The palaeogeographic significance of isotopic age determinations on detrital micas from the triassic of the Stockport-Macclesfield district, Cheshire, England

Palaeogeography, Palaeoclimatology, Palaeoecology Elsevier Publishing Company, Amsterdam-Printed in The Netherlands

T H E P A L A E O G E O G R A P H I C S I G N I F I C A N C E OF ISOTOPIC A G E D E T E R M I N A T I O N S ON D E T R I T A L MICAS F R O M T H E TRIASSIC OF T H E S T O C K P O R T - M A C C L E S F I E L D DISTRICT, CHESHIRE, E N G L A N D

F. J. F1TCH~ J. A. MILLER AND D. B. THOMPSON

Department of Geology, Birbeck College, London (Great Britain); Department of Geodesy and Geophysics, University of Cambridge, Cambridge (Great Britain); Department of Geology, University of Manchester, Manchester (Great Britain) (Received August 12, 1966)

SUMMARY

A trial investigation of the possible geological application of isotopic age determinations to sedimentary studies. Forty-one new potassium-argon age determinations on detrital mica concentrates from the Triassic of the Stockport-Macclesfield district, Cheshire, and two determinations on detrital micas from the Millstone Grit of north Derbyshire are reported. A general discussion of the theoretical problems involved in the interpretation of isotopic results from detrital micas is followed by a detailed analysis of the sedimentation, petrology and subsequent history of the Triassic samples used in the study. It is shown that the isotopic results can be used to supplement stratigraphical and heavy mineral studies and make palaeogeographic interpretation both easier and more certain. In particular, it is concluded that the bulk of the Triassic of the Stockport-Macclesfield district was almost certainly derived from the south, being the deposits of a river that drained the Wessex-Channel Basin via the Worcester Graben. Differences in their isotopic patterns enable a clear distinction to be made between the Bunter and Keuper deposits, and it is suggested that this kind of work may have economic applications.

INTRODUCTION

The principal aim of this research was to discover whether isotopic dating of detrital micas would be of use in studies of sedimentary provenance. The Triassic of the Cheshire Basin in England was selected for a detailed trial investigation because it contains a number of distinctly mica-rich horizons. Various theoretical problems and technical difficulties became apparent as the work progressed, and Palaeogeography, Palaeoclimatol., Palaeoecol., 2 (1966) 281-312

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in this paper these are discussed in conjunction with a preliminary geological interpretation of the results obtained. The samples of micaceous sandstones and shales used were collected from boreholes and outcrops between Stockport and Macclesfield, in the northeast of the Permo-Triassic Cheshire Basin. This sedimentary basin was initiated by regional warping followed by block faulting during the Late Carboniferous and Early Permian Variscan earth movements. It was part of an extensive region of generally negative movement, bounded on the north by the dissected remnants of the Caledonides and on the south by the uplifted foreland and new fold mountains of Armorica. It is now generally thought that the Cheshire Basin was infilled during Permian and Triassic times by material largely derived from the denudation of high areas lying to the south (WILLS, 1951, 1956), accumulating as much as 7,000 ft. of sediment in the central and southern parts of the basin (KENT, 1949, P. E. KENT, personal communication, 1965; PUGH, 1960); but derivation from the north and west, with components from a North Welsh massif, has been suggested by many previous workers (see BONNEY,1880; TRAVISand GREENWOOD,1911 ; HARRIS,1924; SMITHSON, 1931). An outline of the succession, lithologies and proposed environments of deposition of the Permian and Triassic rocks of the Cheshire Basin is set out in Table I. Structurally, the basin consists of a double syncline, aligned northeastsouthwest and shaped rather like a boat, the prow lying in the southwest to the north of Shrewsbury, the rounded stern, complicated by the Wilmslow anticline, lying to the north east in the Stockport-Manchester area. The main synclinal axes trend northeast-southwest, from Macclesfield to a point approximately 8 km west-southwest of Nantwich and from Betley through Prees to a point between Cockshult and Burlton. The folding of the Permian and Triassic rocks is post-dated by powerful, generally north-south, faults which represent the southern portion of a zone of fracturing and subsidence which can be traced from the Atlantic continental margin through Kintyre, Galloway and west Cumberland to Church Stretton and beyond. The age of the folding and faulting is post-Liassic/pre-Pleistocene; most observers suggest a Tertiary age (e.g., TAYLORet al., 1963), although SHACKLETON (1953, p.33) has raised the possibility of Cretaceous movements.

NEW POTASSIUM-ARGONAGE DETERMINATIONS Forty-three new age determinations are presented in Table II. Argon extraction and purifications were carried out in the manner described in MILLER and BROWN (1964). Isotopic ratios were measured either using a conventional 90 ° direct focussing mass spectrometer or with the omegatron-type machine described recentPalaeogeography, Palaeoclimatol., Palaeoecol., 2 (1966) 281-312

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ly by GRASTY and MILLER (1965). Enriched argon-38 was employed as the internal standard (spike). Potassium oxide contents were determined by flame photometry. Several repeat determinations were made on each sample. Errors in radiogenic argon volume arising from uncertainties in the isotopic ratios of the argon sample and spike volume together with those introduced in the determination of potassium oxide content are combined. The error in millions of years associated with each separate age determination is calculated as described in MILLER and FITCH (1964).

GENERALTHEORETICALDISCUSSION Isotopic ages obtained from detrital mica in sedimentary rocks cannot be interpreted satisfactorily until the following factors have been taken into account: (1) The accuracy of the geological age estimates for the sedimentary strata sampled. (2) The adequacy of the sampling in the field and the efficiency of mica separation in the laboratory. (3) The types of sediment sampled, the nature of the sedimentary environment and the possibility of argon loss from micas during the initial weathering, erosion and sedimentation processes. (4) Events in the subsequent history of the sediment (diagenesis, faulting, folding, metamorphism and/or metasomatism associated with mineralization) which could cause argon loss, or could cause dilution of the sample by the introduction of secondary micas. (5) The freedom of the samples from argon loss caused by recent groundwater leaching or surface weathering effects. In order to evaluate these factors fully it is necessary to carry out a detailed geological and petrological analysis of all samples and strata involved. Only after this has been done can the possible effects of initial and subsequent argon loss be estimated. If, after consideration of these factors, it can be shown that the micas from an individual sedimentary sample have not suffered excessive argon loss, then the theoretical situation can be stated thus: "The average of the apparent mica ages obtained from a sedimentary horizon equals the average apparent age of the micas present in the source area when these are mixed in the proportions provided by contemporaneous erosion from the various parts of the source area". The various parts of the source area may be composed of rocks of the same or of different ages. In the simplest case of a homogeneous source area, composed of rocks of uniform age, the apparent ages of the micas in the sediment should match those in the source area. If there have been Palaeogeography, Palaeoclimatol., Palaeoecol., 2 (1966) 281-312

