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

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

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 10-15 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

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.

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

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

Typical of Forced Regression Birdfoot - Low Latitudes Low Tidal Ranges

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

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

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

Typical of Tidal Dominated Deltas Indented Coastal Estuaries High Tidal Ranges

Processes & Classification of Deltas

Processes & Classification of Deltas

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

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

Mouth Bars: River Dominates Waves

Shoestring Sand Facies At Distributary Mouths

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

The End: Chapter 8-Part 2!