Structural Control of the Morphometry of Open Rock Basins, Kananaskis Region, Canadian Rocky Mountains
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5" %~ '.5" :7~, 6[mU ELSEVIER Geomorphology 22 (1998) 313-324 Structural control of the morphometry of open rock basins, Kananaskis region, Canadian Rocky Mountains D.J. Sauchyn a,*, D.M. Cruden b, X.Q. Hu c,1 a Department of Geography, University of Regina, Regina, Saskatchewan $4S OA2, Canada b Department of Civil and Environmental Engineering, University of Alberta, Edmonton, Alberta T6G 2G7, Canada c Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada Received 12 September 1996; revised 18 July 1997; accepted 14 August 1997 Abstract The morphometry of chutes (couloirs), rock funnels, and open cirques are related to the structure of dissected rock masses in the Kananaslds region of the Canadian Rocky Mountains. Data for ten morphometric variables were derived from digital elevation models of 56 open rock basins. The basins were classified structurally according to the relative orientations of bedding planes and the rock slopes. A hypothesis of no differences in morphometry among structural classes is rejected from the results of nonparametric analysis of variance and paired comparisons of rank scores. Basins on dip and overdip slopes have a distinct :size, and those on anaclinal slopes have a distinct width and shape. Variation in morphometry from low compactness and area/relief (chutes) to high compactness and low area/relief (funnels) to high compactness and area/relief (open cirques) corresponds to a change in dominant structure from orthoclinal to dip-overdip to underdip to anaclinal. The dip of bedding planes relative to the slope of rockwalls controls the mode of initial displacement of joint blocks and, thereby, the spatial distribution of the retreat of rockwalls. The angle between the rock slope and the strike of dipping strata determines whether beds of differing stability form chutes and buttresses (orthoclinal slopes), or extend across rockwalls (cataclinal and anaclinal slopes) and retreat at similar rates to form funnels and open cirques. The optimal structure for large compact rock basins is anaclinal, and the least favourable is cataclinal dip-overdip slopes. Topoclimate and other geologic structures may account for variance in morphometry not explained by differences among structural classes. © 1998 Elsevier Science B.V. Keywords: mountains; rock slopes; open rock basin morphometry; geologic structure 1. Introduction received relatively little attention compared to the preoccupation of geomorphologists with coarse de- The geomorphic significance of geologic structure bris landforms in alpine environments (Klimaszew- is implicit in studies of steep mountain slopes (Rapp, ski, 1971; Gerber and Scheidegger, 1973). This dis- 1960; Luckman, 1988). These slopes, however, have parity may reflect the impracticality of field experi- ments on precipitous slopes and the more immediate * Corresponding author. E-mail: [email protected] link between talus slopes and contemporary pro- 1 Current address: Westworth Associates Environmental Ltd, cesses. Even though geologic control on the form of 140, 9405-50st., Edmonton, Alb., T6B2T4 Canada. rock slopes may be intuitive, and obvious from 0169-555X/98/$19.00 f~ 1998 Elsevier Science B.V. All rights reserved. PII S0169-555X(97)00083-4 314 D.J. Sauchyn et al. / Geomorphology 22 (1998) 313-324 cursory observation, rigorous explanation of this con- (e.g. Terzaghi, 1962; Hoek and Bray, 1981). The trol is more elusive. Some control is direct, for relevant rock mechanics and geological factors de- example, planar slopes on bedding planes. Other- pend on the nature and magnitude of the geomorphic wise, it involves the interaction of structure, process, process. Rock control in geomorphology can be sub- and form. divided into resistance to (1) weathering processes, Current understanding of the development of steep (2) detachment of joint blocks from a rock mass, and rock slopes is based largely on the work of geomor- (3) higher-magnitude deep-seated failures, where phologists (e.g. Gerber and Scheidegger, 1973; Selby, residual stress is an important consideration. This 1982) and built on a foundation of rock mechanics paper examines the morphometry of open rock basins ....j Mt. Kidd • open rock basin study sites / • major peaks * Highwood Pass BanffNational t'\ l~ Park ~ r -'-~ \.%]~....~..jFortress •Mtn•~ •'• '\. 1~ < ",,. ~",, ~ :?\':\ m ALBERTA < Peter "~%,,,, \ Mt. ..~',"""~ ~,t Lougheed ~':~\~ •\,- BRITISH '~'., -'-"~ Provincial X,~%,, COLUMBIA t . ....',L.. Park ~*'~ r..,..,,, • f=,, \ } Mt. Black~,,~.,~ ~ ~ Prince ~ Kananaskis Lakes % ), % / - - T - - "7- - -t... I I ! L_ /Study t f .,. t Storm Mt. I II Cs. _ _ "~ I'~-i i k f ,_.t.._J 2--.r--~,_.t t I -r---l_ _~-4 ! 