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Seafloor erosion and sea-level change: Early Jurassic Blue Lias Formation of central England
Palaeogeography, Palaeoclimatology, Palaeoecology 270 (2008) 287–294
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Palaeogeography, Palaeoclimatology, Palaeoecology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p a l a e o
Seafloor erosion and sea-level change: Early Jurassic Blue Lias Formation of central England Jonathan D. Radley School of Geography, Earth & Environmental Sciences, The University of Birmingham, Birmingham B15 2TT, UK
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Article history: Received 4 October 2006 Accepted 24 January 2008 Keywords: Early Jurassic Storm deposition Sea-level change Central England
a b s t r a c t Minor bedforms within the mudstone-dominated Early Jurassic Hettangian Saltford Shale Member (Liasicus up to Angulata Chronozone) of the Blue Lias Formation in central England, indicate weak seafloor erosion in a mid to outer ramp setting. Distal storm flows below maximum storm wave base are proposed as the most likely generative mechanism for silty scour and gutter casts that enclose concentrations of well-preserved schlotheimiid ammonites and arthropod trace fossils. Within the upper part of the Saltford Shale (probably Angulata Chronozone), a discrete layer of reworked and bioencrusted limestone nodules signifies an episode of more persistent seafloor erosion. The immediately overlying strata, transitional to the Hettangian– Sinemurian Rugby Limestone Member, are relatively bioturbated and feature fossils of macrobenthos, as well as shell concentrations resembling relatively proximal storm beds. This suggests that the reworked nodule horizon marks sea-level fall, rather than stratigraphic condensation associated with sediment starvation. The biostratigraphic evidence raises the possibility that this erosional episode correlates with a mid-Angulata Chronozone hiatus documented from the Wessex Basin, southwest England. Equally however, it could be linked to contemporaneous movement on one or more nearby faults, affecting the southern part of the English East Midlands Shelf. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Onshore British Lower Jurassic sediments were deposited in epicontinental marine environments influenced by eustatic and regional sea-level fluctuations and a warm, predominantly humid climate (Hallam, 1975; Hallam and Sellwood, 1976; Hesselbo, 2000; Simms et al., 2004). The ammonite-rich argillaceous formations that dominate the succession are taken to indicate the deepest-water settings (Hallam, 1975, 1997; Hallam and Sellwood, 1976; Hesselbo and Jenkyns, 1998; Simms et al., 2004). Storm events (this account) are being increasingly recognized as a major influence on Early Jurassic deposition (Waterhouse, 1999; Wignall, 2001; Simms, 2004; Sheppard, 2006; Sheppard et al., 2006). The Hettangian up to Early Sinemurian Blue Lias Formation of eastern Warwickshire, central England, includes the Saltford Shale Member (Fig. 1; Hettangian; Planorbis up to Angulata Chronozone; Ambrose, 2001; Radley, 2002) that forms the focus of this paper. Blue Lias deposition was initiated during a phase of global sea-level rise (Hesselbo and Jenkyns, 1998; Hesselbo, 2000; Hallam, 1997, 2001), marking the establishment of the Jurassic sea over much of southern Britain. The eastern Warwickshire succession was deposited in a mud-
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dominated mid to outer ramp setting near the southwestern end of the East Midlands Shelf that covered much of eastern England, northwest of the partly emergent London Platform (Figs. 2 and 3; Donovan et al.,1979; Donovan, 1992; Simms et al., 2004). Over southwestern Warwickshire, the chronostratigraphic equivalent of the lower part of the Saltford Shale is a limestone-dominated member (Fig. 1; Rhaetian (latest Triassic)– Hettangian Wilmcote Limestone Member of the Blue Lias Formation; Ambrose, 2001). This has been taken as evidence for localized shallowing, possibly linked to contemporaneous movement on a nearby structure such as the Princethorpe Fault (Ambrose, 2001) or Vale of Moreton Axis (Fig. 3; Old et al., 1991; Radley, 2003). Above the Saltford Shale, rhythmically bedded limestone–mudstone alternations become well developed within the Rugby Limestone Member (Hettangian– Sinemurian; Angulata up to Semicostatum Chronozone) of the Blue Lias Formation (Fig. 1) and have been related to orbital-climatic (Milankovitch) cyclicity (Weedon, 1986). Southam Cement Works quarry, Long Itchington, Warwickshire, England (Fig. 4; National Grid Reference SP 418629-422632) currently (2007) provides the only complete, representative section through the Saltford Shale Member of central England (Figs. 1,5, and 6). There it is approximately 17 m thick (Fig. 5) and assigned to the Liasicus and Angulata chronozones of the Hettangian Stage on the basis of enclosed ammonite faunas (see below and Figs. 1 and 5). Old et al. (1987), Ambrose (2001) and Radley (2002) provided outline descriptions and Martill (2005) published a summary graphic log of the section. At
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Fig. 1. Lithostratigraphy and biostratigraphy of the Lower Jurassic Blue Lias Formation, Warwickshire, central England. Chronozones are indicated in the left-hand column.
Fig. 2. Outline palaeogeography for the Early Jurassic of Britain. S = approximate position of Southam, Warwickshire. (Adapted from Cox et al., 1999.)
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Fig. 3. Outcrop and structural setting of British Lower Jurassic (Lias Group) strata. S = approximate position of Southam, Warwickshire. (Adapted from Cox et al., 1999.)
