Mixed Siliciclastic-carbonate System Research Articles

Middle Triassic tectono‐sedimentary development of Sichuan Basin: Insights into the cratonic differentiation

The Middle Triassic succession of the Sichuan Basin contains key information about the Early Indosinian Movement, during which it has changed from extensional to compressional. In order to clarify the relationship between the evolution of the Sichuan Basin during the Indosinian Movement, the Middle Triassic tectonostratigraphic environment should be determined. Detailed stratigraphy and facies analysis integrated with structural and basin analysis, allowed a full reconstruction of the tectonic and depositional environments for the Middle Triassic strata of the Sichuan Basin. Six facies associations from two rock systems of: (a) carbonate–evaporite system and (b) a mixed carbonate–siliciclastic system, were characterized. Four depositional stages highlighted by the lithostratigraphic members of the Leikoupo (Lei) and Badong (Ba) formations were divided: (a) stable subsidence and deposition of the Lei‐1/Ba‐1 Member, leading to an embryonic ‘uplifts and sags’ configuration; (b) inherited deposition of the Lei‐2/Ba‐2 Member, with the platform margin belt extended; (c) rapid deposition of the Lei‐3/Ba‐3 Member, characterized by an eastward shift of the depositional centre and formation of the central anhydrite‐rich lagoon; and (d) intensification of cratonic tectono‐sedimentary differentiation during the Lei‐4/Ba‐4 deposition, that resulted in subaerial exposure along the Luzhou–Kaijiang uplift zone and a hydrodynamically restricted depression along the western platform. The Middle Triassic tectonic framework of the Sichuan Basin was dominated by compression in the east and extension in the west. The western margin was a passive continental margin, inheriting an early rifting event, while the eastern area within the craton showed an ‘uplift‐sag’ pattern, which is a response to intracontinental tectonic movement of the Xuefengshan uplift belt. The basin evolved from incipient low‐relief uplifts and sags within the intracratonic depression to subsequent isolated foreland and restricted depression during the Middle Triassic.

Sedimentology of mixed siliciclastic-carbonate system in a structurally constrained Paleogene basin: Eocene Musawa Formation in Eastern Oman Mountains

The Middle Eocene sedimentary succession in Abat Basin contains a thick mixed siliciclastic and carbonate sequence belonging to the upper member of the Musawa Formation. Over 300 m thick deltaic sediments of the Bartonian to Priabonian upper member of the Musawa Formation show two main facies successions over ten km distance, grading from a proximal delta-plain to a distal delta-front succession. Four major lithofacies associations based on their depositional significance are identified in the basin, namely the conglomerate, sandstone, mudstone, and carbonate. Each of these lithofacies associations is further divided into several individual lithofacies based on their depositional characteristics. The lithofacies association assemblage represents fluvially-dominated deltaic sediments that accumulated as a local siliciclastic feature in a carbonate dominated tropical environment. The conglomerate and sandstone lithofacies associations were deposited by channels and shoreface processes, whereas coastal processes in delta-plain setting deposited mudstone lithofacies association (including coal). The carbonate lithofacies association was deposited in an open marine shelf conditions during transgressive events associated with highstands of sea level and delta-lobe switching. The occurrence of thick siliciclastic sediments is interpreted as the result of activation of the North Ja'alan Fault resulting in a transtensional basin during the Middle to Late Eocene.

Seagrass-related carbonate ramp development at the front of a fan delta (Burdigalian, New Caledonia): Insights into mixed carbonate-siliciclastic environments

Past shallow-water carbonate environments of the main island of New Caledonia (NC) have been subject to high terrigenous influx derived from the erosion of ultramafic obducted nappes and are therefore a relevant case study for characterizing neritic carbonate production in mixed carbonate-siliciclastic systems under a tropical climate. More particularly, we focused on Burdigalian carbonate sedimentary records cropping out on the western coast of NC, in the Népoui area. Based on a comprehensive sedimentological study of cores and outcrops, we established a new depositional model of an alluvial fan to carbonate ramp transition. Shallow-water (euphotic) carbonate production was dominated by seagrass-related biota and corals derived from small-sized bioconstructions. Extensional tectonics and associated normal faulting, driven by post-obduction isostatic rebound, favored carbonate ramp aggradation and preservation. The carbonate ramp was incised by conglomerate-filled terminal distributary channels, which indicate that terrigenous inputs remained significant during marine transgression and did not inhibit the development of seagrass and scleractinian carbonate factories. Phytal substrates induced by seagrass and/or macro-algae seafloor colonization strongly controlled the nature of carbonate production and promoted the accumulation of foraminiferal-coralline algal sediments. Seagrass development at the front of the fan delta is interpreted to have controlled the preservation of a diverse and significant carbonate production by reducing water turbidity and by limiting the risk of suffocation for filter-feeding biota in such a high terrigenous influx setting.

