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Saskatchewan Geological Survey 1 Summary of Investigations 2009, Volume 1

Sedimentological and Ichnological Aspects of a Sandy Low-energy Coast: Upper Devonian–Lower Mississippian Bakken Formation,

Williston Basin, Southeastern Saskatchewan

Solange Angulo 1

Angulo, S. and Buatois, L. (2009): Sedimentological and ichnological aspects of a sandy low-energy coast: Upper Devonian–Lower Mississippian Bakken Formation, Williston Basin, southeastern Saskatchewan; in Summary of Investigations 2009, Volume 1, Saskatchewan Geological Survey, Sask. Ministry of Energy and Resources, Misc. Rep. 2009-4.1, Paper A-5, 17p.

and Luis Buatois 1

Abstract The Upper Devonian–Lower Mississippian Bakken Formation hosts one of the most important oil reservoirs in Saskatchewan. Integration of ichnological and sedimentological data indicates that deposition of Bakken strata occurred in two different paleoenvironmental settings: open-marine and brackish-water marginal marine. This paper focuses on the open-marine deposits in the Lower Member, the basal and the upper part of the Middle Member, and the Upper Member. The open-marine deposits embrace shelf, lower and upper offshore, offshore transition, and a transgressive lag facies. The large extension and continuity of the open-marine sedimentary facies point towards deposition in a low-gradient system in a shallow epeiric sea. With the exception of the black shale from the Lower and the Upper Member, all these deposits are characterized by a high bioturbation index and a “distal” Cruziana ichnofacies with Phycosiphon incertum and Nereites missouriensis as dominant elements, and Asterosoma isp., Planolites montanus, and Teichichnus rectus as subordinate elements. Steady high bioturbation index, dominance of fair-weather infauna, and general absence of storm-generated sandstones reflect mainly deposition under a weakly storm-affected epicontinental platform. Scarcity of terrestrial organic matter, absence of soft-sediment–deformation or fluid-mud intervals and a steady high bioturbation index in the marine deposits of the Middle Member suggest a sand-dominated coast. Detailed understanding of the conditions and processes that prevailed during deposition of the open-marine deposits of the Bakken Formation gives a better comprehension of the areal and vertical distribution of the sedimentary facies, which is important to exploration, production, development, and secondary recovery of the oil reservoirs in the Bakken Formation.

Keywords: Bakken Formation, Late Devonian, Early Mississippian, Williston Basin, Western Canada Sedimentary Basin, sedimentology, sequence stratigraphy, ichnology, ichnofacies, sedimentary facies, Saskatchewan, low-energy coast, sandy coast, open marine.

1. Introduction The Bakken Formation is currently one of the most prominent oil-producing units in Saskatchewan. After Bakken discoveries in Montana and North Dakota, industry attention has turned to exploration for Bakken reservoirs in Saskatchewan, resulting in a dramatic increase in the level of drilling activity in this province. In 2004, fewer than 80 wells were producing from Bakken strata in southeastern Saskatchewan, but by June 2009 this number had climbed to over 1,200 Bakken producers (E. Nickel, written comm., 2009).

The Bakken Formation is present in the subsurface of the Williston Basin in northeastern Montana, North Dakota, southwestern Manitoba, and southern Saskatchewan (Christopher, 1961; Meissner, 1978; LeFever et al., 1991; Martiniuk, 1991; Smith and Bustin, 2000; Kreis and Costa, 2005; Kreis et al., 2005, 2006). It is subdivided into three members (Figure 1), the Lower, Middle, and Upper, which comprise an “idealized” petroleum system that includes source rock, reservoir, and seal (Halabura et al., 2007). These units were deposited during the Late Devonian and Early Mississippian, as suggested by conodont biostratigraphy (Hayes, 1985; Karma, 1991). The Devonian-Mississippian boundary (359.2 ma ±2.5 ma; International Commission on Stratigraphy, 2007) is within the Middle Bakken. Bakken strata can be correlated westward to Alberta and British Columbia where the Lower and Middle members of the Bakken are equivalent to the Exshaw Formation, and the Upper Member is equivalent to the lowermost strata of the Banff Formation.

The exceptional utility of ichnology for paleoenvironmental reconstructions is being increasingly recognized. Biogenic structures effectively provide an in situ record of environment and environmental changes, based on

1 Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, SK S7N 5E2.


Figure 1 - Stratigraphic subdivision, composite lithological description and facies of the Bakken Formation (modified from LeFever et al., 1991 and Angulo et al., 2008a). Facies interpreted as open marine in this paper are shaded in blue.

factors that influence benthic organisms. Trace fossils can constrain important environmental variables, such as salinity, oxygen and food supply, which are not commonly recorded in the original sedimentary fabric. Integration of sedimentology and ichnology therefore provides a more accurate picture of depositional conditions. Integration of ichnology with conventional sedimentological analysis suggests that the Bakken Formation was deposited under two different environmental settings: open marine and brackish water (Angulo et al., 2008a). This paper focuses on the open-marine deposits in the Lower Member, the basal and the upper part of the Middle Member, and the Upper Member (Figure 1).

