Clastic Carbonate Research Articles

Syndeposition and epigenetic modification of the strata-bound Pb–Zn–Cu deposits associated with carbonate rocks in western Kunlun, Xinjiang, China

Lead–zinc–copper (Pb–Zn–Cu) deposits in the western Kunlun Orogen of China are mostly distributed along the Keziletao–Kusilafu fault zones in the Palaeozoic clastic–carbonate rocks, which formed in a late Palaeozoic subsidence belt at the south-west margin of the Tarim platform. Geological and geochemical studies were conducted on these deposits including detailed field observation, petrographic study, and geochemical and isotopic analyses of sulfur, lead, strontium, rhenium–osmium, and argon isotopes, as well as fluid inclusion geochemistry. The characteristics of the deposits include: (1) Except for the Tiekelike Pb–Cu deposit associated with mid-Devonian rocks, all of the other deposits were developed in the lower layers of the early Carboniferous strata. These strata mostly comprise sedimentary dolomitic limestone, sandy limestone, black limestone, and calcareous shale. (2) The ore bodies are stratiform and host veins. The stratiform ores are coeval with host rocks (yielding ore-forming ages of 331–337Ma), while the vein ore bodies are mostly massive deposits formed at 206–235Ma. The isotopic compositions of the sulfur and lead reveal that the ore-forming elements originated from various sources. (3) The mineralization in cements of brecciated ore is a veinlet type, while in the breccias it is a disseminated type. The stratoid, disseminated, and framboidal structures of the ore reveal the synsedimentary ore-formation. However, the veins, mesh-veins, and massive structures indicate late stage hydrothermal mineralization. (4) The principal metallic minerals include sphalerite, galena, and pyrite, while the secondary oxide minerals include variable distributions of cerussite, anglesite and limonite. The gangue minerals include dolomite, calcite, quartz, sericite, and gypsum, with minor chert and barite. (5) The original host rocks likely experienced dolomitization, calcitization, and minor silicification. The ore deposits underwent hydrothermal alterations, sub-greenschist facies metamorphism, and brecciation. The ore deposits may have initially formed via syndeposition in a subsidence belt along the southwest margin of the late Paleozoic Tarim platform; these deposits were further modified during later hydrothermal alteration and orogenesis during the Triassic period.

Calcretes and palustrine carbonates in the Oligo-Miocene clastic–carbonate unit of the Farafra Oasis, Western Desert, Egypt: Their origin and paleoenvironmental significance

The origin and paleoenvironmental significance of the calcretes and palustrine carbonates in the Oligo-Miocene clastic–carbonate unit that outcrops at the eastern sector of the Farafra Depression (Western Desert, Egypt) have been discussed based on field and petrographic investigations. The calcretes–palustrine carbonates assemblage occurs above a siliciclastic/distal alluvial-floodplain facies. The calcretes represent the transition from the underlying siliciclastic/distal alluvial-floodplain facies to the overlying palustrine carbonates. The calcrete-host rocks are muddy sandstones and sandy mudrocks.This study reveals the occurrence of groundwater calcretes with an upward gradational maturity pattern, ranging from incipient to nodular and to massive calcretes. The calcretes micromorphological analysis suggests that they were originated in vadose and phreatic diagenetic environments by groundwater through evaporation, degassing with no biological activity. The palustrine carbonates are also recognized above the calcrete horizons. They occur in the form of micritic limestones displaying different features that indicate their modifications during pedogenesis and subaerial exposure. These features include clotted-peloidal texture, fenestral fabric, mottling, pseudo-brecciation, desiccation cracks, pseudomicrokarst, root traces and silicification of the lime mud. The calcretes–palustrine carbonates assemblage records a progressive decrease in the terrigenous supply and a continuous rise of the groundwater table associated with local subsidence in a semi-arid to sub-humid climate. A model is suggested for the development of calcretes and palustrine carbonates in the study area.

Magnetic susceptibility logging of Chicxulub proximal impact breccias in the Santa Elena borehole: implications for emplacement mode

Magnetic susceptibility logging is used to study the impact breccias in the Chicxulub crater. The basic premise is that the high contrasts in magnetic properties can be used to characterize the breccias. The Santa Elena borehole was drilled 110 km radial distance from crater center and sampled a 172 m thick sequence of impact breccias, between 332 and 504 m depth. Breccia units are distinguished from differences in composition, size, and relative contents of clasts, type of matrix and textural and lithological assemblages, which can be resolved in the susceptibility logs. The whole-core log shows characteristic variation patterns with high, intermediate and low susceptibilities. High resolution logging of matrix and clasts records the heterogeneous nature of impactites, with higher variability at smaller spatial scales. Measurements confirm that diamagnetic susceptibilities characterize the carbonate clasts, high susceptibilities the basement granitic clasts and intermediate values the silicate melt-rich and silicate-poor matrix. Intermediate variable susceptibilities characterize breccias rich in melt particles. Correlation of matrix and clast logs with whole-core log shows that signal is controlled by the matrix. Logs for clast shows a discrete distribution with peaks of intermediate to high values, which correlate with large clast distributions. The ejecta blanket includes the fallback suevites rich in silicate melt particles and shocked minerals, the high temperature vapor deposits from ejecta curtain collapse and high velocity basal flows, and the carbonate rich deposits from lateral basal flows and secondary cratering. Late fallback suevites record minor turbulent conditions resulting from progressive cooling of the ejecta plume.

