ROLF S. ARVIDSON*
Department of Oceanography, Texas A&M University, College Station, TX 77843-3146

and

FRED T. MACKENZIE and MICHAEL GUIDRY
Department of Oceanography, University of Hawaii, School of Ocean and Earth Science and Technology
Honolulu, HI 96822

ABSTRACT: In this paper, we consider how long-term tectonic conditions and their effect on the surface environment of the earth interacted with the global carbonate cycle during the hothouse (greenhouse)–icehouse transition of the past 100 million years. Using the recalculated output of the Berner, Lasaga, and Garrels (BLAG) geochemical model as a template (Berner et al., 1983; Lasaga et al., 1985), we computed changes in seawater carbonate chemistry for the past 100 m.y. Experimental dolomite and calcite precipitation rate data as a function of environmental conditions were used to calculate the ratio of rates of dolomite and calcite precipitation rates during this period of time. We conclude from these model calculations that the observed decrease in the ratio of dolomite to calcite in sedimentary carbonates deposited since the Late Cretaceous transgression was a result of changes in the ocean saturation state with respect to carbonate minerals and global surface temperature. Thus, the solution to the classical “dolomite problem” may lie in relatively small but coupled changes in the composition and temperature of the atmosphere and seawater.

*email: arvidson@ocean.tamu.edu

Marine Authigenesis: From Global to Microbial, SEPM Special Publication No. 66
Copyright 2000 SEPM (Society for Sedimentary Geology), ISBN 1-56576-064-6, p. 1-5.

______________________________________________________________________

DAVID T. WRIGHT*
Department Of Earth Sciences, University Of Oxford, Parks Road, Oxford OX1 3PR

Abstract: Conventional hydrologically-driven models of dolomite formation, though popular, often lack empirical support, and encounter fundamental chemical problems related to kinetic impediments in saline solutions: these include the high hydration energy of the magnesium ion, the extremely low activity of the carbonate ion, and the presence of even very low concentrations of sulfate. Although an organogenic dolomite model exists, it has been mainly limited in application to modern, organic-rich, deep marine sediments. However, growing evidence from modern and ancient sediments points to a greatly enhanced and fundamental role for benthic microbial communities in dolomite formation, linked to anoxic organic diagenesis driven by sulfate reduction during which all kinetic inhibitors to dolomite formation are overcome. Magnesium concentrated in cyanobacterial sheaths may be liberated during degradation into sulfate-free solutions of high ionic strength and carbonate ion activity, to become available for dolomite formation. Another possible source of magnesium in modified saline solutions is from dissociation of complexed ions. These processes, which provide the basis for a new organogenic model for dolomite formation, have operated throughout the geologic record on a range of scales, from relatively minor modern occurrences to the thick, extensive platformal dolostones of the Proterozoic which developed in association with microbially-dominated shallow marine environments.
Analyses of lake waters and sediments from ephemeral Coorong lakes of South Australia reveal seasonally high carbonate alkalinities and magnesium concentrations in association with intense bacterial sulfate reduction and cyanobacterial degradation, indicating a genetic link between microbial mediation of ambient waters and dolomite formation. The ecosystem of the Coorong distal ephemeral lakes during late stages of evaporation provides a small scale analogue for biogeochemical processes operating in microbially-dominated shallow marine environments typical of the Proterozoic, where sulfate reduction would have been a major, shelf-wide phenomenon operating just beneath the sediment surface, continually driving biochemical modification of interstitial waters and sustaining high carbonate alkalinity in an underlying sulfate-free medium.
Thin sections of late Archaean platformal Conophyton stromatolites from South Africa reveal progressive disintegration of constituent cyanobacterial sheaths accompanied by the appearance and increasing crystal size of replacive, authigenic dolomite in a calcite matrix, indicating that magnesium was derived from in situ degradation of cyanobacterial sheaths. Total loss of sheath material resulted in a coarse dolospar fabric preserving no evidence of its microbial origin. Petrographic analysis of silicified microbial shelf sediments, including cyanobacterial mat, ooids, peloids and intraclasts, from the Cambrian Eilean Dubh Formation of northwestern Scotland reveals a common diagenetic trend in which sequential anoxic biofabric degradation resulted in the appearance and progressive increase in abundance of dolomite, culminating in a dolospar fabric. Structureless dolostones may thus evolve through a sequence of ephemeral fabrics by authigenic precipitation or replacement linked to microbial degradation, but evidence for their origin depends on the preservation of successive stages in their development from organic-rich sediments to massive dolomites. The predominance of stromatolites and other microbialitic sediments in thick dolostone platformal successions throughout much of the Precambrian
and early Paleozoic, indicates that organic diagenesis has played a major role in dolomite formation from the microbial to the global scale.
in surface seawater are:

*email: davew@earth.ox.ac.uk
Marine Authigenesis: From Global to Microbial, SEPM Special Publication No. 66
Copyright 2000 SEPM (Society for Sedimentary Geology), ISBN 1-56576-064-6, p. 7-20.

_______________________________________________________________________

J. COMPTON*
Dept. of Geological Sciences, University of Cape Town, Rondebosch 7700, South Africa

D. MALLINSON
Dept. of Marine Science, University of South Florida, St. Petersburg, Florida 33701 USA

C. R. GLENN
Dept. of Geology and Geophysics, SOEST, University of Hawaii, Honolulu, Hawaii 96822 USA

G. FILIPPELLI
Dept. of Geology, Indiana University–Purdue University at Indianapolis, Indianapolis, Indiana 46202-5132 USA

K. FÖLLMI
Geological Institute, University of Neuchatel, Ch-2007 Neuchatel, Switzerland

G. SHIELDS
Centre de Geochimie de la Surface, CNRS, 1 rue Blessig, F-67084, Strasbourg Cedex, France

and

Y. ZANIN
United Institute of Geology, Geophysics and Mineralogy, Siberian Branch of the Russian Academy of Sciences, 630090 Novosibirsk, Russia

ABSTRACT: Phosphorus is a critical element in the biosphere, limiting biological productivity and thus modulating the global carbon cycle and climate. Fluxes of the global phosphorus cycle remain poorly constrained. The prehuman reactive phosphorus flux to the ocean is estimated to range from 0.7—4.8 x 10¹²g/yr. Uncertainty in the reactive phosphorus flux hinges primarily on the uncertain fate of phosphate adsorbed to iron oxyhydroxide particles which are estimated to constitute 50% or more of the chemically weathered-phosphorus river flux. Most reactive phosphorus is initially removed from seawater by burial of organic matter and by scavenging onto iron–manganese oxide particles derived from mid-ocean ridge (MOR) hydrothermal activity. Calculation of the oceanic phosphorus burial flux is complicated by early diagenetic redistribution of both oceanic and terrestrial phosphorus. Increased phosphorus input during periods of warm, humid climate is offset to some degree by increased burial rate as productivity shifts to expanded shallow-water estuary and shelf areas where phosphorus is rapidly decoupled from organic matter to form phosphorite. Phosphorus scavenging is greater if high sea levels are associated with increased MOR hydrothermal activity such as during the Late Cretaceous. Less phosphorus is derived from weathering during cool, dry climatic periods but a more direct transportation of phosphorus to the deep ocean, and a shift of productive upwelling regions to deeper water areas allows more phosphorus to be recycled in the water column. Lowered sea level results in less effective trapping of phosphorus in constricted estuary and shelf areas and in an increase in the phosphorus flux to the deep ocean from sediment resuspension. A decrease in MOR spreading rates and the resulting decrease in phosphorus scavenging by iron–manganese oxide particles would result in more phosphorus for the biosphere. Orogeny and glaciation may accelerate chemical weathering of phosphorus from the continents when the increased particle flux is exposed to warm and humid climate. Large, reworked phosphorite deposits may proxy for short-term organic carbon burial and correspond to periods of increased reactive phosphorus input that cannot be accommodated by longterm organic matter and iron-oxide particulate burial.

*email address: compton@geology.uct.ac.za

Marine Authigenesis: From Global to Microbial, SEPM Special Publication No. 66
Copyright 2000 SEPM (Society for Sedimentary Geology), ISBN 1-56576-064-6, p. 21-33.

_______________________________________________________________________

MICHAEL W. GUIDRY*, FRED T. MACKENZIE
Department of Oceanography, University of Hawaii, School of Ocean and Earth Science and Technology, Honolulu, HI 96822

and

ROLF S. ARVIDSON
Department of Oceanography, Texas A&M University, College Station, TX 77843-3146

ABSTRACT: Until very recently, the effect of tectonism on the exogenic phosphorus cycle has been neglected. Currently, the subduction of phosphorus associated with organic matter buried in ocean sediment to the mantle represents a significant flux relative to other fluxes within the exogenic phosphorus cycle. Phosphorus is generally considered to have been the limiting nutrient for oceanic primary production over much of geologic time. Therefore, changes in the total exogenic phosphorus mass control, to a certain extent, the amount of phosphorus available for oceanic production. Over geologic time, the exogenic phosphorus mass is likely governed by plate tectonics; specifically, by the balance between input of newly formed crystalline rock and the output via subduction of oceanic crust and the overlying sediment to the mantle.

*email: mguidry@soest.hawaii.edu

Marine Authigenesis: From Global to Microbial, SEPM Special Publication No. 66
Copyright 2000 SEPM (Society for Sedimentary Geology), ISBN 1-56576-064-6, p. 35-51.

