Davis Creek silt, an Early Pleistocene or Late Pliocene deposit in the ...

NOTES

Davis Creek silt, an Early Pleistocene or Late Pliocene deposit in the Cypress Hills of Saskatchewan W. J. VREEKEN Department of Geography, Queen's University, Kingston, Ont., Canada K7L 3N6

R. W. KLASSEN Terrain Sciences Division, Geological Survey of Canada, Calgary, Alta., Canada T2L 2A7 AND

R. W. BARENDREGT Department of Geography, The University of Lethbridge, Lethbridge, Alta., Canada TIK 3M4

Received March 15, 1988 Revision accepted August 11, 1988 Davis Creek silt is the informal name for a previously unreported loess and its reworked detritus encountered at several locations to the south of the east and centre blocks of the Cypress Hills. This unit intervenes between a pediment with an estimated age of 10 Ma and Late Wisconsinan glacial deposits. Because the unit has reversed magnetization, it is older than 788 ka, the astronomical age of the Matuyama-Brunhes magnetic polarity reversal. The unit also contains an undated volcanic ash from the Pearlette ash family that could represent the Mesa Falls (1.27 Ma) or the Huckleberry Ridge (2.02 Ma) ash bed. Davis Creek silt overlies an oxidized weathering zone and contains large secondaly carbonate nodules near its truncated top that were, in places, reworked into a lag deposit or stone line before accumulation of the glacial overburden. At one location Davis Creek silt is separated from this overburden by a unit of cryoturbated gravelly loam with remnants of a reddish-yellow paleosolic B horizon. Nous dtcrivons ici pour la premikre fois le silt de Davis Creek, nom informel utilist pour dtsigner un dCpBt de loess et de detritus remanit, lequel est observt plusieurs endroits au sud des blocs est et centre des collines du Cypress. Cette unit6 est intercalte entre un ptdiment d'Bge estimt B 10 Ma et des dtp6ts glaciaires datant du Wisconsinien tardif. L'aimantation inverste des mattriaux indique un Bge plus vieux que 788 ka, l'ige astronomique correspondant B la transition magnttique Matuyama-Brunhes. Une couche de cendres volcaniques, non-datte, appartenant ?i la famille des cendres de Pearlette fait partie de cette unitt, pourrait correspondre ?i un lit des cendres de Mesa Falls (1,27 Ma) ou de Huckleberry Ridge (2,02 Ma). Le silt de Davis Creek recouvre une zone oxydte par mtttorisation et pri!s du toit tronqui! le silt contient de gros nodules secondaires de carbonate qui furent remanits, en places, pour former un dtpBt d: fragments abandonnts ou une trainke de dkbris pierreux, et ce avant l'accumulation de la couverture de dtpBts glaciaires. A un endroit le silt de Davis Creek est isole des dtp8ts glaciaires par une unitt de limon graveleux cryoturbt incluant quelques rtsidus de l'horizon B jaune-rougeitre d'un paltosol. [Traduit par la revue] Can. J. Earth Sci. 26, 192-198 (1989)

Introduction Surficial geology studies in the Cypress Hills region of Alberta and Saskatchewan (Fig. 1) begun in 1983, have provided significant new information concerning the stratigraphy and age of its glaciated surfaces (Vreeken 1986; Klassen and Vreeken 1987). Because of poor preservation of preWisconsinan sediments, less is known about the history of the nonglaciated plateaus and pediments. Recently however, we located a number of sites (Fig. 2) with a previously unreported deposit, referred to informally as Davis Creek silt, that intervenes between a pediment surface and its glacial overburden. This stratigraphical unit dates minimally from the Early Pleistocene because it has reversed paleomagnetism. It also contains a Pearlette volcanic ash bed. This note serves to introduce the new evidence and to indicate its relevance to the regional chronological framework. Pnnted in Canada I Impnme au Canada

Previous work Nonglaciated sul3caces The Cypress Hills plateaus form part of the continental divide between Hudson Bay and the Gulf of Mexico and are remnants of the Cypress Plain, the oldest Tertiary landscape level within the Interior Plains (Alden 1932). They are underlain by gravel of the Oligocene - Middle Miocene Cypress Hills Formation (McConnell 1885; Russell 1975; Storer 1975). Most of this gravel originates from the western ranges of the Rocky Mountains but was deposited by northeastflowing braided streams with sources in the Sweetgrass Hills, Bearpaw Mountains, and possibly the Highwood Mountains of northern Montana (Leckie and Cheel, in preparation). The Cypress Plain has been regarded as a relict depositional surface or surface complex (McConnell 1885; Alden 1924, 1932; Vonhof 1969) and as a Miocene erosion surface (Williams and

TES

193

centre blocks may have been glaciated at an earlier time, as depicted by Prest et al. (1968). No indication of this has been found on the west block.

