MAYA BRADFORD SANDSTONES Consist mainly of silicate grains ranging in size from 1/16 to 2 mm, plus cement and matrix grains smaller than 0.03 mm. MINERALOGY AND CHEMICAL COMPOSITION (OF FRAMEWORK GRAINS, CEMENTS, and MATRIX MINERALS): Only a few principal kinds of minerals make up the bulk of all sandstones. Framework grains can be individual mineral grains or clasts of parent rock that have not yet broken down to individual mineral grains. Binding together those framework grains are cements, which are made up of either silicate minerals (such as quartz) or nonsilicate minerals (such as calcite and dolomite). Matrix minerals fill in the interstitial spaces and are largely made up of clay minerals. Mineralogy and Chemical Composition of Framework Grains - Quartz - 50-60% of the framework grains of average sandstones - Superior hardness + chemical stability - Derived from plutonic rock, particularly felsic plutonic rocks such as granites, metamorphic rocks, and older sandstones - Feldspars - 10-20% of the framework grains of average sandstones - K(potassium)-feldspar v. plagioclase feldspar - Softer and chemically less stable than quartz - K(potassium)-feldspars are generally considered to be more abundant overall in sedimentary rocks than plagioclase feldspars; however, plagioclase is more abundant in sandstones derived from volcanic rock - Accessory minerals - <1-2% of the framework grains of average sandstones - Micas (muscovite and biotite): <0.5% of the framework grains of average sandstones, muscovite is more stable and more abundant in sandstones than biotite, micas are derived particularly from metamorphic parents and plutonic igneous parents - Heavy minerals: minerals that have a specific gravity greater than 2.9, include stable (ex. zircon, which can survive multiple recycling episodes and are commonly rounded, indicating the last source was sedimentary) and unstable minerals (ex. magnetite, pyroxenes, and amphiboles, are less likely to survive recycling, are commonly first-cycle sediments that reflect the composition of proximate source rocks). Heavy minerals are useful indicators of sediment source rocks because different types of source rocks yield different suites of heavy minerals - Rock fragments/ clasts - 15-20%* of the framework grains of average sandstones *(the rock fragment content of sandstones is highly variable and ranges from 0 to >90%) - Pieces of the parent rock that have not yet broken down into individual mineral grains - Most common rock fragments in sandstones are clasts of volcanic rocks, volcanic glass, and fine-grained metamorphic rocks such as slate, phyllite, schist, and quartzite - Chert grains are rock fragments Mineralogy and Chemical Composition of Cements - All cements are secondary minerals that form in sandstones after deposition and during burial (see: diagenesis) - Silicate cements (i.e., quartz) - Quartz is the most common silicate mineral that acts as a cement. In most sandstones, the quartz cement is chemically attached to the crystal lattice of existing quartz grains, forming rims of cement called overgrowths, which look like lines of impurities or bubbles marking the surface of an original grain. MAYA BRADFORD - - - In some sandstones, the quartz cement forms a mosaic of very tiny quartz crystals that fill the interstitial spaces among framework silicate grains and is called microcrystalline quartz. - Opal can occur as a cement in sandstones, particularly in volcanic sandstones Nonsilicate cements (i.e., calcite, dolomite) - Carbonate minerals are the most abundant nonsilicate mineral cements in sandstones - Calcite is the most common carbonate cement and is precipitated in the pore spaces among framework grains, forming a mosaic of smaller crystals which adhere to the larger framework grains and bind them together - Dolomite and siderite (iron carbonate) are less common Hematite and limonite, feldspars, anhydrite, gypsum, barite, clay minerals, and zeolite (volcanic) minerals can also make up a sandstone’s cement Minerology and Chemical Composition of Matrix Minerals - Matrix minerals are grains smaller than about 0.03 mm and fill interstitial spaces in sandstones. Usually, matrix grains are largely made up of clay minerals but may include micas, quartz, and feldspars - Clay minerals form principally as secondary minerals during subaerial weathering and hydrolysis, but may also form via subaqueous weathering or during burial diagenesis CLASSIFICATION (mostly) ON THE BASIS OF MINERAL COMPOSITION Descriptive classification of sandstones is based fundamentally on framework mineralogy. Relative abundance of matrix also influences some classifications. At the end of the day, it is very difficult to name one way to suitably classify all types of sandstones — classifications that are all inclusive tend to be too complicated