Subaerial Weathering Research Articles

Geochemistry of aquatic and terrestrial sediments, precambrian shield of Southeastern Ontario

Lake water and sediment samples from approximately 2200 lakes and glacial sediment (sub-solum) samples from about 1800 sites were collected throughout a 38000 km2 rectangular area extending from Georgian Bay east to the Ottawa and St. Lawrence Rivers, Ontario, Canada. Lake water alkalinity and pH patterns are similar to the distribution of carbonate components in glacial drift. Carbonate-rich drift derived from the Paleozoic limestone terrain on the northeast flank of the Precambrian Frontenac Arch has been dispersed in a south-westward direction across a variety of non-calcareous metasedimentary and igneous rocks of the Canadian Shield, providing a buffering capacity to lakes situated in granitic terrain. The distribution patterns of mobile trace and minor elements are influenced by geochemical processes associated with subaerial weathering, ground and surface water transport, and the geochemical environment within the lakes themselves. Although composition of the drift is generally reflected by lake geochemistry, these post depositional processes can cause significant variations between patterns derived from the two sample types. Anions and cations such as SO 4 = , Cl−, Na+, and F− exhibit concentration patterns thought to reflect both anthropogenic inputs and natural variations due to differences in the geology. All regional geochemical patterns may show evidence of local enhancement caused by high concentrations of chemically distinctive minerals in drift or nearby bedrock.

Alumina-rich rocks from the early Precambrian of the Kaapvaal Craton as indicators of paleosols and as products of other decompositional reactions

Within early Archaean to early Proterozoic pelites and volcanics of the Kaapvaal Craton, rocks with above 20% Al2O3 are widely distributed. The alumina-rich nature of these rocks is ascribed to decompositional reactions during which up to about 60% of the original material was leached out of the source rock and removed in solution. Many of these rocks also show diagenetic enrichment in K2O of up to 10%. The decomposition reactions in many cases affected the source rocks without disturbing fine structures such as perlitic devitrification cracks in volcanic glass granules. The origin of the majority of these rocks can be ascribed to decomposition during subaerial weathering. They represent paleosols. For other occurrences the stratigraphic context rules out an explanation as paleosols. The origin of this type is ascribed to subaqueous decomposition of lavas or tuffs, assisted by hydrothermal exhalations. It is shown that alumina-rich rocks in greenschist belts almost exclusively formed subaqueously whereas the younger occurrences mostly are of weathering origin. Geotectonic differences are invoked to explain this contrast. Rocks of similar chemical composition are described from the Canadian Shield, Zimbabwe, Western Australia, India, and the Baltic Shield. They illustrate that the preservation potential of these rocks is high. Only the study of the stratigraphic context will allow a conclusion on whether a particular occurrence is the product of weathering or subaqueous decomposition.

Temporal span of bone accumulations at Olduvai Gorge and implications for early hominid foraging behavior

Bones of mammals exhibit progressive stages of weathering during their time of subaerial exposure. Consequently, the study of bone weathering in fossil assemblages may help to assess the period represented by an accumulation of bones. Stages of bone decomposition due to subaerial weathering have been identified in assemblages of fossil macromammals from Olduvai Gorge, Tanzania. A modern bone assemblage collected by spotted hyenas is used to devise a method for recognizing attritional accumulations of bones from weathering characteristics. This method, which involves study of long bone diaphyses, is applied to Plio-Pleistocene faunal assemblages from Olduvai, 1.70–1.85 ma old. Previous work indicates that early hominids had an important role in the collection of fauna at five of the six sites studied. It is shown that animal bones were accumulated at each site over a period of probably 5–10 yr or more. The length of this period, along with other taphonomic evidence, suggests that the processes of bone aggregation at these sites differed from those at the short-term campsites of modern, tropical hunter-gatherers.

