"Rock of the Month #141, posted for March 2013" ---

"Fossiliferous basalt" sounds like an impossibility: one can reasonably expect a hot silicate lava to decompose 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. The basalt component may have arrived not as lava but as volcanic ash, 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 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.

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.