ISOTOPIC AGE DETERMINATIONS ON DETRITAL MICAS

289

minor amounts of argon loss from the detrital micas, the ages from the sediment will be minimum ages, marginally below those of the source area. Minimum ages should always be expected: excess argon is unlikely to be present. Although secondary muscovite (sericite) and more rarely, biotite, may arise during intense mineralization or later metamorphism and cause low ages to be obtained as a result of dilution of the sample, most discrepantly low ages are from biotite that has lost argon during chemical weathering and subsequent partial alteration. Experience of mica age determination has shown that muscovite is more resistant than biotite to argon loss because it is less prone to alteration. By constructing a histogram of the age results from numerous muscovitebiotite pairs, separated from a sequence of different horizons within a single sedimentary formation, it is possible to get some idea of the spread of ages due to variation of apparent age in the source area and to varying argon loss by the micas in the sediment. Sharp single peaks on the detrital muscovite/biotite histograms indicate homogeneity in the source area over the time-spread of the sampling (see Fig. 1a). Inhomogeneity in the source area will lead to diffuse peaks (see Fig. 1b). The more diffuse the peaks become the less homogeneous is the source area. From a heterogeneous source area successive flushes of sediment will be an ever-changing sample of the micas present, and this must necessarily produce very different apparent ages from each detrital mica sample. A histogram of the ages of detrital micas from such a succession will show a large spread and be inconclusive (see Fig.lc). Separation of the muscovite and biotite peaks on the histogram may mean that argon loss is appreciable in the biotites, or, alternatively, that the two minerals are of different ages in the source area.

EVALUATION OF SAMPLES USED IN THIS STUDY

(1) The accuracy of the geological age estimates for the sediments The lithological boundaries of continental fluvial deposits are notoriously diachronous. The boundary between the Bunter Pebble Bed and the Bunter Upper Mottled Sandstone Formation is taken where pebbles cease to be present, although this may occur at different horizons in different places. The boundary between the Upper Mottled Sandstone and the Lower Keuper Sandstone Formations is taken at the incoming of coarse pebbly sandstones, a horizon that can be more easily defined within a limited region. The junction between the Lower Keuper Sandstone and the Keuper Waterstone Formations is taken at the point where thick sequences of interbedded, flaggy, ripple-bedded, ripple-marked sandstones and shales or mudstones, all with signs of evaporite minerals, appear in the succession; again this may be at very variable levels within even a local sequence. In none of these cases, therefore, save possibly that of the base of the Lower Keuper SandPalaeogeography, Palaeoclimatol., Palaeoecol.,

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F.J. FITCH, J. A. MILLER AND D. B. THOMPSON

stone Formation,

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ISOTOPIC AGE DETERMINATIONS ON DETRITAL MICAS

(2) Adequacy of sampling Samples were collected from the outcrop and from two boreholes but the stratigraphic distance apart of suitable samples was not uniform. In a final stratigraphic analysis of the Permian and Triassic rocks of this part of the Cheshire Basin, it would be advantageous if the Lower Keuper Sandstone and Bunter Upper Mottled Sandstone Formations could be more adequately sampled. Samples were only taken from the upper part of the Bunter Pebble Bed Formation in this study. Normally each sample consisted of several kilogrammes of sediment from which a few grammes of both light and dark mica were separated. Dark micas were concentrated magnetically, light micas were isolated using a vibrating table. Inspection of the mica separates by microscope revealed that perfect separation was never achieved, but that contamination of the muscovite fraction by dark mica and vice versa was never greater than 1 ~ . Assuming minor preferential argon loss from biotite, this contamination would cause slightly lower apparent ages to be obtained from light mica and slightly higher values from dark mica. The biotite fraction was occasionally further contaminated by the presence of recognisable chlorite. An attempt was made to separate some of this material and date it separately, but the effort was abortive.

(3) Environment in area of provenance: weathering, erosion, transportation The environment of the possible source areas can only be judged indirectly on general geological grounds. It is likely that the remnants of the Caledonides and the Mercian Highlands area were dissected upland plateaus (WILLS, 1951, 1956) but that to the south, in Armorica, there were high ranges built of young fold mountains. In view of the redness of the final sediments, the source was probably situated within 30 ° of the equator (COLLINSON, 1966). If it is accepted, as seems likely, that the red or potentially red, ferruginous pigment of the sediments is derived from the soils of these regions, (see FRIEND, 1966) then the climate in the source area was tropical with hot wet and hot dry seasons. Under these conditions, deep soil profiles generally develop on mature landforms as a result of profound chemical and biochemical weathering. A high proportion of oxidized biotite flakes was found in some of the Keuper samples. This may be due to more intense weathering in the source area, resulting from the progressive development of a more mature land surface or a change of climate over the Bunter-Keuper interval. Fresh and only mildly oxidized biotites are common, however, in both the Bunter and Keuper samples, and contrast sharply with the altered types just mentioned. These, and the ubiquitous fresh feldspars found in the rocks, were probably derived directly from bedrock below the soil profile during deep gullying, whilst the altered mineral grains originated from the regolith above. Palaeogeography, Palaeoclimatol., Palaeoecol.,

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A brief description of the inferred transportation agency and depositional environment for each of the horizons from which micas were separated is given below. Bunter Pebble Bed Formation (a) Samples 969, 971,972: within foresets of red medium- and fine-grained cross-bedded sandstones: transporting agent, swift rivers in flood: environment of deposition, lateral accretion deposits of channel bars and alluvial islands within turbulent shifting braided river channels. (b) Sample 970: in flat-laminated siltstones: transporting agent, swift rivers: environment, settling from high flow currents which are relatively swift in comparison to their depth, in cut-off and secondary channels within the main river channel. Bunter Upper Mottled Sandstone Formation (c) Sample 968: within the flat laminae of red coarse siltstones: environment as for b. Lower Keuper Sandstone Formation (d) Samples 664, 780: in the foresets of both very coarse pebbly sandstone and red fine- and very fine-grained cross-bedded sandstone: environment, lateral accretion in shifting bars of flooded rivers. Rivers probably less braided: some evidence of associated floodbasins. (e) Sample 899: in flat thinly laminated silty shale: environment, tranquil flow settling from suspension in swale-fill ponds, and abandoned channels of the rivers described in d above. Keuper Waterstones Formation (f) Samples 772, 773, 774, 776, 777, 778, 779: within beds and ripple stratified sets of red fine-grained sandstone and siltstone: transporting agent, steady meandering rivers of more constant discharge than in a or d and in the distal parts of their course. (g) Sample 775: in thin and laminated beds of fiat-bedded fine-grained sandstone, siltstone and shale: transporting agent, river or lake currents, probably the former, in which case the deposits are likely to be those of large flood basins or backswamps. In all these cases there is little chance of alteration of micas and argon loss during transport, for, although the material would travel more slowly in traction cases a, c, d and f, and for the greater part of their journey more swiftly in suspension in cases b, e and g, in no instance would the period of transportation be long.

