Figure 1, above: a small sample with distinctive heft, remarkable for containing white mollusc (bivalve) shells in a granular matrix. The fossil debris, virtually all bivalve shells, occurs in a very fine-grained, chilled, granular dark brown basaltic host. One 3-mm coiled fragment may be part of a gastropod shell. So, who am I kidding? Well:
"Rock of the Month #141, posted for March 2013" ---
"Fossiliferous basalt" sounds like an impossibility:
one can reasonably expect a hot silicate lava to melt
and absorb carbonate-dominated shells the same way
butter melts on hot peas!
Can we hedge our bets, and
avoid undertaking destructive
sampling of the specimen? The extreme case would be
the shells becoming entrained in rapidly-cooling lava,
cooling before the shells could be destroyed.
Maybe! Perhaps a basalt flow was eroded on the Icelandic coast, and the
disintegration of the lava produced a coarse sand that
incorporated some shells and was then preserved, perhaps as an
interflow sediment armoured by the arrival of a younger flow, which
preserved the rock.
Another alternative is that the sample represents not lava but a
volcanic ash deposit, a layer of basic tuff interbedded with the lavas, and which
engulfed some shells along the shoreline.
A thin section view would provide additional textural evidence.
Now I want to visit Tjornes, too!
"Fossiliferous basalt" sounds like an impossibility: one can reasonably expect a hot silicate lava to melt and absorb carbonate-dominated shells the same way butter melts on hot peas! Can we hedge our bets, and avoid undertaking destructive sampling of the specimen? The extreme case would be the shells becoming entrained in rapidly-cooling lava, cooling before the shells could be destroyed. Maybe! Perhaps a basalt flow was eroded on the Icelandic coast, and the disintegration of the lava produced a coarse sand that incorporated some shells and was then preserved, perhaps as an interflow sediment armoured by the arrival of a younger flow, which preserved the rock. Another alternative is that the sample represents not lava but a volcanic ash deposit, a layer of basic tuff interbedded with the lavas, and which engulfed some shells along the shoreline. A thin section view would provide additional textural evidence. Now I want to visit Tjornes, too!The Tjornes peninsula displays as many as 12 lithological cycles along the coastal sections, each beginning with a diamictite (a poorly-sorted terrestrial sediment, here interpreted as a lithified glacial deposit) overlain by further terrestrial sediments and lava flows. Interbedded fossiliferous marine mudrocks and sandstones indicate repeated marine incursions, followed by further diamictites. Tuff layers are present, as well as volcaniclastic sedimentary layers and lava flows (Eiriksson, 1981).
The whole of Iceland is displayed in series of nine maps at 1:250,000 scale, in both topographic and geological series. The map areas of interest here are sheets 4 and 7, mid-north and northeast Iceland, respectively. The maps viewed for this note were published by the Geodetisk Institute of Copenhagen in 1966. The exact provenance of the specimen is lost to us. It is most probably from the north-facing coast of Sjavarsandur, south of the bay of Skjalfandi, on the west side of the Tjornes peninsula, south of Husavik and northwest of Myvatn (a town, located inland to the south of the peninsula).
The sediments of the Tjornes peninsula have been assigned an upper Pliocene to Pleistocene age (Einarsson et al., 1967). Marine and non-marine sediments, intercalated with basalt flows and, higher up, with tillite layers representing at least 10 glacial episodes, offer a tantalising record of the Ice Age history of the northern hemisphere, more complete than previously noted in North America and Europe. The evidence of as many as 10 glaciations suggests that the Bering Land Bridge, thought critical in the peopling of the Americas, may have been renewed as many as ten times by glacial-eustatic sea level lowerings (Einarsson et al., 1967). Detailed studies of bivalves, echinoids and brachiopods suggest periodic Arctic linkages between the Pacific and Atlantic oceans (Durham and MacNeil, 1967). The molluscs in the Tjornes Beds include bivalve genera such as Glycimeris, found also in the sandy Red Crag deposits of Essex, northwards into east Anglia (the counties of Suffolk and Norfolk), in southeast England. The late Tertiary and Quaternary paleoecology of that region of Britain has been documented in great detail since at least the 19th century, and some of the more-recent work can be found in: Greensmith et al. (1973); Hunt (1989; Head (1998); as well as a non-technical synthesis of Essex geology by Lucy (1999).
The mid-ocean-ridge volcanism, as in Hawaii, has been linked to the upwelling of a deep mantle plume (Schilling et al., 1999). The Tjornes district is seen as a fracture system, part of the larger-scale rifting of Iceland, and the wider Atlantic, bisecting the island in a broadly southwest to northeast direction (Saemundsson, 1974).
