SED 773 Dispatch: 1.2.06 Journal: SED CE: Hari Journal Name Manuscript No. B Author Received: No. of pages: 18 PE: Revathi
Sedimentology (2006) 1–18 doi: 10.1111/j.1365-3091.2006.00773.x
Sedimentology and stratigraphy of a transgressive, muddy gravel beach: waterside beach, Bay of Fundy, Canada F SHAHIN E. DASHTGARD*, MURRAY K. GINGRAS* and KARL E. BUTLER *Department of Earth and Atmospheric Sciences, 1-26 Earth Sciences Building, University of Alberta, Edmonton, AB, Canada T6E 2G3 (E-mail: [email protected]) O Department of Geology, University of New Brunswick, PO Box 4400, Fredericton, NB, Canada E3B 5A3
ABSTRACT O Sediments exposed at low tide on the transgressive, hypertidal (>6 m tidal range) Waterside Beach, New Brunswick, Canada permit the scrutinyR 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 threeP 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 shallowlyD 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-formedE sand and gravel bars and associated deposits include: sediment sourced from low-amplitude, unstable sand bars; gravel deposited from large (up to 5Æ5 m high, 800 m long), landward-migrating gravel bars; and, zones ofT 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 depositionC of mud beds, and muddy clast- and matrix-supported conglomerates in ancient conglomeratic successions. Idealized sections are presentedE 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-grainedR 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 packageR 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,O a coarse lag, and capped by trough cross-bedded sand and gravel. Keywords Beach, conglomerate, macrotidal, mud and gravel, muddy con- glomerate, sedimentology,C stratigraphy, transgressive.
INTRODUCTIONN logical and stratigraphic relationships on modern beaches aids in predicting the extent, thickness, Studies of modern, transgressive gravel beaches and morphology of conglomerates in the sub- provide importantU information regarding facies surface. Waterside Beach is a transgressive, relationships and organization of ancient conglo- muddy gravel beach in the hypertidal Bay of merates. In particular, determining sedimento- Fundy. Because of the area’s extreme tidal range