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The analyses of 248 samples have revealed that the composition and distribution patterns of sediments within the Albemarle estuarine system (AES) represent a complex interaction between multiple sediment sources, basin morphology and evolution, and associated estuarine processes. Three sediment end-member types are dominant: sand, peat, and organic-rich mud (ORM). Throughout the AES, shallow perimeter platforms and associated sediment-bank shorelines are eroded into Pleistocene units. Shoreline recession supplies sand to the platforms and mud to the central basins; these sediments mix with suspended sediment from the fluvial drainages. Swamp forest-peat and marsh-peat shorelines are actively eroding and supply fine organic detritus to produce the dominant ORM sediment in the central basins. Perimeter platform sands grade into ORM on the platform slope. ORM constitutes about 70% of the benthic habitats within the AES and has an average composition of 76.2% inorganic mud, 13.1% sand, and 10.7% organic matter. The characteristics of ORM greatly affect the benthic community structure, chemical quality of the sediments, and the water quality of the estuary. ORM readily moves in and out of the water column in response to natural and anthropogenic activities, affecting water column turbidity and trace and major element geochemistry. Organic matter and clay minerals in ORM are chemically reactive and interact with the water column to adsorb or release contaminants, nutrients, and gases. Thus, ORM acts as both a sink and source for many different chemical constituents in the water column and plays important, but poorly understood, functions in the physical and chemical dynamics of estuarine ecosystems.

Intense storms, including hurricanes, are believed to have an important impact upon the biota and sediment cover on the continental shelf. Hurricane Diana crossed Onslow Bay, North Carolina, during September 1984 and provided an opportunity to examine its effect on the sea floor. An area in the bay transited by the hurricane had been surveyed previously by side-scan sonar, 3.5 kHz echosounding, and underwater television. We re-surveyed the area in November 1984 with the same techniques and collected box cores for sedimentary structure analysis. The post-hurricane survey yielded no unequivocal evidence of hurricane impact on the sea floor and, surprisingly, showed an abundance of seemingly delicate benthic organisms thriving on the impacted area. Sedimentary structures in the box cores included rip-up clasts and cross-laminations that are indicative of a dynamic sedimentary environment, but they could have formed during non-hurricane storm conditions. The lack of a dramatic signature on the sea floor from Hurricane Diana is attributed to the low sediment influx and frequent occurrence of intense storms on the North Carolina shelf. These factors result in an “armored” depositional environment that shows little response to the passage of each successive storm.

