brookfield 2008

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Palaeoenvironments and palaeotectonics of the arid to hyperarid intracontinental latest Permian- late Triassic Solway basin (U.K.) Michael E. Brookeld Institute of Earth Sciences, Academia Sinica, P.O. Box 1-55 Nankang, Taipei 1159, Taiwan ABSTRACT ARTICLE INFO Article history: Received 21 December 2007 Received in revised form 2 April 2008 Accepted 10 June 2008 Keywords: Triassic Permian Solway Basin Hyperarid Palaeoenvironments Palaeotectonics The late Permian to late Triassic sediments of the Solway Basin consist of an originally at-lying, laterally persistent and consistent succession of mature, dominantly ne-grained red clastics laid down in part of a very large intracontinental basin. The complete absence of body or trace fossils or palaeosols indicates a very arid (hyperarid) depositional environment for most of the sediments. At the base of the succession, thin regolith breccias and sandstones rest unconformably on basement and early Permian rift clastics. Overlying gypsiferous red silty mudstones, very ne sandstones and thick gypsum were deposited in either a playa lake or in a hypersaline estuary, and their margins. These pass upwards into thick-bedded, multi-storied, ne- to very ne-grained red quartzo-felspathic and sublithic arenites in which even medium sand is rare despite channels with clay pebbles up to 30 cm in diameter. Above, thick trough cross-bedded and parallel laminated ne-grained aeolian sandstones (deposited in extensive barchanoid dune complexes) pass up into very thick, multicoloured mudstones, and gypsum deposited in marginal marine or lacustrine sabkha environments. The latter pass up into marine Lower Jurassic shales and limestones. Thirteen non-marine clastic lithofacies are arranged into ve main lithofacies associations whose facies architecture is reconstructed where possible by analysis of large exposures. The ve associations can be compared with the desert pavement, arid ephemeral stream, sabkha, saline lake and aeolian sand dune environments of the arid to hyperarid areas of existing intracontinental basins such as Lake Eyre and Lake Chad. The accommodation space in such basins is controlled by gradual tectonic subsidence moderated by large uctuations in shallow lake extent (caused by climatic change and local variation) and this promotes a large-scale layer-cake stratigraphy as exemplied in the Solway basin. Here, the dominant ne-grained mature sandstones above the local basal reg breccias suggest water-reworking of wind-transported sediment, as in the northern part of the Lake Chad basin. Growth faulting occurs in places in the Solway basin, caused by underlying evaporite movement, but these faults did not signicantly affect pre-late Triassic sedimentation and did not expose pre-Permian units above the basal breccias. There is no evidence of post-early Permian rifting anywhere during deposition of the late Permian to middle Triassic British succession although the succession is often interpreted with a rift-basin model. The arid to hyperarid palaeoclimate changed little during deposition of the Solway basin succession, in contrast to Lakes Eyre and Chad: and this is attributed to tectonic and palaeolatitude stability. Unlike the later Mesozoic- Cenozoic, only limited plate movements took place during the Triassic in western Europe, palaeolatitude changed little, and the Solway Basin remained in the northern latitudinal desert belt from early to mid-Triassic times. However, the inuence of the early Triassic impoverished biota on environmental interpretations needs further study. © 2008 Elsevier B.V. All rights reserved. 1. Introduction An intracontinental basin is a sedimentary basin on continental crust within a continent, whatever the age (intracratonic basins rest on Precambrian cratons, while the more general term epicontinental is used for basins on a continent or a continental shelf). The post- Variscan basins of Western Europe are intracontinental basins resting predominantly on eroded Proterozoic to Carboniferous orogens - net additions to continental crust during continental collisions (Dewey, 1982). They consist of early rift basins truncated by overlying saucer- shaped complex basins, the combination having the typical steers- headgeometry, and originate by cooling and subsidence of litho- sphere after initial uplift, rifting and erosion over hot upper mantle (McKenzie, 1978; Ziegler, 1990). Intrabasinal differential movements, however, give rise to sub-basins and swells in almost all such basins (Hartley and Allen, 1994; Russell and Gurnis, 1994; Cohen, 2003, p.41). The Solway Basin is a small part of the extensive post-rifting late Permian to late Triassic desert basin complex which stretched from Sedimentary Geology 210 (2008) 2747 Fax: +886 6 2 27 3493. E-mail address: mbrook@earth.sinica.edu.tw. 0037-0738/$ see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2008.06.003 Contents lists available at ScienceDirect Sedimentary Geology journal homepage: www.elsevier.com/locate/sedgeo

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Solway Basin Paleoenvironments and tectonics (Late P to early Tr)

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Page 1: Brookfield 2008

Sedimentary Geology 210 (2008) 27–47

Contents lists available at ScienceDirect

Sedimentary Geology

j ourna l homepage: www.e lsev ie r.com/ locate /sedgeo

Palaeoenvironments and palaeotectonics of the arid to hyperarid intracontinentallatest Permian- late Triassic Solway basin (U.K.)

Michael E. Brookfield ⁎Institute of Earth Sciences, Academia Sinica, P.O. Box 1-55 Nankang, Taipei 1159, Taiwan

⁎ Fax: +886 6 2 27 3493.E-mail address: [email protected].

0037-0738/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.sedgeo.2008.06.003

A B S T R A C T

A R T I C L E I N F O

Article history:

The late Permian to late Tr Received 21 December 2007Received in revised form 2 April 2008Accepted 10 June 2008

Keywords:TriassicPermianSolwayBasinHyperaridPalaeoenvironmentsPalaeotectonics

iassic sediments of the Solway Basin consist of an originally flat-lying, laterallypersistent and consistent succession of mature, dominantly fine-grained red clastics laid down in part of avery large intracontinental basin. The complete absence of body or trace fossils or palaeosols indicates a veryarid (hyperarid) depositional environment for most of the sediments. At the base of the succession, thinregolith breccias and sandstones rest unconformably on basement and early Permian rift clastics. Overlyinggypsiferous red silty mudstones, very fine sandstones and thick gypsumwere deposited in either a playa lakeor in a hypersaline estuary, and their margins. These pass upwards into thick-bedded, multi-storied, fine- tovery fine-grained red quartzo-felspathic and sublithic arenites in which even medium sand is rare despitechannels with clay pebbles up to 30 cm in diameter. Above, thick trough cross-bedded and parallel laminatedfine-grained aeolian sandstones (deposited in extensive barchanoid dune complexes) pass up into very thick,multicoloured mudstones, and gypsum deposited in marginal marine or lacustrine sabkha environments.The latter pass up into marine Lower Jurassic shales and limestones. Thirteen non-marine clastic lithofaciesare arranged into five main lithofacies associations whose facies architecture is reconstructed where possibleby analysis of large exposures. The five associations can be compared with the desert pavement, aridephemeral stream, sabkha, saline lake and aeolian sand dune environments of the arid to hyperarid areas ofexisting intracontinental basins such as Lake Eyre and Lake Chad. The accommodation space in such basins iscontrolled by gradual tectonic subsidence moderated by large fluctuations in shallow lake extent (caused byclimatic change and local variation) and this promotes a large-scale layer-cake stratigraphy as exemplified inthe Solway basin. Here, the dominant fine-grained mature sandstones above the local basal reg brecciassuggest water-reworking of wind-transported sediment, as in the northern part of the Lake Chad basin.Growth faulting occurs in places in the Solway basin, caused by underlying evaporite movement, but thesefaults did not significantly affect pre-late Triassic sedimentation and did not expose pre-Permian units abovethe basal breccias. There is no evidence of post-early Permian rifting anywhere during deposition of the latePermian to middle Triassic British succession although the succession is often interpreted with a rift-basinmodel. The arid to hyperarid palaeoclimate changed little during deposition of the Solway basin succession,in contrast to Lakes Eyre and Chad: and this is attributed to tectonic and palaeolatitude stability. Unlike thelater Mesozoic- Cenozoic, only limited plate movements took place during the Triassic in western Europe,palaeolatitude changed little, and the Solway Basin remained in the northern latitudinal desert belt fromearly to mid-Triassic times. However, the influence of the early Triassic impoverished biota on environmentalinterpretations needs further study.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

An intracontinental basin is a sedimentary basin on continentalcrust within a continent, whatever the age (intracratonic basins reston Precambrian cratons, while the more general term epicontinentalis used for basins on a continent or a continental shelf). The post-Variscan basins of Western Europe are intracontinental basins resting

l rights reserved.

predominantly on eroded Proterozoic to Carboniferous orogens - netadditions to continental crust during continental collisions (Dewey,1982). They consist of early rift basins truncated by overlying saucer-shaped complex basins, the combination having the typical ‘steers-head’ geometry, and originate by cooling and subsidence of litho-sphere after initial uplift, rifting and erosion over hot upper mantle(McKenzie, 1978; Ziegler, 1990). Intrabasinal differential movements,however, give rise to sub-basins and swells in almost all such basins(Hartley and Allen, 1994; Russell and Gurnis, 1994; Cohen, 2003, p.41).

The Solway Basin is a small part of the extensive post-rifting latePermian to late Triassic desert basin complex which stretched from

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Fig. 1. Early Triassic palaeogeography of the North Atlantic with location of Solway Basin (modified from Ziegler, 1990) with location on western equatorial globe reconstruction forthe mid-Triassic (240 ma, Anisian) (courtesy Ron Blakey).

28 M.E. Brookfield / Sedimentary Geology 210 (2008) 27–47

North America to Central Europe (Fig. 1). The basin fill reflects thermalsubsidence at the end of spreading in the early Permian Greenland-Norwegian- British rifts. It cuts cross the structures and trends of theunderlying Lower Permian rift basins (Brookfield, 1980; Ziegler, 1990;Newman, 1999), and it predates the younger late Triassic-Jurassicrifting phase which began the opening of the Central Atlantic Ocean(Manspeizer, 1988). The succession also has a unique association ofclay minerals that has defied satisfactory explanation for many years(Jeans, 2006). The Solway Basin stretches from Carlisle to the Isle ofMan and is mostly now underwater (Fig. 2). In the easternmostonshore part (known as the Carlisle Basin), the latest Permian toTriassic succession consists of a relatively uniform blanket ofpredominantly sub-mature, very fine-grained red clastics deposited

Fig. 2. A: Geological map of Solway basin Carlisle Basin

in a variety of arid to hyperarid environments (Akhurst et al., 1997;Newman, 1999; Brookfield, 2004; Holliday et al., 2004). The lower-most fine-grained deposits contain continental and disputably marinefloras and faunas attributed to the late Permian (Stoneley, 1958;Clarke, 1965; Pattison, 1970), while the overlying continental red bedspass upwards into latest Triassic (Rhaetian) marine mudstones(Warrington et al., 1980).

