Mid-Cretaceous Orbitolinids, from the Decoy Basin, Newton Abbot (Devon)

C. Nicholas and M.B. Hart MID-CRETACEOUS ORBITOLINIDS, FROM THE DECOY BASIN, NEWTON ABBOT (DEVON) C. NICHOLAS1 AND M.B. HART2 Nicholas, C. and Hart, M.B. 2004. Mid-Cretaceous orbitolinids from the Decoy Basin, Newton Abbot (Devon). Geoscience in south-west England, 11, xxx-yyy. Within the Upper Greensand (mid-Cretaceous) succession of the Decoy Basin cherts are regularly found to contain the benthic foraminiferid Orbitolina. This genus is a large agglutinated taxon that almost certainly lived in warm, shallow waters (by analogy to modern species). South-western England marks the most northerly limit of this genus, which has been recorded elsewhere in Devon and in the glauconitic sands of Cenomanian age found around the Haig Fras granite west of the Scilly Isles. The species identification is difficult as the preservation of the fauna, within chert, means that many of the detailed internal structures (on which the determination must be based) are missing. Previous work has indicated that the species is probably Orbitolina sp. cf. O. concava. This species characterises the lower Cenomanian in France and other regions of Europe and the Middle East. Recent work on Middle Eastern faunas has shown that the external shape of the specimens is controlled by the environment and can be used to place assemblages within a sequence stratigraphical context. The fauna from Zig Zag Quarry appear to indicate deposition in a transgressive sedimentary succession. 1David Roche Geo Consulting, 19 Richmond Road, Exeter, EX4 4JA, U.K. (E-mail: ) 2School of Earth, Ocean & Environmental Sciences, University of Plymouth, Drake Circus, Plymouth, PL4 8AA, U.K. (E-mail: [email protected]). INTRODUCTION The Orbitolinidae are a family of complex agglutinated Zig Zag Quarry (NGR SX878 690) is located immediately foraminifera. While the total range of the family is Jurassic to south-east of Newton Abbot (Figure 1) on the eastern side of Palaeogene, they were most diverse during Early to the Newton Abbot to Torquay main road (A380). It is a worked mid-Cretaceous times (Aptian to Cenomanian). Though most quarry (Harleyford Aggregates), producing ~100,000 tonnes commonly found in sediments of the Tethyan realm, the genus of sand and gravel for the construction industry. The quarry Orbitolina has been regularly described from south-west operations remove both the “sands and gravels” of the Aller England (Hart, 1971, 1973, 1982; Carter and Hart, 1977; Hart Gravel and the “glauconitic sands and cherts” of the Upper et al., 1979; Hart and Williams, 1990; Schroeder et al., 1986; Greensand. The lithostratigraphical status of both these units is Simmons et al., 1991; Simmons and Williams, 1992). Rare still under discussion; see Gallois (2004), for a recent view on individuals have been recorded from the Warminster Greensand the lithostratigraphy of the Upper Greensand Formation. The (Carter and Hart, 1977, p. 98, fig. 5) but reports of specimens Aller Gravels comprise abraded flint gravels with sands, silts from the Hibernian Greensand of Northern Ireland (Hume, and occasional clays. They rest unconformably on the Upper 1897; Hancock, 1961; Reid, 1971) have never been substantiated, Greensand and are, with the overlying Bovey Formation, despite lengthy discussions with the Ulster Museum in Belfast thought to be of Palaeogene age (Edwards, 1973; Edwards and (the supposed repository of the material). Freshney, 1982; Selwood et al., 1984). The greensands are As indicated by Simmons and Williams (1992, p. 21), records typical of the Cullum Sands-with-Cherts Member of the Haldon of Orbitolina in South-West England extend back over 160 Sands Formation (Hamblin and Wood, 1976), comprising years to the work of Godwin-Austen (1842). These early coarse-fine grained glauconitic sands with discontinuous layers records (including Parker and Jones, 1860; Meyer, 1874; of nodular brown, rather glassy, fresh chert. The sands are Jukes-Browne and Hill, 1900) are, in some ways, curiosities and typically unfossiliferous. As a direct result of their diagenetic it was work in the Middle East (Henson, 1948) that paved the origin the cherts often contain relict bedding (see Williams, way for our present understanding of the genus. In recent 1986, 1991), bioturbation (Figure 2A) and shelly fossils years Schroeder has provided an up-to-date understanding of (Figure 2B). The fauna is occasionally dominated by serpulids the taxonomy and evolution of Orbitolina; see Schroeder and (Figure 2C) although, while some are the planispirally enrolled Neumann (1985), Simmons and Williams (1992) and Simmons Rotularia concava, many appear to be uncoiled, variably et al. (2000) for a full assessment of his work. curved tubes. Bivalves, when present, are dominated by indeterminate species of Exogyra. In places (Figure 2D) the ZIG ZAG QUARRY cherts are crowded with specimens of Orbitolina, many showing the typical “Mexican hat” shape in cross-section. In 2003, as part of a “Geodiversity Audit of Active Aggregate Preservation of the internal structures of these individuals is Quarries” in Devon (David Roche GeoConsulting, 2003), Zig poor and, in thin-section, none of the characteristic chamber Zag Quarry near Newton Abbot was described in detail and arrangement is visible. The specimens are encased in glassy, further examples of Orbitolina were discovered in the cherts of fresh chert and lie sub-parallel to the bedding. the Upper Greensand. These specimens provide further indications of both the stratigraphy and depositional setting of the Upper Greensand in S.W. England. 