Department of Petroleum Geology & Sedimentology, Faculty of Earth Sciences, King Abdulaziz University, Jeddah, Saudi Arabia EPS 421 CLASTIC SEDIMENTARY ROCKS Lecture 10: Classification of sandstones, Provenance and Tectonic Setting Prof. Dr. Mahmoud A. M. Aref
Four Major Components Of Sandstone Framework Sand Grains That Are Made Up Crystals Of Two Or More Different Minerals. Sand (And Silt) Size Detrital Grains Matrix Fine-grained (<0.03mm) Material That Are Associated With The Sand Grains. Silt And Clay Size Detrital Material Cement Material Precipitated Post-depositionally, During Burial. Cements Fill Pores And Replace Framework Grains Pores Classification of Sandstones Most sandstone classifications are based on the composition of the rock. Voids Among The Above Components
FOUR COMPONENTS OF SANDSTONE Geologist s Classification 1. Framework 2. Matrix 3. Cement 4. Pores Engineering matrix Note different use of matrix by geologists and engineers PORE CEMENT FRAMEWORK (QUARTZ) MATRIX FRAMEWORK (FELDSPAR) 0.25 mm
CLASSIFICATION OF SANDSTONES Classifications of sandstones are petrographic,, based on microscopic studies and requiring approximate determinations of the modal composition. Most classifications use a triangular diagram with end members of quartz (+ chert), feldspar and rock fragments.. The triangle is divided into various fields, and rocks with a modal analysis falling within a particular field are given a particular name. In this scheme, sandstones are divided into two major groups based on texture, that is whether the sandstones are composed of grains only, the arenites,, or contain more than 15% matrix,, forming the wackes.
Arenites Group Of the arenites, the term quartz arenite is applied to those with 95% or more quartz grains,, a rock-type formerly referred to as quartzite or orthoquartzite. Arkosic arenite refers to an arenite with more than 25% feldspar, which exceeds the rock fragments content,, The arkosic arenites can be divided into arkoses and lithic arkoses. Litharenite is applied where the rock fragments content exceeds 25% and is greater than feldspar. Specific names which have been applied to litharenite are phyllarenite where the rock fragments are chiefly of shale or slate,, and calclithite where the rock fragments are of limestone. Two rock-types are transitional with quartz arenite are subarkose and sublitharenite.
Wackes Group The wackes are the transitional group between arenites and mudrocks. The most familiar is the greywacke. Two types are distinguished: feldspathic and lithic greywacke. The term arkosic wacke is used for arkoses with a significant proportion of matrix. Quartz wackes, not a common rock-type, are dominant quartz plus some matrix.
Failure in the classification of sandstones This classification is primarily concerned with the mineralogy of the sediment and presence or absence of a matrix. It is independent of the depositional environment,, although some lithologies are more common in certain environments. The nature of the cement in arenites is not taken into account. With regard to fine-grained interstitial material (matrix( matrix), a basic feature of the wackes, there is often a problem of origin. Some is deposited along with the sediment grains, or could be authigenic,, a cement,, and some a diagenetic alteration product of unstable grains. The latter is particularly the case with greywackes.
Hybrid sandstones In addition to terrigenous clastic sandstones, there are many hybrid sandstones. These contain a non-clastic component derived from within the basin of deposition. The three main types are calcarenaceous,, glauconitic and phosphatic sandstones. In glauconitic sandstones,, the glauconite occurs as sand-sized sized pellets. With phosphatic sandstones,, the phosphate may be present as a cement, nodules, coprolites or bone fragments. Calcarenaceous sandstones contain up to 50% CaCO 3, present as carbonate grains. The latter are chiefly ooids, often with quartz z nuclei, and carbonate skeletal fragments. Calcarenaceous sandstones are developed in carbonate-producing areas where there is a large influx of terrigenous clastics. They will often pass laterally into limestones or into purer sandstones towards the source of the siliciclastic sediment. Terrigenous clastics cemented by calcite have been referred to as a calcareous sandstones.
