pt 2 Flashcards
0 slip face forms
Sand sheets, stringers, and domes. These are accumulated lumps of sand that tend to form a bell shape. They form in areas with low sand supply.
1+ slipface bedforms
Blowout dunes: sand accumulating ~haphazardly near vegatation
Parabolic dunes: These occur due to plants securing soil at the edges of a barchan dune and reversing the “U” shape so the curve is in the direction of the wind.
2 slipface bedforms
Symmetric Ridge/linear dunes: These are long lines that sometimes undulate. They form due to two subparallel dominant winds. The wind can become helical.
3+ slipface bedforms
These are Star dunes that form due to variable wind directions
Accumulation and preservation space
The accumulation space is the area of elevation available for accumulating material.
There is an erosional line that defines the preservation space by being the space where migration will not erode the material.
Preservation space can change due to compression/subsidence, rising water table, or changes in migration.
Alluvial Fan Depositional Environment
Principle of continuity dominates. The area of flow rapidly increases as slope decreases and competence falls fast. Near the interface, proximal fans, there are debris flow deposits (poor sorting, large clasts, matrix supported conglomerates/breccias). Further down, mid-fan, there is sheetflood deposits and incised channels that vary the deposition from fluvial to find sands down slope. Then there is there distal fan that merges with the alluvial plane below.
Alluvial fan depositional processes
Wet Fans: Channelized confined flows deposit fluvial sediments. This produces scour and fill, imbrication. Sheet floods create well sorted materials that fine outwards
Dry fans: Debris flows and rock avalanches (differ in presence of mud)
Alluvial fan geometry
Parallel to the mountains they are like a bell curve (convex up). Normal to the mountains they are like a channel and are concave up.
Alluvial fan types (4)
wet fans: stream dominated and have fluvial facies
Dry fans: These are arid fans that are dominated by debris flows and sheet floods.
Mega fans: These are low slope, stream dominated fans common in monsoonal areas.
Fan-deltas: Fans that empty into standing water. Common in glacial enviroments.
Alluvial Fan vertical sections
These tend to have autocyclical channel switching and abatement related to tectonic processes and climactic factors. This creates two superimposed cycles of sediments.
On a large scale there are cycles of coarsening upwards due to the progradation of the fan combined with subsidence resetting the cycle and on a small scale there is a fining upwards due to the loss of competence within the flow.
Alluvial Fans
These are conical prisms/wedges of sediments that form at the mouths of canyons and high relief areas with little vegetation. They are flow dominated or stream dominated and typically transport large quantities of sediment into a basin.
They tend to form in either tectonically active or glacial areas.
Alluvial fans distinguishing character
On a transect they have lobes of muddy large flow deposits with gravelly channel incisions. They also have sieve deposits which are gravel dominated subsurface deposits from sub-surface water flow removing mud.
Anastomosing channels
These are channels that form multiple stable channels with prominent levees and vegatated islands. They do not meander.
Anastomosing river deposits
These form on low slopes and generally have greater deposition than erosion. They can avulse (rapidly change course). Vegetation on the levees encourages muds and organics.
When compared to anastomosing rivers they show vertical continuity.
Anatomy of a carbonate platform
These are ~circular platforms of boundstones and fossiliferous rudstones and bafflestones that encapsulate a low energy center area that is filled with micrite and flanked by steep cliffs.
Back Barrier deposits
The sub-enviroments of this area vary with the barrier island configuration but can show signs of being marshy (anoxic, organics) to bidirectional channelized flow that produces lenticular and flaser bedding.
Backshore sediments
These are often eolian deposits or paleosols.
Bajadas
These are distal fans that merge with one another because of adjacent alluvial fans.
Barrier island facies from backshore to offshore
root traces within fine sands. coals and other lagoon deposits
Eolian dunes (trough cross beds, 3d ripples)
Swash-related deposits (planar beds or multidirectional trough cross beds). Well-sorted, mature sediments.
There are increasingly massive beds of coarse sediments as the breaker zone is at a lower depth.
alternating muds and sands grading into fine sands with bioturbation
Barrier Island Facies Model
General shift from eolian to shallow marine.
Barrier Island Reaction to Eustatic SL Shifts
Transgression: The Barrier islands erode and slump into deeper water
Regression: They prograde like shores or dunes. The Back-barrier becomes increasingly brackish and is capped by evaporites.
Barrier Island Subenviroments
There is the subtidal/subaerial barrier-beach complex: Sand islands about 1-20 m thick and long. This protects the coast from high energy waves.
Back-barrier region: calm, swampy/marshy areas aka a lagoon
Inlet-Channel complex: This is the calmish breaks in the barrier islands that allow for water to transfer between the back and front areas.
Beach Deposit Character
Fine grained sands with parallel planar laminae dipping at about 2-3 degrees. Placers are association with upper flow regimes. Within coarse sediment beaches there can be imbrication towards the shore.
Backshore beaches on barrier islands are more likely to be interbedded with muds and hummocks from storm surges.
