Middle and Late Quaternary Depositional History Reconstructed from Two Boreholes at Lågjæren and Høgjæren, SW Norway
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Middle and Late Quaternary depositional history reconstructed from two boreholes at Lågjæren and Høgjæren, SW Norway JURAJ JANOCKO, JON Y. LANDVIK, EILIV LARSEN, HANS PETTER SEJRUP & PER IVAR STEINS UNO Janocko, J., Landvik, J. Y. , Larsen, E. , Sejrup, H. P. & Steinsund, P. l.: Middle and Late Quaternary depositional history reconstructed from two boreholes at Lågjæren and Høgjæren, SW Norway. Norsk Geologisk Tidsskrift, Vol. 78, pp. 153-167. Oslo 1998. ISSN 0029-196X. Two boreholes drilled in the lowland (Lågjæren) and upland (Høgjæren) area of the Jæren region of SW Norway, revealed sedimentary successions of various ages. The Auestad borehole at Lågjæren, doser to the coast, contained only Saalian sediments. The 35-m-long sedimentary succession comprises lill (unit 1), glacioftuvial and ftuvial sediments (unit 2), glaciomarine sediments (unit 3), lill bed (unit 4) and glacioftuvial sediments (unit 5). The Elgane borehole at Høgjæren contained 30 m of Weichselian sediments. The lower part of the succession consists of coarse-grained, probably glacioftuvial sediments (unit l) and till (unit 2). · This is overlain by in situ glaciomarine silts (unit 3), deposited before about 33,000 years ago. The Elgane sedimentary succession is capped by clayey till (unit 4). The difference in stratigraphy between the drilling sites suggests intensive erosion at Lågjæren probably by a north-ftowing glacier. The two successions indicate four glacial episodes ranging in age from Saalian to Late Weichselian. The glacial phases are separated by marine intrusions. When at its highest, sea leve! was at !east 200 m above the present leve! during the Middle Weichselian Sandnes Jnterstadial. Juraj Janocko and Per Ivar Steinsund, University of Tromsø, Department of Geo/ogy, IBG, N-9000 Tromsø, Norway; Jon Y. Landvik, The University Courses on Svalbard (UNIS), PO Box 156, N-9170 Longyearbyen, Norway; Eiliv Larsen, Geological Survey of Norway, PO Box 3006, N-7002 Trondheim, Norway; Hans Petter Sejrup, University of Bergen, Department of Geology, A/legt. 41, N-5007 Bergen, Norway. Introduction diorites and phyllites are upthrusted onto Precambrian granites and granitic gneisses along a boundary approxi The long stratigraphic successions of the Quaternary mately identical with the boundary between Høg- and deposits in the Jæren area have been known since the Lågjæren in the studied area (Fig. l). turn of the century (Bjørlykke 1908; Grimnes 191 O; The presented cores from the two boreholes, at Aue Feyling-Hanssen 1964, 1966, 1971, 1974; Wangen 1968; stad and Elgane, were collected in 1994 as a part of a Østmo 1971; Andersen et al. 1981, 1987; Fugelli 1992; renewed study of the Quaternary stratigraphy in the Fugelli & Riis 1992). Jæren (Fig. l) has a lowland Jæren area (Larsen et al. 1992) in addition to a co ring at character, with a slightly undulating relief rising from the the coastal site of Grødeland (Fig. l) (Janocko et al., in coast towards the east where elevations reach about 250 press). The Auestad borehole was located in the yard of m a.s.i. A pronounced step, generally trending north the old Auestad schoolhouse (UTM 32VLL072011) south, divides the region into two morphological levels. east-southeast of Varhaug. The site is at 75 m a.s.l., and The lower, flat-lying Lågjæren (lower Jæren) extends only 100 m west of the foot of the morphological step from the coast up to ca. 80-100 m a.s.l. The upper leve1, separating Låg- and Høgjæren (Figs. l, 2). The Elgane Høgjæren (upper Jæren), extends from about 180 m borehole was located next to a barn at Skretting farm towards a mountain fringe in the east at some 240 m (UTM 32VLL101017) at an altitude of 208 m in the (Fig. l). Hø gjæren area, east of Varhaug (Figs. l, 6). A flat area An up to 125-m-thick sedimentary succession, consist comprising clayey and gravelly till prevails from the ing mainly of till and glaciomarine sediments, records borehole towards the west. The transition zone between deposition from the midd1e Pleistocene onwards (An Høg- and Lågjæren, consisting here of a step-like mor dersen et al. 1987; Janocko et al., in press). Clayey phology, Iies a bo ut l km west of the borehole locality fine-grained sediments were found at elevations up to 200 (Figs. l, 9). East of the Elgane site, the relief becomes m a.s.l., and their origin has been debated in the litera more dissected and surface sediments consist mostly of ture (Reusch 1895; Bjørlykke 1908; Grimnes 1910; Han till and glaciofluvial deposits. The drilling site is located sen 1913; Feylling-Hansen 1964; Andersen et al. 1987, only 250 m from the Høgemork borehole, described by 1991; Fugelli 1992; Fugelli & Riis 1992). The basement Andersen et al. (1981, 1987). Two sections at Høgjæren consists of Palaeozoic and Precambrian granites, phyl were described earlier: Oppstad (Feyling-Hanssen 1971; lites, mica schists and mica gneisses (Birkeland 1981; Andersen et al. 1981) and Bjorheim (Vistnes in Feyling Jorde et al. 1995). Palaeozoic and Precambrian grano- Hanssen 1964). 