Glacially Overdeepened Troughs on the Labrador Shelf, Canada

Abstract

The continental shelf off Labrador is underlain by a clastic wedge of Early Paleozoic to Cretaceous-Tertiary sandstones, siltstones and limestones which onlap the Precambrian metasediments of the mainland. Regional seismostratigrphic mapping [Josenhans et al., 1986] shows that the contact between Precambrian basement and the clastic wedge is faulted in places. Buried valley fill remnants are recognised on the middle shelf together with faulted sequences at the base of the fluvial sequences. Fluvial down-cutting along fault-weakened zones is suggested to have occured in late Tertiary-Pliocene time [Grant, 1966]. Intense glacial erosion, particularly on the inner shelf near the Precambrian/clastic wedge contact, has down cut the clastic sediments and shaped a marginal trough which connects to a transverse channel (Hopedale Saddle) with depths of up to 800m. The intensity of glacial erosion appears most pronounced at the Precambrian contact and diminishes toward the outer shelf edge as demonstrated by the presence of remnant valleyfill and early glacial progradational deposits. Water depth on the outer shelf of Hopedale saddle/trough is only about 300m suggesting up to 500m of glacial overdeepening on the inner shelf. Note that the upper till lies directly on the bedrock unconformity on the inner shelf although older glacial(?) deposits are preserved on the outer shelf. This is interpreted to result from increased glacial erosion on the inner shelf.