Mixed Siliciclastic-carbonate System Research Articles

A Paleogene mixed carbonate-siliciclastic system in the western Tethys: spectral gamma-ray as a tool for the reconstruction of paleoclimate and transgressive-regressive cycles

The mixed carbonate-siliciclastic ramps of the Dauphinois-Provençal domain in northwest Italy are characterized by a common development process during the Paleogene. Spectral gamma-ray (SGR) analysis indicates the concentration of potassium (K), uranium (U), and thorium (Th) in the total gamma-ray signal, permitting the identification of subtle stratigraphic compositional trends, sedimentary minerals, paleoclimatic conditions, and the individualization of lithofacies characters through the stacking pattern cycles. This study integrates known lithological and fossil content analysis of the section with new independent data from the spectral gamma-ray (SGR) to explore the paleoclimate and stacking patterns for defining large-scale depositional cycles along a Paleogene succession in the southwestern Alps.The differences between the outcropping Microcodium Formation and the Capo Mortola Calcarenite Formation are indicated by the SGR, conventionally shown on a linear scale as American Petroleum Institute (API) units, i.e., Th/U, and Th/K ratios. Our data indicate lithofacies associated with normal marine to continental conditions and the presence of abundant smectite, rather than kaolinite, suggesting that the prevailing climate was characterized by prolonged dry periods. Furthermore, the SGR (API) reconstruction of the overall short cycles reflects a progradational and/or retrogradational stacking geometry that accumulated throughout the whole succession. However, the use of the derivative trend analysis (DTA) of the SGR indicates the establishment of long cycles in both formations. The short cycles observed in the Capo Mortola Calcarenite Formation are of particular importance because they characterize the marine transgression of the middle-upper Eocene, transgressive upon the Upper Cretaceous beds, and permit pinpointing the onset of the drowning ramp, which can be correlated regionally with sections spanning from the French-Italian Maritime Alps to the France-Switzerland border.

Stratigraphic-sedimentary evolution of a mixed siliciclastic-carbonate system in the Huizhou Sag of the Pearl River Mouth Basin, Northern South China Sea

Mixed siliciclastic-carbonate systems are regulated interactively by factors such as siliciclastic sediment supply, carbonate production, sea-level change, tectonism, and climate conditions. These systems record vital information that aids in understanding ancient environments. This study used a merged 3D seismic volume, in conjunction with over 100 industrial wells, to systematically investigate the stratigraphic-sedimentary evolution of such a system within the Huizhou Sag of the Pearl River Mouth Basin, located on the northern continental shelf of the South China Sea. In total, six major sequence boundaries were identified for the Zhujiang Formation within the area, thus subdividing the interval into five typical third-order depositional sequences. Each of these sequences can be divided into a transgressive and a highstand systems tract. Lowstand or falling stage systems tracts were also recognized, the deposition of which was potentially in response to the uplifting process of the Dongsha Rise. During the deposition of the Zhujiang Formation, the Huizhou Sag may have undergone a sequential evolutionary history from delta-shore, to delta-shore-tidal-lagoon, to delta-shore-carbonate, and finally to delta-shore-shallow marine systems. This evolution responded to a varying degree of mixing processes, which was mainly regulated by siliciclastic sediment supply, confined paleomorphology, and local oceanic currents. Furthermore, the deposition of the Zhujiang Formation in the Huizhou Area was time-equivalent with the spreading process after the ridge jump of the South China Sea (23–16.5 Ma), providing valuable insights into sea-level fluctuations, provenance changes, and tectonic evolution. Our results may also shed light on the evaluation of lithologic traps and hydrocarbon sweet spots within mixed siliciclastic-carbonate systems.

Simultaneous seismic inversion study for channel sandstone identification, northern part of the Eastern Shelf, King County, North-Central Texas

