Travertine mounds of the Cave and Basin National Historic Site, Banff National Park

Travertine mounds of the Cave and Basin NationalHistoric Site, Banff National Park1, 2

Stephen E. Grasby, Robert O. van Everdingen, Jan Bednarski,and Dwayne A.W. Lepitzki

Abstract: The Cave and Basin National Historic Site is a fan-shaped travertine deposit associated with four thermalspring outlets. Tentative age dating of the travertine mound indicates growth initiated with onset of the late Holoceneshift to more humid and cool climate conditions and suggests that the flow of thermal waters was limited during theHypsithermal, which in turn places constraints on the evolutionary biology of endemic species in the spring system.Two large caves and one collapsed cave structure are developed within the deposit. Cave development is in response toboth physical erosion of till underlying the travertine and acid gas attack of calcite that makes up the deposit. Thisprocess is buffered by formation of reaction crusts of gypsum on the interior cave walls. Only minor modern travertinegrowth occurs due to historic flow control measures. Understanding the flow of water through the historic site is criti-cal for long-term preservation.

Résumé : Le site national historique Cave and Basin est un dépôt de travertin en forme d’éventail associé à quatreexutoires de sources thermales. Des estimations d’âge du monticule de travertin indiquent que la croissance a com-mencé avec le début d’un changement, à l’Holocène, vers des conditions climatiques humides et fraîches et quel’écoulement des eaux thermales était limité au cours de l’Hypsithermal, ce qui par conséquent impose des contraintessur la biologie de l’évolution des espèces endémiques au système de sources. Deux grandes cavernes et une structurede caverne affaissée sont développées à l’intérieur du dépôt. Le développement de cavernes est en réponse à l’érosionphysique du till en dessous du travertin et à des attaques, par des gaz acides, de la calcite qui constitue le dépôt. Ceprocessus est tempéré par la formation de croûtes de gypse de réaction sur les murs intérieurs de la caverne. Le travertinmoderne ne croît que peu étant donné l’histoire des mesures de contrôle de l’écoulement. La compréhension del’écoulement de l’eau à travers le site historique est essentiel pour sa préservation à long terme.

[Traduit par la Rédaction] Grasby et al. 1513

Introduction

Travertine deposits are common features associated withthermal and mineral springs throughout the CanadianCordillera, however, little work on these structures has beendone to date. Travertine deposits associated with the SulphurMountain thermal springs (Grasby and Lepitzki 2002) areperhaps the best known, as they have been declared aNational Historic Site in recognition of the role they playedin the birth of Canada’s National Parks System. Their discoveryin 1883 by surveyors for the Canadian Pacific Railroad ledto disputes over ownership and eventually the establishmentof Canada’s first national park (a 670 km2 area around theCave Spring that eventually grew to the modern Banff NationalPark). Despite the historical importance, work at this site has

largely focused on the hydrodynamics and water chemistryof the thermal springs rather than the actual deposits thatform the site (Warren 1927; van Everdingen 1972; vanEverdingen et al. 1985; Grasby and Lepitzki 2002). Recentproblems with water seepage into the cave, leading to partialcollapse of the interior wall and acid attack on buildingstructures, have heightened concern over the long-term stabilityof the historic site. Here, based on new data in conjunctionwith previous work, we examine the origin and natural historyof the Cave and Basin National Historic Site to identify thefactors controlling long-term stability. In addition, as thiswork provides one of the first descriptions of a travertine depositin the Canadian Cordillera (see also Bonny and Jones 2003),our results provide valuable information on paleohydrogeologicconditions in the region.

Can. J. Earth Sci. 40: 1501–1513 (2003) doi: 10.1139/E03-058 © 2003 NRC Canada

1501

Received 5 February 2003. Accepted 23 June 2003. Published on the NRC Research Press Web site at http://cjes.nrc.ca on25 November 2003.

Paper handled by Associate Editor B. Chatterton.

S.E. Grasby.3 Geological Survey of Canada, Natural Resources Canada, 3303-33rd St. NW, Calgary, AB T2L 2A7, Canada.R.O. van Everdingen. 2712 Chalice Road NW, Calgary, AB T2L 1C8, Canada.J. Bednarski. Geological Survey of Canada, 9860 West Saanich Road Sidney, BC V8L 4B2, Canada.D.A.W. Lepitzki. Wildlife Systems Research, P.O. Box 1311, Banff, AB T1L 1B3, Canada.

1This article is one of a selection of papers published in this Special Issue on Sedimentology of hot spring systems.2Geological Survey of Canada Contribution 2003170.3Corresponding author (e-mail: [email protected]).

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1502 Can. J. Earth Sci. Vol. 40, 2003

Background

Previous workSatterley and Elworthy (1917) and Elworthy (1918) were

the first to study the thermal springs at Banff, focusing onwater chemistry and levels of radioactivity. Later work byWarren (1927) recognized that the springs originated throughdeep circulation of meteoric water. Haites (1959) providedthe first hydrogeological assessment of the springs and suggestedrecharge was from infiltration along the Bourgeau Thrust.More detailed work on the chemistry and hydrogeology ofthe springs by van Everdingen (1972) shows that outlets arerelated to fracture flow along the Sulphur Mountain Thrustand local transverse structures. Mazor et al. (1983) used noblegas data to show that the spring discharge is consistent withair-saturated water, supporting a meteoric source of recharge.Most recently, Grasby and Hutcheon (2001) and Grasby etal. (2000) examined the springs as part of a regional overviewand showed that water chemistry is controlled by equilibriumreaction with the host rock and that the spring sites are relatedto lateral “necking” of the Sulphur Mountain Thrust sheet inthe Bow Valley area. In addition, they show that the structuralgeometry of the Sulphur Mountain Thrust provides a depthlimit on circulation and thus controls maximum temperatureof the springs. Grasby and Lepitzki (2002) also showed thatthe various springs at Banff are part of the same hydrogeologicalsystem and that seasonal and spatial variations in waterchemistry are a function of the degree of mixing betweenthermal and shallow groundwaters.