Sedimentary characteristics and genetic mechanisms of high-quality reservoirs in a mixed siliciclastic-carbonate system in the Qinhuangdao area, Bohai Sea, China

Mixed siliciclastic-carbonate sediments occur broadly in modern and ancient systems. Studies on mixing processes began in shallow shelf environments; however, the genetic model of marine mixed sediments is difficult to apply to continental rift basins due to the complex palaeogeographic environment. We identified three mixing types in the first and second members of the Palaeogene Shahejie Formation (E2s1–2) in the Qinhuangdao area of the Bohai Sea: (1) mixed fan delta, (2) retrogradation mixed sheet, and (3) mixed sheet without siliciclastic influx. Tectonic stability, arid climate, and saline lakes are prerequisite conditions for the development of mixed sediments, whereas the palaeogeomorphologic unit should be the critical factor. We also concluded that the primary sedimentary material contains near-source coarse terrestrial debris, and the advantageous lithologic facies producing biological components are the foundation for high-quality mixed reservoirs, which are characterized by thick layers and favorable porosities and permeabilities. The micritic coatings and early dolomitization against the background of a saline lake environment favored the preservation of primary pores, whereas the leaching by atmospheric water and organic acid erosion during thermal evolution of the source rock created many secondary pores. In addition, hydrocarbon charging protected the reservoir space from carbonate cementation.

Characteristics of a Tonian reef rimmed shelf before the onset of Cryogenian: Insights from Neoproterozoic Kunihar Formation, Simla Group, Lesser Himalaya

The Kunihar Formation records evolution of a unique rimmed shelf during the Tonian, before the Cryogenian/Marinoan glaciation of Krol Formation, Lesser Himalaya. The mixed siliciclastic-carbonate system of Neoproterozoic Kunihar Formation provokes interest for outcrop based facies analysis and sequence stratigraphy in order to frame a better understanding on the mechanism and control of its deposition. The formation records a 450 m thick mixed siliciclastic-carbonate rimmed shelf. Five facies associations have been identified- Peritidal-FA1, Lagoonal-FA2, Barrier island/Ooidal shoal-FA3, Reef complex-FA4 and Fore reef-FA5 facies associations. Two systems tracts were delineated - transgressive (TST) and highstand (HST) systems tracts. TST is marked by dominant aggradation of stromatolitic carbonates, while siliciclastic incursion surpassed carbonate deposition during highstand. It conforms to the “lower carbonate-upper clastic” model of mixed siliciclastic-carbonates. During the Precambrian, reef rimmed shelves were considered more abundant in the Archaean and Palaeo-Mesoproterozoic. This study determines that development of rimmed shelves was more common during the Neoproterozoic than previously speculated. Neoproterozoic reef rimmed shelves are concentrated more within the Cryogenian and Ediacaran Period. Stromatolitic reef rimmed shelf of Kunihar Formation is the only rimmed shelf recorded in the Tonian Period till date. Increased microbial stromatolitic accretion was propelled by surge in nutrients due to chemical weathering of underlying tholeiitic basalt (Darla volcanics) associated with the rifting of supercontinent Rodinia. With increasing interest of Tonian-Cryogenian transition as a key period in both planetary and organic evolution, Kunihar Formation represents a terminal reef rimmed shelf system that evolved due to microbial stromatolites in the absence of complex organisms. Subsequent to this, Cryogenian and Ediacaran biota took over as the dominant reef builders. • First detailed sequence stratigraphic analysis of Neoproterozoic mixed siliciclastic-carbonate Kunihar Formation. • Sequence stratigraphic model is similar to “lower carbonate-upper clastic” model of mixed siliciclastic-carbonates. • Rimmed shelves were common during the Neoproterozoic with more concentration in Cryogenian and Ediacaran. • Stromatolitic rimmed shelf of Kunihar Formation is the only rimmed shelf recorded in the Tonian Period till date.

