Chapter 8: Learning objectives

Transcription

1 Chapter 8: Learning objectives Understand concept of sedimentary facies Signinifance of lateral and vertical facies associations Walther s Law Understand the sedimentary facies of alluvial fans, eolian deserts, braided streams and meandering streams types of sediments; sedimentary structures; lateral and vertical facies associations; and all other characteristics Strat & Sed, Ch. 8 1

2 Concept of facies Sedimentary facies = sediments or sedimentary rocks that formed within a given depositional environment (i.e., tidal flat facies; deep sea facies) Lithofacies are characterized by physical properties (color, lithology, texture, sedimentary structures) Biofacies are characterized by their unique fossil assemblages Sedimentary environments produce sedimentary facies Strat & Sed, Ch. 8 2

3 Lateral and vertical facies associations Often, the environmental interpretation of a sedimentary facies is equivocal (similar sedimentary deposits can be produced in different environments) e.g., cross-bedded sandstone could be fluvial or nearshore marine Facies associations provide context to aid the interpretation, because adjacent facies are related environmentally e.g., if underlying or overlying units contain peat, fossil roots, leaves, etc., then cross-bedded sandstone probably is fluvial, not marine Strat & Sed, Ch. 8 3

4 Lateral and vertical facies associations Walther s Law: In a conformable vertical sequence of rocks, facies that are adjacent vertically must at one time have been adjacent laterally. Corollary: If a vertical sequence of rocks includes an unconformity, then adjacent facies on either side of the unconformity probably were not adjacent laterally Strat & Sed, Ch. 8 4

5 Walther s Law (prograding shoreline example) The migration of laterally adjacent sedimentary facies produces a predictable sequence of vertically adjacent facies Strat & Sed, Ch. 8 5

6 Continental environments Four main kinds of continental depositional systems: Fluvial (alluvial fans, braided rivers, meandering rivers) Desert Lacustrine (lakes) Glacial Continental deposits are: dominated overwhelmingly by siliciclastic sediments scarcity of fossils complete absence of marine fossils Strat & Sed, Ch. 8 6

7 Alluvial fans Depositional setting of alluvial fans: Areas of high relief, commonly at the base of a mountain range Most common in arid or semiarid climates Characterized by sporadic but high sediment discharge Fans may pass downslope into braided stream, desert floor or playa lake settings Also occur in humid climates Glacial outwash fans May pass downslope into plains or standing bodies of water (fan deltas) Strat & Sed, Ch. 8 7

8 Alluvial fans Fan geometry Cone-shaped, arcuate or lobate as seen from above Convex-upward as seen in transverse profile Concave-upward as seen in longitudinal (radial) profile Upper fan, middle fan, distal fan subdivisions Overall size is variable, usually less than 30 km across Along mountain fronts, individual fans may merge to form a laterally extensive piedmont Strat & Sed, Ch. 8 8

9 Death Valley fan Strat & Sed, Ch. 8 9

10 Strat & Sed, Ch. 8 10

11 Fans feeding braided trunk stream Strat & Sed, Ch. 8 11

12 Fan geometry transverse profile plan view longitudinal profile Strat & Sed, Ch. 8 12

13 Alluvial fan sedimentary processes and products Stream flow Coarse, poorly sorted, linear deposits occur in stream channels Sheetflood deposits occur where sediment spreads out overbank or at the end of a channel onto the fan Better sorted, but variable grain sizes; some cross-bedding Sediment-gravity flow Especially in arid settings with infrequent but torrential rains, debris flows, landslides, mudflows produce very poorly sorted, chaotic deposits which normally lack sedimentary structures Strat & Sed, Ch. 8 13

14 Alluvial fan sedimentary facies Strat & Sed, Ch. 8 14

15 Alluvial fan sediments Upper fan deposits Debris flow processes common to dominant Thick beds, very coarse grained, poorly sorted, few sedimentary structures matrix-rich conglomerates Middle fan deposits Subequal stream flow and debris flow processes Thinner beds, finer-grained sediments (sands and gravels; clast supported conglomerates and sheet sands) Planar and trough cross-beds may be present Distal fan deposits Stream flow and sheet flood processes dominant Still thinner beds, finer-grained and better sorted sediments (sand, silt) Low-angle cross-stratification and troughs Strat & Sed, Ch. 8 15

