Deep Shelf Facies Research Articles

Characteristics and genesis of reef-bank complexes in deep shelf facies: A case study of the Middle-Late Jurassic in the northern Amu Darya Basin, Central Asia

A large-rimmed carbonate platform was developed in the Amu Darya Basin during the Middle-Late Jurassic Callovian-Oxfordian period. What distinguishes it from typical carbonate platforms is that a series of reef-bank complexes had extensively developed in the deep shelf depositional zone of the basin. However, only a few studies have reported on the classification, characteristics, and genesis of these reef-bank complexes in relatively deep water, greatly limiting the development of deep-water carbonate sedimentology. To address this issue, the types and the genesis of reef-bank complexes in the deep shelf environment have been clarified based on the systematic petrography, seismic sedimentology, and geomorphology study of the Callovian-Oxfordian carbonate rocks in the northern Amu Darya Basin during the Middle-Late Jurassic period. The results find that the reef-bank complexes are widely distributed in the deep shelf environment in the study area with laminar, reticulated, and zonal distributions. The reef-bank complexes include barrier-bonding reef-bank complexes, lime-mud mounds (LMMs), and granular shoals (GSs). The deep shelf environment can be further divided into an inner shelf, shelf margin, and shelf slope. The inner shelf and shelf margin have a relatively shallow water body and a strong sedimentary hydrodynamic force, mainly developing reticulated reef-bank complexes and laminar GSs, whereas the shelf slope mostly develops zonal LMM deposits in strips. The scale of the reef-bank complexes is mainly controlled by basement paleogeomorphology and water energy. Relatively high-energy reef-bank complex bodies are developed on the seaward side of the paleouplift limb with relatively turbulent hydrodynamic conditions, whereas low-energy LMMs are mostly developed on the high position of paleouplift and landward side. The obtained findings can deepen our understanding of relatively deep-water carbonate sedimentation and enrich the carbonate sedimentation theory.

Microfacies types and distribution of epicontinental shale: A case study of the Wufeng–Longmaxi shale in southern Sichuan Basin, China

For black shales, laminae and bedding are hard to identify, grain size is difficult to measure, and trace fossils do not exist. Taking the Ordovician Wufeng – Silurian Longmaxi shale in southern Sichuan Basin, China, as an example, the types, characteristics and models of microfacies in epicontinental shale are analyzed by means of full-scale observation of large thin sections, argon-ion polishing field emission-scanning electron microscopy (FE-SEM), and kerogen microscopy. The epicontinental sea develops delta, tidal flat and shelf facies, with black shale found in microfacies such as the underwater distributary channel and interdistributary bay under delta front facies, the calcareous and clayey flats under intertidal flat facies, the calcareous and clayey shelfs under shallow shelf facies, the deep slope, deep plain and deep depression under deep shelf facies, and the overflow under gravity flow facies. Basinward, silty lamina decreases and clayey lamina increases, the grain size changes from coarse silt to fine mud, the silica content increases from about 20% to above 55%, the carbonate and clay minerals content decreases from above 40% to around 10%, and the kerogen type changes from type II2 to type II1 and type I. Provenance and topography dominate the types and distribution of shale microfacies. The underwater distributary channel, interdistributary bay, clayey flat, clayey shelf, and overflow microfacies are developed in areas with sufficient sediment supply. The calcareous flat and calcareous shelf are developed in areas with insufficient sediment supply. The deep shelf shale area is divided into deep slope, deep plain, and deep depression microfacies as a result of three breaks. The formation of epicontinental shale with different microfacies is closely related to the tectonic setting, paleoclimate, and sea level rise. The relatively active tectonic setting increases the supply of terrigenous clasts, forming muddy water fine-grained sediment. The warm and humid paleoclimate is conducive to the enrichment of organic matter. The rapid sea level rise is helpful to the widespread black shale.

