Fluvial Sediments “As a young man, my fondest dream was to become a geographer. However, while working in the customs office I thought deeply about the matter and concluded it was far too difficult a subject. With some reluctance, I then turned to Physics as a substitute.” - Albert Einstein (unpublished letters)

2 Main types: –Bedrock channels: –sediment transport capacity > sediment supply Types of rivers

Types of rivers 2 Main types: –Alluvial channels Channels formed in and by sediment transport Sediment transport capacity <= sediment supply

What to rivers do? They.. –Erode material –Transport eroded material –Deposit transported eroded material -> sediments –Move in space over time

River erosion Leads to first few characteristics of fluvial sediments 1/ larger clasts than eolian sediments 2/ Not necessarily well sorted 3/ Often rounded movie

River transport Transported clasts dependent on flow “strength” “strength” proportional to density, flow velocity, flow depth, slope => deeper or faster or steeper flows can carry larger clasts Leads to next characteristics of fluvial sediments 4/ clast size can be an indication of river “strength”

River sediment deposition Sediments are deposited following change is river dynamics - If river slows down, shallows (slope and depth) -most dramatic example: Alluvial fans

Types of alluvial rivers Straight: sinuosity (path length / length covered) < 1.3 Meandering -> side to side oscillations Braided: flow divides into more than one thread Anabranching: stable braid

Braided channels Virgin River [north fork] Zion National Park Compton (1985)

Meandering rivers “superelevation” Centrifugal force Hydrostatic pressure “Primary circulation”: “Secondary circulation”: Net Result: channel migration

Stratigraphic x-section

Channel x-section Idealized fluvial fining-upward sequence [Walker]

Summary 1/ larger clasts than eolian sediments 2/ Not necessarily well sorted 3/ Often rounded 4/ Clast size can be linked to channel “strength” Shinarump conglomerate

Example of alluvial fan conglomerate [fanglomerate] shed from the Sevier-Laramide uplifts Paleocene Echo Canyon conglomerate, Echo Canyon, Utah

Blocks of Shinarump, Coal Pits wash, Zion National Park

Triassic Chinle Formation [painted desert, mostly paleosols] overlain by Jurassic fluvial and eolian seds – ledge and cliff formers]

Lower Jurassic Moenave Fm. Zion National Park Ledge formers--fluvial channels and related facies Slope formers--paleosols

Eocene Claron Formation in Bryce National Park – mostly fluvial and floodplain

channel facies 

Shinarump Conglomerate member of the Triassic Chinle Formation [basal member]

Permian paleosols [soft, badlands topography, colorful] fluvial channel deposit [resistant cap, ledge former]

Shinarump conglomerate member of the Chinle Formation, Capitol Reef

Shinarump – cross-bedded coarse sandstone with some soft-sediment deformation

Shinarump conglomerate member of the Chinle Formation, outside Zion NP

Lower Jurassic Moenave Fm. Zion National Park

Lower Jurassic Moenave Formation, Zion National Park [trail above campground]

Lower Jurassic Moenave Fm. Zion National Park

Lower Jurassic Moenave Fm. Zion National Park

Lower Jurassic Moenave Fm. Zion National Park

Lower Jurassic Moenave Fm. Zion National Park