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== Upper Old Red Sandstone ==
 
== Upper Old Red Sandstone ==
The Upper Old Red Sandstone in the main part of the Orcadian Basin is characterised by coarser-grained fluvial sediments than the underlying sequence and by the development of sabkha deposits, indicative of marginal marine conditions. The only Upper Old Red Sandstone in the Grampian Highlands lies on the southern side of the Moray Firth, between Fort George in the west and Spey Bay in the east, where they are peripheral to the basinal succession [[Media:P915441.png|(P915441)]].
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The Upper Old Red Sandstone in the main part of the Orcadian Basin is characterised by coarser-grained fluvial sediments than the underlying sequence and by the development of sabkha deposits, indicative of marginal marine conditions. The only Upper Old Red Sandstone in the Grampian Highlands lies on the southern side of the Moray Firth, between Fort George in the west and Spey Bay in the east, where they are peripheral to the basinal succession (P915441).
  
 
Although the relationship with the Middle Old Red Sandstone is generally unconformable, in the southern part of the Orcadian Basin, there is evidence to indicate that the junction largely represents a facies change without any break in sequence. In the Nairn–Elgin district, the basal beds appear to be of late Givetian age and the sedimentary sequence spans the Middle–Upper Devonian boundary. To date, no early Famennian sedimentary rocks have been recognised and there is the possibility of a sequence-break between late Frasnian and late Famennian–?Tournaisian beds around Elgin (Rogers, 1987). Because of lack of exposure, the nature of the boundary on the southern side of the Moray Firth is not resolved, although it is clear that the Upper Old Red Sandstone is diachronous from west to east and that the oldest beds are restricted to the western region (P915442). The overall shallow-water and rare dry-bed conditions are reflected by relatively rapid lateral facies changes and by some overstepping of units along the margins of fault-bounded sub-basins. The Upper Old Red Sandstone is made up essentially of sandstone and the recognised subdivisions are based mainly upon six distinctive fossil-fish assemblages (Traquair, 1896; 1897; 1905; Westoll, 1951; Tarlo, 1961; Miles, 1968) that have correlatives in Baltic, Belgium, Spitzbergen and Greenland successions.
 
Although the relationship with the Middle Old Red Sandstone is generally unconformable, in the southern part of the Orcadian Basin, there is evidence to indicate that the junction largely represents a facies change without any break in sequence. In the Nairn–Elgin district, the basal beds appear to be of late Givetian age and the sedimentary sequence spans the Middle–Upper Devonian boundary. To date, no early Famennian sedimentary rocks have been recognised and there is the possibility of a sequence-break between late Frasnian and late Famennian–?Tournaisian beds around Elgin (Rogers, 1987). Because of lack of exposure, the nature of the boundary on the southern side of the Moray Firth is not resolved, although it is clear that the Upper Old Red Sandstone is diachronous from west to east and that the oldest beds are restricted to the western region (P915442). The overall shallow-water and rare dry-bed conditions are reflected by relatively rapid lateral facies changes and by some overstepping of units along the margins of fault-bounded sub-basins. The Upper Old Red Sandstone is made up essentially of sandstone and the recognised subdivisions are based mainly upon six distinctive fossil-fish assemblages (Traquair, 1896; 1897; 1905; Westoll, 1951; Tarlo, 1961; Miles, 1968) that have correlatives in Baltic, Belgium, Spitzbergen and Greenland successions.
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The oldest unit is the Nairn Sandstone Formation, which comprises an irregular basal reddish conglomerate overlain by red, grey and yellow calcareous cross-bedded and flaggy sandstones containing thin beds of conglomerate and soft or shaly limestone-bearing mudstone. Good exposures occur on the beach north-west of Nairn, in Muckle Burn, at Glenshiel and in the Findhorn area. In the Findhorn area, the sandstone is faulted against Pre-cambrian gneisses (Black and Mackenzie, 1957) and some mudstone beds have been desiccated to clay galls. High in this section, a 3 m-thick calcrete bed named the Cothall Limestone (Parnell, 1983) is succeeded by 2.6 m of red and violet marl containing fossiliferous limestone concretions veined with calcite, cherty dolomite and pyrite; there are also patches of mamillate chalcedony. The fish fauna of the Nairn Sandstone comprises two main faunules. The lower is characterised by ''Asterolepis maxima'', ''Psammolepis tesselata'' and ''P.'' ''undulata'', whilst the upper one, known as the Boghole faunule, is distinguished by ''A. alta'' and ''Eusthenopteron traquairi<nowiki>; species common to both faunules include </nowiki>''Coccosteus magnus, Polyplocodus leptognathus'' and ''Holoptychius'' ''decoratus''. The first appearance of'' Bothriolepis ''has been taken to mark the base'' ''of another unit that bears the name Whitemire Beds (P915442). The Whitemire fauna is first encountered about 7 m above the Boghole faunule (Westoll, 1951) and is transitional in type between the Boghole and the overlying Alves faunas. The diagnostic species is ''Bothriolepis taylori'' and the assemblage includes ''C.'' ex. gr. ''magnus'' and ''H. nobilissimus,'' which are common in the older faunule, and ''Psammosteus taylori, Cosmacanthus, Conchodus'' and ''H.'' ''giganteus ''present in the Alves faunule.
 
