PALEOBIOGEOGRAPHY OF LATE PLIENSBACHIAN (EARLY JURASSIC) AMMONITES FROM THE PENINSULAR TERRANE OF SOUTHERN ALASKA

Early Jurassic ammonites from the Talkeetna Formation (southern Alaska) provide zone level age constraint for two sections in the Talkeetna Mountains, and enable an approximate paleogeographic position for the Peninsular terrane during the Pliensbachian. Genera Amaltheus de Montfort and Fanninoceras McLearn suggest siliciclastic sedimentation took place in the northeastern paleo-Pacific Ocean during the Late Pliensbachian (Kunae Zone). The Talkeetna Formation is a well-known magmatic island arc sequence with interbeded basalt and shallow water siliciclastics, extending throughout much of southern Alaska from the Talkeetna Mountains to as far southwest as Puale Bay (Alaska Peninsula).

Previous studies have shown Early Jurassic ammonites to be particularly useful for biochronology and paleobiogeographic analysis. This is because rapid evolution and steadily increasing endemism among species peaked in the Late Pliensbachian. Amaltheus is a common boreal genus that was restricted globally to northern paleolatitudes, while Fanninoceras was limited to the eastern paleo-Pacific Ocean (only being found throughout the Cordilleran terranes from southern Alaska to South America).

During the Late Pliensbachian, the Peninsular terrane was an extremely dynamic island arc located in the northeastern paleo-Pacific Ocean. A diverse suite of organisms inhabited its surrounding shallow waters. Organisms such as: ammonites, a multitude of bivalves, brachiopods, gastropods, corals, fish (scales), fossil burrows, as well as Cycads and other plant material have been collected. Acquiring an Early Jurassic paleogeographic position for this terrane is important in order to test theories surrounding an amalgamated Peninsular/Wrangellia/Alexander superterrane (known as the Wrangellia composite terrane) in the Mesozoic.