Stratigraphy and Paleoenvironment of Lower Ordovician York Mountain Carbonates, Seward Peninsula, Alaska

D. Jeannie Vandervoort

This Seward Peninsula field study is the first one in the area focused specifically on the nonmetamorphic carbonate rock stratigraphy and paleoenvironment. Field work was conducted on a 26 km stretch of the southwestern coast and in the adjacent mountains.

Carbonates previously reported to be Precambrian are actually in the Lower Ordovician system. Stratigraphy is more structurally complex than previously mapped.

Faunal information, paleogeographic reconstructions, and lithofacies indicate that cyclic shallow subtidal to supratidal deposition took place on a pericontinental shelf in warm marine conditions which persisted at low northern latitudes. Six lithofacies were created. Quiescent offshore deposition yielded the calcareous shale facies (unit 6). Landward, shallowing enabled storm wave base to contact intermittently the sea floor creating grainy parallel planar-laminated or ripple-laminated beds between shaly beds (unit 5, interbedded argillaceous wackestone and lime mudstone facies). Touchdown of normal wave base farther landward caused large- and small-scale ripples, parallel planar laminations, channel scours, flat pebble conglomerates, and gently dipping cross-laminations in the shall w subtidal to intertidal offshore shoaling zone (packstone and grainstone facies, unit 4). Columnar and colonial stromatolites grew in lee of the intertidal shoals. Calm back-shoal lagoonal waters supported invertebrate faunas forming the fossiliferous bioturbated wackestone and mudstone facies (unit 3). Laterally linked hemispheroidal stromatolites, which grew along the marshy shoreline, became thin film mats at the upper intertidal limits (boundstone facies, unit 2). Brecciation, micritization, and mud cracking in adjacent supratidal zones yielded the brecciated boundstone, intraclast grainstone facies (unit 1).

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.