Subsurface Sedimentology, Ichnology and Sequence Stratigraphy of Cambrian Mount Clark and Mount Cap Formations Beneath the Colville Hills, Northwest Territories Matthew J. Sommers1, Murray K. Gingras1, Robert B. MacNaughton2 Karen M. Fallas2 1University of Alberta; 2Geological Survey of Canada – Calgary

Summary

The Lower and Middle Cambrian succession in the Colville Hills region, Northern Interior

Plains, Northwest Territories, consists of the Mount Clark (sandstone), Mount Cap (shale

and carbonate) and Saline River Formations (mudrock, evaporites, and carbonate), which

were deposited in an epicontinental basin. A recently completed core-based high-resolution

facies analysis that included detailed ichnology is the basis for a new sequence

stratigraphic framework for the Mount Clark and Mount Cap Formations. Fifteen distinct

facies were recognized and categorized into four facies associations: FA1 (storm-

influenced shoreface sandstone); FA2 (fairweather shoreface sandstone to sandy

carbonate); FA3 (offshore mudstone); and FA4 (bioturbated dolostone). FA1 tended to

preserved primary sedimentary structures with absent to minimal bioturbation, suggesting

ichnologically stressed environments during deposition. Conversely, FA2 recorded

abundant bioturbation and relatively high ichno-diversity. This presentation focuses on core

Union Mobil Colville D-45 (67° 14’ 08.57”, 125° 09’ 20.87” W; Figure 1) and Tweed Lake A-

67 (66° 56’ 11.60” N, 125° 56’ 18.88” W ;Figure 1), which were chosen for their exemplary

examples of the sedimentary facies and occurrence in characterstic facies associations.

The base of the Cambrian is a regional-scale sequence boundary (the “sub-Cambrian

unconformity”). Although the detailed facies permit recognition of parasequences within

individual wells, parasequences generally cannot be correlated reliably outside their

respective depocentres. However, packages of facies associations can be correlated

across the study area, and permit regional delineation of three T-R sequences within the

Mount Clark Formation and the lower part of the Mount Cap Formation.

Geological Background

The Lower and Middle Cambrian succession in the Colville Hills area, Northwest Territories

consists of the Mount Clark, Mount Cap and Saline River Formations. These units were

described first by Williams (1922, 1923) from outcrops in the Franklin Mountains nearly one

hundred years ago. Mount Clark Formation is dominated by quartz sandstone, Mount Cap

Formation consists of shale and lesser carbonate facies, and Saline River Formation

contains mudrocks, evaporites, and carbonates (e.g., Aitken et al., 1973). In the last fifty

years, considerable efforts have been put forth to better understand these strata in the

subsurface Cambrian Basin and adjacent outcrop belts (MacQueen and Mackenzie, 1973;

Meijer Drees, 1974; Williams, 1987; Hamblin; 1990; Pugh, 1993; Dixon, 1997; Dixon and

Stasiuk, 1998; Janicki, 2004; MacLean, 2011; Herbers et al., 2016), reflecting the known

presence of oil and gas in Cambrian strata beneath the Colville Hills (Hannigan et al., 2011).

The present study incorporates additional subsurface data that were not available to

previous workers.

Strata in the Cambrian Basin were formed in a semi-enclosed, epicontinental environment

as significant quantities of siliciclastics entered a shallow sea. Laurentia (ancestral North

America) was located at the equator, rotated 90 degrees clock-wise from present day, and

had rifted from Rodinia resulting in the basal unconformity of the Sauk I Sequence and

continued subsidence (Sloss, 1963; Dixon and Stasiuk, 1998; Pyle, 2012). Global

greenhouse conditions began in the Middle Cambrian, causing eustatic sea level to rise,

which in conjunction with syndepositional normal faulting, allowed for a large increase in

accommodation space in the Cambrian basin (Bond and Kominz, 1984; Scotese and

McKerrow, 1990; Levy and Christie-Blick, 1991; MacLean, 2011).

Figure 1. Map of Northwest Territories (left) with outline of study area (right) showing well locations; orange stars indicate wells with core that was logged in this study. Cored wells Colville D-45 and Tweed Lake A-67 are indicated by the red circles.

