ICHNOLOGY AND SEDIMENTOLOGY OF A MUD-DOMINATED DELTAIC COAST: UPPER CRETACEOUS ALDERSON MEMBER (LEA PARK FM), WESTERN CANADA

Transcription

1 Journal of Sedimentary Research, 2008, v. 78, Research Article DOI: /jsr ICHNOLOGY AND SEDIMENTOLOGY OF A MUD-DOMINATED DELTAIC COAST: UPPER CRETACEOUS ALDERSON MEMBER (LEA PARK FM), WESTERN CANADA JUSSI HOVIKOSKI, 1 RYAN LEMISKI, 1 MURRAY GINGRAS, 1 GEORGE PEMBERTON, 1 AND JAMES A. MACEACHERN 2 1 Ichnology Research Group, Department of Earth and Atmospheric Sciences, University of Alberta, 1-26 Earth Science Building, Edmonton, AB, T6G 2E3, Canada 2 Department of Earth Sciences, Simon Fraser University, Burnaby, British Columbia, V5A 1S6, Canada juanho@utu.fi 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. INTRODUCTION Mud-dominated, open-coast shorelines form typically as interdeltaic muddy shorefaces, tidal flats, or down-drift deltaic environments (e.g., chenier plains) that are sourced by wave- and/or tide-agitated, hyperpycnal and hypopycnal mud plumes. Compared to their sand-dominated counterparts, mud-dominated coastlines are sedimentologically and ichnologically poorly understood. As a result, their recognition in the geological record is hindered. In fact, despite the growing number of reported present-day examples (e.g., coast of Brazil Guayana, Louisiana [USA], Kerala [India], East China, and Carpentaria [Australia]), documented ancient examples are still rare (see Catskill Formation for an exception) (Beall 1968; Walker and Harms 1971; Rhodes 1982; Rine and Ginsburg 1985; Mallik et al. 1988; Augustinus 1989; Allison and Nittrouer 1998; Bentley et al. 2003; Neill and Allison 2005). Recently, it has been suggested that a parsimonious locale for mud accumulation is actually the coastal zone, and that only a part of mud supply escapes to deeper-water environments in low-gradient and lowenergy settings. This view is based on the observation that few sedimentary processes account for the transportation of fine-grained sediment to distal offshore (below storm-weather wave base) settings (Nittrouer and Wright 1994). Nearshore locales strongly favor the deposition of fine-grained sediment, as they are prone to the formation of fluid mud via various depositional processes (e.g., wave and tide agitation, estuarine flow convergence) and the tendency for nonflocculated (hypopycnal) plumes and fluid mud to move in alongshore directions such as promoted by longshore currents and Coriolis forces (e.g., Nittrouer and Wright 1994; Wright and Nittrouer 1995; Geyer et al. 2004; Khan et al. 2005). Accordingly, it has been proposed that the formation of thick basinal shale units may actually require considerable changes in shoreline position (Dalrymple and Cummings 2005). However, the notion that mudrocks are primarily deposited in nearshore positions, especially in foreland basins, makes the paucity of recognized coastal shale units in the geological record suspicious. Lately, Schieber et al. (2007) demonstrated by flume experiments that mud floccules form under highly variable experimental conditions (water chemistry, clay mineralogy, sediment concentration) and accumulate at flow velocities that transport and deposit sand. Their observations further point out the need to reevaluate published paleoenvironmental interpretations of ancient shale successions (Macquaker and Bohacs 2007). The main problems regarding recognition of shallow-marine mudrocks from the sedimentary record may relate to inferring the depositional energy levels and the initial water depths of shale-dominated strata. The Copyright E 2008, SEPM (Society for Sedimentary Geology) /08/ /$03.00

2 804 J. HOVIKOSKI ET AL. JSR presence of large quantities of high-concentration mud suspensions, coupled with a low-gradient, dissipative shoreline, effectively suppresses wave energy even in unbarred coastal settings (Wells and Coleman 1981; Rine and Ginsburg 1985; Mallik et al. 1988; Huh et al. 2001; Bentley et al. 2003). Depending on the source-rock mineralogy, these suspension plumes or fluid muds may be rich in swelling clays (e.g., Surinam Coast), making detailed study of analogous unlithified strata in the geological record challenging. In addition, shale-on-shale erosional contacts, such as those produced by storm waves, can be subtle and potentially difficult to recognize, especially in core (cf. Schieber 1998b, 2003). Finally, bedform development in muds is more complex than that of sands, in that it is strongly influenced by interparticle cohesion, sediment water content and primary productivity (e.g., Schieber et al. 2007). Even in high-energy muddy coastal settings, the resulting sedimentary structures may consist only of massive mud, micro-cross-lamination, parallel- to weakly nonparallel-laminated mud, lenticular bedding, or small-scale scourand-fill structures (Rine and Ginsburg 1985; Schieber et al. 2007). Consequently, active deposition of mud above fair-weather wavebase of an unbarred coastline can be easily misinterpreted as quiescent, deeperwater sedimentation, if the proportion of sand-size calibers is limited. A key to differentiating between deeper water (below storm-weather wavebase) sedimentation and coastal mud deposition lies in the recognition of various sedimentary processes inshore tidal flux, fairweather wave agitation, and deltaic input. As a result of the interaction of these factors, muddy coastal zones can be subject to: (1) rapid mud emplacement; (2) high depositional turbulence; (3) fluid-rich sediments; (4) shifting, ductile substrates; and (5) possibly low or fluctuating salinities. Therefore, these environments are ecologically challenging, imposing a variety of stresses on burrowing infauna, and making them potentially both sedimentologically and ichnologically distinct from deeper water muddy settings (e.g., Pemberton and Wightman 1992; Rine and Ginsburg 1985; Allison and Nittrouer 1998; Schieber 2003; Bann and Fielding 2004; MacEachern et al. 2005; Schieber et al. 2007; MacEachern et al. 2007a, MacEachern et al. 2007b). The objective of this paper is to describe ichnological and sedimentological characteristics of a shale unit that we interpret to represent offshore and mud-dominated, deltaic coastline deposits. The study focuses on the Upper Cretaceous Alderson Member (Lea Park Fm) of western Canada, which consists of an up to 180-m-thick succession of bentonitic sandy shales. Traditionally regarded as a shelfal unit deposited hundreds of kilometers seaward from the coeval shoreline (e.g., Meijer-Drees and Myhr 1981), the Alderson Member has recently been reinterpreted to display deltaic influence (O Connell 2003; Pedersen 2003). In this study, we demonstrate the influence of waves and onshore tidal processes in much of these strata, and report the presence of recurring root-bearing horizons. The results improve our knowledge of ichnological and sedimentological characteristics of shallow-marine shale units, and are potentially useful for the recognition of similar deposits elsewhere. MATERIALS AND METHODS The Study Area The Campanian Alderson Member is a 130- to 180-m-thick, sandy shale unit that is present in the subsurface of Alberta and Saskatchewan of western Canada (Figs. 1, 2). Together with the co-mingling of Medicine Hat and Second White Specks intervals, these shales host the largest gas reservoir in the Western Canadian Sedimentary Basin (WCSB) (O Connell 2003). This informal unit has traditionally been referred to as the Milk River Formation or the Milk River Equivalent in the earlier literature (e.g., Meijer-Drees and Myhr 1981; Gatenby and Staniland 2004). However, recent reports based on palynological, sedimentological, and radiometric data have suggested that the Alderson Member is (at least for the most part) a few Myr younger than the shallow marine FIG. 1. The study area with major known gas pools shown. Black line indicates the correlation transect of Figure 3. The studied cores are: (1) W3, (2) W3, (3) W3, (4) W3, (5) W3, (6) W3, (7) W3, (8) W3, (9) W3, (10) W3, (11) W3, (12) W3, (13) W3, (14) W3, (15) W3, (16) W3, (17) W3, (18) W3, (19) W3, (20) W3, (21) W3, (22) W3, (23) W3, (24) W3, (25) W3, (26) W3, (27) W3. continental Milk River Formation that is exposed at the Writing-in-Stone Provincial Park, Alberta, and is, instead, time-equivalent to the Lea Park Formation (e.g., Payenberg et al. 2002; O Connell 2003). The age of the basal part of the Alderson has yet to be defined, and it may correlate either with the uppermost member (continental Deadhorse Coulee Member) or the top of the Milk River Formation (cf. Shurr and Ridgley 2002). Payenberg et al. (2002) noted age equivalence between the Alderson Member and the deltaic Upper Eagle Formation of Montana, U.S.A. It has been estimated that the Alderson Member represents ca. 2 My of deposition (Payenberg et al. 2002). Despite the unquestioned economic value of the unit, surprisingly little has been published about its sedimentological, stratigraphic, and ichnological characteristics. This oversight is due to various factors, not least being the high content of swelling clays in these sediments, which makes detailed core logging challenging. Recent interpretations of the Alderson Member include distal, storm-influenced shelf (Gatenby and Staniland 2004) and a prodeltaic unit (O Connell 2003; Pedersen 2003). Several sequence stratigraphic and facies schemes have been proposed in various government reports and conference abstracts. However, no precise data, such as systematic and detailed facies descriptions, have been published to date.

