LATEQUATERNARY STRATIGRAPHY AND PALÆOHYDRAULIC DEVELOPMENTOF AN OUTWASHFAN IN THE NORTHERNVENEZUELAN ANDES
A thesis submitted to the Faculty of Graduate Studies in partial llfilment of the requirements for the degree of
Master of Science
Graduate Programme in Geography York University North York Ontario
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a thesis submitted to the Faculty of Graduate Studies of York University in partial fulfillment of the requirements for the degree of
Master of Science
0 1998 Permission has been granted to the LIBRARY OF YORK UNi- VERSITY to lend or seil copies of this thesis. to the NATIONAL LIBRARY OF CANADA to microfilm this thesis and to lend or sel1 copies of the film. and to UNIVERSITY MICROFILMS to publish an abstract of this thesis. The author reserves other publication rights. and neither the thesis nor extensive extracts from it may be printed or other- wise reproduced without the author's written permission. Abstract
A proglacial environment, located north of La Mucuchache valley in the northern Venemelan Andes, was examined to determine the pakohydraulic conditions under which it was created. The region consists of a sandur, showing morphological charactenstics of an alluvial fan cornpiex, two lateral moraines of Late Mérida (Late Wisconsinan) age (La Mucuchache and El Caballo moraines), and an end moraine complex of pre-Mérida (pre- Wisconsinan) age (Mesa del Caballo). Stratigraphie evidence at the fan site showed six distinctive depositional episodes resulted in the genesis of the complex. Sediments of glaciofluvial ongin were found to be the major constituents of the fan, confirming an outwash origin, with lacustrine silts and clays deposited during times of lower, or absent, fluvial activity. Sehents of large gravel and small boulden suggest flood flows during either high ablation penods, or jokulhlaups spawned by sudden release of meltwater from the palreoglacier. Bedforms within sand and srna11 gravel deposits are rare; however, they suggest nomal fluvial conditions existed within c hannels for some time. Instrumental neutron activation analysis did not identify which watershed (La Mucuchache to the southwest or El Caballo to the est) was the main source for the glaciofluvial sediment. Analysis of the chondrite-nonnalized concentrations of the rare earth elements (REEs) within La Mucuchache moraine, EI CabalIo moraine and the glaciofluvial sediment, showed little difference between the three sets of sediments. Geomorphologically, La Mucuchache moraine exhibits a breach near the apex of the fan, resulting fiom either meltwater erosion or glacial erosion, or both. Overriding of El Caballo moraine is possible; however, a senes of recessional moraines on the fan near the apex Mersuggests La Muchuchache Valley as the source of the meltwater. Due mainly to high surface gradients, flow was catastrophic during deposition of the largest clasts. Maximum palæovelocities were approximately 12 m s-' for flood deposited sediment while various cross-bedded sand deposits suggested slower, yet still turbulent (as reflected by high Reynolds numbers) flow. Sampling limitations prevented the identification of the hydraulic geomeiry of any channels. SEM analysis implies that such flow regimes have minimal effect over short distances on the microtexture of sand grains in transport as either bedload or suspended sediment The fiequencies of fluvial microfeatures on coane quartz sand grains are low compared to other studies; the result of a shorter transport distance, a low concentration of sand in transport, or high flow turbuIence. Table Of Contents
Absmct iv Table of Contents v vii List of Figures .-. List of Tables vi11
1.0 Introduction and Research Objectives 1.1 Introduction 1.2 Research Objectives 1-3 Assumptions Required for Palæohydraulic Estimates 1.3.1 Steady, uniform flow 1.3.2 Small relative bed roughness 1-3.3 Uniform distribution of roughness elements 1.3.4 Low suspended sediment concentration 1-3.5 Maximum sediment availability
2.0 Literature Review 2.1 Introduction 2.2 Definitions 2.3 Physical Roperties of Outwash Features 2.3.1 Hydrology 2.3.2 Morphology 2.3.3 Sediment 2.3.3.1 Particle size and distribution 2.3.3.2 Particle shape and clast fabric 2.4 Stratigraphy and Sedimentary Facies 2.4.1 Lithofacies 2.4.2 DownsIope changes 2.5 Lacustrine Sedirnents 2.5.1 Stratigraphy and sedirnentology 2.6 Provenance and Heavy Minerals
3.0 Study Area 3.1 Introduction 3.2 Climate 3.3 Surfical Geology and Topography 3 -4 Glacial Sediments of the Fan Complex and Surrounding Area 3.4.1 Till 3-4.2 GlaciofluMaI Sediment 48 3.4.3 Geology 49 3.5 Glaciations in the Sierra Nevada de Mérida 50 3.6 hvious Studies Involvùig La Mucuchache Outwash Fan Complex 5 1
4.0 Methods 4.1 Introduction 4.2 Field Methods 4.3 Laboratory Methods 4-3.1 Granulomeüic analysis 4-3-2 Instrumental neutron activation analy sis (INAA) 4.3.3 Heavy Mineral Identification 4.3.3 Scanning electron microscopy (SEM) analysis 4.4 Palæohydmlogical Reconsûuction 4.4.1 Empincal and theoretical methods of palieo hydraulic reconstruction 4.4.2 Tractive force method 4.4.2.1 Criticd shear stress 4.4.2.2 Flow depth 4.4.2.3 PaIæovelocity 4.5 Turbulence indicators
5.0 Results and Discussion 5. 1 Introduction 5.2 Findings and Interpretations 5.2.1 Stratigraphy 5.2.2 Granulometry 5.2.3 Geochemical analysis 5 -2.4 Heavy mineral identification and analysis 5.2.4.1 Metallic bIack heavy minerals 5.2.4.2 White translucent heavy minerals 5 L4.3 Brown heavy minerals 5.2.4.4 Orange heavy minerals 5.2.4.5 Light green heavy minerals 5.2.4.6 Rose (light pink) heavy minerals 5.2.5 Scanning electron microscopy 5.2.6 Palæohydraulic estimations 5.3 Glacial Chronology
6.0 Conclusions 6.1 Conclusion and Summary References Cited Appendix One: Particle Size Distni'butions Appendix Two: Heavy Minerd Concentrations Acknowledgements
List of Figures
Figure 2.1 An outwash fan in the Sierra de Santa Domingo mountain range Figure 2.2 Longitudinal profile and clast variation on numerous sandurs Figure 2.3 Sediment and water transport routes within a glacier Figure 2.4 Flow parameters of the Scott outwash fan, Alaska Figure 2.5 Composite diagram of facies changes on two braided plains Figure 2.6 Typical mode1 of delta formation
Figure 3.1 Sierra de Santo Domingo Mountain Range, Northem Venezuelan Andes Figure 3.2 La Mucuchache and El Caballo watersheds Figure 3.3 La Mucuchache Outwash Fan Complex Figure 3.4 Photo looking south down La Mucuchache Outwash Fan Complex Figure 3.5 Classical alluvial loess exposed on the surface of the fan at site ped 6 Figure 3.6 The breach located between Mesa del Caballo and La Mucuchache moraine Figure 3.7 El Caballo moraine bordenng the noah east side of the fan Figure 3.8 El Caballo valley located to the east of the fan complex Figure 3.9 La Mucuchache Valley located to the south West of the fan Figure 3.10 La Mucuchache outwash fan escarprnent at the north West section of the fan Figure 3.1 1 Stratigraphy of the Pedregal sections with radiocarbon dates Figure 3.12 Fabric of Ped 1, Unit 3
Figure 5.1 Stratigraphic section Fan 1 Figure 5.2 Stratigraphic section Fan 2 Figure 5.3 Stratigraphic section Fan 3 Figure 5.4 Stratigraphic section Fan 5 Figure 5.5 Stratigraphic section Ped 15 and 16 Figure 5.6 Stratigraphic section Ped 17 Figure 5.7 Stratigraphic section Ped 6 Figure 5.8 Stratigraphic section Lag 7 and 8 Figure 5.9 Cross-section of La Mucuchache Outwash Fan Figure 5.10 Unit 2 lacustrine sediments have been exposed on the surface of the fan Figure 5.1 1 The massive grave1 and boulder beds of Unit 3
vii Figure 5.12 Fabric orientation of grave1 at three sites near Fan 1 86 Figure 5.1 3 Temary diagrams for a) glaciofluvial samples, b) lacustrine and à11 samples 9 1 Figure 5.14 Chondrite normalized profile of samples hmLa Mucuchache moraine and El Cabal10 moraine 99 Figure 5.15 Chondrite nodized profile of samples f?om La Mucuchache Outwash Fan Complex I O0 Figure 5.16 Selected coloured grains removed for microanalysis and instrumental neutron activation analysis 104 Figure 5.17 Summary graph of rnicrofeature frequencies on glaciofluvial and till sand grains 115 Figure 5.18 Microfeatures found on coarse glaciofluvial and till quartz sand grains with the SEM 116 Figure 5.19 Relationship between transport distance and a) low relief, b) medium relief, c) rounded features and d) V-shaped percussion cracks 127 Figure 5.20 Relationship between sand content and a) low relief, b) medium relief, c) rounded features and d) V-shaped percussion cracks 129 Figure 5.21 Relationship between flow turbulence and a) low reiief, b) medium relief, c) rounded features and d) V-shaped percussion cracks 131 Figure 5.22 Critical mean palæovelocities predicted by a velocity profile mode1 136 Figure 5.23 Cntical rnean palæovelocities predicted by Manning's equation and a calculated Manning's n 140 Figure 5.24 Critical mean palæovelocities predicted by Manning's equation and a constant Manning's n 142
List of Tables
Table 1.1 The various assurnptions that have to be made in order to estimate pakoflow conditions 7 Table 4.1 The rationale behind the choices of locations of each sample site 58 Table 4.2 Critena used for glaciofluvial lithofacies assignment 59 Table 5.1 Particle size data for La Mucuchache Outwash Fan Complex 89 Table 5.2 Macro- and micro elernent concentrations 92 Table 5.3 Summary statistics of macro- and micro elements 95 Table 5.4 hreearth element concentrations 96 Table 5.5 Surnrnary statistics of rare earth elements 97 Table 5.6 Concentration of magnetic and non-magnetic heavy minerals in six till samples 102 Table 5.7 Macro- and micro elernental concentrations within selected heavy mineral samples 1O8 Table 5.8 Characteristics of various SEM sarnples 1 13 Table 5-9 Summary counts and percentage frequencies of SEM microfeahues on glacio fluvial sand grains 113 Table 5.10 Summary counts and percentage frequencies of SEM microfeatures on till grains 1 14 Table 5.1 1 Summary statistics of till and glaciofluvial microfeatures 114 Table 5.12 Grain shape characteristics of the selected glaciofluvial samples 134 Table 5.13 Estimated palæovelocities of selected sample sites based on a velocity profile mode1 135 Table 5.14 Estimated palæovelocities of selected sample sites using the Manning's equation and a calculated Manning's n 139 Table 5.15 Estimated palæovelocities of selected sarnple sites using the Manning's equation and a constant Manning's n 133 Introduction and Research Objectives Chapter One
1.1 Introduction 1.2 Research Objectives 1.3 Assumptions Within the Pala~ovelocity Estimates 1.3.1 Steady, uniform 8ow 1.3.2 Smatl relative bed roughness 1-3.3 Uniform distribution of resistance elements 1.3.4 Low suspended sedirnent concentration 1 -3-5 Maximum sedirnent availability 1.1 Introduction One of the more important components of palæohydrology is the extrapolation of present-day fluvial or glaciofluvial sediment budgets into the past. Traditionally, researchers have concentrated on stratigraphy and palæoform analysis to complete this initiative: however. there has ken an emerging trend (e.g. Williams 1984, Maizels 1986, 1989. 1995) over the last few decades to reconstruct the processes responsible for these budgets. By doing so. the science of paiæohydrology is begiming to emerge as a distinct discipline within the confines of the Quaternary sciences, geomorphology and geophysics (Clark 1987). Mode1ling the palæohydraulic environment relies on the understanding of present day process systems (see Dingman 1984, Raudkivi 1976) but because of the dificulty in understanding these systerns, palæohydrology seems to be destined for an uncertain future. In 1987, Clark noted that long-term infierence and accurate pakohydraulic estimations seemed to be an aspiration rather then a usefbl descriptive tool; however, progress is being made toward developing useful empirical and theoretical models (e.g. Maizels 1995). With the alpine glaciofluvial sediment system as a modem analogy, the aim of palæohydrology seems to be devising useful, accurate methods rather than confidently accepting the theoretical estimations themseIves (Clark 1987)- The aim of this study is to incorporate both the traditional stratigraphie aspects of palieohydrology and the process dominated endeavours of today. Coupled with the unfolding Quatemary sîmtigraphy and glacial chronology of the northem Andes mountains (e-g. Geigengack and Grauch 1975, Schubert 1994, Mahaney and Kalm 1 W6), reconstmctive process palxohydrology will help define the palxoenvironmental conditions present during the Last Glacial Maximum (LGM)in a srnaIl outwash basin. This study examines a proglacial environment located to the north of La Mucuchache valley in the northem Venezuelan Andes (see Figure 3.2). It consists of an outwash fan, having the morphology of a gently sloping (-5O) alluvial fan complex, and two synchronous LGM moraines (La Mucuchache moraine and El Caballo moraine) emplaced behind end moraines (Mesa del Caballo) of pre-Mérida @re- Wiscominan)age (see Figure 3.3). This study tests the hypothesis that the glacier occupying La Mucuchache valley created meltwater that washed out part of La Mucuchache lateral moraine on the northern side of the valley and then deposited the alluvium creating the fan complex. This meltwater, along with meltwater from a glacier originating in a cirque to the east (within El Caballo valley), created a large proglacial lake that then washed out a portion of Mesa del Caballo end moraine. It is hypothesised that the outwash deposit, the proglacial lake and a meltwater channel leading to the lake date fiom the LGM about 20,000 yr. B.P. At the foot of the fan, tills found in Mesa del Caballo suggest that glaciation has occurred over the complex since at least the late middle Pleistocene (Mahaney and Kalm 1996). In the first part of this research, the glacial and glaciofluvial deposits will be described and their source inferred fiom morphological and sedimentological properties. Sediment analysis, including granulometry and microscopie analysis, will aid in detemining the origin of the material and provide textural information for environmental reconstruction. Geochemical analysis, including the identification of heavy mineral suites within till sarnples will help decipher the source of the sediment as well as begin to offer information into the abundance of economica~lyvaluable rninerals in the northern Venezuelan Andes. Previous research in the southeastem Andes of Peni (Hérail et al. l989), has show that gold placers of "notable quantity
@g. 369)" have ken found in the Ancocala-Ananea Basin. There has been little research into the economic feasibility of extracting valuable minerals hmthe till and glaciofluvial sediments of the northern Venezuelan Andes. The second component of the research will focus on the reconstruction of the processes responsible for the deposition of the fan material. This will include a reconstruction of the palæohydrological regimes that created the meltwater features. Using the grain size characteristics of the sand and grave1 fiactions of the glaciofluvial sediments, the required hydraulic parameters needed to transport the matenal [including mean flow velocity (U), mean depth (d), and turbulence indicators], will be detennined for the outwash fan sediment.
Many studies investigating hydraulic conditions within contemporary channels have concentrated on the relationship between grain size and strearn competence. This is usually achieved by using critical shear stress (r,), mean velocity (U), critical velocity (U,), Stream power (o)and/or sediment transport rate (1,) as the indicator of competence (Gregory and Maizels 1991). However, errors remain when these methods are applied to palæoflows (for example, see Maizels 1983, 1986, Williams 1984). The erroa are usually the result of applying assurnptions about strearn flow to the fluvial conditions of the past. These assumptions will be discussed in section 1.3.
To reduce these errors, the researcher must keep the number of unknown variables to a minimum and use as many independent variables as possible. This can be problematic when
Wng to reconstmct cornpiete palæohydraulic conditions when only grain size is known and the occurrence of bedforms is minimal.
1.2 Research Objectives In order to study the Quatemary history and paloohydraulic development of La Mucuchache outwash fan cornplex, three objectives have been developed. The objectives of this study are:
1) to constmct a Quatemary chronology for the events responsible for the emplacement of the outwash fan. This chronology will be based on stratigraphie evidence of glacial and glaciofluvial deposits resulting from events during the LGM and will not involve subsequent soi1 developrnent during the Holocene. Included in this chronology will be the genesis of the outwash fan and lacustrine deposits, the meltwater channel at the northeast section of the complex, and the breach of the Mesa del Caballo/La Mucuc hac he moraines,
2) to detemine the transport histones of the glaciofluvial material comprising the fan complex. This includes estimating the palæoveloc ities and channel characteristics (e.g. palazodepths) of the meltwater fiow within the basin as well as inferring transport conditions and recognizing changes in the frequency of fluvial-induced micro features on quartz sand grains [via scanning electron microscopy (SEM)].
3) to determine the provenance of the glaciofluvial sediment (and therefore the meltwater) found within the fan cornplex using instrumental neutron activation analysis (MAA). It is hypothesised that the source of the rneltwater was the ice within La Mucuchache valley.
There is less geomorphic evidence (i.e. no visible flow pathway) to suggest that the water originated in El Cabal10 valley to the northeast; however, srna11 fan-shaped hummocks exist on the southeast section of the fan complex suggesting possible meltwater flow fiom that direction;
4) to devise an empirical mode1 to summarize and predict the movement of rneltwater through the study area with regard to the palieovelocity estimates and depth calculations.
1.3 Assumptions Within the Palæovelocity Estirnates Estimating palæohydraulic parameters requires the assumption of numerous conditions within the fluvial palooenvironrnent. Maizels (1986) presents and discusses these assumptions (see her Table 1, pg. 266), while Mahaney and Kalm (1996) summarized the most important assumptions that were used in the estimation of preliminary palæovelocities for La Mucuchache outwash fan complex. The most important assumptions employed in this study are found in Table 1.1. and will be discussed below; however, less relevant ones will be mentioned throughout the text as required. See Maizels (1 983, 1986) for complete descriptions of al1 assumptions.
1.3.1 Sîeady, unl;formflow
The most predominate assurnption in any palæohydraulic reconstruction nudy involves the concept of steady, uniform flow. Steady flow refers to a constant average flow velocity with time, while unifonn flow is achieved if there is no spatial variation in flow velocity. This. in turn, means that there would be no acceleration or deceleration within the channel. The cross- section of the charnel (hydraulic geometry) would then have to remain relatively constant. Maizels (1983) notes that the assurnption of steady, uniform flow has four important consequences. First, it assumes that the former energy gradient of the flow is parallel to the slope of the bed. Because of this relation, it can be assumed that the measurernent of bedslope is an accurate indicator of the former energy gradient. Second, it allows for the use of the continuity equation for strearn discharge (Q=Uxw xd; where Q = discharge, w = width. d = depth of flow and U = velocity) to estimate former regimes. If there were velocity fluctuations within the palæoflow, either temporally or spatially, U would not be constant. Thirdly, because the channel boundary is stable, the flow equations are only applicable to conditions below the threshold of sediment motion. The hydraulic geometry of the channel will change with an increase in the shear stress exerted on the bed (and therefore a decrease in shear resistance). Church and Gilbert (1975) provide flow equations which can be used to approxirnate palæoflow under such changing conditions.
Finally, by assuming steady, uniform flow, any predictions of the former regime will be indicative of representative flow conditions. instantaneous velocity and discharge changes are not accounted for. This can be problematic when trying to accurately portray the palæoenvironrnent within gravel-bed fluvial systems. Grave1 is known to create various forms Parameters Assum~tionsmade energy gradient - steady, uniform flow* - accuracy of meanirements of overall and local gradients - siope gradient is an accurate indicator of energy gradient - no tectonic uplift since the time of palæoflow particle size - selection of sample sites is accurate - sarnple size is representative - measurement procedures and choice of clast diameter are representative - fluvial deposition of al1 sediment - maximum sediment availability* critical shear stress - validity of Shields critical tractive force approach - value of Shields' coefficient is accurate critical flow depth - validity of DuBoys equation - substitution of s, and S into DuBoys equation is accurate - substitution of 4 for R in DuBoys equation is accurate flow resistance - particle resistance is the dominant source of resistance - flow is relatively fiee of suspended sediment* - validity of resistance rnodels for 'n' - low or moderate slope gradients - unifom distribution of resistance elements* - small relative bed roughness* critical mean flow velocity - validity of palæoflow velocity models
Table 1.1: The various assumptions that have to be made in order to estimate palæoflow conditions. Assumptions with an asterisk (*) are discussed in the text. Modified from Mahaney and Kalm (1 996) and Maizels (1986). of fiow turbulence, including flow separation and eddies, of which contribute to residual velocities around the steady, uniform fiow curve.
1.3.2 Small relative bed roughness One of the major controls on the velocity profile of a channel is the relative roughness of the bed expressed as the ratio D/d, where D is grain diameter and d is fiow depth. The flow equations used to estimate palæoconditions al1 assume a small relative grain diameter. Relative roughness values are typically between 0.067 and 0.4 within contemporary channels, with a depth approximately 2.5 to 15 times the grain diameter. As the depth of flow increases, there is a tendency for the velocity profile to resemble those found in smooth, unifom flow conditions. If the depth of flow becomes shallower, the large-scale roughness elements begin to dominate and eddies develop to create non-uniform turbulent fiow. There is no known velocity law to account for this turbulent flow profile (Maizels 1983).
