A delta is a depositional system developed where a river enters a standing water body (sea or lake). Different features will characterize deltas

Transcription

1 DELTAIC SYSTEMS

2 A delta is a depositional system developed where a river enters a standing water body (sea or lake). Different features will characterize deltas formed under different wave or tidal influence, sediment supply and coastal morphology. after Dalrymple et al., 1992

3 The name Greek letter

4 Factors in Delta Development 1. Climate 2. Water discharge 3. Sediment yield 4. River-mouth processes 5. Wave power 6. Tidal processes 7. Aeolian processes 8. Nearshore currents 9. Shelf slope 10. Tectonics of receiving basin 11. Receiving basin geometry

5 Different types of Deltas

6 Processes Important in Delta Development and Maintenance CLIMATE Controls sediment-water yield. Controls in-situ delta deposits. Tropical = large thick accumula=ons of peat. Temperate = thin, high con=nuous peat layers. Arid = complex interfingering of supra=dal and evaporite deposits.

7 Processes Important in Delta Development and Maintenance WATER DISCHARGE 1. ERRATIC DISCHARGE = braided channels with wide lateral con=nuity. Numerous interfingering, fining upward sequences showing highly variable porositypermeability rela=onships. v. irregular sedimenta=on rates. 2. NON-ERRATIC DISCHARGE = stable meandering channels (shoestringing sands).

8 Processes Important in Delta Development and Maintenance SEDIMENT YIELD 1. Primarily func=on of basin area and discharge. 2. High fine grained sediment loads = expansive sub-aqueous deltas with high H 2 O content and unstable clays. Slumping Deforma=onal features Local diapirism 3. Compac=on is high.

9 Processes Important in Delta Development and Maintenance RIVER MOUTH PROCESSES Where a distributary channel enters a water body, most of its sedimentary load is dropped down and redistributed by the overriding sustained flow (tractional conditions) into form a lobate body. After dropping sediment, the riverine-fed flow decreases its density (ρf ), which can be higher, similar or lower than those of the surrounding water (ρw). If ρf<ρw, the flow will override the main water body forming a plume (hypopycnal flow). If ρf~ρw, the flow will mix rapidly with the main water body (homopycnal flow). If ρf>ρw, the flow will glide down under the main water body (hyperpycnal flow). If at a certain depth hyperpycnal flows meet a denser watermass (stratified water bodies), these cases the flow will detach itself from the floor and override the denser water (mesopycnal flow). After Galloway & Hobday, 1996

10

11

12 Processes Important in Delta Development and Maintenance Waves Most important in reshaping deltas. Volume sediment delivered/wave energy. Deposi=onal units = beaches, barriers, etc. Low energy = low profile beaches osen overwash dominated. High energy = higher profiles with high quartz deposits. Subaqueous slope - auenua=on capacity. Low energy 1 x 10 7 ergs/second High energy 20 x 10 7 ergs/second

13 Processes Important in Delta Development and Maintenance Tidal Processes 1. Reduc=on in ver=cal density stra=fica=on, therefore buoyancy insignificant. 2. Bidirec=onal sediment transport (flood and ebb). 3. Marine and fluvial mixing zone is expansive (ver=cal and horizontal). Linear =dal ridges common. Upstream =dal asymmetry resul=ng in significant bedload transport into system.

14 Processes Important in Delta Development and Maintenance Winds Can create wind set-up at coast giving rise to current and liuoral circula=on. Can create significant increases in nearshore wave energy. Offshore winds can cause set-down and therefore reduce wave ac=on significantly. Parallel-to-coast winds can drive longshore currents resul=ng in significant mud deposits downdris of delta.

15 Processes Important in Delta Development and Maintenance Nearshore Currents Driven by: Deep oceanic currents impinging on shelf. Tidal propaga=on. Wind and water. Density currents. Offshore sandbodies sub-parallel/parallel to deposi=onal strike. Sand bodies located significant distances offshore or downdris form ac=ve lobe.

16 Processes Important in Delta Development and Maintenance Shelf Slope High rates of sediment accumula=on and rapid prograda=on resul=ng in low angle slopes. Fric=onal auenua=on of surface gravity waves. Slopes may be ac=vely prograding during modern =me. Submarine canyons = net loss of sediment (Congo, Ganges- Brahmaputra).

