Abstract Limestones and dolomites form the economically important and exceedingly complex family of carbonate rocks. They are set distinctly apart from related rock families by their intrabasinal and highly local origin, their genetic dependence upon organic activity, and their extreme susceptibility to post- depositional modification. The successful classification of carbonate rocks requires detailed knowledge of their multiple com-ponents and genetic processes. Such knowledge has been greatly increased during a period of accelerated investigations since 1940, with the result that the modern classifications are marked improvements over their predecessors. Most of the newer classifications utilize a practical blending of descriptive and genetic parameters. The parameters most commonly used are depositional fabric, particularly the relative abundance of coarser carbonate particles (grains) as compared with the finer grained particles (matrix or micrite); the size and genetic types of the grains or of in-place biotic constituents; the mineralogy; and the nature and degree of post-depositional modification. Secondary parameters include porosity, cementation, the degree of abrasion or rounding of the grains, admixtures of noncarbonate material, and a host of others. The symposium classifications of carbonate rocks and two allied articles of this volume are briefly reviewed and compared. Despite the differences in approach, purpose, and experience among the various authors, the resulting classifications show strong similarities and therefore indicate that a basis of mutual

Abstract Classification of limestones should consider (1) mode of origin of CaCO 3 (chemical [biochemical or physicochemical] and mechanical), (2) form of CaCO 3 (skeletal [secretionary] or nonskeletal [accre- tionary or particulate]), and (3) the processes of deposition and accumulation of units of limestone. The biological and other genetic aspects of limestones are interrelated but commonly difficult to evaluate. Studies of modern carbonate sediments demonstrate that organisms play a dominant role in the forma-tion of skeletal and nonskeletal material. Disintegration of skeletal material to fine sand and smaller size generally renders such material unrecognizable regarding its skeletal nature and biological origin. Nonskeletal carbonate sediments of biological origin resemble, and are difficult to distinguish from, car-bonates of physico-chemical origin. Progressive lithification and diagenetic changes with increasing age of the carbonate rocks com-pound the problems of identifying the nature and origin of carbonate particles. The result is a decrease in the uniformity and accuracy of description and classification of carbonate sediments. The varied concepts of classification of carbonate sediments have resulted from variation in back-ground, interest, methods employed, and purpose of an investigation. The geographic extent and geologic range of the investigation likewise impart a control to the problem of each limestone classification. Ultimately, the classification of carbonate rocks will have to provide for the needs of the field geologist, subsurface geologist, paleontologist, petrographer, and geochemist. Each of these fields of investigation is concerned with only a part of the total evidence available to be used in classification of carbonate rocks. A three-stage procedure of description of carbonate rocks, each with its applicable terminology, is necessary before all descriptions and purposes concerned with carbonate rocks can be satisfied. These stages consist of (1) field description of limestone units, (2) low-power binocular description of samples, and (3) thin-section and geochemical description of samples. Most descriptive data in the procedure are ultimately based on field observations. Generally, it is only after all three stages have been fulfilled that origin can be assigned to ancient carbonate rocks.