1.3.3 Ungorm distribution of resistance elements
It is assumed by most flow equations that the dominant form of flow resistance is grain resistance. ûther foms of resistance include bedform resistance and spi11 resistance by boulders (Maizels 1983). In steady, uniform flow, this assumption tends to be fairly accurate (Maizels 1983).
It is also assumed that grains within the fluvial system are uniforrnly distributed along the bed and that entrainment is a random and stochastic process; however, it has been shown (Maizels 1983) that the resistance grains are not disvibuted randomly because of imbrication. packing and mouring over the bed Fomar (1996) summaries models applicable to these situations]. 1.3.4 Low surpended sediment concentration Most flow equations are derived for conditions with little or no suspended sediment concentration (S.S.C.) during entrainment and deposition (Maizels 1983). Presently active proglacial strearns however, cm show high values of S.S.C. (Drewry 1986) which can drastically alter flow conditions. Komar (1970) has shown that the specific weight of water can increase as much as 18% with an increase in sediment concentration of 9%. Water densities have also been shown to increase in sediment-laden water, Maizels (1983) recommends that the value for water density should be increased to 1.005 g cm-3to estimate palæoflow conditions because of this. Studies on the effect of S.S.C. on flow conditions have been relativety inconclusive
(Raudkivi 1976). However, material in suspension has been shown to dampen turbulence and thereby reduce resistance. Vanoni and Nomicos (1960) showed that on a fixed sand dune. suspended sediment concentrations of 3.64 and 8.08 kg m-3decreased fi-iction by 5 and 28% respectively. Knighton (1984) has noted that data is spse for these types of studies and the general opinion is that S.S.C. has little effect on flow resistance.
1.3.5 Maximum sediment availabilii-y One of the mon important assumptions is that particles of a11 clast sizes are available for transpon. If sediment of ail clast sizes capable of king moved are not available, pakoveiocity estimates woutd be lower than the real values. This assumption is valid in a glacial environment; however, exceptions can almost always be found (Maizels 1983). It also assumes that al1 sediment was deposited by fluvial processes and deposition by other agents. such as the wind, had not occurred, or was infiequent. These assumptions make any reconstruction of the fluvial palsoenvironrnent somewhat approxirnate. When one introduces a high number of variables and equations into the deciphenng of events, more assumptions have to be made. Therefore the results of such midies. including this one, become fargely theoretical, and may represent only one of many different interpretations. Literature Review Chapter Two
2.1 Introduction 2.2 Definitions 2.3 Physical Properties of Outwash Deposits 2.3.1 Hydrology 2.3.2 Morphology 2.3.3 Sediment 2.3.3.1 Particle size and distribution 2.3.3.2 Particle shape and clast fabric 2.4 Stratigraphy and Sedimentary Facies 2.4.1 Lithofacies 2.4.2 Downslope changes 2.5 Lacustrine Sediments 2.5.1 Stratigraphy and Sedimentology 2.6 Provenance and Heavy Minerals 2.1 Introduction La Mucuchache outwash fm complex is one of numerous well-developed outwash features in the Sierra de Santo Domingo basin of the Venezuelan Andes (for an example see
Figure 2.1). It provides information on a period of time during the LGM when landscape evolution was progressing at an accelerated rate, with active erosional and depositional agents working to constnict landforms and with a substantial source of available sediment. Today. the proglacial environment in which the complex was constructed is gone, and constructive landform development is quiescent. The stratigraphic and morphologic nature of the exposed sediment is the only evidence available to deduce the processes that were dominant during that time. To achieve a reliable paltwenviromental reconstruction, a reliance on studies of contemporary outwash features is a necessity. Studies concentrating on the relationship between contemporary fluvial processes and the resulting sedimentological and stratigraphic characteristics of the fe~tures(e.g. Church 1972; Boothroyd and Ashley 1973, rnay begin to offer an analogy to complete Quatemary reconstructions. This chapter will concentrate on the characteristics of outwash fans, as studied within the present-day proglacial environment. and discuss the background needed to decipher past hydraulic regirnes.
A discussion of the properties of outwash fans and sandurs, in ternis of morpho Iogy and hydrology, will help provide a basic understanding of the processes that created La Mucuchache outwash fan complex. Since the fan complex also contains a quantity of lacustrine sediments. a discussion of the theory of lacustrine sedimentation will be offered. The use of instrumental neutron activation analysis in determining the provenance of glacial sediments, and microanalysis of heavy minerals in glacigenic sediment is also discussed. Relevant literature pertaining to scanning electron microscopy is discussed in chapters 4 and 5. Figure 2.1: An outwash fm in the Sierra de Santo Domingo mountain range west of the village of Apartadaros, northern Venemelan Andes (view is to the south). Many of these Quatemary fieanires are found in the Chama Valley as a result of meltwater flow fkom palmglaciers within the upslope cirques. Some of these fans, including La Mucuchache outwash fan cornplex, have been tectonicdy altered by the Bocono hdt (foreground behind Chama River. Denoted by O). 2.2 Definitions Meltwater strearns carrying glacial sediment fiom the snout of a glacier can deposit the material within a fluvial, lacustrine or marine environment. Within a fluvial environment most of the detrital sediment is deposited within large outwash plains which may be constrained within the confining walls of a valley, or spread over large proglacial areas without any confinement. These outwash plains have been tornmonly called sandm or sandurs [Icelandic for sand plains, singular sandur (Smith 1985)]. They rnay be located in the irnmediate proglacial region or downstrearn several kilometres depending on topography. They are typically gently sloping features with numerous braided channels incised into the plain. These channels fiequentiy change position because of the enormous fluctuations in discharge and sediment loads normally associated with proglacial strearns (Smith 1985, Drewry 1986).
A sandur is defined by Goldthwait (1 989) as being "...in front of, on, or even under thin marginal ice, inwash included; fully loaded fluvial deposition, gently sloping plain, steepest and highest near ice, SI ightly concave downvalley; with shallow dividing or braided channels; altemating sand and grave1 beds paralle1 to surface; lenticular channel-and-fil1 . . . " (p. 276).
Numerous authon have recognized at least two different types of sandurs. VaUey sandurs. called 'valley trains' in North Amenca, are accurnulations of outwash located within the confines of local valleys. They are associated with the valley or cirque glaciers. They are usually concave with sediment fining downvalley away fiom the ice. Active and abandoned braided channels are usually found on their surfaces. if erosion has occurred during postglacial periods, matching terraces rnay be found (Smith 1985).
Plain sanaUrs, or 'outwash plains', are unconfined accwnulations of outwash deposited by numerous braided streams. The coalescence of several outwash plains can form very broad alluvial plains. Outwash plains are charactensed by concave shaped longitudinal-foms with sediment fming toward the bottom reaches of the feature (Figure 22). The most cornmon plains are fan-shaped, resembling a syrnmetrical gentle cone (Goldthwait 1989). These fans also usually exhibit horizontal sand and grave1 bedding (Goldthwait 1989). Moa outwash plains are the result of Pleistocene glaciations, while most contemporary sandun are valley trains (Smith 1985). Both valley trains and outwash plains represent major accumulations of glaciofluvial sediment and are well represented within the sedimentological record. Smith ( 1985) notes that the requisites for sandur formation - abundant sediment availability and hydraulic regimes capable of transporting the material - are commonly found within other environments and sedimentological records. For instance, outwash plain sediments cm resemble those of ialluvial fans.' Smith (1985) makes the important note that the simila.rïties between glacial and nonglacial fans far outweigh the differences therefore, without the presence of nearby glacial sediment ancilor feanires, identification or interpretation may not be easy. The area surrounding La Mucuchache and El Cabal10 watersheds are abundant in glacial landforms and sediments (Geigengack and Grauch 1975, Mahaney and Kalm 1996), therefore identification of the fan as a glacial feature is made easier.
2.3 Physical Properties of Outwash Deposits
2.3.1 Hydrology The genesis and morphology of sandurs, fans and outwash plains are directly detemined by the hydraulic nature of the rneltwater. It is therefore important to discuss the nature of the meltwater channels and the hydraulic behaviour of the water that is responsible for the deposition and maintenance of these features. Glaciers act as large reservoirs holding vast amounts of seasonal precipitation which is Figure 2.2: Longitudioal pronle and clast variation on numerous sandurs. Notice the Confederatioo Fiord Sandur does not show a change in clast size downstrea.. Apparently the fan showed no sips of active ag-mdation and was graded (after Church and Gilbert 1975). then released during the wami periods of the year, most commonly during the surnmer months. The amount of glacial meltwater in the proglacial zone is more a result of seasonal fluctuations in air temperature than in long term precipitation (Smith 1985). Because of this, meltwater channels on sandurs are highly seasonal and cm exhibit highly variable discharges. Church (1972) and Church and Gilbert (1975) recognized five different periods of meltwater runoff within sandur environments. Each penod records differing hydraulic regimes and thus represents various stages and processes of sediment deposition. The periods are:
1) breakug: characterised by the beginning of spring snowrnelt and the establishment of a braided channel structure. Discharge is commonly low and sporadic with channel construction becoming complete after a stonn or major mehater event. 2) nival flood: represents the start of an established channel network with abnormally large discharge events resulting nom glacier and mowbank meh (Stenborg IWO). Sediment transport is very high during this time. 3) late surnrner: meltwater flow is contemporaneous with the meltwater generation within the glacier and is typically constant in discharge. Seasonal rainfall during the nival flood and late summer periods increases discharge over short periods of tirne. Temperate glacien rnay show a lag between the rainfall event and meltwater flow in the proglacial zone (Stenborg 1969). 4) freezeback: occurs afier melt has ceased within the glacier. Flow remains in small channelways which rnay drain into the proglacial zone. Outwash fans rnay be typified by shallow, sluggish strearns carrying limited sediment. 5) winter: Water discharge has completely ceased. A small amount of flow rnay be supplied by groundwater.
Superposed on these annual fluctuations are diurnal changes. These tend to be a function of the transit time of meltwater between the ablation zone and the proglacial zone (Maizels
1995). The transit time, in tum, is influenced by the size of the ablation zone and the extent to which the supraglacial, englacial and subglacial channels are developed (Figure 2.3). Because of this change in transit time, it rnay seem that large glaciers. or ones in which rneltwater channels are fully established, do not show significant diumal fluctuations. Glaciers that do exhibit diumal changes rnay show one complete flood record for a particular day (Maizels 1995). She states that these diurnal flood flows are very important in terms of bedload sediment transport and morphologie changes within the proglacial environment. Church and Gilbert (1975), Smith (1985) and Maizels (1995) recognize the importance of irregular meltwater fluctuations, which can be partially recognized in the fan complex by abrupt spatial changes in grain size and bedform structure. However. assigning sediment to specific tirne intervals or flow durations is very difficult within Quatemary deposits. Rainstorms occumng after relatively wann penods, can deliver vast amounts of water which may go unequalled during the rest of the year (0strem 1975). Rothlisberger and Lang ( 1987) found that the greatest floods of a 20-year runoff penod fiom the Aletschglestscher basin in Switzerland were induced by the combination of glacial meltwater and min stom eventç. These events have proved significant in the sediment transport and channel changes within the pmglacial zone. Ashworth and Ferguson (1986) found that a min flood of IO m3s", compared with 5 to 8 m3s" for normal peak flows, generated an order of magnitude increase in the sediment transport rates in a meltwater Stream in northem Norway. Typical sediment loads were 0.3 kg mkl. but increased to 3.0 kg m-'s" dunng the event.
Jokulhlaups are floods of abnormally high discharge (Maizels 1995). They rnay be initiated by a number of factors; however, the most cornmon is the failure of an ice-dam which once held back an ice-marginal lake. Water then escapes at catastrophic magnitudes through a breach or over the dam. Numemus studies have been completed on these, sometirnes disastrous. events (e-g. Clague and Mathews 1973; Rothlisberger and Lang 1987; Mathews and Clague 1993). Other causes, sorne more relevant to La Mucuchache outwash fan complex, include the rupture of subglacial, englacial or supraglacial meltwater sources, breaching of a moraine or rock-dam, the result of ice-avalanches or rockfalls on the glacier, prolonged rain events. glacial surging, andor subglacial geothermal activity (Maizels 1995). Jokulhlaups can account for a vast amount of meltwater activity during the ablation Figure 2.3: Sediinent and water transport routes witliin a glacier. Diumal changes in meltwater and sediment supply to ille proglacial zone is largely the function of the developinent of siibglacial, eiiglacial and supraglacial channel systeins. Glaciers exhibiting large, well-developed chaniiel systeins may not show signi ficant diumal meltwater fliictiiations (afler Rbthlisberger and Lang 1987. pg.234). season. Their discharges far exceed those of the rest of the season by up to several orders of magnitude (Maizels 1995). For exarnple, Church (1972) investigated a Baffin Island jokulhlaup and found discharges to reac h 200 m3s" hmthe normal flow of 20 m's-' . It was also found that this flood represented 10% of the total runoff for that particular season. Other recorded jokulhlaup discharges have ranged from 40 000 m3s-' (Bjomsson 1992) to N.5 x IO6 m's-' (Maizels 1995). It is evident that the proglacial environment is highly intluenced by extreme meltwater discharge events. Figure 2.4 shows the spatial variation of various flow parameters on the Scon and Yana outwash fans in Alaska as a result of varying flow regimes. ïhese temporal fluctuations, occumng for anywhere between a few minutes and several months, each exhibit different capacities to entrain, transport and deposit sediment. Therefore modification of the proglacial zone is inevitable. Maizels (1995) notes that very little work has been completed in trying to decipher which of the meltwater magnitudes are the most influential in the proglacial environment. Estimating palæovelocities within Quatemary deposits based on grain size rnay be able to expand this since these studies involve deposits which are not in the process of formation. The extent of the diffenng grain sizes, and therefore palæovelocities. could then be mapped and calculated. The moa fiequent meltwater magnitudes (as reflected in the extent of the maximum grain sizes) may be deduced from this method however it would be very difficult to determine the duration of the meltwater event responsible for the deposition.
2.3.2 MorphoIogy
Kngstrom ( 1962) recognized three distinct reg ions on the sandur surface. The proximal zone is closest to the glacier and contains the fewest number of channels. It is characterised by rapid sediment deposition with channels that are typically deep and narrow. The incision of the channels into the outwash probably results from slope adjustment to a normal hydraulic and OISTANCE FROM SOURCE KM
Figure 2.4: a) Fiow parameters for the Scott outwash fan, Alaska. Velocity and depth values were obtained dong a profile line runnhg the length of the main huck meam. Measunmeuts were taken dmhg the falling flood stage. b) Fiow parameters for the Yana outwash fan, Alaska. Data were obtained dong the main stream during the rising flood stage (after Boothroyd and Ashley 1975). sediment toad regime (Smith 1985). The intemediate zone, characterised by a network of shallow, braided streams flowing away fiom the apex, responds to changing fluctuations in meltwater discharge. with frequent changes in channe1 position. Smith (1985) noted that abandoned channels are characteristic of many sandur surfaces during normal flow conditions.
Distai zones are characterised by shaliower, ill-defined channeis where flow may transfomi into sheet floods during high flow events. The distal zone exhibits significantly lower surface gradients than proximal zones and a distinct discontinuity can usually be found between the two areas. There have been numerous theories on why this distinctive divide is found (Maizels 1995); a change fiom proximal to distai slopes reflects a change in particle size distributions; proximal zones may contain detritus directly deposited by the ice and not of glaciofluvial origin; the slope may be adjusted to the amount of stream power needed to transport sediment to the system in order to maintain stream equilibrium, or an altemating pattern of glaciofluvial sedimentation and fluvial scour. Church and Gilbert ( 1975) have recognized this changing channel habit on sandurs on Bafin Island.
Except during tirnes of high discharge, it is comrnon to find areas of a sandur inactive. Depending on climatic variables and channel migration, vegetation may stabilize the surface (Boothroyd and Ashley 1975). This is common on older deposits where fluvial processes have been absent for thousands of years. Other sandurs rnay also be subjected to aeolian processes. such as deflation and dune creation, during or after ceasation of fluvial activity.
2.3.3 Sediment 2.3.3.1 Particle size and distribution
Numerous -dies have investigated particle size distributions throughout sandurs (e-g.
Maizels 1989). It is comrnon to find a general fmhg of particle size fiom the proximal zone to the distal zone (see Figure 2.2). Proximal-to-distal particle size changes are related to the rate of decline in gradient, lithology of the sediment and initial clast size (Maizels 1995) and cm be attributed to abrasion and selective sorting within the changing hydraulic conditions (see Maizels 1995 pg. 395 for merdiscussion). Maizels (1995) reports that mean proximal clast sizes can typically range between 50 mm and 1800 mm. Particle size distributions of ouîwash features are highly variable due to changing source matenals (Maizels 1995) and flow regimes. Distributions rnay range From well-sorted, unimodal sands to poorly-sorted, bimodal or polymodal gravel deposits with various matrix sediments. Polymodal gravel sediments are typically found in proximal regions where rapid deposition has resulted in marrix matenal having been trapped amongst the clasts. Sand deposits are found in the channel zones of the distal areas (Maizels 1995). Typical well-sorted unimodal sands show median particle sizes around 0.0 to 2.0@,a sorting index (an expression of the range of grain sizes) of 0.25-0.75 (cf. Folk and Ward 1957), skewness (degree of asymmetry of the partic le distribution) of 0.0 to M.75, and a kurtosis (degree of peakness of the partic le size distribution) value of 0.2 to 0.8. In cornparison, polymodal, poorly-sorted gravel can show values of -2.0 to
-4.04, 2.0-3.5 (very poorly sorted), 0.3-0.8 (fine tail), and 0.2-0.4 (platykurtic), respectively (Maizels 1995). Most outwash sediments contain srnaIl arnounts of silt and clay (-5%); however, pockets of fine matenal may be found in isolated areas, having been deposited under waning-stage flows or by non-fluvial mechanisms.
2.3.3.2 Particle shape and clast fabnc Sediment particles found in the proglacial environment can vary greatly in shape. These variations can be atîributed to transport distance (both within the glacier and the meltwater), clast lithology, resistance to abrasion and transport history (Maizels L 995). In many fluvial environments, plaîy or discoidal clasts are usually aligned perpendicular to fiow with the 'a' plane dipping upstrearn (Maizels 1995). Stable imbrication then develops with a çtrong upstream fabric becoming recognizable. Weak or randorn fabric orientations result if the flow fluctuates or if deposition is too rapid to allow for freely rotating grains (Maizels
1995). When clasts are pushed along the bed rather than rolled, they begin to align parallel with flow.
Fabric orientations on sandurs are highly variable. Boothroyd and Ashley ( 1975) found that a-axis orientation on the Scott outwash in Alaska was largely transverse to flow: while
Church (1972) found that orientations on gravel bars were not stmng in direction or strength. due to changing discharge magnitudes and directions. Jokulhlaup discharges were suspected to be the cause of varying fabric within bars of Breidarnerkursandur, Iceland (Rust 1975). Trends in bar-fabric orientation, after deposition by jokulhlaup floods, were found to differ significantly fiom the underlying channel deposits created during normal flow conditions.
2.4 Stratigraphy and Sedimentary Facies
2.4. I Lithofacies There are typically three distinct facies types found within outwash fans and sandun (Smith 1985): 1) small-scale bedforms found in nearly al1 sandbed streams. and some gravel bed channels; 2) intermediate scale forms found in gravel-based channels, including chutes-and- lobes, gravel sheets and various unit bars; and 3) compound bars with complex depositional histories. Figure 2.5 shows the facies relationships of two outwash fans, one in Alaska and one in Iceland. Nurnerous articles and textbooks summarize small-scale bedform morp hology (e.g. B latt et al. 1980; Allen 1982; Todd 1996). Sandurs and fans usually contain three types of small- scale features: ripples, dunes and plane beds (Smith 1985), each produced under different flow conditions and represented by different sedimentary structures. Todd ( 1996) and Smith ( 1985) note that these thefeatures produce, respectively: npple-drift cross-laminae, cosets of trough- stratification, and horizontal bedding. Al1 of these forms can be found dominating sandur stratigraphy with dune-created cross lamination, most cornmon.