This paper documents the results of a prestack simultaneous seismic inversion conducted to investigate the Wolfcampian lower Tannehill sandstone channels within a mixed carbonate siliciclastic system in the northern Eastern Shelf in King County, North Central Texas. Previous exploration methods for the Tannehill sandstone were based largely on well-log correlation which, because of their widely spaced sample points, are not optimal for channel delineation. We have generated crossplots of P- and S-wave velocity ratios ([Formula: see text]/[Formula: see text] ratios) versus P-impedance to determine lithologic types. We then used 3D seismic data and wireline logs to implement a prestack simultaneous seismic inversion procedure to generate P- and S-impedances, density, and [Formula: see text]/[Formula: see text] ratio volumes to identify the valley-fill Tannehill lowstand systems. Our results find that (1) based on impedance and density, the sandstone-rich channels can be easily separated from the carbonate-rich zones but are difficult to distinguish using [Formula: see text]/[Formula: see text]. (2) Although there appeared to be several channel systems in the area of investigation, two of these, hereby referred to as channel A and channel B, are recognized to be the key contributors of sandstone sediments to the shelf edge, slope, and basin areas. (3) Of these two channels, channel A is the larger and highly sinuous; it is associated with a meander loop, a point bar, and an abandoned channel. At the point-bar location, channel A has a width of 890 m (2920 ft) and a depth of 29 m (94 ft). The point bar is structurally anticlinal and thus constitutes a potential exploration target. In contrast, channel B is less sinuous, without any associated point bar. It has a maximum depth and width of 23 m (75 ft) and 270 m (886 ft), respectively. Finally, the inversion results suggest that other potential Tannehill sandstone reservoirs may still be present elsewhere within King County.

Expression of the Toarcian Oceanic Anoxic Event in the mixed siliciclastic–carbonate system of the Arabian Plate: Insights from rare-earth element geochemistry and statistical analysis

Rare earth elements and yttrium (REEY) are considered powerful indicators of paleo-redox conditions. However, to date, the utilization of REEY remains limited to either pure carbonates or deep marine shales, with little evidence of attempts to test their ability to depict marine anoxia within mixed carbonate–siliciclastic systems. Across the Tethys Ocean, the Toarcian Oceanic Anoxic Event (T-OAE) is widely recorded by black marine shales with strong negative carbon isotopic excursion. Yet, the occurrence and expression of T-OAE across the mixed siliciclastic and carbonate system of the Arabian Plate has not been recorded or even explored. This provides a unique opportunity to test the robustness and viability of REEY, supported by statistical analysis, in revealing marine anoxia in a mixed system. Our results demonstrate a positive Ce anomaly within the middle Marrat, coupled with a positive Eu anomaly and aluminum-normalized Mn enrichment in the underlying lower Marrat carbonates. This could be hypothetically related to volcanic pulses, which triggered the T-OAE. Overall, the REEY geochemical record and detailed statistical analysis of this study indicate, for the first time, the possibility that T-OAE might have extended to the Arabian Plate for brief periods of time during the deposition of the middle Marrat shallow marine red beds.

Prestack Seismic Velocity Ratio Evaluation of a Mixed Siliciclastic–Carbonate Formation: Case Study from the Strawn Group on the Eastern Shelf Texas

Although a mixed carbonate–siliciclastic system of the Strawn Group on the Eastern Shelf in King County, Texas, USA provides excellent hydrocarbon reservoirs, facies variability and reservoir properties within such systems are not well understood. We conducted prestack, simultaneous seismic inversion, and high-level petrophysical analysis to derive elastic properties of rocks to facilitate lithology identification and determination and distribution of the different carbonate facies. Our results show that (1) the Strawn Group in King County is dominated mostly by carbonates and (2) given the ratio of P- and S-wave velocity (Vp/Vs ratio), the carbonates can be separated into three facies: (a) high-Vp/Vs-ratio shelf-edge reef carbonates, in which the Vp/Vs ratio decreases linearly as porosity increases and the Vp/Vs ratio varies from ~2.1 to ≤2.6; (b) moderately low-Vp/Vs-ratio shelf (platform) carbonates, in which the Vp/Vs ratio also decreases as porosity increases and in which the Vp/Vs ratio ranges from ~1.75 to ≤2.15; (c) extremely low-Vp/Vs-ratio slope and basin carbonates, in which the Vp/Vs ratio, although appearing to be almost constant for a wide range of porosity, increases as porosity increases, and in which most Vp/Vs-ratio values appear to range from ~1.5 to ≤2. Results of a through c can be summarized thusly: the Vp/Vs ratio of reef carbonates >the Vp/Vs ratio of platform carbonates and >the Vp/Vs ratio of slope and basin carbonates in the study area.