_______________________________________________________________________

ALBERT S. COLMAN*
Department of Geology and Geophysics, Yale University, P.O. Box 208109, New Haven, CT 06520-8109

and

HEINRICH D. HOLLAND
Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138

ABSTRACT: The marine geochemistry of phosphorus links the burial rate of organic carbon in marine sediments to the oxygen content of the atmosphere and may serve as a major component of the system that controls atmospheric oxygen. The return flux of phosphate from marine sediments to seawater is an important part of the marine phosphorus cycle. This paper examines the relationship between the return flux of phosphate and the oxidation state of marine sediments, a necessary preliminary step in defining the efficacy of the oxygen control mechanism.
The diffusive return flux of phosphate from marine sediments to overlying bottom waters was calculated for 193 published pore water phosphate profiles that met a number of stringent criteria for sediment core and pore water recovery and processing. The phosphate return fluxes, scaled to carbon regeneration fluxes, are significantly greater from highly reduced sediments than from highly oxidized sediments. In highly reduced sediments the return phosphate flux from carbon regeneration is frequently augmented by a large phosphate flux released during the reductive dissolution of ferric (oxy)hydroxides. In highly oxidized sediments the return phosphate flux can be somewhat less than the flux to be expected from carbon regeneration. The missing phosphate is probably adsorbed on ferric (oxy)hydroxides in these sediments. The strong coupling between the oxidation state of marine sediments and the return phosphate flux to seawater suggests that the marine phosphate cycle is indeed an important part of the system that stabilizes atmospheric O₂.
The total preagricultural return flux of P from marine sediments was ca. 12 x 10¹¹ mol P/yr. This rate is more than an order of magnitude larger than the riverine flux of total dissolved phosphorus to the oceans, ca. 0.3 x 10¹¹ mol P/yr. We estimate that the total continental preagricultural flux of reactive P that ultimately appears dissolved in ocean water was ca. 3.5 x 10¹¹ mol P/yr. The large flux of continental P to seawater, via direct input of riverine dissolved inorganic and organic P and via the diagenetic return flux from reactive continental particulate P deposited in marine sediments, indicates that the marine residence time of phosphate with respect to terrigenous inputs is ca. 10,000 years. This figure depends quite heavily on the fraction of terrigenous, particulate-phase phosphate that is released to seawater during diagenesis. Variations in this fraction can significantly affect the marine residence time of phosphate and the relative proportion of detrital versus authigenic phosphate phases in marine sediments.

*email: albert.colman@yale.edu

Marine Authigenesis: From Global to Microbial, SEPM Special Publication No. 66
Copyright 2000 SEPM (Society for Sedimentary Geology), ISBN 1-56576-064-6, p. 53-75.

_______________________________________________________________________

V. SUBRAMANIAN*
School of Environmental Sciences, Jawaharlal Nehru University, New Delhi, 110 067, India

ABSTRACT: Rivers of South Asia carry dissolved inorganic phosphorus (DIP) at levels far greater than many of the world’s rivers such as the Amazon River and European rivers on whose phosphorus data average rates of DIP exported to the oceans are largely based. Similarly, all the major river systems in South Asia have sedimentary phosphorus values that are considerably higher than the world average. Unlike the Amazon or other large river system for which extensive data are available, organic phosphorus contributes very little to the total sedimentary phosphorus pool in South Asian rivers. Trapping of enormous amounts of sediment in the vast alluvial belts in the northern part of South Asia indicate that very little of the sedimentary phosphorus transported by these rivers is likely to be exported to the adjacent marine environment.

*email: subrama42@hotmail.com

Marine Authigenesis: From Global to Microbial, SEPM Special Publication No. 66
Copyright 2000 SEPM (Society for Sedimentary Geology), ISBN 1-56576-064-6, p. 77-88.

_______________________________________________________________________

BIRGER RASMUSSEN*
Centre for Strategic Mineral Deposits, Department of Geology and Geophysics, the University of Western Australia,
Nedlands, 6907, Perth, Australia

ABSTRACT: Early-diagenetic aluminophosphate minerals (mainly florencite) are ubiquitous in ancient (Archean to Cretaceous) marine-deposited sandstones. The crystals are <10 µm in diameter and are mostly associated with thin coatings or pockets of detrital clay particles lining quartz grain surfaces. Aluminophosphate crystals are also found in altered detrital aluminosilicate grains (e.g. feldspar).
The aluminophosphate minerals precipitated in sands deposited in shallow marine environments. Diagenetic textures and the presence of structural
sulfate in the aluminophosphate minerals suggest that the authigenic crystals formed shortly after deposition, probably in the zones of sulfate reduction and methanogenesis. The aluminophosphate minerals formed as the concentrations of phosphate in the pore water increased and phosphate combined with various cations (e.g. calcium, barium, rare earth elements) to precipitate at sites of aluminum availability (e.g. detrital clay particles and aluminosilicate grains).
Although the aluminophosphate minerals are volumetrically minor constituents (<0.05 wt%), the authigenic crystals, which are present in >90% of samples studied, are spatially and temporally widespread in ancient Australian sandstones. Burial flux estimates show that phosphorus is removed at a rate of ~5.6 x 10–7 g/cm²-yr. This value suggests that aluminophosphate precipitation was probably an important sink for oceanic phosphorus in the past, and possibly significant in present-day coastal environments. The discovery of the magnitude of the authigenic aluminophosphate sink has implications for models of the marine phosphorus cycle.

*email: brasmuss@geol.uwa.edu.au

Marine Authigenesis: From Global to Microbial, SEPM Special Publication No. 66
Copyright 2000 SEPM (Society for Sedimentary Geology), ISBN 1-56576-064-6, p. 89-101.

_______________________________________________________________________

GRAHAM SHIELDS*, PETER STILLE
Centre de Géochimie de la Surface, UMR 7517, 1 rue Blessig, 67084, Strasbourg, France

and

MARTIN D. BRASIER
Department of Earth Sciences, University of Oxford, Oxford, OX1 3PR, UK

ABSTRACT: Sedimentary phosphorite formation has occurred episodically over geologic time. Substantial phosphorite deposits were formed during P-giant episodes of which the most widespread and long-lasting were the Precambrian-Cambrian and the Cretaceous-recent. The aim of this review is to reevaluate these two episodes in the light of sedimentary Sr, Nd, S, and C isotopic records. Some, although not all of these phases of phosphorite formation were accompanied by increases in seawater ⁸⁷Sr/⁸⁶Sr unrelated to changes in seafloor spreading, which supports a possible link between orogeny and global weathering rates, P-input, and P-giant formation. Assessing the relationship between d¹³C and phases of widespread phosphogenesis is more complicated due to the counterbalancing effects on seawater d¹³C of productivity/organic matter deposition and subsequent phosphogenesis/early diagenetic carbon oxidation. It is recommended that proposed links between phosphogenesis and global changes of d¹³C or P input continue to be reexamined on a case-to-case basis. The application of secular trends in Nd isotopic ratios to unravelling the often vital role of paleocurrents in phosphorite formation is also discussed. In addition, recent isotopic research is outlined that has led to significant improvements in stratigraphic resolution around the Precambrian-Cambrian boundary. A temporal and causal connection is put forward between metazoan evolution, that is the introduction of bioturbation, fecal pellets, biomineralization, and filter feeders, which would have helped to concentrate mineral phosphate in sediments, and widespread phosphogenesis at this time.

*email: shields@illite.u-strasbg.fr

Marine Authigenesis: From Global to Microbial, SEPM Special Publication No. 66
Copyright 2000 SEPM (Society for Sedimentary Geology), ISBN 1-56576-064-6, p. 103-115.

_______________________________________________________________________

JACQUES LUCAS and LILIANE PRÉVÔT-LUCAS*
Centre de Géochimie de la Surface (CNRS), EOST-Université Louis Pasteur,
1 rue Blessig, F-67084 Strasbourg Cedex, France

ABSTRACT: Calcium carbonate is known to be one of the sedimentary materials occuring with phosphorite in many of the phosphorite deposits. This relationship has been described for various geological periods and at various scales: mineable phosphorite alternating with barren carbonate; calcitic or more often dolomitic phosphorite alternating with phosphatic dolomite or limestone. In the present study, we attempt to explain why there is a link between phosphorite and carbonate. This relationship does not presuppose a genetic link between minerals, although this link does exist
where apatite can be recognized as the result of apatitization of pre-existing calcite. Calcite is a true biomineral that is directly produced by living marine organisms as their skeletons. It is deposited as a solid material before various diagenetic recrystallizations add their imprints. Apatite is an authigenic mineral, precipitated during early diagenesis from an organic matter (OM)-rich ooze. The OM must be furnished predominantly by naked and or siliceous organisms and its degradation processes are controlled by dysoxic conditions. The formation of limestones depends on a predominantly carbonate-producing productivity that is common because of the abundance of carbon. Phosphorite formation requires high quantities of OM related to specific noncarbonate-producing productivity that is less common because the availability of phosphorus is minor compared to that of carbon. The “phosphorite factory,” which produced the phosphorite deposits, and the “carbonate factory” are independent but complementary rather than competing processes;
they do not use the same primary material, nor do they occur at the same phase of the sedimentary cycle. The occurrence of alternations of beds of phosphorite and beds of calcium carbonate is thought to be the record in the sediment of changes in the dominant type of bioproductivity in the water column. Shallow-water shelves without significant detrital influx are the environments where such frequent changes in the type of bioproductivity are known to occur easily in response to slight physical or chemical changes.

