Sedimentology of a tidal point-bar within the fluvial-tidal transition: River Severn Estuary, UK

摘要

5.1 INTRODUCTION Published studies of marine-dominated outer estuaries in the geological record are legion, but the identification of sediment sequences characteristic of the fluvial to tidal (F-T) transition remains difficult (e.g., Ashour et al., 2012; Bhattacharya et al, 2012; Bose and Chakraborty, 1994; Corbett et al., 2011; Flaig et al, 2011; Ghosh et al., 2005; Shanley and McCabe, 1995; Shanley et al., 1992; Van den Berg et al., 2007; Yoshida, 2000). This difficulty is due in part to a relative paucity of studies of modern process-based analogs that link the hydrodynamic processes and the associated sedimentology and stratigraphy of fluvial-influenced tidal deposits (Billy et al., 2012; Chaumillon et al., 2013; Choi et al., 2004; Gingras et al., 1999; Johnson and Dashtgard, 2013; Lambiase, 2013; Nanson et al., 2013; Sisulak and Dashtgard, 2.012; Smith, 1985, 1987, 1988; Vakarelov and Ainsworth, 2013). The zone of the fluvial to tidal transition is subject to the interaction of fluvial, tidal, and wind-wave activity and thus, as a transitional environment, may differ in the sedimentary response when compared with predominantly fluvial or predominantly marine estuarine systems (Dalrymple and Choi, 2007). Identifying the stratigraphic associations of these F-T transitions has economic relevance, as estuary-head locations in the geological record may be favorable for oil and gas reservoirs (Hubbard et al., 2002; Zaitlin et al., 1994). For example, the facies and reservoir models of the Lower Cretaceous McMurray Formation in Alberta, arguably one of the most important hydrocarbon reservoirs worldwide, indicate a marine-influenced, fluvial depositional setting (Fustic et al., 2012; Hubbard et al, 2011; Martinius et al., 2015; Musial et al., 2012). The objective of this study is to use shallow sedimentological borehole data and hydrodynamic data to (i) develop a point-bar conceptual depositional model in the F-T transitional zone of the estuary of the river Severn, UK and (ii) to contrast this deposit to borehole logs within the fluvial zone upstream and the marine zone downstream. The use of pollen (Loboziak et al., 2005) and diatoms (Vos and de Wolf, 1993) as diagnostic tools discriminating terrestrial, transitional, and fully marine deposits is increasingly important in studies of both modern and ancient deposits (Czarnecki et al., 2014) and so a pilot study of these components was included in this investigation. Thus, the study provides a modern example of a fluvial-tidal transitional zone depositional setting.