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.
References
Aitken, J.D., Macqueen, R.W. and Usher, J.L. 1973. Reconnaissance studies of Proterozoic and Cambrian stratigraphy, lower Mackenzie River area (Operation Norman), District of Mackenzie, Geological Survey of Canada Paper 73-9.
Bond, G.C. and Kominz, M.A. 1984. Construction of tectonic subsidence curves for the early Paleozoic miogeocline, southern Canadian Rocky Mountains: Implications for subsidence mechanisms, age of breakup, and crustal thinning. Geological Society of American Bulletin, vol. 95, pp. 155–173.
Dixon, J. 1997. Cambrian stratigraphy of the Northern Interior Plains, Northwest Territories. Geological Survey of Canada Open File Report 3510, pp. 1-38.
Dixon, J. and Stasiuk, L.D. 1998. Stratigraphy and hydrocarbon potential of Cambrian strata, northern Interior Plains, Northwest Territories. Bulletin of Canadian Petroleum Geology, vol. 46, no. 3, pp. 445–470.
Gingras, M.K., MacEachern, J.A. and Dashtgard, S.E. 2011. Process ichnology and the elucidation of physico-chemical stress. Sedimentary Geology, vol. 237, pp. 115-134.
Hannigan, P.K., Morrow, D.W. and MacLean, B.C. 2011. Petroleum resource potential of the northern mainland of Canada (Mackenzie Corridor). Geological Survey of Canada Open File Report Open File 6757, pp. 1-260.
Herbers, D.S., MacNaughton, R.B., Timmer, E.R. and Gingras, M.K. 2016. Sedimentology and ichnology of an Early-Middle Cambrian storm-influenced barred shoreface succession, Colville Hills, Northwest Territories. Bulletin of Canadian Petroleum Geology, vol. 64, no. 4, pp. 538-554.
Janicki, E.P. 2004. Hydrocarbon Pools of the Colville Hills. Northwest Territories Geological Survey, NTGO Publication: 2004-006.
Levy, M. and Christie-Blick, N. 1991. Tectonic subsidence of the early Paleozoic passive continental margin in eastern California and southern Nevada. Geological Society of America Bulletin, vol. 103, pp. 1590–1606.
Macqueen, R.W. and MacKenzie, W.S. 1973. Lower Paleozoic and Proterozoic Stratigraphy, Mobil Colville Hills E-15 well and environs, Interior Platform, District of Mackenzie. Geological Survey of Canada Report of Activities 1973, Paper 73-1, Part B, pp. 183-187.
MacEachern, J.A., Bann, K.L., Bhattacharya, J.P. and Howell, C.D. Jr. 2005. Ichnology of deltas: organism responses to the dynamic interplay of rivers, waves, storms and tides. In: River Deltas: Concepts, Models, and Examples. Giosan, L. and Bhattacharya, J.P. (eds.). Society for Sedimentary Geology Special Publication, vol .83, pp. 49-85.
Meijer Drees, N.C. 1974. Geology of the lower Paleozoic formations in the subsurface of the fort Simpson area, district of Mackenzie, N.W.T. Geological Survey of Canada Paper 74-40.
MacLean, B.C. 2011. Tectonic and stratigraphic evolution of the Cambrian basin of northern Northwest Territories. Bulletin of Canadian Petroleum Geology, vol. 59, no. 2, pp. 172–194.
Pemberton, S.G., Reinson, G.E., and MacEachern, J.A. 1992. Comparative ichnological analysis of late Albian estuarine valley-fill and shelf-shoreface deposits, Crystal Viking Field, Alberta. In: Applications of Ichnology to Petroleum Exploration: A Core Workshop. Pemberton, S.G. (ed.). Society of Economic Paleontologists and Mineralogists Core Workshop 17, pp. 291-317.
Pyle, L.J. 2012. Cambrian and Lower Ordovician Sauk Megasequence of Northwestern Canada, Northern Rocky Mountains to the Beaufort Sea. In: The Great American Carbonate Bank: The Geology and Economic Resources of the Cambrian-Ordovician Sauk Megasequence of Laurentia, AAPG Memoir 98, pp. 675-723.
Scotese, C.R. and McKerrow, W.S. 1990. Revised world maps and introduction. In: Palaeozoic Palaeogeography and Biogeography. W.S. McKerrow, and C.R. Scotese, (eds.). Geological Society of London Memoir, vol. 12, pp. 1–24.
Sloss, L.L. 1963. Sequences in the Cratonic Interior of North America. Geological Society of America Bulletin, vol. 74, pp. 93-114.
Williams, M.Y. 1922. Exploration east of Mackenzie River between Simpson and Wrigley. Geological Survey of Canada Summary Report 1921, part B, pp. 56–66.
Williams, M.Y. 1923. Reconnaissance across northeastern British Columbia and the geology of the northern extension of Franklin Mountains, N.W.T. Geological Survey of Canada Summary Report 1922, part B, pp. 65–87.