Mixed Carbonate-siliciclastic Platform Research Articles

Biostratigraphy and facies around the D/C boundary interval of the Tuye-Darvar section, Eastern Alborz Range, NE Iran

The 1st order mass extinction at the Devonian/Carboniferous transition, known as the Hangenberg Crisis, is characterized by major transgressive/regressive cycles which led to widespread ocean anoxia during the Hangenberg Black Shale Event, as well as to a global major sea-level fall and the worldwide deposition of regressive Hangenberg Sandstone equivalents. The Devonian/Carboniferous transition at the Tuye-Darvar section in the eastern Alborz Range is studied in terms of conodont biostratigraphy, litho-, microfacies and sequence stratigraphy. In order to examine the biostratigraphical framework, forty conodont samples were systematically taken from the studied interval. Thirty-two conodont species belonging to ten genera led to the discrimination of twelve conodont zones, ranging from the Pseudopolygnathus granulosus Zone to the Scaliognathus anchoralis-Doliognathus latus Zone. Due to facies, the conodont record also exhibits some hiatuses. Field observations and sedimentological and microfacies studies led to the identification of thirteen facies types from sub-tidal environments to the fully marine environments, including seven microfacies types. The sediments deposited in a mixed carbonate-siliciclastic platform, revealed four third-order sequences. The Hangenberg Black Shale is not recorded in the Tuye-Darvar section as a result of depositional facies. In comparison with other studied Devonian/Carboniferous Boundary (DCB) sections of the central and northern Iran, the Tuye-Davar section suggests a tectonic position, which is most likely placed on a separate tectonic block.

Facies and stratigraphy of the basal Ediacaran cap carbonate, Naukluft Mountains, Namibia

The termination of the Marinoan Snowball Earth glaciation constitutes a dramatic interval of climate change. This field study seeks to investigate this climate transition by characterizing the basal-Ediacaran cap carbonate succession of the Tsabisis Formation overlying the Blässkranz Formation glacial deposits exposed in the Naukluft Mountains of Namibia by using sedimentologic and stratigraphic field observations supplemented by chemostratigraphic analyses. Measured sections and lateral bed/unit tracing delineate stratigraphic stacking patterns and regional facies changes. In downdip areas Marinoan diamictite transitions upward into dolostone intermixed with sandstone and extrabasinal clasts that is gradually overlain by fine grained laminated dolostone. Updip localities show the diamictite is overlain by intercalated sandstones, gravels, and shales before an abrupt change to laminated dolostone of the cap carbonate. A succession of stromatolites, which become strongly elongate upward, prograde into the laminated dolostone in the updip localities. The stromatolites are overlain by laminated dolostone that grades upward into rhythmite with intercalations of shale. Near the top of the cap, rhythmites may be reworked into tabular intraclast conglomerate, locally intercalated with hummocky cross stratified sandstone, which passes upward into the shale and limestone members of the Tsabisis Formation. The Tsabisis Formation is overlain by the Noab Formation, dominated by sedimentary breccia, laminated dolostone, and sandstone. The lateral and vertical distribution of facies indicate a retreat of the shoreline and glacially sourced siliciclastics near the base of the cap carbonate, a shallowing succession to fair-weather wave base at the top of the stromatolite facies, and a second shallowing succession to storm wave base near the top of the cap carbonate. Maximum flooding occurred soon after the initiation of carbonate deposition and two sequence boundaries mark higher stratigraphic levels within the cap carbonate. The depositional patterns of the Tsabisis Formation cap carbonate resemble that of other Precambrian mixed carbonate-siliciclastic platforms and shows multiple base-level fluctuations that could be attributed to a range of mechanisms. Some of the base-level changes may have been directly coupled to events driven by deglaciation but distinguishing these from conventional controls on accommodation space in the rock record is challenging and multiple depositional models are feasible.

Stratigraphy, isotope geochemistry, seismic stratigraphy and paleogeography of the Lagoa do Jacaré Formation, Bambuí Foreland Basin (Ediacaran-Cambrian), Southeast Brazil

