Abstract
A dropstone horizon is described from lake deposits in a palaeo-valley from the c. 1000 Ma Diabaig Formation, Torridon Group, NW Scotland. Dropstones occur in wave-rippled, fine-grained sandstones and siltstones that contain desiccation and syneresis cracks indicative of fluctuating lake levels. Five locally derived dropstones occur at the same horizon over lateral distance of 250 m and display clear evidence of deflection and penetration of laminae at the base, with thinning, onlap and draping of laminae on to clast margins and tops. Mechanisms of dropstone formation are discussed, with ice-rafting considered the most likely explanation. It is suggested that rafting was promoted by cold winters at 35° S in the early Neoproterozoic, possibly in an upland setting. Interpretation of the dropstones as ice-rafted debris provides the first physical record of evidence for ice at the Earth's surface during the late Mesoproterozoic to early Neoproterozoic.
The Neoproterozoic Torridon Group outcrops throughout NW Scotland (Fig. 1). It unconformably overlies Lewisian Gneiss and Mesoproterozoic Stoer Group sediments and is unconformably overlain by Lower Cambrian quartzites (Stewart 2002; Fig. 2). The Torridon Group comprises four formations: the basal Diabaig overlain by the Applecross, which is in turn overlain by the Aultbea and Cailleach Head formations (Fig. 2). The c. 1000 Ma Diabaig Formation has been studied extensively by previous authors who have provided a framework for understanding both the depositional setting and controls on the distribution of the formation (Stewart 1972, 1988, 2002; Stewart & Parker 1979; Rodd & Stewart 1992; Turnbull et al. 1996; Park et al. 2002; Prave 2002; Kinnaird et al. 2007; Callow et al. 2011; Parnell et al. 2011). The thickness and distribution of the Diabaig Formation is strongly influenced by the underlying palaeo-topography developed on the Stoer Group and Lewisian Gneiss (Stewart 1972; Fig. 2). In contrast the overlying sandstones of the Applecross Formation show little influence of underlying topography on sedimentation and record the development of extensive fluvial systems that drained to the SE across the entire Torridon Group outcrop belt (Stewart 2002).
Location map showing the distribution of the Diabaig Formation in the Torridon area and location of the studied section. Map based on Stewart (2002) and Callow et al. (2011).
Generalized lithostratigraphy of the Torridonian succession in NW Scotland showing the stratigraphic position of the Diabaig Formation. Radiometric dates from (1) Turnbull et al. (1996) and (2) Parnell et al. (2011). Based on Lawson (1965), Stewart (1966, 2002), Williams (1966) and Callow et al. (2011).
In the eastern and southern part of the Torridon Group outcrop, the Diabaig Formation records deposition within a series of palaeo-valleys, which are up to 600 m deep, cut into the Lewisian Gneiss (Stewart 1972, 1988, 2002; Rodd & Stewart 1992; Park et al. 2002; Kinnaird et al. 2007). Along palaeo-valley margins, locally derived angular breccias record alluvial fan/fan-delta development (Stewart 1988, 2002). These proximal deposits pass both laterally and vertically into tabular sandstones and siltstones, which in turn transition into interbedded grey siltstones, claystones and thin, fine-grained, sandstones (Stewart 1988, 2002; Park et al. 2002). The sandstones and siltstones display ubiquitous desiccation cracks and wave ripples, with syneresis cracks and microbial structures also documented (Prave 2002; Callow et al. 2011). The Diabaig Formation is interpreted to record shallow siliciclastic lake deposition with fluctuating water levels (Stewart 1988, 2002; Callow et al. 2011). Here we present new field observations that describe an additional depositional component of the Diabaig Formation, which we interpret to represent ice-rafted dropstones. Following this, we discuss the significance of this interpretation within the context of the early Neoproterozoic climate and highlight the possibility that the dropstones provide the first affirmative physical evidence for ice at the Earth's surface in the Tonian.
Description
The studied section in the Diabaig Formation is exposed on the southern shore of Upper Loch Torridon in Ob Mheallaidh Bay (Fig. 1) at the high tide mark [NG 83055375]. It has been described previously by Stewart (2002) and Callow et al. (2011) and is located in the centre of what is referred to as the Shieldaig palaeo-valley (Fig. 3). The palaeo-valley is oriented north–south and present-day exposure indicates a minimum erosional relief of 200 m during Diabaig deposition, although this depth could be considerably greater as the base of the valley is not exposed.
Annotated Google Earth image (Image Landsat/Copernicus, copyright 2019 Getmapping plc, ©2018 Google) illustrating the Sheldaig palaeo-valley cut into the Lewisian basement and infilled with the Diabaig Formation overlain by the Applecross Formation. The studied section is in the centre of the palaeo-valley. View to south.
