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On the Bedrock Geology of the Tarfala Valley: Preliminary Results of 2003 and 2004 Fieldwork

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On the Bedrock Geology of the Tarfala Valley: Preliminary Results of 2003 and 2004 Fieldwork Tarfala Research Station Annual Report 2003/2004 On the Bedrock Geology of the Tarfala Valley: Preliminary Results of 2003 and 2004 Fieldwork Graham B. Baird Department of Geology and Geophysics, University of Minnesota, 310 Pillsbury Drive SE, Minneapolis, Minnesota 55455-0219, USA [email protected] Abstract. The bedrock of the Tarfala Valley has been mapped as part of geologic investigations into the tectonics of the area and the significance of meso-scale ductile shear zones in the rocks found there. Two summers of fieldwork showed that eight major units could be distinguished and that each unit is probably an individual thrust slice. Most units possess good foliations and lineations, indicating significant deformation. Throughout the valley, fabrics and lithologies are quite planar (though local folding does occur) with SW strikes and gentle to moderate dips to the NW, typically with a down plunge lineation. This work is ongoing and modifications to the following in future publications should be expected as data analysis continues. Introduction and Upper Allochthon, are present (Figure 1; Figure 2; Table 1) based on the descriptions in e.g. Stephens During August of both 2003 and 2004, field investiga- et al. (1985), Silvennoinen (1987), and Page (1993). tions were conducted within the Tarfala Valley to map Bedrock Geology the bedrock lithologies and the associated geologic structure of the valley. Motivating this mapping was: The structurally lower most rocks are Precambrian 1) Though bedrock geological maps of the Tarfala Granitoids of the Baltic shield. Apart from evidence Valley have been published (e.g. Bhattacharyya, 2000; of rare brittle Caledonian deformation, these display Andréasson and Gee, 1989), the detail is limited; 2) Ex- excellent primary igneous textures. The most typical tensive glacio-logical and geomorphological research unit is a pink syenitic pegmatite. The upper surface of is conducted and published on the Tarfala Valley each the granitoids is a major décollement marked by black year, so an accurate account of the type, distribution, “cherty” ultramylonite, that appears to locally contain and structure of the bedrock units is important as the pseudotachylite. The ultramylonite is never more than valley’s geomorphology and glacial geology is influ- a few tens of meters thick but it is also found as frac- enced by the bedrock geology; 3) Distinctive meso- ture fill immediately below the décollement. Along scale ductile shear zones are found within the valley this décollement, at two localities (extreme southern (see Bhattacharyya and Hudle-ston, 2001; Srivastava portion of the valley), a pure quartz quartzite/quartz et al., 1995); these warrant further study and need to be pebble conglomerate can be found associated with the understood in the regional tectonic context. For these ultramylonite. reasons it is important that the bedrock geology of the A black shale unit is present on the southeastern Tarfala Valley be understood. wall of the Tarfala Valley (SH, Figure 1). This is weakly deformed but clearly rests in tectonic contact with the Tectonic Framework underlying granitoid. Associated with and above the The Scandinavian Caledonides are often described as black shale is a gray slate unit that that forms a large having five major tectonostratigraphic units: 1) The fold approximately 500 m. east of Rännan (SL, Fig- Autochthon-Parautochthon; 2) the Lower Allochthon; ure1; Figure 3). The shale, slate, and quartzite/quartz 3) the Middle Allochthon; 4) the Upper Allochthon; pebble conglomerate probably represent the remains and 5) the Uppermost Allochthon. Descriptions of of a sedimentary sequence marginal to Baltica prior to these units are summarized well in e.g. Stephens et al. Caledonian deformation. (1985) and Page (1993). Within the Tarfala Valley eight The first continuous thrust sheet found in the valley major lithologic units, which are thought represent is the Jåkk Mylonite Gneiss (JMG, Figure 1), named for the Autochthon-Parautochthon, Middle Allochthon, its excellent exposure along the “Jåkk” (Tarfalajåkk) 04B1–1 Tarfala Research Station Part B – Research Meters 0 1000 Kaskasatjå Symbols contour interval 100 meters Sydöstra hkagla Contacts; closely located, ciären inferred 27 1500 Foliation and lineation measurement, KDC foliation dip given N Tarfalajaure 31 35 KA 16 20 Kebnepakteglaciären 27 SMG40 35 QPS 21 18 Isfallsglaciären 30 28 Tarfalastationen TA 16 25 29 25 1500 10 KA 32 20 0 200 22 29 Nordtoppen 19 QPS 11 Kebnekaise Storglaciären 19 10 26 13 11 27 33 21 27 27 21 14 Rännan SL 69 21 19 4 KDC 74 30 SMG 7 SH 24 TA Ta 14 22 rfalajåk Units 23 5 k KDC - Kebne