The Bambuí Group is a lithostratigraphic unit that comprises a mixed carbonate-siliciclastic platform deposited in a foreland basin. It is a 1st order sequence that filled the São Francisco Basin during the Ediacaran-Cambrian in southeast Brazil. The Lagoa do Jacaré Formation is the second carbonate unit of the Bambuí Group and is related to a 2nd-order regressive hemicycle composed of dark gray shallow water carbonates with high positive ẟ1³C values (>+10‰) related to the upper part of the Middle Bambuí Excursion (MIBE). We described four outcrops in the center-north of Minas Gerais state (Paraopeba, Curvelo, Montes Claros and Ubaí areas) coupled with ẟ13C and ẟ18O, and SEM-EDS analysis in order to support detailed facies analyses. One seismic line and three well logs in the south part of the Januária High were also used to define the stratigraphic framework of the Lagoa do Jacaré Formation in the Bambuí Foreland Basin system. Eight facies were interpreted from the outcrop data and grouped into five facies associations, that represent peritidal zone (FA1), ooidal sand shoals (FA5), tempestites of mid ramp (FA3), fine-grained carbonate of outer ramp (FA4), and fine-grained siliciclastic from offshore (FA2). The strong presence of terrigenous material (quartz, feldspars, muscovite, biotite, apatite) with some authigenic/diagenetic minerals (pyrite, titanite, rutile) in all facies are the main petrographic aspect of the studied carbonate rocks, related to high sedimentary input and some events during early diagenesis. FA1 with lower mean values of ẟ1³C (+9‰) predominates in the north, meanwhile FA3 is the main association in the south with higher ẟ1³C mean values (+12‰). This difference is probably related to the stratigraphic differences between the basal section (at south) and upper section (at north) of the Lagoa do Jacaré Formation. The Januária High was the main controlling element for the deposition of the carbonate ramp in the north sector of the basin, close to the Ubaí and Montes Claros cities. Probably, this structural high also played an important role as source area in the forebulge sector, uplifted by flexure due to the orogeny of the Brasília and Araçuaí (still in development) Belts (∼540-520 Ma). To the east, a N–S carbonate ramp system was developed in the back-bulge sector according to the main paleocurrents measurements. This ramp is related to a 300 m thickness section of the Lagoa do Jacaré Formation that was firstly identified and correlated to the successions on the surface using well logs, possibly registering at the top the decreasing in the ẟ1³C values, after the main stages of the MIBE probably in close association with Ediacaran-Cambrian boundary, related to the assembling of the Gondwana Supercontinent recorded in this region of southeast Brazil.

Stratigraphy and depositional environment of a mixed siliciclastic-carbonate platform and slope succession of the Paleogene Nisai Group, Pakistan

We document the stratigraphy, lithofacies and depositional environment of the lower Eocene-lower Oligocene Nisai Group, based on petrographic and micropaleontological analysis, and field observations.We rename the whole sedimentary succession as the Nisai Group and divide it into three formations, from bottom to top; Jabrai, Torkhezhe Tsah and Nisai formations. The Jabrai Formation comprises a thick succession of olive gray mudstone and shales with subordinate sandstone, limestone and quartzose conglomerate, interpreted as deltaic and littoral deposits. The Torkhezhe Tsah Formation comprises calciturbidites in its lower part and periplatform oozes and basinal shales in upper part. The shallow-marine carbonate material of calciturbidites possibly derived from a lost platform which was a narrow delta-top-platform developed over deltaic sediments of the Jabrai Formation. The uppermost unit, the Nisai Formation, comprises thick bedded limestone with subordinate shale and nodular reefoid limestone, and shows a shallowing-up succession overlying the slope and basinal deposits of the Torkhezhe Tsah Formation. The presence of Cr-rich spinels in sandstone of the Jabrai Formation, ultramafic-mafic and quartzite lithoclasts in calciturbidite beds as well as paleocurrent data, suggest that siliciclastic sediments of the Nisai Group derived from the Zhob Valley Ophiolites. Moreover, the presence of quartz, igneous and metamorphic extraclasts, and dolomitic horizons within calciturbidite beds, indicate deposition of these calciturbidites in response to lowering of sea level.We propose that emplacement of Zhob Valley Ophiolites served as a drainage divide and supplied deltaic sediments to the Jabrai Formation to its north and the Ghazij Group to the south.

Kilometre-scale coral carpets on mixed carbonate-siliciclastic platforms; a sedimentological study from the Lower Cretaceous of northwestern Africa

