Formation of Marine Phosphate Deposits - Mechanisms and Origins

especially of fish, and coprolites are often associated. On the South West African shelf, the phosphorite occurs within diatom oozes as dispersed phosphate, biogenic fragments, coprolites, pellets and phosphatic nodules. The phosphate content increases with increasing induration. Diatoms, originally composed of opaline silica, have beer replaced by cryptocrystailine. phosphate within nodules. Off Peru and Chile the phosphorite is developing on the upper continental slope as replacements of benthic foraminifera (Manheim et ai., 1975). Obliteration of the foraminifer's structure by the replacement leads to the production of phosphorite pellets. Extensive and valuable phosphate deposits occur in the Upper Cretaceous to Lower Tertiary of North Africa and the Middie East, from Morocco and the Spanish Sahara to Iraq and Turkey (Sheldon, 1964). These bioclastic and pelleted phosphorites, also related to upwelling, accumulated on the continental margin around the southern shore of Tethys. Some ancient bedded phosphorites have been interpreted as formed within confined basins, lagoons or estuaries, where high phosphate values, promoting high organic productivities, do occur. 4. Origin of marine phosphate deposits. Two factors control the formation of marine phosphorites: upwelling and low sedimentation rates (see Fig. below). The importance of upwelling was noted that many modern phosphorites are located in areas where cold, nutrient-rich waters rise from the depths towards the surface. Upwelling currents lead to high organic productivities and phytoplankton growth in surface waters which m turn results in organic-rich (and so phosphate-enriched) sediments and oxygen-deficient waters overlying the seafloor. Mass mortalities of fish occasionally take place in areas of upwelling, particularly as a result of poisoning by phytoplankton blooms. More organic matter, with its combined phosphorus, and skeletal phosphate (bones) are thus contributed to the seafloor during these events. Although it was once thought that phosphorite was precipitated directly from seawater, perhaps as some type of colloid, data indicate that it is being formed within the surficial sediments largely by replacement (Baturin, 1970) The breakdown of organic matter in the sediment hberates phosphate which is precipitated in pellets and coprolites, and replaces siliceous and calcareous skeletons and lime mud, eventually giving rise to nodular masses of phosphorite. The role of the phytoplankton is crucial in transporting the phosphate from the upwelling currents to the seafloor. A further stage in the formation of extensive marine phosphorites is reworkmg. Ocean currents and severe storms remove much of the fine unphosphatized sediment from the seafloor, leaving a concentrate of nodules' pellets and coprolites in various stages of induration and phosphate impregnation, which are then further phosphatized. Even greater reworking of bottom sediment is achieved during sealevel changes; transgressive-regressive events across the shelf off southern Africa have been importam in the formation of the phosphorites there (Birch, 1979). There is a latitudinal control on the formation of phosphorites: they occur in tropical or subtropical arid zones within 50 of the equator (Cook &McElhinny, 1979). The paucity of phosphorites actually forming at the present time, and their greater development in the Tertiary, has been related to slightly higher seawater temperatures at that time (Tooms et ai., 1969). - Summary of origin of sedimentary phosphate deposits. 43

Two types of phosphate deposits occur, continental or non marine phosphate (e.g. Guano is deposited from excrements of birds and bats in caves and small oceanic islands, and marine phosphate which explained by upwelling model. However, geologically, guano itself is not significant. However, downward percolation of solutions derived from guano may cause phosphatization of underlying carbonate sediments and rocks. The upwelling model has been used by several authors to interpreting origin of marine phosphate deposits. Two factors control the formation of marine phosphates: upwelling and low sedimentation rates. (see the figure below). A further stage in the formation of extensive marine phosphorites is reworkmg. Ocean currents and severe storms remove much of the fine unphosphatized sediment from the seafloor, leaving a concentrate of nodules' pellets and coprolites in various stages of induration and phosphate impregnation, which are then further phosphatized. increased organic sealevel productivity photic zone low rates of sediment supply upwelling of cold nutrient- rich waters deposition of phosphate-rich sediments inshore shelf shelf-platform margin deep water/ ocean basin Fig. 22 Upwelling Model for Marine Phosphate 12. Organic-Rich Deposits Introduction Most sedimentary rocks, including rocks of Precambrian age, contain at least a small amount of organic matter consisting of the preserved residue of plant or animal tissue. Rocks with 3-10 percent organic matter are regarded as organic rich. Black shales and other organic-rich and bituminous mudrocks typically 44