17 Processes Important in Delta Development and Maintenance Tectonics of Receiving Basin Rapidly subsiding basins result in overthickening of deltaic bodies. Rela=vely stable result in con=nuous, widespread, laterally con=nuous bodies. Localized differen=al weigh=ng and dewatering of sediments = Subaqueous mass movement. Displaced sediments. Complex slumping.

18 Processes Important in Delta Development and Maintenance Major ConfiguraBons of Receiving Basins I. Skewed deltas due to high current (Sea of Japan). II. Input of sediment from closed end (Gulf of California). III. Downwarped area-sediment movement inland (Niger). IV. Ac=ve subsidence seaward of shoreline (Senegal River). V. Semi-enclosed (Gulf of Mexico).

19 Distributary Channel PaEerns A. High subsidence, low waves & offshore slope, small tidal range, fine grained sediments B. Erratic discharge, intermediate wave energy, high tide range, steep offshore slope C. High waves, high tidal range

20 Types of Delta Switching PaEerns

21 Main types Mississipi Delta: fluvial dominated Rodano Delta: wave dominated Gange Delta: tide dominated

22 Components of a Delta environment After Bhattacharya, 2006 Despite all the possible variations, the basic architecture of a deltaic body shows a delta-plain area (characterized by the presence of distributary channels), a delta front (the steepest part of the system) and a prodelta (where fine deposits are accumulated over a sub-horizontal surface.

23 Postma s classification (1990) is based on feeder systems and water depth at river mouth. This last parameter allow to define two main typologies of deltas: Gilbert-type delta, with a steep front (25-35 ) and shoal-water type delta, with a gently inclined front (5-15 ) after Postma, 1990

24 Independently from water depth or dominant sedimentary processes, the basic building block of the deltaic architecture will be a lobate accumulation developed at the outlet of the distributary channel and called mouth bars.

25 Deposi=onal Environments at a River Mouth

26 Lateral and vertical stacking of mouth-bar units will build up the deltaic body forming a lobate front which will be reshaped by other processes (tides or waves). Modified after Wellner et al., 2005

27 In SHALLOW WATER SETTING, the abrupt flow expansion lead to a sediment dropping at a very high rate, commonly promoting formation of massive, normal-graded deposits, which can be successively removed under tractional/ frictional condition by the overriding sustained flow. after Wellner et al., 2005

28 A mouth bar is commonly fan-shaped, with a flat base and concave upward top. Beds thins both laterally and frontally. The proximal part is commonly characterized by erosive surfaces, indicating a dominant bypass. Feeder-channel levee can be present also underwater in the proximal mouth-bar area. Sedimentary features of mouth bars are mainly defined in shallow-water settings. after Bridge & Demicco, 2008

29 Sedimentary dynamics and deposits in a mouth bar are very similar to those characterizing a crevasse splay in alluvial plain setting. after Fielding, 2010

30 Although, the basic architecture of a mouth-bar lobe is simple, several minor feature can testify the occurrence of peculiar processes of sediment distribution. after Fidolini, 2010

31 In the proximal part of the bar, the coarser fraction occur. The bar front can be inclined up to 15 and massive, normalgraded beds can be accumulated by rapid sediment dumping from turbulent suspensions. after Fidolini, 2010

32 In the proximal part of the bar, sections transverse to the main flow show a mounded geometry of the bar after Fidolini, 2010

33 In the proximal part of the bar, sections transverse to the main flow can also show the geometry of the feeder channel.

34 In the distal part of the bar, fine-grained deposits and tabular geometries will be dominant. Localized scours can also occur. after Fidolini, 2010

35 CU Mouth bar progradation will form coarsening-upward units, from mud and fine sand (distal lobe) to coarse sand and gravels, if the feeder channel is preserved at the top. CU

36 CU

37 Sections parallel to flow will show gently inclined beds (3-10 ) pinching out downcurrent. CU

38 Spatial distribution of deltaic mouth bars is commonly controlled the attitude of the system to fill the available spaces. Normally, a newly prograding bar will tend to fill the space between two pre-existing lobes. The stratal architecture derived from this process is called compensational stacking pattern, and, in outcrops, it will be highlighted by stacked bed packages pinching out toward different directions. B A A B

39 mouth bar3 mouth bar2 mouth bar1

40

41 In case of fluvial-dominated deltas, mouth bar can prograde for long distances from the coast generating deltas with digitated profile. In these cases mouth bar are labeled as finger-bar. after Reineck, 1970

