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Jökull - 01.12.1984, Qupperneq 123

Jökull - 01.12.1984, Qupperneq 123
mer sea levels. He furthermore presented a theory for the deglaciation of the Borgarfjördur region and a scheme for sea level changes. Briefly, Bárdarson’s observations were the following (Figs. 2 and 3): He recognized two clay units in the region, which sometimes were separated by gravel. Bárd- arson described the lower clay unit as a “lamin- ated, distorted clay”, and pointed out that in some sections the surface of the unit is exten- sively eroded. He recognized this stratum only in the area south of the Hafnarfjall-Skardsheidi massif, and found difficulties in correlating between the various sections where it is exposed. On the basis of the fossil fauna composition of the stratum he inferred that the sea temperature was similar to the present temperature off the west coast of Greenland, at least during the deposition of the lowest part of the stratum. He concluded that this stratum was deposited during a period when the glaciers were retreating and the sea transgressing towards the marine limit at 80—100 m above present sea level. Bárdarson postulated that this transgression was interrupted for some time at 50-60 m elevation, but admit- ted that there was no lithostratigraphical evi- dence for such an intermission. Molluscs col- lected from the upper part of the distorted clay show evidence of a low arctic environment during its deposition. Bárdarson explained the lack of distorted clay in the northern part of the region by stating that the lowlands there must have been ice covered at the time. Later, during the period of highest sea level, the glaciers retreated into the tributary valleys of Borgarfjördur, where they deposited terminal moraines before finally melting. Subse- quent to the retreat of the glaciers there followed a period of falling sea level, at least down to the 40-50 m level. Bárdarson obviously found it difficult to explain the presence of gravel in the middle of a marine sequence, and he stated that “it is important to answer the question whether one or two transgressions are registered in the older marine strata of Borgarfjördur” (Bárdarson 1923, p. 65). He further suggested that the ero- sion of the distorted clay unit in the Melabakkar cliffs was the work of bottom currents during a period of relatively low sea level. In a later work (Bárdarson 1927) he seemed to doubt that there had been two transgressions in Late Weichselian times in the Borgarfjördur region. Bárdarson (1923) concluded that the sea level remained at the 40-50 m level for a considerable period and postulated that the upper clay unit was deposited in that time. He described this unit as a “horizontally laminated clay stratum”. He observed this unit throughout the area north of Hafnarfjall-Skardsheidi, where the stratum often constituted the major part of the sequence. Bárd- arson correlated it with similar clays he observed in the Melabakkar and Ásbakkar cliffs and in coastal sections south of Mount Akrafjall. In this stratum Bárdarson found indications of an increasing sea temperature during the deposition of the sediments as the fossil fauna assemblages change towards the top of the sequence from low arctic to boreal. Finally, on the basis of sub- merged bogs near Akranes and Borgarnes, Bárd- arson suggested a regression of the sea down to a level of 4—5 m below the present sea level. In the uppermost part of the Borgarfjördur sequence Bárdarson mapped various units of sand and gravel, but did not discuss their depositional environments. According to Bárdarson’s results, the Late Weichselian in the Borgarfjördur region was characterized by retreating glaciers, except for a late intermission during which the glaciers were extended for some period as far as the entrances to the tributary valleys. Terminal moraines piled up during this period were later levelled by the sea and formed into terraces. Bárdarson did not describe the morainic features in any detail. It is interesting to note, that although Bárdarson rec- ognized the nearby presence of glaciers during the time of deposition of the lower part of the Borgarfjördur sequence, he did not observe tills or other glacial deposits on the lowlands. He postulated the presence of glaciers partly on the basis of negative evidence, i.e. the lack of certain sediments, and partly on faunal evidence. Continued study of the Borgarfjördur sediments Bárdarson’s study of the Borgarfjördur sedi- ments remained unchallenged for nearly half a century, and has been referred to as classical by Ashwell (1975). Kjartansson (1960, 1968) out- lined the distribution of sediments in the Borgar- fjördur region. He recognized outcrops of marine and alluvial sediments, and considered the sur- face sediments of Melasveit to be of glacial ori- gin. Later Sœmundsson and Einarsson (1980) JÖKULL 34. ÁR 121
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