Raised channel systems as indicators of palaeohydrologic change: a case study from Oman

Palaeogeography, Palaeoclimatology, Palaeoecology, 76 (1990): 241 277

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Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Raised channel systems as indicators of palaeohydrologic change: a case study from Oman JUDITH MAIZELS Department of Geography, University of Aberdeen, Aberdeen AB9 2UF (U.K.) (Received August 14, 1989)

Abstract Maizels, J., 1990. Raised channel systems as indicators of paleohydrologic change: a case study from Oman. Palaeogeogr., Palaeoclimatol., Palaeoecol., 76: 241-277. The aim of this paper is to determine the extent to which "raised" channel systems may be used as indicators of longterm paleohydrologic change, and hence of broader paleoenvironmental changes, in arid areas during the Cenozoic. The paper reviews the paleohydrologic significance of former drainage systems in arid areas, and demonstrates the application of paleohydrologic methods to the analysis of the long-term evolution of a complex series of exhumed, multistorey, raised channel systems in interior Oman. The paper discusses the range of possible autocyclic and allocyclic controls on changes in channel behaviour through time, as a means of establishing the paleoenvironmental significance of observed changes in paleochannel pattern, morphology, sedimentology and stability. The paper identifies some of the major allocyclic controls, including climatic changes, and particularly the seasonality, magnitude, frequency and distribution of rainfall events during the Cenozoic, together with the degree of continentality and hence the strength and persistence of arid wind systems. Base-level changes, due to eustatic fluctuations and/or tectonic activity, may also act to control long-term patterns of aggradation followed by large-scale landscaping lowering. However, long-term fan ageing associated with progressive aggradation and steepening of paleochannel systems, may also eventually lead to fanhead trenching, independent of any external controls. In addition, progressive expansion of the drainage network and resulting river capture, can lead to marked increases in water and sediment supplies to the fluvial system. The paper also emphasizes the difficulties involved both in differentiating the wide range of possible controls on paleohydrologic response, and also in developing an accurate and precise chronology for desert paleodrainage systems, and in their correlation with non-fluvial sedimentological sequences. The raised channel systems of interior Oman consist of heavily desert varnished chert-and/or ophiotite-rich gravels, dating from some time during the Plio-Pleistocene period. The paleochannels form sinuous gravel ridges rising < 30 m above the surounding plains. At least 12 successive generations of superimposed paleochannel systems have been identified, representing major periods of fluvial activity, each of varying duration and character. The earlier generations form typically broad, sinuous, single-thread channels, comprising fine-grained, chert-rich gravels, emented by clear crystalline calcite, and associated with bankfull paleodischarges of up to 1400 m 3 s ~. The later generations become increasingly narrow, steep and less sinuous, with single-thread distributary channels which narrow rapidly downfan. The gravels are coarser grained and dominated by increasing concentrations of ophiolites. The latest stage is associated with fanhead entrenchment, and formation of terrace gravel sheets, poorly cemented by CaCO~, and dominated by ophiolites. Peak paleoflows may have exceeded 13,000 m 3 s - 1. The observed changes have resulted from a complex combination of both internally and externally generated environmental changes during the Cenozoic. Progressive extension of the drainage network on to the ophiolite massif of the m o u n t a i n source area resulted in river capture and a concomitant increase both in paleodischarges and in supplies of ophiolitic sediments to the paleodrainage system. The early sinuous channels may have developed under a humid to sub-humid climate, in which perennial or seasonal flows allowed the distinct meander paleochannels to form. Ephemeral flows, characterized by flash flood regimes, were probably associated with the later distributary systems, which may reflect semi-arid to arid climatic conditions. The large-scale deflation of the interfluve sediments, which resulted in the eventual exhumation of the cemented, gravel-capped, multistorey paleochannel sequence, is also likely to have occurred during periods of marked continentality, probably during periods of low sea-level associated with the Quaternary glaciations. 0031-0182/90/$03.50

~b 1990 Elsevier Science Publishers B.V.

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Introduction Major paleodrainage systems have been identified in a number of arid regions, indicating that at some time in the past, surface runoff was on a sufficiently large scale to form river networks and to recharge deep groundwater aquifers. The analysis of these paleodrainage systems can therefore be used to infer not only the former hydrological conditions associated with river flows, but also the nature of the discharge regime, sediment supply and catchment conditions, regional paleoclimatic conditions, and associated global environmental conditions. This paper reviews the paleohydrologic significance of former drainage systems in arid areas, and proposes a framework for the analysis of raised channel systems in particular. The study also demonstrates the application of a range of paleohydrologic methods in assessing the likely hydraulic, hydrologic and climatic conditions associated with the longterm evolution of an alluvial paleodrainage system in a hot desert area. A major problem arising in such analyses of fluvial paleohydrology in arid environments, however, is the development of a precise chronologic framework. The case study is taken from a complex series of exhumed, superimposed paleochannel systems on an extensive piedmont alluvial fan in interior Oman. Paleohydrologic changes identified during progressive fan growth are interpreted in terms of likely climatic and catchment controls on fan hydrology during the Cenozoic, but no precise chronologic control is yet available.

Paleodrainage systems in arid areas and their paleohydrologic significance Evidence of paleodrainage systems and paleochannel deposits has been described from a number of desert areas, ranging from Australia (eg. Schumm, 1968; Van de Graaf et al., 1977) to Arabia (Kassler, 1973; Hotzl et al., 1978a,b; Anton, 1984) and northern Africa (Adamson et al., 1980, 1982; McCauley, 1988), to the arid southwestern U.S.A. (Reeves, 1983). The sig-

J. MAIZELS nificance of these paleodrainage systems in terms of indicating more humid climatic conditions at some time in the past, and their association with changing river courses, has been widely recognised. However, few such studies have attempted to explore the link between paleochannel characteristics and possible changes in hydraulic, hydrologic and climatic conditions over the recent geological past (i.e., the past 3 million years) (e.g. Adamson et al., 1980). Such an approach demands more than just a description of paleochannel sedimentology, morphology, pattern and stratigraphy. Analysis of fluvial paleohydrologic change in desert areas also requires an understanding of present-day relationships between climatic and catchment conditions on the one hand and runoff and channel characteristics on the other hand. These relationships need to be established for both semi-arid and sub-humid river systems, since it is these river systems that are likely to be represented in the paleodrainage record of the arid lands. The dominant catchment control on drainage development is the precipitation regime. Many present-day drainage systems in desert areas are subject to rare, high intensity storm events which generate flash floods. These are characterised by sharply rising hydrograph peaks associated with rapid runoff from poorly vegetated slopes and thin rocky soils, and are accentuated in areas of high relief (e.g. Schick, 1988). The runoff events in arid areas tend to be short-lived, transporting large volumes of washload and bedload sediments (Graf, 1988). Sedimentation is concentrated in distal reaches, reflecting the downstream decrease in river flow with increasing transmission losses through the bed and banks of the channel. Channel systems commonly comprise low sinuosity, coarse-grained "wadi" courses, often braided and forming distributary systems that peter out downstream (e.g. Glennie, 1970; Riley, 1977; Graf, 1988). However, many desert paleochannel systems exhibit highly sinuous, extensive or persistent drainage courses that are likely to reflect more continuous flow

RAISED ('HANNEL SYSTEMS AS INDICATORS OF PALAEOHYDROLOGIC CHANGE

regimes, and hence a more seasonal or even perennial precipitation regime in the past (e.g. Adamson et al., 1980). Similarly, the coarsegrained sediments associated with many modern wadi deposits may contrast with the finergrained materials derived from more extensively vegetated catchment slopes in the past, leading to differences in stream sediment loads, channel bed and boundary materials, channel morphology and channel pattern (eg. Adamson et al., 1980). Hence, present-day relationships between (a) runoff and sediment supply, and (b) channel characteristics, may be extrapolated to the analysis of paleochannel systems, as a means of assessing likely former climatic conditions and extent of vegetation cover in the catchment (cf. Street and Grove, ]976; Adamson et al., 1980; Graf, 1988). The direct interpretation of paleohydrologic conditions from present-day relationships, however, is fraught with problems. The interrelationships are highly complex, and rely on the interaction of a large number of variables whose magnitudes are generally unknown or only poorly known in any paleochannel system. The paleohydrologic significance of former drainage courses in desert areas can be established only where all the major likely controls on channel behaviour through time can indeed be identified. These controls may be dominated by climatic factors such as the precipitation regime, but many other controls have been cited in published analyses of paleochannel change (cf. Alexander and Leeder, 1978; Baker, 1978; Heward, 1978; Flores and Pillmore, 1987; Kraus and Middleton, 1987). These controls fall into two main groups: allocyclic or external controls, and autocyclic or internal controls. In addition, the controls on preservation of the paleochannel deposits themselves, are also significant in any paleoenvironmental reconstruction from alluvial deposits. The possible effects of the main allocyclic and autocyclic controls on a range of paleochannel characteristics, and on the degree to which paleochannel deposits may be preserved in the geological record, are summarised in Fig.1.