Outer shelf and upper slope environments (250 to 525 m) on the East Australian continental margin off northern New South Wales, represent an important iron-rich sediment province. The dominant (up to 30 9c) authigenic-diagenetic components are glauconite and associated goethite, with minor concentrations of carbonate fluorapatite (mean P₂O₅ content = 0.37 %) within a muddy foraminiferal fine sand. The outer shelf and shelf-slope break (250 to 425 m depth) contain a series of small scarp features interpreted to be outcrops of indurated subsurface units. Sedimentologic and biostratigraphic data from these features suggest that they are Miocene hardgrounds composed of ferruginous-phosphate pavements and gravels. Plank tonic foraminiferal assemblages in unconsolidated surface sediments overlying the hardgrounds range in age from late Pliocene through Holocene and are draped across the continental margin as a veneer covering older strata which only locally crop out. Relative concentrations of glauconite and goethite change downslope and downsection as follows. (1 ) Surface sediments at depths from 250 to 340 m are dominated by goethite with subordinant amounts of weathered glauconite grains. (2) Sediments underlying (1) and constituting the surface sediments from 340 to 465 m are dominated by glossy dark green glauconite with subordinant amounts of goethite. (3) Sediments underlying both of the above and constituting the surface sediments from water depths of 465 to 525 m are dominated by glossy dark green glauconite with subordinant concentrations of light green glauconite within foraminiferal tests. Glauconite forms within foraminiferal tests as light green grains and is subsequently altered to glossy dark green grains with accompanying dissolution of the tests. Above the shelf-slope break, dark green glauconite is altered to goethite within the surface sediment. Goethite occurs at the top of multiple depositional cycles in some sediments between 375 and 465 m depth, suggesting either periods of upper slope oxidation or off-shelf sediment transport during sea-level lowstands. Two types of phosphate occur within the sediments of outer shelf and upper slope. Type I (dark reddish-brown, ferruginous phosphate) is found in water depths from at least 200 to 425 m where it forms hardground pavements and associated nodules and lag gravels. Type I phosphates contain 7 9c to 12 9c P205, 10 % to 50 % Fe₂O₃, and are in radioactive equilibrium indicating uranium-series ages in excess of 800 ka (O'BRiENand Veeh, 1980 ; O'Brien et al., 1986). Type II (gray phosphate) occurs in water depths from 340 to 525 m as irregularly shaped nodules and internal molds within macrofossils in muddy sediments with unweathered light and dark green glauconite. Type II phosphates contain 4% to 19 9c P205, 2 9c to 4 9c Fe₂O₃, negligible goethite, and yield uranium-series ages that range from 250 ka to 2.5 ka (O'Brien and Veeh, 1980 ; O'Brien et ai, 1986)., Sur la marge continentale australienne, au Nord de la Nouvelle-Galles du Sud, la partie externe de la plate-forme et la partie supérieure de la pente (profondeur de 250 à 525 m) représentent une importante province sédimentaire riche en fer. Le sédiment, qui est une boue sableuse fine à foraminifères, contient jusqu'à 30 9c de glauconite authigène et diagénétique, et de goethite associée, et une faible quantité de carbonate fluorapatite (0,37 % de P205 en moyenne). La plate-forme distale et le talus (250 à 425 m de profondeur) présentent une série de petits escarpements interprétés comme des affleurements de niveaux indurés de subsurface. Les caractères sédimentologiques et biostratigraphiques de ces niveaux permettent de les attribuer à des hardgrounds miocènes formés d'encroûtements et de graviers ferrugino-phosphatés. Des sédiments de surface non consolidés, à foraminifères planctoniques d'âge Pliocène terminal à Holocène. recouvrent l'ensemble de la marge continentale, formant un placage continu percé localement par des hardgrounds sous-jacents. Les concentrations relatives de glauconite et de goethite changent aussi bien avec la profondeur d'eau qu'avec la profondeur dans le sédiment. (1) Dans les sédiments de surface à une profondeur comprise entre 250 et 340 m, la goethite domine sur la glauconite en grains altérée. (2) Dans les sédiments de surface à une profondeur comprise entre 340 et 465 m, ainsi que dans les sédiments sous-jacents à ceux de la zone (1 ). c'est la glauconite, en grains vert sombre brillants, qui domine sur la goethite. (3) Dans les sédiments de surface à une profondeur de 465 à 525 m, ainsi que dans les sédiments sous-jacents à ceux de la zone (2), la goethite a disparu et de la glauconite vert pâle à l'intérieur des tests de foraminifères s'ajoute à la glauconite vert foncé brillante. La glauconite vert foncé est l'altération, après dissolution des tests, de la glauconite vert pâle. Au-dessus du talus, la glauconite vert foncé est altérée en goethite dans le sédiment de surface. La goethite se rencontre au sommet de plusieurs cycles de dépôt dans les sédiments compris entre 375 et 465 m de profondeur, ce qui suggère soit des périodes d'oxydation sur la pente proximale, soit un déplacement de sédiment pendant des périodes de bas niveau de la mer. Deux types de phosphate se rencontrent dans les sédiments de part et d'autre du talus. Le type I, phosphate ferrugineux noir à brun-rouge, est trouvé à une profondeur d'eau de 200 à 425 m, où il forme des plaques indurées, des nodules et des graviers résiduels. Ce phosphate de type I contient de 7 à 12 % de P₂O₅,de 10 à 50% de Fe₂O₃ ; son contenu en uranium est en équilibre radioactif, ce qui indique un âge supérieur à 800 000 ans. Le type II, phosphate gris, se rencontre à une profondeur d'eau comprise entre 340 et 525 m, sous forme de nodules irréguliers et de moules internes de macrofossiles, dans des boues à glauconite vert clair et vert foncé. Ce phosphate de type 11 contient 4 à 19 % de P₂O₅ et 2 à 4 % de Fe₂O₃, une faible quantité de goethite ; la série uranifère indique des âges compris entre 250000 et 2500 ans., Riggs Stanley R., Snyder Scott W., O'Brien Geoffrey W., Cook Peter. J., Heggie David T. Sedimentology of the Neogene to Modern glauconite-goethite-phosphate system : east australian continental margin between 29° and 32° south latitude. Le système sédimentaire glauconite-goethite-phosphate du Néogène à l'Actuel : la marge continentale orientale de l'Australie entre 29° et 32° de latitude sud. In: Sciences Géologiques. Bulletin, tome 42, n°3, 1989. Apatite et phosphorites, sous la direction de Jacques Lucas, Peter J. Cook et Liliane Prevot. pp. 185-204.

