REGIONAL UPPER TRIASSIC-CRETACEOUS STRATIGRAPHY ALONG THE NORTHERN MARGIN OF WRANGELLIA, SOUTHERN ALASKA: IMPLICATIONS FOR A SHARED HISTORY OF BASIN DEVELOPMENT IN THE ALASKA RANGE SUTURE ZONE

A conformable three-part stratigraphy has been documented for Mesozoic strata in the northwestern Talkeetna Mountains based on new mapping and measured stratigraphic sections. Strata consist of:

(1) Upper Triassic-Lower Jurassic volcanic and marine sedimentary strata informally referred to as the Susitna sequence,

(2) Upper Jurassic-Lower Cretaceous marine strata of the Kahiltna assemblage, and

(3) Cretaceous to younger nonmarine strata informally referred to as the Fossil Leaf formation.

The lower part of this succession is Late Triassic (Norian) based on occurrences of bivalves and hydrozoans in the Susitna sequence. Bivalve occurrence together with new U-Pb detrital zircon ages reveal a Kimmeridgian-Albian age for the Kahiltna assemblage. The Fossil Leaf formation is thought to be Cenomanian to younger based on palynomorphs and fossil leaves. We interpret three distinct stages of basin development based on these data.

STAGE 1 - Initial deposition during Late Triassic time of arc/arc-margin deposits that make up the Susitna sequence. The top of this unit is capped by a thin condensed stratigraphic section of chert-pebble conglomerate (<50 m thick) and is interpreted to represent a period of little to no deposition (>25 m.y. disconformity) during Early Jurassic time.

STAGE 2 - Late Jurassic-Early Cretaceous regional development of submarine fan systems of the Kahiltna assemblage. Deposits consist of a thick succession (up to 3000 m thick) of clastic material.

STAGE 3 - Nonmarine deposition of the Fossil Leaf formation beginning by the end of the Early Cretaceous.

Lithologically similar stratigraphic successions have been described throughout southwestern Alaska with the most notable similarities including the Chilikadrotna Greenstone sequence and Koksetna River sequence in the Lake Clark region, and Tatina River volcanic sequence and Kahiltna assemblage in the McGrath Quadrangle. Tentative correlation with these strata suggests that these areas may share a Mesozoic history of basin development during the collision of the Wrangellia island arc.