GEO Stratigraphy and Sedimentology Mineralogy, Qualitative and Quantitative Description of Clastic Grains

Transcription

1 GEO Stratigraphy and Sedimentology Mineralogy, Qualitative and Quantitative Description of Clastic Grains I. The mineralogy of sedimentary grains is an integral component of classification, and provides significant information for a variety of interpretive applications (provenance, maturity, etc.). Most grains have been subjected to varying degrees of rounding and abrasion, producing small grain size that makes identification of grain mineralogy difficult with hand specimens in field conditions. However, careful examination using a 10X handlens (in the field) or microscope (in the lab) can improve accuracy of identifications of the mineralogy of most clastic grains. For more accurate mineralogical studies of sedimentary grains, grain mounts or rock thin sections are studied using a petrographic microscope to identify mineralogy from optical characteristics of light transmitted through the grains (we will devote lab time to this later). After assessing the variety of minerals composing a sample, the relative volume of each is estimated. Qualitatively, this is done using charts (attached). Quantitatively, this is done by manually sorting each mineral type in unconsolidated sediments, or by conducting "point counts" across a thin section of lithified samples. General physical characters of minerals used in hand specimen identification: A. Form or habit: crystallinity, shape, surface features (ex. mica flakes) B. Cleavage/Fracture:number of cleavage planes (1: micas; 2: feldspars); conchoidal fractures (quartz, garnet) C. Color or streak. D. Transparency/Luster: transparency can be obscured by abrasion (such as in frosted quarts grains of dune sands), look for transparency in fractures or cleavages. E. Hardness: test by scratching with steel or glass (5.5), penny (3.0), fingernail (2.5). F. Other: taste odor, malleability, etc. G. Larger grains can be polymineralic (rocks), so some grains are best described by igneous, metamorphic or sedimentary rock names (ex. chert, basalt, granite) II. Qualitative Description of Clastic Grain Size & Shape A. Grain size: several grain size scales have been proposed, but the Udden-Wentwork (Φ) scale is almost universally accepted by sedimentologists since its inception by Udden in 1898 and subsequent modification by Wentworth in It is a geometric scale (with each division twice as large in increasing size, or half as large in decreasing size. This size field is divided into four general categories: gravel, sand, silt, and clay. (See text, Fig. 5.13, p. 83; and attached comparison chart). Grain size is estimated in the field using a sand grain size chart. After several field seasons geologists can usually identify the grain size without visual aids. Mudstones are identified by eating samples and assessing grittiness (due to silt). Grain size is qualitatively measured by a variety of methods depending on the general size range of grains and the degree of lithification. Gravels are analyzed by manual measurement of each clast. Sands (if unlithified) are sieved or put through settling tube analysis, while lithified sandstones are

2 analyzed in thin section. Mudstones (silt and/or clay) are analyzed after disaggregation in water using a pipette method, photohydrometer or Coulter counter; lithified mudstones are studied by electron microscopy or X-ray diffraction. B. Grain Sorting: estimates of the degree of variability of the sizes of grains. This can be done quantitatively for sands, or by comparison to standard grain sorting charts (attached). These range in qualitative units from very well sorted to very poorly sorted C. Grain Shape 1. Sphericity: Qualitatively this is assessed using charts. Quantitatively this can be measured using a variety of methods (intercept sphericity, maximum projection sphericity, etc.) 2. Pebble shape: equant (sphere), oblate (disk), prolate (roller, bladed). D. Roundness/Angularity: Again this is estimated by comparison to charts (attached), or by mathematical determinations for measurements (these are particularly complex for assessing roundness) III. Cements: The process of lithification involves the compaction of grains (text, p ). Generally with increasing pressure (burial) the grains are reorganized from cubic packing (48% porosity) to rhombohedral packing (26% porosity). Subsequent diagenesis can precipitate cementing minerals between the grains (in the pore spaces). Common cements include: A. quartz: precipitated in optical continuity with the quartz grain on which precipitation began. This results in very firmly cemented rocks. B. calcite: sparry calcite (large crystals), tested in the field by application with dilute HCl. C. hematite (Fe 2 O 3 ) or clay minerals (illite/montmorillonite), commonly derived from alteration of feldspar. Hematite produces red coloration of rocks. PART I. Qualitative analysis of sediment. ************************************** ANALYSIS OF SILICICLASTIC SEDIMENT ************************************** A. Texture grain size or range or grain sizes; for sand and gravel use both Φ units and verbal description (ex. fine sand (3 Φ); or coarse sand (-1 Φ) in a matrix of fine sand (3 Φ); silt; clay. B. Composition 1. Color : 2. Clasts Whole rock % a. Quartz: %

3 b. Feldspar: % c. Rock Fragments: % types: igneous, metamorphic, sedimentary, specifically: d. Clay: % e. Other: % types: mica, amphibole/pyroxene, glauconite, 3. Fossils: % (types: ) C. Sorting: very well-sorted, well-sorted, moderately well-sorted, poorly sorted D. Roundness: very angular, angular, sub-angular, subrounded, rounded, well-rounded E. Maturity: immature, submature, mature, supermature PART II. Quantitative analysis of sediment. The next step is to describe the size distribution of your sample in a quantitative manner. Sand grain size analysis is method of characterizing features of sands. This analytic technique requires the grains to be completely uncemented, or friable enough cement that the grains may be disaggregated. Well cemented sandstones are analyzed for grain size in thin section. The goal of such analyses is to quantitatively characterize the grain size and sorting by statistical means. Remember statistics is always a reduction of data into a more efficient display to illustrate trends or patterns. The size distribution of clastic sediments provides information on the "maturity" of the sediment. With increasing transport (duration & distance) sediments exhibit decrease in grain size and increases in sphericity, roundness and sorting. METHOD Clastic size analysis uses the Wentworth Scale for size ranges (classes). This scale is a geometric progression in grain size between less than 1/256th to greater than 256 mm, allowing increasingly narrower ranges of classes at the finer end of the size range distribution. See text, p. 83. Most modern and ancient sediments are less than 1 mm in diameter, requiring more detailed analysis for grains within this general size fraction. Coarse Clastic Sediment Analysis (Sieve) A. Disaggregate the grains. A variety of procedures can be used to disaggregate grains within a sedimentary rock. Because we are using disaggregated samples, no further manipulation is necessary. B. Use sample splitter to obtain approximately 100 grams of sample (unless sample is a gravel or contains significant clay, see below). When using splitter, pour sample into hopper and stir to mix thoroughly; do not shake because this can locally concentrate fines. C. Weigh split sample to within 0.01 gram. D. We will use screens at 0.5 Φ gradations. Research accuracy requires 0.25 Φ gradations; 0.5 Φ gradations are used in rough work; and 1.0 Φ gradations are too broad to generate useful data. Clean the screens (using wire brush, air hose and/or tapping on table).

