Viséan (Lower Carboniferous) sea-level cycle in carbonate ramp to basinal settings of the Wales-Brabant massif, British Isles

Sedimentary Geology 284–285 (2013) 197–213

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Facies and petrophysical signature of the Tournaisian/Viséan (Lower Carboniferous) sea-level cycle in carbonate ramp to basinal settings of the Wales-Brabant massif, British Isles Ondřej Bábek a, b,⁎, Jiří Kalvoda a, Patrick Cossey c, Daniel Šimíček b, François-Xavier Devuyst d, Simon Hargreaves c a

Department of Geological Sciences, Masaryk University, Kotlářská 2, 61137 Brno, Czech Republic Department of Geology, Palacky University, 17, listopadu 12, 77200 Olomouc, Czech Republic c Faculty of Science, Staffordshire University, College Road, Stoke-on-Trent, Staffordshire, ST4 2DE, United Kingdom d Carmeuse Lime and Stone, Technology Center, 3600 Neville Road, Pittsburgh PA 15225, USA b

a r t i c l e

i n f o

Article history: Received 17 July 2012 Received in revised form 10 December 2012 Accepted 16 December 2012 Available online 23 December 2012 Editor: B. Jones Keywords: Carbonate production Sequence stratigraphy Gamma-ray spectrometry Stratigraphic correlation Lower Carboniferous

a b s t r a c t We studied the relationships between stratigraphic distribution of outcrop spectral gamma-ray, magnetic susceptibility and carbonate facies stacking patterns across the regionally significant transgressive–regressive cycle at the Tournaisian/Viséan boundary (Tn/V, early Carboniferous) in southern Great Britain and Ireland (South Wales, North Staffordshire and Dublin Basin). The Tn/V boundary coincides with a prominent climatic pulse connected with the Late Paleozoic glaciation of Gondwana. The aim was to correlate the gamma-ray and magnetic susceptibility log patterns in carbonate ramp- and basin settings and discuss the global/regional nature and magnitude of this transgressive–regressive cycle. A robust ramp-to-basin correlation was produced based on the log patterns, facies stacking patterns and foraminifer biostratigraphy. The concentrations of K and Th, the “clay” gamma-ray values and, partly, magnetic susceptibility are dependent on facies and show systematic changes along the inferred bathymetric profile from inner ramp to outer ramp and basin. A model of carbonate productivity-driven dilution of fine-grained siliciclastics in CaCO3 as the major control on the petrophysical patterns is discussed. The cleaning-up and cleaning-down petrophysical trends are related to down-dip and up-dip shifts of the carbonate factory with changing relative sea level. In middle-to-outer ramp and basin settings, this generates petrophysical trends just opposite to Paleozoic carbonate shelves where peaks in magnetic susceptibility are known to be associated with peak regressions. A distinct, late Tournaisian to early Viséan regressive-to-transgressive cycle with a prominent sequence boundary located close to the Tn/V stage boundary can be seen in the sections. Glacioeustatic origin of the sequence boundary is inferred from its correlation with Tn/V boundary sections from Europe, carbon isotope data from South China and the glacial deposits in the southern hemisphere mentioned by previous authors. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The depositional profile of carbonate ramps represents a critical factor driving the response of carbonate ramps to relative sea-level changes (Burchette and Wright, 1992; Betzler et al., 1999; Schlager, 2005). The ramp morphology is controlled by the rate of sediment input (carbonate production) and hydrodynamic material dispersal along the bathymetric gradient (Aurell et al., 1998; Pomar, 2001; Wright and Burgess, 2005). Ramps are characterized by relatively low rates of carbonate production, which generally decreases down dip from the inner ramp settings. This pattern can be expressed in mixed carbonate–siliciclastic systems with pure carbonate being ⁎ Corresponding author at: Department of Geology, Palacky University, 17, listopadu 12, 77200 Olomouc, Czech Republic. E-mail address: [email protected] (O. Bábek). 0037-0738/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sedgeo.2012.12.008

deposited on inner ramps and siliciclastic mudstone in outer ramp-to-basin settings. Due to the low depositional relief, relative sea-level changes therefore result in abrupt facies shifts between carbonate-dominated inner ramp facies and mudstone-rich outer ramp and basinal facies, which can produce distinctive signatures on wireline logs (Burchette and Wright, 1992). The main sources of fine-grained siliciclastics into marine depositional systems include atmospheric dust from continents (Hladil et al., 2006; Hladil et al., 2010; Koptíková et al., 2010), plumes of suspended sediment from river deltas and coastal erosion. This transport is by nature diffuse, long-distance and assumed to cover large areas of carbonate sedimentation (McCulloch et al., 2003; Hladil et al., 2006). The proportion of carbonate and fine-grained siliciclastics in ramps is then modulated either by changes in carbonate production and dispersal or by changes in the rate and composition of diffuse siliciclastic influx. Both parameters are closely related to sea-level variations. Petrophysical methods such as gamma-ray

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spectrometry and magnetic susceptibility are widely used as sensitive indicators of carbonate-to-siliciclastic ratio (Ellwood et al., 1999; Rider, 1999; Crick et al., 2001; Ehrenberg and Svana, 2001; Halgedahl et al., 2009; Koptíková et al., 2010). Consequently, they may qualify as excellent proxies of relative sea level, climate and carbonate productivity variations in carbonate ramps. The Lower Carboniferous carbonates of southwest Britain represent classical examples of ancient carbonate ramp depositional systems (Wright, 1986; Burchette and Wright, 1992). Their deep-water counterparts are preserved in central Britain (North Staffordshire Basin) and eastern Ireland (Dublin Basin). All these basins, located on the flanks of the Wales-Brabant massif, were affected by a prominent sea-level fall near the Tournaisian/Viséan (Tn/V) boundary. The facies signal of this sea-level cycle can be correlated between shallow-water- and basinal carbonate settings (Faulkner et al., 1990; Hance et al., 2001). In this paper, we examine facies, the spectral gamma-ray and magnetic susceptibility log patterns in shallow- and deep carbonate ramp settings of central and southern Great Britain and the Dublin Basin. Our aim is to see the petrophysical response and discuss the carbonate productivity variations during a well-documented sea-level fall and rise cycle near the Tn/V boundary. 2. Gamma-ray and magnetic susceptibility proxies in carbonate sediments Spectral gamma-ray and magnetic susceptibility (MS) logs are frequently used as quantitative physical parameters in the stratigraphic analysis of carbonate and mixed carbonate–siliciclastic strata (Crick et al., 1997; Rider, 1999; Thibal et al., 1999; Whalen and Day, 2010). In carbonates, the spectral gamma-ray (concentrations of K, U and Th) and MS data are used as proxies of the dilution of radioactive and magnetic siliciclastic minerals in nonradioactive and diamagnetic calcium carbonate. This assumption is based on the mineralogy of the gamma-ray and magnetic susceptibility carriers, which include common terrigenous minerals. Th and K concentrations usually relate to the presence of illite, kaolinite, sericite, muscovite and potassium feldspars. Uniform ratios between K and Th in a carbonate data set indicate that they are carried by the same source minerals (e.g., illite, kaolinite and other clay minerals), which contain stoichiometric amounts of K in their crystal lattices and tend to adsorb Th on the crystal surface (Rider, 1999; Fiet and Gorin, 2000). Good correlation between K and Th therefore points to a variation in fine-grained siliciclastic admixture in carbonate rocks (Ehrenberg and Svana, 2001; Fabricius et al., 2003; Bábek et al., 2010). Uranium concentrations, in contrast, relate to the presence of organic carbon, calcium phosphates and heavy minerals and/or diagenetic processes such as dolomitization, displaying a very heterogeneous distribution in sediments (Bhattacharya et al., 1997; Rider, 1999; Luczaj and Goldstein, 2000; Lüning et al., 2004). The MS signal in carbonates is thought to be related to detrital paramagnetic grains (clay minerals, micas) (Ellwood et al., 1999; Crick et al., 2001) but authigenic grains may represent important carriers of at least part of the MS signal (Bloemendal et al., 1992; Schneider et al., 2004). The concept of siliciclastic input to carbonate systems was studied especially for MS signals and flat-top carbonate platforms. In such settings, sea-level fall results in decelerated, or even the cessation of, carbonate production and increased eolian/fluvial influx of siliciclastics, which causes positive excursions on MS logs (Hladil et al., 2006; da Silva et al., 2009; Boulvain et al., 2010). Many published studies relate the maxima and minima in MS logs in shallow-water carbonate platform successions to regressions and transgressions, respectively (Ellwood et al., 1999; Zhang et al., 2000; Mabille et al., 2008; Whalen and Day, 2010). On carbonate ramps, however, a sea-level fall usually results in a shift, rather than complete cessation, of the carbonate factory downslope while the depositional profile will be maintained (Wright, 1986; Wright and Burchette, 1996; Bosence

