Appendix Classifications Used in Case Studies

Appendix Classifications Used in Case Studies Rationale Outline The authors contributing to this volume were F our main systems for describing or characteriz asked to use specific classifications of carbonate ing carbonates and their pore systems are used in rocks and associated pore systems. Classifica this book: tions have at least two vital functions. One is to 1. A textural classification by Dunham (1962), provide investigators with the intellectual benefits supplemented for coarse biogenic limestones of organizing data and interpretations into clear, by Embry and Klovan (1971). concise language (Ham and Pray, 1962). The 2. A petrographic-genetic classification of poros other is to supply a means of communicating that ity by Choquette and Pray (1970). information in a "normalized" form so that it can 3. A classification relating "matrix" texture to be compared readily with similar kinds of infor pore size and pore frequency, proposed by mation about other occurrences. Both of these Archie (1952) and modified by Roehl (un benefits apply, of course, to any classification of published). natural systems. For carbonate rocks and their 4. A detailed system of symbols and descriptive pore systems, the rationale and historical perspec procedures developed in the Shell companies tives for classifications have been ably sum for the description and logging of sedimentary marized by others and need no elaboration here. rocks and their constituents, and published Among the more useful treatments of the subject recently by Swanson (1981). are the papers on classification of carbonate rocks in Memoir 1 of the American Association of Pet Some authors in this book chose to employ the roleum Geologists including the lead article by limestone classification of Folk (1959, 1962), in Ham and Pray (1962); a discussion of porosity lieu of or in addition to that of Dunham. All of the types and their origins by Choquette and Pray systems just cited are in relatively widespread use (1970); and a concise summary and comparison by sedimentary geologists in the petroleum indus of many pore-system and carbonate-rock classifi try and elsewhere. The brief summaries that cations by R. Cussey and his associates in Elf follow are intended for readers not closely conver Aquitaine, translated by Reeckman and Fried sant in carbonate sedimentology; more extended man (1982). information can be found in the original publi- 572 Appendix cations (see References), and key terms are tional texture is no longer recognizable, the term defined in the GLOSSARY. crystalline carbonate is used. Carbonate Sediment and The Embry-Klovan Classification Rock Classifications Embry and Klovan (1971) proposed important modifications to the Dunham system, specifically Both of the classifications currently in widespread to accommodate the fact that biogenic ally bound use, by Dunham (1962) and Folk (1959, 1962), carbonates, such as many biogenic reefs and reef recognize that limestones consist in general of mounds (see James, 1983, for definitions) com three kinds of components: "grains", "matrix" or monly contain components much coarser than lime mud (the "micrite" of Folk when indurated), sand size (>2 mm) and may be bound together in and cement. Both classifications also make a different ways. As shown in Figure A-3, this basic distinction between limestones made up classification retains the basic grain-support! dominantly of components bound together bio mud-support terms of Dunham, adding categories genically during deposition, and limestones with as follows: For carbonates not biogenic ally bound mostly unbound components. Neither classifica ("organically" in the usage of Embry and tion provides for clear distinctions between dif Klovan), but containing significant amounts of ferent dominant grain-size characteristics, a fea coarse components, the terms floatstone and ture partly responsible for the system proposed by rudstone were proposed, depending on whether Embry and Klovan (1971). the matrix or coarser fraction is load-bearing. For biogenic ally bound carbonates, the terms baJ]les The Dunham Classification tone, jramestone, and bindstone were proposed. Examples of the kinds of reefal organisms that This system, elegant in its terminological sim can occur in these carbonates also are illustrated plicity, can be used equally well, despite the suffix by James (1983). They might include platy or "-stone" in all of its terms, to name and charac "potato-chip" calcareous algae that collect and terize lime sediments as well as limestones. It is shelter sediment, in a baJ]lestone; robust frame illustrated in Figures A-I and A-2. Dunham sug work builders such as corals or rudistids in life gested two basic textural features, in addition to positions, in