4 E. Nest the screens in order, coarsest at top, PAN on bottom. Pour sample into top screen and shake gently by hand, remove screens that are too coarse to catch any grains. Place cover on the stack of screens. F. Fasten very tightly into sieve shaker, and sieve samples for 10 minutes. G. Place 18" by 18" brown paper or poster board with center crease on table. Place waxed paper that has been creased into center of brown paper sheet. H. Carefully pour grains from coarsest screen onto waxed paper. Invert screen over paper and gently tap with heel of the hand. TAP SCREEN DIAGONAL TO THE MESH OF THE SCREEN. I. Transfer grains to balance pan with paper underneath to catch any spillage. J. Replace waxed paper onto brown paper, invert screen and pound it sharply on the paper. STRIKE THE TABLE EVENLY TO PREVENT DAMAGE TO THE SCREEN. Add any additional grains to the balance pan. Weigh sample to 0.01 grams. Store splits in folded paper. Label with sample number, sieve size and Φ size range. K. Repeat for each sieve. DO NOT FORGET TO WEIGH THE PAN FRACTION. L. Record data into provided data table. Reference: Folk, R.L., 1974, Petrology of Sedimentary Rocks: Hemphill Publishing Company, Austin, TX, 182 p. PART III: Graphic Representation of Sample A. Using Excel or Kaleidagraph, generate a histogram of weight percentages of fractions on report sheet. This is a bar graph. Its utility is as an efficient visual depiction of the size distribution for comparison between samples. See example in text, p. 84. B. Again using graphical software, generate a cumulative frequency (SF, for size frequency) curve. This curve is generated by plotting grain size (as Φ οr mm) on the X-axis, and the cumulative weight percent on the Y- axis. The Y-axis is most commonly an arithmetic scale (equally scaled from 0 to 100%). Most samples plot as an "S" shaped curve on the arithmetic cumulative frequency plot. However, a semi-logarithmic (X-axis arithmetic; Y-axis logarithmic) scale is sometimes used, usually producing a straight-line distribution. The primary advantage of this method is that it is independent of the sieve gradation used, allowing all statistical parameters to be read directly from the curve. Plot both arithmetic and semi-log cumulative frequencies. Make sure you use the semi-log scale for subsequent calculations, because it allows more precise determination of statistical parameters. C. Identify: 1. Median (M) at 50% point of SF curve. 2. Quartiles (Q 1 at 25%; and Q 3 at 75%) 3. Percentiles (P 5 and P 95 )

5 D. Determine millimeter values for points from Φ conversion chart and record them in proper blanks of report sheet. Statistical Characterization of Sample Calculate the following and fill in appropriate blanks of your data sheet using the cumulative curve on semi-log graph paper. See text, p A. Measures of Central Tendency (averageness) 1. Mode (Mo) is the most frequently-occurring grain diameter, defined by the highest point on the frequency curve, but is difficult to determine from cumulative frequency curves. 2. Median (Me) is easy to measure and commonly used. It is the grain size on the 50% line of the cumulative curve (such that half of the weight is finer-grained and half is coarser grained. Disadvantages include that it does not accurately reflect skewed samples or bimodal. Folk (1974) recommends against usage. 3. Graphic Mean (M z ) is most recommended because it closely responds to the mean, and is easier to calculate. M z = (Φ 16 + Φ 50 + Φ 84 )/3 B. Measures of Sorting (Uniformity): Inclusive Graphic Standard Deviation (text, p. 85) C. Measures of skewness (asymmetry): Inclusive Graphic Skewness (Sk I ), (text, p. 85) D. Measures of Kurtosis of Peakedness (K G ); (text, p. 85) 1 = normally peaked, mesokurtic < 1 = excessively peaked, leptokurtic > 1 = deficiently peaked, platykurtic

6 CLASTIC TEXTURE REPORT BY SIEVE ANALYSIS Observer Sample Date Stratigraphic Information Locality Information RECENT UNKNOWN Phi Size MM Sieve No. Weight (g) Individual Wt. % PAN Cumulative Wt. % Dry Wt. Graphic Mean Inclusive Graphic Skewness M z = SK I = Inclusive Graphic Standard Deviation Graphic Kurtosis I = K G = Prepare a 1-2 page report of your sample, integrating your analyses and a reasonable interpretation of your results. The report must include your histogram, two cumulative frequency graphs (arithmetic and semi-log) and results of quantitative analyses. You will present these results to the class (with, I hope, some visual aids) (see text, pp for example). NOTE do not get hung up on rare grains that seem to invalidate your environmental interpretation. Remember that others have used the sieves and that grains often get stuck in the mesh and then get free later on. Your presentation and report are both due on the day indicated on the syllabus.