Research article Journal of the Geological Society Published online March 7, 2017 https://doi.org/10.1144/jgs2016-043 | Vol. 174 | 2017 | pp. 646–654

Climate fluctuations recorded in phreatic and vadose calcretes of the Lower Carboniferous Clyde Sandstone Formation of Machrihanish, Kintyre Peninsula, SW Scotland

Pierre Jutras Department of Geology, Saint Mary’s University, Halifax, NS, B3H 3C3, Canada [email protected]

Abstract: The Galdrings cliffs of Machrihanish, in the Kintyre Peninsula of Scotland, expose a Lower Carboniferous clastic succession that hosts a wide variety of calcretes, including thick and massive host-replacing phreatic calcrete hardpans (HRPCHs), which are geologically rare, and which are the products of a thorough replacement of host minerals by calcite in the mixing zone between fresh and evaporitic groundwaters. Isotopic values of 53 samples from various calcretes distributed in a c. 45 m thick succession provided well-defined trends of covariance between carbon and oxygen isotopes, thus delineating trends of aridification and humidification from the interplay of precipitation and evaporation rates. Although values from HRPCHs are more constrained, all other forms of calcrete in the succession (pedogenic nodules, pillar calcrete, laminar calcrete and invasive phreatic calcrete cement in sandstone) follow similar stable isotopic trends. An aridification trend preceded the formation of each HRPCH occurrence, corroborating studies on modern equivalents that suggest that they develop in the more arid range of calcrete formation. The better constrained isotopic values of HRPCHs compared with those of more common forms of calcrete are interpreted to be indicative of more specific environmental requirements, which may in part explain their rarity in the geological record. Received 17 April 2016; revised 19 January 2017; accepted 19 January 2017

A c. 45 m thick succession of Lower Carboniferous sandstone, Geological setting mudstone and conglomerate is exposed in a cliff near Machrihanish, in SW Scotland (Fig. 1). These rocks host several types of calcrete, The study area is located in the southwestern part of the Kintyre including thick and massive host-replacing phreatic calcrete Peninsula of SW Scotland, south of the town of Machrihanish hardpans (HRPCHs) in which most of the original host material (Fig. 1), in an area referred to as ‘the Galdrings’ by McCallien has been replaced by calcite (Fig. 2). This type of calcrete is rather (1927). The Carboniferous clastic rock succession at the Galdrings rare in the geological record, having been reported only from lies unconformably on highly deformed schist of the Tournaisian rocks of SW Scotland (Jutras et al. 2011; Young & Neoproterozoic to Early Ordovician Dalradian Supergroup Caldwell 2011), Visean rocks of eastern Canada (Jutras et al. 1999, (British Geological Survey 1996). It is constrained to a lenticular 2001, 2007; Jutras & Prichonnet 2002), and Quaternary deposits of area of about 2 km2 in a palaeotopographic low of the irregular post- Western and central Australia (Butt et al. 1977; Mann & Horwitz Caledonian surface that was developed in the Dalradian substrate 1979; Arakel & McConchie 1982; Jacobson et al. 1988; Arakel prior to Carboniferous deposition (McCallien 1927). In Scotland, et al. 1989). It has been proposed that the presence of a nearby, Lower Carboniferous strata are part of fault-bounded successor long-lived evaporitic basin may be necessary for the formation of basins to the Caledonian Orogeny (Read et al. 2002). these thick (>3 m), massive and mineralogically pure (>90% The clastic rocks above the unconformity are sharply overlain by calcite) replacement calcretes. Such basins create a zone in which a succession of basaltic lavas that has been tentatively correlated fresh groundwater mixes with highly alkaline evaporitic ground- with the Clyde Plateau Volcanic Formation of the Midland Valley of water, thus increasing the solubility of silicates while decreasing that Scotland (British Geological Survey 1996). Recent geochrono- of calcite (Jutras et al. 2007, and references therein). logical studies suggest that the volcanic rocks in the Midland Valley The Machrihanish outcrops provide data for the first report of a formed at 334.7 ± 1.7 and 335.2 ± 0.8 Ma (Monaghan & Parrish vertical succession of such calcretes. Because they form below the 2006). Although Monaghan & Parrish considered these ages as water table, in the phreatic zone, stable isotopic data from HRPCHs falling within the Holkerian Substage based on the B timescale of typically present fewer irregularities than data from calcretes that Menning et al. (2000), they would fall within the Asbian Substage were formed in the vadose zone, which are more influenced by the based on more recent work by the International Subcommission on immediate hydric conditions and vegetation cover (Jutras et al. Carboniferous Stratigraphy (Richards 2013). Although the volcanic 2007, 2011). The stable isotope signature of HRPCHs therefore rocks of south Kintyre are unlikely to be an extension of the Clyde better reflects the regional climate at the time of formation. Plateau Volcanic Formation flows in the Midland Valley (Caldwell The succession at Machrihanish includes other types of calcrete, & Young 2011), and although there are some geochemical such as pedogenic nodules, pillar calcrete, laminar calcrete and differences between these two successions (Smedley 1988), they invasive phreatic calcrete cement. All these types of calcrete were are considered to be approximately the same age and are therefore analysed for their carbon and oxygen isotope contents, compared referred to by the same name in this paper. with each other, and placed on a vertical profile to provide clues on The clastic succession that hosts the calcretes below the basalt at climate evolution during deposition of the succession. the Galdrings is mainly characterized by thick bodies of coarse grey