Marine Authigenesis: From Global to Microbial, SEPM Special Publication No. 66
Copyright 2000 SEPM (Society for Sedimentary Geology), ISBN 1-56576-064-6, p. 117-125

_______________________________________________________________________

FRANKLYN B. VAN HOUTEN
Department of Geosciences, Princeton University, Princeton, NJ 08544

ABSTRACT: Chemical components of most ooidal ironstones and phosphorites were initially derived from deeply weathered uplands and deposited in shallow waters of epeiric seas and continental margins that received very little clastic sediments. They were then commonly transformed during diagenesis largely by upwelling seawater and redeposited in winnowed layers.
Ironstones and phosphorites differ, however. Ironstones were first laid down as ferric oxide and kaolinite ooids and peloids, whereas phosphorites were formed first as phosphatic carbonate granules, crusts, and hardground. Ironstones were then transformed to berthierine (or rarely nontronite) during diagenesis, whereas phosphorites were transformed to carbonate fluorapatite (CFA).
In terms of their distribution in the geologic record ironstones and phosphorites are also dissimilar. A few ooidal ironstones were deposited in Early Proterozoic time. Most accumulated during greenhouse phases of the Phanerozoic (Ordovician-Devonian and Jurassic-early Cenozoic). With the exception of the late Cretaceous–early Cenozoic deposits, major phosphorites are notable for their absence in rocks of those ages. In fact, phosphorites were most abundant in the Late Proterozoic and early Cambrian times.

Marine Authigenesis: From Global to Microbial, SEPM Special Publication No. 66
Copyright 2000 SEPM (Society for Sedimentary Geology), ISBN 1-56576-064-6, p. 127-132.

_______________________________________________________________________

KURT O. KONHAUSER*
School of Earth Sciences, University of Leeds, Leeds, UK LS2 9JT

ABSTRACT: Precambrian banded iron-formations (BIFs) are chemical sediments of hydrothermal origin and consist of Fe-rich minerals with alternating layers of chert. Because microorganisms potentially played a role in their precipitation, the study of bacterial-mineral interactions at modern hydrothermal environments may provide small-scale analogues to those conditions under which they accumulated. Interestingly, microbial populations currently growing at hot springs and deep-sea vents are commonly encrusted in iron and silicate minerals. Iron biomineralization occurs either passively through interaction between the reactive sites of the cell and dissolved cationic iron from the hydrothermal
fluid, or actively through chemolithotrophic iron-oxidation by bacteria such as Gallionella genera. Amorphous silica precipitates on individual bacteria through hydrogen bonding between hydroxy groups in the extracellular polymers and hydroxyl groups in dissolved silica, with some colonies becoming completely cemented together within a siliceous matrix up to several micrometers thick. Iron-silicates form due to reactions between dissolved silica and cell-bound iron. In these predominantly nonspecific processes, bacterial cells simply catalyze reactions that are rendered possible by the supersaturated conditions created by the sudden physical and chemical changes induced through venting. Diagenetic reactions, some of which are also catalyzed by microorganisms growing in the sediment, can further alter the mineralogy of these primary precipitates, leading to the formation of secondary magnetite and siderite. In this way, all of the main mineralogical components of BIFs can be associated with microbial activity.

*email: kurt@earth.leeds.ac.uk

Marine Authigenesis: From Global to Microbial, SEPM Special Publication No. 66
Copyright 2000 SEPM (Society for Sedimentary Geology), ISBN 1-56576-064-6, p. 133-145.

_______________________________________________________________________

PAUL W. JEWELL*
Department of Geology and Geophysics, University of Utah, Salt Lake City, UT 84112

Abstract: Barite occurs throughout the geologic record as massive beds, laminations, rosettes, and nodules. The most important scientific and economic occurrences of barite are stratabound and stratiform massive beds from the Early Archean and the Early to Middle Paleozoic. Paleozoic bedded barites are by far the most volumetrically significant deposits in the geologic record. Additional occurrences have been documented in some Middle Proterozoic, Late Proterozoic, and Mesozoic rocks and in several localities on the modern ocean floor. Bedded barite is believed to have formed as emanations from seafloor sediments, as diagenetic replacements of preexisting minerals, or as direct precipitants due to biological fixation of barium in the water column. Direct field evidence to differentiate between these theories is often lacking or contradictory. Geochemical studies, particularly those that have employed d³⁴S and ⁸⁷Sr/⁸⁶Sr analyses of the barite, have proven very useful in understanding bedded barite genesis.
The low solubility of barite relative to other natural salts has helped barite survive as a pseuodomorph of stratiform evaporite minerals in some Archean sedimentary sequences. Other examples of Archean barite appear to have a shallow water detrital or authigenic origin. Very low d³⁴S values of Archean barite are interpreted as indicating a low-sulfate ocean. Large deposits of Paleozoic bedded barite are typically found in fine-grained, organic-rich siliciclastic sequences and are associated with massive and disseminated sulfides, cherts, phosphorites, and less frequently limestones and volcanic rocks. d³⁴S analyses indicate that almost all bedded barite had a seawater sulfate source. The genetic link between Paleozoic bedded barites and sedimentary submarine exhalative Pb-Zn sulfide deposits has been established by field and geochemical study of deposits in western Canada and western Europe. ⁸⁷Sr/⁸⁶Sr analyses suggest that these bedded barites have a continental barium source. Economically important bedded barites in China, Arkansas, and Nevada have no significant sulfides or other hydrothermal manifestations. The clear association of dissolved barium and barite with biological cycles in the modern ocean, associations with phosphorites and cherts, and ⁸⁷Sr/⁸⁶Sr analyses that are comparable to contemporaneous seawater suggest that the Chinese, Nevada, and Arkansas barite deposits formed as biological precipitates on the seafloor. Bedded barite formed by this mechanism holds promise as an indicator of high paleoproductivity and open ocean sulfate reduction during selected periods of the Paleozoic. The lack of world class examples of bedded barite in Mesozoic and Cenozoic black shale sequences indicates a lack of open ocean sulfate reduction during these periods of geologic time.

*email: pwjewell@mines.utah.edu

Marine Authigenesis: From Global to Microbial, SEPM Special Publication No. 66
Copyright 2000 SEPM (Society for Sedimentary Geology), ISBN 1-56576-064-6, p. 147-161.


SECTION II:
PHOSPHORITES, GLAUCONITES AND ASSOCIATED FACIES ON THE MODERN SEAFLOOR

_______________________________________________________________________

W. C. BURNETT*¹, C. R. GLENN², C. C. YEH¹, M. SCHULTZ¹, J. CHANTON¹, and M. KASHGARIAN³
1Department of Oceanography, Florida State University, Tallahassee, FL 32306-3048
2Department of Geology & Geophysics, University of Hawaii, Honolulu, Hawaii 96822-2285
3Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratories, 7000 East Avenue, Livermore, CA 94551-0808

ABSTRACT: Phosphorites on the Peru shelf occur mainly as dark and dense conglomerate (“D”-phosphates) and occasionally as soft, friable (“F”-phosphates) protocrusts, which appear as laterally extensive, centimeter-thick plates of bored, phosphatized sediment. Several undisturbed and well-documented samples of these protocrusts were collected by box coring and submersible operations during an expedition of the R/V Seaward Johnson in 1992. Three of these crusts were analyzed in layer-by-layer fashion for U-series isotopes, while two were analyzed for AMS-¹⁴C, and stable carbon isotopes. Unlike the D-phosphates, which may show a progressive decrease in their d¹³C values (from 0 to –5‰) with a co-linear increase in structural CO₂ contents (from 2–6 wt. %), the protocrusts show a more restricted range with most values from 0 to –2‰ d¹³C and 4.2–4.7 wt.% CO₂, indicative of their precipitation directly on or slightly below the seafloor.
U-series disequilibrium results (²³⁰Th/²³⁴U, ²³¹Pa/²³⁵U) indicate that the protocrusts are very young. They are so young, in fact, that corrections for “common” ²³⁰Th and ²³¹Pa (that is, those amounts not associated with decay of the incipient uranium) are very important, and different interpretations (of growth direction, for example) are possible depending upon which assumptions are applied. AMS ¹⁴C results are very consistent with the U-series ages in the topmost, highest-phosphate portion of these crusts. A few millimeters into the less phosphatic portions of the crusts, uncorrected ²³⁰Th and ²³¹Pa ages diverge from the ¹⁴C ages, probably because of contamination of these daughter nuclides from associated sediment grains. An isochron approach confirms that there is a minimal age difference between the upper and lower layers of the protocrusts. All protocrusts display a distinct trend of decreasing phosphate content with depth into the crust. Based on these observations, we propose a growth model that suggests that phosphatic protocrusts grow upward at rates of 2–9 mm/ky in response to downward diffusion of phosphate from an interstitial phosphate pore water spike just below the sediment-seawater interface. Later exhumation of protocrusts resulted in erosion into the more common and more complicated phosphatic hardgrounds, conglomerates, and nodules found scattered throughout the sediments of the Peru shelf upwelling zone.

*email: wburnett@mailer.fsu.edu

Marine Authigenesis: From Global to Microbial, SEPM Special Publication No. 66
Copyright 2000 SEPM (Society for Sedimentary Geology), ISBN 1-56576-064-6, p. 163-183.