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Figure 1 indicates that while the seasonality, magnitude, frequency and intensity of rainfall events play a significant role in controlling river runoff and sediment supply to the river, changes in runoff and sediment input to a river system can also be instigated through changes in channel and catchment gradients produced, for instance, by regional or local tectonic uplift, subsidence or tilting, or by global eustatic changes. Steeper gradients, for instance, lead to increased runoff energy, resulting in channel incision and increased rates of sediment removal to distal positions of the system. Such changes in river behaviour may therefore occur quite independently of any climatic fluctuation. Similarly, major channel changes can be induced by controls operating internally within the fluvial system (e.g. Schumm et al., 1987). Long-term fan aggradation, for example, can result in the progressive increase of channel gradients, and hence in stream power, sediment transport, erosion and, eventually, in overall incision of the fan (cf Harvey, 1987). River discharge within the system can also be increased independently of climatic controls, by progressive expansion of the drainage network, either through integration of a newly established system or through headward growth and river capture (e.g. Eckis, 1928; Schumm et al., 1987). Channel changes may also occur through local migration, avulsion, bed scour and fill, or bank erosion or accretion. Although such local changes may represent a response to climatic, tectonic or eustatic controls on flow, stream power or sediment transport, they are more likely to reflect channel response to thresholds of local sediment transport, bank stability, or channel geometry. Longterm processes of sediment accretion or removal; channel erosion, migration and avulsion; and subaerial weathering and geochemical alteration by groundwaters, all contribute to the differential burial, preservation, exposure and exhumation of former channel and floodplain deposits. Maximum preservation potential will be achieved by deposits that are subject to rapid burial resulting from high

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RAISED C H A N N E l SYSTEMS AS INDICATORS OF PAI.AEOHYDROLOG1C CHANGE

rates of sediment accretion. In particular, frequent overbank flooding by flows rich in suspended sediments are likely to result in rapid floodplain and channel aggradation, with relatively little erosion of former floodplain deposits. Conversely, fluvial deposits are least likely to be preserved within an unstable, degrading system with shifting channels that erode sediments from both the channel and floodplain. Erosion of buried deposits by fluvial or eolian activity can act to expose paleochannel and floodplain sediments, releasing them for re-working or removal. Where paleochannel deposits have been cemented by calcite or silica-rich groundwater precipitates, these deposits may be selectively preserved and form lines of inverted drainage (eg. Reeves, 1983). Changes in long-term stability of the drainage system produced by alternating periods of' erosion and aggradation will be reflected in the alluvial architecture and stratigraphy of the deposits, as well as in the differential preservation of the fluvial sediments. The interpretation of paleodrainage systems in desert areas must therefore reflect the interaction between a large number of global, regional, catchment and local controls on river behaviour, and the observed paleochannel characteristics. The approach to paleohydrologic analysis of desert paleodrainage systems must consequently rely on the assumption that a number of different mechanisms might. account for observed variations in channel pattern, morphology, sedimentology, and longterm system stability. P a l e o h y d r o l o g i c f r a m e w o r k for a n a l y s i s of raised c h a n n e l s y s t e m s ~Raised" channel systems are alluvial stream deposits t h a t have been cemented and differentially exposed by weathering and erosion to form a series of upstanding linear ridges and sheets representing an area of inverted drainage. Raised channel deposits have been identified in a number of desert areas, where they have been described under a variety of different terms. Miller (1937)

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described raised channels in eastern Saudi Arabia as "suspendritic drainage lines" that stood out in "bas-relief' above the surrounding plains, while Knetsch (1954) employed the unfortunate term "pseudo-esker" to describe the sinuous gravel-ridge features of the Dakka Basin in Saudi Arabia. Other workers have described these "gravel-capped ridges" in the western Transvaal (King, 1942), as "gravel trains" (Holm, 1960; Brown, 1960 in Beydoun, 1980), as "perched wadis" or ~'wadi ridges" in Egyptian Nubia (Butzer and Hanson, 1968), as "suspenparallel drainage" in western Texas and New Mexico (Reeves, 1983), and as ~raised channels" (Warren et al., 1985). Less ambiguous definitions have been provided tbr the vast paleochannel systems which extend across the piedmont alluvial plains of interior Oman. Glennie (1970) describes these paleochannels as ~ridges of exhumed Pleistocene (?) wadi gravels", and Beydoun (1980) as ~exhumed or fossil river channel systems". In this study, the terms "raised channels", ~exhumed channels', and '~paleochannels" are used to describe the Omani paleodrainage systems. However, the Omani paleochannel networks are far more complex than those described elsewhere. The paleochannel courses represent more than one period of fluvial activity. Indeed, over 12 separate generations of paleochannel deposits can be identified, such that the paleochannel ridges intersect with one another with one another and successive paleochannel deposits are superimposed on one another. Hence, the "raised channels" comprise superimposed sequences of successive paleochannel deposits, differentiated from each other by contrasts in elevation, morhology, sedimentology, lithological composition and degree of weathering (see below). The successive generations of paleochannel deposits have been exposed at the surface by progressive exhumation, revealing a complex sequence of multistorey, linear, gravel deposits, each of varying character and representing different paleohydrologic conditions (cf. the '~multistorey" ribbon sandstones of the Oligocene-Miocene in the Ebro basin, Spain

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(Puigdefabregas and Van Vliet, 1978; Friend et al., 1981)).

Approaches to paleohydrologic analysis Paleohydrologic analysis of ancient river systems is based on three fundamental assumptions. First, the basic premise is t h a t the individual elements of a fluvial system represent a direct response to environmental conditions (Fig.l) and are in equilibrium with those conditions. Hence, in paleohydrologic analysis it is assumed that measures of paleochannel characteristics, such as channel sedimentology, morphology, pattern and network parameters (Fig.l), all reflect the response of the fluvial system to local or regional catchment conditions. The second major assumption is t h a t a change in environmental conditions will be of a sufficient magnitude, or of a particular form, to produce a significant and identifiable change in the fluvial system. For example, an increase in aridity may be reflected by a change from seasonal or perennial flows that transport fine-grained sediments from wellvegetated catchment slopes, to a more flashy flow regime involving the transport of large amounts of coarse material by rapid runoff and erosion of poorly vegetated slopes. Major changes may then be observed in paleochannel sedimentology (e.g. sequences become increasingly coarse-grained, with flood facies separated by greater thicknesses of eolian facies), morphology and pattern (e.g. becoming less sinuous, less persistent downstream, and exhibiting higher gradients and width/depth ratios). However, the record of many hydrological events may be absent from the stratigraphic record, having been buried or subsequently removed by erosion. The final assumption, arising from the first two assumptions, is that identifiable changes in the characteristics of paleochannel systems can be used as a reliable means of estimating changes in a wide range of hydraulic, hydrologic and broader environmental conditions. Paleohydrologic analysis therefore involves a series of interpretive procedures; these are

J. MAIZELS

based on empirical relationships and theoretical models t h a t have been derived from present-day fluvial systems. These relationships and models are, in turn, presumed to represent fluvial systems that are directly analogous to the ancient fluvial systems being studied. The environmental conditions under which many paleochannel systems have formed, and in particular raised channel systems such as those in Oman, remain uncertain. Thus, this interpretive stage of paleohydrologic analysis is liable to considerable error where procedures originally derived for present-day rivers, are inappropriately applied to paleochannel systems (cf. Williams, 1984).

