Abstract Recent work has focused on erecting new Seilacherian ichnofacies for depositional environments subject to recurring temporal and spatial variations in physico-chemical stress. In marine deltaic settings, these correspond to the Phycosiphon Ichnofacies for mudstone-dominated prodeltaic deposits and the Rosselia Ichnofacies for sandstone-dominated delta-front successions. The archetypal expressions of these ichnofacies, however, are founded on mixed process (wave- and river-influenced) systems, because the juxtaposition of ambient marine conditions during periods of prolonged wave energy with rapid deposition and physico-chemically stressed conditions during heightened fluvial discharge best expresses the deltaic signal. As deltaic settings shift towards end-member processes (e.g. river domination, wave domination and tide domination), or towards mixed-process conditions other than river and wave influence, the resulting ichnological suites and bioturbation fabrics depart from the recently published archetypes. Using selected studies of marine deltaic deposits, predictable departures from the archetypes can be recognized on the basis of these changing processes and their associated physico-chemical stresses. River-dominated delta deposits and tide-dominated delta successions display the greatest deviation from the published archetypes. River-dominated examples show elevated deposition rates, periods of salinity reduction, slumping and dewatering, elevated water turbidity, flood-induced sediment gravity flows and hypopycnal-generated fluid mud. As a result, river-dominated successions are largely devoid of bioturbation. Evidence of marine conditions is commonly restricted to isolated occurrences of dwelling structures such as Arenicolites , Ophiomorpha or Rosselia in sandstone, and Chondrites , Phycosiphon or Zoophycos in mudstone beds, particularly in prodeltaic intervals. Tide-dominated deltaic successions are markedly heterolithic and typified by highly mobile substrates manifested by incrementally migrating asymmetric bedforms and abundant fluid mud. Such settings are also prone to marked changes in salinity and shifts in the position of the turbidity maximum zone. Successions typically show low intensities of bioturbation and sporadically distributed burrows, as well as deposit-feeding structures, deeply penetrating dwelling structures or fugichnia. Many trace fossil suites consist entirely of facies-crossing elements, making assignment to an ichnofacies impossible. Storm flood-dominated deltaic successions are characterized by tempestites that are typically interstratified with river flood-induced sediment-gravity flow deposits and/or mantled by largely unburrowed mudstone drapes derived from hypopycnal plumes associated with river floods. Where these storm flood cycles are interstratified with ambient fairweather beds, assignment to the archetypal deltaic ichnofacies is straightforward. However, as storm beds become increasingly erosionally amalgamated, the preservation potential of the fairweather beds is reduced and the resulting trace fossil suites are biased towards those recording opportunistic colonization of the event beds. The presence of mudstone layers with low bioturbation intensity (BI) containing small numbers of ichnogenera positively correlated with marine conditions (e.g. Chondrites , Phycosiphon and/or Zoophycos ) may be the only evidence that the suites should be assigned to one of the deltaic ichnofacies. Wave-dominated deltas lacking significant storm influence are typically challenging to differentiate from their archetypal strandplain shoreface counterparts and, correspondingly, the resulting trace fossil suites are broadly comparable to the archetypal Cruziana and Skolithos ichnofacies. Most of the preserved record of wave-dominated delta successions is related to fairweather ambient conditions, and so facies typically show high BI values and uniformly distributed bioturbation. Key to recognizing that the suites should be assigned to one of the deltaic ichnofacies is the presence of rare river-generated mudstone and sandstone beds that display evidence of physico-chemical stress and/or the paucity of domichnia typical of suspension-feeding organisms. In most delta types, the prodeltaic facies are most readily discerned to contain trace fossil suites of the Phycosiphon Ichnofacies, owing to the higher preservation potential of all depositional processes, including marine fairweather beds, river-supplied hyperpycnites and other sediment gravity flow deposits, tempestites and fluid mud derived from river flood-related hypopycnal plumes. Assignment of trace fossil suites to the Rosselia Ichnofacies requires some record of the fairweather conditions, which are generally diminished in river-, tide- and storm-dominated successions. The dominance of structures positively correlated to deposit-feeding ethologies at the expense of those attributed to suspension-feeding strategies may point to elevated water turbidity and assignment of the suite to the Rosselia Ichnofacies. However, in many cases, the ichnological suites of delta fronts are so depauperate that assignment to an ichnofacies is problematic and should be avoided.

