COMPARATIVE ANALYSIS OF MID-PALEOZOIC LAKE SYSTEMS IN SCOTLAND AND ALASKA’S ALEXANDER TERRANE

The Old Red Sandstone (ORS) accumulated in late-orogenic, rapidly subsiding, intermontane basins during Late Silurian–Devonian Caledonide tectonism. “Classic” ORS redbed sequences comprise interfingering alluvial fan, eolian, fluvial, and lake deposits. The most extensive ORS lacustrine deposits occur in Scotland, where lake and lake-margin facies extend from central Scotland (Midland Valley) north to the Shetland Islands (Orcadian Basin). The ORS laminites, studied for more than 100 years, accumulated in shallow, ephemeral lakes and exhibit “classic” lacustrine characteristics: planar, continuous bedding; very fine, even, parallel laminations; rhythmic alternation of light (carbonate) and dark (organic) couplets (average thickness=0.5-0.7 mm); and fish and terrestrial plant fossils.

The Karheen Formation of southeastern Alaska (Alexander terrane) is an 1800 m-thick redbed molasse sequence that formed during the Late Silurian-Early Devonian in the wake of the Klakas orogeny. Comparative analysis with Scotland’s Lake Orcadie deposits shows that the Karheen Formation is strikingly similar in age, thickness, lithology, paleontologic characteristics, and tectonic setting. Previous research interpreted the platy limestone in the Karheen as marine in origin. Yet similarities to the ORS sites include the lack of marine fossils, finely laminated and rhythmically deposited couplets, and abrupt transitions with siliciclastic facies; these confirm that the Karheen laminites formed in a lake. Rapid subsidence, basin fill, and impoundment of fluvial drainage led to the onset of lacustrine sedimentation in sites that experienced similar post-orogenic conditions as the ORS deposits. The Scottish and Alaskan laminites co-occur in transgressive-regressive cycles that reflect the influence of climate and ongoing tectonism on the expansion and contraction of subtropical-tropical lakes that formed close to, but on opposite sides of, the equator.