ALBERTA

CYPIPESS

MONTANA

1WKILOMEIRES SCALE

r so

FIG. 1. Map showing location of the Cypress Hills and outline of area covered by Fig. 2. Dyer 1930) modified by altiplanation during Wisconsinan glaciation (Jungerius 1966). Cyclical lowering of base levels for the northern Great Plains resulted in pediments graded to depositional surfaces or benches that formed below the Cypress Plain or No. 0 bench (Collier and Thom 1918; Alden 1924, 1932). Alden's relative age scheme for these surfaces was used by other workers (Williams 1929; Broscoe 1965; Westgate 1968; Karlstrom 1987) for correlation and additional dating. If the Cypress Plain dates from the Middle Miocene it could be about 15 Ma old. The cycle that resulted in formation of the Flaxville Plain (No. 1 bench) and the No. 1 pediment occurred around 10 Ma ago, since that is the age of volcanic ash from the Flaxville Formation in northeast Montana (Colton et al. 1986). The next cycle ended more than 320 ka ago, before formation of the oldest dated travertine on the No. 2 pediment in central Montana (Szabo and Lindsey 1986). Various benches and pediments have been recognized within the Cypress Hills (Williams and Dyer 1930; St. Onge 1966; Jungerius 1967; Westgate 1968; Vreeken 1986). These have been dated in the relative sense only. Glaciation The Late Wisconsinan Laurentide glacier overrode parts of the preglacial pediments and topped divides between the west, centre, and east blocks (Fig. 2). Meltwater channels and outwash trains issued from northern ice over the south-facing pediments, where they were truncated by deposits from ice that circumvented the west and east blocks. Radiocarbon dates from deglacial and early postglacial sediments range from 14 000 to 12 000 years BP (Klassen and Vreeken 1987) and confirm a Late Wisconsinan age for most of the glacial sediments. Ongoing field studies confirm that parts of the east and

Setting and stratigraphy of the Davis Creek site The Davis Creek site (site 1; Figs. 2, 3a) is at an elevation of 1044 m as1 and is located in lsd. 2, sec. 27, tp. 6, rge. 25 (latitude 49"30118"N, longitude 109"18'O"W). It is a roadcut in the north wall of Frenchman Valley near the confluence with Davis Creek and on the edge of a pediment surface. This pediment bevels the east block plateau (No. 0 bench) at 1200 m as1 and is mantled by till and glaciolacustrine sediments below 1120 m asl. It appears to be a No. 1 pediment because it descends to a surface near Woody Island Creek (Blaine County, Montana) that Alden (1932, p. 18) described as Flaxville Plain. Most of it is underlain by quartzite gravel and sandstone of the Ravenscrag (Tertiary) and Frenchman (Cretaceous) formations as defined by Vonhof (1969) and Whitaker (1976). Stratigraphical observations Three major stratigraphical units were examined, initially from roadside outcrops shown in Fig. 3a and later from backhoe trenches made at stations 3 and 4. Unit I (station 1; Fig. 3a) is quartzite gravel, originating from the Cypress Hills and Ravenscrag formations and redeposited on a surface that was cut more than 160 m below the level of the east block plateau. This unit is about 5 m thick over Frenchman sandstone exposed further downslope. The concealed upper contact of the unit was established from augering at the base of the trench at station 3 (Fig. 3a). Unit 11is Davis Creek silt and is 5.25 m thick. It is a massive brownish-yellow sandy loam (2.25 m) grading into massive very pale brown silt loam (2.25 m) and overlain by an increment of planar-bedded very pale brown sandy loam and silt loam with-a discontinuous pebble band (Fig. 3c). This increment was 1.2 m thick in the intial roadside outcrop at station 3 but only 0.75 m in the backhoe cut made at the same location. Unit I1 is calcareous throughout, and some silt beds are strongly cemented with carbonate. We regard the massive lower part of Davis Creek silt as loess. It lacks the structures normally seen in waterlaid sediments, and there is no older unit composed of similar material from which it could have been derived by fluvial action. It could only have been deposited by the wind. However, the bedded upper part of Davis Creek silt, with its undulating base, probably represents an alluvial deposit composed of reworked loess material. Unit III is 2.1 m thick at station 4 (Figs. 3a, 3b) and has two subunits. The basal 0.9 m (Fig. 3b) is a complex with planar beds composed of reworked material from unit 11 that alternate with beds of material similar to that from the overlying subunit. The latter subunit is a massive, dark grayish-brown calcareous loam with few scattered quartzite, granite, and gneiss pebbles that have north -south-aligned long axes. This subunit is 1.2 m thick at station 4 and 1.7 m thick in an auger hole at station 5, farther away from the valley slope. Scattered over its surface are Laurentide erratics (granite, gneiss, and dolomite) as large as 0.3 m. Unit I11 must have formed in close proximity to glacier ice in an ice-marginal water body. The basal subunit accumulated in the absence of strong currents and the massive upper subunit may represent flow till. We