3 JSR ICHNOLOGY AND SEDIMENTOLOGY OF A MUD-DOMINATED DELTAIC COAST 805 Methods This study is based on sedimentological and ichnological descriptions of 27 cores (Fig. 1). Sedimentological data include documentation of grain size (visual estimation), sedimentary structures, bedding contacts, character of the bedding, soft-sediment-deformation structures, and mineralogical accessories (e.g., pyrite, glauconite, chlorite, etc.). Ichnological data comprises description of the ichnogenera and/or ichnospecies, bioturbation index (BI of Taylor and Goldring 1993), crosscutting relationships, and estimation of tiering depths. Thin sections were collected from each facies, in order to study sedimentological and ichnological fabrics in detail. Finally, a set of cores was correlated to get an insight into changes of relative sea level and facies distributions in the Hatton Abbay Lacadena areas (Fig. 3). The possible sources of error are: (1) The deposits are commonly rich in bentonite, locally hindering detailed observations in these unlithified sediments. Therefore, detailed estimations of bioturbation intensity and particular ichnogenera were not always possible. This is especially the case with Facies 5, which is typically poorly preserved (rubbly); (2) the changes in grain size are commonly subtle and not always clearly inferable from well-log data. Thus, in those parts of the system where grain size changes are minimal and key stratigraphic surfaces appear conformable, well-log correlations become increasingly enigmatic. Moreover, the deposits show strong local variability. Owing to these uncertainties, the sequence stratigraphic approach focuses on detecting major transgression regression trends in this study, and should be considered as tentative (Fig. 3). RESULTS The deposits are divided into seven recurring facies and five subfacies, based on sedimentological and ichnological criteria. These are summarized below. Facies 1 (F1): Bioturbated Shale Description. Facies 1 consists of moderately to pervasively bioturbated (BI 3 6), dark to light gray silty shale. The facies is typically present at the bases of upward-coarsening sedimentary successions, and may gradationally underlie Facies 2A (F2A) or Facies 3A (F3A). Most commonly, it forms decimeter-scale successions. This facies can be subdivided into two subfacies types, based on ichnological criteria: Facies 1A (F1A) consists of a Chondrites-dominated fabric (Fig. 4A), whereas Facies 1B (F1B) is Planolites dominated (Fig. 4B, C). F1A: Chondrites-Dominated Shale F1A is typically present near the tops of major upward-fining intervals (i.e., within the finest-grain-size units of a succession) in the Abbay Lacadena area. It also develops as carbonaceous-detritus-bearing aggradational successions, especially near the top of the Alderson Member (Fig. 5). Subordinate ichnogenera include sporadically distributed Planolites, Phycosiphon, and Schaubcylindrichnus freyi. Trace fossils are diminutive, and are typically overprinted by the more abundant Chondrites. Chondrites are visible only on fresh, dry-cut core surfaces. Bioturbation intensities are very high (BI 5 6). r FIG. 2. An example well log of the Alderson Member and adjacent units. 2 nd WSS 2 nd White Speckled Shale.

4 806 J. HOVIKOSKI ET AL. JSR FIG. 3. Stratigraphy of the Alderson Member in Hatton and Abbay Lacadena areas. Main facies occurrences are shown. Well logs are gamma ray (GR), resistivity (Res) and neutron (N). Blue line flooding surface, Red line maximum flooding surface, Ca-calcium carbonate enrichment in matrix, Gl-glauconite. Interpretation. The pervasively burrowed composite ichnofabric and the fine-grained matrix of FA1 point to low-energy depositional conditions, and slow to moderate depositional rates. F1A occurs near the tops of major upward-fining successions, representing the end of transgression and deposition at the zone of maximum flooding below storm-weather wave base. Well-developed tiering, recorded by small deposit-feeding structures, may point to an abundance of food materials. Terrestrial organic debris likely needs to ferment, prior to its consumption by marine invertebrates; the exploitation of these resources via deeptier deposit-feeding structures is consistent with the presence of buried, bacterially mediated food. F1B: Planolites-Dominated Shale Subfacies F1B consists of Planolites-bearing organic-rich shale, which is locally present in the lower parts of upward-coarsening successions (Fig. 4B, C). Small amounts of sand may be present in the matrix. Planolites may bear a muddy mantle locally. Commonly, Planolites are reburrowed with diminutive grazing structures. Other ichnogenera present include very rare, diminutive Schaubcylindrichnus freyi, Chondrites, and Helminthopsis. Sedimentary accessories include pyrite in burrow infills. Bioturbation intensities are very high (BI 4 6). Interpretation. F1B represents the bases of upward-coarsening successions, thus marking the initial progradation of these successions. F1B contains increased organic-matter contents, variable bioturbation intensities, and, locally, deformed, mud-mantle-bearing burrows. Pyrite in the matrix and burrow infills supports lowered oxygen contents, reducing conditions in the sediment, and the presence of sulfate; these conditions require the presence of marine waters and concomitant accumulation of organic material. Considering that the overlying facies (F3A) bears evidence of wave- and tide-agitated sediment gravity flows (see below), F1B is interpreted as the distal prodeltaic portion of a subaqueous delta. Facies 2 (F2): Bioturbated Sandy Shale Facies 2 is a common and diverse facies that occurs throughout the studied intervals. It comprises dark to light gray, bioturbated sandy shale. The stratigraphic occurrence of the facies depends upon the subfacies (see below). Bentonite and carbonaceous detritus are present in the matrix in varying amounts. Facies 2 is subdivided into three subfacies, based on ichnological criteria: Facies 2A (F2A) is Phycosiphon-dominated; Facies 2B (F2B) bears a mixed-ethology trace-fossil assemblage; and Facies 2C (F2C) is

