Restricted Marine Basin Research Articles

The marine biodiversity impact of the Late Miocene Mediterranean salinity crisis.

Massive salt accumulations, or salt giants, have formed in highly restricted marine basins throughout geological history, but their impact on biodiversity has been only patchily studied. The salt giant in the Mediterranean Sea formed as a result of the restriction of its gateway to the Atlantic during the Messinian Salinity Crisis (MSC) 5.97 to 5.33 million years ago. Here, we quantify the biodiversity changes associated with the MSC based on a compilation of the Mediterranean fossil record. We conclude that 86 endemic species of the 2006 pre-MSC marine species survived the crisis, and that the present eastward-decreasing richness gradient in the Mediterranean was established after the MSC.

Contrasting nutrient availability between marine and brackish waters in the late Mesoproterozoic: Evidence from the Paranoá Group, Brazil.

Understanding the delayed rise of eukaryotic life on Earth is one of the most fundamental questions about biological evolution. Numerous studies have presented evidence for oxygen and nutrient limitations in seawater during the Mesoproterozoic era, indicating that open marine settings may not have been able to sustain a eukaryotic biosphere with complex, multicellular organisms. However, many of these data sets represent restricted marine basins, which may bias our view of habitability. Furthermore, it remains untested whether rivers could have supplied significant nutrient fluxes to coastal habitats. To better characterize the sources of the major nutrients nitrogen and phosphorus, we turned to the late Mesoproterozoic Paranoá Group in Brazil (~1.1Ga), which was deposited on a passive margin of the São Francisco craton. We present carbon, nitrogen and sulphur isotope data from an open shelf setting (Fazenda Funil) and from a brackish-water environment with significant riverine input (São Gabriel). Our results show that waters were well-oxygenated and nitrate was bioavailable in the open ocean setting at Fazenda Funil; the redoxcline appears to have been deeper and further offshore compared to restricted marine basins elsewhere in the Mesoproterozoic. In contrast, the brackish site at São Gabriel received only limited input of marine nitrate and sulphate. Nevertheless, previous reports of acritarchs reveal that this brackish-water setting was habitable to eukaryotic life. Paired with previously published cadmium isotope data, which can be used as a proxy for phosphorus cycling, our results suggest that complex organisms were perhaps not strictly dependent on marine nutrient supplies. Riverine influxes of P and possibly other nutrients likely rendered coastal waters perhaps equally habitable to the Mesoproterozoic open ocean. This conclusion supports the notion that eukaryotic organisms may have thrived in brackish or perhaps even freshwater environments.

Sr–Nd isotopic constraints on carbonates, conodonts, and brachiopods of Early-Middle Pennsylvanian Itaituba and Nova Olinda formations, Amazonas Basin, Brazil

Isotopic signatures presented in carbonates and fossils herein are one of the first attempts to obtain Sr and Nd data for the Early – Middle Pennsylvanian of the Itaituba epeiric sea and to evaluate the paleoseawater and paleotectonic of the Amazonas Basin. 87Sr/86Sr data range from 0.708330 ± 0.000018 to 0.708850 ± 0.000046, with ΔSW varying from −60 to −87 reflecting the marine sedimentation, while ΔSW varying from −33 to −51 suggest a post-depositional alteration. These values are in general agreement with the 87Sr/86Sr global seawater curve evolution for this time. The increasing influx of Sr from landmasses in a restricted marine basin, the Itaituba Epicontinental Sea, reflecting enhanced continental weathering during low stand sea level is a potential explanation for the Amazonas Basin during this time. Plotted in the εNd(t = 310 Ma) isotopic global seawater curve evolution data for the Itaituba and the Nova Olinda formations shift to lower values of εNd(t = 310 Ma), when compared to the Panthalassa Ocean. Nd isotopic signature suggests an incoming of seawater masses also from other oceans (e.g. Paleo-Tethys), instead of a unique Panthalassa Ocean provenance of water during the Pennsylvanian in the Amazonas Basin. 143Nd/144Nd isotopic values showed scattered 0.511608 ± 0.000077 to 0.512270 ± 0.000012. Nd-TDM around 1.5 Ga and εNd present-day values around −15 support an old continental Nd provenance to the ancient seawater, similar to those present in the rocks from Amazonian Craton.

