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Home , Kinnesswood Formation, Old Red Sandstone

Reinterpretation of ’s historic discovery on the as a double masked by a phreatic calcrete hardpan

Pierre Jutras1*, Grant M. Young2, and W. Glen E. Caldwell2 1Department of , Saint Mary’s University, Halifax, Nova Scotia B3H 3C3, 2Department of Earth Sciences, University of Western Ontario, London, Ontario N6A 5B7, Canada

ABSTRACT INTRODUCTION Because it is partly masked by a phreatic calcrete hardpan (PCH), a rare and poorly known The unconformable contact discovered by type of that can transgress stratigraphic boundaries, there has been ongoing contro- James Hutton in 1787 on the Isle of Arran, versy concerning the exact position of James Hutton’s fi rst discovered unconformity on the , was the fi rst of this kind to be for- Isle of Arran in southwest Scotland. The unconformity separates folded Neoproterozoic to mally identifi ed. It played a pivotal role in the lower Paleozoic () metasedimentary rocks from upper Paleozoic . The mas- development of ideas concerning the antiquity sive PCH developed in Late red above the unconformity, but it also of the Earth and therefore has great historical assimilated some of the underlying basement rocks, thus giving the false impression that the signifi cance. It has, however, received much unconformity is at a lower position, as both host materials are almost entirely replaced by cal- less attention than the more photogenic uncon- crete. At Hutton’s discovery site, only a small remnant of the deeply calcretized Late Devonian formity identifi ed later by Hutton at Siccar conglomerate was preserved from erosion prior to being disconformably overlain by lower Point, east of . This is partly because red conglomerate and . Thus, there are two at Hut- the unconformable contact in Arran is obscured ton’s historical site, but the younger has previously gone unnoticed, and the two red suc- by massive calcrete, creating confusion as to its cessions on each side of the disconformity were previously thought to belong to the same unit. precise position. There is no surviving detailed drawing from Hutton’s time that would indicate where exactly he wished to place the contact, but phreatic when Archibald Geikie edited the third volume calcrete of Hutton’s Theory of the Earth in 1899, he hardpan included such a drawing and placed the con- 1m tact at the base of a tabular calcrete unit that is concordant with stratifi cation in the sedi- mentary succession above the unconformity (Fig. 1A). On closer inspection, the structural grain of the Dalradian metasedimentary rocks that form the basement beneath the unconfor- mity can be seen within the fi rst meter of cal- crete. Although this was pointed out by Ander- son (1947) and Tomkeieff (1953), ambiguity PCH(1) PCH(1) concerning the exact position of the contact 10 cm 20 cm persisted, and Tomkeieff (1963) placed the A Newton Point BB CC contact at yet another (third) position. Young

N55°43′05″ 0 5 and Caldwell (2009) placed it higher than both W05°17′17″ D km levels suggested by Tomkeieff (1953, 1963), Lochranza again refl ecting the obscure nature of the con- tact due to the strong calcrete overprint. N55°41′11″ PCH(1) ~~~ ′ ″ W05°10 31 20 cm ~~~ Figure 1. Simplifi ed geological map of study area (modifi ed from British Geological Sur- ~~~ vey, 1987), showing three studied localities. A–C: Hutton’s angular unconformity (classi- N N55°39′03″ Corrie W05°08′37″ E cal site) near Newton Point. A: General view SOUTHERN and interpretive sketch. B: Window of non- SCOTLAND 55°38′ 10 cm calcretized conglomerate and (unit 1, Upper Old Red Sandstone, UORS) in 55°37′ phreatic calcrete hardpan [PCH(1)]. C: Close- map Glasgow up of disconformable contact between area PCH(1) and unit 2 (Kinnesswood Formation). 55°36′ D: Disconformable contact between PCH(1)

5°08′ 5°06′ and unit 2 at Fallen Rocks. E: Boulder of mas- sive calcrete in basal bed of unit 2 at Corrie.

*E-mail: [email protected].