contain 3 to 10 or more percent by weight of organic matter. Some oil shales contain even higher percentages, and coals may be composed of more than 70 percent organic matter. Certain solid hydrocarbon accumulations, such as asphalt and bitumen deposits formed from petroleum by oxidation and loss of volatiles, constitute another example of a sedimentary deposit greatly enriched in organic carbon. Currently, coal and petroleum (including natural gas) are the principal kinds of fossil fuels. Oil shales or kerogen shales contain significant volumes of organic matter that can be converted to petroleum through heating. Organic-Rich Deposits Peat, brown coal (lignite), hard coal and oil shale are the main organic deposits. Principal kinds of organic matter: Three basic kinds of organic matter accumulate in subaerial and subaqueous environments: humus, peat, and sapropel. Soil humus is plant organic matter that accumulates in soils to form a number of decay products such as humic and fulvic acids. Most soil humus is eventually oxidized and destroyed; thus, little is preserved in sedimentary rocks. Peat is also humic organic matter, but peat accumulates in freshwater or brackish-water swamps and bogs, where stagnant, anaerobic conditions prevent total oxidation and bacterial decay. Therefore, some peat that accumulates can be preserved in sediments, e.g. in coals and shales. Sapropel is fine organic matter that accumulates subaqueously in lakes, lagoons, and marine basins where oxygen levels are moderately low. It consists largely of the remains of phytoplankton and zooplankton and of spores, pollen, and macerated fragments of higher plants. Humic organic matter is the chief constituent of most coals, although a few coals are composed of sapropel. The original organic material has been changed by a complex Carbonaceous sedimentary rocks diagenetic process involving chemical and biochemical degradation, yielding an insoluble organic substance called kerogen. kerogen is believed to be the precursor material of petroleum, which is derived from kerogen by natural thermal or thermocatalytic cracking. Kerogen is insoluble in both aqueous alkaline solvents and common organic solvents. Some organic material in sediments is soluble in organic solvents. This material is called bitumen. In ancient shales, kerogen makes up 80 to 99 percent of the organic matter; the rest is bitumen (Tissot and Welte, 1984). On the basis of chemical composition, Tissot and Welte (1984) classify kerogen into three major types. Type I: Algal/Sapropelic Type I kerogens are characterized by high initial hydrogen-to-carbon (H/C) ratios and low initial xygen-to- carbon (O/C) ratios. This kerogen is rich in lipid-derived material and is commonly, but not always, from algal organic matter in lacustrine (freshwater) environments. On a mass basis, rocks containing type I kerogen yield the largest quantity of hydrocarbons upon pyrolysisHydrogen:carbon atomic ratio > 1.25 Oxygen:carbon atomic ratio < 0.15 Derived principally from lacustrine algae, deposited in anoxic lake sediments and rarely in marine environments Composed of alginite, amorphous organic matter, cyanobacteria, freshwater algae, and lesser of land plant resins 45

Formed mainly from protein and lipid precursors Has few cyclic or aromatic structures Shows great tendency to readily produce liquid hydrocarbons (oil) under heating Type II: Planktonic Type II kerogens are characterized by intermediate initial H/C ratios and intermediate initial O/C ratios. Type II kerogen is principally derived from marine organic materials, which are deposited in reducing sedimentary environments. The sulfur content of type II kerogen is generally higher than in other kerogen types.. Although pyrolysis of type II kerogen yields less oil than type I, the amount yielded is still sufficient for type II-bearing sedimentary deposits to be petroleum source rocks. Hydrogen:carbon atomic ratio < 1.25 Oxygen:carbon atomic ratio 0.03 - 0.18 Derived principally