Southam Cement Works quarry the basal Jurassic Planorbis Chronozone has not been recognized (Fig. 5; Old et al., 1987; Ambrose, 2001; Radley, 2002). There, the Saltford Shale rests on an eroded surface of Langport Member (Rhaetian) limestone and comprises grey-coloured, generally finely laminated silty mudstones enclosing bands, lenticles,
and nodules of calcareous siltstone and fine-grained silty limestone. The lower part of the member includes two laterally persistent, structureless, argillaceous limestone beds (Fig. 5). Schlotheimiid ammonites dominate the sparse macrofauna of the Saltford Shale (Radley, 2002) and are identified as Waehneroceras
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Fig. 4. Location map for Southam Cement Works quarry, Warwickshire, central England.
portlocki (Wright) and Schlotheimia angulata (Schlotheim) (D.T. Donovan, personal communication 2001). Fossils of macrobenthos are rare, except within the highest 5 m which include thin mudstone and argillaceous limestone layers packed with bivalve-dominated bioclastic debris (Fig. 5), rarer echinoid remains and displaying the ichnofossil Chondrites. Above the Saltford Shale Member, the Rugby Limestone Member (approximately 25 m preserved at Southam quarry; Figs. 5, and 6) is characterized by the more typical British Blue Lias facies association of rhythmically bedded dark, laminated and organic-rich to pale, massive, calcareous mudstones, and relatively bioturbated and fossiliferous shelly argillaceous limestones (Weedon, 1986; Old et al., 1987; Ambrose, 2001; Radley, 2002). Sedimentological and sequence-stratigraphical interpretations of argillaceous successions in general are hindered by the scarcity of diagnostic sedimentary structures (e.g. as discussed by Schieber, 1989; Brett and Allison, 1998). Reworked concretions, rapidly buried or hydraulically concentrated skeletal accumulations and arenaceous bedforms are particularly significant in this respect, providing insight into depositional rates, events and bathymetry (Aigner, 1982, 1985; Aigner and Reineck,1982; Brett and Allison,1998). Investigations of the Saltford Shale at Southam quarry have revealed occurrences of calcareous nodules, lenticles and shell concentrations that indicate
Early Jurassic (Hettangian) seafloor erosion on the English East Midlands Shelf (Fig. 3). These are summarised herein and their significance assessed within the contexts of palaeoenvironmental reconstruction and sea-level change. Additional observations of the Saltford Shale, made during 2006 at an adjacent smaller excavation (Fig. 4; Spiers's Farm quarry; SP 425638) are reported where applicable. 2. Scour and gutter casts Sharp-based lenticles of pale, calcareous, laminated to structureless siltstone, up to 205 mm in length and 15 mm in thickness, exhibiting patchy superficial pyritisation, are concentrated within laminated silty mudstones approximately 7 m above the base of the Saltford Shale (Figs. 5, and 7A). Many possess relatively convex lower surfaces displaying vague flute-like structures and resemble shallow scour fills. Lower and upper surfaces respectively preserve bilobate protuberances and depressions identified as arthropod traces (Fig. 7B; cf. Isopodichnus; Radley, 2002), as well as minute bivalve shells. Associated mudstones enclose rare, small, discrete, deep-rounded steep-sided gutters (sensu Myrow, 1992), filled with calcite-cemented bivalve debris. Rounded to elongate, straight, occasionally pipe-like nodules of laminated to structureless calcareous siltstone (up to 720 mm long)
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Fig. 6. Section at Southam Cement Works quarry, Warwickshire, central England (southern end of quarry). The sections in the foreground are approximately 2.5 m high and expose micritic limestones of the Rhaetian Langport Member. The main quarry face (approximately 35 m high) exposes dark mudstones of the Early Jurassic (Hettangian) Saltford Shale Member, overlain by alternating mudstones and pale limestones of the Hettangian–Sinemurian Rugby Limestone Member. Arrow marks the approximate position of the Saltford Shale–Rugby Limestone boundary.
(Fig. 8C) and their body chambers commonly geopetally filled with calcite-cemented mudstone and sparry calcite. Inner whorls are crushed in some examples. Sole markings have not been observed. Such nodules are known chiefly as ex situ specimens (Radley, 2002), but may be concentrated approximately 8.5–10.5 m above the base of the member (Martill, 2005 and JDR, personal observations, 2006). An in situ example at Spiers's Farm quarry (see above) was oriented 13°N–193°S. 3. Reworked limestone nodules
Fig. 5. Stratigraphic log and biostratigraphy of the Early Jurassic Saltford Shale Member (Blue Lias Formation) at Southam Cement Works quarry, Warwickshire, central England.
enclose densely packed skeletal concentrations dominated by schlotheimiid ammonites and fish scales and rarely, small bivalves (Fig. 8A–C). These are interpreted as scour and gutter casts, the latter (Fig. 8A,B) equating to the discrete shallow-rounded to deep-rounded morphologies of Myrow (1992). Enclosed ammonites are frequently imbricated
The eroded surface of the Langport Member (Rhaetian) limestone locally preserves a thin veneer of black shelly mudstone enclosing small, subrounded to rounded micritic limestone clasts, some of which are oyster-encrusted or preserve small Gastrochaenolites (bivalve) borings. This marks the base of the Saltford Shale Member (Radley, 2002). Higher in the succession, approximately 12 m above the base of the Saltford Shale, a layer of fine-grained, biogenically encrusted and bioeroded fine-grained limestone nodules caps an interval of shelly, bioturbated mudstone (Fig. 5; Radley, 2002). The associated bioclastic debris includes lagenine foraminifera, bivalve fragments and echinoid debris. The nodules are dominantly subrounded, often broadly discshaped, generally less than 25 mm in thickness and up to around 170 mm in length. Some are superficially pyritised. Small encrusting oysters and serpulids are commonly concentrated on upper surfaces (Fig. 8D). Additionally, the upper and lower surfaces of many nodules exhibit networks of straight to slightly curved scratches and grooves, up to about 10 mm in maximum length. The finer, regular scratches are interpreted as hard-substrate grazing traces (Gnathichnus isp.). Some of the coarser marks resemble the crustacean scratch traces documented from the surfaces of Early Jurassic reworked concretions (Coinstone horizon of the Charmouth Mudstone Formation, Sinemurian) in the Wessex Basin (West Dorset coast, southwest England; Hesselbo and Palmer, 1992). 4. Interpretation and discussion Deposition of the Blue Lias Formation in Southern Britain was initiated during a phase of eustatic sea-level rise in the latest Triassic or earliest Jurassic (Hallam, 2001; Hallam and Wignall, 2004; Hesselbo et al., 2004). Widespread deposition of dysaerobic muds in Southern Britain during Liasicus Chronozone times is indicated by laminated
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Fig. 7. (A) Calcareous siltstone lenticles within laminated silty mudstones, approximately 7 m above the base of the Saltford Shale Member (Blue Lias Formation), Southam Cement Works quarry, Warwickshire, central England. Length of hammer: 30.5 cm. (B) Undescribed arthropod trace (cf. Isopodichnus isp.) on lower surface of calcareous siltstone lenticle. From approximately 7 m above the base of the Saltford Shale Member (Blue Lias Formation), Southam Cement Works quarry, Warwickshire, central England. Warwickshire Museum specimen G15641.