Coral carpets are biostromes that form laterally continuous but low-relief (typically less than 1-metre-high) coral communities with a lack of clear internal zonation. There are few locations that allow analysis of such systems on a regional (tens of kilometre) scale in the rock record. This study describes extensive, well-exposed, coral-rich bodies (coral carpets), deposited on the Moroccan northwest Atlantic shallow-marine margin during the Cretaceous (early Hauterivian). The coral-rich sequence reaches 50 m in thickness, with individual carpets between 10 and 100 cm thick, extending over an area of more than 150 km 2 . Logging and regional correlation, integrated with thin-section analysis, has allowed the evaluation of facies distribution, geometries and temporal evolution. The results provide information on allogenic and/or autogenic driving processes for coral development, and allows the generation of conceptual models for their growth. The Tamanar Fm. corals reflects a shallowing up sequence and the recovery of carbonate productivity following stressed environmental conditions in the late Valanginian. Temporally, coral morphological response relates to high-frequency relative sea level changes that affected water energy and turbidity; thick-branching and massive forms grew in shallow high energy environments whilst platy/flat forms grew in poorly lit and turbid environments. However, local-scale (over hundreds of metres) lateral changes in morphology and facies can be explained by autogenic factors independent of sea level change; varying sedimentation, hydrodynamism and turbidity across the extensive open marine platform. The termination of coral deposition is marked by a hardground surface, indicating slow rates of sedimentation and a transgression, which is followed by an influx of clastic sediments onto the platform, resulting in the deterioration of carbonate platform health, as conditions became intolerable for coral growth. These coral-rich outcrops contribute to the relatively sparse documentation of Hauterivian corals globally and suggest niche palaeoecological and palaeoclimatic relationships that allowed corals to develop despite stressed platform conditions. The low diversity, low-relief coral carpets may provide a proxy for the recovery and demise of corals in response to environmental stress on modern carbonate platforms and future climate change projections. • Kilometre-scale coral-rich outcrops onshore the Moroccan Atlantic Margin. • Non-framework coral carpets show no internal zonation. • Coral morphology and abundance controlled chiefly by water energy and turbidity. • Establishment and demise of corals linked to sea level and climatic change. • Low diversity corals possible proxy for recovery/loss in response to environmental stress.

The paleoecology and depositional model of the Oligo-Miocene coral reefs in the Mohammad Abad Area, the west of Baft, SW Kerman, Iran

This paper deals with the Oligo-Miocene coral reefs in the Mohammadabad Area, W Baft, SW Kerman. The studied coral reefs include 11 families, 25 genera and 33 species of the scleractinian corals. The coral reef successions have deposited in three cycles on a mixed siliciclastic-carbonate platform. To understanding the paleoecology, reef architecture and depositional model of studied reefal successions, statistical analyses based on the coral niches, sedimentology analyses based on the grain size of the marly strata and microfacies analyses on limestone strata have used. Based on the statistical analyses, the coral community includes eight ecotypes. The distribution of these ecotypes indicates that, despite of various thicknesses, in all three coral reef successions, the branching and fast growth ecotypes are present at the basal parts and the massive, placoid and meandroid colonies are recorded in the slope, crest and distal parts of the reefs. The main disruptive phenomenon that stopped the reef development and affected the reef architecture has been the addition of clastic deposits to the carbonate system. This sediment load increase is recorded as marly strata. By The addition of clastic sediment load to the system, the carbonate content of the system has diluted, the salinity, water transparency and photic level is decreased, the favorable attachment hard substrate for coral polyps is limited, the nutrient and oxygen flux is disrupted and the coral reefs are suffocated.

The Siliciclastic Content Controls the Sealing Behavior of Layers and Cave Development in a Mixed Carbonate-Siliciclastic Platform, Salitre Formation, Brazil

The Siliciclastic Content Controls the Sealing Behavior of Layers and Cave Development in a Mixed Carbonate-Siliciclastic Platform, Salitre Formation, Brazil

Los mármoles cálcicos de El Escorial (Complejo Metamórfico Cushamen, Macizo Norpatagónico): caracterización isotópica de 87Sr-86Sr y edad de sedimentación

The El Escorial marbles (Cushamen Metamorphic Complex) along with amphibolites form metamorphic septa within the permian granitoids of the Mamil Choique Formation (261-286 Ma). The metamorphism, determined in granulite facies migmatic gneisses septa cropping out 120 km southwest of El Escorial, occurs at 311 ± 27 Ma (CHIME method in monazite). The marbles are calcitic (calcite > 95%, R.I.: 0.5 to 2.5%) and show 87Sr/86Sr values between 0.70768 and 0.70825 (n = 10). The data provided in this work, added to previous contributions, allow to constraints the sedimentation age of the silicic-carbonate successions of the Cushamen Metamorphic Complex between ca. 385 and 335 Ma. This suggests the existence of a mixed carbonate-siliciclastic platform at least in the southernmost portion of southwestern Gondwana between Middle Devonian and early Carboniferous (Middle Mississippian).