The oldest unit is the Nairn Sandstone Formation, which comprises an irregular basal reddish conglomerate overlain by red, grey and yellow calcareous cross-bedded and flaggy sandstones containing thin beds of conglomerate and soft or shaly limestone-bearing mudstone. Good exposures occur on the beach north-west of Nairn, in Muckle Burn, at Glenshiel and in the Findhorn area. In the Findhorn area, the sandstone is faulted against Pre-cambrian gneisses (Black and Mackenzie, 1957) and some mudstone beds have been desiccated to clay galls. High in this section, a 3 m-thick calcrete bed named the Cothall Limestone (Parnell, 1983) is succeeded by 2.6 m of red and violet marl containing fossiliferous limestone concretions veined with calcite, cherty dolomite and pyrite; there are also patches of mamillate chalcedony. The fish fauna of the Nairn Sandstone comprises two main faunules. The lower is characterised by ''Asterolepis maxima'', ''Psammolepis tesselata'' and ''P.'' ''undulata'', whilst the upper one, known as the Boghole faunule, is distinguished by ''A. alta'' and ''Eusthenopteron traquairi<nowiki>; species common to both faunules include </nowiki>''Coccosteus magnus, Polyplocodus leptognathus'' and ''Holoptychius'' ''decoratus''. The first appearance of'' Bothriolepis ''has been taken to mark the base'' ''of another unit that bears the name Whitemire Beds (P915442). The Whitemire fauna is first encountered about 7 m above the Boghole faunule (Westoll, 1951) and is transitional in type between the Boghole and the overlying Alves faunas. The diagnostic species is ''Bothriolepis taylori'' and the assemblage includes ''C.'' ex. gr. ''magnus'' and ''H. nobilissimus,'' which are common in the older faunule, and ''Psammosteus taylori, Cosmacanthus, Conchodus'' and ''H.'' ''giganteus ''present in the Alves faunule.
  
In the Nairn–Findhorn area, a younger fauna occurs in a sequence of grey to reddish siliceous pebbly sandstones named the Alves Beds, which, in the Elgin area, rests unconformably on metamorphic rocks near Burgie [[Media:P915442.png|(P915442)]]. The diagnostic species are ''B. alvesiensis'' and ''B. gigantea'', with ''H. nobilissimus ''extending upwards into these beds. East of the Rothes Fault, the Alves'' ''succession is represented by two lithological units. The lower Scaat Craig Beds, comprising red and yellow sandstone and fine conglomerate, contain taxa such as ''Cosmacanthus malcolmsoni, Conchodus ostreiformis'' and coccosteomorph arthrodires, which are closely linked to the Whitemire fauna below. The Alves fauna, however, is distinguished by ''P.'' cf. ''falcatus, Traquairosteus pustulatus'' and ''B. paradoxa''. The influence of an active Rothes Fault on Alves sedimentation is'' ''possibly reflected by the deposition east of the fault of a more carbonate-rich succession of pale grey and reddish brown marly sandstones, which overlies the Scaat Craig Beds. This sequence is characterised by sandy cherty calcrete beds representing palaeosols and is named the Cornstone Beds.
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In the Nairn–Findhorn area, a younger fauna occurs in a sequence of grey to reddish siliceous pebbly sandstones named the Alves Beds, which, in the Elgin area, rests unconformably on metamorphic rocks near Burgie (P915442). The diagnostic species are ''B. alvesiensis'' and ''B. gigantea'', with ''H. nobilissimus ''extending upwards into these beds. East of the Rothes Fault, the Alves'' ''succession is represented by two lithological units. The lower Scaat Craig Beds, comprising red and yellow sandstone and fine conglomerate, contain taxa such as ''Cosmacanthus malcolmsoni, Conchodus ostreiformis'' and coccosteomorph arthrodires, which are closely linked to the Whitemire fauna below. The Alves fauna, however, is distinguished by ''P.'' cf. ''falcatus, Traquairosteus pustulatus'' and ''B. paradoxa''. The influence of an active Rothes Fault on Alves sedimentation is'' ''possibly reflected by the deposition east of the fault of a more carbonate-rich succession of pale grey and reddish brown marly sandstones, which overlies the Scaat Craig Beds. This sequence is characterised by sandy cherty calcrete beds representing palaeosols and is named the Cornstone Beds.
  
 
The highest strata in the Upper Old Red Sandstone are preserved in the Elgin area and form the Rosebrae Beds (P915442). They are well exposed in Quarry Wood, west of Elgin, and comprise brownish grey, yellow and reddish sandstone with only scarce pebbles. The fish fauna includes ''Phyllolepis'' cf. ''woodwardi, B. cristata, B. laverocklochensis, Phaneropleuron ''cf.'' andersoni, Rhychodipterus elginensis ''and'' Glyptopomus elginensis''. Associated taxa include'' B. alvesiensis, Conchodus, Eusthenopteron ''and'' H. nobilissimus'', all present in the'' ''underlying units. Palynological evidence (Marshall ''in'' Rogers, 1987) suggests that the Rosebrae Beds are late Famennian in age at their base and that the highest strata may transgress the Devonian–Carboniferous boundary; a non-sequence may separate the Alves and Cornstone beds from the Rosebrae Beds.
 
The highest strata in the Upper Old Red Sandstone are preserved in the Elgin area and form the Rosebrae Beds (P915442). They are well exposed in Quarry Wood, west of Elgin, and comprise brownish grey, yellow and reddish sandstone with only scarce pebbles. The fish fauna includes ''Phyllolepis'' cf. ''woodwardi, B. cristata, B. laverocklochensis, Phaneropleuron ''cf.'' andersoni, Rhychodipterus elginensis ''and'' Glyptopomus elginensis''. Associated taxa include'' B. alvesiensis, Conchodus, Eusthenopteron ''and'' H. nobilissimus'', all present in the'' ''underlying units. Palynological evidence (Marshall ''in'' Rogers, 1987) suggests that the Rosebrae Beds are late Famennian in age at their base and that the highest strata may transgress the Devonian–Carboniferous boundary; a non-sequence may separate the Alves and Cornstone beds from the Rosebrae Beds.

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