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Journal of Sedimentary Research, 2008, v. 78, 803–824

Research Article

DOI: 10.2110/jsr.2008.089

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

JUSSI HOVIKOSKI,1 RYAN LEMISKI,1 MURRAY GINGRAS,1 GEORGE PEMBERTON,1 AND JAMES A. MACEACHERN2

1Ichnology Research Group, Department of Earth and Atmospheric Sciences, University of Alberta, 1-26 Earth Science Building, Edmonton, AB, T6G 2E3, Canada 2Department of Earth Sciences, Simon Fraser University, Burnaby, British Columbia, V5A 1S6, Canada

e-mail: [email protected]

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 sandbodies. Although commonly the case, significant departures from this model may occur when the availability of coarser sedimentcalibers (sand-sized and larger) is limited. This is especially true where alongshore sediment-transport-influenced depositionalsystems are associated with rivers that supply abundant suspended sediments. Underestimating the role of grain-size segregationmay 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 asan ancient analogue for modern offshore and mud-dominated deltaic coasts. Sedimentological and ichnological data collectedfrom 27 cores indicate that much of the sediment volume of the Alderson Member was deposited in relatively shallow waterunder the influence of tidal and wave processes in a deltaic coastal setting. Characteristic features reflecting these depositionalaffinities include: (1) increased proportions of terrestrially derived organic matter; (2) indications of thixotropic to soupysubstrates (e.g., fluid mud) coupled with rapid depositional rates; (3) an impoverished ichnological signal (Planolites-dominatedsuites); (4) micro-laminated shale; (5) shale-on-shale erosional contacts; (6) scour-and-fill structures; and (7) intervals of low-angle cross-stratification. The interpretation of relatively shallow-water settings is also supported by recurring root-bearinghorizons, Glossifungites Ichnofacies-demarcated transgressive surfaces of erosion, and conglomeratic surfaces at particularstratigraphic 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 interdeltaicmuddy shorefaces, tidal flats, or down-drift deltaic environments (e.g.,chenier plains) that are sourced by wave- and/or tide-agitated, hyperpyc-

nal and hypopycnal mud plumes. Compared to their sand-dominatedcounterparts, mud-dominated coastlines are sedimentologically and

ichnologically poorly understood. As a result, their recognition in thegeological 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 foran exception) (Beall 1968; Walker and Harms 1971; Rhodes 1982; Rineand Ginsburg 1985; Mallik et al. 1988; Augustinus 1989; Allison andNittrouer 1998; Bentley et al. 2003; Neill and Allison 2005).

Recently, it has been suggested that a parsimonious locale for mudaccumulation is actually the coastal zone, and that only a part of mudsupply ‘‘escapes’’ to deeper-water environments in low-gradient and low-energy settings. This view is based on the observation that fewsedimentary processes account for the transportation of fine-grainedsediment 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 non-

flocculated (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 coastalshale 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 interpre-

tations 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) 1527-1404/08/078-765/$03.00



presence of large quantities of high-concentration mud suspensions,coupled with a low-gradient, dissipative shoreline, effectively suppresseswave 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 suspensionplumes or fluid muds may be rich in swelling clays (e.g., Surinam Coast),making detailed study of analogous unlithified strata in the geologicalrecord challenging. In addition, shale-on-shale erosional contacts, such as

those produced by storm waves, can be subtle and potentially difficult torecognize, especially in core (cf. Schieber 1998b, 2003). Finally, bedformdevelopment in muds is more complex than that of sands, in that it isstrongly influenced by interparticle cohesion, sediment–water content andprimary productivity (e.g., Schieber et al. 2007). Even in high-energymuddy coastal settings, the resulting sedimentary structures may consistonly of massive mud, micro-cross-lamination, parallel- to weakly

nonparallel-laminated mud, lenticular bedding, or small-scale scour-and-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, deeper-water sedimentation, if the proportion of sand-size calibers is limited.

