pt 2 Flashcards
(150 cards)
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.