Sequence stratigraphy, depositional setting and evolution of the Fahliyan carbonate platform (Zagros fold-thrust belt, SW Iran) in the Early Cretaceous

A regional sequence-stratigraphic framework is proposed for the Berriasian-Barremian shallow platform succession in the Zagros Fold-Thrust Belt (FTB), based on outcrop sections, in combination with subsurface data. The time framework is supported by new biostratigraphic results, using benthic foraminifera, dasycladalean algae and tintinnids, calibrated by Sr-isotope data. We identified and described four large-scale sequences and sixteen small-scale sequences throughout the studied succession. Large-scale sequences occur in the Berriasian-earliest Valanginian, Valanginian, Hauterivian and early to late Barremian. Based on facies associations, a relative water depth has been assigned to distinct depositional environments. Four successive architectures were observed in the platform growth: rimmed platform during the early-middle Berriasian, isolated rimmed platform in the middle Berriasian-earliest Valanginian, non-rimmed open platform to ramp in the Valanginian, and finally a mixed siliciclastic-carbonate platform to ramp in the Hauterivian-Barremian.

Facies analysis of the Upper Ordovician Xiazhen Formation, southeast China: Implications for carbonate platform development in South China prior to the onset of the Hirnantian glaciation

The Upper Ordovician Zhe-Gan Platform was a short-lived carbonate platform that formed near the northern margin of the Cathaysian Land of the South China Block. The Zhe-Gan Platform is less well understood than the adjacent Yangtze Platform due to the absence of detailed sedimentological study as well as the structural complexity of the area. The aim of this study was to reconstruct the Zhe-Gan Platform based on detailed facies analysis of the middle to upper Xiazhen Formation. A total of 24 shallowing-upward depositional cycles (C1–24) are identified and subdivided into three types based on the vertical association of facies: (1) mixed carbonate–clastic subtidal cycles; (2) carbonate subtidal cycles; and (3) peritidal-capped subtidal cycles. The reconstructed depositional model indicates a ramp-type, mixed siliciclastic–carbonate platform, generally similar to other Late Ordovician carbonate platforms characterised by dominant skeletal grains, less common ooids, and an absence of skeletal barrier reefs. The newly developed Late Ordovician ramp-type carbonate platforms could have been induced by the evolution of skeletal organisms and the accompanying palaeoceanographic changes prior to the Hirnantian glaciation. The complex palaeogeography of the South China Block would have influenced the co-occurrence of the Xiazhen carbonate platform and black shales of the adjacent Yangtze Platform on the same block during the Late Ordovician.

Permian mixed carbonate–siliciclastic lagoon coastal system in West-Central Gondwana

Although in recent years mixed carbonate–siliciclastic platforms have been increasingly recognized in the Permian of Western Gondwana, there is no detailed description of these mixed systems in West-Central Gondwana. This study focuses on the Permian of the Sergipe–Alagoas Basin (NE Brazil), with the aim of filling this knowledge gap. Based on sedimentological data and microfacies analysis, six facies associations (FA) were identified: FA1 is an eolian–fluvial system, composed of stratified sandstones, FA2 and FA3 represent a mixed tidal flat–lagoon system composed of microbialites, pelitic/micritic sediments and intraclastic packstones, FA4 represents shoreface wave deposits composed of laminated sandstones and silicified grainstones, and FA5 and FA6 are interpreted as a barrier–tidal inlet system, composed of oolitic/intraclastic grainstones, subtidal microbialites, and mixed rudstones. Facies associations reflect the paleoenvironmental conditions of a coastal environment, with dunes and small rivers adjacent to a mixed carbonate–siliciclastic ramp. This is defined as a lagoon system associated with tidal flats protected from the high-energy region by carbonate shoals and microbial constructions. These deposits represent the eastern portion of a shallow epicontinental sea that covered lowlands in West-Central Gondwana within the arid–semiarid belt, whose climate characteristics are reflected in eolian deposits, desiccation structures, mixed carbonate–siliciclastic facies, and fossil records. The main diagenetic feature observed in petrographic analyses was an early diagenetic silicification. Although the absence of spiculites and shelf configuration did not allow a correlation with the Permian glass ramp model, similarities were observed with Eocene glass ramps of Australia, and a relationship with the Permian Chert Event is suggested.