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Fig. 1. Simplified geology of southern Kintyre (modified from British Geological Survey 1996), location of the studied section at the Galdrings, and rose diagram plotting 22 palaeocurrent vectors obtained from trough orientations in cross- channelized deposits of the Clyde Sandstone Formation in that section. sandstone, with thin greyish-red mudstone intervals and a few grey assemblage (Neves et al. 1973; Stephenson et al. 2004), which is conglomerate channel fills. Although mapped as the Tournaisian late Courceyan (late Tournaisian) to latest Arundian (mid-Visean) Kinnesswood Formation by the British Geological Survey (1996), (Waters 2011). Bearing in mind that the paraconformably overlying these lithologies and intraformational stratigraphic relationships basalt is considered to be Asbian, the Clyde Sandstone Formation more closely resemble those of the Clyde Sandstone Formation can be reasonably constrained to the Chadian to Holkerian interval, (Young & Caldwell 2011, 2012), the uppermost unit of the and most probably Arundian to Holkerian, which makes it Tournaisian to early Visean Inverclyde Group, to which they are penecontemporaneous with the Arundian to Holkerian La Coulée here assigned. Formation of eastern Canada (Jutras et al. 2007), one of only four Although the direct contact with basement rocks is not exposed previously known units to hold thick and massive host-replacing below the Galdrings cliff, field mapping extrapolations suggest that phreatic calcrete hardpans. Based on palaeomagnetic reconstruc- the Clyde Sandstone Formation directly overlies the Dalradian tions (Torsvik et al. 2004), eastern Canada was closely connected to basement, in contrast to adjacent areas of southern Scotland, in Scotland at the time. which this unit is separated from basement by thick Devonian and Tournaisian deposits (Read et al. 2002). Similar to what is observed Calcrete petrology at the Galdrings, the Clyde Sandstone Formation is paraconform- ably overlain by the Clyde Plateau Volcanic Formation in many Five types of calcrete were recognized in the Galdrings section, all other areas of SW Scotland (Read et al. 2002), but in NE Arran and characterized by microsparitic calcite. in central Bute, the two units are separated by undated plant-rich grey mudstone and fine sandstone of the Birgidale Formation Pedogenic calcrete nodules (Young & Caldwell 2011, 2012). No spore-bearing strata have been identified in the Clyde The most ubiquitous form of calcrete, besides pore-filling calcrete Sandstone Formation, but in the Midland Valley, the top of the cement, pedogenic calcrete nodules form in the vadose zone and conformably underlying Ballagan Formation bears a Pu Biozone therefore tend to be at least in part oxidized (Wright & Tucker 648 P. Jutras

Fig. 2. View of the Carboniferous succession at the Galdrings, near Machrihanish. HRPCH, host-replacing phreatic calcrete hardpan; PCC, phreatic calcrete cement.

1991). At the Galdrings, they are mainly found in muddy this type of calcrete is known only from Quaternary sediments of palaeosols, but also in conglomerate (Fig. 3). central and Western Australia, as well as from Lower Carboniferous rocks of eastern Canada and SW Scotland, and they have been Pillar calcrete linked to the presence of a nearby long-lived evaporitic basin, with mineral replacement occurring in the mixing zone of fresh and Developing in mud cracks of the vadose zone, pillar calcretes evaporitic groundwater. Although massive in appearance, cut typically occur exclusively in muddy intervals (Fig. 3). Similar sections reveal areas of karstic autobrecciation from episodic calcretes have been described in Early Carboniferous muddy dissolution pathways developing within the calcrete mass, only to Aridisols on the nearby Isle of Arran (Tandon & Friend 1989). be filled again with new calcrete (this study and Jutras et al. 1999, 2007, 2011). Invasive phreatic calcrete cement Laminar calcrete hardpan Grain-displacing calcrete cement occurs in coarse sandstone throughout the succession and is interpreted as phreatic based on Above the lower HRPCH, laminar calcrete has developed, probably its uniform distribution over a thickness of several metres (sensu following a lowering of the water table, which left the HRPCH as an Wright & Tucker 1991), its alpha (i.e. massive) microfabric (sensu obstacle to downward migrating rainwater in the vadose zone. Wright 1990), and the lack of oxidation, suggesting development Although in direct contact with phreatic calcrete, the laminar below the water table. It is differentiated from simple pore-filling calcrete is interpreted to have formed in the vadose zone, from calcrete cement by its calcrete-supported grain fabric, which rainwater that was periodically accumulating at the top of the suggests an overall expansion of the deposit. These calcretes can previously formed HRPCH. be regarded as the calcitic equivalents of groundwater dolocretes described by Khalaf (1990) and El-Sayed et al. (1991)inlate Stratigraphy and sedimentology of the Galdrings section Palaeogene to Neogene sediments of Kuwait, by Spötl & Wright (1992) in Upper Triassic sediments of central France, and by Colson Approximately 20 m above the Dalradian basement and c. 45 m & Cojan (1996) in Palaeogene sediments of southern France. below the base of the Clyde Plateau Volcanic Formation, the most basal exposure of the Clyde Sandstone Formation at the Galdrings Host-replacing phreatic calcrete hardpan (HRPCH) cliff is characterized by thick, cross-channelized beds of quartzitic sandstone with an invasive phreatic calcrete cement that has partly The Galdrings succession includes three thick intervals in which the displaced the quartz grains (Fig. 3). These rocks are interbedded host sediment was thoroughly replaced by massive calcrete, leaving with greyish-red mudstone palaeosols with calcrete nodules, as well few clues regarding the nature of the original host material. Lacking as with minor quartzose pebble conglomerate up to a level of c. ferric oxides and forming a continuous mass of alpha microfabric of 32 m below the basalt. At this level, the calcretized sandstone shows up to 8.5 m in thickness, these calcretes are interpreted to have an irregular upper contact with the lowermost HRPCH of the involved the entire aquifer. The three intervals are herein referred to succession, which is 3 – 4 m thick, and which is topped by 1.5 m of as the lower, middle and upper HRPCHs (Fig. 3). As noted above, laminar calcrete (Fig. 3). Climate fluctuations recorded in Carboniferous calcretes 649

Fig. 3. Measured section at the Galdrings, with sample locations, δ13C VDPD and δ18O VDPD values, and palaeocurrent vectors presented at their measured depth below the base of the Clyde Plateau Volcanic Formation.