and Wilson, 2002). This mechanism can result in increased carbonate production in down-dip areas and, hence, negative excursions on magnetic susceptibility and spectral gamma-ray logs. In contrast to the common interpretation of magnetic susceptibility logs in carbonate platforms, transgressions and marine flooding may result in opposite petrophysical trends in carbonate ramps (Bábek et al., 2010; Koptíková et al., 2010). Analysis of facies trends coupled with gamma-ray and magnetic susceptibility curves can therefore provide a suitable methodology to assess the role of carbonate production and sea-level history in carbonate ramp systems. 3. Geological setting and stratigraphy 3.1. Geological setting and lithostratigraphy The studied sections are located on the sides of the Wales–Brabant High, a Caledonian structural high, which formed a part of the former East Avalonia microcontinent during the upper Palaeozoic Variscan orogeny (Bless et al., 1980; Ziegler, 1982; Krawczyk et al., 2008) (Fig. 1). On the northern side of the Wales Brabant High, these include Brown End Quarry, Ladyside Wood and Ossom's Hill sections in the North Staffordshire Basin. On its south side, they include Portishead and Three Cliffs Bay on the South Wales–Mendip Shelf. The Rush section is located in the Dublin Basin on the north-western side of the Wales–Brabant High. In the late Tournaisian–early Viséan, the South Wales–Mendip Shelf formed a southerly dipping carbonate ramp (Wright, 1986). The Portishead section, located near Bristol comprises mid ramp Black Rock Limestone and the overlying Black Rock Dolomite. Above this, two prominent paleosols are separated by peloidal packstones and grainstones of the Sub-Oolite Bed. The upper paleosol horizon is overlain by the Gully Oolite, a shallowing upward unit representing subtidal to foreshore environment (Faulkner et al., 1990). The Three Cliffs Bay section (Wright, 1986, 1987) starts in mid- to outer-ramp carbonates (Tears Point Limestone), which is dolomitized in the upper part (Langland dolomite) (Fig. 2). The overlying Caswell Bay

Fig. 1. Location of the studied sections on the flanks of the Wales–Brabant High (modified after Devuyst, 2006).

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Oolite is separated from the dolomites by a sharp basal surface with shell lags, which is interpreted as basal surface of forced regression by Adams et al. (2004). Regarded as the lateral equivalent of the Gully Oolite in the Bristol area (Waters et al., 2009), the Caswell Bay Oolite consists of two, prograding and shallowing-upward shoreface to foreshore units. They are both capped by surfaces with karstic morphology, vadose cements and relics of paleosols indicating subaerial exposure (Wright, 1986; Ramsay, 1987; Burchette et al., 1990). Above this are argillaceous and peloidal limestones and dolostones of the Caswell Bay Mudstone (peritidal deposits), capped by a sharp erosion surface at the base of the High Tor Limestone. (George et al., 1976; Riding and Wright, 1981; Wright, 1986; Ramsay, 1987; Searl, 1989; Adams et al., 2004). The base of the High Tor Limestone is assumed to reflect a prominent transgressive event (ravinement surface) (Riding and Wright, 1981; Wright, 1986). The sections in the North Staffordshire Basin are synrift half-graben deposits (Lee, 1988) dominated by outer ramp deposits of the Milldale Formation (Fig. 2) comprising dark gray cherty micrites, crinoidal calciturbidites, Waulsortian mud-mounds and inter-reef facies that are locally dolomitized (Courceyan–Chadian; Tournaisian–to early Viséan). The Waulsortian carbonate mud-mounds were deposited in a gently sloping carbonate ramp environment (Smith et al., 1985) at estimated water depths of 220 up to 300 m (Lees and Miller, 1985; Cossey and Adams, 2004). These are overlain by gray coarsely bioclastic conglomeratic limestones and thick bedded peloidal and crinoidal bioclastic limestone of the Hopedale and Ecton Limestone Formation (Arundian to Asbian; Viséan) (Gawthorpe et al., 1989; Cossey and Adams, 2004; Waters et al., 2009). The Hopedale and Ecton Limestone are bound by basal erosional surface at some localities (Brown End Quarry; Cossey et al., 1995; Cossey and Adams, 2004).

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The Brown End Quarry section is located close to the village of Waterhouses, Staffordshire, central England. The Ladyside Wood and Ossom's Hill sections are located on the flanks of the River Manifold Valley near Waterhouses, Staffordshire, central England. The Dublin Basin (Figs. 1, 2) is a fault-bound basin, which was reactivated during the Lower Carboniferous (Phillips and Sevastopulo, 1986; Nolan, 1989; Pickard et al., 1994; Strogen et al., 1996). Rapidly subsiding, basinal parts of the Dublin Basin recorded an overall deepening trend, culminating with the Waulsortian type carbonate mounds and deep water calcareous mudrock with rare horizons of impure limestone (Tober Colleen Formation, upper Tournaisian). It is overlain by a thick succession of mixed carbonate–siliciclastic turbidites, shales and debris-flow breccias of the Rush Formation (uppermost Tournaisian– lower Viséan). The Rush section is located south of the Rush Harbour, approximately 23 km north NE of Dublin. Detailed stratigraphy and sedimentology of the section are described elsewhere (Nolan, 1989; Devuyst, 2006; Bábek et al., 2010). 3.2. Biostratigraphy and age control Biostratigraphy of the Tn–V boundary interval (Fig. 2) at the studied sections is based on the zonation of Devuyst and Hance (in Poty et al., 2006) and papers by Devuyst and Kalvoda (2007) and Kalvoda et al. (2011, 2012). The Tn/V boundary is indicated by Eoparastaffella ovalis M2, Eoparastaffella tummida and Eoparastaffella asymmetrica followed by Eoparastaffella simplex in the top part of the Caswell Bay Oolite (Figs. 2 and 3) in the Three Cliffs Bay section (Kalvoda et al., 2012). No important index foraminifers were found at Portishead but the latest Tournaisian age is indicated by the brachiopod Levitusia humerosa in the Sub-Oolite Bed (Ramsbottom, 1973). Based on lithostratigraphic

Fig. 2. Lithostratigraphic, chronostratigraphic and biostratigraphic framework of the South Wales–Mendip Shelf, North Staffordshire Basin and Dublin Basin at the Tournaisian– Viséan interval.