CAN. J. EARTH SCI. VOL. 26, 1989

km

0Unglaciated Bedrock Hummocky Moraine Till Veneer o n Pediment Till Plain

Eroded Slope Meltwater Channel and Outwash Terrain Sites , , Profile

FIG.2. (a) Surface-geology map of the central portion of the Cypress Hills in Saskatchewan. (b) Cross section through the east block along the line A-B shown in (a) (1) Cretaceous shale and sandstone; (2) Tertiary sand and gravel (Ravenscrag and Cypress Hills formations); (3) Late Wisconsinan till and glaciolacustrine sediments, discontinuous veneer; (4) Late Wisconsinan till, blanket.

regard this till as part of the regional Late Wisconsinan drift mantle. Age of Davis Creek silt Paleomagnetic analysis Two samples were collected from unit I11 and 13 from the bedded part of unit I1 (Fig. 4) for paleomagnetic analysis at the Pacific Geoscience Centre, Sidney, British Columbia. Remanence was measured with a Schonstedt DSM-1 magnetometer.

Stepwise alternating-field demagnetization of four pilot specimens was carried out at 10,20, 30,40,50,60,70, 80,90, and 100 mT. Qualitative assessment of changes in the magnetization vector of these specimens led to a choice of 10 and 50 mT cleaning fields for the remaining samples. Three of the specimens were characterized by a single-component remanent magnetization (e.g., sample 1A; Fig. 5D), and one specimen (sample 9A; Fig. 5D) showed a two-component remanent magnetization. As the sampled part of Davis Creek silt

NOTES

195


TABLE1. Major element composition of volcanic glass from Davis Creek and Wascana Creek ash bedsa

Davis Creek

Si02 (wt.%) Ti02 A1203 Cr203 FeOc MnO MgO CaO Na20 K20 Totald An. totale

Wascana Creekb

1

2

3

Avg.

1

2

3

Avg .

78.92 0.23 11.75 0.00 1.17 0.00 0.00 0.41 1.87 5.66 100.00 95.25

78.58 0.28 11.90 0.00 1.13 0.15 0.06 0.45 2.33 5.12 100.00 94.78

78.60 0.17 11.92 0.00 1.34 0.00 0.00 0.45 2.20 5.31 99.99 94.68

78.70 0.23 11.86 0.00 1.21 0.05 0.02 0.44 2.13 5.36 100.00 94.90

78.01 0.24 11.97 0.00 1.61 0.00 0.00 0.55 2.27 5.34 99.99 94.82

78.26 0.06 12.12 0.00 1.27 0.00 0.00 0.45 2.46 5.37 99.99 94.98

78.10 0.11 12.01 0.00 1.37 0.07 0.00 0.47 2.09 5.78 100.00 87.89

78.12 0.14 12.03 0.00 1.42 0.02 0.00 0.49 2.27 5.50 99.99 92.56

"Samples were treated with H202to remove organic matter, with HC1 to remove carbonates, and with Na2S204to remove amorphous coatings of Fe, Al, and Mn. After heavy-liquid separation, sand-size grains were dispersed in epoxy, smeared on a glass slide, polished, and coated with carbon. P. L. Roeder, Department of Geology, Queen's University, analyzed the specimens using an ARL-SEMQ microprobe at 15 kV with a 0.02 pA sample current. bThe Wascana Creek ash sample was collected by A. MacS. Stalker at the type locality near Regina, Saskatchewan, on May 22, 1973. 'Total iron calculated as FeO. dSum of above percentage values. 'Analytical (An.) total is total weight percentage of above oxides from microprobe analysis.