Numerous hydrocarbon discoveries have been made in equivalentstrata in the East Irish Sea Basin (Jackson and Mulholland, 1993;Quirk et al., 1999) and there are many recent studies on the subsur-face late Permian to Triassic succession in the Irish Sea (e.g. Barneset al., 1994; Mitchie and Bowden, 1994; Akhurst et al., 1997; Herriesand Cowan, 1997). Very little, however, has been published on the

with cited localities and location of seismic lines.

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Table 1Stratigraphic terminology for Solway Basin and adjacent areas: West Cumbria fromAkhurst et al. (1997), East Irish Sea from Jackson et al. (1995)

Radiometric dating of biostratigraphic units from Menning (1995) and Gradstein et al.(2004), modified with Hounslow and McIntosh (2003) and Ovtcharova et al. (2006).Note that the Spathian occupies 6 ma out of the total 8 ma for the Early Triassic.

29M.E. Brookfield / Sedimentary Geology 210 (2008) 27–47

onshore surface exposures in the Solway Basin since 1942 exceptfor recent summaries of the outcrop and borehole stratigraphy(Barrett, 1942; Holliday et al., 2001; Brookfield, 2004; Holliday et al.,2004).

The aim of this paper is to summarize the palaeoenvironments ofthe Solway Basin and compare it with its close analogues, the youngerand still active Quaternary arid to hyperarid parts of the intraconti-nental Mesozoic-Recent Lake Eyre and Lake Chad basins; andconversely, to illustrate the importance of arid to hyperarid (asopposed to semi-arid) basin sedimentology in interpreting manyother ancient intracontinental non-marine basins. Arid basins have aprecipitation/potential evapotranspiration ratio (P/PET) of 0.05 to 0.2and mean annual rainfall of 25-200 mm. while hyperarid basins aretrue deserts with a (P/PET) ratio of less than 0.05, where rainfall isextremely low (less than 25 mm/year) and unpredictable in space andtime, and where in some years there is no precipitation at all (Grove,1977; UNEP, 1992). The Sahara desert contains nearly 70 per cent ofthe present hyperarid areas on Earth (UNEP, 1992). Though hyperar-idity can not be directly determined for ancient sediments, criteriaindicating very low rainfall and very low P/PET ratios can, such as thescarcity or absence of plant and animal remains, and the absence ofpalaeosols. The entire Lake Eyre basin is mostly within the 250mm/yr.isohyet and arid to semi-arid. However, the entire central part of thebasin, including Lake Eyre and the lower parts of its northern andeastern tributaries is within the 100 mm/yr isohyet, has a P/PET ratioof 0.03 and is thus arid to hyperarid (Kotwicki, 1986). The Lake Chadbasin similarly varies fromwet in the south to hyperarid in the north.The entire northern desert area (including the Bodele depression) ishyperarid, with average rainfall of b20 mm/year and a P/PET ratio of0.001 (Eugster and Maglione, 1979; Maley, 1981). Both the Lake Eyreand Lake Chad basins have complex sub-basin development but haveaccumulated relatively thin sedimentary successions controlledprimarily by slow tectonic subsidence and climate, with the alterna-tion of periodic large but shallow megalakes with dry desertconditions during the Quaternary (DeVogel et al., 2004; Schusteret al., 2005).

2. Stratigraphy

Brookfield (2004) summarized the history of study, the outcropstratigraphy of the onshore Carlisle Basin, while Holliday et al. (2004)summarized the evolution of the entire Solway basin. Only limitedseismic and borehole information, however, has been published onthe subsurface Solway Basin and adjacent areas (Chadwick et al., 1995;Jackson et al., 1995; Holliday et al., 2001, 2004).

Unconformably above the Lower Permian desert rift basins, thelate Permian to Late Triassic Solway succession is surprisinglyconsistent vertically and horizontally both within the basin andfurther afield in the East Irish Sea Basin (Table 1) (Chadwick et al.,1995; Jackson et al., 1995; Akhurst et al., 1997). The Cumbrian CoastalGroup begins with a very thin and variable breccia and sandstone unit(Basal Clastics) less than 10 metres thick, which is present whereverpre-Permian bedrock is exposed, but is absent above the LowerPermian aeolian Penrith sandstones and correlative units. The brecciapasses rapidly up into a relatively thick fine-grained gypsum/anhydrite evaporite and red shale unit (Eden Shales) which is up to180 metres thick in boreholes, though only the upper clastic part isexposed at outcrop due to solution removal of the evaporites. Inboreholes in the Vale of Eden, to the southeast, four main gypsum/anhydrite levels can be identified (Meyer, 1965, Fig. 2). These passsouthwest, in the centre of the East Irish Sea basin, into a halite-dominated successions (e.g. borehole 112/25A-1, Fig. 2) (Jackson et al.,1987). The Eden Shales pass gradually up into thick tabular, fine-grained sandstones, known as the Sherwood Sandstone Group in thesubsurface where they are up to 850 metres thick. At outcrop, thedominantly fluvial Annan Sandstone Formation is sharply overlain by

the dominantly aeolian Kirklinton Sandstone Formation (Brookfield,2004; Holliday et al., 2004). The Kirklinton Sandstone Formation isrelatively sharply overlain by a second thick very fine-grained unitwith evaporites (Mercia Mudstone Group, known locally as theStanwix Shale Formation), with almost no exposures, though up to725 metres of mudstones and halite occur in boreholes (Newman,1999). The Stanwix Shale Formation passes gradually up into overlyingfossiliferous marine Upper Triassic (Rhaetic) to Lower Jurassic lime-stones and shales (Ivimey-Cook et al., 1995). The succession is uni-form and persistent across the Solway Basin and almost undisturbedby synsedimentary faults (Fig. 3). Those faults which cut the Sher-wood Sandstone Group in the subsurface are most likely related todissolution and/or migration of the evaporites in the underlying EdenShales during deposition of the later Triassic Stanwix Shales sincethese faults cut neither the Carboniferous below, nor the bulk of theStanwix Shales above (Fig. 3) and there are no significant variations inthickness across the faults until the latest Triassic Stanwix Shales unitwhich is also evaporitic (Stuart, 1993; Ruffell and Shelton, 1999;Holliday et al., 2004). The sandstones throughout the Cumbrian andSherwood Sandstone Groups are uniformly moderately to moderatelywell sorted, fine to very fine grained quartz arenites and quartzo-felpathic to sublithic arenites, with only rare medium grained sand(Brookfield, 2004; see also Akhurst et al., 1997; Meadows, 2006).Unlike the underlying Lower Permian, there are no marginal alluvialfan deposits (Brookfield, 1980): all previously described as suchturn out to be thin regolith pavement breccia deposits (Brookfield,2004).

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Fig. 3. Interpreted seismic lines across the Solway Basin (Newman, 1999); location on Fig. 2.

30 M.E. Brookfield / Sedimentary Geology 210 (2008) 27–47

There are still few satisfactory ages and correlations for the latePermian to Triassic red beds of Western Europe (Menning, 1995). Inthe absence of palaeomagnetic reversal data, and lack of plant andanimal fossils, the only way of dating the Solway succession is toassume that its lithological units correlate with analogous lithologicalunits to the south, which is the basis for the dating of Table 1.

3. Sedimentology

3.1. Previous work

Of the various stratigraphic units, only the sedimentology of theSherwood Sandstone Group has been studied in any detail fromboreholes and seismic sections in the Irish Sea (because it is the maingas-bearing unit), where the Group consists of fine- to medium-grained sandstones and rare thin mudstones. The first interpretationwas that the Groupwas deposited by a major braided river system in asemi-arid climate (Colter and Barr, 1975). The recognition of thinaeolian sandstones within the system led to a modified interpretationin which a semi-arid braided system was periodically covered byaeolian sheet sands during drier climates (Cowan, 1993). The secondinterpretation recognized three units, partly based on sonic logs. Alower playa mudflat-sheetflood -braided river unit (St Bees Sand-stone) passes up into a dominantly aeolian fine- to coarse-grainedsandstone unit (Calder Sandstone) which is sharply overlain by a finergrained, but again dominantly aeolian unit with sheetflood andbraided channel sandstones (Ormskirk Sandstone) (Meadows andBeach, 1993; Jones and Ambrose, 1994). A convincing case has beenmade that the Ormskirk Sandstone is dominated by evaporitic clasticsabkhas with interbedded aeolian sands (Herries and Cowan, 1997):an interpretation more in keeping with the lack of biota andpalaeosols and the marine isotopic signature of the sandstones andof the overlying gypsiferous Mercia Mudstone Group (Greenwood andHabesch, 1997). Recently Meadows (2006) studied the sequencearchitecture of the Ormskirk Sandstone from revised well correlationsin the East Irish Sea Basin. He interpreted the succession as incisedvalleys filled with stacked amalgamated fluvial channel sandstones,

based on detrital mineralogical and geochemical correlations of theunits. However, the reconstructed architecture is based on units lessthan 20 metres thick in only four wells spaced between 10 and 40 kmapart along a 60 km long section. The heavy mineral and geochemicalcorrelations of Meadows (2006) could be re-interpreted to outlinebroad convex upward fan accumulations instead of incised valleyssince the playa lake sediment surface used as a datum by Meadows(2006) need not be horizontal. The actual floor of Lake Eyre varies15 metres in altitude over a similar 60 km section (Bye et al., 1978).