Evolution, paleoecology and sequence architecture of an Eocene carbonate ramp, southeast Zagros Basin, Iran

ABSTRACTWe document and discuss the results of detailed fieldwork, facies analysis and the subsequent integration of paleoecological evidence from the Middle Eocene carbonate ramp succession in the southeast Zagros Basin (Jahrum Formation). A combination of a sea-level fall and tectonic and diapiric basement uplift favored the initiation of the Jahrum carbonate platform. The lower portions are affected by pervasive, probably early diagenetic dolomitization, whilst the upper Jahrum consists mainly of limestone. Here, the focus is on the limestone portions of the Jahrum Formation. Based on the abundance, diversity and rapid evolutionary turnover of the alveolinids and nummulitids, the limestone intervals of the Jahrum Formation are interpreted for the upper Middle Eocene (Bartonian). The Jahrum Formation is capped by a major unconformity and overlain by the Lower Oligocene mixed clastic/carbonate Razak Formation.Based on data from field sections, eight facies associations and a series of sub-types have been established, which correspond to inner-, middle-and outer-ramp depositional environments. In their overall context, these data show a southward-dipping inner-ramp-to-basin transect. Towards the Coastal Fars (e.g. Hulur-01 Well) the Jahrum grades laterally into deep-marine Pabdeh foredeep shale units. Based on facies analysis and paleoecological evidence from larger benthic foraminifera, a major transgressive-regressive pattern is recognized in all outcrop sections of the Jahrum. The lowermost stratigraphic units of the formation are here interpreted as a distally steepened ramp. Evidence comes from abundant allochthonous shallow-water facies in the distal, deeper-ramp setting. Shallow-water carbonate clasts were exported towards the basin, a feature that is probably linked to relative sea-level fall control. Furthermore, local to regional basement instabilities by salt diapir-related basement reorganization was arguably of significance. Upsection, evidence is found that the ramp system evolved from a distally steepened to a homoclinal geometry with an overall very gentle slope geometry during the Late Bartonian.The data shown here are significant for those concerned with the Paleogene evolution of the southeast Zagros Basin and provide a well-exposed case example of a Middle Eocene carbonate ramp factory.

Frasnian–Famennian crisis in the Malaguide Complex (Betic Cordillera, Spain)

AbstractDevonian sediments of the Malaguide Complex potentially could include the Frasnian–Famennian boundary, one of the five greatest Phanerozoic biotic crises. Conodont biofacies and microfacies of carbonate clasts from a pebbly mudstone underlying Tournaisian radiolarites allows identification, for the first time in the Malaguide Complex, of Devonian shallow marine environments laterally grading to deeper realms. The clasts yielded Frasnian conodont associations of the falsiovalis to rhenana biozones, with six biofacies that reveal different environmental conditions in their source areas. Source sediments were dismantled and redeposited within the pebbly mudstone, whose origin is tentatively related to one of the events that are associated worldwide with the Frasnian–Famennian crisis. The latter is recorded, in two equivalent Malaguide pelagic successions, by stratigraphic discontinuities, and it was, probably, tectonically and/or eustatically controlled, as in other Alpine‐Mediterranean Paleotethyan margins.

Geology, mineralogy, and geochemistry of the Vazante Northern Extension zinc silicate deposit, Minas Gerais, Brazil