A key to differentiating between ‘‘deeper water’’ (below storm-weatherwavebase) sedimentation and coastal mud deposition lies in therecognition of various sedimentary processes—inshore tidal flux, fair-weather wave agitation, and deltaic input. As a result of the interaction of these factors, muddy coastal zones can be subject to: (1) rapid mudemplacement; (2) high depositional turbulence; (3) fluid-rich sediments;(4) shifting, ductile substrates; and (5) possibly low or fluctuatingsalinities. Therefore, these environments are ecologically challenging,imposing a variety of stresses on burrowing infauna, and making thempotentially 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 sedimento-

logical characteristics of a shale unit that we interpret to represent offshoreand 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 sandyshales. Traditionally regarded as a shelfal unit deposited hundreds of kilometers seaward from the coeval shoreline (e.g., Meijer-Drees and Myhr1981), the Alderson Member has recently been reinterpreted to displaydeltaic influence (O’Connell 2003; Pedersen 2003). In this study, wedemonstrate 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 forthe 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 Saskatchewanof western Canada (Figs. 1, 2). Together with the co-mingling of

Medicine Hat and Second White Specks intervals, these shales host thelargest gas reservoir in the Western Canadian Sedimentary Basin (WCSB)(O’Connell 2003). This informal unit has traditionally been referred to asthe Milk River Formation or the Milk River Equivalent in the earlierliterature (e.g., Meijer-Drees and Myhr 1981; Gatenby and Staniland2004). 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–

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.

FIG. 1.— The study area with major known gas pools shown. Black lineindicates the correlation transect of Figure 3. The studied cores are: (1) 11-24-21-19W3, (2) 6-22-17-28W3, (3) 6-5-18-26W3, (4) 14-17-25-16W3, (5) 14-36-26-29W3,(6) 4-19-15-14W3, (7) 16-31-20-20W3, (8) 6-27-1-23W3, (9) 6-4-4-28W3, (10) 14-09-1-12W3, (11) 8-10-3-28W3, (12) 10-32-1-24W3, (13) 6-9-12-26W3, (14) 14-30-1-12W3, (15) 14-35-21-18W3, (16) 7-11-1-27W3, (17) 16-3-22-18W3, (18) 6-12-22-22W3, (19) 10-36-19-23W3, (20) 06-32-17-27W3, (21) 08-15-16-28W3, (22) 10-27-15-22W3, (23) 16-34-13-28W3, (24) 11-08-18-29W3, (25) 16-24-22-26W3, (26) 14-29-14-29W3, (27) 08-25-15-27W3.

804 J. HOVIKOSKI ET AL. J S R



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.). Ichno-

logical data comprises description of the ichnogenera and/or ichnospecies,

bioturbation index (BI of Taylor and Goldring 1993), crosscuttingrelationships, 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 summa-

rized below.

Facies 1 (F1): Bioturbated Shale

Description.— Facies 1 consists of moderately to pervasively biotur-

bated (BI 3–6), dark to light gray silty shale. The facies is typically presentat 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 distrib-uted 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. 2nd

WSS–2nd White Speckled Shale.

ICHNOLOGY AND SEDIMENTOLOGY OF A MUD-DOMINATED DELTAIC COAST 805J S R



Interpretation.— The pervasively burrowed composite ichnofabric andthe fine-grained matrix of FA1 point to low-energy depositionalconditions, and slow to moderate depositional rates. F1A occurs nearthe tops of major upward-fining successions, representing the end of

transgression and deposition at the zone of maximum flooding belowstorm-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 consump-tion by marine invertebrates; the exploitation of these resources via deep-tier 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, whichis 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 arereburrowed with diminutive grazing structures. Other ichnogenerapresent include very rare, diminutive Schaubcylindrichnus freyi , Chon-drites, and Helminthopsis. Sedimentary accessories include pyrite in

burrow infills. Bioturbation intensities are very high (BI 4–6).

Interpretation.— F1B represents the bases of upward-coarseningsuccessions, thus marking the initial progradation of these successions.F1B contains increased organic-matter contents, variable bioturbationintensities, and, locally, deformed, mud-mantle-bearing burrows. Pyritein the matrix and burrow infills supports lowered oxygen contents,reducing conditions in the sediment, and the presence of sulfate; theseconditions 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 thestudied intervals. It comprises dark to light gray, bioturbated sandy shale.The stratigraphic occurrence of the facies depends upon the subfacies (seebelow). Bentonite and carbonaceous detritus are present in the matrix invarying amounts.

Facies 2 is subdivided into three subfacies, based on ichnologicalcriteria: Facies 2A (F2A) is Phycosiphon-dominated; Facies 2B (F2B)

bears a mixed-ethology trace-fossil assemblage; and Facies 2C (F2C) is

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.




characterized by small-scale burrow mottling (Fig. 6). The two lattersubfacies typically have higher interstitial sand contents.