Facies patterns and depositional processes in two Frasnian mixed siliciclastic-carbonate systems in the Cantabrian Mountains, northwest Spain

Abstract Relative sea level fluctuations during the Frasnian generated two shallow-marine, mixed siliciclastic-carbonate successions in the Devonian Asturo-Leonese Basin. Each system represents a third-order sequence-stratigraphical unit deposited in the same basin during comparable extreme greenhouse conditions without nearby fluvial entry points. Depositional control on the siliciclastic and carbonate distribution was driven by relative sea level fluctuations, basin geometry, availability of sand and the way sediment was distributed by shelf currents. Early Variscan flexural bending of the continental crust changed the basin shape from a shelf with a gradual profile and low dip (early Frasnian) towards a shelf with a steep depositional dip (late Frasnian). Shelf distribution changed from along-shelf transport (early Frasnian) towards offshore-directed gravity flows (late Frasnian). As a consequence, siliciclastic-carbonate distribution changed from a predominance of skeletal carbonate in the proximal shoreface – foreshore area and siliciclastic predominance distally (early Frasnian), to a distribution pattern with proximal shoreface skeletal carbonates, offshore muddy carbonates and a siliciclastic zone in between where gravity flows distributed the siliciclastic sediment down dip (late Frasnian).

Sedimentary and tectonic controls on Lower Carboniferous (Visean) mixed carbonate–siliciclastic deposition in NE England and the Southern North Sea: implications for reservoir architecture

Discovery of the Breagh gas field in the Southern North Sea (SNS) has demonstrated the potential that the Lower Carboniferous (Visean, 346.7–330.9 Ma) Farne Group reservoirs have to contribute to the UK's future energy mix. New biostratigraphic correlations provide a basis to compare Asbian and Brigantian sedimentary cores from the Breagh Field and age-equivalent sediments exposed on the Northumberland Coast, which has proved critical in gaining an understanding of exploration and development opportunities. Thirteen facies associations characterize the mixed carbonate–siliciclastic system, grouped into: marine, delta front, delta shoreface, lower delta plain and upper delta plain gross depositional environments. The facies associations are interpreted as depositing in a mixed carbonate and siliciclastic fluvio-deltaic environment, and are arranged into coarsening- and cleaning-upward cycles (parasequences) bounded by flooding surfaces. Most cycles are characterized by mouth bars, distributary channels, interdistributary bays and common braided rivers, interpreted as river-dominated deltaic deposits. Some cycles include rare shoreface and tidally-influenced deposits, interpreted as river-dominated and wave- or tide-influenced deltaic deposits. The depositional processes that formed each cycle have important implications for the reservoir net/gross ratio (where this ratio indicates the proportion of sandstone beds in a cycle), thickness and lateral extent. The deltaic deposits were controlled by a combination of tectonic and eustatic (allocyclic) events and delta avulsion (autocyclic) processes, and are likely to reflect a changing tectonic regime, from extension within elongate fault-bounded basins (synrift) to passive regional thermal subsidence (post-rift). Deep incision by the Base Permian Unconformity across the Breagh Field has removed the Westphalian, Namurian and upper Visean, to leave the more prospective thicker clastic reservoirs within closure. Thematic collection: This article is part of the Under-explored plays and frontier basins of the UK continental shelf collection available at: https://www.lyellcollection.org/cc/under-explored-plays-and-frontier-basins-of-the-uk-continental-shelf

Middle Eocene mixed carbonate-siliciclastic systems in the southern Caribbean (NW colombian margin)

Middle Eocene carbonates and mixed carbonate-siliciclastic deposits occur in disconnected outcrops in the San Jacinto Fold Belt in northern Colombia. Three lithofacies were distinguished: A) claystones and siltstones with planktonic components; B) packstones/rudstones with red algae and claystone-siltstone rip-up clasts in medium to thick plane-parallel, plane-convex, and channel-shaped beds; and C) poorly-bedded rhodolith rudstones with minor larger benthic foraminifers. The latter are interpreted as rhodolith beds accumulated in middle-ramp settings. Lithofacies B consists of bioclasts from the middle ramp redeposited as sediment gravity flows in individual beds and in small channel-lobe systems in the outer ramp and slope. The autochthonous sedimentation basinward from the middle ramp was dominated by hemipelagic claystones and siltstones (Lithofacies A). In outcrops with exposure of vertical successions, these facies are arranged in a transgressive pattern. The limited continuity of outcrops prevents any assessment of the original extension of ramps and lateral dimensions of channel-lobe systems. Comparison with modern tropical rhodolith beds suggests that the middle Eocene carbonate factories in the southern Caribbean developed on ramps under mesotrophic conditions.