16 Alluvial fan sediments: Summary Fan deposits are characterized by overall textural and compositional immaturity Fan deposits exhibit strong grain size gradient from coarse (proximal) to fine (distal) Abrupt lateral facies changes in down-fan direction and radially away from stream channels Strat & Sed, Ch. 8 16

17 Alluvial fan sedimentary facies Strat & Sed, Ch. 8 17

18 Alluvial fan vertical facies associations Individual beds may show no detectable vertical grain size trends If fan is prograding basinward, vertical sequence will exhibit pronounced upward thickening of beds and coarsening of sediments Strat & Sed, Ch. 8 18

19 Eolian deserts Environmental setting Most deserts found at 10-30º N and S latitude (Trade wind belts) Deserts presently cover 20-25% of Earth s land surface Desert subenvironments include: Alluvial fans, ephemeral streams and lakes, dune fields, interdune regions of sedimentation or deflation (erosion) Strat & Sed, Ch. 8 19

20 Depositional processes and products Wind is the dominant agent of sediment transportation Particles smaller than 50µm (silt) can be transported in suspension; larger particles transported by traction and saltation Deposits are: Wind-blown silt (loess) Sand, typically very well sorted Lag deposits (gravel) the residue that forms deflation pavement Strat & Sed, Ch. 8 20

21 Depositional processes and products Dunes may form extensive dune fields or sand seas Size of dunes can range from < 1m to 450m, up to several km in length ( draas ) Geometry of dunes is governed by sediment supply and prevailing wind direction(s) Unidirectional wind (barchan, transverse ridge dunes) Bi-directional wind from opposite directions (reversing dune) Bi-directional wind from orthogonal directions (seif or longitudinal dunes) Dominant sediment is texturally mature quartz sand Strat & Sed, Ch. 8 21

22 Strat & Sed, Ch. 8 22

23 Dune types Barchan Seif (longitudinal) Strat & Sed, Ch. 8 23

24 Strat & Sed, Ch. 8 24

25 Strat & Sed, Ch. 8 25

26 Strat & Sed, Ch. 8 26

27 Dune deposits Sets of medium- to large scale crossstrata with foresets dipping up to 30-34º in lee direction Sets of planar-tabular cross-strata Horizontal or slightly dipping bounding surfaces between sets of cross-strata Note scarcity of trough cross-strata (little or no scour and fill activity) Strat & Sed, Ch. 8 27

28 Dune deposits bounding surfaces steep foresets Strat & Sed, Ch. 8 28

29 Interdunes Interdune areas occur between dune fields or other eolian deposits (e.g., sand sheets) Interdune may be site of deposition or deflation Deflationary interdune Coarse, granule-size lag sediments; sometimes with rippled surface and inverse grading Depositional interdune Subaqueous or subaerial deposits; finer grained; always characterized by low angle stratification Strat & Sed, Ch. 8 29

30 Sand sheets Sand sheets are flat to gently undulating sand bodies that surround dune fields Deposits include low- to moderatelydipping cross-stratification, sometimes interbedded with ephemeral stream deposits Strat & Sed, Ch. 8 30

31 Interdune and sand sheet deposits Strat & Sed, Ch. 8 31

32 Eolian desert summary No universal model for vertical facies associations (lateral facies are highly variable) Scale is important: Individual sets of cross-beds may be 10-20m high Deposits: High-angle cross-stratification Dominantly quartz, fine grain size, good sorting, good grain rounding, frosted/pitted grains Strat & Sed, Ch. 8 32

33 Size matters! You won t see this in a subaqueous deposit. Strat & Sed, Ch. 8 33

34 Strat & Sed, Ch. 8 34

35 Braided rivers Depositional setting and characteristics of braided river systems: Low sinuosity (i.e., ratio of channel length to river valley length) High gradient (distal alluvial fan, glacial outwash plain, mountainous reaches of river systems) Numerous channels separated by bars and small islands High sediment load Sometimes high, but variable water discharge Strat & Sed, Ch. 8 35

