Facies Characterization of the Lower Cretaceous Sparky ...€¦ · depositional environments....

Saskatchewan Geological Survey 1 Summary of Investigations 2009, Volume 1

Facies Characterization of the Lower Cretaceous Sparky, Waseca, and McLaren Formations (Mannville Group) of West-central

Saskatchewan

Alireza Morshedian 1

Morshedian, Alireza, MacEachern, James A., and Dashtgard, Shahin E. (2009): Facies characterization of the Lower Cretaceous Sparky, Waseca, and McLaren formations (Mannville Group) of west-central Saskatchewan; in Summary of Investigations 2009, Volume 1, Saskatchewan Geological Survey, Sask. Ministry of Energy and Resources, Misc. Rep. 2009-4.1, Paper A-9, 14p.

, James A. MacEachern 1, and Shahin E. Dashtgard 1

Abstract Preliminary results from ichnological and sedimentological facies analysis of 69 cored wells of the Lower Cretaceous (Lower to Middle Albian) Sparky, Waseca, and McLaren formations (Mannville Group) in the Lloydminster area of west-central Saskatchewan yield ten recurring facies that reflect conditions of reduced and fluctuating salinity, typical of brackish-water settings. The successions display physical sedimentological features dominated by abundant organic-rich mudstone drapes of probable fluid-mud origin, syneresis cracks, soft-sediment deformation, carbonaceous detritus, normally graded mudstones, low-angle undulatory parallel laminations, and wave, current and combined-flow ripples. Facies are generally heterolithic, attesting to autogenic fluctuations in depositional processes.

Ichnologically, bioturbation intensities (BI) range widely (BI 0 to BI 5), and facies are characterized by abundant diminutive ichnogenera comprising low-diversity (locally monogeneric) trace-fossil suites. Common ichnogenera include Gyrolithes, Teichichnus, Planolites, Palaeophycus, Cylindrichnus, Skolithos, Thalassinoides, Chondrites, and Lingulichnus. Rare but important secondary elements include Asterosoma, Lockeia, Phycosiphon, Rosselia, and Rhizocorallium. Most suites consist of mixtures of elements characteristic of both the Skolithos Ichnofacies and the Cruziana Ichnofacies. Persistent ichnogenera constitute facies-crossing elements that record permanent to semi-permanent dwellings of deposit-feeding infauna. Additional behaviour indicators include navichnia (sediment-swimming structures) and fugichnia (escape structures), consistent with rapid and episodic deposition of sand as well as mud (e.g., via flocculation). Variations in bioturbation intensity primarily attest to marked changes in sedimentation rates.

The integration of ichnological and sedimentological datasets indicates that deposition occurred within brackish-water embayments, bay-head deltas, tidal flats, and coastal-plain environments. Variations in trace-fossil diversities and the introduction of less markedly facies-crossing ichnogenera (e.g., Asterosoma, Phycosiphon, and Rhizocorallium) may suggest that elevated salinities occurred periodically within the study area.

Keywords: Mannville Group, brackish water, ichnology.

1. Introduction The Lower Cretaceous Mannville Group has been studied since the early 1940s and was first drilled in 1942. It unconformably overlies Paleozoic to Jura-Cretaceous strata, is disconformably overlain by the transgressive Joli Fou marine shale of the Colorado Group (Figure 1), and is composed of: 1) well cemented to weakly consolidated sandstone, 2) siltstone, 3) mudstone, 4) mixed sandstone and mudstone that form heterolithic stratification, and 5) minor coals.

The “Mannville Formation” was originally proposed by Nauss (1945), based on his work on cores from oil and gas wells in the Vermillion area of east-central Alberta. The type section was recognized from the Northwest Mannville No.1 well (01-18-50-8W4). Nauss subdivided the Mannville Formation (from top to bottom) into the O’Sullivan, Borradaile, Tovell, Islay, Cummings, and Dina members. Wickenden (1948) suggested that the Mannville can be divided into basal, middle, and upper units. The most commonly used stratigraphic nomenclature for the Mannville Group in the Lloydminster area of Saskatchewan and Alberta is based on unofficial driller’s terminology and electric log characteristics (Edmunds, 1948). These include (from top to bottom): Colony, McLaren, Waseca, Sparky, General Petroleums, Rex, Lloydminster, Cummings, and Dina, although no formal type section has been

1 ARISE, Department of Earth Sciences, Simon Fraser University, 8888 University Drive, Burnaby, BC V5A 1S6.


established to formalize this nomenclature. The Mannville was elevated to Group status by Badgley (1952). Vigrass (1977) and Orr et al. (1977) used the nine informal stratigraphic subdivisions of Edmunds (1948) except that Vigrass (1977) referred to them as members, whereas Orr et al. (1977) considered them as formations. For the purpose of this study, the units are assigned formation status, a hierarchal level commonly assigned to these units.

2. Study Area and Methodology

The study area is located between Townships 50 and 60, and between Range 19 west of the 3rd Meridian and Range 3 west of the 4th Meridian, for a total area of 12,400 km2 (Figure 2). The cored intervals were evaluated for trace fossils, sedimentary structures, lithological features, and stratigraphic discontinuities. Recurring successions have been assigned interpretations regarding facies relationships, facies associations, and depositional environments. Ichnological appraisal includes ichnogenera identification, trace-fossil size, bioturbation intensity, distribution of burrowing, and ethological interpretation.

3. Facies Description Based on the integration of lithology, primary sedimentary structures, bioturbation intensity, and ichnological suites, ten recurring facies have been identified from the Sparky, Waseca, and McLaren formations of the Mannville Group within the study area (F1 to F10 of Table 1). Evaluating bioturbation intensity has been used to help assess the relative rates of deposition and processes responsible for sedimentation. This technique was originally proposed by Reineck (1963) and involves a semi-quantitative measurement of the intensity of bioturbation in order to describe the destruction of primary bedding by biogenic activity (Reineck and Singh, 1980). Taylor and Goldring (1993) redefined this approach as “bioturbation index” (BI), based on a non-linear scale of BI 0 to BI 6.

a) Facies 1: Mudstone Facies 1 consists of 70 to 98% mudstone (silt and clay) with the remaining components consisting of thin bands of siltstone and sandstone. Based on sedimentological and ichnological features, Facies 1 has been subdivided into three subfacies.