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correlation with its dated counterparts across the South Wales, the overlying Gully Oolite at Portishead is of early Viséan age at least at its top (Waters et al., 2009). Consequently, the Sub-Oolite Bed in Portishead can be regarded as a late Tournaisian age-equivalent of the lower Caswell Bay Oolite in Three Cliffs Bay below the lower exposure surface (Figs. 3 and 4). In the North Staffordshire Basin, the Tn/V boundary is indicated by E. cf. simplex 30 m below section top in Brown End Quarry and E. tummida 16.5 m above the base of the section in Ladyside Wood (Figs. 2, 5, and /INS; 6). Although not precisely indicated, the base of the Ossom's Hill section is located very close to the Tn–V boundary, as indicated by the occurrence of E. asymmetrica

and E. ovalis M2. At the Rush section, Dublin Basin, the Tn–V boundary is indicated by Eoparastaffella cf. simplex (~ 112 m below section top, Fig. 7). The higher Viséan MFZ 10 zone (base of the Arundian stage in Britain) is indicated by first appearance datum of Ammarchaediscus in the Ossoms's Hill section (36.2 m, Fig. 6). No archaeodiscids to indicate the MFZ 10 zone were found in the Ladyside Wood, Brown End Quarry and Rush sections. However, from lithostratigraphic correlation it is suggested that the base of the Arundian is located slightly above the base of the Ecton Formation in the Ladyside Wood section. Higher parts of the MFZ 10 or even lower MFZ 11 are already

Fig. 3. Lithostratigraphy, facies stacking patterns, important biostratigraphic levels (FAD, first appearance data), petrophysical logs (concentrations of K and Th, U/Th ratio, MS), EDXRF element concentrations of K and Fe and sequence stratigraphic interpretation of carbonate ramp succession at Three Cliffs Bay, South Wales–Mendip Shelf (WGS-84: 51°34′14.5″N; 4°7′0.1″W). For facies scheme refer to Table 1. Foraminifer zones after Devuyst and Hance (in Poty et al., 2006).

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Fig. 4. Lithostratigraphy, facies stacking patterns, important biostratigraphic levels (FAD, first appearance datums), petrophysical logs (concentrations of K and Th, U/Th ratio, MS) and sequence stratigraphic interpretation of carbonate ramp succession at Portishead, South Wales – Mendip Shelf (WGS-84: 51°28′8.2″N; 2°47′28.7″W). For facies scheme refer to Table 1. Foraminifer zones after Devuyst and Hance (in Poty et al., 2006).

indicated by the Glomodiscus/Uralodiscus foraminifers in the High Tor Limestone (Fig. 3) at the Three Cliffs Bay section (Kalvoda et al., 2012). The underlying Caswell Bay Mudstone contained only poorly preserved foraminiferal fauna, which did not allow the distinction between MFZ9 and 10 but, traditionally, the Caswell Bay Mudstone is regarded as Arundian (MFZ 10) in age (Riding and Wright, 1981; Wright, 1986; Waters et al., 2009).

4. Material and methods Six sections (measuring 47 to 192 m in thickness) were described, logged and sampled for foraminifer biostratigraphy and thin sectionanalysis. Field gamma-ray spectrometry was measured using an RS-230 Super Spec portable spectrometer (Radiation Solutions, Inc., Canada) with a 2×2″ (103 ccm) bismuth-germanate (BGO) scintillation detector. One measurement with a 180-s count time was performed at each logging point. Time-dependent measurements showed that the concentrations of K, U and Th sufficiently stabilized after 120 to 150 s. For more information about the instrument specifications and field procedure, refer to Šimíček et al. (2012). The sections were logged with 0.25 to 1.0 m thickness interval depending on the total section thickness and average bed thickness. 866 spectral gamma-ray data points were measured. Counts per second in selected energy windows were converted to concentrations of K (%), U (ppm) and Th (ppm) automatically by the instrument, based on calibrations carried out by the manufacturer.

“Clay” gamma-ray values, sometimes used as improved clay volume indicators, were calculated from the spectral values using the formula (Rider, 1999) (1): “clay”gamma ray ½API ¼ Th½ppm  3:93 þ K½%   16:32:

ð1Þ

Fresh rock samples for bulk magnetic susceptibility (MS), weighing from ~6 to ~55 g per sample, were collected at 0.25 to 1.0 m thickness intervals depending on total section and bed thickness. MS was measured using KLY-4 kappabridge (Agico, Czech Republic; magnetic field intensity of 300 Am−1, operating frequency of 920 Hz, sensitivity of 4 · 10−8 SI). The error of measurement did not exceed ±2%. Mass-specific MS data expressed in m 3 kg−1 were used. 826 MS samples were measured in total. To provide a better insight into the relationship between the spectral gamma-ray and MS signatures and the geochemistry of the carbonates, we measured the EDXRF concentrations of selected elements including Ca, Fe and the typical terrigenous elements (K, Rb, and Zr). Element concentration data were measured from flat, fresh rock surface of the MS samples by hand-held, energy-dispersive X-ray fluorescence (EDXRF) instrument Delta Premium (Innov-X, U.S.A.) equipped with large-area SDD (Silicon-Drift Detector), with 120 s and 120 s acquisition time (beam) at 15 kV and 40 kV accelerating voltage, respectively. The error of measurement indicated by the machine ranged between ± 5 and ±19 ppm for Fe; ±38 and ±101 ppm for K; ±0.1 and ±0.5% for Ca and ±0.3 and ±0.5 ppm for Rb.

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Fig. 5. Lithostratigraphy, facies stacking patterns, important biostratigraphic levels, petrophysical logs (concentrations of K and Th, U/Th ratio, MS), EDXRF element concentrations of K and Fe and sequence stratigraphic interpretation of the outer-ramp to basinal carbonate succession at Brown End section (WGS-84: 53°2´59.4”N; 1°52´1.6”W), North Staffordshire Basin. For facies scheme refer to Table 1. Foraminifer zones after Devuyst and Hance (in Poty et al., 2006).

5. Results and discussion 5.1. Facies The inner- and middle ramp carbonates of the Three Cliffs Bay and Portishead sections (South Wales–Mendip Shelf) can be subdivided into seven facies types (F1 to F7; Table 1). They are interpreted as paleosols, peritidal deposits of back-barrier lagoons, upper-shoreface oolitic sands through to subtidal offshore deposits and storm deposits (cf. Wright, 1986; Faulkner et al., 1990; Adams et al., 2004). In the Portishead section, sedimentary structures are poorly visible in the equivalent marine bioclastic facies (Black Rock Limestone) partly due to extensive dolomitization (massive, slightly argillaceous, reddish-brown, saccharoidal dolomites with relics of partly dolomitized, crinoidal packstone). The outer-ramp and basinal carbonates of the North Staffordshire Basin (Brown End, Ladyside Wood and Ossom's Hill sections) can be subdivided into five facies (F8 to F12; Table 1). They are interpreted as high-density turbidites, sandy debris flows, distal carbonate turbidites, bottom current deposits, micritic suspension deposits and Waulsortiantype mud-mounds (cf. Aitkenhead et al., 1985; Cossey and Adams, 2004; Bábek et al., 2010). Sedimentary rocks at the Rush section, Dublin Basin, can be subdivided into six facies (RF1 to RF6, Table 2). They are interpreted as turbidity current deposits (RF2 to RF5), hemipelagic, calcareous, silty shales (RF6) and debris-flow breccias (RF1) (cf. Sevastopulo and Wyse Jackson, 2008; Bábek et al., 2010).