F2A: Phycosiphon-Dominated Sandy Shale

Description.— Facies 2A has a broad stratigraphic range. It is locallygradationally interbedded with Facies 1, 2B, and 3. Additionally, Facies 4

erosionally overlies the subfacies locally. Facies 2A forms decimeter- tometer-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 containssporadically distributed, sharp-based, very fine- to fine-grained sandlenses. Typically, F2A has low proportions of organic matter andinterstitial sand. Other ichnogenera present include small Helminthopsis,rare and diminutive Schaubcylindrichnus freyi , Zoophycos, Chondrites,and Planolites. The distinction between Helminthopsis and Phycosiphon isbased on the presence of spreite in the light-colored, siltier material lyingbetween the paired tubes of the Phycosiphon (cf. Wetzel and Bromley1994) (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-dominat-ed trace-fossil suites, and represents a distal expression of the Cruziana

Ichnofacies (cf. MacEachern et al. 2007a, MacEachern et al. 2007b). Inconcert with the sharp-based, locally present sand lenses, these dataindicate deposition near storm-weather wave base, likely within aproximal 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 studiedexamples also suggest a broad tiering range. As observed by Wetzel andUchman (2001) from the Eocene Beloveza Formation, the Aldersonsediments 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 themiddle and upper portions of the Alderson Member. It typically overliesF2A 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 assemblagesconsisting of medium-sized Asterosoma, Schaubcylindrichnus coronus,Schaubcylindrichnus freyi , Scolicia (Laminites), Arenicolites, Thalassi-noides, Planolites, Chondrites, Zoophycos, Phycosiphon, Helminthopsis,and comparatively robust fugichnia (Fig. 6B, C). Typically, several sizeclasses of each ichnotaxon are present. F2B commonly has sharp-basedsandstone interbeds and/or lenses, and thus it is intergradational withF3B and F4.

Interpretation.— The highest-diversity examples of F2B bear similaritieswith the archetypal Cruziana Ichnofacies. These occurrences show amixed-ethology assemblage that comprises several suites (event, post-event, fair-weather), dominated by deposit-feeding and grazing behaviors,with subordinate escape and suspension-feeding behaviors. Such suitestend to occur within proximal offshore to distal lower-shorefaceenvironments. Most commonly, however, F2B displays lower diversity,

FIG. 4.— A) F1A. Diminutive Chondrites (e.g., white arrow)-dominated shale. Core 2, 472.5 m. B , C) F1B. Planolites-bearing shale. Note the pyrite infill of someburrows. The trace fossils are commonly deformed and/or are reburrowed. Black arrow points to a cross section of a burrow. Core 6, 624 m.







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 619–613 m). Landward, it grades through burrow-

mottled 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, Rhizo-

corallium, 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-feedingstructures 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 mantle-

bearing 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-laminatedmuddy 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 sand-

filled and are surrounded by a muddy mantle (Fig. 8F). Arenicolites are

deeply penetrating (. 10 cm) and very irregular (Fig. 8F). Bioturbationintensities 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 mud-

stone beds that are associated with soft-sediment deformation are best

explained by periods of rapid deposition. The dominance of Planolites-

dominated suites and the paucity of more specialized feeding traces, as

well as structures attributed to suspension-feeding organisms, may pointto 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 bar-

like, 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.







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, orcontain thin intervals of low-angle cross-stratification or heterolithic

planar laminations (Fig. 10C). Locally, the cross-strata onlap theerosional lower contact (Fig. 10A).

The interbedded mudstone lithosomes consist of alternating biotur-

bated, 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 verticaldistance 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 issupported 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 andthinning 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-coarseningsuccessions, especially in the Hatton Field and in southern Saskatchewan