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(4) Subsequent history of sediment (a) Diagenesis Apart from the possibilities of argon loss during erosion as discussed under 3 above, there is a further chance of chemical degeneration and argon loss at the site of deposition. There is evidence, in forms of rolled ventifacts, in case 3(d) that the recently deposited river sediments may have been subject to periods of aridity and fluctuating water table. Alternations of low and high pH/Eh conditions would be experienced in environments of this character. The colour of the sediment suggests conditions of high pH and high Eh in which further decomposition of micas might be considerable. A more important episode of possible argon loss occurs, however, during lithification. Compaction and cementation of the sediment may be accompanied by pressure-induced solution and migration of material, and by the movement of chemically active pore fluids. All of the sediments sampled from the Cheshire Basin have undergone subsequent diagenesis, and some preferential loss of argon by their micas at this time must be expected (see later for details).

(b) Earth movements Folding in the area in question is gentle (post-Middle Liassic/pre-Weichselian, probably Tertiary), involving little plastic deformation. Faulting, of later age, but still within the same period, is more severe. Neither is likely to be the cause of significant argon loss.

(c) Metasomatism associated with mineralization The effects of local mineralization occurring within the same time period as the folding (see conflicting views of MOORBATH, 1962; WARRINGTON, 1965; and F. M. Trotter in: TAYLORet al., 1963), are more difficult to evaluate. The samples at outcrop (Stormy Point and Opencast) are near areas of mineralization, being 250 m and 415 m, respectively, from the main Engine Vein deposit which bears argentiferous galena, small amounts of blende, copper sulphides, and a host of carbonates, oxides, phosphates, arsenates of copper and lead, and within 5 m of smaller fault lines which carry either barytes gangue alone or only a small proportion of ore minerals. The mica samples used in this work were collected from red, as opposed to drab shales and sandstones, the fresh red colour being taken as an indication of the lack of influence of metasomatizing solutions. The likelihood of argon loss from these two samples depends partly upon the interpretation of the environment of mineralization, and this, as always in red beds, is a contentious matter. If the origin were syngenetic (DEWEY and EASTWOOD, 1925) there is less possibility of argon loss, if epigenetic (see WARRIN6TON, 1965, for summary) a considerable chance, for the amount of silver in the galenas could indicate a relatively high temperature in the vicinity of the Engine Vein. It is interesting, therefore, Palaeogeography, Palaeoclimatol., Palaeoecol., 2

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F. J. FITCH,J. A. MILLERAND D. B. THOMPSON

that despite the enhanced chance of argon loss, these two yield the highest apparent ages of all samples. Nevertheless, they must be considered to be minimum ages. The sandstones from the Haymans Farm and Adlington boreholes show little evidence of "mineralization", which is restricted to the presence of thin barytes films in joints and the development of calcareous cements. The former borehole is 1 mile south of the centre of the Alderley mineralization, and the latter 2 miles east of the Kirkley ditch mineralization. Argon loss caused by mineralization from these sources is not therefore likely. There is no evidence in these samples of the development of secondary micas the introduction of which would give rise to low apparent ages.

(5) The effects of recent groundwater leaching and weathering The outcrop samples at 600 ft. A.O.D. were collected well above the water table of the area and could be much affected by the passage of meteoric water. Soil conditions at these heights are podsolic (CHEsWORTH, 1960) and of high Eh and low pH. These conditions might lead to decomposition and argon loss, but the samples were taken from points below the " C " horizon of the pedologists. The samples taken from the boreholes are well below the weathered layer, but within the area of movement of groundwater. Analyses of these groundwaters indicate wholesome water with little dissolved mineral matter. Argon loss from these sources is not likely to be significant. It is now clear that argon losses from factors 3, 4(b) and 5 are likely to be small other than in exceptional circumstances. Losses from factors 4(a) and (e) could be considerable, although factor 4(c) is probably not important in the case of these particular samples. Factor 2 results in less extreme ages. The chemical effects of cement depositing fluids, mineralizing solutions and the circulation of recent groundwater, however slight, act in the same direction as that of initial and recent weathering, and tend to reduce the ages of the micas.

(6) Petrographic analysis A study of thin sections and detrital mounts of the samples analysed confirmed the conclusions reached above. Two major sources of argon loss were suspected: (a) chemical weathering during erosion and sedimentation, and (b) alteration of detrital micas during diagenesis. Further losses caused by subsequent metasomatism may have occurred, but it was impossible to recognise any distinctive evidence of this in the rocks. The rocks examined in thin section ranged from coarse ferruginous, micaceous, feldspathic sandstones to petrographically similar siltstones and shales. Cementing materials included secondary outgrowths of quartz in optical continuity, calcite, barytes, haematite and sparse intergranular films of iron oxides, secondary

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silica, sericitic micas and chlorite. The specimens from the Bunter Pebble Bed Formation were mainly micaceous sandstones of fine- to medium-grain size, with a preponderance of angular and subangular grains. The specimens from the Bunter Upper Mottled Sandstone Formation were, on the whole, coarser grained and contained a much larger proportion of well-rounded grains. The rocks of the overlying Lower Keuper Sandstone Formation, whilst still largely medium-grained micaceous sandstones, contained fewer well-rounded grains. The specimens from the Keuper Waterstones Formation were finer grained, mostly siltstones with thin shale laminae, and contained yet higher proportions of angular grains. The composition of the rocks was variable in detail, but from each formation all of the following detrital components could be recognised: large and small grains of quartz (the larger grains often containing bubble-trains and acicular inclusions, and exhibiting strain effects of the types frequently seen in granite); a great variety of schistose, mylonitized and strained quartz, including numerous composite, sutured grains of strained quartz; vein quartz; quartzites of many kinds, from simple orthoquartzites through greywacke-metaquartzites of all grain sizes to quartz-schists; quartz-chlorite-schists; quartz-muscovite-schists; schorlrock; chert; slates and phyllites; various rhyolitic volcanic rocks; orthoclase; plagioclase; microcline; muscovite; biotite; chlorite; sericitized and kaolinized feldspars; sericitic and chloritic aggregates; tourmaline (green, brown and multicoloured); calcite, apatite; zircon; magnetite; haematite; limonite and leucoxene granules. Less easily, it was possible in some slides to find garnet, topaz, staurolite, microperthite, epidote, ruffle, psammitic- and andalusite-bearing hornfelses and various limestones (some enclosing Foraminifera). The majority of the feldspar grains have a very fresh and unweathered appearance. Plagioclase is very rare in the Bunter rocks, where it is mostly albite. In the Keuper, especially in some of the Waterstone specimens, oligoclase is relatively abundant. Potash feldspar is abundant in all specimens: microcline appears in larger proportion in the Bunter than in the Keuper, but with orthoclase or orthoclase-microperthite, it is present in all specimens. Both light and dark micas are present as thin flakes which have been bent and strained during compaction. The light mica fraction consists of clean fresh muscovite. A little detrital chlorite accompanies the abundant dark mica, which is a strongly pleochroic dark brown and greenish-brown biotite. The biotite flakes often enclose minute granules and filaments of haematite, and occassionally the flakes are interleaved with chloritic or haematitic layers. Part of this haematite in biotite is the result of initial weathering processes, for, rarely, detrital grains of completely oxidized biotite are found alongside the more usual slightly altered grains. Close observation of the processes of diagenesis in these rocks suggests that slight but significant alteration of biotite occurred during compaction and cementation. Sharply bent biotite flakes that have been squeezed between adjacent quartz or other resistant grains have splayed out ends in which chlorite and/or haematitic wedges can be identified. Some argon loss from biotite must be expected as a result Palaeogeography, Palaeoclimat ol., Palaeoecol., 2 (1966) 281-312