Figure 1 - Correlation of Lower Cretaceous strata in eastern Alberta and western Saskatchewan (modified after Christopher, 2003); red dots indicate formations included in this study.

ALBERTA SASKATCHEWAN

ATHABASCA COLD LAKE LLOYDMINSTER

LOW

ER

MID

DLE

UP

PE

R

ALB

IAN

LOW

ER

CR

ETA

CE

OU

S

PELICAN FM VIKING FM VIKING FM

JOLI FOU FM JOLI FOU FMJOLI FOU

FM

GRAND RAPIDSFM

CLEARWATER FM

WABISKAW

A UNIT

B UNIT

C UNIT

D UNIT

MA

NN

VIL

LE G

RO

UP

LOW

ER

CO

LOR

AD

OG

RO

UP

WASECA

SPARKY

GENERAL PETROLEUMS

REX

CUMMINGS

LLOYDMINSTER

DINA

COLONYMcCLAREN

MCMURRAY FM

NE

OC

OM

IAN

-A

PTI

AN

WAB

AMUN

WIN

TERB

URN

WO

ODB

END

BEAV

ERHI

LL L

AKE

DEVONIAN

SOURIS RIVER

DUPEROW

THREE FORKS

SUCCESS FM (S2)


Fig

ure

2 - L

ocat

ion

of st

udy

area

in w

est-c

entr

al S

aska

tche

wan

and

eas

t-cen

tral

Alb

erta

. Red

dot

s on

the

deta

iled

map

indi

cate

loca

tions

of 7

3 w

ells

with

Man

nvill

e G

roup

co

res t

hat h

ave

been

exa

min

ed.

AB

SK

N

T49 T50 T51 T52 T53 T54 T55 T56 T57 T58 T59 T60

W3


Tabl

e 1

- Sum

mar

y of

dep

ositi

onal

faci

es. B

I, bi

otur

batio

n in

dex;

HC

S, h

umm

ocky

cro

ss-s

trat

ifica

tion;

and

SC

S, sw

aly

cros

s-st

ratif

icat

ion.

LIT

HO

LO

GY

PHY

SIC

AL

SE

DIM

EN

TA

RY

ST

RU

CT

UR

ES

BIO

GE

NIC

ST

RU

CT

UR

ES

B I

DE

POSI

ON

AL

PR

OC

ESS

ES

DE

POSI

TIO

NA

L S

ET

TIN

G

1a: F

issi

le

mud

ston

eTh

ick

dark

gre

y to

bla

ck

mud

ston

e, lo

cally

lam

inat

ed

fine-

grai

ned

sand

ston

e/si

ltsto

ne

inte

rlam

inat

ed. S

hale

98%

, sa

ndst

one/

silts

tone

2%

.

Rar

e os

cilla

tion

rippl

e la

min

atio

n.C

hond

rite

s, Pl

anol

ites,

navi

chni

a.0

to 1

Mud

ston

e de

posi

ted

thro

ugh

susp

ensi

on se

ttlin

g or

as f

locc

ulat

ed

mud

; sed

imen

t m

obili

zed

by

osci

llato

ry fl

ow.

Var

ious

dep

ositi

onal

setti

ngs,

aban

done

d ch

anne

l, ch

enie

r pla

in.

1b: B

urro

wed

si

lty

mud

ston

e

Mix

ture

of m

udst

one,

fine

- to

very

fine

-gra

ined

sand

ston

e an

d si

ltsto

ne; m

udst

one

70%

, sa

ndst

one/

silts

tone

30%

.

Rar

e pr

eser

vatio

n of

ph

ysic

al se

dim

enta

ry

stru

ctur

es, l

ocal

ly

osci

llatio

n rip

ples

, mic

ro-

HC

S (th

in-b

edde

d lo

w-

angl

e un

dula

tory

par

alle

l la

min

atio

n).

Teic

hich

nus,

Plan

olite

s, Pa

laeo

phyc

us, A

ster

osom

a,

Hel

min

thop

sis,

Phyc

osip

hon,

G

yrol

ithes

, Cyl

indr

ichn

us,

Skol

ithos

, Cho

ndri

tes,

fugi

chni

a, n

avic

hnia

.

2 to

5M

udst

one

depo

site

d th

roug

h su

spen

sion

settl

ing

or a

s flo

ccul

ated

m

ud;

sand

ston

e de

posi

ted

by

osci

llato

ry fl

ow.

Fully

mar

ine

depo

sitio

n be

low

stor

m

wav

e-ba

se o

r set

tings

shel

tere

d fr

om

stor

ms,

slow

con

tinuo

us d

epos

ition

, ox

ygen

ated

con

ditio

ns.

1c: P

inst

ripe

mud

ston

eD

ark

grey

or b

lack

mud

ston

e w

ith v

ery

thin

inte

rbed

ded

silts

tone

or v

ery

fine-

grai

ned

sand

ston

e.

Pins

tripe

lam

inat

ion,

no

rmal

gra

ding

, syn

eres

is

crac

ks.

Plan

olite

s, C

ylin

dric

hnus

, Sk

olith

os, T

eich

ichn

us,

Gyr

olith

es,

navi

chni

a.

0 to

2M

udst

one

depo

site

d th

roug

h su

spen

sion

settl

ing

or a

s mud

; se

dim

ent m

obili

zed

by o

scill

ator

y flo

w.

Var

ious

dep

ositi

onal

setti

ngs.