Regional geologyThe Cave and Basin National Historic Site lies in the Bow

Valley Corridor, which cuts through the Front Ranges of thesouthern Canadian Rocky Mountains. The Front Ranges hereare characterized by four main northwest-trending, southwest-dipping thrust sheets: the McConnell, Rundle, SulphurMountain, and Bourgeau thrusts (Price 1981). The thrustsheets are generally comprised of resistant Paleozoic carbonatesthat form long linear mountain ranges. Intervening valleysare formed in less resistant Mesozoic shales and siltstones.Thermal springs occur along the Sulphur Mountain, Bourgeau,and Rundle thrusts, with the springs along the Sulphur MountainThrust being the warmest. Thermal springs in the Banff area,including those at the Cave and Basin site, are related to unusualtransverse structures along the Sulphur Mountain Thrust (Grasbyand Lepitzki 2002), where a swarm of steeply dipping,northeast-trending dextral faults, with up to 200 m offset,crosscut the Sulphur Mountain Thrust sheet (Fig. 1) (Price2001). Thermal springs along the Sulphur Mountain Thrustdischarge from the mountain slopes at elevations rangingfrom 1400 to 1584 m above sea level (asl). The Cave andBasin springs are the topographically lowest of this trend(1400–1417 m asl) and closest to the main channel of theBow River (�1350 m asl).

The Banff area was extensively glaciated during thePleistocene, when large glaciers originating near the continentaldivide filled the Bow Valley with ice 500–800 m thick. Previouswork (Rutter 1972) has identified at least two eastward glacialadvances that deposited thick till in the valley bottoms, whichare in turn underlain by thick deposits of stratified outwash

gravels. Higher on the mountain slopes, the till thins andcommonly overlies bedrock. On steep slopes, the till is modifiedor covered by colluvium. Till on the valley sides is composedof angular to rounded rock fragments (�20%) enclosed in acalcareous sandy clay loam. The fine matrix has a highcarbonate content of up to 50%, which tends to cement thematerial. Rock fragments are dominantly dolomite andlimestone, with minor quartzitic sandstone, chert, shale, andpebble conglomerate. Glaciers retreated from the Bow Valleynear Banff as early as 11 000 BP. During deglaciation thevalley would have channelled large volumes of meltwaterfrom the retreating ice, and rivers would have been activelyincising through the valley-bottom sediments.

Postglacial western interior Canada experienced warmingand drying conditions, culminating in severe drought conditionsduring the middle Holocene (ca. 7000–5000 BP; Lemmenand Vance 1999; Schweger et al. 1981). During the Hypsi-thermal, glacier margins in the Canadian Rocky Mountainswere well above present positions (Luckman et al. 1993),and treelines were lower than present (Luckman and Kearney1986). By 5000 BP the interior of Canada was moister andcooler and saw the establishment of modern vegetation(MacDonald 1987; MacDonald and Case 2000). In the CanadianRockies, glacial-fed lakes experienced an increase of glacialsedimentation around this time, marking the onset of theNeoglacial period (Leonard 1986).

Methods

The boundaries of the travertine deposit were mapped basedon airphoto interpretation, visual exposures, and several shallowhand-dug test pits. Actual thickness was measured wherepossible and estimated elsewhere by extrapolation of thetill–travertine boundary assuming a flat contact plane.

For water samples, unstable parameters (pH, H2S, O2,temperature) were measured on site. Samples for chemicalanalyses were passed through 0.45 µm filters and stored inthe dark at 4 °C in high-density polyethylene (HDPE) bottlesuntil analysed. Samples for cation analyses were acidifiedwith ultrapure nitric acid to pH < 2. Major cations and traceelements were determined by inductively coupled plasma –emission spectroscopy (ICP–ES). Anions where determinedby ion liquid chromatography (ILC). Alkalinity was determinedusing an Orion 960 auto-titrator. Analytical error in concentrationmeasurements was estimated to be less than 2%. For pHmeasurements of thin water films on the cave roof, colorpHast®pH paper, with 1 pH unit resolution, was used. H2S concen-trations within the cave were measured using a hand-heldmeter.

Qualitative mineralogy of the samples was determined bypowder X-ray diffraction (XRD) using relative peak intensityon a Phillips 1700 XRD.

14C dating of travertine was conducted on cleaned travertinesamples by Waterloo Isotope Analysts Inc.