Abstract Several groups of limestones are recognized on the basis of their textural differences. Most of them give evidence of mechanical deposition and they are described as clastic textured. The various types within this clastic-textured group owe their characteristic appearances to the kinds and amounts of four textural components—grains, lime mud (micrite), cement, and void (pores). A nonclastic-textured group of limestones is built up of a fifth textural component, the organic framebuilders. A sixth textural component, recrystallization calcite, modifies the basic limestone types. Carbonate grains, which are analogous to sand or silt grains, form the rock framework for the mechani-cally deposited limestones. If grouped together, they are capable of yielding an effectively porous rock. Recognizable carbonate grain types are grouped into five divisions (1) detrital grains—fragments derived from pre-existing rocks, (2) skeletal grains—broken or whole, (3) pellets—grains composed of micritic material, (4) lumps—grain aggregates or composite grains, and (5) coated grains—grains with concentric coating or with rims of calcium carbonate enclosing or encrusting a central nucleus. Important subvarieties of grains are recognized within these gross divisions. The lime mud or micritic material in a limestone is roughly equivalent to the argillaceous material in a “dirty” sandstone or mudstone. Micritic material refers to unconsolidated or lithified ooze or mud of either chemical or mechanical origin, and is given an arbitrary upper size limit of 0.03 mm. Cement is the clear crystalline component that occupies the interstices between grains and is similar in appearance and distribution to silica cement in sandstones. Void spaces have various shapes and distribution, depending in part on other textural features and in part on genesis. Recrystallization calcite refers to those calcite mosaics resulting from grain enlargement or conversion processes, and excludes calcite cement. Basic limestone rock types are recognized by (1) noting the types and amounts of grains or framework builders where present, and (2) estimating the relative proportions of grains, framebuilders, and micritic material. A classification based on these objective and measurable features has been developed. These features are used in the rock name. Other features, such as porosity, cementation, grain size, and color, are restricted to the rock description or are used as modifiers before the rock name. Dolomites must be treated somewhat differently. The system for describing and naming dolomites is based primarily on a compositional grouping into calcareous dolomites and pure dolomites. These groups are modified by appropriate textural terms.

Abstract In the writer’s previous classification of limestones, rocks were divided into three major families. A more sensitive division can be made into eight groups forming a complete spectrum of textural types, representing deposition in environments of different physical energy. Basis for the classification is (1) relative proportion of allochems and carbonate mud, (2) sorting of allochems, and (3) rounding of allo-chems. A complete parallel exists between these limestone types and the sequence of textural maturity in sandstones, even to the existence of textural inversions. However, rounding appears to be accomplished best in environments where the energy level is too great for good sorting.

Abstract Limestone genesis is an important consideration in reconstruction of sedimentary basin history. We have designed a genetic classification of limestones based upon the energy that existed in the depositional environment, and one which permits us to construct geologic models of sedimentary history. The energy spectrum at any depositional site, which is related to wave and current action, may range from quiet water through strongly agitated water. The Energy Index (El) is an operational device for dividing the continuous energy spectrum into discrete energy levels. These steps are designated as limestone types in the El classification as follows: Type I—quiet water sediments; Type II—intermittently agitated water sediments; Type III—slightly agitated water sediments; Type IV—moderately agitated water sediments; Type V—strongly agitated water sediments. The grading spectrum of energy and related water agitation is not directly related to depth, as quiet-water, low-energy sediments may be deposited in very shallow water. We believe limestones are largely of biogenic origin and are comprised of carbonate materials which originate within the depositional basin. Such material may or may not be indigenous to its depositional site. Thus, recognition of energy levels and classification of a limestone in the El spectrum depend upon evidence for mechanical transport of sedimentary particles by wave or current action. This evidence is found in the textural properties and biotic make-up of the granular components and in the textural properties of the finer grained matrix. Carbonate sediments are easily altered by postdepositional processes. Thus, alteration textures must be recognized and understood, as they tend to obscure the depositional features we use for interpretation. In most limestones studied, however, primary features are sufficiently recognizable for successful interpretation of the energy spectrum. A rock classification has a maximum utility if, in addition to its use as a medium of description and communication, it provides a means of constructing geologic models for interpretation of earth history. Our classification has this utility because it allows us to relate limestones genetically on a three-dimensional basis in space and time. Changes in depositional energy at one place as a function of time may be studied by means of an El log. The change of El with time at one locality is related to important developments in basin history such as transgression, regression, basin subsidence, or stillstand. Combination of several El logs permits one to relate these energy-time changes on an areal basis for correlation purposes. Finally, the limestone types, which essentially are lithofacies, provide a genetic framework for delineation of facies related to geomorphic environments of deposition.