_______________________________________________________________________

G. N. BATURIN*
Institute of Oceanology, Russian Academy of Sciences, 36 Prospect Nakhimova, Moscow, 117 851, Russia

ABSTRACT: Phosphatic grains and nodules related to recent diatom oozes on the inner Namibian shelf as well as Pleistocene grains and nodules from the phosphorite deposit on the adjacent outer shelf have been studied by means of scanning electron microscopy combined with several analytical methods.
Recent grains and nodules are being formed simultaneously in the same diatom oozes. Both consist of poorly, moderately, and well-lithified varieties that reflect the process of progressive phosphatization, compaction, and crystallization of phosphatic matter along with dissolution and expulsion of nonphosphatic components. Similar processes are going on during the phosphatization of macro- and microcoprolites. The common varieties of apatite mineral morphologies are colloform masses without distinct texture, globules and globular aggregates with more or less pronounced radial crystallization, and micrometer-sized elongated particles and their aggregates of various shape, which are interpreted as diagenetically formed crystallites.
All of these textures are related to each other, with the globular one becoming predominant in most lithified accretions. Reworked Pleistocene grains reveal textural features quite similar to the recent grains. They contain similar organic remains, which proves their common origin in the same
environment. The chemical evolution of recent accretions in the course of their lithification consists of gradual increases of phosphatic components and decreases of nonapatite components. The most striking feature of their geochemistry is the extremely low concentration of rare Earth elements in recent grains in contrast to their high concentration in Pleistocene grains. This might mean that phosphate absorbs these elements during prolonged contact with the ambient seawater.

*email: batur@geo.sio.rssi.ru

Marine Authigenesis: From Global to Microbial, SEPM Special Publication No. 66
Copyright 2000 SEPM (Society for Sedimentary Geology), ISBN 1-56576-064-6, p. 185-199.

_______________________________________________________________________

KENDALL B. FOUNTAIN* and GUERRY H. MCCLELLAN
Department of Geology, University of Florida, Gainesville, Florida 32611, USA

ABSTRACT: Phosphorite nodules recovered from the upper west Florida slope provide an analog for phosphogenesis under conditions of marginal upwelling that are significantly different from the regional, continental shelf-upwelling models applied to the majority of the southeastern United States. We believe that ferruginous (AI and AII) and nonferruginous (BI and BII) phosphorite nodules record the episodic precipitation of francolite since the mid-Miocene (12–15 Ma) in response to positioning of the Gulf of Mexico Loop Current. It is likely that during sea level highstands, deflection of the Loop Current landward increases the Corg flux, establishing conditions suitable to the concentration of dissolved inorganic phosphorous (DIP) within redox stratified sediments. Francolite containing approximately 6.2% CO₂ (a-values: 9.316-9.324 Å) apparently precipitates in response to the early diagenesis of organic matter and/or Fe-P shuttling under suboxic to anoxic conditions d¹³C:–6.55–0.47‰ PDB, d¹⁸O: –4.92–2.07‰ PDB), forming nonferruginous nodules (< 6% Fe₂O₃) and hardgrounds. Anhedral to subhedral francolite (ovoids, globules, and botryoids) encountered in BI and BII nodules is mineralogically consistent with primary nucleation, and appears to record a microbial component. Subsequent recrystallization of francolite, in response to submarine diagenesis, obliterates such fabrics, producing euhedral, hexagonal crystallites (AI and AII) possessing a decrease in carbonate substitution to 4.9–5.5% CO₂ (a-values: 9.322-9.328 Å).
During sea level lowstands, oxic conditions prevail, favoring Fe-enrichment of nodules (up to 22% Fe₂O₃) at the expense of carbonate minerals and glaucony. This is consistent with the evolution of primary, BII nodules into ferruginous lithotypes (AI and AII). Aragonite dissolution and the dissolution and/or reprecipitation of both calcite and francolite (d¹³C: –1.09–0.67‰ PDB, d¹⁸O: –0.61–1.38‰ PDB) within ferruginous nodules occurs during such intervals in response to changes in pore water redox and pH. Both petrographic observations and major element trends illustrate Fe-enrichment processes associated with loss of fine micritic cements and biogenic carbonate as a result of dissolution and/or replacement by FeOOH precipitates, as well as the oxidation of glaucony.

*email: kendall@ufl.edu

Marine Authigenesis: From Global to Microbial, SEPM Special Publication No. 66
Copyright 2000 SEPM (Society for Sedimentary Geology), ISBN 1-56576-064-6, p. 201-220.

_______________________________________________________________________

B. NAGENDER NATH,* B. RAMALINGESWARA RAO
Geological Oceanography Division, National Institute of Oceanography, Dona Paula, Goa-403 004, India

K. MOHANA RAO
Regional Centre, National Institute of Oceanography, 176 Lawsons Bay Colony, Visakhapatnam-530 017, India

and

CH. M. RAO
Geological Oceanography Division, National Institute of Oceanography, Dona Paula, Goa-403 004, India

ABSTRACT: A sediment core collected from a bathymetric high off Goa on the western continental margin of India has yielded phosphatic nodules at various subsurface depths (at 110, 150, 305, 355, 435, 500, 505, and 525 cm). The nodules are hosted by sediments of Pleistocene age. They are <1–5 cm in size, have carbonate flourapatite (CFA) as a single authigenic mineral phase, are free of detrital inclusions, and have very high P₂O₅ contents (>30%). In addition, soft and hard phosphatic nodules have also been recovered from the eastern continental margin of India. Eight nodules from various subsurface depths of the sediment core from the western continental margin and two nodules from the eastern continental margin were analyzed for uranium and rare-earth elements (REEs) by inductively coupled plasma-mass spectrometry (ICP-MS).
Total REE contents are very low (8–21 ppm) in western continental margin nodules and only slightly higher in eastern continental margin nodules (maximum is 42 ppm). REE abundances relative to shale (REE in sample/REE in shale) are less than 0.22 and are comparable to phosphatic nodules from the Namibian continental margin, and are slightly lower than the nodules from the Peruvian continental margin. Light REEs are depleted, with Lan/Ybn [(Lasample/Lashale)/(Ybsample / Ybshale)] values lower than unity, indicating a minimal contribution from terrestrial sources. The cerium anomaly (Ce/Ce*), which is a measure of Ce fractionation relative to the neighboring REEs, ranges between 0.64 and 1.38 with most close to 1, indicating very little Ce fractionation. Uranium concentrations are very high in nodules from both margins.
Low REE contents and LREE depletion collectively indicate either a seawater or porewater source for these elements. Since Ce is mostly stable in its trivalent state and U precipitates under reducing conditions, the absence of Ce fractionation in association with U enrichment indicates that the nodules may have formed by authigenic precipitation in reducing porewaters. High productivity in association with upwelling might have driven the accumulation of organic matter, which in turn would help enrich phosphate in porewaters, eventually leading to the formation of high-grade phosphorites.
Ce anomaly values in nodules do not correlate with paleoredox conditions in the water column, thus the REEs for these nodules are interpreted to mainly reflect porewater conditions.

*email: nagender@csnio.ren.nic.in

Marine Authigenesis: From Global to Microbial, SEPM Special Publication No. 66
Copyright 2000 SEPM (Society for Sedimentary Geology), ISBN 1-56576-064-6, p. 221-232.

_______________________________________________________________________

M. THAMBAN and V. PURNACHANDRA RAO*
National Institute of Oceanography, Dona Paula - 403 004, Goa, India

ABSTRACT: Investigations on green grains from sediments of the western continental margin of India, between Ratnagiri and Cape Comorin, (water depth 37-330 m) indicate the presence of verdine and glaucony facies. Verdine facies occurs over an area of about 100,000 km2, representing the largest sedimentary basin in the world associated with low fluvial input. Green grains occur as irregular grains, fecal pellets, and infillings/internal molds of microfossils. They are abundant on the shelf off river mouths and their distribution varies with sediment type. Grains vary from dark green to pale green or brownish green. Green grains studied here are a mixture of predominant authigenic green clay and detrital clay minerals and are altered. Both phyllite C and suspected phyllite V- (verdine minerals) associated green grains occur on the continental shelf (between 37 m and 100 m), the former being associated with the transition zone between inner and outer shelf and the latter with relict sands and reefs on the outer shelf. On the continental slope, suspected phyllite V occurs at depths between 100 m and 235 m, followed by phyllite C at 280 m depth and glauconitic smectite of the glaucony facies at 330 m. As these grains are composed of a mixture of predominant authigenic clay, detrital clay, and some altered products, their major element composition differs from those of the green grains (reported elsewhere) that contain pure authigenic clay. The low REE contents and flat shale-normalized REE patterns suggest that the REEs were inherited from the substrate.
We suggest that the size of the verdine deposit is related to the influx of iron rather than the amount of fluvial discharge. The color and morphology of the grains do not reflect the authigenic mineral or its evolution. Green grains on this margin formed at different times when the sea level was at different depths during the Late Quaternary. The distribution of verdine and glaucony facies on the southwestern margin of India is different from those of the distribution along the east coast of India, Senegal, and French Guiana margins, suggesting different paleogeography and subsidence history of the western Indian margin during the Late Quaternary.

*email: vprao@csnio.ren.nic.in

Marine Authigenesis: From Global to Microbial, SEPM Special Publication No. 66
Copyright 2000 SEPM (Society for Sedimentary Geology), ISBN 1-56576-064-6, p. 233-244.