Mixed siliciclastic–carbonate systems and their impact for the development of deep-water turbidites in continental margins: A case study from the Late Jurassic to Early Cretaceous Shelburne subbasin in offshore Nova Scotia

Progradation of deltaic systems that reach the shelf edge is considered a primary mechanism to deliver sand to continental margins. Sediment bypass processes can dominate the outer shelf to upper slope transition, causing poor preservation of reservoir quality sandstones. Turbidites can carry the bulk of the coarse fraction downdip where a thicker sand pile can be deposited once the channel-to-lobe transition is reached. Predicting this lateral and vertical variability along the slope is challenging. This configuration can be further complicated when mixed siliciclastic–carbonate systems are present. The Roseway–Missisauga case study from Nova Scotia is used here to explore potential implications associated with the development of deep-water turbidites that are time equivalent to outer-shelf mixed siliciclastic–carbonate units. Two scenarios are possible: (1) The carbonate factory is dominant, and the development of carbonate reefs and pinnacles on the outer shelf prevents the passage of siliciclastic systems beyond the shelf break; in this case, the siliciclastic component is sequestered within the inner-outer shelf. (2) Favorable conditions for carbonate production gradually deteriorate by the activation of fluviodeltaic systems that prograde outboard reaching the slope region. In this last scenario, low relief and lateral discontinuous carbonate shoals are ubiquitous in the outer shelf representing the last outboard remnants of the carbonate factory. Shelf-edge deltas circumvent or breach these carbonate shoals, establishing sedimentary pathways on the shelf-break region that connect with deep-water turbidites. Observations suggest that this last scenario is the most likely in this part of the Scotian margin during the Late Jurassic to Early Cretaceous.

Late Eocene – Early Miocene facies and stratigraphic development, Taranaki Basin, New Zealand: the transition to plate boundary tectonics during regional transgression

AbstractEight latest Eocene to earliest Miocene stratigraphic surfaces have been identified in petroleum well data from the Taranaki Basin, New Zealand. These surfaces define seven regional sedimentary packages, of variable thickness and lithofacies, forming a mixed siliciclastic–carbonate system. The evolving tectonic setting, particularly the initial development of the Australian–Pacific convergent margin, controlled geographic, stratigraphic and facies variability. This tectonic signal overprinted a regional transgressive trend that culminated in latest Oligocene times. The earliest influence of active compressional tectonics is reflected in the preservation of latest Eocene – Early Oligocene deepwater sediments in the northern Taranaki Basin. Thickness patterns for all mid Oligocene units onwards show a shift in sedimentation to the eastern Taranaki Basin, controlled by reverse movement on the Taranaki Fault System. This resulted in the deposition of a thick sedimentary wedge, initially of coarse clastic sediments, later carbonate dominated, in the foredeep close to the fault. In contrast, Oligocene active normal faulting in a small sub-basin in the south may represent the most northerly evidence for rifting in southern Zealandia, related to Emerald Basin formation. The Early Miocene period saw a return to clastic-dominated deposition, the onset of regional regression and the southward propagation of compressional tectonics.

Shallow-marine depositional sequences in a transgressive mixed siliciclastic-carbonate system: The Early Jurassic Marrat Formation from central Saudi Arabia

A high-resolution sequence-stratigraphic framework is proposed for the Marrat Formation (Early to Middle Toarcian of Saudi Arabia) by presenting new outcrop and shallow core sedimentological data located along a 280 km long N-S transect south of Riyadh and westward subsurface correlations based on gamma-ray wireline logs (200 km Ar Riyadh to Khurais Field). This extended spatial cover allows the identification of a northward thickening of sedimentary successions due to syndepositional differential subsidence. Basically, these successions formed on a low-energy falt-topped platform with limited accommodation space in which mainly retrogradational and aggradational stacking patterns were recorded. Overall, the evolution from continental meandering fluvial deposits at the base of the formation to tidal or wave-dominated siliciclastic and carbonate inner-platform deposits indicate a progressive marine transgression that reached its maximum during the Middle Toarcian bifrons Zone. In details, we identify two 3rd-order sequences that onlap southward onto the Triassic−Jurassic unconformity. Higher-energy siliciclastic shoreline facies were deposited in the upper transgressive systems tract of depositional sequences during periods of high accommodation space. The carbonate units consist of platform mud-dominated facies and are well-developed during the maximum marine transgressions of the two Marrat sequences. Continental red shales and fluvial sandstone deposited in a humid and warm environment during the Toarcian bifrons Zone (sublevisioni Subzone) indicate a higher terrigenous input interrupting the two maximum transgressive sequences. The sequences were likely produced by global sea-level changes but also appear to have been influenced by local climate changes with humid and warm conditions during periods of regression and more arid conditions during transgressions. This study serves as an outcrop analog, provides guidelines for hydrocarbon exploration and improves seismic interpretations. Overall, these new facies analyses, wireline log data, stratigraphic interpretations, and regional correlations contribute to a broader understanding of the Early Jurassic successions of the Arabian Platform.

