Ichnology and Sedimentology of a Mud-Dominated Deltaic Coast: Upper Cretaceous Alderson Member (Lea Park Fm), Western Canada

Abstract

Current depositional models largely promote the perception that all open-coastal distal (sea)-proximal (land) gradients are reflected by upward-coarsening grain-size trends, and that shoreline deposits are represented by prominent sand bodies. Although commonly the case, significant departures from this model may occur when the availability of coarser sediment calibers (sand-sized and larger) is limited. This is especially true where alongshore sediment-transport-influenced depositional systems are associated with rivers that supply abundant suspended sediments. Underestimating the role of grain-size segregation may lead to misinterpretations of energy levels and water depths, especially in some shale-dominated sedimentary units.The Upper Cretaceous Alderson Member (Lea Park Fm) is an up to 180-m-thick, gas-charged shale unit that we interpret as an ancient analogue for modern offshore and mud-dominated deltaic coasts. Sedimentological and ichnological data collected from 27 cores indicate that much of the sediment volume of the Alderson Member was deposited in relatively shallow water under the influence of tidal and wave processes in a deltaic coastal setting. Characteristic features reflecting these depositional affinities include: (1) increased proportions of terrestrially derived organic matter; (2) indications of thixotropic to soupy substrates (e.g., fluid mud) coupled with rapid depositional rates; (3) an impoverished ichnological signal (Planolites-dominated suites); (4) micro-laminated shale; (5) shale-on-shale erosional contacts; (6) scour-and-fill structures; and (7) intervals of lowangle cross-stratification. The interpretation of relatively shallow-water settings is also supported by recurring root-bearing horizons, Glossifungites Ichnofacies-demarcated transgressive surfaces of erosion, and conglomeratic surfaces at particular stratigraphic levels. The deposits are interpreted to include offshore, ''subaqueous deltas,'' muddy shoreface and/or tidal flat, and aggradational muddy coastal plain (chenier plain) sub-environments.The results of this study improve our knowledge of ichnological and sedimentological characteristics of shallow-marine shale units, and are potentially useful for recognition of similar nearshore mud-prone deposits elsewhere. J S R J S R FIG. 7.-Core 12, Southern Saskatchewan, upper Alderson. 812J. HOVIKOSKI ET AL. J S R

Palaeoenvironment of Eocene prodelta in Spitsbergen recorded by the trace fossilPhycosiphon incertum

“…Ichnological and geochemical data: incidence of oxygenation and/or food availability P. incertum traces are interpreted as having been produced by deposit feeders, usually opportunistic, at different tiers in mud and fine sand at variable depths up to 15 cm in sediments deposited in water depths below lower shoreface (Fu 1991;Goldring et al 1991;Wetzel & Bromley 1994;Uchman 1995Uchman , 1999Wetzel & Uchman 2001;Hovikoski et al 2008;Wetzel 2010). Phycosiphon is considered a pascichnial/fodinichnial structure (Ekdale & Masson 1988;Bromley 1996); the absence of spreite was interpreted as reflecting a pascichnia (grazing) rather than a fodinichnia (mining) behaviour (Ekdale & Masson 1988).…”

“…The Phycosiphon producer reworked a relatively small volume of the sediment containing abundant nutritious material. DepthÁsize grading is typical of colonization; after initial colonization, the trace-maker grew in size and burrowed deeper into the sediment (Wetzel & Uchman 2001;Hovikoski et al 2008). Orientation of Phycosiphon spreiten varies from inclined in homogeneous muds to horizontal in laminated deposits, probably reflecting the homogeneous distribution of food material (Wetzel & Uchman 2001;Wetzel 2010).…”

Clay fabric of fluid-mud deposits from laboratory and field observations: Potential application to the stratigraphic record

“…The resuspension of muddy deposits, including deposits that originally formed from fluid mud, also results in the development of fluid mud (Whitehouse et al, 2000) that can be transported along the shore (Allison et al, 1995) and to farther offshore environments as wave-or current-induced sediment-gravity flows (Wright et al, 2001;Wright and Friedrichs, 2006;McAnally et al, 2007). Thus, fluid-mud deposits may form in a variety of depositional systems in coastal and shallow-marine environments, thereby producing a mud-dominated stratigraphic succession (Hovikoski et al, 2008).…”

The Dobrotiv Formation (Miocene) in the Boryslav-Pokuttya and Sambir nappes of the Ukrainian Carpathians: a record of sedimentary environmental change in the development of the Carpathian Foredeep basin

“…Sig nif i cant bioturbation of prodelta sed i ments, with the oc currence of typ i cally ma rine trace fos sils such as Chondrites or Phycosiphon is com mon (Hovikoski et al, 2008). The most con vinc ing ex pla na tion is brack ish con di tions in the ba sin, and maybe ox y gen de fi ciency.…”

“…Field ing et al, 2005), in clud ing many lac ustrine del tas (e.g., Thomas et al, 2006), sug gests a mud-dom inated delta on a mud-dom i nated coast. Such depositional settings are still poorly un der stood, but known from re cent and fossil ex am ples (Augustinus, 1980;Hovikoski et al, 2008). Delta - A -rain drop casts; B -raindrop im prints; C -wind ing in ver te brate bur rows; D, E -bird foot prints; F -Gazellipeda -mam mal foot print; G -mammal trackway Gazellipeda, bird foot prints, rain drop casts -front fa cies in such del tas can be com posed mostly of mud (Tanabe et al, 2003).…”