Poss

ible

hyp

erpy

cnal

and

/or

hypo

pycn

al c

ondi

tions

.

2a: L

entic

ular

-be

dded

m

udst

one

Gr e

y, si

derit

ic si

lty m

udst

one

with

ver

y fin

e to

fine

gra

ined

rip

pled

sand

ston

e, lo

cally

bi

otur

bate

d. M

udst

ones

>7

0%.

Osc

illat

ion

rippl

e cr

oss-

lam

inat

ion,

con

volu

te

bedd

ing,

nor

mal

ly g

rade

d be

ds, s

harp

con

tact

s.

Plan

olite

s, Sk

olith

os,

Cyl

indr

ichn

us, T

eich

ichn

us,

fugi

chni

a, n

avic

hnia

.

0 to

2A

ltern

atio

n be

twee

n tra

ctio

n tra

nspo

rt an

d su

spen

sion

sedi

men

t de

posi

tion.

Var

ious

dep

ositi

onal

setti

ngs,

com

mon

in

shal

low

, sub

tidal

to

inte

rtida

l env

ironm

ents

.

2b: B

urro

wed

m

udst

one

and

sand

y si

ltst o

ne

Het

erol

ithic

, int

erbe

dded

m

udst

one

and

very

fine

- to

fine-

grai

ned

sand

ston

e.

Mud

ston

es >

70%

.

Osc

illat

ion

rippl

e cr

oss-

lam

inat

ion,

loca

l mic

ro-

HC

S, sy

nere

sis c

rack

s, no

rmal

gra

ding

.

Plan

olite

s, Sk

olith

os,

Gyr

olith

es, T

eich

ichn

us,

Thal

assi

noid

es,

Pala

eoph

ycus

, Ros

selia

, Rh

izoc

oral

lium

, Ter

ebel

lina,

C

hond

rite

s, fu

gich

nia,

na

vich

nia.

1 to

5M

udst

one

depo

site

d th

roug

h su

spen

sion

settl

ing

or a

s flo

ccul

ated

m

ud, s

edim

ent m

obili

zed

by

osci

llato

ry fl

ow. C

lay

shrin

kage

fro

m sa

linity

fluc

tuat

ions

.

Dep

ositi

on w

ithin

fair-

wea

ther

wav

e-ba

se. F

luct

uatin

g sa

linity

, low

rate

of

sedi

men

tatio

n, a

nd p

erio

dic-

even

t se

dim

enta

tion.

3a:

Spor

adic

ally

to

mod

erat

ely

burro

wed

in

terb

edde

d sa

ndst

one

and

mud

ston

e

Het

erol

it hic

mud

ston

e an

d sa

ndst

one;

shar

p co

ntac

ts,

loca

lly b

iotu

rbat

ed.

Sand

ston

e/m

udst

one

prop

ortio

ns a

ppro

xim

atel

y su

bequ

al.

Osc

illat

ion

rippl

e, v

ery

rare

cu

rrent

ripp

les,

sand

-fille

d sy

nere

sis c

rack

s.

Plan

olite

s, Sk

olith

os,

Gyr

olith

es, C

ylin

dric

hnus

, C

hond

rite

s, Te

ichi

chnu

s, na

vich

nia,

?Ar

enic

olite

s, fu

gich

nia.

0 to

2M

udst

one

depo

site

d by

susp

ensi

on

settl

ing

or a

s flo

ccul

ated

mud

; sa

ndst

one

depo

site

d by

un

idire

ctio

nal f

low

and

osc

illat

ion,

sa

linity

-indu

ced

and

clay

shrin

kage

.

Dep

osite

d ab

ove

stor

m w

ave-

base

but

be

low

fair-

wea

ther

wav

e-ba

se,

fluct

uatin

g sa

linity

.

3b:

Mod

erat

ely

to

perv

asiv

ely

burro

wed

i n

terb

edde

d sa

ndst

one

and

mud

ston

e

Het

erol

ithic

mud

ston

e an

d sa

ndst

one;

con

tact

s are

bi

otur

bat e

d.

Sand

ston

e/m

udst

one

prop

ortio

ns a

ppro

xim

atel

y su

bequ

al.

Rar

e os

cilla

tion

and

curre

nt

rippl

es.

Plan

olite

s, C

ylin

dric

hnus

, Sk

olith

os, T

eich

ichn

us,

Pala

eoph

ycus

, Th

alas

sino

ides

, Ros

selia

, C

hond

rite

s, As

tero

som

a,

Gyr

olith

es,

navi

chni

a,

fugi

chni

a.

2 to

4M

udst

one

depo

site

d by

susp

ensi

on

settl

ing

or a

s flo

ccul

ated

mud

, sa

ndst

one

depo

site

d by

un

idire

ctio

nal f

low

and

osc

illat

ion.

Dep

osite

d ab

ove

stor

m w

ave-

base

but

be

low

fair-

wea

ther

wav

e-ba

se, s

low

er

depo

sitio

n th

an F

acie

s 3a.

4a:

Inte

rbed

ded

mud

ston

e a n

d rip

pled

sa

ndst

ones

Het

erol

ithic

mud

ston

e an

d sa

ndst

one.

Dar

k m

udst

one

drap

es o

f cen

timet

re to

m

illim

etre

scal

e, o

rgan

ic

detri

tus;

sand

ston

es c

entim

etre

to

dec

imet

re sc

ale

(>60

%).

Osc

illat

ion

rippl

e cr

oss-

lam

inat

ion

s yne

resi

s cra

cks,

mic

ro-H

CS,

mud

ston

e dr

apes

, co n

volu

te b

eddi

ng,

norm

ally

gra

ded

beds

.

Plan

olite

s, Sk

olith

os,

Gyr

olith

es, C

ylin

dric

hnus

, fu

gich

nia,

nav

ichn

ia.

