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Keywords:

  • Beach;
  • conglomerate;
  • macrotidal;
  • mud and gravel;
  • muddy conglomerate;
  • sedimentology;
  • stratigraphy;
  • transgressive

Abstract

Sediments exposed at low tide on the transgressive, hypertidal (>6 m tidal range) Waterside Beach, New Brunswick, Canada permit the scrutiny of sedimentary structures and textures that develop at water depths equivalent to the upper and lower shoreface. Waterside Beach sediments are grouped into eleven sedimentologically distinct deposits that represent three depositional environments: (1) sandy foreshore and shoreface; (2) tidal-creek braid-plain and delta; and, (3) wave-formed gravel and sand bars, and associated deposits. The sandy foreshore and shoreface depositional environment encompasses the backshore; moderately dipping beachface; and a shallowly seaward-dipping terrace of sandy middle and lower intertidal, and muddy sub-tidal sediments. Intertidal sediments reworked and deposited by tidal creeks comprise the tidal-creek braid plain and delta. Wave-formed sand and gravel bars and associated deposits include: sediment sourced from low-amplitude, unstable sand bars; gravel deposited from large (up to 5·5 m high, 800 m long), landward-migrating gravel bars; and zones of mud deposition developed on the landward side of the gravel bars. The relationship between the gravel bars and mud deposits, and between mud-laden sea water and beach gravels provides mechanisms for the deposition of mud beds, and muddy clast- and matrix-supported conglomerates in ancient conglomeratic successions. Idealized sections are presented as analogues for ancient conglomerates deposited in transgressive systems. Where tidal creeks do not influence sedimentation on the beach, the preserved sequence consists of a gravel lag overlain by increasingly finer-grained shoreface sediments. Conversely, where tidal creeks debouch onto the beach, erosion of the underlying salt marsh results in deposition of a thicker, more complex beach succession. The thickness of this package is controlled by tidal range, sedimentation rate, and rate of transgression. The tidal-creek influenced succession comprises repeated sequences of: a thin mud bed overlain by muddy conglomerate, sandy conglomerate, a coarse lag, and capped by trough cross-bedded sand and gravel.