The Lower Lias of Northern Gloucestershire: New Data on Early Jurassic Stratigraphy and Outcrop Patterns from Temporary Exposures
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Proceedings of the Cotteswold Naturalists’ Field Club The Lower Lias of northern Gloucestershire: New data on Early Jurassic stratigraphy and outcrop patterns from temporary exposures. Michael J. Simms Department of Geology, Ulster Museum, Botanic Gardens, Belfast BT9 5AB, Northern Ireland. E-mail: [email protected] 1. INTRODUCTION only a very incomplete picture of the succession in the Northern Gloucestershire lies within the Severn Basin, Severn Basin. sometimes also known as the Worcester Basin or Graben, an elongate north-south trending basin containing a thick Mesozoic succession. Its eastern and western boundaries are well defined by the Vale of Moreton Anticline and the East Malvern Fault respectively, a width of about 50 km. The north and south limits of the basin are less clearly delimited but extend probably from the Kidderminster-Bromsgrove area southward to beyond Stroud, a distance of 90 km or more. Lower Lias (Lower Jurassic, Hettangian- Lower Pliensbachian) strata outcrop extensively throughout Gloucestershire, in the southern part of the basin, and attain a total thickness approaching 300 metres. However, details of many parts of the succession are poorly known. The Stowell Park Borehole penetrated the full succession in the eastern part of the basin and was well documented (Green and Melville, 1956; Melville, 1956; Spath, 1956), with several other boreholes penetrating parts of the Lower Jurassic elsewhere, such as the Hettangian and lowermost Sinemurian at Twyning (Worssam et al., 1989). But boreholes have limitations, for instance in terms of showing broader facies variations and faunal content, compared with surface exposures. Despite the great thickness and extensive surface outcrop of the Lower Lias in this area, there are few permanent exposures. A few low cliffs and foreshore exposures occur along the banks of the River Severn and Severn Estuary downstream of Gloucester but these expose only Hettangian and lowermost Sinemurian strata and most remain poorly documented. Later Sinemurian and most Lower Pliensbachian strata have been exposed only in brief temporary excavations. Within the Severn Basin only two exposures, Hock Cliff and Robin’s Wood Hill Quarry, currently are of sufficient Table 1. Stages, Zones and Subzones of the Lower permanence and scale to have been described in the Lias (largely after Dean, Donovan and Howarth, forthcoming Lower Jurassic Geological Conservation 1961). Letters in brackets indicate the zonal and Review volume (Simms et al., 2004), but they provide subzonal symbols used on the maps. 191 Proceedings of the Cotteswold Naturalists’ Field Club Figure 1. Shelly limestone with large Pararnioceras alcinoeiforme and small body chamber of Euagassiceras. From Sauzeanum Subzone exposed in small trial pit for housing development at Northway, near Tewkesbury (SO 924342), in 1979. Scale in cm. However, in the 19th and early 20th centuries there north of Cheltenham. Other than a few permanent were many more exposures in the form of small exposures, these encompassed small drainage ditches, quarries and brickpits while the development of the pipe trenches, building sites, gravel workings and railway system saw the creation of other temporary landfill excavations. Apart from those in the exposures. Brief accounts of many of these were immediate vicinity of Bishops Cleeve, many were published in numerous papers during this time, a encountered by chance and hence many others must significant number appearing in the Proceedings of have been missed. The specimens collected were the Cotteswold Naturalists’ Field Club during the 19th catalogued and the location, and biostratigraphic level, and early 20th centuries. These accounts of temporary of exposures was marked on 1:25000 Ordnance exposures, particularly those by Linsdall Richardson Survey maps (and the 1:10000 sheet for Bishops (e.g. Richardson 1904, 1918, 1929) and by Thomas Cleeve). From my occasional visits back to Wright (1878-86) are an important source of Gloucestershire, since moving to Northern Ireland in information on the nature of the local Lower Lias 1996, it is evident that temporary exposures of the succession but it is not intended to comprehensively Lower Lias are as common now, if not more so, than review here these earlier records. Instead the present during the main period of my perambulations between paper is intended as a supplement to the earlier 1972 and about 1990. Indeed, during a brief visit in accounts of Richardson and others continuing a long- November 2002 I collected material from four new established, if somewhat lapsed, tradition of field sites, clearly unvisited by any other geologists, that