Thus far in this study, 54 cores from the Bakken Formation in Saskatchewan have been slabbed and described in detail, representing a total length of 1055 m. Forty of these cores are from wells located in the current study area in southeastern Saskatchewan (Figure 2); total length of described cores is 898 m. Core slabbing enhances visibility of many features of the sedimentary facies, and thus facilitates description and interpretation. Two outcrops of the Exshaw Formation in Alberta have also been studied: the type section in Jura Creek and the Crowsnest Lake outcrop.

The Lower Bakken black shale records a regional transgression associated with a eustatic rise in sea level near the end of the Late Devonian (Johnson et al., 1985). During this time, a shallow epicontinental sea covered much of the North American craton (Figure 3) and black marine shales were deposited throughout a series of silled intracratonic basins (Smith and Bustin, 1998; Algeo et al., 2007) near the north-south–trending western shoreline of the Laurasia land mass approximately 5° to 10°N from the Equator (Ettensohn and Barrow, 1981; Van der Voo, 1988; Brand, 1989).

2. Sedimentary Facies and Trace-fossil Content Five sedimentary facies and two subfacies have been defined in the open-marine facies association of the Bakken Formation: black shale facies, highly bioturbated siltstone facies, sandy siltstone and micro-hummocky cross-stratified facies (which is further subdivided into two subfacies: highly bioturbated sandy siltstone subfacies and interbedded highly bioturbated siltstone and micro-hummocky cross-stratified sandstone subfacies), highly bioturbated interbedded sandstone and siltstone facies, and a coquina facies.

For the sedimentological description, lithology, grain size (visual estimation), sedimentary structures and bed boundaries were considered, and ichnological data included evaluation of the trace-fossil content and bioturbation index of Taylor and Goldring (1993). The open-marine environmental classification scheme of MacEachern et al. (1999) was used.

Interbedded highly bioturbated siltstone and micro-hummocky cross-stratified sandstone, mottled, dolomitic, argillaceous, grey-green, locally fossiliferous, disseminated pyrite.

Thinly interlaminated dark grey mudstone and silty sandstone, moderate to low bioturbated, disseminated pyrite, mud drapes, locally current ripples.

Sandstone, internally complex, wavy and flaser bedded silty sandstone current ripples cross laminated silty sandstone with mud drapes, massive trough and planar cross-bedded sandstone. Few brachiopods, disseminated pyrite, buff to green, calcareous, slight to no bioturbation, locally with some oolites.

Highly bioturbated interbedded sandstone and siltstone, moderate to very strong bioturbation disrupting laminae, disseminated pyrite.

Highly bioturbated sandy siltstone, massive, mottled, very calcareous, argillaceous, grey-green, highly fossiliferous, random orientation of fossils, disseminated pyrite.

COMPOSITE DESCRIPTIONSTRATIGRAPHIC SUBDIVISION

A

B1

B2

B3

C

A

B1

B2

B3

B4

A

B

C

1

2

3

Upper Member

Lower Member

LeFever, et al.(1991)

Thrasher (1985)Karma and Parslow

(1989)Christopher (1961)

Smith and Bustin (1996) Angulo, et al. (2008a)

FACIES

Mb

Mb

SMm, SMh

Sw, Sl

Sw, Sl

SMm,SMh

Sf, St, Sr, Lo

Units/Subunits

BB

Units Units/Subunits Subunits

1

2

4

6

9103B

8

7

5

3A

BA

KK

EN

FO

RM

AT

ION

1

MiddleMember

Black, noncalcareous, massive to locally parallel-laminated shale.

Black, noncalcareous, massive to locally parallel-laminated shale.

OF LITHOLOGIES


Figure 2 - Locations of two Exshaw Formation outcrops, and of the study area in southeastern Saskatchewan (after Smith et al., 1995); locations of the described Bakken cores are shown on the inset map.

1617

33

18

1514

1920

21 22

74

5

68

923 38

3712

39

34251335

29

32 31

28

1124

2

3

1

36

10

26

2730

40

R3 R1W3R29 R27 R25 R23 R21 R19 R17 R15 R13 R11 R9 R7 R5 R3W2

T1

T2

T3

T4

T5

T6

T7

T8

T10

T11

T12

T13

T14

T15

T16

T17

N

ALBERTA

NORTH DAKOTA

MONTANA

MANITOBASASKATCHEWAN

EX

SH

AW

FO

RM

ATIO

N

BAKKEN FORMATION

49O

49O

O

Eastern lim

it of deformation

60

110O

??

Jura Creek Exshaw outcrop

Study area

LEGEND

N

SCALE

MILES

KILOMETERS

0 40 60

0 64 128

T9

0 10 20 km

Cored wells included in this study

Crowsnest Lake outcrop

Exshaw


Figure 3 - Paleogeographic map of North America at 360 ma (Late Devonian) [after Blakey (2006)].

a) Black Shale Facies (F1) Description: This facies consists of black, noncalcareous, massive to locally parallel-laminated shale (Figure 4). Locally some shell fragments and pyrite are present. Bioturbation is generally absent, but some cores revealed the presence of Chondrites isp., and Thalassinoides near the top of the Lower Member. Fossils identified include finely ribbed brachiopods, Lingula, local crinoid ossicles, spores, and conodonts (Fuller, 1956; Christopher, 1961; Karma, 1991).