Neoproterozoic extensional basins and its control on the distribution of hydrocarbon source rocks in the Yangtze Craton, South China

• Neoproterozoic source rocks in the Yangtze Craton. • Structural and sedimentary patterns of the extensional basins evaluated. • The source rocks are related to the extensional basins, climate and life evolution. • Favorable areas for the source rock distribution are predicted. Hydrocarbon exploration in the Neoproterozoic extensional basins in the Yangtze Craton of South China has made prominent progresses in the past decades. However, the spatial distribution of source rocks across the craton still remains unclear. Here, we use outcrop observations, borehole data, and reflective seismic profiles to investigate the prototype, distribution and sediment infilling of these basins in the Yangtze Craton during the periods of source rock deposition. The basin prototype is categorized based on its tectonic origin and structural-sedimentary feature as follows. (1) During the deposition of the Datangpo Formation, rift basins extensively developed, band are characterized by a series of graben or half graben structures that consist of basin-controlling normal faults. The source rocks in the Datangpo Formation were deposited as the inner shelf basin or deep shelf facies, obtaining high thickness (i.e., 200 m) in the Wuling rift basin. (2) During the deposition of the Doushantuo Formation, the intracratonic rift basin and passive continental margin basin formed. These basins were controlled by syn-depositional faults that inherited the existing rift fault structures. The source rocks of the Doushantuo Formation were deposited as shelf lagoon, slope to deep-water basin facies. (3) During the deposition of the Dengying Formation, the intraplatform rift basin and passive continental margin basin developed with influence of regional tectonic extension. Not only are these basins affected by the preexisting faults, but also by the newly developed syn-depositional faults. The argillaceous source rocks of the Dengying Formation deposited as slope and deep-water basin facies. Further in-depth analyses revealed that a range of mechanisms may have contributed to the formation and distribution of source rocks in the Yangtze Craton, such as the break-up of the Rodinia Supercontinent, paleaolatitudes, glacial-interglacial geological events, basin extension and sedimentary environments. By integrating the distribution of the source rocks and the late tectonic reworking, this study predicts several targets for the source rock distribution in the Yangtze Craton, including in the central Sichuan, northeastern Sichuan, southern Shanxi, western Hubei, northeastern Guizhou-western Hunan, and the northeastern Guangxi-central Hunan areas.

Re-recognition of “unconventional” in unconventional oil and gas

Taking the Wufeng–Longmaxi shale gas in the Sichuan Basin as a typical example, based on the new progress in exploration and development, this study re-examines the “unconventional” of unconventional oil and gas from two aspects: oil and gas formation and accumulation mechanisms, and main features of oil and gas layers. The oil and gas of continuous accumulation and distribution from integrated source and reservoir is unconventional oil and gas, and the study focusing on shale oil and gas in comparison with conventional oil and gas has made progress in five aspects: (1) Unconventional oil and gas have source-reservoir-in-one and in-situ accumulation; according to the theory of continuous oil and gas accumulation, the accumulation power of oil and gas is overpressure and diffusion; for conventional oil and gas, the source and reservoir are different formations, the trapping accumulation is its theoretical foundation, and the accumulation power is characterized by buoyancy and capillary force. (2) The unconventional oil and gas reservoirs are mainly formed in the low-energy oxygen-anaerobic environment, dominantly semi-deep to deep shelf facies and the semi-deep to deep lake facies, simple in lithology, rich in organic matter and clay minerals; conventional oil and gas mainly occur in coarse-grained sedimentary rocks formed in high-energy waters with complex lithology. (3) The unconventional oil and gas reservoirs have mainly nano-scale pores, of which organic matter pores take a considerable proportion; conventional oil and gas reservoirs mainly have micron-millimeter pores and no organic matter pores. (4) Unconventional shale oil and gas reservoirs have oil and gas in uniform distribution, high oil and gas saturation, low or no water content, and no obvious oil and gas water boundary; conventional oil and gas reservoirs have oil and gas of complex properties, moderate oil and gas saturation, slightly higher water content, and obvious oil, gas and water boundaries. (5) Organic-rich shale is the main target of unconventional oil and gas exploration; the sedimentary environment controls high organic matter abundance zone and organic matter content controls oil and gas abundance; positive structure and high porosity control the yields of shale wells; bedding and fracture development are important factors deciding high yield.