Thickness

m

Unit

MAGNETIZATION DECLINATION(^) INCLINATIONC) & A rn*1)10.2 90 180-90 0 90 10.

0

FIG. 4. Generalized stratigraphic sequence at Davis Creek site showing vertical variations in magnetic declination, inclination, and intensity for samples, which were numbered from the top down. Samples were collected at stations 3 and 4 from the outcrops shown in Fig. 3.

shows evidence of localized secondary carbonate enrichment, the secondary component in sample 9A may relate to this enrichment. Because of the low median destructive field of these specimens, it would appear that detrital magnetite is the predominant remanence carrier. Davis Creek silt shows a weak magnetization, with intensities ranging from 0.005 to 0.001 A m-'. The Late Wisconsinan till (unit 111) has normal polarity (Fig. 4). However, the bedded part of Davis Creek silt has reversed magnetization, and the same applies to samples collected more recently from the massive lower part. Thus the massive and the bedded parts of Davis Creek silt were depos-

ited before the Matuyama-Brunhes magnetic polarity reversal, whose astronomical age of 788 ka serves as the provisional age of the Early Pleistocene - middle Pleistocene boundary (Richmond and Fullerton 1986). Tephra analysis The two lowermost paleomagnetic samples from the base of the bedded part of Davis Creek silt (Fig. 4), collected from the initial roadside exposure, contained abundant volcanic glass. Trace amounts of glass were present in the other samples from the bedded sediment, and no glass was present in the upper metre of the subjacent loess. This suggests that the tephra, although slightly reworked, is confined to a discrete lens that formed soon after a volcanic eruption. The major-element composition of volcanic glass from Davis Creek ash resembles that of Wascana Creek ash from the type locality near Regina, Saskatchewan (Table 1). The similarity coefficient, proposed by Borchardt et al. (1972), has a value of 94% for this tephra pair when calculated from the oxidic contents of Si, Al, Fe, Ca, Na, and K. This is a strong indication that these ashes have a similar origin. Wascana Creek ash occurs in normally magnetized sediments and has the same composition and fission-track age (620 ka) as Lava Creek B ash (formerly Pearlette 0), originating from the Yellowstone caldera complex (Westgate et al. 1977; Izett and Wilcox 1982; Richmond and Fullerton 1986). Although Davis Creek ash probably represents one of the four members of the Pearlette ash bed family defined by Izett and Wilcox (1982), it has reversed magnetization and is therefore too old to represent one of the two Lava Creek ash beds. More likely, it represents either the Mesa Falls (1.27 Ma) or the Hucklebeny Ridge (2.02 Ma) ash. In the former case, Davis Creek silt would date from the Early Pleistocene. In the latter case, part or all of it would date from the Late Pliocene, given that the provisional age for the Pliocene -Pleistocene boundary was set at 1.65 Ma (Richmond and Fullerton 1986).

North 1.0-

~ o r t hvsEast

0.5

-1.0 West}::

(bl

-

0.5 1.0 : I . : : : !East

: :-0.5 : : :

N

20

0

40 60 Alternating Field (mT)

80

-. 9A

100

-1.0- 'N South

Down MEast

GN

*

+ -1 0 W e s t ) ' : :

-0,5 '

I

. . :

"i

1.0 Down

C

.

+

+

'r

+

-VE Incl.

FIG. 5. Magnetic characteristics for samples 1A (unit 111) and 9A (unit 11) from the sequence shown in Fig. 4. (a) Magnetization (MIMO) versus alternating field demagnetization (mT). (b) Orthogonal vector projections, north versus east. (c) Orthogonal vector projections, down versus east. (d) Equal-area plot of remanent magnetic directions.

