<p>Geology 103 Lecture #1 Reading: Boggs, 5th ed., pp. 3-12, 45-63, 110- 114 Sedimentary textures</p><p>I) Field description of sedimentary textures</p><p>- Description is an important tool for scientists - Every experienced geologist should have a similar description - Similarities are based on use of a common “language” of geology - Make observations, then interpretations</p><p>A) Sorting:</p><p>See Figure 3.4 from Boggs 5th edition, p. 51</p><p>- Proper terms for a Geologist to use in a visual description: Very well sorted well sorted moderately sorted poorly sorted</p><p>- Refers to the WHOLE SAMPLE, not just individual grains - Provides an estimate of size distribution - Sorting is essentially a measure of the standard deviation of the population</p><p>B) Roundness:</p><p>See Figure 3.11 from Boggs, 5th edition, p. 58</p><p>Roundness refers to sharpness of the corners of the grains These are the proper terms for a geologist to use in a visual description: Very angular, angular, sub-angular, sub-rounded, rounded, well- rounded Is used to describe individual grains/components in a sample Different grain types may have different roundness</p><p>C) Composition: - Is given for each clast type or mineral present - Is listed as a percent: - Usually to the nearest 10% - Traces can be listed as a few percent - Occasionally juggle percentages to the nearest 5% Must total 100% !!!! D) Size - Is given for each clast type or mineral present - At least 4 common methods of size description</p><p>See Table 3.1 from Boggs, 5th edition, p. 46</p><p>1) Sieve analysis</p><p>2) Wentworth verbal scale for size range</p><p>3) phi scale</p><p>4) mm</p><p>The properties listed above should be described for every clastic sample</p><p>E) Other??</p><p>Cement, stain, fossils, alteration etc.</p><p>1) Grain shape:</p><p>Can be described mathematically using DL, DI, DS</p><p>See Figure 3.9 from Boggs, 5th edition, p. 57</p><p>- Zingg Form diagram:</p><p>- More of a visual/field approach - Plots DI/DL vs. Ds/DI</p><p>See Figure 3.9 from Boggs 5th edition, p. 57</p><p>- Provides good field terms for unusual shaped grains - Terms: Oblate: disk (frisbee) shaped Equant: spherical (bowling ball) shaped Bladed: tabular (book) shaped Prolate (roller): tubular (water bottle) shaped</p><p>- Summary: People have tried to use form for environmental interpretation (ex: disk-shaped pebbles in beach environments), but there is too much overlap!</p><p>2) Surface texture:</p><p>2 - A recent method of particle shape analysis - Uses S.E.M. images to study pitting, striations</p><p>See Figure 3.11 from Boggs 5th edition, p. 61</p><p>- Examples where it works well:</p><p>- Grain frosting = an eolian environment</p><p>- Grain polish = beach environment</p><p>- Grain striations = a glacial environment</p><p>3) Fabric:</p><p>- The third component of grain texture</p><p>- Describes how the grains are put together</p><p>- May be used at a macro scale (conglomerates, gravels)</p><p>See Figure 3.13 from Boggs 5th edition, p. 61</p><p>- Common fabric is imbrication:</p><p>See Figure 3.14 from Boggs 5th edition, p. 61 and Freidman and Sanders figure</p><p>- Tells current direction - Fabric terms may also be used to describe grain contacts at a micro scale</p><p>II) Interpretations:</p><p>- Are based on observations - Should be separate - May include interpretations of transport, weathering, depositional environment etc. - Relate other issues: grain stability, transport distance and transport mechanism, depositional environment - May not be as certain</p><p>III) Sediment maturity:</p><p>Other factors that affect sediment composition and texture:</p><p>3 Transport distance, energy, grain composition Grains may be texturally mature or compositionally mature</p><p>A) Textural maturity: is related to transport distance and energy - Texture changes with transport:</p><p>- Roundness, sorting tend to increase away from source</p><p>- Grain size tends to decrease away from source (grains break down with increasing transport)</p><p>- Finer material is removed (winnowed) in higher energy environments</p><p>See Figure 5.6 from Boggs 5th edition, p. 111</p><p>- A sediment with a high proportion of rounded, well-sorted grains is texturally mature</p><p>B) Compositional maturity: is related to grain composition and stability </p><p>- Resistant grains (quartz, zircon) don't round as easily as less resistant grains</p><p>See Table 1.2 from Boggs, 5th ed., p. 10</p><p>- A sediment with a high proportion of resistant grains is compositionally mature</p><p>- Ex: sandstone composed almost entirely of quartz</p><p>- Quartz is especially resistant to weathering, is used as an index of transport (includes chert)</p><p>See Fig. 4.1 from Boggs, earlier edition? </p><p>- Other rock types break down with transport, too - Mineral fragments remain - This also applies to igneous rocks - ex: Granite breaks into quartz, mica, feldspar with relatively short transport distance</p><p>- A complicating factor: Composition is also related to particle size.</p><p>See Figure 8.21 from Blatt, Middleton and Murray, 1980</p><p>4 IV) Summary: </p><p>- Because these factors are not unique: texture alone is rarely a good indicator of depositional environment.</p><p>- In most cases there is so much overlap between grain sizes, forms and fabrics from different environments, that additional information is needed for environmental interpretations.</p><p>- The big picture is best: include grain size, form, fabric and compositional data</p><p>- Starts to address the concept of maturity</p><p>5</p>