Hettangian marine invertebrates from the Kenai Peninsula near Seldovia, Alaska

Abstract

A sequence of Late Triassic to Early Jurassic marine sedimentary and volcanic deposits compose the sea cliffs west and south of Seldovia on the southwestern portion of the Kenai Peninsula in Alaska. Informally known as the Pogibshi formation and assigned to the island-arc Peninsular terrane, the middle July member of the Pogibshi is an understudied fossiliferous unit perhaps encompassing the Triassic-Jurassic boundary (TJB). A paleontological and geochronological search for the TJB has resulted in the establishment of the earliest Jurassic (Hettangian) pectinid bivalve Weyla (Lywea), solitary stylophyllid scleractinian corals, and gastropods including Pleurotomaria. These fossils are significant because they may be the oldest known Jurassic occurrences in North America. To confirm the age of the fossils, detrital zircons were extracted from two fossiliferous sandstones from an upper potentially Sinemurian location and lower potentially Hettangian location. An analysis of the uranium-lead dates resulted in maximum depositional ages of 198.9 ± 0.62 ± 2.2 and 200.5 ± 2.5 ± 1.8 Ma, respectively. These dates were compared to ages derived from established ammonite zones. The combination of these geochronologic and biochronologic constraints establishes a middle Hettangian age for the fossils occurring at the base of the July member.