42

43 Finger bars progrades over fine sediments, which are commonly intensively loaded. The presence of well-developed channel levee, limits sediment dispersion in the interlobe areas, which are commonly characterized by mud deposition. after Fisk, 1970

44 Where distributary channels will enter relatively deep water, a different type of mouth-bar will develop. Lateral stacking of these mouth-bars will produce a GILBERT-TYPE DELTA. Gilbert deltas are characterized by the presence of an avalanching front. Basin on their architecture, they are divided into four main portions (from base to top): topset, foreset, toeset and bottomset. It s clear that the difference in elevation between topset and bottomset represents the depth of the waterbody where the delta is prograding. toeset topset foreset bottomset

45 TOPSET (tracbon) FORESET (en-mass+tracbon) TOESET (tracbon+en-mass) BOTTOMSET (tracbon) 5 m 500 m

46

47 TOPSET FORESET TOESET

48 FORESET TOPSET

49 Topset/foreset transition Topset deposits are commonly represented by fluvial deposits (with different styles) which can be locally strongly reworked by waves or tidal currents. According to some Authors, gradual transitions between topset and foreset deposits can be indicative of wave influence. Abrupt transitions can be indicative of fluvial dominance. TOPSET FORESET

50 A gradual transition is thought to be indicative of wave reworking.

51 These geometries can also be interpreted in terms of base level oscillations

52

53 Foresets are characterized by a depositional angle ranging between 25 and 30, but they can reach 40 if the sediment is sufficiently coarse. Under these conditions, delta front is unstable and commonly affected by collapses, which can generate slumps, debris flow or even turbidity currents. These processes commonly form linguoid accumulations along the front, where also erosive features (scours or chute cut by currents accelerating along the slope) can be present. after Nemec, 1990

54 Foreset deposits in transverse sections, can locally highlight the mounded geometry of gilbertian, mouth-bar lobes.

55 Lateral shift of gilbertian mouth bars is commonly highlighted by backlap and downlap geometries in parallel sections. DOWNLAP BACKLAP BACKLAP DOWNLAP

56 A peculiar feature of Gilbert deltas is represented by the so called «backset beds», whose development is associated with hydraulic jump conditions.

57 Debris flow Debris flow Debris flow Hydraulic jump occur commonly where the flow expands colliding with an obstacle, which is commonly represented by a debris-flow accumulation. Debris flow

58

59 Toeset deposits represent the transition between the steeply inclined foreset and the subhorizontal bottomset. Because of the significant change in depositional angle, the larger rolling clasts along with several debris flow are trapped in the toeset zone, where also turbulent flows can drop part of their sedimentary load.

60

61 Bottomset are the finest deposits of the Gilbertian system. They are characterized by deposition of turbidity currents (HDTC and LDTC), although, being located (such as toeset) below the wave base, they are commonly interested also by mud settling.

62

63 Sedimentary processes acting in a Gilbert-type delta settins: a summary

64 ORIGIN A OF A GILBERT- TYPE DELTA A deep water setting is required to develop a Gilbert-type delta. Such a conditions can be generated by tectonics (faults) or erosional morphologies. In case of deltas developed on tectonically active escarpments, a different stacking pattern will characterized deltas developed under different extensional regimes from those formed under compressional ones. Normal fault Erosive generated depression Inverse fault

65 Both Gilbert and shoalwater deltas can be dominated by fluvial, tidal or wave processes, but the stratigraphic record of their progradation will be represented by CU successions. after Nichols, 1996

66 WAVE DOMINATED FLUVIAL DOMINATED TIDE DOMINATED da Nichols, 1996

67 Deltas represent probably the major oil and gas reservoirs, and knowledge of their internal architecture is a key point to approach reservoirs. The basic facies models concerning distinction between tide-, wave- and river-dominated deltas are not enough to predict internal architecture and distribution of barriers. Distribution of these properties vary with delta s depositional history, but also with minor local factors, such as topography, sediment supply and depositional processes. Wells placement and reservoir management can be properly assessed only after setting up a detailed facies model for the specific case of study. After Maguregui & Tyler, 1991

68 AKNOWLEDGEMENTS Part of the material contained in this lecture is courtesy of Dr. M. Ghinassi Course on Sedimentology Padova University (Italy) Petroleum Geoscience and subsurface Geology