High-resolution Uniboom, sparker, and 3.5 kHz subbottom seismic profiles, in combination with 9-m vibracores from Onslow Bay, delineate a belt of Neogene sediments which crop out on the North Carolina continental shelf around the east flank of the mid-Carolina platform high. This broad, southeast-trending basement high has controlled the general distribution of Cenzoic sediment sequences within the mid-Atlantic continental margin. The Neogene section includes the Miocene Pungo River Formation, which thickens downdip to the east and southeast off the mid-Carolina platform high. The Pungo River Formation consists of three third-order and at least 21 fourth-order seismic depositional sequences, many of which are abruptly truncated by erosional surfaces and associated channels. These seismic sequences reflect third- and fourth-order eustatic sea-level cycles within the Miocene second-order transgressive sea-level cycle. The fourth-order seismic sequences appear to be equivalent to cyclic lithostratigraphic units characterized by interbedded phosphorite sands, phosphatic foraminiferal muds, calcareous quartz sands, calcarenites, and dolosilts. Some phosphorite sands also occur in the Holocene sand sheet which forms a thin and discontinuous cover on the Pungo River sediments in the Frying Pan phosphate district.Preliminary evaluation of phosphate resources in Onslow Bay has delineated several potential economic prospects. The Pungo River Formation crops out in a northeast-southwest-trending belt which is 150 km long by 25 to 50 km wide and extends into the subsurface to the east and southeast. In two different areas within Onslow Bay, vibracores have penetrated five beds which contain anomalous concentrations of phosphate. Two beds within the Frying Pan phosphate district are estimated to contain 3.75 billion metric tons of phosphate concentrate with an average content of 29.2 percent P 2 O 5 . The other three beds in the northeast Onslow Bay phosphate district are extensive units with low to intermediate phosphate content within the total sediment (up to 9.8% P 2 O 5 . These three "lean" deposits contain at least 780 million metric tons of phosphate concentrate with average P 2 O 5 contents which range from 29.7 to 31 percent. Phosphate resources in the Frying Pan phosphate district have a moderate to high potential for economic development, whereas those in the northeast Onslow Bay phosphate district have a low potential for economic development. This newly discovered continental shelf phosphate province could become economically feasible to mine in the near future when rapidly expanding demand for fertilizers and agricultural products, combined with ever-increasing land-use pressures, produces an unacceptable escalation in the cost of mining land-based reserves.

The structural framework of the Florida peninsula produced an extensive system of coastal, shallow nearshore shelf and platform marine environments which controlled Miocene phosphorite formation, deposition, and accumulation. Microsphorite-intraclast sedimentation was dominant within the inner perimeter belt around the Ocala Upland, and pelletal phosphorite sedimentation in the outer belt, down the depositional slope and offshore from the Ocala Upland. Optimum phosphorite formation took place in the coastal and shallow platform environments. The resulting intraclastic and pelletal phosphate grains were then transported as clastic particles alone and off the shoals, diluted by the associated terrigenous and dolomitic sediment, and deposited in basins on the structural highs, on the flanks of the structures, and in adjacent deeper basinal environments.