These previous interpretations of the succession, however, arebased primarily on boreholes, which have one advantage in thatpetrology can be studied better than at outcrop, but a disadvantage inthat the geometrical relationships of sedimentary strata and struc-tures can only be inferred from imprecise long wavelength seismicsections whose resolution, even under ideal circumstances, ismeasured in decimetres. Also, in fluvial-aeolian systems it isimpossible to determine bedform and channel type, form anddevelopment without large outcrops, and hence impossible to inferthe character of the system (Miall, 1985; North, 1996).

The following three sections describe the individual lithofacies(3.2), some of the problems of interpreting these (3.3), and thelithofacies associations (3.4), which can be compared with cores, andfacies architecture where possible (which can not be adequatelydescribed from cores). I use the dynamic stratigraphic approach ofAigner (1985): where stratinomic analysis gives the process (deposi-tional dynamics) that formed individual layers (lithofacies); analysisof interbedded lithofacies (lithofacies associations) allows environ-ments to be inferred from the associations of processes; and basinanalysis of the distribution of lithofacies associations (environments)allows the basin dynamics to be inferred. This method separates thevarious scales of stratal development and minimizes jumping topremature conclusions about environments and basin development.

The methods used are those of Jackson (1975), Allen (1983) andMiall (1985). Sedimentary layers can be grouped to form hierarchies,separated by bounding surfaces of various scales, representing packetsof genetically related units (Allen, 1983). The hierarchies of unitsrepresent hierarchies of fluid flow - what Allen (1983) called fluid

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31M.E. Brookfield / Sedimentary Geology 210 (2008) 27–47

vector fields. At the lowest scale, individual layers are formed by thesmallest elements of the flow and deposited by the smallest and mostrapidly forming bedforms or microforms, for example current ripplesand planar beds (Jackson, 1975). At the intermediate scale, flowelements deposit larger bedforms or mesoforms, for example dunes,which take longer to form and may include the smaller elements. At alarger scale, flow elements such as individual river channels depositeven larger bedforms or macroforms, for example, point bars. Thesedifferent scales of bedform can be grouped into architectural elementsand interpreted according to their spatial distribution and (inferred)depositional time scale (Miall, 1985; Todd, 1996).

3.2. Lithofacies

Individual lithofacies are distinguished with the facies code ofMiall (1977) used for easy recognition: several additional codedivisions are required to identify large-scale trough cross-beddedsandstones (Stl) and inverse graded laminated sandstones (Sli)interpreted as aeolian lithofacies. The individual lithofacies arebounded by the 0 to 2nd order bounding surfaces separating laminae,ripples and dunes (or microforms to mesoforms) (see Miall, 1996).

The character and interpretation of the lithofacies are summarizedon Table 2 and are supported by the brief descriptions andinterpretations below - which are necessary since the succeedinginterpretations depend heavily on the lithofacies interpretations:simple tabular summaries are inadequate.

3.2.1. Clast-supported limestone breccias (Gp, Gt)These breccias are dominated by angular to subrounded Carboni-

ferous limestone and basalt clasts and are confined to the Basal

Table 2Lithofacies and their interpretation

Clastics (Brookfield, 2004, Fig. 4). The breccias rarely show anychanneling or cut-and-fill (Brookfield, 2004) (Fig. 4) and only occurwhen Carboniferous Limestone immediately underlies, or is adjacentto, the sediments. The matrices range from quartz arenites to sublithicarenites, include well-rounded aeolian quartz sand (Fig. 4C) and aremore mature than the dominant quartzo-felpathic to sublithicarenites of the overlying units (Brookfield, 2004, Fig. 6 - where theBasal Breccia and Kirklinton Sst symbols are wrongly reversed).Current directions are also very variable (Fig. 5). There are no quartzgrains coarser than very fine in the Carboniferous limestone, and thiscan therefore not supply the very well-rounded medium-grainedquartz sand in the breccias (Fig. 4C).

Modern equivalents of this facies occur around pediment lime-stone outcrops in the hyperarid Egyptian western desert wherelimestone regoliths and far-traveled fine aeolian quartz sand arereworked by sheetflows after very rare rain storms (personalobservations 2001-2006) (Fig. 4D).

3.2.2. Sand-matrix-supported limestone breccias (Gm)These breccias differ little from the clast-supported breccias,

except that they have slightly more angular clasts and greaterpercentage of fines. They probably represent local mass flows fromthe same source. Modern equivalent mass flows mobilized during rarerainstorms occur around pediments in the hyperarid Egyptianwesterndesert (Fig. 4D).

3.2.3. Intraclastic (bedrock) massive and parallel laminated tabular fine-grained granular to fine-grained sandstones (Gsm, GSp)

Like lithofacies 1, these sandstones contain very locally derivedlimestone clasts together with mature fine sand, and also represent

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Fig. 4. Basal breccias. A: part of section at Kellhead, metre stick for scale; B: Detail of clast-supported breccia with subrounded limestone clasts, 10 cm of scale showing;C: Photomicrograph (plane polarized light) of breccia matrix, with dominantly silty limestone granules in fine-grained calcareous quartz sand matrix; very well rounded aeoliancoarse sand grain is arrowed; D: modern equivalent of basal breccias - Eocene limestone outcrops weathering and infiltrated by finewind-blown quartz sand and reworked into localdepressions, Bahariya, western Egyptian desert.

32 M.E. Brookfield / Sedimentary Geology 210 (2008) 27–47

fluvial reworking of local regoliths with the fine quartz sand broughtin from elsewhere. Parallel laminated units and the absence of rippleand dune cross-bedding in this relatively coarse-grained facies (meangrain size around 0.5 to 1.5 mm) suggests deposition from thinsheetfloods under upper flow regime conditions. Pebble clusters with15 cm. diameter clasts suggest current velocities exceeding 2.0m.sec-1

at times (Brayshaw et al., 1983).

3.2.4. Thick-bedded trough cross-bedded (10° to 25° maximum dip) fine-grained moderately sorted micaceous sandstones, often with mudstoneintraclasts (St)

These occur as thick-bedded individual units up to 1 m thickforming sets up to 5 m thick, and as the basal parts of sets of trough toripple-drift cross-laminated to parallel laminated fine to very finegrained, moderately sorted sandstones (Fig. 6). Current directionsfrom the cross-beds are unidirectional with over 90° variability withinindividual beds (loc. 23, Fig. 5), and with average flow directionsvarying from southeasterly to westerly among beds (locs. 7, 17, 23,Fig. 5). Intraclastic beds contain angular to rounded red clay pebbleclasts (with fine sand armour) up to 30 cm in diameter (Fig. 7A, B)which normally rest on strongly current scoured surfaces (Fig. 7C).Precursors of angular mud clasts of this type can be seen on variousextensive mudcracked surfaces between the sandstones (Fig. 7D) andanalogous surfaces can be found in dried up channels of modernephemeral streams (Fig. 7E).

Trough cross-bedded units are formed by migrating three-dimen-sional dunes during waning flow in channels in modern streams atmean current velocities during deposition, for the fine sand grain sizespresent, of between about 0.8 to 0.6 m.sec-1 (Miller et al., 1977; Allen,1982). The angularity of the mudstone intraclasts indicates very rapidreworking and deposition of within-channel mudcracked mudstones.Armoured mudballs (like those of Fig. 7B) frequently develop during

short-lived, intensefloods in ephemeral streams (SholokhovandTiunov,2002).

3.2.5. Low angle (less than 10° dip) cross-bedded, parallel laminatedfine-grained moderately sorted micaceous sandstones (SL)

These sandstones occur as individual finely parallel-laminatetabular beds up to 0.3 m. and as 0.5 m thick beds in sets up to 5 mthick and form most of the middle parts of the Annan Sandstones(Fig. 8A). Most beds are lenticular and sometimes convex upwards ona large scale, may stretch for decimetres along outcrops, and areseparated by reactivation surfaces.

Low angle cross-bedded lenticular sheet sandstones are formed byprogradation of linguoid bars inwide shallow channels or on adjacentfloodplains during floods (Fig. 8B). Reactivation surfaces are formed byextensive reworking during flood to low-flow transitions or vice versa(Bridge, 1993).

3.2.6. Ripple drifts cross-laminated fine-grained micaceous sandstones(Sr)

Ripple-drift trough cross-laminated sandstones occur abovelithofacies 4 and below lithofacies 7 and as separate beds withinfiner facies. Ripple wavelengths vary from a few to 20 cm. Ripple-driftcross-lamination is the result of high rates of traction deposition fromdecelerating heavily sediment-charged unidirectional low flowregime currents (Allen, 1982)., Deposition occurred below currentvelocities of around 0.6 to 0.8 m.sec-1 for the fine sand grain sizespresent (Brayshaw et al., 1983).

3.2.7. Parallel laminated fine-grained micaceous sandstones (Sp)These occur above lithofacies 6, as multiple and single beds within

thick sandstone beds, and also as thin separate layers within finer siltand clay facies. Parting lineation is common. There is something of a

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Fig. 5. Palaeocurrents for various localities: all fluvial except Kirklinton Sandstone, loc. 24, which is aeolian cross-bedding.

33M.E. Brookfield / Sedimentary Geology 210 (2008) 27–47

problem in interpreting these parallel laminated sandstones, as theycommonly occur transitionally above ripple-drift cross-laminated unitssuggesting deposition under lower flow regime plane bed transport(Allen, 1982). However, there is no such stability field at the mean fine

Fig. 6. Lithofacies 4 (St). A) Trough cross-bedded, parallel laminated fine-grained sandstonerounded to angular quartz grains.

sand grain sizes present, and parting lineation is characteristic of upperflow plane beds. These parallel laminated sandstones were probablydeposited as upper flow regime plane beds in highly sediment-chargedshallow flows during waning floods (Todd, 1996).

s; B) Photomicrograph of Annan Sandstone (loc. 21) showing fine-grained well-sorted

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Fig. 7. Mudcracks, mudballs and scours: A) angular clay clasts from mudcracks; B) armored mudballs at base of sandstone (armour is only fine sand); C) regular scour pit moulds(on base of sandstone) formed around eroded mud clasts; D) mudcracked mudstone with cracks infilled with overlying sandstone; E) modern mudcracked surface in semiaridephemeral stream, Sahara desert, Mauritania. A,B,C, D, from Annan Sandstone, Cove quarry, loc. 8. Camera lens cap is 8 cm diameter.