Despite the thorough replacement of quartz grains, it is clear from incisive portion, the latter is c. 8.5 m thick (Fig. 3). The mostly sharp the basal relationship of the lower HRPCH with the underlying basal contact suggests that the mudstone acted as a partial aquiclude calcretized sandstone that the latter is the host, at least in its lower during formation of the HRPCH. part. The lower HRPCH and overlying laminar calcrete are sharply The lower part of the uppermost and thickest HRPCH is intruded truncated by a downcutting quartzose pebble conglomerate that by a mafic sill that was probably fed from a large dyke that cuts includes some calcrete clasts as well as some red sandstone clasts. through the succession at the Galdrings (Fig. 2). Whereas the bulk of The upper part of this conglomerate contains calcrete nodules, and it the calcrete is massive and identical in facies to the lower and middle is locally invaded by a large calcrete pillar (Fig. 3). As pillar calcretes HRPCHs, the upper 2.5 m seems to have experienced an episode of typically develop in smectite-rich intervals, along pathways provided partial karstification above the water table, resulting in a nodular to by mud cracks (Tandon & Friend 1989), the pillar that truncates the columnar appearance. A poorly exposed c. 3.5 m interval between conglomerate is interpreted as a downward intruding extension of the upper HRPCH and the basal lava flow of the Clyde Plateau the middle HRPCH, which is identical to the lower HRPCH in terms Volcanic Formation includes a very crumbly red mudrock (lacking of facies, and which is separated from the conglomerate by a c. 1m calcrete) overlain by some sandstone. gap. Based on available exposure, the middle HRPCH is at least 4 m As HRPCHs typically develop a few metres below the surface thick, and possibly 6 m thick (Fig. 3). It leaves no clues regarding (Butt et al. 1977; Mann & Horwitz 1979; Arakel & McConchie the nature of its host sediment, but based on inferences from the 1982), it is possible that the red mudstone is a remnant of the host lower HRPCH and on the lack of penetrative calcretization in the material, above the calcrete. However, in the underlying succession, mudstone and conglomerate intervals of the succession, it is penetrative phreatic calcretization seems to be limited to the porous probable that the host was also a porous, sandy deposit. sandy intervals, and as the top of the upper HRPCH seems to have Above a c. 1 m gap, the middle HRPCH is overlain by two poorly experienced minor weathering and perhaps erosion, it is more likely exposed intervals of greyish-red mudstone with calcrete nodules that the c. 3.5 m interval of red mudstone and minor sandstone was and pillars separated by a thin sandstone that truncates the pillars of deposited after calcrete formation in a climatic setting that was less the lower mudstone interval (Fig. 3). Above a 2 m gap are c. 3mof favourable to calcrete formation. Some thin sandstone beds and red calcretized sandstone and a greyish-red mudstone with calcrete mudstones are also visible at low tide beneath the lavas at the north nodules, which are partly invaded by massive phreatic calcrete end of the Galdrings exposures. These also lack evidence of calcrete extending down from the uppermost HRPCH. Discounting its basal formation. 650 P. Jutras

Palaeocurrents siliciclastic host, and in the absence of significant carbonate rocks in the inferred source area to the south, both C and O contents in the The exposed section at the Galdrings includes cross-channelized various forms of calcrete are assumed to be a mix of atmospheric sandstone and conglomerate bodies that provide palaeoflow and biogenic inputs. The δ13Cv.δ18O results are presented in directions based on the shape of scour-and-fill troughs, with dips Figure 4, and δ13C and δ18O variations up-section are presented in converging downflow. Twenty-two measurements were taken from Figure 3. All samples show δ13C values below −2, which are typical such troughs and indicate that the source area lay to the south (Fig. 1).

Stable isotopes Table 1. Carbon and oxygen isotope data δ13 δ18 A total of 53 bulk samples from various types of calcrete in the Depth* (m) Calcrete type C O Galdrings succession were analyzed for carbon and oxygen isotope −3.6 Upper HRPCH −4.23 −8.37 contents (δ13C VPDB and δ18O VPDB) by dual inlet mass −4 Upper HRPCH −4.25 −8.09 spectrometry at the GEOTOP (Université du Québec à Montréal, −4.25 Upper HRPCH −3.93 −7.92 Canada) using a GV Instruments Multicarb preparation system −4.5 Upper HRPCH −3.70 −7.75 connected to an Isoprime dual inlet mass spectrometer. In a −5 Upper HRPCH −4.21 −8.31 −5.5 Upper HRPCH −3.57 −7.77 −6.5 Upper HRPCH −3.13 −7.50 −7 Upper HRPCH −3.71 −8.15 −7.5 Upper HRPCH −3.94 −7.78 −7.75 Upper HRPCH −2.62 −7.60 −8.25 Upper HRPCH −2.60 −7.46 −8.75 Upper HRPCH −3.40 −7.43 −9.5 Upper HRPCH −3.41 −7.84 −10.25 Upper HRPCH −3.21 −7.08 −10.75 Upper HRPCH −2.80 −7.16 −11 Upper HRPCH −2.70 −7.50 −11.5 Upper HRPCH −3.23 −7.26 −12 Upper HRPCH (late cement) −3.48 −7.72 −12 Upper HRPCH (early cement) −3.47 −7.78 −12.5 Pedogenic nodule −2.60 −6.77 −13 Intrusive upper HRPCH −3.26 −6.69 −13.5 Calcrete cement in sandstone −2.26 −7.09 −14 Intrusive upper HRPCH −2.70 −6.54 −15 Intrusive upper HRPCH −2.90 −6.67 −17.5 Pillar calcrete −3.47 −6.19 −18.25 Calcrete cement in sandstone −3.17 −6.08 −18.5 Pillar calcrete −4.24 −6.91 −19 Pedogenic nodule −3.68 −7.07 −20 Pillar calcrete −3.95 −7.38 −21 Pedogenic nodule −4.16 −7.76 −22 Middle HRPCH −3.99 −8.05 −23 Middle HRPCH −4.80 −8.17 −24 Middle HRPCH −4.07 −8.15 −25 Middle HRPCH −4.69 −8.12 −26.5 Middle HRPCH −4.96 −8.18 −27.5 Intrusive middle HRPCH −5.13 −7.53 −27.75 Pedogenic nodule −4.72 −8.41 −28 Calcrete clast in conglomerate −4.25 −7.54 −29 Laminar calcrete −4.04 −5.44 −29.5 Lower HRPCH −3.66 −8.02 −30.5 Lower HRPCH −4.05 −8.16 −31.5 Lower HRPCH −4.67 −8.87 −32.5 Calcrete cement in sandstone −5.25 −7.37 −33.5 Calcrete cement in sandstone −4.90 −8.23 −34 Calcrete cement in sandstone −6.07 −8.74 −34.5 Calcrete cement in sandstone −6.27 −8.54 Fig. 4. Stable isotopes of carbon and oxygen in various calcretes of the −35 Calcrete cement in sandstone −6.15 −9.47 Galdrings section. Blue (light grey), upper half of the section (less than −35.25 Pedogenic nodule −3.90 −8.39 21.5 m below the basalt); red (dark grey), lower half of the section (more −35.5 Pedogenic nodule −4.20 −8.08 than 21.5 m below the basalt. (a) All calcrete types (see symbol legend in −38.5 Calcrete cement in sandstone −5.62 −11.09 Fig. 3); (b) host-replacing massive phreatic calcrete hardpan (HRPCH) −40 Pedogenic nodule −5.94 −9.75 samples; (c) pedogenic calcrete nodules, pillar calcrete, laminar calcrete, −42.5 Pedogenic nodule −5.66 −7.98 calcrete clast and invasive phreatic calcrete cement in sandstone. Also −44 Calcrete cement in sandstone −5.69 −10.34 indicated are the ranges of previously studied Early Carboniferous HRPCHs from eastern Canada (Jutras et al. 2007) and the Isle of Arran of Raw data were corrected based on standard UQ6 (δ13C = +2.25‰ v. V-PDB; δ18O= SW Scotland (Jutras et al. 2011), as well as the range of Quaternary −1.4‰ v. V-PDB). HRPCH, host-replacing phreatic calcrete hardpan. HRPCHs in central and Western Australia (Jacobson et al. 1988). *Depth from the base of the Clyde Plateau Basalt Formation. Climate fluctuations recorded in Carboniferous calcretes 651 of terrestrial carbonates and unlike normal marine carbonates, and characterized by high evaporation rates during a large portion of which typically show positive δ13C values (Brownlow 1996). the year (Tanner 2010, and references therein). In this environment, Early and late cements in autobrecciated HRPCH were analysed the formation of quartzitic sandstone and conglomerate suggests and compared at the −12 m level (Table 1).They show only sedimentary reworking, as chemical weathering at the source would negligible variations of 0.02 δ13C and 0.06 δ18O. The rest of the have been insufficient to concentrate quartz substantially. Hence, it samples were taken from massive intervals of the HRPCHs. is here concluded that remnants of the Lower Old Red Sandstone, in The HRPCH samples all fall within the range of previously which quartz sands and gravels are already concentrated, were an studied occurrences from the Early Carboniferous of eastern Canada important part of the source area when the Clyde Sandstone (Jutras et al. 2007) and from the Isle of Arran in the Firth of Clyde, Formation was deposited. As noted above, remnants of this unit still to the east of the Kintyre Peninsula (Jutras et al. 2011). The results occupy part of the inferred source area, and lithic clasts in are, however, entirely outside the observed range of Quaternary conglomerate intervals of the Clyde Sandstone Formation at the HRPCHs in central and Western Australia (Jacobson et al. 1988), Galdrings are compatible with such provenance. The Lower Old which is a reflection of the substantial differences in vegetation and Red Sandstone is also discontinuous below the Carboniferous atmospheric composition between the Carboniferous and the succession north of the Mull Slide (Fig. 1), suggesting that it had Quaternary (Fig. 4b). already undergone some erosion prior to deposition of the Clyde As was noted by Jutras et al. (2007, 2011) for other Early Sandstone Formation. Carboniferous occurrences, HRPCH samples from the Galdrings Based on the presence of thick and massive HRPCHs in the (Fig. 4b) have less dispersed values than other forms of calcrete succession, this basin must have connected to an evaporitic basin to (Fig. 4c). The HRPCH samples have minimum δ13C and δ18O the north (Fig. 5), in accordance with the model first proposed by values of respectively −5 and −9, whereas the range of other Arakel & McConchie (1982). The vicinity of a long-lived evaporitic calcretes extends into much lighter values and slightly heavier basin is thought to be necessary to bring the groundwater pH values (Figs 3 and 4), therefore showing a significantly greater range sufficiently high to cause the thorough replacement of silicates by than HRPCHs (Fig. 4). calcite that leads to the formation of HRPCHs (Jutras et al. 2007). Despite their less constrained values, stable isotopes in other North of the Galdrings, in the inferred area of this syndepositional forms of calcrete follow a similar trend to that of the HRPCHs evaporitic basin, the interval corresponding to the Clyde Sandstone throughout the succession, with δ13C and δ18O in all calcretes Formation dips below the surface and is covered by younger irregularly increasing up-section to reach maximum values near deposits until it reaches the NW–SE-trending Kilchenzie Fault, the invasive base of the upper HRPCH before decreasing for the which exposes the pre-Carboniferous basement to the north (Fig. 1). remainder of the section (Fig. 3). Overall, the upper half of the Hence, if evaporites or evidence for the former presence of section tends to have heavier values than the lower half in all calcrete evaporites were to be found, they would most probably be in the types (Fig. 4a). Similar to those of eastern Canada (Jutras et al. subsurface, just to the south of the Kilchenzie Fault. 