UK requirements for small mass spectrometer calibration: Peter Nash, National Laboratory, Teddington, TW11 0LW, UK

The surface chemistry of fresh and weathered historical basalt flows was characterized using surface-sensitive X-ray photoelectron spectroscopy (XPS). Surfaces of unweathered 1987–1990 flows from the Kilauea Volcano, Hawaii, exhibited variable enrichment in Al, Mg, Ca, and F due to the formation of refractory fluoride compounds and pronounced depletion in Si and Fe from the volatilization of SiF4 and FeF3 during cooling. These reactions, as predicted from shifts in thermodynamic equilibrium with temperature, are induced by diffusion of HF from the flow interiors to the cooling surface. The lack of Si loss and solid fluoride formation for recent basalts from the Krafla Volcano, Iceland, suggest HF degassing at higher temperatures.Subsequent short-term subaerial weathering reactions are strongly influenced by the initial surface composition of the flow and therefore its cooling history. Successive samples collected from the 1987 Kilauea flow demonstrated that the fluoridated flow surfaces leached to a predominantly SiO2 composition by natural weathering within one year. These chemically depleted surfaces were also observed on Hawaiian basalt flows dating back to 1801 AD. Solubility and kinetic models, based on thermodynamic and kinetic data for crystalline AlF3, MgF2, and CaF2, support observed elemental depletion rates due to chemical weathering. Additional loss of alkalis from the Hawaiian basalt occurs from incongruent dissolution of the basalt glass substrate during weathering.

Paleocene to lower Eocene clay sedimentation in the Bay of Biscay

A study of the mineralogy of red clay sequences overlying basalt in the Bay of Biscay revealed two successive units: unit one (lower unit) indicates that clay minerals are the result of subaerial weathering of acid or basic continental rocks, occurring in the Galice province (northwest Spain); and unit two (upper unit) consists of fine-grained particles which suggest distinct volcanic activity. Clay materials from the first unit (Paleocene) are deposited in shallow environments, whereas the second unit materials (lower Eocene) Occur in deeper environments followed by neoformation of clay Minerals and concentration of chemical elements (cobalt, nickel, lead, zinc, and barium).

Petroleum Reservoirs in Silurian Dolomite of Western Illinois: ABSTRACT

The basal Silurian Alexandrian carbonates, sandwiched between the Ordovician-Silurian and Silurian-Devonian unconformities, have produced more than 3 million bbl of oil from the Buckhorn East and Brooklyn fields on the western margin of Illinois. These fields are shallow (450-670 ft), remote from the most-productive portion of the Illinois basin, and potential exists for additional production from this unconformity play. Three lithofacies were recognized in Alexandrian carbonates in detailed core and wireline studies that included thin-section, x-ray, and SEM analyses. These carbonates are regressive sequences composed of shallow-shelf, restricted-subtidal, and intertidal facies. The shallow-shelf facies is primarily biosparite composed of crinoid and brachiopod debris; t is facies is dolomitic in places. The restricted-subtidal facies consists of both bioturbated and undisturbed gray micrites containing stylolites, green chloritic clay, and glauconite; sparry clacite fills vertical cracks. The intertidal facies is tan, mottled, vuggy dolomite composed of algal mats, anhydrite-filled birdseye structures, burrows, and scattered marine fossils. Petroleum reservoirs apparently developed as a result of dolomitization in intertidal and shallow-shelf carbonates. Vuggy porosity, a possible result of karst leaching, is found in pay zones within the dolomitic intertidal facies. Evidence of emergence and subaerial weathering include the presence of vadose silt, anhydrite, and nontectonic breccia. End_of_Article - Last_Page 306------------

Lower Eocene red-clay sedimentation on the George Bligh/Rockall Channel

Core 7710, recovered on the George Bligh/Rockall Channel, is composed of fine grained ocher-colored sediments rich in ferriferous smectite (montmorillonite, beidellite). These sediments were deposited during early Eocene in a shallow marine environment. Their mineralogic, chemical and isotopic constituents indicate subaerial weathering of lower Eocene intermediate igneous rocks (ferrodiorite, marscoiite) eroded from the still-emergent George Bligh and Rockall Banks. Lack of any post Eocene Sediments may be due to major morphologic and hydrologic changes forming sedimentation gaps.