The local geology has been well-researched, being such an ever-present, active thread in the fabric of Icelandic life. The wild landscapes of Iceland and Scandinavia influenced Old Norse mythology (Bergstrom, 1989). To this day the scenery captivates and draws tourists from around the world (e.g., Winter, 1997; Hill, 2011; Kunzig et al., 2012).
An Icelandic contribution to geology, beyond the ubiquitous volcanic features and hot springs, is the sandur (plural sandar), alluvial outwash plains deposited by streams of meltwater flowing away from the front of a glacier (Bluck, 1974; Maizels, 1993; Zielinski and Van Loon, 2003). They are known elsewhere in the world, including Poland and (Church, 1972) Baffin Island in the Canadian Arctic.
Postscript: two polished thin sections were prepared from an offcut sliver of the sample, as shown in the following images.
Figures 3-4, below: two close-up views of crystals (clear, tabular plagioclase feldspar; red altered olivine, etc) in an oxidized, granular matrix. Photomicrographs in plane-polarized transmitted light, nominal magnification 50X, long-axis field of view circa 1.7 mm.
Acknowledgements: The sample, maps and memories were all proffered by Doug Astill, who explored this fascinating land in the summer of '69! The thin section, of a tricky material, was rendered in expert fashion by Anne Hammond.
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Bluck,BJ (1974) Structure and directional properties of some valley sandur deposits in southern Iceland. Sedimentology 21, 533-554.
Carmody,RW, Shanks,WC, Young,E and Rumble,D (1992) Conventional and laser oxygen isotope results on rocks and minerals of the Slaufrudalur stock, S.E.Iceland: hydrothermal and source material effects. Program and Abstracts, V.M.Goldschmidt Conference, Reston, Virginia, 134pp., 17.
Church,M (1972) Baffin Island Sandurs: a Study of Arctic Fluvial Processes. Geol.Surv.Canada Bull. 216, 208pp. plus folder of figures.
Durham,JW and MacNeil,FS (1967) Cenozoic migrations of marine invertebrates through the Bering Strait region. In `The Bering Land Bridge' (Hopkins,DM editor), Stanford University Press, 495pp., 326-349.
Einarsson,T, Hopkins,DM and Doell,RR (1967) The stratigraphy of Tjornes, northern Iceland, and the history of the Bering land bridge. In `The Bering Land Bridge' (Hopkins,DM editor), Stanford University Press, 495pp., 312-325.
Eiriksson,J (1981) Lithostratigraphy of the upper Tjornes sequence, north Iceland: the Breidavik Group. Acta Naturalia Islandica 29, 37pp., Reykjavik.
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Head,MJ (1998) Pollen and dinoflagellates from the Red Crag at Walton-on-the-Naze, Essex: evidence for a mild climatic phase during the early late Pliocene of eastern England. Geol.Mag. 135, 803-817.
Hill,A (2011) A volcanic wonderland. Globe & Mail, L6, 09 March.
Hunt,CO (1989) The palynology and correlation of the Walton Crag (Reg Crag Formation, Pliocene). J.Geol.Soc.London 146, 743-745.
Kunzig,R, Haarberg,O and Haarberg,E (2012) Iceland's resilient beauty. National Geographic 221 no.5, 104-125, May.
Lucy,G (1999) Essex Rock, a Look Beneath the Essex Landscape. Essex Rock and Mineral Society, 128pp.
Maizels,J (1993) Lithofacies variations within sandur deposits: the role of runoff regime, flow dynamics and sediment supply characteristics. Sed.Geol. 85, 299-325.
Saemundsson,K (1974) Evolution of the axial rifting zone in northern Iceland and the Tjornes fracture zone. Bull.Geol.Soc.Amer. 85, 495-504.
Schilling,J-G, Kingsley,R, Fontignie,D, Poreda,R and Xue,S (1999) Dispersion of the Jan Mayen and Iceland mantle plumes in the Arctic: a He-Pb-Nd-Sr isotope tracer study of basalts from the Kolbeinsey, Mohns, and Knipovich ridges. J.Geophys.Res. 104 no.B5, 10543-10569.
Winter,S (1997) Iceland's trial by fire. National Geographic 191 no.5, 58-71, May.
Zielinski,T and Van Loon,AJ (2003) Pleistocene sandur deposits represent braidplains, not alluvial fans. Boreas 32, 590-611.
Visit the Turnstone "Rock of the Month" Archives!
or, Visit Essex, England and see "Pliocene Glycimeris shells" !