Outer Platform Research Articles

An early diagenetic deglacial origin for basal Ediacaran “cap dolostones”

The beginning of the Ediacaran Period (∼635 Ma) is marked by conspicuous dolostone units that cap Marinoan glacial deposits worldwide. The extent and sedimentary characteristics of the cap dolostones indicate that anomalous carbonate over-saturation coincided with deglacial sea-level rise and ocean warming. However, the geochemical variability within cap dolostones, both between continents, across single continental margins, and within individual stratigraphic sections has been difficult to reconcile with depositional models. Using a compilation of new calcium and magnesium isotope measurements in Marinoan cap dolostone successions worldwide, we show that the geochemical variability can be explained by early diagenetic dolomitization of aragonite along a spectrum of fluid- and sediment-buffered conditions. Dolostones from the outer platform formed under fluid-buffered conditions, whereas dolostones on the inner platform and foreslope environment formed under sediment-buffered conditions. This spatial pattern of dolomitizing conditions is consistent with buoyant recirculation of glacial seawater within carbonate platforms driven by the deglacial sea-level rise and development of a meltwater surface ocean. Using a numerical diagenetic model to evaluate the geochemical differences between sediment- and fluid-buffered cap dolostone units, we constrain the chemical and isotopic composition of both the dolomitizing fluid (glacial seawater [δ13C ∼ 0–2‰]), the meltwater lens (δ13C ∼−11‰), and the primary aragonite sediment (δ13C ∼−6 to −3‰). These model end-members do not imply that primary geochemical variability did not exist but demonstrates that it is not necessary to change the chemistry of seawater to explain the global stratigraphic variability in the geochemistry of basal Ediacaran cap dolostones. Our results provide a novel framework for understanding the geochemical variability of cap dolostone units, including large excursions in carbon isotopes, and how this variability is the product of local diagenetic processes expressed globally in continental margin environments following the last Snowball Earth.

STATES OF PRESERVATION AND ROLE OF DASYCLADAL ALGAE IN MISSISSIPPIAN CARBONATE MOUNDS

The upper Visean–lower Serpukhovian Roque Redonde Formation in the southern part of the Montagne Noire, SW France, contains carbonate mounds with predominant micropeloidal facies tens of meters thick representing paleotopographic highs. Calcareous green algae, including representatives of the Dasycladales and Bryopsidales, are relatively common within the mounds (varying in percentages from 1% to 15% of the components, exceptionally reaching up to 40%). Six morphological types of algal remains are recognized within the mounds, and represented by (1) fragmented material, (2) discrete segments, (3) semi-articulated elements, (4) articulated elements, (5) stacked blades, and (6) bushes. Analysis of their preservation state suggests that they mostly constitute autochthonous or parautochthonous assemblages, which were living during the growth of the predominant micropeloidal/clotted fabrics of the carbonate mounds. As such, the carbonate mounds in the Roque Redonde Formation would have developed in an outer platform setting under dysphotic-euphotic conditions. Green algae in the carbonate mounds of the Mississippian confirm that they do not have analogs within the modern deep-water/bathyal carbonate mounds. The abundant green algae in the Montagne Noire is an exceptional case for any geological time period.

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.

The lower Maastrichtian chalk in the Ghezala-Mateur area, northern Tunisian domain: Sedimentary environments and early diagenetic features

The lower Maastrichtian chalks of the Ghezala-Mateur area, northern Tunisian domain, were investigated with regard to their palaeogeographic implications. Two lateral facies have been identified (i) the white chalk facies occurs to the NW, NE and SE strikethrough of the late Cretaceous synsedimentary El Aouana Graben and, (ii) the chalky limestone facies occupied the central part of the Graben.Sedimentary features, fossil content and trace fossils distribution reveal that sedimentation occurred in a slightly reducing shallow-water environment, probably corresponding to an outer platform setting, characterized by frequently interrupted sedimentation and conspicuous activity of erosional currents. Both features seemingly contributed to the genesis of numerous hiatal surfaces marked by the widespread activities of Thalassinoides trace fossils. The genesis of hiatal surfaces was probably promoted by recurrent burial-early diagenesis-exhumation cycles in which Thalassinoides burrows were preferentially affected by early diagenetic processes. Nodular chalky limestone, intraformational conglomeratic chalky limestone and knoby chalky limestone are the most common lithologies resulted from the early diagenesis of Thalassinoides-omission surfaces.