and unwieldy for general use, and classifications that are oversimplified do not convey enough information Methods for Classifying Sandstones - Step one: Is it a sandstone? Textural nomenclature to name a rock of mixed sediments: - - The first way to classify sandstones is to use your grain scale card to see what are the sizes of the particles making up your rock and then use triangular texture diagrams to see if the ratio of sediment sizes in your rock make it a sandstone, or another rock like a mudstone, shale, conglomerate, etc… Step two: Mineralogical classification of sandstones: MAYA BRADFORD - - - - Most sandstone classification schemes involve a QFR or QFL plot, which is a triangular diagram on which quartz (Q), feldspars (F), and rock fragments (R or L) are plotted at the ends of the triangle. The book recommends the Gilbert model: Sandstones with <5% matrix are classified as quartz arenites, feldspathic arenites, or lithic arenites based on the relative abundance of QFL components - Arkose is an informal term for any feldspathic arenite that has an abundance of feldspars >25% Sandstones with 5% matrix are classified as quartz wackes, feldspathic wackes, or lithic wackes - Graywacke is another informal term for any matrix-rich sandstones that have undergone deep burial, have a chloritic matrix, are dark gray to dark green, and are very hard and dense (controversial) Another step: Classification of sandstone maturity: - Compositional maturity: refers to the relative abundance of stable and unstable framework grains; a sandstone composed mainly of quartz is considered compositionally mature while a sandstone composed mainly of unstable minerals (i.e., feldspars) or unstable rock fragments is compositionally immature - Textural maturity: determined by the relative abundance of matrix and the degree of rounding and sorting of framework grains Major Classes of Sandstones MAYA BRADFORD QUARTZ ARENITES (~15% of all sandstones) FELDSPATHIC ARENITES (~15-20% of all sandstones) LITHIC ARENITES* (~50% of all sandstones) COMPOSITION COLOR MATURITY ASSOCIATED SEDIMENTARY STRUCTURES ORIGIN AND DEPOSITIONAL ENVIRONMENT EXAMPLE >90% siliceous grains that may include quartz, chert, and quartzose rock fragments commonly white or light gray but may be stained red, pink, yellow, or brown by iron oxides most are texturally mature to supermature. wackes uncommon cross-bedding and ripples may be abundant, fossils are rarely abundant Jurassic Navajo Sandstone, Colorado; <90% quartz, more feldspar than unstable rock fragments, and minor amounts of other minerals such as micas and heavy minerals some are pink or red because of k-feldspars and iron oxides; others are light gray to white commonly texturally immature or submature not characterized by any specific kinds of structures could be first-cycle deposits derived from primary crystalline or metamorphic rocks, with vigorous unstable chemical-eliminating weathering conditions. more likely the product of multiple recycling of quartz grains from sedimentary source rocks. typically deposited in stable cratonic environments such as eolian, beach, and shelf environments although some feldspars may survive recycling from a sedimentary source, it’s unlikely that sedimentary source rocks can furnish enough feldspar to produce a feldspathic arenite or arkose. mostly derived from granitictype primary crystalline rocks (i.e., coarse granite or other rocks abundant in k-feldspar). feldspathic arenites that are abundant in plagioclase feldspar are derived from igneous rocks (i.e., quartz diorites) or from volcanic rocks. deposited in very cold or very arid climates (where chemical weathering processes are inhibited and large quantities of feldspar remain), or in warmer, more humid environments where marked relief of local uplifts allows rapid erosion of feldspars before they can be decomposed >90% quartzose grains and contain more unstable rock fragments than feldspars all kinds of gray texturally immature to submature all kinds of structures originate under conditions favoring the production and deposition of large volumes of relatively unstable materials. probably derived from rugged, high-relief source areas. may be deposited in nonmarine settings nearby alluvial fans or other fluvial environments, in marine foreland basins adjacent to fold-thrust belts, or in deltaic or shallow shelf environments Pennsylvanian Pottsville Formation, Appalachians Cretaceous Dakota Sandstone, Colorado Carboniferous Old Red Sandstone, Scotland *Volcaniclastic sandstones are a special kind of lithic arenite composed primarily of volcanic detritus and are made up largely of pyroclastic materials. They are characterized by the presence of euhedral feldspars, pumice fragments, glass shards, and volcanic rock fragments, and generally have a very low quartz content