An Ichnological and Sedimentological Facies Model for Muddy Point-Bar Deposits

Abstract

Detailed analysis of fine-grained clastic deposits associated with the macrotidal estuarine mouth of the Shepody River, New Brunswick, Canada, concentrated on identifying the ichnological and sedimentological characteristics of tidally dominated point bars and their adjacent tidal flats. The aim of the study was to establish an ichnological facies model for similar deposits.Within the study area, the distribution of ichnological structures and sedimentary characteristics such as grain-size distribution and total organic content are associated with bar elevation (i.e., upper, middle, or lower intertidal), tidal-bank slope, and the local hydraulic processes. The distribution of animal traces is also influenced by the duration of intertidal exposure and sedimentation rates. Polykladichnus-and Skolithos-like traces characterize upper-subtidal and lower-intertidal zones of the point bars; Arenicolites-, Diplocraterion-, Polykladichnus-, Palaeophycus-, and Planolites-like forms are pervasive in middle-intertidal zones; and, Siphonichnus-and Polykladichnus-like burrows typify the upper-intertidal point bars and the tidal flats. The size and diversity of the burrowing fauna are affected by the chemistry of the depositional waters and by seasonal variations in composition and temperature of those waters. The extreme seasonality of the area favors opportunistic fauna and thereby contributes to an impoverished, brackish-water trace assemblage.Geomorphologically, the area is dominated by point-bar and tidal-flat deposits, which comprise rhythmic bedding, composed of interlaminated to thinly interbedded silty and sandy mud. Point-bar bedding dips channelwards and represents muddominated inclined heterolithic stratification (IHS). The IHS alternates between burrowed and laminated beds. Laminated beds contain rhythmic lamination. Cyclic variations in laminae thickness are attributed to neap-spring variation in tidal-current strength. The burrowed interbeds exhibit high degrees of bioturbation that eradicate the preexisting lamination. The intercalation of laminated and burrowed beds represent seasonal variations in the depositional system: laminated beds characterize early winter and early spring sedimentation, and the bioturbated beds represent late spring through fall deposits.

Seasonal Controls On the Development And Character of Inclined Heterolithic Stratification In A Tide-Influenced, Fluvially Dominated Channel: Fraser River, Canada

“…Deposits of such strata typically range in thickness from 1-30 m, with dip angles commonly ranging from 5-15°. The development of IHS is generally attributed to lateral accretion of bar deposits in channelized settings (e.g., point bars, in-channel bars), where the sedimentological and ichnological variability reflects the wide range of fluvial (Jackson, 1981;Calverley, 1984;, mixed tidal-fluvial (Smith, , 1988Thomas et al, 1987;Lettley et al, 2007;, and tidal channels (Choi et al, 2004;Pearson and Gingras, 2006;Choi, 2010) in which IHS is developed. Prior to the terminology proposed by Thomas et al (1987), inclined heterolithic stratification was referred to by several different terms, including: longitudinal oblique bedding (Reineck, 1958), epsilon cross stratification (Allen, 1963), and lateral accretion structures (Collinson, 1978;Allen, 1982;De Mowbray, 1983).…”

“…In the macrotidal polyhaline Shepody River, New Brunswick, Canada, Pearson and Gingras (2006) report a wide variety of traces from muddy point-bar IHS (e.g., Arenicolites, Diplocraterion, Palaeophycus, Planolites, Polykladichnus, Siphonicnus, Skolithos). These traces are constructed by opportunistic fauna that can tolerate an environment with extreme temperature stress (warm summers, freezing winters) and moderate salinity fluctuation (range: 16-27 psu).…”

“…These traces are constructed by opportunistic fauna that can tolerate an environment with extreme temperature stress (warm summers, freezing winters) and moderate salinity fluctuation (range: 16-27 psu). By documenting the bioturbation intensity of the succession, Pearson and Gingras (2006) were able to identify a pattern of alternating burrowed and laminated beds ( Fig. 1.15).…”

“…IHS is generally attributed to lateral accretion of bar deposits in channels (e.g., point bars, channel bars), and is particularly abundant in tide-influenced settings Smith, 1988;Thomas et al, 1987;Choi et al, 2004;Hovikovski et al, 2005;Pearson and Gingras, 2006;Hovikovski et al, 2008;Choi, 2010). In both modern and ancient point bar examples, several recurrent fining trends have been documented, including: 1) overall vertical fining upward; 2) along strike, proximal-to-distal fining (downstream fining); 3) up-dip fining within individual inclined units; and, 4) fining-upward rhythms within individual members (Thomas et al, 1987).…”

“…Despite sharing the fundamental characteristics of IHS, work in modern environments has shown that there are differences in the style of IHS produced in fluvial (Calverley, 1984;Smith et al, 2009;Smith et al, 2010), and tide-dominated channels (Choi et al, 2004, Pearson andGingras, 2006;Choi, 2010). Fluvial IHS in 8 m thick point bar deposits of the upper delta plain of the Athabasca River Delta consist of fining-upward sand beds, with mud beds of irregular thickness occurring in irregularly spaced intervals (Calverley, 1984;.…”