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r . J . FITCH, J. A. MILLER AND D. B. THOMPSON

of these diagenetic processes. Very occasionally, especially in the Keuper Waterstones, muscovite can be seen to have suffered similar partial degeneration during diagenesis, but the argon losses to be expected from muscovite as a whole will obviously be very much smaller than those from biotite. The petrographic evidence does not indicate severe argon loss from any of the analysed mica fractions. In the majority of the samples there is no reason to suspect significant losses from muscovite, but minor argon loss from biotite must be expected in all cases, partly the result of chemical weathering during erosion and sedimentation, but principally due to readjustments occurring within the rocks during diagenesis. No major petrographic differences between the four formations were revealed by the thin section analysis. All are of continental red-bed facies and show a variability in composition and cementation typical of such rocks. The composition of the specimens from the Keuper Waterstones Formation was not significantly different from that of the Lower Keuper Sandstone Formation. Both the Keuper rocks and those from the Bunter Pebble Bed Formation contained a larger proportion of chlorite- and sericite-bearing rock fragments (from slates to schists and micaceous aggregates) than do the rocks from the Bunter Upper Mottled Sandstone Formation. The evidence suggests that the ultimate areas of provenance included muscovite-biotite alkali-granites with their aureole rocks, including areas that had undergone extensive tourmalinization, kaolinization and greisening; sedimentary rocks including cherts, limestones and orthoquartzites; a range of low-grade metamorphic rocks including slates, phyllites, greywacke-metaquartzites and low-grade schists and a group of high-grade metamorphic rocks such as could have supplied the staurolite, garnet and intensely strained quartzose fragments. The absence of evidence suggesting large areas of basic rocks undergoing erosion is notable. The Keuper rocks contain less microcline, chert and orthoquartzite, and more schists, rhyolites and chloritic and sericitic rock fragments than do those from the Bunter, and in addition they carry fair amounts of oligoclase in place of the very rare albite seen in the Bunter. These differences may not be significant because of the small size of the sample, but they could be interpreted as suggesting that erosion was biting deeper into a mountain system in the source area as time progressed. The lower proportion of detrital grains containing unstable minerals, and the greater rounding of the quartzose fragments seen in the Bunter Upper Mottled Sandstone specimens, might be taken to indicate less active erosion in the source area during this period.

INTERPRETATION

The search for source areas was carried out by a systematic process of elimination, involving all possible sources, both near and far, in all directions. For the rocks Palaeogeography, Palaeoclimatol., Palaeoecol., 2

(1966) 281-312

ISOTOPIC AGE DETERMINATIONS ON DETR1TAL MICAS

297

of the Cheshire Basin there would appear to be seven possible areas of provenance, which may be grouped conveniently as follows: Areas lying mainly to the north and west

(1) The core of the ancient north Atlantic continent. From what is known of the ages of rocks from the Lewisian of northwest Scotland from Greenland and the Canadian Shield, detrital micas from these sources would normally average over 1,000 million years. (2) The metamorphic Caledonides of Ireland and the Highlands of Scotland. The ages of metamorphic micas from this source would most probably fall within the range 400-550 million years. Micas from the Caledonian Newer Granite suite would have ages between 375 and 420 million years. (3) The non-metamorphic Caledonides of Ireland, Wales, the Lake District and the southern Uplands. Apart from limited exposures of the ancient rocks which form the core of the Irish Sea Land Mass (e.g., Anglesey, from which partially overprinted detrital micas might have ages similar to those discussed under 2 above), there is otherwise nowhere in this region where metamorphic micas of the present size and quantity could have been derived. Micas from Newer Granite suite intrusive rocks in the non-metamorphic Caledonides could provide micas of an age 375~,20 million years, as noted above. Any detrital micas derived from Lower Devonian volcanic rocks in the area of the Caledonides would have ages in the same range as those of the Newer Granite suite.

Areas lying ma#zly to the north and east

(4) Those areas from which reworking of Carboniferous, Avonian, Namurian and Westphalian micas could be achieved. The only likely horizons in the Carboniferous from which detrital micas of the types found in this study could arise, would be those of the Namurian and Westphalian, where grits and sandy shales are present. There are good grounds for believing that the Millstone Grit micas are largely derived from the metamorphic Caledonides (see GILLIGAN,1919 a n d SHACKLETON, 1962). In order to substantiate these views two samples of micaceous sandstone (samples 823 and 824 of Table II) from the Namurian grits of the Hayfield area were dated. The large difference between the apparent muscovite and biotite ages most probably indicates that the muscovite fraction is distinctly older than that of biotite. The actual age pattern obtained suggests an origin in the metamorphic Caledonides. The biotite fraction is probably dominated by micas derived from the Newer Granite suite, whilst the muscovites include older, possibly even pre-Caledonian metamorphic fractions. The results, therefore, support and con-

Palaeogeography, Palaeoclimatol., Palaeoecol., 2 (1966) 281-312

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F.J. FITCH, J. A. MILLERAND D. B. THOMPSON

firm previous hypotheses. Detrital micas derived from Westphalian rocks are likely to have come from similar original source areas.

Areas lying mainly in the near south and southeast (5) The areas from which micas could be derived include rocks of Charnwood and Malvern type, and red beds of Devonian, Late Carboniferous and New Red Sandstone ages. F r o m areas in which rocks of Charnwood type would have been exposed in New Red Sandstone times, only small quantities of mica could be derived, and their apparent potassium-argon ages are likely to lie in the range 400-600 million years. Biotites from the granites of Mount Sorrel type would yield Newer Granite ages. In the Malverns, micaceous migmatites and granites of Precambrian age are exposed, but most micas from the Malvernian yield overprinted apparent ages in the Caledonian age range, and only a very few retain Late Precambrian ages. Micas derived by reworking of red bed Devonian strata would be generally very much smaller than the present specimens, and they could not yield ages less than Caledonian. Those of Late Carboniferous and Early New Red Sandstone, though doubtless locally rich in detrital mica derived from the first hand and second hand sources already described above, might have had an important additional source in the volcanic belt that was active along the outer Variscides, and may contain many detrital micas with ages between 270-320 million years derived from these rocks (FITCH and MILLER, 1964; LAMING, 1965).