The Lower Member black shale conformably overlies greenish grey shale of the Big Valley Formation in southeastern Saskatchewan and lies unconformably upon interbedded weathered dolostone, dolarenite, dolomitic mudstone, and minor anhydrite of the Upper Devonian Torquay Formation as far east as Range 31 West of the First Meridian, where it ends as an erosional edge in southeastern Saskatchewan (Christopher, 1961; Kreis et al., 2006). The upper contact of the lower black shale with the highly bioturbated siltstone facies (between the Lower and the Middle Member) is sharp. However, no evidence of erosion has been recognized. The contact has therefore been interpreted in this study as conformable. The basal contact of the Upper Member shale with the underlying interbedded highly bioturbated siltstone and micro-hummocky cross-stratified sandstone sub-facies of the Middle

Study area


Figure 4 - Black Shale Facies (F1). Note the absence of bioturbation.

Member, and the upper contact with the Lodgepole Formation are both sharp, but are considered to be conformable (Christopher, 1961).

In the type section of the Exshaw Formation (Figure 2), this facies includes a matrix-supported pebble and cobble conglomerate at the base. The matrix is composed of fine- to medium-grained sand mud chips and abundant shell fragments that are replaced by pyrite. Concretions and sandstone lenses occur locally. In the Crowsnest Lake outcrop (Figure 2), Chondrites isp., Phycosiphon incertum, Zoophycos isp., and fish remains occur within the same stratigraphic level.

Distribution: This facies occurs in the Lower and Upper members of the Bakken Formation, and is equivalent to facies Mb of Smith and Bustin (1996) and Facies 1 of Angulo et al. (2008a) (Figure 1).

Interpretation: This facies records suspension-fallout deposition in a low-energy setting without the influence of waves and currents. The dark colour, lack of bioturbation, the scarcity of fauna, and thin lamination suggest anoxic conditions. According to Thrasher (1985), however, the presence of in situ or locally reworked benthic macrofauna remains near the basal and upper contacts of the Lower Member suggests brief periods of more hospitable conditions on the sea floor during the initial and final stages of deposition. This interpretation is also consistent with the local occurrence of bioturbation. Schieber (2003) demonstrated how careful examination of black shales can reveal more common bioturbated intervals reflecting periods of oxygenated bottom waters in black laminated shales. In addition, these fine-grained sediments may have been characterized by high water content, and their degree of consolidation likely affected biogenic-structure preservation, which is very low in soupy substrates (Ekdale, 1985). Further

research needs to be done in order to explore the interplay of oxygen and substrate conditions in the Bakken black shales.

This facies is interpreted as having been deposited in a shelf environment below storm wave-base as indicated by the absence of oscillatory structures (Figure 5).

b) Highly Bioturbated Siltstone Facies (F2) Description: This facies consists of greenish grey, commonly calcareous (generally dolomitic), pyritic siltstone, with fragments of brachiopods and crinoids. It is highly bioturbated (bioturbation index 5 to 6) and characterized by a burrow-mottled texture. Estimation of the bioturbation index and recognition of discrete ichnogenera are difficult due to the absence of lithological contrast (Figure 6). Abundant Phycosiphon incertum can, however, be locally recognized. This facies sharply overlies the black shale facies and gradationally passes upward into the highly bioturbated sandy siltstone subfacies.

Distribution: This facies occurs in the lower half of unit A (sensu LeFever et al., 1991) of the Middle Member, and corresponds to facies SMm of Smith and Bustin (1996) and Facies 2 of Angulo et al., 2008a (Figure 1).

Interpretation: The highly bioturbated siltstone facies records suspension-fallout deposition in a low-energy setting mostly in the absence of waves and currents and is characterized by deposit-feeder trace fossils (Phycosiphon incertum). The high bioturbation index and common shell fragments suggest oxic conditions. Although the sedimentary processes involved during deposition of this facies are similar to those in the black shale facies, the presence of siltstone instead of mud and oxic conditions, revealed by the high bioturbation, suggest deposition in a lower offshore environment (Figure 5) immediately above storm wave-base.

1 cm

Well: 10-01-02-19W2Depth: 2273.25 m


Figure 5 - Distribution of the open-marine sedimentary facies in an idealized log of the Bakken Formation. A) 07-12-07-11W2, 1707.5 m; B) 07-11-08-10W2, 1593 m; C) 01-25-07-5W2, 1493.6 m; D) 07-11-08-10W2, 1597.15 m; E) 07-32-03-11W2, 1996.59 m; F) 02-05-32-27W3, 866.6 m; and G) 15-31-03-11W2, 1970 m. Phycosiphon incertum (Ph), Nereites missouriensis (Ne), Asterosoma isp. (As), Planolites montanus (Pl), and Teichichnus rectus (Te). Facies nomenclature after Angulo et al. (2008a).

enira

Mne

pO

enira

Mne

pO

Marginal Marine

Shelf

Shelf

UpperO�shore

O�shoreTransition

Lower O�shore

UpperO�shore

1 2 3 4 5 60

Lower

reb

meM

rep

pU

reb

meM

reb

meM

eld

diM

NAI

NO

VE

DN

AIP

PIS

SIS

SIM

AGE

MEM

BERS

TRAC

EFO

SSILS

BIOTU

RBAT

ION

INDEX

SEDIMEN

TARY

ENVIRO

NMEN

T

?