Beach Morphology
Moving seaward:
- Eolian dunes (backshore) grade into the beach that starts with the foreshore this area is below the high tide line
- This grades into a steeper subaqueous zone that is the surf zone
- The base of the surf zone is at the low tide line where another steep dip occurs and this is the breaker zone
- There is then another sloe and that becomes the transition zone and grades into offshore
- Nearshore describes the areas in the surf, breaker, and transition zone.
Bioherms
These are hills that form from reefs. They depict the progradation of reefs are generally underlain by a massive boundstone lump flanked by massive framestones with beds of micrite in the center.
Braided channels
These are channels that have many sub-channels. They are highly unstable with many unvegetated islands aka sandbars. They tend to not have levees but instead are incised into the landscape.
Braided River Deposits
These channels have more gravel-like sediments, less fossils/organics, less mud, and pervasive longitudinal bars (tear-drop shaped lobes that migrate downstream and coarsen upwards)
Breaker Zone sediments
These have isolated trough cross stratification and finer sediments. They often have planar horizontal laminae with many vertical burrows.
Carbonate ramps
These are similar to continental ramps but with carbonates. This occurs on the inside of Florida. They differ from reefs sedimentologically by not having framestones or boundstones.
Channel Form Factors (4 total)
Sinuosity: the channel’s deviation from a straight line
of channels: Degree of subdivision
Braiding: Prevalence of bars/islands
anastomosing: The prevalence of flood-plains
Channel formation variables
Flow character, slope, sediment source, discharge, continuity, bed roughness, and human activity.
Generally as discharge and slope increases channels move from anastomosing to meandering, to braided.
Clast age on alluvial fans.
Because alluvial fans most often form at the interface of rising mountain faces the youngest clasts are at the base of fan because these are eroded first. The oldest clasts are eroded last and are on the top of the fan.
Coastal Sand Belt
These are barrier islands. They form in wave dominated coastal systems.
Coastal Sedimentology
Sources: These are generally fluvial or reworking from continental shelves
Progradaiton: There is episodic deposition
Keep in mind that all the current processes we observe are in an age of major trangression
Common subenviroments within deltas
Transgressive: tidal flats, estuaries, lagoons,
Regressive: tidal flats, deltaic sands/muds, and strandplains.
Correlation chart
Compares one or more stratigraphic section to time and not to depth.
Crevasse Splay deposits
These are lobes of coarse sands or gravels. They have climbing ripples because of the principle of continuity. They may show flaser bedding.
Deflation pavement
Lag deposits of large gravels. These are the materials that remain after small material is eroded away.
Delta
This describes a deposit of any subaerial or subaqueous deposit formed by fluvial sediments prograding into a standing water body.
The transgression we are currently within enable delta progradation. During regression the channels erode the deltas.
Delta Classification
Deltas are either river, wave, or tidal dominated. This is along the spectrum of mixed mud/siilts, fine sand, sand, and gravel. This second axis is dependent on how close the delta is to the source.
Deltaic Facies
These broadly have marine sediments that grade into marshy muds, organics, and channel sands. This grades into fluvial sediments. This is because regressive sequences are more likely to be preserved.
At the base of prograding deltas there are often turbidites and beach sands. These are overlain by marshy sediments and fluvial sediments.
Deltaic sub-enviroments and deposits
Overall they are heterogeneous and complex.
Overbank deposits are like crevasse splays. These will likely subside to marshy sediments.
Interdistributary bays/marshes: These form inbetween the lobes of a river dominated delta. They have high organic content, bioturbation, fine grained muds, oftentimes with oyster beds.
Distributary mouth bars: These are sub-aqueous sand/mud bars from the rapid loss of competence when entering the water.
Subaqueous levees: These also form due to the rapid loss of competence and underflow of denser material.
Entrenchment
This describe the lenses of fining upwards strata within alluvial fan deposits that reflect incision within alluvial fans.
Eolian facies model
These have high angle bedset with small-scale trough cross beds. They often have grain fall/flows. This is adjacent to low angle translatent ripple laminae. This is above fine grained silts, muds, clays, evaporites with bioturbation and fossils from interdune periods.
Eolian strata
These have grainfall slides, lag gravels, erosional bases with gravels, kinetic sieving, trough cross stratification.
Eolian structures
Excellent sorting, large scale trough cross beds.
Compared to fluvial systems they have translatent ripple stratification which describes that the planar beds are more likely to be preserved than the cross-strata of fluvial deposits. On a large scale they form sub-parallel parallel bedsets with lenses of trough cross bedding.
Epicontintental Seaways
These are like pericontinental seas but they are within the continent and are surrounded by land on both sides. They differ from normal seas because of limitted fetch. This results in a large amount of organic shales similar to lakes. They also can have storm dominated coastlines.
Ergs
These are areas that are larger than 125 km2 they encompass about 20% of modern deserts or 6% of earth’s surface.
They require ample sand and wind.