Areas lying mainly to the far south (6) The new fold mountains of Armorica. Igneous and metamorphic micas derived from the newly formed Variscan mountain ranges of France and southern Britain would have ages in the range 250-350 million years (see Fig.2a, b, and c). In the outer ranges through Cornubia and southern England, the Channel and northernmost France, the igneous and methamorphic micas mostly have ages within the range 250-320 million years, whilst further south, in central France and eastwards across Europe, the average mica ages are older and largely fall between 300 and 350 million years. (7) Small areas of reworked and partly overprinted Caledonian and Precambrian rocks within the Variscides. These older areas would have been exposed in the cores of the Variscan ranges as erosion progressed. Mica ages from areas of this type (the Lizard, inliers in Brittany, etc.) are shown separately in Fig.2a and b. It is now possible to examine and interpret the results of the present study Fig.2. Histograms of mica ages. a = histogram of 144 biotite ages from the Variscan mountains of Britain and France; b = histograms of 27 muscovite ages from same area as a; c = histogram of 53 mica ages from Cornubian granites; d = histogram of 18 new mica ages from detrital micas in the Bunter of the Cheshire Basin; e, f, g = histograms of 21 new mica ages from the Keuper of the Cheshire Basin.

299

ISOTOPIC AGE DETERMINATIONS ON DETRITAL MICAS

~

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Palaeogeography, Palaeoclimatol., Palaeoeeol., 2

(1966) 281-312

300

F.J. FITCH, J. A. MILLERAND D. B. THOMPSON

in relation to the seven source regions. It is necessary to discuss the Bunter and the Keuper results separately, because their sediments belong to two distinct major cycles of sedimentation which may have formed in different environments and drawn upon somewhat different source regions. On the histogram Fig.2d, constructed from Bunter results, there are two sharp peaks, one for biotite around 260 million years and another for muscovite between 280 and 290 million years. By reference to the theoretical arguments which are illustrated in Fig. 1, it can be suggested that the Bunter micas were derived from a homogeneous source area, and that the difference between the two peaks might be explained in terms of preferential argon loss from biotite. From a study of the factors that might produce argon loss in these particular samples, it has already been concluded that the Bunter biotites have suffered minor argon losses. Interpretation of these results along the lines discussed in the section "General theoretical discussion" requires that the possible source area should be homogeneous and bear micas of an age range 280-300 million years. The on|y possible source area that would fulfil these requirements would lie within area 6, i.e., within those outer ranges of the Armorican mountains where rocks which might reasonably be expected to have regional age patterns closely similar to those found in Cornubia today were exposed to erosion. Interpretations of the palaeogeography of the Devon New Red Sandstone basin (THOMAS, 1902; MARTIN, 1908) suggest that it was drained towards the east, but there is evidence that the larger Cornubian granite masses like that of Dartmoor, which could have supplied the necessary micas, were not uncovered in New Red Sandstone times (GROVES, 1931 ; BtJTCHER, 1961 and HUTCHINS, 1963). The Triassic sediments on either side of the Bristol Channel indicate a late stage expansion of the depositional basin into the area south of the Welsh Uplands during the Keuper, and it is clear that a route for micas from western Cornubia to Cheshire via the general line of the Bristol Channel is unlikely in Bunter times. Nevertheless, the area of provenance must be sought to the south of the Armorican mountain front at a latitude at least as far south as that of Cornubia. If direct northward derivation from the latter is excluded as a possibility, the most likely alternative area would be to the south and east of Cornubia, still within the outer Armorican ranges, probably very largely south of the present coast line of the British Isles, but north of the distinctly older internal Variscan ranges which outcrop further to the south in central France. A major river carrying sediment from this possible source area could have passed across southern England to the east of the now exposed part of the New Red Sandstone basin of Devon, which the authors believe is merely a western part of the Wessex-Channel Basin. Support for this ultimate source region can be found in the presence of the following in the Bunter Pebble Beds of England: (a) Exotic derived faunas of Ordovician and Devonian age, which can best be matched with those of a Palaeozoic belt of the Gr6s de Mai and the Gr6s Armoricain Palaeogeography, PalaeoclimatoL, Palaeoecol., 2 (1966) 281-312

ISOTOPIC AGE DETERMINATIONS ON DETRITAL MICAS

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of Normandy and Brittany of the Ordovician of Gorran Haven in Cornwall (see LAMONT, 1946). (b) Abundant pink quartzites which might come from a similar area. (c) Certain specific pebbles; including distinctive felsites, quartz-porphyrites and a host of tourmalinized rocks, which many observers have linked superficially with those rocks which would have formed the hypabyssal and volcanic superstructure of areas like Cornubia. Some of these workers have attempted to match the peculiarities of the present Cornish Peninsula; CAMPBELLSMITH(1963) reports marked similarities with the rocks of the Maker Peninsula and Withenoe Down areas. A pebble of vein quartz with cassiterite found in the Burton-on-Trent area (STEVENSON and MITCHELL, 1955, p.45) strengthens this general association. These facts in particular suggest that the main through river may have had important western left bank tributaries flowing parallel with the general mountain trend and receiving debris draining southwards and eastwards from the Cornubian Highlands. (d) Specified heavy minerals, especially tourmaline, garnet and staurolite, which, though in small quantities in the Stockport-Macclesfield district, become more common in the basins towards the south (SMITHSON, 1931). Tourmaline and some garnet are likely to have been derived from areas like Cornubia, whilst staurolite and further tourmaline and garnet could have been derived from the regionally metamorphosed Variscides of northern France and from the granites intruded into these ranges. (e) Cross-bedding palaeocurrent data for the Bunter Pebble Bed Formation collected by RICE(1939) in the Wirral, by one of the present authors (D. B. Thompson) in the Irwell Valley, Manchester, Stockport and Mow Cop areas of the northeast and east of the Cheshire Basin, show an absence of readings in the south and southeastern sectors of the compass, implying currents from those general directions. It is now possible to consider these ideas in the context of the present knowledge of Triassic sedimentation, and in particular of that of the Bunter Pebble Bed Formation. A review of published work on the provenance of the Triassic formations of the Cheshire Basin and surrounding areas reveals an unreconciled difference between conclusions reached from heavy mineral analysis and those obtained from a study of the pebble contents. Three opposing views on the origin of the Triassic detritus of central England, especially that of the Bunter Pebble Bed Formation, were current at the beginning of the 20th century. Many workers, including BONNEY (1880) and BURTON(1917) concluded that the Triassic rocks were derived from a northerly quadrant via a great south-flowing river that had its headwaters in the Caledonides. Others such as HULL(1860) for the Keuper Basement Beds and HARRISON (1882) for the Bunter Pebble Beds regarded the conglomerates of the Midlands as representing old shingle beaches formed around the shores of an inland sea washPalaeogeography, Palaeoclimatol., Palaeoecol., 2 (1966) 281-312