3A

4

5

2

1

10

3B

1

9

8

7

6

Ph

TeNe

Pl

Ne

PlTe

Ne

Ph

Ph

SEDIMEN

TARY

FACIES

Ne Te

As

As

Pl

Ne

Ph

1 cm

Ph

NeAs

As

Ph

1 cm

Ph

1 cm

1 cm

A

E

F

G

1 cmNe

B

Ne

1 cm

C

1 cm

Ne

Ne

D

Asterosoma isp.Nereites missouriensisPlanolites montanus

Phycosiphon incertum Teichichnus rectusTraceFossils

Chondrites isp.

Brackish-Water


Figure 6 - Highly Bioturbated Siltstone Facies (F2). Although discrete bioturbation is not recognizable, the mottled aspect indicates a high degree of bioturbation. Note fragments of crinoids (Cr). Figure 7 - Highly Bioturbated Sandy Siltstone Subfacies

(F3A). The ichnofauna consists of Phycosiphon incertum (Ph), Nereites missouriensis (Ne), Teichichnus rectus (Te), and Asterosoma isp. (As). c) Sandy Siltstone and Micro-hummocky

Cross-stratified Facies (F3A and F3B) This facies has been divided into two subfacies: (i) highly bioturbated sandy siltstone (Figure 7) and (ii) interbedded highly bioturbated siltstone and micro-hummocky cross-stratified sandstone (Figure 8).

Highly Bioturbated Sandy Siltstone Subfacies (F3A)

Description: This subfacies consists of light grey or greenish grey, burrow-mottled, sandy siltstone to very fine-grained silty sandstone, commonly calcareous (generally dolomitic), pyritic, locally with shell remains and discontinuous thin laminae of shale. Discrete beds are absent or extremely rare. These deposits have a bioturbation index of 5 to 6 and are dominated by Phycosiphon incertum and Nereites missouriensis; subordinate ichnotaxa are Asterosoma isp., Teichichnus rectus and Planolites montanus (Figure 6). In outcrops of the Exshaw Formation, Nereites missouriensis and Phycosiphon incertum are clearly preserved. Both the lower and the upper contact of this subfacies with the underlying black shale facies and the overlying highly bioturbated interbedded sandstone and siltstone facies are gradational, and the changes from one facies to the other are so subtle that locating facies boundaries is often difficult. In some cores, however, the upper contact is represented by an erosive surface that has been interpreted as a sequence boundary which separates this facies from the overlying brackish-water deposits (see Angulo et al., 2008a).

Distribution: This subfacies occurs in the upper half of unit A (sensu LeFever et al., 1991) of the Middle Member, and corresponds to facies SMh of Smith and Bustin (1996) and Subfacies 3A of Angulo et al., 2008a (Figure 1).

Well: 05-31-06-13W2Depth: 1717.83 m

1 cm

Cr

Cr

Well: 14-15-02-23W2Depth: 2156.8 m

1 cm

Ne

Te

Ph

Ph

Ne

As


Figure 8 - Interbedded Highly Bioturbated Siltstone and Micro-hummocky Cross-Stratified Sandstone Subfacies (F3B). The ichnofauna consists of Nereites missouriensis (Ne) and Teichichnus rectus (Te).

Interpretation: The thoroughly bioturbated deposits and the overwhelming dominance of a deposit-feeding ichnofauna imply deposition under low-energy conditions. The interpreted environmental scenario is an upper offshore setting below fair-weather wave-base, but above storm wave-base (Figure 5). Absence of distal storm deposits (tempestites) is attributed to the high biogenic reworking.

Interbedded Highly Bioturbated Siltstone and Micro-hummocky Cross-Stratified Sandstone Subfacies (F3B)

Description: This subfacies consists of interbedded dark grey, highly bioturbated siltstone, and light grey, very fine-grained sandstone with micro-hummocky cross-stratification and very thin parallel lamination. In some cases, wave ripples occur on top of micro-hummocky beds. The bioturbation index is highly variable. In the siltstone, the bioturbation index is generally 5, while in the sandstone it ranges between 0 and 1. However, estimation of the bioturbation index and recognition of discrete ichnogenera are difficult due to the absence of lithological contrast, especially in the intervals deposited under fair-weather conditions. The dominant ichnofauna in the siltstone is Phycosiphon incertum and/or Nereites missouriensis, whereas Teichichnus rectus is dominant in sandstone beds (Figure 8). Overall, the basal contact of this subfacies is gradational with brackish-water marginal-marine facies (see Angulo et al., 2008a), while the upper contact is sharp but conformable with the black shale facies. Locally it may gradually pass upward into the highly bioturbated siltstone facies.

Distribution: This subfacies occurs in the upper half of unit C (sensu LeFever et al., 1991) of the Middle Member, and corresponds to facies SMh of Smith and Bustin (1996) and subfacies 3B of Angulo et al. (2008a) (Figure 1).