A high-TOC shale in a low productivity world: The late Mesoproterozoic Arctic Bay Formation, Nunavut

The latest Mesoproterozoic Arctic Bay Formation (Borden Basin, Nunavut, Canada) is up to ∼1130 m-thick and contains a significant proportion of unusually organic-rich black shale (up to 12.3 wt% total organic carbon). Insofar as increased biological productivity is related to organic matter burial, this organic-rich succession is seemingly incongruent with the low biological productivity world hypothesised for much of the Proterozoic. To better understand the conditions leading to development of this organic-rich unit, we explore the redox geochemistry of the Arctic Bay Formation using a multi-proxy approach (nitrogen isotopes, iron speciation, total organic carbon, total sulphur, and trace metal abundances). Redox proxy data support a stratified water column, with oxic surface waters underlain by intermittently euxinic waters, which are in turn underlain by persistently ferruginous deeper waters. The highly alkaline, restricted marine basin in which the Arctic Bay Formation was deposited may have allowed for rapid sequestration of highly reactive iron in carbonate minerals, resulting in an ‘excess’ of sulphur that resulted in sulphurisation of organic matter. Estimates for organic matter burial rates during deposition of the Arctic Bay Formation suggest that they were perhaps ∼5–6 times mid-Proterozoic average values (although there are permissible scenarios in which it was extremely productive), underscoring that such organic-rich sedimentary rocks could be produced in a low productivity world.

Reviews of the Mesoproterozoic to Neoproterozoic sedimentary sequences and new constraints on the tectono-sedimentary evolution of the southern margin of the North China Craton

The North China Craton (NCC) is thought to experience multi-stage extensional events after the breakup of the Columbia (Nuna) Supercontinent, and even during the assembly of the Rodinia Supercontinent. In recent years, volcanic and detrital zircon U-Pb dating ages provided new insights into the Mesoproterozoic to Neoproterozoic tectonic framework in the NCC. As the Mesoproterozoic Xiong’er Group in the southern NCC is regarded as the oldest volcanic-sedimentary succession after the final amalgamation of the NCC, we reviewed the sedimentary environments and provenances of Mesoproterozoic to Neoproterozoic sedimentary sequences above the strata in order to understand tectono-sedimentary evolution of the southern NCC during this period. The early Mesoproterozoic sedimentary rocks were deposited in alluvial-fan to fluvial and (fan-) delta-coastal environments suggesting an initial stage of basin development. Fine-grained clastic sedimentary rocks interlayered with carbonate deposition formed on the southern NCC later, and implied a broad subsidence before ca. 1611 Ma. After that, depositional breaks caused by regional tectonic uplifts led to different sedimentary sequences and lateral migration of the depocenter from the north to the south (to the current co-ordinates). The unconformably overlying sedimentary rocks in the north of the southern NCC were deposited in restricted marine basins during late Mesoproterozoic to Neoproterozoic as a whole. However, the enlargement of shallow-marine shelf led to the thick accumulation of carbonates in the southernmost NCC characterized by a deepening-upward trend from the late Mesoproterozoic sequence. The Neoproterozoic sedimentary sequence is typified by sharp increasing terrigenous supplies and frequent changes in environment, and could be deposited in a tectonically active basin in consideration of its close association with volcanism. Detrital zircon grains from the oldest clastic sequence (i.e., the Bingmagou Formation) overlying the Xiong’er volcanic-sedimentary rocks and from the earliest Paleozoic sedimentary rocks (i.e., the Xinji Formation) are used to track changes of provenances. Detrital zircon grains from the Bingmagou Formation display two dominant age peaks at ca. 1850 and 2500 Ma and three secondary peaks at ca. 2100, 2350 and 2700 Ma. In contrast, detrital zircon grains from the Xinji Formation have three major age peaks at ca. 1850, 2200 and 2500 Ma and three subordinate populations of 950, 1100 and 1450 Ma. In combination with the uplifts during the Mesoproterozoic to Neoproterozoic, Neoproterozoic magmatism, and a large amount of detrital zircon derived from Rodinia-related orogenesis in the southern NCC, it is very likely that the major changes of basin configuration could be related to significant tectonic readjustment along with the evolution of the Rodinia Supercontinent.

Palynostratigraphic, palynofacies, organic geochemical and palaeoenvironmental analysis of the Silurian Tanezzuft Formation in the Ghadames Basin of north-west Libya

Two palynofacies associations are documented from the Silurian Tanezzuft Formation in the Ghadames Basin. These are characteristic of the basal ‘Hot Shale’ and the overlying deposits, referred to here as the Cold Shale. The former reflects deposition in distal suboxic anoxic conditions and is dominated by highly oil-prone amorphous organic matter (AOM) typical of deposition in generally anoxic, restricted marine basins. Only a few acritarchs, prasinophyte algae and chitinozoans occur in association: virtually no spores or cryptospores were recorded. Thick-walled prasinophytes are most numerous in this part of the Tanezzuft Formation in both the Ghadames and Murzuq basins, suggesting enhanced surface water productivity. Deposition took place after the melting of the Late Ordovician ice sheets, which led to a major marine transgression. The palynofacies recorded from the overlying ‘Cold Shale’ deposits indicate deposition in distal shelf and basin conditions that were also relatively anoxic. They contain more palynomorphs, especially acritarchs, and generally less AOM. The phytoplankton assemblages are dominated by simple and thin-walled prasinophyte algae (leiospheres), suggesting dysoxic–anoxic conditions. Overall the middle and the upper parts of the Tanezzuft Formation are regarded as being deposited in distal dysoxic–anoxic shelf, distal dysoxic–oxic shelf and distal suboxic–anoxic basin respectively. Because chitinozoans are very rare, age determinations of the samples investigated are based mainly on acritarchs. The Hot Shale is dated as early-mid Rhuddanian (early Llandovery) whereas the rest of the formation is considered to late Rhuddanian–Telychian in age. As documented previously from other samples of the Tanezzuft Formation in both the Ghadames and Murzuq basins, the Hot Shale has a very high TOC content and excellent source potential for liquid hydrocarbons, whereas rest of the formation is less rich in organic matter with larger terrestrial and oxidized components and hence reduced potential for sourcing hydrocarbons (both oil and gas).