mudstone-dominated successions such as the Lavernock Shale of South Wales and St Audrie's Shale and Saltford Shale of southwestern and central England (Donovan et al., 1979; Hallam, 1995; Warrington and Ivimey-Cook, 1995; Hesselbo and Jenkyns, 1998; Simms et al., 2004). At Southam, central England, following non-deposition (Planorbis and/or Early Liasicus Chronozone) and generation of a basal reworked clast bed (see above and Fig. 5), the laminated and benthos-poor nature of much of the Saltford Shale confirms an overall dominantly low-energy dysaerobic environment (Wignall and Hallam, 1991). Significantly, Planorbis Chronozone ammonites have been identified within the
basal Saltford Shale 10 km to the north of Southam (Old et al., 1987). This diachroneity has been taken to indicate transgressive onlap of Lower Jurassic sediments onto the northwest flank of the London Platform (Figs. 2 and 3; Donovan et al., 1979). At Southam, thin calcareous beds in the lower part of the member (Fig. 5) possibly signify relatively oxygenated phases. Occurrences of bioturbated, benthos-rich mudstones in the upper part of the Saltford Shale (Fig. 5) similarly indicate increased benthic oxygenation (Hallam, 1995; Radley, 2002), presaging deposition of the richly fossiliferous and relatively bioturbated Rugby Limestone (Old et al., 1987; Ambrose, 2001). Regionally, comparable transitions from argillaceous to relatively calcareous strata of Angulata Chronozone age have been interpreted in terms of increased oxygenation (see above) and changes in sediment supply, whether or not connected to sea-level change (Hesselbo and Jenkyns, 1998). Significantly, Hallam (1997) and other workers (e.g. Howard, 1984; Van Buchem and Knox, 1998; Sheppard et al., 2006) have drawn attention to the significance of bedforms attributable to storm deposition (notably hummocky cross-stratified beds and gutter casts) for bathymetric reconstructions of British Lower Jurassic successions. Hallam (1997) compared Early Jurassic (Pliensbachian) facies developments and storm beds in the Cleveland Basin (northeast England; Fig. 3) to those of the present-day German Bight (Southern North Sea) and Norton Sound (Bering Sea). In offshore mud facies of the German Bight, thin distal storm-flow silts, deposited by offshore gradient currents, wedge out below maximum storm wave base at depths in the order of a few tens of metres (Aigner and Reineck, 1982; Aigner, 1985). The thin, discontinuous and fine-grained (silt-grade) nature of the scour casts that occur 7 m above the base of the Saltford Shale (Figs. 5 and 7) indicate weak current influence, distal to sediment source. Howard (1984) attributed thin, silt-filled scours within Pliensbachian mudstones of the Cleveland Basin to low-velocity distal storm flow below storm wave base. A comparable origin is envisaged for the examples noted herein, given the absence of structures attributable to modification by oscillatory currents (e.g. wave ripples; Aigner and Reineck, 1982; Aigner, 1985) within the scour and gutter cast horizons. The fossiliferous gutter and scour casts (Fig. 8A–C) similarly indicate linear erosion of cohesive sediment by gradient currents, presumably more-or-less perpendicular to a prevailing palaeo-shoreline (Myrow, 1992; Pérez-López, 2001). The limited evidence from a single in situ gutter cast (see above) is consistent with the accepted scenario of a shoreline fringing the London Platform to the south (Figs. 2 and 3; Donovan et al., 1979; Donovan, 1992). Following erosion, scours and gutters were filled with silt, waterlogged ammonite shells and other skeletal debris with similar hydrostatic properties (Maeda and Seilacher, 1996). In some instances the ammonite shells became sufficiently concentrated to allow interference and imbrication (Fig. 8C). Relatively deep gutters (Fig. 8A,B) would have been prone to rapid degradation following incision (Myrow, 1992), even in cohesive sediment. This suggests that such examples are products of single storm events. Preservation of these distal storm deposits was facilitated by generally low oxygenation and minimal infaunal biomass, recorded by the widespread lamination in the scour and gutter cast horizons. As in the Southern North Sea (Aigner and Reineck, 1982; Aigner, 1985), they are taken as a record of exceptionally powerful storms, rather than storm frequency. The bioeroded and bioencrusted limestone nodules approximately 12 m above the base of the Saltford Shale at Southam quarry (Figs. 5 and 8D) indicate the presence of a ‘shale on shale’ erosion surface (Baird,1976). A phase of net seafloor erosion following increased benthic oxygenation, is indicated by their association with bioturbated shelly sediment. This suggests heightened current and/or wave influence, initially raising benthic oxygenation and culminating in erosion of an unknown thickness of sediment. An episode of relative sea-level fall leading to increased storm influence is taken as the most plausible mechanism (Hallam, 1999), rather than condensation due to sediment
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Fig. 8. (A) Silty limestone nodule (side view) enclosing concentrated schlotheimiid ammonites. Saltford Shale Member (Blue Lias Formation), Southam Cement Works quarry, Warwickshire, central England. Warwickshire Museum specimen G15665/1. (B) Detail of lower surface of silty limestone nodule enclosing schlotheimiid ammonites. Saltford Shale Member (Blue Lias Formation), Southam Cement Works quarry, Warwickshire, central England. Warwickshire Museum specimen G15665/2. (C) Silty limestone nodule enclosing imbricated schlotheimiid ammonites. Saltford Shale Member (Blue Lias Formation), Southam Cement Works quarry, Warwickshire, central England. Warwickshire Museum specimen G15577/2. (D) Upper surface of argillaceous limestone nodule encrusted by small oysters and serpulids. From approximately 12 m above the base of the Saltford Shale Member (Blue Lias Formation), Southam Cement Works quarry, Warwickshire, central England. Warwickshire Museum specimen G15748.