5 JSR ICHNOLOGY AND SEDIMENTOLOGY OF A MUD-DOMINATED DELTAIC COAST 807 FIG. 4. A) F1A. Diminutive Chondrites (e.g., white arrow)-dominated shale. Core 2, m. B, C) F1B. Planolites-bearing shale. Note the pyrite infill of some burrows. The trace fossils are commonly deformed and/or are reburrowed. Black arrow points to a cross section of a burrow. Core 6, 624 m. characterized by small-scale burrow mottling (Fig. 6). The two latter subfacies typically have higher interstitial sand contents. F2A: Phycosiphon-Dominated Sandy Shale Description. Facies 2A has a broad stratigraphic range. It is locally gradationally interbedded with Facies 1, 2B, and 3. Additionally, Facies 4 erosionally overlies the subfacies locally. Facies 2A forms decimeter- to meter-scale successions. It occurs most commonly in the lower parts of upward-coarsening successions, and consists of Phycosiphon-dominated, light gray, silty to sandy shale (Fig. 6A, B). Locally, F2A contains sporadically distributed, sharp-based, very fine- to fine-grained sand lenses. Typically, F2A has low proportions of organic matter and interstitial sand. Other ichnogenera present include small Helminthopsis, rare and diminutive Schaubcylindrichnus freyi, Zoophycos, Chondrites, and Planolites. The distinction between Helminthopsis and Phycosiphon is based on the presence of spreite in the light-colored, siltier material lying between the paired tubes of the Phycosiphon (cf. Wetzel and Bromley 1994) (Fig. 6A, B). Bioturbation intensities range from localized and limited to pervasive and intense (BI 2 6). Interpretation. Facies 2A is dominated by grazing behavior-dominated trace-fossil suites, and represents a distal expression of the Cruziana Ichnofacies (cf. MacEachern et al. 2007a, MacEachern et al. 2007b). In concert with the sharp-based, locally present sand lenses, these data indicate deposition near storm-weather wave base, likely within a proximal offshore to lower shoreface (above storm-weather wave base) setting. In earlier studies, Phycosiphon has been regarded as a shallow-tier (Wetzel and Uchman 2001), medium-depth (Ekdale and Bromley 1991), and a deep-tier trace fossil. The crosscutting relationships of the studied examples also suggest a broad tiering range. As observed by Wetzel and Uchman (2001) from the Eocene Beloveza Formation, the Alderson sediments also seems to display a general increase in tiering depth as the size of the structure grows larger. F2B: Mixed-Ethology Assemblage-Bearing Sandy Shale Description. Facies 2B is moderately common in the western part (Hatton) and the southern end of the study area, particularly in the middle and upper portions of the Alderson Member. It typically overlies F2A and occurs below Facies 2C (F2C) or Facies 3B (F3B). It consists of bioturbated, light to dark gray, (very fine to fine) sandy shale (Fig. 6B, C). F2B has high interstitial sand contents and is moderately to intensely burrowed (BI 2 6), with moderately diverse trace-fossil assemblages consisting of medium-sized Asterosoma, Schaubcylindrichnus coronus, Schaubcylindrichnus freyi, Scolicia (Laminites), Arenicolites, Thalassinoides, Planolites, Chondrites, Zoophycos, Phycosiphon, Helminthopsis, and comparatively robust fugichnia (Fig. 6B, C). Typically, several size classes of each ichnotaxon are present. F2B commonly has sharp-based sandstone interbeds and/or lenses, and thus it is intergradational with F3B and F4. Interpretation. The highest-diversity examples of F2B bear similarities with the archetypal Cruziana Ichnofacies. These occurrences show a mixed-ethology assemblage that comprises several suites (event, postevent, fair-weather), dominated by deposit-feeding and grazing behaviors, with subordinate escape and suspension-feeding behaviors. Such suites tend to occur within proximal offshore to distal lower-shoreface environments. Most commonly, however, F2B displays lower diversity,

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7 JSR ICHNOLOGY AND SEDIMENTOLOGY OF A MUD-DOMINATED DELTAIC COAST 809 has higher shale contents, and ichnological suites appear more stressed than those characteristic of the archetypal Cruziana Ichnofacies. Considering the presence of normal marine suites that contain trace fossils such as Schaubcylindrichnus, Scolicia, Asterosoma, Chondrites, Helminthopsis, and Phycosiphon, salinity fluctuation does not seem to be a plausible limiting factor. Instead, the physiological and colonization stresses may be related to suspended sediment input and elevated water turbidity (lack of elements of the Skolithos Ichnofacies; cf. Moslow and Pemberton 1988; MacEachern et al. 2005; MacEachern et al. 2007a). F2B forms a part of progradational offshore coastline successions (e.g., Fig. 7; interval m). Landward, it grades through burrowmottled sandy shale (Facies 2C) to root-bearing shale (Facies 5). This suggests that unlike that of a normal, wave-dominated offshore foreshore succession, wave energy becomes less prominent towards the foreshore environment in these successions. Thus, the occurrence of F2B most likely extends to shallower-water environments in the Alderson Member such as muddy lower shoreface and/or muddy shoreface. F2C: Burrow-Mottled Sandy Shale Description. Facies 2C preferentially occurs near the top of the Alderson Member in the southern part of the study area. It consists of decimeter-scale beds of light-dark gray sandy shale (Fig. 6E, F). Stratigraphically, it gradationally overlies F3 or F4. Upward, it grades into Facies 5 (F5; root-bearing shale) or is erosionally overlain by Facies 6 (F6; conglomerates). F2C is typically rich in organic matter. F2C is burrowed with a monospecific to low-diversity trace-fossil suite, consisting mostly of indistinct burrow mottling. Recognizable trace fossils include pervasive and diminutive Planolites. Subordinate elements include muddy mantle-bearing Thalassinoides, Palaeophycus, Teichichnus, Rhizocorallium, subvertical Diplocraterion, and small meniscae-bearing trace fossils (?Taenidium) (Fig. 6F). Grazing structures are locally present. Trace fossils are typically deformed. Bioturbation intensities are very high (BI 5 6). Rhizoliths commonly crosscut this ichnofabric. Interpretation. The low diversity of trace fossils, the small sizes of the ichnofossils, the dominance of morphologically simple deposit-feeding structures such as Planolites, and the persistent absence of more specialized feeding traces are typical features of stressed settings (e.g., MacEachern et al. 2007a). Facies 2C occurs below a root-bearing horizon (Facies 5). F2C F5 transitions represent a change from (shallow) subaqueous sediment accumulation to a subaerially exposed setting. The high bioturbation intensities and the lack of wave-generated sediment structures at the coastline may point to generally low-energy sediment accumulation. The local presence of Diplocraterion and equilibrium structures (Teichichnus), however, indicates that events of high-energy sediment accumulation also occurred. Deformed and muddy mantlebearing burrows indicate low substrate consistency. In view of the aforementioned sedimentary features and stratigraphic occurrence, F2C is interpreted to record episodic accumulation of mud and very fine sand on a dissipative shoreline (e.g., slingmud). Facies 3 (F3) Heterolithic Bedding Facies 3 is a commonly occurring facies throughout the study area. Laterally it can be followed up to tens of kilometers (Fig. 3). It typically is present in 4 8 m-thick successions. Facies 3 can be divided into two subfacies, based on lithology and the character of bioturbation: F3A is mud-dominated and bears unburrowed massive mudstone laminae; whereas F3B is sandy and typically bioturbated (Figs. 8, 9, 10). F3A: Muddy Heterolithic Bedding Description. Facies 3A consists of mud-dominated heterolithic bedding, predominantly characterized by interlaminated sand and shale (Fig. 8A F). The mud laminae commonly consist of 1 10 mm thick, massive to micro-laminated dark gray shale (Fig. 8C). Parallel-laminated muddy sandstone interbeds or mud-draped symmetric ripples are also present. Double mud drapes are observed locally. Unlike F3B, the tops of the sand units are normally unburrowed. Micro-faults and load structures are commonly present (Fig. 8A, B). The most mud-rich examples of F3A contain very low-diversity suites of sand-filled Planolites. The burrows are commonly deformed and exhibit a muddy mantle (Fig. 8D). Diminutive Chondrites, Teichichnus and Phycosiphon or Helminthopsis are locally present. Bioturbation intensities are variable but typically low (BI 0 3). Sandier examples of F3A contain a moderate- to low-diversity trace-fossil suite, including diminutive Phycosiphon, small Schaubcylindrichnus freyi, Thalassinoides, Arenicolites, and Helminthopsis. Thalassinoides and Arenicolites are sandfilled and are surrounded by a muddy mantle (Fig. 8F). Arenicolites are deeply penetrating (. 10 cm) and very irregular (Fig. 8F). Bioturbation intensities can reach up to 60% (BI 3 4). In core 17, an interval of interbedded root-bearing mudstone and bioturbated to heterolithic deposits is sharply overlain by an occurrence of ca. 10-cm-thick, evenly laminated, unidirectional cross-stratified lamina-set (, 10u) of mud and fine-grained sand (Fig. 11). This interval further grades upward into interbedded massive mud and interlaminated mud and sand. The interlaminated set displays opposing, low-angle inclinations (, 1u). Interpretation. The unbioturbated, massive to micro-laminated mudstone beds that are associated with soft-sediment deformation are best explained by periods of rapid deposition. The dominance of Planolitesdominated suites and the paucity of more specialized feeding traces, as well as structures attributed to suspension-feeding organisms, may point to physicochemical stresses attributable to elevated water turbidities and possibly lowered salinities (cf. MacEachern et al. 2005; MacEachern et al. 2007a). The deformed, mantle-bearing burrows and loading structures support high interstitial water contents in the sediments (e.g., Lobza and Schieber 1999; Schieber 2003). In concert, these data are consistent with accumulation of fluid mud. Considering the wide spatial distribution of the facies, and that it may grade upward into root-bearing mud, these strata are more likely related to a wave- and tide-agitated coastal mud wedge ( subaqueous delta ) than direct river-delta sediments (see Discussion). The evenly cross-laminated to interbedded massive mud and interlaminated mud and sand unit (Fig. 11) that overlies the mud-flat and coastal-marsh deposits is interpreted to represent a longshore barlike, sandy mudcape (see Discussion). F3B: Sand-Dominated Heterolithic Bedding Description. F3B is common in the Alderson Member, occurring near the top of upward-coarsening successions. It comprises sandier, bioturbated, lenticular bedding. Typically, the sandstone lithosomes consist of 1 5 cm-thick, normally graded, very fine- to fine-grained sand lenses or beds (Fig. 10A). The lower contacts of the sand layers are locally r FIG. 5. Core 17, Abbay Lacadena, lower middle Alderson Member. See Figure 7 for explanation of sedimentological and ichnological symbols.