Geochemical characteristics of Upper Jurassic – Lower Cretaceous platform carbonates in Hazine Mağara, Gümüşhane (northeast Turkey): implications for dolomitization and recrystallization

The Upper Jurassic – Lower Cretaceous Berdiga Formation of the Eastern Pontides, Turkey, represents a carbonate platform succession composed of pervasively dolomitized intra-shelf to deep-shelf facies. In this area, polymetallic deposits occur as veins and lenses within the Berdiga Formation in close proximity to its upper contact with the overlying formation. Three different types of replacive dolomites occur in the formation: (i) microcrystalline dolomite, (ii) fabric-preserving dolomite, and (iii) fabric-destructive dolomite. Replacive dolomites are Ca rich and nonstoichiometric (Ca56–58Mg42–44) and are characterized by a pronounced negative shift in oxygen (–11.38‰ to –4.05‰ Vienna Pee Dee Belemnite (VPDB)), δ13C values of 0.69‰ to 3.13‰ VPDB, radiogenic 87Sr/86Sr ratios (0.70753 to 0.70884), and extremely high Fe (2727–21 053 ppm) and Mn (1548–27 726 ppm) contents. All dolomite samples have low Y/Ho ratios (23–40), and they also contain highly variable contents of rare earth elements (REE) (7–41). REE patterns of dolomites normalized to Post-Archean Australian shale show a distinct positive Eu anomaly (1.3–2.1) and slightly flattened Ce anomalies (0.8–1.1). Integration of petrographic and geochemical studies reveals the history of a variety of diagenetic processes highly affected by hydrothermal alteration, which include dolomitization, recrystallization, dissolution, silicification, and pyrite mineralization associated with the emplacement of the polymetallic mineralization.

Ypresian Nummulites and their stratigraphic significance from El-Guss Abu Said plateau, Farafra Oasis, Western Desert, Egypt

El-Guss Abu Said plateau lies on the western scarp of Farafra Oasis. The studied succession consists of Esna Formation (shale) at the base, which is overlain by well-bedded alveoline marl (Nusf Formation) with occasional limestone ledges rich in nummulitic fauna. This unit ends with varved-like thin beds of marl and limestone before it is truncated by limestone unit of the Farafra Formation (limestone). Esna Formation is deep shelf facies as it is rich in planktonic foraminifera that assign it to Early Eocene in age. The first occurrence of nummulitic fauna was recorded in the lower part of Nusf Formation. Two species of Nummulites were identified; Nummulites solitarius de la Harpe 1883 and Nummulites fraasi de la Harpe 1883. These two species date this unit to Early Eocene time (Ypresian) that ranges from SBZ5 to SBZ6 of the standard Serra-Kiel et al., 1998, shallow benthic zonal scheme. The shift from deep shelf facies of Esna Formation to the carbonate platform facies of Nusf and Farafra Formations could point to a tectonic uplift that occurred in Early Eocene time.

Microstructural investigation of gas shale in Longmaxi Formation, Lower Silurian, NE Sichuan Basin, China