Interpretation: The highly bioturbated siltstone records quiet-water sediment fallout during fair-weather conditions, while the micro-hummocky, cross-stratified, very fine-grained sandstone beds were generated by oscillatory flows during storm events. Wave ripples overlying the micro-hummocky zone indicate temporary reworking by waves during waning storms (Dott and Bourgeois, 1982). Fair-weather deposits are clearly dominated by a deposit-feeding ichnofauna. The predominance of siltstone with episodic tempestites suggests deposition in an upper offshore environment (Figure 5).

d) Highly Bioturbated Interbedded Sandstone and Siltstone Facies (F4)

Description: This facies consists of interbedded light grey, massive, very fine-grained sandstone and siltstone

with diffuse bed boundaries. Deposits are generally slightly to moderately calcareous and have locally continuous shale laminae. The bioturbation index is between 4 and 5. Dominant ichnofauna are Nereites missouriensis, particularly in the very fine-grained sandstone beds, and Planolites montanus; subordinate ichnotaxa are Phycosiphon incertum, present in the siltstone layers, and Asterosoma isp. (Figure 9). This facies gradationally

Well: 07-12-07-11W2Depth: 1694.8 m

1 cm

Te

Ne


Figure 9 - Highly Bioturbated Interbedded Sandstone and Siltstone Facies. Note the presence of Nereites missouriensis (Ne) and subordinate Phycosiphon incertum (Ph).

overlies the highly bioturbated sandy siltstone subfacies. Upward, it grades into a sand-dominated unit (Facies 5 of Angulo et al., 2008a), or is separated from the overlying unit by a sequence boundary.

Distribution: This facies is present in the lower half of subunit B1 (sensu LeFever et al., 1991) of the Middle Member, and corresponds to facies Sw of Smith and Bustin (1996) and Facies 4 of Angulo et al., 2008a (Figure 1).

Interpretation: The presence of intensely bioturbated siltstone interbedded with very fine-grained sandstone beds suggests open-marine conditions, below fair-weather wave-base and above storm wave-base, in an offshore-transition zone (Figure 5). The lack of sedimentary structures suggests significant biogenic reworking by a deposit-feeding infauna.

e) Coquina Facies (F10) Description: This facies consists of a poorly sorted coquina with sandy matrix that passes upward into highly bioturbated, massive and micro-hummocky cross-stratified sandstone with localized pyrite (Figure 10). The basal contact is sharp. The thickness of this facies is generally 8 to 12 cm. Typically, it sharply overlies brackish-water marginal-marine deposits and passes gradationally upward into the interbedded highly bioturbated siltstone and

micro-hummocky cross-stratified sandstone subfacies.

Distribution: This facies occurs in the lower half of unit C (sensu LeFever et al., 1991) of the Middle Member and corresponds to Facies 10 of Angulo et al., 2008a (Figure 1).

Interpretation: This facies occurs above brackish-water deposits (Figure 5) and is interpreted as a transgressive lag that formed during wave ravinement. These transgressive deposits pass upward into upper

Figure 10 - Coquina Facies (F10). Note abundant shell fragments.

Well: 05-31-06-13W2Depth: 1703.62 m

1 cm

Well: 03-09-06-16W2Depth: 1990.05 m

1 cm

Ne

Ne

Ne

Ne

Ph


offshore facies. This facies records high-energy ravinement during drowning of the brackish-water deposits.

3. Sedimentological and Ichnological Characteristics of a Sand-dominated Low-energy Marine Succession

The open-marine facies association comprises a progradational basal succession which includes the Lower Member and the basal part of the Middle Member (Unit A and subunit B1 of LeFever et al., 1991), and an upper retrogradational interval which encompasses the upper part of the Middle Member (unit C defined by LeFever et al., 1991) and the Upper Member (Figures 1 and 5). These two discrete packages of open-marine deposits are separated by an interval of brackish-water deposits (LeFever et al., 1991 subunits B2 and B3). The open-marine sedimentary facies recognized are arranged along a depositional profile from the offshore transition to the shelf (Figure 11).

The basal open-marine succession starts with the black shale of the Lower Member that has been interpreted as suspension-fallout deposits in a shelf, below the storm wave-base. Dark colour, rare local bioturbation, scarce fauna, thin lamination, and the high organic-matter content (an average of 8% total organic carbon [TOC] according to Smith and Bustin, 1998) suggest that anoxic to dysoxic conditions prevailed during deposition of the Lower and Upper members.

The black shales of the Lower and Upper Bakken members, as well as other Devonian-Mississippian organic-rich marine black shales [e.g., Exshaw Formation (Western Canada Sedimentary Basin), Chattanooga Shale (Appalachian Basin), Antrim Shale (Michigan Basin), New Albany Shale (Illinois Basin), Woodford Shale (Iowa-Oklahoma Basin)] were deposited in the North American Epicontinental Seaway into a series of deeper basins, separated by shallower sills, resulting in conditions favouring development of benthic anoxia within the deeper waters of the basin centres (Algeo et al., 2007). Smith and Bustin (1998) suggested that climatic conditions and the semi-enclosed, isolated geography of the Williston Basin may, at the western edge of the North American craton, have produced a two-layer pattern of estuarine-like water circulation between basin water and the open-ocean conditions that included upwelling in the basin, causing anoxic conditions.

