Sedimentology Lecture 2. continental depositional systems
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Continental depositional systems accumulate and distribute sediment far from marine environments via gravity and water flows. The most common systems are colluvial, alluvial, and fluvial. Colluvial systems involve downslope sediment movement via rainwash, sheetwash, or creep. Alluvial systems form distinctive fan patterns as streams emerge from mountains. Fluvial systems create complex depositional patterns as rivers transport sediment from mountains to basins via braided, meandering, anastomosing, or straight channel patterns.
Sedimentology Lecture 2. continental depositional systems
- 2. 2 Continental depositional systems are those where sediment accumulation and distribution occur far from marine environments. Most processes are generated by gravity and water (flows), transferring masses of loose sediments towards morphological depressed areas on the emerged real or towards lacustrine basins. Most common continental systems are: (1) colluvial systems; (2) alluvial systems and (3) fluvial systems. Gawthorpe & Leeder, 2000
- 4. Colluvium (also colluvial material or colluvial soil) is a general name for loose, unconsolidated sediments that have been deposited at the base of hillslopes by either rainwash, sheetwash, slow continuous downslope creep, or a variable combination of these processes. Colluvium, or hillwash, is both an erosive deposit and a preserving medium for buried surfaces. The term encompasses slope deposits moved by shallow surface flow (or slope wash) or by mass movement (or creep or slide). Colluvium is soil- and/or sediment-derived material that accumulates on lower slopes. It is poorly sorted and heterogeneous, composed of any size grade from clay to coarse sand plus rock rubble, and can be up to several meters thick. Bedding and stratification are often poor and particularly hard to identify in the field (as opposed to in thin section), and colluvium may also contain a variety of artifact inclusions brought down from upslope. Colluvium may occur any place that possesses more than two degrees of slope, even beneath woodland. French C.A.I. (2017) Colluvial Settings. In: Gilbert A.S. (eds) Encyclopedia of Geoarchaeology. Encyclopedia of Earth Sciences Series. Springer, Dordrecht
- 5. Eluvial vs. colluvial process POROSITY Ancient example E L U V I A L P R O C E S S a n d D E P O S I TPOROSITY Modern example C O L L U V I A L P R O C E S S a n d D E P O S I T
- 6. Blikra & Nemec, 1993
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- 10. Longhitano et al., 2014
- 11. Longhitano et al., 2014
- 12. Blikra & Nemec, 1993 Møre-Romsdal region, western Norway
- 13. Nemec & Kazanci, 1999 west-central Anatolia, Turkey
- 14. ALLUVIAL SYSTEMS
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- 16. Blikra & Nemec, 1993
- 17. 17 When streams emerge from mountains, they often spread out and deposit sediment in a distinctive pattern known as an alluvial fan
- 20. 10 Km
- 21. Longhitano et al., 2014
- 22. Longhitano et al., 2014
- 23. Alluvial fan systems When streams emerge from mountains, they often spread out and deposit sediment in a distinctive pattern known as an alluvial fan Depositional environments in an alluvial fan (less easy in a colluvial fan) can be distinguished on the base of their respective relationships of proximality and distality from the ‘point of sediment source’ (apex). Sediment grain size, sorting and internal structures are highly varying depending on their relative distance. Proximal fan Mid fan Distal fan
- 24. Types of alluvial fan: debris-flow dominated, sheetflood and stream-channel types – mixtures of these processes can occur on a single fan. Nichols, 2009
- 26. FLUVIAL SYSTEMS
- 27. Rivers form complex depositional systems and are tought to represent the most efficient type of sediment transport/transfert from the mountains towards the sedimentary basins
- 28. Miccadei et al., 2018
- 30. TYPES OF RIVERS AND CHANNEL PATTERNS 30 Braided Anastomosing Straight Fluvial deposits are sediments that are transported and deposited by rivers in a continental environment. There are several types of fluvially derived deposits, including: (1) braided-river deposits, which form at and beyond the bases of mountains, where the gradient of the ground surface is relatively steeply inclined; (2) meandering-river deposits, which form on more gently inclined floodplains; (3) anastomosing-river deposits, where channels are slightly sinuous but laterally stable; (4) straight-channel river deposits which fill quasi rettilineous valleys. Meandering
- 31. meanders (slope gradient = 0.025%) braided channels (slope gradient = 0.87%) The Rhone River Clastic Reservoirs Master Course 2014 - S. Longhitano 31 Clastic depositional systems Continental systems: fluvial systems
- 32. 32 Continental systems: fluvial systems Oligocene Ussana Fm., SW Sardinia
- 33. BASAL DISCONTINUITY CHANNEL FILL TOP OF THE CHANNEL Channels are often the most diagnostic features in ancient rivers FLUVIAL CHANNELS 33
- 34. CHANNEL FILL BASE OF THE CHANNEL 34
- 35. erosion deposition erosion deposition Crosssection Fluvial channels are elongate erosional/depositional features. Meandering channels are common in rivers, as they reproduce specific characteristics that help in detecting them in modern and ancient settings. FLUVIAL CHANNELS and POINT BARS In the rock record, meandering channels potentially represent good reservoirs, because they are represented by sandbodies encased into mudrocks. In more detail, the cross-section view of a meandering channel fill is markdly asymmetrical and filled by clinostratified coarse sandstone or conglomerate. 35
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- 38. 38 TYPES OF RIVERS AND CHANNEL PATTERNS Braided Meandering AnastomosingStraight CHANNEL-FILL: FINING- UPWARD SEQUENCE
- 39. A 3-D perspective view of a subsurface structure map, undeveloped offshore West Africa reservoir, draped with a 2-D spectral decomposition image. The reservoir is a sandy turbidite filling an erosional valley at a depth of about 3,100 meters. The draped image also conveys a 3-D perspective by showing three different slices through the reservoir section with different color bars (red, green and blue). Understanding the vertical stacking patterns of flow units is important for determining optimum drainage.
- 41. Miocene channel systems within the Congo MegaSurvey (courtesy of PGS Exploration)