Paleomagnetic Indications for a Late Age for Part of the , Williston Basin, Southern Saskatchewan

1 1 2 Randolph J Enkin , Judith Baker , and K. G. Osadetz

Enkin, R.J., Baker, 1. , and Osadetz, K.G. (2001): Paleomagnctic indications for a Late Paleozoic age for part of the Watrous Formation, Williston Basi n, southern Saskatchewan; in Summary of In vestigations 200 1, Volume I, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 2001-4.1. l. Introduction Saskatchewan, Manitoba, and the adjacent United States of America (Figure I). The basin contains The Williston Basin is a Phanerozoic epicratonic basin several significant petroleum provinces and mineral about 800 km in diameter underlying southern deposits (e.g. Burrus et al., 1996), and has thus been the focus of detailed exploration. SASKATCHEWAN MANITOBA However, there remain profound uncertainties concerning the origin, tectonics, and even _..-. Petroleum ..... Province stratigraphy of the basin. These uncertainties hamper the Depth to base of understanding of its petroleum (m) systems.

The Watrous Formation (Milner and Thomas, 1954) is composed of red bed siliciclastics and anhydrites (Christopher, 1984; Edwards et al., 1994 ). The formation onlaps a major regional underlain by the deeply eroded Paleozoic carbonate succession (Figure 2). These usually " tight" rocks are commonly cap rocks to the Mississippian subcrop oil fields. The formation has been a minor ----- oil producer, from more sandy porous lithologies, as typified by the Waskada Field in southwestern Manitoba that was discovered in 1980.

The Watrous Formation is informally split into "upper" and " lower" members. The upper Middle member is dominated by '""'"''~ anhydrites and is considered to be of age (Christopher, 1984). The lower member has been correlated to the Lower Figure 1 (detail) Jurassic upper which crops out in North Dakota. Others suggest this member may be as young as Middle Jurassic (see Edwards et al., 1994). Figure 1 .. Regional setting, major tectonic elements, and petroleum provinces ofthe Williston Basin. Basi11form-lines are indicated by the depth to the base ofth e Carbo11iferous .

I Geological Survey of Canada - Pacific, 9860 West Saanich Road, POB 6000, Sidney, BC V8L 482. 2 Geological Survey of Canada, 3303 - 33rd Street NW, Calgary, AB T2L 2A7.

72 Summary ofInv estigations 200/, Volume I (km) Fonnation is underlain by the 0 100 200 300 Charles Fonnation of the Carboniferous Madison Group. NNE SSE N s 2. Methods We drilled 2.5 cm diameter minicores perpendicular to the core axis. Since the original core _o was almost vertical, our E minicores were taken parallel to ~ the near- horizontal strata. Where c .2 the rock was massive and red­ j .!! pigmented, we sampled every 2 w to 3 ft (60 to 90 cm).

Magnetic remanence was JURASSIC measured on an Agico JR5-A spinner magnetometer. Magnetic susceptibilities were measured using a Sapphire SI2-B meter. We measured both full core and minicore susceptibility. Thermal demagnetizations were performed with an ASC TD-48 furnace, over 12 to 15 steps up to 700°C. Imperial* Herald Magnetic remanence directions 1-31-1-20W2 were determined by Principal .~I:. =- I. Triassi<;. .. Component Analysis, using programs developed by the first lower tJ r-re~ian author (available on request). Watrous { 'VVVV'\, tf] I~:~~sylvanian 3. Results The core of the lower Watrous - - consists mostly of red to orange Figure l - Section of Williston Basin illustrating the Phanerozoic stratigraphic mudstone to siltstone with rare succession after Burrus et al. (1996). Location ofthis section is indicated in Figure I. beds of sandstone. Common The interpreted stratigraphic position ofthe lower member ofthe Watrous in the ripple marks and cross bedding analyzed core is shown in the inset in the lower left-hand corner ofthe diagram. indicate shallow-water deposition. Reduction spots are In an attempt at better determining the age of this non­ common, and usually have sharp fossiliferous formation, we sampled core from an oil edges. They are early diagenetic features, and exploration well for paleomagnetic study. The apparently have not been altered by subsequent fluid observed magnetic inclination gives the paleolatitude at motions. which the rocks were situated at the time of magnetization. By comparing the paleolatitude history Characteristic demagnetization curves are plotted in of the site, derived from North America's apparent Figure 3. The natural remanent magnetizations (NRM) polar wander path (APWP), with the observed are typical of red sediments (geometric mean paleolatitude, we can detennine the age of the 5.34 mA/m ±48%). The samples are relatively magnetization. If the magnetization is primary, we can efficiently magnetized (Koenigsberger ratio KN= date the age of sediment deposition. NRM/induced magnetization = 1.34 ±45%) compared to similar rocks which usually have KN < 1. However, We sampled the Imperial Herald core (1-31-1-20W2; about half of the magnetization is a drilling induced 49.1 °N, I 04.5°W, approximately 160 km south of overprint. This component, roughly parallel to the Regina), because it lies near the centre of the Williston direction ofminicoring, is removed by heating to 150° Basin and has excellent recovery of the lower member or 200°C. The remaining magnetization remains very of the Watrous Fonnation. The Watrous Formation was stable to about 500°C. The dominant magnetic carrier picked between depths of 5206 and 5593 ft (1586.8 is clearly hematite (with a Curie Temperature Tc = and 1704.7 m), and the bottom 110 feet (33.5 m) were 680°C), as most of the stable magnetization unblocks almost continuously recovered. The Watrous between 600° and 700°C heating, well above the Tc of magnetite, 580°C. The remanence direction held by the hematite is easily determined by line-fitting this high