C, O, and Sr isotopic variations in Neoproterozoic-Cambrian carbonate rocks from Sete Lagoas Formation (Bambuí Group), in the Southern São Francisco Basin, Brazil

ABSTRACT: High-resolution chemostratigraphic data of carbonates from the Sete Lagoas Formation (Bambuí Group) show large variations on the C, O, and Sr isotope compositions. Impure limestones at the base show primal δ13C values between -1.0 and 0‰, and δ18O values between -12.0 and -8.0‰. However, some dolostones demonstrate δ13C values varying from +2.8 to -6.8‰, highly radiogenic 87Sr/86Sr ratios (>0.7111), and low Sr concentrations (<350 ppm) related to post-depositional processes. In contrast, pure limestones at the top show very positive δ13C values between +8.3 and +12.8‰, δ18O values between -10.0 to -6.0‰, and 87Sr/86Sr ratios from 0.7073 to 0.7086, with high Sr concentrations (>900 ppm). They are linked to depositional controls on the carbonate platform, such as fluvial and/or submarine water influx, in which carbonates deposited on the proximal sector exhibit significant Sr isotopic variations and those on the distal sector were not subject to such controls, resulting in very homogeneous Sr isotope profiles. However, 87Sr/86Sr ratios of the distal carbonates are less radiogenic than carbonates expected for late Ediacaran (~0.7085). This discrepancy suggests a restricted marine basin without Sr isotopic homogenization with contemporary oceans and, in this case, global correlations based on Sr isotope stratigraphy are not reliable.

Definition of new trace-metal proxies for the controls on organic matter enrichment in marine sediments based on Mn, Co, Mo and Cd concentrations

Trace metal enrichments in sedimentary deposits are of prime interest because they are governed by processes that also control the production and preservation of organic matter. Consequently, trace metals have been used in reconstructions of the (palaeo)depositional environment of organic-rich deposits, but most of these studies have primarily focused on hydrographically restricted basins and the response of trace metals to changing redox conditions whereas the role of trace metals in the nutrient cycle and primary productivity in upwelling settings remains relatively unexplored.In this study we present a comprehensive database of published trace metal concentration data in modern organic-rich deposits from a variety of marine settings. Scrutiny of the compiled dataset has resulted in the development of novel trace metal based proxies that allow the distinction between two marine end-member depositional settings that are associated with enhanced organic carbon burial: open marine settings on the continental margin associated with upwelling (e.g. Namibian Margin) and hydrographically restricted marginal marine basins (e.g. Black Sea).It is shown that high Cd/Mo values are typical for sediments from continental margin upwelling settings whereas Co and Mn concentrations (expressed as Co∗Mn values) are high in samples from hydrographically restricted marine basins. The Cd/Mo ratios are thought to track the relative importance of productivity versus preservation with high values in productivity driven systems attributed to the transfer of Cd to the sediments from re-mineralised plankton biomass. Co∗Mn values, on the other hand, are believed to reflect the supply and reactive behaviour of Co and Mn and can be used to assess circulation patterns in the water column and the relative contribution of deep versus surface/river water influx to the basin. It is demonstrated that the combined use of the Cd/Mo and Co∗Mn proxies provides a highly effective way to distinguish modern/recent marine sedimentary environments, which holds promise for its use in palaeo-environmental reconstructions.

