Lithostratigraphy, chronostratigraphy, geochronometry, Cainozoic of north-east Scotland
From: Merritt, J W, Auton, C A, Connell, E R, Hall, A M, and Peacock, J D. 2003. Cainozoic geology and landscape evolution of north-east Scotland. Memoir of the British Geological Survey, sheets 66E, 67, 76E, 77, 86E, 87W, 87E, 95, 96W, 96E and 97 (Scotland).
Classification of deposits
The Drift editions of most BGS 1:50 000 scale maps covering north-east Scotland show deposits that have been classified using a morpho-lithogenetic scheme and identified by standard colours and symbols (Figure 5), for example ‘Glaciofluvial ice-contact deposits’ in pink or crimson and ‘Peat’ in brown. This method of classification has proved to be a practical means of mapping deposits cropping out at the surface and it is particularly appropriate for air photo interpretation. The Quaternary deposits classified in this way are described systematically in Chapter 6.
The symbols have been embellished on more recently published maps, such that lithostratigraphical map codes are added as superscripts, lithological codes as prefixes, chronostratigraphical qualifiers as subscripts and inferred depositional environments as suffixes (Figure 5). More subcategories are generally found on the detailed 1:10 000 or 1:10 560 ‘clean copies’ that are available for large parts of the district (see Information sources). The symbol scheme has been modified over the years with the result that there are variations in presentation between sheets, although the differences are largely semantic (Table 3).
The morpho-lithogenetic scheme does have some failings. For example, it does not easily accommodate complicated sequences of deposits or bodies of sediment that contain a mix of lithologies. In order to overcome these difficulties, the more recently published maps depict some deposits that have been classified lithostratigraphically in accordance with internationally agreed codes (Hedberg, 1976). The Neogene deposits have also been classified in this way. It is beyond the scope of this publication to inaugurate a formal lithostratigraphical framework that systematically encompasses all the Quaternary deposits occurring across the district, but a start has been made with the glacigenic deposits (Chapter 8). For example, deposits laid down in association with ice that flowed out of the Moray Firth are placed within the Banffshire Coast Drift Group (formerly the ‘blue-grey series’), whereas deposits laid down by more local ice from the Grampian mountains are allotted to the East Grampian Drift Group (formerly the ‘inland series’). The former ‘red series’ is divided so that deposits laid down by ice flowing through Strathmore (Mearns Drift Group) are distinguished from those deposited by ice moving onshore from the North Sea (Logie-Buchan Drift Group). Deposits laid down by ice entering the district from the Monadhliath mountains and the Spey valley are placed in the Central Grampian Drift Group. Subdivisions at formation, member and bed level are described in Chapter 8 and the type localities of most units are described in Appendix 1. The original names of units are retained whenever possible.
A new mapping-related lithostratigraphical framework for the Quaternary of Britain has been proposed recently by McMillan and Hamblin (2000). For example, fluvial, lacustrine, estuarine, coastal and aeolian deposits are defined geographically within a series of Catchment Groups defined by major river drainage systems. It is proposed to change the ranking and naming of the glacigenic groups described here slightly. The Central Grampian Drift Group may be subdivided in order to identify the sandy tills of the Inverness area, which extend towards Elgin.
Chronostratigraphy is the definition of internationally agreed boundaries to units of strata (systems, series and stages) that correspond to intervals of geological time (periods, epochs and ages). The Cainozoic era is divided here into three periods, the Palaeogene, Neogene and Quaternary (the first two formerly known as the ‘Tertiary’; Table 2). The Quaternary period embraces two epochs, the Pleistocene (‘Ice Age’) and the Holocene (the last 10 ka), but the latter is really the last of a series of relatively short-lived, warm interglacial stages separating longer glacial stages. The Quaternary has been divided traditionally into climato-stratigraphical stages because climatic change has had a dominant influence on sedimentation. The British chronostratigraphy embraces an alternating sequence of glacial and interglacial stages that have been defined principally in East Anglia (Figure 6; Table 1), but problems have arisen because the geological record there is incomplete. The more comprehensive north-west European scheme has been adopted offshore. The placing of the base of the Quaternary is discussed in Chapter 5. In northern Britain, glacial, periglacial (tundra) and boreal (like central Scandinavia) environments existed during the glacial stages, and both boreal and temperate environments occurred at times during the interglacial stages.
