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GEOL 332 Lab 5 Sedimentary Rock Identification II Name: _____________________________________________ Date: _______________ Sedimentary Rocks: Carbonates, et al. Lab Equipment List: hand lens, ruler, pencil, and eraser. Objectives 1) to become familiar with the properties important in recognizing and classifying sedimentary rocks 2) to become familiar with the textures characteristic of sedimentary rocks; Carbonate Sedimentary Rock Classification In a simple model for the evolution of sedimentary rocks we find that if weathering, transportation, and sorting go to completion all that remains are three end member rock compositions. Siliciclastic Rocks: Quartz Arenite / Shale Carbonate Rocks: Limestone / Dolomite Limestones are not single composition rocks but a group of related rocks all composed of CaCO3 and reacting with dilute HCl acid. Limestone [CaCO3] is also chemically related to Dolomite [CaMg (CO3)2]. Because all these rocks have CO3 in common they are called the Carbonates. The composition of most Carbonates is derived from a combination of biological and chemical components. Two Carbonate classification systems are used today, one by R.L. Folk and the second by R.J. Dunham. The Dunham system is based on depositional texture (that is, the amount of matrix surrounding the grains at the time of deposition). It uses such names as Mudstone, wackestone, packstone, grainstone, and boundstone. Carbonate rock names (Limestones and Dolomites) consist of a conjunction of two names, one describing the ALLOCHEMS, the large pieces, the other describing the INTERSTITIAL MATERIAL. Allochems are equivalent to gravel, sand, lithics or feldspars in the siliciclastics. Interstitial material is equivalent to Clay or cements in clastics. There are four kinds of allochems: 1. Fossils ‐ may be whole fossils, or broken and abraded fossils; all are called "bio" fragments 2. Oolites ‐ small spheres 3. Pellets ‐ fecal Pellets produced by invertebrate animals; look superficially like Oolites but are dull 4. Intraclasts ‐ chunks of eroded Limestone deposited as a Conglomerate 1 | Page GEOL 332 Lab 5 Sedimentary Rock Identification II Interstitial Material Micrite is "lime mud", the dense, dull‐looking sediment made of Clay sized crystals of CaCO3. Micrite forms from the breakdown of calcareous algae skeletons. It is not clear if all ancient Micrites formed in the same way. Many Carbonates are composed of nearly 100% Micrite. Such rocks are simply called Micrites. With Carbonates containing allochems the question is whether Micrite is present or absent as an interstitial material, and if present, by how much. If Micrite is present during deposition then it fills the spaces between the allochems and the rock will be given a name which describes the allochems in a Micrite matrix. For example, a rock with fossil fragments embedded in Micrite is called a "Biomicrite". If the depositional environment has strong currents, only allochems may be deposited. If we could see the sediment during deposition and all the allochems would be loose, like a pure sand or gravel. This is analogous to a 100% siliciclastic sand on a beach with no silt or Clay. In this case, Micrite would be clay‐ sized and would be washed away. The rock formed is then composed only of allochems, held together by clear to translucent Calcite crystals with rhombohedral cleavage (called SPAR or SPARITE) acting as a cement. The spar is precipitated from fresh or marine water percolating through the sediment after deposition, but before final cementation. Classification of Carbonates The classification of Carbonates using the allochem/interstitial material system (the Folk System) is very systematic and straight forward. The allochem name is combined with the interstitial name (Micrite or spar). The table below shows the major categories of Carbonate rocks based on their allochems and interstitial material. But what happens if there is more than one allochem in the rock, or there is a mixture of Micrite and spar? You can easily build your own descriptive rock names. The name is built up by stringing together 2 | Page GEOL 332 Lab 5 Sedimentary Rock Identification II all the allochem names in order from least to most abundant, and then adding the interstitial material name ("matrix" below for short). For example, a rock like this: Oolites + Fossils + Spar matrix = Oo bio sparite o The name is written as one word, Oobiosparite. Another example (again allochems from least to most abundant): Pellets + Oolites + Fossils + Micrite matrix = pel oo bio Micrite o The name is written as one word, Peloobiomicrite. But what if there is both Micrite and spar matrix? The system is the same; just list them from least to most abundant. Fossils + Spar matrix + Micrite matrix = bio spar Micrite This system goes through other levels of refinement, such as in the table below where the abundance of allochems is indicated. Other modifiers may deal with different sizes of allochems. A classification such as this one works well if you want to construct rock names from observations. The