Chemostratigraphy of the lower Bambuí Group, southwestern São Francisco Craton, Brazil: insights on Gondwana paleoenvironments

ABSTRACT: The Bambuí Group, the most extensive carbonate-siliciclastic cover on the São Francisco craton, has been a matter of debate because of its potential correlations to global glacial events. Unfortunately, most available chemostratigraphic data came from samples collected on surface rock exposures, ever susceptible to the aggressive chemical weathering that characterizes the southeastern Brazil. On the other hand, we present here high-resolution chemostratigraphic studies based on C, O and Sr isotopic data from 53 samples collected along a weathering-free, continuous, 175 m thick sedimentary succession. This succession was recovered by borehole drilling in the southwestern São Francisco craton, where occur the Carrancas and Sete Lagoas formations, the lowermost units of the Bambuí Group. The drill cores reveal extremely irregular contacts between the basal diamictite and its basement, an Archaean foliated granodiorite. Geochronological and sedimentological data strongly suggest that the diamictite represents a lodgement till. This glaciogenic deposit is covered by a limestone succession which starts with impure carbonates showing aragonite pseudomorph fans and thin bands of black shale. The limestone pile grades to a marl-mudstone interval, which turns to a carbonate with biological components, succeeded by stromatolitic dolomite at the top. C and O isotopic signatures (referred to V-PDB) allow to the subdivision of the lower carbonate-pelite section into three intervals. The first isotopic interval corresponds to a cap carbonate, and displays negative values of δ13C (c . -4‰), and a large oscillation of the δ18O (-6 to -15‰). The Interval II shows a striking homogeneity in δ13C and δ18O, around 1‰ and -7‰, respectively. At the top, Interval III shows a large positive excursion of the δ13C (up to 8‰) and δ18O (-8 to -3‰) values. Unaltered 86Sr/87Sr ratios range from 0.7075 to 0.7077, mainly at the top of the section. The geochemistry of the carbonates is controlled by their terrigenous content (mostly quartz and clay minerals) which is concentrated in the lower units. Samples free of terrigenous contamination show Y/Ho ratios ranging from 25 to 50, suggesting a freshwater input during carbonate deposition. It is concluded that the diamictite has a glaciogenic origin and is covered by a cap carbonate. This pair has been identified along the basin and is related to one of the main Neoproterozoic glaciations. Discrepancy between the 86Sr/87Sr values and the global variation curves can be related to freshwater input during the carbonate deposition. Based on the regional tectonic context, the Bambuí Basin may have been a restricted marine basin, totally or partially surrounded by mountain ranges within Gondwana, in the Neoproterozoic/Paleozoic boundary. In its early stages, the sedimentation was influenced by a global glacial event, whose melting phase was responsible by freshwater input in the basin. The gradual rise of the temperature was followed by an increase of the biological activity. Finally, a sudden increase in the biological activity could have been driven by paleogeographic changes caused by the active tectonic.

Organic petrology and geochemistry of Eocene Suzak bituminous marl, north-central Afghanistan: Depositional environment and source rock potential

Organic geochemistry and petrology of Eocene Suzak bituminous marl outcrop samples from Madr village in north-central Afghanistan were characterized via an integrated analytical approach to evaluate depositional environment and source rock potential. Multiple proxies suggest the organic-rich (TOC ∼6 wt.%) bituminous marls are ‘immature’ for oil generation (e.g., vitrinite Ro < 0.4%, Tmax < 425 °C, PI ≤ 0.05, C29 ααα S/S + R ≤ 0.12, C29 ββS/ββS+ααR≤0.10, others), yet oil seeps are present at outcrop and live oil and abundant solid bitumen were observed via optical microscopy. Whole rock sulfur content is ∼2.3 wt.% whereas sulfur content is ∼5.0–5.6 wt.% in whole rock extracts with high polar components, consistent with extraction from S-rich Type IIs organic matter which could generate hydrocarbons at low thermal maturity. Low Fe-sulfide mineral abundance and comparison of Pr/Ph ratios between saturate and whole extracts suggest limited Fe concentration resulted in sulfurization of organic matter during early diagenesis. From these observations, we infer that a Type IIs kerogen in ‘immature’ bituminous marl at Madr could be generating high sulfur viscous oil which is seeping from outcrop. However, oil-seep samples were not collected for correlation studies. Aluminum-normalized trace element concentrations indicate enrichment of redox sensitive trace elements Mo, U and V and suggest anoxic-euxinic conditions during sediment deposition. The bulk of organic matter observed via optical microscopy is strongly fluorescent amorphous bituminite grading to lamalginite, possibly representing microbial mat facies. Short chain n-alkanes peak at C14–C16 (n-C17/n-C29 > 1) indicating organic input from marine algae and/or bacterial biomass, and sterane/hopane ratios are low (0.12–0.14). Monoaromatic steroids are dominated by C28 clearly indicating a marine setting. High gammacerane index values (∼0.9) are consistent with anoxia stratification and may indicate intermittent saline-hypersaline conditions. Stable C isotope ratios also suggest a marine depositional scenario for the Suzak samples, consistent with the presence of marine foraminifera including abundant planktic globigerinida(?) and rare benthic discocyclina(?) and nummulites(?). Biomarker 2α-methylhopane for photosynthetic cyanobacteria implies shallow photic zone deposition of Madr marls and 3β-methylhopane indicates presence of methanotrophic archaea in the microbial consortium. The data presented herein are consistent with deposition of Suzak bituminous marls in shallow stratified waters of a restricted marine basin associated with the southeastern incipient or proto-Paratethys. Geochemical proxies from Suzak rock extracts (S content, high polar content, C isotopes, normal (αααR) C27–29 steranes, and C29/C30 and C26/C25 hopane ratios) are similar to extant data from Paleogene oils produced to the north in the Afghan-Tajik Basin. This observation may indicate laterally equivalent strata are effective source rocks as suggested by previous workers; however, further work is needed to strengthen oil-source correlations.

