GEOG376 Geomorphology I Spring 2005

Transcription

1 6.2 Physical properties of sediments physical properties of sediments (e.g., mineralogy, texture, sorting) or bulk properties relating to arrangements in a deposit or landform (e.g., porosity, shear strength, imbrication, fabric, structures) can control the rate & type of geomorphic processes Grain size & texture texture: size and shape of sediment particles including surface features on grains and their bulk arrangement (e.g., fabric, packing, orientation, porosity ). a fundamental property of sediments useful description for classification & interpretation purposes measured using the Udden-Wentworth size scale (Fig. 1): a geometric scale using a log 2 transform of grain sizes (D in mm) in phi (φ) classes, where φ = -log 2 D mm... as such, the -ve log makes φ positive for small (most abundant) particle sizes the log 2 transform means that classes are ½ or 2x the mm value of the next class avoids using fractions of mm & log-normal graph paper terms such as sand, gravel, silt, clay-loam all have specific textural contexts Table 1: common size classes and φ intervals (see also fig. 2) size class D phi boulder > 256 mm < pebble 64-4 mm to -2.0 medium sand mm 1.0 to 2.0 medium clay mm 9.0 to 10.0 an excellent online text on the properties of sediments and sedimentary rocks published by a leading sedimentologist/geomorphologist, Robert Folk (1999). Sedimentary Petrology. Check it out it s free! 376_manualS05.doc Dr. Ian J. Walker 70

2 Figure 1: Udden Wentworth grain size scale (source: Lewis & McConchie, 1994: fig. 5.1) 376_manualS05.doc Dr. Ian J. Walker 71

3 Figure 2: Qualitative (verbal), metric (mm), Wentworth (φ) & graphic representations of grain size scale (source: Boggs 2001: fig. 3.2) 376_manualS05.doc Dr. Ian J. Walker 72

4 the texture triangle below is a graphical way of expressing the % sand silt clay in any sediment or soil sample Figure 3: Soil texture triangle (source: Christopherson, 1999) 376_manualS05.doc Dr. Ian J. Walker 73

5 variation in grain sizes of a sediment sample can be quantified & summarized in tabular format by size groups (fig. 4A) or using a frequency distribution (figs. 4B-C) where frequency is expressed as a weight of sediment in individual size classes (4b) or cumulative weight of size classes (4c,d) otherwise known as the grain size distribution or gsd Figure 4: Methods for summarizing grainsize data (source: Boggs 2001: fig. 3.3) 376_manualS05.doc Dr. Ian J. Walker 74

6 Figure 5: Normal and cumulative frequency distributions of grainsizes (source: Boggs 2001: fig. 3.6, 3.4) 376_manualS05.doc Dr. Ian J. Walker 75

7 6.2.2 Sorting & skewness various moment statistics can be derived from the frequency distribution either graphically (Folk & Ward, 1957; see fig. 5, 6) or statistically (Krumbein & Pettijohn, 1938) to describe sediment properties 1 st moment = mean, 2 nd = standard deviation (SD, σ φ ), 3 rd = skewness (sk φ ), 4 th = kurtosis (k φ ) mean (first moment): decribes the average size of sediment particles in the sample In general, mean grain size indicates energy of the transporting or depositing environment telling either of the competence of the process (i.e., can move a certain size of sediment 14 ) or the fall velocity below which that sediment can no longer remain in transport standard deviation (SD or σ φ, 2 nd moment) or sorting is the range of grain sizes in a sediment sample & how widely scattered these are about the mean σ φ sorting < 0.35 φ very well sorted φ well sorted φ moderately well sorted φ moderately sorted φ poorly sorted φ very poorly sorted > 4.00 φ extremely poorly sorted in reality, most natural sediments are not normally distributed (i.e., mean, median and mode do not coincide) most are asymmetrical & have coarse or fine tails to the distribution so we need some other measure of grain size distribution shape 14 provided that the maximum size is available! 376_manualS05.doc Dr. Ian J. Walker 76

8 skewness (sk φ, 3 rd moment) describes the shape of the grainsize distribution e.g., is it skewed toward larger or smaller values (i.e., tails of fine or coarse sediments)? o reflects sorting toward the tails in the distribution e.g., positive skewness = mode of coarser sediments (- φ) with a tail of finer sediments (+ φ) or the distribution is skewed toward +φ values (see fig. 6A) Figure 6: Skewness of grainsize distributions (source: Boggs 2001: fig. 3.7) sk φ skewness > +0.3 strongly fine skewed fine skewed near symmetrical coarse skewed < -0.3 strongly coarse skewed What would cause tails of fine or coarse sediment in nature? it is also possible to have bimodal distributions (i.e., two peaks in the gsd) that reveal clues about the sediment history (i.e., source, subsequent reworking) and possibly, the contemporary processes acting in a sedimentary system o e.g., coarse lag or deflation deposits in deserts are negatively skewed with few coarser sediments larger particles (fine gravels delivered by fluvial action) are left behind as they exceed the competence of the fastest winds grain size and texture variations reflect modes of transport, deposition and changes in these processes through time (e.g., stratigraphic sequences, upward fining of lacustrine deposits) and/or space (e.g., grainsize variations on a beach from fine to coarse to fine, etc.) (Figs. 7,8,9) sorting reflects variations in the energy of the transporting or depositing environment e.g., most aeolian dune sands are well-sorted because wind action continuously winnows away finer particles most fluvial sands are poorly sorted why? 376_manualS05.doc Dr. Ian J. Walker 77

9 Figure 7: Texture as a function of transport modes & environments of deposition (source: Lewis & McConchie, 1994: fig. 5.5) Figure 8: Bivariate plot of skewness vs. SD showing regions typical for beach & fluvial sands (source: Boggs, 2001: fig. 3.8) 376_manualS05.doc Dr. Ian J. Walker 78