Gravel tends to be found sporadically over horizontal surfaces in shallow flows (Smith 1985). This results in crude horizontal bedding features. Gravel dunes rnay form in deep fiows; however, ripples are only comrnon in sedirnent with particles less than 0.6 mm in diameters
(Smith 1985). Inactive sandun do not usually preserve antidune features in sand or gravel. However, Koster (1978) has interpreted transverse ribs as relict antidunes. 'Transverse ribs' rnay be found within facies as low, equally-spaced ndges of gravel oriented transverse to fiow. Wavelengths are usually in the order of a few decimeters long, with amplitude only a few centimetres high (Smith 1985). Gravel-bed bars tend to 1) fine downstream, 2) fine upward and 3) consist of finer grained matenal than adjacent channels (Ballantyne 1978; Bluck 1982; Smith 1985). These bars typically consist of massive to slightly bedded gravel with sand and silt matrices created by percolation (Smith 1985). Slip-face margins, with low-sloping riffles or horizontal surfaces rnay be present on accreting bars. Low angle cross-strata rnay result fiom the last two processes. and high-angle planer tabular cross sets usually results fiom the former. ney are usually found in units deposited by deeper flows [where the depwgrain size ratio (d/D) > 10 (Smith I985)]. Smith (1985) makes the important note that cross-strata usually comprise only subordinate components of gravel sandurs. In the sand-rich areas of a fan, migrating bars rnay produce tabular sets of planer cross beds. Thin deposits of small-scale bedforms rnay then be superimposed on these features (Smith 1985) by the migration of sand over the bars to the slipfaces. Compound bars are rare features that do not contain predictable sedimentological assemblages. They have not been adequately studied according to Smith (1985). Most sandur deposits record changing discharge regimes. These changes can be preserved in many ways; however, the most cornrnon is the transition of grain sizes and sedimentary structures through a vertical profile (Smith 1985). These features represent temporal fluctuations in fiow depth and velocity. Scour surfaces and fine suspension deposiü
(fine sand, silt and clay) are usually always found. Fine deposits are typically preserved as lenticular beds and could have easily been eroded during subsequent high discharge events. When deposition is reduced during low flow stages, graded deposits can preserve an entire flow stage (Smith 1985). This process is also evident in matrix-free and matrix-filled gravel couplets. Upon low flow conditions, sand travelling in suspension is deposited within the previously deposited gravel to create these layea. Thin, discontinuous, open-work gravel beds can sometimes be found in fan deposits if the settling sand creates an impermeable layer on the gravel (Smith 1985). Smith (1985) notes that these sometimes occur as recurring couplets as a result of cyclic discharge fluctuations.
2.4 2 Do wnslope Cha~ges Because of the depositional and stratigraphie differences between sand and gravel deposits, it is not surprising to see distinct variations within sedimentary architectures between proximal and distal regions of sandun (Figure 2.5). Despite a debate over the definition of
'proximal' (see Smith 1985, pg. 1 17 for discussion), sandurs show distinct changes downstream. The coarsest material is usually found at the region closest to the glacier (at the apex of an outwash fan), and is usually deposited within cornpound and unit bars of the braided meams. Grave1 dunes may form in deeper flows (Smith 1985). Beds are typically massive to siightly horizontal. Downslope, average grain size decreases, sandy deposits become greater in frequency and size, and transverse bars become more abundant. Small-scale bedforms and planer cross beds become more abundant in the distal regions as well. ln sandurs where aggradation is high, al1 grave1 deposition rnay occur in the proximal region and the entire distal zone comprises sand and finer sediment (Smith 1985). Boothroyd and Nummedal(1978) note that non-fluvial and overbank facies may be evident in lower reaches of the distal zone. Studies on the distal reg ions of Giji and Skeidara sandar in Iceland (Boothroyd and Nummedal 1978) have shown the existence of aeolian dune fields and wind-tidal flats, while the Scott outwash fan in Alaska contains peat and marshy wetlands in its proximal zone (see Figure 2.5). In areas with silt and clay, it may also be possible to fmd meandering and anastomosing channel patterns. instead of the common braided channel system (Smith 1985).
2.5 Lacustrine Sediments
The arnount of meltwater contained in a proglacial lake is dependant on its distance from the glacier (Smith and Ashley 1985). Mon midies on glacial lakes are completed on present day water bodies; however, sedimentology and stratigraphy have been cornpleted on older lake-beds where water no longer exists. Two types of lakes can be found within the proglacial environment. "Ice-contacf' lakes are found in direct contact to an ice body (Ashley 1995) while "distal lakes" ("non-contact lakes") are not in contact with ice, and may be located several kilornetres from the glacier.
Many of todays lakes can be considered glacial lakes; however, they may no longer be fed by glacial meltwater (Smith and Ashley 1985). La Mucuchache outwash fan complex contains deposits from an LGM 'distal Me,' therefore 'distal lakes' and their sediments will be stressed here.
2.5.1 Smtigraphy and Sedirnentology Sedimentation within distal lakes is highly influenced by fluctuating infiow discharges. low salinity, low temperatures and high sediment loads (Ashley 1995). These Mes may fom within topographically low mas and may last thousands of years; however, distal lakes impounded by moraines or other sediment dams, rnay only exist for less than a few hundred years (Ashley 1995).
Deposition in distal lakes takes place primarily in deltas (Figure 2.6) (Ashley 1995: Smith and Ashley 1985). Deltas fomed within the proglacial environment differ linle from those formed in non-glacial areas. They form where fast flowing water converges with the stagnant water of the lake. Coarse sediment canied by the Stream suddenly experiences a decrease in velocity, and the cornpetence of the stream to cany the material dirninishes. Sediment is deposited subaqueously on the shore of the lake and achieves the angle of repose for the material (typically 300-35'). Fine sediment rnay be carried away from the delta and deposited over large areas of the lake bottom.
Typical deltas in a distal lake have steep upper delta-front slopes rnainly due to the high bedload to suspended sediment ratio (Smith and Ashley 1985). These couse-grained steep upper slopes give way to more gradual, finer-grained lower faces. Gilbert (1 890) divided the delta face into three specific regions: 'topsets,' representing areas of alluvial distributaries on the top of the feature; 'foresets,' the usually corne-grained, steep upper-delta face; and
'bottomsets,' the finer-grained, gently sloping basin floor. Deltas exhibiting this type of morphology are typically called "Gilbert deltas" (Smith and Ashley 1985) (Figure 2.6). Delta topsets are typically sandy in character with distx-ibutary streams exhibiting medium-sized trough cross-bedding (Brodzikowski and van Loon 199 1 ). These cross-beds pass into horizontally saatified sets and as the charnel reaches the margin, rippled sands begin to form (Brodzikowski and van Loon 1991). If grave1 is common in the charnels, sets of tabular cross-bedding may replace the nppled sands. Most delta topsets, especially if they are fine grained, show evidence of cyclic tlooding. Stream Lake I Surface STREAM FLOW
O' - - , .:Anvout 10
\ \ \ \ \ALAKE BOTTOM
Figure 2.6: Typical mode1 of delta formation Note the three important deposits within a lacustrine environment: a) topsets beds, where giaciofluvial sediments are deposited; b) foreset beds, where fier mataial is deposited from bedload; and, c) iake bottom deposits consistingof bottomset beds, prodelta and lake bottom Sediments (der Drewry 1986)- Most delta foreset deposits are sands or finer material (Brodzikowski and van Loon 1991); however, gravel rnay be present. Foresets always show distinct inclination with individual beds typically 5-20 cm thick (Brodzikowski and van Loon 1991). A number of different sedimentary structures can be found, including scour and fil1 structures. horizontal (to the inclined plane) laminae and cross-bedding.
Sediment in the upper-delta foresets are usually deposited as rhythmically bedded. fining upward, sand and gravel deposits (Church and Gilbert 1975; Ashley 1995). Ashley (1 995) believes that the rhythmites are not ususally the result of seasonal discharge fluctuations but separate depositional episodes during the melt season. Changes in the seasonal events may be the result of a change in discharge, or a change in the location of the distributary streams (Ashley 1995). Deposits in the mid-delta and lower delta foresets are finer grained and may exhibit lower dip angles ( Ashley (1 995) to be the distal equivalent of lower-delta foreset rhythmities. Most deposits are rippled or laminated sands, or laminated or massive muds (Brodzikowski and van Loon 199 1). The lamination is usually of the low-energy type found parallel to the sedimentary surface. Sediments near an incoming Stream typically consist of stratified sands that show evidence of deposition hmsuspension downflow in the form of climbing ripples. These tend to pass into larninated muds (Brodzikowski and van Loon 1991) merinto the lake. Fine sand and silt dominate the deposits with clay and medium sand common in small amounts. Sorting of the material has aimady taken place before the sediment reaches the lake and very Iittle improvement occurs once in the lake (Brodzikowski and van Loon 1991). Grading-upward of the units may be present if deposition hmsuspension has occwed after an influx of sediment; however, varve-like features and revened grading are rare (Brodzikowski and van Loon 1991). 2.6 Provenance and Heavy Minerais Debris transported by glaciers and/or meltwater may have originated by one of three processes: a) erosion of substratum or valley walls @rewry 1986), b) deposition from nunataks as a result of, for example, frost weathering (Drewry 1986), or c) aeolian input. Erosion from valley walls and floors account for most of the transported sediment while sedirnent derived from aeolian processes are rare (Brodzikowski and van Loon 199 1). Most of the sedirnent that is eroded fiorn the substratum is transported at the base of the glacier; however, shearing and thsting may induce the incorporation of debris into the englacial, and possibly supraglacial zones, of the ice mass. Regardless of the origin of the material, al1 sediment becomes mixed within the ice, therefore resulting in the diamictic character of till deposits (Brodzikowski and van Loon 1991). Tills that are fonned within the ablation zone shortly after the entrainment of the sediment, and sediment transponed by meltwater within channels or conduits, are the only glacigenic deposits that may show some degree of sorting. One of traditional methods to determine the provenance of glacial rnatenal is through detailed mapping exercises (e.g. distinguishing ice lobal patterns and the extent of glacial deposition and erosion); however, Evenson et aL(1979) point out that many aspects of past glacial flow can only be identified through more detailed provenance investigations. One of the early methods to determine provenance was through pebble counting (Dreimanis and Reavely 1953, Arnernan and Wright 1959), although it was more commonly used to detemine sediment transport distances in areas of continental glaciation. Glacial provenance studies in alpine areas using pebble counting have been rare; however. Clague (1975) used the technique with favourable results in the Southem Rocky Mountain Trench of British Columbia. Heavy mineral analysis has also been used to determine source areas. Numerous researchers, including Gravenor (1 95l), Dreimanis and Reavely ( 1953), W illman et ai. ( 1 963) and Frye et al. (L969), have used this technique to source and correlate glacial deposits with the parent bedrock based on the similarity of their heavy mineral suites. Evenson et al. (1979) conducted a detailed provenance study in the Idaho Rocky Mountains based on boulder and pebble counts and heavy mineral analysis, while Pasquini (1 976) perfonned a similar investigation in the Copper Basin of the same mountain range. Based on the lithology of boulders and pebbles collected at 161 different sites and the identification of various heavy minerals taken fiom till matrix, Pasquini's (1976) data showed distinctive trends and groupings which identified source areas. Statistical analysis confirmed the recognized patterns. Evenson et al. (1979) note that successfiil provenance studies should be coupled with field mapping. In this case, Evenson et al. (1979) argued that flow peculiarities such as up- canyon flow, divide crossing, readvances, ice-rafting and j6kulhlaup activity, could be more easily recognized. 3 3 Provenance studies can dso involve the determination of rare-earth element (REE) concentrations (Hancock et al- 1988). Hancock et al. (1988) displayed REE concentrations in chondrite-nomialized plots for samples in the WeUsch Vailey, Saskatchewan. They showed an overall geochemical uniformity between samples and they attri'buted this to the geochemical homogeneity of the underlying bedrock, whic h tills O &en resemble rnineralogically and chemically (Drewry 1986). Hancock et al. (1 988) did acknowledge that the different tills at the site were apparently the result of glaciations that originated korn different locations on the Canadian Shield. Mahaney and Hancock (1993) have also utilised REE concentrations to determine the mineralogical differences in till successions within the Scarborough Bluffs, Scarborough, Ontario Canada. They determined the genesis of the till and the degree of weathering during post-glacial time. The provenance of the deposits was determined by examining whether chlorite was present in the diamicton. The younger HaIton diamicton lacked chlonte, while the O lder Sumybrook diamicton contained mostly trace to moderate amounts. Through this analysis, it was determined that the source for the diamictons was the Canadian Shield. Clay mineral analysis confirmed these findings. Study Area Chapter Three 3.1 Introduction 3.2 CIimate 3.3 Surficial Geology and Topography 3.4 Glacial Sediments of the Fan Complex and Surrounding Area 3.4.1 Till 3.4.2 GIaciofluvial sediments 3.4.3 Geology 3.5 Glaciations in the Sierra Nevada de Mérida 3.6 Previous Studies Involving La Mucuchache Outwash Fan Complex 3.1 Introduction The fan complex is located in the northern Venezuelan Andes (8°47'00n N., 70 O 5 1 'OOnW .) (Figure 3.1 ). It is part of the Sierra de Santo Domingo Mountain Range which is an extension of the larger Sierra Nevada de Mérida (Figure 3.2). The head of the complex lies appmximately 3700 m a.s.l., and the base at 3450 m a.s.1. (Geigengack and Grauch 1975). It is situated approximately 40 lan northeast of the city of Mérida, near the small village of Apartadms and southwest of Laguna de Mucubaji (Lake Mucubaji). Geigengack and Grauch ( 1975) refer to La Mucuchache outwash fan complex as "the sediment-filled basin, (pg. 266)" while Mahaney and Kalm (1996) refer to it as "the outwash plain (pg. 59)" and "the Pedregal site (pg. 59)." The outwash fan is referred to here as "La Mucuchache outwash fan complex," after the nearby valley of the same name. 3.2 Climate The Intertropical Convergence (ITC), located south of the equator during the winter, controls the climate of the Venezuelan Andes (Geigengack and Grauch 1975). The wam, dry air of the NE Trade Winds blow across the Caribbean, the Orinoco Basin, and the Llanos to finally cool adiabatically as they nse over the Andes. Minor precipitation may result in the winter. Midday temperatures are usually cornfortable while night temperatures can get quite low. Temperature variation is not as pronounced during the rainy season (May - July) as it is in the winter (November to April). Spring bnngs the transgression of the ITC over the Andes from south to north. As it progresses northward, precipitation increases with the first heavy rains occurring in the south. As summer (May to October) advances, rain becomes heavier in the north, and tapers off in the south. Winds are still hmthe northeast in the northern Andes during the rainy season; however, Figiirc 3.1: The Sierra de Santo Domingo Mountain Range is a siiiall extension of the l~rgerSierra Nevada de Merida range in the northern Veneziielan Andes. It extends from the Coloiiibian border to east of Lac Maracibo. La Miiciiclinche Outwasli W 4 Fan Complex is located to the east of the village of Apsnaderos. Figure 3.2: La Miiclicicache Valley waterslied extends in the southeasterly directioii to an elevation of approxiinately 4000 m a.s.1. El Caballo Valley extcnds to an elevatioii of 3800 m ad. Note qiiarc grid is 2 km x 2 kiii. the SE trades can be felt in the south (Geigengack and Grauch 1975). The Amazon Basin modifies the wind flow considerably. Geigengack and Grauch (1975) describe a typical rainy season day as having clear skies in the morning hours; however, by noon rain clouds begin to travel down the mountain range to the southwest. This is the result of the cooling of the moisture-laden trade winds. Rain usually begins in early afternoon and may continue dlnight. This pattern is repeated the following day after the sun wamthe air of the Llanos. This weather pattern was highly evident in the study area during data collection for this research. The Sierra Nevada de Mérida mountain chah consists of two parallel ranges with a major valley in between. They trend northeast to southwest. Since the airflow is predominately hmthe east, the ridge to the southeast receives much more precipitation than the northwest range (Geigengack and Grauch 1975). Overcast skies may not develop on the northwest ridge until late afiernoon. As of 1975, there were three mountains in the Andes which yield permanent snow: Pico Bolivar (5007 m), Pico La Concha (4992 m) and Pico La Corono (compnsing Humboldt (4942 m) and Bonpland (4893 m) peaks). Al1 of these mountains are east of the city of Ménda. 3.3 Surficiai Geology and Topography La Mucuchache outwash fan complex consists of an outwash plain, morphologically resembling an alluvial fan complex, with a small area of lacustrine sedimentation at its base (Figure 3.3). The fan appean to be the product of a washout of meltwater through (or over) either La Mucuchache moraine (Figure 3.4) separating La Mucuchache valley fiom the outwash, or El Cabal10 moraine to the northeast. A layer of classical alluvial loess, approximately 1 metre thick covers the outwash in some places (Mahaney and Kalm 1996) (Figure 3.5). Lateral moraines (LG.M.: End moraine (Pre-Mérida tacustrine sediments Outwash fan Fault Pond Site lacations Figure 3.3: La Mucuchache Outwash Fan Cornplex, Sierra de Santo Domingo Moutain Range. Sample sites are indicated. Figure 3.1: Photo looking south on La Mucuchache outwash fan cornplex. La Mucuchache moraine is in the right background with hydro towers (indicated by 0). Degrees ùidicate azimuth fiom due north. Figure 3.5: Classical alluvial loess exposed on the surface of the fan at site Ped 6- To the north of the outwash fan complex is Mesa del Cabailo, cornprising a series of end moraines believed to be of pre-Mérida Age (Mahaney and Kalrn 1996). The western end of the moraine appears to have been washed away by meltwater during the LGM (Figure 3.6). Bordering the fm complex to the east is El Caballo lateral moraine (Figure 3.7), a result of ice advances down the EI Caballo valley (Figure 3.8). Quebadra El Caballo (EI Caballo Creek) now occupies this valley. To the West is La Mucuchache valley (Figure 3.9). The right lateral moraine, La Mucuchache moraine, is a single ridge which may be comprised of older buried sediments, a remit of numemus advances during the Quaternary. No evidence exists to support this; however older soils are present on the distd fIank (Mahaney 1998, pers. comm.) The moraine extends hmthe apex of the fan to the outwash breach near Mesa del Caballo. The valley is now incised by Rio La Mucuchache (Mucuchache River). Quebadra La Canada (Figure 3.3) flows through the eroded landscape between the fan and Mesa del Caballo. Quebadra Los Bijinos lies on the opposite side of Mesa del Caballo, within a spillway bordered to the north by Mesa del Julih (Geigengack and Grauch 1975). Geigengack and Grauch (1975) note that the lithology of Mesa del Juliiin does not correlate with that of La Mucuchache valley. Therefore Mesa del Juliin's glacial sediments orïginated Eom another source area. Geigengack and Grauch (1975) suggest it came fiom the Chama valley; however, the drainage of Aguila Peak to north may be an alternative source. Mahaney and Kalm (1996) showed that Mesa del Caballo consists of at least two separate moraines: one of older pre-Mérida till, and one to the south consisting of younger pre- Mérida till. The older deposit can be divided into two separate till sheets, one that drapes over the edge of the ridge towards Mesa del Julih and another younger moraine remnant that exhibits weathered clasts and a palæosol with a textural B horizon that may represent weathering during an unspecified interglaciation. An embayrnent about 0.4 km by 0.1 km along the Figure 3.6: The breach located between Mesa del Caballo and La Mucuchache moraine. Figure 3.7: EI Caballo moraine bordering the northeast side of the fan. Figure 3.8: El Cabaiio Valley located to the east of the fan cornplex- Figure 3.9: La Mucuchache Valley located to the south and southwest of the fan cornplex. northem flanks of Mesa del Caballo exposes a deep red soi1 profile with weathered biotite gneiss and ironstone lithology that has provided the red colour to the palæosol. It is believed that this palieosol, one of a series of red palæosols, fomed primarily as residual regolith in the Brunhes Chron (Mahaney, pers. comm. 1997). The younger pre-Mérida moraine is better defined with constructional topography and contains more surface boulders. It appears that La Mucuchache moraine was once continuous with Mesa del Caballo, now separated by the breach along the Bocono Fault. The Bocono Fault transects the field area transverse to palæoice flow (Figure 3.3). it lies on the southem Bank of the younger pre-Mérida sediments of Mesa del Cabal10 and cuts through the breach to the west Schubert and Sifontes (1970) have calculated the strike-slip displacement of the fault since the LGM as 66 rn in a right-lateral sense. The influence on the moraines of the area is discussed in chapter 5. Evidence of minor vertical dispiacement is also apparent. 