0 to

2M

udst

one

depo

site

d m

ainl

y fro

m

susp

ensi

on se

ttlin

g, o

scill

ator

y flo

w,

poss

ible

hyp

erpy

cnal

and

hyp

opyc

nal

cond

ition

s. C

lay

shrin

kage

from

sa

linity

fluc

tuat

ions

.

Fair

wea

ther

and

rare

stor

m

cond

ition

s. Fl

uvia

l sed

imen

t inp

ut in

th

e co

asta

l reg

ime.

4b:

Bio

turb

ated

in

terb

edde

d m

udst

one

and

rippl

ed

sand

ston

es

Het

erol

ithic

mud

ston

e an

d sa

ndst

one.

Dar

k m

udst

one

drap

es c

entim

etre

to

mill

imet

re sc

ale,

org

anic

de

tritu

s; sa

ndst

ones

cen

timet

re

to d

ecim

etre

scal

e (>

60%

).

Osc

illat

ion

rippl

e cr

oss-

lam

inat

ion

syne

resi

s cra

cks,

mic

ro-H

CS,

mud

ston

e dr

apes

, co n

volu

te b

eddi

ng,

norm

ally

gra

ded

beds

.

Plan

olite

s, Sk

olith

os,

Gyr

olith

es, C

ylin

dric

hnus

, C

hond

rite

s, Te

ichi

chnu

s, Pa

laeo

phyc

us, L

ocke

ia,

Thal

assi

noid

es,

fugi

chni

a,

navi

chni

a.

2 to

4M

udst

one

depo

site

d fro

m

susp

ensi

on, s

ands

tone

ent

rain

ed b

y w

aves

. Cla

y sh

rinka

ge fr

om sa

linity

flu

ctua

tions

.

Fair-

wea

ther

and

rare

stor

m w

aves

. V

aria

ble

wat

er sa

liniti

es, p

ossi

ble

fluvi

al se

dim

ent i

nput

.

Fine

-gra

ined

sand

ston

e,

orga

nic

detri

tus,

loca

l m

udst

one

drap

es.

Wav

y pa

ralle

l lam

inat

ion.

Skol

ithos

, Cyl

indr

ichn

us,

Cho

ndri

tes,

fugi

chni

a.0

to 2

Sedi

men

t mob

ilize

d by

wav

es.

Sedi

men

t dep

osite

d ab

ove

fair-

wea

ther

wav

e-ba

se.

Ver

y fin

e-gr

aine

d, e

rosi

onal

ly

amal

gam

ated

sand

ston

e, ra

re

mud

ston

e d r

apes

.

HC

S, S

CS,

osc

illat

ion

rippl

e cr

oss-

lam

inat

ion.

fugi

chni

a0

to 1

Eros

iona

l am

alga

mat

ion,

pro

duce

d by

stor

m w

aves

.H

CS

and

SCS,

dep

osite

d un

der

stric

tly o

scill

ator

y st

orm

con

ditio

ns.

S edi

men

t dep

osite

d ab

ove

fair-

wea

ther

wav

e-ba

se o

r bel

ow fa

ir-w

eath

er w

ave-

base

, but

abo

ve st

orm

w

ave-

base

.Fi

ne- t

o m

ediu

m-g

rain

ed

sand

ston

e w

ith so

me

inte

rstit

ial m

ud a

nd

biot

urba

ted

fabr

ic, a

ltern

atin

g w

ith th

in, s

harp

-bas

ed

lam

inat

ed sa

nd b

eds.

Loca

lly H

CS

and

osci

llatio

n rip

ples

Dom

inat

ed b

y bi

ogen

ical

ly

mot

tled

fabr

ic;

Plan

olite

s, Sk

olith

os, P

alae

ophy

cus,

Aste

roso

ma,

Cho

ndri

tes,

?A

reni

colit

es, C

ylin

dric

hnus

.

0 to

5Se

dim

ent m

obili

zed

by o

scill

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Facies 1a: Fissile Mudstone

This facies consists of massive dark grey to black mudstone, locally laminated. Siderite-cemented bands or nodules occur sporadically throughout the facies. Very fine-grained sandstone and siltstone comprise 3 to 5% of the facies, and locally show normal grading, and oscillation ripples (Figures 3A and 3B). Facies 1a displays BI 0 to BI 1 and is sporadically bioturbated. Trace fossils include Planolites, Chondrites, and navichnia (mantle-and-swirl structures).

We interpret that muds have been deposited through suspension settling or as flocculated mud, and silts and sands transported by oscillatory waves. The graded beds indicate sediment emplacement by sudden, waning-energy density underflows. Low bioturbation intensities are probably the result of high sedimentation rates. The presence of diminutive Chondrites suggests that marine conditions prevailed.

Facies 1b: Burrowed Silty Mudstone

Facies 1b consists of up to 70% mudstone with intercalated layers of fine-grained sandstone/siltstone. It is pervasively bioturbated, although rare preservation of physical sedimentary structures such as oscillation ripples and micro-hummocky cross-stratification occurs within some siltstone and sandstone layers. Carbonaceous detritus is locally common. Bioturbation intensities within the facies commonly reach BI 5 and locally range down to BI 2 where primary sedimentary structures are preserved (Figures 3C and 3D). Trace fossils include Teichichnus, Planolites, Palaeophycus, Asterosoma, Helminthopsis, Phycosiphon, Gyrolithes, Cylindrichnus, Skolithos, Chondrites, fugichnia (escape structures), and navichnia.

We interpret Facies 1b to have been deposited in a well oxygenated and more normal marine setting, probably lying below fair-weather wave-base and largely sheltered from storms. Facies 1b contains distinctively marine trace fossils (e.g., Asterosoma, Helminthopsis, and Phycosiphon) and corresponds to a slightly stressed expression of the Cruziana Ichnofacies.