The late Cenomanian maximum flooding Neolobites bioevent: A case study from the Cretaceous of northeast Egypt

Abstract Late Cretaceous environmental changes are manifested in several bioevents, which have been used as important tools for basin analysis, sequence stratigraphic interpretation and global correlations. One of these events is the hitherto poorly known late Cenomanian Neolobites bioevent. The present study introduces the first detailed documentation for this bioevent from northeast Egypt. The Neolobites bioevent consists of three limestone–marly limestone couplets with autochthonous biogenic concentrations. Within the study area, the bioevent displays a wide geographic distribution in Sinai and the Eastern Desert with a thickness range from 8 to 4.5 m. The Neolobites bioevent is characterised by diagnostic faunal change, condensed deposits and rich molluscan content. In addition to abundant oysters, cephalopods are present and are very commonly represented by the ammonite Neolobites vibrayeanus (lower upper Cenomanian index of the bioevent) and the nautiloid Angulithes mermeti. Sequence stratigraphic analysis of the studied succession shows that the occurrence of the Neolobites bioevent formed within late the transgressive to early highstand interval of a third-order depositional sequence and not during a transgressive stage as previously described. Therefore, the bioevent is considered here as a maximum flooding bioevent, which commonly occurs in mixed siliciclastic–carbonate systems. Palaeoecology and sedimentary facies of the bioevent indicate shallow marine conditions (deep lagoon to shallow subtidal) during a maximum rise of sea-level interval, with euhaline, moderate water energy and well-oxygenated stable sea floor sediments. The marker ammonite Neolobites vibrayeanus shows a wide palaeogeographic distribution in western Tethys and North Atlantic oceans. The maximum flooding position of the Neolobites bioevent provides a high potential for an excellent interbasinal correlation.

Tectonic and eustatic controls on the spatial distribution and stratigraphic architecture of late early Cambrian successions at the northern Gondwana margin: The siliciclastic-carbonate successions of the Lalun Formation in central Iran

Abstract The lower Cambrian Lalun Formation of the Tabas Block, Central Iran, allows to generate a sequence stratigraphic model which characterises the sedimentary evolution and improves palaeoenvironmental interpretation of sedimentary successions along the northern margin of the Prototethys. This formation provides a key for understanding processes that took place during the late early Cambrian in Iran and is the best representative for the complex mosaic of mixed siliciclastic-carbonate systems which illustrate the development and evolution of fluvial and tide-dominated estuarine systems adjacent to an open coast during this interval in the region. Detailed facies analysis allows to differentiate five vertically-stacked groups of facies associations (fluvial facies associations; estuarine facies association; open coast tidal flat facies association; carbonate ramp facies association; and delta facies association) in the uppermost interval of the Lalun Formation. The conceptual depositional models for the siliciclastic-carbonate successions of the Lalun Formation are summarised in a sequence stratigraphic framework, including three third-order depositional sequences (braided fluvial systems; incised-valley fill; and carbonate ramp and tide-influenced delta systems). The data suggest that the Central Iranian Microcontinent broke up during latest Neoproterozoic–early Cambrian times, resulting in the Proto-/Palaeotethys rifting, with considerable differential subsidence recognizable from the reactivation of faults in the early Cambrian which led to new syn-rift successions. The mosaic pattern of deposition observed among the studied sections suggests a compartmentalization of the depositional setting as a result of the interplay between tectonics, sediment supply, and eustasy. Abrupt lateral changes in facies and thickness within the sequences depositional sequences suggest that deposition was controlled by repeated tectonic downdrop and uplift as a consequence of local syndepositional, extensional fault movements. Tectonic activities in the studied region controlled sedimentation and accommodation as well as the position of depocentres within the basin, and the stratal geometries within the sequences. However, the stratigraphic architectural style of the depositional sequences and the asymmetrical trend of the transgressive and regressive second-order sequences imply that the deposition of the late early Cambrian successions is at least partly a result of eustatic sea level changes and cannot be linked exclusively to differential subsidence due to fault movements. Our study indicates that stratigraphic relationship between carbonate and siliciclastic facies in the depositional sequences was also controlled by subsidence/uplift in the hinterland and the areas of siliciclastic input rather than climatic changes. The late early Cambrian depositional environments evolved from braided rivers and an incised valley system (lowstand) to transgressive incised-valley-fill successions which are capped by transgressive open coast tidal flat and carbonate open marine facies. Depositional history of the formation terminated with a regression that ceased with progradation of tide-influenced delta systems. The stratigraphic evolution described in this study documents an eustatic sea-level fall interposed between the top of the Lalun Formation and the base of transgressive tidal bars of the overlying Fasham Formation reflecting the complex (sub-)globally recognizable regressive–transgressive event at the lower–middle Cambrian boundary interval, which led to the establishment of an extensive middle–late Cambrian carbonate platform in present-day northern and central Iran.