34 M.E. Brookfield / Sedimentary Geology 210 (2008) 27–47

3.2.8. Wavy and irregularly cross-laminated, very fine-grainedmicaceous sandstones (Sw)

These occur in the upper parts of the Kirklinton Sandstone wherevery fine-grained low-angle cross-bedded feldspathic arenites, in 5 to20 cm thick lenticular units, show abundant small-scale cut-and-fillwith small angular redmudstone clasts occasionally lining their bases.They also occur irregularly interbeddedwith siltstones andmudstones(Fl) throughout the Eden and Stanwix Shales, forming flaser and wavy

Fig. 8. Lithofacies 5 (Sl). A) low angle cross-bedded, parallel laminated fine sandstones (Brashowing low relief bars.

bedded units. Current directions from the low-angle cross bedding arevary between southeast and west. Lithofacies 8 is very different fromthe superficially similar lithofacies 6 (Sr), being altogether on a muchsmaller scale with abundant complex small-scale scour-and-fills withsmall angular mudstone chips. A few rare surface exposures havesmall symmetrical wave ripples of small wavelength.

At first sight, much of this facies looks like an aeolian sand driftfacies. However, the presence of clay clasts and fine mica, lack of well-

mpton Old Quarry, loc. 33); B) Warburton river east of Lake Eyre during waning flood

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rounded medium sand (present in facies 9), and wave ripples, is moreindicative of shallow and irregular water flow. Wavy and flaser-bedding is typical of frequent alternations of current velocity, and ischaracteristic of reversing currents in tidal situations (Reineck andWunderlich, 1968), though such alternations can also occur withrepetitive and divergent shallow flows (sheetfloods) on floodplainsand sabkhas (Martin, 2000). The short wavelength symmetricalripples suggest wave action in very shallow water. The lack of anyevidence of subaerial exposure (such as mudcracks and palaeosols)and lack of trace fossils is curious but may indicate hypersalineconditions. This facies was formerly attributed to sheetfloods on thedistal parts of fans where they interfinger with aeolian and playa basinfacies (Cowan, 1993; Meadows and Beach, 1993). Herries and Cowan(1997) re-interpreted it as a sabkha facies which seems more likely,especially in view of its association with evaporite-bearing lithofacies(Elm).

3.2.9. Small to large-scale trough cross-bedded moderately to well-sortednon-micaceous fine-grained sandstones (Stl)

These are confined to the Kirklinton Sandstones where they consistof moderate reddish orange, subangular to subrounded, fine to rarelymedium grained sublithic arenites and subarkoses, with no mica andno clays clasts (Fig. 9). These sandstones are coarser and slightly morepoorly sorted than the trough and parallel laminated sandstones oflithotypes 4 and 5 (Brookfield, 2004). Rare internal bounding surfacesof variable inclinations form sets between 2 and 5 metres thick andbound variably orientated asymptotic cross-stratified beds. Palaeo-winds from the foresets are very variable, ranging from SSW throughSE to NE (Fig. 5, loc. 24).

The cross-bedding, together with the absence of mica and clayclasts, and presence of well-rounded medium sand, indicates winddeposition; while the variably orientated bounding surfaces withtrough cross-beds, lack of obvious grain flow and dominance ofgrainfall (?) and rippleform strata, the fine grain size and lack of coarselags suggest deposition by three-dimensional coalescing bedforms(without erosional interdunes) in which slipfaces are a minor part ofthe system (Brookfield, 1979). The lack of grainflow strata, even in thesmaller units, may be due to variable winds reworking slipfaces. Andthis is also indicated by the palaeowind pattern from the large-scalecross-beds (Fig. 5, loc.24) which is trimodal: such patterns occur incomplex linear dune fields (Fryberger, 1979).

3.2.10. Parallel laminated micaceous silty sandstones and sandysiltstones (SSip)

These occur throughout the succession, usually interbedded withfacies 8, 11, and 12. They consist of fine- to very fine- grained, poorly

Fig. 9. Lithofacies 9 (Stl). A) smaller scale cross-bedded fine-grained sandstone (River Eden, lowell-rounded (arrowed) quartz grains, lamina shown by black line (River Eden, loc 26).

sorted silty sandstones and sandy siltstones with muscovite flakeslining laminae (Table 2). Parallel laminated silty sandstones andsiltstones represent traction deposits of shallow gentle water flows(low flow regime). Facies 10 is different from the finely laminatedaeolian sandstones described from cores of the Ormskirk Sandstone inboreholes from the Irish Sea (Herries and Cowan, 1997) which consistof very regularly laminated finely-striped clean sandstones inter-preted as climbing translatent strata formed by wind ripples (Hunter,1977).

3.2.11. Irregularly laminated sandy and silty gypsiferous mudstones andmudstones (Fl)

These occur as thicker units inter-bedded with facies 10 and 12within the Eden and Stanwix Shales (Table 2) (Fig. 10A) and have raresmall molds and nodules of copper carbonate (Brookfield, 2004). Thesediments very rarely show disrupted plasmic and clotted texturestypical of soils (FitzPatrick, 1993) (Fig. 10B) and in cores contain lenses,rosettes and nodules of gypsum (Holliday et al., 2001). The plasmicand clotted textures and the absence of any other soil characteristicssuch as rootlet casts and horizonation suggests disruption by solutionof evaporites rather than by organisms, although some of the clayaggregates may have been transported as clay pellets as in CooperCreek, central Australia (Nanson et al., 1986). The sandy lenses,especially those containing well-rounded coarse sand, have beeninterpreted elsewhere as wind-laid deposits on irregular water-laidsilty mudstones with an irregular surface possibly disturbed by saltefflorescence (Akhurst et al., 1997). The combination of massivegypsiferous mudstones and irregular wind-deposited lenses suggestsa saline sabkhas like those now forming on the coasts of the ArabEmirates (Alsharhan and Kendall, 2003). The lack of preserveddesiccation structures such as mudcracks (common in lithofacies 12below) is a puzzle but the sediment surface is frequently stabilized byalgal mats in such environments (Noffke et al., 2003), or salineporewaters may have limited desiccation contraction. Similar sedi-ments form in both marine sabkhas and at the margins of fluctuatingsaline lakes, where the alternate growth and solution of evaporitesgives cracked, puffy, ‘popcorn’ textures (Fig. 10C).

3.2.12. Interbedded massive reddish brown siltstone and mudstone (Fm)These often form mud-cracked clay drapes on top of stacked

tabular sandstones of lithofacies 5, and form thicker units, sometimesfilling large channels associated with lithofacies 4, 6, and 7 (see faciesassociation 3). Alternating desiccation cracked mudstones andsiltstones indicate suspension deposition in quiet water followed bydrying out. The absence of the evaporitic characteristics of lithofacies11 (Fl) suggests fluvial environments above saline groundwater levels.

c. 26); B) thin section of fine-grainedmoderatelywell-sorted sublithic arenite, angular to

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Fig. 10. Lithofacies 10 (Ssip), 11 (Fl) and 12 (Fm).A) detail of pelletal mudstone (Cadgill Burn, loc. 59); B) section of pelleted sandy, silty mudstone; C) puffy pelletal evaporiticmudstone, brackish Lake Sitra ,Qattara Depression, Egypt.

36 M.E. Brookfield / Sedimentary Geology 210 (2008) 27–47

3.2.13. Gypsum/anhydrite (Elm)Persistent beds of variously nodular and laminated anhydrite,

partially or completely hydrated to gypsum, occur in the lower 30m ofthe Eden Shales in boreholes, but have been dissolved at outcrop(Meyer, 1965; Holliday et al., 2001). The best descriptions are from thefour main evaporite horizons in the Vale of Eden boreholes(Arthurton, 1971; Arthurton and Hemingway, 1972; Burgess andHolliday, 1974; Arthurton and Wadge, 1981). These contain variousevaporitic types ranging from algal mat anhydrite through layeredanhydrite to laminated anhydrite. Several beds show cyclical alterna-tions of these types suggestive of upward-deepening (Arthurton,1971). Gypsum is precipitated as an evaporite mineral in hypersalineconditions and as nodular and lensitic beds by saline groundwaters.The interbedding with fine clastics deposited by thin sheet flows andwith adhesion and desiccation structures indicates a sabkha toshallow hypersaline environment (Schreiber and El Tabakh, 2000).

3.3. Problems of interpretation

First, the uniformly fine-grained nature of the sandstones tendsto limit the variety of fluvial bedforms possible to ripples andupper plane beds, with only limited dune cross-bedding (Allen, 1982,Figs. 2–4, 7-4; Mantz, 1978). Also, the effects of high suspendedsediment concentration on bedform development are poorly known,though there is some evidence that high concentrations suppressbedform development due to increased viscosities and dampenedturbulence (Best, 1996). The dominant ripple-drift cross-lamination(and minor dune cross-bedding) in the sandstones may have beenformed by three-dimensional bedforms in sediment-laden, shallow,vigorous and turbulent, rather than deep andweak, flows (Allen,1982;Dade and Friend, 1998). Plotting the mean grain sizes of the sandstonelithofacies on a Hjulstrom diagram shows that most of the sand wouldbe in suspension in even relatively slowly moving flows of 0.5 m/sec.or less, and the Solway sand grain size ranges are those most readilyeroded. Both experimental studies and field observations of thetransport and deposition of such fine-grained sediments are rare,which makes hydrodynamic interpretation of the lithofacies difficult(Reid and Frostick, 1987).