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9 JSR ICHNOLOGY AND SEDIMENTOLOGY OF A MUD-DOMINATED DELTAIC COAST 811 erosional, whereas the upper margins are gradational, and bioturbated with robust Helminthopsis (or mud-filled?chondrites), causing a laminated to scrambled ( lam-scram ) fabric (Figs. 10B, 12) to be developed. Sandstone lenses are symmetric-ripple cross-laminated, or contain thin intervals of low-angle cross-stratification or heterolithic planar laminations (Fig. 10C). Locally, the cross-strata onlap the erosional lower contact (Fig. 10A). The interbedded mudstone lithosomes consist of alternating bioturbated, micro-laminated, and unburrowed massive sandy shale (Fig. 9C, D). In places, burrowed, shale-on-shale erosional contacts are apparent (Fig. 9B). The bioturbated intervals consist either of F2A or F2B (Fig. 10E). In order to distinguish F2A and F2B from F3B, a vertical distance of less than 15 cm between two successive sand lenses was employed in order to designate the unit as heterolithic. Interpretation. The erosionally based, homolithic sandstone lenses are best interpreted to record intervals of increased wave activity. This is supported by the presence of normal grading and a lam-scram fabric. Locally, strata onlap the lower erosional contacts of scour-and-fill structures, suggesting the presence of gutter casts (Fig. 10A). Actively deposited massive to microlaminated shale intervals point to a limited availability of sand. Finally, heterolithic planar lamination bears evidence of tidal influence, such as double mud drapes and cyclic thickening and thinning of sand mud couplets (Fig. 10C, D). The rhythmite series in Figure 10C is too short for statistical analysis. Facies 4 (F4): Low-Angle Cross-Stratified Sandstone Description. Facies 4 occurs near the tops of major upward-coarsening successions, especially in the Hatton Field and in southern Saskatchewan (Townships 1 4). It consists of low-angle cross-stratified, fine- to mediumgrained sandstone (Fig. 10F). The facies is typically cm thick, and locally sideritic (e.g., the Hatton Area). Typically, these intervals are unburrowed, but locally their tops are burrowed with low-density suites of vertically oriented, morphologically simple trace fossils (BI 0 2). Interpretation. As with F3B, low-angle cross-stratification in the sandstones is interpreted to record intervals of increased wave activity. The presence of siderite is most consistent with a paucity of ocean-derived sulfate, and can be associated with the presence of freshwater or postdepositional groundwater influx. Facies 4 is attributed to inshore locales, most commonly associated with the late phase of progradation (see Discussion). Facies 5 (F5): Root-Bearing Shale or Sand Description. Facies 5 is present in the middle and upper parts of the Alderson Member. It typically gradationally overlies F2C or F3A, and is gradationally overlain by Facies 6 (F6) or Facies 7 (F7). Facies 5 typically consists of successions, 10 cm to several meters thick, of root-bearing, gray, massive-appearing, bioturbated (BI 5 6) or heterolithically laminated to bedded (BI 1 3), rubbly shale (Fig. 13A C). Root-bearing intervals occur sporadically, and they are interbedded with intervals of bioturbated mud or heterolithic bedding. Thin sandy interlaminae, dark Fe-rich concretions, partially dissolved shell hash, articulated gastropods and bivalves, and abundant disseminated terrestrial organic matter as well as coal fragments are commonly intercalated. Pedogenic slickensides occur locally in thin intervals (Fig. 13C). Detailed ichnological and sedimentological observations are locally difficult to achieve, due to the poor preservation of the facies. Another expression of F5 is tan to reddish root-bearing sand. In those instances the ichnofabric is AMB ( adhesive meniscate burrow ) dominated (Fig. 13D). Interpretation. Roots and sporadic slickensides point to subaerial exposure and local incipient pedogenic alteration. The pedogenic features are interbedded with bioturbated mud, heterolithic bedding, or shell hash or shelly sandy mud, and are distributed throughout the succession, suggesting alternating periods of exposure and subaqueous deposition. Periodic subaerial exposure and several-meters-thick occurrences of this facies further attest to aggrading conditions and moderate sediment supply. F5 occurs near the tops of progradational coastal successions and develops most commonly in bioturbated mud (F2) and heterolithic bedding (F3A), further demonstrating that the shoreline was muddominated. The rare, tan to reddish sand-dominated occurrences of F5 with abundant AMB trace fossils are interpreted to represent well-drained paleosols. F5 is interpreted to have formed downdrift from a sediment point source(s), and is interpreted as an aggradational muddy coastal plain in chenier plain-like setting (see Discussion). Facies 6 (F6) Conglomerate and Massive Coarse-Grained Sand Description. Facies 6 occurs as discrete conglomerate or coarsegrained sand layers overlying sharp basal surfaces in the lower and upper part of the Alderson Member. Laterally, the facies can be followed at least, 20 km in the lower Alderson. It forms thin (centimeters-scale), clast- or matrix-supported extraformational conglomeratic layers of undefined composition (Fig. 14A). Glauconite is locally present in the sandy matrix. The matrix is commonly sideritic (e.g., the Hatton area). Clasts are moderately rounded to subangular, and are up to 10 mm in diameter. The lower contact of the facies is erosional with Facies 5 or F2C. The upper contact is gradational with F2. F6 is unburrowed. Interpretation. Facies 6 punctuates or terminates progradational successions, recording erosional truncation of the underlying, rootbearing shale, lagoonal, or deltaic shoreline deposits. The local presence of glauconitic sand, coupled with apparent deepening across the surface, indicate transgressive reworking. Correspondingly, some occurrences of the facies are interpreted to overlie a transgressive surface of erosion (TSE), probably generated by wave or tidal-scour ravinement (Fig. 7, 619 m). Facies 7 (F7): Glossifungites Ichnofacies-Bearing Sandy Shale Description. Facies 7 occurs west from 20W3, between townships 16 and 25 (Fig. 12, 419 m). Laterally it can be followed at least 20 km. It r FIG. 6. A, B) F2A. Phycosiphon-dominated sandy shale. Black arrows point to paired dark cores within silty mantle; White arrow spreite caused by a shift in burrow s position. Core 6, m and Core 1, m, respectively. C) Scolicia in bioturbated sandy shale (F2B). White arrows point to examples of meniscate backfill. Background bioturbation includes Chondrites, Helminthopsis, and Planolites. Core 16, m. D) Moderately diverse ichnological suite in sandy shale interbedded with low-angle cross-stratified sandstone (F2B and F3B). White arrow indicates Schaubcylindrichnus; black arrows point to Asterosoma. Background ichnofossils include Planolites, Schaubcylindrichnus freyi, and Phycosiphon. Core 16, m. E) Burrow-mottled sandy shale (F2C). Core 10, 291 m. F) Higher-diversity example of F2C. Deformed and mantled Diplocraterion crosscut burrow-mottled sandy shale. Recognizable (undeformed) spreite is preserved only in places (black arrows). Small Teichichnus (white arrow) is visible in the top of the photo. Core 19, 423 m.