Paleohydrologic indicators derived from raised channel systems Several important questions arise in the paleohydrologic analysis of raised channel systems in desert areas. First, the hydrologic conditions under which channel formation occurred need to be established. The likely mean annual, bankfull, or peak flows need to be estimated, together with some assessment of the annual flow regime. Of particular value is the prediction of whether flows were perennial, seasonal or ephemeral in occurrence, as their relative duration is a critical indicator of paleoclimatic conditions. In addition, the nature of sediments being transported through the system need to be determined, as their composition, texture, facies type and geometry can provide important indications of (a) former source materials and transport routes, (b) stability of catchment slopes and patterns of production of sediments for stream transport, (c) the nature of stream sediment loads and associated channel types, and particularly whether the streams were suspended-load or bed-load types, and (d) and the patterns of downstream change in stream behaviour in response to changes in gradient, sediment size, and stream power. Second, the mechanisms of channel change within the former fluvial systems need to be identified. These changes may extend both

RAISE[) CHANNEL SYSTEMS AS INDICATORS OF PALAEOHYDROLOGIC CHANGE

spatially over the whole river basin, from source to mouth, and temporally, as vertical variations in observed paleochannel characteristics. Downstream changes in sediment size and sorting, and in paleochannel geometry, dimensions and morphology, can provide important clues about the processes of sediment sorting (eg. Frostick and Reid, 1980), and downstream changes in stream discharge. In many fluvial distributary systems in arid and semi-arid areas, discharge decreases significantly downstream, and flows may even become non-existent, as a result both of transmission losses into the channel bed and/ or repeated distributary offtakes (e.g. Riley, 1977). Hence, downstream changes in selected hydraulic and sedimentary parameters can reflect the relative degree of catchment aridity during development of the river system. Vertical changes in alluvial sedimentology are particularly significant in the analysis of long-term paleohydrologic change and system stability. Vertical changes in facies type, channel facies locations, and channel facies dominance within the stratigraphic sequence can indicate relative stability in channel pattern and form, and in location of the active zone, during long-term fluvial sedimentation. In addition, these factors can reflect the relative rates of flood-plain and channel accretion, the relative frequency of erosional episodes, and the significance of periods of longer term stability associated, for example, with paleosol development or accumulation of eolian sands or loess. Third, the processes of preservation, burial, exposure and exhumation can themselves reflect a complex sequence of post-depositional hydrological and climatic conditions. Cementation of fluvial deposits, for example, is likely to occur during periods of relatively high water tables, associated with rapid rates of evapotranspiration from subsurface waters, and precipitation of mineral salts within the gravel sequence (e.g. Glennie, 1970; Stalder, 1975; Goudie, 1983). According to James (1985), the distribution of carbonate cement in Quaternary gravels in Indiana, U.S.A., is limited to

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within 7 m of the ground surface, although this may vary with angle of bedding, grain size and sorting, and composition of the framework grains. Cementation of deposits enhances their resistance to erosion, and particularly to deflation, promoting the development of inverted relief. Burial of fluvial deposits may be achieved by a number of processes, including further accretion of channel or overbank facies, or deposition of eolian sediments if channels have been abandoned or if the climate has become increasingly arid. Exposure or exhumation of fluvial deposits may also be accomplished by fluvial, eolian or marine activity. Accelerated fluvial erosion may result from tectonic uplift, fan entrenchment, increased humidity, rainfall intensity or duration, or depleted sediment supplies. Exhumation by eolian deflation of fine-grained and/or non-cemented sediments is likely to occur during periods of increased aridity. Finally, the autocyclic controls on channel change need to be identified as far as possible, and differentiated from the likely allocyclic controls. The differentiation of the controls on observed paleohydrologic changes within an ancient fluvial system presents one of the major difficulties in any study of raised channel systems, partly because of the complex interaction between a wide variety of possible controls (Fig.l), and partly because of equifinality in the system (e.g. Heward, 1978; Kraus and Middleton, 1987). Thus, changes in runoff and sediment supply can be produced by a range of different controls, both allocyclic and autocyclic in nature. In most large and complex fluvial systems, however, major hydrologic changes are likely to reflect a combination of various controls, operating interdependently but exerting responses over different timescales and with varying impact in different parts of the system (e.g. Schumm's, 1977 "complex response" phenomenon).

Description of paleochannel systems Paleohydrologic analysis of former channel systems relies on the initial identification and

248 description of 5 main categories of paleochannel response to allocyclic and autocyclic controls. These categories are: the nature of the paleochannel network (e.g. channel extent and density; distributary development); paleochannel pattern (e.g. degree of channel multiplicity and sinuosity; channel wavelength, amplitude, and degree of meander curvature); paleochannel morphology (e.g. channel width and width/depth ratio; cross-sectional shape; and gradient); fluvial sedimentology (e.g. sedim e n t size, bedding structures, and facies type; particle shape and composition; fabric); and sedimentary architecture and stratigraphy (e.g. vertical and lateral facies distribution; stratigraphic relationships with non-fluvial sediments). The full range of relevant descriptive parameters for paleochannel systems are given at the bottom of Fig.1. The detailed rationale for selection of these variables, and the problems associated with their measurement and interpretation are discussed more fully elsewhere (Maizels, in press b). A wide range of methods is required in order to obtain descriptive measures for the many paleochannel parameters listed. These methods include the mapping of paleochannel networks, channel patterns and planform variations uaing satellite imagery, aerial surveys and field mapping techniques (depending on the scale of the paleochannel system itself). Sedimentological analysis is crucial for understanding the nature of paleohydraulic conditions, the sources and relative volumes of sediment supply to the system, the pathways of sediment transport, the degrees of weathering, and the changes in these factors through time. Facies and lithofacies types, bedform assemblages, particle size variations, and sedimentary architecture, together with morphological data, can be used as inputs both to facies models, and to paleohydraulic models (see below) for estimating paleoflow conditions, and particularly paleodischarge and sediment transport rates. In addition, stratigraphic surveys are vital for determining the alluvial architecture, the interrelationships between different facies types and with deposits of

J. MAIZELS different origin, and the longer-term sedimentological history of the fluvial system. A further essential element in any study of paleohydrologic change is the development of a precise chronology at the required temporal resolution over the relevant timescale. Numerous methods of establishing an absolute and/or relative framework for dating paleochannel systems have been described in the literature (e.g. Starkel and Thornes, 1981). In desert areas, however, dating techniques may be limited to analysis of archaeological remnants; development of desert varnish and desert pavement; degrees of relative weathering, and extent of soil development; paleomagnetic dating, and, rarely, 14C dating, and possibly TL dating of eolian sediments (e.g. Hunt and Mabey, 1966; Shlemon, 1978; Christenson and Purcell, 1985; Rutter, 1985; Gerson and Amit, 1987; Grossman and Gerson, 1987; Kraus and Brown, 1988).

Paleohydraulic interpretation of former channel systems The paleohydraulic interpretation of former channel systems involves the application of numerical models based on empirical and theoretical relationships established for present-day rivers that allow calculation of a range of former flow parameters. Despite the many problems associated with the derivation and application of these models to ancient river deposits, many models do appear to provide estimates to well within one order of magnitude of such paleoflow parameters as flow depth, mean flow velocity (and hence Froude number), bed shear stress, bankfull or mean annual discharge, and peak discharge (e.g. Ethridge and Schumm, 1978; Williams, 1984, 1988; O'Connor and Webb, 1988). Paleoflow parameters, and particularly former river discharges, can be estimated in a number of ways. Details of the methods, assumptions and problems associated with the different techniques have been dealt with in some detail elsewhere (Ethridge and Schumm, 1978; Andrews, 1983; Maizels, 1983, 1986, in

RAISED CHANNEl, SYSTEMS AS INDICATORS OF PALAEOHYDROLOGIC CHANGE

press b; Williams, 1984, 1988). The two most commonly adopted approaches are first, that based on hydraulic-sediment relations in rivers, and second, that based on empirical morphological relations for single-thread, sinuous channels. The first approach is based on initial estimates of the critical shear stress at which bed particles begin to move. This computation uses Shields' (1936) function, modified for differential entrainment of mixed bed sediments (eg. Andrews, 1983), and allows subsequent calculation of former flow depth using the Du Boys equation, and flow-resistance and velocity using the Colebrook-White, Keulegan, and Darcy-Weisbach-type equations (e.g. Maizels, 1983, 1986; Church et al., in prep.). This approach requires input measurements of the Ds0 and Ds4 percentiles of particle size distribution, as well as of former channel gradients and flow widths. The main assumptions of this critical shear stress approach are the former existence of steady uniform flow; the validity of the shear stress model itself, and the impact of packing and imbrication on the value of Shields coefficient; the absence of local bedform or coarse particle clusters; and the availability of all particle sizes for transport by competent Newtonian fluid flows (e.g. see Costa, 1984). The second approach is based on the extrapolation of empirical discharge-form relations established for samples of present-day meandering rivers, to sinuous paleochannel systems. These relations require determination of meander wavelength, radius of curvature, and bankfull channel width. Hence, major errors arise where former channel morphology is only partially or poorly preserved (eg. Ethridge and Schumm, 1978; Rotnicki, 1983), and in the application of established relations to channels that are likely to have formed under significantly different environmental conditions (e.g. relations for sand-bed channels applied to gravel-bed paleochannels). The sources for the relations adopted in this particular study in the prediction of paleoflow parameters for the raised channel systems in Oman are summarised in Table II.