Four basic types of macroscopic phosphorite have been recognized within the Florida stratigraphic section. Orthochemical and allochemical phosphorite are primary marine sediments formed within the environments of deposition. The authigenic microcrystalline phosphorite mud (microsphorite) which precipitates in situ either biochemically or physicochemically is orthochemical phosphorite. If the microsphorite mud is subsequently modified into discrete clastic particles, then it is considered to be allochemical phosphorite. The muds may be torn up by biological or physical processes to produce the intraclastic allochems or ingested and excreted by organisms to form the pelletal phosphorites; if there is sufficient energy, the muds may also aggregate around a nucleus grain to form the oolites or pseudo-oolites. A fourth type of allochemical grain is the fossil skeletal material which rains into the sediment system. The bulk of the phosphate macrograins deposited during the Miocene phosphogenic system consisted of these orthochemical and allochemical phosphorites. Subsequent processes modified some of the primary phosphorites to produce the other two varieties of macroscopic grains. Any phosphorite which is later reworked into a younger sediment system is called lithochemical phosphorite. Subaerial weathering processes chemically and mineralogically change the phosphorite to produce the metachemical grains.Microscopically, most of the phosphate macrograins are true aggregates composed of a complex mixture of various mineralogical and biological components. The primary component is the cryptograined carbonate fluorapatite matrix with various types and amounts of disseminated cryptograined coloring matter. Also, there is a multitude of micrograined included material which represents a mixture of everything that was in the environment at the time of precipitation of the orthochemical mud. The inclusions consist primarily of bacteria-like rods and rod aggregates, microorganism fossil hash, dolomite rhombs, and terrigenous sand and clay. Consequently, each phosphate macrograin is a complex sedimentary rock, the chemical composition and physical characteristics of which are totally dependent upon the specific types of inclusions and their abundances.

Some phosphatic sediments occur in many places in the geologic column and form in many environments on the sea floor in response to "normal" conditions and processes of sedimentation. However, that portion of the Miocene which contains the extensive phosphorites and associated anomalous mineralogies of the Hawthorn Group represents a very "abnormal" period of sedimentation. This phosphogenic system was characterized by a specific tectonic setting, structural framework, and an abnormal set of environmental conditions; the consequences represent one of the most extensive and important phosphate deposits in the world.The structural framework which controlled the formation and deposition of the phosphorites of the Florida Miocene was a series of arches or topographic highs associated with the major Ocala Upland and Sanford High. Extensive coastal, shallow nearshore shelf, and platform environments occurred around the highs and were the sites of major phosphorite sedimentation--the phosphate machines and associated entrapment basins. The perimeter phosphogenic belt is situated around the Ocala Upland and was dominated by microsphorite-intraclast sedimentation. The outer phosphogenic belt occurs downdip from the Ocala and in the offshore areas around the Sanford High; this area was characterized by pelletal phosphorite sedimentation. Phosphorite precipitation took place as the cold, chemically supercharged and somewhat toxic upwellings moved across the shallow platforms and into the coastal environments. The biologically stressed shallow-water environments received the bacterially precipitated microcrystalline phosphorite mud, or microsphorite, as well as the other biologically produced phosphate grains. This orthochemical microsphorite mud, along with all of the included microorganism hash, dolomite mud, and fine terrigenous sediment, then responded to the local environmental energy conditions and biological processes. The muds, populated by a benthic community characterized by high environmental tolerances and a low diversity index, injested and excreted the muds as fecal pellets. Under low-energy conditions the muds settled out, became indurated, and were subsequently broken up by biological and physical processes producing intraclasts. Very locally, under high-energy conditions, some of the mud was aggregated to produce pseudo-oolites. The resulting phosphorite allochems (phosphorite gravel, sand, and clay) were then transported as clastic particles along and off the shoals by periodic high-energy conditions. They were diluted by the associated terrigenous and carbonate sediment systems and were deposited and accumulated in the adjacent entrapment basins and on the flanks of the structural highs. Thus, wherever the phosphorus sources were adequate, the physical current and the geochemical systems were appropriate, and the shallow marine environments had the proper geometry, then the "phosphate machine" produced and supplied clastic phosphorites to the associated "entrapment basins." The ultimate magnitude of phosphorite deposition was then dependent upon the size and extent of the structural system, the duration of the phosphogenic system through geologic time, and the volume and rate of terrigenous or carbonate diluent sedimentation. Subsequent fluvial erosion and subaerial weathering severely modified the most updip portions of the Hawthorn phosphorites following emergence.