The facies are stacked into distinct transgressive (retrogradation) and regressive (progradation) stratigraphic packets in all measured sections (see Sections 5.4 and 5.5). The most prominent examples of progradational patterns are visible in the Three Cliffs Bay section between ~40 and 129 m (Fig. 3), in Portishead between ~40 and 0 m (Fig. 4) and in Rush between ~150 and ~92 m (Fig. 7). Examples of retrogradational facies stacking patterns can be seen in Three Cliffs Bay, between ~2.5 and ~20 m (Fig. 3), in Brown End between ~25 and ~11 m (Fig. 5), in Ladyside Wood between ~27 and ~53 m, in Ossom's Hill between ~10 and ~30 m (Fig. 6) and in Rush between ~62 and ~8 m (Fig. 7). 5.2. Spectral gamma-ray and magnetic susceptibility logs: proxies of siliciclastic input in carbonates The concentrations of Th are highly correlated with K (linear regression coefficient, R 2 = 0.925) but not with the concentrations of U (Fig. 8). This indicates that the typically terrigenous elements (K and Th) vary proportionally to each other, this variability being driven by their dilution in CaCO3. The EDXRF concentrations of K and Rb (Three Cliffs Bay and Brown End sections) are highly correlated (R2 = 0.901 to 0.962) whereas the correlation of these two elements with Zr and Fe is less significant (Fig. 9). K, Rb, Zr and Fe are all negatively correlated with Ca, particularly in the Brown End section (R2 = 0.92, 0.9, 0.85 and 0.67 for K, Rb, Zr and Fe, respectively). At the Three Cliffs

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Fig. 6. Lithostratigraphy, facies stacking patterns, important biostratigraphic levels, petrophysical logs (concentrations of K and Th, U/Th ratio, MS) and sequence stratigraphic interpretation of carbonate ramp succession at Ossom´s Hill (WGS-84: 53°5´49.5”N; 1°51´40.4”W) and Ladyside Wood sections (WGS-84: 53°5´24.1”N; 1°51´34.4”W), North Staffordshire Basin. For facies scheme refer to Table 1. Foraminifer zones after Devuyst and Hance (in Poty et al., 2006).

Bay section, the correlation is low or absent (R2 = 0.11, 0.14, 0.34 and 0.41 for K, Rb, Zr and Fe, respectively), possibly due to replacement of Ca by Mg during dolomitization. The EDXRF concentrations of K show positive correlation with the gamma-ray spectrometric concentrations of K and even slightly better correlation with the “clay” gamma-ray index (Fig. 10). The correlation is not high (R2 = 0.407 to 0.681) but this can be expected if we consider the different volume of a gamma-ray (several cubic decimetres) and EDXRF (several cubic centimetres) sample and the heterogeneous distribution of siliciclastic admixture in carbonates. To the contrary, the EDXRF concentrations of K and Fe show variable correlation with magnetic susceptibility (R2 = 0.116 to 0.751; Fig. 10). Compared to the gamma-ray spectrometry,

this correlation should be much better as identical samples were used for both the methods, but, surprisingly, Fe concentrations failed to correlate with MS at the Three Cliffs Bay section (R2 = 0.116). Our spectral gamma-ray data are consistent with the common interpretation of K and Th in wireline logs as the “shale content” or “shaliness” indicator in carbonates (Rider, 1999; Ehrenberg and Svana, 2001). The K/Th ratios in the carbonates of the South Wales (mean 2.48 × 10 3, standard deviation/σ/: 1.77 × 10 3) are similar to those in the deep-water carbonates of the North Staffordshire Basin (mean 2.86 × 10 3, σ: 0.92 × 10 3) and the Dublin Basin (mean: 2.95 × 10 3, σ: 0.53 × 10 3). This suggests that the three areas were supplied from a similar siliciclastic source located in the Wales–Brabant

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Fig. 7. Lithostratigraphy, facies stacking patterns, important biostratigraphic levels (FAD, first appearance datums),** petrophysical logs (concentrations of K and Th, U/Th ratio, MS) and sequence stratigraphic interpretation of the mixed carbonate – siliciclastic turbidite succession in Rush (WGS-84: 53°31´21˝ N; 6°04´44˝ W), Dublin Basin. For facies scheme refer to Table 2. Foraminifer zones after Devuyst and Hance (in Poty et al., 2006).

High. The K/Th ratios in both basins plot within the illite field in the K: Th crossplot of Rider (1999). This suggests that the terrigenous supply was dominated by clay minerals, which are usually dispersed by suspended-load transport. MS in our data set can also be regarded a proxy of siliciclastic admixture in carbonate rocks, consistently with the interpretation of previous authors (Ellwood et al., 1999; Hladil et al., 2006; da Silva et al., 2009). However, the problems with the correlation with EDXRF data (see above) indicate that MS has significant limits. In addition, there is a poor correspondence between the gamma-ray and MS data while the maximum MS values coincide with pure carbonate facies with very low “clay” gamma-ray values (Fig. 8; section Portishead). Partly, this is a function of the sampling techniques (sample volume measured by the gamma-ray spectrometry is several orders of magnitude higher than with the MS, see above). However, factors other than the carbonate-to-siliciclastic ratio (Ellwood et al., 1999; Crick et al., 2001) such as the diagenetic phase transformations between iron-bearing magnetic carriers may also influence the MS signals. In particular, authigenic magnetic grains may influence the total MS values (Bloemendal et al., 1992; Schneider et al., 2004). Notwithstanding the source of this discrepancy between the gamma-ray spectra and MS interpretation, the former provides a much better proxy of the fine-grained siliciclastic admixture than the latter, at least in our data set.

5.3. Facies dependence of spectral gamma-ray and magnetic susceptibility signals and carbonate productivity The spectral gamma-ray and MS signals are facies-dependent and there is a distinct gradient in their values between shallow-waterand deep-water facies (Fig. 11, Tables 1 and 2). The beach-ridge oolitic sands (F3) and inner- and middle ramp subtidal facies (F4 and F5) have the lowest K and Th concentrations. The K and Th concentrations progressively increase towards deeper mid-ramp and outer ramp facies (F6 and F7) and even further to the basinal turbidites (F8, F9, and F10) and hemipelagic facies (F11). Up-dip from the oolite (F3, F4, and F5, cf. Wright, 1986) the K and Th concentration increase rapidly towards the restricted lagoonal facies (F2a and F2b). K and Th concentrations in paleosols are higher than in inner- and middle ramp facies but lower than in the peritidal facies. The mudmound facies have K and Th concentrations distinctly lower than the adjacent basinal facies of the North Staffordshire Basin. With the exception of a thin conglomerate horizon (Caldon Low Conglomerate) located on top of the Milldale Limestone formation at some localities (Chisholm et al., 1988) the South Wales–Mendip Shelf and North Staffordshire Basin are predominantly carbonate depositional systems. No siliciclastic wedges or significant admixture of detrital sand grains was reported from their upper Tournaisian to lowermost Viséan successions. This suggests that

Table 1 Upper Tournaisian to lower Viséan (Chadian to Arundian) carbonate facies of the South Wales–Mendip Shelf (F1 to F7) and the North Staffordshire Basin (F8 to F12) and their selected petrophysical characteristics. Modified after Ramsbottom (1973), Riding and Wright (1981), Wu (1982), Aitkenhead et al. (1985), Wright (1986), Gawthorpe (1989), Faulkner et al. (1990) and Cossey et al. (1995). Bed thickness/geometry

