Upper Tertiary Non-Marine Environments and Climatic Changes Zone on the Reykjanes Peninsula, SW-Iceland
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Stefán Arnórsson, Gudni Axelsson, and Kristján Saemundsson Reviewed research article Water-Rock Interaction 205–208, Balkema, Rotter- In: Kristjánsson, L. ed. Geodynamics of Iceland and dam. the North Atlantic Area. NATO ASI Series, 177–188 Tómasson, J. and H. Franzson 1992. Alteration and tem- (ISBN 90 277 0505 4). perature distribution within the margin of the volcanic Weisenberger, T. 2005. Zeolite facies mineralization in the Upper Tertiary non-marine environments and climatic changes zone on the Reykjanes peninsula, SW-Iceland. Water- Hvalfjördur area, Iceland. Unpubl. Diploma thesis, Rock Interaction (eds. Y. K. Kharaka and A. S. Maest), Albert Ludwigs University, Freiburg, 142 pp. in Iceland 1467–1469. Balkema, Rotterdam. White, D. E., L. J. P. Mufer and A. H. Truesdell 1971. Vapour-dominated hydrothermal systems compared Walker, G. P. L. 1960. Zeolite zones and dike distribution 1,2 1 in relation to the structure of basalts of eastern Iceland. with hot-water systems. Econ. Geol. 66, 75–97. Friðgeir Grímsson and Leifur A. Símonarson J. Geol. 68, 515–528. World Geothermal Congress, proceedings 1995. Florence, 1Institute of Earth Sciences, University of Iceland, Askja, Sturlugata 7, 101 Reykjavík Italy, 3028 pp. 2 Walker, G. P. L. 1963. The Breiddalur central volcano, Icelandic Institute of Natural History, Hlemmur 3, 105 Reykjavík eastern Iceland. Q. J. Geol. Soc. London 119, 29–63. World Geothermal Congress, proceedings 2000. Kyushu- [email protected]; [email protected] Walker, G. P. L. 1974a. Eruptive mechanisms in Iceland. Tohoku, Japan, 4111 pp. In: Kristjánsson, L. ed. Geodynamics of Iceland and World Geothermal Congress, proceedings 2005. Antalya, the North Atlantic Area. NATO ASI Series, 189–201 Turkey. (ISBN 90 277 0505 4). Abstract — Palaeontological research on the Miocene and Pliocene oras and faunas of Iceland date back Walker, G. P. L. 1974b. The structure of eastern Iceland. to the 1770’s. At that time, plant fossils from Iceland were considered belonging to a single ora, and in the early 20th century scientist considered this ora of Eocene age. During the past few decades systematic palaeontology, palaeoclimatology, stratigraphy, and palaeogeography have been linked to results from isotopic (K/Ar and Ar/Ar) absolute age determinations and palaeomagneticmeasurements of Cainozoic lavas in Iceland. As a result, numerous fossil oras and a few faunas identied from terrestrial sediments in Iceland are now known to give a contiguous record dating back 15 Ma. The oldest oras are therefore Middle Miocene in age. Younger oras are quite different from the older ones and their composition changed as the Miocene came to an end. Gradual cooling that occurred on Iceland since 12 Ma, when the climate was mild with no dry season and warm summers, and continuous isolation of Iceland, in the northern North Atlantic, had its affect on the Icelandic ora. Thermophilous plants soon became extinct and more cold tolerant species became prominent. INTRODUCTION (1868). The global cooling that has occurred on Earth since the Middle Miocene is well documented in the The fossil record of Iceland extends back as far as its record of fossil oras in Iceland, owing to the reg- oldest rocks, which have been radiometrically dated ular spacing of plantbearing sedimentary formations to about 15 Ma (McDougall et al., 1984; Hardarson within the lava pile (Figure 2). Unfortunately, well et al., 1997). The Icelandic lava pile formed almost preserved remains of terrestrial faunal assemblages exclusively above sea level and as a result the fos- are rarely found in these formations. sil bearing strata are primarily of terrestrial origin. A Iceland is the only landmass in the northern North number of sedimentary formations containing identi- Atlantic that has a fairly continuous fossil record able plant remains occur interstratied with the lavas from the early Middle Miocene to the present. The (Figure 1). Studies of their fossil oras were initiated geographical position of Iceland, between Greenland more thana hundredyears ago and a Mioceneage was and continental Northwest Europe, and its rich fossil correctly assigned to the oldest of them (Heer 1868). record makes it an interesting place to study possi- This was later rejected in favour of an Eocene age and ble transcontinental migration of plants and animals that view persisted into the 1960’s (see Kristjánsson, between North America/Greenland and Europe/Asia 1993). Radiometric dating of the lavas, which started during the Late Cainozoic and to interpret and re- in the mid-1960’s, veried that the oras could be no construct palaeoclimate and palaeoenvironment in the older than Miocene, as originally proposed by Heer northern North Atlantic based on terrestrial proxies. 