system, however, does not lend itself well to constructing keys for classification. A key requires the establishment of arbitrary categories of rocks, and a system like the one above deals with all the myriad combinations that are possible. Most limestones are classified by Folk allochemical rocks if they contain over l0% allochems (transported carbonate grains). Based on the percentage of interstitial material, the rocks may be further subdivided into two groups: sparry allochemical limestones (containing a sparry calcite cement of clear coarsely crystalline mosaic calcite crystals) and microcrystalline allochemical limestone (containing microcrystalline calcite mud, micrite, which is subtranslucent grayish or brownish particles less than about 5 microns in size). Further subdivision is based on the allochem ratios of Folk (1962) are shown in Scholle & Ulmer‐Scholle (2003). Thus Folk's classification is most suited for thin section study. Remember that he terms rocks with appreciable matrix as micrites while matrix‐free rocks that contain sparry calcite cement are termed sparites. As you can see sparites and micrites are further subdivided by means of their most common grains. 3 | Page GEOL 332 Lab 5 Sedimentary Rock Identification II In contrast, Dunham's classification (figures above) and its modification by Embry and Klovan (1971) and James (1984) deals with depositional texture. For this reason, his scheme may be better suited for rock descriptions that employ a hand lens or binocular microscope. For example, if the grains of a limestone are touching one another and the sediment contains no mud, then the sediment is called a grainstone. If the carbonate is grain supported but contains a small percentage of mud, then it is known as a packstone. If the sediment is mud supported but contains more than 10 percent grains, then it is known as a wackestone, and if it contains less than 10 percent grains and is mud supported, it is known as a mudstone. If one compares the two classifications, a rock rich in carbonate mud is termed a micrite by Folk and a mudstone or wackestone by Dunham. Moreover, a rock containing little matrix is termed a sparite by Folk and a grainstone or packstone by Dunham. The wide range of percentage of mud matrix that a carbonate may have and still be termed a packstone by Dunham sometimes reduces the utility of this classification. Embry has modified Dunham's classification and Klovan (1971) to include coarse grained carbonates (above figure). In their revised scheme, a wackestone in which the grains are greater than 2mm in size is termed a floatstone and a coarse grainstone is called a rudstone. Both terms are extremely useful in description of limestones. Embry and Klovan to more graphically reflect the role that the organisms performed during deposition also modified the boundstone classification of Dunham. Terms such as bafflestone, bindstone, and framestone are useful in concept but are extremely difficult to apply to ancient limestones where diagenesis and sample size limit one’s ability to assess an organism’s function. The last two pages of this lab includes a key to identify rocks based on their allochems and interstitial material. Just be aware that its main weakness is that there are always rocks that do not fit easily into its simple categories. There is also a chemical and biochemical rock identification key. 4 | Page GEOL 332 Lab 5 Sedimentary Rock Identification II Folk Sedimentary Rock Classification: Folk’s (1959, 1962) classification of limestones, which uses prefixes to indicate the framework grains present (bio‐ for fossils, pel‐ for peloids, oo‐ for Ooids, and intra‐ for intraclasts) and stems to indicate whether the interstitial calcite is micritic or sparry. If the rock is originally bound together (as in a reef rock), it is a biolithite. 5 | Page GEOL 332 Lab 5 Sedimentary Rock Identification II Textural maturity classification of limestones proposed by Folk (1962). Textural maturity classes are based on the percentage of allochems present, their degree of sorting, and the extent of rounding (a function of abrasion history). (Folk, 1965) The Dunham classification of carbonate sedimentary rocks (Dunham 1962) with modifications by Embry & Klovan (1971). This scheme is the most commonly used for description of limestones in the field and in hand specimen. Rock Identification Step 1, fill out the table for the known sedimentary rock samples. (15 pts) Step 3, fill out the table for the unknown sedimentary rock samples. (15 pts) 6 | Page GEOL 332 Lab 5 Sedimentary Rock Identification II (Dunham, 1962) 7 | Page GEOL 332 Lab 5 Sedimentary Rock Identification II 8 | Page GEOL 332 Lab 5 Sedimentary Rock Identification II References: Dunham, R. J., 1962, Classification of carbonate rocks according to depositional texture. In: Ham, W. E. (ed.), Classification of carbonate rocks: American Association of Petroleum Geologists Memoir, p. 108‐ 121. Embry, AF, and Klovan, JE, 1971, A Late Devonian reef tract on Northeastern Banks Island, NWT: Canadian Petroleum Geology Bulletin, v.
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