High-resolution sequence stratigraphy of a cretaceous mixed siliciclastic-carbonate platform succession of the Sergipe–Alagoas Basin, NE Brazil

The Sergipe–Alagoas Basin, in northeastern Brazil, has extensive exposures of carbonate rocks that may be studied as analogues to hydrocarbon reservoirs. The outcrops are equivalent to the pre-salt and post-salt reservoirs of the Campos and Santos basins, the most important hydrocarbon exploration targets nowadays in Brazil. Outcrops and well logs of the Riachuelo Formation, the most important for analogue studies of Cretaceous marine carbonate reservoirs of the Campos Basin due to the great variability of facies, are described and interpreted to identify the depositional cycles and stratigraphic sequences. Three unconformities formed during the initial drift phase of the basin are recognized. The depositional setting of the studied section changes from a siliciclastic-dominated continental to shallow-marine mixed siliciclastic-carbonate at the base to a shallow-marine carbonate platform in the upper part. The transgressive–regressive (T–R) sequence model was used for the stratigraphic analysis, leading to a stratigraphic framework of three third-order T–R sequences, with an estimated duration of approximately 2.0, 4.5 and 1.5 Ma, respectively. Consequently, the higher frequency cycles identified in the well logs represent a timing of circa 140,000 years, and are interpreted as fourth-order depositional cycles.

Evolution of the Urgonian shallow-water carbonate platform on the Helvetic shelf during the late Early Cretaceous

Urgonian platform carbonate (Cretaceous; Barremian, Aptian) forms an important lithostratigraphic unit in the Helvetic fold- and thrust unit of the northern Swiss Alps. Its widespread distribution and ubiquity allow for an integrated high-resolution study of macro- and microfacies, benthic foraminiferal biostratigraphy, sequence stratigraphy, and carbon-isotope and phosphorus records. The resulting data confirm the importance of environmental forcing and sea level change on the style of carbonate production and accumulation along the margin of the northern Tethys during the late Early Cretaceous. Stratal geometries, the succession of microfacies, the identification of major emersion surfaces, and biostratigraphy observed and analysed in twelve sections through the inner, middle, and outer platform, and in the panorama of the Churfirsten range, permit subdivision of the analysed succession into eight depositional sequences. The succession starts with a phase of sedimentary condensation (Altmann Member (Mb); late Hauterivian – late early Barremian; sequences H7, H8, and B1), followed by the deposition of hemipelagic sediments (Drusberg Mb; restricted to sequence B2 of the late early Barremian on the inner platform; and covering the late early Barremian to the middle late Barremian on the outer platform, and to the early Aptian on the outer shelf, thereby showing an important diachroneity of its upper boundary related to the inception and progradation of the Urgonian platform), and the development of predominantly lagoonal carbonate (Schrattenkalk Formation (Fm); early late Barremian – early Aptian; sequences B3, B4, B5, and A1). The Schrattenkalk Fm documents important progradation and aggradation of the carbonate platform, and the change from a ramp-like to flat-topped geometry. The oldest, allochthonous remains of the shallow-water carbonate platform were identified intercalated in and on top of the lower Barremian Drusberg Mb (sequence B2). Sequence boundary (SB) B3 resulted from an important regressive phase near the early-late Barremian boundary, which led to the emersion of the hemipelagic sediments of the Drusberg Mb in the inner part of the shelf and to the deposition of a lowstand systems tract at the base of the Lower Schrattenkalk Mb (late Barremian; sequences B3–5) in intermediate and distal domains. Deposition of in-situ platform carbonate started during the following transgressive phase in the middle late Barremian, which flooded the entire investigated area. The associated faunal assemblages and phosphorus contents indicate a concomitant increase in nutrient input, which led to a mixed photozoan-heterozoan platform association dominated by annelids and flat orbitolinids, and the formation of a condensed phosphate-rich bed on the outer shelf (Chopf Bed; middle late Barremian). The subsequent sea-level highstand allowed for the deposition of the first typical Urgonian carbonates rich in corals and rudists. This depositional sequence (B3) terminated by the important infilling of accommodation space combined with sea-level fall of at least 15 m. Later on, close to the Barremian-Aptian boundary, a further, major emersion phase (SB A1) was triggered by sea-level fall, estimated here as at least 16 m, which terminated this first phase in the deposition of rudist and coral-rich platform carbonates covering the middle late to latest Barremian (B3–B5). The overlying Rawil Mb (lowermost Aptian; transgressive systems tract A1) resulted from progressive deepening and document a phase of increasing eutrophication of the depositional environment, resulting in a mixed siliciclastic-carbonate platform build-up, characterized by sea-grass facies and the massive occurrence of Palorbitolina lenticularis. The overlying Upper Schrattenkalk Mb (lower Aptian; highstand systems tract A1) records recovery of the rudist-rich photozoan Urgonian platform. Its subsequent demise occurred well before the Selli oceanic anoxic episode (OAE 1a). It was initiated by emersion of the platform due to high-amplitude sea-level fall (SB A2), followed by eutrophication during the subsequent transgressive phase.The carbon-isotope records show an increase towards more positive values during the Lower Schrattenkalk Mb and the base of the Rawil Mb, interrupted in most sections by an excursion to lower values near the Barremian–Aptian boundary. A shift to lower values occurred also in the uppermost part of the Rawil Mb, followed by variable trends in the Upper Schrattenkalk Mb. These long-term trends are well correlated with the basinal record (Angles, La Bédoule). Deviations in the correlations are related to the influence of facies and microfacies, primary mineralogy, emersion phases, and post-depositional alteration.