APPLICATION OF 18O AND 13C STABLE ISOTOPES COMPOSITION OF THE CARBONATE ROCKS OF THE JERIBE FORMATION EASTERN IRAQ; AN APPROACH TO DEFINE THE PALEO TEMPERATURE AND PALEO DEPTH

Stable isotopes 18O/16O and 13C/12C in the carbonate rocks of the Jeribe Formation are examined here to define the depositional characters in the basin includes paleo temperatures and paleo depth. The Jeribe Formation comprises a transgressive unit belongs to the Latest Eocene-Recent Megasequence (Ap11). Complete sections of the formation were studied in the core of Himreen structure in eastern part of Iraq. The Himreen structure lies in the outer platform margin of unstable shelf of the Arabian plate. The stable isotope (18O/16O and 13C/12C) of Jeribe Formation provides the first records of paleotemperature for the Neogene (middle Miocene Transgression) of the Southern Teythes, which indicate cooling during deposition. Microfacies study imply that the skeletal grains are composed of green and red algae, stromatolite and verities of benthic foraminifera. The non-skeletal grains are pellets and intraclasts. Analysis of microfacies suggests lagoon, reef, shoal and open marine environments. The depletion of both 18O and 13 C indicated shallowing at the contact with overlying and underlying formations in the shoal and lagoonal facies respectively. While in the middle part in the coral reef facies reveals elevation of the sea surface temperature. These suggest that the enrichment of both 18O and 13 C at the cool water temperature associated with upwelling and arise of sea level represented by reef and open marine facies.

A Late Cretaceous epeiric carbonate platform: the Haftoman Formation of Central Iran

An integrated study of the litho-, bio-, and microfacies of several sections has greatly improved the knowledge on the stratigraphy and depositional setting of the Coniacian to Campanian Haftoman Formation in the Khur area of the northern Yazd Block, Central Iran. Generally, the Haftoman Formation rests on a major tectonic unconformity and commences with a basal conglomerate followed by up to 900 m of shallow-water carbonates with local red sandstone intercalations. Five different depositional environments (from distal to proximal) characterize the facies associations (FA) of the Haftoman Formation: silty, spiculitic wackestone (proximal basin, FA I), bio-/intraclastic wacke-, pack-, and grainstone (marginal shoals, FA II), bioclastic rud-/float-/boundstone (outer platform, FA III), silty mud-/wackestone (lagoonal inner platform, FA IV), and sandstone/sandy limestone (areas close to the mouth of ephemeral streams, FA V). The litho-, micro-, and biofacies of the Haftoman Formation are typical for an epeiric carbonate platform characterized by an arid climate and lagoonal circulation, resulting in nutrient-poor waters, warm temperatures, and high salinities. The Haftoman Platform was attached to an emergent arid hinterland formed by the Anarak Metamorphic Complex to the west and southwest of the study area. Unconformity-bounded depositional units indicate sea-level changes that may correspond to 400-kyr high-frequency sequences but further studies are needed to fully exploit the potential of sequence stratigraphy for regional and inter-regional correlation of the Haftoman Formation.

The start‐up of the Dolomia Principale/Hauptdolomit carbonate platform (Upper Triassic) in the eastern Southern Alps

AbstractWide carbonate platform environments developed on the western passive margin of the Tethys during the Late Triassic, after a major climate change (Carnian Pluvial Episode) that produced a crisis of high‐relief microbial carbonate platforms. The peritidal succession of this epicontinental platform (Dolomia Principale/Hauptdolomit, Dachstein Limestone) is widespread in the Mediterranean region. However, the start‐up stage is not fully understood. The original platform to basin depositional geometries of the system have been studied in the north‐eastern Southern Alps, close to the Italian/Slovenian boundary where they are exceptionally preserved. Sedimentological features have been investigated in detail by measuring several stratigraphic sections cropping out along an ideal depositional profile. The analysis of the facies architecture allowed reconstruction of the palaeoenvironments of the Dolomia Principale platform during its start‐up and early growth stages in the late Carnian. The carbonate platform was characterized by an outer platform area, connected northward to steep slopes facing a relatively deep basin. Southward, the outer platform was connected to inner sheltered environments by a narrow, often emerged shelf crest. Behind this zone, carbonate sedimentation occurred in shallow lagoons and tidal flats, passing inward to a siliciclastic mudflat. The Dolomia Principale platform was initially aggrading and able to keep pace with a concomitant sea‐level rise, and then prograding during the late Carnian. This stratigraphic interval was correlated with the Tuvalian succession of the Dolomites, allowing depiction of the depositional system on a wide scale of hundreds of kilometres. This large‐scale depositional system presents features in common with some Palaeozoic and Mesozoic carbonate build‐ups (for example, the Permian Capitan Reef complex, Anisian Latemar platform), both in terms of architecture and prevailing carbonate producers. A microbial‐dominated carbonate factory is found in the outer platform and upper slope. The recovery of high‐relief microbial carbonate platforms marks the end of the Carnian Pluvial Episode in the Tuvalian of Tethys.

