Abstract

Grand Bank is a shallow, flat-topped bank more than 100,000 km2 in area that is separated from the Avalon Peninsula of southeastern Newfoundland by the deeper Avalon Channel. The channel and innermost bank are covered by a gravel facies; petrologic evidence shows that this facies was deposited by the late Wisconsin Avalon ice cap, which extended part way onto the bank. A seaward-fining quartz sand facies covers the rest of the bank and extends beyond the shelf break; petrologic evidence shows that this facies was derived by erosion of the underlying Mesozoic-Tertiary coastal plain succession during postglacial marine transgression.

Skeletal carbonate (mainly barnacles) has been accumulating on Grand Bank since the Holocene rise in sea level cut off its supply of all but minor ice-rafted terrigenous sediment. The Holocene carbonate fraction is in hydraulic equilibrium with the older terrigenous fraction, which, along with textural, petrologic, and other features, show that these palimpsest sediments are being reworked and mixed in response to the modern hydraulic regime. The seaward-fining textural pattern probably has also developed in response to modern processes, possibly by storm-induced bottom currents.

The results show that (1) combinations of source lithologies and depositional processes have interacted to produce a “classical” systematic decrease in grain size and corresponding increase in grain roundness and compositional maturity across the bank, (2) sediments of different ages and origins are readily mixed on this shelf, and (3) processes not related to glaciation form sedimentary deposits on a higher latitude continental shelf.

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