SEDIMENTOLOGYSTRATIGRPHY GEOL 103A
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This 7 page Class Notes was uploaded by Gloria Quigley on Monday October 5, 2015. The Class Notes belongs to GEOL 103A at California State University - Sacramento taught by Timothy Horner in Fall. Since its upload, it has received 38 views. For similar materials see /class/218811/geol-103a-california-state-university-sacramento in Geology at California State University - Sacramento.
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Date Created: 10/05/15
Geology 103A Lecture 1 Reading Boggs pp 6573 582584 130131 Importance of sedimentary rocks begin grain textures I Field description of sedimentary rocks Description is an important tool for scientists Every experienced geologist should have a similar description Similarities are based on use ofa common language of geology Make observations then interpretations A Sorting See Figure 33 from Boggs p 58 Proper terms for a Geologist to use in a visual description Very well sorted well sorted moderately sorted poorly sorted 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 B Roundness See Figure 310 from Boggs p 67 Proper terms for a Geologist to use in a visual description Very angular angular subangular subrounded rounded well rounded Is used to describe individual grainscomponents in a sample Different grain types may have different roundness 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 l D Size Is given for each clast type or mineral present At least 4 common methods of size description See Table 31 from Boggs p 53 l Sieve analysis 2 Wentworth verbal scale for size range 3 phi scale 4 mm E Other Cement stain fossils alteration etc The properties listed above should be described for every clastic sample F Interpretations 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 Last time talked about field description of sedimentary rocks texture Today s topic Mathematical methods of describing or measuring textural properties II Sedimentary texture and laboratory description of sedimentary rocks Sedimentary texture smallscale features of the rock Primary textural properties Size includes sorting sorting is just the size distribution Shape form roundness surface texture Fabric how the grains are put together Secondary textural properties are derived from the primary properties Density Porosity Permeability Textural properties include A Grain size We have already covered much of grain size B Grain shape Has 3 aspects form roundness surface texture 1 Particle form The most important aspect of grain shape 2 common ways to measure form sphericity Zingg shape classi cation a Sphericity a mathematical approach Is different from roundness Roundness refers to the angularity of the comers of the grains Sphericity compares the shape of a particular grain to the shape of a similar circumscribed sphere See Figure 38 from Boggs 4th edition p 65 Generally equations for sphericity use the long intermediate and short aXis measurements of the particle DL D1 Ds Ll psi the symbol for sphericity A problem When you look at shape this way at grains can have the same sphericity as round grains But at grains and round grains don t settle at the same rate Solution maximum projection spherici accounts for at grains DLDI Ds b Zingg Form diagram More of a visualf1eld approach Also uses measures of DL D1 D5 to describe grain shape Plots DIDL vs DSDI See Figure 39 from Boggs 4th edition p 66 Provides good eld 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 Summary People have tried to use form for environmental interpretation eX diskshaped pebbles in beach environments but there is too much overlap 2 Roundness Is different from sphericityH Roundness refers to sharpness of the comers of the grains Can be described With visual terms very angular angular subangular subrounded rounded well rounded Wadell roundness A mathematical approach Lab method Roundness is measured by comparing the diameter of an inscribed circle to the diameter of circles that t the comers of the grains See Figure AA2 from Boggs p 584 Formula Where r radius of small circles RW Z r that t in comers RN R radius of large circle N number of comers 3 Surface texture A recent method of particle shape analysis Uses SEM images to study pitting striations See Figure 311 from Boggs 4th edition p 69 Examples Where it works well Grain frosting an eolian environment Grain polish beach environment Grain striations a glacial environment C Fabric The third component of grain texture Describes how the grains are put together May be used at a macro scale conglomerates gravels See Figure 312 from Boggs 4th edition p 70 Common fabric is imbrication See Figure 313 from Boggs 4th edition p 71 and Freidman and Sanders Tells current direction Fabric terms may also be used to describe grain contacts at a micro scale III Sediment maturity Other factors that affect texture Transport distance energy grain composition Grains may be texturully mature or compositiouully mature A Textural maturity is related to transport distance and energy Texture changes with transport Roundness sorting tend to increase away from source Grain size tends to decrease away from source grains break down with increasing transport Finer material is removed winnowed in higher energy environments See Figure 56 from Boggs 4th edition p 131 A sediment with a high proportion of rounded wellsorted grains is texturally mature B Compositional maturity is related to grain composition and stability Resistant grains quartz zircon don t round as easily as less resistant grains See Table 12 from Boggs 4th ed p 11 A sediment with a high proportion of resistant grains is compositionally mature EX sandstone composed almost entirely of quartz Quartz is especially resistant to weathering is used as an indeX of transport includes chert See Fig 41 from Boggs earlier edition 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 A complicating factor Composition is also related to particle size See Figure 821 from Blatt Middleton and Murray 1980 IV Summary Because these factors are not unique texture alone is rarely a good indicator of depositional environment 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 The big picture is best include grain size form fabric and compositional data Starts to address the concept of maturity
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