SECTION III:
SEAMOUNT PHOSPHORITES AND FE-MN DEPOSITS

_______________________________________________________________________

LAURA M. BENNINGER*
Geology Department, University of California, Davis, California, 95616, USA

and

JAMES R. HEIN**
U.S. Geological Survey, MS 999, 345 Middlefield Rd., Menlo Park, California, 94025, USA

ABSTRACT: Phosphorite, limestone, basalt, and breccia/conglomerate occur on the flanks of central Pacific Cretaceous seamounts. Phosphatization occurred during the Tertiary, at or immediately below the sediment-water interface, within thin bodies of porous carbonate sediment and within all rock types that occur on the seamounts. The phosphorites are composed of carbonate fluorapatite (CFA), which lines pores and replaces carbonate and silicate protoliths; protolith fabric is locally preserved in fine detail. Phosphatization is typically most pervasive adjacent to pores, fractures, and the outer surfaces of samples.
In order of decreasing abundance, diagenetic minerals associated with seamount phosphorites (SMP) include: CFA, Fe-Mn oxyhydroxides, palagonite, smectite, phillipsite, and barite. This mineral assemblage indicates that CFA formed under conditions consistent with low-temperature, opencirculation seafloor alteration of basalt. The common association between CFA and precipitation of Fe-Mn oxyhydroxides indicates that SMP formed under oxic to suboxic conditions.
Phosphatization is characterized by mineralization fronts; the angular superposition of those fronts (successive mineralizing fluids coming from different directions) locally leads to irregularly shaped domains that differ in pervasiveness of phosphatization, degree of fabric preservation, and abundance of secondary minerals. Phosphatization is, therefore, controlled by a variety of factors including protolith mineralogy and fabric, degree of pore-fluid saturation with respect to CFA, rate of pore-fluid/seawater circulation (rock-water ratio), and number of previous episodes of phosphatization.

*email: benninger@geology.ucdavis.edu
**email: jhein@octopus.wr.usgs.gov

Marine Authigenesis: From Global to Microbial, SEPM Special Publication No. 66
Copyright 2000 SEPM (Society for Sedimentary Geology), ISBN 1-56576-064-6, p. 245-256.

_______________________________________________________________________

MIRIAM A. BERTRAM* and JAMES P. COWEN
Department of Oceanography, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii, 96822-2285, USA

ABSTRACT: An experimental approach was used to investigate in-situ metal-oxide accumulation on artificial substrates. Mn-containing (ferromanganese-oxide, rhodochrosite) and Mn-free (CaCO₃, basalt) substrates were deployed on Cross Seamount (18°40’N, 158°17’W) at a number of depths (800–2000 m) and time intervals (1–42 months). Chemical and biological alterations to the experimental surfaces were documented by scanning electron microscopy and energy dispersive X-ray fluorescence spectroscopy.
Geochemical alterations of the experimental substrates included the formation of Mn-oxide coatings on some Mn-containing substrates and an apparent loss of Mn from ferromanganese-oxide substrates. Temporal and spatial patterns in Mn-oxide deposition were related to environmental changes and the presence of a particular benthic foraminifera. These patterns, as well as morphological features of the Mn-oxide deposits, suggest that diagenetic alteration of ferromanganese crusts can occur and may be associated with surface water productivity and benthic organisms. These data suggest that suboxic diagenesis, within a microenvironment created by the deposition of organic-rich particulate matter, may contribute to the formation of the narrow bands of Mn and Fe oxides observed within naturally occurring ferromanganese-oxide crusts and nodules.

*Current address: Pacific Marine Environmental Laboratory, NOAA, 7600
Sand Point Way, N.E., Seattle, WA 98115-0070, USA.
email: bertram@pmel.noaa.gov

Marine Authigenesis: From Global to Microbial, SEPM Special Publication No. 66
Copyright 2000 SEPM (Society for Sedimentary Geology), ISBN 1-56576-064-6, p. 257-269.

_______________________________________________________________________

E. HEINEN DE CARLO*, X.Y. WEN, and J.P. COWEN
Department of Oceanography, School of Ocean Earth and Technology,
University of Hawaii, Honolulu, HI 96822-2285

*email: edecarlo@soest.hawaii.edu

_______________________________________________________________________

DENYS L. VONDERHAAR* and GARY M. MCMURTRY
School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, Hawaii 96822, USA

DIETER GARBE-SCHÖNBERG
Geologisch-Paläontologisches Institut, Universität Kiel, Olshausenstrasse 40, 2300 Kiel 1, Germany

DORIS STÜBEN
Institut für Petrographie und Geochemie, Universität Karlsruhe, Kaiserstrasser 12,
76128 Karlsruhe, Germany

and

BRADLEY K. ESSER
Isotope Sciences Division, Lawrence Livermore National Laboratory, Livermore, CA 94551, USA

ABSTRACT: Many of the oldest and most depositionally complete hydrogenetic ferromanganese crusts are recognized to be enriched in Pt, but the source of this enrichment is debated. Despite the presence of numerous Fe-, Ni-, and Pt-rich spherules in the ferromanganese crusts studied, ratios of the platinum group elements (PGE) in the ferromanganese material from one sample indicate that Pt is greatly enriched compared to the other PGE and slightly enriched (about two times) with respect to seawater ratios. Osmium isotopic ratios also indicate a nonchondritic source for the PGE. Much of the crust Pt and Ir correlates with Fe-rich layers. Results of seawater-ferromanganese oxide adsorption experiments using ¹⁹¹Pt and ¹⁹²Ir suggest that Pt is preferentially scavenged by FeOOH when compared to Ir, suggesting a seawater source for the enrichment. Platinum, Fe, Mn, Ni, and Co all co-vary within the Pt-rich zone in stratigraphic transect 1 of this crust. The more detailed transect 2 results differ somewhat from those of transect 1. These data are compared with Pt and Ir data obtained by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses at ~1-mm intervals. Redox-sensitive elements such as Fe, Mn, Co, Cr, the derivative Ce and Eu anomalies, and Sm/Yb ratios from transect 2, when compared with the Pt profiles, suggest that localized redox conditions associated with either O₂ minimum zone processes or with the formation of carbonate fluorapatite (CFA) are responsible for the Pt enrichments. Analyses of two other crust samples from the same seamount and several other ferromanganese crusts from other areas of the Pacific suggest that the PGE enrichment is more closely tied to the formation of the CFA than to O₂ minimum zone processes, carbonate ion variations in the water column, or cosmogenic sources.

*email:denys@iniki.soest.hawaii.edu

Marine Authigenesis: From Global to Microbial, SEPM Special Publication No. 66
Copyright 2000 SEPM (Society for Sedimentary Geology), ISBN 1-56576-064-6, p. 287-308.


SECTION IV:
STRATIGRAPHIC, SEQUENCE STRATIGRAPHIC AND CHRONOSTRATIGRAPHIC STUDIES OF MARINE AUTHIGENESIS


_______________________________________________________________________

KEVIN G. TAYLOR*
Department of Environmental and Geographical Sciences, Manchester Metropolitan University,
Chester Street, Manchester, M1 5GD, United Kingdom

and

JOE H. S. MACQUAKER
Department of Earth Sciences, University of Manchester, Oxford Road, Manchester, M13 9PL, United Kingdom

ABSTRACT: Early diagenetic minerals are common within mudstone-dominated shallow marine sediments. However, our understanding of how these early diagenetic assemblages vary in a spatial and temporal sense across sedimentary basins is poorly developed. Such an appreciation is important, as early diagenetic mineral assemblages show a clear relationship with stratigraphic architecture and stratal surfaces.
In this study we present integrated stratigraphic and geochemical data for four differently aged mudstone-dominated successions (the Lower Jurassic Scunthorpe and Brant Mudstone Formations of eastern England, the Lower Jurassic Cleveland Ironstone Formation of eastern England, the Triassic Westbury Formation of South West England, and the Upper Cretaceous Mesaverde Group of Book Cliffs, Utah). Both spatial and temporal variability in early diagenetic mineral assemblages can be recognized in these successions, taking the form of ooidal ironstones, phosphates (apatite-rich units and bone-beds), concretionary carbonates, and pyritic mudstones. These variations are interpreted to result from spatial and temporal variations in physical and geochemical conditions in the sediments close to the sediment-water interface. Specifically, the physical and geochemical conditions required for the formation of these cemented units occur at key stratal surfaces (marine flooding surfaces, maximum flooding surfaces, and down-dip equivalents of sequence boundaries).
The physical conditions that occur at sequence boundaries (low accommodation, high energy, and frequent sediment reworking) can lead to the predominance of suboxic diagenesis and the formation of berthierine- and siderite-rich ooidal ironstones. Conditions of low net sediment accumulation and oxic/suboxic diagenesis upon major transgressive surfaces and maximum flooding surfaces leads to the formation of phosphate-rich units. Breaks in sediment accumulation at marine flooding surfaces lead to enhanced early diagenesis and the formation of laterally extensive carbonate cements and concretionary horizons. Sulfide-dominated early diagenesis (that dominated by sulfate reduction) is predominant during “normal” periods of shallow marine sedimentation and appears to mark the bulk of sediments within systems tracts. This study emphasizes the importance of integrating stratigraphic and geochemical data in developing quantitative and predictive models for early diagenesis.

*email: k.g.taylor@mmu.ac.uk

Marine Authigenesis: From Global to Microbial, SEPM Special Publication No. 66
Copyright 2000 SEPM (Society for Sedimentary Geology), ISBN 1-56576-064-6, p. 309-323.

