Reinterpretation of James Hutton’s historic discovery on the Isle of Arran as a double unconformity masked by a phreatic calcrete hardpan Pierre Jutras1*, Grant M. Young2, and W. Glen E. Caldwell2 1Department of Geology, Saint Mary’s University, Halifax, Nova Scotia B3H 3C3, Canada 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 rock 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 Scotland, 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 (Dalradian) metasedimentary rocks from upper Paleozoic red beds. The mas- development of ideas concerning the antiquity sive PCH developed in Late Devonian red conglomerate 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 Edinburgh. This is partly because Carboniferous red conglomerate and sandstone. Thus, there are two unconformities at Hut- the unconformable contact in Arran is obscured ton’s historical site, but the younger has previously gone unnoticed, and the two red bed 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 quartz pebbles (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 Famennian 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 pebble 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 for :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 Australia 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.