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F.J. FITCH, J. A. MILLERAND D. B. THOMPSON

ing against the base of an Archean ridge that extended across the site of central England. An examination of the pebble content of the Triassic rocks of Devon, the Midlands and Cheshire convinced SHRUaSOLE(1903) and MATLEY(1914), however, that the Triassic rocks of England were fluviatile and very largely derived from the south. The general diminution in size and abundance of pebbles from south to north is in accord with this conclusion. The results of a detailed petrographic study of the Triassic rocks of the Wirral were published by TRAVIS and GREENWOOD in 1911 and 1915. They concluded that the Trias of the Wirral was derived from a source area of granitoid and sedimentary rocks. Because of differences in the physical state of the constituent grains they thought that the Keuper was derived direct from an igneous source and the Bunter from the same general source but via the intermediary stage of an earlier arenaceous sediment. The presence of a peculiar blue tourmaline, which they believed to be distinct from the tourmaline found in the Trias of the Midlands and southwestern England, and their failure at that time to identify garnet or staurolite in the Triassic rocks of Wirral, convinced Travis and Greenwood that the source could not be to the south. Later, GREENWOOD (1916) suggested that the Triassic rocks of Britain could be separated into a number of provinces each with a different provenance, and that the Trias of the Liverpool district, including that of the Wirral, southwestern Lancashire, Cheshire and the Vale of Clwyd, constituted one such province with a common westerly or southwesterly derivation. JONES (1917) studied the pebble content of the Bunter Pebble Bed Formation of the Liverpool district and concluded that there was an undeniable resemblance between the pebble content of these rocks and those of the Midlands Trias. He maintained that all the heavy minerals described by Travis and Greenwood, including the peculiar variety of tourmaline they noted, could be found in the rocks from which the pebbles were derived. GREENWOOD published a study of the Triassic rocks around Macclesfield in 1916. In this paper he concludes that the Millstone Grit was only a minor source of reworked heavy minerals. He records that garnet and staurolite are present in the Trias of both east and west Cheshire, but that they are very rare in the west, whilst the peculiar blue tourmaline found in the Wirral is rare in the east. lie concluded that the Trias of east Cheshire was derived from the east, from the same source as that of the Midlands, and that the Trias of west Cheshire is an admixture with components derived from both west and east. He was the first and only worker to study the pebbles of the Mow Cop-Stockport area in detail, but unfortunately in his conclusions, and in some of his descriptions, he did not differentiate between Keuper and Bunter. He suggested that some pebbles had undergone intense strain, rupture and recementation by silica before or after pneumatolysis, and that they had formed part of a conglomerate which was subject to contact metamorphism and cataclasis before being supplied to the Trias. HARRIS (1924) made a detailed petrographic study of the Triassic rocks of Palaeogeography, Palaeoclimatol., Palaeoecol., 2 (1966) 281-312

ISOTOPIC AGE DETERMINATIONSON DETRITAL MICAS

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southwest Lancashire and concluded that no definite distinction could be made between the Bunter and Keuper in the area under consideration. F r o m a comparative study she suggested that the Triassic deposits of Arran, Antrim, Lancashire and Cheshire had only insignificant differences, but that, as a whole, they were sharply differentiated from the Triassic rocks of the Midlands. The general absence or extreme rarity of staurolite in their heavy mineral concentrates led her to conclude that they are all largely of northwesterly derivation with only minor additions from local sources. To explain such evidence as that of JONES (1917), she suggested some connection with the Midlands through Cheshire with occasional incursions of coarse detritus from that direction. Further work by 5ONES (1926) on the coarse fragments of the Keuper Basement Bed of Wirral, which contains, amongst other components, large fragments of orthoclase, did not lead to any firm conclusions regarding ultimate provenance. Jones believed that the material was derived from many sources over a wide but undefined drainage area. The work of DOUBLE (1926) on the petrography of the Trias (?Lower Mottled Sandstone) of the Vale of Clwyd confirmed the conclusions already reached by Harris. Double showed that the detritus forming the Trias of Lancashire, North Wales, Antrim, Arran and Carlisle is very similar in many respects (including rarity of staurolite) and he suggested that they probably formed part of a single basin of deposition having a northern provenance. Further petrographic work was undertaken by Harris (ALTV, 1926) on 670 ft. of a borehole which penetrated the Bunter Upper Mottled Sandstone Formation at Wilmslow in the Stockport-Macclesfield district (erroneously referred to as the Keuper in her paper). Like GREENWOOD (1916), she identified both garnet and staurolite in the rocks of this part of the Cheshire Basin, and concluded that communication between the Triassic basins of the Midlands and the Irish Sea area across east Cheshire had resulted in the rocks of the Stockport-Macclesfield district being intermediate in character between those of the Midlands and southwestern Lancashire. JONES (1927) summarized the conclusions of this period of research as follows: "The investigations, as far as they have been carried, reveal a general resemblance between all the exposures associated with the Irish Sea basin, including those of Antrim and Arran, and indicate a common source for the material. As far as is at present known, however, there appears to be in some localities an absence of staurolite and a scarcity of garnet which distinguish the beds from those of the Midlands, and negative the theory of a southern origin of the material. On the other hand, a study of the pebbles of the conglomeratic sandstone proves an undeniable relationship to those of the Midland beds, and appears to necessitate a c o m m o n derivation for both. I f those of the Midlands have been derived from the south, there seems to be no escape from the conclusion that those of our own area have also come from the same direction, whatever difficulties may be in the way of Palaeogeography, Palaeoclimatol., Palaeoecol., 2 (1966) 281 312

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F . J . FITCH, J. A. MILLER AND D. B. THOMPSON

its acceptance. Of late years a southern origin has been favoured for the Midland Trias, and in their recent work on "The Geology of Yorkshire", KENDALLand WROOT (1924) have suggested transport from the Armorican Highlands by a river flowing northwards, a theory directly opposed to that of the late Professor Bonney, who imagined a river from the north bringing down material from the northwest Highlands of Scotland. It seems more probable, however, that the immense volume of material forming the Triassic deposits was derived from many directions and many formations, and in the complete theory of the origin of the Trias which may some day be formulated, it may well be that a place will be found, if not for both rivers, at any rate for torrents and rivers in flood rushing down from the highlands enclosing the basin on the west, and distributing their burden of stones and sand over the central plains to the east. As already suggested, a not inconsiderable proportion of the material may have been derived from denudation within the area itself." The Triassic and older red beds of the Midlands were studied by FLEETin a series of papers (1923, 1925, 1927, 1929 and 1930). His conclusions can be summarized as follows: (a) An influx of new sedimentary material, derived from freshly denuded crystalline rocks, began in Bunter Lower Mottled Sandstone times and reached its acme in the Bunter Pebble Beds. The provenance of this detritus was different from that of the Downtonian, Carboniferous or Permian red beds of the Midlands. (b) The evidence suggests that the rocks of the Bunter Upper Mottled Sandstone Formation were derived mainly from the re-distribution of earlier Bunter sediments. (c) The rocks of the Keuper Lower Sandstone Formation are distinct from those of the Bunter, and reveal evidence of a fresh source of sediments similar in general character to that which supplied the Downtonian. This latter conclusion was based, not on a change in mineral composition, but on variation in the proportion of the various species present, and as such has a doubtful validity. A general discussion of the evidence from the heavy mineral composition of the English Trias was included in a paper by SMITHSONin 1931. He concluded that the abundance of staurolite, apatite, tourmaline and fresh orthoclase in the Trias (especially in the Bunter Pebble Bed Formation) of Devon, the Midlands, the Stockport-Macclesfield district and south Yorkshire indicated first-hand derivation from an area of granites and schists. He pointed out that the absence of hornblende and hypersthene from rocks which contain so much fresh apatite indicates that few basic rocks could have been exposed in the source area. He also concluded that the absence or extreme scarcity of staurolite and garnet from all but exceptional areas of the Triassic rocks of the Irish Sea area and north Yorkshire indicated that these areas had a different provenance. His palaeogeographic map (1931, p. 148) suggests derivation of the Trias of Britain from two main sources, one in Normandy and Palaeogeography, Palaeoclimat ol., Palaeoecol., 2 (1966) 281-312