starvation. Further indication of shallowing during deposition of the uppermost Saltford Shale is provided by centimetre-scale, sharp-based sheets of densely packed, angular, bivalve-dominated bioclastic debris within a fine-grained limestone bed approximately 3 m below the top of the member at the adjacent Spiers's Farm quarry. These invite comparison with ancient shell concentrations modeled as proximal storm beds deposited between fair weather- and maximum storm wave base (e.g. Aigner, 1982; Fürsich and Oschmann, 1993). A significant hiatus has been invoked within the mid-Angulata Chronozone of the Blue Lias Formation in coastal sections on the Dorset–
Devon border (Wessex Basin, southwest England; Fig. 3), on the basis of regional correlation of presumed Milankovitch cycles (Smith, 1989) and vertical trends in clay mineral abundances (Deconinck et al., 2003). Although the biostratigraphic evidence from Southam quarry is inconclusive, it is feasible that the erosion surface marked by reworked, bioencrusted limestone nodules is of comparable age. If confirmed this would raise the possibility of a late Hettangian episode of sea-level fall of regional scale. Equally however, the erosive event demonstrated at Southam could have been due to localized shallowing, linked to movement on one or more nearby faults (see above).
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5. Conclusions Storm-current influence on deposition of the largely dysaerobic Saltford Shale Member of the Early Jurassic Blue Lias Formation near the southwestern edge of the East Midlands Shelf, central England, is indicated by occurrences of scour and gutter casts. Consideration of the generally accepted depositional model for onshore British Early Jurassic facies developments (e.g. Hallam, 1975, 1997) suggests that these formed in an offshore environment influenced by weak storm flows, below maximum storm wave base. Actualistic comparisons with modern storm-dominated shelf settings (Aigner, 1985; Hallam, 1997) suggest that there is no need to invoke depths much in excess of a few tens of metres for this setting. Near the top of the Saltford Shale Member, shelly horizons confirm increasing oxygenation. Amongst the shelly strata, a layer of reworked, bioencrusted limestone nodules establishes a link between increased oxygenation and relative sealevel fall that culminated in net erosion in a wave and/or currentinfluenced setting. Acknowledgements This paper is dedicated to the memory of Roland Goldring, who commented on an early draft during the summer of 2005. Gratitude is also expressed to Paul Wignall (University of Leeds), Keith Ambrose and Huw Sheppard (British Geological Survey) who commented on a later draft, and to Dan Bosence (Royal Holloway University of London) and anonymous referees for supplying further constructive comments on the manuscript. Peter Blake (Bridgwater, Somerset) is acknowledged for introducing the author to the Southam quarry, and staff of Cemex are thanked for permitting access to the site. Desmond Donovan (University College London) kindly identified several ammonites from Southam quarry during 2001. Figs. 2 and 3 are reproduced by permission of the British Geological Survey (© NERC. All rights reserved). References Aigner, T., 1982. Calcareous tempestites: storm-dominated stratification in Upper Muschelkalk limestones (Middle Trias, SW-Germany). In: Einsele, G., Seilacher, A. (Eds.), Cyclic and Event Stratification. Springer-Verlag, Berlin, pp. 180–198. Aigner, T., 1985. Storm Depositional Systems. Springer-Verlag, Berlin. Aigner, T., Reineck, H., 1982. Proximality trends in modern storm sands from the Helgoland Bight (North Sea) and their implications for basin analysis. Senckenbergiana Maritima 14, 183–215. Ambrose, K., 2001. The lithostratigraphy of the Blue Lias Formation (Late Rhaetian–Early Sinemurian) in the southern part of the English Midlands. Proceedings of the Geologists' Association 112, 97–110. Baird, G.C., 1976. Coral encrusted concretions: a key to recognition of a ‘shale on shale’ erosion surface. Lethaia 9, 293–302. Brett, C.E., Allison, P.A., 1998. Paleontological approaches to the environmental interpretation of marine mudrocks. In: Schieber, J., Zimmerle, W., Sethi, P. (Eds.), Shales and Mudstones I. E. Schweizerbart'sche Verlagsbuchhandlung, Stuttgart, pp. 301–349. Cox, B.M., Sumbler, M.G., Ivimey-Cook, H.C., 1999. A Formational Framework for the Lower Jurassic of England and Wales (onshore area). British Geological Survey Research Report RR/99/01. Deconinck, J., Hesselbo, S.P., Debuisser, N., Averbuch, O., Baudin, F., Bessa, J., 2003. Environmental control on clay mineralogy of an Early Jurassic mudrock (Blue Lias Formation, Southern England). International Journal of Earth Sciences 92, 255–266. Donovan, D.T., 1992. Early Hettangian. In: Cope, J.C.W., Ingham, J.K., Rawson, P.F. (Eds.), Atlas of Palaeogeography and Lithofacies. Geological Society, London, Memoir 13, pp. 108–109. Donovan, D.T., Horton, A., Ivimey- Cook, H.C., 1979. The transgression of the Lower Lias over the northern flank of the London Platform. Journal of the Geological Society (London) 136, 165–173. Fürsich, F.T., Oschmann, W., 1993. Shell beds as tools in facies analysis: the Jurassic of Kachchh, western India. Journal of the Geological Society (London) 150, 169–185. Hallam, A., 1975. Jurassic Environments. Cambridge University Press, Cambridge. Hallam, A., 1995. Oxygen-restricted facies of the basal Jurassic of north west Europe. Historical Biology 10, 247–257.
