LACUSTRINE ORIGIN OF KARHEEN FM. PLATY LIMESTONE LINKS ALASKA’S ALEXANDER TERRANE TO THE CALEDONIDES AND OLD RED SANDSTONE CONTINENT IN THE LATE SILURIAN-EARLY DEVONIAN

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. In the Karheen’s type section, polymictic conglomerate, planar and trough cross-bedded sandstone, siltstone, and alternating red and green mudstone display complex facies relationships with thick sequences of thin, platy limestone. Previous research interpreted the Karheen’s platy limestone as marine in origin. We describe new evidence, including a lack of marine fossils, that indicates the platy limestone, which comprises bipartite, calcareous- and organic-rich laminae, formed in deep, permanently stratified lakes in cyclic association with siliciclastic, lake-margin facies.

Five transgressive-regressive cycles record the sudden expansion of a lake, rapid deepening, and the accumulation of finely laminated, varved sediment. In each cycle, the platy limestone exhibits abrupt transition with underlying siliciclastic facies (alluvial fan conglomerate, fluvial sandstone, and mudstone overbank deposits). The platy limestone grades upward to marly carbonate deposits that are rhythmically interbedded with paper-thin lime shale and then abruptly overlain by coarse siliciclastics. This sequence reflects progressive lake shallowing and the progradation of alluvial plain facies during regressive phases in each cycle.

Comparative analysis with coeval Lake Orcadie deposits in Scotland shows that the Karheen Formation is strikingly similar in age, thickness, lithology, paleontologic characteristics, and tectonic setting to the Old Red Sandstone. Thus, the Karheen’s platy limestone and interbedded facies, together with other published geologic data from southeastern Alaska, anchor the Alexander terrane to a specific part of the Uralian Seaway—the N. Atlantic-Caledonide region and the Old Red Sandstone continent—during the mid-Paleozoic.