Late Pliocene glaciation and landscape evolution of Vestfirðir, Northwest Iceland

Vestfirðir, Northwest Iceland, has been extensively modified by glacial activity, but the timing and landscape impact of pre-Last Glacial Maximum (LGM) glacial activity is not well known. We present new geochemical and chronological data from two tephras, Óþoli and Hjallanes, which constrain the age of high altitude ice-dammed lake sediments near the plateau surface of Skagafjall, located on the outer westernmost fjords of Vestfirðir. Our findings provide the first dated evidence of Late Pliocene glaciation on Vestfirðir. The 2.26±0.11 Ma fission-track age of the airfall rhyolitic Óþoli tephra is compatible with reversed palaeomagnetic polarity data and suggests an ice-dammed lake existed on Skagafjall at broadly the same time as a major and well-documented Late Pliocene downturn in global climate. We assess the likely impact of Late Pliocene and subsequent glacial activity on Vestfirðir landscapes and propose that: (1) geomorphological features and sedimentary deposits on or near the Skagafjall plateau surface could have survived beneath relatively low basal velocity areas and/or relatively non-erosive areas of glacial maxima ice sheets on Vestfirðir at glacial maxima, and (2) the heavily incised fjord network, the principal macroscale landscape feature of Vestfirðir, existed in the Late Pliocene.