Neogene evolution and demise of the Amapá carbonate platform, Amazon continental margin, Brazil

The Amazon continental shelf hosted one of the world's largest mixed carbonate-siliciclastic platforms from the late Paleocene onwards - the Amapá carbonates. The platform architecture, however, remains poorly understood and causes and timing of the cessation of carbonate deposition are still controversial. Here we present a stratigraphic analysis of the Neogene succession of the Amapá carbonates, based on a grid of 2D/3D seismic data correlated to revised micropaleontological data from exploration wells. The results provide improved constraints on the age of the transition from predominantly carbonate to siliciclastic sedimentation, which is shown to have varied through time across three different sectors of the shelf (NW, Central and SE). Four Neogene evolutionary stages of carbonate deposition could be defined and dated with reference to the new age model: (1) between ca. 24 and 8 Ma a predominantly aggrading mixed carbonate-siliciclastic shelf prevailed across the entire region carbonate production gave way to siliciclastic sedimentation across the Central and SE shelves; (2) between 8 and 5.5 Ma carbonate production continued to dominate the NW shelf, as deposition was able to keep up with base level oscillations; (3) between 5.5 and 3.7 Ma (early Pliocene), sediment supply from the paleo-Amazon River promoted the progressive burial of carbonates on the inner NW shelf, while carbonates production continued on the outer shelf (until 3.7 Ma). Longer-lasting carbonate sedimentation on the NW shelf can be explained by a lesser influx of siliciclastic sediments due to the paleo-geography of the Central shelf, characterized by a 150-km-wide embayment, which directed most terrigenous sediments sourced from the paleo-Amazon River to the continental slope and deep ocean; (4) from 3.7 Ma onwards, when the Central shelf embayment became completely filled, continuous sediment supply to the NW shelf resulted in the final transition from carbonate to siliciclastic-dominated environments on the entire Offshore Amazon Basin.

Diagenesis and reservoir quality of carbonates rocks and mixed siliciclastic as response of the Late Carboniferous glacio-eustatic fluctuation: A case study of Xiaohaizi Formation in western Tarim Basin

Abstract Late Carboniferous-Permian was a longest-lived and most widespread ice age. The depositional environment, diagenesis and reservoir quality of carbonate rocks as response to the glacio-eustatic changes was analyzed in this study. The Upper Carboniferous Xiaohaizi Formation in the western Tarim Basin is an important oil reservoir. The dolostones from the Xiaohaizi Formation have very good physical properties, with maximum porosity and permeability of 16.6% and 214mD, respectively. The sedimentary facies of the Xiaohaizi Formation consist of carbonate shoals and lagoon on a mixed siliciclastic-carbonate platform. There was a strong heterogeneity of carbonate rocks deposited on shoals. Porous dolostones always alternate with tight sparry allochemical limestones. Based on the petrographic observation and stable isotope measurement, the controlling factors for the heterogeneity and reservoir quality were revealed. The palaeosol, vadose silt, karst breccia, detrital kaolinite, darkening of dolostones and enrichment of 18O indicate the short-lasting subaerial emergence during the glacial period. During the glacial sea level fall, the mixed siliciclastic and carbonate allochems deposited on bioclastic and intraclastic shoals, while dolomitization of calcic grains occurred under a relatively low-temperature. Through the correlation analysis of physical property, lithology, rock components, it's suggested that the reservoir quality of the Xiaohaizi Formation was controlled by mixed sedimentation and dolomitization associated with glacio-eustatic fluctuation. At the burial diagenesis, the effect of hydrothermal fluid on physical properties of dolostones with relative high primary porosity was positive, whereas resulted in the poikilotopic calcite cementation. The assembly of fluorite, dickite and authigenic pyrite were indicators for hydrothermal fluid accumulation. The control of glacio-eustatic fluctuations on sedimentation and diagenesis in carbonates could be used to forecast the reservoir quality.