© 2011 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, February February 2011; 2011 v. 39; no. 2; p. 147–150; doi: 10.1130/G31490.1; 3 fi gures; Data Repository item 2011065. 147 In this paper, the petrogenesis, nature, and Hutton’s Unconformity Near Newton Point Although the calcrete appears to be massive age of the calcrete at Hutton’s Unconformity on At the classic locality of Hutton’s Uncon- in outcrop, petrographic investigation reveals the Isle of Arran are reinvestigated. We demon- formity in north Arran, ~650 m northeast of three distinct phases of formation. The earliest strate that Hutton’s fi rst unconformity is in fact a Newton Point (Fig. 1), the section begins with phase is dark gray micritic calcrete, which is threefold structure involving a Devonian uncon- the Dalradian (unit 0) strata, which dip ~55° locally replaced by a lighter gray micritic cal- formity, a latest Devonian–earliest Carbonifer- toward the southeast and have a relatively sharp crete, and in part autobrecciated and incorpo- ous lithodemic contact with a phreatic calcrete but irregular upper contact with a tabular mass rated into the latter. The third phase is white spar hardpan, and a previously undetected early Car- of light gray calcrete that dips ~25° toward the distributed in millimetric veins. Such complex boniferous disconformity. northwest (Fig. 1A). textures are typical of PCHs and are thought to The thickness of the calcrete varies later- be due to alternating phases of precipitation and FIELD RELATIONSHIPS ally from 2 to 2.75 m, but it is sharply eroded dissolution in laterally circulating groundwater The Isle of Arran is one of a few localities in at the top and therefore incomplete. It includes that becomes increasingly obstructed as the cal- southwest Scotland where the contact between rare windows of partially preserved host mate- crete is sealed (Arakel and McConchie, 1982; rocks of the Upper Old Red Sand- rial, including in situ basement host rocks in Jutras et al., 2007). stone and the lower Carboniferous Kinness- its lowest 75–150 cm. In the basal 50–100 cm, In terms of stable isotopes, samples from wood Formation can be observed. The contact these windows preserve the structural grain and various levels of the calcrete [PCH(0,1)] at is obscure because all of the rocks were depos- color of the basement host rocks, but fragments Newton Point (samples a and b in Fig. 2) show ited in a similar arid continental setting, which of basement material are randomly oriented a well-constrained range of δ13C and δ18O val- resulted in only subtle petrographic differences and increasingly oxidized in the succeeding ues, although there is a tendency for the light between the two units (Read et al., 2002). Both 25–50 cm (Fig. 1A). The in situ basement mate- gray late phase (b samples) to have lighter val- include polymictic red conglomerate and cal- rial that is host to the calcrete probably repre- ues than the dark gray early phase (a samples). crete, but the Upper Old Red Sandstone tends to sents a poorly developed regolith. The vein-like third phase is highly contaminated be coarser and more quartz rich. It also contains In the uppermost 75–125 cm of the calcrete, with small fragments of the earlier phases, and fewer and less strongly developed calcretes. in situ basement rocks are absent, but rare win- was therefore not analyzed for stable isotopes. The transition between the two units is thought dows of preserved host material are made up A sharp, slightly erosive contact is observed to correspond approximately to the Devonian- of coarse clastic sedimentary rocks with well- between PCH(0,1) and the concordantly over- Carboniferous boundary, but the lack of datable rounded pebbles and small cobbles. Locally, a lying lower Carboniferous red beds of the Kin- material makes this interpretation uncertain grayish-red, coarse sandy matrix is preserved nesswood Formation (unit 2) (Fig. 1C). This (Read et al., 2002). At Fallen Rocks, in north- (Fig. 1B), indicating that the host material is a disconformable contact was not previously east Arran, the Kinnesswood Formation is con- matrix-supported polymictic conglom- recognized because of its near-planar nature, formably overlain by