Sedimentary features and their controls in a mixed siliciclastic–carbonate system in a shallow lake area: An example from the BZ‐X block in the Huanghekou Sag, Bohai Bay Basin, Eastern China

Mixed siliciclastic–carbonate sediments were well developed in the BZ‐X block as a subunit in the Huanghekou Sag in the southeastern part of the Bohai Bay Basin (China) during the Member 1 of the Shahejie Formation. An integrated approach based on detailed microscopic observation and core descriptions was applied to characterize the lithofacies types, facies associations, and depositional environment of the mixed sediments system. A detailed sedimentary facies analysis suggests that various lithofacies types can be grouped into four facies associations: mixed bioclastic sandy beaches (FA1), mixed clastic bioclastic shoal (FA2), mixed sandy oolitic shoals (FA3), and mixed distal bioclastic sandy beaches (FA4). These facies associations are interpreted to be deposited in varying depositional environments ranging from foreshore to shoreface setting in a shallow lake setting. An interplay of the climate regime, nearshore carbonate productivity, and current activity was proved to control the lithofacies types, dispersion and depositional environment change of mixed sediments in different depositional phases. Meter‐scale mixed cycles were found in this shallow lake setting and their features seem to differ from the classical mixed sediment models. It is suggested that these particular cycles in the stratigraphic record might indicate a typical mark of mixed sediment system in shallow areas of the lacustrine basin.

Ordovician and Silurian ichnofossils from carbonate facies in Estonia: A collection-based review

Trace fossils are common in the Ordovician and Silurian shallow marine carbonate succession of Estonia, with 45 ichnofossil genera and five bioclaustration structures identified, representing 31 categories of architectural designs and nine categories of ethological classification. Diverse soft sediment traces, bioerosional traces and bioclaustrations occur both in the Ordovician and Silurian. Diversity of trace fossils is similarly high in the Late Ordovician and Silurian, but markedly lower in the Middle Ordovician. This could be explained by the fact that during the Late Ordovician, Baltica drifted to the subtropical climatic zone where ichnofauna is usually more diverse than in temperate climatic settings. In addition, the Great Ordovician Biodiversification reached its peak in the Late Ordovician for many groups of organisms, which further contributed to the increase in ichnodiversity. Distribution of trace fossils is also controlled by the type of sedimentation, so that the mixed carbonate-siliciclastic systems prevailing in the Late Ordovician and Silurian have higher ichnodiversity than the pure carbonate settings of Middle Ordovician age. Feeding and locomotion traces are relatively rare in the Ordovician and Silurian of Estonia with the exception of the feeding structure Arachnostega, which is formed inside of protective shells and therefore has abundant occurrences. Bioerosional trace fossils may be extremely common in places, with a large number of different genera in the Upper Ordovician, supporting the idea of the Ordovician Bioerosion Revolution.