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Paleohydrologic analysis The use of paleochannel characteristics as indicators of more general hydrologic and climatic conditions within the former catchment is far more problematic than the interpretation of former stream flow conditions. For example, no reliable empirical relationships have yet been established for predicting catchment runoff response, sediment yields, or precipitation regime. Schumm (1965) has presented generalised relations between annual precipitation totals, mean annual temperature and annual sediment yields on a global basis, but these relations cannot be used for accurate prediction of catchment hydroclimatology from paleochannel systems. Channel pattern and sediment type have been used more widely to suggest likely catchment conditions. Semi-arid regions, for example, with sparse vegetation cover and subject to intense rainfall events produce high sediment yields, which are commonly transported within broad, shallow, braided channel systems (e.g. Graf, 1988), while areas with a more equable climate and well vegetated slopes yield only fine sediments forming cohesive banks and narrow, sinuous, single-thread channels (e.g. Schumm, 1960). However, as yet no greater accuracy can be achieved in the prediction of paleo-environmental conditions within the catchment, and only general inferences can be proposed at this stage.

Raised channel systems in Oman: evidence for long-term paleohydrologic change

Geological background The raised channel systems extend across vast, low gradient alluvial fans, for a distance of up to 250km from the Eastern Oman Mountains southwards towards the Arabian Sea (latitudes 21-23~'N) (cf. Gezira plains, Adamson et al., 1980, 1982). The mountains comprise a central ophiolite massif (upper Semail), flanked in the south by low, W E trending ridges of limestones and cherts (lower

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Hawasina) (Glennie et al., 1974; Hopson et al., 1981). Narrow gaps have been cut through these ridges by the south-draining wadis Adam (west), Andam and Matam (east) (Fig.2). The main source rocks for the raised channels therefore include a wide variety of cherts and limestones from the southern ridges, and basics and ultrabasics from the more northerly ophiolite outcrops. The fan deposits overlie the continental Mio-Pliocene "Fars" Group to thicknesses of over 280m in the north, and Miocene limestones in the southwest. These stratigraphic relations indicate that the earliest possible date for commencement of gravel accumulation was early Pliocene. The latest date of formation is defined by the age of worked chert lithics lying on the older, lower paleochannel ridges. According to Edens (in press) the artefacts are likely to date from between 4 and 7 ka, representing the latest period during which paleochannel exhumation could have taken place. This study concentrates on the Wadi Andam drainage system which, unlike any other drainage system in the southern piedmont zone, extends northwards through the flanking limestones and cherts of the Hawasina to drain the ophiolites. The Wadi Andam system extends for over 250 km, and drains an area of c. 4500km 2. The catchment currently experiences an arid climate, with rains only once every 3-6 yr which produce rapid runoff and flash flood of up to 1000 m 3 s 1 (Curtis, 1985; Maizels and Anderson, in press). The present Wadi Andam forms a low sinuosity, broad (<700 m wide) channel, bounded by terrace gravels and cliffs of the Barzaman Formation. The wadi floor comprises extensive transverse gravel bars, traversed by a narrow, more sinuous, silt-mantled, thalweg path. Maximum flow depths during flash floods rarely exceed 2 m. (Maizels and Anderson, 1989).

Chronologic framework for evolution of raised channel systems N o dates are yet available for the oldest paleochannel systems, but the high degree of

J. MA|ZELS

surface weathering and desert varnish development, the extent of chemical alteration of the ophiolites, and the large volume of material (30-40 km 3) removed by deflation from the fan suggest that these deposits are likely to date from the Pliocene (cf. HStzl et al., 1978a,b; Anton, 1984). Anton (1984) suggests that in parts of the Arabian peninsula, savannah and forest vegetation prevailed during the PlioPleistocene, associated with considerably more humid conditions than those of today. The hypothesis of seasonal river flows and high water tables is further supported by the generally more humid climates within the (now) arid tropics, and global high sea levels during the Pliocene, and during Pleistocene interglacials (e.g. Van Campo et al., 1982). The formation of the youngest raised channels, the period of entrenchment, and extensive land-surface lowering by deflation, appear to date from later episodes of falling watertables and increased aridity. These paleochannel and landscape features may therefore have formed during arid and semi-arid phases of the midand late-Pleistocene. In the Arabian peninsula, the mid-late Pleistocene (1.1 M a - 2 5 k a ) appears to have been associated with alternating arid and semi-arid phases, giving rise to steppe and savanna vegetation, gravel accumulations and local torrential erosion (Anton, 1984). The latest period of landsurface lowering and paleochannel exhumation is likely to have been during the last glaciation (25-10 ka B.P.) (cf. McClure, 1978), when global sealevels were up to 130m lower than at present, winter northeast trade winds were stronger and more persistent, and aridity and continentality were markedly increased (Glennie, 1970; Street and Grove, 1979; Prell et al., 1980; Van Campo et al., 1982). Many of the gravels form a thin pebble lag at the surface and are underlain by fine-grained, loose, sands and silts. This subsurface horizon (B-horizon) is likely to represent the long-term accretion of wind-blown, atmospheric dust (Gerson and Amit, 1987). In the Negev, Gerson and Amit (1987) found that a gravel-free, siltrich B-horizon, comprising gypsum and salts,

251

RAISPA) C H A N N E l , S Y S T E M S AS INDICATORS OF P A L A E O H Y D R O I X ) G I C C H A N G E

UPPER

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Fig.2A. L o c a t i o n of r a i s e d c h a n n e l s y s t e m s on a l l u v i a l fans of i n t e r i o r O m a n , i n d i c a t i n g m a j o r c a t c h m e n t a r e a s of m a i n wadi s y s t e m s . B. P a l e o c h a n n e l s y s t e m s in t h e p i e d m o n t i n t e r i o r fan, B a r z a m a n area.

252 had reached a thickness of 5-40 cm on surfaces exceeding 50 ka in age. However, data from the Mohave suggests t h a t B-horizon thicknesses of < 1 m have developed in the past 20 ka, representing high rates of accretion in an area located directly downwind of a sediment-rich playa. Measurements of B-horizon thicknesses in the subsurface segments of the Oman paleochannel deposits indicate highly variable amounts, ranging between 5cm and 1.1m, although commonly averaging between 10 and 50 cm. Using Gerson and Amit's estimate of minimum dust accretion rates in the Negev (0.001-0.003 mm a 1), a B-horizon thickness of 1.1 m could require between 0.37 and 1.1 Ma to develop. Hence exhumation of the gravels could even have been completed well before the last glaciation. The presence of human lithics (dated at 7-5 ka B.P.) mantling some of the lowest raised channel ridges (i.e. amongst the last to have been exhumed), indicates that paleochannel exhumation had certainly ceased by the early to mid-Holocene. Rates of paleochannel exhumation appear to have diminished rapidly by the early Holocene, in response both to the decreasing strength and persistence of winds, and to increasing humidity associated with global warming and rising sea-levels (cf. Gardner, 1986). Rates of deflation in the later Holocene have probably remained relatively low, compared with those of full glacial times. Indeed, the high degree of desert varnish development on the oldest paleochannels suggests t h a t most exhumation is likely to have been completed much earlier during the Pleistocene.

Channel systems and channel patterns Detailed morphological mapping from aerial photographs (at scales of 1:100,000 and 1:30,000) (Maizels and McBean, in press), combined with stratigraphic investigations of sediment sequences at a number of paleochannel intersections, has allowed identification of a succession of at least 12 major paleochannel generations exposed on the piedmont fan

J. MA1ZELS which extend southwards for over 200 km from the Eastern Oman Mountains (Fig.2). Although the older generations are the most poorly preserved, they appear to have been the most extensive, with paleodrainage lines stretching for tens of kilometers beyond those of the younger generations (Fig.3) (Table I). The later stages of drainage development therefore appear to have been more limited, possibly representing flows that diminished downstream. The maximum degree of paleodrainage inversion is c. 30m, where wadi incision has cut through both older and younger paleochannel generations at c. 50 km downstream (cf. Abrams et al., 1988). Local paleochannel relief progressively diminishes downstream to heights of only 1 2 m. The complexity of the channel network represented by successive generations also apears to increase with time. The number of active channel zones that appear to have been contemporaneous (i.e. formed during a single generation) increased from only 3 or 4 in the older generations (spaced c. 4 5 km apart), to 6 or 7 in the younger generations (spaced 3.5-4km apart) (Table I). In addition, the number of contemporaneous, active channel zones in these younger generations increases substantially in the downstream direction. This downstream increase reflects the progressive development of a distributary channel system, producing a series of distinct channel bifurcations associated with the fanning out of drainage radially from the single drainage outlet at the mountain edge (Fig.2) (cf. Harvey, 1989). By contrast, the youngest channel deposits in the system do not have distinct linear paleodrainage courses, but instead, form broad sheets and spreads, many of which bound the present wadi courses. However, these deposits extend for only c. 40 km beyond the drainage gap, at which point the drainage system curves towards the southeast and disappears into (and beneath) the present sand sea of the Wahiba desert. Schumm et al. (1987) found from experimental studies that fanhead trenches similar to this latest wadi system here were also typically curved.