Results

Distribution of travertine and springsThe Cave and Basin site is best defined as the area underlain

by a large triangular travertine deposit, covering � 26 000 m2



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Grasby et al. 1503

on the south bank of the Bow River at the north end of SulphurMountain (Fig. 2). Towards the apex, the travertine depositforms two distinct terraces with a maximum thickness of atleast 6.5 m (Figs. 2, 3). Farther downslope, as the travertinedeposit fans out, there are several other smaller terraces, andthe deposit thins to approximately 1 m in areas where exposed.The entire travertine deposit is heavily overgrown with brushand trees. There is no evidence of modern-day travertineformation except within the cave of the Cave Spring site,where modern flowstone deposits are forming at two locations:(i) where natural discharge flows over a ledge into the pool,and (ii) where artificially pumped water runs down the interiorback wall in efforts to aid recovery of the endangered BanffSpring Snail (Remigio et al. 2001; Lepitzki 2002; Grasbyand Lepitzki 2002).

There are four distinct thermal spring outlets at the Cave

and Basin site. The highest (Upper Pool Spring, �1417 m asl)discharges from the apex of the travertine deposit, at thebase of a steep-walled depression. Jumbled blocks of travertineat that location suggest that the Upper Pool Spring dis-charges through a collapsed cave. The Lower Pool Springemerges into a small cave, 7.5 m lower than the Upper PoolSpring. This cave has been sealed by Parks Canada and is nolonger accessible. Previous visits by one of us (vanEverdingen) indicated that the base of the cave is till,whereas the top is a dome-shaped roof formed within thetravertine. The site has a number of small inflows that feedinto a gravel-bottom pool, which in turn discharges to theoutside at the base of the second terrace of the travertinemound (Figs. 2, 3).

The Cave Spring emerges in a large cavern (�1400 m asl),which forms the original “discovery site,” and is part of a

Fig. 1. (A) Local geology (after Price 2001) and spring locations in the Banff area. Location of Cave and Basin site is indicated. Theup to 200 m wide deformation zone along the Sulphur Mountain Thrust is shown as the hatched area between the two fault lines. Notethat all the thermal springs within the Banff area fall within this fault zone. Contours in metres. (B) Simplified cross section throughSulphur Mountain, showing flow system (after Grasby and Lepitzki 2002).



public self-guided tour of the Cave and Basin National Historicsite. An entrance tunnel from the visitor centre was constructedfor easier access and is not natural (original access was via aladder through a hole in the cave roof). Similar to the LowerPool Spring, the Cave Spring is floored by till with adome-shaped roof formed within the overlying travertinedeposit. The till–travertine boundary appears to be preferen-tially eroded, as material along this boundary is removed upto 1 m back from the main cave structure itself (Fig. 3).Within the main cave, the till underlying the travertine islargely covered by flowstone structures, with the exceptionof the east wall where till is fully exposed (Fig. 4).

In 2001, clogged drainage pipes caused discharge fromthe Lower Pool Spring to form a pond on the first terrace,immediately above the Cave Spring (Frank’s Pool, Figs. 3).Shortly after, water was observed to discharge through theexposed till on the interior east wall of the cave, and portionsof the till and overlying travertine began to collapse into thecave. Subsequent drainage of Frank’s Pool eliminated thewater seepage and the till wall appears to have stabilized.Historical records indicate that, at the time of discovery, theoutflow from the Lower Pool Spring flowed across the firstterrace and then over a waterfall into the Basin Spring (Renaudand McAllister 1988).

The fourth spring (Basin Spring) occurs in a subaerialpool at the base of a steep travertine wall at approximatelythe same elevation as that of the Cave Spring. This pool hasbeen largely modified in the past for bathing and, morerecently, as part of the redevelopment of the Cave and BasinNational Historic Site.

All of the aforementioned spring outlets have been modifiedto some extent in an attempt to control the flow of dischargethrough the site. The discharge is constrained in constructedpools or runs through constructed channels and is eventually

captured into a pipe network. Given this, the current conditionsare not representative of the natural state at the time ofdiscovery. The lack of historic records, however, makes ituncertain exactly how, and to what degree, the natural flowhas been altered.

Previous estimates of discharge rates (van Everdingen 1972)give flows of 680 L/min for the Basin Spring, 1136 L/minfor the Cave Spring, and 680 L/min for the Upper Pool andLower Pool springs combined. Although it was not possibleto install a recording device for this study, visual observationsof water height in flow channels suggest that flow rates donot vary significantly through the year, unlike those of springshigher on Sulphur Mountain (Grasby and Lepitzki 2002).

Water chemistrySpring waters discharging from the Cave and Basin site

are all Ca–SO4–HCO3 type (Table 1). Monitoring of thechemical and physical properties for the Cave, Basin, andUpper Pool springs shows that there is only minor seasonalvariability (Figs. 5, 6), unlike thermal springs higher on SulphurMountain that show greater variability in flow (van Everdingen1970; Grasby and Lepitzki 2002). Temperatures are fairlyconstant, with the Cave Spring averaging 30 °C and the Basinand Upper Pool springs 33 °C. The pH is also relatively constantthrough the year, with the Cave Spring having a slightlyhigher average (7.3) than the Upper Pool and Basin springs(7.0). The total dissolved solids (TDS) of the discharge watersshow 7%–14% variability through the year, with the lowestvalues tending to be during the spring and early summer.The Upper Pool and Cave springs have similar TDS levelsand composition (�1050 mg/L). In contrast, the TDS of theBasin Spring is �60% higher and shows a slightly higher ratioof SO4 to HCO3 than the other sites (Fig. 6). Grasby andLepitzki (2002) show that variations in both TDS and the ratioof SO4 to HCO3 between spring outlets along SulphurMountain, as well as temporal variations at individual outlets,are a function of mixing of thermal waters with shallowgroundwater. The Basin Spring thus represents the most “pure”end-member thermal water. Average dissolved HS– levelsrange from 1.3 mg/L (Cave) to 4.4 mg/L (Basin). Gas dischargefrom the springs is dominantly nitrogen (97%–98%), withminor components of H2S, CO2, and CH4. Previous work byMazor et al. (1983) indicates that noble gas ratios in thespring waters are consistent with air-saturated water, withthe exception of enrichment of He due to radiogenic decay.Atmospheric H2S levels measured within the cave were highlyvariable, dependent largely on airflow through the site affectedby the opening and closing of the visitor centre doors. Duringclosed hours, the atmospheric H2S level within the Cavereached up to 13 ppm, whereas it was below detection duringdaytime hours.