Petrography and origin of principal sandstone types The four common types of sandstone are quartz arenite, arkose, litharenite and greywacke. They are frequently typical of a particular depositional environment, but because of the provenance control on sandstone composition, they are not restricted to that particular depositional setting. Four categories of sandstone as seen in thin section under the microscope. Diameter of field of view is about 4 mm.
1. QUARTZ ARENITES Sandstones with at least 95% quartz grains are the most compositionally mature of all sandstones. In addition, they frequently consist of well-rounded rounded and well-sorted grains so that textural maturity is also very high. Cements are typically quartz overgrowths, but calcite is also common. Monocrystalline quartz grains dominate. Common heavy minerals are tourmaline, zircon and ilmenite.
ARKOSES-1 Arkoses contain more than 25% feldspar, much quartz and some rock fragments. Detrital micas are also present and some fine-grained matrix. The feldspar is chiefly potassium feldspar and much of this is microcline. m The feldspar is usually fresh although some may be altered to kaolinite and sericite. Polycrystalline quartz and quartz/feldspar rock fragments are common. Arkoses are typically red or pink, through the feldspar's color, but also through the presence of finely disseminated hematite, since many arkoses occur in red-bed sequences.
ARKOSES-2 Arkose texture is typically poorly-sorted to well-sorted, with very angular to sub-rounded grains, the precise texture dependent on the degree of transportation. Grain-supported arkoses are cemented by calcite or quartz, while others are cemented by a matrix, often containing much kaolinite.
3. LITHARENITES Litharenites are characterized by a rock fragment content which is in excess of feldspar. They range widely in composition, both in terms of mineralogy and d chemistry, depending largely on the types of rock fragment present. These are a chiefly fragments of mudrock and their low grade metamorphic equivalents; ; other components are flakes of mica, some feldspar and much quartz. There is little primary matrix, otherwise they are similar to greywackes in composition and in fact they have been referred to as subgreywackes. Cements are usually either calcite or quartz.
4. GREYWACKES As indicated by the name, greywackes are dark grey or black rocks, often resembling dolerite The characteristic feature of greywackes is the fine-grained matrix, which consists of an intergrowth of chlorite, sericite and silt-sized sized grains of quartz and feldspar. Of the sand fraction, quartz dominates over rock fragments and feldspar. f Many different rock fragments are usually present. Feldspar grains are e chiefly sodic plagioclase and these are usually fresh in appearance. The origin of the matrix has been referred to as the 'greywacke problem'. There are two possibilities: fine grained sediment deposited along with h the sand fraction (primary( origin), and diagenetic alteration of unstable rock fragments (diagenetic origin).
Genetic Implications of Sandstone Composition a)maturity of a sandstone i) Textural Maturity ii) Compositional Maturity b) Provenance of a sediment i) Tectonic setting ii) Climate
Provenance of a sediment Provenance: where something originated. The Provenance of a sediment is determined by aspects of composition that reflect the source rock and tectonic and climatic characteristics of the source area for the sediment. i) Tectonic setting The source rock of a sediment and the tectonic setting are closely linked: the tectonic setting determines the relative abundance of different types of rock that is available for weathering and the production of clastic sediment. e.g., An arkosic sandstone (rich in feldspars) would have a source area that is rich in granites.
ii) Climate Climate exerts a strong control on the type of weathering that takes place in the source area of a sediment; this, in turn, influences composition. Cold, arid climate: predominantly physical weathering, producing abundant detrital grains (unaltered mineral grains and rock fragments). Sandstones produced in such settings will be relatively immature, depending on the source rocks.
Provenance & Tectonic setting of Quartz Arenites In many cases, quartz arenites are the product of extensive periods of sediment reworking, so that all grains other than quartz have been en broken down. The majority of the quartz grains are second cycle, derived from pre-existing existing sediments. Quartz arenites of this type, often deposited on shallow low-marine shelves, with little tectonic movement and the basin subsides very slowly.