Abstract Three textural features seem especially useful in classifying those carbonate rocks that retain their depositional texture (1) Presence or absence of carbonate mud, which differentiates muddy carbonate from grainstone; (2) abundance of grains, which allows muddy carbonates to be subdivided into mudstone, wackestone , and packstone ; and (3) presence of signs of binding during deposition, which characterizes boundstone. The distinction between grain-support and mud-support differentiates packstone from wackestone—packstone is full of its particular mixture of grains, wackestone is not. Rocks retaining too little of their depositional texture to be classified are set aside as crystalline carbonates .

Abstract Jurassic carbonate reservoir rocks of northeastern Saudi Arabia contain productive oil at several levels. For more than 20 years, samples and other data have been collected on the main oil reservoir—the Upper Jurassic Arab-D zone. Diamond cores from 12 wells with high Arab-D recovery were selected for detailed petrologic examination as part of the present study. Nearly 4,000 thin sections have been cut at closely spaced intervals and examined in detail; of these, 1,200 were analyzed by point counting. Particle size, particle type, authigenic constituents, and visual porosity were recorded. Calcareous algae, stromatoporoids, and Foraminifera proved to be the main skeletal elements; aggregate pellets, “algal” nodules, and “fecal” pellets are the most important nonskeletal particles. Examination of Arab-D cores, equivalent rocks on outcrop, and modern calcareous Persian Gulf sediments has shown that these carbonates can best be understood by considering them to be products of mechanical deposition with certain environments being characterized by specific particle sizes and types. Arabian carbonates can be divided according to original particle size and sorting or obliteration of original texture into five groups—(1) aphanitic or fine-grained limestone, (2) calcarenitic limestone, (3) calcarenite, (4) coarse carbonate clastic, and (5) dolomite. Classification of Arab-D rocks according to this scheme has permitted (a) recognition of distinctive stratigraphic units for correlation and reservoir zonation, (b) delineation of original environment-sedimentation patterns, and (c) relation of reservoir properties to original textures and secondary changes. Arab-D rocks represent the transition from continuous carbonate deposition to precipitation of nearly pure anhydrite. The lower part of the reservoir consists of mixed mud and nonskeletal sand. Near the middle, a thin persistent unit of aphanitic limestone records an episode of muddy deposition over a wide area. Widespread shallow-water conditions during upper Arab-D time are suggested by a pronounced increase in skeletal sand and clean-washed calcarenite derived in large part from dasyclad algae and stromatoporoids. Many features known to be persistent on outcrop have proved equally con-tinuous in the subsurface, indicating that changes which affected sedimentation must have operated on a large scale. Regionally, reservoir units show gradual lateral change. The picture to emerge is that of a broad shelf with finer lagoonal sediments being deposited in the west, dominantly calcareous sand in the form of offshore bars accumulating near the present coast, and presumably deeper water mud with sparse sand being laid down in the east. The sediment patterns cut across and are apparently unrelated to modern structure. Of the original textural elements, carbonate mud matrix exerts a dominant control on reservoir rock behavior. As mud content increases relative to sand content, porosity and permeability uniformly decrease. Original rock textures have been altered in at least six ways. Addition of dolomite (which exhibits strong affinity for mud-size particles, little for calcarenites) has the most pronounced effect on reservoir properties. In any textural group, porosity and permeability (a) progressively decrease as dolomite increases from 10 to 80 per cent, (b) increase where dolomite forms 80 to 90 per cent of the rock, and (c) again decrease as dolomite exceeds 90 per cent.