_______________________________________________________________________

KURT A. GRIMM*
Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada

ABSTRACT: Understanding the relationship between stratigraphic architecture, sedimentary processes, and the accumulation of economic phosphorites is fundamental to paleogeographic reconstruction and phosphorite mining concerns, and is the primary focus of this paper. Shelfal biosiliceous sediments from the middle Tertiary of Baja California Sur, Mexico, preserve a suite of features recording stratigraphic condensation, a distinctive association of coarse-grained, nearshore-derived sediments atop extensively condensed horizons, and the amalgamation of peloidal phosphorite event strata to form thick economic phosphorite. A disconformity-based, allostratigraphic approach is used to elucidate stratigraphic packaging and origin of these sediments.
Prominent rock types include silt-rich biosiliceous mudrock, phosphatic sandstone, peloidal phosphorite, and coquinoid silty limestone. Sedimentological data suggest mixed hemipelagic (suspension) and tractional event deposition in a muddy marine forearc that was influenced by coastal upwelling and marked shoaling in its uppermost part. Paleontological data and geochemical considerations suggest that concretionary horizons and a diversity of dolomitic hardgrounds are attributable to stratigraphic condensation; these features are described and ranked in relative complexity, a hierarchy interpreted as a proxy for relative intensity of each condensation episode. Extensively bored dolomitic hardgrounds in the Timbabichi Formation are attributed to maxima in stratigraphic condensation, and are overlain by a very coarse lithofacies (VCL) that includes very coarse lithic arenite, coquinoid limestone, bored dolostone pebble conglomerate, and peloidal
phosphorite. Each horizon juxtaposing the VCL atop an extensively condensed marine hardground is interpreted as a Type 1 sequence boundary, attributable to an abrupt decrease in accommodation volume; each of three such episodes resulted in seaward downlapping of coarse-grained, nearshore-derived sediments and redeposition of condensed sediments into more distal environments that were formerly settings of extensive stratigraphic condensation.
The relationship between allostratigraphic packaging and thick phosphorites is illuminating but problematic. The association of some phosphatic grainstones with cross-stratified barnacle coquinas suggests a nearshore origin for phosphorites that was adjacent to hard substrates suitable for barnacle proliferation. Conspicuous burrows produced by event-redeposited callianassid tracemakers in some phosphatic turbidites (Grimm and Föllmi, 1994) suggest that peloidal phosphorites were sourced from shoals that hosted these burrowing crustacea. Depositional and biological amalgamation of phosphatic event strata (shelfal turbidites) formed thick economic phosphorites. These conclusions provide essential insights for improving phosphorite exploration, reserve assessment, and production programs.

*email: kgrimm@eos.ubc.ca

Marine Authigenesis: From Global to Microbial, SEPM Special Publication No. 66
Copyright 2000 SEPM (Society for Sedimentary Geology), ISBN 1-56576-064-6, p. 325-347.

_______________________________________________________________________

MARC S. HENDRIX*
Department of Geology, University of Montana, Missoula, MT 59812, USA

and

CHARLES W. BYERS
Department of Geology and Geophysics, University of Wisconsin-Madison, Madison, WI 53706, USA

ABSTRACT: A well preserved record of phosphatic sedimentation across an ancient continental ramp is contained in Upper Leonardian - Guadalupian (Upper Permian) strata of the Phosphoria and Park City Formations exposed along the flanks of the Uinta Mountains, northeastern Utah. The Uinta Mountains exposures trend approximately down depositional dip and, unlike exposures of the type Phosphoria in the Idaho - Wyoming overthrust belt, are not structurally telescoped.
This study is based on twelve detailed measured sections and hundreds of samples collected and studied macroscopically and microscopically from the uppermost Grandeur Member (Park City Formation), Meade Peak Member (Phosphoria Formation), and Franson Member (Park City Formation). The uppermost Grandeur Member is characterized by local carbonate hardgrounds which are partially reworked in a transgressive lag at the base of the Meade Peak. Strata from the Meade Peak and Franson are described in terms of five lithofacies: 1) organic-rich dolomite which is fine-grained, laminated, and present only in distal sections of the Meade Peak; 2) granular phosphorite packstone and grainstone which is mainly associated with the Meade Peak and consists of peloids, phosphatic shelly debris, intraclasts, molluscan steinkerns, and local phosphatic coated grains; 3) gray shale and 4) chert, which are most common at the transition zone between the Meade Peak and Franson Members; and 5) phosphatic dolowackestone which comprises most of the Franson and which contains calcite-filled molds after anhydrite nodules and supratidal fabrics in more shoreward sections.
Stratal architecture and stacking patterns, as well as the sedimentologic character and degree of bioturbation of individual facies suggest that the Meade Peak and Franson members together are unconformity bounded and record a single major transgressive-regressive cycle. As such, they are interpreted as a stratigraphic sequence. In addition, detailed sedimentologic relations suggest that repetitively interbedded phosphorite, gray shale, and phosphatic dolowackestone of the transition zone between the Meade Peak and Franson were deposited in response to small scale fluctuations in sea level, likely with associated changes in climate which affected the ramp.
The nature and style of phosphate accumulation were not uniform across the ramp, but varied down depositional dip. Poorly oxygenated basinward positions were marked by deposition of organic-rich carbonate in which phosphate is interpreted to have grown authigenically in situ. In contrast, the inner ramp was current winnowed, well oxygenated, and was the site of phosphate condensation, as suggested by the presence of only minor amounts of interstitial mud, the ubiquity of bioturbation, and the abundance of granular phosphorite, respectively.

*email: marc@selway.umt.edu

Marine Authigenesis: From Global to Microbial, SEPM Special Publication No. 66
Copyright 2000 SEPM (Society for Sedimentary Geology), ISBN 1-56576-064-6, p. 348-368.

_______________________________________________________________________

STANLEY RIGGS*, SCOTT SNYDER, DOROTHEA AMES
Department of Geology, East Carolina University, Greenville, North Carolina 27858, USA

and

PETER STILLE
Centre de Géochimie de la Surface (CNRS), 1, rue Blessig, F-67084 Strasbourg, France

ABSTRACT: Integration of biostratigraphic and Sr isotope data constrain the ages of four third-order depositional sequences within the Aurora and Onslow Embayments of the Miocene Pungo River Formation. Within the Aurora Embayment the microsphorite surface capping the Oligocene and marking the base of the Pungo River Formation is ~28 Ma. The bulk of Unit A, which constitutes the Aurora Embayment Sequence (AES), is 23.2–21.6 Ma. Unit B, at 19.1–17.0 Ma, is largely contemporaneous with the Frying Pan Sequence (FPS) in Onslow Embayment. Units C and D, at 16.4–14.8 Ma, are largely contemporaneous with the Onslow Bay Sequence (OBS) in Onslow Embayment. The microsphorite surface at the contact of unit D and the Pliocene Yorktown Formation is 12.7 Ma. Within the Onslow Embayment the microsphorite at the unconformity between the
Oligocene and the Miocene Pungo River Formation is 21.0–19.5 Ma. The remainder of the Frying Pan Sequence (FPS) is 20.5–16.7 Ma. Most of the Onslow Bay Sequence (OBS1–OBS3) is 15.4–14.8 Ma, with OBS4 being slightly younger. The lower units of the Bogue Banks Sequence (BBS1–BBS5) are 12.5–8.6 Ma. BBS6-BBS8 have not been constrained biostratigraphically; however, Sr ages suggest that the phosphate was reworked from pre-existing units and deposited between 8.6–7.1 Ma.
Interpretation of depositional patterns in phosphate-rich sediments of North Carolina has led to the development of a sedimentological model for continental margin phosphogenesis. Sedimentation patterns in an idealized phosphogenic episode reflect various stages of sea-level change. Sea-level lowstand produces an erosional unconformity across previously deposited sediment sequences. Marine transgression is characterized by a finingupward sequence of fine siliciclastic sediments with intermixed phosphate grain types. During early-stage transgression, a basal microsphorite crust forms and erodes to produce rip-up intraclasts that are then reworked into and dominate the lower portion of the subsequent depositional sequence. Mid-stage transgression is dominated by authigenic phosphate, including skeletal grains and fine sand-sized peloids, which form as disseminated grains below the sediment-water interface in response to the degradation of organic matter in nutrient-rich, suboxic shelf environments where there is little or no sediment winnowing or reworking. During late-stage transgression the phosphogenesis is terminated as oxygen-depleted environments migrate landward across the shelf with the formation of organic-rich dolosilts or foraminifer-rich/diatom-rich muds with little to no phosphate. The sea-level highstand is characterized by carbonate-rich sediments deposited under the influence of normal marine, open-shelf conditions. During the subsequent regression, sediments of these shallower depositional environments are often eroded and reworked to form beds of extensively reworked phosphate-rich sediments.
Four episodes of phosphate sedimentation characterize the North Carolina continental margin. Three episodes (I, II, and III) are dominated by primary phosphate formation within the Miocene, whereas one episode (IV) consisting of the Pliocene-Quaternary sediments contains totally reworked phosphates derived from sediments of Episodes II and III. Episode I occurred during the Aquitanian (23.3–21.6 Ma), forms the basal sequence of the Pungo River Formation in the Aurora Embayment, and represents a mid-stage transgression that is only partially preserved. Episode II preserved all sediment facies of an entire sea-level cycle that produced the main Miocene phosphogenic event, which took place during the Burdigalian and Langhian (21.0–14.8 Ma) and extended throughout the Aurora and Onslow Embayments. Episode III was deposited in eastern Onslow Embayment during the uppermost Serravallian and Tortonian (12.7 Ma and likely continuing through 7.1 Ma). Low sea stand during the Messinian led to severe erosion and weathering of Episode III sediments, limiting their distribution and modifying their composition. Episode IV represents many sea-level events that produced Pliocene-Quaternary deposits with local and variable concentrations of reworked phosphate of many types. The four phosphate episodes recognized in North Carolina closely correspond to the Upper Cenozoic phosphorite episodes identified globally by Riggs and Sheldon (1990).