ISOTOPIC AGE DETERMINATIONS ON DETRITAL MICAS

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Brittany and the other in Scotland. He further suggested that interdigitation of sediment from these two sources occurred in the Cheshire Basin and in south Yorkshire. Later work by SMITHSON(1947) in north Staffordshire does not deny these views. More recently, in the palaeogeographic reconstructions of WILLS (1948, 1951, 1956) a southerly origin for the bulk of the Trias of Cheshire and the Midlands (see also THOMSON, 1953) has been favoured, although Wills concluded that there was no direct connection between the Midlands and the Devon Basin to the south. The Geological Survey Memoir for the present area suggests that the rudaceous material was derived from the south (TAYLORet al., 1963). As previously noted, conclusions, derived from cross-bedding palaeocurrent data of the Bunter Pebble Bed Formation in the northwest, northeast and east of the Cheshire Basin tend to support Wills' view and deny the northerly, westerly or southwesterly origin suggested by other workers. The present writers envisage that the micas found in the Bunter sediments of the Stockport-Macclesfield district travelled north in a major river, which in its course through the Worcester Basin, was flanked by the Malvern fault scarp on the west and by rather more hypothetical scarps on the east. They do not agree with Wills that the course of this river south of Worcester would be so short and restricted. They consider that the evidence of the micas and their age pattern, as well as that of much of the pebble evidence, is best explained by the continuance of the valley southwards into the Wessex Basin. The existence of this basin in New Red Sandstone times is strongly suggested on geophysical grounds (KING, 1954). The catchment area of the river in the mid- and south-Channel may have been quite large, as is suggested in Fig.3. WILLS(1956, footnote to p.113) does believe that the ultimate source of much of the Bunter Pebble Bed material was in this area, but he prefers an interpretation in which it is not derived first-hand via a long river, but rather by second-hand erosion of gravels which accumulated in marginal foredeeps and basins on the northern side of the Armorican mountains. The petrography of the rocks and the very remarkable internal consistency of the Bunter mica age results we have obtained from the Stockport-Macclesfield district does not favour any complex history of re-sedimentation, and Will's two-stage interpretation is considered, therefore, to be unlikely. The Keuper results are illustrated on three histograms (Fig.2e, f and g). On Fig.2e all the results are plotted on one histogram, while on Fig.2g and fthose of the Lower Keuper Sandstone and Keuper Waterstones are shown separately. Although the results from the Lower Keuper Sandstone are not sufficiently numerous to make an acceptable statistical sample, the indications are that the pattern for this formation is not significantly different from that of the Keuper Waterstones, and that an interpretation can be based upon the combined histogram Fig.2e. Comparison of Fig.2e with the theoretical models (Fig.l) suggests that the Palaeogeography, Palaeoclimatol., Palaeoecol.,

2 (1966) 281-312

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E.J. FITCH~J. A. MILLERAND D. B, THOMPSON

AREA

~ INTERNAL

REA

RANGES

AND

BASINS

TO

THE

SOUTH

Fig.3. Diagram to illustrate proposed directions of sediment transFort in Middle Bunter times.

Keuper sediments were derived from a complex heterogeneous source area of the type shown in Fig.lc. The general age pattern and some individual results are not too different from those of the Bunter to recognize that some of the Keuper micas may have come from the same sources. This probably implies that the general palaeogeographic framework of the Bunter was maintained. In part, however, the results indicate that a large fraction of the Keuper micas must have been derived from older source rocks. These may have included: (A) Areas which could yield reworked micas of older ages. Some of these areas were described previously, and those of regions 4 and 5 are most likely. Micas derived from Namurian and Westphalian sediments have given muscovite ages of 492 and 530 million years. Since, however, even amongst the muscovites of the Keuper there are no ages over 400 million years, it is unlikely that Early Caledonian primary or secondary sources were greatly represented. That there is some representation is testified by the fact that amongst the pebbles of the Lower Keuper Sandstone, there are a few pebbles of feldspathic grit like those of the Namurian Rough Rock (GREENWOOD, 1918), and one large, worn, gannister pebble with Stigmariaficoides, found during the present study, is best derived from Namurian or Westphalian outcrops within 5 to 15 miles of Alderley Edge. It was originally, no doubt, exposed along the line of what is now the west Pennines, between Poynton and Mow Cop. Reworked sedimentary micas of Devonian age would be small in size, unlike those of the present samples.

Palaeogeography, Palaeoclimatol., Palaeoecol., 2 (1966) 281-312

ISOTOPIC AGE DETERMINATIONS ON DETRITAL MICAS

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Again, since the Keuper muscovite ages do not reach 400 million years, it is not likely that rocks of Malvern type made any significant contribution. On the other hand, contributions of primary mica of Newer Granite age from areas like Charnwood appear more likely, but in the type area muscovite is rare, and borings in the northern fringes of the Mercian Highlands, though few, are sufficient to suggest that the occurrence of large areas of rocks of this type are not likely. Geophysical work in the Edge Hill and Banbury area (W. Bullerwell in: EDMONDSet al., 1965) shows that in that part of the Mercian Highlands likely Palaeozoic plutons are hundreds of feet below the base of the New Red Sandstone. (B) Inner ranges of the Variscan mountains with ages in the range 300-350 million years. (C) Inliers of ancient rocks well within the Variscide chain which had been laid bare by long periods of erosion during and following uplift. There are several other matters which are pertinent here and require comment. Firstly, it is possible that some of the micas in the Lower Keuper Sandstone Formation are reworked from rocks of Bunter Pebble age. The fact that a large proportion of the pebbles of the Lower Keuper Sandstone are similar to those of the Bunter Pebble Beds (HULL, 1864, p.67) was confirmed during the present study, and this, together with some of the heavy mineral evidence, particularly the increase of roundness shown by staurolite, suggests that this possibility is likely. A suitable area of provenance, where Keuper Sandstone rests immediately above Bunter Pebble Beds, is an uplifted block forming a ridge bounded approximately by Alton and Burton on the north, Sandon on the west, and Birmingham on the south. Ages of reworked Bunter micas would be the same as those described previously. This source of mica would not be available in the later period represented by the Keuper Waterstones Formation, since during this later period there is evidence that there was overlap: (a) onto the area now represented by the Pennines (PococK et al., 1906); (b) onto the Charnwood massif (BosWORTH, 1912); and (c) onto the north side of the Mercian Highlands (WILLS, 1948). It is also possible that some individual samples from the Keuper, especially that from which the lowest ages were obtained (sample 773; 233 and 258 million years for dark and light mica, respectively), may have been derived from atypical parts of the source area. It is apparent, therefore, that the area of provenance of the Keuper of the Stockport district, although generally similar to that of the Bunter, must have included areas from which older micas could be derived. Some, but not all, of these may have been local areas of old rocks. If it is assumed that the margins of the Wessex-Channel basin are the most likely source for the Bunter micas, then it is possible that continued erosion in these areas might have exposed larger inliers of partially overprinted older rocks, from which a significant fraction of the Keuper micas could have been derived. A further possible source would be the older structural zones ot the Variscides lying further to the south. Acquisition of micas Palaeogeography, Palaeoclimatol., Palaeoecol., 2 (I 966) 281-312