Facies 1c: Pinstripe Mudstone

Facies 1c is characterized by abundant millimetre- to centimetre-thick, sharp-based, locally normally graded siltstone or very fine-grained sandstone layers interlaminated with fissile mudstone. Interlaminae within the pinstripes are commonly parallel laminated and locally are wavy rippled. Syneresis cracks and siderite nodules are common elements of the facies. Bioturbation intensities at the bed scale range from BI 0 to BI 2 (Figures 3E and 3F). Trace-fossil diversities are typically low. Trace-fossil suites are dominated by deposit-feeding structures, with subordinate dwelling structures of inferred suspension-feeding organisms. Trace fossils include Planolites, Cylindrichnus, Skolithos, Teichichnus, Gyrolithes, and navichnia.

The low bioturbation intensity of the facies suggests that sedimentation rates were high. Reduced trace-fossil diversities, diminutive ichnogenera, and locally common syneresis cracks indicate that the salinities were variable and commonly brackish (Beynon and Pemberton, 1992; Gingras et al., 2005; MacEachern and Gingras, 2007).

b) Facies 2: Mudstone-dominated Heterolithic Facies Facies 2 comprises mudstone-dominated heterolithic intervals wherein mudstone comprises at least 70% of the facies, with the remaining components consisting of siltstone and sandstone layers. Facies 2 has been subdivided into two subfacies on the basis of sedimentological and ichnological features.

Facies 2a: Lenticular-bedded Mudstone

Facies 2a comprises dark grey to black mudstone interbedded with abundant oscillation-rippled siltstone and sandstone stringers. Mudstone and siltstone bedsets are typically 1 to 3 cm thick and may reach up to 5 cm; the surfaces between them are commonly sharp and undulating. Facies 2a shows low bioturbation intensities (BI 0 to BI 2, with BI 1 most common); this weakly burrowed character distinguishes it from Facies 2b. Biogenic structures are typically diminutive, and suites show low diversity (Figure 4A). Trace fossils include Planolites, Skolithos, Cylindrichnus, Teichichnus, fugichnia, and navichnia.

Facies 2a is interpreted to have been deposited due to autogenic alternations between traction-sediment reworking by waves and suspension-sediment deposition. Low-diversity trace-fossil suites, diminutive ichnogenera, and a predominance of facies-crossing elements suggest an environment in which water salinities were persistently brackish (e.g., Beynon and Pemberton, 1992; Gingras et al., 2005; MacEachern et al., 2005; MacEachern and Gingras, 2007). The trace-fossil suite contains elements of both the Skolithos and Cruziana ichnofacies. Facies 2a corresponds to sediments deposited in brackish-water bays, and bay-head deltas.


Figure 3 - Mudstone facies (Facies 1): A) Fissile mudstone (Facies 1a) with locally millimetre-scale silt laminations; trace fossils include Planolites (P); well 07-03-054-25W3, 443.6 m. B) Unburrowed fissile mudstone from Facies 1a; well 08-30-051- 19W3, 478.1 m. C) Facies 1b showing pervasively bioturbated (BI 5) sandy mudstone; trace-fossil suite includes Skolithos (S), Teichichnus (T), Planolites (P), Cylindrichnus (Cy), Palaeophycus (Pa), and Chondrites (Ch); well 07-03-054-25W3, 490.2 m. D) Pervasively bioturbated (BI 4 to BI 5) silty mudstone from Facies 1b with vestiges of oscillation-ripple lenses; trace fossils include Planolites (P), Teichichnus (T), Chondrites (Ch), Asterosoma (As), Helminthopsis (H), Palaeophycus (Pa), Cylindrichnus (Cy), and fugichnia (Fu); well 13-01-052-27W3, 472.8 m. E) and F) Pinstripe mudstone from Facies 1c; note the thin lenses of fine-grained sandstone/siltstone; trace fossils include Planolites (P), Chondrites (Ch), Cylindrichnus (Cy), Gyrolithes (G), Teichichnus (T), and navichnia (Na); well 11-20-051-19W3, 486.5 m and well 15-35-058-21W3, 315.5 m.


Figure 4 - A) Weakly burrowed lenticular-bedded mudstone from Facies 2a; note the syneresis cracks (syn) in the middle of the photograph; trace fossils include Planolites (P); well 07-08-052-23W3, 444.2 m. B) Facies 2b showing oscillation-rippled sandstone and siltstone encased in mudstone; trace-fossil suite includes Planolites (P), Cylindrichnus (Cy), and Rhizocorallium (Rh); well 07-29-051-25W3, 511.5 m. C) Facies 2b showing heterolithic laminated to burrowed silty mudstone interbedded with thin wavy parallel-laminated sandstone; trace fossils include Planolites (P), Cylindrichnus (Cy), Skolithos (S), Gyrolithes (G), Asterosoma (As), Chondrites (Ch), Palaeophycus (Pa), and Phycosiphon (Ph); well 05-06-051-20W3, 452.2 m. D) Moderately bioturbated mudstone interbedded with sandstone of Facies 2b; note the disruption of sandstone due to bioturbation; trace-fossil suite consists of Planolites (P), Cylindrichnus (Cy), Skolithos (S), Chondrites (Ch), and Teichichnus (T); well 13-14-050-25W3, 529.8 m. E) Sporadically bioturbated interbedded mudstone and sandstone of Facies 3a showing oscillation ripples; note the syneresis cracks (syn); trace-fossil suite includes Cylindrichnus (Cy), and Planolites (P); well 07-10-052-23W3, 421.6 m. F) Interbedded mudstone and oscillation-rippled sandstone of Facies 3a; trace-fossil suites are dominated by Gyrolithes (G), which are mud filled and penetrate both mudstone and sandstone with rare, isolated Cylindrichnus (Cy) and Arenicolites (Ar); well 04-16-050-19W3, 455.8 m.