A unified framework for modelling sediment fate from source to sink and its interactions with reef systems over geological times

Understanding the effects of climatic variability on sediment dynamics is hindered by limited ability of current models to simulate long-term evolution of sediment transfer from source to sink and associated morphological changes. We present a new approach based on a reduced-complexity model which computes over geological time: sediment transport from landmasses to coasts, reworking of marine sediments by longshore currents, and development of coral reef systems. Our framework links together the main sedimentary processes driving mixed siliciclastic-carbonate system dynamics. It offers a methodology for objective and quantitative sediment fate estimations over regional and millennial time-scales. A simulation of the Holocene evolution of the Great Barrier Reef shows: (1) how high sediment loads from catchments erosion prevented coral growth during the early transgression phase and favoured sediment gravity-flows in the deepest parts of the northern region basin floor (prior to 8 ka before present (BP)); (2) how the fine balance between climate, sea-level, and margin physiography enabled coral reefs to thrive under limited shelf sedimentation rates after ~6 ka BP; and, (3) how since 3 ka BP, with the decrease of accommodation space, reduced of vertical growth led to the lateral extension of reefs consistent with available observational data.

Expanding the spectrum of shallow‐marine, mixed carbonate–siliciclastic systems: Processes, facies distribution and depositional controls of a siliciclastic‐dominated example

AbstractMost of the present knowledge of shallow‐marine, mixed carbonate–siliciclastic systems relies on examples from the carbonate‐dominated end of the carbonate–siliciclastic spectrum. This contribution provides a detailed reconstruction of a siliciclastic‐dominated mixed system (Pilmatué Member of the Agrio Formation, Neuquén Basin, Argentina) that explores the variability of depositional models and resulting stratigraphic units within these systems. The Pilmatué Member regressive system comprises a storm‐dominated, shoreface to basinal setting with three subparallel zones: a distal mixed zone, a middle siliciclastic zone and a proximal mixed zone. In the latter, a significant proportion of ooids and bioclasts were mixed with terrigenous sediment, supplied mostly via along‐shore currents. Storm‐generated flows were the primary processes exporting fine sand and mud to the middle zone, but were ineffective to remove coarser sediment. The distal zone received low volumes of siliciclastic mud, which mixed with planktonic‐derived carbonate material. Successive events of shoreline progradation and retrogradation of the Pilmatué system generated up to 17 parasequences, which are bounded by shell beds associated with transgressive surfaces. The facies distribution and resulting genetic units of this siliciclastic‐dominated mixed system are markedly different to the ones observed in present and ancient carbonate‐dominated mixed systems, but they show strong similarities with the products of storm‐dominated, pure siliciclastic shoreface–shelf systems. Basin‐scale depositional controls, such as arid climatic conditions and shallow epeiric seas might aid in the development of mixed systems across the full spectrum (i.e. from carbonate‐dominated to siliciclastic‐dominated end members), but the interplay of processes supplying sand to the system, as well as processes transporting sediment across the marine environment, are key controls in shaping the tridimensional facies distribution and the genetic units of siliciclastic‐dominated mixed systems. Thus, the identification of different combinations of basin‐scale factors and depositional processes is key for a better prediction of conventional and unconventional reservoirs within mixed, carbonate–siliciclastic successions worldwide.

Ecological tolerances of Miocene larger benthic foraminifera from Indonesia

To provide a comprehensive palaeoenvironmental reconstruction based on larger benthic foraminifera (LBF), a quantitative analysis of their assemblage composition is needed. Besides microfacies analysis which includes environmental preferences of foraminiferal taxa, statistical analyses should also be employed. Therefore, detrended correspondence analysis and cluster analysis were performed on relative abundance data of identified LBF assemblages deposited in mixed carbonate-siliciclastic (MCS) systems and blue-water (BW) settings. Studied MCS system localities include ten sections from the central part of the Kutai Basin in East Kalimantan, ranging from late Burdigalian to Serravallian age. The BW samples were collected from eleven sections of the Bulu Formation on Central Java, dated as Serravallian. Results from detrended correspondence analysis reveal significant differences between these two environmental settings. Cluster analysis produced five clusters of samples; clusters 1 and 2 comprise dominantly MCS samples, clusters 3 and 4 with dominance of BW samples, and cluster 5 showing a mixed composition with both MCS and BW samples. The results of cluster analysis were afterwards subjected to indicator species analysis resulting in the interpretation that generated three groups among LBF taxa: typical assemblage indicators, regularly occurring taxa and rare taxa. By interpreting the results of detrended correspondence analysis, cluster analysis and indicator species analysis, along with environmental preferences of identified LBF taxa, a palaeoenvironmental model is proposed for the distribution of LBF in Miocene MCS systems and adjacent BW settings of Indonesia.