Sedimentary structures

F1

0.5 to ~3.2 m

F2a

5 to 60 cm

F2b

5 to 120 cm

F3

2 to 16 m

Rhizocretions, nodular structure, clay fissures, columnar calcite cements, spherulites, fascicular microspar, speleothems?, bioturbation Mudstone to calcisilitite/lime Calcispheres, ostracods, forams, Algal lamination, ripple-cross lamimudstone, wackestone dasycladacean algae, peloids, ooids nation, fenestral structures, gypsum pseudomorphs Wavy lamination, parallel lamination Calcisilitite, f-grained calcarenite/lime Calcispheres, forams, crinods, mudstone, wackestone, packstone dasycladacean algae, peloids, ooids Cross bedding, stylolites m-to-c-grained calcarenite/grainstone, Ooids, peloids, crinoids, brachiopods, packstones echinoid spines

F4

30 to 75 cm/amalgamated

F5

35 to 90 cm

F6

1.2 to 3.0 m/massive

F7

5 to 30 cm/amalgamated

F8

25 to 380 cm

F9

5 to 35 cm/locally amalgamated

F10

3 to 60 cm

F11

2 to 40 cm

Massive, chert nodules

F12

~2 m and more

Massive, geopetal cavity fills, stromatactis

Trough cross bedding, cross-lamination, shell lags, in places structureless Bedding-parallel grain alignment, sometimes faintly graded Stylolites, bioturbation

Wavy lamination, HCS, graded bedding Parallel lamination, cross lamination, rarely graded bedding, erosive bases Graded bedding, parallel lamination, ripple-cross lamination, Zoophycos trace fossils, scours, chert nodules Massive, rare parallel lamination, chert nodules

Grain size/Dunham classification

Allochems

Depositional process/depositional setting

“Clay” gamma-ray (API) (mean/σ)

Magnetic susceptibility (10−7 m3kg−1) (mean/σ)

Paleosols

12.9/3.9

−0.1/2.2

Peritidal deposits

36.5/8.3

63.8/51.1

Peritidal deposits

15.3/2.8

65.6/31.3

Oolitic beach ridge plain deposits/ above wave base up to upper shoreface environment Subtidal deposits, below/close to normal wave base

3.1/2.4

−1.4/3.1

6.2/3.0

3.7/3.8

3.0/2.6

7.9/0.63

8.2/4.1

5.5/4.7

10.1/2.6

7.6/3.9

m-to-c-grained calcarenite/packstone, wackestone

Rugose corals, brachiopods, crinoids

Calcirudite, breccia/rudstone, floatstone c-grained calcarenite to f-grained calcirudite/packstone, grainstone

Brachiopods, bivalves, gastropods, corals Crinoids, productid brachiopods, bryozoans, dasycladacean algae, rugose corals, forams Crinoids, brachiopods,

Storm-winnowed lags/above storm wave base Inner-to mid-ramp, storm-dominated, above storm wave-base Storm beds/near storm wave base

Echinoderms, corals and brachiopods, peloids

High-density carbonate turbidites, sandy debris flow deposits

18.7/9.3

4.8/10.8

Echinoderms, corals and brachiopods, trilobites, ostracods, peloids

Carbonate turbidites

24.5/9.8

4.9/6.7

Echinoderms, sponge spicules, trilobites, ostracods, peloids

Distal carbonate turbidites, bottom current-reworked outer ramp deposits Suspension deposits

29.8/9.7

9.4/1.3

35.9/7.1

4.4/0.7

Deep water „knoll reefs“ (Waulsortian mudmounds)

9.3/1.2

1.8/1.4

m-to-c-grained calcarenite/ wackestone, packstones vc-grained calcarenite to f-grained calcirudite/packstone, floatstone, rudstone c-grained calcarenite, calcirudite/ packstone, wackestone Calcilutite, calcisiltite, f-grained calcarenite/lime mudstone, wackestone Mudstone, marls Calcilutite/lime mudstone, wackestone

Echinoderms, sponge spicules, ostracods,peloids Brachiopods, fenestrate bryozoans, crinoids, ammonoids, trilobites, nautiloids, forams, spicules, ostracodes

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Facies code

205

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Table 2 Upper Tournaisian to lower Viséan (Chadian to Arundian) facies of the Rush section, Dublin Basin, and their selected petrophysical characteristics. Facies Bed thickness/ code geometry

RF1

RF2

RF3

Sedimentary structures

~2.5 to ~5.5 m Inverse grading at base, ± normally graded top, projecting clasts on top of beds, sometimes turbidite caps Normal graded bedding, ~0.5 to projecting clasts on top of beds, ~1.4 m, outsized clasts soft-sediment lens-like deformation geometry

Normal graded bedding, wavy lamination, basal scours, rip-up shale lithoclasts, outsized clasts, soft-sediment deformation Normal graded bedding, ripple-cross lamination, parallel lamination Massive, gradational bases and tops

RF4

~0.2 to ~1 m, lens-like to sheet-like geometry ~2 to ~25 cm

RF5

~8 to ~25 cm

RF6

Tens of meters Massive

Grain size/Dunham classification

Allochems

Depositional process/ depositional setting

“Clay” gamma-ray (API) (mean/σ)

Magnetic susceptibility (10−7 m3kg−1) (mean/σ)

Coarse rudite; limestone clasts of boulder size, muddy-to-silty matrix

Crinoids, bryozoans, intraclasts, quartz, feldspars, lithic grains/corals, foraminifers, brachiopods Crinoids, bryozoans, intraclasts, quartz, feldspars, micas, lithic grains/corals, foraminifers, calcispheres, brachiopods, peloids, ooids

Debris-flow breccia

43.7/11.1

97.9/47.3

High-density 47.3/14.6 turbidity current deposits, sandy debris-flows Turbidites 54.7/17.3

97.6/32.7

Turbidites

73.0/17.6

109.8/34.8

Turbidites (?)

75.0/18.0

131.2/47.9

Suspension deposits

65.8/16.0

97.0/31.6

Rudite, clast-supported, arenitic matrix/grainstone, packstone

c-grained arenite to f-grained rudite/ grainstone, packstones m-to-c-grained arenite/ grainstone, packstone f- to c-grained calcarenite/ grainstone

Siltstone-mudstone

siliciclastic detrital supply was low and presumably limited to suspended load of clay and fine silt material dispersed in water column while the bed-load siliciclastic transport was strongly suppressed. This is supported by the uniform K/Th ratios, which indicate illite as the predominant carrier of the spectral gamma-ray signal. Consequently, the increase in K and Th concentrations and the “clay” gamma-ray values from shallow-water to deep-water facies (Fig. 11) is related to the decreasing ratio of carbonate to fine-grained siliciclastic (clays). The carbonate-to-siliciclastic ratio is controlled by dilution of terrigenous siliciclastics by marine calcium carbonate (cf. Rider, 1999; Ehrenberg and Svana, 2001). We conclude that the down-dip decreasing carbonate production drives the CaCO3 dilution rather than the changes in siliciclastic supply along the bathymetric profile. The “clay” gamma-ray signal can be therefore regarded a suitable proxy of carbonate productivity and dispersal along the ramp depositional profile (Fig. 12). The much lower K and Th signal in the