Research on Simulation System of Missile and Arrows Based on Unity3D

Aiming at the demand of visualization system development in flight simulation, a projectile missile flight simulation system is designed and developed. Using Unity3D to build the 3D model of the simulation object and reproduce the real environment design semi physical simulation system. By interfacing with Google Earth API platform and calling function, the 3D terrain modeling is realized by 3D Terrain Modeling Technology. And the database-based outer trajectory simulation platform simulation program data link, receive the simulation data of the ballistic simulation program, the whole process of the real-time three-dimensional animation reproduction of the reaction trajectory simulation, and add the bomb selection, playback settings, perspective tracking, explosion effects and other functions. The actual engineering shows that the system can be realistic to reflect the process of flying ammunition and has good stability and versatility.

A geomorphic and tectonic model for the formation of the flight of Holocene marine terraces at Mahia Peninsula, New Zealand

At Table Cape, Mahia Peninsula, North Island, New Zealand, four marine terraces have been uplifted coseismically during the past 3500years. Detailed facies assessment of the terrace coverbed sequence coupled with identification of modern analogues on the active shore platform were used to infer the process of marine terrace formation and to estimate the timing and amount of past uplift events (earthquakes). The modern platform can be subdivided into seven depositional zones: subtidal, outer platform, intertidal sand pockets, inner platform, high-tide, mid-storm, and storm beach. Terrace coverbeds were characterised from two trenches excavated across the full width of the uplifted terrace sequence. Off-lapping packages of high tidal, mid-storm, and storm beach sediments were most common. Outer platform sediments occurred only rarely near the base of some uplifted shore platforms. Overlying the marine sediments were near-horizontal terrestrial deposits of airfall tephra (on the two highest terraces), subsoil, topsoil, rare wedges of colluvial sediment (slopewash) shed from terrace risers, and an anomalous deposit possibly emplaced by a tsunami. Fifty-one radiocarbon ages, obtained from molluscs in the marine coverbeds, showed a general pattern of seaward-younging across the coastal plain and across each terrace and a less pronounced pattern of decreasing age upward in each coverbed sequence. The distinctive stepped geomorphology of the terraces, the facies and age structure of the terrace deposits and historical earthquake causation of similar terraces elsewhere in New Zealand provided the data to invoke an earthquake-driven model for terrace formation. Marine terrace development following an uplift event involved rapid cutting of a new intertidal shore platform and generally regressive deposition of high-tide to storm beach deposits. Following further uplift, the platform became a geomorphic terrace (above marine influence) and was then mantled by terrestrial sediments. On the two highest terraces at Table Cape, airfall tephras mantling the marine coverbeds provided a minimum age for terrace uplift. The youngest radiocarbon ages from high-tide deposits high in the stratigraphy and near the seaward edge of each terrace provided the best estimates for the timing of uplift. Based on the new radiocarbon ages and the constraining airfall tephra ages, we revised the earthquake ages to 3530–3350, 1810–1730, 1560–1300 and 300–100cal. YBP. Associated best estimates of the coseismic uplift amounts were 2.1, 1.4, 1.8, and 3.1m respectively, once we accounted for eustatic sea level changes through the late Holocene.