Second, the scarcity of medium and coarse sand is difficult toexplain since such sand is abundant in underlying and adjacentCarboniferous and earlier sandstone bedrock, and the large mudintraclasts in the channel sandstones suggests that velocities weremore than capable of transporting it. Medium and coarse sands musthave somehow been removed before deposition of the sands of theSolway Basin. Recent basins with bedrock drainages always havecoarser sand particles in fluvial deposits: even the recent arid LakeEyre basin in central Australia has ephemeral streams carrying in

coarse sand from sources over 500 kilometres away (Bourke andPickup,1999). So, someway of removing the coarser grains is required.The only way of doing this fluvially is to have a lake in which coarsersediment can be trapped. Such an analogy still does not explain thefine sand means of the Permo-Triassic sandstones and brecciamatrices, because silt and clay are more likely to be the mainsediment transported further by such a system. However, wind canreadily separate various grain sizes, with fine sand and coarse silt(loess) being deposited in different areas (e.g. the Loess plateau ofnorthern China - Chen, Li and Zhang, 1991) than coarse sand lags (e.g.the Selima Sand Sheet in Egypt - Maxwell and Haynes, 2000) and withfiner dust being removed entirely from the system. These fine sandsand coarse silts can then be reworked by local rains in an internallydrained system (e.g. Orfeo, 1999); especially as fine sand in the 0.1 to0.05mm range is themostmobile grain size (Miller et al., 1977). Such awind-dominated system now operates in the western EgyptianSahara; though this area is currently only at the erosional piedmontstage (El-Baz et al., 2000). A closer analogue may be the northern LakeChad basin (Bodele depression) or the southern Gobi desert area ofwestern China where ephemeral streams drain basins entirelycomposed of wind-blown fine sand and silt (Pye, 1987) and wherefiner dust is transported further (Goudie and Middleton, 2001).

Third, due to solution of evaporites at outcrop, diagenetic featureshave to be studied from borehole cores. Such diagenetic features areimportant since early diagenesis involved the precipitation ofanhydrite cements in the Basal Clastics and sandy units of the EdenShales, and dolomite in the St Bees and Calder Sandstones (Annan andKirklinton Sandstone equivalents). These early events were followedby the formation of authigenic quartz, K-felspar and albite (Stronget al., 1994). In the northeastern Irish Sea boreholes, the breccias andsandstones now have significant (20-25%) secondary porosity causedby solution of the early evaporitic cements (Strong, 1993). Andsolution of evaporites probably also accounts for some of thedisrupted structure of some units of the Eden Shales and the lateTriassic normal faulting.

Fourth, the lack of body and trace fossils is very significant since.Even in arid to semi-arid ephemeral stream - lacustrine systems,Recent biotas readily colonize the environments during rare favour-able periods (Timms, 2001) and caliche soils are well developed(Retallack, 2005). For example, a relatively diversemacro-invertebratefauna and flora (including gastropods, bivalves, arthropods and algae)colonizes the lower reaches of the rivers entering Lake Eyre duringand after floods (Madden et al., 2002; Costelloe et al., 2005). This lackof fossils and palaeosols (even in fine-grained facies) is the bestevidence for a hyperarid as opposed to an arid or wetter system (seepalaeoclimate section). Conditions during deposition of the Solwaybasin sediments appeared to have been so extreme over such a large

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area that temporary colonization by organisms from outside duringrare favourable conditions was practically never possible. At present,the arid-hyperarid transition on Earth marks a shift from biotic tolargely abiotic conditions (Warren-Rhodes et al., 2006). Underhyperarid conditions, biological effects, silicate transformations andleaching loss are no longer the dominant forces in pedogenesis. Whilein semiarid to arid soils, carbonate retention increaseswith decreasingrainfall due to decreased leaching in generally biotic soils (Retallack,2005), in hyperarid soils, carbonate retention decreases with decreas-ing rainfall and nitrate increases (Ewing et al., 2006). These trends arethe result of the near-absence of both biology and dissolved losses inhyperarid soils, making them fundamentally different from other soilson Earth. Such hyperarid processes may also help in the interpretationof the peculiar clay mineral assemblages of these early Triassicsediments which are dominated by ferric (?pedogenic) and recycledmicas with minor Fe-rich chlorite (Jeans, 2006).

3.4. Lithofacies associations and lithofacies architecture

Many of the lithofacies occur in repetitive associations whichdefine specific environments and sub-environments. Generally, thelithofacies associations are defined on vertical associations and can bedefined without the large two- and three-dimensional outcropsrequired for lithofacies architecture. Strata between boundingsurfaces are adequately covered by the terms laminae, foreset, setand coset of McKee and Weir (1953).

3.4.1. Tabular limestone breccia and granular sandstone associationThis association consists of lithofacies 1, 2, and 3, in generally fining

upwards units (Brookfield, 2004). It is confined to the Basal Clasticsunit and shows extreme vertical and lateral variability of coarserclastics in very small areas, which indicates very small, local systemsof sediment transport and deposition (Fig. 4). The thicker brecciaassociation at Kellhead has up to 11 main units separated by majorbounding surfaces (Brookfield, 2004). Here, current directions fromthe two main sets of cross-beds in unit 1 are towards the northeastand south-east (Fig. 5, loc. 2). This is at right angles to the adjacentfluvial flow patterns higher up, but is consistent with local derivationfrom a limestone outcrop formerly quarried a mere 20 m to the west.One section of the top 5 metres is well-enough exposed for beds andbounding surfaces to be traced and their variability to be evaluated(Fig. 11). The sets of tabular sheet breccia units are cut by one low-relief channel (unit 2) and represent very local sheetflood depositswith only local channeling.

Modern equivalents of facies association 1 occur in the low-relief,hyperarid areas of the Sahara where sporadic limestone basementoutcrops supply angular clasts to be locally reworked by very rarerainstorms (El-Baz et al., 2000; pers. obs. 2001 - 2005) (Fig. 5D).

3.4.2. Multistorey trough cross-bedded intraclastic sandstone (St) -tabular sandstone (Sl) - thin sandstone-siltstone-mudstone associations(SSip, Fm)

This association dominates the middle part of the AnnanSandstones and it was formerly extensively quarried for buildingstone, so a number of large (though mostly overgrown) sections are

Fig.11. Lithofacies association 1. Basal Clastics at Kellhead. Units 1 to 4 of planar to low-angle coccurs below unit 2.

available where multiple sandstone sets can be up to five metres thick(Fig. 12A,B). The vertical sections show repetitions of relatively thickcycles. Intraclastic trough cross-bedded fine-grained sandstones oflithofacies 4(St), often with erosional bases and very divergentpalaeocurrents are overlain by multistory cosets of large-scale over-lapping lensitic thick tabular low-angle cross-bedded parallel-lami-nated slightly finer-grained sandstones of lithofacies 5 (Sl) withconsistent southward palaeocurrents (Fig. 12A, B). Individual sets inboth are draped by thin silty mudstones (Fm) and lithofacies 5 (Sl) isoverlain by fine-grained facies of lithofacies 7, 10 and 12 (Sp, Ssip, Fm)at 5- 10 metre intervals (Fig. 12A, B). Lithofacies 5 (Sl) may alsodominate entire decimetre-thick quarry sections as at Brampton(Fig. 8A), or may be almost absent as in the lower parts of CoveQuarry where the associations is transitional to facies association 3(Fig. 12A).

There are basically only three main types of bounding surface.Major bounding surfaces are draped by thicker units of very fine-grained facies (Fig. 12A at 25 metres and 14B at 10 metres). Lessextensive bounding surfaces are gently convex-up surfaces which: a)separate trough cross-bedded (St) sets, intersect, and pass intoconformable laminations (Fig. 13A); and b) enclose lensitic tabularsets of low angle cross bedded parallel laminated sandstones (Sl)representing lateral and downstream accretion on low-relief bars(circles on Fig. 13C). Very rare tabular foresets draped with siltymudstones possibly represent bar migration into ponds duringwaning flow, as described by Stanistreet and Stollhofen (2002) fromthe Namib desert, SWAfrica. Rare current reversals in lithofacies 5 (Sl)may originate in the sameway. On the other hand, the rare occurrenceof high-angle cross-bedding, the dominance of low-angle cross-bedding and the abundance of ripple-drift cross-lamination closelyresemble the linguoid bars of fine sand described by Williams (1971)from ephemeral streams entering Lake Eyre from the west.

In the Irish Sea boreholes, such multistory fluvial units (thereinterbedded with minor aeolian sandstones) were interpreted as thedeposits of a permanent westward flowing river system (Herries andCowan, 1997). However, such a system necessitates a high water tableand would also involve significant overbank flooding, deposition ofcrevasse-splay and other floodplain facies, some reduction of ferriciron at least in abandoned channels, and more pertinently someorganic life - none of which are present in the Annan Sandstones.

Two major peculiarities need explaining for this Annan Sandstonefacies association 2. First, it basically does not change in character atoutcrop across the entire northeastern Carlisle Basin, a distance of40 kilometres, and from seismic and borehole records the main unitdoes not change significantly over thousands of square kilometres. Isthis due to almost basin-wide fluvial changes or to overlapping facieslenses? Second, fine overbank facies are very rare, as is any sign ofmuch channeling at the decimetre-scale and persistent (at outcropscale) thin tabular units of finer facies separate the main multistoryfluvial units. Though reworking of floodplain sediments can beinvoked to explain the scarcity of silt and clay, there is no evidenceof channeling above the metre-scale which occurs in modernephemeral streams carrying fine sediment (Reid and Frostick, 1997;Tooth, 2000). Most of the fine sediment was carried away to bedeposited in facies associations 3 and 4.

ross-laminated beds are separated bymajor bounding surfaces (bold). Minor channeling

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Fig. 12. Lithofacies associations in vertical section of Annan Sandstones with lithofacies labeled (arrows are cross-bedding palaeocurrent directions with north at top -see directioncross at bottom right). Lithofacies association 2: A) Cove Quarry section (loc. 8); B) Glinger Burn section (locs. 88-94). Lithofacies association 3: C) Lower Annan Sandstones (AnnanRiver section, loc. 3), 5 fining upwards channels in top 5 metres,; D) Warmanbie Quarry section (loc.5), note mudstone-filled channels at 2 - 4 metres; E) River cliff (loc. 3), inferredbank collapse below clay drape.

38 M.E. Brookfield / Sedimentary Geology 210 (2008) 27–47

The overwhelming dominance of fine sands suggest depositionfrom thin sheet flows in very wide and shallow fluvial channels or onvery widely inundated flood plains. Large floods sporadically occur inboth African and Australian deserts and are responsible for most of thedeposition outside channels in the lower reaches of the streams. Forexample, enormous floods extend for tens to hundreds of kilometresout from lowland ephemeral streams feeding Lake Eyre in Australiaand Lake Chad in Africa (Durand,1995; Tooth,1999). They occur wherechannelized flow ceases and floodwaters spill across adjacent alluvialsurfaces to eventually dissipate into dune fields, sabkhas or playas(Bull, 1997: Tooth, 1999).

