CONSTRUCTION OF SUBGLACIAL PILLOW RIDGES: INSIGHTS FROM COMPOSITIONAL VARIATIONS IN A 3-D EXPOSURE, UNDIRHLITHAR QUARRY, SOUTHWEST ICELAND

Undirhlíðar quarry, located on the Reykjanes Peninsula in southwest Iceland, exposes the three-dimensional internal structure of an early Weischelian subglacial ridge, allowing us to examine the eruptive and intrusive events that formed the basal pillow unit of a subglacial tindar. In July 2010, we mapped and sampled the NW-SE trending walls of Undirhlíðar, which are dominated by three distinct units of pillow lava that comprise individual pillows ranging from 0.5 m to 3 m in diameter. The lower pillow unit is separated from the middle pillow unit by a stratified layer of tuff-breccia, consistent with formation during a pause between cycles of pillow effusion. The upper pillow unit is clearly fed from a dike and separated from the middle pillow unit by another lense of tuff-breccia. Thin section analyses reveal that the pillow units are generally similar in texture and mineralogy: highly vesiculated (~35% vesicles) with fine-grained plagioclase laths (<0.2 mm) set in an intersertal groundmass with only slight variations in the relative proportions of glass (2-6%) and augite (20-32%).

Three dikes and a columnar-jointed sheet are interpreted to have formed in at least two different intrusive events. Two dikes (~2.5 m and 0.25 m wide) cut the middle and lower pillow units on the western edge of the wall while the sheet, which is located on a knob ~50 m north of the central part of the wall, appears to intrude the middle pillow unit. The two dikes and the sheet are the only units that contain olivine phenocrysts (0.5-1.0 mm in diameter, <1%) and seem to be related to the same intrusion event. The third dike at the eastern edge of the wall, which cuts the primary pillows and feeds the upper pillows, has no olivine and appears to have intruded during a different event. The margin and interior of the third dike differ significantly in texture and vesicularity; the margin is coarser grained (5% microcrystalline groundmass) and less vesiculated (~11% vesicles) than the interior (33% microcrystalline groundmass and ~23% vesicles). This corresponds to the unusual pillow-like appearance of the interior of the third dike.