10 812 J. HOVIKOSKI ET AL. JSR FIG. 7. Core 12, Southern Saskatchewan, upper Alderson.

11 JSR ICHNOLOGY AND SEDIMENTOLOGY OF A MUD-DOMINATED DELTAIC COAST 813 typically comprises bioturbated sandy shale containing robust, passively infilled palimpsest Thalassinoides (Fig. 14B). The burrow fill consists of glauconitic fine-grained sand. In some zones, the matrix is glauconitic as well. The burrow walls are commonly siderite cemented. The ichnofabrics that have been crosscut by the palimpsest suite correspond to F2. In places, the underlying facies also bears roots. Interpretation. F7 terminates a major progradational succession, and demarcates a transgressive surface of erosion (Fig. 3). The palimpsest trace-fossil suite is attributable to the Glossifungites Ichnofacies, characteristic of firmground conditions. DISCUSSION Distinction of Inner-Shelf Shale Units Current depositional models predict that open-coastal, distal (sea) proximal (land) gradients are reflected by upward-coarsening grain-size trends, and that shoreline facies are represented by prominent sand bodies. Although commonly the case, significant departures from this model may occur where the availability of sand-size sediments is limited. This is especially true where the alongshore-sediment-transport-influenced marine depositional system is sourced by rivers carrying a significant suspended-sediment load. As a result of grain-size segregation, dissipative, low-gradient shoreline and onshore processes, even opencoast coastlines, can become mud-dominated. Modern examples of such coasts include the Louisiana, Kerala, East China Sea, and Brazil Guayana coasts (e.g., Wells and Coleman 1981; Rine and Ginsburg 1985; Mallik et al. 1988; Allison and Nittrouer 1998; Neill and Allison 2005). In addition, mud-dominated shallow marine depositional systems commonly develop in coastal embayments and shallow epicontinental seas, such as the Gulf of Carpenteria and the Adriatic Sea (Rhodes 1982; Cattaneo et al. 2003). Distinction between shallow-water and deep-water shales may be problematic, owing to difficulties in discerning the depositional energy levels within sand-starved depositional systems (Schieber 1998a; Schieber et al. 2007). If the impact of limited grain-size availability is underestimated, shallow marine shale deposits can easily be misinterpreted as quiescent deeper marine sediments. This is unfortunate, because many shale units (e.g., gas shales) are economically important, and misinterpretations as to their depositional settings may lead to flawed interpretations of the facies architectures and geometries, consequently leading to errors in mapping and inaccurate reservoir calculations. Common features for modern mud-dominated coastal sedimentary systems and a requirement for their formation are the presence of a fine-grained riverine source(s) and along-coast dispersal of the sediment. Suitable river systems include complexes of minor rivers and/or large, suspension-load-dominated rivers that are associated with wide, lowgradient alluvial plains (e.g., Amazon, Mississippi Atchafalaya, Yangtze, Ganges, and Po rivers). Due to sediment concentration dilution (large water mass) and sediment trapping in alluvial plains, such rivers typically do not produce rapid seaward dispersal of sediments as hyperpycnites (Mulder et al. 2003). A requirement for long-distance alongshore sediment dispersal is turbid coastal waters that keep mud in a nearbottom, resuspended state, and prevent sea-bed consolidation (Geyer et al. 2004). Sufficient turbidity is typically reached by wave and tide agitation, which can be further enhanced by coastal winds, monsoons, cyclones, hurricanes, or storms (Geyer et al. 2004; Kineke et al. 2006). In particular, these episodic, high-energy events may be a significant driving force in nearshore to inshore mud accumulation (e.g., Bentley et al. 2003). Due to aforementioned depositional dynamics, there are several sedimentological and ichnological properties that are characteristic of mud-dominated coastlines. These include: (1) content of continentderived organic matter is high; (2) deposition rates are high and/or variable. Consequently, bioturbation intensities are low and/or fluctuating (e.g., Rine and Ginsburg 1985; Neill and Allison 2005); (3) substrate consistency is commonly soft and interstitial water contents are high. As a result, soft-sediment-deformation and fluid-mud intervals are common. Deformed, mantle-and-swirl trace fossils resulting from organisms moving or swimming through soupy sediment may occur (cf. Lobza and Schieber 1999; Schieber 2003); (4) turbid sedimentation leads to reduced or variable trace-fossil diversity. Morphologically simple trace fossils such as Planolites may be dominant locally, but are commonly interstratified at the facies level with specialized structures generated by organisms deemed intolerant of physicochemical stresses (MacEachern et al. 2005; MacEachern et al. 2007a); (5) event and post-event suites are impoverished due to heightened water turbidities and development of soupy substrates. Notably, the proportion of trace fossils attributable to suspension-feeding and/or filter-feeding behaviors may be strongly reduced (Moslow and Pemberton 1988; MacEachern et al. 2005); (6) the range and type of sedimentary structures may be restricted to various types of heterolithic bedding. If sand-size sediments are not available, active wave and/or tide agitation may cause micro-laminated shale, massive shale, weakly nonparallel-laminated shale, and shale-on-shale erosional contacts (Rine and Ginsburg 1985; Schieber 1993, 1998b; Neill and Allison 2005; Schieber et al. 2007); and (7) shallow-water shales can be enriched with shelly material (shell hash, articulated mollusks) as compared to the deeper-water shales (e.g., Neill and Allison 2005). Fluid-mud gravity flows can also occur in distal offshore and deeper marine environments, especially in energetic shelves (Wright and Friedrichs 2006) and submarine canyon systems seaward of small mountainous rivers that periodically achieve hyperpycnal states (cf. Mulder et al. 2003). In those cases, rapid seaward sediment dispersal or deep storm-weather wave base may allow clastic material to escape the reach of various mechanisms (e.g., Coriolis, coastal winds) that otherwise force most of the sediments to flow in alongshore directions (Nittrouer and Wright 1994; cf. Varban and Plint 2008). It has been suggested that wave-agitated, fluid-mud sedimentation in such settings is the main across-shelf sediment dispersal mechanism (Wright and Friedrichs 2006). Recently, distal (250 km from shore) mud accumulation has been documented from strongly storm-dominated, shallow and low-gradient shelf in the Cretaceous Kaskapau Formation (Varban and Plint 2008). Intuitively, deep-water mud accumulation could be distinguishable from coastal fluid muds by: (1) their more episodic nature of sedimentation; (2) the presence of hyperpycnites; (3) differences in vertical facies successions; (4) differences in the nature of post-event bioturbation; (5) lack of onshore tidal signatures; (6) lack of wavegenerated structures (i.e., owing to deposition below storm-weather wave base); and (7) facies architecture. Because they are commonly derived directly from a point source, deep-water fluid muds may be spatially more confined and prograde seaward (i.e., reflecting across-shelf vs. alongcoast sediment dispersal). Sedimentological and Ichnological Characteristics of Coastal Mud Accumulation in the Alderson Member In the Alderson Member, fluid-mud accumulation is commonly reflected by unburrowed massive or microstratified shale (lamina or bed scale), indications of both low substrate consistencies and high depositional rates (e.g., loading and fluid-escape structures) (Fig. 8). Characteristic ichnological properties attesting to these depositional affinities include reduced and/or fluctuating bioturbation intensities, reductions in trace-fossil sizes, and locally monospecific, Planolitesdominated suites (cf. MacEachern et al. 2005; MacEachern et al. 2007a). A very distinctive feature is also the common occurrence of deformed, mantled burrows indicating high interstitial water contents in the sediments (Figs. 6F, 8D, F). Finally, trace fossils typical of sandy high-

