NAME:___________________________________ Introduction to Sedimentary Petrology: Common Sedimentary Particles The purpose of this lab is to get you acquainted with using a petrographic microscope and identifying some of the most common sedimentary particles in rocks. Chapter 5 in Boggs, especially Figures 5.1, 5.2, and 5.13, will be useful references for what these grains and cements look like in thin section. First, we will review how to use the microscope and all the terminology used by sedimentary petro logists. Part I. Clastics: Quartz Quartz is one of the most abundant minerals in sedimentary rocks. Quartz is recognized in thin section by these properties: low relief, clear color in plane-polarized light, 1st order birefringence (grey to white color in cross polarized light), concoidal fracture, and commonly, undulose extinction. Quartz comes in different forms, as grains and cements. a) Examine THREE of the following thin sections to identify which forms of quartz are in each of these thin sections: CH-3; CH-19.1, 278.4; 296-ADM; 280-ABQ; 278.11. All of these thin sections contain at least one form of quartz. Use a Y for yes, N for no. CH-3 Monocrystalline quartz clasts Polycrystalline quartz clasts Chert clasts Chalcedony cement Quartz overgrowths Chert cement CH-19.1 278.4 296-ADM 280-ABQ 278.11 b) Sketch examples of each of the following types of quartz in the space provided below. Label the magnification of the field of view you used to make your sketch. A high magnification may be useful to make you sketches. Use the same thin sections as in part (a) to make your sketches. Quartz Overgrowth Cement Around Monocyrstalline Quartz Grain Magnification _____________ Chert Clast Magnification _____________ Polycrystaline Quartz Grain Magnification _____________ Part II: Clastics: Feldspars Feldspars are also common sedimentary particles found in sedimentary rocks. In thin section, quartz and feldspars share many of the same optical properties. Both have low relief and 1 st order birefringence. However, unlike quartz, feldspars have cleavage and twinning. Your handout has some sketches of the different types of twinning. Twinning in feldspars is described as the appearance of symmetrical planes (twin lamellae) parallel to a crystal face and usually identical with the compositional plane. Twinning is recognized only in cross -polarized light. 1. Sample 316-MPC is a feldspathic arenite. Identify and sketch an example of potassium feldspar and plagioclase feldspar in XPL. Label the twinning style for both grains and the magnification you used to make your sketch. Potassium Feldspar Twinning Style ___________________ Magnification _____________ Plagioclase Feldspar Twinning Style ___________________ Magnification _____________ Part III: Lithic Fragments Not all lithic fragments are polycrystalline. Some lithic fragments are monocrystalline such as muscovite, biotite, and chlorite. Polycrystalline lithic fragments tend to be fine-grained because coarser-grained rocks would break up into individual mineral grains . 1. Sketch examples of each of the following types of lithic fragments using samples 304-AK or 335-AKG Schist Fragment Magnification _____________ Rhyolite Fragment Magnification _____________ Mica (list which one _______________) Magnification _____________ Quartzite Clast Magnification _____________ Part III: Carbonate Rocks in Thin Section There are many important differences between carbonate and siliciclastic rocks. First, most carbonates are “born,” not made. Siliciclastic material can be transported great distances whereas most carbonate particles remain in or near the same place it was created. As a result, the grain size of the majority of carbonate rocks is a function of the type of skeletal particle or non-skeletal grain size, not hydraulics. Secondly, carbonate rocks are only common in clear, shallow, warm, tropical to subtropical marine environments (in the “carbonate factory”). Limestones make up approximately 25% of sedimentary rocks in the stratigraphic record. The common carbonate minerals fall into three main groups: aragonite, calcite, and dolomite. Although these minerals have similar chemical formulas, their structures are very different and can be distinguished in thin section. Aragonite can be distinguished from calcite and dolomite because of its lack of rhombohedral cleavage. Dolomite has a higher index than calcite. Table 1 (Boggs, p.160) lists a number of features that can be used to differentiate these three carbonate minerals. Most of the samples you will see in today’s lab are made up of calcite. There are a number of thin section photomicrographs in Boggs (pp. 175, 178, and 179) that will be good references. I. The Orthochems There are two major types of othochems: micrite and sparite. Micrite is microcrystalline carbonate (super fine-grained), which means that the edges of the individual crystals are not visible. Spar refers to the larger crystals (coarser-grained than micrite). Spar is usually more translucent (clear). Typical spar is 0.02-0.1 mm in diameter. Spar can form in a variety of cementation fabrics such as fibers, blades, and equant crystals. Compare the two types of orthochems. Look at samples 668-TE-4 and UNK and sketch an example of both micrite and sparite in the appropriate circles below. Sample Number ____________________ Sample Number ____________________ Magnification Magnification ____________________ Orthochem type ____________________ ____________________ Orthochem type ____________________ II. The Allochems – Non-skeletal Carbonate Grains Most non-skeletal grains are coated except for peloids, intraclasts, and extraclasts. Coated grains include ooids, pisoids, and oncoids. Figure 1 depicts some diagrams of each carbonate grain type. The best way to identify the grain type is by the size of the grain, shape of the grain, and the characteristics of the laminae inside the grain. Because coated grains have a polygenetic origin, processes of formation are poorly understood. Choose one sample from each group to sketch and describe in the space below. For each sample, indicate the scale of your sketch of the grain. For all your chosen samples, name the grain type. Name the rock according to Folk. If the grain is a coated grain, indicate what the composition of the nucleus is. Indicate the one probable depositional environment for each sample. How do you think these grains formed? Be specific. Look up the answers in Boggs. Explain in the space provided below for each group. Group 1: Group 2: 601-JAR, FOI 758-AHO, 665-APK Sample ____________________________ Sample ___________________________ Magnification ______________________ Magnification _____________________ Grain type _________________________ Grain type ________________________ Folk ______________________________ Folk ______________________________ Nucleus composition _________________ Nucleus composition ________________ Depositional Environment ____________ Depositional Environment ___________ ____________________________________ __________________________________ Mechanism of Formation: Mechanism of Formation: Group 3: Group 4: 690-APP 682-AOL Sample ____________________________ Sample ___________________________ Magnification ______________________ Magnification _____________________ Grain type _________________________ Grain type ________________________ Folk _______________________________ Folk ______________________________ Nucleus composition _________________ Nucleus composition ________________ Depositional Environment ____________ Depositional Environment ___________ ___________________________________ __________________________________ Mechanism of Formation: Mechanism of Formation: III. The Allochems – Skeletal Carbonate Grains Skeletal grains are the most abundant and important kinds of grains that occur in limestones. These grains will be either whole fossil organisms or fragments that have been rounded to various degrees by abrasion. Fossils yield vital information about environmental conditions such as water depth, salinity, turbidity, and energy levels because the organisms were adapted to specific ecological tolerances. Tables and figures have been provided for you to help guide you through fossil identification. We’re only looking at a few marine fossils in thin section. 1. Sample 669-T-5 Bivalves (clams) are abundant in limestones, especially in the Mesozoic and Cenozoic. The wall structure provides the most effective means to differentiate bivalve shells form the other shelly fauna. Make a sketch of the shell in the space provided below. Magnification _______________________ Orthochem Name ____________________ Based on the orthochem type and the taphonomic state of the shell (Is the fossil abraded? Is it a whole fossil?), what is the energy of deposition of this rock? High, medium or low? Why? Cite specific reasons for your interpretation. 2. Sample 617-PRO. This fossil is an important time marker because this group of foraminfera is characteristic of the Late Carboniferous and Permian. The have coiled "shell" forms called tests that are similar in shape to gastropods (snails) and ammonites. But, there are a few major differences between those coiled forms and foraminfera. Name one. Make a sketch of one of the foraminfera: . Magnification _________________________ 3. Sample S1017 There are three common marine allcohems in this sample: bryozoans, brachiopods, and echinoderms. Make a sketch of each one in the space provided below. It is best to look at two of the thin sections to see the variety of shapes. Bryozoan Magnification Brachiopod ____________________ Orthochem type ____________________ Magnification ____________________ Orthochem type ____________________ Echinoderm Magnification ____________________ Fossil name ________________________ Name the rock according to Dunham _______________________________ 4. Sample ACC Look closely at the absence of any internal structure of this fossil. They are often found fragmented in the fossil record. These fossils are now extinct. This fossil group provides an excellent time marker for rocks. They only existed during the Paleozoic. Make a sketch of one below. Magnification ____________________ Fossil Name ________________________ Part 3: Sandstone Provenance Analysis We will be learning a method of semi-quantitative analysis of sandstone composition and then we will use these analyses to interpret different sandstones' provenances. Recall that sandstones are detrital sedimentary rocks-they result from the physical and chemical breakdown of preexisting rocks, followed by the transport, deposition, and lithification of the resulting sediments. Therefore, if we are able to account for the different susceptibilities of different rock types to weather in the sedimentary environment, we can estimate the composition and/or tectonic environment of the parent rock(s) (i.e., the provenance) from which the younger detrital sedimentary rock was derived. Our methods have two parts: First, you will learn to conduct a point-count of the quartzose, feldspathic and lithic components of a number of detrital sedimentary rocks. Second, you will plot the resulting datasets on an accompanying QFL ternary diagram and then compare the plot with a) the nomenclature QFL ternary diagram and b) Dickinson and Suczek's (1979) sandstone provenance QFL ternary diagram. Choose 1 sample from Groups I, II, III and, using the hand sample, give each a name. Then conduct a point count on each sample and plot the samples on a QFL ternary diagram. Now give the name according to the point count. It will likely be different. When all samples are plotted, sketch the provenance fields over the sample points. Group 1 Group 2 Group 3 218-PCA 407-TNY 328-AKW 269-AKL 311/310-TEW ACA 261-KSU 299-AJN 309-KBC 112-APB 313-TDC 325-AKN Sample Number: Rock name from hand sample: Point count tallies Quartz: Feldspar: Lithics: Textural and Mineralogical comments Quartz: Feldspar: Lithics: Rock name from point count: Provenance field: Comments: Sample Number: Rock name from hand sample: Point count tallies Quartz: Feldspar: Lithics: Textural and Mineralogical comments Quartz: Feldspar: Lithics: Rock name from point count: Provenance field: Comments: Sample Number: Rock name from hand sample: Point count tallies Quartz: Feldspar: Lithics: Textural and Mineralogical comments Quartz: Feldspar: Lithics: Rock name from point count: Provenance field: Comments: How to conduct a point count: 1. Choose magnification such that ~30 grains are in field of view. Make sure that you are looking at grains and not cement. 2. Pick 5 nonintersecting areas on the sample slide. 3. Divide field of view into quadrants, then count quartz, feldspar, and lithics in each. 1 2 3 4 4. Continue for each area until 150 grains have been counted. Then divide the count for each grain type by the total number of grains counted to get percentages for quartz, feldspar, and lithics. 5. Plot in QFL space on ternary graph sheet provided. Some notes on provenance fields: Stable Continental Craton: not much elevation – rivers, beaches, etc. Sediments are intensely reworked. Block Faulted Continental Basement: Mainly samples upper crust, so high percentage of quartz and feldspar. Recycled Orogen: Complicated setting, samples older/deeper rocks. Dominated by quartz and lithics (metamorphic fragments). Magmatic Arc: High percentage of lithics (volcanic fragments) and K-spar relative to quartz.