Chemical changes during progressive subaerial palagonitization of a subglacial olivine tholeiite hyaloclastite: A microprobe study

The palagonite and parent sideromelane of the 6–7-mm thick reddish-brown subaerial weathering zone (W.Z.) as well as the less-altered palagonite/sideromelane inside the W.Z. of an olivine tholeiite hyaloclastite have been investigated by XRD, SEM and the microprobe. The light reddish-brown to deep yellow palagonite rinds around the sideromelane grains in the outer part of the W.Z. is ∼ 4–5 times thicker than the dark-brown to nearly black palagonite (∼ 6 μm thick) inside the W.Z. Relative to the parent sideromelane the palagonite in the W.Z is considerably more depleted in SiO 2 and MgO, slightly more depleted in CaO, and considerably more enriched in Al 2O 3 and TiO 2 than the palagonite inside the W.Z. Na 2O is consistently depleted, whereas FeO t shows only minor variations in either direction. K 2O shows the largest variations by being strongly depleted in the outer part of the W.Z, and strongly enriched inside the W.Z. No authigenic minerals were observed in the palagonite and intergrain pores of the hyaloclastite. The chemical variations within a palagonite rind, or across the analysed profile are therefore assumed to have taken place in the amorphous state of palagonite, through interaction with freshwater environment.

Genesis of Englacial Debris within the Lower Glacier De Tsidjiore Nouve, Valais, Switzerland, as Revealed by Scanning Electron Microscopy

Previous analysis has failed to establish unequivocally the genesis of englacial debris bands within the Glacier de Tsidjiore Nouve. Although extraglacial sources were presumed to be dominant, some of the debris appeared to be derived either from a subglacial source or had not been passively transported. Evidence obtained through the application of scanning electron microscopy reinforces the view that the englacial debris was incorporated within the glacier via crevasses in the firn basin, and indicates that it is derived from (1) supraglacial debris, resulting from subaerial weathering of rock-faces above the ice in the firn basin, and (2) subglacial debris derived from the base of ice standing above these rock faces. The results of this investigation demonstrate the value of scanning electron microscopy in the study and interpretation of complex sediment assemblages on and within glaciers.

Carbonate-Dominated Shelf Cycles in Late Pennsylvanian of Mid-Continent: Intrabasinal and Extrabasinal Controls on Sedimentation and Early Diagenesis: ABSTRACT

The Upper Pennsylvanian (Missourian) Lansing-Kansas City Groups in the subsurface of western Kansas consist of a dozen cyclothems of carbonate and terrigenous clastics deposited on a platform that was gently tilted southward toward the more rapidly subsiding Anadarko basin. Maps of the study area covering western Kansas, derived from several thousand well logs and over 30 cores, describe four cyclothems in the Kansas City Group. Regressive carbonates developed in each of these cyclothems are major petroleum reservoirs in this region and are the focus of this examination. The regressive carbonates thicken southward (basinward) at rates controlled by the tilt of the platform. Local and subregional variations in thickness and facies distribution are affected by local differential subsidence on the shelf, particularly along broad positive areas that closely correspond with previously active uplifts. Relatively thin carbonates having restricted shallow-marine facies are abundant over these positive areas. Subtle flexures along the shelf, especially where the slope increased basinward, were loci for ooid shoal development caused by wave and current action during shallow-water deposition. Facies patterns interpreted from core and log-derived mapping demonstrate that neither the flexures nor the broader positive areas of the shelf were consistently active throughout the deposition of all four cycles studied. Hence, despite remarkable similarities, there are distinct variations between the cycles. Furthermore, not all cycles cover the study area to the same extent because (1) in some cycles, regressive carbonates pinch out along the northern (landward) shelf; (2) shallow restricted marine facies can be displaced southward; (3) the marine black shale, the deepest water phase of the cycle, can be missing or only poorly developed; and (4) intense local variations can occur in the early meteoric freshwater diagenesis that affects all cycles over much of the shelf. Terrigenous clastics in the cycles are composed of thin layers of silty shale and claystone that prograded southward over much of the northern half of Kansas during the regressive phase of each cycle. Clastics from the Ouachitas, important components in cycles in eastern Oklahoma and southeast Kansas, never reached western Kansas until the Virgilian because these sediments were trapped by a relatively deep basin in western and central Oklahoma. Extensive and prolonged subaerial weathering and associated freshwater diagenesis, marked and sharp vertical changes in the lithofacies, and broad repetitive facies patterns in relatively thin carbonates and shales can be explained by glacial eustatic changes in sea level. The Gondwana glacially influenced eustatic sea level changes had a periodicity and magnitude comparable to those of the Pleistocene. Variations in thickness, carbonate facies patterns, and diagenesis were also strongly influenced by second-order intrabasinal processes, including differential subsidence over positive areas and along breaks in slope. End_of_Article - Last_Page 567------------