Abstract Four case histories of textural and reservoir analyses of selected Paleozoic carbonate cycles and reef complexes of the Western Canada basin have been utilized in the formulation of a carbonate rock classification chart. This chart is presented to illustrate the relationships of grain, matrix, and cement variants of carbonate rocks, to porosity and permeability determinations, and should satisfy the requirements of an oil geologist or reservoir engineer. Large stratigraphic accumulations of oil have been discovered at or near the Paleozoic subcrop of the Mississippian “Midale” carbonate cycle in southeastern Saskatchewan. Apart from scattered, vuggy, algal encrusted strand-line deposits, most of the carbonates of the “Midale” producing zone consist of sorted, silt-sized material and skeletal or nonskeletal limestones which have a very finely comminuted, commonly dolomitized, limestone matrix with intergranular and chalky porosity. Effective reservoir porosity is controlled by the relative distribution, grain size, and sorting of this matrix. Major hydrocarbon (oil and gas) reserves have been found in the Mississippian “Elkton” carbonate cycle, both in the foothills belt and along the subcrop, in southwestern Alberta. Effective reservoir material of this cycle was found to consist mainly of the dolomitized equivalent of originally coarse, uncemented skeletal limestone, and skeletal limestone with a variable amount of generally porous, finely comminuted (granular) skeletal matrix. Primary porosity was very important in the control of dolomitization, which began with the replacement of this matrix by euhedral rhombohedrons, and finally affected the coarse skeletal material (now generally indicated by leached fossil-cast outlines). These porous dolomites grade laterally in a predictable way into tight, relatively nondolomitized, well sorted, coarse skeletal (locally oolitic) limestones with original high interfragmental porosity now completely infilled with clear crystalline calcite. This lithification by cementation took place early in the history of carbonate sedimentation of this area and before secondary dolomitization processes took effect. The transgressive, reef-fringed, limestone banks or platforms of the Upper Devonian Beaverhill Lake formation in the Swan Hills region of Alberta have been found to contain major reserves of oil and gas. Successive rims of organic lattice, stromatoporoidal and algal, atoll-like “buildups,” with granular matrix, separate generally medium to dark brown pelleted lime muds containing abundant amphiporids, and intercalated lighter colored lagoonal carbonates, from open marine shales and nodular, argillaceous, crinoid- and brachiopod-rich limestones. The most effective reservoir material along the reef fronts or terraces consists of vuggy organic lattice, algal encrusted amphiporids (minor developments), uncemented skeletal or nonskeletal limestone, and reworked stromatoporoidal, algal, and amphiporid material with intra- organic vugs, embedded in a porous, well sorted, micro to finely granular matrix. Matrix grain size and sorting studies are essential to exploration and secondary recovery problems, as the granular material grades laterally into chalky or micrograined limestones, which were laid down under lower energy conditions. Matrix granularity ratio outlines are considered to be superior to ecological maps (percentage of algal and stromatoporoidal material) in the prediction of shoal areas. The highly productive Nisku, regressive, dolomitized biostromal-evaporite complex of the Edmonton and Red Deer areas of Alberta, contains numerous stratigraphic and/or structural traps. Zonation of Nisku dolomites has been accomplished by crystal size and shape studies in combination with identification of vestiges of original organic structures and skeletal or nonskeletal grain outlines. The morphological expression of the underlying Leduc reef platforms and carbonate buildups in the Duvernay formation strongly influences facies variations of the Nisku carbonate-evaporite unit. An algal, stromatoporoidal, coralline, organic lattice chain associated with generally coarse, dolomitized, clastic carbonates with a porous, granular matrix is developed on the Rimbey-Meadowbrook trend, and forms a front to a shale- limestone facies deposited under open marine conditions to the northwest. To the east of this barrier, a complex pattern of fringing organic and clastic carbonate shoals separate locally silled, lagoonal deposits of evaporites and brown carbonate muds containing abundant secondary anhydrite-replaced amphiporids. The shoal and lagoonal carbonates throughout most of this area are overlain by anhydrite or anhydride dolomite sheets, which were precipitated in the wake of an overall regressive Nisku sea.