Facies 2b: Burrowed Mudstone and Sandy Siltstone

The heterolithic units of Facies 2b comprise 70% mudstone that is commonly bioturbated. Individual mudstone beds range from 1 to 10 cm thick. Thin, intervening siltstone and fine-grained sandstone beds (1 to 3 cm thick) contain rare oscillation ripples, small-scale hummocky cross-stratification, and current ripple laminae. The facies locally contains syneresis cracks and normal grading. Bioturbation values vary at the bed scale from BI 1 to BI 5 but typically average BI 3 (Figures 4B, 4C, and 4D). Trace-fossil diversities are typically low and are characterized by diminutive forms. Trace fossils include Planolites, Skolithos, Gyrolithes, Teichichnus, Thalassinoides, Palaeophycus, Rosselia, Rhizocorallium, Terebellina, Chondrites, fugichnia, and navichnia.

We interpret Facies 2b to reflect deposition in a standing body of water within fair-weather wave-base. The environment was characterized by a low (though variable) rate of deposition subject to periodic-event sedimentation. Salinity appears to have fluctuated but was generally higher than in Facies 2a. Trace-fossil suites are dominated by deposit-feeding structures and lesser dwellings of inferred filter-feeding organisms, and corresponds to a stressed expression of the archetypal Cruziana Ichnofacies.

c) Facies 3: Subequal Sandstone and Mudstone Heterolithic Facies Facies 3 consists of regularly interbedded sandstone and mudstone, forming heterolithic composite bedsets. Sandstone and mudstone each constitute approximately 50% of the facies. Facies 3 has been subdivided into two subfacies on the basis of sedimentological and ichnological features.

Facies 3a: Sporadically to Moderately Burrowed Interbedded Sandstone and Mudstone

Facies 3a consists of low-angle, parallel-laminated to oscillation-rippled sandstones intercalated with moderately bioturbated mudstone. These form composite bedsets that correspond to wavy bedding. Soft-sediment deformation structures and syneresis cracks are locally present. Internal laminae are locally marked by carbonaceous detritus or spherulitic siderite. Bioturbation intensities range from BI 0 to BI 2, and trace fossils are sporadically distributed (Figures 4E and 4F). Mudstone beds commonly possess higher bioturbation intensities. Ichnogenera include Planolites, Skolithos, Gyrolithes, Cylindrichnus, Chondrites, Teichichnus, Arenicolites, navichnia, and fugichnia. Locally within this facies, suites are monogeneric and dominated by one of Gyrolithes, Cylindrichnus, or Planolites.

Facies 3a is interpreted to have been deposited subaqueously, above storm wave-base but below fair-weather wave-base. Low-diversity trace-fossil suites, diminutive ichnogenera, and locally monogeneric suites suggest an environment in which water salinities were persistently brackish (e.g., Beynon and Pemberton, 1992; Gingras et al., 2005; MacEachern et al., 2005; MacEachern and Gingras, 2007). Low-intensity bioturbation may indicate a combination of stressed conditions and elevated deposition rates. Such settings are typical of shallow, sheltered, brackish-water bays with river-sediment input along their landward margins.

Facies 3b: Moderately to Pervasively Burrowed Interbedded Sandstone and Mudstone

Facies 3b is sedimentologically similar to Facies 3a, but is more pervasively burrowed (Figure 5A). Bedding contacts are pervasively bioturbated and only locally sharp. Bioturbation intensities range from BI 2 to BI 4. Trace fossils include Planolites, Cylindrichnus, Skolithos, Teichichnus, Palaeophycus, Thalassinoides, Rosselia, Chondrites, Asterosoma, Gyrolithes, navichnia, and fugichnia.

Facies 3b is interpreted to have been deposited subaqueously above storm wave-base but below fair-weather wave-base, similar to Facies 3a, but in higher salinity environments. This is particularly well indicated by the presence of Asterosoma, Chondrites, and Rosselia, as well as the higher diversity suites. The elevated bioturbation indicates generally slower rates of deposition than for Facies 3a. Facies 3b is interpreted to have been deposited in shallow bay settings that were subject to less pronounced depositional stresses. Such bays are generally less restricted, and/or have fewer fluvial channels draining into their landward margins.

d) Facies 4: Sandstone-dominated Heterolithic Facies Facies 4 comprises sand-dominated heterolithic successions. Sand beds constitute significantly greater than 50% (generally 60 to 80%) of the interval. The facies is, therefore, intergradational between Facies 3 and Facies 5. It can be separated into two subfacies, which have distinctive ichnological characteristics but are sedimentologically similar.


Figure 5 - A) Sandstones and interbedded mudstones of Facies 3b largely reworked by Planolites (P), Thalassinoides (Th), Cylindrichnus (Cy), and Palaeophycus (Pa); note the syneresis cracks (syn) at the top of the photograph; well 13-02-051-22W3, 480 m. B) Sporadically bioturbated rippled sandstone mantled by mudstone (Facies 4a); note the syneresis cracks (syn) in the middle of the photograph; trace-fossil suite consists of Skolithos (S), Gyrolithes (G), Cylindrichnus (Cy), Planolites (P), Palaeophycus (Pa), and Thalassinoides (Th); well 12-24-052-27W3, 441.4 m. C) and D) Wavy parallel-laminated sandstone interbedded with moderately to intensely bioturbated muddy sandstone beds (Facies 4b). Trace-fossil suite comprises Skolithos (S or Sk), Gyrolithes (G), Planolites (P), Cylindrichnus (Cy), Palaeophycus (Pa), Teichichnus (T or Te), Asterosoma (As), Phycosiphon (Ph), and Helminthopsis (H); well 13-02-052-27W3, 431.5 m and 429.2 m. E) Erosionally amalgamated oscillation-rippled sandstone (Facies 5); trace fossils include Cylindrichnus (Cy); well 13-02-052-27W3, 433.2 m. F) Hummocky cross-stratified sandstone (Facies 6); note the escape structure (fu); well 07-03-054-25W3, 480.2 m.