The highly bioturbated siltstone of the lower offshore sharply but conformably overlies the shelf black shale. Presence of siltstone, lighter colours, and high degree of bioturbation, indicate a clear increase in oxygen level, probably due to sporadic storm events above the storm wave-base or by sediment progradation overtopping the basin sill. The lower offshore siltstone gradually passes into upper offshore silty sandstone. Bioturbation degree is high, and the absence of sedimentary structures from the primary fabric is attributed to biogenic reworking. Upper offshore deposits are gradually overlain by highly bioturbated interbedded sandstone and siltstone of the offshore transition. Diffuse bed boundaries are characteristic of these deposits. Sand content gradually increases upward.

Figure 11 - Distribution of Bakken open-marine sedimentary facies along depositional profile. Facies nomenclature after Angulo et al. (2008a). Not to scale. General orientation is southwest to northeast. Off. Tra, offshore transition.

F1 F2 F3 F4

Shelf Shoreface Foreshore BackshoreOff. TraOffshoreL U

1 cm 1 cm 1 cm 1 cm1 cm

Southwest Northeast

Well: 15-31-03-11W2Depth: 1970 m

Well: 05-31-06-13W2Depth: 1717.83 m

Well: 12-05-11-5W2Depth: 1417.38 m

Well: 16-10-07-15W2Depth: 1720.1 m

Well: 07-32-03-11W2Depth: 1996.59 m

F1 F2 F3A F3B F4


The upper part of the Middle Member and the Upper Member record the re-establishment of open-marine conditions after deposition under brackish-water conditions. Above the transgressive lag, upper offshore interbedded highly bioturbated siltstone and micro-hummocky cross-stratified sandstone occurs. As the transgression proceeded, the black shale of the Upper Member was deposited in an anoxic shelf environment; Upper Member TOC values average 10% (Smith and Bustin, 1998).

With the exception of the black shale of the Lower Member, the open-marine succession, including the lower offshore, upper offshore, and offshore-transition deposits, are characterized by a high bioturbation index (4 to 6) and a “distal” Cruziana ichnofacies (Figure 12). Dominant elements are Phycosiphon incertum and Nereites missouriensis, while subordinate elements are Planolites montanus, Asterosoma isp. and Teichichnus rectus. Locally, rare Chondrites isp. are present. Phycosiphon incertum is present preferentially in the siltier intervals, being dominant in the lower and upper offshore deposits and locally in the siltier intervals of the offshore-transition facies. In contrast, sandier intervals of the offshore transition are dominated by Nereites missouriensis.

According to MacEachern and Pemberton (1992), a “distal” or “outer” Cruziana assemblage is characteristic of lower offshore deposits, while a diverse Cruziana is more common in upper offshore and lower shoreface deposits. In contrast, in the Bakken, the “distal” Cruziana ichnofacies seems to have spread over large areas of the depositional profile, from the lower offshore to the offshore-transition deposits. It has been speculated that the Late

Figure 12 - Selected core intervals illustrating the “distal” Cruziana ichnofacies. Phycosiphon incertum (Ph), Nereites missouriensis (Ne), Asterosoma isp. (As), Planolites montanus (Pl), and Teichichnus rectus (Te).

Well: 14-15-02-23W2

Depth: 2156.8 m

1 cm

Ph

Te

Ne

Ph

As

Well: 09-18-04-22W2

Depth: 2048.75 m

1 cm

Ne

AsPh

Te

Well: 09-18-04-22W2

Depth: 2049.1 m

1 cm

Ph

Ph

Ne

Well: 07-32-03-11W2

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Devonian mass extinction event provided an opportunity for the shoreward expansion of the “distal” Cruziana ichnofacies (Angulo et al., 2008b).

Although the bioturbation degree is high in the open-marine facies association of the Middle Member, the ichnodiversity is relatively low. A taphonomic effect could be invoked in this case. With a low sedimentation rate, the deep-tier fauna would have enough time to rework sediments, destroying shallower tier structures. There is a paucity of primary sedimentary structures in the open-marine facies of the Middle Member due to high biogenic reworking; only wispy lamination and, very locally, some parallel lamination are present in the highly bioturbated interbedded sandstone and siltstone facies. A burrow-mottled texture is characteristic of the open-marine facies.

The large areal extent and continuity of the open-marine sedimentary facies point towards deposition in a low-gradient system for the Bakken marine succession. Black shale facies and highly bioturbated sandy siltstone subfacies, for example, have been identified in the type section of the laterally equivalent Exshaw Formation in Alberta (more than 800 km away from the study area). Shelf, offshore, and offshore-transition deposits are recurrent in the whole area.

No open-marine deposits shallower than the offshore transition have been recognized in the Bakken Formation. The absence of identifiable nearshore deposits may reflect intense cannibalization of open-marine strata during the sea-level drop associated with the Devonian-Mississippian boundary. This boundary probably occurs in the Middle Bakken at the transition from open-marine deposits to brackish-water marginal-marine deposits (Angulo et al., 2008a).