Abstract This study integrates ichnological, sedimentological and stratigraphic analyses of the Lower Cretaceous (Albian) Viking Formation in west-central Alberta, facilitating the recognition of sand- and mud-prone heterolithic central basin deposits in wave-dominated, estuarine incised valley fills. Core descriptions from 110 wells in four fields (26 wells from Crystal, 4 from Cyn-Pem, 50 from Willesden Green, and 30 from Sundance-Edson) comprise the data set. Central basin settings diverge from open marine settings based on conditions of reduced and fluctuating salinity. Indications of brackish-water conditions can be subtle, often leading to the misidentification of central basin deposits as open marine “Regional Viking” parasequences. Ichnology is ideally suited to assist in the identification of such brackish-water deposits. Ichnological suites, bioturbation intensities, and physical sedimentary structures are used to differentiate five recurring brackish-water, central basin facies associations. Although central-basin deposits are not significant hydrocarbon producers, associated sand-prone facies of incised valley-fills are lucrative. Central basins comprise the most volumetrically extensive deposits of the valley fills and, as a result, exploration and early development wells are most likely to intersect these heterolithic successions. Facies Association CB1 is mud-dominated, and contains trace fossil suites indicative of the most marine conditions of all the central basin deposits. Facies of CB1 are interpreted to record deposition in marine-influenced bays of incompletely barred estuaries. Intervals encompass BI 3-5, with abundant Planolites , Teichichnus , Palaeophycus , Chondrites , “Terebellina” ( sensu lato ), and Thalassinoides , and subordinate Ophiomorpha , Helminthopsis , Phycosiphon , Asterosoma , Skolithos , Lockeia , Cylindrichnus , Rhizocorallium , Arenicolites and fugichnia. CB1 bay deposits are particularly common to the Sundance-Edson valley system. Facies Association CB2 is also mud-dominated, but generally lacks those ichnogenera considered to be restricted to fully marine settings (e.g., Phycosiphon , Asterosoma , Rhizocorallium , and Helminthopsis ). Intervals display variable and generally reduced bioturbation intensities (BI 1-4). Suites comprise Planolites , “Terebellina” ( sensu lato ), Teichichnus , and secondary Cylindrichnus , Ophiomorpha , Rosselia , Palaeophycus , Diplocraterion , Arenicolites , Skolithos and fugichnia. Very rare occurrences of Chondrites and Lockeia are locally present. Facies of CB2 reflect accumulation in low-energy, strongly brackish bays of well-barred estuaries, and is common to Willesden Green and some Crystal successions. Facies Association CB3 comprises sand-prone heterolithic successions, deposited along shallow bay margins of well-barred estuaries (where it grades upwards from CB2), and/or adjacent to bay-head deltas. The facies displays BI 0-3, with a low-diversity suite of Planolites , Teichichnus , “Terebellina” ( sensu lato ), Ophiomorpha , diminutive Palaeophycus , and fugichnia. Rarely, Rosselia , Cylindrichnus , Thalassinoides , Diplocraterion , Arenicolites , and Skolithos , and very uncommon occurrences of Chondrites , Rhizocorallium , and Phycosiphon are present. Facies of CB3 are common to the Willesden Green valley and landward portions of the Crystal valley. Facies Association CB4 corresponds to sand-dominated heterolithic intervals. The facies display BI 1-4, with robust ichnogenera and high diversity suites (e.g., Planolites , Teichichnus , fugichnia, Palaeophycus , Ophiomorpha , Thalassinoides , Rosselia , Arenicolites , Cylindrichnus , Diplocraterion , Skolithos , Lockeia , Chondrites , Phycosiphon , Siphonichnus , Taenidium , Asterosoma , and “Terebellina” ( sensu lato ). Facies Association CB4 is interpreted to represent bay-margin positions of incompletely barred estuaries (e.g., the Sundance-Edson valley, where it grades upwards out of Facies CB1), as well as late-stage bay infill associated with the early stages of transgression (e.g., all Viking incised valleys studied). Facies Association CB5 encompasses sandstones deposited along the seaward edges of central basins, adjacent to the estuary mouth (e.g., associated with flood-tidal deltas and storm-washover fans). This facies displays BI 0-3, with a strongly marine-influenced trace fossil suite. Ichnogenera comprise Planolites , Ophiomorpha , Palaeophycus , Skolithos , and fugichnia, with secondary Teichichnus , Diplocraterion , Arenicolites , Rosselia , “Terebellina” ( sensu lato ), Thalassinoides , Chondrites , Phycosiphon , Asterosoma , Bergaueria , and Conichnus . Facies of CB5 are common to Willesden Green and Crystal valley successions. Ongoing research seeks to identify characteristic geophysical well-log signatures for central-basin deposits, in order to enhance recognition of estuarine incised-valley fills. Bay-fill signatures in wave-dominated estuaries are commonly misidentified as open-marine parasequences, as bays are generally characterized by sanding-upward successions. Central basins constitute the most areally extensive subenvironments of wave-dominated estuaries, and their deposits are the most likely to be encountered during drilling. Consequently, reliable identification of central basins could facilitate future discoveries of incised valleys. Estuarine incised valley fills are under-represented in the Viking Formation, given the abundance of forced regressive, lowstand, and transgressively incised shoreline trends that have been identified. Integrating ichnology with sedimentology and stratigraphy will assist in the recognition of incised valley fills in the rock record.