Sedimentology Grain Parameters Reading Assignment: Boggs, Chapter 3 Medium grained turbidite sandstones with crinoid stems in the Brushy Canyon Fm., West Texas Sedimentology Grain Parameters Reading Assignment: Boggs, Chapter 3

Key Concepts Grain-size analysis & statistical parameters Sediment / Rock textural classification Grain shape Grain roundness Textural maturity Grain surface features Sediment / rock fabric

Medium grained turbidite sandstones with crinoid stems in the Brushy Canyon Fm., West Texas Sedimentology Grain Parameters (1): Grain Size Reading Assignment: Boggs, Chapter 3

Why do Grain-Size Analysis? Sediment / rock description = communication Sediment / rock name = classification = communication Information on source area, transport & depositional environment

Grain Size: The Udden-Wentworth Scale Weathering products in transport include: Sediment from physical weathering Sediment from chemical weathering Ions from chemical weathering Wentworth Scale addresses sediment Sediment ranges from clay to boulder Uses geometric scale because grain size ranges from <0.004 mm to m Establishes size-classes (cobble, sand, silt, etc.) Standard sieve sizes for silt to pebble Also uses a Phi Scale to make units of equal value F = -log2D (D=grain diameter)

How do we measure grain size?

Gravel Sand Mud F = -log2D Size range (metric) Wentworth Size Class <−8 >256 mm Boulder −6 to −8 64–256 mm Cobble −2 to −6 2–4 mm Pebble -1 to -2 2 - 4 Granule 0 to −1 1000–2000 µm Very coarse sand 1 to 0 500–1000  µm Coarse sand 2 to 1 250–500  µm Medium sand 3 to 2 125–250 µm Fine sand 4 to 3 62.5–125 µm Very fine sand 8 to 4 3.9–62.5 µm Silt 20-10 0.98–3.9 µm Clay Gravel Sand Mud

conversion of unconsolidated sediments into rock Lithification conversion of unconsolidated sediments into rock F = -log2D Size range (metric) Wentworth Size Class <−8 >256 mm Boulder −6 to −8 64–256 mm Cobble −2 to −6 2–4 mm Pebble -1 to -2 2 - 4 Granule 0 to −1 1000–2000 µm Very coarse sand 1 to 0 500–1000  µm Coarse sand 2 to 1 250–500  µm Medium sand 3 to 2 125–250 µm Fine sand 4 to 3 62.5–125 µm Very fine sand 8 to 4 3.9–62.5 µm Silt 20-10 0.98–3.9 µm Clay Conglomerate (rounded clasts) Breccia (angular clasts) Gravel Sand Sandstone Mud Mudstone

How To Present The Grain-Size Data Record weights Always plotted as Coarse to Fine Histogram Frequency Curve Cumulative Semi-Log Plot

End of Part 1: Grain Size

Medium grained turbidite sandstones with crinoid stems in the Brushy Canyon Fm., West Texas Sedimentology (2): Grain Size Distributions Grain Parameters Reading Assignment: Boggs, Chapter 3

Statistical Treatment of Grain-Size Data “Average” grain size Mode - most frequently occurring size class Median – 50% Mean – arithmetic average Sorting Skewness Kurtosis

“Average” Grain Size Mode Median f84 f16

Why Sample at 16 & 84%; 5 & 95%? 84% 16% 68% 1 sd 1 sd 95% 5% 90% 2 sd

Significance of Average Grain Size Source Area – What size did the grains start at? Weathering history? Transport History Grain size directly correlates to energy level Selective transport Proximal to Distal fining Depositional Environment Energy level Calculation of flow speed in water & air from grain size Depositional process of sediment layers Segregation of grain populations on bedforms Bedload vs. suspended load

Sorting – How Uniform Is The Population?

Sorting in Real Sediment

Visual Estimates of Sorting Fig 3.3, p. 58.

Significance of Sorting Reflects source area – original range in population Transport history Selective sorting Depositional environment Rate of deposition (e.g., rapid vs. slow) Type of processes (e.g., glacial till vs. beach sand)

Skewness – Population Symmetry

Normal & Skewed Populations p. 59, fig 3.4, fig. 3.5 p. 60. Symmetrical population Positive-Skewed Fine-Skewed Negative-Skewed Coarse-Skewed

Significance of Skewness How does the grain population depart from a normal population? Usually reflects depositional processes in removing/concentrating fine or coarse fraction Beach sand is typically coarse-skewed as wave reworking removes finer sediment and carries it offshore River sand is typically coarse-skewed because fine sediment is carried in suspension distally Lag surfaces (as in deserts) are coarse-skewed as fine sediment is deflated Suspension deposits (lakes, offshore, aeolian loess) tend to be fine- skewed as they represent most distal transport

Kurtosis

Significance of Kurtosis Similar to sorting is showing dominance of center or ends of population As with sorting – product of source area, transport history & depositional processes Leptokurtic sediment are usually well sorted with concentration of population at mean Platykurtic sediments are usually poorly sorted with coarse & fine ends of population over- represented Bimodal or Polymodal distributions are platykurtic, typically reflecting mixing of environments

End of Part 2: Sorting

Medium grained turbidite sandstones with crinoid stems in the Brushy Canyon Fm., West Texas Sedimentology Grain Parameters (3): Sediment Texture Reading Assignment: Boggs, Chapter 3