4. Discussion Based on the shoreface variability of Cretaceous successions of the Western Interior Seaway of North America, MacEachern and Pemberton (1992) proposed three principal types of lower and middle-shoreface deposits: those that are strongly storm dominated (“high energy”), those that are moderately affected by storms (“intermediate energy”), and those that are only weakly affected by storms, or are dominated by fair-weather deposition (“low energy”). Strongly storm-dominated shorefaces typically consist of hummocky cross-stratified or swaly cross-stratified, fine- to very fine-grained sandstone characterized by minimal bioturbation; non-bioturbated thick intervals are common. Where bioturbation is present, fugichnia or trace fossils of opportunistic pioneers are dominant. Moderately storm-dominated shorefaces are typified by storm-laminated sandstone intervals interstratified with more thoroughly bioturbated sandstone beds. Commonly, the upper portion of the storm beds is bioturbated, reflecting post-storm colonization of the substrate by the pioneer opportunistic community. With the return of fair-weather conditions, the resident fair-weather trace-fossil assemblage replaces the opportunistic suite, cross-cutting the latter. Finally, weakly storm-affected shorefaces are characterized by more thoroughly bioturbated muddy sandstones. Fair-weather trace-fossil assemblages dominate the succession, and where thin storm-generated sandstones are deposited, these tend to be thoroughly bioturbated or obliterated by biogenic reworking.

Although MacEachern’s and Pemberton’s (1992) classification is based on shoreface variability, their basic concepts can be extrapolated into offshore and offshore-transition deposits. Accordingly, the ichnological and sedimentological features of the open-marine deposits of the Bakken Formation point toward deposition in a weakly storm-affected setting under low-energy conditions. In general terms, with the exception of the black shales from the Lower and Upper members of the Bakken Formation, open-marine deposits are typified by a high bioturbation index (4 to 6), resulting from the activity of fair-weather infauna. The scarcity of sedimentary structures generated during storm events in the open-marine deposits also reflects deposition in a weakly storm-affected setting, where fair-weather conditions prevailed most of the time. Storms may have occurred only sporadically with resulting sedimentary structures only locally preserved or totally destroyed by thorough bioturbation.

Tempestite preservation depends on net sedimentation rate, the biogenic mixing rate, and the magnitude of physical reworking (Pemberton and MacEachern, 1997; Pemberton et al., 2001). Wheatcroft (1990) proposed a model for tempestite preservation focused on time scales, relating transit time (defined as the time necessary to bury an event beyond the reach of the burrowing infauna) to dissipation time (defined as the time required to biogenically destroy an event bed). In addition, Pemberton and MacEachern (1997), and Pemberton et al. (2001) noted the low preservation potential of hurricane-induced tempestites in low-latitude settings as a reflection of the infrequent nature of such events. Under these conditions, the storm layer is exposed to long periods of biogenic colonization and modification before burial below the reach of infauna. In contrast, these authors also mentioned that many high-latitude settings are characterized by cyclic variations in storm activity whereby winter storm seasons are typified by both frequent and high-magnitude storms, and summer fair-weather seasons are characterized by less frequent and less intense storms. Under such conditions, tempestites accumulate rapidly during winter seasons with little or no time for biogenic reworking, while summer fair-weather seasons favour biogenic colonization and modification of the tempestites. The weakly storm-affected nature of the Bakken Formation, shown by the absence of wave/storm-dominated sandstones, suggests deposition in tropical latitudes.


Upper offshore deposits in the Bakken Formation reflect two different textures depending on tempestite preservation (highly bioturbated sandy siltstone subfacies and interbedded highly bioturbated siltstone and micro-hummocky cross-stratified sandstone subfacies). Upper offshore sandy siltstone from the lower part of the Middle Member is characterized by a high bioturbation index (between 4 and 5), producing total homogenization of this subfacies. The upper offshore interbedded siltstone and micro-hummocky cross-stratified sandstone from the upper part of the Middle Member, however, present a variable degree of bioturbation. The very fine-grained sandstone beds exhibit a low bioturbation index (0 to 1), while the bioturbation index in the interbedded fair-weather siltstone is high (4 to 5), reflecting the activity of the climax, resident infauna. While high biogenic activity has reworked the sediment and totally obliterated any storm deposits in the highly bioturbated sandy siltstone subfacies, tempestites have been preserved in the interbedded highly bioturbated siltstone and micro-hummocky cross-stratified sandstone subfacies (Figure 13).

Applying Wheatcroft’s (1990) model to the Bakken Formation, preservation of storm events in the interbedded highly bioturbated siltstone and micro-hummocky cross-stratified sandstone subfacies would be explained as a response of a short transit time and/or a long dissipation time, while the total reworking of tempestites due to burrowing in the highly bioturbated sandy siltstone subfacies may have resulted from a long transit time and/or a short dissipation time. One factor that could have influenced the different transit time recorded in these two intervals may be sedimentation rate. A lower sedimentation rate for the basal progradational open-marine deposits and a higher sedimentation rate for the upper retrogradational open-marine deposits would explain the total obliteration of the storm events in the former (due to a long transit time) and their preservation in the latter (as a result of a short transit time). However, this explanation is hard to reconcile with the sequence-stratigraphic context. Higher sedimentation rates would be expected in the highstand systems tract and not in the transgressive systems tract. Another alternative is, therefore, to consider that frequency and intensity of storm events differentially affected the basal and upper marine deposits of the Bakken: the total reworking of storm events in the progradational basal open-marine deposits may have resulted from less intense and frequent storms during the highstand, while tempestites were preserved in the transgressive upper open-marine deposits as a consequence of more frequent and intense storms. Increased frequency of storms has been recorded in connection with postglacial sea-level rise in Holocene deposits (Blum and Price, 1998; Anderson et al., 2004).