1999, 2007), the lowermost two HRPCHs show increasing values The successive development of synsedimentary HRPCHs in the up-section (Fig. 3). In contrast, the uppermost HRPCH shows succession is indicated by the truncation of the lowermost decreasing values up-section (Fig. 3). occurrence by fluvial conglomerate with calcrete clasts. Groundwater conditions therefore only intermittently promoted Palaeogeography HRPCH formation, but the abundance of other forms of calcrete and the absence of preserved organic matter between the HRPCH In the non-orogenic setting of Early Carboniferous deposition in occurrences suggest that the climate remained consistently arid to SW Scotland, the relative coarseness of the succession, which semi-arid during deposition of most of the succession. The absence includes some pebble conglomerates, suggests that it was sourced of calcrete in the uppermost beds, just below the basalt, may point to from sharp relief associated with a fault scarp that could not have the introduction of more humid conditions some time before the been more than a few kilometres away. Based on this and available eruption of mafic lavas. This is consistent with the presence of plant- palaeocurrent data, this fault scarp was probably associated with rich mudstone of the Birgidale Formation between the Clyde movement on the east–west-striking Mull Slide (Fig. 1). The Sandstone and Clyde Plateau Volcanic formations in some areas of occasional presence of red sandstone clasts in the quartzose the Firth of Clyde (Young & Caldwell 2011). conglomerates is compatible with the presence of remnants of the Lower Devonian Arbuthnott Group (Lower Old Red Sandstone) SE Palaeoclimatic evolution of that fault (Fig. 1). According to Coward (1993) and Caldwell & Young (2013), SW Because of the absence of C4 plants during the Carboniferous, both Scotland was experiencing nearly north–south shortening in Early δ13C and δ18O values tend to systematically increase with aridity Carboniferous times. In this tectonic setting, the east–west-trending and decrease with humidity in calcretes of that period, as degassing Mull Slide would have acted as a reverse fault, which is also and evaporation preferentially remove 12C and 16O, and as suggested by its very irregular fault line (British Geological Survey precipitation and vegetation conversely concentrate them (Stiller 1996). Hence, it is here interpreted that the Clyde Sandstone et al. 1985; Cerling 1991; Rossinsky & Swart 1993; Andrews et al. Formation at the Galdrings was sourced from a fault scarp to the 1998; Hsieh et al. 1998; Pentecost 2005). In a succession that south that was generated by reverse movement along the east–west- includes calcretes and that shows evidence for a steady palaeogeo- trending Mull Slide (Fig. 5). The hilly topography that this unit is graphical setting, such as that of the Galdrings, trends of covering suggests that the region rapidly evolved from an elevated aridification and humidification from the interplay of precipitation source area to an irregular basin floor. and evaporation rates can be determined by the covariance of δ13C and δ18O values. As the stable isotopic data in this study are from Palaeoenvironments diagenetic material that does not follow perfectly the principle of superimposition, their relationship with depth only approximately Alternations between fluvial conglomerates, coarse cross-channe- corresponds to timelines. However, this offset is never more than a lized sandstone and minor mudstone suggest a sandy to gravelly few metres, and trends can therefore be identified at the scale of the braidplain environment, and the abundance of pedogenic and section. The absence of datable material in the succession and the phreatic calcretes indicates that the environment was relatively arid unpredictability of sedimentation rates in fault-controlled clastic 652 P. Jutras

Fig. 5. Depositional and eodiagenetic model for the HRPCH-bearing Clyde Sandstone Formation in the study area. systems preclude the establishment of a precise time frame for these burial of the host sediment below the water table. This hypothetical climatic trends at this stage, but the latter may prove to be useful in succession of vadose calcrete precursors may have had decreasing subsequent correlation efforts. Better time constraints may also values up profile in response to climate change. However, previous eventually come from the development of a magnetostratigraphic studies on HRPCHs have shown that the stable isotopic signature of framework for Carboniferous rocks of the region, which is at present carbonate hosts has negligible influence on that of the calcretes that lacking. replace them (Jutras et al. 2007), although other geochemichal characteristics such as rare earth element contents may be preserved (Jutras et al. 1999). Observed trends Gradual decrease in aridity may have eventually caused phreatic calcretization to end, as suggested by the presence of calcrete-free δ18 Values below the lower HRPCH are dispersed, especially O beds between the uppermost HRPCH and the overlying basalt. A values from invasive phreatic calcrete cement in sandstones (Fig. 3), similar decrease in δ13C and δ18O values from early to late calcrete in which some contamination from the breakdown of sand grains cement was observed in the HRPCH at the base of the Tournaisian may have occurred. However, an overall increase of stable isotope succession at Newton Point (site of Hutton’s Unconformity) on the values is observed up from the base of the section (Figs 3 and 4a). Isle of Arran (Jutras et al. 2011). It was also interpreted to reflect an This suggests that the onset of formation of HRPCHs, which are evolution towards conditions that no longer promoted HRPCH best developed in the upper half of the section, may have occurred formation. However, at Newton Point, early and late calcrete only after the climate had become sufficiently arid, with resulting cements are intermingling at the same stratigraphic level as a result δ13 δ18 − − C and O values above 5 and 9, respectively (Table 1; of