> 60 > 60 > 60 > 60 > 60 > 60 > 60 ~ 40 ~ 40 ~40 ~40 40 ~ 30

1 2 3 4 5 6 7 8 9 10 11

1.17 1.21 pr ox 1.16 d i s t 1.47 1.09 1.20 1.18 1.09 1.07 1.04 1.07 1.07 1.07 1.09

Mean sinuosity

2.8 4.7 4.2 5.9 2.9 4.5 4.4 3.3 2.7 3.2 3.2

Mean meander wavelength (km)

1.3 3.6 1.9 3.4 1.2 3.2 2.3 1.5 1.3 1.2 1.4 > 6.5 > 6.5

Amplitude of m e a n d e r belt (kin)

~Sample d i s t a n c e s too s m a l l to be r e p r e s e n t a t i v e . (After Ma i z e l s , in pre s s b).

12

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Paleochannel generation

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881 2185

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136 105 102 241 164 164 140 122 75 34 23 866 1114

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Mean channel w i d t h (m)

Max. 2239 1142 4180 7055 1180 4649 1368 2883 2117 2750 2750 14440 30720

Min. 594 424 408 1249 485 483 553 1755 663 83 59 5750 9140

Bankfull paleodischarge e s t i m a t e s (m3s 1)

SH H H H SH SA A SA SH H SH H SA SA A SA A SA A SA A SA A

Paleohydrologic interpretation SH/H - subhumid/humid S A /A = s e m i a r i d / a r i d

M o r p h o l o g i c a l c h a r a c t e r i s t i c s of s u c c e s s i v e p a l e o c h a n n e l g e n e r a t i o n s , a n d t h e i r p a l e o h y d r a u l i c a l i n t e r p r e t a t i o n , O m a n r a i s e d c h a n n e l s y s t e m s

TABLE I

~T

254

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255

RAISED CHANNEl, SYSTEMS AS INDICATORS OF PALAEOHYDRO1,O(;IC CHANGE

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A distinctive feature of the paleochannel systems is that they are dominated by sinuous, single-thread channel courses. It seems particularly significant, however, that the older generations tend to be highly sinuous, with maximum sinuosities exceeding values of 1.7, while the younger generations tend to exhibit low sinuosity courses (values < 1.1) (Table I). This overall decrease in paleochannel sinuosity during long-term drainage evolution is reflected also in an overall decrease in meander wavelength from over 4 km to under 3 km, and in the width of the meander belt from 3 km to under 1.5 km (Table II). Multiple or braided channels appear to have existed only locally within the single-thread courses, while the youngest channel deposits (in the most easterly part of the system), exhibit more highly braided networks as they approach the edge of the Sand Sea.

Channel morphology A major feature of paleochannel morphology is the downstream narrowing of preserved channel widths in the later paleochannel generations. In generations 7-9, for example,

J. MAIZELS

proximal paleochannel widths are 1.2-2.6 times greater than distal widths. Many of these later paleochannels ultimately taper off and disappear downstream. The oldest channels are not only more persistent downstream, but also tend to be significantly wider, with proximal widths exceeding 300m compared with 60-175m in the younger generations (except for generation 8). The youngest spreads and sheets of fluvial deposits bounding Wadi Andam, for example, reach widths exceeding 2 km (Table I). A further significant contrast between the older and younger generations is that the former exhibit markedly undulating crestlines, while the younger raised channels and fluvial spreads follow smooth longitudinal profiles, apparently representing the original depositional gradient (cf. Figs.4 and 5). Calculations of former gradients from the longitudinal profiles over 8 10 km transects indicate that gradients varied between c. 1.5 and 2.3 m km -1 (at 50km downstream), but no clear relation is apparent with the relative age of the channels. These gradient values compare closely with the present wadi gradient of c. 1.9m km -1.

Fig.4. Proximal raised channel ridges of relatively young generation (Generations 11-12), comprising coarse-grained, rounded, ophiolite cobbles overlying the Barzaman Formation. (photo)

RAISE[) (!HANNEL SYSq'EMS AS INDICATORS OF PALAEOHYDROI~OGIC

259

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Fig.5. Distal raised channel ridges illustrating recent generation (Generations 10 11), ophiolite-rich, plane-bedded gravels capping thick sequences of the Barzaman formation. (photo)

Paleochannel sedimentology Facies types Six main facies types have been identified within the paleochannel deposits. Facies 1 4 dominate the older paleochannel generations, while the younger generations and gravel spreads are dominated by Facies 5 and 6. Facies 1 comprises fine-grained, large scale, trough cross-stratified, sand and gravel units up to 3 m thick, comprising individual sets of 15 cm to 1.6m thickness. The cross-bedded sets fine upwards from poorly sorted basal gravels, comprising well-rounded clasts < 20 cm diameter, to well-sorted fine gravels. Facies 1 sediments form the major component of the older paleochannel deposits (Fig.6). Facies 2 comprises thin units ( < 0 . 5 m thick) of poorly structured, massive, clast-supported cobble gravels, containing well rounded clasts <20 cm in diameter, but only weakly imbricated. Facies 3 comprises lenses of thin, horizontally bedded, medium sand, and occurs on only a local, minor scale. Facies 4 is the most extensive and thickest facies, which underlies all other facies types, and is locally

interbedded with lenses and channels of gravels. This facies comprises a whitish-pink, massive, indurated, fine-grained, dolomitic material known as the Barzaman Formation (Maizels, 1987, in press a,b). Much of the Barzaman Formation is rich in clay minerals, and exhibits reddish-brown and orange mottling, which Glennie et al. (1974) interpret as representing "ghosting" or the altered remnants of former ultrabasic clasts (Fig.7). Borehole evidence indicates that the Barzaman Formation may be up to 285 m thick (Aubel, 1983; Jones, 1986). Facies 5 dominates the youngest raised channel deposits, and comprises crudely bedded, poorly sorted, clast-supported pebblecobble gravel units up to 2 m thick. Clasts reach 0.5 m in diameter in the proximal deposits, are moderately well rounded, and locally exhibit a strong preferred fabric. Facies 6 consists of poorly sorted, planar and trough cross-stratified, clast-supported, gravel units at least 3 m thick. Individual sets range from 20 to 80 cm thickness, fining upwards from pebble gravels into fine gravel and coarse sand.

260

J. MAIZELS

Fig.6. Facies 1 sediments, indicating part of large-scale trough cross-bedded units, cutting into trough infill sediments, and comprising cemented sand, gravel and pebbles of varied lithology. Dark clasts are mostly chert. (photo)

Fig.7. Facies 4 gravels within the Barzaman Formation showing weathered ophiolite pebbles (mostly troctolites), "ghosting", and fine gravels in a silty, doolomitic matrix. (photo)

Clast size variations T h e r e are significant differences in the m e a n m a x i m u m clast sizes (measured from the b-axis of the 10 l a r g e s t stones w i t h i n e a c h horizon) b e t w e e n the older and y o u n g e r channel deposits, w i t h the y o u n g e r deposits being m a r k e d l y coarser. F o r example, gravels of the

older, m o r e sinuous, p a l e o c h a n n e l generations 1-5 a v e r a g e c. 7.7 cm in diameter, comp a r e d with 10.8 cm for g e n e r a t i o n s 7 9, at the same site (Maizels, 1989). Clast sizes also d e c r e a s e significantly d o w n s t r e a m , but the r a t e s of d o w n s t r e a m d i m i n u t i o n are concomit a n t l y m u c h lower in the older raised chan-

RAISED CHANNEL SYSTEMS AS INDICATORS OF PALAEOHYDROLOGIC CHANGE

nels ( ~ l . 0 1 m m km 1) than in the younger (~4.1 mm k m - 1).

Lithological composition Significant differences also occur in the lithological compositions of the raised channel gravels of different relative age. The relative proportions of the three dominant lithological groups in the paleochannel deposits, namely the ophiolites, cherts and limestones, change systematically through the alluvial sequence. The oldest paleochannel generations exhibit the highest concentrations of cherts, ranging between 65 and 9 7 0 , while the younger generations become increasingly dominated by the ophiolites, with chert percentages ranging between only 19% and 0% (the latter in terrace deposits bounding the wadi) (cf. Figs.8, 9). Changes in lithological composition also occur downstream of paleochannel intersections, where older gravels appear to have been incorporated into younger deposits. Significant spatial variations in lithological composition are also apparent, such that concentrations of limestones become particularly high (up to 80~o) in the most westerly and southerly gravels, west of Wadi Adam and south of Wadi Matam, respectively (Fig.2).