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Palaeogeography, Palaeoclimatology, Palaeoecology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p a l a e o
Seafloor erosion and sea-level change: Early Jurassic Blue Lias Formation of central England Jonathan D. Radley School of Geography, Earth & Environmental Sciences, The University of Birmingham, Birmingham B15 2TT, UK
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Article history: Received 4 October 2006 Accepted 24 January 2008 Keywords: Early Jurassic Storm deposition Sea-level change Central England
a b s t r a c t Minor bedforms within the mudstone-dominated Early Jurassic Hettangian Saltford Shale Member (Liasicus up to Angulata Chronozone) of the Blue Lias Formation in central England, indicate weak seafloor erosion in a mid to outer ramp setting. Distal storm flows below maximum storm wave base are proposed as the most likely generative mechanism for silty scour and gutter casts that enclose concentrations of well-preserved schlotheimiid ammonites and arthropod trace fossils. Within the upper part of the Saltford Shale (probably Angulata Chronozone), a discrete layer of reworked and bioencrusted limestone nodules signifies an episode of more persistent seafloor erosion. The immediately overlying strata, transitional to the Hettangian– Sinemurian Rugby Limestone Member, are relatively bioturbated and feature fossils of macrobenthos, as well as shell concentrations resembling relatively proximal storm beds. This suggests that the reworked nodule horizon marks sea-level fall, rather than stratigraphic condensation associated with sediment starvation. The biostratigraphic evidence raises the possibility that this erosional episode correlates with a mid-Angulata Chronozone hiatus documented from the Wessex Basin, southwest England. Equally however, it could be linked to contemporaneous movement on one or more nearby faults, affecting the southern part of the English East Midlands Shelf. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Onshore British Lower Jurassic sediments were deposited in epicontinental marine environments influenced by eustatic and regional sea-level fluctuations and a warm, predominantly humid climate (Hallam, 1975; Hallam and Sellwood, 1976; Hesselbo, 2000; Simms et al., 2004). The ammonite-rich argillaceous formations that dominate the succession are taken to indicate the deepest-water settings (Hallam, 1975, 1997; Hallam and Sellwood, 1976; Hesselbo and Jenkyns, 1998; Simms et al., 2004). Storm events (this account) are being increasingly recognized as a major influence on Early Jurassic deposition (Waterhouse, 1999; Wignall, 2001; Simms, 2004; Sheppard, 2006; Sheppard et al., 2006). The Hettangian up to Early Sinemurian Blue Lias Formation of eastern Warwickshire, central England, includes the Saltford Shale Member (Fig. 1; Hettangian; Planorbis up to Angulata Chronozone; Ambrose, 2001; Radley, 2002) that forms the focus of this paper. Blue Lias deposition was initiated during a phase of global sea-level rise (Hesselbo and Jenkyns, 1998; Hesselbo, 2000; Hallam, 1997, 2001), marking the establishment of the Jurassic sea over much of southern Britain. The eastern Warwickshire succession was deposited in a mud-
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dominated mid to outer ramp setting near the southwestern end of the East Midlands Shelf that covered much of eastern England, northwest of the partly emergent London Platform (Figs. 2 and 3; Donovan et al.,1979; Donovan, 1992; Simms et al., 2004). Over southwestern Warwickshire, the chronostratigraphic equivalent of the lower part of the Saltford Shale is a limestone-dominated member (Fig. 1; Rhaetian (latest Triassic)– Hettangian Wilmcote Limestone Member of the Blue Lias Formation; Ambrose, 2001). This has been taken as evidence for localized shallowing, possibly linked to contemporaneous movement on a nearby structure such as the Princethorpe Fault (Ambrose, 2001) or Vale of Moreton Axis (Fig. 3; Old et al., 1991; Radley, 2003). Above the Saltford Shale, rhythmically bedded limestone–mudstone alternations become well developed within the Rugby Limestone Member (Hettangian– Sinemurian; Angulata up to Semicostatum Chronozone) of the Blue Lias Formation (Fig. 1) and have been related to orbital-climatic (Milankovitch) cyclicity (Weedon, 1986). Southam Cement Works quarry, Long Itchington, Warwickshire, England (Fig. 4; National Grid Reference SP 418629-422632) currently (2007) provides the only complete, representative section through the Saltford Shale Member of central England (Figs. 1,5, and 6). There it is approximately 17 m thick (Fig. 5) and assigned to the Liasicus and Angulata chronozones of the Hettangian Stage on the basis of enclosed ammonite faunas (see below and Figs. 1 and 5). Old et al. (1987), Ambrose (2001) and Radley (2002) provided outline descriptions and Martill (2005) published a summary graphic log of the section. At
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Fig. 1. Lithostratigraphy and biostratigraphy of the Lower Jurassic Blue Lias Formation, Warwickshire, central England. Chronozones are indicated in the left-hand column.
Fig. 2. Outline palaeogeography for the Early Jurassic of Britain. S = approximate position of Southam, Warwickshire. (Adapted from Cox et al., 1999.)
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Fig. 3. Outcrop and structural setting of British Lower Jurassic (Lias Group) strata. S = approximate position of Southam, Warwickshire. (Adapted from Cox et al., 1999.)