dissolution and recystallization processes. Fig. 3). A general decrease in isotopic values is observed above the lower HRPCH, but it is followed by a re-increase into the next HRPCH Regional implications (Fig. 3). The climate seemingly became increasingly arid above the middle HRPCH, based on gradually increasing δ13C and δ18O The gradual increase and following decrease in aridity that is values (Fig. 3), but the muddy material of this interval may have not inferred from stable isotopic signatures in calcretes of the Clyde been porous enough to host a laterally moving aquifer, which would Sanstone Formation may not be easily detected in sections of this have been necessary for HRPCH formation. This may also unit that do not include calcretes, although they may eventually help correspond to a time when the inferred evaporitic basin had dried explain certain patterns in the sedimentation style. The observed down to produce a playa environment. Stable isotope values are trend suggests that climate humidification between this unit and the fairly high at the base of the upper HRPCH, as well as in calcrete subsequent Birgidale Formation may have occurred gradually. nodules and cement from the units that it partly invades, but these Despite the poor time constraints provided by the undated Clyde values gradually decrease up the profile (Fig. 3). Sandstone Formation, which is tentatively considered Arundian to As noted above, δ13C and δ18O values in the two lowermost Holkerian, it is clear from the cumulative thickness and maturity of HRPCHs increase from the base to the top of the profiles (Fig. 3). the calcretes involved that the inferred climate variations may span a Similar increases of stable isotope values up an HRPCH profile few million years. Hence, apart for shorter sub-cycles such as that have been attributed to increasing evaporation and degassing rates between the lower and middle HRPCHs (Fig. 3), this general trend towards the top of the water table (Jutras et al. 1999, 2007). probably occurred beyond the range of orbitally controlled cycles However, the covariant decreasing values in the uppermost profile and most probably corresponds to tectonically controlled climate (Fig. 3) are less typical and suggest that aridity was decreasing change. In Atlantic Canada, based on palaeomagnetic studies by during formation of this calcrete. They also suggest that the HRPCH Scotese & McKerrow (1990) and Ziegler et al. (2002), the transition was formed in stages from the gradual rise of the water table. In from aridity in the Visean to more humid conditions in the other words, although HRPCHs are formed by laterally moving Serpukhovian is interpreted to be the result of ‘continental drift’ groundwater, the saturated aquifer may have never been as thick as from the sub-tropics to the equatorial zone (Allen et al. 2011; Jutras the resulting calcrete body, which would have been built et al. 2015). Because Atlantic Canada and Scotland are thought to incrementally by the gradual rise of a thin aquifer that was in step have been juxtaposed at the time (Torsvik et al. 2004), a similar with deposition and that was constrained at the bottom by explanation is proposed to explain dry conditions in the Visean impermeable calcrete. Also possibly influencing the HRPCH data Clyde Sandstone Formation and more humid conditions subse- is a calcrete cement precursor that may have developed prior to quently. Available data suggest that this climatic transition occurred Climate fluctuations recorded in Carboniferous calcretes 653 earlier in Scotland, which could be explained by an earlier crossing References into the equatorial zone, but which could also be explained by Allen, J.P., Fielding, C.R., Gibling, M.R. & Rygel, M.C. 2011. Fluvial response microclimatic conditions related to volcanism in the Midland Valley to paleo-equatorial climate fluctuations during the late Paleozoic ice age. – of Scotland. Geological Society of America Bulletin, 123, 1524 1538. Andrews, J.E., Singhvi, A.K., Kailath, A.J., Kuhn, R., Dennis, P.F., Tandon, S.K. & Dhir, R.P. 1998. Do stable isotope data from calcrete record Late Pleistocene monsoonal climate variation in the Thar Desert of India? Quaternary Concluding remarks Research, 50, 240–251. Arakel, A.V. & McConchie, D. 1982. Classification and genesis of calcrete and Although the presence of calcretes of any kind is in itself an gypsite lithofacies in paleodrainage systems of inland Australia and their indication of aridity, they are known to form in a rather wide range relationship to carnotite mineralization. Journal of Sedimentary Petrology, 52, 1149–1170. of hydrological conditions (Semeniuk & Searle 1985), and their Arakel, A.V., Jacobson, G., Salehi, M. & Hill, C.M. 1989. Silicification of climatic signature can therefore be refined by investigating their calcrete in paleodrainage basins of the Australian arid zone. Australian stable isotopic contents. For this purpose, data from mature and Journal of Earth Sciences, 36,73–89. well-homogenized phreatic calcretes are especially useful, but their British Geological Survey 1996. Campbelltown, Scotland Sheet 12. Solid and Drift Geology. 