12 814 J. HOVIKOSKI ET AL. JSR

13 JSR ICHNOLOGY AND SEDIMENTOLOGY OF A MUD-DOMINATED DELTAIC COAST 815 FIG. 9. Thin sections of selected facies. A) Bioturbated sandy shale (F2A). Note the dark, hook-shaped grazing structures (black arrow). Core 10, m. B) Burrowed shale-on-shale erosional contact (dashed white line) (F3A). Core 10, m. C) Micro-laminated to massive-appearing shale (F3A). Micro-laminated interval is indicated by a white bar. Core 17, m. D) Massive sandy shale (F3B). White bar indicates the massive interval. Core 10, m. E) Lenticular bedding (F3B). Core 10, m. energy environments such as Diplocraterion occur locally in muddominated substrate in the Alderson (Fig. 6F). In addition to hyperpycnal fluid-mud accumulation, hypopycnal buoyant sedimentation appears to have been a widespread phenomenon during deposition of the Alderson Member. As a result, much of the depositional system was prone to both turbidity-induced stress and an abundance of terrestrially derived organic detritus. This causes a characteristic ichnofaunal composition: (1) deposit-feeding and grazing structures, such as Phycosiphon, Helminthopsis and Chondrites, are prolific, and occur as facies-crossing elements throughout the depositional system. Grazing structures are not confined only to the shallowest tiers, but also occupy mid-tier positions as well (. 5 cm depth in postevent suites). In examples where sedimentation rates were inferably low, the tiering structure developed further, and composite, diminutive Chondrites-dominated ichnofabrics were established; (2) complex feeding structures such as Zoophycos can be locally observed in facies reflecting shallow water depths, owing to the abundance of food resources and the availability of unexploited niches (subdued diversity). Consequently, ichnofossil suites attributable to distal expressions of the Cruziana Ichnofacies may occur in shallower water than would be expected otherwise. Such suites may be interbedded with Planolites-dominated fabrics, especially where fluid-mud facies are well developed. The presence of wave and onshore tidal indicators suggests a coastal affinity for portions of the studied strata. Wave influence is indicated by parallel-laminated scour-and-fill structures, combined-flow ripples, and thin intervals of low-angle cross-stratification. Moreover, thinsection analysis reveals that shale-dominated intervals also bear micro-laminae and shale-on-shale erosional contacts, suggesting active (bedload transport) mud deposition under wave and current influence. These data also suggest limited availability of sand. The distribution of wave-generated structures indicates that most of the cored intervals of the Alderson Member were deposited above storm-weather wave base. Finally, tidal influence in these strata is indicated by the presence of mud-draped ripple foresets and double mud drapes, and by tentatively identified rhythmic thickening and thinning of sand mud couplets. r FIG. 8. F3A. A) Interlaminated sand and shale. Arrow 1 points to deformed interlamination; arrow 2 indicates burrow mottling; arrow 3 highlights massiveappearing mud lamina/bed; arrow 4 indicates a deformed burrow. Core 17, m. B) A close-up of loading structures. Core 17, m. C) Close-up of interlaminated shale and sand. Arrows indicate apparently unburrowed mud laminae. Core 17, m. D) Deformed Planolites. Core 17, m. E) Lenticular bedding with minor bioturbation. Core 4, m. F) Bioturbated lenticular bedding (proximal F3A). White dashed line follows a deeply penetrating, irregular, muddy mantle-bearing Arenicolites. Arrow points to Thalassinoides. Core 1, m.

