STRATIGRAPHY AND GEOLOGIC HISTORY OF VATNSDALSVATN, ICELAND

Combined data analyses of sub-bottom profiles (SBP), bathymetric maps, and sediment cores provide evidence for past geologic events that might be missed by single-method approaches. The utility of this method is demonstrated for an Icelandic lake, Vatnsdalsvatn, as part of a larger study on climate. Presented here are four litho-facies and corresponding geophysical facies: 1) laminated and non-laminated mud sediments; 2) mud sediments containing dropstones; 3) ice-contact deposits; and 4) basaltic bedrock.

The distribution of laminated muds is not uniform throughout the lake and may be controlled by stream-delta deposits. Highly erosional soils and steep valley walls are also possible contributors to these processes. Lack of deposition in the deeper parts of the lake may be due to high energy resulting from inlet/outlet flow. Several dropstones, indicative of periods of glacial rafting, were extracted from cores and interpreted as isolated and grouped hyperbolic reflections in the SBP data. Till deposits dominate the stratigraphy of the lake in the form of both ground and end-moraines. Three end-moraines, identified using SBP and bathymetric data as well as topographic maps and aerial photographs, are evidence of a glacial retreat that probably occurred in steps. The country rock is composed of Tertiary basalt that crops out on the valley walls but was not observed in cores or SBP data.

These data allowed for the division of Vatnsdalsvatn’s evolution into six stages. The most recent stage is represented by laminated and non-laminated sediments likely controlled by deposition of upland eroded sediments, stream-delta deposition, lake energy states, and recent lacustrine deposition. Three earlier stages encompass separate periods of ice-contact deposition, glacial retreat and stagnation indicated by a basin-wide till sheet, three identifiable end-moraines, and mud sediments with dropstones. An older stage involves the carving of the current lake-depression by multiple Quaternary glaciations. The earliest stage describes the genesis of the Tertiary basalt bedrock.