The change from channels with very divergent palaeocurrents tomore uniform unchannellized flow can be inferred for the streamsdraining into the eastern part of Lake Eyre, Central Australia (Kotwicki,1986). There, during a major flooding of Cooper Creek in 1974,floodwaters extended over the floodplains and playas for a width of60 kilometres and the discharge was 4000 m3.sec-1 at a mean flowvelocity of 0.85 m3.sec-1 at Innamincka with a depth of 16 metres inthe deepest section of the channel. The result was that the floodedare was covered with unidirectional linguoid bars, underlain andoverlain by anastomosing channels formed during less extreme con-ditions which reworked a small part of the floodplain sediments

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Fig. 13. Lithofacies association 2. Facies architecture. A: River Lynne bank face (loc. 23); B) Cove Quarry lower face (loc. 8); C) Old Brampton Quarry face (loc. 33). Major throughgoingbounding surfaces (circled): in B) and C) often with clay drapes and cut less important bounding surfaces without clay drapes. Thin lines are low-angle laminations with arrowsshowing dip directions, north at top (see right of ‘B’). Alignment of quarry surfaces shown above faces in degrees.

39M.E. Brookfield / Sedimentary Geology 210 (2008) 27–47

(Fig.14A). Suchdeposits cover vast areas northeast and east of Lake Eyre,and are deposited by many ephemeral streams in which wide flood-plains and playas alternate (Fig. 14B). The inferred result being thealternation of channelized, multistory sand and finer-grained units seenin facies association 2. However, the overbank mud facies is much lessprominent in the Solway Triassic compared with the Lake Eyre basin.

Apart from a facies architecture analogous to that inferred for thearea north and east of Lake Eyre, facies association 2 resembles, indecreasing order of study and knowledge, the flood deposits of the1965 major Bijou Creek flood in Colorado (McKee et al., 1967), thelaminated sheets sands described by Tunbridge (1981) and thedeposits of ephemeral streams draining the area northeast of LakeChad (Durand, 1995; personal observations in 2001-2005). The Bijouflood spread extensive locally thick deposits over the adjacentfloodplain, dominated by parallel laminated sands in flat to low-angle beds up to about 50 cm. thick, which were deposited by thin,upper flow regime flows. In the channels, sediments were dominantlytrough cross-bedded, ripple-drift cross-laminated and parallel lami-nated sands, up to 50 cm. thick, with minor tabular cross-laminatedunits with dips up to 25°, deposited by migrating dunes and bars.These flood deposits of sand, up to 4 metres thick, were deposited inonly a few hours. Bijou Creek sands, however, include coarse andmedium sands not found in this study, and the geometry of sets couldnot be observed.

The cyclical variations of facies association 2 could be caused byintrinsic changes in the basin, such as sporadic flooding and lateralmigration of channels, or by base-level changes due to fluctuations in

lake levels. The former case is the standard interpretation of suchcycles. In the latter case, major floods gradually extend the lake,causing deepening in river channels and the deposition of successivelymore extensive overbank deposits. Themajor cycles vary from a few toover 25 metres and show no consistent thickness variation. Underuniform subsidence and channel migration, they should be at leastwithin one order of magnitude. If the latter lake level concept applies,then synchronous changes should occur throughout the basin, withsporadic megafloods causing filling and extension of playa lakes anddeposition of extensive correlatable fine-grained facies units. There isno data available at present to test this idea, which requires very goodtemporal correlation currently unobtainable in the strata here.

Facies association 2 is practically identical to those described fromsome fluvial and deltaic facies in the Mesozoic of the western U.S.A.(Miall, 1988a,b; Stephens, 1994; North and Taylor, 1996) where, ashere: a) there is no obvious large-scale cyclicity, b) there is no gradualdecrease in flow energy, individual depositional units are cappedabruptly by thin mud drapes, c) there is no extensive ‘overbank’ facies,and d) most deposition occurred on lateral, oblique and downstreammacroforms analogous to accreting bars (Bridge, 1993). Other ancientfluvial clastics with identical geometries (including steepening setinclinations and intersecting low angle sets) were described by Willis(1993) and compared with simulated point bar models.

3.4.3. Fining upwards intraclastic sandstone - mudstone associationFining upwards sets of intraclastic and trough cross-bedded to

ripple-drift cross-laminated to parallel laminated sandstones of

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Fig. 14. Modern analogues of lithofacies association 2: A) floodplain of Cooper Creek, east of Lake Eyre showing extensive bars separated by anastomosing channels; B) alternatingplaya and braided/anastomosing reaches of Eyre Creek and Diamantina Creek, about 100 kmNWof Copper Creek., arrow show creek flowdirections. Inset shows Solway Basin (Fig. 2)to same scale.

40 M.E. Brookfield / Sedimentary Geology 210 (2008) 27–47

lithofacies 4, 6, 7, and 8 (St, Sr, Sp and Sw) are interbedded withrelatively thick very fine silty and sandy lithofacies 10, 11, and 12 (Ssip,Fl, Fm). Associations of this type (with individual channel sandstoneunits less than three metres thick) are typical of the lower and upperparts of the Annan Sandstones (Fig. 12C) and are thus transitional (ifWalther's Law applies) between facies associations 2 and 4: suchtransitions can be seen in the lower part of Cove Quarry (Fig. 12A at10 metres). Occasional channels filled with fine-grained facies occur(Fig. 12D), and there are signs of contemporaneous channel bankcollapse in places (Fig. 12E).

The regular vertical facies change within each tabular sand unitmarks individual depositional (possibly flood) events with strongscouring changing to rapid deposition fromwaning sediment-chargedflows. In most modern environments, this association is characteristicof seasonal fluvial systems with well-defined migrating channelsseparated by, and cutting into, extensive overbank deposits (Bristow,1996). However, the regular tabular units often persist laterally,without the low-angle inclined surfaces and lateral variation found insuch bars (Willis, 1993) and current directions among units are verydivergent (Fig. 12C). Furthermore, the individual associations are lessthan 3 metres (and often less than one metre) thick, which indicatesthe maximum channel depth. This, plus the thinness of the channelunits, the scarcity of multistory channel units (only three occur inFig. 13C) and the abundance of large angular mudstone clasts inalmost all channels (rapidly reworked and deposited from desicca-tion-cracked mudstones) suggest that each sandstone was the resultof one flood, rather than a gradually migrating channel. Rarely, tabularunits of trough cross-bedded sandstones without mudstone laminaeand interbeds suggest migrating dunes in major channels whilethe interbedded fine-grained facies (10,11,and 12) have the character-istics of saline sabkhas rather than fluvial floodplains (Thompson andMeadows, 1997).

Small channel- sabkha associations of this type occur in bothmarginal marine and lacustrine hypersaline environments such as theEmirate coast of the Persian Gulf, Lake Eyre in Australia, and the ChottRharsa in Tunisia (Magee et al., 1995; Alsharhan and Kendall, 2003;Swezey, 2003). In these, relative changes of water (sea or lake) level,caused by tides, seiches, storms and floods, control sabkha aggrada-tion and channel cutting. Without accurate dating it is impossible todecide whether the changes seen in facies association 3 are the resultof short-term changing lake level fluctuations in an arid environment,or longer term changes due to fluctuating relative sea-level or lake-level changes. The interpretation depends in whether facies associa-tion 4 (below) is considered as marine or lacustrine. However, thecomplete absence of body and trace fossil makes a marginal marineenvironment most unlikely. In the Lake Eyre area of Australia, theweakly incised meander belts of ephemeral streams feeding the lake

from the east and northeast are periodically inundated by rising lakelevels during large floods, depositing lacustrine sabkha and evaporiticfacies (Fl, Fm, Elm) in the channels (Wells and Callen, 1986). Thechannels consist of trough cross-bedded fine sands (St)(sometimesclay-pellet sands) with a basal conglomerate of lithic clasts (heredominantly gypsum and pedogenic carbonate) derived from theunderlying Tertiary. The gypcreted dunes found in this Recentenvironment, however, appear unrepresented in facies association 3.

3.4.4. Interbedded fine clastics and evaporitesThis association is confined to the Eden Shale Formation and the

Mercia Mudstones Group. It can really only be studied in boreholes,though some idea of the clastic facies can be obtained from the limitedoutcrops transitional to facies associations 3 and 5 where thesediments consist of lithofacies 7,8,10,11.

Along the northern edge of the basin, the Eden Shales of theChapelcross boreholes consists of 80 metres of interbedded siltygypsum/anhydrite and gypsiferous siltstones and fine sandstones(lithofacies 10, 11, 12, 13) which lie directly above the basal brecciasand pass upwards into 10 to 30 metres (depending on where thecontact with the Annan Sandstone is taken) of interbedded planarfine-grained sandstones and interbedded mudstones (lithofacies 10,11, 12) (Fig. 15) (Holliday et al., 2001). Forty kilometres south-east theLangwathby and Lounthwaite boreholes (Fig. 2) have 100-120 metresof interbedded fine gypsiferous clastics and gypsum/anhydrite similarto the Chapelcross borehole, with the basal clastics resting on LowerPermian aeolian sandstones (Burgess, 1965). However, here, fourdistinct gypsum/anhydrite beds (A to D) can be recognized, of which Band C are persistent and D , which dies out to the north has a thinunderlying dolomite. Towards the west the anhydrite/gypsum anddolomite layers become thicker at the base of the St Bees (=Eden)Shales at St. Bees and in borehole #112/19 north of the Isle of Man(Fig. 2) (Akhurst et al., 1997; Newman, 1999) and pass into halite-dominated successions to the south (up to 200 metres thick arerecorded in offshore boreholes 112/25A and 113/27-1, see Fig. 2):though further south andwest these pass back into anhydrite/gypsumand dolomite (Jackson et al., 1995). The variety of subfacies of thegypsum/anhydrite (Elm) lithofacies (and biofacies) can be used torefine the saline lake/marginal marine model in some detail (and willbe discussed elsewhere). Here a simple comparison is made withsome possible modern equivalents.

