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90 Cards in this Set
- Front
- Back
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Define Alluvial Fans
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localized areas of sedimentation downstream of points where laterally confined flows expand.
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What do you need to form alluvial fans?
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you need mountains; can form syntectonically; steep slopes (1-12degrees)
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What are the conditions proximal to alluvial fans?
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channelized; coarsest seds
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What are the condition middle to alluvial fans?
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braided; intermediate seds
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What are the conditions distal to alluvial fans?
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sheet flow; finest seds
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What conditions are most preferred for the generation of an alluvial fan?
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along linear mountain fronts; sides of valleys; glacial Margins
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What is the importance of alluvial fans in the geological record?
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most important environment is in thrust- or normal-fault bounded basins where episodic faulting enables subsidence and thus fan preservation active base margin.
Reflects proximal tectonic activity and provenance. |
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What is the relationship between basin drainage size and fan size?
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as drainage size increases so does the fan size.
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Define pediment
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Area at the foot of a mountain
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Define Delta Fan
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A fan that progrates into water.
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What are the processes in order for alluvial fans?
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infiltration → rapid deposition → sediment buildip → avulsion (as a channel deposits sediments, it will change to a quicker/steeper path)
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What are the physical trends as you move down an alluvial fan?
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mean grain size ↓
bed thickness ↓ channel depth ↓ sediment sorting ↑ debris flows → mud flows → braided stream deposits → sheet floods |
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How do you define debris flow?
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grain support mechanism = strength of mud matrix
lots of matrix characterizes the flow deposits |
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Define diamictite
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Mix of all Grain Flow
has become associated with glacial environments but does not refer only to glacial sediments |
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What is imbrication used for?
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Can be used to infer stream flow and paleo-current direction
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Give a definition of sheet flood environments?
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"water flows over the surface like a sheet"
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Name some Secondary Processes / Modifications.
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Sieve Deposits = increadibly clast supported, no matrix, fine grains filter down; found in proximal parts of the fan
Gullying Deflation Soils - calcium carbonate fossil soils; cobbles coated in CaCO3 |
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What are some examples of distal environments in alluvial fans?
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Playas.
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What is the alluvial fan face model?
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Proximal = thick beds, massive
Intermediate = x-beds; horizontal beds; stream flow Distal = no channels; maybe ripple x-stratification |
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What are some general features you find in delta systems?
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transitional environment
clastic coastlines reflect interplay of fluvial currents and basinal energy Delta = distinct shoreline protuberence point source of sediments |
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What are the Effluent Behavior and Depositional Patterns of delta systems?
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Depends on (1) inertia and friction of inflowing water and (2) buoyancy processes at river mouth
which derive from (1) density of river vs basin water (2) sediment concentration & grain size (3) water depth, discharge, and velocity |
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Define and Characterize hypopycnal flow:
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Buoyancy Dominated
"often, where a river enters salt water, ρ of freshwater + sediments < saltwater = hypopycnal flow " seperation of bedload and suspended load Result = Buoyant Plume, low depositional slope effluent less dense(more buoyant) than basinal |
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Define and Characterize Hyperpycnal Flow:
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Inertia Dominated
effluent more dense than basinal density underflows, bypasses shoreline, deposits on prodelta inhibits progradation |
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Define and Characterize Homopycnal flow
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Friction Dominated
effluent same density as basinal thorough mixing |
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Define and Characterize Gilbert Deltas
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Abrupt discontinuity of slope at or near shoreface → steep foresets @ angle of repose (25-35°)
abundant sediment gravity flows Classic topset-foreset-bottomset geometry |
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Define and Characterize Delta Sub Environments
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Upper Delta Plain = Fluvial Processes dominate; above Mean High Tide
Lower Delta Plain = Fluvial & Marine Processes dominate; Between Mean High Tide and Mean Low Tide; fluvial distributary channels; interdistrubtary bays are highly variable Subaqueous Plain = Delta Front and Prodelta |
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Define and Characterize Delta Front
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Marine Processes
below Mean Low Tide Distributary mouth bars reworking |
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Characterize Prodelta
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Low Energy Marine
Fines from suspension settling storm reworking flood deposition gravity flows and slumps |
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What is the importance of Avulsion in producing Facies Succession?