Scanning Electron Micrographs of Quartz Sand Grains from Cold Environments Examined Using Fourier Shape Analysis

ABSTRACT The use of grain shape to identify sedimentary environments assumes that grain morphology reflects environmental history. Scanning electron microscopy permits detailed examination of quartz sand grain surface morphology. Particle shape from gross form to surface texture is quantified using closed-form Fourier analysis of whole grain outlines from micrographs of 158 quartz sand grains representing subglacial transport and subaerial weathering in cold environments. Mean harmonic amplitude spectra and roughness coefficients are calculated, and three problems are investigated yielding the following results. Findings are preliminary due to the relatively small number of grains analyzed. 1) Two-dimensional particle micrographs are a suitable surrogate for three-dimensional grain roundness a d texture. 2) The particle size of a sand grain sample affects its roundness and texture, and only samples of similar size range should be compared. 3) Fourier shape analysis distinguishes significant differences between basal and englacial grains, the latter being more angular and rough in texture. This finding is relevant to the definition of debris transport paths through glaciers. The technique may also prove useful in the interpretation of marine sediments from cold regions, and pre-Quaternary diamictites of controversial or unknown origin.

Geological Development, Origin, and Energy Mineral Resources of Williston Basin, North Dakota

The Williston basin of North Dakota, Montana, South Dakota, and south-central Canada (Manitoba and Saskatchewan) is a major producer of oil and gas, lignite, and potash. Located on the western periphery of the Phanerozoic North American craton, the Williston basin has undergone only relatively mild tectonic distortion during Phanerozoic time. This distortion is largely related to movement of Precambrian basement blocks. Sedimentary rocks of cratonic sequences Sauk through Tejas are present in the basin. Sauk (Cambrian-Lower Ordovician), Tippecanoe (Ordovician-Silurian), and Kaskaskia (Devonian-Mississippian) sequence rocks are largely carbonate, as are the major oil- and gas-producing formations. Absaroka (Pennsylvanian-Triassic) and Zuni (Jurassic-Tertiary) rocks have more clastic content, but carbonates are locally important. Clastics of the Zuni sequence (Fort Union Group) contain abundant lignite. Tejas (Tertiary-Quaternary) sequence rocks are not significant in the production of minerals or energy, although glacial sediments cover much of the region. Oil exploration and development in the United States portion of the Williston basin since 1972 have given impetus to restudy basin evolution and geologic controls for energy-resource locations. Consequently, oil production in North Dakota, for instance, has jumped from a nadir of 19 million bbl in 1974 (compared to a previous zenith of 27 million in 1966) to 32 million bbl in 1979 and 40 million bbl in 1980. Geologic knowledge of carbonate reservoirs has expanded accordingly. Depositional environments throughout Sauk, Tippecanoe, and Kaskaskia deposition were largely shallow marine. Subtidal and even basinal environments were developed in the basin center, but sabkha deposits were abundant near the basin periphery. Evidence of subaerial weathering was commonly preserved in structurally high areas and on the basin periphery, especially in upper Kaskaskia rocks. Some pinnacle reefs were developed in Kaskaskia deposition, morphologically similar to the Silurian pinnacle reefs of the Michigan basin. Clastic sediments were transported into the southern part of the basin during Absaroka sequence deposition, a product of erosion of Ancestral Rocky Mountain orogenic structures. Continental and shallow marine clastic sediments were deposited during Zuni sedimentation until Cretaceous deeper marine environments were established. Laramide orogenesis to the west provided detritus that was deposited in fluvial, deltaic, and marginal marine environments, regressing to the east. Major lignite deposits are part of this postorogenic regressive rock body. Major structures in the basin, and the basin itself, may result from left-lateral shear along the Colorado-Wyoming and Fromberg zones during pre-Phanerozoic time. Deeper drilling in the basin has revealed several major structures and given indications of others. Most structures probably resulted from renewed movement or tensing of pre-Phanerozoic faults. Meteorite impact events have been suggested as the origin for one or more structures.