*email: riggss@mail.ecu.edu

Marine Authigenesis: From Global to Microbial, SEPM Special Publication No. 66
Copyright 2000 SEPM (Society for Sedimentary Geology), ISBN 1-56576-064-6, p. 369-385.

_______________________________________________________________________

JOHN D. KRONEN Jr. and CRAIG R. GLENN*
Department of Geology and Geophysics, SOEST, University of Hawaii, 1680 East-West Road, Honolulu, HI 96822

ABSTRACT: Thin, well-defined condensed sections containing authigenic green minerals of the verdine facies occur in forereef depositional sequences of the Great Barrier Reef (GBR), Queensland, Australia. Pristine and reworked verdine grains within these sections formed in a mixed carbonatesiliciclastic depositional setting that accumulated during the last 1.5 m.y. in relatively warm waters. Seismic reflection, physical properties, and oxygen isotope data reveal that cyclic deposition occurred at ODP Site 821. Authigenic verdine grains are concentrated and most abundant along maximum flooding surfaces at the top of transgressive systems tracts and are also concentrated along marine flooding surfaces within transgressive systems tracts. These are notably “pristine” and commonly occur as relatively well-preserved green clay minerals that infill primary intraparticle porosity, especially within foraminifer tests. There are also verdine grains that show the effects of reworking and transport. These are usually not contained within foraminifer tests like the pristine verdine, but are abraded to fragmented, less abundant, and typically occur within highstand systems tracts. The reworked nature of these particles is attributed to a higher-energy depositional setting where loss of accommodation space, seaward progradation, and sediment reworking occur. Chemical data from electron microprobe analyses reveal low K₂O concentrations and X-ray diffraction data possibly indicate variations in the degree of verdine mineral maturity.
Verdine is ferric-rich, but because it contains iron in both its oxidized and reduced states, we suggest that it forms in association with mildly reducing, suboxic solutions from which both ferric and ferrous iron may be supplied. Evolution of verdine is terminated shortly after burial. The relatively rapid formation of verdine facies minerals may have implications on reverse weathering and may play an important role in large scale biogeochemical cycling in the oceans. This facies may provide a sink for reverse weathering reactions involving Fe, Mg, and K in clay mineral formation. Although it has been previously suggested that in situ formation of verdine minerals is rare in water depths exceeding ca. 60 m and, due to its poor preservation potential, is rare in pre-Holocene sediments, we document here its formation at paleo-water depths of about 200 m within repetitive moderately condensed sections spanning the past ~1.5 m.y.

*email: glenn@soest.hawaii.edu

Marine Authigenesis: From Global to Microbial, SEPM Special Publication No. 66
Copyright 2000 SEPM (Society for Sedimentary Geology), ISBN 1-56576-064-6, p. 387-403.

_______________________________________________________________________

ALESSANDRO AMOROSI* and MARIA CARLA CENTINEO
Dipartimento di Scienze della Terra e Geologico-Ambientali, Via Zamboni 67, 40127 Bologna, Italy

ABSTRACT: Detailed examination of the lowermost Cenomanian, glaucony-rich deposits of Cap Blanc-Nez (Northern France) reveals a hierarchy of depositional units within a stratigraphically condensed succession, providing a fundamental tool for its sequence stratigraphic interpretation.
Condensation is obvious in the highly bioturbated basal glaucony-rich unit (Strouanne Formation), where glaucony is predominantly autochthonous, highly evolved (> 8 % K₂O), and generally constitutes 60–90 % of the sand fraction. Glaucony concentration decreases markedly upwards (to less than 10 %) in the overlying rhythmically bedded marls and chalks (Petit Blanc-Nez Formation), where the green grains are allochthonous and comparatively less evolved (6.0–6.5 % K₂O). Glaucony distribution shows a vertical cyclic pattern within the condensed interval. Maxima of glaucony concentrations are invariably recorded at intensively burrowed omission surfaces, interpreted as bases of either parasequences or parasequence sets.
The characterization of glaucony attributes indicates that a prolonged break in sedimentation, on the order of 0.5–1 Ma, occurred in the study area during the Early Cenomanian. The highly condensed nature of the basal glaucony-rich deposits suggests that either one or two of the third-order Lower Cenomanian depositional sequences recognized in the coeval, less condensed successions of Western Europe are missing at Cap Blanc-Nez. Glaucony maturity in the overlying marl-chalk couplets is consistent with interpretation of these deposits as Milankovitch-scale cycles.

*email: amorosi@geomin.unibo.it

Marine Authigenesis: From Global to Microbial, SEPM Special Publication No. 66
Copyright 2000 SEPM (Society for Sedimentary Geology), ISBN 1-56576-064-6, p. 405-413.


SECTION V:
ORIGIN OF PHOSPHORITES, GLAUCONITES AND ASSOCIATED FACIES THROUGH TIME: CASE STUDIES


_______________________________________________________________________

DAVID SOUDRY*
Geological Survey of Israel, 30 Malkhe Yisrael St., Jerusalem 95501, Israel

ABSTRACT: Study of the carbonate micrite phase of laminated pelletal phosphorites from various phosphate fields of the Negev reveals that this phase was largely formed in situ, and that bacterial mats are principally the frame-builders of this phase. These mats are composed of vertically stratified, calcite-mineralized assemblages of bacterial bodies of various shapes, sizes, and organizations. This biostratification is believed to represent the fossilized record of time-successive consumer bacterial populations that degraded planktonic detritus, and mats of filamentous and coccoid microphytes, which colonized the Mishash seafloor during phosphorite formation and were partly preserved in these rocks as apatitic sheaths and envelopes. Carbonate mineralization of the bacterial consumers probably took place during anaerobic degradation of the matted sediments, as indicated by low d¹³C values in many of these carbonates. The diagenetic fate of the filamentous and coccoid Mishash bottom-dwelling microphytes (apatite mineralization versus turnover as bacterial carbonates) was dependent on the balance between the rate of organic matter accumulation and the extent of bottom sediment ventilation, which together controlled the redox conditions of organic matter decomposition and pore-water carbonate ion concentrations. This may explain why primary (pristine) apatite beds are common in condensed proximal Mishash sections around Negev Campanian highs, whereas they are more sporadic in basinal carbonate-rich Mishash sections. It might also explain why the coccoid and filamentous microphyte structures, which matted the Mishash seafloor and are now preserved in many of the Negev phosphorite rocks, were found to be mineralized only by phosphate and never by carbonate.

*email: david.soudry@mail.gsi.gov.il

Marine Authigenesis: From Global to Microbial, SEPM Special Publication No. 66
Copyright 2000 SEPM (Society for Sedimentary Geology), ISBN 1-56576-064-6, p. 415-426.

_______________________________________________________________________

S. BENALIOULHAJ AND J. TRICHET
Laboratoire de Géochimie Organique, Université d’Orléans, 45067 Orléans cedex 2, France

N. BENALIOULHAJ
Department of Geology, Mohammadia School of Engineering, Agdal, Rabat, Morocco

J. LUCAS
Institut de Géologie, Université Louis Pasteur, 1 Rue Blessig, 67084 Strasbourg cedex, France

B. C. SCHREIBER*
Department of Geology, Appalachian State University, 195 Rankin Science Building, Boone, NC, 28608, USA

and

R. P. PHILP
School of Geology and Geophysics, University of Oklahoma, 100 East Boyd St., Norman OK, 73019-0628, USA

ABSTRACT: The sedimentary subbasins of Oulad Abdoun and Timahdit (Morocco) were part of a single marine gulf that extended across Morocco for more than 300 km (west to east) from Late Cretaceous through the Eocene. The differential structural development of this basin resulted in the presence of a stable shelf in Oulad Abdoun and a subsident orogenic zone in Timahdit. This structural asymmetry explains the sedimentary asymmetry of the Maastrichtian sequence that consists of a thin, 5-m thick, phosphorite sequence interbedded with dolomitic limestone, marl, and shale near Oulad Abdoun, and an oil shale, 200-m thick, in Timahdit.
Widespread reducing conditions that prevailed in the basin resulted in preservation of large amounts of organic matter. Quantitative and qualitative analysis shows an identical phytoplanktonic origin for all of the organic matter in both of the two subbasins, and the organic matter still remains in an immature state. The subsequent history of this organic matter, however, reflects the specific environments of the two related but disparate depositional settings.
In each part of the basin, the organic material associated with marls, limestones, and shales contains kerogen, whereas in the phosphate-rich strata the organic matter is in two forms: humic compounds inside the phosphatic grains and kerogen in the matrix. Humic compounds from different lithologies have a similar composition, whereas the kerogen in samples from the phosphate-rich strata shows the effect of oxidation (during synsedimentary reworking), increasing with the amounts of P₂O₅ of the total rock. These changes in the phosphate-rich strata have allowed the crystallization of apatite and the genesis and preservation of humic compounds trapped inside these phosphatic grains. In the oil shale, large amounts of associated clay resulted in a rapid and total unsolubilization of the organic matter and its evolution toward kerogen. The study of the organic matter, which is associated with a wide variety of facies from both parts of the basin, shows the extreme sensitivity of organic compounds to processes that have affected them from their deposition through all phases of diagenesis. The fate of the organic matter during early diagenesis is related to the presence or absence of clays: the presence of clays led to the formation of black shales and the absence of clays led to the formation of phosphorites.