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Results in a Vertical Facies Succession
controls are autocyclic or external |
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What are the controls on the clastic shoreline?
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Morphology controlled by (1) sediment supply (2) wave vs tidal energy detacched vs attached
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What are the origins of the tides?
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the gravitational attraction of the moon causes the oceans to bulge out in the direction of the moon
another bulge occurs on the opposite side, since the Earth is also being pulled toward the moon (and thus away from the water on the opposite side) |
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What are the different tidal magnitudes and why?
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Spring Tides at new moon & full moon phases are large because gravity of the moon and sun pulls in the same direction
Neap Tides at first and third-quarter phases are weak because gravity of the sun pulls at right angles to that of the moon |
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what are some Tidal Processes and Deposits?
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Flow varies in direction & velocity
Relative abundance of Mud Dominant Current Located in Tidal Systems Sed Structures = Herringbone X-Stratification |
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Define and Characterize the subtidal enviroment:
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submerged
highest current velocities channels and sand waves |
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Define and Characterize the Intratidal enviroment:
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between low & high tide
mixed sediment load intermittent exposure |
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Define and Characterize the supratidal enviroment:
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mostly exposed
channels, low energy areas climate - marsh vs. evaporites |
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What the typical facies model for tidal flats?
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Geometry
Bedding structures Facies Fossils Typical Vertical Sequence = Fining Up - Progradational |
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Characterize Foreshore
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sandy substrate
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Characterize offshore
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Muddy Substrate
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Characterize Beaches and Barrier Islands:
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High Energy
generally lower tidal ranges waves , storms , longshore currents |
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Characterize wave shoreline environments:
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Backshore
Foreshore Shoreface Offshore |
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What influences wave energy?
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wave motion is influenced by water depth and shape of the shoreline
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Characterize shoreface:
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Subtidal - mean low tide to fair weather wavebase
Bidirectional & Longshore Currents Gradation - upper → lower Sandy , Crossbedding , Ripples , Vertical Burrows finer grained in lower shoreface |
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Characterize Foreshore:
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Intertidal - swash zone
high velocity & low water depth grades landward & seaward Sand (mostly) , Plane Beds , Antidunes , Seaward Dipping Laminations , Rarely Burrowing |
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Characterize Backshore:
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coastal dunes
marsh coastal Plain wash over fans |
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Characterize Barrier islands:
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lagoon
tidal channels |
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Explain the different shelf systems:
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Shelf = shoreward of shelf break (<200 m depth)
Pericontinental = around margins on continents Epicontinental = ("epeiric") marine atop continental crust |
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What are the processes effecting shelves?
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Tides - forming linear sand ridges
Ocean Currents - causing winnowing; Ex - Gulf Stream Density Currents - sediment plumes, nepheliod layers, forming mud drapes Stormwind Currents (HCS) Bilogical Activity |
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Define Bathymetry:
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Average depth = 3.5 km
continents > 1km abyssal plain >40000m composes 80% of ocean |
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Define Resedimentaition:
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Moving Sediments to Deep Water
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Describe Turbidity Flows
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generated on outer shelf & slope
Triggering Mechanism - variable |
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Describe Grain Flows
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localized deposits within canyons
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Where are debris flows important?
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Within canyons.
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Where is gravity sliding important?
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can be very large volume @ active margins
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What are distinctive facies of deep water?
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Classical Tubidites (thick monotonous successions of graded beds; scour & flute marks
Structurless Sandstones - dish structures; probably also deposited by turbidity currents Peddbly Sandstones - commonly graded Conglomerates - graded or structureless Pebbly Mudstones |
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What are characteristic of modern canyons?
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debris flow
sand-waves (big dunes) grain flows |
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Briefly describe channels:
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braided or meandering
Levees Often very sand rich Avulsions = lobe abandonment |
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Overbank regions:
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areas between canyons
sand, silt, and clay |
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what is characteristic of Levees, Overbanks?
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thin-bedded turbidites
slumping flows tend to be sandier downflow, owing to levee spillover of muddier material |
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What is a Frontal Splay/ Terminal Lobe?