The open-marine deposits of the Bakken Formation are characterized by fine grain size, ranging from shale to very fine-grained sandstone. However, these deposits are not considered to have formed in a mud-dominated coastal environment. According to Hovikoski et al. (2008), mud-dominated coastlines are typified by: 1) high content of terrestrial organic matter; 2) high and/or variable deposition rates such that bioturbation intensities are low and/or fluctuating; 3) high interstitial water content and, commonly, soft substrate consistency, which make soft-sediment–deformation and fluid-mud intervals common; 4) reduced or variable trace-fossil diversity due to turbid sedimentation; 5) impoverished event and post-event suites due to heightened water turbidites and development of soupy substrates; 6) restricted range and type of sedimentary structures to various types of heterolithic bedding; and 7) enrichment of shallow-water shales with shelly material. Sediments of the open-marine facies association in the Bakken Formation do not reflect any of these features. Terrestrial organic matter is scarcely found in the offshore, or offshore-transition deposits of the Bakken Formation, no soft-sediment–deformation or fluid-mud intervals were identified, and bioturbation was steadily high during deposition of the marine sedimentary facies, with the exception of the anoxic black shale of the Lower and Upper members. Integration of sedimentological and ichnological data indicates that the Bakken open-marine succession records deposition in a sand-dominated coastline under low-energy conditions.

5. Conclusions Integrated ichnological and sedimentological observations suggest that two environmental settings, open-marine and brackish, prevailed during deposition of the Bakken Formation. The open-marine succession includes two intervals, a basal one which comprises the Lower Member and the basal part of the Middle Member; and an upper one which encompasses the upper part of the Middle Member and the Upper Member (unit A, subunit B1, and unit C defined by LeFever et al., 1991). The basal interval consists of a progradational succession, while the upper one encompasses a retrogradational succession. These deposits consist of shelf black shale, lower offshore highly bioturbated siltstone, upper offshore highly bioturbated siltstone and interbedded highly bioturbated siltstone and micro-hummocky cross-stratified sandstone, offshore-transition highly bioturbated interbedded sandstone and siltstone, and a coquina which has been interpreted as a transgressive lag.

The “distal” Cruziana ichnofacies typifies the offshore and offshore-transition deposits, with Nereites missouriensis and Phycosiphon incertum as dominant elements, and Planolites montanus, Asterosoma isp., and Teichichnus rectus as subordinate ichnofauna.


Figure 13 - Upper offshore deposits (highly bioturbated sandy siltstone and interbedded highly bioturbated siltstone and micro-hummocky cross-stratified sandstone subfacies) showing two different textures depending on tempestite preservation. While storm deposits have been totally obliterated by biogenic reworking in highly bioturbated sandy siltstone subfacies, tempestites have been preserved in the highly bioturbated siltstone and micro-hummocky cross-stratified sandstone subfacies. Phycosiphon incertum (Ph), Nereites missouriensis (Ne), and Teichichnus rectus (Te) are present.

With the exception of the black shales of the Lower and Upper members, open-marine sedimentary facies are characterized by a high bioturbation index (from 4 to 6), but relatively low ichnodiversity. This could be the result of a taphonomic effect in which deep-tier fauna would have enough time to rework sediments destroying shallower structures due to a low sedimentation rate.

High bioturbation index resulting from the activity of the fair-weather infauna and absence of wave-generated sedimentary structures suggest a weakly storm-affected epicontinental platform during sedimentation of the open-marine deposits of the Bakken Formation. Depending on tempestite preservation, offshore and offshore-transition

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deposits show two different textural patterns. Deposits in the basal open-marine unit reflect steady thorough bioturbation in which tempestites, if present, were totally obliterated by biogenic reworking (Unit A of LeFever et al., 1991). Deposits in the upper open-marine unit preserved thin storm-generated sandstones (Unit C of LeFever et al., 1991). These two patterns suggest different frequencies and intensities of storm events rather than overall different sedimentation rates for the lower progradational-basal marine deposits and the upper transgressive-marine deposits (transit times).

Scarcity of terrestrial organic matter, absence of soft-sediment-deformation or fluid-mud intervals, and a steady high bioturbation index in the marine sedimentary facies of the Middle Member suggest deposition along a sandy coastline.

6. Acknowledgments Financial support for this project is provided by the Saskatchewan Ministry of Energy and Resources and the University of Saskatchewan. Additional funds have been provided by Shell Canada and AAPG (2008 and 2009 Grant). We would like to thank Kim Kreis and Erik Nickel for reviewing this paper and the Geological Subsurface Laboratory staff for their assistance in core slabbing and displaying material. Special thanks go to Melinda Yurkowski, Daniel Kohlruss, Erik Nickel, Chris Gilboy, Kim Kreis, Chao Yang, and John Lake for all their support and help.

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