Isotopic Data (Sr, S, and 0) from Anhydrites in the Northern Williston Basin, and Pennsylvanian Ages for the Watrous and Amaranth Formations of Saskatchewan and Manitoba I 2 3 4 R.E. Denison , HR. Krouse , and TP. Poulton Denison. R.E., Krouse, H.R., and Poulton, T.P. (2001): Isotopic data (Sr, S. and 0) fr om anhydrites in the northern Williston Basin, and Pennsylvanian ages for the Watrous and Amaranth formations of Saskatchewan and Manitoba; in Summary of Investigations 200[ , Volume 1, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 2001-4.1. 1. Introduction from other evaporite units in the northern Williston Basin. We have determined 109 strontium isotope Strontium, sulphur, and oxygen isotopic ratios provide ratios of core samples of anhydrite from 19 widely important infonnation regarding the marine character, separated boreholes in Saskatchewan, Manitoba, and freshwater influence, circulation characteristics, and North Dakota (Figure I) and two outcrop localities of age of deposition of anhydrites and gypsums. This gypsum from South Dakota. Sulphur and oxygen report presents the first data indicating an age (Late isotope ratios were determined on 36 of these Pennsylvanian) for the previously undated upper anhydrite/gypsum samples. Most of the samples come members of the Watrous and Amaranth evaporitic from the Upper Watrous Member, but other units were formations in southern Saskatchewan and Manitoba. It sampled as well. also presents preliminary results from a more limited examination of Devonian through Jurassic(?) samples 2. Watrous and Amaranth Manitoba Formations The Watrous Formation and its correlative in Manitoba, the Amaranth Formation, are subdivided into lower and upper members. Neither has been the subject of detailed sedimentologic analysis. The lower members of each formation consist of red and red-brown, partly dolomitic mudstones and siltstones, with minor sandstones with pitted grains. They contain minor anhydrite laminae, blebs and fracture fills (see descriptions by Milner and Thomas, 1954; Stott, 1955; Barchyn, 1982; Christopher, 1990, p677; 0 McCabe, 1990, pl 1-12; Kreis, 9-139-100 1991 ). On well logs, the red beds are characterized by a high . '-·- ·- ·- ·- ·- ·- _,_. - - - - - gamma ray signature. This red­ 105" 100· bed unit infills irregularities of the underlying weathered Figure I - Location ofwells from which samples were analyzed. Three outcrop erosional surface on Devonian to samples from two locations in South Dakota are not shown. Locations ofcross sections W-E, NW-SE, and E-E' are shown. The wells yielding typical Upper Watrous Mississippian rocks in strontium values indicating Pennsylvanian ages are shown ;,, black. The 0-edge Saskatchewan and progressively showing the limits ofthe Upper Watrous (heavy dashed line) and the 20 m and 40 m older rocks down to Precambrian isopachs (light dashed lines) are approximated from electronic data bases offormation basement eastward in Manitoba. picks. The Lower Watrous reaches at least 30 m in thickness. I Geological Survey of Canada Contribution No. 2000280. 1 Geoscicnces. The University of Texas at Dall as. Box 830688, Richardson, TX 75080. ' Department of Physics. University of Calgary, 2500 University Drive NW. Calgary. AR T2N I N4. • Geological Survey of Canada. 3303 - 33rd Street NW, Calgary, AB T21. 2A 7. Saskatchewan Geological Survey 77 The Lower Watrous red beds are continuous southward (Christopher, 1990, p677), and its basal contact with into northern Montana and northwestern North Dakota the Lower Watrous is sharp to apparently gradational. (Francis, 1956; Lefever et al., 1991), where they are The anhydrite occurs as small nodules or in massive considered to be a northern extension of the upper part pure beds up to I .5 m thick, in which it has a "chicken­ of the Spearfish Formation and are generally assigned wire" texture (Kreis, 1991, p l 0) consisting of a Triassic age (see Dow, 1967; Carlson, 1968, 1993; coalesced nodules. No extended sequences of Mclachlan, 1972) (Table I). laminated beds, nor enterolithic, teepee or sabkha-like breccias have been observed. The Upper Watrous (Evaporite) Member of southern Saskatchewan contains varying proportions of The upper boundary is apparently gradational with the anhydrite, more than 50% in some wells, interbedded lowest Gravelbourg mudstones and dolomitic with buff or grey argillaceous limestone, dololutite and limestones. However, beyond the northeastern limit of calcareous or dolomitic mudstone. Some of the buff the basal beds of the Lower Gravelbourg, the Upper dololutites are cryptalgal-laminated. The Upper Watrous is brecciated and contains secondary anhydrite Watrous overlaps the margins of the lower red-bed unit fracture fills below the unconformity at its top (Kreis, around the northern edges of the Williston Basin 1991 ). The Upper Watrous exceeds 40 m in thickness. Table 1 - Table offorma tions, showing conventional age assignments (left), revised age assignments (right), and proposed correlations (bold). Columns are adapted f rom The correlative Upper Amaranth Bluemle el al. (1986), Kreis (1 991), LeFever et al. (1991), Poulton et al. (1994), and anhydrite unit in Manitoba Richards et al. (1 994). contains interbeds of shale and dolostone, and chert concretions (fl are present at the top (McCabe, SOUTHERN SOUTHERN (JJ(JJ > f:l~ NORTH DAKOTA w -o 1990, p l t ). Descriptions of the ~> SASKATCHEWAN MANITOBA ~ J: :::> <( w <( t:.t; 0:: Upper Watrous and Amaranth e:. W. CENT. N.CENT formations are available in INYAN KARA FM. UPPER SUCCESS (S2) SWAN RIVER FM. L. CRET. L CRET. - - - - - Milner and Thomas ( 1954 ), Stott ( I 955), Barchyn ( 1982), LOWER SUCCESS (S1) 0:: 0:: w Christopher, ( 1990), McCabe w 0. 