3.4 GIacial Sediments of the Fan Complex and Surroundhg Area 3.4.1 Till According to Geigengack and Grauch (I975), Mesa del Cabal10 and La Mucuchache moraine, both contain Apartaderos Diamicton. This diamicton consists of unsorted, poorly consolidated, unstratified cobble to boulder sized clasts with a matrix of almost al1 clay. It is charactensed by deep weathering on the clasts and rnaûix. The lithology consists of gneiss and granites. The Apartaderos Diamicton exhibits a range of particle sizes and can be found in close proximity with younger sediments. Mahaney (pers. cornm. 1997) notes, however, that weathenng associated with this diamicton is not present in La Mucuchache moraine, but is present in moraines of Mesa del Caballo. Teminal monines in the lower La Mucuchache valley contain Mucubaji Till (Geigengack and Grauch 1975). These moraines are considered LGM sediments by Mahaney and Kalm (1996). This till is found in al1 major terminal loops found within the Rio Charna and Rio Santo Domingo valleys. All of these LGM deposits contain numerous surface boulders, thin weathering rinds, and near-perfect presewation of morainic morphology (Geigengack and Grauch 1 975). The closest exposures of Munibaji Till to the fan complex lie in the adjacent Mucubaji valley to the northeast. Here monhic loops of the till enclose Laguna de Mucubaji. In the upper Santo Dorningo valley, Geigengack and Grauch (1 975) recognize Sanzo Dorningo Till. It is characterized by a high content of cobbles and phyllite boulders fiom the El Aguila Formation. Giegengack and Grauch (1975) note that the occurrence of these boulders in the lower section of the valley is rare. Despite being found in poorly preserved morainal deposits, Giegengack and Grauch (1975) consider this a diamicton. These sediments, and their glacial origin are not recognized by Mahaney and Kalm (1996). 3.4.Glaciofluvial sediments The Charna Valley contains nurnerous masses of glaciofluvial material, each originating from quebadras draining fkom the valley sides. At some locations these masses coalesce into a large deposit where the gradient becomes comparable (i.e. direction and slope) to that of the Rio Charna. Giegengack and Grauch (1975) note that the city of Mérida is built upon one of these such 'terraces. ' The study of outwash fans within the Sierra de Santo Domingo has concentrated on their relationship with these valley fills and 'terraces.' Comprehensive analysis of the sediments within these outwash features has been minimal despite their abundance down the length of the valley. Giegengack and Grauch (1975) have determined three important characteristics about the fans (or 'terraces'): 1) sediment within the outwash is composed of locally derived material. This suggests that meltwater flow, and therefore deposition, was lateral rather than axial in direction. They do not state how this was determined. 2) outwash slope is steep both down valley and towards the river. Deposition laterally downvalley is therefore unlikely, even under torrential or catastrophic conditions. 3) both height and dope of the outwash surfaces Vary downvalley. Geigengack and Grauch (1975) therefore conclude that these characteristics are either a result of tectonic activity shifting the features, or more likely, the accumulation of valley fill fkom the mountain quebadras. They state that there is no evidence that the alluvial deposition is a result of changing flow regimes of the Rio Chama. Mahaney and Kalm (1996) have identified outwash fan matenal at La Zerpa; however, detailed analysis of the glaciofluvial material is unpublished. The geology of the area surrounding La Mucuchache outwash fan complex, including the two neighbouring watersheds, is not well known. However, the primary rnineralogy of sarnples fiom the site were identified by Mahaney and KaIm (1996) by x-ray diffraction and include orthoclase, plagioclase, quartz, mica and traces of talc, serpentine and amphibole. Analysis of the PED 1 site (see section 3.6) showed that orthoclase and plagioclase comprise approximately 4/5ths of the primary minerals in equal amounts. Lower units showed only traces of quartz in the 0.2pm fraction. Mahaney and Kalm (1996) showed that clay rninerals within the outwash reflect the local bedrock composition, with some influence of diagenesis and postsedirnentation weathering. Generally, the clay minerals consist of 10A micas, MA chlorites and mixed-layer minerals such as verrniculite-illite and chlorite-illite. Five samples of soi1 parent materid taken near Laguua de Mucubaji in Sierra Nevada National Park to the northeast of the study site, show traces to mal1 amounts of quartz, mica and plagioclase (in the <2 Pm. hction onlyl. Amounts of quartz and mica increased within two Cu and Cox horizons in two srnaller, upvalley moraines. Orthoclase was not measured (Mahaney and KaIm 1996). 3.5 Glaciations in the Sierra Nevada de Mérida Evidence of only three glaciations can be found near the field area. The early stade of the Mérida glaciation (dates unknown, but probably lOOKa to -50Ka ago) is conternporaneous with North American eurb Wisconsinan or European Riss, and lefi morainic deposits between 2600-2800 m a.s.1. (Schubert 1994). The later stade (late Wisconsinan) is more widespread between 2900-3500 rn a.s.1. Deposits of the early stade were Iargely overrun by advances of the ice during the late stade (LGM). Schubert (1994) considered the youngest moraines in the Mucubaji region, to date fiom 12 650 130 yr B.P., (date on peat within glaciofluvial matenal 3 km up valley fiom outer end moraine complex), while glacial outwash (with peat) in La Mucuchache outwash fan escarpment yielded "C dates of 19 080 * 820 yr B.P., (at the base of an exposure) and 16 500 * 290 yr B.P. (at the top). Schubert (1994) suggested that these dates provide evidence that glaciers were actively eroding and depositing sediment between 2900-3500 m a.s.1. during this interval. These dates are inconsistent with dates fiom Mahaney and Kalm (1996) (see section 3.6), and newly obtained, but unpublished [% dates as old as 55 000 yr B.P. (Mahaney pers. comrn. 1998). Geigengack and Grauch (1 975) also obtained radiocarbon dates for sites within Mucubaji valley and also in La Mucuchache valley. Organic samples were taken from alluvial and lacustrine outcrops on the valley floors, up valley of the terminal loops. They concluded that the glacier within La Mucuchache valley had melted to at least 3770 m a.s.1. by 12 700 110 yr. B .P. and the glacier within Mucubaji valley melted to 3660 m a.s.1. by 1 1 465 * 1 10 yr. B.P. These represent minimum ages. Evidence of pre-Mérida glaciation is scarce in the Sierra de Santo Domingo. Mahaney and Kalm (1996) have dated till in La Canoa drainage to greater than 80,000 yr. by thennoluminesence dating and there is ample weathering evidence to suggest at least middle Pleistocene ages from tik below La Mucuchache outwash fan (Mesa del Caballo). Recessional moraines are also evident at higher elevations (Schubert 1994) between 3600 m a.s.1. and 4200 m a.s.l., while moraines of the "Little Ice Agen can be found at -4700 m ad.(Mahaney and Kalm 1996) below the Bonpland lobe of the Humboldt glacier. 3.6 Previous Studies Involving La Mucuchache Outwash Fan Complex The only study to concentrate on the outwash fan cornplex is Mahaney and Kalm ( 1996). Sarnple sites were examined on the fan escarpment (Figure 3.10) and within La Mucuchache moraine (PED 1-5,7, Il, 13) to gain insight into the genesis and age of the features. Analyses of particle size, primary mineralogy, clay mineralogy, fabric, radiocarbon dating and palæohydrology were undertaken in the study. Six sedimentary units were distinguished by Mahaney and KaIm (1996) (Figure 3.1 1 ) within the river cut along quebadra La Caiiada. They are: Unit 1 (1-2-0.3 m. thick): massive glaciolacus&ine sandy siIt with aeolian silt - transition fiom Unit 2 is gradua1 - massiveness is contributed to frost action, soi1 formation and/or aeolian input Unit 2 (max. thickness: 4.7 m.): rhythmically bedded glaciolacustrine sandy silt and clay with peat layers - pro-delta dope sediments deposited as a sequence of ripple-drift altemating with undulatory beds Unit 3 (max. 7 m. thick): poorly sorted glaciofluvial pebbly grave1 and glacial diamicton re 3.10: La Mucuchache outwash ih escarpment at the northwest section of tbie fàn. The escarpment was created by fluvial erosion in La Cafiada Quebadra during the Holocene. Photo lookhg east kom northern end of La Mucuchache moraine. PEDZ PED3 PEDS PED4 PED7 PED Il Figure 3.1 1: Stratigraphy of the Pedregal sections with radiocarbon dates. [from Mahaney and Kalm ( 1W6)]. - shows flow till characteristics in PED 2 - transition from Unit 2 to Unit 3 is comprised of deformed silt and flow till inclusions unit 4 (0.2-0.6 m. thick): rhythmically bedded glaciolacustrine sandy silt and clay with contorted beds - possibly represents ice retreat during the LGM Unit 5: glaciofiuvial pebbly grave1 and glacial diarnicton - represents an ice retreat during the early LGM Unit 6: glaciofluvial sand with rhythmically bedded glaciolacustrine sandy silt and clay, with peat Iayers - upper part is contorted at PED 3 Figure 3-12: Fabric of PED 1, Unit 3. North at top. Calculation method: frequency Class interval: 15 degrees Data type: unidirectional population: 36 maximum percentage: 13.9 mean percentage: 7.1 standard deviation: 3 -03" vector mean: 3.67" confidence interval: 2 1-06" (fiom Mahaney and Kalm 1996) chlorites and rnixed-layer rninerals such as vdculite-illite and chlonte-illite, al1 of which are compatible to a source in the gneissic terrain to the south. Kaolinite is found throughout the sections; however, quantities are small (Mahaney and Kalm 1996). For a complete description, see Mahaney and ffilm ( 1996 pg.6 1). Fabric analysis of Unit 3 was undertaken at PED 1 (Figure 3.12). The fabric shows flow from the south or southeast. A fabric transverse to the main flow orientation was also found. Radiocarbon dates for the five samples taken hmthe PED sections (see Figure 3.1 1 for locations) are shown in the table (fiom Mahaaey and Kalm 1996): Sarnple Material Date ("C ane) PED 1-1 finely disseminated peat 14579 * 260 PED 1A-2 finely disseminated peat 12950 * 450 PED 4 finely disseminated peat 22710 460 PED 5-1 finely disseminated peat 27150 400 PED 5-2 finely disseminated peat > 25800 Palæohydraulic analysis was completed on selected samples fiom the glaciofluvial beds within the sections. Using the tractive force method outlined by Maizels (1986)- and D,, as the indicator of flow cornpetence, palæovelocities were calculated. The various assumptions that were made are surnrnarized in their Table 1 1 @g. 64). Pal~ovelocitiesranged from 0.74 m s-' to 1.96 m s-' for the coarsest beds. However, calculations found in their Table 12 @g. 65) are suspect Of the seven samples examined, four are labelled till and a fifth (PED 1 glacfluv+Sm) may be either till or glaciofluvial material. Therefore, the only valid calculations would be fiom the remaining hvo samples. Also Manning's 'n' may not have been accurately calculated since not al1 of the needed grain sizes were known. Despite the shortcomings in the method used, the palæovelocities may be of some use. Although the calculated velocity may be incorrect, the magnitude difference between any two values will not differ if the same method was used to calculate each velocity. For example, 0.73 m s-' is approximately one half of 1.56 m s" and that can be valuable in deciphering the magnitude of fluvial events in relation to each other. It was concluded that the outwash fan complex contained various sedimentary beds and therefore an important history of varying flow conditions. Each of the glaciofluvial deposits could be attributed to major ablation episodes of the source pakoglacier and hence provide an important palæoclimatic record. Radiocarbon dates were used to create a chronology of glacial events which could easily be applied to other areas within the Sierra de Santo Domingo. Methods Chapter Four 4.1 Introduction 4.2 Field Methods 4.3 Laboratory Methods 4.3.1 Gmulometnc Analysis 4.3.2 Instrumental Neutron Activation AnaIysis (INAA) 4.3.3 Heavy Mineral Identification 4.3.4 Scanning Electron Microscopy (SEM) Analysis 4.4 Palawhydrological Reconstruction 4.4.1 Empirical and Theoretical Methods of Palæohydraulic Reconstruction 4.4.2 Tractive Force Method 4.4.2.1 Critical Shear Stress 4.4.2.2 Flow Depth 4.4.2.3 PalæoveIocity 4.5 Turbulence Indicators 4.1 Introduction The methodology is divided into three important components: a) field methods, b) laboratory analysis, and c) palæohydraulic parameter estimation. The field methods involve an evaluation of potential exposures, measuring selected exposures and pis, sedirnent description (lithostratigraphy) and sample retrieval. Laboratory methods include water content calculation, granulometric analysis, instrumental neutron activation analysis, heavy minerai identification and analysis and scanning eiectron microscopy (SEM). Palsohydraulic estimation consists of the calculations of nominal diameter, critical shear stress (r,),critical shear velocity (U.,,). cntical flow depth (&), Manning's roughness coefficient (n), critical mean flow velocity (U,) and effective roughness height m. Turbulence indicators, including both Reynolds number (Re) and Froude number (Fr), may then be estimated to provide insight into the magnitudes of the fluvial processes during part of the Late Quatemary. A background to palæohydraulic research and rnethods is also offered. 4.2 Field Methods Stratigraphie investigations and sediment sampling were undertaken in the field at exposed sections and in pits (see Figure 3.3). See TabIe 4.1 for the rationale behind the c hoice of locations. The only location where exposures are present is at the base of the fan (see Figure 3.10). Three stratigraphical sections were exposed within the escarpment separating the lacustrine beds from the fan (Fan 1, Fan 2 and Fan 3 on Figure 3.3). These sections complement sections sarnpled previously along the escarpment by Mahaney and Kalm (1996). Three additional sample pits were dug spanning the trajectory of the fan (Ped 6, Ped 16, Ped 17). One additional pit was sarnpled fiorn the site of the rneltwater breach in La Mucuchache moraine (Ped 15), whereas two additional pits were dug within the El Cabal10 moraine near the apex of the fan Sample Site Rationale Lag 7 - to test whether the fan material comes fiom El Caballo valley Lag 8 - to test whether the fan material comes fiom El Caballo valley Ped 15 - to test whether the fan material comes fiom Mucuchache valley Ped 16 - to contrast with sample sites Lag 7, and 8 and Ped 15 to test where fan material originated - to gain sedimentological properties of the material near the apex of the fan - to compare and contrast flow characteristics with sample site Ped 17, Ped6, Fan 1, Fan2and Fan 5 Ped 17 - to contrast with sample sites Lag 7, and 8 and Ped 15 to test where fan material originated - to gain sedimentological properties of the material near the middle of the fan - to compare and contrast flow characteristics with sampie site Ped 16, Ped 6, Fan 1, Fan 2 and Fan 5 Ped 6 - to contrast with sample sites Lag 7, and 8 and Ped 15 to test where fan material originated - to gain sedimentological properties of the material near the middle of the fan - to compare and contrast flow characteristics with sarnple site Ped 16, Ped 17, Fan 1, Fan 2 and Fan 5 Fan 1 - to contrast with sample sites Lag 7, and 8 and Ped 15 to test where fan material originated - to gain sedimentological properties of the materiat near the foot of the fan and of previous lacustrine episodes - to compare and contrast flow characteristics with sample site Ped 16, Ped 17, Ped 6, Fan 2 and Fan 5 Fan 2 - to contrast with sample sites Lag 7, and 8 and Ped 15 to test where fan material originated - to gain sedimentological properties of the material near the foot of the fan and of previous lacustrine episodes - to compare and contrast flow characteristics with sample site Ped 16, Ped 17, Ped 6, Fan 1 and Fan 5 Fan 5 - to contrast with sample sites Lag 7, and 8 and Ped 15 to test where fan rnaterial originated - to gain sedimentological properties of the rnaterial near the foot of the fan and of previous lacustrine episodes - to compare and contrast flow characteristics with sample site Ped 16, Ped 17, Ped 6, Fan 1 and Fan 5 Table 4.1: The ratiode behind the location of each sarnple site. Code Gms Massive, matrix supported gravel None Gm Massive or crudely bedded gravel Horizontal bedding; imbrication Gt Gravel, stratified Trough crossbeds GP Gravel, stratified Planer crossbeds St Sand, medium to coarse, rnay Solitary or grouped trough crossbeds contain pebbles s P Sand, medium to coarse, may Solitary of grouped planer crossbeds contain pebbles Sr Sand, very fine to coarse Ripple marks of al1 types Sh Sand, very fine to very coarse, rnay Horizontal lamination, parting or contain pebbles streaming lineation SI Sand, fine Low angle ( Na)crossbeds Se Erosion scours with interclasts Crude cross bedding Ss Sand, fine to coarse, rnay contain Broad, shallow scours including cross- pebbles stratification F t Sand, silt, mud Fine lamination, very small ripples Fsc Silt, mud Larninated to massive Fcf Mud Massive with freshwater mohscs Fm silt, mud Rootlet traces Table 4.2: Critena used for glaciofluvial lithofacies assignment (modified from Graham 1988). (Lag 7, Lag 8). In the first analysis, sediment was classified as diarnicton, lacustrine and glaciofluvial material and subdivided into variow lithofacies. Classification of the glaciofluvial sediment will follow Miall's (1977) lithofacies codes (Table 4.2). Lithofacies identification is a preliminary attempt at distinguishing particle size, bedding characteristics, sorting and grading. Lithostratigraphy will be cofinned in the laboratory. Till fabric analysis, which mesures the direction in which the long-ais (.a') is oriented. will yield valuable information regarding pala~oflowdirection. Clast orientation parallel or perpendicular to the palæoflow would suggest a glaciofluvial origin for the fan and it helps reveal the source of the meltwater (Goudie 198 1). Previous fabric analysis (Mahaney and Kalm 1996) has revealed a northem orientation within the glaciofluvial sediment of the escarprnent (Figure 3.1 1). Fabric was measured at Fan 1, the only sample site where clasts were large enough. The 'a' axis, representing the longest axis, 'b,' representing the maximum diameter at right angles to the 'a' axis, and the 'c' axis, the maximum diameter of the third right-angle plane (Briggs 1977) was measured in the Iaboratory on the five largest particles within selected sedimentary sarnples taken from the glaciofluvial deposits. Nominal diameter was then calculated [D= (axbxc)'"] (Briggs 1977) for palieohydraulic estimation. Upon completion of the field methods, samples were removed at various intervals down the stratigraphie column (Folk 1968). Samples were removed in a vertical line from the top to the base. At Fan 2, sarnples were taken frorn the bottom of the exposure towards the top. The complete exposure was not sampled due to machable heights of the section. Sample pits were dug (approxirnately 1 m x 1 m with varying depths) in areas where exposures are not present. The intervals at which the sarnples were removed depended on the height of the exposure (Folk 1968) and the organisation of the sedimentary beds. To incorporate al1 of the necessary laboratory analysis, sample sizes were approximately 130 g in mass. If the glaciofluvial sediment contained clast sizes too extreme for a representative sample to be taken, the coarsest matenal was measured in the field using calipers. Standard sieve analysis was used for the finer particles (see below). 