36 Braided rivers depositional processes and products Bars and braiding form as a consequence of Rapid, large fluctuations in water discharge High load, high supply of coarse sediment Sparsely vegetated, noncohesive banks Formation of bars: 1. Coarsest sediment chokes channel and is deposited during brief episodes of high water discharge 2. As current energy wanes, bar material remains in place while channel breaching (entrainment of finer material) occurs between bars Strat & Sed, Ch. 8 36

37 Braided rivers depositional processes and products Bars Longitudinal bars (mid-channel) Long axis is parallel to current flow Mostly coarse material, but grain size decreases toward the top and at the downstream end Massive or crude horizontal bedding indicative of deposition under high flow regime Transverse bars (dunes) Long axis is normal to current flow Crest may be straight or linguoid Well developed tabular and trough cross-stratification Mostly sand-size sediment Lateral bars (analagous to point bars) Large bars form in low energy setting along sides of channel Strat & Sed, Ch. 8 37

38 Braided river anatomy Strat & Sed, Ch. 8 38

39 Braided rivers depositional processes and products Overall summary of deposits: Gravelly and sandy sediment with little mud Dip direction of foresets is variable, but unidirectional in downstream direction Lateral migration of the river and vertical aggradation produces a sheet-like deposit Strat & Sed, Ch. 8 39

40 Braided rivers vertical facies associations In general, vertical facies associations are highly variable, depending on sediment size and variability of stream discharge End members, from (1) coarse sediment/high discharge setting to (2) finer sediment/seasonal discharge setting Possibly no distinct pattern or vertical change in grain size Or, stacked fining-upward cycles Strat & Sed, Ch. 8 40

41 Braided rivers vertical facies associations Decreasing sediment size Increasing seasonality of discharge Strat & Sed, Ch. 8 41

42 Meandering rivers Depositional setting and characteristics: High sinuosity Low gradient Finer sediment load Single channel Vegetated, cohesive banks Steady, less variable rate of discharge Sometimes a downstream continuation of braided river; often discharge into a delta system Strat & Sed, Ch. 8 42

43 Meandering rivers Anatomy of a meandering river Main meander channel Point bars (inside bend of meander loop) Natural levee Flood basin or floodplain (beyond levee) Oxbow lakes Abandoned cut-off meanders Strat & Sed, Ch. 8 43

44 Strat & Sed, Ch. 8 44

45 Meandering river anatomy Strat & Sed, Ch. 8 45

46 Meandering river processes and products Current velocity is greatest along the outer bend of meander loops Helical flow within channel creates a gyre that moves bottom water away from the cut-bank to the point bar, and surface water away from the point bar to the cut-bank Result is erosion of cut-bank, deposition on point bars, and lateral and downstream migration of meanders Strat & Sed, Ch. 8 46

47 Meandering river processes and products Strat & Sed, Ch. 8 47

48 Meandering river processes and products Channel deposits Coarse lag of sand and gravel Indistinct bedding Thin, discontinuous Point bars Coarse material on lower part, finer material on upper part Large-scale trough cross-bedding (lower) to small-scale trough cross-bedding (upper); some plane-bed parallel laminations (flood) Paleocurrent indicators highly variable, but overall in downstream direction Strat & Sed, Ch. 8 48

49 Meandering river processes and products Natural levee Vertical accretion of fine sediment during overbank (flood) deposition Thicker and coarser near the bank; thinner and finer toward flood basin Cross-bedded and horizontally stratified sands; laminated muds Flood basin or floodplain Laminated muds that settled from suspension May contain plant material, evidence of bioturbation Crevasse-splay Graded deposit that forms when floodwaters breach a natural levee Strat & Sed, Ch. 8 49

50 Meandering river processes and products Strat & Sed, Ch. 8 50

51 Meandering river vertical facies associations Classical finingupward sequence produced by lateral migration of meanders Coarse channel lag deposits overlain by fining-upward point par deposits overlain by muddy overbank Strat & Sed, Ch. 8 51

52 Meandering river summary Characteristic finingupward cycles may be stacked vertically Overall meandering river system produces a mosaic of shoestring sand bodies encased in muddy overbank deposits Strat & Sed, Ch. 8 52