1. Readings from Sediments & Basins: (8:1-67)
Sedimentary Geology Geos 240 – Chapter 8 Depositional Environments Part 2 Barrier Islands & Deltas
Dr. Tark Hamilton
Readings from Sediments & Basins: (8:1-67)
Camosun College

3. See Notes River Dominated –Forced Regression
Tide/Storm Dominated “Forced Transgression”
Tides and Storm waves have different effects on outer exposed and inner estuarine coasts and different long term effects depending on gradually rising or falling sea levels. Transgressions, tectonic subsidence or rising systems tracts cause the coastline to recede as the sea comes ashore. Along flat, exposed coasts like the Gulf and Atlantic coasts of the USA, the beach is continually swept ashore by storm waves, often forming barrier islands as sea level continues to rise. Tide flats are generally narrow, especially for muddy facies which are easily eroded and worked down basin. Rising sea levels (transgressions) flood river valleys and produce sheltered, warm shallow brackish estuaries. These are sheltered from chaotic storm waves but subject to strongly funnelled tidal bores that move and deposit tractive sands as tongues heading into the estuary. This progresses with the development of barrier islands and longshore bars. Muddy facies are more typically restricted to the back barrier lagoons during rising systems tracts. Conversely, during regressions, coastal uplift or falling systems tracts, the increased gradient causes erosion, ravinement and outbuilding of the coast. Deltas build the Birdfoot shape, tide flats widen and wave dominated coasts from broad flat strandplains. For increased sediment supply, the deltas build out anyway in a situation called forced regression.
See Notes

4. Ravinement: Transgressive Facies Progression
Erosion > Deposition ~10m
Barrier Island/Cay
If sea level were stable for a long period, the mature coast would be straight and the shelf would be flat at m below mean low tide over much of the shelf width. During transgressions or active tectonic subsidence, storm wave erosion cuts an inclined surface as the coastline receeds and sea level raises. This erosional feature, like an inclined unconformity is termed a ravinement surface. In a vertical section the overlying facies become more basinal, but this might include either upwards coarsening or fining because lagoonal muds and shelf muds are separated by a sandy beach or barrier bar. These are often called “Cays” or Keys in the case of Florida where Americans can neither pronounce nor spell common English words. The original word was Quay for a finger of land that extended into the sea.
L1, L2
Contemporaneous Facies
S1, S2
See Notes

5. Effects of Hurricane Carla 1964 on Galveston Island Mud Marsh Sand
Old channels reopened and new ones were cut. Net erosion and removal of mud from tide flats and marshes. Lost of productive habitat and fringing marsh.

6. River Dominated Delta, Mississippi
Typical of Forced Regression
Birdfoot - Low Latitudes
Low Tidal Ranges

7. & Wave Dominated Deltas Smooth Arc - Low Tidal Ranges
Typical of Current
& Wave Dominated Deltas
Smooth Arc - Low Tidal Ranges

8. Typical of Forced Regression Birdfoot - Low Latitudes
Low Tidal Ranges

9. & Wave Dominated Deltas Smooth Arc - Low Tidal Ranges
Typical of Current
& Wave Dominated Deltas
Smooth Arc - Low Tidal Ranges

10. Indented Coastal Estuaries
Wave Dominated Deltas
Indented Coastal Estuaries
High Tidal Ranges

11. & Wave Dominated Deltas Smooth Arc - Low Tidal Ranges
Typical of Current
& Wave Dominated Deltas
Smooth Arc - Low Tidal Ranges

12. Typical of Tidal Dominated Deltas Indented Coastal Estuaries
High Tidal Ranges

13. Processes & Classification of Deltas

14. Processes & Classification of Deltas

16. Avusion Nodes & Distributary Switching
This is a perspective drawing.
The blue lines are all on one plane at sea level,
as is the green profile of the seaward edge of the delta.
The grade (gray) is a gradual decline in a perpendicular
plane above the central blue dashed line.
A-C is the shortest & steepest path to the coast & SL.
Delta builds outwards from old coastline and upwards above sea level
The Path from A to B becomes longer and slower due to a gentler slope.
The path from A to C remains both shorter, steeper and faster.
If the river finds a shorter path to C during overbank flooding, it will switch the river to the shorter course and abandon the old delta lobe. The old lobe will compact, erode and sink as wave and tidal processes exceed river flow and sediment supply fails. The New delta lobe will grow until the cycle starts over again.
This channel switching process also occurs in marshes and anastomosing streams. There it is also called avulsion, as flow directions and channels switch. There as at delta / river mouths, the avulsion node and channel switching occurs at flood times. This enhances the gradient, speed, competence and erosive power of flowing water.
See Notes

17. Small Distributary Deltas, Mississippi
Historic Development:: C) 1838, D) 1862, E) 1871, F) 1891

18. Mouth Bars: River Dominates Waves

19. Shoestring Sand Facies At Distributary Mouths

20. Prograding Delta Environments Foreset, Clinoform Silts
Topset, Flat Sands
Foreset, Clinoform Silts
Bottomset, Flat Clays

21. The End: Chapter 8-Part 2!