Quaternary interglacial and associated deposits in southwest Newfoundland

Abstract

A coastal cliff section near Codroy, Newfoundland exposes five distinctive lithologic units, which are defined lithologically and structurally as colluvial, lacustrine, or glacial in origin. They have been preserved in a gypsum karst depression, but continuing karst evolution has disturbed their original attitudes.The two oldest units, 5 and 4, were formed by sidewall slumping of local bedrock and earlier glacial material from the failing rim of a newly formed sinkhole. Unit 3 represents tranquil sedimentation of thin sand and silt–clay laminae, first in a freshwater sinkhole pond, then in brackish marine waters that entered after breaching of its seaward wall, and finally again in fresh water as sea level fell below the sill of the small embayment. Units 2 and 1 are glacial deposits laid down over older units after a second major sinkhole collapse. Collapse has continued to the present day.Pollen and spores from unit 3 indicate that during its deposition regional vegetation changed from tundra (Zone A) to boreal forest (Zone B), and back to tundra (Zone C), through an interglacial cycle of vegetation change. Balsam fir wood from Zone B is radiocarbon-dated to > 40 000 BP (I-10203).Foraminifera from unit 3 indicate that brackish marine conditions prevailed before Zone B times, and that nearshore fully marine conditions briefly prevailed in Zone B times. Zone B is thus assigned full interglacial status, based on pollen and foraminiferal evidence, and is tentatively assigned to the Sangamonian for want of indications to the contrary and for economy of interpretation. The overlying glaciogenic units 1 and 2 are thus probably of early Wisconsinan age.Calcite crystals, possibly pseudomorphous, found within Zone B of unit 3, were radiocarbon-dated at 30 000 ± 1450 BP (UQ-115). δ18O values of −6.8 to −7.5‰ (versus PDB) indicate replacement by calcite under conditions similar to present. δ13C values of −24.00 to −26.24‰ (versus PDB) indicate that carbonate carbon was supplied by decay of local organic matter. The date is thus a minimum and supports the contention that the sediments are of at least last interglacial age.Gypsum karst has thus been evolving in this area since before the last interglacial and has preserved a record of environmental change for that period that is unique in the Atlantic Provinces of Canada. Unit 3 is proposed as the stratotype for the "Codroy Interglacial Beds" (new name).