Fossiliferous basalt

from the north coast of Iceland


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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:
  • The term "fossil": pretty youthful by fossil standards - the whole of Iceland and adjacent ocean floor is geologically recent, a part of the Tertiary Volcanic Province formed as the north Atlantic ocean opened. The shells are of Pliocene (5.3-2.6 million years) or more likely Quaternary (Pleistocene-Holocene, <2.6 million years) age. Nevertheless, a most impressive mode of preservation! A relatively "old" rock from Iceland is the late Miocene, silicic Slaufrudalur stock, age dated at 6.6 Ma (Carmody et al., 1992). Even the oldest rocks found on Iceland, generally on the west and east coasts furthest from the main rift zone, may be only 16 million years or so in age, erupted / deposited in early to middle Miocene time.
  • The term "basalt": the host rock to the shells is very granular, and possibly an extremely immature, poorly sorted sediment of grains derived from basalt volcanism, composed of dense, dark minerals such as pyroxenes, magnetite and ilmenite. The sample is not glassy in appearance, and does not seem to be a quenched basaltic melt formed from submarine eruption into sea water. It may represent a lithified basaltic tuff, an ash fall that engulfed the shells.
  • The sample, "PB14", was collected from the Tjornes area by Doug Astill in August 1969. 7.0x4.5x2.0 cm in size, it is less magnetic than one might expect of fresh basalt, possibly by alteration of magnetite during cooling (?). Bulk magnetic susceptibility is about 1.9x10-3 SI units. When first received it was weighed at 52.22 grams. However, tipped off that it seems to absorb water readily, it was heated for 30 minutes at about 115°C (240°F) and reweighed at 51.67 g, a loss of 1 percent. Within 24 hours it had regained almost all the loss, to 52.15 g.


"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!

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.

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Figure 2, above: a thin section, viewed in transmitted light. The lamellar structure of the mollusc (bivalve) shell fragments is prominent, within the fine granular groundmass. The maximum lateral dimensions of the rock slice (circa 0.03 mm thick) are 40x23 mm.

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.

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The two photomicrographs display the shell fragments in a fine-grained groundmass of igneous crystals, mostly 0.1 mm or less in diameter. These include olivine, clinopyroxene and plagioclase feldspar in an oxidized matrix. Some of the olivine is altered to dark brown "iddingsite". Most notably, the shells are not the only foreign ingredients: the rock also contains rounded carbonate grains and scattered grains of quartz, which is out of chemical equilibrium with the olivine of the host rock. The nature of the groundmass which cements the crystals and shells is critical to the interpretation. If it is a devitrified glass then the rock is properly a lava. However, if the scattered voids are vesicles (gas bubbles) and the matrix a very fine-grained dust or silt, then the rock may be a tuffaceous deposit.

Upon further examination, the sample contains about 80% magmatic components, including what appears to be a partially devitrified basaltic glass (palagonite) and abundant sheet silicate minerals, including serpentine replacing olivine, as well as various magmatic minerals found in basalt, such as pyroxenes, olivine, plagioclase feldspar and Fe-Ti oxides. The remainder of the rock seems to have been scoured from a beach or older beach deposit: quartz (sand grains), shells, and rounded carbonate blobs, perhaps derived from broken shells. The quartz remains more angular than the carbonate, perhaps because it is more refractory, and slower to alter in proximity to lava. So, if the identification of a quenched glass is sound, this is indeed a "fossiliferous basalt", and at the least, a "fossiliferous basaltic tuff".

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.

References

Bergstrom,J (1989) Incipient earth science in the Old Norse mythology. GFF (Geologiska Foreningens Forhandlingar) 111 part 2, 187-191.

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.

Greensmith,JT, Blezard,RG, Bristow,CR, Markham,R and Tucker,EV (1973) The Estuarine Region of Suffolk and Essex. Geologists' Association Guide 12, 41pp.

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.

Graham Wilson, 20-22 December 2012, last edited and extended 15-17 October 2013, 15 March 2014, and 25 September 2016.

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