ABSTRACT: Silurian Trace Fossils in Carbonate Turbidites from the Alexander Arc of Southeastern Alaska

Constance M. Soja

Early to Late Silurian (Wenlock-Ludlow) body and trace fossils from the Heceta Formation are preserved in the oldest widespread carbonates in the Alexander terrane of southeastern Alaska. They represent the earliest shelly benthos to inhabit a diversity of marine environments and are important indicators of the early stages in benthic community development within this ancient island arc. The trace fossils are significant because they add to a small but growing body of knowledge about ichnofaunas in deep-water Paleozoic carbonates.

Proximal to medial carbonate turbidites yield a low-diversity suite of trace fossils that comprises five distinct types of biogenic structures. Bedding planes reveal simple epichnial burrows (Planolites), cross-cutting burrows (Fucusopsis), and tiny cylindrical burrows. These and other casts, including chondrites(?)-like burrow clusters, represent the feeding activities (fodinichnia) of preturbidite animals. Hypichnial burrows and rare endichnial traces reflect the activities of postturbidite animals. Broken and offset traces indicate that infaunal biota commenced burrowing before slumping and subsequent soft-sediment deformation.

The abundance and density of trace fossils increases offshore in the medial turbidites associated with a decrease in the size and amount of coarse particles and with an increase in mud and preserved organic material. Although diversity levels are similar in the proximal and medial turbidite facies, they are much lower than in Paleozoic siliciclastic turbidites. This may reflect unfavorable environmental conditions for infaunal biota or paleobiogeographic isolation of the Alexander terrane during the Silurian. A greater use of trace fossils in terrane analysis will help to resolve this issue and should provide new data for reconstructing the paleogeography of circum-Pacific terranes.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990