Southam Cement Works quarry the basal Jurassic Planorbis Chronozone has not been recognized (Fig. 5; Old et al., 1987; Ambrose, 2001; Radley, 2002). There, the Saltford Shale rests on an eroded surface of Langport Member (Rhaetian) limestone and comprises grey-coloured, generally finely laminated silty mudstones enclosing bands, lenticles,
and nodules of calcareous siltstone and fine-grained silty limestone. The lower part of the member includes two laterally persistent, structureless, argillaceous limestone beds (Fig. 5). Schlotheimiid ammonites dominate the sparse macrofauna of the Saltford Shale (Radley, 2002) and are identified as Waehneroceras
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Fig. 4. Location map for Southam Cement Works quarry, Warwickshire, central England.
portlocki (Wright) and Schlotheimia angulata (Schlotheim) (D.T. Donovan, personal communication 2001). Fossils of macrobenthos are rare, except within the highest 5 m which include thin mudstone and argillaceous limestone layers packed with bivalve-dominated bioclastic debris (Fig. 5), rarer echinoid remains and displaying the ichnofossil Chondrites. Above the Saltford Shale Member, the Rugby Limestone Member (approximately 25 m preserved at Southam quarry; Figs. 5, and 6) is characterized by the more typical British Blue Lias facies association of rhythmically bedded dark, laminated and organic-rich to pale, massive, calcareous mudstones, and relatively bioturbated and fossiliferous shelly argillaceous limestones (Weedon, 1986; Old et al., 1987; Ambrose, 2001; Radley, 2002). Sedimentological and sequence-stratigraphical interpretations of argillaceous successions in general are hindered by the scarcity of diagnostic sedimentary structures (e.g. as discussed by Schieber, 1989; Brett and Allison, 1998). Reworked concretions, rapidly buried or hydraulically concentrated skeletal accumulations and arenaceous bedforms are particularly significant in this respect, providing insight into depositional rates, events and bathymetry (Aigner, 1982, 1985; Aigner and Reineck,1982; Brett and Allison,1998). Investigations of the Saltford Shale at Southam quarry have revealed occurrences of calcareous nodules, lenticles and shell concentrations that indicate
Early Jurassic (Hettangian) seafloor erosion on the English East Midlands Shelf (Fig. 3). These are summarised herein and their significance assessed within the contexts of palaeoenvironmental reconstruction and sea-level change. Additional observations of the Saltford Shale, made during 2006 at an adjacent smaller excavation (Fig. 4; Spiers's Farm quarry; SP 425638) are reported where applicable. 2. Scour and gutter casts Sharp-based lenticles of pale, calcareous, laminated to structureless siltstone, up to 205 mm in length and 15 mm in thickness, exhibiting patchy superficial pyritisation, are concentrated within laminated silty mudstones approximately 7 m above the base of the Saltford Shale (Figs. 5, and 7A). Many possess relatively convex lower surfaces displaying vague flute-like structures and resemble shallow scour fills. Lower and upper surfaces respectively preserve bilobate protuberances and depressions identified as arthropod traces (Fig. 7B; cf. Isopodichnus; Radley, 2002), as well as minute bivalve shells. Associated mudstones enclose rare, small, discrete, deep-rounded steep-sided gutters (sensu Myrow, 1992), filled with calcite-cemented bivalve debris. Rounded to elongate, straight, occasionally pipe-like nodules of laminated to structureless calcareous siltstone (up to 720 mm long)
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Fig. 6. Section at Southam Cement Works quarry, Warwickshire, central England (southern end of quarry). The sections in the foreground are approximately 2.5 m high and expose micritic limestones of the Rhaetian Langport Member. The main quarry face (approximately 35 m high) exposes dark mudstones of the Early Jurassic (Hettangian) Saltford Shale Member, overlain by alternating mudstones and pale limestones of the Hettangian–Sinemurian Rugby Limestone Member. Arrow marks the approximate position of the Saltford Shale–Rugby Limestone boundary.
(Fig. 8C) and their body chambers commonly geopetally filled with calcite-cemented mudstone and sparry calcite. Inner whorls are crushed in some examples. Sole markings have not been observed. Such nodules are known chiefly as ex situ specimens (Radley, 2002), but may be concentrated approximately 8.5–10.5 m above the base of the member (Martill, 2005 and JDR, personal observations, 2006). An in situ example at Spiers's Farm quarry (see above) was oriented 13°N–193°S. 3. Reworked limestone nodules
Fig. 5. Stratigraphic log and biostratigraphy of the Early Jurassic Saltford Shale Member (Blue Lias Formation) at Southam Cement Works quarry, Warwickshire, central England.
enclose densely packed skeletal concentrations dominated by schlotheimiid ammonites and fish scales and rarely, small bivalves (Fig. 8A–C). These are interpreted as scour and gutter casts, the latter (Fig. 8A,B) equating to the discrete shallow-rounded to deep-rounded morphologies of Myrow (1992). Enclosed ammonites are frequently imbricated
The eroded surface of the Langport Member (Rhaetian) limestone locally preserves a thin veneer of black shelly mudstone enclosing small, subrounded to rounded micritic limestone clasts, some of which are oyster-encrusted or preserve small Gastrochaenolites (bivalve) borings. This marks the base of the Saltford Shale Member (Radley, 2002). Higher in the succession, approximately 12 m above the base of the Saltford Shale, a layer of fine-grained, biogenically encrusted and bioeroded fine-grained limestone nodules caps an interval of shelly, bioturbated mudstone (Fig. 5; Radley, 2002). The associated bioclastic debris includes lagenine foraminifera, bivalve fragments and echinoid debris. The nodules are dominantly subrounded, often broadly discshaped, generally less than 25 mm in thickness and up to around 170 mm in length. Some are superficially pyritised. Small encrusting oysters and serpulids are commonly concentrated on upper surfaces (Fig. 8D). Additionally, the upper and lower surfaces of many nodules exhibit networks of straight to slightly curved scratches and grooves, up to about 10 mm in maximum length. The finer, regular scratches are interpreted as hard-substrate grazing traces (Gnathichnus isp.). Some of the coarser marks resemble the crustacean scratch traces documented from the surfaces of Early Jurassic reworked concretions (Coinstone horizon of the Charmouth Mudstone Formation, Sinemurian) in the Wessex Basin (West Dorset coast, southwest England; Hesselbo and Palmer, 1992). 4. Interpretation and discussion Deposition of the Blue Lias Formation in Southern Britain was initiated during a phase of eustatic sea-level rise in the latest Triassic or earliest Jurassic (Hallam, 2001; Hallam and Wignall, 2004; Hesselbo et al., 2004). Widespread deposition of dysaerobic muds in Southern Britain during Liasicus Chronozone times is indicated by laminated
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Fig. 7. (A) Calcareous siltstone lenticles within laminated silty mudstones, approximately 7 m above the base of the Saltford Shale Member (Blue Lias Formation), Southam Cement Works quarry, Warwickshire, central England. Length of hammer: 30.5 cm. (B) Undescribed arthropod trace (cf. Isopodichnus isp.) on lower surface of calcareous siltstone lenticle. From approximately 7 m above the base of the Saltford Shale Member (Blue Lias Formation), Southam Cement Works quarry, Warwickshire, central England. Warwickshire Museum specimen G15641.