Glacio-isostatic control on hypoxia in a high-latitude shelf basin

In high-latitude continental shelf environments, late Pleistocene glacial overdeepening and early Holocene eustatic sea-level rise combined to create restricted marine basins with a high vulnerability to oxygen depletion. Here we show that ongoing glacio-isostatic rebound during the Holocene may have played an important role in determining the distribution of past hypoxia in these environments by controlling the physical exchange of water masses and the distribution of large-scale phosphorus (P) sinks. We focus on the Baltic Sea, where sediment records from a large, presently oxic sub-basin show evidence for intense hypoxia and cyanobacteria blooms during the Holocene Thermal Maximum. Using paleobathymetric modeling, we show that this period was characterized by enhanced deep-water exchange, allowing widespread phosphorus regeneration. Intra-basin sills then shoaled over a period of several thousand years, enhancing P burial in one of the sub-basins. Together with climate forcing, this may have caused the termination of hypoxia throughout the Baltic Sea. Similar rearrangements of physical and chemical processes likely occurred in response to glacio-isostatic rebound in other high-latitude shelf basins during the Holocene.

The Geological Characteristics for Pre-Cambrian Shale Gas Accumulation in China

China has huge shale gas resources potential. The exploitation of shale gas can effectively reduce atmosphere pollution and relieve the energy shortage in China. Organic rich shale in pre-Cambrian period mainly lies to North China and Yangtze areas. The shales were developed in continental shelf and restricted marine basin. The distribution is stable with the thickness ranging from 10m-150m. The geochemistry characteristics are favored for shale gas accumulation for pre-Cambrian shale. The shale mineral is mainly quartz and feldspar. The pre-Cambrian formations have favored geological condition for shale gas accumulation. The favored intervals and regions for development of shale gas are Doushantuo shale in Upper Yangtze areas and Hongshuizhuang shale in northern Hebei Province or Jianchang Basin in western Liaoning Province.

Shale Gas Potential of the Major Marine Shale Formations in the Upper Yangtze Platform, South China, Part III: Mineralogical, Lithofacial, Petrophysical, and Rock Mechanical Properties

The marine black shale formations on the Upper Yangtze Platform, South China, are currently exploration targets for shale gas. Here, we report on the mineralogy, lithofacies, petrophysics, and rock mechanics of samples collected from the Ediacaran (Upper Sinian), Lower Cambrian, and Lower Silurian black shale intervals. All three formations are composed of high proportion of quartz, low content of clay, and rare or nonexistent content of carbonates. The Ediacaran and Lower Cambrian shales deposited in restricted deep water marine platform to marine basin environments are characterized by a higher quartz content and lower clay content than the Lower Silurian shales that were deposited in a more restricted marine basin environment. The carbonate content varies from 0 to over 50%, with the higher values measured in the Lower Silurian samples. These stratigraphic units were formed during bottom water anoxic conditions; therefore, they were rarely influenced by bioturbation. Lithologically, laminated and nonlaminated siliceous mudstones predominate, with minor contributions of other lithotypes. Pores generally have diameters in the nanometer (nm) to micrometer (μm) range, and numerous pores occur in organic matter. Most of the measured samples have porosities less than 4%, although a few samples show porosity in excess of 10%. Pores with radii less than 50 nm contribute significantly to total pore volume and total porosity. Permeability is extremely low, and helium permeability coefficients (Klinkenberg corrected permeability coefficient) are less than 20.2 nD (nano-Darcy, ∼2 × 10–20 m2). The rock mechanical properties of the samples are characterized by high brittle behavior, which coincides with their high compressive and tensile strengths and elastic properties. The Lower Cambrian shale is generally more brittle than the Lower Silurian shales, which possess a relatively higher content of clay minerals. The rock mechanical properties of the measured samples, however, depend on the overall mineral compositions and physical properties.