The Pleistocene Series is divided formally into Lower, Middle and Upper as shown on Figure 33. The Quaternary Period is divided here into Early, Middle and Late as shown on Table 1. Although capital letters are used here for these divisions of the Quaternary, the precise definition and formal status of many of them are not agreed internationally.
The terrestrial record of the Quaternary is fragmentary because later glaciations have removed most of the evidence of earlier events. In contrast, cores taken from deep ocean floors provide a more continuous record. A complete chronostratigraphy has been based on the analysis of calcareous microfossils preserved in these sediments. The changing microfaunal assemblages preserve a record of fluctuating oceanic water temperature (Ruddiman and Raymo, 1988) and the relative proportions of the two common isotopes of oxygen contained in the tests provide a proxy record of global ice volume and sea level (Imbrie et al., 1984). During glacial periods, water is lost from the oceans to form ice sheets. The oceans consequently become relatively enriched in water containing the heavy isotope of oxygen (O18). The oscillating O18 content of ocean water (as determined from the carbonate tests of organisms that lived in it) can thus be used as an index of ice sheet growth and decay. The oxygen isotope stages thus defined now provide a universal means of dividing the Quaternary (Emiliani, 1954; Shackleton and Opdyke, 1973). The even numbered stages generally refer to cold periods and the odd numbered stages to the warm part of each global glacial–interglacial cycle. An exception is Stage 5, which is broken down into 5a–5e where only 5e is generally accepted to represent a full interglacial (Figure 7). The oxygen isotope stages (OIS) are used in this publication in association with the ‘long’ British stratigraphy onshore and the north-west European scheme offshore (Figure 6), although many correlations with oxygen isotope stages remain uncertain and are the subject of active research.
Radiocarbon dating is the principal method for determining the age of organic materials from the present to about 60 000 years ago. The method takes advantage of the natural occurrence of a radioactive isotope of carbon (14C), which is produced in the upper atmosphere by the interaction of cosmic ray neutrons with nitrogen-14. The carbon-14 is taken up by plants during photosynthesis and then passes up the food chain to other organisms. Once an organism dies the 14C in its structure is gradually lost by radioactive disintegration back to 14N. Carbon14 decays by beta particle emission with a half life of 5730 years. Conventional radiocarbon dating involves the measurement of beta particle transformation and is primarily limited by the amount of raw carbon that is available for counting. The Accelerator Mass Spectrometer (AMS) technique involves counting the carbon atoms rather than the beta particles emitted during decay. AMS radiocarbon dating is generally superior to conventional methods and, importantly, only very small samples (from 2 mg to 5 mg) are required for dating.
Dates quoted in this memoir in the style 12.5 ka* BP are conventional (uncalibrated) radiocarbon years before present (taken as 1950). Dates on shell material should have been adjusted to take into account the ‘marine reservoir effect’ for British waters, which involves subtracting 405 ± 40 years from the conventional radiocarbon age. It should be noted that calendar (sidereal) ages based on historical records, annual layering in ice cores, tree rings, varve counting etc are somewhat older than conventional radiocarbon ages. If ages quoted in this publication have been calibrated to take this disparity into account, the Calib 3.0 radiocarbon calibration program (Stuiver and Reimer, 1993) has been used, and they are quoted as ‘cal yr BP’.
Amino-acid dating has been used widely in north-east Scotland to date deposits older than the last glaciation. The technique mainly involves the analysis of proteins locked-up in the shells of certain molluscs and tests of foraminiferids (Sykes, 1991). Several time-dependent chemical reactions occur upon death that provide a means for relative dating. The most useful reaction is racemisation, where L-isomers of individual amino acids transform to the D-configuration. North-east Scotland has been important in the development of British aminostratigraphy, where amino-acid ratios have been used to rank fossils and their associated sediments according to relative age (for example Figure 43).
Luminescence dating is the collective term that covers a variety of dating methods, specifically thermoluminescence (TL) and optically stimulated luminescence (OSL). These methods are based on the principle that naturally occurring minerals like quartz and feldspar can act as dose meters, recording the amount of nuclear radiation that they have been exposed to (Miller, 1990). The total radioactive dose can be calculated by making a series of luminescence measurements, either where the sample is heated (TL) or exposed to a monochromatic or narrow band source of light (OSL). The methods are particularly applicable for dating loess and wind-blown sand, but less reliable for glaciofluvial and glaciolacustrine deposits. Several dates have been obtained from the sequences at Kirkhill Leys, Teindland and Howe of Byth (Appendix 1).
* ka thousand years