The purpose of this paper is to probe into the nature and variability in vertical microstructure within Longmaxi gas shales of well WX2, Lower Silurian, located in the NE Sichuan Basin of China. To understand the pore system of these rocks, the total porosity, pore size distribution (PSD), surface area, organic geochemistry, and mineralogy, some image analyses by electron microscopy are performed. The obtained results indicated the following. (1) Gas shales have a complex pore structure. Total porosity from helium pycnometry ranges between 0.75% and 4.62%. Total organic content ranges between 3.4 wt.% and 11 wt.%, and vitrinite reflectance measured between 2.5% and 3.1%. The gas shales are quartz and clay rich. (2) Focused ion beam milling and field emission scanning electron microscopy provide high-resolution images of pores distribution and types. Three primary types of pores, organic matter (OM)-hosted pores, mineral-hosted pores, and micro-fractures, and 11 subgroups have been recognized. (3) The N2 isotherm curves are grouped into two types, II b as well as IV a, with the hysteresis loop belonging to Type B, reflecting the morphology of slits pores, and Type A, presenting cylinder pores. Combination of N2 and CO2 isotherm curve shows bimodal in two diameter sections, 0.3–1 nm and 2–5 nm, revealing meso-pores play important role in pore volume, whereas the micro-pores have great influence on pore specific surface area. (4) Total organic carbon (TOC) is considered to be the dominant contributor to total porosity and pore types, with the proportion of OM-hosted pores following the upward decreasing tendency of TOC. In plane, pore types are controlled by sedimentary facies. The OM-hosted pores are more prevalent in deep shelf facies within lower part of shale other than upper formation, instead of shallow shelf facies.

Subtropical to temperate facies from a transition zone, mixed carbonate–siliciclastic system, Palaeogene, North Carolina, USA

AbstractPalaeogene passive margin sediments on the US mid‐Atlantic coastal plain provide valuable insight into facies interaction and distribution on mixed carbonate–siliciclastic shelves. This study utilizes well cuttings, outcrop, core, and seismic data to document temporal and spatial variations in admixed bryozoan‐rich skeletal carbonates and sandy siliciclastic units that were deposited on a humid passive margin located in the vicinity of a major marine transition zone. This zone was situated between north‐flowing, warm waters of the ancestral Gulf Stream (carbonate dominated settings) and south‐flowing, cold waters of the ancestral Labrador Current (siliciclastic dominated settings). Some degree of mixing of carbonates and siliciclastics occurs in all facies; however, siliciclastic‐prone sediments predominate in nearshore settings, while carbonate‐prone sediments are more common in more open marine settings of the inner shelf break and deep shelf. A distinctive dual‐break shelf depositional profile originated following a major Late Cretaceous to Palaeocene transgression that drowned the earlier shallow platform. This profile was characterized by prominent mid‐shelf break dividing the shallow shelf from the deep shelf and a major continental shelf/slope break. Incomplete filling of available accommodation space during successive buildup of the shallow shelf preserved the topographic break on this passive margin. Storm wave base also contributed to the preservation of the dual‐break shelf geometry by beveling shallow shelf sediments and transporting them onto and seaward of the mid‐shelf break. Sediment fines in deep shelf facies were produced in place, transported downdip from the shallow shelf by storm ebb currents and boundary currents, and reworked from adjacent areas of the deep shelf by strike‐parallel boundary currents. Regional climate and boundary currents controlled whether carbonate or siliciclastic material was deposited on the shelf, with warmer waters and more humid climates favouring carbonate deposition and cooler, more arid conditions favouring glaucony and siliciclastic dominated deposition. Continuous wave and current sweeping of the shallow shelf favoured deposition of mud‐lean facies across much of the shallow shelf. Skeletal components in much of the carbonate‐rich strata formed in warm, nutrient‐rich subtropical waters, as indicated by widespread occurrences of larger benthic foraminifera and molluscan assemblages. These indicators of warm water deposition within the bryozoan‐mollusk‐rich carbonate assemblage on this shelf provide an example of a warm water bryomol assemblage; such facies generally are associated with cooler water depositional settings.