Provenance & Tectonic setting of Arkoses Arkoses are derived from granites and gneisses and vary from in-situ weathering products, to stratified and cross-bedded arkoses where there has been substantial sediment transport. Arkoses are clearly derived from feldspar-rich rich rocks but apart from an appropriate provenance geology, climate and source area relief are also important factors. Under humid conditions, feldspars weather to clay minerals,, so that semi-arid and glacial climates favor arkose formation. If erosion is very rapid, however, i.e. the source area has a high relief, then arkoses can accumulate in spite of an adverse climate. Many arkoses were deposited in fluviatile environments.
Geologic environment in which an arkose would be deposited.
Provenance & Tectonic setting of Litharenites Litharenites account for some 20 to 25% of all sandstones. Their immature composition implies high rates of sediment production followed by short transport distances. Many fluviatile and deltaic sandstones are litharenites.
Provenance & Tectonic setting of Greywackes Many greywackes were deposited by turbidity currents in basins of various types, often off continental margins and in association with volcanics. Frequently greywackes are transported by masses of water highly charged with suspended sediment. Because of the suspended matter, the mass is denser than surrounding water and moves along the sloping sea floor or down submarine canyons as a turbidity current. Greywacke sediment characteristically accumulates in deep-sea fans at the base of the continental slope.
Provenance & Tectonic setting of Sandstones Sedimentary Rock Source area lithology Paleoclimate Tectonic Activity Energy Levels Time Quartz ss, well sorted, well rounded Arkose, poorly sorted, poorly rounded Quartz ss, angular, muddy, poorly sorted Arkose, well sorted, well rounded Litharenite, poorly sorted, no mud, angular Basalt, Gabbro, Gneiss, other mafic rock
Provenance & Tectonic setting of Sandstones Sedimentary Rock Source area lithology Paleoclimate Tectonic Activity Energy Levels Time Quartz ss,, well sorted, well rounded Humid Arkose, poorly sorted, poorly rounded Quartz ss,, angular, muddy, poorly sorted Arkose, well sorted, well rounded Arid Humid Arid Litharenite, poorly sorted, no mud, angular Basalt, Gabbro, Gneiss, Arid or Temperate
Provenance & Tectonic setting of Sandstones Sedimentary Rock Source area lithology Paleoclimate Tectonic Activity Energy Levels Time Quartz ss, well sorted, well rounded Humid Passive Arkose, poorly sorted, poorly rounded Quartz ss, angular, muddy, poorly sorted Arkose, well sorted, well rounded Litharenite, poorly sorted, no mud, angular Basalt, Gneiss, other mafic rock Arid Humid Arid Arid or Temperate Active Passive Active Active
Provenance & Tectonic setting of Sandstones Sedimentary Rock Source area lithology Paleoclimate Tectonic Activity Energy Levels Time Quartz ss, well sorted, well rounded Humid Passive High and consistent Arkose, poorly sorted, poorly rounded Quartz ss, angular, muddy, poorly sorted Arkose, well sorted, well rounded Litharenite, poorly sorted, no mud, angular Basalt, Gneiss, other mafic rock Arid Humid Arid Arid or Temperate Active Passive Active Active Inconsistent Inconsistent High and consistent Inconsistent
Provenance & Tectonic setting of Sandstones Sedimentary Rock Source area lithology Paleoclimate Tectonic Activity Energy Levels Time Quartz ss,, well sorted, well rounded Humid Humid Passive High and consistent Long Arkose, poorly sorted, poorly rounded Quartz ss,, angular, muddy, poorly sorted Arkose, well sorted, well rounded Arid Arid Humid Arid Arid Active Passive Active Inconsistent Inconsistent High and consistent Short Short Long Litharenite, poorly sorted, no mud, angular Basalt, Arid or Arid or Gneiss, other Temperate Temperate mafic rock Active Inconsistent Short