The Attikamagen–Ferriman Transition in Part of the Central Labrador Trough

Abstract

The sequence of the Labrador geosyncline is subdivided in three cycles, each beginning with deposition of shallow-water quartzites and precipitates and culminating with the deposition of thick shale–graywacke–basalt suites. This paper describes the stratigraphy and sedimentology of the formations deposited during the gradual re-stabilization of the basin at the end of cycle I.At the end of cycle I, red, green, and gray pro-deltaic shales and sandstones prograded into the basin from the west. They interfinger eastwards and upwards with the Dertaull Dolomite. In the west laminated dolomite was deposited on a deltaic platform; brecciated and conglomeratic dolomites developed at the slope margin, and grade into alternating laminated dolomite, conglomeratic dolomite, and pelite in the basin. Slumping and turbidity currents were important transport mechanisms. Lenses of massive dolomite deposited in relatively deep water occur in the center of the trough and a platform extended in the eastern, eugeosyclinal zone. Upper horizons of the Denault Dolomite grade into the Attikamagen IV basinwards. The basal member of the Attikamagen IV consists of relatively deep-water shales; these grade upwards into red silstone and fine-grained sandstones of deltaic or pro-deltaic origin. The Fleming Breccia overlies the Denault Dolomite at the western margin of the trough. This unit is probably derived from a bedded sequence of siltstone, sandstone, and dolomite, that has been profoundly altered by complete replacement of dolomite by chert, followed by pene-contemporaneous brecciation and slumping. The Wishart Formation overlaps all older units with a marginal unconformity, but has gradational contacts with the latest deposits of Cycle I (uppermost Attikamagen IV and perhaps Fleming) closer to the basin center.