Difference between revisions of "Loch of Park - locality, Cainozoic of north-east Scotland"

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From: Merritt, J W, Auton, C A, Connell, E R, Hall, A M, and Peacock, J D. 2003. Cainozoic geology and landscape evolution of north-east Scotland. Memoir of the British Geological Survey, sheets 66E, 67, 76E, 77, 86E, 87W, 87E, 95, 96W, 96E and 97 (Scotland).

Loch of Park

The organic sediments preserved within the large ice-scoured rock basin at Loch of Park, north-east of Banchory (Map P915380), provide evidence of typical climate and vegetation changes that have occurred within the district since deglaciation. Pollen data and radiocarbon dates from the site indicate that a ‘tripartite’ Late-glacial sequence is preserved that passes upwards into Holocene deposits.

The Loch of Park (P915327) is the site of a freshwater lake that was drained during the 19th century and is now a local nature reserve. It occurs at the south-eastern end of the basin, which extends for about 4 km along the northern margin of Sheet 66E Banchory and across the southern margin of Sheet 76E Inverurie. The basin was formed by ice that flowed eastwards from the Grampian Highlands, scouring the underlying decomposed Crathes Granodiorite. The ice deposited a thin spread of till on the basin floor. It also laid down morainic mounds at the eastern margin of the basin during its westward retreat. These moraines blocked the subsequent drainage of the lake eastwards and an interbedded sequence of organic muds, peats and fine-grained lacustrine sediments were laid down on top of the basal till. Mounds and narrow terraced spreads of sand and gravel were also laid down around the southern margin of the lake.

The organic sediments in the basin were first described by Vasari and Vasari (1968), who analysed the pollen content from a narrow diameter piston-cored borehole. The borehole, at about 70 m above OD, near the eastern end of the former lake (P915327 a), penetrated interbedded peats, clays and organic muds to a depth of 5.6 m. The lowermost 1.70 m of the core was recorded as bluish clay, almost entirely devoid of pollen. Above this unit occurred 0.5 m of organic mud, 0.35 m of nutrient-rich silty peat (gyttja) and 0.53 m of clay. The upper clay was overlain by peat, which became increasingly sandy upwards. The three units above the basal blue clay (and their pollen assemblages) were interpreted as being of Late-glacial age and the upper peat as being Holocene.

The pollen spectrum (P915327 b), between 2.10 and 3.85 m depth was assigned to the classical Late-glacial pollen zones I (cold), II (warm), III (cold), a combined zone III–IV, coinciding with the transition from cold to warm conditions, and the Postglacial (Holocene) IV (warm). The Late-glacial sediments, which were subdivided in to three subzones, Ia–Ic, are characterised by a dominance of non-arboreal pollen and the Holocene sediments by a marked increase in the proportion of Betula (birch), Pinus (pine) and Juniperus (juniper) pollen. A corresponding decrease was recorded in the proportions of grass and herb pollen in the Holocene deposits. Subzone Ia (mainly almost barren blue clay) and subzone Ib, (the organic mud) were thought to reflect progressive climatic amelioration. This was followed by a colder episode (subzone Ic).

The sequence was subsequently reinvestigated in 1972 (Vasari, 1977), when samples of organic sediment were taken for radiocarbon dating from a second boring. The borehole was sited immediately adjacent to the first and a similar stratigraphical sequence was proved. The lower sample, HEL–417, taken at the boundary between the top of subzone Ic and the base of zone II (Empetrum {crowberry} rise/Rumex {docks} fall), yielded a radiocarbon age of 11 900 ± 260 BP (P915348). This was said by Vasari (1977) to correspond closely with the accepted radiocarbon age for the I/II boundary for much of north-western Europe. The upper sample, HEL–416 (at the combined zone III–IV boundary; between the Rumex and Empetrum maxima), yielded a radiocarbon age of 10 280 ± 220 BP. This was seen by Varasri (1977) as compatible with the generally accepted radiocarbon age of the Late-glacial/Holocene boundary in Britain (about 10 250 BP).


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