(Townships 1–4). It consists of low-angle cross-stratified, fine- to medium-grained sandstone (Fig. 10F). The facies is typically 10–20 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 thesandstones 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 inshorelocales, 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 isgradationally 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 lami-

nated to bedded (BI 1–3), rubbly shale (Fig. 13A–C). Root-bearingintervals 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 thepoor 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 subaerialexposure 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 thisfacies 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 mud-

dominated. The rare, tan to reddish sand-dominated occurrences of F5

with abundant AMB trace fossils are interpreted to represent well-drainedpaleosols. 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 coarse-

grained sand layers overlying sharp basal surfaces in the lower and upperpart 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 progradationalsuccessions, recording erosional truncation of the underlying, root-

bearing 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 16and 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 inburrow’s position. Core 6, 643.30 m and Core 1, 295.90 m, respectively. C) Scolicia in bioturbated sandy shale (F2B). White arrows point to examples of meniscatebackfill. Background bioturbation includes Chondrites, Helminthopsis, and Planolites. Core 16, 315.90 m. D) Moderately diverse ichnological suite in sandy shaleinterbedded with low-angle cross-stratified sandstone (F2B and F3B). White arrow indicates Schaubcylindrichnus; black arrows point to Asterosoma. Backgroundichnofossils include Planolites, Schaubcylindrichnus freyi , and Phycosiphon. Core 16, 318.16 m. E) Burrow-mottled sandy shale (F2C). Core 10, 291 m. F) Higher-diversityexample of F2C. Deformed and mantled Diplocraterion crosscut burrow-mottled sandy shale. Recognizable (undeformed) spreite is preserved only in places (blackarrows). Small Teichichnus (white arrow) is visible in the top of the photo. Core 19, 423 m.




FIG. 7.—Core 12, Southern Saskatchewan,upper Alderson.




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 aswell. The burrow walls are commonly siderite cemented. The ichnofabricsthat have been crosscut by the palimpsest suite correspond to F2. Inplaces, the underlying facies also bears roots.

Interpretation.— F7 terminates a major progradational succession, and

demarcates a transgressive surface of erosion (Fig. 3). The palimpsesttrace-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-sizetrends, and that shoreline facies are represented by prominent sand

bodies. Although commonly the case, significant departures from thismodel may occur where the availability of sand-size sediments is limited.This is especially true where the alongshore-sediment-transport-influ-enced 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 open-coast coastlines, can become mud-dominated. Modern examples of suchcoasts 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). Inaddition, mud-dominated shallow marine depositional systems common-ly develop in coastal embayments and shallow epicontinental seas, such as

the Gulf of Carpenteria and the Adriatic Sea (Rhodes 1982; Cattaneo etal. 2003).

Distinction between shallow-water and deep-water shales may beproblematic, owing to difficulties in discerning the depositional energylevels within sand-starved depositional systems (Schieber 1998a; Schieberet al. 2007). If the impact of limited grain-size availability isunderestimated, shallow marine shale deposits can easily be misinter-

preted 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, consequentlyleading to errors in mapping and inaccurate reservoir calculations.

Common features for modern mud-dominated coastal sedimentarysystems—and a requirement for their formation—are the presence of afine-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, low-gradient alluvial plains (e.g., Amazon, Mississippi–Atchafalaya, Yangtze,Ganges, and Po rivers). Due to sediment concentration dilution (largewater mass) and sediment trapping in alluvial plains, such rivers typicallydo not produce rapid seaward dispersal of sediments as hyperpycnites

(Mulder et al. 2003). A requirement for long-distance alongshoresediment dispersal is turbid coastal waters that keep mud in a near-

bottom, resuspended state, and prevent sea-bed consolidation (Geyer etal. 2004). Sufficient turbidity is typically reached by wave and tideagitation, which can be further enhanced by coastal winds, monsoons,cyclones, hurricanes, or storms (Geyer et al. 2004; Kineke et al. 2006). Inparticular, these episodic, high-energy events may be a significant drivingforce in nearshore to inshore mud accumulation (e.g., Bentley et al. 2003).

Due to aforementioned depositional dynamics, there are severalsedimentological and ichnological properties that are characteristic of mud-dominated coastlines. These include: (1) content of continent-derived organic matter is high; (2) deposition rates are high and/or

variable. Consequently, bioturbation intensities are low and/or fluctuat-

ing (e.g., Rine and Ginsburg 1985; Neill and Allison 2005); (3) substrate

consistency is commonly soft and interstitial water contents are high. As aresult, 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 andSchieber 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 atthe facies level with specialized structures generated by organisms deemedintolerant of physicochemical stresses (MacEachern et al. 2005; MacEa-

chern 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 (Rineand 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 thedeeper-water shales (e.g., Neill and Allison 2005).