The Vazante Northern Extension orebodies (850kt at 16% Zn) are the continuation of the structurally controlled, hypogene, non-sulfide zinc orebodies of the Vazante deposit (18Mt at 19% Zn), Minas Gerais, Brazil. The Vazante orebodies are hosted in the dolomitized carbonate rocks of the Serra do Poço Verde Formation that were deposited in an intertidal to supratidal environment. The Serra do Poço Verde Formation is part of the Neoproterozoic Vazante Group, which is located in the Brasília Fold Belt along the western margin of the São Francisco Craton. The Northern Extension orebodies occur within major tectonic-hydrothermal breccias controlled by a NE-trending fault zone. The faults and the breccias are interpreted to be formed during the so-called D2 deformation event, which is related to the Late Proterozoic Brasiliano Orogeny. The faults were reactivated during their evolution and are cut by later transcurrent faulting.Four types of tectonic-hydrothermal breccia occur in the North Extension zone. Type 1 breccia is distal from the willemite ore and is comprised of weakly brecciated dolostone with Fe-carbonate alteration, whereas type 2 breccia exhibits strong Fe-bearing carbonate alteration (dolomite with 1.5 to 3.3wt.% FeO and 0.3 to 1.25wt.% ZnO) of the clasts, Fe-carbonate veinlets, a more intense brecciation of the clasts, and has overprinted the type 1 breccia. Type 3 breccia is composed mainly of hematite that replaced previously altered dolomite clasts and filled the breccia matrix. Type 4 breccia, which is typically proximal to type 3 breccia, is the willemite ore breccia which contains three generations of hypogene willemite as well as hematite, franklinite and minor amounts of quartz and ferroan (~1.99wt.% FeO) and zincian (~2.65wt.% ZnO) dolomite. Type 4 breccia commonly occurs in lenses surrounded by the type 3 hematite breccia, and willemite partially replaces hematite and altered carbonate clasts and fills breccia matrix and veinlets. In the southern part of the deposit, late sulfide-rich veinlets with galena, sphalerite, native silver, covellite, and stromeyerite cut the hypogene zinc silicate mineralization.Lithogeochemical analysis and mass-balance of the hydrothermal breccia were used to evaluate metals added during the hydrothermal period. The data show that the ore-related elements have been added to the system since the formation of the earliest breccia (Type 1 breccia). However, the type 3 breccia contains the highest average concentrations for As, Ba, Be, Bi, Fe, Ge, In, Mo, Ni, Sb, U, V, and W. Type 4 breccia is also enriched in the same elements and contains the highest average concentrations of Ag, Cu, Hg, Pb, S, Se, SiO2, and Zn. The evidence that the ore-related elements were added to the system during the formation of types 1 and 2 breccias, suggests that the metalliferous fluids were interacting with the host rocks, but were not able to effectively precipitate the ore-related elements. Important changes in the hydrothermal system would have increased its fugacity and pH allowing for formation, respectively, of type 3 hematite-rich breccia and type 4 willemite ore breccia, which are enriched in the other ore-related elements (e.g. As, Ba, Be, Bi, Cu, Fe, Ge, Hg, In, Mo, Ni, Pb, Sb, U, V, and W). These observations are consistent with a formation model proposed for the Vazante orebodies involving early interaction of metalliferous brines with the host rocks, followed by mixing of the metalliferous fluids with more oxidizing fluids, most likely meteoric water, leading to the precipitation of abundant hematite and willemite. Late, more reduced fluids formed the Pb, Cu and Ag-rich sulfide veinlets that cut the willemite mineralization.

Three‐Dimensional Investigation of Petrocalcic Materials: Insight into Pedogenic Processes and Future Applications

Computed tomography (CT) imaging was applied to Mormon Mesa petrocalcic and related materials to observe the location, arrangement, and interconnectedness of void spaces and to visualize density differences that may define pedogenic mineral relationships. Density variations visualized by these scans reveal features that may result from long developed pedogenic processes within these unique soil materials. Voids are present as cracks (shown to interconnect three‐dimensionally), elongated spaces filled with unconsolidated materials, and oblate‐spheroidal shapes that appear to be associated with detrital carbonate clasts. Density variations viewed through CT imaging may show the products of mineral neoformation, mineral packing differences during crystal growth, and/or older, denser petrocalcic materials eroded and incorporated via subsequent recementation (pisoliths). Computed tomography scanning can be used to understand unique forms and processes that are vital to our understanding of dissolution/precipitation and other pedogenic processes at work in these mature petrocalcic soils. We encourage the application of CT imaging to similar soil materials.

The relationship between carbonate facies, volcanic rocks and plant remains in a late Palaeozoic lacustrine system (San Ignacio Fm, Frontal Cordillera, San Juan province, Argentina)

The San Ignacio Fm, a late Palaeozoic foreland basin succession that crops out in the Frontal Cordillera (Argentinean Andes), contains lacustrine microbial carbonates and volcanic rocks. Modification by extensive pedogenic processes contributed to the massive aspect of the calcareous beds. Most of the volcanic deposits in the San Ignacio Fm consist of pyroclastic rocks and resedimented volcaniclastic deposits. Less frequent lava flows produced during effusive eruptions led to the generation of tabular layers of fine-grained, greenish or grey andesites, trachytes and dacites. Pyroclastic flow deposits correspond mainly to welded ignimbrites made up of former glassy pyroclasts devitrified to microcrystalline groundmass, scarce crystals of euhedral plagioclase, quartz and K-feldspar, opaque minerals, aggregates of fine-grained phyllosilicates and fiammes defining a bedding-parallel foliation generated by welding or diagenetic compaction. Widespread silicified and silica-permineralized plant remains and carbonate mud clasts are found, usually embedded within the ignimbrites. The carbonate sequences are underlain and overlain by volcanic rocks. The carbonate sequence bottoms are mostly gradational, while their tops are usually sharp. The lower part of the carbonate sequences is made up of mud which appear progressively, filling interstices in the top of the underlying volcanic rocks. They gradually become more abundant until they form the whole of the rock fabric. Carbonate on volcanic sandstones and pyroclastic deposits occur, with the nucleation of micritic carbonate and associated production of pyrite. Cyanobacteria, which formed the locus of mineral precipitation, were related with this nucleation. The growth of some of the algal mounds was halted by the progressive accumulation of volcanic ash particles, but in most cases the upper boundary is sharp and suddenly truncated by pyroclastic flows or volcanic avalanches. These pyroclastic flows partially destroyed the carbonate beds and palaeosols. Microbial carbonate clasts, silicified and silica-permineralized tree trunks, log stumps and other plant remains such as small branches and small roots inside pieces of wood (interpreted as fragments of nurse logs) are commonly found embedded within the ignimbrites. The study of the carbonate and volcanic rocks of the San Ignacio Fm allows the authors to propose a facies model that increases our understanding of lacustrine environments that developed in volcanic settings.

