BIG RIVERS WORLDWIDE Paul Edwin Potter and W Kenneth Hamblin
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BIG RIVERS WORLDWIDE Paul Edwin Potter and W Kenneth Hamblin BRIGHAM YOUNG UNIVERSITY GEOLOGY STUDIES Volume 48. 2006 Big Rivers Worldwide Part 1 Origins BY Paul Edwin Potter and W. Kenneth Hamblin CONTENTS Abstract ..............................................................1 Introduction ...........................................................1 Methods ..............................................................2 Drainage and Tectonics ..................................................2 Plateconvergence ...................................................3 Plateseparation .....................................................9 BasinsandArches ..................................................12 ShearsandMegashears ..............................................12 Special Events in Big River Evolution .....................................14 River Diversions ...................................................14 RoleofLakes .....................................................17 Resurfacing .......................................................19 Desertification .....................................................20 Ancestry of Modern Rivers .............................................. 21 Dating River Systems ...............................................21 Gondwana Breakup .................................................24 Eurasian Megasuture ................................................26 Relic Tethyan Drainage ..............................................26 Miocene Worldwide Tectonism ........................................ 26 Post-Miocene Rivers ................................................27 LongSurvivingRivers ..............................................27 Conclusions ..........................................................27 Acknowledgments .....................................................28 References Cited ......................................................31 Appendix1 ...........................................................37 PartIIAnnotatedBibliography ........................................... 45 BYU GEOLOGY STUDIES 2005, VOL. 48 A Publication of the Department of Geology Brigham Young University Provo, UT 84602 Editor Scott Ritter Brigham Young University Geology Studies is published by the Department of Geology. This publication consists of graduate student and faculty research within the department as well as papers submitted by outside contributors. Each article submitted is externally reviewed by at least two qualified persons. ISSN 0068-1016 Big Rivers Worldwide-Part I Origins Big Rivers Worldwide Part 1 Origins PAUL EDWIN POTTER and W. KENNETH HAMBLIN ABSTRACT The geologic characteristics and origins of big nver systems-35 In Eurasia, 15 in the Americas, eight in Africa, and two in Australia-are the result of plate tectonics, climate, volcanism and changes in relative base level. Collectively, these four, but mostly plate tectonics, determine the location, size, shape, and orientation of a large watershed, its longevity and most of its pat- tern since early in Earth history. Big nver systems are, in broad terms, relatable to the Wilson Cycle of initial hot spot + dome + continental rift -+ arm of the sea + small ocean + large ocean and passive margin + small (closing) ocean wlth an active margin (ocean-continent or micro-continent or continent-continent convergence) + new direction of continental tilt and drainage asyrn- metry. Thus, big river longevity, like global landscape evolution, operates on the same time scale as tectonic cycles. We found the breakup of Gondwana and the closing of the Tethyan Ocean by many small microplates plus Africa, Arabia, and India to be responsible for most of the drainage pattern of Eurasia. The closing of the Tethyan Ocean created most of the moun- tain chains and basins of southern Eurasia Erom Spain to Vietnam, a distance of some 13,000 km. In Eurasia, the divide between north-flowing nvers and rivers flowing to the Pacific and Indian Oceans closely follows the northern limit of Gondwana. In North and South America, ocean-to-continent convergence and accretion of microcontinents to the Cordilleran and Andean orogens are responsible for much of the major drainage pattern of the Americas. A second important result is the number of these rivers that came into existence, essentially in their present form during the Miocene, even though a few have ancestors traceable back to the Carboniferous and perhaps even earlier. The key to the long-term survival of a large river is location on a long-lived craton or passive margin and persistence of continental t~lt,all without intenuption by desertification, continental glaciation or volcanism. Conversely, orogenies both destroy existing big rivers and form new ones. INTRODUCTION beginnings inspired Hadding (1929) to speculate about the first rains. So from many points of view, rivers, and Big river systems are an intergral and essential part especially big rivers, merit our full attention. of Earth dynamics. Examples of the role of big rivers There is a vast, diverse literature about rivers and in Earth dynamics include geomorphology at all scales, the search engine GEOREF alone has 31,183 entries tectonics (erosion induced uplift and isostatic rebalancing about "rivers." Of these, however, only a tiny fraction are of the crust), the filling of sedimentary basins (thick about big rivers and their geologic origins. We focused sandy and muddy fills, mixed terrigenous-carbonate fills, our efforts on two key questions, "What are the long- mostly carbonate fill or starved basins), and geologic term geologic processes that control modem big river history (giant alluvial fanslpaleo Amazons) plus all the systems?'and "What is known about the longevity of global geochemical cycles dependent upon the transfer modem rivers?" Answers to both questions should go of continental mass (solids and chemical compounds in far to improve our understanding of the great variability solution) to the ocean. In addition, the big deltas and of rivers large and small (Schumm and Winkley, 1994, thick alluvial fills of big rivers are rich in petroleum and p.5). We hope these answers will also provide useful coal. Rivers, large and small, also play a role in plant insights to those who work with more immediate human and animal geography and have long been a key element concerns such as flooding and irrigation, navigation, land in human affairs; for example, transportation, flooding use, power, river training and restoration, biology, and and irrigation, political boundaries, and inspiration for sedimentation as well as those who explore for petroleum writers, painters, and musicians.] Rivers go far back in and placer mineral deposits. Excluded from our discussion time (Mojzsis et al., 2001; Wilde et al., 2001) and their are short-term riverine dynamics and changes on the 1 Riverine titles of music abound: think of Moonlight on the Wabash, the Blue Danube Waltz, and the tone poem the Moldau. And there is even a short symphonic piece by Tobias Picker in 1986 that totally captures the spirit of our efforts called Old and Lost Rivers! BYU GEOLOGY STUDIES 2005, VOL. 48 scale of Pleistocene glacial-interglacial changes and their ones, whose essential characteristics are summarized in effects on river discharge and load, riverine morphology Appendix 1 (p. 41) of Part 1. We also collected some (braided vesus meandering), avulsion, etc. Nor do we studies of ancient river systems identified in basin studies, consider the role of catastrophic floods. Instead we focus which added an important dimension to our results. In on the factors that have controlled big rivers through the addition, our search of the literature identified some 23 long sweep of geologic time and use 60 modem rivers as papers that demonstrate that the study of big river systems, our data base. These are scattered across the globe: 35 in although always outside the main themes of geology, has Eurasia, 15 in the Americas, eight in Africa, and two in indeed had a long history (Table 1). Australia (Fig. 1). We used satellite imagery, geologic maps, colored digital shaded-relief maps, and radar imagery to study METHODS details of river patterns and their relation to watersheds and landforms. This "field work" helped us to generate We approached the origin of big river systems ideas and better evaluate the literature. The present by combining an analysis of a widely scattered world availability of such maps for Russia and China, which literature in English, German, French, Spanish, collectively cover about 31 percent of the Earth's land Portuguese, Russian, and Chinese with study of world surface excluding Antarctica, added much. maps and imagery. The scant literature on the origin and Most of the data about river length, watershed area, evolution of big river systems contrasts with the vast and discharge used in our paper come from Allen (1997, flood of articles on the geography of rivers, their ecology, Table 3.4). For the spelling of foreign place and river engineering problems (river training) and hydrology, names, we followed those used in the 1992 edition of the geomorphology, and sedimentology. Literature relevant OxfordAtlas of the World. to the origin of big river systems is widely scattered and well concealed (Potter and Hamblin, Part 2, this volume) DRAINAGE AND TECTONICS so much patience and persistence was required to find it. We used GEOREF and references collected from more As we progressed with our study, we developed than 25 years of joint interest in the origins