marginal marine beds of erate with nearly 50% coarse sand-size mate- and because it is masked by the presence of the upper Tournaisian Ballagan Formation (Brit- rial. Although poorly sorted, the conglomer- calcite deposits along the planar, joint-con- ish Geological Survey, 1987) and was therefore atic host rock of the calcrete has well-rounded trolled faces of the bedrock exposure. Hence, probably deposited well into the Tournaisian. clasts and is compositionally mature, with Stratigraphic relationships near the base of the nearly 90% quartz pebbles. The petrography 0 Observed range fo r :Tournaisian vadose calcrete Kinnesswood Formation were studied at Hut- and stratigraphic position of this material cor- the Viséan La Coulée :Tournaisian phreatic calcrete hardpan Calcrete (based on 46 ton’s Unconformity near Newton Point (north respond to that of the Upper Old Red Sand- samples from across eastern Canada (data from Jutras et al., 2007)) Arran), at Fallen Rocks, and at Corrie (both in stone (unit 1). Observed range for the Quaternary phreatic calcrete northeast Arran) (Fig. 1). Because the stratigra- Apart from these rare windows, most of the hardpans of Au stralia ce d d e aa (data from Jacobson phy at that interval is rendered complex by over- host material, including the siliciclastic frame- –5 et al., 1988) C (‰ VPDB) e b cc 13 a b b a prints of regolith and calcrete within the succes- work, was thoroughly replaced by massive δ b b g fg sion, we simplify the reading by using numbers calcrete. Such thoroughness of mineral replace- Observed range fo r the Tournaisian phreatic calcrete hardpans of Arran (this study) and acronyms that segregate lithostratigraphic ment over several meters in thickness is diagnos- gf –15 –10 –5 0 units from lithodemic or pedostratigraphic units. tic of a PCH, the genesis of which involves an δ18O (‰ VPDB) • Unit 0: low-grade metasedimentary rocks entire aquifer (Jutras et al., 2007, and references of the Dalradian Supergroup (Neoproterozoic therein). The massive structure from base to Figure 2. Stable isotopes of carbon (δ13C Vi- to Early ) that were deformed by the top and the sharp lower contact with competent enna Peedee belemnite, VPDB) and oxygen δ18 to Devonian Caledonian orogeny, and basement rocks (which is thought to correspond ( O VPDB). In phreatic calcrete material [PCH(0,1)] at localities of Newton Point (a— that form the basement of upper Paleozoic suc- to the impermeable base of a paleoaquifer) are early phase; b—late phase), Fallen Rocks cessions on Arran (Read et al., 2002). also typical of such calcretes, which have only (c), Corrie (d—in situ; e—reworked calcrete • Unit 1: the Devonian Old Red Sandstone. previously been identifi ed from the Quaternary boulders in basal Kinnesswood, unit 2). In • Unit 2: the early Carboniferous Kinness- of central Australia (Arakel and McConchie, pre-Kinnesswood vadose calcrete from regolith [VC(0)] below Hutton’s Unconfor- wood Formation. 1982) and from the Visean of Atlantic Canada mity (f—new, low-tide site), and in clast from • PHC(0), PHC(1), and PHC(0,1): phreatic (Jutras et al., 2007). In this context of pervasive the basal Kinnesswood conglomerate above calcrete hardpan developed in, respectively, mineral replacement by phreatic calcrete, with (g; see Table DR1 in GSA Data Repository1). units 0, 1, or both; PCH(0,1) indicates that two only a small fraction of the original gravelly lithostratigraphic units (0 and 1) now belong to framework preserved, the presence of small 1GSA Data Repository item 2011065, Table DR1 one single lithodemic unit. remnants of more easily replaced red sandstone (carbon and oxygen isotopes in phreatic and vadose • VC(0): vadose calcrete developed in rego- matrix, however rare and dispersed, suggests calcrete material at the Devonian-Carboniferous boundary on Arran), is available online at www lith of unit 0. that the Upper Old Red Sandstone host material .geosociety.org/pubs/ft2011.htm, or on request from • VPC(1) and VPC(2): vadose and pedogenic (unit 1) was already in part cemented at the time [email protected] or Documents Secretary, calcretes developed in units 1 and 2, respectively. of calcrete formation. GSA, P.O. Box 9140, Boulder, CO 80301, USA.

