The mode of emplacement of Neogene flood basalts in Eastern Iceland: Facies architecture and structure of the Hólmar and Grjótá olivine basalt groups

Abstract

Hólmar and Grjótá are two stratigraphically distinct transitional alkaline olivine basalt lava groups within the westward-dipping Neogene flood basalts of eastern Iceland. The Hólmar olivine basalt group, separated from the overlying Grjótá olivine basalt group by only a few tholeiite flows, can be traced over 80km north–south, with thicknesses varying from ~250m where thickest to ~30m where thinnest. The Grjótá group can be traced over 50km also north–south, reaching thicknesses of ~250m and thinning down-dip to ~10m. In contrast to other groups in eastern Iceland that thicken down-dip, the studied olivine basalt groups thicken up-dip. The groups filled topographic confinements and formed aprons around central volcanoes. We have estimated the minimum volumes to be ~119km3 for Hólmar and ~86km3 for Grjótá. Scoria cones are found in the Hólmar group, and two thick olivine dolerite sills cross-cut the Hólmar group and probably belong to the plumbing system that fed the Grjótá group. The architecture of the lava groups are near identical. The architecture is compound, with lobes stacked horizontally and vertically, varying from 1–15m thick and 2–200m long, but do also encompass a number of thicker (15–20m) and more extensive (>1km long) lava lobe in the stacks. Filled lava tubes are commonly observed within the lava flows. The constituent lobes of the flows are often directly emplaced or welded together, suggesting rapid buildup, but are also found interbedded with redbeds and thicker tuff deposits, and occasionally preserve tree molds. The internal structure follows the characteristics for lava lobe morphology in general, with an upper vesicular crust forming half to one third of the total thickness, a massive core with abundant vesicle cylinders, and a thin basal vesicular crust. Flow tops are of the pahoehoe type, seldom found with scoria or clinker. Inflation structures such as tumuli and inflation clefts were identified in the flows. The eruptions were dominantly fissure-fed, with a few vents evolving to short-lived point source volcanism. The pre-existing topography exerted control on the advancement of the flows and influenced the final architecture of the groups at each location. It is evident that the large and dense spatial distribution of these groups, including numerous thick lava units, suggests large volcanic episodes. Together, the groups imply a period with higher production of olivine basalts in Iceland and form well-defined stratigraphic markers. The transitional alkaline olivine basalt groups are interpreted to delineate a fossil southward propagating rift flank volcanic zone located east of a now extinct and buried mature rift zone.