Dyke Complex KA - Kebne Amphibolite JMG SMG - Storglaciären Mylonite Gneiss 9 TA - Tarfala Amphibolite 5 10 QPS - Quartzo-feldspathic gneiss and pelitic schist PC 1000 1000 JMG - Jåkk Mylonite Gneiss 29 17 SL - Slate SH - Shale PC - Precambrian Granitoids Figure 1. Bedrock geology of the Tarfala Valley. See text for unit descriptions. Basemap, Holmlund and Schytt (1987). upstream from Rännan. The Jåkk Mylonite Gneiss is a unit and likely represents the undeformed protolith for quartz and feldspar rich ultramylonite to augen mylo- most of the Jåkk Mylonite Gneiss. A quartzo-feldspatic nite with strong and pervasive foliation and lineation gneiss and pelitic schist (QPS, Figure 1) occurs locally defined by flattened and stretched mineral aggregates. in the center of the valley between the Jåkk Mylonite Large pods of nearly undeformed medium grained Gneiss and the higher units. Most lithologies within syenite (tens of meters thick and extending possibly the quartzo-feldspatic gneiss and pelitic schist are hundreds of meters laterally) can be found within this mica rich and may be metamorphosed and deformed 04B1–2 Tarfala Research Station Annual Report 2003/2004 sediments of similar origin to the slate, shale and quartzite/quartz pebble conglomerate described above. On the ridge east of the valley there is a small area of outcrops with lithologies largely consistent with the Kebne quartzo-feldspatic gneiss and pelitic schist within the Dyke valley and is though to be the same unit. Due to lack Complex of exposure along portions of the eastern ridge, some unit contacts may actually be different than currently mapped. Shear Zones Folding The Tarfala Amphibolite (TA, Figure 1) and Kebne (Upper Allochthon Amphibolite (KA, Figure 1) outcrop over most of the Seve Nappe valley and are only distinguished from each other by Kebne their tectonostratigraphic position (Figure 2). Both are Amphibolite a medium grained hornblende rich amphibolite with lesser to equal portions of plagioclase. Garnet can be seen in most outcrops but is locally absent. Gneissic Storglaciären ) layering is common and is defined by changes in the Mylonite hornblende to plagioclase proportions. Foliation and Gneiss lineation are moderate to weak and are defined by the alignment of hornblende crystals. Complex outcrop- Quartzo- scale folding of the gneissic layering can be found most Tarfala feldspathic Amphibolite commonly in the uppermost portions of the Kebne gneiss and Amphibolite. pelitic schist Separating the two amphibolite units (Tarfala Amphibolite and Kebne Amphibolite) is the Storgla- Locally slate, Allochthon ciären Mylonite Gneiss (SMG, Figure 1). Typically the Middle shale, and Jåkk Ultra- Storglaciären Mylonite Gneiss is a quartz-feldspar rich quarztite Mylonite mylonite gneiss with lesser amounts of garnet and mica. At most Gneiss localities it is augeniferous and almost always pos- Parautochthon sesses strong foliation and lineation defined by aligned Autochthon mica, feldspar augen, and stretched mineral aggregates. Precambrian The Storglaciären Mylonite Gneiss probably represents Granitoids highly deformed meta-sediments. The Kebne Dyke Complex (KDC, Figure 1), structurally the highest unit Figure 2. Schematic tectonostratigraphy of the Tarfala Valley with in the valley, is starkly different from the lower in the context of the regional tectonostratigraphy framework, units of the valley as it largely lacks deformation. see Stephens et al. (1985). Original igneous textures are common (see Andréas- son and Gee, 1989) including the chilled margins of similar to what is characteristic of the two amphibolite the individual dolerite dyke sheets that comprise the units lower in the valley, is observed. unit. Though lacking deformation, there is significant In the Tarfala Valley the Seve Nappe, one of evidence, at the microscopic scale, of metamorphism a number of thrust sheets that make up the Upper and recrystallization. The contact between the Kebne Allochthon, is made up of the two amphibolite units, Amphibolite and the Kebne Dyke Complex is a décol- the quartzo-feldspathic gneiss and pelitic schist, the lement, indicated by the high degree of deformation Storglaciären Mylonite Gneiss, and the Kebne Dyke manifested in meso-scale ductile shear zones and the Complex . The Jåkk Mylonite Gneiss and slate(?) com- complex folding of the Kebne Amphibolite. prise the Middle Allochthon. The shale and quartzite/ Both the Tarfala Amphibolite and Kebne Amphi- quartz pebble conglomerate are part of the Autohthon- bolite are likely to be deformed and metamorphosed Parautohthon, while the Precambrian Granitoids are equivalents of the dolerite dykes (KDC), and their of course part of the Autochthon (Figure 2; Table 1). tectonostratigraphy may well be the result of thrust Structural Geology duplication. On the north side of Storglaciären, near the equilibrium line, a transition
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