Saskatchewan Geological Survey 73 lower Watrous Group A: 5515 feet attempted to orient the core NN • Lower N 6.25mA/m using the faint present-fi eld eo nupper overprint which survived the drilling overprint. The resolution is low, but we find north-west declinations in the A group, and southerly declinations in the B group. This indicates that the A group is truly normal polarity, while the B group has reverse o , oo 300 soo 1oo·c polarity and is not a normal polarity magnetization from lower Watrous Group A: 5546 feet when North America was NN situated in the southern eo 3 75mA/m hemisphere.

The mean inclination of group A is 25.1° ±4.1° , N = 49, Fisher precision k = 14.9 (method of Enkin and Watson, 1996). This 6 75'C observed inclination is consistent with the expected inclination derived from the North American APWP for ages between 209 to 3 1 1 Ma lower Watrous Group B: 5570 feet (Pennsylvanian through Triassic) NN ea 6.1mA/m (Figure 5). Note, in particular, that North America drifted rapidly northward during latest Triassic and Early Jurassic time, and that the expected incl ination fo r all more recent times is far steeper than what we observe. 675'C Thus we can be sure that these rocks did not suffer a post­ 0 100 300 500 Triassic remagnetization. lower Watrous Group B: 5576.5 feet f he observed polarity is normal, NN eo 5.92mAlm and during most of Pennsylvanian and Permian times, the geomagnetic field has persistent reverse polarity, a period known as the Kiaman Reverse Chron. Thus we conclude that the age of this interval of the Watrous Formation is between latest Pennian to Triassic time (260 to 209 Ma), likely during one Figure 3 - Typical demagnetization characteristics oflo wer Watrous Formation nonnal polarity chron. specimens, illustrated as orthogonal projections, stereographs, and magnetic intensity as a/unction oftherma l demagnetization step. The B group has significantly shallower inclination (- 13 .5° temperature segment. About 20% of the samples were ±3.8°, N = 25, k = 34.7). This result is consistent with unsuitable, since the remanence was demagnetized at the North American APWP between the ages of 351 much lower temperatures and gave either incoherent and 303 Ma (Mississippian th rough Pennsylvanian) directions, or only drilling overprint directions. (Figure 5). There are reversals before the Kiaman Reverse Chron, so it is not possible to constrain the age The observed remanence directions clearly separate using magnetic polarity as confidently as with th e A into two groups (Figure 4). The A group, from depths group. But since we observed no reversals in this of5484 to 5560 ft (167 1.5 to 1694.7 m), has positive interval of the core, we consider it likely to have post­ (down) inclinations, while the 8 group, from 5563 to dated the start of the Kiaman at about 3 15 Ma. 5583 ft ( 1695.6 to 1701.7 m), has negative (up) inclinations. The declinations are random. However, from core segments which we could piece together, we

74 Summary of Investigations 200 I, Volume I concerning the age and correlation of that formation, N and in this respect the present report is no exception". •Lower They go on to say, " We maintain the tradition". While A Upper recognizing the preliminary nature of our study, we feel confident in saying that the lower Watrous 1, recovered in the Imperial Herald core was not • deposited after Triassic time, and is most likely latest • Permian to Triassic in age. Furthermore, a period of • • tens of millions of years is missing near the bottom of • the Watrous in this core. The lowest 20 ft (6. 1 m) is of • ) Paleozoic, probably mid-Pennsylvanian, age.

+ Our study on sub-Watrous portions of the core • • indicates that only the red sediments retain stable • • magnetizations. The possibility exists that the Charles I • • • Formation has suffered pervasive remagnetization during Permian time. This observation was also • • • ••• suggested from the paleomagnetic study of Devonian • • Prairie Formation evaporites in Saskatchewan (Koehler r et al., 1997). Clearly further study on core material •• from around the Williston Basin is essential to delineate the stratigraphic and hydrothermal history of • •• the region. lower Watrous Group A 5484-5560 feet 5. Acknowledgments A We thank the staff at the Subsurface Geological IP, Laboratory of Saskatchewan Energy and Mines for AA help accessing the core, and Glenda Rathwell and Jennifer Porter for excellent laboratory assistance. A

A 6. References A Besse, J. and Courtillot, V. (1991 ): Revised and synthetic apparent polar wander paths of the + African, Eurasian, North American and Indian plates, and true polar wander since I 00 Ma; J. Geophys. Resear., v96, p4029-4050.