Sedimentology and facies analysis of Miocene mixed siliciclastic–carbonate deposits of the Dam Formation in Al Lidam area, eastern Saudi Arabia

The Burdigalian mixed siliciclastic–carbonate deposits of the Dam Formation are well-exposed in Al Lidam area, in the eastern province of Saudi Arabia. They represent a shallow part of the Arabian plate continental margin. The Dam Formation is correlatable to the Miocene reservoirs in both Iran and Iraq. Therefore, studying the Dam Formation lithologic heterogeneity in a small distance with high resolution could help in further work related to pattern prediction of the Miocene reservoir properties. High-resolution sedimentological investigation was carried out through six outcrops. The facies parameters (lithology, sedimentary structures, main fossils, paleocurrent patterns and geometries of the sedimentary bodies) were described. The results revealed 15 lithofacies that have been further grouped into 7 lithofacies associations 5 of which are carbonates and include (1) interbedded dolostone and evaporates, (2) microbialite buildup, (3) ooid-dominated grainstones, (4) burrowed skeletal peloidal wackestone–packstone and (5) mollusc-dominated wackestone–packstone. The remaining two associations are of siliciclastics and include (6) intertidal siliciclastics and (7) wave-dominated siliciclastics. These lithofacies were interpreted to reflect deposition in a mixed siliciclastic–carbonate ramp system that includes subtidal, shoreface, intertidal, foreshore, supratidal and estuarine deposits in a shallowing-upward succession. Each one of these lithofacies association has distinct geometry and architecture pattern. Oolites and heterozoan lithofacies occur as sheets and show great continuity along the strike direction. Oolites pass laterally in the dip direction into more skeletal- and peloid-dominated zones, whereas heterozoan lithofacies stay continuous in the dip directions and change from siliciclastic to carbonate heterozones. In contrast, microbialite lithofacies lack continuous beds and occur as localised bioherms and biostroms. Channelised lithofacies are restricted laterally into isolated channel bodies and vertically in the contact boundary between siliciclastic and carbonate lithofacies, whereas the interbedded dolostone and evaporite lithofacies form distinct, relatively thick continuous layers. With continuous exposures in both strike (1.2 km) and dip (0.15 km) directions, the outcrops in the Al Lidam area provide unique opportunity to study the heterogeneity among lithofacies of the mixed siliciclastic–carbonate system of the Dam Formation. Such study may provide insights to predict occurrence and distribution of lithofacies bodies in their equivalent reservoirs which are important for reservoir characterisation.

Types of mixing and heterogeneities in siliciclastic-carbonate sediments

Mixed siliciclastic-carbonate deposits consist of a suite of different types of mixing between the two components, from bed (core-plug) to stratigraphic (seismic) scales, producing a high vertical and lateral lithological variability. Mixed deposits results from the interaction of siliciclastic input and coeval carbonate production controlled by temporal and/or spatial factors. Although mixed deposits are very diffuse in the geological record, studies about these deposits are scrappy and not well encoded. Accordingly, mixed deposits represent a labyrinth for researchers who want to investigate them for the first time.In this paper, different types of mixing (compositional versus strata) controlled by different allocyclic (e.g. sea-level, climate) and/or autocyclic (e.g. depositional processes) factors that operate at different scale are documented. Mixing is recognized and described at three main scales of observation: bed/core-plug scale; lithofacies/well-log scale; and stratigraphic/seismic scale. (i) Compositional mixing reflects the contemporaneous accumulation of the two heterolithic fraction in space and time. This type of mixing is observable at lamina to bed scale, locally producing depositional structures diagnostic for particular depositional environments. (ii) Strata mixing results from the alternation of the two heterolithic fraction in time. This type of mixing is observable at lithofacies to stratigraphic scale and can be related to depositional processes, climatic variations and/or relative sea-level changes.A correct identification of these different types of mixing and the scale of their occurrence is crucial in revealing (i) physical processes that control the sedimentation, (ii) environmental factors that influence the carbonate factory related to the siliciclastic dispersal mechanisms, and (iii) internal heterogeneity of the resulting sedimentary deposit. Furthermore, the petroleum industry is interested to unravel new insights about internal properties of mixed siliciclastic-carbonate systems (e.g., porosity, permeability) and to reconstruct predictive 3D models for the related reservoirs. The correct prediction of internal heterogeneity and the recognition of lateral and vertical compartmentalization have an important impact on hydrocarbon exploration and exploitation.