Estuaries
This is a term that describes drowned river valleys. They only occur during transgression because they fill with sediment and become a delta causing a transgressive lag of progradation elsewhere. This includes both fluvial and marine sediments. They can be classified by being wave, tide, or river dominant.
Estuaries facies
Because of ample accumulation space these deposits are often preserved. They are similar to deltas with a general shift from fluvial sediments to marine muds but differ in that the lowest energy area is in the middle of the estuary which is reflected in the progradation too.
Facies
This is a recognizable lithology or group of lithologies. It is the “sum of a sedimentary rocks lithologic character and acts as a class of deposit type.”
They are not dependent on locale only on the nature of the rocks. They can be lumped together to create sequences of facies (Bouma Sequence).
These are objective observations of rocks that do not vary with time. interpretations vary with time.
Facies features (10)
This includes grain size/maturity/composition, beforms, structures, geochemical character, diagenetic alternations, fossils, or geophysical character (polarity or magnetic susceptibility)
Facies of storm dominated Coastlines
These generally have hummocky shelf muds with fine sands and storm based “lag gravels” This occurs along usually passive margins where the fair-weather waves do not transport or impact sediment.
Fan Deltas
These are coastal prisms of alluvial sediments that are derived from density flows.
Ferestrae
These are fossil algea with holes.
Fetch
This is the surface area that wind is able to blow along the interface of a standing body of water and the air. Limitted fetch limits the turbidity of the surface and decreases the amount of waves that are seen on the surface.
This is why beaches are sandy but lake shores are mucky.
Floodplain Deposits
These are bioturbated muds with lots of organics.
They often include crevasse splay deposits which represent a break in the levee with similar character to a Bouma sequence.
Floodplains
These are natural flats adjecent to the flow that are periodically flooded during avulsions. They are particularily prevelant in single channel systems.
Fluvial channel deposits
There is a gradual fining upwards of sediments that represent the decreased competence of the flow near the top. The outer corner is erosional, the inner is depositional. The bases has trough cross stratification which grade into 2d ripples.
Fluvial System Deposits
These will be large U-shaped valleys with fine laminated planar muds that are intercepted by troughs of coarser normally graded sands with epsilon cross beds that shift throughout the basin due to the changing position of the channel/s. These also have “shoestring” sands that are from point bars and levees. There are abundant paleosols.
Hadley Circulation
This is the rising warm air (equator) and falling cold, dry air (@ +-30 degrees).
This is the reason deserts form at +-30 degrees. This helps paleogeographers understand continental shifts by finding how large sand seas shifted with time.
Helical Flow
This describes that when fluids are moving around a bend like in an anastomosing channel the outer edge has a higher elevation than the inner which causes a helical flow.
Lagoon facies models
When compared to an estuary there are less indicators of freshwater aka restricted flow and circulation. They tend to have massive deposits of silty muds that grade into a tidal inlet and eventually offshore deposits. They are similar to lakes in that they fine upwards as they fill their accumulation space. They have limited biodiversity because of their unique chemistry.
Orbital Velocity
This is the cyclical change in velocity of the wave’s velocity. It is significant because it peaks towards the shore and can only cause traction transport towards the shore. An integral of the oscillation yields zero.
Physical Processes within Lakes
Entering flow either floats has a middling (where there is a density gradient in the lake) or dense turbid underflow The first two create slow precipitation of particles the last creates turbidity flows.
Bioproduction also creates carbonates, shells, other carbon material that settles slowly to the lake base.
Reef facies models from inner to off-reef
The inner reef is the calmest enviroment with shifting salinity. There are few corals and mainly micrite.
The outer back-reef are skeletal grain/packstones with some bioturbation.
The Reef crest is made of boundstones, framestones, and bafflestones that are resistant to waves. This is the area of optimal growth because it faces the ocean and the ocean chemistry is most constant.
The deeper fore reef is made from gravity flows off the reef crest and will be more likely to have slumps of rudstone that may end up in deep water.
Shelf Break cause
There is usually a basement ridge that fills with sediment off the shores of continents that creates the continental shelf. This can be from tectonics, reefs, faulting, volcanics, diapars or folds.
Currently, because of transgression the shelf on the East coast is not depositing sediments which is why people are able to fish mammoth bones.
Storm Geomorphology and Impacts
Low pressure cells create storm urges/swell that are very longwave and deep waves which erode beaches, create muddy sediment plumes, and flood the shores. This creates hummocks because of the changing flow conditions.
Tidal Flat Anatomy
There is the subtidal zone below the mean low tide level. It has the highest velocity of tidal shifts and will have bedload deposits. These often have herringbone cross stratification and 2D ripples.
intertidal zone: this is the area within the average tidal flux where there is a mix of bedload and settling. This creates wavy shales, flaser and lenticular beds.
supratidal zone: This is the area only flooded at extreme tides and can form sabkahs in arid regions. This will range from organic rich shales to evaporites. They tend to have high bioturbation.
fluvial systems chart