Burrus, J. , Osadetz, K.G. , Wolf, S., Doligez, B., Visser, K., and Dearborn D. ( 1996): A two-dimensional A regional model of Williston Basin hydrocarbon systems; Amer. Assoc. Petrol. Geol. Bull., v80, p265-29 l.

Christopher, J.E. (1984): Depositional pattern and oil lower Watrous Group B field trends in the Lower Mesozoic of the northern 5563-5583 feet Williston Basin, Canada; in Lorsong, J.A. and Figure 4 - Stereographs ofthe lower Watrous Groups A Wilson, M.A. (eds.), Oil and Gas in Saskatchewan, and B. The declinations are random, but the polarities and Sask. Geo!. Soc., Spec. Pub I. No. 7, p83-102. inclinations are well defined. Cumming, A.O. ( 1956): The Watrous strata in Saskatchewan; in First International Williston Basin Symposium, N. Oak. Geol. Soc./Sask. Geol. 4. Discussion and Conclusion Soc., Sask. Geol. Soc., p 165-1 69. The basic stratigraphy of a region must be established before questions concerning the regional setting of Edwards, D.E., Barclay, J.E., Gibson, D.W., Kvill, deposition, subsequent deformation and hydrocarbon G.E., and Halton, E. ( 1994): Chapter 17, Triassic generation can be answered. Watrous stratigraphy is strata of the Western Canada Sedimentary Basin; both complicated and uncertain. Edwards et al. ( 1994, in Mossop, G. and Shetsen, I. (comp.), Geological p273) quote Cumming (1956): " It is almost traditional Atlas of the Western Canada Sedimentary Basin, that papers on the Watrous should express doubt Can. Soc. Petrol. Geol./Alta. Resear. Counc., p259-275.

Saskatchewan Geological Survey 75 80 I 70N N. Americap AP P I I 60N ! I I 50N 60 ... ,I . I .. ., .. .. i 40N i I I I 30N 40 I I· I 20N

,...... , 20 10N ...... 0 r c .....0) 0 ;:::.: ::; c ro 1 jl__ . lj a. 0 1 0 CD c d~ ,...... () 1 ,__,0 c I I Ill 1 11 I I- i -20 Jt ..l. "~ ~·t -' 10S

11,; I I !C'C I. ~ l ·:s: ~ ! ,_I - -'--- --i 20S -40 I· Kiaman j Reverse ! Chron i I , 30S Ord Sil Dev Miss Tr -60 Jur Cret Pg /Ng 40S 500 400 300 200 100 0 Age [Ma] Figure 5 - The paleomagnetic inclination history ofthe Williston Basin (calculated at 49°N, 105°W, using the North American apparent polar wander path compiled by Van der Voo (1990), Wynne e t al. (1992), and Besse and Courtillot (1991)), compared to the observed inclinations ofth e two levels ofl ower Watrous Formation sampled at the Imperial Herald well: A 5484 to 5560 ft (16 71.5 to 1694. 7 m), and B 5563 to 5583ft (1695.6 to 1701.7 m). The uncertainty bands show 95% confidence intervals.

Enkin, R.J. and Watson G.S. ( 1996): Statistical Van der Voo, R. ( 1990): Paleozoic paleomagnetic analysis ofpaleomagnetic inclination data; J. poles from Europe and North America and Geophys. Intemat., v126, p495-504. comparisons w ith continental reconstructions; Rev. Geophys., v28, p l67-206. Koehler, G., Kyser, T.K., Enkin, R.J. , and Irving, E. {1997): Paleomagnetic and isotopic evidence for Wynne, P.J., Irving, E., Schulze, D.E., Halls, D.C., and the diagenesis and alteration of evaporites in the Helmstaedt, H.H. ( 1992): Paleomagnetism ofthree Paleozoic Elk Point Basin, Saskatchewan, Canada; Canadian Rocky Mountain diatremes; Can. J. Can. J. Earth Sci., v34, p 16 19- 1629. Earth Sci., v29, p35-47.

Milner, R.L. and Thomas, G.E. (1954): Jurassic System in Saskatchewan; in Western Canada Sedimentary Basin- a Symposium, Amer. Assoc. Petrol. Geol., Ralph Leslie Rutherford Memorial Volume, p250-267.

76 Summary of /11 vestigatio11s 200/ , Volume I