148 GEOLOGY, February 2011 the lowermost red beds (unit 1; host to the that, in turn, unconformably overlies Dalradian phreatic calcretization event. The age of the PCH) and the disconformably overlying, litho- basement rocks (unit 0). In contrast with the calcretization event is possibly latest Famen- logically similar succession of unit 2 were both Newton Point occurrence, the 1–1.5-m-thick nian, but because it occurs in a Famennian con- thought to belong to the Kinnesswood Forma- calcrete remnant below the early Carboniferous glomerate (unit 1) that shows evidence of pre- tion (British Geological Survey, 1987; Young disconformity at Fallen Rocks has a more dif- vious weathering at Fallen Rocks, and because and Caldwell, 2009), although both were pre- fuse lower contact, indicating that the base of the overlying Kinnesswood Formation (unit 2) viously assigned to the Upper Old Red Sand- the paleoaquifer was more poorly constrained could be as young as upper Tournaisian, it is stone (Gunn and Geikie, 1947). above imperfectly cemented material. The cal- more likely early or middle Tournaisian. The Kinnesswood Formation (unit 2) at crete is developed in a polymictic, but quartz Such PCHs (or their dolocrete equiva- Newton Point is dominated by red sandstone pebble–rich Upper Old Red Sandstone con- lents; Colson and Cojan, 1996, and references with abundant vadose or pedogenic calcrete glomerate (unit 1) that was weathered prior to therein) are rare in the geological record. They nodules and pillars [VPC(2)] (Young and calcrete formation, as suggested by the destruc- are only known to develop in hyperarid cli- Caldwell, 2009), which are typical of this unit tion of sedimentary structures just below the mates by the mixing of fresh and salty ground- in southwest Scotland (Read et al., 2002). The calcrete and by the fragmented nature of weak waters in the discharge zone into contempora- basal bed is a granular to pebbly sublitharenite clasts. In terms of stable iso- neous evaporitic basins, where the solubility of with rare calcrete nodules, the dearth of which topes, PCH(1) at Fallen Rocks is very near the silicate minerals increases while that of calcite suggests that the erosional surface developed range of PCH(0,1) at Newton Point, albeit with takes a sudden drop (Arakel and McConchie, on PCH(0,1) was well washed prior to its slightly lighter δ18O values (sample c in Fig. 2). 1982; Colson and Cojan, 1996; Jutras et al., burial by unit 2. The top of PCH(1) at Fallen Rocks displays a 2007). Tournaisian evaporites are well docu- Although the paleosurface below unit 2 sharp, eroded surface beneath unit 2 (Fig. 1D), mented in Scotland (Read et al., 2002), and is only mildly undulating, with local surface which begins with a red pebbly sandstone with therefore the model of Arakel and McConchie topography of <30 cm (Fig. 1C), the deeply no calcrete clasts. Just as at Newton Point, unit (1982) may also apply to the PCHs on Arran. sculpted nature of the disconformity can be 2 consists mainly of brick-red sandstone with inferred from a new, low-tide exposure of abundant vadose or pedogenic calcrete nod- Stable Isotopes Hutton’s Unconformity that was identifi ed by ules, pillars, and hardpans [VPC(2)]. All samples of the Tournaisian(?) PCH on Young and Caldwell (2009) ~360 m west of Arran have δ13C and δ18O values that are close the original site. There, PCH(0,1) is missing Corrie Locality to, or within, the range of values obtained from between units 0 and 2. Below the unconfor- At the Corrie locality, ~4.5 km south-south- the Visean La Coulée Calcrete in eastern Can- mity, the uppermost 1–2 m of basement rocks east of Fallen Rocks, there is an erosional con- ada (Jutras et al., 2007), although they have an (unit 0) are deeply weathered, oxidized, and tact between units 1 and 2, but with a much overall tendency toward lighter values. They contain many calcrete nodules and veins. The more pronounced stepped paleotopography show no overlap with values from Quaternary structurally controlled distribution of calcrete than at the other localities, and with massive occurrences of central Australia (Jacobson et suggests a nonpedogenic origin from ground- calcrete [PCH(1)] that is only preserved in the al., 1988). This may be due to the very dif- water, but diffuse and incomplete assimilation least-deeply eroded areas of unit 1. PCH(1) ferent nature of Quaternary vegetation, which of the well-developed and thoroughly oxidized from this area has δ13C and δ18O values (sample directly infl uences the δ13C and δ18O values of regolith points to calcrete formation in the d in Fig. 2) very similar to those of PCH(0,1) groundwater even in areas of very