Calculated saturation indices indicate that the spring watersare typically slightly oversaturated with respect to calcite(Table 1; Fig. 7). The waters of the Upper Pool and Basinsprings tend to have lower overall saturation and can beundersaturated with respect to calcite during the spring andsummer months. For the three sites in general, saturation indicestend to be highest during late winter and lowest during springand summer. The springs maintain a fairly constant under-saturation with respect to gypsum throughout the year. TheCave and Upper Pool springs have a lower degree of gypsum

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Fig. 2. Distribution of travertine at the Cave and Basin site andlocation of spring outlets.



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Grasby et al. 1505

saturation than the Basin Spring due to the lower sulphatecontent.

Travertine compositionXRD results indicate that the travertine deposit at the Cave

and Basin site is almost 100% calcite, with trace amounts ofquartz (Table 2). In contrast, sediment in the discharge poolsfor the Cave and Basin springs is mostly quartz (>93%),with minor amounts of carbonates and gypsum. XRD resultsalso confirmed that white crystalline material coating the interiorcave walls was gypsum. Previous work by van Everdingen et al.(1985) shows δ34SCDT (CDT, Cañon Diablo troilite) andδ18OSMOW (SMOW, standard mean ocean water) values ofthe gypsum to be –5.2‰ and –23.3‰, respectively. Gypsumin the pool sediments has δ34S and δ18O values of –1.8‰ and–14.9‰, respectively.

Exposed tillTill and the till–travertine contact are well exposed along

the interior east wall of the cave. Along the south and westwalls of the cave the till–travertine contact is obscured, asthe till has been overgrown by travertine flowstone structures.Here the flowstone coating the till forms an apron that is �1 mthick. Where broken from the wall, these flowstone structuresare composed of fine laminations (Fig. 8). Where uncoatedtill is exposed along the east wall, the till at the contact ismoist and friable, whereas 20 cm below the contact the till ismore indurated. Recent collapse into the cave exposed an�50 cm horizontal depth profile through the till. Examinationof the till indicates that the outer �30 cm was relatively wellcemented, but behind this the till was unconsolidated. XRDanalyses of the unconsolidated matrix material indicated 75%gypsum and 20% quartz. Given the general carbonate-cemented

Fig. 3. Profile through the upper portion of the travertine mound at the Cave and Basin site (line of section in Fig. 2). Note that thelower portion of the caves sits in till underlying the travertine.

Fig. 4. Photographs of interior of the cave at the Cave Spring site: (A) view across the old swimming pool looking at the indentedtravertine–till boundary (a waterfall on the right forms an actively growing flowstone structure overhanging the pool); and (B) close-upof the east wall, showing exposed till underlying travertine deposits.



© 2003 NRC Canada


character of the tills in the Banff area, this suggests that theoriginal till matrix in the Cave has been greatly altered.

Discussion

Travertine precipitationPrevious work by Grasby and Hutcheon (2001) and Grasby

and Lepitzki (2002) showed that the Banff thermal springsoriginate by deep circulation of meteoric water associated

with anomalous transverse structures along the SulphurMountain Thrust in the Bow Valley area. TheCa–SO4–HCO3 chemistry of the waters is consistent withmeteoric water flowing through, and reacting with, the carbon-ate bedrock. The high SO4 content suggests that dissolutionof anhydrite is also important. Although not exposed at thesurface, the Devonian and Mississippian host rocks areknown to have evaporite sequences in the subsurface to theeast (Mossop and Shetsen 1994) and in outcrop to the west

Calculatedsaturation index (SI)

Sampling date pH T (°C)H2S(mg/L)

TDS(mg/L)

Mg(mg/L)

Ca(mg/L)

Na(mg/L)

K(mg/L)

SO4

(mg/L)Cl(mg/L)

HCO3

(mg/L) Calcite Gypsum

Upper Pool Spring27 Apr. 2000 7.1 34 2.7 1061 49 230 6.7 4.8 614 5.9 136 0.02 –0.6217 May 2000 7.1 33 3.4 1032 37 240 6.0 4.6 589 5.8 137 0.05 –0.607 June 2000 7.1 33 3.3 1004 37 230 5.8 4.4 562 5.6 146 0.01 –0.6328 June 2000 7.1 33 3.5 1026 37 240 6.0 4.6 584 5.8 135 0.01 –0.6119 July 2000 7.1 33 3.5 1033 37 240 6.0 4.6 600 5.8 126 –0.03 –0.602 Aug. 2000 7.0 33 3.8 1057 37 240 6.0 4.6 623 5.8 127 –0.14 –0.5813 Sept. 2000 6.7 34 3.6 1032 37 230 5.9 4.5 604 5.9 132 –0.56 –0.5912 Oct. 2000 7.2 34 3.4 1045 37 230 5.9 4.5 623 4.1 128 0.06 –0.608 Nov. 2000 7.1 33 2.8 1053 43 240 6.0 4.6 630 5.9 111 –0.05 –0.595 Dec. 2000 7.1 34 2.8 1072 43 240 6.0 4.7 623 6.0 137 0.06 –0.594 Jan. 2001 7.1 34 — 1125 45 220 6.9 4.7 644 5.5 186 0.15 –0.6131 Jan. 2001 7.2 33 3.2 1115 45 220 6.6 4.7 633 5.5 187 0.21 –0.6227 Feb. 2001 7.2 34 2.4 1138 46 220 6.5 4.7 655 5.5 187 0.18 –0.6128 Mar. 2001 7.1 34 — 946 45 220 6.3 4.7 650 6.0 — — –0.60