Depositional processes of the mixed siliciclastic-carbonate system in the Lower Cambrian Xiannüdong Formation in the northern Sichuan Basin, South China

Mixed siliciclastic-carbonate systems contain significant heterogeneity of facies associations which complicates the interpretation of depositional environments and platform evolution. The Lower Cambrian Xiannüdong Formation in the northern Sichuan Basin was deposited in a mixed system which is characterized by the non-skeleton grain (ooids and microbialites) dominated carbonates mixed with siliciclastic sediments. Such a depositional system provides a template to decipher the facies variability of mixed depositional systems and the evolution of platform geometry. Based on the facies analysis in outcrops and drilling wells, seventeen lithofacies were identified and grouped into seven lithofacies associations which represents a shallow-marine environment dominated by wave and storm activities. The vertical stacking pattern of lithofacies in the Xiannüdong Formation archives a regressive cycle which can be subdivided into four sub-cycles (i.e., S1 to S4). All the four sub-cycles exhibit the characteristic of the shallowing-upward successions, which are overlain by a flooding unit. The spatial distribution of lithofacies suggests an eastward-inclined platform. The migrating ooid shoals together with the storm activities may prevent the platform from evolving into a rimmed shelf. The lateral and vetical distribution of the terrigenous materials indicates that the provenance of terrigenous sediments were mainly sourced from the Kangdian and Bikou massifs, which indicates that the mixed system and the provenance area were separated by the Mianyang-Changning trough. Therefore, the transport of siliciclastic materials was related to the available accommodation space on the shelf, and was controlled by the eustatic sea-level fluctuation, with increased input of siliciclastic material occurred during phases of relative sea-level rise. The input of siliciclastic sediments brings nutrition for the growth of algal–archaeocyathan mounds, and therefore the mounds were mainly developed in the lower part of Xiannüdong Formation. Moreover, high siliciclastic sediments influx would also influence the growth of ooids by serving nuclei and promoting abrasion, which resulted in smaller ooid size compared to those of the relatively pure oolite. This study provides clues to understanding the facies variability in mixed systems and servers as a valuable reference for studies of other similar platforms.

Temporal and spatial variations in calcium carbonate deposition in a mixed siliciclastic–carbonate deep marine system: the Ediacaran Deeside Limestone Formation, Aboyne, Scotland

The Deeside Limestone Formation (DLF), outcropping in western Aberdeenshire, comprises the eastern margin of a distinctive Upper Dalradian mixed siliciclastic–carbonate system of Ediacaran age which may be correlated southwestwards for more than 400 km to County Donegal in Ireland. A reconstructed stratigraphic column suggests three broad vertical sequences (S1–S3), each comprising a general upward proportionate increase in calcium carbonate with respect to quartz, locally capped by metalimestones (L1–L3). A simple explanation for this upward change lies in the differing physical response of the two heterolithic components under the same hydrodynamic process, with the heavier quartz grains preferentially enriching the bedload and the finer carbonate mud fraction, the suspended load. The three metalimestone intervals are observed only in central/eastern parts of the field area, suggesting a lateral facies variation. The predominance of calcareous–siliciclastic bed-scale compositional mixing within the DLF suggests sedimentation on the Dalradian shelf comprised contemporaneous mixing of the siliciclastic and carbonate fractions prior to subsequent re-mixing during transportation downslope into the deep water, punctuated by pulses of point-sourced siliciclastic input.

Evaluation of distinct soft-sediment deformation triggers in mixed carbonate-siliciclastic systems: Lessons from the Brazilian Pre-Salt analogue Crato Formation (Araripe Basin, NE Brazil)

Soft-sediment deformation structures (SSDSs) are ubiquitous in several depositional sedimentary environments and can be triggered by autogenic- and allogenic-related mechanisms. SSDSs identification in the geological record is more frequent in siliciclastic deposits, from which the most accepted models were developed. Given the fact that carbonate rocks have rapid diagenesis, which results in changes in the rheological behavior of deformational processes in short time, these are excellent facies for exploring past deformational mechanisms in a given sedimentary basin. Here, we describe distinct SSDSs developed in mixed carbonate-siliciclastic successions deposited in an Early Cretaceous lacustrine depositional system (carbonate-dominated) subjected to recurrent epicontinental marine ingressions (siliciclastic-dominated). Deformation occur in some siliciclastic intervals but it is more common in carbonate beds. Facies associations analysis combined with detailed outcrop description of the SSDSs allowed the recognition of seismic- and non-seismic-generated structures. Two km-scale, laterally continuous carbonate beds, characterized by deformation-bearing horizons, are interpreted to be the result of seismic shocks. Conversely, most of the recognized SSDSs cannot be confidentially interpreted as seismites. Autogenic triggers and criteria for accessing the origin of deformation mechanisms in both carbonate- and siliciclastic-dominated settings are alternatively proposed. Assessing such criteria is fundamental for differentiatte the timing and range of deformation and are here discussed in the context of the offshore petroleum reservoirs in the Brazilian Pre-Salt successions, where lacustrine carbonates were probably subjected to a similiar superposition of syn-sedimentary seismicity and later events of salt-tectonics and hydrotermalism.