Sequence Stratigraphic Evolution and Cyclicity of An Ancient Coastal Desert System: The Pennsylvanian-Permian Lower Cutler Beds, Paradox Basin, Utah, U.S.A

Abstract The lower Cutler beds comprise a 200-m-thick succession of at least 12 repeating cyclic packages of strata, each 10–18 m thick, that are of mixed eolian, fluvial, and shallow marine origin. These accumulated during the latest Pennsylvanian to early Permian in the Paradox foreland basin of southeast Utah. Each depositional cycle comprises a lower unit composed of a bioclastic wackestone or a bioclastic sandy calcarenite facies, both of shallow marine origin. The upper units in each cycle are of nonmarine eolian (dune and interdune) and fluvial (channel and floodplain) origin. The mixed nonmarine and shallow marine nature of the inferred paleoenvironments of the lower Cutler beds is significant because the preserved expression of the nonmarine eolian and fluvial units in each cycle demonstrates an external climatic control on system development, whereas the marine units in each cycle (together with associated transgressive and regressive surfaces) demonstrate repeated episodes of relative sea-level change. Key stratigraphic surfaces have been traced from the continentally dominated northeastern portion of the basin into apparently contemporaneous, shallow-marine dominated sections in the southwest of the basin. Interdependent changes in both climate and relative sea-level change are shown to have generated the preserved cycles, which are interpreted to represent high-frequency sequences that are bounded by erosional unconformities (sequence boundaries) and paraconformities (diastems). Relative sea-level lowstand was coincident with climatic aridity at which time exposure of a broad, low-relief coastal plain enabled the generation of a substantial eolian sediment supply that was available for transport by a paleowind aligned parallel to the trend of the paleocoastline, resulting in dune-field construction and accumulation along a coastal belt. Episodes of relative sea-level rise were coincident with a transition to a more humid paleoclimate, during which the eolian sediment supply was greatly reduced, resulting in dune-field deflation and associated expansion of the fluvial system across the deflated former dune-field surface. Marine transgression occurred first in a series of estuarine embayments via the back-flooding of broad but shallow fluvially incised valley systems. Later transgression culminated in marine incursion over the former low-relief floodplain. Episodes of relative sea-level highstand were coincident with maximum climatic humidity. The high-frequency sequences make up a composite sequence set, which itself records a longer-term relative sea-level cycle. At this larger scale, a lowstand systems tract is recorded in the lower part of the studied lower Cutler beds succession, whereas a substantial transgressive systems tract is developed in the upper part. A regionally extensive maximum flooding surface defines the top of the lower Cutler beds. The preserved thickness and number of high-frequency cycles, together with their relation to the long-term subsidence trend of the Paradox Basin, records a periodic switching of both climate and relative sea level, the temporally linked nature of which demonstrates that glacio-eustasy exerted a dominant control on the origin of the preserved succession. The lower Cutler beds preserve evidence for the linkage and feedback between combined climatic and eustatic allogenic controls on sedimentation, and their role in generating high-frequency Milankovitch-style cycles in mixed clastic–carbonate successions.

The Miocene Sommières basin, SE France: Bioclastic carbonates in a tide-dominated depositional system

The Miocene Sommières Basin in SE France is a semi-enclosed depression that was connected to the Mediterranean Sea by a flooded paleo-incised valley and then filled by a suite of sediments comprising carbonate grains coming from temperate factories that were largely deposited in tidal-dominated paleoenvironments. The strata are partitioned into two sequences that reflect repeated flooding of the incised valley system, one of several similar situations in this region of France. The carbonate grains are mostly bioclasts, namely from barnacles, bryozoans, coralline algae (encrusting, branching, and rhodoliths), echinoids, and benthic foraminifers (large and small) with ostracods, sponge spicules and planktic foraminifers prominent in muddy facies. Particles were produced by shallow water carbonate factories on hard substrates (valley walls in particular), associated with subaqueous dunes, and in deeper water basinal settings. Each depositional sequence is underlain by an eroded and bored hard surface that is progressively overlain by TST subaqueous tidal dunes or storm deposits that grade up, in one case, into HST marls (the HST of the upper sequence has been removed by erosion). The lower sequence is ebb tide dominated whereas the upper sequence is flood tide dominated. The succession is interpreted to represent a TST whose tidal currents were focused by the narrow valley and a HST that reflected flooding of the overbanks. This stratigraphic and depositional motif is comparable to that in other spatially separated Neogene paleovalleys that are filled with tide-dominated clastic carbonates in the region. Together with other recently documented similar systems, these limestones constitute an important new group of carbonate sand bodies in the carbonate depositional realm.