Cave Spring27 Apr. 2000 7.2 31 1.5 — 45 240 6.2 4.8 631 6.0 127 0.09 –0.5917 May 2000 7.3 31 — 1054 44 240 6.1 4.7 612 5.8 128 0.12 –0.597 June 2000 7.3 31 — 1037 37 230 5.8 4.5 585 5.6 156 0.21 –0.6228 June 2000 7.2 31 1.3 1037 37 230 5.9 4.5 610 5.8 131 0.09 –0.6019 July 2000 7.3 31 1.4 1053 44 240 6.1 4.7 605 5.8 135 0.17 –0.602 Aug. 2000 7.3 30 1.0 1048 37 240 6.0 4.6 616 5.8 125 0.15 –0.5813 Sept. 2000 7.4 31 1.2 1048 44 240 6.0 4.6 612 5.7 123 0.06 –0.5712 Oct. 2000 7.4 30 1.2 1055 43 240 6.0 4.6 617 5.9 125 0.29 –0.598 Nov. 2000 7.4 30 1.3 1043 44 240 6.1 4.7 633 6.0 96 0.11 –0.585 Dec. 2000 7.4 30 1.4 1084 44 240 6.2 4.7 636 6.0 134 0.26 –0.584 Jan. 2001 7.4 30 1.1 1115 45 220 6.2 4.7 643 5.5 178 0.38 –0.6131 Jan. 2001 7.4 30 1.2 1120 45 220 6.2 4.6 650 5.5 176 0.31 –0.6127 Feb. 2001 7.5 29 1.4 1124 45 220 6.2 4.6 654 5.6 175 0.40 –0.6028 Mar. 2001 7.3 29 1.2 1107 45 220 6.1 4.6 638 5.6 174 0.29 –0.61

Basin Spring27 Apr. 2000 7.0 34 4.6 1630 67 400 7.5 6.5 1001 6.2 127 0.08 –0.3017 May 2000 7.0 34 4.7 1660 65 390 7.1 6.3 1049 6.0 123 –0.03 –0.297 June 2000 7.0 34 4.6 1656 65 380 7.0 6.1 1023 5.8 155 0.04 –0.3128 June 2000 6.9 34 6.4 1672 65 390 7.1 6.2 1047 6.0 137 –0.02 –0.2919 July 2000 7.0 33 4.2 1700 65 380 7.1 6.2 1083 6.0 138 0.09 –0.292 Aug. 2000 7.0 35 6.0 1647 66 380 7.1 6.3 1039 6.0 128 –0.03 –0.3013 Sept. 2000 7.0 35 4.3 1668 67 400 7.3 6.5 1042 5.9 125 0.01 –0.2812 Oct. 2000 7.1 33 4.3 1649 68 400 7.4 6.6 1049 6.0 98 0.05 –0.298 Nov. 2000 7.1 33 2.9 1669 67 400 7.4 6.5 1047 6.1 121 0.10 –0.295 Dec. 2000 6.9 34 2.6 1724 67 400 7.3 6.5 1064 6.1 160 0.08 –0.294 Jan. 2001 7.1 33 4.0 1756 71 400 7.6 6.5 1132 5.7 118 0.13 –0.2731 Jan. 2001 7.1 33 3.9 1732 71 400 7.5 6.4 1107 5.8 121 0.14 –0.2727 Feb. 2001 7.2 33 — 1743 71 400 7.5 6.5 1131 5.7 107 0.13 –0.2728 Mar. 2001 7.0 34 — 1809 70 410 7.4 6.4 1103 5.7 193 0.26 –0.27

Table 1. Seasonal variability of chemical data for three springs at the Cave and Basin site collected approximately every 3–4 weeksover 1 year.



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Grasby et al. 1507

(Leech 1958). The δ34S and δ18O values of dissolved sul-phate are consistent with values for Devonian anhydriteunits (Claypool et al. 1980).