14 816 J. HOVIKOSKI ET AL. JSR FIG. 10. Facies 3B and 4. A) A scour-and-fill structure (F3B). Note that the strata onlap the lower contact. Core 4, m. B) A lam-scram unit (F4). The top of the sand unit is burrowed with large Helminthopsis or mud-filled Chondrites. Core 2, m. C) Heterolithic planar lamination. Core 2, m. Double mud drapes and cyclic thickening and thinning of sand mud couplets are present. D) Thickness variation of sand mud couplets of the previous photo. Note the rhythmic variations consistent with tidal variations. E) Robust fugichnia in bioturbated heterolithic bedding. Core 12, m. F) Sideritic, low-angle cross-stratification (F4). Core 1, 328 m. The Depositional System Offshore and Mud-Dominated Deltaic Coast The facies of the Alderson Member in the study area are interpreted to reflect: (1) subaqueous deltas ; and (2) successions consisting of offshore, muddy shoreface or tidal flat, and muddy coastal plain (or chenier plain). These are summarized below. Subaqueous Deltas We apply the term subaqueous delta in a broad sense to laterally extensive, upward-coarsening prodelta-like successions that cannot be followed to a fluvial point source (cf. Cattaneo et al. 2003). In the Alderson, such strata are widespread (facies can be followed several tens

15 JSR ICHNOLOGY AND SEDIMENTOLOGY OF A MUD-DOMINATED DELTAIC COAST 817 of kilometers; Fig. 3), occurring especially in Abbay Lacadena and south Saskatchewan (Lower Alderson; e.g., Core 10). Similar successions also occur in Hatton, but those intervals are more amalgamated and/or erosionally truncated, and fluid-mud intervals are typically less prominent. We apply term muddy shorefaces to these fluid-mud-influenced successions to differentiate them from fluid-mud-dominated subaqueous deltas (Figs. 15, 16). The subaqueous deltas also contain abundant evidence of wave and tide reworking, inasmuch as both tide- and wavegenerated structures can be encased in fluid-mud beds. Moreover, decimeter-scale hyperpycnite units attributable to river floods have not been recognized. Such successions also grade into root-bearing mud locally. In concert, these data suggest that the deposits likely represent alongshore- redistributed mud wedges rather than direct, seawardprograding river deltas. The width of the mud belt (at least some tens of kilometers) suggests that across-shelf oriented sediment transport also occurred, and was likely caused by storm scours. The across-shelf sediment transport component was less extreme than, e.g., in the Kaskapau Fm. (Varban and Plint 2008), apparently due to shallow wave base in the Alderson. Typical subaqueous deltaic successions are present in the Lower Middle Alderson (Abbey Lacadena area) and ideally consist of the following components (e.g., Fig. 5, m): 1. The succession may overlie fully bioturbated, diminutive Chondritesdominated shale (F1A; distal prodelta). The early stages of progradation are typically marked by increasing organic-matter contents and depositional rates (diminished tiering, fluctuating BI values). 2. Ichnologically, the deposits grade into shales or sandy shales with grazing-dominated suites or Planolites-dominated fabrics (F2A, F1B; low concentration [hypopycnal] and high concentration [hyperpycnal] prodelta, respectively). 3. Upward, bioturbation intensities progressively fluctuate, as unburrowed interlaminated shale and sand, and muddy parallellaminated sets (attributed to wave- and tide-agitated fluid-mud flows) appear (F3A; proximal prodelta distal delta front). 4. Farther upward, wave influence becomes increasingly apparent as scour-and-fill structures and thin intervals of low-angle cross stratification become more common. Typically, these strata show subdued ichnological signals and high shale contents in the fairweather deposits. These deposits are likely tidally influenced, as demonstrated by local double mud-drapes bearing combined-flow ripples and sedimentary rhythmites (F3B, F4; wave- and tideinfluenced delta front). The burrowed mud interlaminated mud and sand interbedded sand and mud succession is remarkably similar to the subaqueous delta successions in the Louisiana coast (Neill and Allison 2005). The wide geographical distribution of these successions in the Alderson may also resemble the deposition of subaqueous deltas of the Adriatic and Yellow seas, where alongshore migrating, continuous mud wedges develop near the coastlines (Cattaneo et al. 2003; Yang and Liu 2007). The locus of mud accumulation in these systems lies in slightly deeper water (. 20 m) than in the Louisiana coast. Shore Margin and Coastal Plain The interpreted shore-margin strata are variable, and developed differently in various parts of the Alderson. In the upper Alderson r FIG. 11. Evenly cross-laminated set of mud and silty sand that grade upward into interbedded massive and interlaminated sand and mud. White arrow point to cross-lamination. See text for discussion.

16 818 J. HOVIKOSKI ET AL. JSR

17 JSR ICHNOLOGY AND SEDIMENTOLOGY OF A MUD-DOMINATED DELTAIC COAST 819 FIG. 13. A) Close-up of a subvertical, bifurcating root (F5). Note the variable thickness and the oxidized halo around the root. Core 12, m. B) A vertical root in rubbly, organic-rich shale (F5). Core 8, m. C) Pedogenic slickensides (F5). Core 17, m. C) Paleosol with abundant AMB trace fossils (F5). Core 25. r FIG. 12. Core 2, Hatton, lower to middle Alderson.

18 820 J. HOVIKOSKI ET AL. JSR FIG. 14. A) A conglomerate-mantled surface (F6). Core 2, m. B) Thalassinoides with glauconitic infill, corresponding to the firmground Glossifungites Ichnofacies (F7). Core 2, 418 m. (especially Townships 1 4), the deposits comprise 6 12-m-thick successions that consist ideally of organic-rich, bioturbated shale (F1 and F2 muddy offshore to offshore transition), heterolithic bedding (F2B, F3B, and F4 muddy shoreface or mud flat) and root-bearing sandy shale (F2C and F5 muddy coastal plain). In particular, the shoreface successions demonstrate considerable variability in ichnofossil content and bioturbation intensity, degree of wave influence, and sand/mud ratios. The most mud-prone successions demonstrate decreasing wave energy toward the foreshore, and symmetric in terms of grain size (mud heterolithic sand and mud mud) offshore foreshore or tidal flat successions suggesting a low-gradient, dissipative shoreline. This trend is also observable in successions that display little evidence for tidal dominance. The sandiest examples, on the other hand, form nearly normal asymmetric, upwardcoarsening offshore to foreshore successions. These sediment series demonstrate the highest trace-fossil diversities, and the largest variability in ethological strategies and in range in size of the trace fossils. Trace fossils such as Schaubcylindrichnus, Scolicia (Laminites), and Asterosoma are present in these strata but are typically missing from loci of mud accumulation. Moreover, the event suites are also better developed, and bear dwelling structures such as Arenicolites and robust fugichnia. These data are in line with observations from modern mud-dominated coastlines, where wave dissipation is strongly controlled by the presence of fluid mud (e.g., slingmud) (e.g., Augustinus 1980; Wells and Coleman 1981; Rine and Ginsburg 1985; Mallik et al. 1988; Huh et al. 2001; Bentley et al. 2003). The coastal areas where fluid muds are absent (e.g., inter-mudbank areas) display more wave energy reaching the shore, and coarser-grained shoreline facies develop as a result (e.g., Dolique and Anthony 2005). A more than 10-m-thick shore-margin facies succession (Fig. 5; m) occurs in the mid Alderson (Abbay Lacadena). These strata include root-bearing bioturbated mud (F2) and heterolithic bedding (F3A) that are intercalated with similar but non-root-bearing strata (subtidal to intertidal flat marsh alternations). Locally, shelly intervals, which occur as thin, partially dissolved shell-hash laminae or as shellhash-bearing sandy shale are present. In places, articulated gastropods and coal fragments also occur. These strata are also characteristically rich in bedding-plane-oriented terrestrial organic debris. Furthermore, evenly cross-laminated heterolithic sets (, 10u) that grade into interbedded massive mud and interlaminated sand and mud (Fig. 11) are distinctive elements of the facies. Interlaminae display gently (, 1u) opposing directions of inclination. This bedform is interpreted to represent migration of a shallow-water sandy mud bank, similar in character to washover deposits. In particular, the massive mud and interlaminated mud (with discontinuities) are reportedly distinctive features of the FIG. 15. Interpreted distribution of facies along a mud-dominated deltaic coast. SWWB, storm-weather wave base; FWWB, fair-weather wave base.