mudstone-dominated successions such as the Lavernock Shale of South Wales and St Audrie's Shale and Saltford Shale of southwestern and central England (Donovan et al., 1979; Hallam, 1995; Warrington and Ivimey-Cook, 1995; Hesselbo and Jenkyns, 1998; Simms et al., 2004). At Southam, central England, following non-deposition (Planorbis and/or Early Liasicus Chronozone) and generation of a basal reworked clast bed (see above and Fig. 5), the laminated and benthos-poor nature of much of the Saltford Shale confirms an overall dominantly low-energy dysaerobic environment (Wignall and Hallam, 1991). Significantly, Planorbis Chronozone ammonites have been identified within the
basal Saltford Shale 10 km to the north of Southam (Old et al., 1987). This diachroneity has been taken to indicate transgressive onlap of Lower Jurassic sediments onto the northwest flank of the London Platform (Figs. 2 and 3; Donovan et al., 1979). At Southam, thin calcareous beds in the lower part of the member (Fig. 5) possibly signify relatively oxygenated phases. Occurrences of bioturbated, benthos-rich mudstones in the upper part of the Saltford Shale (Fig. 5) similarly indicate increased benthic oxygenation (Hallam, 1995; Radley, 2002), presaging deposition of the richly fossiliferous and relatively bioturbated Rugby Limestone (Old et al., 1987; Ambrose, 2001). Regionally, comparable transitions from argillaceous to relatively calcareous strata of Angulata Chronozone age have been interpreted in terms of increased oxygenation (see above) and changes in sediment supply, whether or not connected to sea-level change (Hesselbo and Jenkyns, 1998). Significantly, Hallam (1997) and other workers (e.g. Howard, 1984; Van Buchem and Knox, 1998; Sheppard et al., 2006) have drawn attention to the significance of bedforms attributable to storm deposition (notably hummocky cross-stratified beds and gutter casts) for bathymetric reconstructions of British Lower Jurassic successions. Hallam (1997) compared Early Jurassic (Pliensbachian) facies developments and storm beds in the Cleveland Basin (northeast England; Fig. 3) to those of the present-day German Bight (Southern North Sea) and Norton Sound (Bering Sea). In offshore mud facies of the German Bight, thin distal storm-flow silts, deposited by offshore gradient currents, wedge out below maximum storm wave base at depths in the order of a few tens of metres (Aigner and Reineck, 1982; Aigner, 1985). The thin, discontinuous and fine-grained (silt-grade) nature of the scour casts that occur 7 m above the base of the Saltford Shale (Figs. 5 and 7) indicate weak current influence, distal to sediment source. Howard (1984) attributed thin, silt-filled scours within Pliensbachian mudstones of the Cleveland Basin to low-velocity distal storm flow below storm wave base. A comparable origin is envisaged for the examples noted herein, given the absence of structures attributable to modification by oscillatory currents (e.g. wave ripples; Aigner and Reineck, 1982; Aigner, 1985) within the scour and gutter cast horizons. The fossiliferous gutter and scour casts (Fig. 8A–C) similarly indicate linear erosion of cohesive sediment by gradient currents, presumably more-or-less perpendicular to a prevailing palaeo-shoreline (Myrow, 1992; Pérez-López, 2001). The limited evidence from a single in situ gutter cast (see above) is consistent with the accepted scenario of a shoreline fringing the London Platform to the south (Figs. 2 and 3; Donovan et al., 1979; Donovan, 1992). Following erosion, scours and gutters were filled with silt, waterlogged ammonite shells and other skeletal debris with similar hydrostatic properties (Maeda and Seilacher, 1996). In some instances the ammonite shells became sufficiently concentrated to allow interference and imbrication (Fig. 8C). Relatively deep gutters (Fig. 8A,B) would have been prone to rapid degradation following incision (Myrow, 1992), even in cohesive sediment. This suggests that such examples are products of single storm events. Preservation of these distal storm deposits was facilitated by generally low oxygenation and minimal infaunal biomass, recorded by the widespread lamination in the scour and gutter cast horizons. As in the Southern North Sea (Aigner and Reineck, 1982; Aigner, 1985), they are taken as a record of exceptionally powerful storms, rather than storm frequency. The bioeroded and bioencrusted limestone nodules approximately 12 m above the base of the Saltford Shale at Southam quarry (Figs. 5 and 8D) indicate the presence of a ‘shale on shale’ erosion surface (Baird,1976). A phase of net seafloor erosion following increased benthic oxygenation, is indicated by their association with bioturbated shelly sediment. This suggests heightened current and/or wave influence, initially raising benthic oxygenation and culminating in erosion of an unknown thickness of sediment. An episode of relative sea-level fall leading to increased storm influence is taken as the most plausible mechanism (Hallam, 1999), rather than condensation due to sediment
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Fig. 8. (A) Silty limestone nodule (side view) enclosing concentrated schlotheimiid ammonites. Saltford Shale Member (Blue Lias Formation), Southam Cement Works quarry, Warwickshire, central England. Warwickshire Museum specimen G15665/1. (B) Detail of lower surface of silty limestone nodule enclosing schlotheimiid ammonites. Saltford Shale Member (Blue Lias Formation), Southam Cement Works quarry, Warwickshire, central England. Warwickshire Museum specimen G15665/2. (C) Silty limestone nodule enclosing imbricated schlotheimiid ammonites. Saltford Shale Member (Blue Lias Formation), Southam Cement Works quarry, Warwickshire, central England. Warwickshire Museum specimen G15577/2. (D) Upper surface of argillaceous limestone nodule encrusted by small oysters and serpulids. From approximately 12 m above the base of the Saltford Shale Member (Blue Lias Formation), Southam Cement Works quarry, Warwickshire, central England. Warwickshire Museum specimen G15748.