The interbedding of fine clastics showing signs of exposure withprimary and early diagenetic gypsum/anhydrite is characteristic ofboth marine and non-marine sabkhas (Schreiber and El Tabakh, 2000)and playa lakes. Modern analogies to this lithofacies association arepresent in and around Lake Eyre (Australia), Lake Chad (Africa) and theAral Sea (Central Asia), where great fluctuations in lake extent cause

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Fig. 15. Lithofacies association 4 with lithofacies labeled. A) Chapelcross borehole;B) upper Eden Shale transitional to Annan Sandstone (Robgill, loc 13). Legend on Table 2.

Fig. 16. Lithofacies association 5 architecture. A) Kirklinton Sandstone (Cliff house, loc.24). Undulating major bounding surfaces separating cross-bedded sets with occasionalreactivation surfaces; B) modern analogy; 3-dimensional barchanoid dune complexesmigrating over sabkhas (Libyan desert, Libya); truck for scale.

41M.E. Brookfield / Sedimentary Geology 210 (2008) 27–47

interbedding on a very extensive scale of beds and layer of evaporites,‘tidal flat’ fine clastics and fluvial sheet sandstones. Fluctuations can beextremely rapid. For example, in Lake Eyre, filling by inflows of theorder of 5,000 - 10,000 m3/sec. takes only a few months, and if thereare no further inflows drying up takes only one to three years (Magee,1993). In the Solway basin, there are no diagnostic evaporite mineralswhich can be used to distinguish precipitation from marine orcontinental saline waters (Bryant et al., 1994) and the otherlithological associations indicate continental deposition.

The fossil biota found in the southern areas of the Solway-East IrishSea basin is not diagnostic of marine or lacustrine environments, andconsists of plant spores, algae, foraminifera linings, probable fungi,gastropods, and bivalves (Pattison, 1970; Arthurton and Hemingway,1972). The lithofacies distributions suggest a basin centre evaporiticlake or estuary in the Irish Sea depositing mainly halite surrounded bya marginal highly fluctuating sabkha area in which gypsum and fineclastics accumulated (Jackson et al., 1987). The lack of any fossil biotain the Solway evaporite association remains a problem. Recentepisodic saline lakes are rapidly colonized from adjacent areas whenthey fill. For example, Lake Torrens (south of Lake Eyre) filled for thefirst recorded time in March, 1989, and was rapidly colonized by 29taxa of aquatic organisms (dominantly arthropods), before the waterevaporated completely by the end of 1989 (Williams et al., 1998). Thelack of biota in most of the Solway evaporite deposits suggest that anypossible colonization source was either absent or very long distances

away. However, lake faunas have changed markedly through time(Park and Gierlowski-Kordesch, 2007).

3.4.5. Large-scale cross-bedded sandstone - wavy bedded siltstone/mudstone association

Large-scale cross-bedded sandstones (lithofacies 9) interbeddedwith units of parallel laminated sandstones (lithofacies 10), wavybedded siltstones (lithofacies 11) and mudstones (lithofacies12) arecharacteristic of the Kirklinton Sandstone Formation and thislithofacies association starts abruptly above the Annan Sandstones(Brookfield, 2004; Holliday et al., 2004). There are only a handful ofsections suitable for architectural analysis. The best section at Cliffbridge (loc. 23, Brookfield, 2004) shows lensitic units of trough cross-bedded sandstone bounded by undulating erosional boundingsurfaces mostly dipping upwind and bounded by dominantly planarbedded aeolian sandstones (Fig. 16A). Such units represent fields ofsimple three-dimensional barchanoid or linear dunes migratingacross dry intervening interdune accretion deposits; there is noevidence of more complex draa bedforms (Rubin, 1987; Brookfield,1992). The association of these dune sandstones elsewhere withthicker deposits of water-laid occasionally gypsiferous fine sedimentssuggests an aeolian sabkha rather than a continental dune field, asnoted by Herries and Cowan (1997). In Libya, aeolian barchanoid dunecomplexes migrating across dry interdunes could form similarsuccessions and geometries (Fig. 16B). Thicker gypsiferous fine-grained units may represent amalgamated interdunes or wide-spreaddune-free sabkhas (Simpson and Loope, 1985). In the Solway basinthere is, however, no sign of the damp/wet interdune strata recordedfrom the upper units of the Sherwood Sandstone in Cheshire(Mountney and Thompson, 2002).

4. Modern analogues

There have been few detailed studies of fluvial processes in arid tohyperarid areas, largely due to the difficulty of direct observation.Most studies are limited to observing the sedimentary effects longafter flow events. Observation of the sedimentary record shown byarid to hyperarid streams suggests that their long-term historieshave been dominated by repeated large floods, and that the streamchannels are very sensitive to the effects of large or catastrophic floods(Tooth, 2000). For example, rain falls only infrequently on the Gilf

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Kebir Plateau in southern Egypt; and the estimated 50-100mm/year isconcentrated in only a few days of a year (it does not rain every year),causing very large floods extending some distance into the surround-ing hyperarid desert (pers. obs. 2001-2005). Such floods erodestrongly because the sandy banks are weak, especially in the absenceof vegetation (Pickup, 1991). The closest analogies to the faciesassociations recognized above in the Solway basin are in such largearid inland basins with internal drainage into saline lakes in whichgreat fluctuations of lake extent (but not depth) cause very rapid and’layer cake’- type vertical facies changes, but with limited fluvialchannel incision (Magee and Miller, 1998; Maley, 2000). Climaticfluctuations in such lake basins are also widespread and can lead tomajor vertical changes from, for example, dominantly ephemeralstream-lake to dominantly aeolian facies associations, as has hap-pened many times during the Quaternary in the Lake Eyre and LakeChad basins (Durand, 1995; Alley, 1998). The facies associationsdefined above can be recognized in various parts of the Recent LakeEyre Basin; though unfortunately only the areal distributions havebeen studied, with relatively little on the stratigraphy (Alley, 1998).However, the sand being supplied by ephemeral streams is relativelycoarse and immature in the Lake Eyre Basin and the late Cenozoicsection is thin because. tectonic subsidence has practically ceased(Wells and Callen, 1986). The Chad Basin is perhaps a better analogythan the Lake Eyre Basin for the fine-grained alluvial clastics of theAnnan and Kirklinton Sandstone. The northern part of the Lake Chadbasin (the hyperarid Bodele region) contains the Fya sub-basin (Louis,1970) in which most of the fluvial sediment occasionally suppliedfrom the Tibesti and Ennedi Precambrian massifs during rarerainstorms is trapped (Kusnir, 1995) and reworked by the wind tosupply a large proportion of the world's dust (Goudie and Middleton,

Fig.17.Dust storm deflating northern Chad basin; fine sand brought in from Libyan desert acrel Gazal drainage. Inset shows Solway Basin (Fig. 2) to same scale.

2001) (Fig. 17). Very occasionally, flows in the Bahr El Gazal waditransport fine sediment from this area southwestward towards LakeChad (Olivry et al., 1996). The Lake Eye Basin is perhaps a betteranalogy for the lacustrine facies of the Eden Shales, since Lake Chad isnot hypersaline as it is supplied by the Chari and Logi rivers withfreshwater from much wetter savannah areas to the southeast(N1000 mm rainfall)(Maley, 1981). Both the Lake Eyre and LakeChad basins are, however, are onmuch grander scale than the Irish SeaBasin of which the Solway Basin is simply an extension - scale isimportant (Fig. 18). In fact the early Triassic palaeogeography of theBritish Isles almost exactly mimics the present Lake Chad basin: inboth northwesterly flowing river systems end in evaporitic (orpotentially evaporitic) basins; both have tectonically-controlled sub-basins; and both are flanked on the north by hyperarid deserts(compare Fig. 18B with C).

5. Ancient analogues

Using ancient analogues to interpret other ancient deposits simplyleads to circular reasoning. Nevertheless, there are many other an-cient deposits which have the same characteristics as the SolwayBasin and may repay re-investigation.

Red bed successions of this type are found throughout the Permianand Triassic of Western Europe and elsewhere. Akhurst et al. (1997)made reconstructions of the main Permo-Triassic formations ofCumbria which greatly resemble the reconstructions of the faciesassociation above, except that they were related to a positive LakeDistrict block and within the framework of rift basins. Clemmensen(1978, 1985) described almost identical facies to those described here(apart for the aeolian sandstones) and provided a reconstruction of

oss lowdivide between Tibesti and Ennedi massifs and position of very intermittent Bahr

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Fig. 18. Maps and structural cross-sections of: A) Lake Eyre (Alley, 1998); B) Lake Chad (Burke, 1976) basins, small black box on A and B is the Solway Basin (Fig. 2) to the same scale;C) Triassic basins of UK to same scale showing main paleoflow directions (from Ziegler, 1990).

43M.E. Brookfield / Sedimentary Geology 210 (2008) 27–47

lithofacies associations which would serve equally well for the SolwayBasin, except that facies boundary changes are much more horizontalin the Solway Basin. Analogous evaporitic lake-ephemeral stream-aeolian facies in intracontinental basins have been described from thelate Permian of Central Europe (Gaupp et al., 2000), the Permian of thesouthern North Sea (Sweet, 1999) and Northwest China (Wartes et al.,2000) and the early to mid-Triassic of Spain and Germany (Lopez-Gomez and Arche, 1993; Vecsei and Mandau, 2002; Ulicny, 2004). Thefacies of the early Jurassic Kayenta Formation of western NorthAmerica is practically identical to those of the Annan and KirklintonSandstones (Luttrell, 1993).

6. Palaeotectonics

The Solway basin developed in the south-centre of the northernpart of Pangaea (Golonka and Ford, 2000) between about 27°N (latePermian) to 38°N (late Triassic) (Knott et al., 1993), over 1500 kmnorthwest of the Tethyan shelf margin, as a post-rifting intraconti-nental basin (Fig. 1).