10 Mid-Cretaceous orbitolinids Bullers Hill Quarry Telegraph Hill R i R v i e + v r e + r + E + x e + T e Great + ig + n + Haldon + Little N Haldon B Smallacombe ov Babcombe & ey Sands Copse Ba sin Teignmouth legend River Teign alluvium Newton Abbot clay, gravel etc. U. Greensand Wolborough Zig Zag Quarry Palaeozoic etc. + + + + granite sample site 0 4Km Figure 1. Cretaceous localities in the Haldon Hills and the Bovey and Decoy basins. Superfical gravels and clays have been omitted from this figure for clarity. Modified from Carter and Hart (1977). 11 C. Nicholas and M.B. Hart Figure 2. Field photographs from ZigZag Quarry. A. Specimen of chert showing signs of original bedding and presence of bioturbation. B. Specimen of chert showing presence of shelly fossils. C. Specimen of chert showing presence of abundant serpulids. D. Specimen of chert showing presence of abundant orbitolinids that are parallel or sub-parallel to bedding. Note the high width:height ratio of many of the specimens. CONE SURFACE embryonic area chamber layers embryonic apparatus VERTICAL SECTION chamber layer marginal zone Figure 3. Internal structures of the genus Orbitolina, after radial chamber passage Schroeder (1975). HORIZONTAL SECTION central zone radial partition aperture 12 Mid-Cretaceous orbitolinids (1) (2) (3) (4) AGE /ZONE Sea Level Wolborough Haldon Hill S.E. Devon Coast E. Devon /W.Dorset +- n a CBB i A.J. n . a A . m . T o n . R . e C . A C T LB e l d M. dixoni d i . i S l M . H l WiS - . e CSC M t y . n M l r a C PP . a ? ? ? ? m E N EG ? ? ? ? TS . P . AG M r a CB CB p s i . d R . S M WL WS Bs/Fx n . a A i . THS FX C b l A e . t V a . H L m u t a . l f O n . i H . M . Ammonite data C . D Orbitoline data Figure 4. Chronostratigraphic summary chart for the Late Albian to Middle Cenomanian of south-west England. The ammonite zonation is after Owen (1984), Wright et al. (1984) and Hancock (1989). The lithostratigraphical abbreviations are: WL – Wolborough Limestone; THS – Telegraph Hill Sands Member; WS – Woodlands Sands Member; AG – Ashcombe Gravels Member; CSC – Cullum Sands-with-Cherts Member; Fx – Foxmould Sands; CB – Chert Beds; TS – Top Sandstones; PP – Pounds Pool Member; H – Hooken Member; LB – Little Beach Member; Bs – Blackdown Sands; EG – Eggardon Grit; WiS – Wilmington Sands; CBB – Chalk Basement Bed. The sea level change graph is generalised and shows that the flattened forms of Orbitolina sp. cf. Orbitolina sefini in the Wolborough Limestones are part of a transgressive systems tract and that the occurrence of Orbitolina sp. cf. Orbitolina concava in the Cullum Sands-with-Cherts is also associated with a period of sea level rise. TAXONOMY AND PALAEOBIOLOGY OF ORBITOLINA It is now accepted that the internal structure of orbitolinids little in the way of internal structures that are species (Figure 3) is the most important guide to their identification; diagnostic. see, for example, Schroeder (1962, 1963, 1975), Hofker (1963, Orbitolinids are large, internally complex, foraminiferids and 1966) and Schroeder and Neumann (1985). In S.W. England by analogy to modern environments are thought to have this has been a problem as the embryonic apparatus contained algal symbionts. Many workers (e.g. Banner and (comprising the protoconch and first few chambers) is often Simmons, 1994) consider that water depths of 10-50 m typify badly preserved (if at all). In the current material no complete forms such as Palorbitolina lenticularis and most (e.g. Masse, embryonic apparatus has been seen in thin section, although 1976) regard the genus as having a free, epifaunal, mode of life. the approximate measurements that have been made indicate Recently Simmons et al. (2000) have drawn attention to an that the specimens from the Decoy Basin would fall close to intra-specific variation in shape with large, flat, orbitolinids the range recorded by Simmons and Williams (1992, fig. 3) for characteristic of transgressive deposits while more conical forms specimens collected from Babcombe Copse (see Figure 1). are more evident in shallow-water settings. Simmons et al. These plot very close to material collected from Ballon (Sarthe, (2000, fig. 3) have attempted to relate these shape variations to France), within the succession of the ‘type’ Lower Cenomanian, sequence stratigraphy.

Load more