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form where levee height can no longer confine sand-prone turbidity current
upstream of this sand and mud increase downstream sand and and mud decrease |
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What are lobes?
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regions of shallow, distributary channels
sheet like sedimentation classic turbidites |
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What are characteristic of turbidites?
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product of turbidity current
Head, Body, & Tail each turbidite = single event Typically display predictable sequence of lithology and structures Bouma Sequence |
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What is the classic bouma sequence?
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e - pelagic and hemipelagic mud
d - laminated silts c - Cross laminated sands; ripples; LFR b - parallel laminated sands; UFR a - Massive sand and granules; rapidly deposited under UFR scoured base with tool marks, flutes, etc |
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How do you recognize turbidite currents?
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Weak/Moderate Debris Flow deposits
both UFR and LFR fines "Blown back" |
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what are the facies model of a submarine fan?
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Lithology - sand, mud
Scale Geometry - 100-1000 km's sq; 1000's m's thick; wedge/lens Structures - Graded Beds; Ripple X-Bedding; Scour marks, flutes Fossils - something and pelagic |
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Describe upper submarine fan:
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debris flows
coarse conglomerates thin bedded levee turbidites (cde or de) |
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Describe mid submarine fan:
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Distributary Channel - Levee Dominated
massive channel sands Classic (complete) Bouma Sequences Levee |
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Describe lower submarine fan:
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Unconfined Flow
thin-bedded base-cut-out turbidites |
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Describe pelagic sedimentation:
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source of sediment = wind, biologic activity, reworking from shelf/slope
Types of sediment = calcite-shells; biogenic silica- diatoms; red-clay- clastic; organics Distribution contolled by = depth, climate, & geography |
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What are the channel properties of fluvial systems?
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Channel Dimensions - Q= u d w
shear stress = ρ g h S(slope) |
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What is key in fluvial systems?
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Channelized nature of sandbodies, and evidence for continental setting
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How do you classify fluvial systems?
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Morphology - Sinuosity, Braiding, Anastomosing
Sediment Load - Bedload, Suspended Load, Mixed Load Hydrology - Ephemeral, Perennial |
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What are fluvial end members?
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Braided River (bedload) Systems
Meandering River Systems |
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What is characteristic of braided rivers?
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Low sinuosity
Steeper Gradients Multiple Channels Coarser sediments Noncohesive banks |
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What are braided river settings?
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Mid/Lower Alluvial Fan
Mountainous Regions Glacial |
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What are braided river processes?
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Bar Formation
Migration - - Channel widening |
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Type of braided river bars:
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Lateral
Longitudinal Transverse |
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What is the facies model of braided rivers?
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Geometry
Bedding Structures Primary Facies Fossils |
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characterize battery point model:
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Floodplain Mud
Bar-top Tabular X-Beds In-Channel Trough X-Beds Coarse Channel Lag |
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What is characteristic of meandering rivers?
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High Sinuosity
Flatter Gradients Singl Channel Finer Sediments Cohesive Bank |
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What are the meandering river settings?
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coastal plains
fine-grained sediment production |
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What are the meandering river processes?
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Balance between erosion & deposition
cohesive banks |
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Facies model of a meandering river:
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Geometry
Bedding Structures Primary Facies Fossils |
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Describe the Point bar model:
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chute bars and modification
Meander migration / Avulsion Meander Flow |
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What is a channel bar?
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sand-prone, whereas channel and interfluves are mud-prone
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What is a formation?
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a lithologically distinct unit large enough in scale to be mappable (or traceable)
Members = Little Groups = Big |
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What is a nonconformity?
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A surface of Missing time
Nonconformity - strata overlie igneous or metamorphic rocks Disconformity - a period of erosion seperates episodes and deposition Angular Unconformity - underlying layers are not parallel to strata above |
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What is base level?
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level below which sediment cannot erode
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What are the different relative sea level changes and descriptions?
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Eustatic - Glacioeustasy - Ice Volume
Eustatic - Tectonoeustasy - Ocean Volume Tectonic - regional uplift and subsidence Rate of Sediment Supply |
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Importance of Subsidence for Accumulation
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Preservation of strata
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