Crinoids, bryozoans, dasycladacean algae, intraclasts, quartz, feldspars, micas, lithic grains/corals, foraminifers, calcispheres, brachiopods, peloids, ooids Crinoids, bryozoans, foraminifers, intraclasts/ dasycladacean algae, ostracods, calcispheres, brachiopods, peloids, ooids, quartz N/A

96.2/30.8

mud-mound facies (Fig. 11), compared to the adjacent basinal facies, can be explained by enhanced production of carbonate by the mud-mound factory (cf. Schlager, 2003, 2005), which dilutes the suspended terrigenous clay fraction in the carbonate fraction. We assume that this dilution effect is likely to have been accentuated because of upward mudmound growth. Uranium concentrations show different patterns to those of the K and Th (Fig. 11). Uranium tends to increase from the oolitic sands towards the restricted lagoonal facies as well as towards the basinal facies, being presumably carried by the siliciclastic admixture. Interestingly, however, the highest U concentrations were recorded in the subtidal mid-ramp and outer ramp facies (F4 to F6, Three Cliffs Bay, partly dolomitized) and in the dolomites at the Portishead section, which all have low K and Th contents. Uranium is known to be highly mobile during diagenesis being associated with groundwater movement and diagenesis of organic matter under oil-window conditions

Fig. 8. Bivariate plots of K, U, Th concentrations, “clay” gamma-ray and mass-specific magnetic susceptibility (MS) values from the measured sections. Note the good correspondence between K and Th in all sections and a lack of covariance between “clay” gamma-ray and MS values.

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Fig. 9. Bivariate plots of EDX-ray fluorescence concentrations of K, Rb, Zr and Fe from the Three Cliffs Bay (white triangles) and Brown End (gray dots) sections. Note the good covariance between terrigeneous elements (K and Rb).

(Cuney and Mathieu, 2000; Mossman et al., 2005). Such diagenetic migration can lead to precipitation of U at lithological (fluid-flow) barriers such as subaerial exposure surfaces (Kotzer and Kyser, 1995; Ehrenberg and Svana, 2001). Dolomitization of shallow-water carbonates can also be associated with a significant increase (up to 12 to 29 ppm) in U concentrations (Bhattacharya et al., 1997; Luczaj and Goldstein, 2000). Uranium, at any rate, shows less predictable facies dependence patterns than the K and Th. Likewise, the mudmound facies, which has low K and Th values, has, conversely, markedly high uranium concentrations that parallel those of the most distal outer ramp and basinal facies.

The magnetic susceptibility data reveal less systematic variations between proximal and distal ramp facies (Fig. 11). The restricted lagoonal facies and the oolitic sands have the highest and the lowest MS values, respectively. However, the MS values fail to show the gradient from the subtidal middle ramp to basinal facies (F5 to F11) which is clearly visible on the K and Th plots. In addition, the MS values in the paleosols (F1) are very low as compared to the K and Th concentrations. Unlike the “clay” gamma-ray signal, MS cannot be regarded a particularly good proxy of carbonate/siliciclastic ratio in carbonate ramp depositional settings.

Fig. 10. Bivariate plots of gamma-ray data (concentrations of K and “clay” gamma-ray), mass-specific magnetic susceptibility (MS) and element concentrations measured by EDXRF. Note lack of covariance between Fe measured by EDXRF and MS values from the Three Cliffs Bay section.

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Fig. 11. Distribution of spectral gamma-ray (K, U and Th concentrations) and magnetic susceptibility data in carbonate facies along proximal-to-distal carbonate ramp to basin bathymetric profile. Data are aggregated from all sampling points are represented as sample means +/− standard deviation (σ). For facies scheme refer to text and Tables 1 and 2.

5.4. Petrophysical log correlation and sequence stratigraphy of the South Wales–Mendip Shelf A prominent, late Tournaisian to basal Viséan, transgressiveto-regressive (T-R) cycle spanning the Brofiscin Oolite, Tears Point Limestone and Caswell Bay Oolite can be seen at the Three Cliffs Bay section both in the facies stacking patterns (cf. Wright, 1986, 1987) as well as in the K, Th and MS trends (Fig. 3). In the lower part of the section (~2.5 to ~ 20 m), the gamma-ray and MS values gradually increase, which appears to be consistently with a deepening-upward facies trend (transgressive systems tract, TST). High K, Th and MS values are reached between ~13 m (Th peak) and ~22 m (K peak) above the

section base (lower parts of Tears Point Limestone). This interval, which coincides with a tempestite succession deposited close to storm-wave base is regarded as a maximum flooding interval (Goldhammer et al., 1990; Montanez and Osleger, 1993). There is another distinct MS and gamma-ray peak at ~46 m, but this is superimposed on a decreasing K and Th (cleaning-up) trend (see below). The K, Th and MS values then systematically decrease up-section towards the Tn–V boundary (126.6 m) and above this, there are extremely low values between ~105 m and ~129 m (Caswell Bay Oolite). These log patterns coincide with the regressive succession from subtidal tempestites (upper parts of the Tears Point Limestone) into the oolitic grainstones (Caswell Bay Oolite). Two subaerial exposure surfaces are associated with this

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the paleosol beds between 16 and 21 m, which contain K and Th in higher concentrations. Based on its lithology and the K and Th log shapes, this succession can be correlated with the regressive succession (HST to FSST) from the Tears Point Limestone to the Caswell Bay Oolite at the Three Cliffs Bay (Figs. 3, 13). Age constraints for this correlation are provided by the brachiopod Levitusia humerosa in the Sub-Oolite Bed (Ramsbottom, 1973). The K and Th concentrations show an increasing upward trend above the Gully Oolite in Portishead, which, as in the Three Cliffs Bay succession, may indicate a return to transgression. However, the extensive dolomitic recrystallization, poor facies control and the lack of biostratigraphic data in the top parts of the Portishead section make such an interpretation speculative. 5.5. Petrophysical log correlation and sequence stratigraphy of the North Staffordshire and Dublin Basin

Fig. 12. Simplified model showing the relationship between “clay” gamma-ray values, detrital supply from continent and carbonate production and dispersal along the ramp bathymetric profile. “Clay” gamma-ray data are aggregated from all sampling points are represented as sample means +/− standard deviation (σ). The “clay” gamma-ray data can be used as a proxy of the depth-dependence of the rate of carbonate accumulation (see text). Mississippian relative production curve adopted from Tucker and Wright, 1990. For facies scheme refer to Table 1.

regressive succession, one located within the Caswell Bay Oolite and the other at its top (Wright, 1986). This regressive succession, which is interpreted as highstand systems tract (HST) to falling-stage systems tract (FSST), is capped by sequence boundary at 129 m (earliest Viséan). The base of the Caswell Bay Oolite, associated with shell-lag deposits at ~90 m can represent a basal surface of forced regression (sensu Hunt and Tucker, 1992; cf. Adams et al., 2004). Several higher-order K, Th and MS cycles are superimposed on this regressive trend, which, alongside with the long-term gamma-ray, MS and facies trends, are interpreted as higher order T–R cycles (parasequences). The K, Th and MS values quickly increase above the sequence boundary (Caswell Bay Mudstone, 129 to 135 m) reflecting a facies shift from the oolitic sands into peritidal lagoonal facies. However, fully open-marine conditions, indicating renewed sea-level rise, were restored as high as at the base of the High Tor Limestone at ~135 m and this is indicated by the shift from peritidal to subtidal facies (transgressive surface). This boundary corresponds to a significant drop in K, Th and MS values back to levels comparable with the Tournaisian subtidal marine carbonates. Foraminifer biostratigraphy of the base of the High Tor Limestone (FAD of Glomodiscus/Uralodiscus indicating the higher part of the MFZ 10 to MFZ 11, Arundian) and the inferred Arundian (MFZ 10) age of the Caswell Bay Mudstone suggest that the sequence boundary at its base corresponds to a considerable timespan encompassing almost the whole MFZ 9. The Caswell Bay Mudstone is therefore interpreted as a depositional pulse deposited during a long-term, low-order sea-level lowstand (LST). A similar regressive trend in the uppermost Tournaisian ranging from the subtidal facies of the Black Rock Limestone and Dolomite to the Gully Oolite, and interrupted by two paleosol horizons, is recognisable in the Portishead section, between ~40 and 0 m (Fig. 4). The K, Th and MS values show a gradual decrease in this interval, with the exception of

