The Avonian of the Tytherington-Tortworth-Wickwar Ridge, Gloucestershire

Abstract

The Author describes and maps, in terms of Vaughan's system of zonal classification, the northern portion of the horseshoe-shaped wooded ridge of Avonian rocks which partly surrounds the synclinal basin of the Bristol Coalfield lying north-east of Bristol. The area described links together the researches of two former papers, and completes the Avonian zonal map of the horseshoe ridge. The chief zonal characteristics may be summarized as follows:— Cleistopora Zone.—Lithologically the zone consists of shales. grits, and thinly-bedded muddy limestones, with the usual fossils. The Bryozoa Bed (α) has been recognized, and the zonal fossil is unusually abundant in the Tytherington-Grovesend railway section. Zaphrentis Zone.—Limestones of the ‘Petit Granit’ type, with intervening shales in Z 1 , form the major part of the sequence. At the base of Z 1 in the Tytherington-Grovesend railway section is an interesting band of oolite. No chert has been found. Zaphrentids are very abundant, and the ‘Z 2 Fish-Bed’ occurs in the northern part of the area. Horizon γ cannot be differentiated, owing to the absence of Caninia immediately above the konincki subzone. Syringothyris Zone.—The following sequence occurs in ascending order:— (1) Thickly-bedded, nearly unfossiliferous. blue-grey dolomites ( laminosa dolomites), apparently formed by the dolomitization of crinoidal limestones. (2) A series of fossiliferous crinoidal limestones (Sub-Oolite Bed): non-oolitic in the lower, but oolitic in the upper portions. (3) A thick bed of current-bedded white oolite ( Caninia Oolite). This was probably immediately followed by a short period of subaërial denudation. (4) Unfossiliferous thinly-bedded dolomites, with subordinate shales ( Caninia Dolomites). Series (2) and (4) a.re considerably thicker than their eqnivalents in the Avon Section. Seminula Zone.—The zone consists of massive limestones, including examples of , Seminula Pisolite, china-stones, and Seminula Oolites merging into sandstones and conglomerates of a ‘Millstone Grit’ phase of deposition. A calcareous sandstone (Firestone) occurs near the base of S2 (formerly described from the top of Sl). A calcareous development of the Dibunophyllum Zone is absent in the area described. It is thus established, from the evidence of previous writers and that set forth in the present paI)er, that during Avonian times there was a progressive movement of the northern shore-line of Vaughan's ‘ South-Western Channel ’ in a southerly direction. This movement is registered by noting the level in tile Avonian sequence at which ‘ Millstone Grit’ conditions were established. The maximum thickness of the calcareous Avonian beds is only 1280 feet, as compared with 2540 feet in the Avon section. The Tytherington-Grovesend railway provides a complete section of the Avonian rocks of the area.