Hematite and goethite in Telychian marine red beds of the East Baltic

Hematite and goethite were determined by X-ray diffractometry using natural goethite and hematite for calibration. The subtraction of diffractograms before and after ignition at 500[ddot]C was applied for goethite identification and quantification, and ZnO was used as internal standard for the matrix correction. Red terrigenous claystones from deep shelf facies revealed co-occurrences of hematite and goethite, on average 1.7% hematite and 0.6% goethite. Grey claystones from the onshore facies of deep shelf contained yellow goethitic layers with the goethite content from 1.6 to 12.5%. Red metabentonites, layers of altered volcanic ash, contained only hematite, on average 2.7%, and did not reveal goethite above the detection limit. The occurrence of red metabentonites points to the early diagenetic origin of hematite in volcanic ash beds as well as in red terrigenous host rock. Several pathways of hematite and goethite formation are considered for different rock types.

Geochemistry of the Mesoproterozoic Lakhanda shales in southeastern Yakutia, Russia: implications for mineralogical and provenance control, and recycling

Shales of the Lakhanda Group of Late Mezoproterozoic age (1050–1000 Ma) from the southeastern Siberian craton in Russia have been analyzed for major elements and a number of trace elements, including the REE's. Shales along the Maya River formed as platform sediments in a deeper shelf facies, whereas, shales along the Belaya River formed in more active and open environments of an upper shelf carbonate ramp. The log of most elemental compositions to Al 2O 3 ratios are the same in the Maya and Belaya River samples, suggesting a similar source rock composition for rocks in the two areas. The log of SiO 2, MgO, Na 2O, K 2O, Rb, Ba, and Ni to Al 2O 3 ratios are significantly higher and the log of TiO 2 to Al 2O 3 ratios are significantly lower in shales from the Belaya River than the Maya River sections. The CIA (chemical index of alteration) is thus significantly lower in the Belaya shales than the Maya shales, suggesting less weathering of the in the Belaya shales than the Maya shales. The ICVs (Index of Compositional Variability=Fe 2O 3+K 2O+Na 2O+CaO+MgO+TiO 2/Al 2O 3) of the Lakhanda shales are less than 1, suggesting that they are compositionally mature and were likely dominated by recycling. Several samples have ICV>1, suggesting some first cycle input. The low K 2O/Al 2O 3 ratios of these shales suggest that minimal first cycle alkali feldspar was present in the initial source. Most shales of the Lakhanda plot parallel and along the A–K line in A–CN–K plots suggestive of intense chemical weathering (high CIA) and do not indicate any clear-cut evidence of K-metasomatism or direct weathering back to the original source. If K-metasomatism produced these rocks, then they could have formed from tonalites to basalts. If weathering produced these rocks then they could have been produced from varied amounts of mostly granodiorite to granite. Elemental ratios critical of provenance (La/Sc, La/Cr, La/Co, Th/Sc, Th/Cr, Th/Co, and Eu/Eu*) are not significantly different between the Maya River and Belaya River shales, and the ratios are similar to fine-fractions derived from the weathering of mostly granitoids and not basic rocks. The Eu/Eu*, Th/Sc and low K 2O/Al 2O 3 ratios of most shales suggest weathering from mostly a granodiorite source rather than a granite source, consistent with a source from old upper continental crust. Some samples at the bottom of the Belaya River section contain very low Eu/Eu* (0.35), suggesting significant input of first cycle detritus from highly differentiated granitoids similar to those from the Aldan Shield.

Mud-Flat Cycles, Incised Channels, and Relative Sea-Level Changes on a Paleocene Mud-Dominated Coast, Ellesmere Island, Arctic Canada