10 km k ~'\.J L-_L_ -"~'~,l - -'----i-.I (1' Fig. 1. The Kananaskis study area and open rock basin study sites. D.J. Sauchyn et aL/ Geomorphology 22 (1998) 313-324 315 formed by the mass wasting of joint blocks from tion of large talus slopes and open rock basins, and rockwalls in a high mountain landscape, the estimated rates of periglacial weathering and rockfall Kananaskis region of the southern Canadian Rocky (Gray, 1972; Soderman, 1980; Olyphant, 1983), sug- Mountains (Fig. 1). The controlling structural fea- gest high spatial variability in the denudation of tures are bedding planes and joints. The important mountain rockwalls. Once weathering along fractures structural parameters are the dip and strike of discon- eliminates cohesion between rock blocks, the stabil- tinuities relative to the rock face. ity of rockwalls is governed by properties of the Without reference to process, morphometric stud- fractures: roughness, fill, width, frequency, and ori- ies are of limited va~[ue (Thorn, 1988). Therefore, a entation (Selby, 1982). Although angles of sliding primary objective of this study is to relate the mor- friction of the carbonate rocks in the study area phometry and structure of open rock basins to the range from 21 ° to 41 ° , they are usually less than 30 ° processes that remove rock blocks from cliff faces. (Cruden and Hu, 1988). Therefore, the orientation of Rockfall is the dominant geomorphic process on penetrative discontinuities relative to the gradients of mountain rockwalls, according to field observation rockwalls is a major control on the stability and (Gardner et al., 1983; Luckman, 1988) and as im- differential mass wasting of rock masses. plied in models of the cliff-talus geomorphic system Sauchyn and Gardner (1983) demonstrated a link (Statham, 1976; Olyphant, 1983; Whalley, 1984). between the morphometry and aspect of the rock Blocks of rock move exclusively by falling only basins examined here. However, structure and when unsupported. ,Other processes usually trigger topoclimate vary consistently with aspect. Subse- rockfalls by projecting the blocks away from confin- quent research by Cruden (1988) on the structural ing support. The lite:rature on rock mechanics makes fabric of the Canadian Rockies, and its relationship an explicit distinction between rockfall and the pre- to catastrophic mass wasting, provides a conceptual ceding release of rock blocks by sliding or toppling framework for a systematic analysis of structural along or from joints and bedding planes (Hoek and control on the morphometry of open rock basins. The Bray, 1981). Geomorphologists (Rapp, 1960) clas- classification of rock mass structure is based on the sify rockfall as primary or secondary, where the later relative dip and strike of penetrative discontinuities, requires the initial impact of snow avalanches, falling such as bedding or schistosity (Fig. 2). Dip slopes rock, or water. dip in the direction of the discontinuity and scarp (or reverse) slopes face in the opposite direction. Dip 2. Conceptual framework The landforms examined here are chutes (couloirs), rock funnels, and open cirques formed by A t.EJ j j \ "-. differential mass wasting of the rockwalls that bound ~'~/ """1 ///I OVERDI "- major ridges and peaks. They are an assemblage of steep rock slopes, with gradients exceeding the angle of repose of coarse angular clasts (Carson, 1977), ,, and lesser slopes where rock debris may be stored temporarily. Collectively these landforms are called open rock basins (Sauchyn and Gardner, 1983). They are distinct from glacial cirques; coarse sediment is 0--I ~' /- , FOLIATION ° \ ", o,P transferred from open rock basins, usually to a subja- J J30 cent talus cone. Various scales and morphologies of rock basin are integrated or nested spatially and ANACLINAL 1 CATACLINAL functionally within 1the alpine cliff-talus geomorphic Fig. 2. A classification of rock slopes in strike valleys by the system. The size, distribution, and shape reflect vari- orientationof the dominantpenetrative discontinuity (from Cruden, ations in the strength of rock masses. The juxtaposi- 1988). 316 D.J. Sauchyn et al. / Geomorphology 22 (1998) 313-324 slopes may be subdivided into overdip and underdip mulate on benches and reverse slopes formed in slopes which are steeper and less steep, respectively, resistant beds. than the dip of the discontinuity. Some ambiguities Whereas the relative strike determines the expo- occur in this terminology; to avoid them, terms sure of resistant and recessive beds across rockwalls, originally introduced by Powell (1875) can be used. the relative dip of discontinuities determines the On cataclinal slopes, the penetrative discontinuity mechanism by which rock blocks are released, as dips in the same direction as the slope; on anaclinal