The Old Red Sandstone outliers of Gamrie and Rhynie, Aberdeenshire

Abstract

The Gamrie and Rhynie Outliers are the major outcrops of Old Red Sandstone in north-east Scotland. The stratigraphy of these deposits is reviewed and the Crovie and Findon Groups recognised at Gamrie, whilst the Rhynie Group is proposed at Rhynie. The existing stratigraphical subdivision has been modified, expanded, and the sediments described in detail. The resulting stratigraphy allows speculation as to the relationships between the numerous isolate coastal outcrops, but does not as yet allow correlations between the Gamrie and Rhynie Outliers. A study of the sedimentary facies forms a basis of the stratigraphy, and it is concluded that the Gamrie Outlier existed as an almost enclosed intermontain basin surrounded by relatively low mountains from which a variety of alluvial fan environments debouched onto an extensive Piedmont floodplain/Playa surface. Ephemeral stream processes dominate the sedimentary record, with sheets of sediment eventually splaying out onto the floodplain surface which at times supported temporary lakes. The climate at this time was probably arid/semi-arid allowing the extensive accumulation of pedogenic carbonate in thick calcrete profiles. Sedimentation trends indicate that following the development of the basin a period of fan recession and floodplain aggradation occurred, but renewed tectonism is evident (ultimately causing the Middle Old Red Sandstone unconformity) and eventually caused re-advance of alluvial fans at a tire when the 'Orcadian Lake' to the north was rapidly expanding. In the highest deposits of the Garnrie sequence, the Orcadian Lake temporarily extends into the Garnrie Outlier. Paleocurrent and source rock studies allow detailed source area reconstruction and help to confirm the above mentioned trends and indicate that an early south east/south supply of granite-rich detritus was eventually dominated by a south west/west supply of slate material. At Rhynie the picture is clouded by poor exposure but the sedimentation trends are analogous to the early period of fan recession at Gamrie, evident from early sheetflood conglomerates being replaced by braided stream and finally floodplain sediments. The floodplain sediments at Rhynie allowed the development of an extensive, primitive land flora, now preserved in excellent detail in fossil 'peats' of the Rhynie Cherts. Paleocurrents and source area evidence strengthen the hypothesis that the_Rhynie Outier drained northwards to the Orcadian Basin, and not south to the Midland Valley as has been suggested. A probable link between the Rhynie and Gamrie Outliers is considered. The Lower Old Red Sandstone of both Outliers (originally considered 'Barren') offers a surprising collection of trace fossils including possible lungfish or annelid burrows, insect tracts and worm burrows, and helps to build up a more complete picture of the Lower Old Red Sandstone fauna and environment. The clastic sediments have been studied by a variety of techniques including reflected and transmitted light microscopy, scanning electron microscopy and x-ray diffraction; their chemistry by a combination of atomic absorption, x-ray fluorescence and electron· microprobe techniques, whilst thermal/magnetic techniques have allowed an insight into the location and genesis of iron oxides. Overall, it is concluded that the climatic conditions were conducive to the formation of primary red-beds. but the normally accepted mechanism of formation are regarded as incomplete. In the present case, instead of biotite liberating iron by the ~ accepted mechanism of in-situ chemical degradation, this study concludes that biotite has in fact concentrated iron and liberated clay sized haematite principally by the mechanical degradation of biotite in floodplain sites during transport. In-situ degradation and remobilisation of iron is considered to be minimal •