Geochemical characterization and redox signals from the latest Barremian to the earliest Aptian in a restricted marine basin: El Pui section, Organyà Basin, south-central Pyrenees

The Organyà Basin located in the south-central Spanish Pyrenees developed in the northern part of the Iberian Peninsula mainly during a Barremian to early Albian phase, when Iberia rifted and rotated counter-clockwise away from Europe. Extension was followed by strong subsidence in the basin resulting in the development and accumulation of carbonate platforms and deeper-water hemipelagic sediments.Here we present the multiproxy results of a high-resolution lithostratigraphic and geochemical study of the lowest 85m of the El Pui section in the Organyà Basin, Catalunya, Spain. The sequence comprises series of limestone and marlstone interbeds spanning the latest Barremian and the earliest Aptian. Our results show that intermittent dark limestone and marlstone layers associated with extremely low diversity and scarce benthic fauna, low bioturbation index (0–3) and high TOC (up to 1.7wt.%), indicate recurrent oxygen-deficient conditions within the lowest 31m of the section and more uniform oxygenation in the upper 54m. SEM analyses reveal high abundance of calcareous nannofossils as the source of carbonate throughout the section. EDS analyses confirmed the presence of clastics (mainly aluminum silicates) in the matrix. Sustained high primary productivity was the result of constant supply of nutrients, especially biolimiting elements (P, Fe), from terrestrial fluxes as indicated by excellent linear correlation with Al, Si and Ti. δ13Corg values within the high TOC intervals show excursions of up to 2.0‰ suggesting important isotopic changes in the carbon reservoir. Relative high enrichment of Ni, and minor of U and Cr with respect to the average shale concurrent with high Ni/Co ratios, and punctuated high U/Th values, suggest the existence of intermittent reducing benthic conditions in these intervals. However, V is depleted, indicating that reducing conditions were weak with the absence of an anoxic/euxinic phase.In the present study we document that basin physiography along with the nature of the adjacent geologic terranes surrounding the Organyà Basin induced sustained nutrient supply, thereby enhancing constant productivity, which intensified OM respiration in the water column. Density stratification may have also temporarily exacerbated oxygen deficiency (without an anoxic phase) that enhanced OM preservation from the late Barremian to the earliest Aptian.

Chronology of the Pueblo Viejo epithermal gold–silver deposit, Dominican Republic: formation in an Early Cretaceous intra-oceanic island arc and burial under ophiolite

The Pueblo Viejo deposit (production to 1996: 166 t Au, 760 t Ag) is located in the Dominican Republic on the Caribbean island of Hispaniola and ranks as one of the largest high-sulfidation/acid-sulfate epithermal deposits (reserves in 2007: 635 t Au, 3,648 t Ag). One of the advanced argillic ore bodies is cut by an inter-mineral andesite porphyry dike, which is altered to a retrograde chlorite–illite assemblage but overprinted by late-stage quartz–pyrite–sphalerite veins and associated low-grade Au, Ag, Zn, Cd, Hg, In, As, Se, and Te mineralization. The precise TIMS U–Pb age (109.6 ± 0.6 Ma) of the youngest zircon population in this dike confirms that the deposit is part of the Early Cretaceous Los Ranchos intra-oceanic island arc. Intrusion-related gold–sulfide mineralization took place during late andesite–dacite volcanism within a thick pile (>200 m) of carbonaceous sand- and siltstones deposited in a restricted marine basin. The high-level deposit was shielded from erosion after burial under a late Albian (109–100 Ma) ophiolite complex (8 km thick), which was in turn covered by the volcano-sedimentary successions (>4 km) of a Late Cretaceous–Early Tertiary calc-akaline magmatic arc. Estimates of stratigraphic thickness and published alunite, illite, and feldspar K-Ar ages and closure temperatures (alunite 270 ± 20°C, illite 260 ± 30°C, K-feldspar 150°C) indicate a burial depth of about 12 km at 80 Ma. During peak burial metamorphism (300°C and 300 MPa), the alteration assemblage kaolinite + quartz in the deposit dehydrated to pyrophyllite. Temperature–time relations imply that the Los Ranchos terrane then cooled at a rate of 3–4°C/Ma during slow uplift and erosion.

Spatial Distribution Patterns of Sulfur Isotopes, Nodular Carbonate, and Ore Textures in the McArthur River (HYC) Zn-Pb-Ag Deposit, Northern Territory, Australia