The Brown End Quarry section represents a succession with good biostratigraphic control and clear petrophysical log patterns. The lower portion of the Brown End Quarry section (Fig. 5) shows a sequence of bioclastic turbidites (calcarenites to calcirudites) alternating with gray to reddish marls (65 to 47 m). The overlying succession consists of calcisiltites, fine-grained calcarenites with thin marl interlayers and scarce graded crinoidal calciturbidites (F10, F9; 47 to 30 m), which are capped by massive beds of coarse-grained calcarenites and fine calcirudites (F8; 30 to 25 m). The section from ~45 m upward, this part of the section shows progradation that is amplified between ~31 and ~25 m. This interval is characterized by decreasing K and Th concentrations in spectral gamma-ray logs (cleaning-up succession), which is again amplified between ~31 and ~25 m. This is interpreted as an upper Tournaisian to lowermost Viséan (Tn/V boundary indicated by FAD of E. simplex at 32 m) regressive succession (HST to LST) from distal outer-ramp facies with distal turbidites and bottom-current reworked facies (F10 and F9) into coarse-grained turbiditic facies (F8). The maximum K and Th values coinciding with the distal F10 facies at ~45 m may represent the maximum flooding surface/interval. The sudden onset of regression associated with the most prominent facies shift at ~31 m (lowermost Viséan) is assumed to represent the basal surface of forced regression (Hunt and Tucker, 1992). The overlying strata are then interpreted as falling stage to lowstand systems tracts (FSST to LST). A sequence boundary is located within this package, following the sequence stratigraphic nomenclature (Hunt and Tucker, 1992; Catuneanu, 2006, p. 170), but it cannot be precisely located based on facies and gamma-ray data. This sequence boundary can be correlated with the earliest Viséan sequence boundary in the Three Cliffs Bay section (Fig. 13). Biostratigraphic data at both the sections (Tn/V boundary located very close to the inferred sequence boundary) support this correlation. The overlying lower Viséan fining-upward trend between ~25 and ~11 m, associated with increasing K and Th concentrations represents a prominent turnover to transgressive conditions (TST) following the earliest Viséan lowstand. The maximum K and Th concentrations and the most distal facies between ~11 and ~8 m are again assumed to represent the most prominent interval of maximum flooding, although there are several K and Th concentration maxima (see below). The base of the Hopedale Formation at ~8 m is characterized by a sudden facies shift to coarse-grained, turbiditic facies (F8) associated with basal erosion and a rapid drop in the K and Th concentrations. This level is interpreted as another sequence boundary. The Ladyside Wood succession (Fig. 6) shows a somewhat different facies distribution and petrophysical log response but some of its important features are correlatable with the Brown End Quarry section. A marked drop in the K and Th concentrations at ~27 m above section base coincides with a prominent facies shift from more distal (F11 and F10) to more proximal facies (F8 and F9) can be correlated with similar patterns in the Brown End section at ~30 m level. The following retrogradational facies transition from F8 into F10 (~27 to ~53 m), associated with the increasing K and Th concentrations, is capped by

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Fig. 13. Sequence-stratigraphic correlation of the South Wales–Mendip Shelf, North Staffordshire Basin and the Dublin Basin based on the “clay” gamma-ray logs and foraminifer biostratigraphy.

another marked shift into more proximal facies and a drop in K a Th concentrations at ~53 m level (base of the Ecton Limestone). Consistently with the Brown End section, the facies and gamma-ray signal shifts at ~27 and ~53 m levels are interpreted as basal surfaces of forced regression. The near-Tn/V sequence boundary is then located within the overlying FSST to LST succession between ~27 and ~34 m.. The maximum flooding surface is difficult to locate due to the highly irregular facies and gamma-ray patterns. The lower part of the section (0 to ~27 m) shows similar facies trends but somewhat different gamma-ray patterns than the corresponding succession (~30 to ~60 m) in the Brown End. This is probably caused by the presence of mudmound facies, which shed pure carbonate into the adjacent basin thus increasing the net carbonate-to-siliciclastic ratio. The biostratigraphic Tn/V boundary in Ladyside Wood is located about 10 m below the inferred sequence boundary as opposed to the Brown End section where it is located just below it. Besides the uncertainty with the precise location of FAD of the index foraminifer taxa, which is inherent to resedimented facies, it is possible that the lowermost Viséan depositional sequence was associated with erosion of underlying strata at the Brown End section. This

explanation is supported by predominance of more proximal facies in Brown End as compared to more distal facies in Ladyside Wood. A distinct facies shift, followed by retrogradational succession of carbonate turbidites is visible in the Ossom's Hill section, between ~ 10 and ~ 30 m. It is associated with a sudden drop and then gradually increasing values of K and Th (Fig. 6). The base of the Ecton Limestone at ~ 31 m is again associated with a distinctive facies shift and a drop in K and Th concentrations, which is correlatable with the Brown End Quarry and Ladyside Wood sections (Figs. 5, 6, and 13). These two intervals of facies shifts and decrease of K and Th concentrations at ~ 10 and ~ 31 m are interpreted as basal surfaces of forced regression, which are followed by sequence boundaries located within the overlying FSST and LST packages (see above). Just below the upper one, the distal turbidite and hemipelagic facies (F11 and F10) with the maximum K and Th concentrations are correlatable with their counterparts in Brown End and Ladyside Wood, interpreted as TST or HST. The Rush section, Dublin basin, shows very similar facies stacking patterns and spectral gamma-ray patterns near the Tn/V boundary