Incised valleys and tidal seaways: the example of the Miocene Uzès-Castillon basin, SE France

Abstract The sedimentology and stratigraphy of the Miocene deposits of the Uzès-Castillon basin are revisited. This basin, located in SE France at the junction between the perialpine foreland basin and the W Mediterranean margin, sits in a syncline that formed during the latest Cretaceous Pyrenean tectonic phase. It records the succession of shallow-water mixed siliciclastic to dominantly bioclastic carbonates that alternate with shelf marls. The clastic carbonates were accumulated as a stack of subtidal dunes and bars that were formed by tidal currents channelized in a seaway following the syncline axis. The marls indicate deposition in more protected and locally deeper waters, as interfluves of the sea-way were drowned. Borehole data suggest that the marls are encased over tens of meters in the underlying bioclastic deposits, thus pointing to incised-valley fills. Contrarily to what is observed in the main Rhodanian basin, the molassic deposits are not restricted to transgressive systems tracts but may also correspond to forced regressive systems tracts. Four depositional sequences are identified, ranging from the Lower Burdigalian to the Langhian. They constitute a transgressive-regressive sequence set which might express the uplift of the area starting in the Late Burdigalian. This is consistent with the incision of the Middle Miocene deposits into the Lower Miocene ones as observed in other places of the main Rhodanian basin.

Exhumation of subcontinental mantle rocks: evidence from ultramafic-bearing clastic deposits nearby the Lherz peridotite body, French Pyrenees

Abstract In the eastern Pyrenees, the lherzolites always outcrop as restricted bodies scattered across Mesozoic sedimentary units along the North Pyrenean fault. In this article, we focus on the sedimentary processes associated with the tectonic exhumation of the mantle rocks during the Lower Cretaceous and we provide a new detailed analysis of the ultramafic-bearing sedimentary sequences associated with the Lherz body. The Lherz body, 1.5 km long only, belongs to a series of ultramafic bodies of restricted size (a few m to some hundred of m), occurring within sedimentary formations composed mostly of carbonate breccias originating from the reworking of Mesozoic platform limestones and dolomites. The carbonate clasts include marbles displaying different evolutions in relation with the Cretaceous thermal metamorphic event. The clastic formations also include numerous layers of polymictic breccias reworking ultramafic clasts. These layers are found far from any peridotitic body, implying that ultramafic clasts have been transported far away from their sources by sedimentary processes after their disaggregation on the seafloor. In addition, clasts of ultramafic-rich breccias and sandstones are also observed which indicates rapid sedimentation and diagenesis within a tectonically active environment. We discuss the significance of these deposits and their bearing on our understanding of the tectono-sedimentary processes associated with mantle exhumation in a context of development of narrow intracontinental basins suffering extreme crustal thinning.

Production Optimization in the Campos Basin: A Case for Robustness in Waterflooding