4.3 Laboratory Methods 43.1 Grandometric Analysis Sediment sarnples were analysed in the Geomorphology and Pedology Laboratory in Atkinson College, York University. Samples were air dried, and sedirnent G mm was removed for analysis using a 2 mm sieve. Water content was determined by heating 5 g air dried samples ovemight at 1 10°C. The weight loss was recorded and the moistue content calculated. The air dried equivalent of 50 g of an oven dried sample was weighed into beaken and subjected to 30% hydrogen peroxide to remove organic matter and sodium pyrophosphate to disperse clay and silt. Wet sieving was employed to separate clay and silt From the sand [using 63 pm as the silt/sand boundary (Folk 1968)l. The coarse fraction was oven dried and sieved using sieve intervals of 1,000, 500,250, 125 and 63 pm. The finer matenal was subjected to sedimentation analysis using the hydrometer (Day 1965) deragitation by a mechanical stirrer for 30 seconds and sonification for approximately 3 minutes. The clay/silt boundary of 2 pm follows the Soil Survey Staff (1 975). Obtained phi values and cumulative percentages were used to construct particle size curves. In their simplest form, these distributions indicate mean grain size, grain size of each percentile, and the degree and fom of the spread around that mean (PVFManus 1988). Data was plotted on a Wentworth probability scale diagram to enable the application of both graphic and moment statistical anaiysis. Included in the particle size measurements of the sand, silt and clay fractions. are the measurements of the tive largest clast sizes (D,,) within the glaciofluvial beds. Using the methods outlined by Wolcon and Church (1 987), in which they feel the largest stone of a fluvial or glaciofluvial sample must represent no more than 1% of the total sample weight in order for the particle size distribution to be accurate, sampling in the field would have proved problematic when dealing with sedirnents with D,oo of approximately 25 cm. This resulted in particle size distributions only produced for the sand, silt and coarse clay fractions. Therefore, direct measurements of D,, ('a' axis, 'b' axis and 'c' axis) were completed in the laboratory and in the fieid using calipers and tape measures. Graphic means, standard deviations and other statistical moments will only reflect the distribution between 2 mm and 2pm. 4.3.2 Instrumentai Neutron Activation Anaiysàs (NXA) Samples of glaciofluvial sediment, lacustrine sediment and tills were subjected to instrumental neutron activation anaiysis (TNAA) at the SLOWPOKE Reactor Facility, University of Toronto. This analysis required a sarnple mass of between 600 and 850 mg (Mahaney 1990). To detemine the concentration of elements that produce short-lived radioisotopes (AI. Ca. Cl. Dy, Eu, Ga, K, Mg, Mn, Na, Ti, U and V), irradiation of the sarnple occurred for 5 minutes at a neutron flux of 1.O x 10" n se'. Measurement of the concentrations were achieved by on- site gamma-ray spectmmeters. The samples were then irradiated for 16 hours at a neutron flux of 2.5 x 10" n s" to determine the concentrations of As, Ba, Br, Ce, Co. Cr, Cs, Eu. Fe. Hf. La, Lu, Na, Ni, Rb, Sb, Sc, Sm, Ta, Tb, Th, U and Yb. Any geochemical differences between the Ped 1S/Ped 16, and Lag 7Lag 8 sarnple sites may indicate different elemental composition within the two valleys (La Mucuchache and El Caballo). This is based on chondrite-nonnalized concentration differences of the rare-earth elements (Mahaney 1996, pers. comm.) and on the distribution of macroelements. 4.3.3 Heavy Mineral Identzficafion Investigation of heavy mineral suites in selected wunples was undertaken to help account for the anomalies within the MAA abundances. Samples exhibiting abnormally high or low concentrations of particular elements were analysed. Air dned samples were panned (Mertie 1954, Peuraniemi 1987) using a conical gold pan. Two concentrations of heavy minerals were produced (primary and secondary) and each were separated into sieve fractions of >l .O mm, 0.5 mm, 0.25 mm, 0.125 mm, 0.063 mm and <0.063 mm. Each size fraction was weighed to determine their relative abundance within the suites- The rnagnetic minerais of each size fraftion were removed using a weak hand-release magnet (Jones and Fleming 1965). Weakly magnetic grains were then separated using a small. stmng hand rnagnet Each sieve fiaction was subjected to short-wave ultraviolet light to identify fluorescent minerals. The magnetic ûaaion (magnetite) was we ighed and the concentration was determined. The identification of various coloured minerals was cornpleted with a light microscope. Three to four grains of each colour were removed fiom various samples and mounted on an adhesive scanning electron microscope mont. Chernical analysis on each grain was completed using an X-ray microanalizer [(energy dispersive spectrometer (EDS)]attached to a JEOL 840 Scanning Electron Microscope. Analysis was completed in the Geology Department at the University of Toronto. 4.3.4 Scanning Electron Microscopy (SEM) Analysis SEM analysis consisted of the examination of microfeatures on individuai quartz sand grains (2 mm - 63 pm), with the objective of identiving fractures, abrasion microfeatures. precipitation and weathering. Considerable information is known about the microfeatures of sand grains and their correlation with glacial thickness and transport history (Mahaney 1995b: Whalley 1995); however, its value towards palæoenvironrnental reconstruction is not well established (Mahaney 1995a). This study may provide tùrther insight into the usefulness of this type of research. The presence of glacial and glaciofluvial microfeatures on such grains confirm that the sand grains came into contact with the bedrock in the basal ice layer (Mahaney 1995a). or in contact with each other (which may occur within the fluvial or glaciofluvial system). Identification of various microfeatures (eg. high relief, sharp-angular features arnongst others) will suggest a glacial origùi for the glaciofluvial sediments and help infer their transport history (cf. Mahaney I995b). Appmximately 300 grains were randomly chosen fiom the coarse sand fractions (Whafley 1995). Approximately 30 grains from selected glaciofluvial and till sarnples sites were examined in detail with the Iight microscope. About 10 individual grains per sample were examined and photographed using the SEM - based on the method outlined in Mahaney ( 1990). There was no EDS available for mineral identification. Microfeatures were exarnined under magnifications of approximately 40- 1,000~while small-scale feanires were examined under magnifications reaching 10,000~. Microfeature identification on photographs was also completed. 4.4 Palsshydrological Reconstruction 4.4. I Empirical and Theoretical Methoh of PalœohydrauIic Reconsn~rction The accepted method of reconstructing geomorphic processes is to first identify and classi@ deposits and then to assume that former geomorphic processes occurred in the sarne way as they act today (C hurch 1 978). Many studies in palæohydraulic reconstnict ion corn b ine the physics of sediment transport and deposition, with the sedimentological propenies of alluvium. to detemine the magnitude of the geornorphic processes. This detemination is usually carried out using empirical or theoretical hydraulic formulae (Church 1978). Reconstruction of Quatemary environments can easily benefit from the results of palæohydraulic estimation. Church ( 1978) notes that abundant fluvial landforms and deposits are available in areas that were once glaciated as well as areas where meltwater drains frorn a melting glacier. In most cases, these deposits have not been significantly altered to prevent detailed analysis of the sediment and sedimentary structures (Church 1978). The basis of many palæohydraulic reconstructions are found in using well-known fluvial equations and flow laws to infer past conditions. It is not uncornmon to find equations such as the Manning's equation (Williams 1984; Drewry 1986; Maizels l983), or the Darcy- Weisbach equation (Williams 1984; Maizels 1983), used to calculate needed palieohydraulic variables. However, over the last two decades the concentration has shifted to developing models of simple regression equations to simp1iQ and minimise the calculation procedures (Williams 1984). Fo llowing recent developments in palæohydrology, and especially palzeohydraul ics. nurnerous authors have attempted to sumarise the various methods that are now available to reconstmct the fluvial environment. Jopling's (1 966) attempt at summarising current methods concentrated on the use of fluvial and glaciofluvial bedding and sedimentary structures as the key component to reconstnicting depth and width components. Sediment characteristics such as grain size, were then linked with these data to provide the needed hydraulic parameters. Once these characteristics have been investigated, then velocity, discharge, Reynolds Number and Froude number could easily be calculated. Jopling's (1966) summary is considered by many to be the first senous attempt at summarising palsohydraulic methods. Williams' (1 984) paper summarised over 100 empirical and theoretical equations that have been used in palæohydraulic reconstruction studies. He also exarnined researchers' errors and offered plausible alternatives in areas that require re- examination. His study provided the bais for the rnethodological rationale. The palrwhydraulic component of this study will focus on estirnating the following parameters: critical mean 80w velocity &), cntical shear stress (4). critical shear velocity (U.,), Reynolds number (Re) and Froude number (Fr). Nurnerous other parameters are calculated to achieve these goals and will be explained as required. Sediment concentration has proven to be an important parameter in palæoflow conditions as well (Maizels 1989). Costa (1984) outlined the use of sediment concentration to differentiate Newtonian flows (e.g. clear water), hmdebris flows and hyperconcentrated flows 65 (usuaily non-Newtonian). He noted that some glaciofluvial facies types (e.g. Gms, see Table 4.2; page 60) cm be a result of nich non-Newtonian flows. Such deposits are usually characterïzed by the presence of abundant cIay and silt (Costa 1984, Maizels 1989). Since much of the understanding of glaciofluvial processes cornes fkom alpine glaciers where fine sediment is often low due to high slopes and resistant bedrock (Lord and Kehew I987), sediment concentration was assumed to be Iow in this proglacial environment. This results in the need to utilise Newtonian flow equations. The primary objective in alpine fan analysis is to estimate a reasonable veiocity that reflects the true conditions within the glacio£Iuvial environment at the time of deposition. Maizels (1983; 1986) stressed the point that estimated palæovelocities and discharges contain so many flow assumptions that any estimated value may offer little more than a comparative variable fiom which only magnitude differences could be valuable. Many studies, therefore, use a range of velocities to indicate a reasonable enveIope of possible values. 4-42 Tractive Force Method One approach to determining palæovelocity is by the îractive force method (Maizels 1983). mis involves the calculation of paIæovelocity fiom previous calculations of depth and critical shear stress. Pala~ove1ocit.yis then estimated from Manning's equation after values for energy gradient and Manning's roughness coefficient (n) are inserted. Mean flow velocity is then directly reIated to the instant the measured grain size begins to move. 4.4.2.1. Critical Shear Stress Shields' equation (equation 4.1) (Shields 1936) to determine shear stress at the initiation of movemenf is the moa common rnodel used to predict critical shear stress (Maizels 1983: 1986; Williams 1984). It takes the fom of: 66 where r, = critical shear stress (N m'l) p, = sediment density ( 2650 kg m") p, = fluid density (1000 kg m-') g = gravitational acceleration (9.8 1 m s-') D = grain diameter (m) 0, = Shields' entrainment function Studies by Miller et al. (1977) and Yalin and Karahan (1979), show that for coarse grains and high grain Reynolds numben (approx. >50), Shields entrainment fùnction (OJ is approxirnately constant at 0.045. Based on Shield's equation, this number differs considerably fkom Shields' calculated value of 0.06 for uniform sized sediment. Other studies (e.g. Maizels 1986) use the intemediate value of 0.056, although shear stress, and therefore depth and mean flow velocity. may be over or under estimated because of this (Maizels 1986). 4.4.2.2 Flow Depth Most palæohydraulic studies calculate flow depth by using hydraulic equations. even though other sedimentoIogical and geomorphological methods are available. Maizels ( 1983) noted that hydraulic equations "provide the most accurate estimate of flow depth" (pg. 1 15) since they are directly related to the condition of sediment entrainment of the grains being studied. Sediment entrainment occun when the critical shear stress r, equals the shear stress exerted on the sediment by the flowing water (t) (Dingman 1984). Knowing that s = pgDS (DuBoys equation for shear stress), the relationship where S = slope of the fan [assumed equal to the energy gradient of the pakoflow = 0.05 (fiom topographie maps)] . can be solved to estirnate a critical depth of flow at which the critical shear stress is generated. The slope of the energy gradient during the time of deposition is unknown therefore a constant value of 0.05 was recorded fiom topographic maps. Local dope changes were not recorded in the field and therefore not accomted for in the calculations (see Section 1.3 for further discussion). 4.1.2.3 Paiœoveiociiy The calculated critical flow depth was then inserted into Manning's equation to estirnate the cntical mean flow velocity. This calculated depth, having been calculated using the critical shear stress, is the minimum depth needed in order to initiate movement of D,,. Therefore the velocity calculated using Manning's equation is the minimum velocity of the flow present when D,, was being transported. Manning's equation takes the form of: where n = Manning's roughness coefficient Manning's roughness coefficient can be calculated at least three different ways (Maizels 1986): however, since complete particle size distributions are unknown in this study only equations that do not involve measurement of particle sizes other than Dl, can be used. The rnethod used in this study is the Stickler equation (Stnckler 1923; Clague 1975; Maizels 1 986): n = 0.039 Dl, O." where D,, is measured in m. A different approach to estimating velocities is through a velocity profile distribution rnodel. Upon calculation of the critical shear stress (Q, cntical shear velocity (U.3 [Le. fkiction velocity (Dingman 1984)l was calculated using u., = Adpf where U,, is measured in ds. The shear velocity is related to the mean flow velocity through equation (Dingman 1984): U = 2.5 U., [ Ln @/yo) - 1 ] where y, (roughness length) = 0.033 D,, (for turbulent flows). The mean velocity may be calculated for a range of depths (the criticai depth caiculated with equation 4.2, to 1.0 m). These velocities are theoretical without consideration of the energy gradient, unlike Manning's equation (4.3). 4.5 Turbulence Indicators Along with the above hydraulic characteristics, Reynolds number (Re) and Froude nurnber (Fr) were also estimated to help establish turbulence characteristics at each selected sarnple site. The accepted equations for Re and Fr are: Re = (UD)/v where v = kinematic viscosity (1.8 x 1 o4 mld at -O OC) and, FI= U2/(@) (4.8) Reynolds numbers in excess of 2000, indicate turbulent flow conditions whereas 69 laminar flow occurs if Re is under 500 (Summerfield 1991). Re between 500 and 2000 indicates larninar and turbulent flows are both present in the flow. Froude numben less than I .O indicate a subcritical (or tranquil) flow regime, whereas greater than 1 .O indicates supercritical flow (Summerfield 199 1). Results and Discussion Chapter Five 5.1 Introduction 5.2 Findings 5.2.1 Stratigraphy 5.2.2 Grandometry 5.2.3 Geochemical analysis 5.2.4 Heavy mineral identification and adysis 5.2.4.1 Metallic black heavy minerals 5.2.4.2 White translucent heavy minerals 5.2.4.3 Brown heavy minerals 5.2.4.4 Orange heavy minerals 5.2.4.5 Light green heavy minerals 5.2.4.6 Rose (light pi&) heavy minerals 5.2.5 Scanning electron microscopy 5.2.6 Paiæohydraulic estimations 5.3 Glacial Chronology 5.1 Introduction Ten different sample sites were investigated, with the results of the stratigraphy, grain size, SEM analysis, geochemistry, heavy mineral identification and palæovelocity estimations presented here. An overall summary, based on these analyses, is offered describing the events of the LGM at the study area. 5.2 Findings and Interpretation 52.1 Stratigraphy Ten stratigraphic exposures were investigated in the field and samples were collected from them. Four of the sample sites (Fan 1.2.3.5: see Figure 3.1) are naniral exposures within an escarpment cut into the fan. Of these four, one (Fan 3) is a soi1 profile where abundant fan matenal is not present. Each section was partitioned into various lithostratigraphic units and their corresponding sedimentary units (after Mahaney and Kalm 1996). Results can be found in Figures 5.1 to 5.8 for each respective section. Coupled with the stratigraphic sections in Mahaney and Kalm (1 W6), the preceding information has produced a schematic cross-sectional look at the components of the fan complex (Figure 5.9). Over 250 m, six sedimentary units cm be identified. Unit 1 represents a thin covenng of glaciofluvial sediment consisting of fine sands, silts and limited amounts of clay. Mahaney and Kalrn (1996), state that this unit represents glaciolacustrine sedimentation with some aeolian silt. However, sites Fan 1 and Fan 5 both exhibit fine glaciofluvial sand with pebbles. At Fan 1 (Figure 3.2), these deposits tend to exhibit inverse grading with corner pebbles near the top and a lower distinct, sharp boundary with Unit 2 glaciolacustrine silts. Fan 5 shows a series of altemating coarse and fine beds within this unit (Figure 5.4). The first fine silt bed occurs approximately 0.96 m nom the surface and extends to approxirnately 1.5 m. Above this Fan 1 Litho facies fie send sll. befaminebon gmv~massive. mam supwrted wnmg tlov monts fine send. fine leminalion Grns grever. rnesave. mem maporteu Sh -Sh-FI Sh SI-FI Gms prevel, send metnx SI Sand Sh came send. oxidued SI w send Figure 5.1: Stratigraphie section Fan 1. Units follow Mahaney and Kalm (1996) and Figure 3.1 1 . Fan 2 Genetic lnterpretation al bmrnated; mene sanu ienses Fsc gfwatîuvtal. nigher tlow regrme SR iamrnetetï. merse gmei ienses Figure 5.2: Stratigraphic section Fan 2. Fan 3 Sample Genetic Number lnterprefation Fsc Gm mer. cnrdely litm~neted. giaciorTuvrer. ntgner sanu mm no^ regime Figure 53: Stratigraphie section Fan 3. Fan 5 FI SP mawsand, cfcssaQded SP merse çend, crossMded SP Sh SP Gms FI Gms gjlofluvial. utper flow regma Fsc FI on end sana. finelv lamrneled Gm Gm grevel; inverse gradmg Grns Figure 5.4: Stratigraphie section Fan 5. Ped 17 m'a- mer; messm, UM2 Cox l Cox 2 Figure 5.6: Stratigraphie section Ped 17. Ped 6 SR. mefflve. Wh me and ceerse sencty iwn Figure 5.7: Stratigraphie section Ped 6. Lag 7 Lag 8 Soi1 Horizon Figure 5.8: Stratigraphic sections Lag 7 and 8. lacustrine lm] glaciolluvial diamictori unknown 2 - sedirnentation unit (after Mahaney and Kalm 1996) 200 300 met res Figure 5.9: Cross section of La Mucuchache outwasli fan cornplex b~sedoii stratigraphie evidence and Mahaney and Kahn (1996). East is to the left, West is to the right. bed lies Unit 1 glacioflwial sand and gravel which altemate as massive beds. One bed of sand exhibits slight cross lamination. Particle size analysis (see section 5.2.1.1) shows distinct glaciofluvial characteristics. Fan 3 shows slight lamination of silt housing very little gravel within Unit 1 (Figure 5.3), and Ped 6 displays a thick deposit of aeolian silt with coarse gravel beds approximately 10 cm thick undemeath. Ped 6, however, exhibits a very smooth transition from loess into glaciofluvial sands at approximately 0.75 m. This glaciofluvial unit extends to roughly 2.9 rn whereupon coarse glaciofluvial gravel becornes evident. The glaciofluvial sands altemate within six layea of coarse gravel, each coarse bed no more than 10 cm thick. The only distinction between these layen and the unit above is the coarseness of the sands. The altemating layers of sand and gravel may support seasonal discharge fluctuations in the meltwater supply. A massive fining upward sequence of flood deposits, containing cobbles exceeding IO cm. can be found in the underlying coarse glaciofluvial layer. The fim lacustrine silt bed occun at approxirnately 3.5 m fiom the surface. These observations bring into question the classification of Mahaney and Kalm ( 1996). As show in Figure 3.10, Mahaney and Kalm (1996) classify Unit 1 as "glaciofluvial sand and fine gravel" @g. 58); however, in their discussion of the characteristics of the unit, it is explained as "glaciolacusaine sandy silt ..." @g. 57). If this is indeed a glaciolacustrine unit. then it is proposed that it be included with the underlying Unit 2. Sample sites Ped 6, Fan 1 and Fan 5 al1 show the uppermon Iayer king glaciofluvial sand and fine gravel. Unit 1 should be considered the latest glaciofluvial episode to have occurred in the study area. Unit 2 lacustrine fine silts and sands are present extensively throughout the fan complex. Al1 four of the sample locations within the escarpment exhibit Unit 2 beds in varying thicknesses. The thickest deposits are found in the northeast section of the escarpment and extend into the eroded creek walls (Figure 5.9). Silt and fine sand deposits are also found on the surface of the fan whem erosion by humans and animals have removed the glaciofluvia1 sands of Unit 1 (Figure 5.10). This is most evident towards the middle of the complex . Most of the exposed Unit 2 silts are slightly larninated. Fan 5 shows altemating fine sand and silt beds and sands and gravel of glaciofluvial origin. Seasonal fluctuations in meltwater discharge, coupled with the location of the section at the possible convergence of a meltwater Stream and the probable location of the palsolake. suggest that this area was extensively modified by changing hydraulic conditions. Once an input of meltwater, probably fiom El Cabal10 valley, deposited the glaciofluvial sediments. conditions would have waned to produce a stagnant water body and suspended sediment deposition. The lacustrine silt deposits are the result. With warmer weather, another influx of meltwater would produce beds of glaciofluviai sand and gravel. This may have been repeated to produce the bedding at the site. The exposure is not extensive enough to detemine if the bedding represents top or boaom set beds; however, deltaic conditions may have been prevalent at this location. The glaciofluvial sands and grave1 of Unit 3 (Figure 5.1 1) can be found as a relatively thin (- 6 m) bed extending along most of the escarpmenf only disappearing near the centre. Fan 1. 