sparse cover, vadose zone [unit VC(0)]. The stable isotopic at Newton Point. At this site, large boulders of such as deserts. The tendency toward lighter signature of this vadose calcrete material is massive calcrete with a similar δ13C and δ18O values in the PCHs of Arran than in those of very different from that of the PCH at the clas- signature (samples e in Fig. 2) form the bulk eastern Canada may refl ect a slightly less arid sic site of Hutton’s Unconformity, with lighter of the basal conglomerate of unit 2 (Fig. 1E). setting for the former, as evaporation tends to δ13C and substantially heavier δ18O values (f Thus, the depositional surface was steeply dis- concentrate heavy isotopes of both carbon and samples in Fig. 2). sected prior to and during deposition of unit 2, oxygen (Dever et al., 1987). The earliest cal- The partly calcretized regolith [VC(0)] is leaving resistant knobs of PCH(1) to be scav- crete phase in the PCH at Newton Point has sharply truncated by unit 2, which starts with enged by streams. heavier values than the younger one (Fig. 2), a brick-red pebble conglomerate containing suggesting that the climate may have become abundant clasts of the local basement and DISCUSSION decreasingly arid during the time of calcrete rare calcrete clasts. The contact is an erosive The uppermost host material of the PCH formation, culminating eventually in termina- surface with a stepped paleotopography that at the classic Newton Point locality is litho- tion of the phreatic calcretization process. displays irregularities as much as 75 cm deep. logically similar to sandy conglomerate of the The only calcrete clast retrieved from this basal local Upper Old Red Sandstone (unit 1) and is Inferred Sequence of Events at Hutton’s conglomerate (sample g in Fig. 2) shows δ13C interpreted to be a small remnant of that unit, Unconformity in North Arran and δ18O values similar to those of the underly- spared from early Carboniferous erosion due to 1. Post-Caledonian erosion and weathering ing vadose calcrete, and was probably derived the resistant nature of such calcrete under arid left truncated folds capped by poorly developed from it. climatic conditions (Arakel and McConchie, regolith in Dalradian basement rocks (unit 0) 1982; Jutras et al., 1999, 2007). Although only (Fig. 3A). Fallen Rocks Locality at Newton Point is the calcrete that is discon- 2. This surface was buried by red fl uvial The disconformable contact between formably below the Kinnesswood Formation deposits of the lower to upper Devonian Old PCH(1) and unit 2 is well exposed on a wave- (unit 2) suffi ciently well exposed to be fi rmly Red Sandstone (unit 1, Fig. 3B), which var- cut platform beside the southernmost headland interpreted as a PCH, massive calcrete occur- ies tremendously in thickness across faults on at Fallen Rocks (Fig. 1D), ~8 km southeast of rences at Fallen Rocks and Corrie are at the Arran. Newton Point. This disconformity is above a same stratigraphic level, have similar δ13C and 3. According to petrographic evidence thick succession of Devonian red beds (unit 1) δ18O values, and are probably part of the same at Newton Point and at Fallen Rocks,

GEOLOGY, February 2011 149 ~1.5 m below the Caledonian unconformity ACKNOWLEDGMENTS into basement rocks (unit 0) (Fig. 3C). This led We thank the Natural Sciences and Engineering to the formation of a massive phreatic calcrete Research Council of Canada for fi nancial support, and N. James and V.P. Wright for constructive formal hardpan [PCH(0,1)], which assimilated rego- reviews. lith from both the basement and from the basal part of unit 1, leaving only rare dispersed rem- REFERENCES CITED nants of the host materials (Fig. 3C). Anderson, J.G.C., 1947, Dalradian rocks of Arran: 5. The absence of PCH(0,1) at the new, low- Geological Society of Glasgow Transactions, v. 20, p. 264–286. tide site near Newton Point (Fig. 1F) suggests Arakel, A.V., and McConchie, D., 1982, Classi- that this location represents a deeper erosional fi cation and genesis of calcrete and gypsite level than that seen at Hutton’s original site, lithofacies in paleodrainage systems of inland an inference that is supported by the presence Australia and their relationship to carnotite of coarser deposits in the basal Kinnesswood mineralization: Journal of Sedimentary Petrol- ogy, v. 52, p. 1149–1170. Formation (unit 2) at this new site. The ero- British Geological Survey, 1987, Solid geology of sional event resulted in complete removal of Arran: Scotland Special Sheet: London, British unit 1 (Upper Old Red Sandstone) material Geological Survey, scale 1:50 000. that was above the water table (and therefore Colson, J., and Cojan, I., 