Magnetic fabric characteristics of bioturbated wave‐produced grain orientation in the Bridport‐Yeovil Sands (Lower Jurassic) of Southern England

Abstract

There is little visible primary hydrodynamic lamination preserved in the Bridport‐Yeovil Sands as a result of intense bioturbation. Where lamination is present, it exhibits wave‐produced characteristics, although current ripple lamination is also found. The grain orientation of a variety of bioturbated and non‐bioturbated fine‐grained sandstones has been determined by measuring the magnetic susceptibility anisotropy. The magnetic fabric is of a primary style and preserves two lineation directions approximately 90° apart in azimuth. These lineation directions are interpreted as the result of grain long‐axis orientations produced by wave and current processes. The magnetic fabric is dominantly carried by a small proportion of paramagnetic minerals, thought to be largely detrital chlorite and micas. This magnetic fabric has been acquired by depositional alignment of the detrital phyllosilicates and by reorientation of the phyllosilicates during the early stages of compaction. The magnetic fabric of the intensely bioturbated sandstone is not significantly different in magnitude characteristics or in the preservation of lineation directions from that of the non‐bioturbated sandstone.

A sequence stratigraphic model for an intensely bioturbated shallow‐marine sandstone: the Bridport Sand Formation, Wessex Basin, UK

“…Ham Hill Quarry, Fig. 1; Davies, 1969; Hounslow, 1987). The association comprises three facies (Table 1) which have gradational boundaries and are stacked vertically into upward‐coarsening successions of interbedded siltstones and sandstones (facies T3), overlain by flaser‐bedded sandstones (facies T2) and ripple‐laminated, weakly bioturbated sandstones (facies T1).…”

“…Previous workers have interpreted the BSF as the deposits of submerged, storm‐dominated shelf bars (Bryant et al. , 1988), the distal part of a storm‐dominated shoreface and inner shelf (Colter & Havard, 1981; Hounslow, 1987), and a migrating barrier shoreline and back‐barrier lagoon (Davies, 1969). The regional extent and geometry of the unit are also poorly constrained and poorly understood (Hesselbo & Jenkyns, 1995; Ainsworth et al.…”

Sedimentologic Character of Ancient Muddy Subaqueous-Deltaic Clinoforms: Down Cliff Clay Member, Bridport Sand Formation, Wessex Basin, U.K.

“…The palaeogeographic context of the Down Cliff Clay Member is unclear, and a major fluvial source(s) of sediment to the coeval shoreline has not been identified. Instead, the Bridport Sand Member represents a wave-dominated shoreface with localised, subordinate tidal influence (Davies 1969;Hounslow 1987;Morris et al 2006). As a result, we infer that fluvial sediment influx to the Bridport Sand Member and Down Cliff Clay Member was probably via a number of minor rivers, such as the Apennine rivers that act as a "line source" for the western Adriatic subaqueous clinoform (Cattaneo et al 2003(Cattaneo et al , 2007.…”

“…2A, 4A). The outcrop and core data analysed by Morris et al (2006) and Hampson et al (2015) and corresponding data used in previous sedimentologic studies (Davies 1967(Davies , 1969Hounslow 1987;Bryant et al 1988;Pickering 1995) were collected from the upper, sandstone-rich part of the Bridport Sand Formation. In this paper, we focus on the Down Cliff Clay Member (i.e.…”