LOWER SILURIAN SEQUENCE STRATIGRAPHY AND SEA-LEVEL HISTORY OF THE HUDSON-BAY PLATFORM

Trends in relative sea-level change, shown by curves drawn from drill core and outcrop sections, were used to correlate individual parasequences among widely separated localities in Llandoverian carbonates of the Hudson Bay Platform. Correlations show that two sequences, bounded by regional disconformities, are present: (1) the Severn River Formation and (2) the Ekwan River and Attawapiskat formations, and possibly part of the Kenogami River Formation (listed in ascending order). Initial onlap of marine facies at the base of the Severn River Formation progressed from north to south, occurring first in the Hudson Bay Basin then in the Moose River Basin; final retreat of the seas at the end of Attawapiskat time was in the opposite direction. The Moose River Basin was always shallower than the Hudson Bay Basin, but both basins were remarkably shallow and flat. The most extensive inundations occurred during Ekwan River and Attawapiskat depositional times. For the remainder of the Silurian following Attawapiskat time, only supratidal facies or subaerial exposure surfaces are recorded on the platform, indicating that relative sea level was very low at that time. Correlations based on matching trends in relative sea-level change show that strata of the upper Ekwan River Formation in the southern Moose River Basin are time equivalents of the Attawapiskat reefs in the northern part of the basin and that reefal outcrops along the Attawapiskat River represent one interval of reef growth. Large-scale trends in the relative sea-level curves delineate four major Early Silurian sea-level highstands (late Rhuddanian, late Aeronian, early Telychian and late Telychian) also recognized in other basins across North America.