Calculated saturation indices indicate that Cave and Basinspring waters are slightly oversaturated with respect to calciteand overpressured in CO2 relative to average atmosphericvalues (�10–3.5). Lower temperature systems oversaturatedwith CaCO3 tend to preferentially precipitate calcite (Busenbergand Plummer 1986; Folk 1994), which is consistent with theabsence of aragonite in the Cave and Basin travertine mound.Precipitation of calcite typically requires a critical degree ofsupersaturation (5–10 times saturation; Reddy et al. 1981;Stumm and Morgan 1996) to overcome nucleation barriers.This is often reached at points along the flow path where thewater is agitated (e.g., waterfalls, steep gradients), causingdegassing of CO2 and increase in saturation through thefollowing reaction:

[R1] Ca2+ + 2HCO3– → CaCO3(s) + H2O + CO2(g)�

In contrast, heat loss from thermal waters as they dischargeinto open air will decrease water temperature, leading tohigher solubility of calcite. The relative impacts of degassingand temperature drop on calcite solubility for the Cave Springwas modeled using SOLMINEQ.88 (Wiwchar et al. 1988).Results indicate that degassing of CO2 has a far greater effecton calcite solubility than temperature (Fig. 9). The total potentialprecipitation of calcite was calculated with Geochemist’sWorkbench®, using a sliding fCO2

(CO2 fugacity) reactionpath down to atmospheric equilibrium, while suppressingdolomite and aragonite precipitation. In effect, CO2 was titratedfrom solution to reach equilibrium with atmospheric pressures,and CaCO3 was allowed to precipitate to also reach equilibriumwith calcite. Results suggest that up to 76 mg/L of calcitecould precipitate in response to CO2 degassing. Combinedwith a total flow rate of 1700 L/min for the Cave and Basinsite, this gives � 68 000 kg/year potential travertine deposition(this is an extreme end-member, as it assumes that pH issolely related to equilibrium with atmospheric CO2, that CO2degassing is complete, and that all the CaCO3 precipitates).Even if we divide this potential by a factor of 10, assume anaverage thickness of 1 m, and use the measured density fortravertine samples of 1 g/cm3, then �3800 years are requiredto form the travertine deposit. This indicates that the dischargewaters have the potential to form a fairly large travertine depositin a relatively short period of time.

The deposit age was constrained through 14C age datingof travertine from the interior of caves at the Cave andLower Pool sites (Table 3). The inherent problem with thismethod is a high contribution of “dead” carbon from dissolutionof limestone along the flow path giving artificially old dates.Ages were thus calibrated by setting dates of freshly formedtravertine samples in flow channels to 5 BP (cannot be olderthan 100 BP) and using the difference as a correction factorfor other samples. Results from the interior of the CaveSpring gave adjusted age dates ranging from 3276 to 5296 BP,whereas the Lower Pool Spring gave an age of 928 BP.Although needing to be confirmed by more accurate

Fig. 5. Ternary plots of seasonal data for the Cave, Upper Pool,and Basin springs. Note that the Basin spring has slightly higherSO4 and that all springs show only minor seasonal variability inbulk chemistry.

Fig. 6. Plot of total dissolved solids (TDS) versus the ratio ofSO4 to HCO3.

Fig. 7. Plot of saturation indices (SI = log IAP/K) for calcite andgypsum. IAP, ion activity product; K, equililarium constat.



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230Th/234U dating methods, we feel the results are reasonablegiven the similarity in age to other deposits (Bonny andJones 2003) and the timing of regional groundwater events(see later in the text). Of interest here is the lack of moderntravertine formation. Historical records of the Cave and Basinsite are sparse, and it is unclear if active travertine formationwas occurring at the time of discovery. Historical photographsindicate an unforested hillslope with sparse vegetation andexposed travertine, suggesting more active growth. This mightrelate to greater usage of the site (a “hotel” had initiallybeen built on the travertine mound near the hole in the CaveSpring roof), however. The modern-day Cave and Basin siteis largely overgrown with vegetation, and, with minorexceptions, no active travertine growth is observed. The lackof modern travertine formation seems contradictory toobservations that (i) waters emerge overpressured with respectto CO2; (ii) the waters have a capacity to deposit significantamounts of travertine in a short period of time; and (iii) whereartificially disturbed, waters can be made to degas CO2 anddeposit travertine quite readily. Lu et al. (2000) show thathydrodynamics conditions in travertine pools control spatialpatterns of pH and thus calcite saturation. They indicate thatalthough waters flowing over travertine dams show significantincreases in pH and calcite saturation, waters at the innerdam wall show a pH minimum. Thus subtle variations inflow can cause significant variations in calcite saturationboth spatially and temporally. Even though the Cave andBasin waters are slightly oversaturated, it has been shownthat low levels of supersaturation can inhibit CaCO3 precipi-tation (Suarez 1983). Given this, we argue that the lack of

Source Calcite Dolomite Gypsum Anhydrite Quartz Albite

Travertine mound 100 TraceConsolidated till matrix 11 34 2 49 2Unconsolidated till matrix 75 5 20Coating on cave wall 40 1 59Material at till–travertine contact 99 1Sediments in Cave Spring pool 4 3 Trace 93Sediments in Basin Spring pool 4 2 Trace 94

Note: Mineralogy is based on qualitative assessment of relative peak intensity.

Table 2. XRD data for samples from the Cave and Basin site.

Fig. 8. Photograph of laminated flowstone from interior of thecave at the Cave Spring site.

Fig. 9. Calculated change in calcite saturation as a function ofdegassing CO2 (PCO 2

, partial pressure of carbon dioxide; 1 bar =100 kPa) to reach equilibrium with atmospheric values. Calculationsshown for outlet temperatures and for temperature drops of 5 and10 °C.

