Journal of the Geological Society, London, Vol. 150, 1993, pp. 67-75, 7 figs. Printed in Northern Ireland Geochemical and mineralogical variations in the upper Mercia Mudstone Group (Late Triassic), southwest Britain: correlation of outcrop sequences with borehole geophysical logs A. B. LESLIE l'3,B. SPIRO 2&M.E. TUCKER l 1Department of Geological Sciences, Durham University, South Road, Durham DH1 3LA, UK :NERC Isotope Geoscience Laboratories, British Geological Survey, Nicker Hill, Keyworth, Nottingham NG12 5GG, UK 3Present address: School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK Abstract: The Mercia Mudstone Group comprises up to 1200m of predominantly red mudstones and siltstones laid down in rift-related basins during a period of regional subsidence. The mudstones are commonly dolomitic and contain horizons of sulphate and halide salts. Up to 150 m of undifferentiated red mudstones of Norian and Rhaetian age (Late Triassic) were examined in coastal outcrops in Devon and Somerset, southwest Britain, and mineralogical and stable isotopic studies were carried out in order to identify any subtle changes in the succession. Gamma-ray measurements were taken at outcrop to provide correlation with published borehole logs in which the Mercia Mudstone Group has been subdivided on the basis of gamma-ray and sonic response. One of the borehole subdivisions was identified at outcrop within a sequence of otherwise undifferentiated red mudstones and was related to a transition in clay mineral assemblage and stable isotopic composition over 20 to 30 m of section. The red mudstones below this transition have a higher proportion of magnesian clay minerals and enriched carbonate oxygen isotope compositions, indicating deposition from Mg2+-rich marine-derived waters. Clay mineral assemblages in the succeeding red mudstones are dominated by illite, and oxygen isotopic compositions are relatively depleted, indicating a greater influence of K÷-rich continental-derived waters. The Mercia Mudstone Group (Warrington et al. 1980) consists of up to 1200m of red and green mudstones and siltstones Palaeozoic basement which accumulated in a series of graben and half-graben (Whittaker 1975). In southwest Britain these flanked the major faults southern and eastern sides of the Welsh uplands and extended south and east into Devon and Somerset (Fig. 1). Inland, exposure is poor but good coastal outcrops permit the examination of most of the upper Mercia Mudstone succession (Fig. 2). N During the late Triassic there was negligible extension and fine-grained sediments were laid down in passively sub- siding basins (Chadwick 1985; Holloway 1985). The Mercia Mudstone Group in the study area is Ladinian to Rhaetian in age (Warrington et al. 1980) and reaches a maximum thickness of 800 m in the central Somerset basin. The expo- sures of interest, on the Devon and Somerset coasts, comprise beds in the uppermost 170 m of the Group which are Norian and Rhaetian in age. Lack of continuous exposure made sampling of underlying mudstones not possible. The expo- sures studied are marginal to the thicker basinal deposits of the Inner Bristol Channel, Somerset, and Devon/Dorset basins (Fig. 1). The deposition environment in which the mudstones formed Fig. 1. Location map showing the position of the two cliff sections has been the subject of discussion and several interpretations. samples in this study with respect to the major Upper Triassic Deposition of the mudstones in a hypersaline sea with basins in southwest Britain. restricted circulation of marine waters was proposed by Sherlock (1928), Evans et al. (1968), Warrington (1970) and Jeans (1978); whereas deposition in a playa-alluvial The transport of mud as sand-sized pedogenic aggregates in environment was favoured by Audley-Charles (1970) and fluvial systems has been proposed as one possible process Tucker (1977, 1978). Others have suggested that periods of forming extensive red mudstones (Nanson et al. 1988; Rust & extensive halite precipitation were the result of marine Nanson 1989). There is little evidence for the continual activity incursions into the basins (Wills 1970; Arthurton 1980; Taylor of fluvial systems in the Mercia Mudstone Group, and 1983; Holloway 1985). although such pedogenic processes may have provided some 67 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/150/1/67/4891875/gsjgs.150.1.0067.pdf by guest on 27 September 2021 68 A.B. LESLIE ET AL. Subdivision of the Mercia Mudstone Group has been carried out in boreholes on the basis of variations in gamma-ray and Base / - Blue Anchor sonic log response (Lott et al. 1982; Penn 1987), and l formation correlations continued through several basins (Fig. 4). In particular, boundaries were identified within the mudstone units themselves which are not the result of obvious m lithological changes. These are of most interest in this study 20-. D 20- D since they could, in theory, be identified at outcrop using a ~D i portable gamma-ray spectrometer. ' 'O Methods 40- ' " 40- Up to 150 m of red mudstones were examined from coastal outcrops in west Somerset and south Devon. Sections were logged for gamma-ray (m) (m) variations and samples for clay mineral and stable isotopic analyses were taken every one or two metres. 