Other sites with Davis Creek silt Two other sites on the south-facing pediment provide further clues about the relative chronological placement of Davis Creek silt. Site 2 (Fig. 2) is at an elevation of 1097 m as1 and is located in lsd. 1, sec. 19, tp. 7, rge. 25 (latitude49"34'1lt'N, longitude 109"22'12"W). Loess at this site is directly underlain by an oxidized zone in the sandy matrix of thin, loose quartzite gravel on carbonate-cemented gravel, possibly from the Ravenscrag Formation. Thus a weathering interval elapsed between formation of the pediment and loess accumulation. A subsequent weathering interval is evident from the presence of white, fist-size carbonate nodules within the loess and from a thin lag deposit or stone line composed of fragmented carbonate nodules that intervenes between the loess and the till overburden. Site 3 (Fig. 2) is at an elevation of 1029 m as1 and is located in lsd. 13, sec. 25, tp. 6, rge. 26 (latitude 49"30t25"N, longitude 109"23'16"W). At this site a unit of cryoturbated gravelly loam with inclusions of reddish-yellow paleosolic material intervenes between Davis Creek silt and an over-

burden composed of glaciolacustrine silt rhythmites overlain by till. The clasts in this unit are devoid of Laurentide erratics, and most have a subvertical orientation. The reddish loam inclusions have relict pedogenic structure with oriented clay on peds and resemble material from a paleosolic B horizon. A similar cryoturbated unit with reddish-yellow paleosolic material encountered on a glacially overridden pediment along the north flank of the west block was described by Vreeken (1986). The reconstructed sequence of events is as follows: (i) Davis Creek silt accumulated on a weathered pediment surface. (ii) During a subsequent erosional interval it was locally buried beneath gravelly loam derived from upslope portions of the pediment. (iii) Upon relative stabilization of the landscape, a soil with reddish-yellow B horizon formed from this unit. (iv) This soil mantle was cryoturbated before it was buried beneath Late Wisconsinan glacial deposits.

Conclusions Davis Creek silt accumulated, during a reversed magnetic


198 I I

polarity chron, o n a weathered pediment surface whose age is estimated at 10 Ma. The loess component of this unit accumulated before emplacement of an as yet undated Pearlette volcanic ash, but the fluvially reworked component accumulated during and after its emplacement. If this ash represents one of the four major Pearlette ash beds it could b e either 1.27 o r 2.02 M a old. Davis Creek silt has imprints of weathering and is separated from the glacial overburden by a cryoturbated unit with remnants of a paleosolic B horizon. If this represents the cryoturbated unit identified just to the north of the west block (Vreeken 1986), the new evidence provides for a significant extension of the reconstructed history of the entire Cypress Hills region.

Acknowledgments This study was financed by Natural Sciences and Engineering Research Council of Canada (NSERC) grants A3870 (WJV) and A0581 (RWB) and by the Geological Survey of Canada (Project 830024) (RWK). Dr. P. W . Roeder (Queen's University), Dr. E. Irving (Pacific Geoscience Centre), and Dr. J. A. Westgate (University of Toronto) commented o n various aspects of the study. Victor Levsen and Thomas McCulloch assisted in the field. ALDEN,W. C. 1924. Physiographic development of the northern Great Plains. Bulletin of the Geological Society of America, 35: 385-424. 1932. Physiography and glacial geology of eastern Montana and adjacent areas. United States Geological Survey, Professional Paper 174. P. J., and MILLARD,H. T., JR. BORCHARDT, G. A . , ARUSCAVAGE, 1972. Correlation of the Bishop ash, a Pleistocene marker bed, using instrumental neutron activation analysis. Journal of Sedimentary Petrology, 42: 301 - 306. BROSCOE,A. J. 1965. The geomorphology of the Cypress Hills Milk River canyon area, Alberta. In Guidebook. Part 1. Cypress Hills Plateau. Alberta Society of Petroleum Geologists, 15th Annual Field Conference, pp. 74 -84. COLLIER, A. J., and THOM,W. T., JR. 1918. The Flaxville gravel and its relations to other terrace gravels in the northern Great Plains. United States Geological Survey, Professional Paper 1085, pp. 179-184. COLTON,R. B., NAESER,N. D., and NAESER,C. W. 1986. Drainage changes in eastern Montana and western North Dakota during late Cenozoic time. Geological Survey of America, Abstracts with Programs, 18: 347. IZETT,G. A . , and WILCOX,R. E. 1982. Map showing localities and inferred distributions of the Hucklebeny Ridge, Mesa Falls, and Lava Creek ash beds (Pearlette family ash beds) of Pliocene and Pleistocene age in the western United States and southern Canada. United States Geological Survey, Miscellaneous Investigations Map 1-1325. JUNGERIUS, P. D. 1966. Age and origin of the Cypress Hills plateau surface in Alberta. Geographical Bulletin, 8: 307 -3 18. 1967. The influence of Pleistocene climatic changes on the