Age (years BP)

Source 14C (% modern) Uncorrected Adjusted

Cave SpringFresh travertine 45.9 6 437 ± 160 5Travertine mound 30.9 9 708 ± 170 3276Travertine mound 24.2 11 728 ± 210 5296Travertine mound 27.2 10 762 ± 140 4330Travertine mound 29.8 10 008 ± 140 3576

Lower Pool SpringFresh travertine 36.9 8 242 ± 190 5Travertine mound 36.8 8 264 ± 250 27Travertine mound 33.0 9 165 ± 470 928

Table 3. 14C dating of travertine samples from the Cave and Basinsite, giving uncorrected and adjusted ages (see text).



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modern travertine deposition is related to flow control measuresinstalled in the last 100 years that prevent waters runningnaturally over the slope of the travertine deposit, inhibitingCO2 degassing and thus development of highly supersaturatedconditions needed for calcite precipitation. Diverted springwaters are mixed and discharged into outflow channels northof the pavilion (Fig. 2), where they flow into the marsh andare diluted by surface waters, inhibiting further calciteprecipitation.

Cave developmentThe terraced profile of the travertine mound at the Cave

and Basin site is a common feature of travertine deposits(e.g., Julia 1983; Chafetz and Folk 1984; Guo and Riding1998; Fouke et al. 2000). An unusual feature at this site isthe development of interior caves within the travertine. Theobservation that the caves are excavated along the till–travertineboundary suggests that the underlying till is preferentiallyeroded. This is consistent with recent observations of erosionof the till due to influx of water from the overlying Frank’sPool. In addition, XRD analyses of till material within thecave indicate that the matrix is dominantly gypsum, whereasthe till matrix away from the travertine deposit is verycalcareous. As discussed later in this section, acid gases(H2S) released from the spring waters can readily alter calciteto gypsum, suggesting that the till matrix cement is being alteredand weakened by exposure to the spring water. This wouldincrease the susceptibility of the till to physical erosion.

Although physical erosion appears to play an importantrole in the initial development of the cave, evidence showsthat chemical corrosion is also important (van Everdingen etal. 1985). High relative humidity levels within the cave,when not controlled by mechanical air flow and filtrationdevices, lead to moisture coating the interior walls and ceiling.Once vapor condenses on the cave wall, atmospheric gaseswould then readily dissolve into the water film. During periodsof high atmospheric H2S levels (up to 13 ppm), extremelyacidic water can then be generated by oxidation of dissolvedHS–:

[R2] HS– + 4H2O → SO42– + 9H+

This, in turn, reacts with the calcite of the travertine mound:

[R3] CaCO3 + 2H+ → Ca2+ + CO2 + H2O

The Ca2+ liberated by acid attack could then react with thedissolved sulphate to form gypsum:

[R4] Ca2+ + SO42– + 2H2O → CaSO4·2H2O

This process, known as H2SO4 speleogenesis, has beenshown to be an important influence on the development ofmajor cave systems, including the Carlsbad Caves in NewMexico (Polyak and Provencio 2001, and references therein).Stable isotope data provide support that chemical corrosionof the travertine mound at the Cave and Basin site is an activeprocess. A plot of isotope data from van Everdingen et al.(1985) and Grasby et al. (2000) shows that both the gypsumcrust coating the cave walls and gypsum in the pool sedimentplot within the theoretical sulphide oxidation field of vanStempvoort and Krouse (1994) (Fig. 10), implying that thesulphate in the gypsum is formed through sulphide oxidation.

This is supported by the similarity of δ34S values of the gypsumcoating and H2S from the spring (oxidation of H2S to SO4has minimal fractionation; Fry et al. 1988). In contrast, dissolvedsulphate and gypsum from bedrock samples plot well abovethe sulphide oxidation field and have δ18

SOO4values along

with δ34S values consistent with a primary Paleozoic anhydritesource (Figs. 10, 11).

Given the aforementioned findings, it is interesting to notethat water films on the cave walls typically show extremelylow pH values (<1). This suggests that the formation of agypsum crust on the cave walls inhibits further acid attack ofthe travertine mounds, and without the buffering effect ofcalcite low pH levels develop through oxidation of H2S gasin the water film. That being said, the presence of gypsum inthe pool sediments with stable isotope values similar to thoseof gypsum on the cave wall suggests that this crust periodicallydelaminates from the cave wall, which would expose a freshsurface to acid attack. The total rate of acid dissolution isthus difficult to estimate as the process is buffered by therate at which gypsum crusts form and become dislodgedfrom the surface.

Implications for regional groundwater dynamicsTentative age dates for the travertine mounds (3200–5300 BP)

suggest that the onset of travertine formation is coincidentwith the shift from warm and dry conditions of the Hypsithermalto relatively more cool and moist conditions characteristic ofthe modern climate. Studies of peatlands suggest that theearly Holocene of western Canada had thermal-season aridityindices that were –8.3% to –18.4% less than those of presentday along with 13.5% less precipitation (Zoltai and Vitt1990). In addition, studies in the central prairies indicate thatduring the mid-Holocene regional groundwater tables mayhave been on the order of 6–15 m lower then present (Remendaand Birks 1999). Around 6000 BP a dramatic increase inregional peatland development occurred which is interpretedto represent a transition to climate systems more similar tothose of the present (Zoltai and Vitt 1990) with establishmentof shallow groundwater tables. In addition, around 5000 BPregional groundwater tables are noted to rise across interiorwestern Canada (Lemmen and Vance 1999). During this time,the Rocky Mountains also experienced cooler and wetterconditions marked by the expansion of glaciers, the Neoglacialinterval (Porter and Denton 1967).