Sedimentary and tectonic evolution of the southern Qiangtang basin: Implications for the Lhasa‐Qiangtang collision timing

AbstractThe Mesozoic stratigraphic record of the southern Qiangtang basin in central Tibet records the evolution and closure of the Bangong‐Nujiang ocean to the south. The Jurassic succession includes Toarcian‐Aalenian shallow‐marine limestones (Quse Formation), Aalenian‐Bajocian feldspatho‐litho‐quartzose to feldspatho‐quartzo‐lithic sandstones (shallow‐marine Sewa Formation and deep‐sea Gaaco Formation), and Bathonian outer platform to shoal limestones (Buqu Formation). This succession is truncated by an angular unconformity, overlain by upper Bathonian to lower Callovian fan‐delta conglomerates and litho‐quartzose to quartzo‐lithic sandstones (Biluoco Formation) and Callovian shoal to outer platform limestones (Suowa Formation). Sandstone petrography coupled with detrital‐zircon U‐Pb and Hf isotope analysis indicate that the Sewa and Gaaco formations contain intermediate to felsic volcanic detritus and youngest detrital zircons (183–170 Ma) with εHf(t) ranging widely from +13 to −25, pointing to continental‐arc provenance from igneous rocks with mixed mantle and continental‐crust contributions. An arc‐trench system thus developed toward the end of the Early Jurassic, with the southern Qiangtang basin representing the fore‐arc basin. Above the angular unconformity, the Biluoco Formation documents a change to dominant sedimentary detritus including old detrital zircons (mainly >500 Ma ages in the lower part of the unit) with age spectra similar to those from Paleozoic strata in the central Qiangtang area. A major tectonic event with intense folding and thrusting thus took place in late Bathonian time (166 ± 1 Ma), when the Qiangtang block collided with another microcontinental block possibly the Lhasa block.

Spatial and temporal distribution of microbial carbonates, skeletal and non-skeletal grains in a Pennsylvanian carbonate platform (Valdorria, Northern Spain)

The Valdorria carbonate platform, in northern Spain, features a well-preserved and continuous outcrop of a Bashkirian platform to basin transect. During the Asatauian (late Bashkirian), the platform growth changed gradually from a progradational to an aggradational mode, recording 10 most likely short-eccentricity-controlled cyclothems. The facies of the platform-top and slope have been mapped, and the architecture of each cyclothem schematically reconstructed. The distribution of microbial carbonates (microbially mediated precipitates), skeletal (grazers/burrowers, corals/filter feeders, algae, foraminifera, Osagia-like oncoids and Thartarella-Terebella worm tubes) and non-skeletal grains (faecal pellets) has been quantified for the transgressive–regressive periods of deposition (transgressive, maximum flooding, early and late regressive intervals) of each cyclothem.This study shows that the distribution of microbial carbonates, skeletal and non-skeletal grains along a carbonate platform transect is variable through time and mainly governed by a set of measurable interconnected factors regulating the local palaeoenvironments: the water depth and the wave energy (facies) along the platform profile (inner, outer, break, slope) during periods of sea-level fluctuations (transgressive to regressive intervals of deposition). Auloporid corals, siliceous sponges, phylloid algae and Osagia-like oncoids are characteristic of a low-energy environment situated from estimated palaeo-water depths of 25 to 80metres below sea level (mbsl onward) that formed in the outer platform during the maximum-flooding intervals. Anthracoporella–Archaeolithoporella boundstone is characteristic of the moderate-energy environments created in the horizontal inner-platform during the transgressive intervals. Microbially mediated precipitates are reliable indicators of the slope and Masloviporidium(?) indicator of low-energy environments of the upper slope. On the platform-top, Ungdarella and Donezella are more abundant during the early regressive intervals than during any other interval. Stacheoids are mainly present in the water depth range of 10 to 60mbsl, tournayellids from 15 to 35mbsl and archaediscids from 15 to 75mbsl, all in low- to moderate-energy conditions. Rugose corals are common in the water depth range of 5 to 35mbsl, either during the transition from the early to the late regressive intervals, as solitary forms or forming boundstone, or during periods of higher sea-level stand, as solitary forms in floatstone. Fenestellids, fistuliporids and trilobites occur mainly in the slope, but are also common in the platform top during the maximum flooding and/or the early regressive intervals. Archaeolithoporella is a reliable indicator of the platform top deposits, but its abundance is closely linked to the presence of adequate substrates. A high abundance in the assemblage of gastropods, endothyrids, palaeotextulariids, bradyinids and fusulinids characterizes the platform top shallow-water coated-grain grainstones formed under high-energy currents during the late regressive intervals. Tetrataxids act as encrusters on microbial precipitates of the slope or on phylloid algae and Archaeolithoporella of the platform top, but are also observed as nuclei of coated grains. Brachiopods are more abundant in low- to moderate-energy and 35 to 55mbsl environments that prevailed during the transgressive intervals. Faecal pellets were commonly observed in low-energy environments, from 50 to 325mbsl, at the platform break and in the slope, and the same applies for Thartarella-Terebella worm tubes that are virtually absent in high-energy deposits. Finally, crinoids, lasiodiscids and tuberitinids are randomly distributed and are not indicative of any specific environment.The here well-constrained distribution patterns of microbial carbonates, skeletal and non-skeletal grains in Valdorria outcrop hold the potential to serve as a reference for interpretation of depositional environments of other platforms using biota associations and depositional texture, particularly in regions where the platform geometry is not preserved or is not fully exposed.