Facies 4a: Interbedded Mudstone and Rippled Sandstones

Sandstones within Facies 4a commonly contain oscillation-ripple lamination, and less commonly possess hummocky cross-stratification. Intervals comprise composite bedsets that range from wavy bedding to flaser bedding. Mudstone layers are thin and may contain normal grading and syneresis cracks. They commonly drape the rippled sandstone beds. Many are dark and unburrowed (Figure 5B). Facies 4a intervals are weakly and sporadically burrowed, showing BI 0 to BI 2. Trace-fossil suites are low diversity, commonly diminutive, and generally dominated by facies-crossing structures such as Planolites, Skolithos, Gyrolithes, Cylindrichnus, fugichnia, and navichnia.

Facies 4a reflects the interplay of fluctuating wave-energy conditions and variable sedimentation rates in a subaqueous environment. The dominance of sand and low bioturbation intensities within the subfacies suggests generally high sedimentation rates. Mudstone deposition appears to have occurred under low-energy conditions by suspension settling and clay flocculation, but also at rates sufficiently high to impede larval recruitment of endobenthos. The domination of facies-crossing trace fossils, diminutive sizes of the ichnogenera, and the presence of syneresis cracks indicate persistently (and probably fluctuating) brackish-water conditions. Trace-fossil suites are dominated by dwellings of suspension-feeding and surface-detritus-feeding organisms, typical of stressed expressions of the Skolithos Ichnofacies, and common to salinity-stressed environments (MacEachern and Gingras, 2007).

Facies 4b: Bioturbated Interbedded Mudstone and Rippled Sandstones

Facies 4b generally displays higher BI values than Facies 4a, although their physical sedimentological characteristics are similar. It locally contains syneresis cracks, soft-sediment deformation features, and some normally graded mud drapes. Bioturbation intensities, though high, are nevertheless variable, and typically range from BI 2 to BI 4 at the bed scale (Figures 5C and 5D). Ichnogenera include Planolites, Skolithos, Gyrolithes, Cylindrichnus, Chondrites, Teichichnus, Palaeophycus, Lockeia, Thalassinoides, fugichnia, and navichnia.

Facies 4b reflects fluctuating wave-energy conditions in a subaqueous setting, with variable sedimentation rates. Sand-bed emplacement occurred under higher energy and higher deposition rates than those for the intervening muds. Muds may, however, record both rapid clay flocculation and slow suspension-sediment settling. The generally higher bioturbation intensity attests to the reduced sedimentation rates compared to Facies 4a. The trace-fossil suite contains numerous facies-crossing elements, characteristic of environmental stress, but with intercalated ichnogenera typical of higher salinity conditions (e.g., Chondrites). The subfacies probably indicates shallow brackish-bay depositional environments which were less restricted, and/or were subject to reduced freshwater input along the bay margins.

e) Facies 5: Wavy and Planar Parallel-laminated Sandstone The sandstone beds within the Facies 5 are dominated by thin zones of low-angle planar parallel lamination, interpreted as hummocky cross-stratification and micro-hummocky cross-stratification, with associated oscillation-ripple lamination. Sand beds are typically erosionally amalgamated (Figure 5E) and generally range from 20 cm to 1 m in thickness. Mudstone layers are typically less than 1 centimetre thick and comprise less than 10% of the facies. Soft-sediment deformation is also present as convolute bedding and rare micro-faults, especially where thin beds of mud are intercalated. Bioturbation intensities are typically BI 0 to BI 2 at the bed scale. Trace fossils include Skolithos, Cylindrichnus, Chondrites (in the muds), and fugichnia.

Facies 5 is interpreted to have been deposited subaqueously above fair-weather wave-base where prevailing energy levels were high. Oscillatory processes both during fair-weather shoaling and during storms dominate the preserved expression of the facies. Mud layers, though rare, typically reflect drapes on the laminated sand indicating rapid clay flocculation and settling onto the sandy substrates. The paucity of burrowing in the muds indicates rapid deposition or accumulation under conditions of wave shoaling (e.g., mobile fluid mud). The trace-fossil suite comprises a low-diversity and probably stressed expression of the Skolithos Ichnofacies, but is dominated by facies-crossing dwelling structures that may reflect both suspension-feeding and surface-detritus-feeding organisms. Such suites are typical of higher energy sandy bays exposed to storms and wave energy, and with locally turbid water columns. Environments that possess such features occur in sandy bays and delta fronts of bay-head deltas.

f) Facies 6: Low-angle Undulatory Parallel-Laminated Sandstone Facies 6 consists of erosionally amalgamated very fine-grained, low-angle undulatory parallel-laminated sandstone beds. Laminae are delicate (millimetre scale) and form both concave upward and convex upward inclinations (Figure 5F). Rare oscillation-rippled sandstones are locally interbedded. Bioturbation is absent or very low (BI 0 to BI 1).


Facies 6 is interpreted as hummocky cross-stratification to swaly cross-stratification deposited in a subaqueous setting and reflecting sediment emplacement during storms. The amalgamated nature of sandstone beds in Facies 6 suggests increased storm frequency and heightened storm intensity, which reduced infaunal colonization windows for larvae and favoured erosional removal of fair-weather beds, respectively. Such conditions are typical of deposition above fair-weather wave-base in a subaqueous setting exposed to strong storms. They occur in more open sandy bays and in storm/wave-dominated delta fronts and bay-head delta fronts. The overall marine character of this facies can only be assessed in the context of the overlying and underlying facies with which it is associated.

g) Facies 7: Bioturbated Sandstone Facies 7 comprises fine- to medium-grained sand, in which primary stratification has been disrupted due to the activity of infaunal organisms (Figure 6A). Locally, oscillation ripples and small-scale tempestites (wavy-parallel laminae attributable to hummocky cross-stratification) are preserved within the facies. At the bed scale, bioturbation intensities range from BI 0 to BI 5, averaging BI 3. Common ichnogenera include Planolites, Skolithos, Palaeophycus, Asterosoma, Chondrites, Arenicolites, Cylindrichnus, and fugichnia.