0.. SWIFT WASKADA / 0. ( 1990), and Kreis ( 1991). !l. 7 ::, ::, FORMATION MASEFIELD FORMATION f-- 0 / ' - The Upper Watrous- Amaranth O::o.. ,.,.- ? <::, =>o / evaporite unit correlates CJ er ROSERAY \ z southward with the Poe Member ~CJ RI EROON ::,. 8:;i FORMATION RUSH I ffi IL UPPER of the lower Piper and th e lower LAKE I ir z u 0 Nesson Formation in northern ii\ FM. I ~ w North Dakota (Lefever et al. , z :. .J &"' BOWES cc 1991 ). Regional distributions of ::, 0 UPPER\ I- 0 Cl -, UJ LL --- - - - --- Cl 3j these strata are described in UJ ::,t u..~ ,0: .J FIREMOCN PIPER "' < LOWER\. \h :; Peterson (1972), Imlay (1980), 0 LIMESTONE FM . :r: ">U.. UJ 0 (F) 0 :. and Anna ( 1986). 5' lU LOWER ~ 0:: TAMPICO 5 UPPER_\ ~~ w u. KLINE I. UPPER LOWER MEMB ER 3. Age Constraints on the I ~ ~ RESTON FM. PICARD 1z LOWER " Watrous and Amaranth 1£ ? POE :. I~ Cl) UPPER MEMBER u.. UPPER MEMBER Formations MEMBER IZ "O ~ ::, :r: I- I ci - ~::; z Determining the age of TRIASSIC i IL ~ deposition for evaporite units is <:. z (?) § SAUDE ·- ~ ~ -OWER MEMBER OWERMEMBE~ < one of the great geologic u.. " 0 < z ~ PI NI: SALT)! ~,~5 ! ' <( challenges. Because evaporites :J; BELFIEL~ ~ ;:' ::i z >- generally lack definitive flora or <( MINNEKAHTA Cl er, 1 Cl) 5' 1\ - ..:;:"' I z fauna, their age is usually I!: z OPECHE I UJ w ~ 0.. determined by lateral correlation Cl. .Q I with open marine rocks or, as is ~ ::, BROO~ : ...,0.. I the case with the Watrous i AMSDEN z ~ CJ '. I evaporites, by stratigraphic w z Cl. ~ TYLER bracketing (Holser, 1992). Pocock ( 1972) reported z JOTTER~ OTT&< z <( < palynomorphs from both the ii: gi~ KIBBEY KIBBEY ii: 0.. t:::::=::7 ~ 0.. Lower and Upper Watrous, vi ""' vi Cl) =-7 CHARLES ) z CHARLES cHK.' Cl) cii 0 ui including a minor marine 0. MISSION if) Cl) Q~ C) d MISSION CANYON !!)~ MISSION CANYON CANYON - ~ component ofmicroplankton ~ i i LODGEPOLE ~ LODGEPOLE LODGEPOLE 78 S11mmary of Investigations 2()() I. Volume I from the upper member, but they do not permit precise Assuming these ages to be correct, the Kline age assignments. (limestone and dolostone), Picard (red shale), and Poe Evaporite (gypsum) members of the Nesson Formation The youngest rocks underlying the Watrous and are undated. Amaranth formations of western Canada are Mississippian (but mid-Pennsylvanian strata underlie equivalents in adjacent North Dakota) and the oldest 4. Strontium Isotopes in Seawater and overlying rocks are Middle Jurassic. The latitude of Evaporites these constraints has allowed the Watrous to be assigned a variety of ages, most recently Middle Strontium isotope ratios in seawater varied with time. Jurassic(?) for the upper part and Triassic(?) for the This variation is recorded in calcium-bearing fossils lower (Poulton et al., 1994; Edwards et al., 1994 ). and minerals precipitated from seawater (e.g., Burke et al. , 1982; McArthur, 1994; Veizereta/., 1999). The youngest dated rocks below the Watrous Because the mixing time of the oceans is thousands of Formation are Serpukhovian (Mississippian)- the years and the residence time of strontium is mill ions of Otter Formation of the Big Snowy Group (Richards et years, the open oceans contain perfectly mixed al., 1994 ). In adjacent northe rn North Dakota and strontium isotopes. The 87Sr/86Sr ratio curve is northeastern Montana, the youngest rocks underlying constructed by determining the isotope ratio in samples the Spearfish-Nesson-Piper succession are of known age th at have retained the original seawater DesMoinesian, and possibly Early Missourian ratio (reviewed by Veizer et al., 1997). The quality of (Pennsylvanian) - the uppermost Minnelusa Formation the age assignment and selection of samples that have (Mallory, 1972). retained the original ratio are crucial in the construction of the curve. By convention, the seawater The oldest dated strata above the Upper Watrous in curve is said to be rising or increasing (ratio increasing Saskatchewan (Upper Graveibourg and Shaunavon with decreasing age) or falling or declining (ratio formations) are Middle Jurassic, possibly Late decreasing with decreasing age). Bajocian based on contained faunas and on indirect dates from the Piper in adjacent Montana (Wall, 1960; Evaporites represent a special challenge because they Paterson, 1968; Poulton, 1984 ). These formations are a can be precipitated from brines derived from seawater, shallow marine, limestone-rich, heterogeneous unit continental water, or mixtures of the two (hybrid containing sandstones and shales.