1996, Groundwater dolo- cretes in a -marginal environment; an alter- not subjected to phreatic calcretization), par- native model for dolocrete formation in conti- tial removal of PCH(0,1) at Hutton’s original nental settings (Danian of the Provence Basin, site, and complete removal in the case of the France): Sedimentology, v. 43, p. 175–188, new, low-tide site (Fig. 3D). Such calcretes are doi: 10.1111/j.1365-3091.1996.tb01466.x. typically 10–12 m thick in complete sections Dever, L., Fontes, J.C., and Riché, G., 1987, Isotopic approach to calcite dissolution and precipita- (Arakel and McConchie, 1982; Jacobson et al., tion in soils under semi-arid conditions: Chem- 1988; Jutras et al., 1999). ical Geology, v. 66, p. 307–314. 6. The vadose calcrete and its red regolith Gunn, W., and Geikie, A., 1947, Geological map of host [VC(0)] at the low-tide site are inter- Arran: Scotland (second edition): Geological Survey of Great Britain, scale 1:50 000. preted to have been produced during a weath- Hutton, J., 1899, Theory of the Earth: With proofs ering event that followed erosion of PCH(0,1) and illustrations: Volume III (A. Geikie, ed.): (Fig. 3D), as they would have been unlikely to London, Geological Society of London, 278 p. survive the phreatic calcretization event if they Jacobson, G., Arakel, A.V., and Chen Yijian, 1988, had developed previously. The central Australian groundwater discharge zone: Evolution of associated calcrete and 7. The new regolith was eventually subjected gypcrete deposits: Australian Journal of Earth to erosion and partly incorporated within basal Sciences, v. 35, p. 549–565, doi: 10.1080/ beds of the Kinnesswood Formation (unit 2) 08120098808729469. (Fig. 3E). Jutras, P., Prichonnet, G., and von Bitter, P., 1999, The La Coulée Formation, a new post-Acadian continental clastic unit bearing groundwater CONCLUSIONS calcretes, Gaspé Peninsula, Québec: Atlantic At the classic locality of Hutton’s Uncon- Geology, v. 35, p. 139–156. formity in north Arran, a complex geological Jutras, P., Utting, J., and McLeod, J., 2007, Link history occurred between Caledonian defor- between long-lasting evaporitic basins and the development of thick and massive phreatic cal- mation and the burial of folded and truncated crete hardpans in the Mississippian Windsor Dalradian rocks by the early Carboniferous and Percé groups of eastern Canada: Sedimen- Kinnesswood Formation. Development of an tary Geology, v. 201, p. 75–92, doi: 10.1016/j erosion-resistant PCH allowed the preserva- .sedgeo.2007.04.008. Read, W.A., Browne, M.A.E., Stephenson, D., and tion of some of that history. However, because Upton, B.G.J., 2002, Carboniferous, in Trewin, they may transgress and mask lithologic N.H., ed., The : London, Figure 3. A–E: Inferred sequence of events boundaries, PCHs can also create problems Geological Society of London, p. 251–299. for formation of Hutton’s Unconformity near in stratigraphic interpretations (e.g., Jutras et Tomkeieff, S.I., 1953, Hutton’s Unconformity, Isle Newton Point (see text for discussion). PCH— al., 2007, and references therein). The Newton of Arran: Geological Magazine, v. 90, p. 404– 408, doi: 10.1017/S0016756800065924. phreatic calcrete hardpan; VC—vadose cal- Point occurrence is no exception, with its cryp- crete; UORS—Upper Old Red Sandstone. Tomkeieff, S.I., 1963, Unconformity—An historical tic angular unconformity between Dalradian study: Geologists’ Association Proceedings, metasedimentary rocks and upper Devonian v. 73, p. 383–417. conglomerate (placed at three different posi- Young, G.M., and Caldwell, W.G.E., 2009, A new look at an old unconformity: Field and geo- conglomerate representing the youngest pre- tions successively by Tomkeieff, 1953, 1963, chemical data from James Hutton’s original served Upper Old Red Sandstone (unit 1) on and Young and Caldwell, 2009), a lithodemic unconformity on the Isle of Arran, Scotland: Arran was poorly cemented and subsequently overprint by phreatic calcrete, which led A. Geologists’ Association Proceedings, v. 120, weathered prior to phreatic calcretization (the Geikie (see Hutton, 1899) and subsequent gen- p. 65–75. next event). erations of geologists to place the unconform- 4. The resulting regolith developed in unit able contact 1.5 m too low, and a previously Manuscript received 18 June 2010 Revised manuscript received 7 September 2010 1 was calcretized by saturated phreatic water unidentifi ed disconformity between the PCH Manuscript accepted 12 September 2010 (Fig. 3C). At Hutton’s Unconformity near and early Carboniferous red beds, all within a Newton Point, the saturated aquifer penetrated section of <3 m. Printed in USA

150 GEOLOGY, February 2011