Methods The study focused primarily on the Colville Hills region, Northwest Territories, centered

approximately on the Colville Lake town site. Additional subsurface control was

incorporated from the broader Mackenzie Corridor, bounded between latitudes 62.5°N to

69°N and longitudes 122°W to 132°W, spanning more than 300,000 km2. Nine Cambrian

cores were logged and described using AppleCORE© logging software. For subsurface

correlation, forty-five wells with wireline log data that penetrated the Precambrian were

used in geoSCOUT©. Process sedimentology, detailed ichnology and T-R sequence

stratigraphy were applied to the study of these strata, with a particular focus on the Mount

Clark and lower part of the Mount Cap Formations.

Results and Interpretations

A high-resolution facies analysis utilizing detailed sedimentological and ichnological

observations documented fifteen distinct facies. These can be grouped into three facies

associations (FA). FA1 consists of sandstones that have been subjected to fluctuating wave

energy from storms and to episodic sedimentation, limiting bioturbation across the

shoreface profile; primary sedimentary structures such as hummocky cross-stratification

(HCS) are common throughout and ichnofauna tends to be diminutive where present. FA2

consists of sand-dominated facies also deposited along a shoreface profile. In contrast to

FA1, however, diverse ichnological assemblages point towards robust food supplies, low

sedimentation rates and intense sediment reworking by infauna. Upper shoreface facies

tend to be coarser-grained and associated with monospecific Skolithos assemblages

resulting in ‘piperock’, whereas comparatively distal facies display a diverse ichnological

assemblage produced by deposit-feeding organisms in a fully-marine environment, as

based, for example, on the presence of Asterosoma (Pemberton et al., 1992; MacEachern

et al., 2005; Gingras et al., 2011). FA3 consists of transgressive mudstone facies deposited

in offshore environments where low to absent dissolved oxygen levels severely hindered

sediment colonization. FA4 consists of a sandy, nodular, bioturbated dolostone deposited

along a shoreface profile during conditions when carbonate supply exceeded clastic input.

The sharp contrast between fairweather (FA2) and storm-influenced (FA1)

paleoenvironments observed within the Mount Clark and lower Mount Cap Formations can

be in part attributed to the great size of the basin and correspondingly large spacings

between wells. Each depocentre within the overall basin has similar stacking patterns at

the facies association level. At the facies level, howevever, stacking patterns are too

variable to permit reliable correlations.

Facies Association

Facies Depositional

Influence Sub-Environments

Dominant Lithology

FA1 3, 6, 11, 12, 15

Storm-Influenced Shoreface

Lagoon to Distal Lower Shoreface

Cross-bedded, sparsely bioturbated sandstone

FA2 4, 5, 7,

9, 10, 14 Fairweather Shoreface

Upper Shoreface to Distal Lower Shoreface

Intensely Bioturbated, heterolithic sandstone

FA3 1, 2, 8, 13 Offshore Upper to Lower

Offshore

Low to moderately bioturbated mudstone and shale

FA4 9 Shoreface Middle to Lower

Shoreface

Bioturbated nodular dolostone

Table 1. Summary table of facies, facies associations with corresponding depositional enviornments.

Within the Mount Clark Formation, an aggradational to progradational succession of storm-

influenced FA1 was deposited first, followed by a <5 m thick package of green bioturbated

shales, followed by dark grey, largely barren shale (FA3). The remainder of the formation

consists of either FA1 or FA2 (or a combination of both) packaged in two transgressive-

regressive (T-R) cycles. The base of the Mount Cap is marked by the first occurrence of

bioturbated dolostone facies (FA4). A basal clastic succession composed mainly of FA2

facies is overlain by a predominantly transgressive package of FA3. Four regional

carbonate beds (FA4) can be correlated in the lower Mount Cap Formation; the top of the

fourth carbonate bed marks the boundary between the lower and upper Mount Cap

Formation, and the top of the third T-R cycle.

Acknowledgements

This study was funded by the Research Affiliate Program (RAP) of Natural Resources

Canada (Geo-mapping for Energy and Minerals Program), the University of Alberta and

NSERC.

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