Shelf Margin Environments Research Articles

Early Cretaceous shifting of Zoophycos in the Ouarsenis Mountains (northwestern Algeria)

Early Cretaceous succession of the Oued Fodda Formation in the Ouarsenis Mountains (northwestern Algeria) is mainly composed of marl–limestone alternations, which are subdivided into four informal units (Units 1 to 4), based on distinct lithological, stratonomical, and ichnological features. The ichnological analysis reveals a low diversity of the trace-fossil assemblage, which is exclusively reported from Units 2 and 3. The ichnoassemblage contains six ichnotaxa (Chondrites intricatus, Ophiomorpha isp., Planolites isp., Thalassinoides isp., Zoophycos brianteus, and Zoophycos cauda-galli), among which Zoophycos and Chondrites are the most common elements of the assemblage and occur in distinct mud-rich substrates showing different bioturbation intensities. The development of Zoophycos in the middle part of Unit 2 shows a high degree of bioturbation (bioturbation index (BI) = 4). Zoophycos specimens are of large size, between 45 cm and 75 cm in width, which were interpreted to have formed in a lower offshore environment where the oxygenation amount was optimal, the sedimentation rate was low, and the benthic food was abundant on the seafloor. Toward the upper part of Unit 2, Zoophycos-bearing levels exhibit a less intense degree of bioturbation (BI between 1 and 2) in contrast to Planolites- and Chondrites-bearing levels which have a bioturbation index (BI) between 3 and 4. At these levels, Zoophycos displays relatively small, coiled to U-shaped spreiten, probably in response to stressful and dysoxic conditions prevailing in the water bottom. With improved oxygenation in a quiet lower offshore to shelf margin environment in Unit 3, the benthic organisms recovered, as represented by medium to large size Zoophycos in association with Ophiomorpha and scarce Chondrites burrows, even if the overall bioturbation intensity is very low. The combination of trace-fossil assemblage and lithofacies of the Oued Fodda Formation indicates relatively stable outer shelf environments below the storm wave base, which corresponds classically to the lower offshore to shelf edge environments, and the prevailing palaeoecological conditions are optimal and stressful for the benthic organisms.

Multi dating approach of long marine core sediments from the south-eastern continental shelf of Korea: Comparison of SAR OSL, TT-OSL and pIRIR dates

Marine sediments constitute important archives of past oceanic, climate, and sea-level changes. However, it is difficult to derive accurate chronological models due to the absence of long sediment cores and limitations of dating methods. To investigate the feasibility of applying optically stimulated luminescence (OSL) ages to establish high-resolution chronologies in the East Sea of Korea, we obtained three cores with lengths up to 120 m from the eastern continental shelf of Korea. Most of the core sediments were homogeneous or crudely laminated mud, with some sand and gravel bed. Multiple dating methods, including fine-grained quartz single-aliquot regenerative-dose (SAR) OSL, thermally transferred OSL (TT-OSL) from fine-grained quartz, and single-grain K-feldspar post-infrared (IR) infrared stimulated luminescence (IRSL) (pIRIR), following the above description, were applied to overcome the age limitation and create an accurate age model through cross-checking among the different dating methods. We also investigated the applicability of the standardised growth curve (SGC) to TT-OSL signals in continental shelf environments. The equivalent dose (D e ) values from the SGC were consistent with those from the SAR TT-OSL procedure, which suggests that the SGC is valid for long core sediments on the continental shelf. A detailed chronology derived from the radiocarbon, U/Th, and multiple OSL age dating methods was compared. Especially, in core 19ESDP-101, a comparison between TT-OSL ages from fine-grained quartz and pIRIR ages from coarse K-feldspar showed agreement to within 10% at most depths for which ages were measured. We demonstrated that OSL dating from long marine core sediments in a continental shelf margin environment can provide an accurate and precise chronology in undatable regions.

Can deepwater bottom currents generate clinothems? An example of a large, asymmetric mounded drift in Upper Jurassic to Lower Cretaceous sediments from northwestern Australia

Abstract Clinoforms and clinothems are ubiquitous in shallow marine and shelf margin environments, where they show remarkable seaward progradation trends. Consensus holds that these features do not form in deepwater settings. This study describes an example of a large, asymmetric mounded deposit formed in Upper Jurassic to Lower Cretaceous sediments along the Exmouth Plateau (offshore northwestern Australia). Although it formed in deepwater environments, the deposit has previously been interpreted to reflect either a deltaic or shelf margin system based on clinoform and clinothem geometries. We support that this deposit shares similarities with a delta drift that evolved into a large, mounded drift (~180 km in length, ~120 km in width, and up to ~1.7 km in sedimentary thickness) that exhibits two migration trends: one westward and the other northeastward. Three evolutionary phases are proposed: (1) an onset drift stage (ca. 146.5–143.5 Ma); (2) a growth drift stage (ca. 143.5–138.2 Ma); and (3) a burial stage (ca. 138.2 Ma), which marks the completion of the drift and a shift in depositional style. The drift asymmetry and clinoform orientations indicate the influence of a northward-flowing water mass with two main cores. Our analysis thus suggests that bottom currents can create complex deposits with geometries that resemble clinothems in deepwater environments.