Fluid-mud gravity flows can also occur in distal offshore and deepermarine 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 ordeep 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 (Nittrouerand 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 beendocumented 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 wave-generated 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 moreconfined and prograde seaward (i.e., reflecting across-shelf vs. along-

coast sediment dispersal).

Sedimentological and Ichnological Characteristics of Coastal Mud

Accumulation in the Alderson Member

In the Alderson Member, fluid-mud accumulation is commonlyreflected 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, Planolites-

dominated 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 thesediments (Figs. 6F, 8D, F). Finally, trace fossils typical of sandy high-







energy environments such as Diplocraterion occur locally in mud-dominated substrate in the Alderson (Fig. 6F).

In addition to hyperpycnal fluid-mud accumulation, hypopycnalbuoyant sedimentation appears to have been a widespread phenomenonduring deposition of the Alderson Member. As a result, much of thedepositional system was prone to both turbidity-induced stress and anabundance of terrestrially derived organic detritus. This causes acharacteristic ichnofaunal composition: (1) deposit-feeding and grazingstructures, such as Phycosiphon, Helminthopsis and Chondrites, areprolific, and occur as facies-crossing elements throughout the deposi-tional system. Grazing structures are not confined only to the shallowesttiers, but also occupy mid-tier positions as well (. 5 cm depth in post-event suites). In examples where sedimentation rates were inferably low,

the tiering structure developed further, and composite, diminutive

Chondrites-dominated ichnofabrics were established; (2) complex feedingstructures such as Zoophycos can be locally observed in facies reflectingshallow water depths, owing to the abundance of food resources and theavailability of unexploited niches (subdued diversity). Consequently,

ichnofossil suites attributable to distal expressions of the Cruziana

Ichnofacies may occur in shallower water than would be expectedotherwise. Such suites may be interbedded with Planolites-dominatedfabrics, especially where fluid-mud facies are well developed.

The presence of wave and onshore tidal indicators suggests a coastalaffinity for portions of the studied strata. Wave influence is indicatedby parallel-laminated scour-and-fill structures, combined-flow ripples,and thin intervals of low-angle cross-stratification. Moreover, thin-section analysis reveals that shale-dominated intervals also bearmicro-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 distributionof 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 presenceof mud-draped ripple foresets and double mud drapes, and bytentatively identified rhythmic thickening and thinning of sand–mudcouplets.

r

FIG. 8.— F3A. A) Interlaminated sand and shale. Arrow 1 points to deformed interlamination; arrow 2 indicates burrow mottling; arrow 3 highlights massive-appearing mud lamina/bed; arrow 4 indicates a deformed burrow. Core 17, 350.68 m. B) A close-up of loading structures. Core 17, 331.27 m. C) Close-up of interlaminated shale and sand. Arrows indicate apparently unburrowed mud laminae. Core 17, 350.27 m. D) Deformed Planolites. Core 17, 348.46 m. E) Lenticularbedding with minor bioturbation. Core 4, 488.81 m. F) Bioturbated lenticular bedding (proximal F3A). White dashed line follows a deeply penetrating, irregular, muddymantle-bearing Arenicolites. Arrow points to Thalassinoides. Core 1, 324.31 m.

FIG. 9.—Thin sections of selected facies. A) Bioturbated sandy shale (F2A). Note the dark, hook-shaped grazing structures (black arrow). Core 10, 291.18 m. B)Burrowed shale-on-shale erosional contact (dashed white line) (F3A). Core 10, 409.18 m. C) Micro-laminated to massive-appearing shale (F3A). Micro-laminatedinterval is indicated by a white bar. Core 17, 332.34 m. D) Massive sandy shale (F3B). White bar indicates the massive interval. Core 10, 408.06 m. E) Lenticular bedding(F3B). Core 10, 397.68 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

FIG. 10.—Facies 3B and 4. A) A scour-and-fill structure (F3B). Note that the strata onlap the lower contact. Core 4, 497.87 m. B) A ‘‘lam-scram’’ unit (F4). The top of the sand unit is burrowed with large Helminthopsis or mud-filled Chondrites. Core 2, 419.36 m. C) Heterolithic planar lamination. Core 2, 424.67 m. Double mud drapesand cyclic thickening and thinning of sand–mud couplets are present. D) Thickness variation of sand–mud couplets of the previous photo. Note the rhythmic variationsconsistent with tidal variations. E) Robust fugichnia in bioturbated heterolithic bedding. Core 12, 617.12 m. F) Sideritic, low-angle cross-stratification (F4). Core 1,328 m.




