triassic floodplain facies, were gradually transgressed with marginal marine to eventual fully marine environments (Ambler 1989). Carbonate-rich, shallow marine to marginal marine environments dominated the Hettangian to early Sinemurian (Broadford Beds Formation), with high variability of lithofacies and thicknesses resulting from lithospheric extension and pre-existing Permian-Triassic topography (Ambler 1989;Searl 1992;Morton 2004). From middle Sinemurian to early Toarcian times, active rifting was followed by thermal subsidence.…”

“…In the MB there is a less pronounced divide between the proximal and distal facies, with thin sandstones encountered by the Minch 1 well, located in the centre of the basin (BP 1990). The reservoir potential of Lower Jurassic sandstones is considered to be poor due to their low initial depositional porosities and permeabilities, resulting from the sandstones high carbonate and clay rich detrital compositions, compounded by high levels of cementation (Searl 1992;Searl 1994). The sandstones beds are also observed to be predominantly localised, causing difficulty with correlation of individual beds (Searl 1994).…”