*email: geologo@aol.com

Marine Authigenesis: From Global to Microbial, SEPM Special Publication No. 66
Copyright 2000 SEPM (Society for Sedimentary Geology), ISBN 1-56576-064-6, p. 427-444.

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ANNA M. CRUSE,* TIMOTHY W. LYONS
Department of Geological Sciences, University of Missouri, Columbia, Missouri 65211, USA

and

DAVID L. KIDDER
Department of Geological Sciences, Ohio University, Athens, Ohio 45701-2979, USA

ABSTRACT: Four distinct shale-normalized rare-earth element (REE) patterns are preserved in the black and gray phosphatic shale facies of the Upper Carboniferous of Midcontinent North America. Two of these patterns [flat and middle REE (MREE) enriched] are observed in both nodular phosphate and the shale host, but MREE-enriched patterns are more common in the nodules. MREE-depleted patterns were observed only in the shales. Subtle negative Ce-anomalies are rare and occur only in samples of nodular phosphate.
MREE-enriched patterns in phosphate are interpreted to have formed during very early diagenesis. Coprecipitation of phosphate and REE may have depleted pore waters in MREE to the extent that host shales, which lithified later, are characterized by MREE depletions. Phosphate diagenesis can thus be a critical factor controlling REE patterns preserved in shales. Flat REE signatures characterize gray shale samples that have low concentrations of phosphate and organic carbon. Such samples were thus not affected by processes of remobilization and scavenging of REE. Black shale samples that have phosphate concentrations intermediate of those in samples characterized by MREE-enrichments and -depletions also display flat patterns. Such patterns likely reflect a composite signal of MREE-enriched phosphate and MREE-depleted shale. Conversely, flat patterns in phosphate nodules could reflect later diagenetic migration of REE from MREE-depleted pore waters into later cement phases in the nodules. Subtle Ce-depleted patterns in phosphate nodules likely reflect the minor exchange of Ce with seawater during episodes of sediment reworking.

*Present address: Woods Hole Oceanographic Institution, Department of
Marine Chemistry and Geochemistry, MS#4, Woods Hole, MA 02543,
email:acruse@whoi.edu

Marine Authigenesis: From Global to Microbial, SEPM Special Publication No. 66
Copyright 2000 SEPM (Society for Sedimentary Geology), ISBN 1-56576-064-6, p. 445-453.

_______________________________________________________________________

CARLOS FONSECA*
Geological and Environmental Sciences Department, Stanford University, Stanford, CA 94305

ABSTRACT: Sediments of the diatomaceous and laminated Marca Shale provide information on the interaction between anoxia and the occurrence of phosphatic nodules in organic-rich sediments along the Late Cretaceous continental margin of California. Sedimentological observations and geochemical analyses permit differentiation of dysoxic, suboxic, and anoxic facies within the Marca Shale. Characteristics of dysoxic facies include massive mudstones, localized macrobioturbation, occurrence of glauconite, and calcareous concretions. Suboxic facies are predominant in the Marca Shale and characterized by submillimeter laminated sediments, absence of macrobioturbation, occurrence of low-oxygen tolerant benthic foraminifers, episodic occurrence of pristine phosphatic sediments, preferential accumulation of vanadium, and modest content of total organic carbon and hydrogen-rich organic matter. Denitrifying conditions prevailed in the suboxic facies and could be associated with the modest preservation of organic matter in the Marca Shale sediments. Pristine phosphates occur episodically in laminated sequences of the suboxic facies and are locally related to intervals with disrupted laminations, soft-sediment deformation, low siliciclastic content, local abundance of biogenic components, and accumulation of lanthanide elements. Constant rates of pelagic sedimentation, fast burial, and oxygen-deficient environments characterized laminated sequences. In contrast, low sedimentation rates, long-term exposure, and fluid exchange of sediments with ambient sea water are associated with the occurrence of pristine phosphatic nodules and concentration of diagnostic trace metals. Anoxic facies are restricted to laminated sediments and characterized by high values of organic carbon, hydrogen index, and episodic precipitation of sulfides under reducing conditions. The occurrence of late Maestrichtian marine sediments associated with vigorous coastal upwelling along the California margin suggests a change in the paleoceanographic and climatic conditions at the end of the Cretaceous at 65 Ma.

*Present address: Shell International Exploration and Production B.V., Research
and Technical Services, P. O. Box 2080 AB Rijswijk, The Netherlands
email: c.fonseca@siep.shell.com

Marine Authigenesis: From Global to Microbial, SEPM Special Publication No. 66
Copyright 2000 SEPM (Society for Sedimentary Geology), ISBN 1-56576-064-6, p. 455-480.

_______________________________________________________________________

TOBIAS SCHWENNICKE
Departamento de Geología, Universidad Autónoma de Baja California Sur, Apdo Postal 19-B, 23080 La Paz, B.C.S., Mexico

HENDRIK SIEGMUND*
Institut und Museum für Geologie und Paläontologie, Universität Tübingen, Herrenberger Str. 51, 72070 Tübingen, Germany

and

CAROLINE JEHL
Laboratoire H.E.A., OSTROM, 911 Avenue Agropolis, BP 5045, 34032 Montpellier cedex 1, France

ABSTRACT: In an attempt to summarize the characteristics and importance of shallow marine phosphogenetic settings we describe three examples from fossil and recent environments. These are the lowermost Cambrian Zhongyicun Formation phosphorites from China; the Tertiary Salada Formation phosphorites from Baja California, Mexico; and recent atoll phosphorites from French Polynesia. Common features of these settings, which range from clastic-dominated to pure carbonate, are a lagoonal or inner shelf to nearshore environment with somewhat restricted water circulation and the influence of an increased supply of nutrients for higher bioproductivity. Organic matter is considered as the major source of phosphorus, but in settings with clastic influence other sources like continental input are feasible. The study of modern atoll phosphorites enabled the introduction of a new phosphogenetic model involving cyanobacterial mats, which provide an environment favorable for phosphogenesis and serve as nucleation sites for apatite crystals. This model is also valid for the Cambrian phosphatized microbial mats and stromatolites. However, the Salada Formation phosphorites lack discernible microbial mat structures. Here the generation of a phosphogenetic environment may be due to fine-grained sediment layers, which prevent free access of oxygenated water into the sediment. Although only one of the above shallow marine phosphogenetic settings leads to the formation of economic phosphorites, we argue that more time for phosphate accumulation and concentration may have produced such deposits in the other localities as well.

*email: h.siegmund@em.uni-frankfurt.de

Marine Authigenesis: From Global to Microbial, SEPM Special Publication No. 66
Copyright 2000 SEPM (Society for Sedimentary Geology), ISBN 1-56576-064-6, p. 481-498.

_______________________________________________________________________

AGUSTÍN MARTÍN-ALGARRA*
Departamento de Estratigrafía y Paleontología and IACT, Facultad de Ciencias, Universidad de Granada, 18071-Granada, Spain

and

ANTONIO SÁNCHEZ-NAVAS
Departamento de Mineralogía y Petrología and IACT, Facultad de Ciencias, Universidad de Granada, 18071-Granada, Spain

ABSTRACT: Facies, fossils, morphology, lamination features, microstratigraphy, and composition of phosphate and iron-manganese nodules and crusts found in condensed limestones associated with stratigraphic discontinuity surfaces in Mesozoic successions of the Betic Cordillera of southern Spain, demonstrate that these structures are not diagenetic concretions, but biosedimentary microbial accretions (stromatolites) that grew rhythmically at the sediment-water interface in pelagic environments. These structures accreted by bacterially mediated precipitation of authigenic minerals, by trapping and binding of fine-grained, pelagic, siliciclastic, and carbonate particles, and by encrustation of benthic foraminifera. SEM examination of stromatolite laminae reveals the presence of dense accumulation of spheroidal, ovoidal, and sausage-shaped strings of micrometer-sized bodies with a bacteria-like size and shape. XRD and microprobe (BSE and X-ray images) studies show the presence of authigenic minerals such as francolite, goethite, hematite, barite, pyrite, and glauconite, among others. Authigenic clays and complex poorly crystalline Fe-Mn-Al- (Si)-oxyhydroxides are always closely associated with these minerals. Marine authigenesis in stromatolites of the pelagic realm was achieved through microbially mediated synsedimentary precipitation of poorly crystalline or amorphous phases that preceded crystallization of the observed minerals. Deposition of finegrained metals and silicate precipitates on bacterial surfaces favored preservation of microbial remains as external moulds. After degradation of bacterial organic matter, other components, such as francolite, nucleated within empty bacterial cells and other voids from bacterially precipitated amorphous precursors. Texture, mineralogy, and chemistry indicate the vertical and temporal evolution of the physicochemical conditions during the synsedimentary bacterial precipitation of amorphous precursors and the oscillations of the oxic-anoxic boundary (usually from oxic to postoxic conditions, although sometimes sulfidic conditions have been registered, allowing pyrite precipitation) during which the maturation of authigenic mineral phases took place. In conclusion, periods of very low sedimentation rate, now seen as sequence boundaries and/or transgressive surfaces in Mesozoic Alpine-Mediterranean paleomargins, favored accretion of stromatolites in the pelagic realm and bacterially mediated precipitation of a wide spectrum of authigenic minerals that provide abundant information about paleoceanography, sedimentation, and early diagenesis in open marine mesozoic environments.

*email: agustin@goliat.ugr.es

Marine Authigenesis: From Global to Microbial, SEPM Special Publication No. 66
Copyright 2000 SEPM (Society for Sedimentary Geology), ISBN 1-56576-064-6, p. 499-525.