Jonathan D. Radley

Atmospheric pCO2 and depositional environment from stable-isotope geochemistry of calcrete nodules (Barremian, Lower Cretaceous, Wealden Beds, England)

Abstract: Nodular soil carbonates (calcretes) are present in overbank facies of Lower Cretaceous, non-marine Wealden Beds (Wessex Formation) of southern England. Field evidence suggests that these calcretes formed mostly under semi-arid Mediterranean-type climatic conditions. Typical calcrete fabrics, identified petrographically, include floating detrital grains, corroded grain margins and circumgranular cracks defining peds. Localized alteration of primary micrites is mainly associated with large cracks where early non-ferroan diagenetic cementation and neomorphism was focused. Diagenetic ferroan calcites occur as void fills and yield relatively light carbon-isotope and oxygen-isotope compositions (δ13C= −15.0‰; δ18O= –6.3‰) compared to well-preserved micrite (δ13C= –10.2‰; δ18O= –4.0‰). Precise definition of δ13C values for well-preserved micrites allow estimation of partial pressure of atmospheric CO2 (pCO2) for the early Barremian of 560 ppmV using a published diffusion-reaction model. The data suggest that atmospheric CO2 was low during the mid-Early Cretaceous before rising to a previously defined mid-Cretaceous high. Data from calcretes in the Weald Clay highlight the need for selection of appropriate material and careful evaluation before pCO2 calculations are attempted. The Weald Clay samples come from marshy palaeoenvironments where ingress of atmospheric CO2 into the soil-zone was either reduced or prevented.

Grazing Bioerosion on Oyster Concentrations (Lower and Middle Jurassic, England)

Southern British Jurassic strata were deposited in epicontinental, mainly shallow-marine environments, influenced by eustatically controlled sea-level fluctuation, regional tectonism and a warm, predominantly humid climatic regime (Hallam, 1975; Hallam and Sellwood, 1976). Representing the deepest-water environments, mudstone-dominated formations commonly coarsen up into shelly, bioturbated and/or crossbedded sandstones, limestones and ooidal ironstones indicating shallower, typically nearshore settings (Hallam and Sellwood, 1976). Marginal-marine environments affected by fluctuating salinities are locally indicated by sediments containing euryhaline faunas (Hallam, 1975). Amongst Jurassic shells, oysters commonly preserve original calcitic shell structure and high-resolution surface detail including growth lines, abrasion features and bioerosion traces (Stenzel, 1971). They are therefore of outstanding importance for paleoenvironmental reconstruction. The author conducted a brief study of the surface textures of gryphaeid oyster shells (Gryphaea spp.) in the Lower Jurassic Lias Group of central and western England, revealing previously undocumented invertebrate grazing traces (Radley, 2003). Subsequently, Gryphaea concentrations were sampled across the Lower-Middle Jurassic bathymetric spectrum in central and eastern England, augmented by collections of the euryhaline oyster (Praeexogyra hebridica (Forbes)) that characterizes the marginal-marine Middle Jurassic (Bathonian) Great Oolite Group on the western flank of the London Platform (Hudson and Palmer, 1976; Cox and Sumbler, 2002; Fig. 1). The sampling has confirmed that grazing bioerosion on oyster shells, characterized by partial to total loss of external growth banding, erosion of bioencrusters and shell margins, is widespread within the Lower-Middle Jurassic succession. Rarely, skeletal concentrations were encountered in which intense grazing bioerosion has reduced shells to pebble-like bio-

Discussion on palaeoecology of the Late Triassic extinction event in the SW UKJournal, Vol. 165, 2008, pp. 319–332

Jonathan Radley writes: The Penarth Group (of Late Triassic and possibly ranging to Early Jurassic age) of the southern UK marks a marine transgression and the establishment of a shallow epicontinental seaway (Hallam & El Shaarawy 1982; Warrington & Ivimey-Cook 1992), influenced by regressive–transgressive pulses and characterized by rapid facies changes (Hallam & Wignall 2004; Hesselbo et al . 2004). A well-documented invertebrate macrofauna includes corals, brachiopods, molluscs and echinoderms (Swift & Martill 1999). Facies developments range from storm-influenced, shallow-marine mudrocks (Westbury Formation) upwards into calcareous mudstones, siltstones and essentially fine-grained carbonates (Lilstock Formation) demonstrating extremely shallow-water conditions: wave ripples, storm beds, desiccation and omission surfaces (Swift 1999 a ; Wignall 2001; Hallam & Wignall 2004). The low-diversity macrofauna of the Lilstock Formation has frequently been attributed to salinities deviating from that of normal seawater within shallow-water, partly landlocked settings, isolated from the open sea (Hallam & El Shaarawy 1982; Swift 1995; Allison & Wright 2005). However, widespread occurrences of stenohaline marine macrofossils such as corals and echinoderms (Swift & Martill 1999) suggest that salinity fluctuations were at most slight (Wignall 2001). Mander et al . (2008) have presented high-resolution benthic macrofaunal data, principally from molluscan macrofossils, for significant palaeoecological change within the Penarth Group of two key sections in the SW UK. Their detailed results demonstrate loss of taxonomic richness within the uppermost Westbury Formation–lower Lilstock Formation (Cotham Member) interval (latest Rhaetian), followed by a poorly fossiliferous interval represented by the latest Rhaetian or early Hettangian upper Lilstock Formation (upper Cotham Member–lower Langport Member). They attributed this pattern to an extinction event followed by a post-extinction ‘Dead Zone’; the latter coinciding with the onset of a negative organic carbon-isotope excursion, and an extinction event recorded in the terrestrial palynoflora. The relatively diverse shelly …

The Triassic and Jurassic of Warwickshire: report of a 2002 Weekend Field Meeting

Triassic and Jurassic strata were examined in southern Warwickshire and the county town of Warwick. The succession demonstrates a range of non-marine and marine palaeoenvironments. The field sessions were augmented by a visit to the Warwickshire Museum.

The Middle Jurassic of Warwickshire: Field Meeting of the Warwickshire Geological Conservation Group, 16 September, 2001

An itinerary is described, allowing examination of contrasting Middle Jurassic sediments and landscapes on either side of the Vale of Moreton Axis in southern Warwickshire.