2 and 5 support evidence that this massive poorly-sorted layer results fiom high discharge events during deglaciation. Mahaney and Kalm (1996) interpret the beds as flow till. ïhis rnay be the case as layen of till have been found at site Ped 16 representing a late advance of ice over the breach of La Mucuchache moraine during Stage 2 (see section 5.3); however, massive sand and gravel beds are cornmon sedimentary structures in glaciofluvial ancilor debris flow derived landforms (Brodzikowski and van Loon 1991, Williams 1984). Fans having a dope greater than 0.0 1 may exhibit debris flow as the dominate flow mechanism in proximal zones (Brodzikowski and van Loon 1991). Moa gravel clasts show distinct roundness in Fan 1 supporting fluvial transport. Fabric orientation can be found in Figure 5.12 for three sites near Fan l . The results show a varying orientation pattern, which indicate changing hydraulic conditions within this Figure 5.10: Unit 2 lacustruie sediments have been exposed on the sdace of the fan in areas between the escarpment and Ped 6 by erosion fiom humans and animais. Figure 5.1 1: fhe massive grave1 and bodder beds of Unit 3, located here at Fan 1. are the coarsest of the giaciofluviai beds and represent the strongest flood flows over the cornplex. sedimentary unit, It is concluded that this unit therefore, is not the result of a single flood event, but many individual episodes of varying magnitude. If deposition resulted hmice ovemding the breach of La Mucuchache moraine then it occurred no later than 12,500 yr. B.P. (Mahaney and Kalm 1996). However, the timing of this event is hard to recognize due to the lack of lacusaine bedding in that area of the fan to accurately "C date. If Unit 2 lacustrine beds were found and correlated to Mahaney and Kalm's (1 996) 14C date for their Ped 1A lacustrine sediments, a clearer date may be determined. A small area of diamicton (after Mahaney and Kalm 1996) is located approximately 100 m from the northem end of the escarpment closest to site Fan 3; however, its extent is very minimal. If this is interpreted as till and not debns flow, then it supports evidence that the El Caballo moraine was created in stages with the western amis created before the substantially larger morainic complex. A plausible alternative is the ovemding of the largest El Caballo moraine in order for the till to be deposited. The diamicton correlates well with srnaIl hummocky ndges near the apex of the fan, discemable only in the field, which were likely deposited as recessional moraines. Further investigation into the depositional rnechanisms of the till is diil required. Small pockets of lacustrine silt and fine sands are discernible in the area of Fan 5. Another srnall pocket is found near the Fan I site. These correspond to a srnaller lacustrine episode, Unit 4, perhaps kinga shorter period of time than the later event. Evidence of the ponding that produced these beds can be found in three significant (>40 cm thick) lacustrine silt and fine sand beds between 9.0 m and 14.0 rn within Fan 5. Again, the interspened glacio- fluvial sands could result from topset deposition. Fine sands 1 1.22 m fiom the top of Fan 1, could be interpreted as lacustrine Unit 4 deposits; however, field conditions prevented an extensive investigation. Following Mahaney and Kalm (1 W6), they may also be considered Unit 6 lacustrine sediments (see below). 1 Fabric Orientation at Three Sites on Figare 5.12: Fabnc orientation at three lacations in Unit 3 gravels at Fan 1 were formd to Vary mauily due to altemating flow regimes. Typical a-ais orientation in upper flow regimes is transverse to flow direction (Todd 1996). The major glaciofluvial deposits in the fan are found within Unit 5. These sediments predominate fiom Fan 5 to approximately 25 m west of Fan 1. Unit 4 lacustrine sediment separates Unit 5 hmUnit 3 at Fan 1; however, Mahaney and Kalm (1 996) have shown that this separation is not found in areas close-by. A direct contact cm be found in their Ped 4 site (see their Figure 12, pg. 58), suggesting that ponding was less extensive in this area. or. more probable, that escarpment conditions prevented MerUnit 4 deposits from king exposed. The sands and fine gravels of Unit 5 are found horkontally cross-bedded at Fan 5. The fmal exposed Unit of the complex is Unit 6. This unit is visible fiom approximately 200 m to 400 m in the cross-section. The only section that reveals the fuie sands and silts of this unit is Fan 1 where particle size analysis reveals increasing arnounts of fine sand down section. The differences in grain sizes reveal an unsettled lacustrine environment with changing hydraulic conditions and sediment influxes. These beds probably represent bottomset beds. pro- delta deposits or lake bottom deposits where suspended sediment deposition was dominate. Again, exposures were not large enough to gather further data. The presence of unconfomities within the cross-sectional profile suggest tectonic activity has been present since deposition. The right-lateral strike-slip movement of the Bocono fault has been rneasured at 66 m since the Iast glaciation (Schubert and Sifontes, 1970). and thus since the Iast depositional episodes that occurred on the fan complex. The Iack of Unit 4 lacustrine deposits in the western section of the fan rnay be a result of this movement. as may be the lateral placement of Unit 3 on Unit 5 glaciofluvial sediments near Fan 1. Figure 5.9 also shows convexing slopes to the sedimentary units; a feature not resulting from normal lacustrine and fluvial deposition. 5.2.2 Granulomehy There is no solution yet to distinguish palieoenvironrnents from particle size alone (McManus 1988); however, differentiation of the various sedimentary beds and depositional processes may be accomplished. See Table 5.1 for the graphic statistics, Figure 5.13 for sand/silt/clay percentages of the Fan, Ped and Lag samples and Appendix 1 for the grain size distribution cwes of sediment < 2 mm. Units 2 and 4 contain lacustrine silt beds which exhibit the largest concentrations of clay particles with the lowea corresponding sand percentages. However, being lacustrine samples, the sand percentages are relatively high (Brodzikowski and van Loon 1991). The high proportion of sand within these sample sites (especially within Fan 1, sarnple 4) indicates that hydraulic conditions were unstable for long penods of lacustrine deposition, or that they represent deposition close to the delta slope where the highest proportions of coarse materiai can be found (Le. in foresets or bottomsets). Seasonal fluctuation in meltwater discharge can be inferred; however, sediment avaitability may have played a rninor role. The glaciofluvia1 beds of Unit 1, Unit 3 and Unit 5 show particle size distributions consistent with glaciofluvial sediments (Le. high sand content with low arnounts of silt and clay). Fan 1- 17 supports evidence of a transition fiom coarse sand into the Unit 6 silts of Iacustrine origin. 5.2.3 Geochernical analysis Geochemical analysis of the < 2 mm fiaction of 53 selected samples was completed and the concentrations of the rnacro- and microelements can be found in Table 5.2. (Hancoc k 1 984; Hancock et al. 1988; Mahaney 1990). Only those elernents with concentrations over detection Iimits are usefui for analysis and therefore are the only concentrations presented. The concentrations of the rare earth elements (REEs) can be found in Table 5.4. The purpose of the analysis is to differentiate between the two possible source areas (La Mucuchache valley and El Cabal10 valley) in ternis of elemental concentrations and assign the glaciofluvial sediment to one or both of the valleys. Table 5.1: Partic le size data of the coane clay, silt and sand fractions, La Mucuchache outwash fan complex. Missing values are due to missing percentiles in the particular size distribution. In phi (@) units. Fan 1-1 Fan 1-2 Fan 1-3 Fan 1-4 Fan 1-5 Fan 1-6 Fan 1-7 Fan 1-8 Fan 1-9 Fan 1-10 Fan 1-1 1 Fan 1-12 Fm 1-13 Fan 1-14 Fan 1-15 Fan 1-16 Fan 1-17 Fm 2-1 Fan 2-2 Fan 2-3 Fan 24 Fari 2-5 Fan 2-7 Fan 2-8 Fan 2-9 Fan 2-10 Fan 2-1 1 Fan 2-12 Fan 3-1 Fan 3-2 Fan 3-3 Fan 5-1 Fan 5-2 Fan 5-3 Fan 5-4 Fan 5-5 Fan 54 Fan 5-7 Fm 5-8 Fan 5-9 Fan 5-10 Fan 5-1 1 Fan 5-12 Fan 5- 13 Fan 5-14 Fan 5-15 Fan 5-16 Ped 6-1 Sam?& nvdton lll~~ srLrwwss sortirtip kurtosis Ptd 6-2 Pd 6-3 Ptd 6-4 Ped 6-5 Pcd 6-6 Ped 6-7 Ped 6-8 Ptd 6-9 Ptd 6-10 Pcd 15 0, Ptd 15 O? Pcd 15 Cox 1 Ped 15 Cox 2 Ped 15 Cu 1 (UII) Pcd 16 0, Ped 16 0, Pcd 16 Cox 1 Ped 16 Cox 2 Pcd 16 Cox 3 Ptd 16 Cu (till) Pcd 17 0, Ped 17 0: Pcd 17 Cox 1 Pcd 17 Cox 2 Ped 17 Cu I Ped 17 Cu 2 1.9 2.1 6.9 2.9 16 Silt % Sand % Sand % Figure 5.13: Summary temary diagrams for a) glaciofluvial samples, and b) lacustrine and till samples from La Mucuchache outwash fan complex. Al Ca Fe K Mg %%%%% Ped 15 O! 6.85 0.29 2.88 1.66 1.65 Pd 15 02 7.15 0.42 3.49 1.60 1.76 Ped 15 Cox l 7.61 0.57 3.43 1.84 1.71 Ped 15 Cox 2 7.69 0.55 3.34 1.68 1.81 Ped 15 Cu Ped 16 Cu Lag 7 Ah Lag 7 Cu Lag 8 Ah Lag 8 Cox Lag 8 Cu Fan 1-1 Fan 1-2 Fan 1-3 Fan 14 Fan 1-5 Fan 1-6 Fan 1-7 Fan 1-8 Fan 1-9 Fan 1-10 Fan 1-1 1 Fan 1-12 Fan 1-13 Fan 1-14 Fan 1-15 Fan 1-16 Fan 1-17 Fan 2- 1 Fan 2-2 Fan 23 Fan 2-4 Fan 2-5 Fan 2-7 Fan 2-8 Fan 2-9 Fan 2-10 Fan 2-1 1 Pan 2-1 2 Fan 3-1 Fan 3-2 Fan 3-3 Pcd 16 01 Pd 16 02 Ped 16 Cor 1 Ped 16 Cox 2 Pcd 16 Cox 3 Ped 1701 Ped 17 02 Ped 17 Cox 1 Pcd 17 Cox 2 Ped 17 Cu 1 Ped 17 Cu 2 Table 5.2: Macro- und micro elenieni conceritratioris within Lu Mucuchuche outwusli fun cornplex. Bolded vulues indicaie glaciofluvial sedinients. Non-bolded saniples are lacustrine, cieoliun or peut smples. Conceniritions in purts per niillioii (ppni) iinless otlierwise stuted. The REE (La, Ce, Nd, Sm,Eu, Tb, Yb and Lu) concentrations did not differ substantially between the two valleys (Table 5.5). Six La Mucuchache moraine samples, indicative of La Mucuchache valley bedrock, were analysed (Ped 15 O,, O,, Cox 1. Cox 2, Cu and Ped 16 Cu) with Ce being the most abundant element (mean = 9 1.5 pprn with o = 16.2 or 17.8%) with La and Nd also showing high concentrations (means = 48.9 17.5% and 36.1 pprn * 33.7%, respectively). The other REEs showed concentrations < 9 ppm. Concentrations OF Lu were below detection lirnits. Lag 7 (Ah and Cu) and Lag 8 (Ah, Cox and Cu) samples were analysed from El Cabal10 moraine and the concentrations resemble the same trend as at Ped 15 and 16. La, Ce and Nd show the highest concentrations with similar os, and again, al1 other REEs are <9 ppm. Trends in micro- and macroelement concentrations within the source area sarnples (Ped 15 and 16, Lag 7 and 8) are also similar (Table 5.3). Most of the mean concentrations are comparable; however, Ca, Na and Ti means are different. Student t-tests (two-tailed, cc = 0.05) show that Ca, Na and Rb mean concentrations are significantly different between the source areas. Concentrations of Rb are below the detection limit in sarnples from the Lag sites and the mean is 88 pprn 18% in the Ped sarnples. Concentrations of Ti are not significantly different according to the t-test; however, the t-probability value (0.17) does closely approach the significance level(0.15). Al, Ca, Fe, K and Mg are the most abundant elements in both source areas, with concentrations generally between OS- 7.5%. Concentrations of the REEs were detemined for 42 sarnples from the outwash fan (Table 5.4), of which 23 were fiom glaciofluvial beds. As with the two source areas. Ce rnean concentrations are highest, around 90 ppm * 24 % and La and Nd concentrations are between 35 ppm and 50 ppm in the glaciofluvial beds. The remaining REEs al1 had concentrations < 8 PPm. Ped to Lag mean ratios range fiom 0.86 to 1.O 1 for each of the elements (Table 5.5) and Al Ca Fe K hlg Ba Br %%%%% Pcd. srmpln mean randard deviarion tandard devia (ion ndard deviarions Lrg. srmplcr mean randard deviarion randard deviarion ndard deviath glaclofluvirl aih fan rrrnplcr mean tandard devlaflon ndard deviations ndard deviariotu ratio ot mcani Pcdliag Pc&ou(wash Lagloutwash mtin rallos 1 Pcdilig 0.87 Pc&ouiwash 0.118 lagloutwasli I .O\ Table 5.3: Suniniary statistics of'iiiücro- and niicro elenieiit conceniratioiis. Pcd suniples are culculuted froni fkd 15 sartiples ürid Ped 16 C'u. Lüg süniples art: Lüg 7 arid 8. Glüciofluviul outwusli tir11 suiiiples arc ~ülculiiicdfimi glüciofltiviul süniples (bolded values iii 'l'able 5.2) oiily. have a mean of 0.94. Pedoutwash values yield a mean of 1 .O0 and Lag/outwash ratios average 1.08. The mean standard deviations of the individual areas are too high (Ped = 20.6%, Lag = 12.3% and glaciofluvial outwash = 28.7%) to conclude that they are different. Glaciofluvial samples hmthe outwash fan showed substantially higher concentrations of 14 micro- and macroelements than the Ped or Lag samples (Table 5.3). Concentrations of AI. K, Mg, Ba, Co, Cs, Dy, Mn, Rb, Sc, Sr, Ta, Ti and U are higher in the glaciofluvial fan sediment than in the source area matenal. Ratios of the means show that the fan material closely resembles the Lag samples more than the Ped samples (Pedoutwash = 0.88, Lag/outwash = 1.O 1); however, as are too high to make a significant differentiation (mean a = 29.2% for the outwash). The concentrations of the REE have been normal ized to the chondrite values pu bl ished by Taylor and Gorton (1977). The chondrite normalized profile of till fiom La Mucuchache moraine and El Cabal10 moraine are shown in Figure 5.14. The general pattern of the concentration are similar between the areas with little change in concentrations down-section in each of Ped 15, Lag 7 and 8. The glaciofiwial sarnples from the fan showed the same trend and no discemible difference can be made between them and the source areas (Figure 5.1 5). 5.2.4 Heavy Minerai Identzfication and Analysis High concentrations of certain elements within till samples of the fan cornplex (see Table 5.2) were investigated by examining the heavy mineral suites. By exarnining the heavy mineral concentrations, the minerals responsible for the high values couid be identified. Mer panning the primary and secondas, heavy mineral concentrates, the samples were separated into magnetic and non-magnetic grains using a hand magnet. The concentrations of each were then detemined (Table 5.6). Selected coloured grains (Figure 5.1 6) were sub- sampled fiom the groups of heavy rninerals (rose, white opaque, orange, light green, brown and La Mucuchache and El CabaUo Valley Till Chondrite Normaiized Profde ElCaballo Till rare earth element Figure 5.14: Chondite- normalized profile of samples from La Mucuchache moraine and El Caballo moraine. La Mucuchache Outwash Fan Chondrite Normaked Profile 1 2 3 4 5 6 7 8 9 10 II 12 13 rare earth element Figure 5.15: Chondite- normalized profile of samples from La Mucuchache outwash fan complex. black) and were analysed by an x-ray microanalyser @DS) on a SEM. Of these six coloured groups, four were subjected to neutron activation to compliment the microanalysis. In the prirnary heavy minerai suites of the 6 samples, the highest magnetic minerai concentrations were found in Ped 15 Cu and Lag 7 Cu where concentrations were over 30%. It is important to note that both of these samples are frorn tills, which are unwashed deposits. The towest concentrations are found in Ped 16 Cu where values failed to reach 15%. 5.2.4.1 Metallie Black Heayv Minerals Four individual grains (al1 fiom Ped 15 Cu) were investigated and the graphical results of the elemental analysis can be found in Appendix 2. AI1 grains were magnetic and with significant arnounts of iron (Fe). One grain contained titaniurn (Ti) in approximately a 1:3 Ti:Fe ratio. A second grain showed a ratio of approximately 1:6. The third grain only showed trace amounts of Ti. Magnetite tends to be titaniferous in mafic igneous rock (Klein and Hurlbut Jr. 1985) which would reflect the igneous nature of the bedrock surrounding La Mucuchache outwash fan cornplex. Al1 three grains showed very small amounts of Al, Si and Mn. The fourth grain had a green gangue coating consisting of Si, Al and significant amounts of K (3:2: 1 ratio). which may suggest the presence of muscovite (Klein and Hurlburt 1985) with minor concentrations of Na and Mg. Si amounts were slightly greater than the Fe amounts. Neutron activation analysis of imperfect minera1 separates dominated by black grains showed very hi& concentrations of Fe and very low concentrations of Al (Table 5.7). Moderate amounts of Ti and Mn were also found, Due to the very high Fe counts (and subsequent magnetic susceptibility) and their black colour (Figure 5.16), it is concluded that the grains are of the spinel fmily, moa likely magnetite (FqO,); however, grains containing Ti rnay be ulvospindel (FqTiO,) (Klein and Hurlbut 1985) which contains Fe and Ti in equal proportions. Ilmenite (FeTiO,) rnay be present SECONDARY non-magnctlc magnctlc magnetic non-magnetlc total ntms in g. nirrss in g. 'Ota1 1 ntcrrs iti g. PO) mnss iti g. mnss in g. Ped 15 Cor 1 PdIS Cor 1 > 1.O nrnr > 1.0 ntni 2.05 2 .O9 0.51-1.0ttirtr 0.51-1.0nrm 1.22 1.27 0.126-0.5ninr 0,126-0.5ninr 2.26 2.38 0.064-0.125ttini 0.064-0.125nini 3.80 3.95 < 0.063 nrnr < 0.063 ttini 1.53 1.68 Pd15 Cox 2 Ped IS Cor 2 > 1.0 rtini > 1.0 ntni O. 79 0.82 0.51-1.0ntnr 0.51-1.0nittr 0. 74 0,75 0.126-0.5ntnt 0.126-0.5mm 1.19 1.2 1 0.064-0.125ntnr O.O64-O.l2Snint 1.55 1 S8 < 0.063 ttint < 0.063 nini 0.93 1 .O0 Ped 15 Cu Ped 15 Cu > 1.0 trrttr > 1.0 mni 1.12 1.22 0.51-1.0ntni 0.51-1.0nrni 1.14 1.21 O. 126-0.5ttitri 0.126-0.5nini 2.69 2.70 0.064-0.12.5 ntni 0.064-0.125nini 2.7 1 2.72 1.22 1.23 Table 5.6: Concentratioii of niagiletic üiid lion-iiiagnetiç Iicavy niiiierals in six till saiiiyles. See Figure 3.3 for location OS sûiiiple sites. Figure 5.16: Selected coloured grains removed for rnicroanalysis and instrumental neutron activation analysis. in some of the grains also. The highest arnount of magnetite in the examined samples occurs in sample Ped 15 Cu (see Table 5.6). However Fe concentrations are lower than other source samples (e-g. Lag 7 and 8). High concentrations of Fe in Lag 7 Cu, Lag 8 Cox and Lag 8 Cu, could be explained by the high percentages of magnetite (Table 5.6). Ped 16 Cu contains high amounts of Fe, however relatively low amounts of magnetite, suggeaing it's Fe content is derived from minerals other than magnetite. 5.2.42 White Translucent Heavy Minerah Four grains were examined (fkom Ped 15 Cu, Ped 16 Cu, Ped 17 Cox 1 and Lag 8 Cu) and the results cm be found in Appendk 2. Al1 of the grains showed substantial amounts of Al and Si, a11 in diflerent proportions. The Ped 15 Cu grain showed a ratio approximately 2:3 (Si:Al) while the Lab 8 Cu and Ped 16 Cu grains showed almost equal proportions of the two elements. Al1 four of the grains showed substantial amounts of K. whereas Fe and Ti were observed in small quantities. The white opaque grains showed high counts of K when subjected to neutron activation analysis (Table 5.8). Very high levels of Ba, Ce, Dy, Eu, Hf, La, Lu, Nd, Ni, Sm, Tb,Th, U and Yb were also found and may have been the result of contamination of the sample by heavy minerals of different composition. One grain of transparent white colour (fiom Lag 7 Cu) was found to have a A1:Si ratio of 2: 1 with trace amounts of K. Due to the very high Si and Al levels, moderate K arnounts, opaque white colour. it is suggested that the grains are possibly a AI,SiO, polymorph. Examination under the light microscope revealed a prismatic structure closely resembling that of andalusite (Gribble 1985). None of the samples examined for heavy minerals showed abnomal concentrations of AI or K when exarnined by iNAA (Table 5.3)- 5.2.4.3 Brown Heavy Minerals Two grains fiom Ped 15 Cu were exarnined and the results can be found in Appendix 2. Both grains showed hi& counts of Si and Al, both at approximately 6:7 AlSi ratios. Small arnounts of Fe and Ti were found in both samples while K occurred at approxirnately 1 :3 ratios with Al in both grains. Having similar chemical composition to opaque white grains, these light brown grains appear to be muscovite (KAI,[Si,AIO,,](O~), a member of the mica group. Cornmon within low-grade metamorphic rocks, it remains in composition when the grade increases. It is therefore kquent in schists and gneisses. At high temperatures, muscovite tends to break down giving K-feldspar, quartz and the A12Si05polymorph sillimanite. 