This article, written by Editorial Manager Adam Wilson, contains highlights of paper OTC 22560, ’Production Optimization in a Campos Basin Reservoir: A Case for Applying Robustness Measures to a Waterflood Project from Subsurface and Operational Design to Execution,’ by Ozan Arslan, SPE, Kyle Koerner, SPE, Stephen Knapp, Nate Biddle, Alwin Bok, SPE, and Paulo Chaves, BP, prepared for the 2011 Offshore Technology Conference Brasil, Rio de Janeiro, 4-6 October. The paper has not been peer reviewed. A successful cross-discipline workflow used in a Campos basin multiproducer-system waterflood program encompassed subsurface characterization, well planning, data acquisition, well controls, and surveillance to support efficient oil recovery. Drilling from a central platform, extended-reach wells were necessary to access a structured series of near-shore, laterally amalgamated channel sands separated by variable barriers posing significant subsurface-modeling, well-design, and drilling challenges. A cross-discipline-team approach employed robustness measures to find a near-optimal solution with controlled risk management. Introduction The Polvo complex is a system of clastic (Upper Cretaceous) and Macae carbonate (Lower Cretaceous) reservoirs in the Campos basin with 18–22°API oil. The clastics have been on production for 3 years with natural waterdrive and water injection. Development drilling started in mid-2008 for the Cretaceous sand system (CSS) where wells were completed with openhole gravel packs and electrical submersible pumps. Robust waterflood design and execution require a proactive, risk-minimizing reservoir-management strategy. The team used a cross-discipline workflow to achieve a near-optimal performance in the Polvo C_3 reservoir (Fig. 1). The workflow ensured the communication of constraints and raised potential decision points for Reservoir characterization Numerical modeling and uncertainty assessment Well planning (constraints and data acquisition) Well completion (integrity and sustained injectivity) Operational issues and surveillance tactics Remediation and optimization Robustness of proposed solutions is particularly critical in terms of reservoir characterization where certain geological outcomes, such as fault transmissibility or sand/sand connectivity, can be discrete in nature (sealing or not sealing). Therefore, capturing key geological assumptions in the CSS and communicating the critical decision points and possible outcomes across the team played pivotal roles in the workflow. Reservoir Characterization and Waterflood Design Detailed descriptions from core data indicated that channels and lagoons are the primary depositional environments for these reservoirs. Analysis of all the data collected on the CSS reservoirs suggested uniform layering, where each sand-prone layer exhibits a high degree of internal lateral continuity. Each sand-prone interval is capped by a shale layer interpreted as a sea-level rise shifting the reservoir facies northwest. The vertical rise in sea level coupled with the local dip on the Macae surface will control the lateral offset distance of reservoir facies from layer to layer, which, in turn, affects the local vertical communication between reservoir layers. The expectation is that there will be vertical permeability barriers, at least locally.

Spatio-temporal evolution of a Tertiary carbonate platform margin and adjacent basinal deposits

The variability in low to moderate energy carbonate platform margins is poorly known from the geological record. Here, the spatial and temporal evolution of platform margin and adjacent basinal deposits is evaluated from the little known Tertiary Kedango Limestone that developed in a semi-enclosed marine embayment in SE Asia. The hypothesis here is that platform margin development will reflect regional and perhaps global influences, such as tectonics, eustasy or biotic change, rather than windward-leeward effects and storms that typically impact strongly upon open oceanic platforms.The development of the carbonate platform was determined through logging, petrography, facies evaluation, provenance and high-resolution dating studies. Eleven carbonate facies were identified from the 30km long western margin of the >600m thick platform and its adjacent slope and basinal deposits. Larger benthic foraminifera and coralline algal packstones and wackestones dominated in shallow waters. During the Oligo-Miocene, coral patch reef-related floatstones, rudstones and less commonly boundstones were also present on the platform top. Perhaps surprisingly for a low energy platform there was considerable variation along the platform margin and much reworking of material into slope and basinal deposits during the Oligo-Miocene. Reworked material includes shallow water bioclasts, clasts from older siliciclastics, fresh feldspars, lithified slope and platform top carbonate clasts, some of the latter showing evidence for karstification. The western platform margin varied laterally over a few kilometres from a gently sloping unrimmed platform, to a probable bank top, with in places coral-fringed, bypass and erosional faulted escarpment margins. Eustasy may have influenced shallowing and deepening trends on the platform top, but apparently had little impact on mass wasting. Instead platform margin development was strongly impacted by tectonics (including active faulting), terrestrial runoff, together with changes in carbonate producers and local variability on the platform top.

Heating-induced changes in the anisotropy of magnetic susceptibility of impact breccias, Chicxulub Crater (Mexico)

Initial results of a thermal treatment study on the anisotropy of magnetic susceptibility (AMS) of impact breccias from Chicxulub crater are used to investigate the nature of the magnetic fabrics. Chicxulub impact breccias are heterogeneous materials, with carbonate, basement and melt clasts within carbonate-rich or melt-rich matrix. Samples studied come from the carbonate-rich basal unit Lower Suevite in the Yaxcopoil-1 borehole impactite sequence (core depth interval: 885–895 m). The Lower Suevite is characterized by mixed prolate and oblate ellipsoids with shallow to steep principal susceptibility axes, which had been related to emplacement as an excavation flow with ground-surge components during the early cratering stages. Thermal treatment results in changes in the fabrics with a tendency to oblate fabrics. Stepwise thermal treatment up to 700°C reveals different behaviors for the oblate, neutral and prolate fabrics marked by changes in AMS parameters and principal susceptibility axis orientations. A sample with oblate fabrics and vertical minimum axes showed an increase of magnetic susceptibility at high temperatures, indicating formation of secondary magnetite and fabric enhancement. A sample with neutral ellipsoid showed heating-induced changes towards oblate fabrics and vertical minimum susceptibility axes. Samples characterized by prolate ellipsoids with horizontal maximum axes showed no directional changes. In a sample with apparent intermediate or inverse fabrics, vertical maximum axes showed changes to horizontal inclinations, with the intermediate and maximum axes switching positions. Changes induced by stepwise thermal treatment appear useful to characterize the fabrics of impact lithologies. Further investigation of heating-induced effects in mineralogy, grain size and textural changes is, however, required to relate the different behaviors observed after stepwise thermal treatment with the magnetic mineralogy and emplacement mode of the breccias.

