Depositional Environments (10-11)

Question 1

4 Sedimentary Basins

Answer 1

• Rift Basins - extentional boundaries
• Passive Margins - continent to ocean
• Foreland Basin - compressional/collision boundary
• Pull-apart - Strike-slip/transform boundary

Question 2

Alluvial Fans

Answer 2

• Close to the source alluvial sediments
• Cone-shaped deposits formed due to flow expantion as it causes a rapid decceleration of water at the fan apex
• Often on tectoniclly active areas as requies a steep juxtaposition of highlands and lowlands
• 3 parts of a fan:
  - Proximal - Conglomerates
  - Mid-fan - alternating conglomerates and sandstones
  - Distal - cross stratified ripple laminated sandstones pinching out into mudflows
• Poorly sorted and subrounded
• Sediments may prograde or regradedepending on climate and tectonics

Question 3

Fluvial Sediments

Answer 3

• Fining upwards succession
• Coarse-grained Bottom river sediments, basal lag
• Cross-stratified sanstones deposited on point bars
• Overbank (floodplain) mudstones show evidence of root formation

Question 4

Coastal/Marine Depositional Environments

Answer 4

Deltas - River flowing into sea or lake carrying a lot of sediment that is deposited at the river mouth in order, fining outwards, deltas prograde out to sea forming a coarsening upwards succession
Estuaries - River flowing into the sea depositing fewer sediments than sea-level rise therefore they do not form depositional deltas
Beaches - Boundary between land and sea that often prograde (coarsening upwards succession), these have wave and hummocky cross strata
Oolite Shoals - Hot wavy lagoons form oolites and often show bi-directional paleocurrent with tides
Continental shelf - Temestites are storm deposits and often show hummocky cross-strata
Reefs - Rigid wave resistant bound by organisms, grow upwards, formed by different organisms through geological time
Deepwater sediments - ash, plankton, dust (all fine-grained muddy)
Turbidites - Deep-sea marine sediments are mostly deposited by density currents of sediments falling off the continental shelf in avalanches, they are horizontal, laterally continuous deposits
- Bouma sequence of turbidite beds is A - Graded Coarse sand, B - Parallel laminated fine/medium sand, C - Current ripple laminated fine sand with silt, D - Laminated Clay and Silt, E - Hemipelagic muds