5.2.4.4 Orange Heavy Minerals Three grains, al1 fiom Ped 15 Cu, were exarnined and the results can be found in Appendix 2. The most abundant elexnent is Si which occurs with Al at a ratio of approximately 3:2 in al1 grains. Fe is present in al1 grains; however, one sample only shows small quantities. Mn occurs in two samples at moderate levels while Ca is present in small to trace amounts in a11 three grains. The neutron activation analysis complimented the microanalysis with identical elernent counts in cornparison to other coloured grains (Table 5.8). Relatively Iow values of Ti and Mn corresponded to the microanalysis results. The orange grains were the only coloured minerals analysed that contained measurable amounts of Sr or Cs. Due to the Si:AI ratio, the presence of Fe, Mn and Ca, and its orange colour, it is concluded that the grains could be spessartine (Mn3A1,Si30,J, grossular (Ca,A,Si,O,,) or almandine (Fe,AI,Si,O,,) gamets. High concentrations of Mn cm be found in the glaciofluvial and ti11 beds of Ped 16 and Ped 17 (i.e. >1,000 ppm) (Table 5.2). Other high anomalies can be found throughout the fan mainly within Fan 1 and Fan 2 lacustrine beds. These may be explained by the presence of spessartine. Concentrations of Ca fluctuated within the outwash fan, with the highest counts found the lacustrine beds of Fan 2 (Table 5.2). These may be explained by the presence of the grossular gamets, 5.2.4.5 Light Green Heavy Minerals Three grains of light green colour were exarnined from Ped 15 and Ped 16 Cu samples (Appendix 2). Si and Al occurred in al1 three grains at ratios of approximately 2:3. Two grains featured Ca in 1 :2 ratios with Si. The third grain did not contain Ca. K was found in two grains in moderate amounts and in trace amotints in the third grain. The K is probably part of a clay coating. Fe was found in moderate amounts in al1 three grains and Ti was found in one grain at a ratio of approximately 1 :3 with Si. Grains with abundant amounts of Ca couId be cIassified as a Ca-rich amphibole (GribbIe 1988). This group includes ferroactinolite (C+Fe,[Si,0,1(OH)2 which is commonly green or brown in colour (Gribble 1988). 5.2.4.6 Rose (Light Pink) Heavy Minerals Three grains of light pink or rose colour were examined from Ped 16 Cu samples. The results can be found in Appendix 2. Si was present in al! three grains at ratios of approximately 4:3 with Al. All grains featured levels of Na, Mg and Ca at various levels with a Na:Mg ratio about 1:2. Ca was found in small to trace arnounts. Al1 grains showed levels of Fe in approximately 1 :2 ratios with Si and moderate amounts of Mn. Rose coloued grains showed ver-high levels of Al and Mn after iNAA. corresponding to the microanalysis. High contents of Sc that were not detected by the microanalysis, were recorded by MAA. Due to the presence of Si, Al. Mn, Ca, Na and Mg, these grains were interpreted as being the gamet spessartine (Mn,AI,Si,O,J which exhibits a rose colour when Mn-rich (Gribble 1988). As discussed in section 5.2.4.4, high concentrations of Mn may be the result of spessartine gamets within glaciofluvial lacustrine sediments of Ped 16 and 17. 5-25Scanning Elecîron Microscopy Four representative sarnples were taken hmtill (Ped 16, Ped 15, Lag 7 and Lag 8) and 1 1 sarnples fiom various glaciofluvial beds of the fan complex and selected coarse (2mm-63pm) quartz sand grains were investigated by Iight microscopy and scanning electron rnicroscopy. Table 5.8 shows the characteristics of those samples. Tables 5.9 and 5.1 0 show the results. whereas Table 5.1 1 contains summary statistics. Only three of the four till sarnples (Ped 15 Cu. Ped 16 Cu, Lag 7 Cu and Lag 8 Cu) cm be attributed to proven source areas: Ped 15 Cu to La Mucuchache valley and Lag 7 and Lag 8 Cu to El Cabal10 valley. Ice thickness is unknown in the respective valleys; however, thicker ice may have been present in La Mucuchache valley. A high degree of relief is common to sands fiom a glacial source and is a consistent trait arnong the till samples (Figure 5.17,s. 18). On average, approximately 55% (*23%, where 23% is Io with n = 4) of al1 till grains showed high relief, while the remaining grains were alrnost evenly split between low and medium relief, a characteristic found more on the fluvial transported grains (Mahaney 199Sa; Whalley 1995). Sharp angular features are also comrnon on till sarnples from the lateral and recessional moraines (-75%, * 13%) while subparallel linear features frequent approximately 70% (* 10%) of the glacial grains. Straight and cwed grooves are also common with a frequency around 55%; however, their respective standard deviations are high (* 20%?24%). Arc-shaped steps and mechanically up-tumed plates are also fiequent (-55%) but their os were low (* 10Y0 and 7% respectively). Average a for the glacial features within the till samples was * 1 1.6%. Of the microfeatures that are found in both glacial and fluvial environments, weathered crushing features and abrasion features are the most common; however, frequencies were low (-30%. * 4% and 13% respectively). Ali other such microfeatures fiequent -20% of the grains with relatively small standard deviations (mean a = 5.2). The till samples generally lack fluvial microfeatures; however, they are present on sorne grains. Frequencies of rounded feahrres and V-shaped percussion cracks are approximately 20% and are generally consistent between samples (O = 6% and 10% respectively). Average u for fluvial features on till samples is * 10%. Grains fiom the glaciofluvial beds of the fan complex show lower fiequencies of glacial rnicrofeatures (see Figure 5.17); however, both adhering particles and curved grooves increase in frequency over the glacial grains. Adhering particles surprisingly increased 16% on average (with n = 1 1 for the glaciofluvial samples), while curved grooves increased 13% on average on fluvial grains. Mechanically up-tumed plates, arc-shaped steps and high relief showed the greatest drop in hquencies (-29%, -25% and -40%, respectively). Sharp angular features and subparallel linear feaîures showed decreases of only - 10%. Standard deviations were roughl y identical to the till samples for the glacial features but the mean o was slightly higher at I 13.5%. Three of the five microfeatures found within glacial and fluvial environments were more fiequent within the glaciofluvial grains. These include solution etching, ahrasion faces and pre- weathered surfaces al1 of which show increases, on average, of +20%, +5% and +1% respectively. Their a's ranged fiom 1 1- 14%. Weathered crushing features and precipitation features showed declines (-9% and -3% respectively). IO~relief FI - medium relief rounded feanues V-shaped percussion - solution etching abrasion faces pre-wcathend - weathmd mhing - prccipitation feanuc~ - sharp anpular fcatures I & sub-parallcl linear fam* - wnchodial fractm u maight groovcs a curvcd groovcs -a cresentric gouges I Figure 5.18: a) Graiii froiii Lag 7 sliowing iiewer cnisliiiig siirfaces witli linear fractiires (riglit side), and adlieriiig panicles on the lefi. 1-ligh relief aiid sliarp, aiigiilar featiires siigpst glacial origiii with little or no fluvial iiiodificatioii. b) Lag 8 graiii exliibits soiiie edge roiiiidiiig and an older siirface witli a raiidoiii pitted siirface iicar the bottoin. e) Linear and coiiclioidal fractiircs are foiiiid oii tliis grain fmin Rn 1 - 1 represeciliiig glacial coiiditioiis (Malianey 1990). Fliivid activity deposited tlie grain; Iiowever. fl~ivialinicrofeatiire evidence is scrrce. Adlieiiiig particles and Iiigh relief are evideiit. f') Older preweatliered f~actiireface (inid to lower portion) gives wriy to dissoliitioii fecitures aiid frrctiii*escaiised by abrasioii on the top of tlie graiiiii. Sample rill Ped 15 Cu massive till; lateral moraine Ped 16 Cu massive till; recessional or ground m. (?) Lag 7 Cu massive till; lateral moraine Lag 8 Cu massive till; lateral moraine glaciofïuvial Fan 1-1 sand; laminated, inverse grading Fan 1-4 matrix supported gravel; massive Fan 1-14 gravel with sand matrix Fan 2-3 laminated gravel, normal grading Fm 2-8 gravel wl sand rnatrix; cmde lam. Fm 4-6 gravel lens in finely Lam. sand Fm 4-8 laminated gravet; normal grading Fan 5-8 horiz. cross-bedded coarse sand Fan 5-13 planer cross- bedded sand and gravel Ped 17 Cu massive coarse sand and gravel Ped 16 Cox 2 massive sand and gravel Table 5.8: Characteristics of the various SEM samples taken fiom La Mucuchache outwash fan complex. The greatest contrasts between the microfeature frequencies came within the fluvial microfeature categories. Al1 rnicrofeatures showed increases in relative fiequency with low relief having the smallest increase. Low relief increased to 40% (k 17%), representing an increase of 20% over the till grains. The other three microfeatures each increased 27% to approximately 50%. Standard deviations for al1 three were -14% - 20%. If it is assumed that fluvial microfeature genesis within glacial charnels or conduits was minimal or non-existent, the differences in fiequencies between till and glaciofiuvial grains should represent transport within pro-glacial channels. Glacial cmshing and grinding has produced hctured surfaces which are typically high in relief and exhibit sharp angular features within the till samples. The kquency of such featurrs can be attributed to the periodicity with which fracture lines appear (Mahaney 1995b) and are usuaily created in the sub-glacial zone. Whailey (1995) States, however, that sharp angular grains with conchoidal Fractures (which are low in both the till and the glaciofluvial sarnples) can also be generated by the initial release hmthe bedrock. Krinsley and Takahashi (1 962)- Whalley and Krinsley ( 1974), Eyles ( 1 978) and Gornez and Small(1983) have largely confmed that there is no one particular texture that can be attributed solely to glacial action. However, glacial grains have ken shown to have high relief, shaq angular features, concoidial and subparallel linear fractures and straight and curved grooves (Mahaney 19954, the latter king a particularly important glacial indicator. As a result grains exhibiting such features may have been transported subglacially, supraglacially or not transported by glaciers at ail. The fact that grains fiom this study area do exhibit a high frequency of microfeatures of this sort, helps in comparative analysis with those which underwent fluvial transport iater on. Frequencies of high relief, sharp-angular features and subparallel linear features on the till grains may reflect either glacial crushing or bedrock release. The fluvial features (eg. v- shaped cracks) that are present within some of the till samples can therefore be attributed to glaciofluvial meltwater flow within glacial channels or conduits. The fiequency of abrasion faces on till grains closely resembles those of the fluvial microfeatures therefore, assuming the abrasion was generated by flowing water (Mahaney 1995a) it can be concluded that glaciofluvial action was present before final deposition had occurred. Whalley (1 995) notes that abrasion cm be created directly by a glacier; however, these features are 'limited' @g. 368), and only created if the sediment was transported within a thin film between the ice and the bed and if shear stresses are large enough, which may not occur if till layers are thick or have high water content. Moa till microfeature's fiequencies follow the same trend as most studies (e.g. Mahaney 1995 a and b). Mahaney (1995a) showed that crescentric gouges may be present in till sarnples: however, these tend to be found in thicker 0500 m) glaciers. Conchoidal fractures do tend to be low, oniy king exhibited in -19% ( * 7%) of the till sarnples, but again, they have a tendency to be found in till deposited by thicker ice. While in motion, particles transported by water tend to collide with each other at high frequencies within the water column, and widi larger particles on the bed while in saltation. V- shaped percussion cracks and subsequent abrasion are usually the result and are kquently found on fluvial transported grains (Whalley 1995). Abrasion also produces surfaces of lesser relief. Grains fiom the fan complex show higher frequencies of these fluvial features; however. frequencies only rarely attain 50%. Although there is a decrease in the fkequency of 'glacial' feanires on the glacioflwial grains (in cornparison with the till samples), a high kquency (-55- 60%) do retain angular edges, linear hctwes and cwed and straight grooves. One of three situations is hypothesised to be responsible for this: a) a Iack of suspended or bedload sediment to collide with while in transport (or lack of sediment of suficient sue); b) a lack of flow turbulence to produce the collisions; andlor c) a lack of suficient transport distance. Figure 5.19 shows the relationship between the fiequency of fluvial microfeatures versus the transport distance. The distance was measured From approximately 150 metres fiom the apex to the smple site. Relationships are very weak in al1 cases with the greatest correlation between medium relief feahues and distance (r = 0.46); however, scatter is high. Sand content also exhibits weak relationships with the frequencies (Figure 5.20); however, fiequency versus rounded features do show somewhat of a relationship (r = 0.63). This could initiate more abrasion. The relationships between the frequencies and turbulence (as measured by Reynolds nurnber) (Figure 5-21), cm be considered fairly stmng but most are inverse relationships. This may suggest that with an increase in turbulence, the number of collisions, and therefore fluvial microfeatures, may decrease. 52.6 PaIœohyCaauiic Estimations The palæohydraulic cornponent of îhis snidy involves the estimation of critical mean flow velocity (U3, critical shear stress (TA, critical shear velocity (U.,), critical flow depth (a), Reynolds number (Re) and Froude number (Fr) for the areas once covered with flowing meltwater. As discussed earlier, the maximum clast diarneter (D,,) was measured in selected glaciofluvial beds within the fan cornplex. However, the diarneten were not contained within the particle size distributions (see section 4.2.1). By using D,, as the indicator of flow cornpetence, any velocity value that is estirnated would represent the maximum flow velocity. Arguments for the use of smaller grain sizes have been presented in the literature (e-g. Jopling 1966); however, complete particle size distributions are not known, and therefore D,, must be used. Estimates may be slightly higher than traditional methods might estimate for the same sample. The palæohydraulic flow estimates represent conditions that may be considered rare and extreme. The uniformity of the larger grave1 and boulder beds suggests that most flow events did not fluctuate greatly in magnitude over short durations. Therefore, a range of velocities from zero to the estirnated value, cm be assmed. Field investigations failed to recognize evidence for channel flow; however, as the flow waned after each successive flood event, a braided pattern surely would have developed. Thirty five samples were exarnined. From each sample, five of the largest clasts had their a axis, b axis and c axis measured. Nominal diameter was then calculated. From these five clasts, a mean size was calculated (the mean D3(see Table 5.12). Of the 35 samples, 15 were exarnined nom Unit 1; 1O hmUnit 3; and 3 fiom Unit 5. Unit 2 sarnples were also exarnined in Fan 5 despite the unit being considered a lacustrine deposit. Glaciofluvial intlow occurred resulting in a 1 rn thick bed within the ponding stage (see Figure 5.4). Unit 4 lacustrine deposits were examined in Fan 1; however, their origin may be debated (see section 5.4). Transport Distance vs. Frequency of Low Relief Features O ------. - - -- . - - - ,- O 200 400 600 800 1000 1200 transport distance (rn) Tram port Dis tance vs. Fre quency of M edium Re lie f Fe atures O --- O 200 400 600 800 1 O00 1200 1400 transport dis tance (m ) Pigure 5.19: Relationshîp between transport distance with a) low relief, b) medium relief, c) rounded features and d) V-shaped percussion cracks Transport Distance vs. Frequency of Rounded Features transport distance (m) Transport Distance vs. Frequency of VShaped Percussion Cracks 100 - 90 - 80 - r = 0.44 1 transport distance (m) Percentage Sand Content vs. Frequency of Low Reiief Featmes % sand content Percentage Sand Content vs. Frequency of Medium Relief 100 Features 90- r=-0.111 80 - % sand content ------. Figure 5.20: Relationship between sand content with a) Iow relief, b) medium relief, c) rounded features and d) V-shaped percussion cracks Percentage Sand Content vs. Frequency of Rounded % sand content Percentage Sand Content vs. Frequency of VShaped Percussion Cracks 100 - r=0.161 90 , O/r sand content Flow Turbulence vs. Frequency of Low Re tief Features 100 - O ------A O 100000 200000 300000 400000 500000 600000 Reynolds Nnmber Flow Turbulence vs. Frequency of Medium Relief 100 - Features Reynolds Ntxmber Figure 5.21: Relationship between flow turbulence with a) low relief, b) medium relief, c) rounded features and d) V-shaped percussion cracks Flow Tubdence vs. F~quencyof Rounded Featores Reynolds Yumber Flow Turbulence vs. Frequency of V-Shaped Percussion Cracks Reynolds Number The velocity profile mode1 was used first to determine the palæovelocities. Results can be found in Table 5.13. Various depths were inserted into the mode1 and values are ploned in Figure 5.22. As can be seen in the plot, palæovelocity is directly proportional to nominal grain size and beds showing the largest clasts yield the highest velocities. Fan 1, Unit 3 deposits shows velocities that are the highest in al1 sarnples examined. Velocities here probably ranged from 1.5 to 4.0 m/s if the flow depth is assumed to be 1 metre. The ratio of highest to lowest velocities in Unit 3 samples (including Fan 5 samples) increases with depth, and ranges from 3.40 for a depth of 1 m to 2.18 for a 10 cm depth. This is a result of the logarithmic relations between nominal pinsize and palæovelocity within the rnodel. Unit 1 gravels show paligovelocities nom 3.4 m/s in Ped 6 to 1.2 m/s in Fan 5 beds at 1 m flow depth. This, again, reflects a high range of variability. However, only 6 of the 15 samples showed velocities > 2.0 m/s. Unit 5 samples fiorn Fan 5 exhibit velocities ranging fiom 1.3 m/s to 2.3 m/s for 1 metre depths while uni& 4 and 6 lacustrine deposits showed velocities around 1.3 m/s - 2.3 m/s for 1 m depths. The highest palaovelocity estimations are found within the massive beds of sand and gravel within Unit 3. They suggest that it was during this penod of the fan's formation, that the most catastrophic flood conditions occurred. Overall, the genesis of the fan was govemed by a high energy environment, unlike today's conditions where minimal geomorphic activity prevaiIs. Velocity estimates indicate that climatic conditions fluctuated through the LGM which resulted in variable melt rates, and therefore meItwater discharges. Finer sand sediments overlying Unit 3 sand and gravel beds reveal waning stages of the flow and the transition to a lower energy environment. The velocity profile rnodel predicts mean flow veloci~of ths entire flow column based on shear stress conditions at the bed. The shear stress is reflected in the shear velocity value. Snmple a axb (mm) b as& (mm) c prir (mm) iiomind Dlamcttt (m) Fa 1-1 16.0 9.8 6.8 Fan 1-2 19.0 12-6 8.8 Fan 1-4 236.7 nia n/a Fan 1-7 170.0 da nia Fa1-8 181.7 da nia Fm 1-10 16.6 13.4 7.6 Fan 1-1 1 27.0 20.4 14.8 Fan 1-13 29.4 252 18.2 Fan 1-14 1310 n/a nia Fan 1-15 11.0 7.8 5.6 Fan 1-16 20.8 16.4 8.2 Fan 1-17 7.8 5.4 3.6 Fan 2-3 24.0 18.6 10.2 Fan 2-8 37.6 24.8 18.8 Fan 5- 1 28.2 12.2 16.6 Fan 5-2 5.6 4 2.8 Fan 5-3 4.4 3 2.4 Fan 5-4 11.0 8.6 6.6 Fan 5-6 5.0 3.8 3 Fan 5-8 7.8 52 3.8 Fan 5-10 21.4 16.8 9.2 Fm 5-13 29.0 21.8 15.2 Fan 5-14 7.4 5 4 Fan 5- 16 35.2 25 14.8 Pd 16 cox 1 13.0 9.8 4 Pcd 16 Cox 2 56.0 nia rua Pcd 16 Cox 3 18.4 12.8 6.8 Ped 17c0~I 17.4 9.4 5.4 Pai l7Cox 2 25.4 21.4 II Pd 17Cu 128.0 tua wa Pcd 65 19.0 12.4 8.2 Pd66 22.4 16.6 9.8 Pd 67 262 17.4 9.8 Pd68 26.6 17.4 8 Pd6-9 552 38.8 16.8 Table 5.12: Grain shape characteristics and diameter of the selected glaciofluvial clasts used for reconstruction. 'a' mis, 'b' axis 'cl ais and nominal diameter are averages of the five largest clasts within the sarnple. Only 'a' axis were measured in the field at Fan 1-4, 1-7, 1-8, 1-14, Ped 16 Cox 2 and Ped 17 Cu. rnean mean mean mean mean velodty dadty vdocity velocity veloclty ifd = 1.0 m ifd =O.Sm ifd = 0.25 m if d = 0+2m tfd =O.lm Fan 1-1 1.æ 1.fil 1.35 1.XI 1.13 Fan 1-2 1.a2 1.64 1.45 1.39 1.20 Fan 14 4.05 3.39 2.72 2.51 1.85 Fan 1-7 3.70 3.14 258 2.40 t .83 Fan 1-8 3.77 3.19 261 242 1.84 Fan 1-10 1.78 1.60 1.42 1-36 1.18 Fan 1-11 213 1-90 1-66 1.59 1.35 Fan 1-13 226 200 1.75 1.67 1.41 Fan 1-14 3.44 295 245 229 1.80 Fan 1-15 1.53 1.38 1-24 1.19 1.O4 Fan 1-16 1-88 1.69 1.49 1.43 1.23 Fan 1-17 1.33 1.21 1.09 1.O5 0.93 Fan 2-3 1-99 1-78 1-57 1.50 1.29 Fan 2-8 232 206 1.79 1.71 1.44 Fan 51 219 1.95 1-70 1.63 1.38 Fan 5-2 1.19 1-09 0.98 0.95 0.85 Fan 5-3 1.O9 1-00 0.91 0.88 0.78 Fan 5-4 1.S8 1.42 1-27 1.22 1.O7 Fan 5-6 1.18 1.O7 0.97 0.94 0.84 Fan 5-8 1.33 1.21 1.09 1 .O5 0.93 Fan 5-10 1.92 1.72 1.52 1.45 1.25 Fan 513 2.17 1.93 1.69 1.61 1.37 Fan 5- 14 1.32 1.20 1-08 1.O5 0.93 Fan 516 2.25 1.99 1.74 1.66 1.41 Ped l6Cox 1 1-54 1.39 1.24 1.20 1 .O5 Ped 16 Cox 2 2.47 2.18 1-89 1.a0 1.50 Ped 16 Cox 3 1.76 1.59 1Al 1.35 1.17 Ped l7Cox 1 1-64 1.48 1.32 1.27 1.11 Ped l7Coxî 206 1-83 1.61 1.54 1 .a2 Ped 17 Cu 3.41 2.92 2.43 2.28 1.79 Pd6-5 1.80 1.62 1.44 1.38 1.19 Ped 6-6 1.94 1.74 1.53 1.47 1.26 Ped 6-7 1.99 1.78 1.57 1.50 ? .