A syn‐depositional age for Earth’s deepest δ13C excursion required by isotope conglomerate tests

Terra Nova, 24, 318–325, 2012AbstractThe most negative carbon isotope excursion in Earth history is found in carbonate rocks of the Ediacaran Period (635–542 Ma). Workers have interpreted the event as the oxidation of the Ediacaran oceans [Rothman et al., Proc. Natl. Acad. Sci. USA100 (2003) 8124; Fike et al., Nature444 (2006) 744; McFadden et al., Proc. Natl. Acad. Sci. USA105 (2008) 3197], or as diagenetic alteration of the δ13C of carbonates (δ13Ccarb) [Knauth and Kennedy, Nature460 (2009) 728; Derry, Earth Planet. Sci. Lett.294 (2010) 152]. Here, we present chemo‐stratigraphic data from the Ediacaran‐aged Wonoka Formation (Fm.) of South Australia that require a syn‐depositional age for the extraordinary range of δ13Ccarb values (−12 to +4‰) observed in the formation. In some locations, the Wonoka Fm. is 700 metres (m) of mixed shelf limestones and siliclastics that record the full 16‰δ13Ccarb excursion. In other places, the Wonoka Fm. is host to deep (∼1 km) palaeocanyons, which are partly filled by tabular‐clast carbonate breccias that are sourced from eroded Wonoka canyon‐shoulders. By measuring the isotopic values of 485 carbonate clasts (an isotope conglomerate test), we show that canyon‐shoulder carbonates acquired their δ13Ccarb–δ18Ocarb values before brecciation and redeposition in the palaeocanyons.

Diagenetic history of the Surma Group sandstones (Miocene) in the Surma Basin, Bangladesh

This study examines the various diagenetic controls of the Miocene Surma Group sandstones encountered in petroleum exploration wells from the Surma Basin, which is situated in the northeastern part of the Bengal Basin, Bangladesh. The principal diagenetic minerals/cements in the Surma Group sandstones are Fe-carbonates (with Fe-calcite dominating), quartz overgrowths and authigenic clays (predominantly chlorite, illite–smectite and minor kaolin). The isotopic composition of the carbonate cement revealed a narrow range of δ 18O values (−10.3‰ to −12.4‰) and a wide range of δ 13C value (+1.4‰ to −23.1‰). The δ 13C VPDB and δ 18O VPDB values of the carbonate cements reveal that carbon was most likely derived from the thermal maturation of organic matter during burial, as well as from the dissolution of isolated carbonate clasts and precipitated from mixed marine–meteoric pore waters. The relationship between the intergranular volume (IGV) versus cement volume indicates that compaction played a more significant role than cementation in destroying the primary porosity. However, cementation also played a major role in drastically reducing porosity and permeability in sandstones with poikilotopic, pore-filling blocky cements formed in early to intermediate and deep burial areas. In addition to Fe-carbonate cements, various clay minerals including illite–smectite and chlorite occur as pore-filling and pore-lining authigenic phases. Significant secondary porosity has been generated at depths from 2500 m to 4728 m. The best reservoir rocks found at depths of 2500–3300 m are well sorted, relatively coarse grained; more loosely packed and better rounded sandstones having good porosities (20–30%) and high permeabilities (12–6000 mD). These good quality reservoir rocks are, however, not uniformly distributed and can be considered to be compartmentalized as a result of interbedding with sandstone layers of low to moderate porosities, low permeabilities owing to poor sorting and extensive compaction and cementation.

Thermomagnetic Analyses of the Permeability-Controlling Minerals in Red and White Sandstones in Deep Tight Gas Reservoirs: Implications for Downhole Measurements

Summary Our recent work on deep tight gas reservoirs containing red and white sandstones (Potter et al. 2009) has suggested that the presence of small amounts of hematite in reservoir samples can have a dramatic effect on permeability. Such conclusions were made using laboratory-based low- and high-field magnetic-susceptibility measurements on reservoir-rock samples and by comparing these measurements with the permeability data. These rapid, nondestructive magnetic measurements have been applied previously in clastic reservoir samples (Potter 2007; (Ivakhnenko 2006; Ivakhnenko and Potter 2008; Potter and Ivakhnenko 2008) and carbonate reservoir samples (Al-Ghamdi 2006; Potter et al. 2011). However, such laboratory-based analyses are not representative of the downhole in-situ conditions, especially in deep gas reservoirs where the temperature can reach quite high values. Typical tight-gas-reservoir depths can reach approximately 4000 m (Abu-Shanab et al. 2005) and 6000 m (Tang et al. 2008), and the equivalent temperatures would measure 131 and 192°C, respectively, if one assumes the normal geothermal gradient (Mayer-Gurr 1976). This paper investigates the in-situ magnetic properties of deep tight gas reservoir samples (containing permeability-controlling reservoir minerals hematite and illite) by means of laboratory experiments to model downhole temperature conditions. We perform magnetic hysteresis measurements at various temperatures in order to identify and quantify mineralogy and model changes in the magnetic behavior of these minerals at in-situ downhole conditions. From these measurements, we are able to show whether the mineralogy or domain state of the permeability-controlling minerals is likely to change with temperature in deep gas reservoirs. These changes can potentially have a major effect on permeability. We also demonstrate that there are strong correlations between core-permeability and magnetic-susceptibility data in these tight-gas-reservoir samples. The permeability is low in red sections of the core wherever there is hematite.