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 promi-nent. 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 wave-

generated 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 mudlocally. In concert, these data suggest that the deposits likely represent

alongshore- redistributed mud wedges rather than direct, seaward-

prograding 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 thefollowing components (e.g., Fig. 5, 362–324 m):

1. The succession may overlie fully bioturbated, diminutive Chondrites-dominated shale (F1A; distal prodelta). The early stages of prograda-

tion 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 un-

burrowed interlaminated shale and sand, and muddy parallel-

laminated sets (attributed to wave- and tide-agitated fluid-mudflows) 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 showsubdued ichnological signals and high shale contents in the fair-weather 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 tide-

influenced delta front).

The burrowed mud–interlaminated mud and sand–interbedded sandand 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 Yellowseas, 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 developeddifferently in various parts of the Alderson. In the upper Alderson

r

FIG. 11.—Evenly cross-laminated set of mud and silty sand that grade upwardinto interbedded massive and interlaminated sand and mud. White arrow point tocross-lamination. See text for discussion.







r

FIG. 12.—Core 2, Hatton, lower to middle Alderson.

FIG. 13.— A) Close-up of a subvertical, bifurcating root (F5). Note the variable thickness and the oxidized halo around the root. Core 12, 613.68 m. B) A vertical rootin rubbly, organic-rich shale (F5). Core 8, 706.51 m. C) Pedogenic slickensides (F5). Core 17, 282.96 m. C) Paleosol with abundant AMB trace fossils (F5). Core 25.




(especially Townships 1–4), the deposits comprise 6–12-m-thick succes-

sions 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 bioturba-

tion 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 sandand 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, upward-

coarsening 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 Coleman1981; 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; 288–

272 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 shell-

hash-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, evenlycross-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. 14.— A) A conglomerate-mantled surface (F6). Core 2, 462.28 m. B)Thalassinoides with glauconitic infill, corresponding to the firmground Glossifun- gites Ichnofacies (F7). Core 2, 418 m.

FIG. 15.—Interpreted distribution of faciesalong a mud-dominated deltaic coast. SWWB,storm-weather wave base; FWWB, fair-weatherwave base.




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 grainsizes.

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 Suter1989). The cheniers typically consist of sandy or shelly material that

accumulate in response to spring tides, storms, or other autocyclicprocesses 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 subtidaland 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 itcomprises 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 muddeposits). No definite cheniers, however, have been identified in thesestrata 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 orshelly alongshore bars (Augustinus 1980). In that context, the aforemen-

tioned migrating sandy mud bank or mud cape that is intercalated withbioturbated 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 Aldersoncoastal 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, preventchenier 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, todate.

Future work on the Alderson should include delineation of the

morphology of the sedimentary units, reconstruction of the paleogeog-raphy of the area, and assessment of permeability distribution in these

strata. We speculate that the coastal mud wedge may have progradedfrom NW to SE in the gas-prolific, northernmost part (Abbay) of the

study area, and that detailed well-log correlations may reveal subtle, low-

relief 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 andGiosan 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 coastalenvironments 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 lenticular-

bedded 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, mud-

dominated 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 sediment-

swimming 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 interpre-tations 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 theSubsurface 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 Vakarelovand Sam Bentley, Associate Editor Janok Bhattacharya, and Editor Colin P.North for their constructive comments that considerably improved the paper.







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r

FIG. 16.— A) Various expressions of root-bearing shale and sand. AMB-adhesive meniscate burrow, Di, Diplocraterion; MS, ‘‘mantle-and-swirl.’’ B) Examples of faciessuccessions interpreted to reflect fluid-mud-influenced (‘‘muddy shoreface’’) and fluid-mud-dominated (‘‘subaqueous delta’’) coastal sedimentation. Ichnofossilabbreviations: Ar, Arenicolites; As, Asterosoma; Ch, Chondrites; He, Helminthopsis; Pa, Palaeophycus; Ph, Phycosiphon; Pl, Planolites; Sch, Schaubcylindrichnu; Te,Tchaubcylindrichnus freyi (‘‘Terebellina’’), and Th, Thalassinoides.




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Received 6 September 2007; accepted 28 May 2008.


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