î9 Ped 6-8 1.95 1.74 1-54 1.47 1 .Z7 Ped 6-9 2.51 2.22 1.92 1.82 1 .52 Table 5.13: Estimated palaeovelocities for the selected sarnple sites based on the velocity profile mode1 using arbitrary depths. Velocities in mis. This model does not take Uito account the energy gradient of the fiow. The estimated pal;9ovelocities are therefore the critical velocities needed to produce the particular shear velocity (therefore shear stress) which in tuni is needed to initiate movement of D,, used to predict shear stress. These values are the theoretical minima needed to move the clasts rneasured within each sample bed. To estimate what the palæoflow conditions were on the fan at the time just before deposition, Manning's equation (equation 4.3), which takes grain roughness and energy slope into account, was used. Results can be found in Table 5.14. By using Manning's equation, a calculated depth was needed. This depth represents the minimum possible depth needed for the initiation of motion of the clast size in question. This calculated depth yielded a critical Dld ratio of 0.54 for each of the samples. Therefore, flow depth had to be approximately twice as deep as D,, is high, for transport to begin. Critical depth mged from 30 cm for the largest ciasts to 7 mm for the smallest. The palæovelocities calculated using this method resemble most of the values calculated using d = 0.5 rn in the velocity profile method. This suggests that the energy gradient and/or grain roughness both had a profond influence on the flow velocity of meltwater on the fan. Velocities were determined for d =1 .O m, 0.75 m and 0.5 m and these are plotted in Figure 5.23. Palæovelocities for 1 metre flow depth, for instance, were substantially higher than those using the velocity profile method. The magnitude of the difference relies on the clast size. The energy gradient was constant for al1 samples investigated and because of a relatively large value (0.05) and its direct relationship with velocity to the power of %, velocity values increased significantly. Velocities calculated by the profile model predicted mean flow velocity 4.0 m/s for the largest clasts in Fan 1, while Manning's equation predicted values to be approximately 10.0- 12.0 m/s for the same clam. However, the direct relationship between nominal grain size and palæovelocity disappears with this approach. Manning's roughness coefficient 'n,' is directiy related to the grain diameter, and based on this method, so is flow depth. However, since flow depth is directly related to the calculated velocity by the exponent %, 'n' has a more pronounced effect on velocity. An increase in grain size will increase the grain roughness exerted on the flow if depth is kept constant. As illustrated in Figure 5.23, if grain size increases, mean flow velocity will decrease over a certain critical velocity (approximately 4.0 - 5.0 mis). This pattern is not evident while using the critical flow depths calculated by DuBoys equation. This suggests a limitation of the tractive force method which has not been addressed in other studies (e.g. Maizels 1986). Also, the Manning equation approach may not accurately estimate Manning's &n'if only because it is based on the nominal diameter of the clasts that are to be transported. It should be assumed that the particles used to calculate 'n' should be in motion, and therefore not on the bed creating resistance. If the particle in question is king introduced to the hidy reach by the flow. the bed material size would theoretically be a) larger than the clast since the flow would not be fast enough to move it (which could then make those particles Dl, in the sedirnentary unit); or b) smaller than D,, but unmovable due to sheltering or armouring effects of other clasts (Komar 1996). The grain size distribution of the bed material before deposition of Dl,in al1 likelihood, is unknown. If Manning's 'n' is assumed to be constant over al1 sarnple sites (which is very unlikely), critical mean flow velocity (at the cntical depth) would follow a parabolic curve as shown in Figure 5.24. However, if depths greater than the critical depth value are used, velocities become constant (Table 5.15)' since only the critical depths incorporate the changing values of Dl, through the DuBoys equation. It is suggested that pa1;eovelocities follow Manning's approach for the critical depth values. This incorporates slope, grain size, and critical shear stress; however, the limitations in the calculation of 'n' should be noted. Since depths were probably never at these critical levels during the maximum flow needed to transport the clasts, to predict velocities at different depths, Sh~rSirnr (N mZ) Urpih (m) ir (nus) 11 (ln's) LI (nilS) (ifd - I.UO ni.] (ifd - 0.75 ni.] (ifd a ulc'dl Fan 1-1 12.6 10.4 O 9 !:an 1-2 12.2 10.0 1.O Fui 1-4 8.0 6.6 3.6 FM 1-7 11.4 7 .O 3.O Fm 1-8 8.3 6.9 3. l Fui 1-10 12.3 10.2 1.O FM 1-1 1 11.3 9.3 1.3 FM 1-13 11.0 9.1 1.4 Fin 1-14 8.8 7.3 2.7 Fm 1-15 13.2 10.9 0.8 FM 1-16 12.0 9.9 1.O FM 1-17 14. I 11.6 0.6 FM 2-3 11.7 9.6 1.1 Fui 2-8 10.8 8.9 1.4 FM 5- 1 11.1 9.2 1.3 Fui 5.2 14.8 12.2 0.6 Fan 5-3 15.4 12.7 O. 5 Fan 5-4 13.0 10.7 0.8 Fm 5-6 14.9 12.3 O. 5 FM 5-8 IC 1 11.6 0.6 Fm 5-10 11.9 9.8 1.1 FM 5-13 11.2 9.2 1.3 FM 5-14 14.1 11.6 0.6 PM 5-16 11.0 9.1 1.4 Pcd I6 Cox 1 13.2 10.9 0.8 Pcd 16 Cor 2 10.5 8 7 1.6 Pcd 16 Con 3 12.4 10.2 1.0 l'cd t 7 Cor I 12.11 10 5 0.9 Pcd 17 Con 2 11.5 9.5 1.2 PrJ 17 Cu n.ll 7 3 2.6 l'cd 6-5 12 2 10 I l .O l'cd b.b 11 11 9 7 I I I'cd 6-7 11 7 ') b 1.1 I'cd 6.H I1.H 9 7 1.1 l'cd b-9 IO 4 n O 1.a l'able 5.14: Esiiiiiuied pulucovclociiics tor the sclcclcd siiiiiplc silcs if Muniiitig's 'II' 1s culciiluiccl uiid urbiirury dcpths, us wcll us itic culculuicd dçptli, urc uscd. the velocity profile method should be used. These values would reflect the critical mean velocities needed to rnove Dlm without regard to slope and are therefore applicable in any circumstance where hydraulic conditions are similar. If slopes were to increase, and D,, remains constant, mean fl ow velocity would increase depending on the magnitude change. but critical mean flow velocity would remain the sarne. 5.3 Glacial Chronology The results discussed earlier can help formulate a general chronology of the Last Glacial Maximum in the area of La Mucuchache outwash fan. Geochemical analysis has provided evidence for the origin of the glaciofluvial sediment, whereas the SEM microfeature analysis reveals the effect of meltwater transport on fluvial sediment Based on the findings within this report, a glacial chronology starting no earlier than approximately 27,000 yrs. B.P. (based on Mahaney and Kalm 1996) can be established. Sediment pre-dating 27,150 k 400 yr. B. P. has not ken found within the fan Fowever recent findings have indicated sediment of -55.000 yr B.P. may be present (Mahaney pers. comrn. lW8)]. The only geologic activity at the study area that can be confimned before this time is the movement of the Bocono Fault and the placement of Mesa del Caballo into its present position. Stage I (Pre-Méri& Glaciation): Stratigraphie evidence can not be found of any glacial advance during the pre-Mérida within the fan. However, sediments of this age are present in the neighbouring Mesa del Cabal10 (Mahaney and Kalm 1996). Therefore, the construction of the end moraine results nom pre-Mérida glaciations within La Mucuchache valley. With a lateral displacement of 66 m/10,000 yrs. (Schubert and Sifontes, 1970), the Bocon6 Bult has shifted the moraine to its present position. Using a displacement of approximately 1 km, Mesa del Caballo can be dated to Ca. 150 ka. Stage 2 (Lote Mérida), first phase: The stratigraphy of the fan suggests three separate Sample Unlt M 'O U (mls) u (ml$) Froude (if d-1.00 m) (if dPO.75 m) Numbcr Fan 1-1 7.26 1.90 Fan 1-2 7.26 1.97 Fm II 7.26 3.01 Fan 1-7 726 2.85 Fa 1-8 7.26 1.88 Fan 1-10 726 1.95 Fan 1-11 7.26 2.12 Fan 1-13 726 2.18 Fan 1-14 726 2.3 Fan 1-15 726 1.81 Fan 1- 16 726 1.00 Fan 1-17 7.26 1.'O Fan 2-3 716 2.06 Fan 2-8 726 -.-7 71 Fan 5- 1 7.26 2.15 Fan 5-2 726 1.62 Fan 5-3 7.26 1 S6 Fan 5-1 7.26 1.34 Fan 5-6 7.26 1.61 Fan 5-8 726 1 70 Fan 5-10 7.26 2.02 FUI 5-13 7.26 2.14 FUI5-14 7.26 1.70 Fan 5-16 7.26 2.18 Pd16 Cox 1 7-26 1 .S? Pd 16 Cox 2 7.26 2.29 Pd16 Cox 3 7.26 1.94 Pd17 Cox 1 7.26 1.S8 Pcd 17 Cox 2 7.26 1.09 Pd17 Cu 7.26 1.71 Pd6-5 7.26 1.96 Pd6-6 7.26 2.03 Pcd 6-7 7.26 2.05 Pd6-8 7.26 2.03 Ped 6-9 7.26 2.3 1 Table 5.15: Estimated palaeovelocities, Reynolds and Froude numbers based on Manning's equation using a constant ln,' and arbitrary and calculated depths. fan generating episodes have taken place and the possibility of a fonh (represented by Unit 1. Unit 3 and Unit 5 sediments). The first episode is believed to have occurred during the fim phase of the Late Mérida During rnelting of an ice lobe within La Mucuchache valley, meltwater began to flow over La Mucuchache moraine and pond against Mesa del Caballo. ïhere are no glaciofluvial deposits to document the flow of rneltwater down the fan at this tirne: however they may be buried (and would then be considered Unit 7). The breach at the ju.nction of Mesa del Caballo and La Mucuchache moraine had not opened up by this time and therefore the drainage route fiorn the lake is unknown. A small depression can be viewed cutting across the top of Mesa del Caballo draining toward Quebadra Los Bijinos; however, the elevation of the channel does not correspond with the probable elevation of the palæolake level. Water within the lake probably dned up after a period of tirne. Stage 2 (Late Mérida), second phase: Sometime after the phase one palæolake disappeared, ice advanced down El Caballo valley and La Mucuchache valley. Meltwater flow within their proglacial areas began upon melting. Meltwater flowed over the breach at the top of La Mucuchache moraine depositing the glaciofluvial sediments of Unit 5. It is not known whether meltwater fiom El Caballo valley contributed to the fan creation. Stage 2 (ulte Mérida)).thirdphase: This rneltwater ponded in times of cooler climate. which, as evident by the thin sediment thicknesses of Unit 4, did not last as long as the previous or subsequent pondings. It is probable that the events of phase 2 repeated themselves when climatic conditions changed; thus, resulting in the deposition of Unit 3 and 2. Due to the large thicknesses of the glaciofluvial bed, and the large size of the clasts found within the unit. the deposition of Unit 3 sediments came at the tirne of rapid ablation and perhaps the warmest climate of the late Mérida Glaciation. This resulted in the high velocity estimates calculateci within this unit. It is unlikely that the large clast sizes in this unit are a result of higher sedirnent availability. Flow till found in the northern section of the fan complex may represent a srnall advance of ice over the area from El Caballo valley, but an accurate date is unknown. No till from the general direction of La Mucuchache moraine cm be found on the fan leading to this sediment corresponding to this age (just before or just after the deposition of Unit 3). However. fabric analysis hmPed 1 (Mahaney and Kalm 1996) shows ice flow from south to north, indicating the ice originated fi-om La Mucuchache valley (see Figure 3.12). There are two partially buried moraines in Unit 1, just above and below Ped 16, suggesting ice came down across the fan sornetime near the end of the fans emplacement. Unit 2 lacustrine sediments mark the last ponding event in the cornplex. Silts and fine sands of this unit extend from the escarpment upslope to Ped 6. It is unknown if they can be found higher upon the fan. Ped 6 is located at a small escarpment winding fiom east to West across the surface of the fan. Geomorphic investigations suggest that this small ridge (approximately 2.5 m high), may represent the former shoreline of the last palæolake. However. an aeolian silt cap consisting of classical alluvial loess is found at this site. If water eroded the shoreline into this feature, and since it is impossible for silt to be deposited in such a vertical face, this may not be the shoreline. If it is, the loess must have been deposited before the lake formed, which is highly unlikely since the loess covers the lacustrine sediments at Ped 6 (see Figure 5.7). This 'shoreline', however, corresponds to the former elevation of the La Mucuchache and Mesa del Caballo moraines before the breach was created. This would suggest that as water filled the lake to this 'shoreline,' it would begin to ovefflow the moraines and gradually erode thern to create this outlet. Tectonic weakening may have accelerated this erosion. Stage 2 (Late Mérida), forth phase: The 1st fan deposition episode occurs sornetime after the disappearance of the palsolake that deposited Unit 2 glaciolacustrine silts and clays. Ice advances down the two basins and over the breach at the top of the complex. Recessional moraines on the fan surface, supported by till within Ped 16, support an ovemding of La Mucuchache moraine by the ice and advancement dom the fan ares- Melting of the ice resulted in the deposition of Unit 1 sand and gravel. The water was probably not too extensive as shown by low velocities and thin sand beds. Drainage would have been through the breach. Stage 3 (Holocene): A Iow energy environment has persisted since deglaciation. Mahaney and Kalm (1996) note that the escarpment is a result of Holocene Stream erosion. Aeolian silt deposition and development of Ah/Cox soils over the complex represent the major geomorphic activity over the las 12,000 years. Conclusion and Summary Chapter SUE 6.1 Conclsion and Summary 6.1 ConcIasion and Summary A proglacial environment, located north of La Mucuchache valley in the northem Venezuelan Andes, was examined to determine the palæohydraulic conditions under which it formed. The region consists of an outwash fan, showing morphological characteristics of an alluvial fan complex two lateral moraines of Late Mérida (Late Wisconsinan) age (La Mucuchache and El Caballo moraines), and an end moraine complex of pre-Mérida (pre- Wisconsinan) age (Mesa del Caballo). The fan preserves evidence of varying environmental conditions during the LGM, that cm be seen in the characteristics of fluvial, lacustrine and till sediments. The stratigraphy at the fan site shows evidence of six distinctive depositional episodes in the evolution of the complex. Sedirnents of glaciofluvial origin were found to be the major constituents of the fkn, confirming an outwash origin, with lacustrine silts and clays deposited during times of lower, or absent, fluvial activity. Sediments consisting of large gravel and small boulders, suggest flood flows during either high ablation periods, orj6kuhlaups spawned by sudden release of meltwater fiom the palaaoglacier. A pattern of more consistent flow was responsible for the smaller clasts and sand matrix matenal being deposited within charnels. This is confirmed by the presence of bedforms within sand and srnaIl gravel deposits. Prolonged flows then created a succession of Iakes at the bottorn of the feature. Instnunental neutron activation analysis was used to study the provenance of sediment fiom two source areas. This analysis showed that the bedrock composition of the area is relatively consistent, with the exception of variations in Na and did not conclusively identify which watershed (La Mucuchache to the southwest or El Caballo to the east) was the main source for the glaciofluvial sediment. Analysis of the chondrite-normalized concentrations of the rare earth elements (REEs) within La Mucuchache moraine, El Caballo moraine and the g1aciofluviaI sediment, showed little diffaence between the three sets of sediments. Geomorphologically, La Mucuchache moraine exhibits a breach near the apex of the fan, resulting fiom either meltwater erosion or glacial erosion, or both. Overrïding of El Caballo moraine is possible; however, a series of recessional moraines on the fan near the apex further suggests La Mucuchache valley as the source of the meltwater. Estirnating palæovelocities and other hydraulic parameters fiom on1y grain size can be very dubious. Based on theory, the observations and interpretations offered in this study, are only some of the possible explanations that could be made. With most sediment transport studies focussing on the actual process of transport, most researchers tend to lirnit their study of deposition. This, argues Bremand (1994), is one of the disadvantages of this type of study. The estimations of the palæoveIocity of the mehater flow down the outwash fan reflected the change in cIast size between sedimentary units. Palæovelocity estimations showed fluctuations over the surface of the fan and through a cross-section which can be attributed to a change in depth, bed surface resistance, slope, channel width and/or the pattern of braided streams (i.e. tributary influence). Due mainly to high surface gradients, flow was catastrophic during deposition of the largest clasts. Maximum palæovelocities were greater than 12 m s-l (at a depth of 1 metre) for flood deposited sediment whiIe various cross-bedded sand deposits suggested slower, yet still turbulent (as reflected by high Reynolds numbers) flow. A more detailed sampling grid would be required to identie the hydraulic geometry of any channels. In light of the assumptions and limitation outlined in chapter 1, if tme conditions of the flow were to be estimated, fiow models based on non-uniform flow would be needed. Research in this area has been Iacking, and experimental as wel1 as field work is needed in this area. SEM analysis implies that such flow regimes have minimal effect over short distances on the microtexture of sand grains in transport as either bedoad or suspended sediment. 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Appendix One Particle Size Distributions MATERML QOûOp - REPRESEKTATNE TEST SAMPCt MATERIAL QOOOp - REPRESENTATIVE TEST SAMPE SmEANALYSIS SEMMENIATK)(J ANALYSIS MATERLAL Qûûûp - REPRESENTA'TNETEST SAMM SlEVEANKYSlS SEoiMENTAm AMîYSIS MATERIAL ~2000~- REPRES ENTATIM TEST SAMPLE 0 SEVE ANALYSIS SEOIMENTATION ANALYSIS 2000 1000 500 250 125 ô3 312 15.6 7.0 39 1.95 OS0 0.4s 0.24 0.12 0.04 WENTWoRTH GRADE SCALE (MiCRONS) MATERIAL 4000~- REPRESENTATIVE TEST SAMPLE PED 15 - 0 1 ----- PED15-02 - PED15-Cox 1 - PED 15 - COX2 ------ 250 125 63 312 15.6 7.0 39 1.95 OgS 0.49 024 0.12 0.04 WENlWORTH GRADE SCALE (MICRONS) MATERIAL 4Wûp - REPRESENfATlM TEST SAMPLE SlEVE ANALYSIS SEDIMEMATION ANALYSIS PED 16 - 0 1 ------PED 16-02 PED 16 - COX 1 -- PED 16-COX2 PED 16 - COX3 ------1 PED 16-Cu --1 I WENTWORTH GRAOE SCALE (MICAONS) MATERIAL PED 17 - 0 2 ---- PED 17-COX 1 PED 17 - COX 2 -- PED 17 -CU 1 ------ i OB 0.49 0.24 0.12 O. WENTWO#TH GRADE SCALE (MiCRONS) MATERIAL ~2000~- REPRESENTATIVE TEST SAMPLE WENTWoRTH GAADE SCALE (MICRONS) Appendix- Two Heavy Mineral Concentrations There are many people who deserve th& for ail their help and support over the last two years. First, and foremost, sincere thanks to supervisory cornmittee members Dr. William C. Mahaney, Dr. André Robert and adjunct professor Michael W. Milner for their invaluable assistance in completing this thesis. First, 1 would like to thank Bill for allowing me to go to Venezuela and complete this research. Without Bill's advise in the field and in the laboratory, many of the face& of this thesis wouId have been overlooked. André's coments and suggestions helped me greatly in working out sorne of the problems 1 encountered dong the way and added considerable substance to the final ciraft. Mike offered considerable advise and comments on previous cirafts of this thesis and provided much needed assistance with the heavy mineral and geochemical aspects of the research. Thank you al1 for the help along the way. There are numemus people I'd like to thank for their suggestions and help while in Venezuela. Dr. Maximilliano Bezada (Deparnent of Earrh Sciences, Universidad Pedogogica Erpenmental Libertador, Caracas, Venezuela) and Dr. Volli Kalm (Institute of Gedogy, Tartu University, Tartu, Estoniu) were a great help in the field and kindly offered assistance when asked. Special th& also to Dr. Marek Zreda (Department of Hydroiogy and Water Resources. University of Arizona) for the interest he took in my studies and trying to keep that fire going at night! Dr. Chalmers Clapperton (University of Aberdeen. Aberdeen, Scotlana') also offered many helpful suggestions about field methods. 1 would also like to thank Dr. Ron Hancock and Ms. Susan Aufreiter for their help at the SLOWPOKE Reactor Facility at the University of Toronto. Ms. Mary Lou Ashton, of the Biology Department, York University, was a great help with the SEM. Ms. Janet Allin, Ms. Carol Randal1 and Ms. Carolyn King (York University) drafied the figures and maps and receive a well-deserved thank you. 1 thank Mr. John Dawson (York University) who produced the great photos. I would also like to thank Dr. Martin Kellman, Director, Department of Graduate Geography, York University, for the tremendous help he has given me over the past year. The completion of this thesis was greatly assisted by his efforts and 1can't express how appreciative 1 am for al1 his help. Alongside Dr. Kellman, 1 would also like to thank Dr. Alan Hill and Dr. Kathy Young, Department of Geography, York University, for taking tirne out of their busy schedules to help me along the way. Financial support for this project was supplied by the Facuity of Graduate Studies, York University and the Graduate Student Association, York University. I would like to thank both for the support. Many of my fkiends here at York, and beyond, are worthy of many thanks. It would have been a lot more difficult without them. Finally, I'd Iike to thank my family for al1 their support, and Paîricia Robinson and her 'dirty little hands,' for al1 her help, both with the sieving and the final production of the thesis. IMAGE EVALUATION TEST TARGET (QA-3) 1 SOmm APPLIED A IMAGE. lnc - 1653 East Main Street ----. - Rochester. NY 14609 USA ---- Phone: 71 W82-0300 ------Fax: 71 6/28&5989