Vertical Sequence and Lateral Transitions in the Facies of a Barred Nearshore Environment

Abstract

ABSTRACT Nine tube cores 7 cm in diameter and up to 150 cm long from a nearshore, crescentic bar in Kouchibouguac Bay, New Brunswick, Canada reveal the vertical sequence and lateral transitions of facies previously identified only in shallow box cores. The facies are a) inner-shelf-shoreface transition: bioturbated fine sands and silts with remnant primary structures; b) seaward slope: planar lamination in medium-to-fine sands dipping gently seaward and composite bed sets of planar-to-ripple cross-lamination in fine sand (chevron lamination is rare); c) bar crest: low-angle, planar lamination dipping both landward and seaward and medium-scale trough cross-lamination in medium-to-coarse sands; d) landward slope: planar lamination dipping landward with small-scale, la dward dipping trough cross-lamination (occasionally small-to-medium-scale trough cross-lamination is found with seaward dips); e) trough: massively bedded lags of coarse, gravelly sands with abundant shell fragments contrast with isolated, very thin units of massive silt and units of well-sorted sands. The latter display subhorizontal planar lamination and small-to-medium-scale trough cross-lamination; polymodal dips indicate bed-form migration landward, alongshore, and offshore. Vertical sequences and lateral transitions seaward of the bar crest suggest that three wedge-shaped units (inner-shelf--shoreface transition, seaward-slope bar crest) interdigitate in an offlap-onlap sequence. To landward the trough and landward slope facies interfinger complexly, while the crest facies forms a simple onlap over landward slope facies. Total reworking of sediments within the bar-trough system is expected every 5 to 6 years, and long-term preservation is limited to the inner-shelf-shoreface transition, seaward slope, and trough facies.