Sedimentological–ichnological model for tide‐dominated shelf sandbodies: Lower Cambrian Gog Group of western Canada

Abstract

Dunes and bars are common elements in tide‐dominated shelf settings. However, there is no consensus on a unifying terminology or a systematic classification for thick sets of cross‐stratified sandstones. In addition, their ichnological attributes have hardly been explored. To address these issues, the properties, architecture and ichnology of compound cross‐stratified sandstone bodies contained in the Lower Cambrian Gog Group of the southern Canadian Rocky Mountains are described here. In these transgressive sandstones, five types of compound cross‐stratified sandstone are distinguished based on foreset geometry, sedimentary structures and internal heterogeneity. These represent four broad categories of subtidal sandbodies: (i) compound‐dune fields; (ii) sand sheets; (iii) sand ridges; and (iv) isolated dune patches; tidal bars comprise a fifth category but are not present in the Gog Group. Compound‐dune fields are characterized by sigmoidal and planar cross‐stratified sandstone in coarsening‐upward and thickening‐upward packages (Type 1); these are mostly unburrowed, or locally contain representatives of the Skolithos ichnofacies, but are intercalated with intensely bioturbated sandstone containing the archetypal Cruziana ichnofacies. Sand‐sheet complexes, also composed of compound dunes, cover more extensive subtidal areas, and comprise three adjacent subenvironments: core, front and margin. The core is characterized by thick‐bedded sets of cross‐stratified sandstone (Type 2). A decrease of bedform size at the front is recorded by wedges of thinner‐bedded, low‐angle and planar cross‐stratified sandstone (Type 3) exhibiting dense Skolithos pipe‐rock ichnofabric. The margin is characterized by interbedded sandstone and mudstone, and hummocky cross‐stratified sandstone. Sand‐sheet deposits exhibit clear trends in trace‐fossil distribution along the sediment transport path, from non‐bioturbated beds in the core to Skolithos ichnofacies at the front, and a depauperate Cruziana ichnofacies at the margin. Tidal sand ridges are large elongate sandbodies characterized by large sigmoid‐shaped reactivation surfaces (Type 4). Sand ridges display clear ichnological trends perpendicular to the axis of the ridge, with no bioturbation or a poorly developed Skolithos ichnofacies in the core, a depauperate Cruziana ichnofacies in lee‐side deposits, and Cruziana ichnofacies at the margin. While both tidal ridges and tidal bars migrate by means of lateral accretion, the latter occur in association with channels while the former do not. Because tidal bars tend to occur in brackish‐water marginal‐marine settings, their ichnofauna are typically of low diversity, representing a depauperate Cruziana ichnofacies. Isolated dune patches developed on sand‐starved areas of the shelf, and are represented by lenticular sandbodies with sigmoidal reactivation surfaces (Type 5); they typically lack trace fossils, but the interfingering muddy deposits are intensely bioturbated by a high‐diversity fauna recording the Cruziana ichnofacies....

Environmental variability of Macaronichnus ichnofabrics in Eocene tidal-embayment deposits of southern Patagonia, Argentina

“…; Desjardins et al . ). Elements of the ichnofabric may be attributed to a number of controls, in particular to the tiering pattern and manner of colonization (Taylor et al .…”

“…; Desjardins et al . ). The textural maturity, the dominance of medium‐ to large‐scale cross‐bedding and the scarcity of mudstone drapes imply deposition in a dominantly high‐energy, shallow to marginal marine setting characterized by periods of reduced sedimentation and energy (Pollard et al .…”

“…; Desjardins et al . ). In the upper trough areas, food particles were kept in suspension promoting the opportunistic settlement of these areas by hemi‐sessile, suspension‐feeding infauna (Fig.…”

“…Facies 1 and 2 were formed in a forward-migrating, flow-transverse, sub-tidal compound-dune complex in a tide-dominated coastal embayment. The medium-to large-scale cross-stratification is interpreted as the product of sub-aqueous compound dunes migrating in the regional flood direction, that is, westward (Bern e et al 1993;Yoshida et al 2004;Sixsmith et al 2008;Desjardins et al 2012). The textural maturity, the dominance of medium-to large-scale cross-bedding and the scarcity of mudstone drapes imply deposition in a dominantly high-energy, shallow to marginal marine setting characterized by periods of reduced sedimentation and energy (Pollard et al 1993;Yoshida et al 2004).…”

“…It is the record of primary depositional conditions, original endobenthic community structure and subsequent taphonomic history (Taylor & Goldring 1993). Different parameters from an endobenthic community are incorporated, such as rates of biogenic activity (bioturbation intensity), burrow ordering, tier profile and the recognition of different ichnoguilds (Taylor & Goldring 1993;Taylor et al 2003;M angano & Buatois 2004;McIlroy 2004;Desjardins et al 2010;Desjardins et al 2012). Elements of the ichnofabric may be attributed to a number of controls, in particular to the tiering pattern and manner of colonization (Taylor et al 2003).…”