Exploration Along Lower Cretaceous Shelf Margin--Golden Lane, Mexico, to Louisiana: ABSTRACT

Exploration for hydrocarbons from Lower Cretaceous shelf-margin carbonates began more than 40 years ago and continues today along the Gulf Coast of Mexico, Texas, and Louisiana. The early discovery of the oil-rich Golden Lane and Poza Rica fields of Mexico provided encouragement for exploration for similar fields in Texas and Louisiana. Exploration along the shelf margin of the U.S. Gulf Coast has resulted in the discovery of smaller gas fields in south Texas. Detailed core studies along the shelf-margin indicate the existence of a nearly continuous trend of rudist banks and associated tidal bars. Seaward, argillaceous carbonate mud was deposited in deeper water; landward, shallow-water lagoon and shelf sediments were diverse and resulted in the accumulation of a wide variety of facies. In the Mexican fields, extremely high early secondary porosity in the Golden Lane is believed to have formed when the carbonates were exposed to subaerial weathering during early stages of the Laramide orogeny. In the Poza Rica field, production is from intercrystalline dolomite porosity. Along the U.S. Gulf Coast, most porosity is primary within the preserved rudist shells and permeability is very low. Originally high intergranular porosity in the grainstone facies was destroyed by several layers of calcite cement deposited during subsequent burial. The source rocks for the hydrocarbons of the entire area are probably the argillaceous, dark-colored mudstones and wackestones of the Gulf basin. The Mexican fields have been prolific and have produced a billion barrels of oil. In contrast, the south Texas fields have produced 150 bcf of dry gas and are estimated to have reserves of 1 to 1.5 tcf of gas. Major differences in postdepositional burial history account for these extreme differences in hydrocarbon production. End_of_Article - Last_Page 548------------

CALCRETE DEVELOPMENT AT THE JUNCTION BETWEEN THE MARTIN LIMESTONE AND THE RED HILL OOLITE (LOWER CARBONIFEROUS), SOUTH CUMBRIA

SUMMARY The strata exposed at the junction between the Martin Limestone and Red Hill Oolite (Lower Carboniferous) from around the Leven Estuary, south Cumbria are described in detail. There is evidence that these rocks underwent a complex depositional and early diagenetic history. The sequence begins with a slight angular unconformity, previously undescribed. There follows up to 2 m of varied limestone including features diagnostic of calcretes and indicative of subaerial weathering. Evidence is presented to suggest that there was continued accumulation of sediments in the terrestrial environment while the calcretes were forming. The overlying limestone breccia, previously interpreted as marine, also contains evidence of subaerial weathering and is re-interpreted as a terrestrial deposit, perhaps as a result of mudflow. The sequence at the Martin Limestone/Red Hill Oolite boundary around the Leven Estuary is compared with sections elsewhere in south Cumbria and the conclusion is reached that local tectonic movements were responsible for the period of emergence described.

Processes controlling the characteristics of the surficial sand sheet, U.S. Atlantic outer continental shelf

A review of recent data on the velocity of bottom currents, the frequency of bottom-sediment movement, the kinds and amounts of suspended sediments in near-bottom waters, and the acoustic and sedimentary features of subbottom strata indicates that the characteristics of the ubiquitous sand sheet on the Atlantic outer continental shelf of the United States have been controlled by a variety of past and present processes. Although these processes collectively have had a widespread effect on the characteristics of the sand sheet, the relative importance of each process changes geographically. On Georges Bank, late Pleistocene glaciations along with modern tidal currents and the regional circulation pattern have played a dominant role. On the Middle Atlantic shelf, ancestral rivers, former near-shore processes, and modern wind- and wave-generated currents are important factors. On the South Atlantic shelf, the sediments reflect subaerial weathering, erosion or nondeposition over or near hardgrounds, and the production of biogenic carbonate. Other processes such as the movement of water masses, bioturbation, and bottom fishing probably have affected the sediments in all areas. A knowledge of the various factors affecting the sand sheet is fundamental to an understanding of its general geologic history and to the paleoenvironmental interpretation of ancient sand strata.

Surface lowering and landform evolution on Aldabra

Aldabra landforms are the result of the karstification of carbonate rocks distributed on surfaces which have been exposed to erosion for varying lengths of time. Morphometric analysis (which is of interest in both geomorphological and botanical contexts) suggests that the most well developed karst features (closed depressions) occur on what appear to be the oldest surfaces. Morphology also varies with lithology. Measurements of present-day erosion rates suggests that weakly cemented rocks and the most soluble mineral components are eroding most rapidly. The evolution of a dissected morphology is related to lithological heterogeneity in coralline rocks or, in the case of the more homogeneous rocks, to the short residence time of waters on the rock surface (the more rapidly dissolving mineral grains eroding faster). Dissolution also proceeds in fresh water pools, but this may be offset by precipitation in some cases. The surface is mostly case hardened, except under deep organic soils where erosion rates are much higher than in other areas. A mean erosion rate measured at 0.26 mm /a appears to make it feasible that large erosional features, such as the lagoon, could have been formed during periods of emersion as suggested by research workers who have hypothesized that an atoll shape may be substantially derived by subaerial weathering.