302 JÖKULL No. 58, 2008 JÖKULL No. 58, 2008 303 F. Grímsson and L. A. Símonarson Upper Tertiary non-marine environments in Iceland PALAEOENVIRONMENTS From the shoreline, vast lowlands with meander- The geology of Iceland is fairly well known ing rivers and streams, oxbow lakes, marshlands, (see Saemundsson, 1979; Steinthórsson and Thorar- oodplains and terraces, stretched towards the high- insson, 1997; Thordarson and Höskuldsson, 2002). lands. Volcanoes with an altitudinal range up to At present, extensional forces that are driven by 2000 m (stratovolcanoes) marked the landscapes, with up-owing magma along the Mid-Atlantic Ridge smaller shield volcanoes, spatter ring craters, scoria cause many of the geological features of the island cones, cinder craters, explosion craters (maars), tuff (Steinþórsson, 1981). The volcanic and tectonic envi- rings, tuff cones, and calderas. Rivers and streams ronments of present-day Iceland have been the same eroded canyonsand valleys. V-shaped valleys, gorges, for millions of years, or since the mantle plume be- ravines, gullies, and steep canyons cut into the high- came positioned under proto-Iceland, between Green- land and high-lying valleys that were surrounded by land and Scandinavia (Vink, 1984). volcanic mountains. Lakes were numerousand of var- During the Miocene and Pliocene Iceland was, as ious types, and formed, as some of the present Ice- it is now, characterized by volcanic systems with large landic lakes, by volcanic activity or extensional rift central volcanoes and surrounding ssure swarms and forces. Large lakes could have been present in rift crater rows (Walker, 1959, 1964; Hald et al., 1971). valleys, calderas, and following the closure of a lava Eruptions were frequent (Saemundsson, 1979; Mc- ow at the mouth of a valley. Other lakes could have Dougall et al., 1984, 1987) and of various types as been present in small explosive craters reaching below evident by various sorts of igneous rocks in the geo- groundwater level. logical formations of the island (Walker, 1959). These environments were full of life as evident Even though modern day Iceland is characterized from the fossil record. Vegetation thrived from by U-shaped fjord landscape this was not so during the lowland to highlands (Grímsson et al., 2007) Figure 1. Geological map of Iceland with localities mentioned in the text (modied from Jóhannesson and the Miocene and Pliocene. The fjord landscape of and freshwater animals (Friedrich, 1966; Friedrich Sæmundsson, 1989). – Jarðfræðikort af Íslandi með fundarstöðum steingervinga sem nefndir eru í texta (byggt north-western and eastern Iceland (Figure 3A), was et al., 1972; Sigurðsson, 1975) occupied lakes and á korti Hauks Jóhannessonar og Kristjáns Sæmundssonar, 1989). formedby the glaciers onlyduringthe last ice age, and streams. Insects were common in various environ- the hyaloclastite (palagonite) mountains and ridges ments (Friedrich et al., 1972; Heie and Friedrich, 1971) and small terrestrial mammals, herbivores, along the active rift zone crossing the island were also loading and burial (Roaldset, 1983). The uppermost separating two successive lava ows (Figure 3B). The were grazing in the undergrowth (Símonarson, 1990). formed only during the last ice age (Pjetursson, 1901) parts of sedimentary units have often been subjected red colour of some sedimentary units is partly be- and date back no further than 3 Ma (Geirsdóttir and to thermal metamorphism by overlying lava. All Ter- lieved to be the result of surface weathering in a warm Eiríksson, 1994). THE MIOCENE–PLIOCENE tiary sedimentary units have subsequently been sub- and moist Miocene climate (Kristjánsson, 1973). La- Sedimentary rocks in the Tertiary formations of jected to burial diagenesis and low-grade metamor- custrine sedimentary rocks are often present, usu- Iceland give critical information on various environ- SEDIMENTARY ROCKS phism to zeolite facies (Roaldset, 1983). Loose par- ally having rather limited distribution but consider- ments during the Miocene and Pliocene. Sediments Upper Tertiary sedimentary rocks in Iceland are quite ticles have therefore become compact, cemented and able thickness. The lacustrine rocks typically con- of volcanic origin, lake, river and lagoon sediments, variable. The most prominent are of volcanic origin, lithied, forming hard glassy sedimentary rock. sist of thin-bedded shales, mudstones and siltstones as well as various other types suggest diverse en- ranging from thin ash layers (very ne tuff) to thick interngered with turbidites (Figure 3D) and over- vironments and changing landscape (Walker, 1959; pyroclastic formations and ignimbrites. The grain Clastic sedimentary rocks in the Miocene– lain by coarser deltaic deposits of sandstone and con- Friedrich, 1966;