THE INFLUENCE OF CALEDONIDE OROGENESIS ON SILURIAN–DEVONIAN EVOLUTIONARY ECOLOGY IN ALASKA’S ALEXANDER TERRANE

The Alexander terrane (AT) originated as a Neoproterozoic island arc and accreted to North America in the Mesozoic. Diagnostic Silurian-Devonian species in the AT constrain the paleogeographic setting of the AT to the N. Atlantic-Caledonide region in the mid-Paleozoic and record enormous ecological change induced by the rising Caledonides. Late-stage (Scandian) tectonism generated significant terrestrial influx, reduced habitats for normal-marine metazoans, elevated nutrient loads, and, ultimately, fueled extensive clam and microbial “blooms” in the AT.

During the Early-Late Silurian, the 3000 m-thick Heceta Limestone accumulated in the AT within 14° of the equator. Diverse, normal-marine benthic species colonized an expanding carbonate platform built on top of a volcanic edifice. Those biotas were decimated during mid-Silurian orogenic pulses, which resulted in the deposition of thick, polymictic conglomerate in nearshore sites. As carbonate sedimentation resumed in the Late Silurian, Pycinodesma megalodont bivalves—preserved with terrestrial clay infill—formed dense concentrates associated with nearshore microbial communities. These unusual deposits record the adaptations of eurytopic taxa to fluctuating conditions in shallow-marine habitats. At the platform margin, a distinctive microbial-sponge consortium—Hecetaphyton, Ludlovia, and Sphaerina (calci-microbes) in association with Aphrosalpinx (sponge) and Fistulella (problematic hydroid)—built extensive stromatolite reefs in the absence of diverse metazoans. These microbe-dominated reef communities record widespread cyanobacterial “blooms” fueled by elevated nutrients delivered via terrestrial influx. During culminating phases of the Klakas-Caledonide orogeny in the Late Silurian-Early Devonian, the shelf margin foundered and the reefs collapsed. The AT biotas experienced regional extinction during marine regression and progradation of an 1800 m-thick clastic wedge (Karheen Formation).