The HYC Zn-Pb-Ag deposit at McArthur River is the largest known and least deformed Australian sedimenthosted stratiform base metal deposit. A study of mineralogical, geochemical, and isotopic zonation through the deposit reveals concentric distribution patterns in (1) the occurrence of nodular carbonate, (2) the δ34S composition of sphalerite, and (3) microscopic ore textures. The correlation of previously documented lateral metal zonation with these other zonation patterns precludes exclusive postsedimentation mineralization and provides some insight into the mechanisms and controls on synsedimentary to early diagenetic mineralization. This mineralization timing is also supported by new carbon and oxygen isotope analyses, recognition of two stages of texturally and isotopically distinct sphalerite, and reinterpretation of complex sulfide textures. Dolomitic carbonate nodules occur in the south, southwest, northwest, and northern fringes of the deposit and are interpreted to have surrounded the high-grade ore lenses prior to structural and erosive truncation. The carbon and oxygen isotope composition of these carbonate nodules is similar to the sedimentary dolomite within the ore lenses. There is no evidence for incorporation of light carbon derived from oxidation of organic material under closed-system conditions and the nodular carbonate probably formed under open-system conditions in communication with seawater. These nodules displace siltstone laminae and, therefore, likely formed in the very shallow subsea-floor environment. In ore lens 3, the δ34S of laminated early sphalerite (sp1) changes from a mean value of 5 per mil in the deposit center to a mean of 1.7 per mil in the extreme fringes. We interpret this to be the result of sulfide precipitation in a restricted marine basin. Later sphalerite (sp2) associated with nodular carbonate has a mean δ34S value of 9.8 per mil, whereas laminated early sphalerite (sp1) has a mean δ34S value of 3.8 per mil. This isotopic separation of the two paragenetic stages of sphalerite is found in immediately adjacent aggregates. Concentration of 34S in late sphalerite (sp2) is likely the result of closed-system conditions in the sediment pore fluid. Textures of laminated sphalerite change from strongly anastomosing in the central part of the deposit to plane laminar and patchy in the deposit fringes. Mass-balance calculations preclude substantial carbonate dissolution and a stylolitic origin for these textures. Instead we propose that rapidly precipitated sphalerite coagulated and trapped pelagic silt and early pyrite (py1) as it was deposited on the sea floor. In contrast, paragenetically late sphalerite (sp2) and pyrite (py2) must be diagenetic as they overprint and are pseudomorphous after the carbonate nodules. We propose a repeating paragenetic sequence of galena/sphalerite (sp1) → pyrite (py1) → nodular carbonate → sphalerite (sp2) → pyrite (py2), which accounts for all the textural complexity and isotopic disequilibrium between sulfide phases. The data presented in this paper suggest that base metal sulfides formed both in the water column and in the uppermost sediment pile. Biological and thermochemical sulfate reduction probably occurred simultaneously in different parts of a complex physicochemical system in which stratification of the marine environment is seen as the primary control on the lateral distribution of the mineralized facies. We propose a stratified water body in which sharp internal chemical gradients separate a surficial oxic layer, an anoxic layer, and a basal hypersaline brine pool. Asymmetric metal zonation across the deposit reflects individual pulses of metalliferous fluid that were introduced into the basin as a bottom-hugging dense current.

U-Pb zircon and geochemical evidence for bimodal mid-Paleozoic magmatism and syngenetic base-metal mineralization in the Yukon-Tanana terrane, Alaska

New SHRIMP (sensitive, high-resolution ion microprobe) U-Pb zircon ages and trace element geochemical data for mafic and felsic metaigneous rocks of the pericratonic Yukon-Tanana terrane in east-central Alaska help define the tectonic setting of mid-Paleo-zoic magmatism and syngenetic hydrothermal Zn-Pb-Ag mineralization along the ancient Pacific margin of North America. We compare data from similar greenschist-facies sequences of bimodal volcanic and subvolcanic rocks associated with carbonaceous and siliciclastic marine sedimentary rocks, in the Wood River area of the Alaska Range and the Salcha River area of the Yukon-Tanana Upland, and from amphibolite-facies augen gneiss and mafic gneiss (amphibolite) in the Goodpaster River area of the upland. Allowing for analytical uncertainties, igneous crystallization age ranges of 376–353 Ma, 378–346 Ma, and 374–358 Ma are indicated by 13 new SHRIMP U-Pb dates for the Wood River, Salcha River, and Goodpaster River areas, respectively. Bimodal magmatism is indicated by Late Devonian crystallization ages for both augen gneiss (371 ± 3 and 362 ± 4 Ma) and associated orthoamphibolite (369 ± 3 Ma) in the upland and by stratigraphic interleaving of mafic and felsic rocks in the Alaska Range. Metabasites in all three study areas have elevated HFSE (high field strength element) and REE (rare earth element) contents indicative of generation in a within-plate (extensional) tectonic setting. Within-plate trace element signatures also are indicated for peralkaline metarhyolites that host the largest volcanogenic massive sulfide deposits of the Bonnifield district in the Wood River area and for metarhyolite tuff interlayered with the carbonaceous Nasina assemblage, which hosts sedimentary exhalative sulfide occurrences in the Salcha River area. Most of the other felsic metaigneous samples from the Alaska Range and the Yukon-Tanana Upland have geochemical signatures that are similar to those of both average upper continental crust and continental-margin arc rocks generated in thick continental crust. Given the absence in our study areas of intermediate-composition magmatic products generally found in most arcs, and the presence of bimodal magmatism, the alkalic within-plate chemistry of the mafic rocks and some of the felsic rocks, and the widespread occurrence of interlayered carbonaceous sedimentary rocks indicative of deposition within a restricted marine basin or submerged continental margin, we consider it most likely that this prolonged Late Devonian to Early Mississippian magmatic episode resulted from attenuation of the ancient continental margin of western North America, rather than development of an arc, as proposed by many others.

