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Other Biogenic Sedimentary Rocks Chapter 13 Chapter 13- Other Biogenic Sedimentary Rocks • In this chapter we will discuss other biogenic deposits that are not carbonate: – Siliceous sediments and chert – Phosphates sediment and phosphatic rock – Organic-rich materials such as coal and oil, which are concentrated in sediments and sedimentary rocks. Chert and Siliceous sediment • Chert is a fine-grained, hard sedimentary rock composed of cryptocrystalline fibrous chalcedony, lesser amounts of microcrystalline and cryptocrystalline quartz, and amorphous silica. • It is the product of organic and inorganic precipitation. Two major types of cherts are found in the geologic record: • Bedded (primary) cherts and; • nodular (replacement) cherts. Each exhibits a distinctive suite of physical characteristics, has its own mode of origin, and occurs within a distinctive lithological and tectonic setting. Bedded cherts • Most bedded chert is produced when silica-rich organic oozes deposited on the seafloor are recrystallized. • Bedded cherts occur as individual bands, layers, or laminae that range in thickness from a few millimeters up to several meters. • The thicknesses of individual chert layers are often laterally and vertically uniform (ribbon cherts), but they can vary. Ribbon chert Red ribbon cherts from Marin County, CA. Red radiolarites from the Gondran cirque, Chenaillet ophiolite near Briançon, France. • Internal sedimentary structures are rare. • Many bedded cherts occurs as part of assemblages of oceanic crustal rocks and associated deep-sea sediments (ophiolite sequences). – Ex. Bermeja Complex, southwestern Puerto Rico Two categories of bedded chert are recognized based on the presence or absence of fossils. • Bedded fossiliferous cherts contain more or less obvious remains of such siliceous organism as diatoms, radiolarians, and sponge spicules. They form when siliceous oozes recrystallize. • Nonfossiliferous bedded cherts contain no visible skeleton remains. Most are siliceous oozes so extensively altered that all fossils have been destroyed. When these nonfossiliferous bedded cherts are etched some fossils are seen so few if any bedded cherts are truly inorganic in origin. The image shows the siliceous skeletons of diatoms (tiny planktonic organisms) preserved in lake sediment, Lake Toskaljarvi, Finland. Radiolarians Radiolarians are single-celled protistan marine organisms that distinguish themselves with their unique and intricately detailed glass-like exoskeletons. During their life cycle, radiolarians absorb silicon compounds from their aquatic environment and secrete well-defined geometric networks that comprise a skeleton commonly known as a test. The radiolarian tests are produced in a wide variety of patterns, but most consist of an organized array of spines and holes (pores) that regulate a network of pseudopods useful in gathering food. When observed with an optical microscope, radiolarian tests are found to be low contrast light-scattering objects that are best viewed using Rheinberg illumination, darkfield illumination, phase contrast, or differential interference contrast (DIC) microscopy techniques. The diversity and beauty of radiolarian tests was first captured and revealed in 1862 by Ernst Haeckel's monograph, Die Radiolarien (Rhizopoda Radiaria), based on specimens gathered from the ocean by the Challenger research cruises of Alexander von Humboldt. The work features 35 exquisite copper plates illustrating hand-drawn radiolarians that still have not been surpassed in quality by modern optical and electron microscopy techniques. Sponges structure is based on microscopic interlocking spicules which are built up from cell secretions. Depending on the species, these may take the form of pointed hard calcareous hooks, or flexible nets. The spicules of some tropical species can cause severe skin reactions if the sponge is touched or handled. Silica Geochemistry • Most of the silica dissolved in seawater is derived from decomposing rock, but very little of it is produced by the solution of quartz because quartz is almost insoluble in natural waters with pH values of less than 11. • Most of the dissolved silica is amorphous silica. This silica is the product of chemical weathering, especially hydrolysis, that occurs as feldspars is converted to clay minerals. • Other sources are submarine weathering of basalts and submarine geysers that pump additional silicarich fluid. • Diatoms, silicoflagellates, and radiolarians use up nearly all the silica as soon as it is available. • Striking variations in the amount of silica in seawater clearly reflect this organic activity. • Very low values of less than 0.01 ppm in surface waters coincide with the photic zone and increases progressively to a maximum of 11 ppm where water depth exceeds 2 km. • Microplankton extract dissolved silica to construct minuscule endoskeletons of opaline silica. What destroys the skeletons • The skeletons of living silica-secreting organism are covered with a carapace of organic matter that keeps them from dissolving. • Solutions begins immediately after death, as the skeletal remains fall slowly to the abyssal ocean floor and continues after burial. Most of the silica that is dissolve is immediately reprecipitated leaving no trace of the fossil. Nodular Cherts • Nodular cherts, also called secondary or replacement cherts, occurs as first-shaped, spherical, sub spherical, and ovoidal masses of opal, chalcedony, and quartz disseminated mainly in shallow-water limestone and dolostone. • Nodules vary in size from a few millimeters to a few centimeters. • Individuals nodules are often linked together, forming roughly planar bands that create anastomosing networks and lenses of chert. • Fossil remains of such silica-secreting organism as sponges occur along with the silicified remains of organism that secrete calcareous shells. Nodular cherts are clearly of diagentic origin • They form when silica dissolves, migrates, and reprecipitates elsewhere, replacing older material. Phosphorites • Almost all sedimentary rocks contain minor amounts of phosphate. • For example, mudrocks and limestone typically contain a fraction of a percent of P2O5. • Sedimentary phosphate deposits (phosphorites), on the other hands are very rare. Phosphorites • They are characterized by an abnormally high concentration of P2O5 (20% or more). • Phosphates in sedimentary rocks occurs as a variety of minerals, but fluorapatite, Ca5(PO4) 3F, is the principal species. • Hydroxyapatite, Ca5(PO4) 3OH, is the principal mineral in vertebrate bones and teeth. • In most sedimentary rocks the minor phosphates typically occurs as detrital clasts of the mineral apatite, as organic fecal matter, or as transported bone fragments. Sources of phosphates in phosphorites • Exceptional circumstances are required to form the few phosphorites known. • Phosphate is derived either directly from hydrothermal veins or by chemical decomposition of such phosphate minerals as fluorapatite in igneous and metamorphic rocks. • Phosphates is essential for organisms because it is an integral component or RNA and DNA. Principal types of phosphorite deposits • 1- Concentration of nodular phosphorite lie scattered on the floor of some modern outer continental shelves. Ancient analog occur in Morocco and North America. • 2- Placer concentrations of transported and reworked organic clasts (bone beds) occur rarely. • 3- diagentically produced by the process of phosphatization. Phosphate-rich fluids leached from guano are concentrated and reprecipitated in limestone. Origin of modern deposits • Zones of upwelling results in high productivity. • Cold, nutrient-rich ocean water wells up onto warmer shallow water in shelf areas. • The abrupt influx of nutrients leads to extremely high organic productivity at the base of the food chain. Burst of phytoplankton activity propagate up the food chain to produce large volumes of organic material (waste and remains). Origin of modern deposits • Bone beds are placer concentrations of organic skeleton from which other sediments has been removed. • Bones, coprolites, and other organic materials are reworked, and the organic remains are concentrated as current lag deposits. Dissolution of some phosphate and its recrystallization as cement produce solid phosphorite that replaces mudrock and limestone. • Flourishing communities of birds or bads confined to islands and peninsulas or caves generate meters-thick piles of layered guano.