from marine plankton and algae Produces a mixture oil and gas under heating Type II-S: Sulfurous Similar to type II but with high sulfur content. Type III: Humic] Type III kerogens are characterized by low initial H/C ratios and high initial O/C ratios. Type III kerogens are derived from terrestrial plant matter, specifically from precursor compounds including cellulose, lignin (a non-carbohydrate polymer formed from phenyl-propane units that binds the strings of cellulose together); terpenes and phenols. Coal is an organic-rich sedimentary rock that is composed predominantly of this kerogen type. On a mass basis, Type III kerogens generate the lowest oil yield of principal kerogen types. Hydrogen:carbon atomic ratio < 1 Oxygen:carbon atomic ratio 0.03 - 0.3 Has low hydrogen content because of abundant aromatic carbon structures Derived from terrestrial (land) plants Tends to produce gas under heating (Recent research has shown that Type III kerogens can actually produce oil under extreme conditions)[35][c'toto needed] Type IV: Inert/Residual Type IV kerogen comprises mostly inert organic matter in the form of polycyclic aromatic hydrocarbons. They have no potential to produce hydrocarbons.[36] Tissot and Welte (1984, p. 155) identify also a residual type of organic matter characterized by abnormally low H/C ratios associated with high O/C ratios. The elemental chemistry of organic matter is relatively simple. It consists dominantly of carbon, with lesser amounts of hydrogen, oxygen, nitrogen, and sulfur. Type I kerogen tends to be high in hydrogen and relatively low in oxygen, whereas Type III kerogen is high in oxygen and relatively low in hydrogen. Type II kerogen has high oxygen and intermediate hydrogen abundance. Thermal maturation or coalificaton of organic material results in a relative loss of oxygen and hydrogen and a relative enrichment in carbon. Coals 46

Coals are a carbon-rich type of carbonaceous sediment. They are composed dominantly of combustible organic matter but contain variable amounts of impurities (ash) that are largely siliciclastic materials. Coal is a readily combustible rock containing carbonaceous material, formed from compaction or induration of variously altered plant remains similar to those of peaty deposits Most coals are humic coals, although a few are sapropelic coals that are made up mostly of spores, algae, and fine plant debris. Differences in the kinds of plant materials (type), in degree of metamorphism (rank), and range of impurities (grade), are characteristic of the varieties of coal. They occur in sedimentary rocks ranging in age from Precambrian to Tertiary, and peat analogs of coal occur in Quaternary sediments. Rank The term rank refers to the level of organic metamorphism of a coal; a number of properties, such as carbon and volatile content, can be used to measure rank, but these require laboratory analysis. It is a function of the degree of coalification (increase in organic carbon) attained by a given coal owing to burial and metamorphism. Humic coal, those formed through in situ organic growth, chiefly in swamps, marshes and bogs Peat consists of unconsolidated, semicarbonized plant remains with high moisture content and the vegetal material may still be recognisable in hand-specimens. Lignite or brown coal is the lowest-rank coal. Lignites are brown to brownish black coals that have high moisture content and commonly retain many of the structures of the original woody plant fragments. They are dominantly Cretaceous or Tertiary in age. Bituminous coals are hard, black coals that contain lower amounts of volatiles and less moisture than lignite and have a higher carbon content. They commonly display thin layers consisting of alternating bright and dull bands. Bitumen and other semi-solid/solid hydrocarbons do occur rarely within sandstones and limestones, and along fault and joint planes. Subbituminous coal has properties intermediate between those of lignite and bituminous coal. Anthracite is a hard, black, dense coal that contains more than 90 percent carbon. It is a bright, shiny rock that breaks with conchoidal fracture, such as the fractures in broken glass. Bituminous coals and anthracite are largely of Mississippian and Pennsylvanian (Carboniferous) ages. Cannel coal and boghead coal are non-banded, dull, black coals that also break with conchoidal fracture; however, they have bituminous rank and much higher volatile content than does anthracite. Cannel coal is composed of conspicuous percentages of spores. Boghead coals are composed dominantly of nonspore algal remains. Bone