Grasby and Lepitzki (2002) show that the flows of theUpper Hot Spring and Kidney Spring along the SulphurMountain Thrust are highly susceptible to variations inprecipitation, where a decrease in average precipitation willresult in a drop in the piezometric surface along the SulphurMountain Thrust the following year, leading to winter flowcessation of the topographically highest springs. Given thissusceptibility of the flow of thermal water to climate variability,it is feasible that during the prolonged dry period of themid-Holocene the Sulphur Mountain thermal springs hadlittle or no flow. The coincidence of the age date for onset oftravertine formation at the Cave and Basin site with risinggroundwater levels may then be a record of the initiation ofthe modern thermal flow system at Banff. This, in turn, impliesthat more regional studies of travertine deposits may record



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the response of groundwater systems to changing climateconditions.

A 5300 BP age for onset of thermal spring dischargewould also have interesting implications for the evolutionarybiology of endemic species that have evolved from cold-waterrelatives to be adapted to live in the warm thermal waters.Studies of the endangered Banff Springs snail (Physellajohnsoni) that are restricted to waters at 30–36 °C show lowlevels of mitochondria DNA divergence, suggesting it is arecently evolved species (Remigio et al. 2001), with timingassumed to be constrained by deglaciation � 10 000 BP.Ages of the travertine may constrain onset of evolutionarydivergence to �5300 BP. Although feasible, this needs to betested by examination of a faster evolving mitochondria DNAgene region than previously studied (E. Remigio, personalcommunication, 2003).

Origin of the Cave and Basin NationalHistoric Site

Based on the our findings and previous work, we presenta model for the formation of the Cave and Basin site inFig. 12. In response to changing climate conditions and risinggroundwater levels associated with the end of the Hypsithermal,sustained flow of thermal waters was established at SulphurMountain, with discharge through bedrock and overlying glacialtill (Fig. 12A). Degassing of CO2 at surface initiated a travertinedeposit (Fig. 12B). Tentative age dating suggests that theterraced profile of the travertine mound forms through thebackstepping model of Fouke et al. (2000). With time,successive layers of travertine formed a terraced mound thatgrew to reach the highest point of discharge (Upper PoolSpring; Fig. 12C). The poorly consolidated nature of the till,in conjunction with acid attack on the matrix cement, madeit easy to erode, leading to water flow along the traver-

tine–till boundary (Fig. 12D). As water flow carved out a cav-ity beneath the travertine, spring pools formed, allowing degas-sing of H2S and the initiation of acid attack of the interior ofthe travertine deposit and leading to the formation of thepresent cave structure (Fig. 12E). This process has weak-ened the travertine deposit to the point of collapse at the Up-per Pool site.

Conclusions

The Cave and Basin National Historic site is characterizedby a 26 000 m2 fan-shaped travertine mound formed byprecipitation of calcite from discharging thermal springs.Discharge waters have a Ca–SO4–HCO3 composition andare typically oversaturated with respect to CaCO3. Althoughdischarge waters can be made to readily precipitate calcitewhen artificially agitated along the flow path, historical flowcontrol measures appear to inhibit modern growth of thetravertine mound.

Flow of thermal waters along the base of the travertinemound has preferentially eroded underlying till, allowingthermal waters to pool. H2S degassing from these pools reactswith overlying calcite of the travertine deposit. This processis buffered by the formation of reaction crusts of gypsum.Evidence indicates, however, that these crusts periodicallyfall into the underlying pool, suggesting that the gypsumcrust formation is a rate-limiting step in the long-term corrosionof the travertine. Evidence at the Upper Pool site suggeststhat the process of corrosion eventually leads to collapse ofthe travertine caves. If allowed to follow a natural course, theother cave structures at the Cave and Basin National HistoricSite would be expected to collapse at some point in the future.Activity at the site has both positive and negative effects onthis natural course of evolution. An influx of air through thevisitor centre reduces the atmospheric H2S levels in the cave,and thus the rate of corrosion. Historical flow control measuresinhibit modern calcite precipitation, however, and thus growthof the travertine mound, while at the same time minimizingphysical erosion of the underlying till. A better understanding

Fig. 10. Plot of δ18SOO

4versus δ18

H OO2

from dissolved sulphateand gypsum at the Cave Spring site (data from van Everdingen et al.1985, and Grasby et al. 2000). Also shown is the theoretical sulphideoxidation field from van Stempvoort and Krouse (1994).

Fig. 11. Range of δ34S values for solids and dissolved sulphurspecies at the Cave Spring site.



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of the movement of water through the travertine mound wouldaid the long-term preservation of this national historic site.

Age dating of the travertine mound at this site providesinsight to the response of deep circulating thermal systemsto changing climate conditions through the Holocene. Furtherstudies of travertine deposit dates could provide valuableinformation on the response of regional groundwater systemsto changing climate conditions.

Acknowledgments

Parks Canada provided research permits and the Cave and

Basin National Historic Site allowed access to the facilities.The paper was improved by helpful comments provided byHenry Chafetz and two other anonymous reviewers.

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Fig. 12. A series of schematic diagrams illustrating the development of the Cave and Basin site. Original discharge through till (A)leads to initiation of travertine formation (B) and eventually a multi-terraced travertine deposit (C). Physical erosion of till divertedflow under the travertine mound (D), leading to cave formation, along with acid attack of travertine and eventual collapse of the cavestructure at the Upper Pool (E).



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Documents

Travertine mounds of the Cave and Basin National Historic Site, Banff National Park