Tertiary sediments and structure of the Bovey Basin, south Devon

Abstract

The Bovey Basin is filled by a thick (about 1200 m) sequence of Tertiary kaolinitic clays, lignites and sands, the Bovey Formation. This paper presents for the first time a detailed stratigraphy of the upper part of the formation (the lower 900 m being unexposed), and suggests the environments of deposition of its component members. Three major and four lesser subdivisions of the upper Bovey Formation are recognised. The oldest of the major units exposed in pits, the Abbrook Clay and Sand, comprises siliceous clays and sands, and is overlain by a brown clay and lignite sequence the Southacre Clay and Lignite. The Blatchford Sand, which rests on the Southacre Clay and Lignite, includes thick beds of coarse to gravelly sands. Marginal facies in the basin are represented by the Ringslade Clay, which includes a mottled-clay facies containing Devonian slate fragments. The stratigraphical position of the Chudleigh Knighton Clay in the north-east of the main basin is uncertain, but it is thought that it may occupy a position intermediate between the Abbrook Clay and Sand and the Southacre Clay and Lignite. The results of the present work indicate that sedimentation in the Bovey Basin is likely to have occurred largely in flood-plain conditions, with probable lacustrine and alluvial fan components, taking place in an enclosed, subsiding, partly fault-bounded basin. It is thought unlikely that the Bovey Basin was ever the site of a large ‘permanent’ lake. Only the upper 350 m or so of the Bovey Formation sequence is at present known from surface exposures and boreholes; the unknown sediments beneath are considered from indirect evidence to have been deposited in a wholly fault-bounded subsiding trough, while the later sediments (Abbrook Clay and Sand and younger units) are thought to have resulted from a phase when sedimentation spread out from the initial fault-bounded basin to overlie older rocks with unfaulted contacts, except in the west and south of the main basin.