The Neogene stratigraphic section of the southeastern U.S. continental shelf-coastal plain system is characterized by (i) a series of major regional phosphogenic episodes; (ii) a strong spatial relationship between the structural or topographic framework and phosphate deposition; and (iii) distinct cyclical and regional patterns of deposition of the terrigenous, carbonate, and phosphate lithofacies. The complex depositional patterns are explained by a paleoceanographic model based upon the interaction of glacial eustatic sea-level fluctuations, associated changes in climate, and the dynamics of the Gulf Stream in response to the bathymetric configurations of the continental margin during the past 20 million years.

Various types of phosphate deposits occur within the marine environment. The location, geometry, physical and chemical characteristics of each type of deposit varies as a function of extreme differences in their genesis. The exploration, development and resource potential of each type of deposit will consequently also differ. Thus, any resource assessment strategy for phosphate must take first into account its origin. The following brief summary of major types of deposits, accompanied by Table I, outlines five categories of marine phosphate occurrences.

Regional sedimentologic and stratigraphic studies of Miocene phosphorites on the continental margin of the southeastern United States demonstrate a strong structural control over the formation and deposition of major concentrations of phosphorite in the Hawthorn and Pungo River Formations. The first-order structures controlled the regional limits of the phosphogenic system and provided the necessary depositional environments. Major phosphate sedimentation was concentrated along the nose and flanks of the Ocala arch and Carolina platform and decreased to a minimum into the intervening Southeast Georgia and Hatteras Embayments. Recent discoveries of Miocene phosphorite on the North Carolina continental shelf, in combination with the extensive lag deposits of phosphate on th Blake Plateau, represent a major phosphogenic system which occupies a position relative to the Carolina platform that is analogous to that of the major central and south Florida phosphogenic province relative to the Ocala arch. Superimposed upon the regional structural framework is a series of second- and third-order structural highs and adjacent basins. Each structural high producing a platform or shoaling environment with the necessary associated accumulation basins, may contain one or more phosphate deposits. The specific location, size, and geometry of the resulting deposit is dictated by the folding or faulting, subaerial or submarine erosion, primary depositional processes, groundwater solution and collapse, etc. The location of the second- and third-order structures and the first-order structural highs dictates the subsequent depositional-erosional history. The latter determines the ultimate preservation and degree of secondary alteration of each phosphate deposit. End_of_Article - Last_Page 1669

Shallow subsurface Tertiary sediments of mid-latitude, modern continental margins of the world contain a tremendously extensive and poorly known Neogene sediment sequence that has a high theoretical potential for containing major deposits of unconsolidated phosphate sediments. Reworked phosphates occurring in thin, Holocene surface sediments are important tracers for these shelf deposits which may crop out or more commonly occur in the shallow subsurface. This hypothesis should be tested by combining techniques of high-resolution seismics with detailed networks of vibracores on continental shelf areas with anomalous phosphate concentrations in the surface sediments. Ultimately, deeper core drilling is required to adequately understand the third dimension. Future mining and production will undoubtedly combine existing technology from the offshore petroleum industry with new technology being developed within the mineral industry such as hydraulic slurry mining.

High-resolution, side-scan sonar data collected from the middle continental shelf of Onslow Bay, North Carolina, before and after an intense storm (Hurricane Diana, September 11-13, 1984) were used to investigate the impact on this shelf surface. Wave hindcasting predicts that significant wave heights and periods were 6 m and 10 s during the storm9s passage, and horizontal water velocity at the sea floor was 125 cm/s. Comparison of prestorm and poststorm sonographs revealed no measurable sea-floor changes. Distinct spatial patterns representing textural variations between Miocene muddy sand and coarse Holocene clean sand, as well as extensive areas of loose rock debris, appeared similar. The storm-dominated nature of this shelf surface results primarily from its sediment-starved existence and the equilibrium that has been reached with the region9s ambient, high-energy setting.