The studied section is exposed in the centre of the palaeo-valley over a distance of 300 m along the shoreline and comprises 2.2 m of very fine- to fine-grained sandstones interbedded with siltstones and subordinate red claystones (Figs 3 and 4). Occasional thin (up to 3 mm) lenses of medium sandstone are present in some horizons. Sedimentary structures are dominated by short-wavelength, symmetrical to asymmetrical wave ripples (3–7 cm wavelength), 0.5 to 4 cm thick and millimetre-scale horizontal to slightly undulose laminae. Occasional desiccation cracks and syneresis cracks are visible on bedding plane surfaces. The logged section is interpreted to record deposition in a relatively low-energy, shallow, siliciclastic lake with fluctuating lake levels, as indicated by the syneresis and desiccation cracks (Callow et al. 2011).
Logged section and photograph of the section illustrating the sedimentological character of the Diabaig Formation. Note the presence of the dropstone.
Dropstones
A 20 cm thick package of very fine- to fine-grained, wave-rippled sandstones 1.2 m above the base of the section contains at least five outsized clasts (Fig. 4). Clast bases show clear evidence of deflection and penetration of laminae with thinning, onlap and draping of laminae on to clast margins and tops, with sediment compaction strain shadows at clast edges (Fig. 5). The clasts are isolated, restricted to a single horizon and occur over a lateral distance of 250 m. Clast long axes range in length from 3.5 to 9 cm. Of the clasts, four have long axes that lie parallel to bedding, with one clast long axis oriented at 60° to bedding (Fig. 5). Clasts range in shape from subangular to subrounded. Two clasts comprise Lewisian gneiss, two comprise amphibolite and one includes both lithologies. The amphibolite occurs as dykes within the gneiss in the adjacent area (Sutton & Watson 1950). The clast compositions, size and shape suggest local derivation.
Examples of dropstones located at the same horizon across the outcrop over 250 m. Note the deflection of laminae above and below the clasts and the variations in size, shape and composition.
Interpretation of dropstones
A dropstone is defined as a clast of anomalous size, and/or lithology, indicative of vertical or oblique introduction into the host sediment (Bennett et al. 1996; Le Heron 2015). The outsize nature and occurrence within very fine to fine-grained, wave-rippled sandstone, the depression and piercing of laminae beneath clasts and the thinning, onlap and draping of laminae above the clasts suggest that they represent dropstones.
Four main processes of transport and formation of dropstones have been documented (Bennett et al. 1996): biological rafting, flotation, ice rafting and projectiles. Whilst algal mat development has been identified in the Diabaig Formation and the older Stoer Group (Upfold 1984; Prave 2002; Callow et al. 2011), it seems unlikely that floating algal mats would be capable of transporting large pebbles of relatively dense amphibolite and gneiss. Similarly, the clast types are too dense for floating to be an appropriate transport mechanism, which is normally associated with coral (Kornicker & Squires 1962), pumice (Whitham & Sparks 1986) or very small, flat, shale clasts (Nordenskiold 1900; Hume 1964). An origin as projectiles in the form of volcanic bombs or meteorites is also inappropriate given the clast composition.
An additional alternative explanation for dropstone formation, which was not considered in the review of Bennett et al (1996), is as possible out-runner clasts that occur at the front of mass flow deposits (Postma et al. 1988; Nemec 1990). These have been described from subaqueous debris fall and flow deposits (Postma et al. 1988; Nemec 1990) and occur at the base of steep slopes in relatively deep water (below storm wave-base) and result in clasts often with load structures isolated within finer-grained sediment. They are commonly related and/or interbedded with coarse-grained debris flow deposits (e.g. Eyles & Januszczak 2007). Whilst it is possible that coarse-grained material could have been introduced into the Diabaig palaeo-valley via gravity flow processes, particularly as breccias are commonly described from the base and margins of the Diabaig palaeo-valleys (Stewart 1988, 2000; Kinnaird et al. 2007), we do not favour this interpretation for the following reasons. The dropstones occur within fine-grained, shallow-water lake deposits and no coarse-grained or debris flow deposits occur within, or are present above or below the section elsewhere in the bay (see Stewart 2002 for a detailed description of the Torridonian succession in the area). The fine-grained character and lack of syn-sedimentary deformation, such as preferred slump or flow directions, in the lake deposits (e.g. Moretti & Sabato 2007) also suggests there was no significant depositional slope at the time of deposition.