Cenozoic anatexis and exhumation of Tethyan Sequence rocks in the Xiao Gurla Range, Southwest Tibet

In order to advance our understanding of the suturing process between continental landmasses, a geologic and geochronologic investigation was undertaken just south of the India–Asia suture in southwestern Tibet. The focus of this study, the Xiao Gurla Range, is located near the southeastern terminus of the active, right-lateral strike-slip Karakoram fault in southwestern Tibet. The range exposes metasandstone, phyllite, schist (locally of sillimanite facies), calc-gneiss and marble, paragneiss (± pyroxene), quartzite, metagranite, and variably deformed leucogranite. These metamorphic rocks are exposed in the footwall of a domal, top-to-the-west low-angle normal (detachment) fault, structurally beneath Neogene–Quaternary basin fill and serpentinized ultramafic rocks of the Kiogar-Jungbwa ophiolite. The detachment is interpreted to be the northeastern continuation of the Gurla Mandhata detachment fault system that bounds metamorphic rocks of the Gurla Mandhata Range ~ 60 km to the southwest. U–Pb geochronology on five detrital zircon samples of schist, phyllite, and quartzite yielded maximum depositional ages that range from 644–363 Ma and age probability distributions that are more similar to Tethyan sequence rocks than Lesser Himalayan sequence rocks. A felsic gneiss yielded a metamorphic zircon age of 35.3 ± 0.8 Ma with a significant population of early Paleozoic xenocrystic core ages. The gneiss is interpreted to be the metamorphosed equivalent of the Cambro-Ordovician gneiss that is exposed near the top of the Greater Himalayan sequence. Leucogranitic bodies intruding metasedimentary footwall rocks yielded two distinct U–Pb zircon ages of ~ 23 Ma and ~ 15 Ma. Locally, rocks exposed in the hanging wall of this fault and of the southward-dipping, northward-verging Great Counter thrust to the north consist of serpentinite-bearing mélange and conglomerate of inferred Paleogene age dominated by carbonate clasts. The mélange is intruded by a 44 Ma granite and the stratigraphically highest conglomerate unit yielded detrital zircon U–Pb ages similar to Tethyan sequence rocks. We attribute the middle Eocene magmatism south of the suture to break-off of the Neo-Tethyan oceanic slab. In addition, our observations are consistent with the late Eocene shortening and crustal thickening within the Tethyan Himalayan sequence, early-middle Miocene leucogranite emplacement being related to underthrusting and melting of the Greater and possibly Lesser Himalayan sequences, and late Miocene arc-parallel extension in the hinterland of the southward propagating Himalayan thrust belt.

Basin architecture and growth folding of the NW Zagros early foreland basin during the Late Cretaceous and early Tertiary

Abstract: We present and use the chronostratigraphy of 13 field logs and detailed mapping to constrain the evolution of the early Zagros foreland basin, in NW Iran. Large foraminifera, calcareous nannofossil, palynological and 87 Sr/ 86 Sr analysis supplied ages indicating a Campanian–early Eocene age of the basin infill, which is characterizd by a diachronous, southwestward migrating, shallowing upwards, mixed clastic–carbonate succession. Growth synclines and local palaeoslope variations indicate syndepositional folding from Maastrichtian to Eocene time and suggest forelandward migration of the deformation front. We also illustrate the basin architecture with a synthetic stratigraphic transect. From internal to external areas, time lines cross the formation boundaries from continental Kashkan red beds to Taleh Zang mixed clastic–carbonate platforms, Amiran slope deposits and basinal Gurpi–Pabdeh shales and marls. The foreland basin depocentres show a progressive migration from the Campanian to Eocene ( c . 83–52.7 Ma), with rates of c . 2.4 mm a −1 during the early–middle Palaeocene ( c . 65.5–58.7 Ma) increasing to c . 6 mm a −1 during the late Palaeocene–earliest Eocene ( c . 58.7–52.8 Ma). Coeval subsidence remained at c . 0.27 mm a −1 during the first 12.7 Ma and decreased to c . 0.16 mm a −1 during the last 4.2 Ma of basin filling. Finally, we integrate our results with published large-scale maps and discuss their implications in the context of the Zagros orogeny. Supplementary material: Tables with dating results are available at http://www.geolsoc.org.uk/SUP18439 .