Karstification and Tectonic Effects on the Drainage Trend in the Southwestern Part of Iraq

The extreme southwestern part of Iraq forms a flat terrain, but densely dissected by valleys that flow in different directions with different types. The exposed rocks are horizontally lying, mainly of carbonates of Ratga Formation; Eocene in age; however, eastwards Cretaceous rocks are exposed of different formations; also mainly carbonates with some intercalations of fine clastics and marls. The concerned area is intensely karstified; exhibiting different karst forms and sizes. It is also intensely parted by lineaments of different orientations, which have controlled the drainage style and flow direction. Tectonically, the concerned area lies within the Outer Platform (Unstable Shelf) of the Arabian Plate. Structurally, apart from two main normal faults of E-W trend and intense jointing and lineaments; no other features exist in the area. The intense karstification, presence of lineaments and neotectonic activities has controlled the trends of the valleys forming very complex water shades and basins that locally interfere with each other. These abnormal drainage style and different flow directions of the valleys are studied and correlated with the regional tectonic frame of the region. Moreover, we have used subsurface data; such as groundwater flow directions and geophysical data to indicate the reasons for existing of abnormal drainage style and flow directions of surface water.

Molluscan death assemblages from uplifted Holocene terraces, Mahia Peninsula, New Zealand, interpreted from present-day intertidal ecology

Beu, A.G., August 2016. Molluscan death assemblages from uplifted Holocene terraces, Mahia Peninsula, New Zealand, interpreted from present-day intertidal ecology. Alcheringa 41, xxx–xxx. ISSN 0311-5518.Macrofossils from cover beds of marine terraces at Table Cape, Mahia Peninsula, uplifted coseismically ca 250, 1400, 1850 and 3500 years ago are compared with the fauna living on the nearby 200-m-wide intertidal rock platform. The inner platform is dry at low tide, apart from shallow pools containing Diloma aethiops and abundant but unexpected specimens of Zeacumantus subcarinatus and Cominella glandiformis exposed to the sun; all are common fossils in the terrace cover beds. The outer platform is carpeted densely with the alga Hormosira and shelters a diverse fauna, again all common fossils in the terrace cover beds. Sand and gravel containing shell fragments and supporting Zostera sea-grass turf unexpectedly covers ca 5% of the platform in low areas along the east coast. The fossil fauna is listed from 47 samples from two trenches excavated east–west and north–south through the terrace cover beds. Death assemblages of 158 molluscs, including 24 new records as fossils, and 11 taxa in other phyla delimit sediment derived from four shell accumulation sites identified around the present cape: storm beach, high-tidal strand line, hollows in the rock platform and Zostera flats. Up to 32 of the 47 samples (68%) possibly were deposited in Zostera flats; the rest were sand beach accumulations deposited at and near the high-tidal strand line. All fossils common in the cover beds lived on the rock platform. Differences between the deposits in the two trenches result from two factors. Protection along the east coast by protruding tephra beds and an outer rampart armoured with the macroalga Durvillaea prevented erosion of Zostera-supporting sediment which, therefore, was retained after each uplift event. The exposed eastern position of Table Cape also caused sediment to be transported predominantly westwards, onshore on the east coast but away from Table Cape along the north coast.A.G. Beu [a.beu@gns.cri.nz], Paleontology Department, GNS Science, PO Box 30368, Lower Hutt 5040, New Zealand.