If biological, projectile, floating and out-runner clast origins are excluded as an origin for the Diabaig dropstones, the most likely explanation for their presence is that they represent ice-rafted material. Ice-rafted dropstones are well known from Quaternary glacial lake (Carrivick & Tweed 2013) and shallow-marine glacial deposits (e.g. Lønne & Mangerud 1991) and in the rock record from shallow-water glacio-marine (e.g. Eyles 1988) and proglacial or ice-contact lake deposits (e.g. Limarino & Cesari 1988; Buatois & Mangano 1995; Netto et al. 2009). Studies of the Late Carboniferous and Permian glacial lakes of Argentina are particularly noteworthy in that they contain dropstones that occur in wave-rippled and desiccated shallow-water siliciclastic glacial lake deposits that infill a series of palaeo-valleys (Limarino & Cesari 1988; Netto et al. 2009).
Discussion
If an ice-rafted origin for the Diabaig dropstones is accepted, then the implications of ice presence in the Diabaig palaeo-valleys in the early Neoproterozoic requires explanation. The absence of any associated glacial deposits, together with the small number (density) of dropstones and the early Neoproterozoic age of the Diabaig Formation, suggest that the dropstones do not record a period of extensive glaciation. Whilst extensive glacial deposits are known from the Neoproterozoic, they occur some 300 million years after deposition of the Diabaig Formation in the Cryogenian (e.g. Spence et al. 2016), and the early Neoproterozoic Tonian time period is considered to be devoid of glacial activity (Young 2018). Whilst the precise age of the Diabaig Formation is uncertain – the only available date of 994 ± 48 Ma being derived from a Rb–Sr whole-rock isochron on diagenetic phosphate (Turnbull et al. 1996) – it is likely that dropstones in the Diabaig Formation record the only direct evidence for ice presence at the Earth's surface between the late Mesoproterozoic (Stenian) and early Neoproterozoic (Tonian).
An alternative ice-rafted interpretation for the dropstones could be related to cold winter temperatures and formation of ice floes on the Diabaig lakes (Fig. 6). In this scenario, such outsized clasts could be incorporated into lake ice either by basal freeze-on near the lake shore or supplied by snow and debris avalanches reaching the frozen lake surface (Luckman 1975). This debris-rich ice floe, during melt season, could be moved by winds and waves releasing dropstones in more distal parts of the lake.
Schematic diagram illustrating seasonal ice-rafting mechanism for dropstone emplacement in the Diabaig Formation: (a) summer/autumnal – background deposition, no lake ice; (b) winter – marginal to extensive lake ice formation with clast incorporation; (c) spring – melting of seasonal ice, ice-rafting and flotation, dropstone deposition in deep lake.
During deposition of the Diabaig Formation, NW Scotland is considered to have been positioned in the southern hemisphere at a latitude of c. 35° S, although this is poorly constrained and could range between 30° S and 50° S (Stewart 2002). The presence of topographic relief of up to 600 m at the base of the Diabaig Formation could indicate that lacustrine deposition took place in an upland setting at altitudes favouring winter ice formation.
Conclusions
Dropstones are described from a palaeo-valley in the Diabaig Formation of the Torridon Group located on the southern shore of Upper Loch Torridon. The dropstones occur within very fine- to fine-grained, wave-rippled sandstones displaying desiccation and syneresis cracks and which are interpreted to represent shallow lacustrine sedimentation. The origin of the dropstones is ascribed to ice-rafting because alternative explanations such as out-runner clasts at the end of debris flows, flotation, biological rafting and projectiles can be excluded. Whilst the dropstones are considered to record ice rafting, they are not thought to represent a period of glaciation due to the lack of associated glacial deposits and the absence of late Mesoproterozoic to early Neoproterozic glacial deposits in general. It is suggested that the dropstones provide a transient record of cold winters at 35° S in the late Mesoproterozoic to early Neoproterozoic promoting ice-rafting within valley confined (upland?) lakes.
Acknowledgements
We thank Daniel Le Heron and an anonymous reviewer for their constructive comments.
Funding
This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.
Author contributions
AH: conceptualization (equal), investigation (equal), writing – original draft (lead); BK: conceptualization (equal), investigation (equal), writing – original draft (equal); JP: conceptualization (equal), investigation (equal), writing – original draft (equal); JA: conceptualization (equal), investigation (equal), writing – original draft (equal)
Scientific editing by Romesh Palamakumbura
- © 2020 The Author(s). Published by The Geological Society of London for EGS and GSG. All rights reserved
References
Ice-rafting in lakes in the early Neoproterozoic: dropstones in the Diabaig Formation, Torridon Group, NW Scotland
