Characteristics of a young lava-hyaloclastite sheet, Snaebylisheidi, Iceland

Extensive sheets of hyaloclastite volcaniclastic debris, coupled with and intruded by largely underlying layers of coherent basalt, are common in the Sida area of southeastern Iceland. They were initially interpreted as submarine deposits, but have recently been re-interpreted as nonmarine deposits formed in the presence of glaciers. Detailed interpretation of the units has been challenging, because their source areas are not preserved. A younger deposit of the same type forms an elongate flat-topped ridge in the Snaebylisheidi area. Its volume of ca. 35 cubic km is similar to that of the larger Sida units, its source area is preserved, and parts of the deposit remain unlithified. Our initial investigation reveals that the source area is dominated by clastic deposits. There is no evidence for a source edifice of pillow or sheet lavas, but there are extensive low-level intrusions near the base, and a plexus of smaller high-level intrusions showing evidence of high viscosities during emplacement. Isolated pillows and other fluidal juvenile clasts near the source lie within matrices of highly vesicular ash and lapilli, or of mixed vesicular and dense glassy fragments. Downstream in the unit, deposits are dominated by dense clasts, and these can in places be demonstrated to have been derived locally from the underlying to intruding basalt sheet. Larger dense clasts are commonly highly irregular, vuggy, and composite; in places many are rolled into subspherical forms enclosing matrix material comprising dense angular glass fragments. The clastic part of the unit has an upper subunit dominated by well-developed bedding in complex geometries with multiple internal truncation surfaces. Lower subunits include thick structureless to alignment-bedded layers, along with intrusion-dominated zones. Soft-sediment deformation is ubiquitous along the edges of the deposit, with many layers broken and tilted to subvertical inclinations. Taken together, these features indicate that little or no lava accumulated at the source area during eruption, but that much basalt was intruded into unconsolidated volcaniclastic deposits. Coherent basalt sheets extended downslope from the source, perhaps largely as intrusions into earlier-deposited tephra, and produced much of the downstream clastic material by local fragmentation of the advancing sheet. Thick beds reflecting high accumulation rates are intercalated with groups of thinner beds formed by multiple depositional pulses. Deformation along deposit tops and edges records pervasive slumping of the unconsolidated deposits. The characteristics overall are suggestive of an initially explosive subglacial origin, with much of the unit emplaced subglacially. More work is underway to better understand the source eruption, and the way in which the basalt sheet was emplaced and associated volcaniclastic deposits produced and deposited.