VOLCANICLASTIC STRATIGRAPHY OF NORTHERN VIDIDALSFJALL: AN ABANDONED ICELANDIC RIFT

In the context of a Keck Undergraduate Consortium research project in Iceland focused on the abandoned Snæfellsnes rift, we studied a 530 m section of volcaniclastic rocks exposed in the rift axis in northern Vididalsdjall. The section is bound on the east by a major west-dipping normal fault and on the west by a series of intrusions. The section consists primarily of volcanicalstic rocks intercalated with basaltic lava flows, and cut by dikes. The base of the section is not exposed. The lower 270 m of the section is dominated by very coarse matrix-supported volcaniclastic rocks having a maximum clast size of 1.5 m. These are interpreted as lahars, and there is at least one thin lahar with a silicic matrix. This lower unit is altered and pervasively mineralized with ubiquitous pyrite. The next 80 m of the section consists of finer, generally clast-supported, volcaniclastic rocks with several lava flows. The clasts in this portion of the section are generally 2-8 cm in diameter and of variable composition, dominated by basalts but also including rhyolites and banded rhyolitic pumice. The upper 184 m of the section consists primarily of fine grained basaltic ash with subordinate epiclastic volcanic sandstone. Lavas in the section and dikes cutting the section are uniform in compostion, being evolved (MgO 4.5-5.0 wt.%) tholeiitic basalts. In the context of the complete group data set, these lavas can be interpreted to have evolved from primitive magmas by fractional crystallization of olivine, joined by clinopyroxene at 5.5 wt.% MgO, and plagioclase beginning at about 3 wt.% MgO. This stratigraphy, and its structural setting, is consistent with the development and filling of a well defined rift-basin in the waning phases of the Snæfellsnes rift immediately prior to rift abandonment at about 7 Ma.