Platform drowning leading to cool-water carbonate deposition: evolution of a Late Ordovician (Turinian–Chatfieldian) mixed-sediment platform within the Taconic orogen (Long Point Group, Newfoundland Appalachians)

Late Ordovician (Turinian–Chatfieldian) drowning of a mixed carbonate–siliciclastic platform within the Taconic Orogen (Newfoundland Appalachians) is recorded by net deepening of an initial warm, shallow-water platform succession (Lourdes Formation) culminating in a metre-scale thick condensed interval that characterizes a drowning succession punctuated by storm deposits. Composition of transported material suggests that seaward drowning was coupled with back-stepping of a high-energy carbonate factory related to hinterland uplift and erosion that would eventually lead to drowning of the outer platform beneath marine-transported siliciclastic sediments (Winterhouse Formation). In the new offshore shelf setting, a sparse reciprocal stratigraphy of fine- to very coarse-grained phosphatic carbonate and mixed sediment is interpreted to document gravity-flow deposition downgradient from either a sustained or episodically developed high-energy cool-water carbonate source along the inner shelf. Transported carbonate was cemented rapidly at temperatures no warmer than 16 °C–23 °C, possibly within a seasonal oceanic thermocline. An upsection decrease in abundance of carbonate by the early Edenian is associated with a dramatic increase in siliciclastic supply. The Turinian–Edenian succession of platform drowning, oceanographic transition to cool-water carbonate production, and, later, its termination by increased siliciclastic supply reflects a first-order tectonic control proximal to uplift within the Taconic Orogen. Similar structural and oceanographic changes along the contemporary distal Laurentian margin provides the basis, with improved biostratigraphic control, for future analysis of the significance of proximal–distal stratigraphies in response to regional foreland tectonism.

A Cambrian mixed carbonate–siliciclastic platform in SW Gondwana: evidence from the Western Sierras Pampeanas (Argentina) and implications for the early Paleozoic paleogeography of the proto-Andean margin

The Western Sierras Pampeanas (WSP) of Argentina record a protracted geological history from the Mesoproterozoic assembly of the Rodinia supercontinent to the early Paleozoic tectonic evolution of SW Gondwana. Two well-known orogenies took place at the proto-Andean margin of Gondwana in the Cambrian and the Ordovician, i.e., the Pampean (545–520 Ma) and Famatinian (490–440 Ma) orogenies, respectively. Between them, an extensive continental platform was developed, where mixed carbonate–siliciclastic sedimentation occurred. This platform was later involved in the Famatinian orogeny when it underwent penetrative deformation and metamorphism. The platform apparently extended from Patagonia to northwestern Argentina and the Eastern Sierras Pampeanas, and has probable equivalents in SW Africa, Peru, and Bolivia. The WSP record the outer (deepest) part of the platform, where carbonates were deposited in addition to siliciclastic sediments. Detrital zircon U–Pb SHRIMP ages from clastic metasedimentary successions and Sr-isotope compositions of marbles from the WSP suggest depositional ages between ca. 525 and 490 Ma. The detrital zircon age patterns further suggest that clastic sedimentation took place in two stages. The first was sourced mainly from re-working of the underlying Neoproterozoic metasedimentary rocks and the uplifted core of the early Cambrian Pampean orogen, without input from the Paleoproterozoic Rio de la Plata craton. Sediments of the second stage resulted from the erosion of the still emerged Pampean belt and the Neoproterozoic Brasiliano orogen in the NE with some contribution from the Rio de la Plata craton. An important conclusion is that the WSP basement was already part of SW Gondwana in the early Cambrian, and not part of the exotic Precordillera/Cuyania terrane, as was previously thought.

Facies variability in mixed carbonate–siliciclastic platform slopes (Miocene)

Miocene tropical carbonate platform slopes in southern Spain contain classical reef-slope facies distribution but also an unexpected abundance of serpulid-rich facies, locally forming build-ups. Two sections from the Miocene Sorbas and Nijar Basins were mapped and analyzed petrographically in order to identify the factors determining this facies variability. Reef-slope facies is intercalated with serpulid-rich facies and siliciclastic bodies. Serpulids are the pioneers colonizing the substrate in zones of quiet hydrodynamic conditions after hydrographical changes such as eventual river discharge. The interplay of sea-level changes and hydrographical conditions, together with episodic terrestrial influx, control lateral and along-slope facies variability as well as the facies distribution across the carbonate platform. Neither a deterministic distribution of facies belts nor a stochastic partitioning of facies in mosaics can accurately explain the facies distribution. A new model is proposed to explain facies variability in the context of intrinsic and extrinsic factors.