a framestone; or encrusting coralline the presence or absence of organic binding of algae, serving to bind corals together in a bind sedimentary particles, as a basis for classifying stone. The relationships between these terms of carbonates: (1) the presence or absence of car Embry and Klovan and those of Dunham are bonate mud, which differentiates muddy car shown in Figure A-3. bonates from essentially mud-free grainstones; and (2) the relative abundance of grains and lime The Folk Classification mud, which determines whether the load-bearing components are grains or mud (grain support" vs. The main elements of this classification are "mud support"). "Grains" mean any sedimen shown in Figures A-4 and A-5 from Folk's two tary particles larger than 0.02 mm for Dunham articles on the subject (1959, 1962). The basis for (or 0.03 mm for Embry and Klovan), i. e., par this system is threefold: (1) relative proportions ticles of coarse silt size and larger. Muddy car of carbonate grains or allochems versus carbonate bonates are called packstone, wackestone, or mud or micrite; (2) sorting of allochems, and mudstone depending on their lime-mud content; (3) rounding of allochems. Allochems include packstone is grain-supported, whereas wacke ooids (oolites in Figs. A-4 and A-5A), pellets, in stone and mudstone are mud-supported. Mud traclasts, and bioclasts. These four types supply content is one indication of whether and how ef the prefixes for rock-textural categories that are fectively winnowing was operative in the depo dependent mainly on the proportiions of allo sitional environment, and can be a critical in chems and micrite, and on whether the allochems fluence in the course of later diagenesis and are cemented by sparry calcite (-sparite) or lime pore-space evolution. Biogenically bound sedi mud (-micrite). ments are called boundstones. Where deposi- As an example, using the chart in Figure A-4, a Appendix 573 DEPOSITIONAL TEXTURE RECOGNIZABLE DEPOSITIONAL TEXTURE NOT RECOGNIZABLE Original Components Not Bound Together During Deposition Original components were bound together during deposition ... CRYST AlUNE CARBONATE Contains mud as shown by intergrown Lacks mud (particles of clay and fine silt size) skeletal matter, lamination and is contrary to gravity, or grain- (Subdivide according to Mud-supported Grain- sediment-floored cavities classifications designed to supported that are roofed over by supported bear on physical texture organic or questionably or diagenesis.) Less than More than organ ic matter and are too 10 percent grains 10 percent grains large to be interstices. MUDSTONE WACKESTONE PACKSTONE GRAINSTONE BOUNDSTONE Fig. A-I. The textural classification of carbonates by Dunham (1962). Published by permission, American Association of Petroleum Geologists. LIME MUDSTONE LIME WACKESTONE LIME PACKSTONE LIME GRAINSTONE Fig. A-2. Diagrams illustrating terms proposed by Dunham (1962) for those carbonates in , CARBONATE GRAIN which components are not organically bound D CALCITE MUD together. Published by permission, American Association of Petroleum Geologists. ~ SECONDARY SPARRY CALCITE limestone with an allochem content of more than regardless of which allochems were dominant. In 25 percent ooids (oolites) would be either an 00- Folk's more recent (1962) version of his system, sparite, an oomicrite, or an oolite-bearing micrite particular recognition was accorded to textural depending on whether allochems made up more maturity in terms of sorting and abrasion of than 10 percent and were calcite-cemented or had allochems as well as winnowing or bypassing of a micrite matrix, or only 1 to 10 percent, with a micrite, as illustrated in Figure A-5B. Several of predominance of micrite. Limestones with less those terms are more fully defined in the than 1 percent allochems would be called micrite Glossary. 574 Appendix ALLOCHTHONOUS LIMESTONES AUTOCHTHONOUS LIMESTONES ORIGINAL COMPONENTS NOT ORGANICALLY ORIGINAL COMPONENTS ORGANICALLY BOUND DURING DEPOSITION BOUND DURING DEPOSITION GREATER THAN BY LESS THAN 10% > 2mm COMPONENTS 10%>2mm BY BY COMPONENTS ORGANISMS ORGANISMS ORGANISMS NO CONTAINS LIME MUD« .03mm) LIME MUD WHICH WHICH WHICH MATRIX ACT ENCRUST BUILD MUD SUPPORTED COMPONENT GRAIN SUPPORTED SUPPORTED AS AND A RIGID SUPPORTED LESS THAN GREATER 10%GRAINS THAN 10% BAFFLES BIND FRAME- (>.03mm<2mm) GRAIN WORK MUD- WACKE- PACK- GRAIN- FLOAT- RUD- BAFFLE- BIND- FRAME- STONE STONE STONE STONE STONE STONE STONE STONE STONE Fig. A-3. The textural classification of carbonates proposed by Embry and Klovan (1971) using elements of the Dunham classification (left side). This expanded scheme allows the differentiation between types of coarse grained (>2 mm) carbonates as well as between different

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