19 JSR ICHNOLOGY AND SEDIMENTOLOGY OF A MUD-DOMINATED DELTAIC COAST 821 migrating mud banks of the Brazil Guyana Coast (Rine and Ginsburg 1985). Clinoforms (foresets) are also reported from these banks, but typically they are only acoustically visible, due to their uniform grain sizes. In concert, this Alderson shoreline complex is interpreted to bear similarities with the chenier-plain coastlines. In general, chenier plains constitute shore-parallel, fine-grained environments lying down-drift of deltas, which comprise transgressive beach ridges (i.e., cheniers) that are separated by prograding silty or clayey deposits (e.g., Penland and Suter 1989). The cheniers typically consist of sandy or shelly material that accumulate in response to spring tides, storms, or other autocyclic processes that activate oblique landward migration of coarser-grained material making up the longshore bars (Augustinus 1980, 1989; Rhodes 1982). The interbedded, progradational strata typically comprise subtidal and intertidal muds or marsh deposits. A critical feature for chenier development is the availability of sand or shelly material. If availability of sand or shells is low, winnowing and progressive sorting result in thin lenses of coarse debris only, which is insufficient for chenier development (e.g., Augustinus 1989; Wang and Ke 1989). The Alderson succession is similar to chenier plains in the sense that it comprises a fine-grained, downdrift shoreline complex associated with deltaic mud wedge, and displays evidence of alternating phases of progradation (e.g., marsh deposits) and transgression (e.g., subtidal mud deposits). No definite cheniers, however, have been identified in these strata so far. A characteristic feature of many cheniers (e.g., Surinam coast) are obliquely landward-dipping washover foresets that are overlain by gently (, 1u) inclined backslope lamination developed on sandy or shelly alongshore bars (Augustinus 1980). In that context, the aforementioned migrating sandy mud bank or mud cape that is intercalated with bioturbated sandy shale (mud shoal) records a depositional process similar to that of developing cheniers. It is possible that the availability of coarse-grained material (sand, shells) was not sufficient in the Alderson coastal regime for true chenier development. In that case, the deposits could resemble the Amapa Coast of Brazil, where limited availability of coarse-grained material, coupled with wide tidal mud flats, prevent chenier development (Allison and Nittrouer 1998). Alternatively, considering that there is currently only one core that covers this interval in its entirety, it is possible that the true cheniers have been overlooked,to date. Future work on the Alderson should include delineation of the morphology of the sedimentary units, reconstruction of the paleogeography of the area, and assessment of permeability distribution in these strata. We speculate that the coastal mud wedge may have prograded from NW to SE in the gas-prolific, northernmost part (Abbay) of the study area, and that detailed well-log correlations may reveal subtle, lowrelief clinoforms oriented perpendicular to this direction of progradation (Fig. 17). If that is true, better quality reservoir sands may be situated farther north from the Abbay Lacadena area (cf. Bhattacharya and Giosan 2003). CONCLUSION The Upper Cretaceous Alderson Member (Lea Park Formation) of western Canada is a gas-charged shale unit, up to 180 m thick, that we interpret as an offshore to mud-dominated deltaic coastline succession. Sedimentological and ichnological data suggest that much of the sediment volume of the Alderson Member was deposited energetically in coastal environments as fluid-mud deposits. Diagnostic features reflecting these depositional affinities include deformed, muddy mantle-bearing burrows, soft-sediment-induced loading, fluid-escape structures, unburrowed massive to micro-stratified shale laminae, and shale-on-shale erosional contacts. Moreover, the relatively shallow-water conditions in certain Alderson Member stratigraphic levels are also demonstrated by wave reworking (scour-and-fill structures, low-angle cross stratification), onshore tidal sediments (semidiurnal tides with slack-water intervals), Glossifungites Ichnofacies-demarcated transgressive surfaces of erosion, conglomerate-mantled erosional surfaces, increased amounts of terrestrial organic matter, and root-bearing horizons. Commonly, discernible wave energy decreased toward foreshore environments in shale-dominated (fluid-mud) successions. The Alderson strata (e.g., Abbay Lacadena Pool) are typically organized in to 3 8-m-thick, upward-coarsening units that consist of bioturbated (sandy) shale, interlaminated sand and mud, and lenticularbedded successions. At particular stratigraphic levels, similar strata are overlain by a complex association of root-bearing sandy mud interbedded with thin intervals of partially dissolved shell hash and interlaminated sand. These successions bear important similarities with the subaqueous delta mud flat open coast marsh (or chenier plain) successions of the modern Louisiana coast (e.g., Neill and Allison 2005). Given the wide geographical distribution of the fluid-mud deposits in the Alderson Member, abundant evidence for wave and tide agitation, and general paucity of hyperpycnites assignable to river floods, these deposits were likely redistributed alongshore by basinal processes (cf. Cattaneo et al. 2003). Consequently, for the most part, the Alderson Member is interpreted to represent a fossilized expression of a deltaic, muddominated shallow marine shale unit, similar to the modern Brazil Guyana, and Louisiana coasts (cf. Rine and Ginsburg 1985; Neill and Allison 2005). The coastal inner-shelf portions of the Alderson Member were shaped by several distinct physicochemical stress factors that controlled the resulting ichnofaunal composition of these strata. Such properties may help to distinguish similar strata elsewhere. Characteristic features include: (1) reduced and generally fluctuating bioturbation intensities; (2) decreases in trace-fossil diversities; (3) decreases in ichnogenera sizes; (4) introduction of monospecific Planolites-dominated suites in fluid mud facies; (5) deformed, muddy mantle-bearing burrows of sedimentswimming organisms (cf. Lobza and Schieber 1999; Schieber 2003). In areas of hypopycnal, turbulent accumulation, deposit-feeding and grazing structures such as Phycosiphon, Helminthopsis, and Chondrites are particularly widespread and occur as facies-crossing elements. The results of this study support some recent findings from modern environments and flume experiments, which have highlighted largely overlooked complexities in shale deposition, and demonstrate that much shale accumulation may actually occur in coastal energetic settings (e.g., Nittrouer and Wright 1994; Geyer et al. 2004; Dalrymple and Cummings 2005; Schieber et al. 2007). Indeed, there is a great need to develop facies models of shallow-marine depositional systems that address sand-starved, mud-dominated coastlines. Re-evaluation of the depositional history of many shale units is also particularly timely, since gas-shale systems such as the Alderson Member are emerging as one of the most important new hydrocarbon plays in North America. Understanding facies variability and depositional setting of mud-prone systems is critical in order to predict the permeability variations in such hydrocarbon reservoirs. Accordingly, misinterpretations in depositional style of shale units (e.g., along-shore vs. across-shelf progradation) may lead to flawed interpretations of the facies architectures and geometries, consequently leading to errors in their mapping and inaccurate reservoir calculations. ACKNOWLEDGMENTS We are grateful for Melinda Yurkowski and Chris Gilboy from the Subsurface Geological Laboratory of Regina for their kind cooperation. Mark Labbe and Don Resultay are thanked for preparing the thin sections. Finally, we would like to express our gratitude to reviewers Boyan Vakarelov and Sam Bentley, Associate Editor Janok Bhattacharya, and Editor Colin P. North for their constructive comments that considerably improved the paper.

20 822 J. HOVIKOSKI ET AL. JSR