Haco’s Ness lavas
Isolated outcrops of fine- to medium-grained basic igneous rocks associated with thin beds of tuff have been recorded in the Middle Old Red Sandstone (~390 Ma) Eday Flags on Shapinsay. At Haco’s Ness, volcanic rocks are well exposed in a narrow strip along this part of the shore and in a vertical cliff section. What we see here is very similar to what we would see on a visit to the coastal lava sequences found on Iceland today.
What follows below is based on Ted Kellock’s excellent description.
The intrusive rocks and the lavas are alkaline olivine-dolerites and basalts which contain interstitial analcime, natrolite and alkaline feldspar. Where the upper parts of the igneous sheets are seen, they exhibit the vesicular, scoriaceous or blocky surfaces characteristic of lava flows. Additional evidence that the igneous units are lavas rather than sills is (i) a depositional contact between the igneous units and the overlying sediments; and (ii) the presence of igneous fragments in the overlying sediments. Moreover, the crumbling, chloritic sediments overlying the lavas are markedly different from the blue, possibly slightly baked, flagstones underlying the flow at The Burg.
At the head of the inlet 250 m N.N.E. of The Burg, some rotted basic igneous rock is seen in a small E.W. fault. From the foreshore here southwards to within about 100 m of The Burg, an interrupted irregular gash cutting through both lava and sediments is filled by highly carbonated agglomerate containing sedimentary and igneous fragments. This is a volcanic vent which was the conduit for small, explosive eruptions.
In thin sections held in the collection of Stromness Museum, the Shapinsay lavas are of fine to fine-medium grain, and of intergranular texture. Fresh olivine was not found but euhedral and fragmented serpentine or carbonate pseudomorphs up to c. 1 mm diameter are plentiful as are plagioclase phenocrysts, largely replaced by white mica. They are typically laths c. 2 X1 mm in size but with a few euhedral plates up to 4 mm in diameter; locally, olivine and phenocryst plagioclase form clots. The ground mass is of plagioclase laths c. 0-5 mm long (up to an average of c. 0-7 mm in some examples) with subordinate brownish subhedral clinopyroxene granules c. 0-2 mm in diameter.
Ash beds and lava flows
The cavities are vesicles left from gas bubbles.
Calcite minerals infill the vesicles indicating later burial of the lava flows and geothermal cooking. The minerals suggest a burial depth of at least several hundreds of metres. That fits with the estimated thickness of ~ 700 m for the overlying Middle ORS on Shapinsay.
Kellock, E., 1969. Alkaline basic igneous rocks in the Orkneys. Scottish Journal of Geology 5, 140-153.