Cap carbonate platform facies model, Noonday Formation, SE California

The Neoproterozoic outcrop belt of the Death Valley region, California, preserves an oblique cross section of the Noonday Formation, a mixed carbonate-siliciclastic platform that hosts distinctive basal Ediacaran cap carbonate−affiliated sedimentary structures, stromatolite textures, and δ^(13)C_(carb) values. The Noonday platform encompasses two depositional sequences that reveal two cycles of relative sea-level change within strata conventionally considered to record a single, rapid, postglacial sea-level rise. In updip localities, facies of the first depositional sequence record the transition from a carbonate ramp to a stromatolite-bearing, “tubestone”-textured, reef-rimmed platform; downdip, localities seaward of the reefal escarpment variably preserve a thin and condensed onlapping foreslope wedge. Base-level fall exposed the reef crest to karstic dissolution and propagated submarine incised valleys into the seaward margin of the reef. Overlying strata record the backfilling of a submarine incised valley and reestablishment of a back-stepping, carbonate-dominated ramp prior to a second subaerial exposure event that defines the contact between the Noonday and Johnnie formations. We address the relative contributions of syndepositional tectonism and recovery from low-latitude deglaciation in dictating Noonday platform architecture and the intra−Noonday Formation sequence boundary. Noonday Formation deposition coincided with extension of the Laurentian margin during disaggregation of the Rodinian supercontinent. Within this framework, previous work has suggested that the intra−Noonday Formation sequence boundary records growth faulting that reinforced differential topography, uplifting reef-rimmed horsts—exposing the reef crest to karstic dissolution—and downdropping grabens. However, we trace the intra−Noonday Formation sequence boundary seaward of the reef crest and demonstrate that, for a time, wave base was situated downdip of the reef escarpment on putatively downdropped fault blocks. Thus, if the Noonday margin were undergoing extension, then the creation of the intra−Noonday Formation sequence boundary required a concomitant decrease in accommodation due, perhaps, to postglacial isostatic uplift attendant with low-latitude deglaciation. We speculate that Noonday Formation sequence architecture records (1) immediate deglacial flooding, (2) shoaling and exposure due to isostatic rebound induced by either a hiatus in meltwater flux or rapid ice-sheet collapse against a background of global deglaciation, and (3) resumed flooding following complete deglaciation. As rift-related tectonism could amplify or counter glacial isostasy, inferences of the amplitude of local postglacial sea-level change will require robust estimates of syndepositional extension across the Noonday margin.

Effects of a regional décollement level for gravity tectonics on late Neogene to recent large-scale slope instabilities in the Foz do Amazonas Basin, Brazil

Extra sets of 2D multi-channel seismic and chronostratigraphic data allowed us to undertake analyses of source to sink processes and triggering mechanisms of the gigantic megaslides previously documented off the NW and SE steep slope settings of the Foz do Amazonas basin. As these megaslides comprise two sets of stacked allochthonous masses within the Upper Miocene-Quaternary sedimentary record, they are now described as Mass-Transport Complexes (MTCs) and have been renamed the Amapá Megaslide Complex (AMC) and the Pará-Maranhão Megaslide Complex (PMMC). Individual megaslides of both MTCs can mobilize up to kilometre-thick sedimentary series as allochthonous masses with distinct flow directions, degrees of sediment disruption and internal coherence. Megaslides spread downslope over areas as large as thousands of km2, attaining dimensions comparable to the world's largest mass-transport deposits. Among all the megaslides, the basal and largest AM1 megaslide (AMC) stands as a quite unique example of mass-transport deposit in the basin, interpreted as a dominant carbonate allochthonous mass sourced from a mixed carbonate-siliciclastic platform. The instability was probably triggered between the late Miocene and the end of the Early Pliocene by the gravitational collapse of the mixed platform under its own weight, after successive subaerial exposures which generated karstification processes. Siliciclastic-type megaslides, on the other hand, are all sourced from large upslope slide and/or rotated blocks (up to 60 km wide in the case of the PMMC). The detachment of upslope blocks was triggered by structurally-induced seabed movements during the Mid Pliocene-Pleistocene (in the case of the PMMC) and during the Pleistocene (in the case of the AMC). All mapped features support the interpretation of sedimentary blocks which have undergone long lasting deformation, having been variably folded and faulted by the sliding along an overpressured condensed section (H3 horizon). H3 horizon equally acts as the upper décollement level for the gravity tectonic system that operates on the regional scale of the Foz do Amazonas basin. In such a context, the results of this study evidence complex links between variable modes and scales of gravity processes (gravity tectonics and MTCs emplacements), all induced by instability created from a condensed section prone to produce pore fluid overpressure. And yet, seismic attributes of the décollement H3 across the slope, characterized by a reflector of negative polarity of high amplitude, indicate that the potential for similar large scale sediment failures continues to exist across the AMC and PMMC.