Only in the early Permian and late Triassic are there signs of rifting;the first associated with the rifting between Greenland and Europe(Brookfield, 1980), and the second associated with the early stages ofthe opening of the Central Atlantic Ocean, south of Britain (Ziegler,1990) and affecting the later Triassic Mercia Mudstones, wherethicknesses across faults vary significantly (Knipe et al., 1993; Shelton,1997); whereas thicknesses of the Sherwood Sandstone Group andunderlying sediments do not vary across these faults (Musgrove et al.,1995). Despite periodic claims that present structural highs (theSouthern Uplands, the Lake District, and the Pennines) affected latePaleozoic sedimentation (e.g. Fraser and Gawthorpe, 1990; Ziegler,1990), there is in fact little evidence from the sedimentary successionfor this and the amount of pre-Cenozoic burial has been under-

estimated (Bray et al., 1992). Thermal histories for these areasreconstructed from fission track dating and vitrinite reflectance datasuggest that regional Cenozoic uplift erosion in the North AtlanticIgneous Province, and consequent erosion, has removed between 0.8to 1.3 km from the EastMidlands shelf, at least 2 km from the Pennines(Green, 1986; Bray et al., 1992), and 3 km from the East Irish Sea basin(Lewis et al., 1992; Rowley and White, 1998). The present topographyis the result of differential Cenozoic uplift and erosion unrelated to latePaleozoic- early Mesozoic topographies.

In view of the inadequate dating, subsidence curves for individualunits in the Solway Basin would be inaccurate at best. The entiresuccession, however, from late Permian to late Triassic (271-204ma, or67 ma duration) at the centre of the Solway Basin has a maximumthickness of around 1600 metres (Quirk et al., 1999). The averagesedimentation rate is thus 24 mm/ka. Finer-grained sections havelower sedimentation rates due to differential compaction and therehas probably been significant loss of evaporites. The Mercia MudstoneGroup, affected by synsedimentary faulting shows less than half thecompacted sedimentation rate of the Sherwood Sandstone Group,unaffected by synsedimentary faulting (Ruffell and Shelton, 1999). Forexample, in the Larne 2 borehole in northern Ireland, using compactedsediment thicknesses, the Sherwood Sandstone Group is about650 metres thick and appears to have been deposited entirely withinthe Scythian Stage (~10 ma)(Penn, 1981). If, so, then the rate ofsedimentation is 65mm.ka-1. TheMerciaMudstone Group in the sameborehole is 950 m thick and, over its 30 ma period (Ladinian-Norian),accumulated at 32 mm.ka -1. However, since muds compact to about50% of their depositional thickness, the sedimentation rate increasesto 64 mm.ka -1, about the same as the sandstones. The Solway Basinmight have been below sea-level for much of the period in which casethe accumulation rates are simply a function of sediment supply,fluctuations in lake level and extent, and possibly wind deflation,

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rather than marine base-level control (Ken Glennie, pers. comm.). Thebasin could simply be filling up with only isostatic subsidence tocontrol the accommodation space.

The tectonic history of the Solway Basin (and adjacent areas)closely resembles that of the Chad Basin. Both began with uplift andrifting; in the Solway Basin during the earliest Permian (~295 ma), inthe Chad Basin in the Lower Cretaceous (~125 ma). After erosion,thermal subsidence began in Solway Basin in the latest Permian(~255), in the Chad Basin in the late Cretaceous (~85 ma): i.e. bothtook 40 ma from initial rifting to thermal subsidence. The SolwayBasin changed into part of a passive margin shelf with the start ofopening of the Atlantic and the marine transgression of the latestTriassic (~205 ma), i.e. after 50 ma of thermal subsidence. The ChadBasin is still an intracontinental basin after 85ma but has accumulatedless than 1 Km of sediment since the early Miocene (25 ma) - asedimentation rate of 40 mm/ka (Hartley and Allen, 1994). This islower than the 65 mm/ka average for the clastic sediments of theSolway Basin (Sherwood Sandstone and Mercia Mudstones above),but is in keeping with the longer history and asymptotically decliningsubsidence rates of the Chad basin.

7. Palaeoclimate

The arid to hyperarid climate claimed for the Solway basin, byanalogy with lakes Eyre and Chad, lasted for possibly 65 million years(Table 1). This requires explanation since lakes Eyre and Chad sedimentsonly showarid to hyperarid conditionsperiodically during the 2million-year-long Quaternary ice-age climatic fluctuations (Durand,1995; Alley,1998). Their earlier sediments were deposited under much morevariable and generally wetter climatic conditions. The rapid northwarddrift of Australia during the Cenozoic has carried it through severalclimatic zones and the climates of both lakes have also changed duringthe world-wide Cenozoic climatic deterioration. How did the Solwaybasin climate remain so arid for such a long time? The explanation lies inthe stability of both tectonics and climate during the Triassic. From latePermian to late Triassic, Britain moved only about 1200 km (about11degrees of latitude) northwards (Knottet al.,1993) fromabout27°N to38°N. This is almost entirely within the 15° to 35° latitudinal desert belt,even if the late Permian to late Triassic latitudinal temperature gradientwas warmer and weaker from pole to equator, with warm climatepalaeosols developing up to 85° latitude (Retallack, 1999; Kidder andWorseley, 2004). In the late Triassic eastern North American rift basins,palaeolatitudinal variation in facies points to a stable climatic config-uration,with awetequatorial beltflankedbydesert facies comparable tothe present, though steeper (Mutti andWeissert, 1995; Kent and Olsen,2000) and similar conditionsprevailed in the late Permian (Fluteau et al.,2001). Monsoon circulation across the northern Tethyanmargin causedseasonal precipitation on the southern Variscan hills (reaching heightsof possibly 2 km in places), whose northward drainage by seasonalstreams provided sediment to the semi-arid to arid southern andmidland areas of England (Steel and Thompson, 1983; Fluteau et al.,2001; Chumakov and Zharkov, 2003). The desert conditions areconfirmed by the widespread xeromorphic Zechstein (late Permian)and Voltzia (middle Triassic) floras found elsewhere in Eurasia, thoughthere is a floral break during the early Triassic (Dobruskina, 1987)after a massive and global Permian die-back of coniferous vegetationwhich did not recover until the mid-Triassic (Looy et al., 1999). EarlyTriassic coals are unknown (Retallack et al., 1996). Furthermore, landanimals (including, uniquely, the insects) also suffered devastatingloss of diversity during the end Permian extinction and did not recoveruntil the mid-Triassic (Labandeira and Sepkoski, 1993; Benton et al.,2004). How significant these unique early Triassic ecosystem character-istics (especially the almost complete loss of land vegetation) are forexplaining some of the peculiarities of the Sherwood Sandstone Group(and other early Triassic sediments) is unknown but require furtherinvestigation (see Retallack, 1995). They certainly affected the Permo-

Triassic river characteristics of South Africa, which changed frompredominantly meandering to predominantly braided (Ward et al.,2000).

8. Conclusions

The thirteen non-marine lithofacies identified in the late Permian-mid-Triassic of the Solway basin can be arranged into five mainlithofacies associations which are comparable with the desert reg, aridephemeral stream, sabkha, saline lake and aeolian sand dune environ-ments of the Recent arid to hyperarid areas of existing intracontinentalbasins such as Lake Eyre and Lake Chad. The fine-grained submaturesandstones above the local basal reg breccias suggest water-reworkingof wind-transported sediment, as in the northern part of the Lake Chadbasin. Sediment accumulation in such basins is primarily controlled bylarge fluctuations in lake extent superimposed on tectonic subsidenceand this promotes a large-scale ‘layer-cake’ stratigraphy as exemplifiedin the Solway basin. There is no evidence of post-early Permian riftinganywhere during deposition of the late Permian to mid-Triassic Britishsuccessions, although these successions are often interpreted with arift-basin model (e.g. Akhurst et al., 1997). The arid to hyperaridpalaeoclimate changed little throughout deposition of the Solway basinsuccession, in contrast to Lakes Eyre and Chad. This is due to tectonic,palaeolatitudinal and palaeoclimatic stability as the Solway Basinremained in the northern latitudinal desert belt from late Permian tolate Triassic times.

The extensive tabular arrangement of lithofacies and their greatextent in the Solway Basin and adjacent areas are far more readilyexplained with models based on Recent arid to hyperarid sedimenta-tion in the Mesozoic-Recent intracontinental basins of Australia andAfrica, than to rift-based models (Hartley and Allen, 1994; Jacksonet al., 1995; see Ruffell and Shelton, 1999). Even though Wills (1970)briefly compared British Triassic sediments with the Recent sedimentsof the Australian interior, only Talbot et al. (1994) have recently madedirect comparisons (of the topmost Mercia Mudstone Group) with theAustralian interior where arid streams spread very extensive clasticblankets of sediment over vast areas. Furthermore, the aridity tohyperaridity of these basins has also not been sufficiently emphasized.These conclusions for the Solway basin possibly also apply to the restof the late Permian to late Triassic basins of Western Europe andperhaps to other similar red bed successions elsewhere. In fact theentire assemblage of late Permian to mid-Triassic basins of WesternEurope may simply be sub-basins within a larger Chadian-typeintracontinental mega basin stretching from central Europe to easternNorth America (see Fig. 1). However, the effects of the peculiar globalstate of the world biosphere on early Triassic environments alsorequire much more evaluation.

Acknowledgments

All my research was funded by NSERC Canada to whom I amindebted for over 35 years of funding (ended in 2006). This study ispart of an informal collaborative study with the Geological Survey ofScotland on the Quaternary and Triassic of the Carlisle basin. Iespecially appreciate the collaboration, and field and office discus-sions, with Andrew McMillan and Maxine Akhurst and constructivecriticism of earlier versions of the manuscript by Andrew Cohen, KenGlennie, Doug Holliday, Peter Martini, Neil Meadows, Andrew Miall,Nigel Mounteney and Colin North.

My knowledge of Lake Eyre, apart from the literature, comes from afield trip in 1986 led by RodWells, Roger Callen and colleagues, andfielddiscussions with Brian Rust. For Lake Chad and other basins in theSahara, I am indebted to expeditions led (ororganized)byAndrás Zborayand Dabuka Expeditions (Egypt, Sudan), Suzanne Leroy and Pedro Costa(Mauritania), and Pointe Afrique (Algeria, Niger, Chad), and to the localinhabitants for food and accommodation (where available).

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