Carnian reefal blocks in the Slovenian Basin, eastern Southern Alps

The Carnian Amphiclina beds of the eastern Southern Alps mostly consist of shale and sandstone deposited in the deep-marine Slovenian Basin, which was located near the western margin of the Meliata Ocean. In the vicinity of Cerkno (Slovenia), blocks of massive or crudely bedded limestone crop out within a succession of clastic rocks that are several hundred meters thick. Sponge-microbialite boundstone and coral pillarstone are the predominant facies within these blocks. Oncoid floatstone-rudstone and moderately sorted peloid packstone form crudely layered parts of the blocks, whereas intraclast-cortoid packstone and poorly sorted peloid packstone occur locally in cross-stratified thin beds, truncated at block margins. Detailed mapping further shows that limestone blocks form units at discrete stratigraphic levels within shale and that these units are variable in thickness. Whereas the largest blocks mostly lie concordant to the bedding, smaller blocks are poorly sorted and randomly oriented. All of this evidence suggests that the limestone blocks are olistoliths, derived from an outer platform margin and/or uppermost slope.

A facies distribution model controlled by a tectonically inherited sea bottom topography in the carbonate rimmed shelf of the Upper Tithonian–Valanginian Southern Tethyan continental margin (NW Sicily, Italy)

The Upper Tithonian–Valanginian shallow-water carbonates outcropping in the Palermo Mts (NW Sicily) consist of several facies associations reflecting different depositional environments of a carbonate rimmed shelf, pertaining to the Southern Tethyan continental margin. The reconstructed depositional model, based on the sedimentological features, cyclic facies arrangement and biota distribution, shows that a wide protected lagoon, dominated by algae, molluscs and scattered patch reefs, was bordered landward by a tidal flat, where stromatolitic algal mats were cyclically subaerial exposed, and seaward by a marine sand belt and reef complex. Oolitic packstone-grainstone lithofacies, cyclically subjected to subaerial exposure, suggests the occurrence of a barrier island, located nearly to the lagoonal carbonate shoreline, allowing the development of narrow embayments with restricted circulation. In the outer platform, the oolitic lithofacies of the marine sand belt pass landward into the protected lagoon, where washover oolite sands occur, and seaward into a high-energy zone (back-reef apron) gradually merging in the reef complex. In the latter, coral framestone occupied the inner sector (reef flat), while the facies association dominated by boundstone with Ellipsactinia sp. developed in the outer sectors (reef wall), adjacent to the fore-reef and upper slope environments. Stratigraphic evidence, associated with the recognized facies associations, helped to reconstruct the geo-tectonic setting of the carbonate platform, where the distribution of the depositional facies along the shelf and their extension were influenced by the tectonically-inherited sea bottom topography. In a regime of extensional tectonics, localized and thin succession of high-energy prograding oolite sand belt depositional facies occupied structural highs (footwall uplift), while the largely diffused and thick low energy aggrading peritidal-to-lagoonal depositional facies developed in subsiding hanging wall. The reef complex and the fore-reef depositional facies, exhibiting lateral migration and prograding geometries with clinoforms and downlap stratal terminations, developed on a subsident stepped margin, adjacent to a faulted shelf-break. This morphostructural setting can be comprised in a model, where the tilted and faulted-blocks (inner platform) and stepped margin (outer platform) paleo-topography, inherited from the rifting tectonics, was the most important factor controlling the geometry and depositional architecture of a carbonate platform growing on the Southern Tethyan passive margin.

High-resolution biochronology and diversity dynamics of the Early Triassic ammonoid recovery: The Dienerian faunas of the Northern Indian Margin

A new high resolution biozonation based on the Unitary Association (UA) method is constructed for the Dienerian ammonoid succession of the Northern Indian Margin. It includes 12 UA-zones and leads to the subdivision of the Dienerian into three parts (early, middle and late). The corresponding diversity analyses, coupled with results previously obtained for the early Smithian ammonoids of the same regions, highlight the four following phases: (1) a first modest peak of diversity in the early Dienerian; (2) a very low diversity persisting throughout the middle Dienerian; (3) a slow increase of diversity during the late Dienerian, and (4) a marked diversification in the early Smithian. Turnover rates are very high during this entire time interval, and the boundaries between early–middle and middle–late Dienerian are emphasised by complete renewals of the ammonoid faunas. The low diversity values in the middle and early late Dienerian are concomitant with an anoxic event on outer continental shelves and coincide with warmer temperatures than those of the early Dienerian and early Smithian. This diversity pattern stands in strong contrast with the credo of a protracted or stepwise recovery following the end-Permian mass extinction. Together with the end-Smithian extinction, the middle and early late Dienerian diversity crises were likely both radical setbacks in the recovery of Early Triassic ammonoids. However, these two diversity crises do not necessarily imply identical environmental triggers that ultimately led to anoxic bottom waters on outer continental platforms in both cases.

