GEOL 440 Sedimentology and stratigraphy: processes, environments and deposits Lectures 16 & 17: Deltaic Facies

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1 GEOL 440 Sedimentology and stratigraphy: processes, environments and deposits Lectures 16 & 17: Deltaic Facies

2 What controls morphology and facies of the Nile Delta? Basic processes Classification Facies

3 Early descriptions of deltas Gilbert, 1885 Pleistocene Lake Bonneville, Utah Barrell, 1911: Devonian, Appalachians

4 Progradation and time Bhattacharya,2006

5 Classification of deltas River-dominated Nile delta MEKONG ORINOC O GANGES Wavedominated Tidedominated A basic scheme we ll use at first... BUT modification by basinal processes is not the only parameter controlling the morphology of deltas

6 River-dominated deltas ESA, 6 th Feb. 2007

7 Modes of interaction between sediment-laden river water and basin water 1a. Homopycnal flow r(inflow)=r(basin)

8 Modes of interaction between sediment-laden river water and basin water 1b. Inertia-dominated deposition downstream fining Gilbert-type delta channel lunate mouth bar mouth channel (almost) flat bar back bar crest foresets steeper bar front bottom sets

9 Gilbert-type delta top sets foresets bottom sets

10 Modes of interaction between sediment-laden river water and basin water 2a. Hyperpycnal flow r(inflow)>r(basin) Basin Plane jet bottom flow Plan view Basin

11 Modes of interaction between sediment-laden river water and basin water 2b. Friction-dominated deposition Curved levees New bars & bifurcation during progradation Channel bifurcation Rapid flow deceleration & lateral expansion Formation of triangular mouth bar, rapidly fining downstream

12 Modes of interaction between sediment-laden river water and basin water 3a. Hypopycnal flow r(inflow)<r(basin) Basin Plane jet near watersurface flow Plan view Settling Basin

13 Modes of interaction between sediment-laden river water and basin water 3b. Buoyancy-dominated deposition Do you expect this type of deposition to occur at low or high river discharge?

14 Summary: Controls on Sedimentation Bhattacharya,2006

15 River dominated: birds-foot delta (Mississippi-type) DELTA PLAIN N 10 km

16 m Prograding mouth bar succession Clean sand, low- & high-angle 3D strat. representing beach & dune deposits Well-sorted sand, planar & ripple x-lam., tangential x-bedding (bar crest); poorly sorted sand/silt, cut & fill, small-scale trough x-bedding (bar back); large channel fills near top (distributary channel) Interbedded mud/sand, wave & current ripple x-lam., parallel/lenticular lam., bioturbation, contorted bedding, possible cut & fill representing distal mouth bar deposit Finely laminated mud, bioturbation, marine fauna, contorted bedding representing prodelta deposit Silty/sandy slump sheet representing redeposited (distal) mouth bar sediment Homogenous mud, finely laminated or bioturbated, marine fauna representing shelf deposit Mississippi delta

17 Distributary channel fill facies Ripple-laminated, fine-grained sand- & siltstone, plant-rich & with palaeosol-coal units representing filling of shallow channel after avulsion Carboniferous, SW Wales 15 Trough & planar x-bedded sandstone with unidirectional palaeocurrents representing migrating dunes/bars in channel 10 5 Massive, locally x-stratified sandstone with internal erosion surfaces & some soft-sediment deformation representing main channel subject to flooding Basal coarse lag deposit above erosional surface representing channel avulsion & erosion into delta front progradation succession 0-5 m

18 Interdistributary Bay filling Bhattacharya,2006

19 2 to 10 m Interdistributary bay facies (sandy, proximal) Overbank flooding Crevasse splay FU s : Alternating deposition from sand-laden overbank flows & fine suspended load CU: Levee progradation Erosional, FU sand

