THE KARHEEN FORMATION IN SE ALASKA: DISMEMBERED “OLD RED SANDSTONE” FACIES LINKS THE ALEXANDER TERRANE TO NW BALTICA–LAURENTIA CALEDONIDES IN THE MID-PALEOZOIC

The Old Red Sandstone (ORS) was first identified in the early nineteenth century as a km-thick sequence of Devonian redbeds in the UK. ORS deposits (conglomerate, sandstone, siltstone, shale, and minor limestone) formed in the Late Silurian–Early Devonian in a variety of terrestrial–shallow marine environments as erosional debris distributed along the Caledonide front. Where it occurs in ne U.S. and Canada, Greenland, Svalbard, Norway, southern England, Wales, and Scotland, the ORS serves as a diagnostic mega-marker bed. Distinctive and remarkably widespread, it denotes Laurussia’s consolidation via suturing of Baltica and Laurentia during Caledonide orogenesis along the southern margins of the Uralian Seaway.

ORS deposits have not been identified previously in western North America; cratonic portions of Alaska and western Canada were too distant from the Caledonides to receive ORS sediment. Yet the Upper Silurian–Lower Devonian Karheen Formation in the Alexander arc terrane (AT) of se Alaska comprises 1800 m-thick terrigenous redbeds and nearshore facies, which conformably overlie shallowing-upward, argillaceous marine limestone (Heceta Formation). Together these represent a classic flysch-molasse sequence that accumulated during the Late Silurian–Early Devonian Klakas orogeny. New investigations of the Karheen Formation and the ORS (Pembrokeshire, UK) reveal similar conglomerate, cross-bedded and pebbly sandstone, shale, and platy limestone that formed as co-eval fanglomerate and fluvial sediment in coastal alluvial fans and intermontane late-orogenic, rapidly subsiding basins. These data—together with Karheen detrital zircons that best match those from western Baltica (northern Caledonides), Late Silurian­–Early Devonian fossil alliances with Uralian Seaway-associated biotas, and paleomagnetic evidence that places the AT within 14° of the paleoequator in the Early Devonian—prompt three conclusions: 1. the AT was located along the southern margin of the Uralian Seaway near nw Baltica in the Late Silurian–Early Devonian, 2. the Klakas event in the AT was related to Caledonide orogenesis, and 3. the Karheen Formation is a manifestation of the ORS, which was dismembered from the Laurussian margin and accreted as part of the AT to northwestern Laurentia millions of years later.