Characterization of the Devonian Kharyaga carbonate platform (Russia): Integrated and multiscale approach

ABSTRACT The Devonian carbonate platform (Obj-2) of the Kharyaga field (Russia) has been operated by Total since 1999 and produces 42° API oil, rich in H2S. A multidisciplinary (sedimentological, structural, stratigraphic, and geomechanical modeling) and multiscale study was made of the field, integrating seismic, micro-resistivity log, and core data to characterize the reservoir. The Kharyaga carbonate platform is characterized by complex relationships among sea-level variations, subsidence, and sedimentation rates. The field is divided into a platform edge (barrier zone) in the southern area, and back barrier and lagoon environments to the north, each characterized by different facies distribution and dynamic behavior. The platform shelf and the reef flat show dome-shaped thrombolites; the back barrier is characterized by grainstone shoals associated with columnar stromatolites, whereas the lagoon comprises peloidal wackestones and packstones associated with laminated fenestral stromatolites. Significant and prolonged subsidence, in both the lagoon and the platform edge, is associated with partial emergence of the platform reef, as shown by karst development. Seismic interpretation highlighted both underlying north-south structures and syndepositional east-west collapse faults along the southern barrier edge. Two-dimensional and three-dimensional sedimentological modeling of the Obj-2, calibrated by well data and core analyses, allowed the simulation of alternative scenarios to explain the evolution of the carbonate platform during the Devonian and suggested the observed facies distribution is compatible with the presence of a fixed outer platform edge represented by east-west faults. The carbonate platform of the Kharyaga field exhibits complex behavior; hydrocarbon (HC) accumulation and flow is controlled by the interplay of primary porosity, karst distribution, and structural discontinuities (faults and fractures). Besides the role of the karstification, this paper mainly focuses on the role played by tectonic structures on the evolution of the carbonate platform and on the HC flow. From drill stem test data, east-west faults and karsts are considered as major permeability heterogeneities, but interference tests showed more complex fluid pathways at the smaller scale. Microresistivity images show dominant northeast-southwest and northwest-southeast fracture sets in the central barrier, forming an interconnected flow network with the seismic faults. Geomechanical modeling suggests that these fractures are subseismic structures linked to inherited, deep-seated faults active at the time of fracturing, in addition to the listric reservoir faults.

An enigmatic kilometer-scale concentration of small mytilids (Late Miocene, Guadalquivir Basin, S Spain)

Upper Miocene heterozoan carbonates crop out extensively in a NE–SW-trending belt (42 km long and 1.5–8 km wide) along the so-called El Alcor topographic high, from Carmona to Dos Hermanas (Seville, S Spain). These carbonates formed at the southern active margin of the Guadalquivir Basin, the foreland basin of the Betic Cordillera. They change to marls basinward (NE) and to sands landward (SE and SW). Therefore, carbonate production was constrained to a limited area in an otherwise siliciclastic shelf. The carbonates (up to 40 m thick) overlie a gradually coarsening-upward succession of marls followed by silts and sandstones. The carbonate sequence can be divided into three subunits corresponding, from bottom to top, to lowstand, transgressive, and highstand system tract deposits. The lower subunit, exhibiting extensive trough cross-bedding, is interpreted as a shallow-water bar deposit. The intermediate subunit onlaps underlying sediments and was deposited in deeper, low-turbulence conditions. The upper subunit deposits accumulated in a well-oxygenated outer platform based on benthic foraminiferal assemblages. The presence of hummocky and swaley cross-stratification in these latter deposits suggests that they were affected by storms. Pervasive fluid-escape structures are also observed throughout the carbonates. The three subunits consist of bioclastic packstones to rudstones made up of abundant fragments of small mytilids. Isotopic data from serpulid polychaete Ditrupa tubes show 13C-depleted values (up to − 16.1‰), whereas δ18O yields normal marine values. Additional isotopic data on shells of scallops, oysters, and small mussels, as well as bulk sediment, show diagenetic alterations. Based on actualistic examples of massive concentrations of mussels, the nearly monospecific composition of the El Alcor deposits, together with negative δ13C values of Ditrupa tubes, indicates that cold seeps presumably promoted carbonate formation. However, the absence of typical features of cold-seep deposits, such as authigenic carbonates mediated by anaerobic bacterial activity and the typical chemosynthetic shelly organisms, makes the large carbonate body of El Alcor an unusual cold-seep deposit.