Questions to ask What are the primary and secondary lithologies (gravel, sand mud)? Gives a sense of range in particle sizes What are the shapes of the grains? How round are they? Why to we care? Tell us a little about source area primarily reflects type and rate of depositional processes – a natural progression

Sediment/Rock TEXTURAL Classification

Grain Shape Determined by relative lengths of long, intermediate & short axis Zing (1954) classification Largely inherited from parent mineralogy reflecting crystal form or mineral cleavage Plutonic Quartz has an spherical shape Biotite has a disk shape But is influenced by transport history Significant in transport – think Mica vs. Quartz

Grain Shape & Environment River gravel – concentration of rollers Beach gravel – concentration of blades & disks

Grain Shape and Roundness Measure of abrasion of grain edges Reflects transport history – intensity and/or duration Strongly influenced by mineral hardness and mineral cleavage planes Grain shape and roundness are independent

Grain Roundness Rounded Subangular Angular

Textural Maturity

Textural Maturity Flow Chart Used in Lab Clay content Sorting (σφ) < 0.5φ Roundness < 5% Rounded > 0.5φ Angular > 5% Immature Sub-mature Mature Super-mature

Significance of Textural Maturity Reflects source material…. BUT primarily reflects type and rate of depositional processes – a natural progression Is the clay content (suspended load) removed? How well is the traction load sorted? Have the grains been in transport long enough to become rounded? Textural Inversion – mix of textural parameters Example: well rounded grains in a mud matrix Usually a mixing of environments (e.g., aeolian sand blown into lagoonal mud)

Grain Surface Features Frosted Aeolian Sand Large field of grain surface textures Common descriptions: unmodified, polished, frosted, pitted, scratched Origin in transport, chemical weathering, precipitation (diagenesis) SEM-scale Distinctive frosting of sand with aeolian transport

End of Part 3: Texture

Medium grained turbidite sandstones with crinoid stems in the Brushy Canyon Fm., West Texas Sedimentology Grain Parameters (3): Sediment / Rock Fabric Reading Assignment: Boggs, Chapter 3

Sediment/Rock Fabric Grain orientation Current orientation of elongate (rod-shaped) grains Imbrication – most stable configuration Random / choatic patterns with suspension settling

Sediment/Rock Fabric Clast support vs. Matrix support Grain packing in sand 26-47 % original porosity determined by grain shape with same grain size Porosity largely independent of grain size (given the same sorting) Permeability decreases with grain size as pores get smaller and friction increases Both Porosity & permeability decreases with sorting as smaller grains fill pores between larger grains

Significance of Grain Size & Sorting in P&P River Bottom Fine sediment Coarse sediment Field of view 30 m by 10 m How a fluid moves through a sedimentary body is a function of fabric, which is determined by sedimentary processes.

Deposit Porosity = (1 – volume concentration of sediment) = void space fraction Porosity of a granular deposit is primarily a function of grain sorting, NOT grain size.

Permeability controlled by both grain size & sorting. Permeability = measure of a deposit to transmit interstitial fluid (e.g., fluid flow through the pore network) Beard, D.C., and Weyl, P.K., 1973, Influence of texture on porosity and permeability of unconsolidated sand: AAPG Bulletin, v. 57, p. 349-369. Permeability controlled by both grain size & sorting.

End of Part 4: Fabric

Questions You Should be Able to Answer How would you determine the grain size in a river bed of cobbles, aeolian dune sand, or clay sediment from a lake? How would you determine grain size in a sandstone? Shale? What are the 3 basic weathering products? What range of these does the Wentworth Size Scale address? Why is a geometric scale used in the Wentworth Chart? What is the phi scale? What are the basic size classes in the Wentworth Scale? What is the mm size break for each of these? What are the 4 basic graphic forms in which to display grain-size data? What is the mode, median and mean as used in grain-size data? Why is a grain population typically sampled at the 5, 16, 50, 84 and 95% points? What determines the average (mean) grain size? What sorts of information can you derive from the mean grain size? Why does sediment typically fine from proximal to distal?

10. What does sorting measure in a sample 10. What does sorting measure in a sample? What factors determine sorting? 11. What does skewness measure in a grain sample? What factors influence skewness? In what settings would you expect to find a fine-skewed population? A coarse-skewed sample? 12. What does kurtosis measure in a grain sample? What factors influence kurtosis? In what settings would you expect to find a leptokurtic population? Platykurtic population? 13. What are bimodal and polymodal distributions? Under what conditions would they occur? 14. What is the basis for the tertiary grain-size classification of sediment/rock? Officially, what is gravel? Mud? Sand? What is the difference between, say, a muddy gravel and a sandy gravel? 15. What determines grain shape? How is it measured? What are the basic grain shapes? What is the shape of typical river cobbles? Beach cobbles? 16. What is grain roundness? What factors determine grain roundness? What does grain roundness tell you? 17. What is the textural maturity of a sediment?

18. What is the typical progression in textural maturity in a sediment 18. What is the typical progression in textural maturity in a sediment? Why does it occur? What is a textural inversion? 19. What surface features are common on grains? How do grain surface features develop? Why are aeolian grains commonly frosted? 20. What is sediment/rock fabric? Why does it develop? What sorts of information does it yield? 21. What is imbrication? How does it indicate flow direction? 22. What is the difference between matrix- and clast-supported fabric? 23. What is porosity? What is permeability? 24. What is the relationship between porosity/permeability and grain size? Why? 25. What is the relationship between porosity/permeability and sorting? Why? 26. What is the range of depositional (original) porosity in sand? What factors influence depositional porosity?