Why is it easy to form high-quality reservoirs in a mixed siliciclastic-carbonate system? Evidence from diagenetic characteristics

Mixed siliciclastic-carbonate reservoirs are considered as new important high-quality exploration targets because giant hydrocarbon accumulations have continually been discovered in these reservoirs in continental rift basins. This study used a combination of reservoir geological and geochemical evidence to investigate the characteristics and formation mechanisms of mixed siliciclastic-carbonate reservoirs in the first and second members of the Shahejie Formation in the Bozhong Sag, Bohai Bay Basin, China. The results show that bioclastic-dominated mixed rocks represent high-quality reservoirs because they did not only preserve a amount number of primary pores but also preserved a fair proportion of secondary pores that contain organic matter fabrics or associated secondary pores. The main diagenetic processes recognized in the mixed sediment reservoirs are micritization, cementation, dissolution and compaction. Micritization in the contemporaneous stage, dolomitization in the penecontemporaneous stage and leaching of meteoric freshwater in the early diagenetic process are the main controlling factors leading to the formation of high-quality reservoirs. The micritic envelope effectively resists compaction, preserving a high amount of primary porosity, and the early leaching of meteoric freshwater can produce a large amount of secondary dissolution pores. This study does not only reveal the genetic mechanism of high-quality reservoirs of mixed rocks in continental basins but also provide significant guides that can be utilized to search and produce from new types of high-quality reservoirs.

Regional stratigraphy and proximal-to-distal variation of lithology and porosity within a mixed carbonate-siliciclastic system, Mississippian strata of northern and central Oklahoma

Mississippian carbonate and silica-rich reservoirs of northern and central Oklahoma formed along a regionally extensive carbonate ramp to basin transect. The stratigraphy, lithology, and porosity characteristics of the Mississippian Meramec and Osage series significantly vary as older ramp carbonates prograde southward and transition into younger calcareous and quartz-rich silt deposits of the Anadarko Basin. Lithofacies identified within the northern carbonate-dominated portion of the system commonly include altered chert, skeletal grainstones, peloidal packstones-grainstones, bioturbated wackestones-packstones, bioturbated mudstones-wackestones, glauconitic sandstones, and siliceous shale. Lithofacies within the southern siliciclastic-dominated portion of the system include structureless to bioturbated sandstones, siltstones, and laminated mudstones, each with varying degrees of carbonate content. We have grouped these core-based lithofacies into dominant lithologies/rock types, which tie to well-log properties. Electrofacies classification methods including the artificial neural network (ANN) and k-means clustering predict lithologies in noncored wells. ANN yielded the highest overall prediction accuracy of 85% for lithologies. Core, well log, and lithology log data establish the regional stratigraphic framework. The Mississippian interval of interest subdivides into 16 stratigraphic zones. A depositional-dip-oriented cross section and the associated reservoir models illustrate the proximal to distal and stratigraphic variability of the lithology and porosity. Lithology trends moving from north to south, from older to younger strata, reveal a carbonate-dominated succession capped by diagenetically altered chert northward shifting into a siliciclastic-dominated interval, which increases in clay content southward. Northward, prospective conventional reservoirs developed near cycle tops within diagenetically replaced cherts and cherty limestones associated with subaerial exposure and sea-level fluctuations. Southward, higher total porosity associates with increased clay content linked to the suppression of calcite cement, forming prospective unconventional targets near the bases of depositional cycles.