ABSTRACT The Upper Paleocene Cape Pillsbury Member on west-central Ellesmere Island, consisting of thick (600+ m), cyclical, calcareous mud-flat and laterally associated mixed mud-sand shelf deposits, provides an example of deposition on a mud-dominated coast. Mud-flat cycles are characterized by laminated mudstone. Cross-bedding is conspicuously rare. Normal grading of mudstone laminae suggests that deposition from suspension was significant. Storms also were common on this mud-dominated coast, as evidenced by beds of locally derived mud-chip conglomerate that commonly are intercalated with the laminated mudrocks, and by hummocky cross-stratification in the deeper shelf facies. Soft-sediment folding and microfaulting is ubiquitous in the mudrock facies. The deformation is analogous to flowsli es in muddy tidal flats now accumulating on the exposed northeast coast of South America. The flowslides are the only potential indicators of ebb-flood tidal reversals in the entire mudstone succession. Most of the mudrock cycles are incised by sand-filled channels. The channels were excavated during a drop in relative sea level (that terminated the preceding cycle). Subsequent filling of channels began when sediment flux in the channels exceeded the rate of sea-level fall. Thin coal seams capping some of the sand-filled channels may be the first indications of early transgression. The next cycle of laminated mudstone accumulated during the succeeding transgressive/progradational phase, but the distinction between these two components is not clear. The mud-flat and shelf facies were mostly progradational, accumulating during successive rises and high stands of relative sea level.

Clinal variation in the evolution of Ectenocrinus simplex

The crinoid Ectenocrinus has unusual trimeric columnals which make it instantly recognizable. In northern New York State such columnals first appear in the deep shelf facies of the lower Trenton Group (the Sugar River Limestone). In these strata the columnals display nearly triangular shapes with triangular lumina and represent a new species, E. triangulus. A small minority of the columnals of these early forms are round with five-pointed lumina.A definite cline had developed by middle Trentonian time. Deep-water columnals display the ancestral triangular form, whereas adjacent, shallow-water forms are rounder and have the five-pointed or pentagonal lumina. These round types came to be the beneficiaries of middle Trentonian facies changes. This was a time when the eastern bank margin steepened and narrowed. The deep shelf habitat shrank to the east and disappeared. Trapped in this shrinking sea floor setting, the populations of the deep-water members of the cline dwindled and disappeared before late Trentonian time. Only the round form with a pentagonal lumen survived. This form, Ectenocrinus simplex, was a great success; its numbers increased and its range expanded throughout the remainder of Trentonian time.The transition from Ectenocrinus triangulus to E. simplex is seamless. No boundary, other than an arbitrary one, can be recognized. A simple and gradual shift of phenotype abundance characterized the event.

Environment of deposition of conglomerates from the Silurian of north Galway, Ireland

ABSTRACTConglomerates from the Lettergesh Formation in the Silurian sequence of N Galway, Ireland, demonstrate palaeocurrents from the NE. Fabric studies demonstrate that the conglomerates were emplaced largely by traction currents and not by down-slope sliding or dispersive currents. Stratigraphic relationships demonstrate that the conglomerates were deposited in an environment which was adjacent to fluviatile, shallow-marine and deeper shelf facies at different times. A fan-delta environment of deposition is suggested, which requires a modification of previous Silurian sea-level curves at, or near, the base of the Wenlock.

Stratigraphy and bibliography of the Waitemata Group of Auckland, New Zealand

Abstract The Waitemata Group comists of flysch and related strata, of uppermost Oligocene to Lower Miocene age, outcropping widely around Auckland. It is redefined to exclude the Manukau Breccias, which are included with the Waipoua Basalt and Omapere Conglomerate of Hokianga in Hayward's new Waitakere Group. Also excluded are the co-eval marine strata of Hokianga (Hay's Otaua Group) and North Cape (Leitch's ParengarengD Group), The group includes a central body of flysch extending from Papakura to Brynderwyn (Warkworth Subgroup), It is flanked to the north by a shelf facies (Bream Subgroup); to the west by a distal-volcanic deep shelf facies (Kaipara Subgroup); and to the south by a shelf facies (Meremere Subgroup), To the east, the group overlaps an uneven surface on Torlesse metagreywackes. Both upper and lower contacts of the group are unconformable. There is a substantial volcaniclastic content in some formations, but no in-situ volcanics occur. The existing formations are defined, and the following ...