starvation. Further indication of shallowing during deposition of the uppermost Saltford Shale is provided by centimetre-scale, sharp-based sheets of densely packed, angular, bivalve-dominated bioclastic debris within a fine-grained limestone bed approximately 3 m below the top of the member at the adjacent Spiers's Farm quarry. These invite comparison with ancient shell concentrations modeled as proximal storm beds deposited between fair weather- and maximum storm wave base (e.g. Aigner, 1982; Fürsich and Oschmann, 1993). A significant hiatus has been invoked within the mid-Angulata Chronozone of the Blue Lias Formation in coastal sections on the Dorset–
Devon border (Wessex Basin, southwest England; Fig. 3), on the basis of regional correlation of presumed Milankovitch cycles (Smith, 1989) and vertical trends in clay mineral abundances (Deconinck et al., 2003). Although the biostratigraphic evidence from Southam quarry is inconclusive, it is feasible that the erosion surface marked by reworked, bioencrusted limestone nodules is of comparable age. If confirmed this would raise the possibility of a late Hettangian episode of sea-level fall of regional scale. Equally however, the erosive event demonstrated at Southam could have been due to localized shallowing, linked to movement on one or more nearby faults (see above).
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5. Conclusions Storm-current influence on deposition of the largely dysaerobic Saltford Shale Member of the Early Jurassic Blue Lias Formation near the southwestern edge of the East Midlands Shelf, central England, is indicated by occurrences of scour and gutter casts. Consideration of the generally accepted depositional model for onshore British Early Jurassic facies developments (e.g. Hallam, 1975, 1997) suggests that these formed in an offshore environment influenced by weak storm flows, below maximum storm wave base. Actualistic comparisons with modern storm-dominated shelf settings (Aigner, 1985; Hallam, 1997) suggest that there is no need to invoke depths much in excess of a few tens of metres for this setting. Near the top of the Saltford Shale Member, shelly horizons confirm increasing oxygenation. Amongst the shelly strata, a layer of reworked, bioencrusted limestone nodules establishes a link between increased oxygenation and relative sealevel fall that culminated in net erosion in a wave and/or currentinfluenced setting. Acknowledgements This paper is dedicated to the memory of Roland Goldring, who commented on an early draft during the summer of 2005. Gratitude is also expressed to Paul Wignall (University of Leeds), Keith Ambrose and Huw Sheppard (British Geological Survey) who commented on a later draft, and to Dan Bosence (Royal Holloway University of London) and anonymous referees for supplying further constructive comments on the manuscript. Peter Blake (Bridgwater, Somerset) is acknowledged for introducing the author to the Southam quarry, and staff of Cemex are thanked for permitting access to the site. Desmond Donovan (University College London) kindly identified several ammonites from Southam quarry during 2001. Figs. 2 and 3 are reproduced by permission of the British Geological Survey (© NERC. All rights reserved). References Aigner, T., 1982. Calcareous tempestites: storm-dominated stratification in Upper Muschelkalk limestones (Middle Trias, SW-Germany). In: Einsele, G., Seilacher, A. (Eds.), Cyclic and Event Stratification. Springer-Verlag, Berlin, pp. 180–198. Aigner, T., 1985. Storm Depositional Systems. Springer-Verlag, Berlin. Aigner, T., Reineck, H., 1982. Proximality trends in modern storm sands from the Helgoland Bight (North Sea) and their implications for basin analysis. Senckenbergiana Maritima 14, 183–215. Ambrose, K., 2001. The lithostratigraphy of the Blue Lias Formation (Late Rhaetian–Early Sinemurian) in the southern part of the English Midlands. Proceedings of the Geologists' Association 112, 97–110. Baird, G.C., 1976. Coral encrusted concretions: a key to recognition of a ‘shale on shale’ erosion surface. Lethaia 9, 293–302. Brett, C.E., Allison, P.A., 1998. Paleontological approaches to the environmental interpretation of marine mudrocks. In: Schieber, J., Zimmerle, W., Sethi, P. (Eds.), Shales and Mudstones I. E. Schweizerbart'sche Verlagsbuchhandlung, Stuttgart, pp. 301–349. Cox, B.M., Sumbler, M.G., Ivimey-Cook, H.C., 1999. A Formational Framework for the Lower Jurassic of England and Wales (onshore area). British Geological Survey Research Report RR/99/01. Deconinck, J., Hesselbo, S.P., Debuisser, N., Averbuch, O., Baudin, F., Bessa, J., 2003. Environmental control on clay mineralogy of an Early Jurassic mudrock (Blue Lias Formation, Southern England). International Journal of Earth Sciences 92, 255–266. Donovan, D.T., 1992. Early Hettangian. In: Cope, J.C.W., Ingham, J.K., Rawson, P.F. (Eds.), Atlas of Palaeogeography and Lithofacies. Geological Society, London, Memoir 13, pp. 108–109. Donovan, D.T., Horton, A., Ivimey- Cook, H.C., 1979. The transgression of the Lower Lias over the northern flank of the London Platform. Journal of the Geological Society (London) 136, 165–173. Fürsich, F.T., Oschmann, W., 1993. Shell beds as tools in facies analysis: the Jurassic of Kachchh, western India. Journal of the Geological Society (London) 150, 169–185. Hallam, A., 1975. Jurassic Environments. Cambridge University Press, Cambridge. Hallam, A., 1995. Oxygen-restricted facies of the basal Jurassic of north west Europe. Historical Biology 10, 247–257.
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