New additions to the Dalsheiði sheet-like formation in Þórisdalur, SE-Iceland

Sheet-like sequences may form in relation to subglacial volcanism. Such sequences are thought to be of two types: Mount Pinafore and Dalsheiði-type. The Mount Pinafore-type presumably forms under thin ice cover or some 150-200 m, whereas the Dalsheiði-type is thought to form under much thicker ice cover or probably ˃ 1000 m. Dalsheiði-type sequences are well known in Iceland and were first described by Walker and Blake for Dalsheiði in SE-Iceland. They assumed the pillows and breccia developed through escape upwards and sideways from the basalt into the overlying glacier or its meltwater. A considerably more widespread Dalsheiði sequence was later mapped by Bergh in the Síða-Fljótshverfi of S-Iceland.

Walker and Blake (1966) mapped the Dalsheiði sequence near the assumed eruptive source by Lambatungnajökull (at 628 m a.s.l.) over a 20 km distance down to heath Dalsheiði (at 200 m a.s.l.). New work by Dalsheiði area has revealed that the sequence is more widespread than previously mapped, extending 5 km further down slope to Þórisdalur, or to 20 m a.s.l. I, the sequence differs drastically from the upper segment in that pillows and breccia are rare but extensive sheet-like units dominate. Here, the sills and pillows are typically nonvesicular and dense. Near the edge of the sequence one sheet is seen to intrude into underlying sedimentary rock as evidenced by “fingered” units where the sill intrudes a hydrothermally altered sediment, a host that presumably had brittle to ductile properties. Most Dalsheiði sequences have a thick hyaloclastite body above a sheet of basalt that presumably intruded the ice/bedrock boundary. The lack of a hyaloclastite body by Þórisdalur and the abundant sill-like intrusive sheets there strongly suggests that the first phase in sheet-like sequence formation are intrusions into the ice sheet/bedrock boundary.

By Þórisdalur the sheets have 1-2 m thick colonade at the base, grade into some 6 m thick entablature fracture part, and at the top a 4-5 m segment with still finer fractures and white secondary minerals. Total absence of vesiculation in the lowest sequence of the Dalsheiði formation by Þórisdalur supports that the intruding magma was confined below thick ice that suppressed magma-water explosivity. The interface sills by Þórisdalur may the best such examples discovered to date and in excellent agreement with the theoretical model proposed by Wilson and Head (2002) for injection of such magma sills at the ice sheet lower base. Furthermore, the dominating sill-like character of the Dalsheiði formation by Þórisdalur and intrusion into underlying sedimentary rock, is in contrast with the Sída formation of S-Iceland where a sill-like unit typically interacts with a thick overlying hyaloclastite unit. At Þórisdalur the hylaoclastite phase is missing suggesting that that pillows, breccias and hyaloclastite form at a later stage on top of the sill, perhaps when a meltwater lense has accumulated there, and that this later stage may not have taken place in Þórisdalur.