The sediments of Facies 7 are interpreted to have been subaqueously deposited above fair-weather wave-base, influenced by waves and storms, but with slow and generally continuous accumulation of sand. Trace-fossil suites within this facies are characterized by a mixture of sessile deposit-feeding structures and dwelling structures attributed to suspension-feeding organisms, surface-detritus feeders and/or carnivores. The low-diversity suite attests to generally reduced salinities, but which were nonetheless sufficiently high to permit the presence of diminutive Asterosoma and Chondrites. This is consistent with sheltered open-bay environments with little or no direct fluvial sediment influx.

h) Facies 8: Massive (Apparently Structureless) Sandstone Facies 8 comprises very fine- to medium-grained, massively bedded (structureless) sandstone displaying high degrees of oil saturation and a paucity of visible sedimentary structures. Locally, low-angle planar lamination may be detected in some zones. Bioturbation is either absent, or the intense oil saturation has obscured the biogenic structures (Figures 6B and 6C).

Interpretation of Facies 8 is problematic. The apparently structureless nature of the sandstone may be due to oil saturation which has obscured primary physical sedimentary structures and biogenic structures. Alternatively, the sands may have been deposited very rapidly without forming stratification (e.g., Bhattacharya and MacEachern, 2009). Such a mechanism would require high-energy density underflows capable of transporting the sand in suspension (e.g., grain flows, hyperpycnites, or turbidites). Intercalated low-angle planar lamination supports the possibility that oil saturation masks the structures, although interpretation of the facies is best made in the context of over- and underlying facies.

i) Facies 9: Current-ripple to Trough Cross-stratified Sandstone Facies 9 encompasses fine-grained to medium-grained sandstones that are dominated by current-generated structures. The most common structures are trough cross-stratification and current-ripple lamination (Figures 6D and 6E). Combined flow ripples and aggradational current ripples are present locally. Bioturbation intensities are generally low, and range from BI 0 to BI 2 at the bed scale. Trace-fossil suites consist of Skolithos, Cylindrichnus, Planolites, and fugichnia.

Deposition of Facies 9 reflects conditions of quasi-steady unidirectional flow with high sedimentation rates, and high- to moderate-energy hydrodynamic regimes. The bulk of the facies reflects asymmetric bedforms of dune and ripple scale that migrate in response to current flow. The trace-fossil suite is low diversity and markedly consists of facies-crossing forms. The paucity of burrowing should not be taken as an indication of reduced salinity, however, as mobile bedforms such as dunes are difficult to colonize unless they are moribund for extended periods of time. Facies 9 probably records channel environments (e.g., estuarine, tidal, distributary, etc.), but could also correspond to proximal delta-front settings (approximate upper shoreface equivalents) of bay-head deltas or open-coast deltas. The context of the intervals with over- and underlying facies is required in order to make such determinations.

j) Facies 10: Coal Facies 10 comprises coal and highly carbonaceous mudstones. Facies 10 units tend to be black to dark grey in colour, depending upon the amount of interstitial silt and sand (Figure 6F). This facies is locally associated with underlying rooted zones, confirming its in situ condition. No trace fossils are associated with this facies.

Facies 10 is interpreted to have been formed as a result of accumulation of plant material and organic matter on the coastal plain, probably as swamps, bogs and forested zones. Micro-lithotype analysis would be required in order to


Figure 6 - A) Thoroughly bioturbated (BI 4 to BI 5) muddy sandstone (Facies 7), with Planolites (P), Chondrites (Ch), Cylindrichnus (Cy), and Palaeophycus (Ph); well 11-26-055-26W3, 540.3 m. B) Apparently structureless sandstone (Facies 8); lack of visible structure is probably due to high oil saturation; well 04-24-053-24W3, 431.2 m. C) Structureless sandstone with abundant organic detritus (Facies 8); well 13-01-052-27W3, 480 m. D) Cross-stratified sandstone indicating dune-scale bedforms generated by current flow (Facies 9); note the siderite cementation; well 13-11-054-25W3, 482.4 m. E) Current-rippled sandstone (Facies 9); note the erosional contacts of ripples and a climbing ripple in the lower part of the photograph; well 11-20-051-19W3, 491.4 m. F) Well-bedded coal (Facies 10); well 05-06-051-20W3, 443.1 m.

A


ascertain the particular coal-forming environment. In coastal regimes, the preservation of coal is commonly associated with elevated and/or rising water tables. As such, coals may mark the early onset of transgression in the coastal regime.

4. Conclusions Preliminary study of ichnology and sedimentology from the Lower Cretaceous (Lower to Middle Albian) Sparky, Waseca, and McLaren formations of the upper Mannville Group in west-central Saskatchewan has resulted in the recognition of ten recurring facies. Sedimentary structures record a predominance of oscillatory depositional regimes (shoaling waves and storms) with limited influence by currents. This supports reduced tidal energies in the setting, and widespread dominance of subaqueous conditions. Low-diversity suites, the prevalence of facies-crossing elements and the diminutive character of ichnogenera within the recognized facies indicate that the environment was generally subject to physico-chemical stress. The most likely stress was one of reduced and fluctuating salinity, although the magnitude of salinity reduction was variable. The local occurrence of more marine ichnogenera (e.g., Asterosoma, Phycosiphon, Chondrites, Rhizocorallium, and Helminthopsis) within the succession may suggest that elevated salinities occurred periodically within the study area.

This interpretation suggests that the facies within the study area correspond to mainly coastal-margin subaqueous settings (e.g., shallow bays), with local bay-head deltas and estuaries feeding into them. Within the formations, the Sparky and McLaren show more diverse suites, whereas the Waseca Formation shows less diversity and is more heterolithic. Storm-generated structures such as hummocky cross-stratification to swaly cross-stratification are more common in the Sparky and Waseca formations as are current-generated structures.

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