recording in this area. has been incorporated into this account. It seems that although there are now many more collectors My own fossil collecting began in the late 1960s and accumulating material from temporary excavations from the early 1970s my main interest was focussed across the Severn Basin, currently there is no on the Lias, often neglecting my schoolwork in order systematic recording of such data comparable to that to collect extensively from exposures within cycling which I compiled even though much effort is distance of my parents home at Bishops Cleeve, just expended on the re-excavation and documentation of 192 Proceedings of the Cotteswold Naturalists’ Field Club old sites. This is a failing which needs to be rectified echinoderms), were donated to Bristol City Museum if significant further progress is to be made in in 1989, with the Liassic echinoderms donated to the understanding the geological history of the Severn Natural History Museum in 1988. Much of the data on Basin. the Lower Lias succession remains unpublished other than brief mention in several BGS Open File Reports My own records (mostly annotated maps) and or the Tewkesbury Sheet Memoir (Worssam et al., extensive collection of ammonites from these 1989). Unfortunately, existing summaries of the temporary exposures were donated to the British Lower Lias succession compiled for the Severn Basin Geological Survey in 1989 and were used in the (e.g. Getty in Cope et al., 1980) contain serious errors remapping of both the Tewkesbury and the Moreton- which have been perpetuated, in some instances, for in-Marsh 1:50,000 sheets (sheets 216 and 217). Much more than 150 years. One of the purposes of this of this present account is based upon this material, paper is to correct these misconceptions and provide a now held at BGS Keyworth. Some ammonites, and summary of the succession based on more than 30 most non-ammonite material (excluding years of fieldwork in this area. Figure 2. Sketch map of the Cheltenham-Tewkesbury area showing distribution of ammonite zones proven in temporary exposures (see Table 1 for key to letters used). Light shading = urban areas; stippled grey = Dyrham Formation; dark grey = Whitby Mudstone Formation; blocks = Inferior Oolite Group. Area enclosed by dashed line corresponds to that in Figure 4. 193 Proceedings of the Cotteswold Naturalists’ Field Club 2. LITHOSTRATIGRAPHY AND FACIES the Severn Estuary (SO 705094); Schlotheimia Data obtained from the few permanent exposures and extranodosa and S. similis indicated the presence of from temporary exposures across the Severn Basin both subzones, but the exposure is now obscured by over a more than 30 year period (M.J.Simms, builders’ rubble. In northern Gloucestershire only two unpublished observations) have helped to build up a sites are tentatively assigned to this zone. A small fairly clear picture of the stratigraphy of this thick pyritised schlotheimiid from a cementstone nodule at Lias Group succession. Typical Penarth Group Stoke Orchard (SO 91602826) may indicate this zone, mudstones with thin sandstones and limestones are although other exposures nearby are Semicostatum exposed in the river cliff at Wainlode Hill north of Zone. Excavations for bridge foundations on the River Gloucester (description summarised in Macfadyen, Chelt near Boddington (SO 895259) exposed thinly 1970) and are succeeded by poorly exposed (dm-scale) interbedded limestones and mudstones alternating mudstones and limestones of the basal Lias containing small Gryphaea arcuata similar to Group ‘Pre-planorbis Beds’. Some of the limestones examples from the Angulata Zone elsewhere, but no in this part of the succession are laminated and have confirmatory ammonite evidence was found. yielded rich insect faunas from these sites and from 19th Century quarries. The mid- to upper Bucklandi Zone is superbly exposed at Hock Cliff, on the Severn Estuary, and is The Planorbis Zone is seldom well exposed but is redescribed in the forthcoming Lias GCR volume developed in Blue Lias Formation facies of limestones (Simms et al., in press) following the earlier, in parts and rather thicker mudstones. The ‘Pre-planorbis erroneous, descriptions of Richardson (1908) and Beds’ and lower part of the Planorbis Zone is still Henderson (1934). Glass Cliff, about 1 km upriver exposed, albeit poorly, in a line of old quarries at from Hock Cliff, exposes the top Angulata to mid- Brockeridge Common, north of Tewkesbury (SO Bucklandi zone, with zonal/subzonal indices well 887385) and during the 19th Century yielded a rich represented if fragmentary. The section, somewhat fauna of fossil marine reptiles (Wright 1878-86). faulted, shows a transition from closely spaced Similar strata were also described from two small limestone-mudstone alternations, in the Angulata quarries on Sarn Hill, west of Tewkesbury