Depósitos de plataforma mista, Neocarbonífero da Bacia do Amazonas, região de Uruará, estado do Pará

Rochas carbonáticas são amplamente registradas na região oeste e centro-oeste do Estado do Pará, borda Sul da Bacia do Amazonas, correspondentes ao período Neocarbonífero. Esses depósitos são incluídos na sequência pensilvaniana-permiana correspondente a uma super sequência de segunda ordem representada pelo Grupo Tapajós. A Formação Itaituba corresponde à porção transgressiva que precedeu a expressiva continentalização e formação de Pangeia. Essa unidade é constituída de carbonatos fossilíferos, arenito com estratificação cruzada e subordinadamente folhelhos e evaporitos de ambiente marinho costeiro. Dezenove fácies e microfácies foram agrupadas em cinco associações de fácies (AF) correspondentes a depósitos de planície de maré (AF1), canal de maré (AF2), laguna (AF3), barra bioclástica (AF4) e plataforma externa (AF5). A AF1 compreende ciclos de exposição subaérea com formação de gretas de contração e deposição subaquosa, composta de brecha dolomítica, dolomito fino laminado e dolomudstone com terrígenos separados por arenito maciço piritoso e dolomito fino silicificado. A AF2 consiste em arenito fino com laminação cruzada de base reta e recobrimento pelítico nos foresets, siltito argiloso com laminação cruzada, marga com laminação cruzada cavalgante e arenito grosso com estratificação cruzada com recobrimento pelítico nos foresets. Falhas sinsedimentares afetam os arenitos e siltitos. A AF3 é constituída de siltito maciço avermelhado, mudstone fossilífero, floatstone com braquiópode e folhelho negro piritoso maciço. A AF4 é representada por grainstone oolítico fossilífero e grainstone com terrígenos e fósseis de braquiópodes, equinodermas, bivalves, moluscos e subordinadamente artrópodes, briozoários e foraminíferos. A AF5 associa-se a uma plataforma composta de wackestone fossilífero, wackestone fossilífero com terrígenos e mudstone maciço. A assembleia fossilífera é representada por foraminíferos, braquiópodes, bivalves e, subordinadamente, briozoários e equinodermas, que atestam um ambiente costeiro de salinidade moderada e profundidade baixa com intensa proliferação de organismos bentônicos. O modelo deposicional da Formação Itaituba, na região de Uruará, foi atribuído a um sistema lagunar e de planície de maré conectado a um sistema de plataforma marinha carbonática. As lagunas eram bordejadas por planície de maré na porção mais continental e periodicamente supridas por aporte de terrígeno fino (silte) que inibia a precipitação carbonática. Barras bioclásticas cortadas por canais de maré (inlet) separavam a laguna da plataforma rasa com maior abundância de fósseis marinhos bentônicos, registrando o início de uma transgressão marinha neocarbonífera na Bacia do Amazonas advinda de NNW.

Frasnian (Late Devonian) conodonts and environment at the northern margin of the Algerian Sahara platform: the Ben Zireg section

AbstractThe Ben Zireg anticline NW of Bechar yields the currently most-important Frasnian succession on the northern margin of the Algerian Sahara. It represents 26.5 m of calcilutites which are attributed to the middle–late Frasnian. As the early Frasnian is not represented, the succession rests conformably on undifferentiated, probably late Givetian, substrate. Fine-scaled conodont biostratigraphy reveals a continuous sequence of Montagne Noire Zones 5–13, superseded by the earliest Famennian LowertriangularisZone. The deposits are organized into 15 sedimentary cycles that are initially condensed and become dilated upwards. Conodont biofacies indicate an overall deepening towards the top of the section with an episode of slight shallowing at the MN11/12 transition marked by an increase of ancyrodellids, which is also seen in the Marhouma section of the Ougarta region (SW Algeria). Homogeneous calcareous microfacies with only a few shaly intercalations determine a predominantly oxygenated depositional environment on an outer platform, submarine rise or ramp setting. This is also indicated by relatively low sedimentation rates which are similar in selected sections from the Tafilalt platform, and contrast with those from the Marhouma trough and the Maider basin. Unlike other Frasnian successions south of the Atlas Fault, but similar to the Moroccan Meseta, the Upper Kellwasser horizon is clearly discriminated at Ben Zireg by an outstanding occurrence of black shales on top of the oxygenated latest Frasnian deposits.