Degree of cementation and weathering All the sand and gravel units of the raised channel deposits have been cemented by calcite, but the nature of the cement differs significantly between the older and younger gravels. The older gravels are cemented by clear, sparry calcite, which forms crystalline crusts around individual clasts and represents the main matrix material in the gravel (Fig.10A) (cf. Moseley, 1965; Vondra and Burggraf, 1978). The presence of a calcite matrix suggests that original clastic grains have been replaced or altered during the cementation process (cf. James, 1985). By contrast, the younger gravels are cemented by an opaque, clay-rich, impure and friable calcite matrix containing high proportions of unaltered fine gravel particles (Fig.10B). The contrasts in

261

composition and degree of induration of the older and younger carbonate cements may reflect a wide range of factors. In particular, increased purity and induration of the cement may reflect more prolonged periods of higher humidity, rates of evapotranspiration and capillary rise (cf. Moseley, 1965), but no firm interpretation of these contrasts is yet possible (Glennie, pers. comm.) All the gravels exposed at the surface of the raised channel deposits exhibit evidence of a wide variety of subaerial weathering processes. The degree of weathering reflects the period of exposure since exhumation, and hence an inverse relationship might be expected between degree of weathering and relative age. However, no such relationship has yet been established. The extent to which clasts exhibit evidence of desert varnish, case hardening, weathering rind development, ventifacting and faceting, solution rilling and pitting, appears to vary with lithology, rather than with relative age of the deposit. Indeed, the reverse pattern of weathering with relative age may exist, since the youngest ophiolitic gravels also exhibit evidence of widespread disintegration, marked by exfoliation, spalling and in situ splitting, while the older gravels appear to have been reduced to sparse patches of heavily varnished cherts, only one particle thick, scattered across the widely exposed Barzaman Formation.

Alluvial architecture Assessment of alluvial architecture has been based on mapping of raised channel courses and active channel zones, analysis of morphologic relations between successive raised channel deposits, and of the sedimentologic and stratigraphic relations between different fluvial facies, and between fluvial and non-fluvial facies. The paleochannel courses form linear gravel deposits 6 0 - 3 0 0 m + wide, <8 m thick, and 40 60 km + in length. These gravels are separated spatially by areas of low relief, 4 7 kln wide for a given paleochannel generation,

262

J. MAIZELS

Fig.8A. A e r i a l p h o t o g r a p h s of s u p e r i m p o s e d , e x h u m e d p a l e o c h a n n e l i n t e r s e c t i o n ("Triple J u n c t i o n " site). Paleoflow w a s from top to b o t t o m (N to S). W i d t h of p h o t o a r e a = c. 2.2 km.

RAISED CHANNEl. SYSTEMS AS INDICATORS OF PALAEOHYDROI,OGIC CHANGE

263

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Fig.SB, Interpretation of "Triple Junction" paleochannell intersection site, indicating at least 5 successwe ~enerations of raised ehanne]s, each with distinctive concentrations of cherts. (After Maizels, in press b)

264

Fig.SA.

J. MAIZELS

R A I S E D C H A N N E L S Y S T E M S AS I N D I C A T O R S O F P A L A E O H Y D R O L O G I C

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Fig.9A. Aerial photograph of superimposed, exhumed paleochannel systems ("Pink Cliffs" site). Paleoflow was from top to bottom (NNW to SSE). Crosses are 0.6 km apart. Present day Wadi Andam truncates the paleochannels in the bottom right hand corner. B. Interpretation of "Pink Cliffs" paleochannel systems, indicating at least 8 successive generations of raised channels, with progressively increasing amounts of cherts (oldest) with increasing relative age. (After Maizels, in press b)

266

J. MAIZELS

i!i~i

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i ¸¸ %

Fig.10A. Crustose cementation of chert-rich, fining-upward gravels, characteristic of the older paleochannel generations. B. Friable and gravel-rich cementation characteristic of the younger, ophiolite-rich cobble gravels. l e a v i n g the p a l e o c h a n n e l s as u p s t a n d i n g ridges. T h e g r a v e l s a r e s e p a r a t e d s t r a t i g r a p h i cally by v a r i a b l e t h i c k n e s s e s of t h e B a r z a m a n F o r m a t i o n . T h e fine-grained t e x t u r e of the B a r z a m a n F o r m a t i o n s u g g e s t s t h a t m u c h of it r e p r e s e n t s o v e r b a n k fluvial facies, i n t e r b e d d e d w i t h lenses of flood g r a v e l s a n d s u b s t a n t i a l t h i c k n e s s e s of e o l i a n silts a n d s a n d s (see below). H o w e v e r , t h e t o p o g r a p h i c r e l a t i o n s between successive paleochannel generations at i n t e r s e c t i o n s do n o t reflect a c o n t i n u o u s s t r a t i g r a p h i c s e q u e n c e of c h a n n e l a n d over-

b a n k facies. Hence, a l t h o u g h m a n y of the y o u n g e r deposits a r e h i g h e r in the s e q u e n c e and o v e r l i e the older deposits, t h e r e are a n u m b e r of exceptions. A t m a n y i n t e r s e c t i o n s t h e y o u n g e r deposits lie at t h e same, or e v e n lower, e l e v a t i o n t h a n t h e older deposits, indicating that long-term alluvial sedimentation w a s p e r i o d i c a l l y i n t e r r u p t e d by episodes of incision. P a l e o c h a n n e l deposits of v a r y i n g age c a n t h e r e f o r e o c c u r a t s i m i l a r or i n v e r s e l y l o c a t e d s t r a t i g r a p h i c levels, a l t h o u g h t h e i r l a t e r a l d i s t r i b u t i o n c a n v a r y from 0 to 7 km,

RAISE[) CHANNEL SYSTEMS AS INDICATORS OF PALAEOHYDROLOGIC CHANtIE

and their vertical spacing from 0 to at least 30 m. The relative age ~f successive paleochannel deposits within the stratigraphic sequence can therefore be identified from the nature of sedimentary contacts and from the lithological composition of the different paleochannel gravels.

Paleohydrologic interpretation of the raised c h a n n e l deposits

Fluvial facies and depositional environment Major changes in facies type and associated depositional environment occur within the alluvial raised channel sequence. The older, more sinuous raised channels are associated with fine-grained, large-scale, trough crossbedded, ribbon sandstones and conglomerates, reflecting the downstream migration of large fluvial dunes along the bed of a meandering channel. The younger, low sinuosity channels, by contrast, are associated with coarsegrained, cobble deposits forming thin, tabular, planar cross-bedded units, representing longitudinal bars (e.g. Miall, 1978; Rust, 1978). The finegrained overbank and floodplain deposits are represented by a variety of fluvial mudstones, silts and sands, cobble and pebble lenses, eolian silts (Stanger, pers. comm.), and indistinct sub-horizontal bedding, characteristic of partially-vegetated, low relief, flood plains (cf. Yaalon and Dan, 1974; Vondra and Burggraf, 1978). The thin gravel spreads forming the wadi terraces accumulated as sheet gravels, probably during flood flows within braided river systems which extended across the former wadi floor (cf. Miall, 1978; Rust, 1978; Ramos and Sopena, 1983) (Fig.ll).

Paleohydraulics of raised channel deposits Major contrasts in paleohydraulic conditions appear to have occurred in association with the observed changes in channel pattern, morphology and sedimentology through the alluvial raised channel sequence. Flow in the earlier paleochannel generations reached esti-

21~7

mated maximum depths of between 1.2 and 4.5 m, with peak velocities averaging between c. 3 and 7m s ~, and discharge ranging between c. 160 and 1400m 3 s ~, the higher values of depth and velocity occurring in the younger, narrower paleochannels. By contrast, flows within the youngest raised channel and terrace deposits, in particular, reached discharges averaging between c. 13,000 and 31,000 m 3 s 1, i.e. exceeding flows in the earlier generations by about one order of magnitude (Maizels, in press b; Table I and Fig.ll). Although the paleohydraulic estimates are subject to some error (see above), the large contrasts in predicted flows between the older and younger paleochannel systems are likely to reflect a real increase in peak flow magnitudes during the latter stages of paleochannel activity. Comparison of the paleodischarge estimates with present-day wadi floods subgest that the earlier paleochannels accommodated similar peak flows to those recorded in recent flash floods in wadi Andam (i.e.c. 1000 m 3 s t) (Curtis, 1985; Maizels and Anderson, 1989).