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from this source could possibly have been accomplished by the tapping of hitherto enclosed drainage basins. Support for these interpretations of the Keuper data can be found in other lines of enquiry: (a) Sample studies of cross-bedding azimuths for the Lower Keuper Sandstone made around the margins of the Cheshire Basin, in the Maer Hills (Staffordshire), at Fools Nook (south of Macclesfield), at Mottram, Alderley, Styal, Timperley, Partington, Lymm, Stockton, Daresbury, Frodsham, Helsby, Peckforten and Grinshill, and even in the far north at Ormskirk and in the Wirral (RICE, 1939), generally show an absence of readings in the south and southeast sectors of the compass, and suggest that the flow in the fluvial channels was from these directions. Studies of the cross-bedding, ripple-bedding and asymmetric ripple marks of the Keuper Waterstones and Lower Keuper Marl, complete in the Alderley-Styal areas, but less detailed in the rest of the north of the basin, indicate that the main directions of the palaeocurrents remain the same in these later time periods. (b) Pebble content analyses of the Keuper Sandstone, suggest that the pebbles, whether they are reworked from the Bunter Pebble Beds, or derived first-hand from the same general area of provenance as those of the Bunter Pebble Beds, mostly have a southern derivation. Rare pebbles and granules of the same type persist in the cross-beds which sometimes form the base of Keuper Waterstone cycles. (c) Studies of the heavy minerals of Keuper Sandstones, Keuper Waterstones and Lower Keuper Marl in the Alderley-Macclesfield area (GREENWOOD, 1918) show that heavy minerals similar to those of the Bunter are present, and that these are most likely derived from southern areas such as described already for the Bunter Formations. (d) Petrographic studies suggest that there was no significant difference in the character of the area being eroded in Bunter and Keuper times. In the Bunter, the indications are of a relatively simple source area composed of sedimentary and metamorphic rocks intruded by alkali-granites. The difference between the Keuper and the Bunter rocks may indicate that erosion had reached lower levels of the mountain ranges in the source area, but the overall indications of provenance are very similar. Comparison of the present palaeogeographic interpretation and its implications with previous views is somewhat difficult, since no palaeogeographic diagrams for Lower Keuper Sandstoae or Keuper Waterstones times have been published. If the source of Wills' Budleighensis River (WILLS, 1951) were extended southwards into the Wessex-Channel Basin in Bunter times, then there would be close agreement between this interpretation and that proposed in this paper. WILLS' (1948, 1951) palaeogeographic map of the Keuper Marl shows a general overlapping of sedimentation onto the margins of all basins and their final connection to form a widespread plain. Isopachytes drawn by the Geological Survey (WoRssAM, 1963) for the present (residual) thickness of the Keuper Marl in the Palaeogeography, Palaeoclimatol., Palaeoecol., 2 (1966) 281-312

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south of the Worcester Basin, show great thicknesses in the south of that graben, and this suggests a considerable thickness and continuity of sedimentation between the Worcester and Wessex Basins in this later period. The present interpretation, which envisages that this connection was strongly established from Bunter Pebble Bed to Keuper Waterstones times, appears reasonable.

CONCLUSIONS

This research has shown that, in suitable circumstances, isotopic age studies on detrital micas can produce significant results of value to palaeogeographers and sedimentologists. Isotopic studies may enable a firm conclusion to be reached regarding sedimentary provenance that was not possible on the basis of conventional petrographic analysis alone. The area chosen for such a study should be compact, the rocks sampled should be free from the likelihood of severe argon loss and the number of analyses undertaken should be sufficient for a statistically viable interpretation. It is essential that the stratigraphy, petrology and subsequent history of the sediments, and of their possible source areas, be known in considerable detail if interpretation is to have any value. The results of the trial investigation reported in this paper can be summarized as follows: (a) The detrital micas of the Bunter Pebble Bed and Upper Bunter Mottled Sandstone Formations of the Stockport-Macclesfield district of Cheshire were derived from a homogeneous source area of crystalline rocks with ages in the range 280-300 million years. (b) The most likely location for the Bunter source is to be found in the Variscan mountains bordering an intermontane basin in the Wessex-Channel area between England and France. (c) The detrital micas of the Lower Keuper Sandstone and Keuper Waterstones Formations of the same district were derived from a heterogeneous source area, still dominantly Variscan in its age pattern, but no longer homogeneous and simple as was the Bunter source. (d) The most likely source for the Keuper rocks is to be found in an extension of the earlier Bunter source area to include a number of inliers of older crystalline rocks, with some additional material from local sources, including the reworking of earlier Triassic, Permian and Carboniferous sediments. It must be emphasised that these conclusions only apply to the Triassic rocks of the Stockport-Macclesfield district of the Cheshire Basin. Inherent in this interpretation, however, is the acceptance of a southerly provenance, ultimately from the Wessex-Channel Basin via the Worcester Graben, for much of the Trias of the English Midlands; and the suggestion, yet to be proven, that this source may have Palaeogeography, Palaeoclimatol., Palaeoecol., 2 (1966) 281-312

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been an important element in the provenance of Triassic accumulations elsewhere in Britain, and especially to the north and west of the present area. It is already clear from this preliminary investigation that further work on the Triassic rocks of the Irish Sea area would produce valuable new evidence and probably enable a final resolution of the question of provenance. Promising fields for research may be present in other New Red Sandstone basins, in the Torridonian and Devonian rocks and amongst the less-metamorphosed Lower Palaeozoic sediments of Britain. In addition to its application to studies of sedimentary provenance and palaeogeography, isotopic dating of detrital micas could have other important applications. The age patterns found for the Bunter and Keuper Formations of the Stockport-Macclesfield district are quite distinct, and certainly that of the Bunter may be unique. Within the confines of one sedimentary basin these features might be of value in stratigraphical correlation, especially between boreholes. The correlation of shoestring sandstones, in particular, should be greatly facilitated by the application of these methods. The combination of isotopic studies with conventional heavy mineral analysis could well revive interest in the scientific and economic value of sedimentary petrology.

ACKNOWLEDGEMENTS

The authors wish to record their indebtedness to Miss D. Bate, Mrs. J. M. Brown, Mrs. F. J. Fitch, Mrs. J. Pickard, Mrs. D. B. Thompson and Mr. J. Esson for their assistance and advice during the preparation of this manuscript. The illustrations were prepared under the direction of Mr. G. Davenport and the thin sections made by Mr. M. J. Fleming in London and Mr. J. Henderson in Manchester.

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