GEO 416S 2024 L8.pdf

GEO416S – Week 4: Rivers and Deltas 1  GEO 416S – Week 4 Announcements Labs: Meet in usual lab location and then walk over to water tank with TA No lab next week First exam Thursday, September 26th in class Exam on paper No phones allowed  Clas...

GEO416S – Week 4: Rivers and Deltas 1  GEO 416S – Week 4 Announcements Labs: Meet in usual lab location and then walk over to water tank with TA No lab next week First exam Thursday, September 26th in class Exam on paper No phones allowed  Class Outline – Sept 14, 2023 Last week: Siliciclastic sediment transport: How fluids move solids Today: Bedform stability Environments of deposition Rivers and deltas Next week: Coastal environments The stability of bedforms can be thought of as a function of grain size and flow velocity River (fluvial) review 5 Overview of Various Channel Types Patterns, Bed Load & Stream Power Galloway and Hobday, 1996 Rivers respond to topography (gradients) and change along a given landscape Galloway and Hobday, 83 Drainage off SE Himalaya, India  Meandering Rivers Key Attributes: confined to one major channel characterized by cohesive banks (difficult to erode) generally, high sinuosity, lower gradients and finer sediment load typical Mississippi River (Satellite) Meandering Rivers Architectural Elements and Processes: Currents, Erosion, and Sedimentation Important Process: Lateral accretion on pt bar (inner bend) erosion on cut-bank (outer bend) helical flow decreasing energy up point bar Ex: South Saskatchewan River Don’t forget about the muddy banks/overbanks! Coal bearing swamps Great places to find fossils (esp. dinosaurs) Cover of Geotimes Idealized river profile sketch 13 Meandering River Facies – Lateral Accretion Surfaces Lateral Accretion Kayenta Fm., CO 14  Meandering Rivers Facies Distribution: Strongly controlled by lateral channel migration Idealized Fluvial Sequence Bedform stability: ripple and dunes controlled by grain size Harms et al, 1975 Cant & Walker, 1984 As a river migrates laterally, it will inevitably leave a sequence of fining-upwards deposits Meandering Rivers Facies Distribution: Strongly controlled by lateral channel migration Galloway & Hobday, 1983 South Saskatchewan River Braided Rivers Key Attributes: numerous channels separated by bars/islands characterized by banks with low cohesion (easily erodable) generally, low sinuosity, higher gradients and coarser sediment load typical Drainage off SE Himalaya, India Topographic control: tectonics Discharge control: monsoon 1. Rivers: Braided Architectural Elements and Processes Channel Morphology Bridge, 1993 1. Rivers: Braided Barforms and Facies Flow and growth patterns for typcial barforms - dashed lines indicate deposition (Smith, 1974) Braided Rivers Variability In Braided Channel Deposits Miall, 1977  Some Notable Characteristics of Fluvial Systems and Their Deposits: no marine fossils/trace fossils present commonly associated with roots, mudcracks, and animal track ways often red in color downstream decrease in grain size typical paleocurrents variable, but generally indicate downstream flow: Miall, 1994 Co-evolution of life (land plants) and river morphology on Earth Lelpi et al 2022 23 24 Coastal Environments: Deltas Mississippi Delta  Deltas form where rivers empty their water and sediment into another body of water, such as an ocean, lake, or another river Nile River Delta (photo credit: SERC) 26  Global distribution of major river deltas Roberts et al 2012 Many factors involved in delta construction Fraser, 89 Classification of Deltas Bhattacharya & Giosan (2003) Coastal Environments: Deltas Classification of modern deltas based on dominant sediment dispersal process (fluvial, wave or tide) and grain size Orton and Reading, 1993 Deltas: River-dominated Lena River delta Mississippi delta Deltas: Tide-dominated Mahakam River Delta Ganges-Brahmaputra River Delta Fly River Delta Deltas: Wave-dominated Yukon River delta Grijalva River delta Senegal river delta  Delta Environments Yukon Delta, Alaska Prodelta Delta Plain: Dominated by fluvial processes (subaerial) Delta Front: Fluvial and Delta front marine processes (subaerial and submarine) Lower delta plain Prodelta: Marine processes Upper delta plain Image: NASA Observatory  River-dominated system Bifurcating distributary channels and mouth bars Saskatchewan Delta, Canada (Cedar Lake, Manitoba)  River-dominated system Bifurcating distributary channels and mouth bars Mississippi River Delta  Tidal Influence Tidal mixing weakens density gradients and the effects of buoyancy Can provide greater sediment- transporting energy – bidirectional sediment transport Affect land-sea interface and position of subaerial- submarine interactions Remember: energy levels are changing daily – rhythmic sedimentation and stressful living conditions Wright (1977)  Tide-dominated system Tidal (ebb) inlet channels normal to shoreline Fly River Delta, Papua New Guinea  Wave Influence Represented by arcuate to cuspate lobes Wave action reworks sediment into bars and beaches Longshore drift and currents redistribute sediment along the shore  Wave-dominated system Beach ridges by redistribution of sand by wave reworking and long-shore drift Sao Francisco Delta, Brazil What kind of delta and why? What kind of delta and why? What kind of delta and why? Deltas: progradation & lobe-switching Deltas: progradation & lobe-switching (Shumaker et al., in prep.) Deltas: progradation & lobe-switching Facies Model Sea Level Delta Front Sandstone Delta Front Siltstone Pro-Delta Shales Offshore Clay (Shumaker et al., in prep.) Several factors influence morphology and internal facies distribution: 1. Density of river outflow a. hyperpycnal (denser) b. hypopycnal (less dense) c. homopycnal (equal density) 2. Interaction with marine processes (waves, tides, storms, currents, bioturbation, etc.) 3. Position of delta (relative to shelf edge) 4. Grain size of fluvial and marine systems 5. Depositional slope 6. Reworking during progradation 7. Post-depositional erosion (e.g. wave truncation on abandoned lobe) Wright (1977) & Bhattacharya (2010) Coastal Environments: Deltas Characteristics: fluvial sediments build into a standing body of water generally, a distal fining of grain size is observed range significantly in size, from < 1 km2 to > 100,000 km2 Allen, 1970; Sugarman et al., 1993 Coarsening upward succession of mud (?), silt, and sand Image: Moenkopi Fm., Utah (courtesy of T. Schwartz) Classic Method: Example – progradational and coarsening upward Lithology Prediction Facies Model Prediction Crevasse Splay Sea Level Channel Fill Proximal Distributary Mouth Bar Distal Delta Front Sandstone Distributary Delta Front Siltstone Mouth Bar Pro-Delta Shales Delta Front Offshore Clay Prodelta