coal is very impure coal with high ash content Oil shales are a diverse group of rocks which contain organic material that is mostly insoluble in organic solvents, but that can be extracted by heating (distillation). The organic matter is largely kerogen, but some bitumen may occur. The quantity of oil that can be extracted ranges from about 4% to more than 50% of the weight of the rock, Oil shales contain a substantial amount of inorganic material consisting largely of quartz silt and clay minerals. Some oil shales are really organic-rich siltstones and mudrocks whereas others are organic rich limestones In many oil shales the remains of algae and algal spores are common and so much of the organic matter is assumed to be of algal origin. Fine-grained higher- plant debris and megaspores also may be an important constituent. The kerogen 47

in oil shales is largely type I, that is it has a high H/C and low O/C ratio. Some kerogen in oil shales may be type III, formed from vascular plant debris. Oil shales were deposited in lacustrine and marine environments. There is considerable interest in oil shales because they are a source of fossil fuel and they may help to offset the expected exhaustion of petroleum reserves. Petroleum is derived largely from the maturation of organic matter deposited in fine-grained marine sediments. Organic-rich sediments can be deposited in anoxic silled basins, on shelf margins in association with upwelling and the oxygen-minimum zone, and on the sea floor at times of oceanic anoxic events. Many marine hydrocarbon source rocks formed at times of high organic productivity of marine plankton, coinciding with transgressive events and highstands of sea-level. Peritidal microbial mats and organic matter in reefs and ooids may be other sources. Lacustrine source rocks also are important. Diagenesis of the organic matter begins very early at shallow burial depths, and substantial quantities of methane can be produced through bacterial fermentation. This marshgas normally escapes into the atmosphere, but it can be trapped. Burial diagenesis of the deposited organic matter leads to the formation of kerogen, the type depending on the nature of the organic input. Burial to temperatures of 50-80 C causes thermocatalytic reactions in the kerogen, and in types I and II, cycloalkanes and alkanes are generated, two of the main constituents of crude oil. When this process takes place, the source rock is said to be mature. With increasing temperature, more and more oil is generated until a maximum is reached, and then the quantity decreases and an increasing amount of gas is formed (see Fig. 8.8). The principal phase of oil generation takes place at temperatures around 70-100 C; in areas of average geothermal gradient this is at depths of 2-3.5km (this is the oil window). The gas produced is wet initially, but above 150 C only methane (dry gas) is generated.Time also is a factor in source-rock maturation; higher temperatures/greater burial depths are required for oil generation from younger rocks, compared with older rocks, which can thus reach maturity at lower temperatures (Fig. 8.9). Burial of type-III kerogen and coal into the catagenesis realm leads to the generation of much gas, mostly methane, and little oil. Some organic compounds in organic matter, the porphyrins, for example, are very resistant to diagenetic alteration and are found in hydrocarbon source rocks as well as crude oils. These geochemical fossils or biomarkers can be very useful in correlating an oil with its source rock. At higher temperatures, the biomarkers begin to break down so that they can also give an indication of the maturity of the source rock. This is important for calculating the amount of oil that has been generated. In the search for petroleum, use can be made of the colour of pollen and spores (palynomorphs) in the source rocks to see if the stage of petroleum generation has been reached. With rising temperature and higher level of organic metamorphism, palynomorphs change colour from yellow to brown when crude oil is evolved, and to black when dry gas is generated. An indication of burial temperature also can be obtained from the vitrinite reflectance of phytoclasts and the colour of conodonts. Other tests for source-rock maturity are the determination of H/C and O/C ratios, UV fluorescence and pyrolysis. The study of petroleum is a science in itself and for further information reference should be made to petroleum geology textbooks, such as Tissot & Welte (1984), Selley (1988), Bjorlykke (1989), North (1990) and Gluyas & Swarbrick (2001). 48