development of the polygenetic pediments in the Cypress Hills area, Alberta. Geographical Bulletin, 9: 218 -231. KARLSTROM, E. T. 1987. Stratigraphy and genesis of five superposed paleosols in pre-Wisconsinan drift on Mokowan Butte, southwestern Alberta. Canadian Journal of Earth Sciences, 24: 22352253. KLASSEN, R. W., and VREEKEN, W. J. 1987. The nature and chronological implications of surface tills and post-till sediments in the Cypress Lake area, Saskatchewan. In Current research, part A. Geological Survey of Canada, Paper 87-lA, pp. 111 - 125. LECKIE,D. A., and CHEEL,R. J. In preparation. The Cypress Hills Formation (Upper Eocene to Miocene): a semiarid braidplain deposit resulting from intrusive uplift. MCCONNELL, R. G. 1885. Report on the Cypress Hills, Wood Mountain and adjacent country. Geological Survey of Canada, Annual Report (New Series), Vol. 1, Report C, pp. 1 -85. PREST,V. K., GRANT,D. R., and RAMPTON,V. N. 1968. Glacial map of Canada. Geological Survey of Canada, Map 1253A, scale 1 : 5 000 000. RICHMOND,G. M., and FULLERTON, D. S. 1986. Introduction to Quaternary glaciations in the United States of America. In Quaternary glaciations in the United States of America. Edited by G. M. Richmond and D. S. Fullerton, Quaternary Science Reviews, 5: 3-10. RUSSELL,L. S. 1975. Revision of the fossil horses of the Cypress Hills Formation (Lower Oligocene) of Saskatchewan. Canadian Journal of Earth Sciences, 12: 636 -648. ST. ONCE,D. A. 1966. Cypress Hills east area, Saskatchewan. In MClanges de gkographie offertes i M. Omer Tulippe. Editions J. Duculot, S. A . , Gembloux, Belgium, pp. 72-79. STORER,J. E. 1975. Middle Miocene mammals from the Cypress Hills, Canada. Canadian Journal of Earth Sciences, 12: 520-522. SZABO,B. J., and LINDSEY,D. A. 1986. Estimating limiting age for Pleistocene erosional surfaces in central Montana by uraniumseries dating of associated travertines. Earth Surface Processes and Landforms, 11: 223 -228. VONHOF,J. A. 1969. Tertiary gravels and sands in the Canadian Great Plains. Ph.D. thesis, University of Saskatchewan, Saskatoon, Sask. VREEKEN, W. J. 1986. Quaternary events in the Elkwater Lake area of southeastern Alberta. Canadian Journal of Earth Sciences, 23: 2024 -2038. WESTGATE, J. A. 1968. SurFicial geology of the Foremost - Cypress Hills area, Alberta. Research Council of Alberta, Bulletin 22. WESTGATE, J. A., CHRISTIANSEN, E. A., and BOELLSTORFF, J. D. 1977. Wascana Creek Ash (middle Pleistocene) in southern Saskatchewan: characterization, source, fission track age, palaeomagnetism, and stratigraphic significance. Canadian Journal of Earth Sciences, 14: 357 -374. WHITAKER, S. H. 1976. Geology and groundwater resources of the Cypress area (72 F), Saskatchewan Research Council, Geology Division, Map 22. WILLIAMS, M. Y. 1929. The physiography of the southwestern plains of Canada. Transactions of the Royal Society of Canada, 23, sections I and 11, IV: pp. 61 -79. WILLIAMS,M. Y., and DYER,W. S. 1930. Geology of southern Alberta and southwestern Saskatchewan. Geological Survey of Canada, Memoir 163.