Paleohydrologic interpretation of flow regime Evidence for the type of paleoflow regime may be sought in a combination of factors, and particularly in the channel pattern and morphology, and the degree of downstream channel persistence. However, since no unequivocal relationships have yet been established between channel pattern or morphology and associated flow regimes for present-day river systems, the interpretation presented here still require further geomorphological testing. The relatively sinuous and persistent channels of the earliest generations of raised channels are likely to represent markedly different flow regimes from those of the low sinuosity, narrowing channels of the later distributary systems; and these in turn may differ from flows producing the extensive terrace gravels. For example, the development of an equilibrium relationship between the meandering channel planform of earlier generations and associated dominant flows is likely

268

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to require a minimum flow duration (Bridge, 1985) of at least 2 - 3 m o n t h s per year (see Jackson, 1978; Arche, 1983). Hence, the earlier g e n e r a t i o n s of s i n u o u s p a l e o c h a n n e l s probably developed under at least seasonal flow conditions characteristic of a sub-humid to semi-arid e n v i r o n m e n t (Fig.ll). These seasonal flows acted to create: and s u s t a i n an extensive c h a n n e l system t h a t exhibits little c h a n g e in its dimensions for m a n y tens of kilometers. S e a s o n a l bankfull flows w o u l d have averaged c. 4 0 0 - 8 0 0 m 3 s -1, while the w h o l e c h a n n e l system a c c o m m o d a t e d flows of c. 1000-2000 m 3 s -~. The e x t e n s i v e growth of distributary c h a n n e l s during the later generations, characterised by their low sinuosity, coarse texture, and ultimate downstream disappearance, suggests t h a t t h e y may h a v e been associated with

more ephemeral flows typical of more arid or semi-arid c o n d i t i o n s t h a n the earlier c h a n n e l s ( F i g . l l ) (e.g. Friend, 1978; Steel and Aasheim, 1978). Estimated bankfull discharges were h i g h e r t h a n in the earlier systems, with total flows r a n g i n g between c. 2700 and 9800 m 3 s 1 in proximal zones and c. 800 and 7000 m 3 s 1 in more distal zones. Short-lived flow events would result in rapid transmission losses into the dry, adjacent gravels, while additional volumes of water could be lost by high rates of evapotranspiration (cf. Butcher and Thornes, 1978; Stear, 1983). Indeed, the downstream decreases in sediment size and c h a n n e l size are characteristic of the semi-arid "terminal" distributary systems described by Nichols (1987) and Parkash et al. (1983) (and cf. Rachocki, 1981). Increased humidity during development of

RAISEDCHANNEl,SYSTEMSAS INI)ICATORSOF PALAEOHYI)ROI,O(HCCHANGE

the earlier channel systems may also be invoked to explain the high degree of cementation of the early channel gravels. Rapid cementation of the channel gravels occurred through precipitation of calcite cement, probably soon after flow in the main channels, during periods of seepage, and evaporation from a high water table and flows of underground water (cf. Moseley, 1965; Glennie, 1970; Sta|der, 1975). The disappearance of the easterly paleochannels into the Wahiba Sands, and subsequent entrenchment and terrace formation along the Wadi Andam course, are

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(1) Initial fan aggradation following formation of Miocene continental Fars Group. (2) Formation of older paleochannel generations still buried within the Barzaman Formation. A. Concentration of paleochannels along persistent active meander belts. B. Contemporaneous active meander belts. C. Isolated channels. D. Accumulation of overbank fluvial sediments (fine gravels, sands, silts and clays). E. Accumulation of eolian silts in interfluve zones and in abandoned channels. (3) Formation of intermediate and younger paleochannel generations, F. Broad, continuous, chert-rich meander channels (stippled shading on figure); exposed as asymmetric cross-secUons in wadi sections. G. Intermediate paleochannel generations, some forming multistorey sequences, especially along long-established meander belts. H. Younger distributary paleochannels, exhibiting narrow, long sinuosity, tapering courses, rich in ophiolites. (4) Overall aggradation to maximum fan elevation, and accumulation of uppermost fan gravels (/). (5) Deflation of fine-grained interfluve sediments of the Barzaman Formation to reveal exhumed paleochannel sequence. (6) Fluviatile incision and terrace formation along main wadi courses (surfaces J and K), culminating in modern wadi formation (L).

270

J. MAIZELS

through the paleochannel sequence is accompanied by a progressive change from chertdominated to ophiolite-dominated lithologies (Fig.13). The decrease in the chert: ophiolite ratios through time must reflect a major longterm change in the availability of these two lithologies within the catchment. This change can be explained by the progressive headward extension of the drainage system northwards, initially through the limestone and chert outcrops of the Hawasina (associated with the early paleochannel generations), and subsequently reaching the Semail ophiolites which lie farther to the north (associated with the later paleochannel generations). In addition, the northward migrating headwaters of Wadi Andam could well have captured the headwaters of the neighbouring Wadi Adam system lying to the west, a process that would have provided a new and abundant influx of ophiolites to the youngest paleochannel systems (Fig.2). This pattern of river capture would have increased the catchment area of Wadi Andam by c. 30%, all of which would lie within the runoff source areas. Hence, signifi-

Hence, the sequence of raised channel deposits represents a record of an overall, long-term change from seasonal, persistent flows to ephemeral, flashy and catastrophic flood flow regimes.

Paleohydrologic interpretation of catchment conditions The relatively fine-grained nature both of the earlier paleochannel generations and of the Barzaman Formation suggest that sediment supply to these early systems may have been derived from limited erosion of mountain slopes, flood-plain deposits and channel banks that were relatively well vegetated, or formed savanna-type landscapes (cf. Van Campo et al., 1982). The increased coarseness of entrained gravels during successive paleochannel generations may well reflect a change to more arid conditions, resulting in an impoverishment of the vegetation cover, and the added effectiveness of high intensity storms and floods in removing coarse sediment from the catchment. The increased coarseness of the gravels

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RAISED CHANNEl. SYSTEMS AS INDICATORS OF PAIJAEOHYDROLOGIC CHANGE

cant increases in runoff volume would also have been generated, and could have contributed to the high peak flows estimated for the later paleochannels and for the periods of entrenchment.

D i s c u s s i o n and conclusion: modelling paleohydrologic c h a n g e in raised c h a n n e l systems The four main questions raised earlier in this paper as proving critical to understanding paleohydrologic changes within raised channel systems, are also fundamental in the construction of any models of such changes. The first requirement is the interpretation of paleofiow conditions at each stage of channel development, and the variability of these conditions in different parts of a single paleochannel system. The case study from Oman illustrates how a range of paleoflow parameters may be estimated on the basis of numerical models, and how contrasts in flow can be estimated (a) for different distributary channels, and proximal and distal channels, of a contemporaneous network, and (b) for paleochannels of different generation. In addition, changes in fluvial style and depositional environment can be identified from channel pattern and planform mapping, and from analysis of lithofacies assemblages and facies types. Hence in the Oman example it has been possible to identify an overall longterm change from fine-grained, single-thread, meandering channels accommodating low or moderate seasonal flows, to coarse-grained, low sinuosity, distributary channels occupied by infrequent, high magnitude, "flash floods". The final stage of development of the fluvial system involved major entrenchment, probably associated with catastrophic floods, interrupted by periods of deposition of wadi gravels. This overall change in paleohydrologic characteristics is interpreted in terms of a long-term trend from more humid to more arid conditions during the Cenozoic, although many climatic fluctuations associated with the Pleistocene glaciations are superimposed on this overall pattern of change.