1:50,000 Provisional Series. British Geological Survey, covariance with data from vadose calcretes (pedogenic nodules, Keyworth, Nottingham. laminar calcrete and pillar calcrete) in this study suggests that the Brownlow, A.H. 1996. Geochemistry, 2nd edn. Prentice–Hall, Upper Saddle δ13C and δ18O signatures of all calcrete types were strongly River, NJ. Butt, C.R.M., Mann, A.W. & Horwitz, R.C. 1977. Regional setting, distribution influenced by regional climatic variations. Calcretes formed in the and genesis of surficial uranium deposits in calcretes and associated sediments vadose zone are more strongly influenced than their phreatic in Western Australia. CSIRO, Mineral Research Laboratory Report, FP 16. counterparts by local hydrological conditions and the nature of the Caldwell, W.G.E. & Young, G.M. 2011. The Early Carboniferous volcanic immediate vegetation cover; a distortion that can be partly removed outliers of Little Cumbrae and south Bute: implications for westward δ13 δ18 attenuation of the Clyde Lava Plateau. Earth and Environmental Science by excluding samples that show C/ O ratios that differ Transactions of the Royal Society of Edinburgh, 102,59–76. substantially from the overall stratigraphical variation. Caldwell, W.G.E. & Young, G.M. 2013. Structural controls in the western In the Galdrings, an up-section trend towards high δ13C and δ18O offshore Midland Valley of Scotland: implications for Late Palaeozoic regional tectonics. Geological Magazine, 150, 673–698, https://doi.org/10. values is observed below the base of each of the three host-replacing 1017/S0016756812000878 phreatic calcrete hardpans (HRPCHs), which do not include values Cerling, T.E. 1991. Carbon dioxide in the atmosphere; evidence from Cenozoic below −5 δ13C and −9 δ18O, in contrast to other, more common and Mesozoic Paleosols. American Journal of Science, 291, 377–400. forms of calcrete (Figs 3 and 4). This suggests that HRPCHs may Colson, J. & Cojan, I. 1996. Groundwater dolocretes in a lake-marginal environment; an alternative model for dolocrete formation in continental need a more arid climate to develop, which corroborates studies on settings (Danian of the Provence Basin, France). Sedimentology, 43, 175–188. modern equivalents of such calcretes (Mann & Horwitz 1979). Coward, M.P. 1993. The effect of late Caledonian and Variscan continental However, as they need the presence of an aquifer, HRPCHs may not escape tectonics on basement structure, Palaeozoic basin kinematics, and subsequent Mesozoic basin development in NW Europe. In: Parker, J.R. (ed.) be able to form in the most arid range of calcrete formation, where Petroleum Geology of Northwest Europe: Proceedings of the 4th Conference. precipitation rates may be sufficiently high to allow vadose calcretes Geological Society, London, 1095–1108, https://doi.org/10.1144/0041095 to slowly form, but not enough to sustain an aquifer and therefore El-Sayed, M.I., Fairchild, I.J. & Spiro, B. 1991. Kuwaiti dolocrete; petrology, – phreatic calcretization. geochemistry and groundwater origin. Sedimentary Geology, 73,59 75. Hsieh, J.C.C., Chadwick, O.A., Kelly, E.F. & Savin, S.M. 1998. Oxygen isotopic In the case of invasive phreatic calcrete cement, it is very unlikely composition of soil water: Quantifying evaporation and transpiration. that it represents the embryonic stage of HRPCHs, as early Geoderma, 82, 269–293. cementation would impede the water circulation that is deemed Jacobson, G., Arakel, A.V. & Chen, Y. 1988. The central Australian groundwater discharge zone: Evolution of associated calcrete and gypcrete deposits. necessary for silica to be removed from the system while being Australian Journal of Earth Sciences, 35, 549–565, https://doi.org/10.1080/ replaced by calcite. Hence, pore-filling and invasive phreatic 08120098808729469 calcretization may occur when the climate is arid enough for the Jutras, P. & Prichonnet, G. 2002. Stratigraphy, depositional setting and diagenetic aquifer to become saturated, but in a context where groundwater pH history of the Saint-Jules Formation (Upper Devonian or Mississippian), a newly identified post-Acadian red clastic unit in the southern Gaspé Peninsula, is not sufficiently high for silicate minerals to be replaced. As this Québec. Canadian Journal of Earth Sciences, 39, 1541–1551. type of calcrete shows values that can be either lower or greater than Jutras, P., Prichonnet, G. & Von Bitter, P. 1999. The La Coulée Formation, a new those for HRPCHs (Fig. 4), it can be inferred that, in the context of a post-Acadian continental clastic unit bearing groundwater calcretes, Gaspé Peninsula, Québec. Atlantic Geology, 35, 139–156. stable palaeogeographical setting, they could form when the climate Jutras, P., Prichonnet, G. & Utting, J. 2001. Newly identified Carboniferous units is either too arid to sustain a water basin or not arid enough to sustain (the Pointe Sawyer and Chemin-des-Pêcheurs formations) in the Gaspé a highly alkaline evaporitic basin. Peninsula, Quebec; implications regarding the evolution of the northwestern – In conclusion, although a much larger database would be needed sector of the Maritimes Basin. Canadian Journal of Earth Sciences, 38,1 19. Jutras, P., Utting, J. & McLeod, J. 2007. Link between long-lasting evaporitic to fully determine this, HRPCHs may be regarded as features that basins and the development of thick and massive phreatic calcrete hardpans in develop only fairly high in the aridity range of calcrete formation, the Mississippian Windsor and Percé groups of eastern Canada. Sedimentary but perhaps not in the highest range. These greater climatic Geology, 201,75–92, https://doi.org/10.1016/j.sedgeo.2007.04.008 Jutras, P., Young, G.M. & Caldwell, W.G.E. 2011. Reinterpretation of James constraints, paired with the necessity of a nearby evaporitic basin, Hutton’s historic discovery on the Isle of Arran as a double unconformity would partly explain why HRPCHs are so much less common than masked by a phreatic calcrete hardpan. Geology, 39, 147–150. other forms of calcrete in the geological record. Jutras, P., Mcleod, J. & Utting, J. 2015. Sedimentology of the lower Serpukhovian (upper Mississippian) Mabou Group in the Cumberland Basin of eastern Canada; tectonic, halokinetic and climatic implications. Canadian Journal of Earth Sciences, 52, 1150–1168, https://doi.org/10.1139/ Acknowledgements I wish to thank G. Young and G. Caldwell for cjes-2015-0062 bringing my attention to the phreatic calcretes at the Galdrings, for fruitful Khalaf, F.I. 1990. Occurrence of phreatic dolocrete within Tertiary clastic discussions, and for providing comments on an earlier version of this paper. I also deposits of Kuwait, Arabian Gulf. Sedimentary Geology, 68, 223–239. wish to thank J.