Abstract

In Glacier Bay, Alaska, Silurian limestones record the development and demise of a stromatolite reef complex in the Alexander terrane. These microbial deposits are of regional and paleontological significance because they contain paleogeographically distinctive biotas and yield important insights into Phanerozoic stromatolites that inhabited normal-marine subtidal environments. Willoughby limestones exposed on Drake Island reveal that stromatolite growth at the platform margin influenced platform dynamics with the protection of peritidal and lagoonal habitats behind a reef-fringed rim, which experienced early lithification by the precipitation of synsedimentary marine cements. Relatively low-energy subtidal conditions in a restricted, shallow-marine lagoon are implied by the peloidal and mollusk-rich wackestones, packstones, and grainstones. At the platform margin, stromatolite boundstones and cementstones capped a reef-like mound by forming a thick microbial-cement crust on a core of outer lagoonal sediments. Well laminated, fenestral mudstones and peloidal wackestones-grainstones have an abrupt lateral contact with the microbial boundstones, indicating their formation in peritidal flats adjacent to the stromatolite reefs. Foreslope breccias consist of clasts derived from shallow subtidal environments on the shelf and at its rim. Cyclic repetition of these deposits indicate that sea level fluctuations and tectonic instability in the Alexander terrane contributed to periodic shedding of shallow-marine detritus to the upper slope, followed by the rebuilding of stromatolite reefs at the platform margin. Culminating stages in the Klakas orogeny induced large-scale, catastrophic collapse of the carbonate platform. Silurian stromatolites characterized by similar habitats and microbial-sponge biotas in southwestern Alaska and the Ural Mountains, Russia, together with paleomagnetic data, detrital zircon evidence, and the timing of late-stage Caledonide events, firmly establish genetic and temporal links of the Alexander terrane with other areas along the Uralian Seaway.

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