Lithofacies, depositional, and diagenetic controls on the reservoir quality of the Mississippian mixed siliciclastic-carbonate system, eastern Anadarko Basin, Oklahoma, USA

In the eastern Anadarko Basin of central Oklahoma, the variability of Mississippian lithofacies, diagenetic products, and reservoir quality is critical for reservoir development. We have investigated lithofacies variability based on sedimentological characteristics and diagenetic alteration through integration of core and thin sections by using optical microscope, scanning electron microscope, energy dispersive X-ray diffraction spectroscope, and electron probe microanalyzer-cathodoluminescence analyses. Based on detailed descriptions of five cores (approximately 260 m [approximately 850 ft]) and analysis of 34 thin sections, we concluded that the Mississippian strata consists of eight lithofacies that represent wave-influenced nearshore with restricted embayment (lagoon) and channel or lobe deposits. We observed diagenetic alteration including calcite cementation, mechanical compaction, albitization, quartz cementation, silicification, dolomitization, Fe-dolomite cementation, pyritization, and dissolution. A paragenesis scheme suggests that quartz cementation occurred earlier compared to albitization and Fe-dolomite cementation. The Fe-dolomite is the latest authigenic mineral formed, whereas the quartz and calcite cement can be attributed to earlier diagenesis. The calcite, quartz, and Fe-dolomite cementation might have potentially increased the brittleness index and frackability of the rocks. The reservoir quality is relatively good in the channel or lobe deposits and is generally poor in the upper shoreface to upper offshore environments. The reservoir quality is significantly reduced by clays, calcite cement, and mechanical compaction. However, the dissolution of calcite cement and detrital grains tends to improve reservoir quality by forming secondary pores. We prove that understanding the characteristic of lithofacies variation, depositional environments, and diagenetic alterations of the Mississippian strata is crucial for optimal development of the Mississippian reservoirs in the eastern Anadarko Basin. We develop a predictive framework that aids in reservoir characterization.

Neogene to Quaternary evolution of carbonate and mixed carbonate-siliciclastic systems along New Caledonia's eastern margin (SW Pacific)

Neogene and Quaternary shallow-water carbonate records surrounding New Caledonia main island, Grande Terre, provide a good example for understanding the stratigraphic architecture of tropical mixed carbonate-siliciclastic systems. Due to a southeastern tilt of the eastern margin, the eastern shelf of Grande Terre has been better preserved from erosion than the western part, favouring the development and preservation of shallow-water carbonates. Based on the integration of bathymetric and seismic data, along with paleoenvironmental and biostratigraphic constraints derived from dredged carbonate rocks, a comprehensive geomorphological and architectural characterization of the offshore eastern margin of Grande Terre has been made. During the Mio-Pliocene, a wide, up to 750 m-thick carbonate build-up developed and extended over at least 350 km from north to south. This Mio-Pliocene build-up, currently lying at 300 to 600 m water depths, is overlain by a Pleistocene-Holocene barrier reef-lagoon complex and associated slope deposits. The switch from aggrading Neogene carbonate banks to backstepping Quaternary platforms likely reflects an increase in accommodation due to a high subsidence rate or to relative sea-level rise, and/or results from a switch in carbonate producers associated with global environmental changes. The internal architecture of the Quaternary barrier reef-lagoon complex is highlighted, especially the development of lowstand siliciclastic prisms alternating with transgressive shallow-water carbonate sequences. This pattern agrees with the reciprocal sedimentation model typically invoked for mixed sedimentary systems. This stratigraphic pattern is well developed in front of the Cap Bayes inlet in the north of our study area, yet it is not observed southward along the eastern margin. This difference suggests that other factors than relative sea-level variations directed the architecture of the margin, such as low terrigenous inputs, lagoon paleo-drainage networks or sediment by-pass towards deep basins.

Sedimentary facies, depositional environments and conceptual outcrop analogue (Dam Formation, early Miocene) Eastern Arabian Platform, Saudi Arabia: a new high-resolution approach

