Part IV: Clastic Depositional Environments.

A. Alluvial Fans.

B. Fluvial systems (rivers and streams).

C. Eolian (Aeolian) depositional systems.

D. Deltas (Chp. 9):

1.) Both constructional processes (fluvial input) and erosional processes (marine-coastal erosion) (see Fig. 9.1).

2.) Controls:

a. Sediment supply
b. Wave action
c. Tidal flux

3.) Deltas build up (aggrade) and out (prograde).

4.) Progradation of distributary cannel sands over prodelta muds produces coarsening-upward stratigraphic packages (Fig. 9.2).

E. Clastic Marine Environments:

1.) Coasts can be erosional or constructional

2.) Beaches (p. 260-267 in Boggs)

a. Shoreline washed by wave action

b. Long, narrow sand bodies.

- Important Petroleum Reservoirs

c. Characteristics:

i.) Well-sorted, rounded clasts typical

ii.) Low-angle cross bedding that dips toward the open water (2-10°)

d. Beaches divided into (Fig. 9.21):

i.) Backshore: behind berm or ridge.

- Water only reaches during storm surge
- Wind transports sand from berm and forms dunes

ii. Foreshore (zone between high and low tides)

iii. Upper shoreface (mean Low tide point down to fair weather wavebase).

 

3.) Shelf Environments (Chp. 10)

a. Sediment size = f(shelf geometry [angle and width] and wave energy)
b. Wave energy decrease with increasing depth.

i.) As water depth increase a point is reached where surface wave energy no longer causes water movement. This point = wave base f(wave amplitude, length and period).

ii.) Wave base during calm weather = fair weather wavebase (typically 5-10 m).

iii.) During storms storm wavebase (typically 50-100 m)

c. In general, clastic shelf deposits:

i.) Ancient clastics shelf environments can be hard to characterize.

• Fossils (body and trace) are extremely helpful.

• Chemical sediments:

- Glauconite: low O2
- Phosphate: elevated nutrients, slow sedimentation
- Glendonites: cold water, glacio-marine
- Carbonate cements: slow sedimentation, warm water

ii.) Primary controls on sedimentation:

- Sediment transport- hydraulic conditions (wave energy, storm, etc.)
- Relative sea level
- Sedimentary supply

iii.) Secondary controls:

- Climate
-Tectonism of the shelf
-Biological/sediment interaction—binding (sea grass)

iv.) Present continental shelves may not be a good model for the ancient.

• Holocene glacio-eustatic S.L rise (-10m/1000yrs) = rapid transgression over former erosional 9in some cases) out equilibrium.

d. Clastic shelf deposits prograde (see Fig. 9.25).

     4.) Deep-water environments: (continental slope to deep basin) (p. 292-305 in Boggs)

a. Slow sedimentation rates 1-60mm/1000 yrs)

b. Sediment Sources and Processes:

i.) Sources:

- Fine-grained
- Eolian “dust”
- Fine-grained shelf sediment stirred up by storms
- Fine-grained glacial sediment
- Volcanic dust
- Cosmic dust

c. Benthic organisms: sponges, echinoderms, worms, arthropods (crabs and amphipods)

i.) Nereites and zoophycus ichnofacies

d. Oxygen levels:

i.) Anoxic (anaerobic) [O2 = 0mL/L]

ii.) Dysoxic (dysaerobic) [0.2-1.0 mL/L O2]

iii.) Oxic (aerobic) [>1.0mL/L O2]

e. Hemipelagic sediments: deep shelf to continental rise

i.) Fine-grained with up to 75% biogenic sediment. (as little as 25% siliciclastics)

f. Pelagic Sediments: fine-grained sediment deposited on the slope to deep basin.

Mostly on abyssal plain.

i.) Mostly biogenic planktonic) sediment with <25% non-biogenic (siliciclastic) [>75% biogenic sediment]

g. Gravity-driven sedimentation (p. 300-302):

i.) Mass wasting on shelf, slope and rise, includes slumps, underwater landslides, density currents (turbidity currents).

ii.)Turbidity currents = high-density fluid (clays, silts and fine sand mixed with water).

• Triggered by storms, earthquakes, slope failure due to sedimentation
• Deposit = Turbidite: has characteristic stratigraphic packaging = Bouma sequence (repeating graded beds)

h. Characteristic Stratigraphic packaging for hemipelagic to pelagic sediments.

i. Biogenic Pelagic- hemipelagic sediments

i.) Silica (all opal)

• Diatoms = plants, phytoplankton
• Radiolarians = animals, zooplankton

Produce: siliceous ooze

ii.) CaCO3

• Coccolithophorids (coccoliths, forams, pteropods)
• ”Inorganic” precipitation = whitings

Sediment produced = calcareous ooze

 

 

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