-F. Hélie for the isotope analyses, R. Corney for preparing thin- Mann, A.W. & Horwitz, R.C. 1979. Groundwater calcrete deposits in Australia: sections, as well as S. J. Jones and an anonymous reviewer for their constructive Some observations from Western Australia. Journal of the Geological Society review of this paper. of Australia, 26, 293–303. McCallien, W.J. 1927. Preliminary account of the post-Dalradian geology of Kintyre. Transactions of the Geological Society of Glasgow, 18,40–126. Funding This project was supported by a research grant of the Natural Menning, M., Weyer, D., Drozdzewski, G., Van Amerom, H.W.J. & Wendt, I. Sciences and Engineering Research Council (NSERC) of Canada. 2000. A Carboniferous timescale 2000: Discussion and use of geological parameters as time indicators from central and western Europe. Geologisches Scientific editing by Stuart Jones Jahrbuch, A156,3–44. 654 P. Jutras

Monaghan, A.A. & Parrish, R.R. 2006. Geochronology of Carboniferous– Midland Valley of Scotland, and elucidation of intra-Dinantian unconfor- Permian magmatism in the Midland Valley of Scotland: Implications for mities. Proceedings of the Yorkshire Geological Society, 55, 131–143, https:// regional tectonomagmatic evolution and the numerical time-scale. Journal of doi.org/10.1144/pygs.55.2.131 the Geological Society, London, 163,15–28, https://doi.org/10.1144/0016- Stiller, M., Rounick, J.S. & Shasha, S. 1985. Extreme carbon-isotope 764904-142 enrichments in evaporating brines. Nature, 316, 434–435. Neves, R., Gueinn, K.J., Clayton, G., Ioannides, N.S. & Neville, R.S.W. 1973. Tandon, S.K. & Friend, P.F. 1989. Near-surface shrinkage and carbonate Palynological correlations within the Lower Carboniferous of Scotland and replacement processes, Arran Cornstone Formation, Scotland. Sedimentology, Northern England. Transactions of the Royal Society of Edinburgh, 69,24–53. 36, 1113–1126, https://doi.org/10.1111/j.1365-3091.1989.tb01545.x Pentecost 2005. Travertine. Springer-Verlag, Berlin and Heidelberg. Tanner, L.H. 2010. Continental carbonates as indicators of paleoclimate. In: Read, W.A., Browne, M.A.E., Stephenson, D. & Upton, B.G.J. 2002. Alonso-Zarza, A.M. & Tanner, L.H. (eds) Carbonates in Continental Carboniferous. In: Trewin, N.H. (ed.) The Geology of Scotland. Geological Settings: Geochemistry, Diagenesis and Applications. Developments in Society, London, 251–299. Sedimentology, 62. Elsevier, Amsterdam, 179–214. Richards, B.C. 2013. Current status of the International Carboniferous Time Scale. Torsvik, T.H., Cocks, L. & Robin, M. 2004. Earth geography from 400 to 250 Ma; In: Lucas, S.G., Nelson, W.J. et al. (eds) The Carboniferous–Permian Transition. a palaeomagnetic, faunal and facies review. Journal of the Geological Society, New Mexico Museum of Natural History and Science, Bulletin, 60,348–353. London, 161,555–572, https://doi.org/10.1144/0016-764903-098 Rossinsky, V.J.R. & Swart, P.K. 1993. Influence of climate on the formation and Waters, C.N.A. 2011. Revised Correlation of Carboniferous Rocks in the British isotopic composition of calcretes. In: Swart, P.K., Lohmann, K.C., McKenzie, Isles. Geological Society, London, Special Reports, 26. J. & Savin, S. (eds) Climate Change in Continental Isotopic Records. Wright, V.P. 1990. A micromorphological classification of fossil and Recent American Geophysical Union, Geophysical Monographs, 78,67–75. calcic and petrocalcic microstructures. In: Douglas, L.A. (ed.) Soil Scotese, C.R. & McKerrow, W.S. 1990. Revised world maps. In: McKerrow, W. Micromorphology; a Basic and Applied Science. Developments in Soil S. & Scotese, C.R. (eds) Paleozoic Palaeogeography and Biogeography. Science, 19, Elsevier, Amsterdam, 401–407. Geological Society, London, Memoirs, 12,1–21, https://doi.org/10.1144/ Wright, V.P. & Tucker, M.E. 1991. Calcretes: an introduction. In: Wright, V.P. & GSL.MEM.1990.012.01.01 Tucker, M.E. (eds) Calcretes, Reprint Series Volume 2 of the International Semeniuk, V. & Searle, D.J. 1985. Distribution of calcrete in Holocene coastal Association of Sedimentologists. Blackwell Scientific, Oxford, 1–22. sands in relationship to climate, southwestern Australia. Journal of Young, G.M. & Caldwell, W.G.E. 2011. Early Carboniferous stratigraphy in the Sedimentary Petrology, 55.1,86–95. Firth of Clyde area: new information from the Isle of Bute. Scottish Journal of Smedley, P.L. 1988. The geochemistry of Dinantian volcanism in south Kintyre Geology, 47, 143–156, https://doi.org/10.1144/0036-9276/01-431 and the evidence for provincialism in the southern Scottish mantle. Young, G.M. & Caldwell, W.G.E. 2012. The Northeast Arran Trough, the Corrie Contributions to Mineralogy and Petrology, 99, 374–384. conundrum, and the Highland Boundary Fault in the Firth of Clyde, SW Spötl, C. & Wright, V.P. 1992. Groundwater dolocretes from the Upper Triassic Scotland. Geological Magazine, 149, 578–589. of the Paris Basin, France; a case study of an arid, continental diagenetic facies. Ziegler, A.M., Rees, P.M. & Naugolnykh, S.V. 2002. The Early Permian floras of Sedimentology, 39, 1119–1136. Prince Edward Island, Canada: Differentiating global from local effects of Stephenson, M.H., Williams, M., Monghan, A.A., Arkley, S. & Smith, R.A. climate change. Canadian Journal of Earth Sciences, 39, 223–238, https://doi. 2004. Palynomorph and ostracod biostratigraphy of the Ballagan Formation, org/10.1139/e01-075