This paper presents the facies and depositional environment of the early Miocene Dam Formation, Eastern Arabian platform, Saudi Arabia. Deposition of Dam Formation (Fm.) was considered as a restricted shallow marine deposition. Few studies suggest the role of sea-level change in its deposition but were without decisive substantiation. Here, we describe the facies and high-resolution model of Dam Fm. under varying depositional conditions. The depositional conditions were subjected to changing relative sea level and tectonics. High-resolution outcrop photographs, sedimentological logs, and thin sections present that the mixed carbonate–siliciclastic sequence was affected by a regional tectonics. The lower part of Dam Fm. presents the development of carbonate ramp conditions that are represented by limestones and marl. The depositional conditions fluctuated with the fall of sea level, and uplift in the region pushed the siliciclastic down-dip and covered the whole platform. The subsequent rise in sea level was not as pronounced and thus allowed the deposition of microbial laminites and stromatolitic facies. The southeast outcrops, down-dip, are more carbonate prone as compared to the northwest outcrop, which allowed the deposition of siliciclastic-prone sedimentation up-dip. All facies, architecture, heterogeneity, and deposition were controlled by tectonic events including uplift, subsidence, tilting, and syn-sedimentary faulting, consequently affecting relative sea level. The resulting conceptual outcrop model would help to improve our understanding of mixed carbonate–siliciclastic systems and serve as an analogue for other stratigraphic units in the Arabian plate and region. Our results show that Dam Fm. can be a good target for exploration in the Northern Arabian Gulf.

Mixed carbonate–siliciclastic tidal sedimentation in the Miocene to Pliocene Bouse Formation, palaeo‐Gulf of California

AbstractMixed carbonate–siliciclastic deposits provide unique insights into hydrodynamic processes that control sedimentation in tidal systems. This study presents sedimentological and ichnological data from the upper Miocene to lower Pliocene Bouse Formation, which accumulated during regional transgression at the margin of a tidal strait near the north end of the ancestral Gulf of California. The basal carbonate member of the Bouse Formation records deposition in a tide‐influenced, compositionally mixed carbonate–siliciclastic system dominated by salt marsh, tidal flat and channel environments. The basal carbonate member is an overall deepening up succession of facies associations comprising: Facies Association 1 – siliciclastic‐rich heterolithic facies, lime mudstone with desiccation cracks, and plant debris rich carbonate silt interpreted as siliciclastic‐rich tidal flats; Facies Association 2 – well‐sorted gravels, siliciclastic‐rich sandy strata, lime mudstone with desiccation cracks, and sandy microbial micrite interpreted as tidal‐channel deposits; Facies Association 3 – carbonate‐rich heterolithic lime mudstone to well‐sorted, cross‐bedded bioclastic grainstone interpreted as intertidal to shallow subtidal deposits; and Facies Association 4 – lime mudstone interpreted as shallow to deep subtidal low‐energy deposits that record the end of tidal conditions in the basin. Trace fossils include marine forms Gyrolithes, Teichichnus and Thalassinoides, and non‐diagnostic forms Arenicolites, Cochlichnus, Conichnus, Lockeia, Planolites, Skolithos and Treptichnus (known from marine, brackish and freshwater environments). The diminutive size of trace fossils reflects brackish conditions created by mixing of freshwater and seawater. This study provides evidence for a late Miocene to early Pliocene humid climate in south‐western North America, in stark contrast to the modern hyperarid climate. Factors that controlled the relative percentage of mixed carbonate and siliciclastic sediment include siliciclastic input from local rivers, in situ carbonate production, current energy, degree of tidal mixing and relative sea level. Pronounced facies variability at bedform, outcrop and basin scale documented in this study appears to be an important characteristic of mixed carbonate–siliciclastic deposits in tidal depositional systems.

Syn-rift carbonate platforms in space and time: testing and refining conceptual models using stratigraphic and seismic numerical forward modelling

AbstractUnderstanding and predicting architecture and facies distribution of syn-rift carbonates is challenging owing to complex control by climatic, tectonic, biological and sedimentological factors. CarboCAT is a three-dimensional stratigraphic forward model of carbonate and mixed carbonate–siliciclastic systems that has recently been developed to include processes controlling carbonate platform development in extensional settings. CarboCAT has been used here to perform numerical experiment investigations of the various processes and factors hypothesized to control syn-rift carbonates sedimentation. Models representing three tectonic scenarios have been calculated and investigated, to characterize facies distribution and architecture of carbonate platforms developed on half-grabens, horsts and transfer zones. For each forward stratigraphic model, forward seismic models have also been calculated, so that modelled stratal geometries presented as synthetic seismic images can be directly compared with seismic images of subsurface carbonate strata. The CarboCAT models and synthetic seismic images corroborate many elements of the existing syn-rift and early-post-rift conceptual model, but also expand these models by describing how platform architecture and spatial facies distributions vary along-strike between hanging-wall, footwall and transfer zone settings. Synthetic seismic images show how platform margins may appear in seismic data, showing significant differences in overall seismic character between prograding and backstepping stacking patterns.

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.