20 Delta lobe progradation-abandonment-reworking Bhattacharya,2006

21 Deltas and sea-level change Porebski and Steel,2006

22 Wave-dominated deltas

23 Rhône delta, France Continuous cuspate/ arcuate beach shoreline

24 Wave-dominated delta front succession U. Cret. Dunvegan Fm., Alberta Medium-grained sand, plane-parallel lam. & lowangle x-strat., passing into sand/mud with plant roots... representing beach to non-marine deposit Fine- to medium-grained, cross-bedded sand... representing wave-modified mouth bar deposit on upper shoreface Fine-grained, bioturbated sand with HCS & plane-parallel lamination... representing wave-swept lower shoreface deposit 10 5 Transgression CU mud to sandy mud to sand with pervasive bioturbation, storm beds & hummocky x-strat.... representing regressive shelf deposit Similar to prograding beach-ridge strand-plain succession due to weak fluvial imprint 0 m

25 Tide-dominated deltas Tidal flood currents may reach far into channel & influence deposition Funnel-shaped distributary channel with bi-directional sediment transport & mud flocculation during slack water Redistribution of mouth bar sand into elongate tidal current ridges & channels with preferred ebb- & flood-dominated pathways

26 Jamuna Himalayans Brahmaputra Ganges-Brahmaputra delta, Bangladesh Extensive tidal flats and marshes dissected by numerous tidal channels DELTA

27 Channel Delta plain (non-tidal) Delta plaintidal flat Tidal sand bar Tidal channel Tidal channel deeps Ganges-Brahmaputra delta, Bangladesh 100 km

28 20-60 m Tide-dominated delta facies succession Alternating sand-silt-mud, x-lam., flaser-linsen bedding, small channel fills, brackish-water fauna... representing tidal flat facies on delta plain Erosive, FU sand with herringbone x-strat., mud drapes, tidal bundles, ripple x-lamination... representing tidal channel on delta plain Ord River delta, Australia Bidirectional x-bedded sand, mud drapes, tidal bundles, reactivation surfaces, scours/channel fills... representing migrating tidal current ridges & inter-ridge channel fills on delta front Bioturbated mud passing into interbedded mud-silt-sand, marine fauna... representing open shelf to prodelta succession

29 Tidal flats and channels in Ord River delta

30 Influence of grain size Bhattacharya,2006

31 Deltaic facies and architecture depend on: Density difference between inflow water and basin water River processes and modification by waves and tides but also on: Type of alluvial feeder or distributary system Grain size of transported sediment Gradient of inflow system Basinal water depth Type of sediment diffusion

32 Types of subaerial feeder system A Very steep gradient (< 20º-30º), gravel-dominated alluvial systems: ephemeral, unconfined streams with mass flows small radius along mountain front and fjord margins B Steep gradient (±0.4º), gravelly alluvial systems: multiple, highly mobile, bedload streams line source along delta front proglacial outwash plains, braided streams at head of fjords/lakes C Moderate gradient, gravelly-sandy alluvial systems: multiple, stable streams acting as line source mouth bar development at head of fjords and lakes D Low gradient, sandy to muddy alluvial systems: widely spaced, highly stable, suspension load streams inflows act as point source and prograde in isolation birdfoot-type, mouth-bar deltas at margins of coastal lowlands

33 Classification of deltas..coarse grained deltas Postma, 1990

34 Deltas and hydrocarbons Source facies mainly marine mud (and lacustrine mud) Reservoir facies mainly delta front sand (mouth bar & bar-finger sand) relatively clean sand adjacent to marine source rock capped by interdistributary mud sand body trend ^ coastline in fluvial- & tide-dominated deltas sand body trend // coastline in wave-dominated deltas other: barrier/beach sand shelf sand channel sand

35 Hydrocarbon traps Main stratigraphic traps: sand pinchouts in areas of maximum interfingering between delta front sand and marine mud isolated sand bodies in mud Main structural traps: synsedimentary growth faults, induced by progradation of delta front sand over prodelta mud, resulting in oversteepening then failure Th (downthrown side) = 5-10 x Th (upthrown side) salt domes

36 Summary Rivers, waves, tides and influence on sediment dispersal Homo, hyper and hypo pycnal flows Patterns of sediment dispersal Vertical sequences Grain size/gradient/depth influences? Soft-sediment deformation? Think about the allo cyclic controls on deltaic sedimentation Reading: B&D: Chapter 15 Boggs; Chapter 9 Bhattacharya, New Facies Models Revisited Reading, Chapter 6 Leeder, Chapter 22