154 J. Janocko et al. NORSK GEOLOGISK TIDSSKRIFT 78 (1998) Norwegian Sea NORTH SEA Areas covered by Quaternary sediments .. Areas wi1h exposed bedrocl< lill Morphologlcal step be1ween L.øg- and Høgjæren l Drlllings 6 Other studied trv-·nlitio�r - Roads Fig. l. Localization of Auestad and Elgane boreholes and other sites discussed in this paper. In the close-up, the morphological step between Lågjæren and Høgjæren is shown. The distribution of Quatemary sediments and bedrock is after Andersen et al. (1987). NORSK GEOLOGISK TIDSSKRIFT 78 (1998) Quaternary depositional history, Jæren, Norway 155 The objective of the two new corings is to obtain a detailed sediment stratigraphy from both Lågjæren and !føgjæren. The drilling sites were located in the proxim tty of an existing seismic profile (Andersen et al. 1987), the old borehole at Høgemork (Fig. l) (Andersen et al. 1987) and a sediment section at Skretting (Fig. l) (Stalsberg 1995). In _this pape� we present a stratigraphic description and mterpretatton of the sedimentary successions at Lågjæren and Høgjæren including some results of bio stratigraphical and chronological work. These data were used for interpretation of depositional environments and palaeoenvironmental reconstructions. Methods � Fig. 2. Detailed localization map of the Auestad borehole. Caption as in Fig. 6. T e _bor�hol�s were drilled using a truck-mounted rotary ?nlhng ng wtth a wire-line device for sampling, described counted at 2 cm intervals (Grobe 1987). Using the thick m more detail by Janocko et al. (in press). The general _ ness of X-rayed sediment, a calculation for outsized clast run mterval during sampling was 150 cm, but in some frequency per 10 cm3 of sediment was made. The micro intervals it was reduced to 75 cm in order to improve structure of the sediment was studied in thin sections recovery. Core recovery in some units was low (see Figs. following procedures described by Merkt (1971) and 3, 7) due to loss of sorted sediments from the core Murphy (1986). Foraminifera were prepared and catcher, or because large clasts blocked the opening in analysed following procedures by Meldgaard & Knudsen the drill bit during penetration. Usually, a small amount (l �78). Amino-acid samples were prepared according to of sediment held in the core catcher was recovered Mtller et al. (1983) and analysed on an automatic ion (core-catcher sample) even if the sampling tube was exchange amino-acid analyser at the Bergen Amino Acid empty. Bouldery diamict units were penetrated without Laboratory. Two shell fragment samples from Elgane sampling using a hard metal drill bit. In intervals with no were AMS radiocarbon dated at the Laboratory for sediment recovery, sediment flushed up by the drill mud Radiological Dating in Trondheim. �as caught on a screen (drill cuttings), and drill penetra tton rate was logged. In order to gain higher recovery from the sorted sedimentary units, parallel corings were made at both Sedimentary successions sites ca. 3 m north of the first boreholes. The individual The sediments recovered at both drilling sites are mainly boreholes are named Al, A2 (Auestad) and El, E2 related to glacigenic environments. Lithofacies and litho (Elgane), respectively (see Figs. 3, 7). facies associations (Figs. 3, 7, Table l) suggest glacial, After the cores were split and a macroscopic descrip . _ glaciomarine, glaciofluvial and fluvial environments. tion carned out, about 1-cm-thick sediment slices were Glacial sediments are represented by two distinct fa transferred to plexiglass trays and X-rayed. The cores cies. Diamictic facies (Dmm) consists of matrix-sup were sampled for grain size, water content, total carbon ported gravelly and bouldery diamicton. The matrix is (TC), total organic carbon (TOC), carbonate content unsorted and consists of a clay-silt-sand mixture. Grav (CaC03), foraminifera and pollen. Undrained shear elly and bouldery clasts are usually subangular some strength was measured by fall-cone test. For outsized times with secondary fractures. Diamicton beds have an clast counting, X-radiographs were placed on a light erosive base. Glacial origin is also ascribed to massive silt table and particles larger than 2 mm in diameter were (facies Fm) with outsized clasts recovered in the upper Table l. Facies codes, modified after Eyles et al. (1983), used in the text and part of the Elgane borehole. The deposit shows defor lithostratigraphic logs in this paper. mation typical of subglacially formed sediments (see detailed description below). Dmm Diamicton, matrix-supported, massive D Diamicton, matrix and clasts-supported, massive Glaciomarine sediments are mostly represented by Gmm Grave!, matrix-supported, massive massive silt (facies Fm) and laminated silt (facies Fl) G Grave!, undistinguished containing outsized clasts interpreted as dropstones. Sm Massive sand s Sand undistinguished Both facies are thought to have been deposited by sus Sd Sand, deformed, tectonized pension fall-out.