Mineralogy and geochemistry of early-formed deep marine dolomite in the Castagnola Formation (Oligocene-Miocene, Northern Italy)

A dolomitic bed of the Castagnola Formation (Aquitanian, Northern Italy) with on average 75 weight % dolomite is interbedded with marine siliciclastic turbidites and hemipelagites with very low dolomitic content. A detailed sampling of the bed (22 samples, 1 cm spacing) enabled the characterization of mineralogical and geochemical features of the dolomitic bed. The main mineralogical peculiarities of the dolomite within the studied bed lie in the cell parameters, in the low crystallographic ordering and in the thermal behaviour (TG, DTG, DTA) that strongly differ from well crystallized dolomites. The high contents of Fe, Na, Ba and Sr are also peculiar. The concentration of some accessory minerals is also anomalous for the Castagnola Formation basin, mainly because of the enrichments in Ca-phosphates and pyrite. Even though pyrite is abundant, the texture of framboidal aggregates indicates the oxic character of the water column, so that a post-depositional formation of sulphides can be supposed. The occurrence of diatom remnants and of high Si/Al ratio in some samples indicate variable carbonate/siliceous productivity, suggesting variations in the climate or water circulation, because the energy of the environment was rather constant, according to the grain size of detrital quartz and feldspars. Since the 87 Sr/ 86 Sr ratio of dolomite is typical of the Aquitanian seawater, the formation of dolomite long after sediment deposition during deep burial is unlikely, as well as direct precipitation, because the burrowing is pervasive. An early and rapid formation of dolomite after burial agrees with the small grain size of dolomite crystals and with the chemical and mineralogical features observed. Such dolomite crystallization is interpreted as the response of the system to a bloom in biogenic sedimentation; in this case the system is represented by early buried sediments, deposited in a restricted marine basin.

The transition from Tethys to the Himalaya as recorded in northwest Pakistan

Early Cenozoic sedimentary rocks exposed in the Kohat Plateau of northwestern Pakistan record tectonic closure of the Tethys sea and development of a restricted marine basin that formed during Himalayan collision between India, Asia, and a series of microplates. In the Paleocene, initial subsidence of the basin was caused by the downward deflection of the Indian plate in response to loading of the Asian plate. The western margin of the early Eocene basin was dominated by deposition of shale, sandstone, and conglomerate derived from microplates located to the north and west of the Indian continental margin. The eastern margin of the basin was a carbonate shelf and sabkha flat. Salt deposition occurred subaqueously in the central parts of the basin. In the late early Eocene, redbeds derived from the northwest were deposited by a fluvial and/or deltaic system. This influx of clastic sediments marks the earliest record of terrestrial foreland-basin deposition in northwest Pakistan. During the middle Eocene, the basin was reflooded and a carbonate shelf developed. Relative sea-level rise may reflect subsidence of the Indian plate in response to continued crustal loading in the Himalayan suture zone. Uplift and erosion occurred between the late Eocene and Miocene, possibly related to a peripheral bulge south of the Himalayan suture zone. The main phase of Himalayan foreland-basin fluvial deposition began in the Miocene. Renewed uplift related to final collision of India and Afghanistan during the Pliocene is recorded by a thick sequence of conglomerate in the western Kohat Plateau. Correlation with Eocene sedimentary rocks from southern Pakistan to northern India delineates the depositional systems that developed as the Tethys sea closed during Himalayan convergence.

Facies analysis and palaeogeography of the Rytteråker Formation (lower Silurian, Oslo region, Norway)

The Llandovery Rytteråker Formation is a carbonate-dominated lithologic unit which extends over the whole Oslo Region. Eight composite sections reveal five characteristic facies associations. These represent different, laterally coexisting depositional settings of an incipient, storm-dominated carbonate platform, which developed out from the transition zone between an open and a restricted marine basin. A mixed carbonate/siliciclastic shelf-lagoonal unit, a bioclastic shoal/barrier unit, a bioherm unit, a tidal delta/shallow shelf association and a mud-dominated deeper neritic unit are presented with their typical inventory of sedimentary structures. A possible tidal inlet fill sequence is reported from the Skien district where it is incised into bioclastic shoal deposits. Biostratigraphic correlation points to diachroneity of facies units and reveals their lateral and vertical association as being due to the migration of depositional belts. General palaeogeographic implications of the observed facies pattern are discussed and an attempt is made to place them into their global palaeoclimatic and geotectonic context.