Abstract The terms reef, bank, bioherm, and biostrome have been used in many different ways by geologists. This varied usage has led to misunderstanding among geologists and, very probably, to misinterpretation of the origin of many limestone deposits. The writers have reviewed past usage of these terms and, from this review, propose a threefold classification of skeletal limestone deposits based on their organic content, shape, and origin. Skeletal limestones are defined as rocks which consist of, or owe their characteristics to, the in-place accumulation of calcareous skeletal material. Askeletal limestone deposit is classified according to the organism primarily responsible for its formation, and its shape is described as biohermal or biostromal, following the definitions proposed by Cumings and Shrock. Following the concept of Lowenstam, a skeletal limestone deposit is classified as a reef or bank, depending upon the ecologic potential of the organisms to build a topographic wave-resistant structure. This classification is proposed not as a substitute for, but rather as a supplement to, detailed descriptions of skeletal deposits. By means of this method of classification the geologist may describe an in-place accumulation of organic remains even if only part of the desired information can be acquired. Later, if all the information is obtained, the geologist may complete his description and arrive at an understanding of how the skeletal limestone deposit may have formed.

Abstract An optimum empirical classification is defined as one in which there is exactly one category for each group of samples separated from other groups by discontinuities in the ranges of their observed properties. A statistical scheme for identifying discrete sample groupings (if they exist) in a set of data is developed which gives equal weight to any number of properties, and treats nonhomogeneous properties simultaneously. Essential features of the scheme are (1) representation of each sample as a vector in an n -coordinate system, where n is the number of attributes considered, (2) use of the angle of separation between sample vectors as an inverse measure of similarity, (3) application of factor analysis to determine the minimum number of coordinates necessary to express main features of the data efficiently, and (4) inspection of the resulting vector array for discrete vector clusters. Applied to 200 samples of modern Bahamian carbonate sediments studied by Purdy, this scheme identifies five discrete sample groups (oolite, oolitic, grapestone, coralgal, and lime mud facies) whose discrete character is not apparent if only a small number of attributes are considered simultaneously. The same 200 samples are treated according to Folk’s limestone classification, and the results compared with the optimum classification scheme. Application of Folk’s criteria yields nine named categories belonging to Type I (sparry allochemical limestone) and Type II (microcrystalline allochemical limestone), although it is difficult to make the I–II distinction in many samples. In general, correspondence between the two classifications is good in Type I, and poor in Type II.

Abstract Limestones and dolomites form the economically important and exceedingly complex family of carbonate rocks. They are set distinctly apart from related rock families by their intrabasinal and highly local origin, their genetic dependence upon organic activity, and their extreme susceptibility to post- depositional modification. The successful classification of carbonate rocks requires detailed knowledge of their multiple com-ponents and genetic processes. Such knowledge has been greatly increased during a period of accelerated investigations since 1940, with the result that the modern classifications are marked improvements over their predecessors. Most of the newer classifications utilize a practical blending of descriptive and genetic parameters. The parameters most commonly used are depositional fabric, particularly the relative abundance of coarser carbonate particles (grains) as compared with the finer grained particles (matrix or micrite); the size and genetic types of the grains or of in-place biotic constituents; the mineralogy; and the nature and degree of post-depositional modification. Secondary parameters include porosity, cementation, the degree of abrasion or rounding of the grains, admixtures of noncarbonate material, and a host of others. The symposium classifications of carbonate rocks and two allied articles of this volume are briefly reviewed and compared. Despite the differences in approach, purpose, and experience among the various authors, the resulting classifications show strong similarities and therefore indicate that a basis of mutual

Abstract A general survey of the geology of the Wichita Mountains is presented here as a means of broadening the geologic background for structural and stratigraphic interpretations in subsurface studies of southern Oklahoma. Detailed descriptions of the common Precambrian igneous rocks, along with measured sections and important faunal markers of Upper Cambrian and Lower Ordovician strata, are taken partly from published sources and partly from unpublished studies by the writers. New mapping of the conglomerates in the eastern part of the region by Chase shows clearly that they are part of the Wichita formation of early Permian age, thus indicating the date of the latest significant orogenic movement in the Wichita Mountains.