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(Fig. 7). The monotonous succession of calcareous shale of the Tober Colleen Formation ends up in a zone of the highest K and Th concentrations at ~ 152 m, which can be interpreted as a maximum flooding surface (Fig. 4). It is overlain by a prograding succession of turbidites interlayered with shale between (~ 152 and ~ 110 m), which show cleaning-upward K and Th trends and which is interpreted as HST. A sudden decrease of the K and Th concentrations at ~ 110 m, coinciding with the onset of the debris-flow breccias is interpreted as basal surface of forced regression. It is overlain by a FSST dominated by the debris-flow deposition, between ~ 110 and ~ 92 m. The overlying succession (~ 92 to ~ 24 m) of turbidites and debris flow breccias showing cyclic, slightly increasing K and Th concentrations is interpreted as LST. The sequence boundary (sensu Hunt and Tucker, 1992; Bosence and Wilson, 2002; Catuneanu, 2006) can be then located at ~ 92 m (Fig. 13). The relatively sudden shift to more distal turbidite facies and higher K and Th concentrations at ~ 24 m is assumed to represent a transgressive surface. 5.6. Regional correlation and possible steering factors of the T–R cycles The studied sections can be correlated using facies stacking patterns, petrophysical log patterns and foraminifer biostratigraphy. The most prominent correlative tie line is the sequence boundary, which is located slightly above the biostratigraphic Tn/V boundary at all the three areas (Fig. 13). The HST to FSST deposits underlying the sequence boundary in the Three Cliffs Bay and Portishead innerto middle ramp settings show clear progradational patterns and are relatively thick (~100 m in Three Cliffs Bay). In the outer ramp- to basinal settings of the North Staffordshire Basin, the thickness of the latest Tournaisian HST is not known due to unclear stacking patterns and the presence of mud-mound facies, but it is certainly much thinner (bb30 m). The inner to middle ramp deposits of the South Wales accumulated much more rapidly than the outer ramp and basinal deposits of the North Staffordshire Basin, which is in accord with their low K and Th values reflecting the relatively high rates of carbonate production in their shallow-water carbonate factory. Above the Tn/V sequence boundary, an early Viséan depositional sequence developed in the outer-ramp and basinal deposits of the North Staffordshire Basin, which is bounded on top by another sequence boundary close to the base of the Ecton and Hopedale Limestone Formations. This sequence correlates with the MFZ 9 foraminifer Zone (Fig. 13). This early Viséan sequence completely correlates with the erosional surface between the Caswell Bay Oolite and Caswell Bay Mudstone in the Three Cliffs Bay section (mid-Avonian unconformity) (Kalvoda et al., 2012). This indicates a prominent basinward shift of the earliest Viséan carbonate factories on the flanks of the Wales Brabant Massif. There is good evidence that a forced regression produced the sequence boundary close to the Tn/V boundary. The South Wales – Mendip Shelf sections show evidence of multiple subaerial exposure surfaces; paleokarsts associated with paleosols with rhizocretions, calcretes and vadose cements (Riding and Wright, 1981; Wright, 1986; Faulkner et al., 1990). Such features are generally considered as evidence for forced regression as compared to normal regression (Schlager, 2005). Likewise, the accelerated progradation and the onset of debris-flow breccias at ~110 m level in the Rush section speak for relative sea-level fall rather than just a normal regression. This rapid shallowing trend, which has regional significance around the Wales–Brabant Massif, in Belgium and Ireland (Hance et al., 2001), can be biostratigraphically correlated with the exposure and karstic surfaces in the South Wales –Mendip Shelf. Petrophysical log patterns and facies trends in the Dinant synclinorium (Salet and Sovet sections) and Dublin Basin (Lane coastal section) are also correlatable and indicate a turnover from regression to transgression just above the Tn/V boundary (Bábek et al., 2010; Kalvoda et al., 2011). The near Tn/V boundary sea-level fall can be correlated with the boundary between D1c and D2a (Tournaisian/Viséan boundary) mesothem of

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Ross and Ross (1985), which can be correlated from North America and Great Britain through to Russian platform. Glacioeustatic sea-level fall near the Tn/V boundary is documented from the distribution of glacial deposits in the southern hemisphere (cf. Fielding et al., 2008; Rygel et al., 2008). The positive δ13C isotopic excursions in Tournaisian/Viséan boundary carbonate successions at Penchong section (Viséan GSSP, South China) are interpreted as resulting from glacioeustatic sea-level fall related to the organic carbon burial (Qie et al., 2011). In summary, the inter-basin correlations based on petrophysical data presented in this paper provide important additional support to the glacioeustatic interpretation of the uppermost Tournaisian to lower Viséan regressive to transgressive cycle. Biostratigraphic data suggest that the sequence boundary on top of the Gully Oolite in the Three Cliffs Bay inner ramp developed whilst a complete lowermost Viséan (MFZ 9) depositional sequence was deposited in the North Staffordshire Basin and turbidite deposition continued at the Rush section. The lowermost Viséan sequence in the North Staffordshire Basin can be assumed as a “higher order” cycle developing during a “lower order” falling-stage to lowstand sequence set producing the prolonged hiatus in the South Wales – Mendip Shelf (mid-Avonian unconformity). Given the radiometric age constraints of the Viséan stage boundaries (346.7 to 330.9 Ma, Gradstein et al., 2012) and its subdivision into seven foraminifer (MFZ) zones, both the, the “higher order” and “lower order” cycles span several million years and rank in the classical third-order depositional sequences.

6. Conclusions Tournaisian to early Viséan carbonate ramp and basin successions of the South Wales Mendip shelf, the North Staffordshire basin and the Dublin Basin on the flanks of the Wales–Brabant Massif can be correlated using spectral gamma-ray and magnetic susceptibility signals, facies stacking patterns and foraminifer biostratigraphy. The outcrop gamma-ray signals, namely the concentrations of K and Th and the “clay” gamma-ray signal, are strongly dependent on facies and show systematic changes along the bathymetric profile from inner ramp to relatively deep basin. These changes are caused by the dilution of the terrigenous K and Th elements by CaCO3. The “clay” gamma-ray parameter is considered a useful proxy of primary carbonate production and carbonate dispersal along the shelf equilibrium profile on the background of the suspended load of fine-grained siliciclastic supply from the continent. Magnetic susceptibility shows less predictable trends in terms of its distribution along the ramp bathymetric profile than the gamma-ray spectrometry. Diagenetic transformation of iron-bearing magnetic carriers and other factors may also influence the MS signal, which may potentially limit its use in stratigraphic analysis of carbonate sediments. A prominent, late Tournaisian to basal Viséan, regressive-totransgressive cycle can be seen in the key sections, which manifests itself in the gamma-ray signature and, partly, MS trends. The sequence boundary, which is located close to the biostratigraphic Tn/V stage boundary (MFZ 8/MFZ 9 zone), was associated with a prominent forced regression. This is indicated by the correlatable Tn/V trends around the Wales–Brabant Massif, paleokarstic features and paleosols in the South Wales–Mendip Shelf and its temporal coincidence with the glacial deposits in the southern hemisphere mentioned by previous authors. A complete lowermost Viséan (MFZ 9) depositional sequence developed in the North Staffordshire Basin during the prolonged exposure of the South Wales–Mendip Shelf (mid-Avonian unconformity at the Three Cliffs Bay section). This sequence is interpreted as a part of a lower order falling-stage to lowstand sequence set, which points out to a distinct hierarchy in the relative sea-level changes and provides additional support to the early Mississippian glacioeustatic sea-level fluctuations.

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Acknowledgments This research was supported by Czech Science Foundation grant projects GACR 205/09/1257 (Bábek) and GACR 205/08/0182 (Kalvoda). Useful comments by three journal reviewers, Andrew Barnett, Peir Pufahl and Malcolm Wallace are gratefully acknowledged. References Adams, A.E., Wright, V.P., Cossey, P.J., 2004. South Wales–Mendip Shelf. In: Cossey, P.J., Adams, A.E., Purnell, M.A., Whiteley, M.J., Whyte, M.A., Wright, V.P. (Eds.), British Lower Carboniferous Stratigraphy: Geological Conservation Review Series, 29, pp. 393–476. Aitkenhead, N., Chisholm, J.I., Stephenson, I.P., 1985. Geology of the country around Buxton, Leek and Bakewell. Memoir of the Geological Survey of Great Britain, Sheet 111 (England and Wales). (178 pp.). Aurell, M., Badenas, B., Bosence, D.W.J., Waltham, D.A., 1998. Carbonate production and offshore transport on a Late Jurassic carbonate ramp (Kimmeridgian, Iberian Basin, NE Spain): evidence from outcrops and computer modeling. 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