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 Ó 2006 The Authors. Journal compilation Ó 2006 International Association of Sedimentologists 1 2 S.E. Dashtgard, M.K. Gingras and K.E. Butler (up to 12 m), foreshore (and shoreface equivalent) foreshore sediments (Clifton, 1981; Massari & sediments are exceptionally well exposed at Parea, 1988). spring low tide. This provides an opportunity to Waterside Beach, New Brunswick, Canada is a assess the sedimentological characteristics of transgressive, muddy gravel beach in the hyper- conglomerates deposited at water depths equival- tidal Bay of Fundy. It is considered that the ent to the upper and lower shoreface (i.e. depos- structures and morphology of the intertidalF ited because of shoaling, breaker, surf, and swash deposits partly result from depositional processes processes). In this paper: (1) sedimentologically (shoaling, breaker, surf, and swash zone pro- distinct deposits are reported; (2) mechanisms for cesses) and water depths equivalent toO the upper depositing mud beds and muddy conglomerates and lower shoreface. Examination of these mod- on gravel beaches are described; and, (3) inferred ern deposits, therefore, provides insights into the stratigraphic successions of transgressive gravel facies and facies relationshipsO of hydraulically beaches are proposed. reworked shoreface conglomerates. Sedimentological descriptions of modern, wave-dominated gravel foreshores and back- Study area R shores are common in geological literature. The original model presented by Bluck (1967) recog- Waterside Beach is located on the New Bruns- nized distinct, shore-parallel zones based on wick coastline of ChignectoP Bay (Fig. 1). Oriented clast-shape selection. This shore-normal zonation northwest–southeast the beach is perpendicular is observed from gravel beaches around the world to the dominant southwest winds (Amos & (Carr, 1969; Carr et al., 1970; Maejima, 1982; Hart Asprey, 1979). During winter cyclones (mainly & Plint, 1989; Postma & Nemec, 1990; Bartholoma¨ November through January) it experiences peak et al., 1998; Bluck, 1999) and may be considered significant waveD heights of 3 m and wave periods typical of high-energy, wave-dominated shore- of 10 sec (Amos et al., 1991). Overall, most lines with a limited fluvial- or marine-sediment significantE waves heights (79%) are below supply. The above model, however, is limited 1Æ25 m with periods of 7 sec or less (Amos et al., to a narrow (average 100–200 m wide) beach- 1991). Waterside Beach experiences a mean tidal normal zone that includes the steeply dipping rangeT of 9 m. Vertical tidal range varies from 6 m beachface (foreshore), berm, and backshore during neap tides to 12 m during spring tides (Bluck, 1967; Carr et al., 1970; Kirk, 1980; Mae- resulting in exposure of up to 1200 m of intertidal jima, 1982; Postma & Nemec, 1990). Modern Czone at low tide. Additionally, up to 650 m of nearshore (shoreface) and more basinal conglo- beach sediments occur sub-tidally (Fig. 2). The merate facies have been described (Hart & Plint, toe of the beach is demarcated by a step that is 1989), but are generally poorly understood.E locally steep (1°), but generally weakly defined. Shoreface conglomerate models are therefore, On the landward edge, backshore and beachface mainly derived from outcrop and core (Clifton, deposits abut either salt marsh or bedrock cliffs 1981, 1988; Massari & Parea, 1988; HartR & Plint, (Fig. 1C). 1989, 2003; Caddell & Moslow, 2004; Zonneveld At the northwest end of the beach, sand with & Moslow, 2004). As a result, most modern minor gravel is the dominant sediment; whereas, depositional models for conglomeratesR are a gravel is present near the mouth of Long Marsh composite of modern foreshore deposits, and Creek (Figs 1B and 2). A maintained dike backs ancient shoreface and more basinal deposits the beach in the southeast (Fig. 1B and C). The (Bourgeois & Leithold, 1984). O dike has significantly hindered transgression, yet Shoreface conglomerates are broadly sub- does not appear to interrupt beach sedimentation divided as transgressive and regressive (Wescott patterns in the intertidal and sub-tidal zones & Ethridge, 1982; Bourgeois & Leithold, 1984; (Fig. 2). Postma & Nemec, 1990).C Transgressive conglom- erate successions encountered in the rock record Methods tend to lack backshore and foreshore deposits as a result of erosion duringN transgression (Bourgeois Fieldwork on Waterside Beach was undertaken in & Leithold, 1984). Regressive (progradational) 2003 and 2004. Beach-normal and beach-parallel gravel beaches tend to be characterized by transects were conducted to establish beach repeating sequencesU of coarsening-upward con- zonation and morphology. Line-and-level meas- glomerates with internal erosional surfaces (Bour- urements were used to document changes in geois & Leithold, 1984), and by the preservation of slope and to establish major changes in morphol- Ó 2006 The Authors. Journal compilation Ó 2006 International Association of Sedimentologists, Sedimentology, 1–18 Sedimentology, stratigraphy of muddy gravel beaches 3 ′ ′ A 65 63 trenches dug mainly perpendicular to deposi- P.E.I. Moncton tional strike. Box cores were collected at most NEW 46′ stations for X-ray imaging. BRUNSWICK High-resolution, single-channel seismic pro- Study Area Chignecto Bay files were acquired in 2003 and 2004. TheseF Saint John surveys were used to map out the toe of the beach NOVA SCOTIA (Fig. 2), but otherwise are not presented in this U.S.A. paper. In 2005, a grab-sampling program was BAY OF FUNDY Halifax undertaken to sample sub-tidal beachO and off- shore sediments. Samples collected during this 0 50 100 ATLANTIC OCEAN program are incorporated into the grain-size data ′ 44 44′ N kilometers and are used to map out the horizontal distribu- ′ ′ ′ 67 W 65 63 O tion of sediments in the sub-tidal zone (Fig. 2). B Grain-size distribution on Waterside Beach was determined using one of three techniques: (1) grid Dennis Bea 915 R Long Marsh sampling, (2) bulk-sample dry sieving, and (3) X- Waterside Bea Creek ch ray absorption. (1) Grid sampling (Wolman, 1954; Rice & Church, 1996; Hoey, 2004) was employed ch P for deposits with