On the geochemistry and origin of the D Tree, Wonarah, and Sherrin Creek phosphorite deposits of the Georgina Basin, northern Australia

The early Middle Cambrian phosphorites of the Georgina Basin were deposited in shallow nearshore marine environments varying from lagoonal, estuarine, littoral, to intertidal. Some deposits show penecontemporaneous erosion by stream channels in addition to subaerial weathering during periods of regression. There are three distinct types of phosphorite: mudstone phosphorite, replacement phosphorite, and pelletal phosphorite, the latter two were formed by diagenetic phosphatization of carbonate skeletal sands, bioclastic and micritic limestones, and dolomites. By comparison, the mudstone phosphorite, which predominates in the three deposits studied, shows little textural evidence of such an origin and has been accepted as an orthochemical sediment. However, this paper proposes a diagenetic origin identical to the other types.Some fifteen known deposits occur over a distance of about 1,000 km along the periphery and insular portions of the basin. The geochemistry of two of these deposits, D Tree and Sherrin Creek, which lie 75 km apart on the eastern edge of the basin, has been studied and compared with that of the Wonarah deposit, which lies within the basin 200 km to the west. Fifty-eight analyses reveal four element associations: apatite group (Ca, P, F), clay group (Al, K, Ti, Cr, Zr), heavy minerals group (Fe, Mn, Zn, Ni, Cu), and a leached group. Although the elements Na, S, CO 2 , Y, La, Ce, Sr, U, and Th correlate with apatite in the little-leached Sherrin Creek deposit, they exist as a separate leached group, or correlate with clays, in the more leached D Tree and Wonarah samples. Ba, V, Cu, and Pb do not behave to a consistent trend. The strongest correlations in the leached deposits are at a lower level of significance than at D Tree.Intense Cenozoic weathering extends beyond the maximum known depth of phosphorite. It, together with certain though unquantifiable Cambrian subaerial weathering effects, has been responsible for leaching, increasing in the order Sherrin Creek, D Tree, and Wonarah. Uranium, Th, La, Ce, Y, Ba, Sr, Na 2 O, SO 3 , and CO 2 are leached from the system to the extent of 45 to 80 weight percent. Comparison of the composition of leached and unleached phosphorites suggests there were discernible differences in the geochemistry of waters in the different embayments at the time the phosphorites were formed.

Petrology of the Tertiary phosphorite system of Florida

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.

Phosphorite sedimentation in Florida; a model phosphogenic system

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 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 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 machines and associated 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 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 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.

Conceptual models in exploration geochemistry

This is the fourth in a series of “Conceptual Models” volumes reviewing geochemical exploration practice in different areas of the world. This volume provides an overview of geochemical exploration for base and precious metals in Australia. The subaerial weathering history of most of the continent probably dates from the Permian including, broadly, an early period of widespread lateritization followed by a late trend to aridity. This gave rise to the formation of a deep weathering profile which may subsequently have been partially or wholly eroded. It differs markedly, therefore, from the terrains described in previous volumes in the series. Consequently, emphasis has been placed throughout on the long weathering history, and the influence it has had on element dispersion and on the surface expression of mineralization in a wide range of sample media - ironstones, gossans, weathered bedrock, soils, transported overburden, stream sediments, waters, plants and atmospheres. The nature, use and geochemical characteristics, including multi-element data where possible, are described for each of these sample media. A set of eight idealized models has been derived using data from fifty-two case histories and previously published literature. The models are based on generalized landform settings, classified hierarchically according to: 1. (1) degree of preservation of the deep weathering profile, i.e. complete, partly stripped or absent; 2. (2) relief, i.e. low, moderate or high. A further model describes the landform situation within which calcrete uranium deposits are formed. The case histories present, for the most part, previously unpublished data and include examples not only of successful exploration but also situations where surface anomalies are spurious.