SILURIAN MEGALODONT BIVALVES AND SILURIAN(?)–DEVONIAN PLATY LIMESTONE: NEW MID-PALEOZOIC LINKS BETWEEN ALASKA’S ALEXANDER TERRANE, THE OLD RED SANDSTONE CONTINENT, AND N. ATLANTIC–CALEDONIDE TECTONICS

The Alexander terrane (AT) of Alaska–British Columbia originated as a Proterozoic–early Paleozoic island arc that underwent Mesozoic accretion to western North America. Silurian megalodont bivalves (Heceta Formation) and Upper Silurian(?)–Lower Devonian platy limestone (Karheen Formation) add new evidence that links the AT to the N. Atlantic Caledonides in the mid-Paleozoic.

Pycinodesma megalodont bivalves are AT endemics that predominate in Ludlow-age, shallowing-upward sequences characterized by depauperate faunas. Low fossil diversity and richness (rare Amphipora, gastropods, brachiopods, ostracodes, corals, and crinoids) are associated with abundant micritized skeletal grains, fenestrae, and thrombolitic fabric, indicating that Pycinodesma outcompeted stenohaline taxa under peritidal conditions. The Pycinodesma concentrates share closest similarities with Canadian Arctic and Ural Mountain Silurian megalodonts.

The Karheen Formation comprises 1800 m-thick, Upper Silurian(?)–Lower Devonian redbeds and platy limestone. Together with the underlying Heceta Formation, these represent a classic flysch-molasse sequence that accumulated during the Klakas orogeny. The platy limestone comprises knife-sharp bedding, cyclical deposition of light and dark couplets, and a general lack of fossils. Interbedded polymictic conglomerate (alluvial fan deposits), pebbly sandstone (alluvial plain), and associated facies indicate that the limestone formed primarily in stratified, ephemeral lakes on a vast coastal plain; rare brachiopods in green mudstone indicate periodic marine inundation.

The Karheen redbeds and platy limestone, Karheen paleomagnetic data that place the AT within 14° of the paleoequator, and Hf isotopic evidence that Devonian detrital zircons had a northern Caledonide source indicate a high degree of similarity between the AT and the Old Red Sandstone Continent (Laurussia). Together with the Pycinodesma concentrates and other Late Silurian­ similarities with Uralian Seaway–associated biotas, these new data help confirm that: 1. the AT was located along the southern margin of the Uralian Seaway near Baltica in the mid-Paleozoic, 2. the Klakas orogeny was a Caledonide event, and 3. the Karheen Formation is depositionally related to the Old Red Sandstone.