60- 60- Both areas provide relatively continuous coastal outcrop of the upper 100 m of the Mercia Mudstone Group. In Devon, the majority of samples were collected from Haven and Seaton Cliffs (SY275895- SY236893). Some samples were also taken from the coast between Coxe's Cliff (SY185881) and Berry Cliff (SY198882) to the west of 80- 80- Branscombe. The gap in vertical section between the Seaton and Branscombe sections was estimated by Jeans (1978) and Warrington & 85 metres Scrivener (1980) as approximately 55 m, based upon borehole evidence Base of Haven/Seaton from Lyme Regis 16 km to the east. In Somerset, a continuous section Cliffs Section was sampled from St Audrie's Bay (ST104443-ST119437). In the lower 100I mudstones in St Audrie's Bay there is minor faulting but this has not A B disrupted the continuity of the section. Fig. 2. Sketch logs showing the distribution of green mudstone beds (black) within the red mudstones below the base of the Blue I OOl (IOA) Anchor Formation. D, massive dolomite-rich beds. (a) South ~ POOl (10.4A) Devon. (b) Somerset. / So0~l122A)/ fine-grained sediment to the basin (Wright et al. 1988; North 1989), it is unlikely to be a volumetrically important source. The relatively abundant (> 10%) silt-sized grains in the Mercia Mudstone Group suggest that there may have been a substantial aeolian input. Aeolian processes have been I suggested as a source of the carbonate within arid red bed environments (Allen 1986) and it is likely that significant /~ S-C (g)(14-15A) volumes of aeolian sediment were supplied to the Triassic s-c /1 cA,,,; ,/ basins (North 1989). • ~'~, i I Correlation within the Mercia Mudstone Group The Mercia Mudstone Group in the study area comprises an uppermost Blue Anchor Formation, which is between 10 to 40m in thickness, and underlying undifferentiated red mudstones (Warrington et al. 1980). The red mudstones are generally unfossiliferous, poorly bedded and contain few ,1 sedimentary structures. Neither chrono- nor litho- stratigraphical correlation has been carried out in the mudstones on a regional scale, although local intrabasinal names have been introduced for sandstone and halite beds (Warrington et al. 1980). In the upper Mercia Mudstone Group prominent sandstone units have been identified in several basins (the Weston Mouth Sandstone Member in Devon, and ~. air-dried the Arden Sandstone member in Worcestershire, for example) glycolated and dated as late Carnian (Warrington & Williams 1984). The 550°C for 4hours overlying red mudstones, the subject of this study, have yielded no datable fossils but are assigned to the Norian and lower Fig. 3. X-ray diffraction peaks used in the identification of clay Rhaetian stages since they underlie the Blue Anchor mineral species. I, illite; C, chlorite; P, palygorskite; S, sepiolite; Formation which has yielded palynomorphs of Rhaetian age S-I, mixed-layer smectite-illite; S-C, mixed-layer smectite-chlorite. (Warrington & Whittaker 1984). (g), glycolated trace used in identification. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/150/1/67/4891875/gsjgs.150.1.0067.pdf by guest on 27 September 2021 MERCIA MUDSTONE GROUP 69 SEABOROUGH BURTON ROW BRISTOL CHANNEL SOUTH DEVON SOMERSET 103/18-1 GR .-.- ~- BHCS Formation i P° lOOm r, D L DD 200m - 300m - Fig. 4. Regional gamma-ray/sonic log 400m - correlation of the Mercia Mudstone Group in southwest Britain. PG, Penarth Group; SS, Sherwood 500m Sandstone Group. Mercia Mudstone Group units A to F taken from Lott et 21°3 o2 al. (1982). 1, taken from Lott et al. (1982); 2, taken from Penn (1987); 3, taken from British Geological Survey unpublished data. GR, gamma-ray intensity, increasing from left to right; BHCS, borehole compensated sonic log, increasing from right to left. Gamma-ray logging was carried out using a Geometrics peak/background ratio was far superior to that achieved when the (G410A Spectrometer with a GPX 21 NaI (TI) detector. The sample was smeared onto a glass slide. ideal surface required for measurement (a planar surface Mudstone samples for stable isotopic analyses were crushed and perpendicular to bedding with a radius of 1 m) was rarely treated with 100% phosphoric acid at 55°C (Rosenbaum & Sheppard exposed naturally and sample spacing varied between 1 and 1986) for up to 24 hours, after the composition of the carbonate fraction 4m depending on outcrop suitability. Duplicate readings were was determined using XRD. Analyses were carried out using standard taken at every tenth station, and standard errors were under methods (Craig 1957). All samples were analysed in the British 10%. For the purpose of correlation the total gamma-ray count Geological Survey isotope geochemistry laboratories at Gray's Inn values were visually calibrated with standard API units in Road, London (now relocated at Keyworth, Nottingham).