271

The second important question addresses the mechanisms of channel change within a raised channel sequence. Two main mechanisms of channel change may be most readily identified as representing change within a contemporaneous network of raised channels. The first mechanism is that of progressive meander migration within an active channel zone (cf. Puigdefabregas and Van Vliet, 1978). Morphological mapping of the Omani channel systems, for example, shows a number of distinctive point bar deposits associated with meander channels, and lithological analysis confirms that these deposits date from the same period of fluvial activity as the adjacent meandering channel. The second mechanism is that of avulsion, in which a new channel has been created (or a former channel re-occupied) by overbank spillage, probably during seasonal floods (cf. Leeder, 1978; Bridge and Leeder, 1979; Gole and Chitale, 1986; Bristow, 1987; Wells and Dorr, 1987), and these new channels have acted to link adjacent active channel zones, or to create new ones. Again, examples of such changes have been identified in the Omani distributary channel systems. Identification of the processes of temporal hydrologic change associated with the longterm succession of raised channel deposits is a far more complex problem. The processes that can be identified from paleochannel evidence include (a) the periodicity of fluvial activity; (b) the long-term occupancy of active channel zones; (c) the occurrence of erosional episodes; and (d) non-fluvial processes that influence subsequent change in the fluvial system. First, the periodicity of fluvial activity can be assessed from the lithological composition of successive paleochannel generations at channel intersections (cf. Fig.13). For example, marked differences in the lithological composition of different paleochannel generations at many intersections within the Omani sequence indicate that significant time intervals are likely to have occurred between successive periods of channel activity. In contrast, the absence of a significant difference in lithological composition suggests that a relatively

272 short time interval is likely to have elapsed between the respective periods of channel activity. Indeed, Schumm et al. (1987) found that most distributary channels have a very limited lifespan. Analysis of differences in the lithological composition of raised channel deposits at intersections suggests that at least 6 distinct, major periods of fluvial activity occurred during formation of the observed raised channel sequences (Fig.13). These discrete periods of fluvial activity, each comprising a number of episodes of paleochannel formation, are likely to reflect relatively prolonged periods with repeated runoff events. Such flows could be produced either during more humid climatic conditions or from repeated storms occurring under more arid of semi-arid conditions. Further analysis of the controls on periodicity and duration of these episodes of fluvial activity must await the construction of a more precise chronologic framework. Second, the degree to which active channel zones were repeatedly occupied by successive paleochannel courses can be readily identified from morphologic and stratigraphic evidence. For example, mapping of Omani paleochannel systems of similar generation suggests that up to 8 major channel routeways could have coexisted during periods of fluvial activity (Fig.3). Comparison of the location of these routeways for successive paleochannel generations indicates that these zones were normally re-occupied during successive periods of fluvial activity. Repeated occupation of active channel zones suggests that many of the meandering channel systems wew relatively stable, and that flows tended to remain along established routeways for long periods, rather than migrating across the whole fan surface (cf. Leeder, 1978; Puigdefabregas and Van Vliet, 1978; Bridge and Leedre, 1979; Bridge, 1985). Hence, the exhumed paleochannel routeways now represent well preserved, multistorey, fossil meander belts (cf. Puigdegabregas and Van Vliet, 1978; Stear, 1983). Third, the occurrence of erosional episodes within an overall aggradational, alluvial se-

J. MAIZELS quence can be determined from stratigraphic and morphologic relations between different paleochannel generations. For example, a number of younger paleochannels in the Omani study are incised into the older channel deposits, and occur at similar or even lower elevations (Fig.12). This evidence indicates that the Omani piedmont plain was not formed as a continuous, long-term aggradational sequence. However, it has not yet been possible to determine the frequency of erosional episodes, compared with the number of episodes of fluvial activity. It is likely that the whole alluvial sequence represents a highly complex pattern of alternating periods of erosion and aggradation, each period being of varying character and duration. The final mechanism of change within raised channel systems relates to those processes operating between periods of fluvial activity that would affect patterns of subsequent fluvial activity. These processes include induration of channel deposits, deflation of interfluve and floodplain sediments, and accumulation of eolian sediments. The location of subsequent generations of river courses will depend on the degree of channel cementation, interfluve deflation, or infilling, damming or burial of former channel courses or channel zones by eolian deposits (cf. Glennie, 1970; Williams, 1975). Hence, repeated occupation of active channel zones probably reflects the absence of any significant interfluve deflation or channel burial between periods of fluvial activity. Such conditions might arise if the inter-fluvial periods are relatively short-lived and/or if sediment supplies are limited (e.g. through persistence of vegetation cover or extensive cementation of surficial sediments). The majority of paleochannels in generations 5-7 (fluvial period IV) and 8 and 9 (fluvial period V), for example, follow the routes established by earlier paleochannel courses. Hence, within a given period of fluvial activity, inter-fluvial episodes are likely to have been significantly shorter than those separating periods of fluvial activity. Thus where successive channels occupied new route-

IIAISED CHANNEL SYSTEMS AS INDICATORS OF PALAEOHYDROLOGIC CHANGE

ways, rates of interfluve deflation or landscape burial by eolian deposits is likely to have been high, probably reflecting more arid or more prolonged time intervals between periods of fluvial activity. The third question concerns the processes of paleochannel preservation and subsequent exhumation. Clearly, preservation will be enhanced where channel cementation and induration is achieved mosat rapidly, where fluvial aggradation is at a maximum, and where accumulation of eolian (and other non-fluvial) deposits is greatest between periods of fluvial activity. Hence, the extensive preservation of vast areas of the Omani raised channel systems suggests that rates of both fluvial and eolian aggradation tended to remain high throughout development of the alluvial sequence in response to abundant sediment supplies both from mountain headwater sources and from earlier alluvial fan sediments. Indeed, major entrenchment appears to have occurred only in the final stages of fan evolution (cf. Harvey, 1984). Similarly, since late-stage, land-surface lowering has been largely accomplished by deflation of fine interfluve sediments, the resistant channel deposits have been left relatively untouched by erosive processes (cf. Yair and Gerson, 1974). The final consideration in understanding and modelling paleohydrologic change in raised channel systems involves identification of the likely autocyclic and allocyclic controls on longterm channel changes. The range of possible controls has already been discussed in relation to the model illustrated in Fig.1. At this stage, the requirement is to suggest the likely dominant controls on the observed longterm channel and landscape changes within the Omani raised channel systems, as a means of providing a model that can be tested in other raised channel systems. The resulting model suggests that the observed paleohydrologic changes have resulted from a complex combination of both autocyclic and allocyclic controls (cf. Schumm et al., 1987). The main autocyclic controls include long-term alluvial aggradation and increased catchment area

273

associated with drainage extension. Longterm aggradation could have resulted in progressive steepening of the paleochannel courses, allowing coarser sediments to be transported in more steeply graded channels hence promoting the development of channels of lower sinuosity. Once gradients reached critically steep slopes, excess stream energy would have been available for largescale entrenchment. The large increases in estimated peak flows and the dramatic changes in lithological composition cannot be explained by this aggradational mechanism, however. Instead, these changes can be attributed to progressive headward extension of the drainage network on to the ophiolite outcrops, and the concomitant capture of the adjacent headwater system. Finally, although major changes in paleohydraulic characteristics have been recorded through the geologic record on the alluvial fan, the absence of an independent chronologic control precludes precise interpretation of these changes in terms of specific allocyclic or autocyclic mechanisms (Fig.l). Hence, it can only be hypothesized at this stage, that the observed changes in channel morphology, pattern and extent, and the associated changes in flow during formation of the raised channel sequence resulted from a complex combination of both (a) large-scale, regional, external controls and (b) more local, within-catchment controls. It can be concluded that the major changes in paleohydrology identified from the raised channel sequence are likely to reflect major changes in climatic aridity, continentality and sea-level during the Cenozoic, in turn affecting flow regime, sediment supply and rates of accretion and/or incision. In addition, network expansion and river capture would have promoted fan entrenchment, while increased continentality would have led to large scale landscape deflation and exhumation of the paleochannel sequence. The detailed model for paleohydrologic change, and the specific controls on each of these changes, for this as well as for other raised channel systems awaits further testing in the future.

274

Acknowledgements T h e a u t h o r w o u l d l i k e to e x p r e s s p a r t i c u l a r a p p r e c i a t i o n to Dr. K. G l e n n i e for m a n y i n v a l u a b l e d i s c u s s i o n s . T h a n k s a r e a l s o ext e n d e d t o P. C o n s i d i n e , J. F. J o n e s , Dr. G. S t a n g e r a n d H. W e i e r , a l l of t h e P u b l i c A u t h o r i t y for W a t e r R e s o u r c e s , S u l t a n a t e o f O m a n , a n d to B. D u f f a n d Dr. M. H u g h e s C l a r k e o f P e t r o l e u m D e v e l o p m e n t O m a n for their helpful and stimulating discussions. T h a n k s a r e a l s o d u e to C. M c B e a n for v a l u a b l e field a s s i s t a n c e a n d to J. C u t l e r a n d J. McI n t o s h for h e l p w i t h t h e g r o u n d s u r v e y s . T h e logistic s u p p o r t of the S u l t a n of O m a n ' s A r m e d F o r c e s a n d t h e C o a s t a l S e c u r i t y F o r c e is a l s o gratefully acknowledged. The author would l i k e to t h a n k a l l m e m b e r s o f t h e R o y a l Geographical Society Eastern Sands Project for t h e i r e n t h u s i a s m a n d s u p p o r t d u r i n g t h e progress of this research. F i n a l l y , the a u t h o r w o u l d l i k e to a c k n o w l e d g e a s s i s t a n c e w i t h l a b o r a t o r y a n a l y s e s f r o m Prof. I. P a r s o n s , Dr. G. W a l k d e n a n d M r s . M. L a m b .

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