STRUCTURAL GEOLOGY & GEOPHYSICS

Structural Geology & Geophysics



Structural Geology & Geophysics

Introduction

The area studied here includes the Bristol and Somerset coal fields and the Mendip Hills and it extends from Wellsin the south to Tortworth in the north in the counties of Avon and Somerset. This area is covered by the BGS special 1: 63,360 scale map of the Bristol district, the Sheets 280, Wells and 281, Frome at 1 : 63,360 scale, and the 1 : 50,000 Weston-Super-Mare sheet. The rocks of the area include Cambrian and Silurian sediments at Tortworth; Siluria annde site and stuffs in the Beacon Hill area; Devonian of Old Red Sandstone (ORS) facies and Lower and Upper Carboniferous rocks: all of which are involved in Variscan deformation. Permo-Triassic and younger rocks lie unconformably above this Variscan 'basement'. A remarkably perceptive account of the stratigraphy and structure of the Mendips was given by De la Beche (1846) in Geologic Survey map and memoir (Godwin-Austen 1856, 38-46).

The structural complexities of the Mendips began to be understood asearlyas 1824 when Buckland & Conybeare described over folded and greatly contorted Coal Measures in the east Mendips (Johnson 1980, 69-96). The faulted 'inliers' of Carboniferous Limestone in the east Mendips were the source of much debate. These 'inliers' were considered to be either autochthonous, block-faulted fragments or allochthonous over thrust pieces of Lower Carboniferous rocks, completely surrounded by Upper Carboniferous rocks. Johnson (1980) successfully interpreted the Vobster 'inlier' as a klippe due to large scale over thrusting of Lower Carboniferous above Upper Carboniferous rocks. Elliott (1976) adopted the over thrusting suggestion in interpreting the structures of other parts of the Mendips (Elliott 1976, 289-312).

More recent work has culminated in the publication of BGS maps and associated memoirs (Curtis 1955, 3-9). The crustal structure to the east of the Mendips, based on deep seismic reflection profiles, has been elucidated by Elliott 1976 and re-evaluated by Williams & Brooks (1985). General accounts of the geology of the Bristol-Mendip area have been published by Coward & Smallwood (1984). These, coupled with our own field observations, have enabled us to carry out detailed studies of individual thrust faults and to elucidate the geometry of the Mendip thrust system using balanced cross-sections (Coward & Smallwood 1984, 89-102).

Summary of stratigraphy

The oldest exposed rocks in the Bristol-Mendip area are the Tremadocian shales of the Tort worth inlier (Chapman & Williams 1984, 121-8). Presumed Silurian volcanic rocks occur in the core of the Beacon Hill periclinein the south of the Mendips. The marine Silurian is overlain by a 600 m-thick conformable sequence of Lower Devonian, Old Red Sand stone continental facies which is unconformable overlain by an Upper Devonian Old Red and sand stone sequence varying in thickness from 500 m in the south to 150m in the north (Cave 1963, 38-9).

Several significant non-sequences are present in the Dinantian succession of the Mendips, but the lithostratigraphy, based particularly on oolitic marker beds, can be correlated successfully from the Mendips in the south to the Avon Gorge ...