Middle Ordovician trilobites from the Houping Formation of South China: implications for palaeoecology and palaeobiogeography

ABSTRACT Seventeen trilobite species belonging to 16 genera and 13 families are systematically documented from the Houping Formation (upper Dapingian-lower Darriwilian, Middle Ordovician) at the Datangkou and Dacao sections, Chengkou, northern Chongqing. Of these, a new species, Mioptychopyge chengkouensis sp. nov., is established and described. New morphological information is provided for previously known species on the basis of new specimens. Two trilobite associations based on faunal composition and dominant forms are distinguished from the Houping Formation, in ascending order, the Agerina Association (from the lower and middle members) and the Nileus-Illaenus Association (from the upper member). The Agerina Association lived in an inner shelf margin environment with carbonate substrate above storm wave base (<70 m deep), whereas the Nileus-Illaenus Association developed in a shallow outer shelf environment below storm wave base (>70 m deep), indicating a rise of relative sea level corresponding to the early Darriwilian transgression. The cluster and nonmetric multidimensional scaling analyses of the trilobites in the formation show that the fauna had a strong endemicity to South China and a close biogeographic connection with east Peri-Gondwana, especially Tarim, Annamia and central Iran.LSIDurn:lsid:zoobank.org:pub:E4E6AD0B-417F-4522-83E3-4DC44B647760

SEQUENCE STRATIGRAPHY, DEPOSITIONAL ENVIRONMENTS AND RESERVOIR CHARACTERIZATION OF SA'DI FORMATION IN EAST BAGHDAD AND HALFIYA OILFIELDS

Turonian-Early Campanian sediments (AP 9) are restricted to area of Mesopotamian and the Jezira Subzone. It was deposited in relatively narrow seaway that occupies the area of the salman, Mesopotamian, Foothill and high folded zones. The lower part of this megasequence is revealed by the Khasib, Tanuma, Sa'di and Kometan formations. This study is based on samples from two wells; the first is in East Baghdad field, Sa'di Formation about 146.7 m thick, and the second is in Halfiya field, Formation about 100 m thick. The Sa'di Formation consists of two members, they are Upper member, which is composed of bitty bioclastic, planktonic foraminiferal chalky limestone, with some calsicphers, and lower member of the formation, which consists of homogenous, rich planktonic foraminiferal chalky limestone,in additionto small amounts of benthonic foraminifera and bioclastic grains. The Sa'di Formation is overlain the Tanuma Formation, and underlain the Hartha Formation. The Sa’di Formations is composed of four main facies subscribe as S1, S2, S3 and S4. The facies S2 appeared in the upper member, whereas the others are spread in the lower member, which were deposited in deep shelf margin environment. The Gamma Ray log is used to calculate the volume of shale of Sa'di Formation after removing the shale effect due to the porosity equation correction. Depending on the result of porosity values for both wells, the Sa'di Formation successions have been divided into members, they are; the lower and upper members. The lower member separated into two zones, the first have poor to fair porosity, and the second have fair to very good porosity. Whereas the upper member divided into two zones and the porosity wobble between good to very good.

Acanthomorphic acritarchs of the Doushantuo Formation from an upper slope section in northwestern Hunan Province, South China, with implications for early–middle Ediacaran biostratigraphy

Doushantuo-Pertatataka acritarchs (DPAs) are abundant and well preserved in the Ediacaran Doushantuo Formation in South China. Not only do they provide insights into the marine ecosystem immediately after a Neoproterozoic global glaciation, they also afford us an ideal tool for Ediacaran stratigraphic subdivision and correlation. However, previous reports of DPAs in South China are mostly from the Yangtze Gorges area in intra-shelf basin and from Weng’an in shelf margin environments, while data from localities in slope and basinal facies are lacking. Here we present lithostratigraphy, carbon and strontium isotope chemostratigraphy, and acritarch biostratigraphy of the Doushantuo Formation from an upper slope section at Lujiayuanzi in northwestern Hunan Province, South China. Five DPA genera and eight species are preserved in the Doushantuo Formation chert nodules, with the uppermost two DPA horizons occurring above a δ13C negative excursion that is correlated with the EN3/Shuram excursion based on integrated litho- and chemostratigraphic correlation. Our new findings reveal a broader temporal and spatial distribution of DPAs in South China, and further support their biostratigraphic potential in the early–middle Ediacaran subdivision and correlation.

Sequence stratigraphy of Paleocene–Eocene successions in selected oil wells, Kirkuk area, northern Iraq

The Middle Paleocene-Eocene Megasequence AP10 was deposited during a period of renewed subduction and volcanic arc activity associated with the final closure of the Neo-Tethys . It is subdivided into two sequences according to the regional unconformity at the base of the Middle-Eocene : the Paleocene-Early Eocene, in which the Aaliji and Sinjar Formations were deposited in the studied area and Middle_Late Eocene sequences, in which the Jaddala Formation was deposited in the studied area. The distinguishable facies throughout Aaliji and Jaddala Formations deposits indicated to deep marine environments that lack benthonic foraminifera in contrast with the high percentage of planktonic foraminifera, especially Morozovella and Acarinina reflect the deep marine environment with tropical to subtropical climate condition. The Sinjar Formation characterizes by abundance of the Nummulite and bioclastic-lime wackestone subfacies indicate shallow inner shelf of carbonate shelf margin environments, followed by shoaling upward sequence, with chance to aerial exposure. The systematic investigation of planktonic foraminifera enabled us to identify thirty–four species and subspecies in the studied section, the stratigraphic distribution of these planktonic foraminiferal assemblages permit the recognition of seven biozones, two biozones (P4 and P5) have been established in Aaliji Formation (Late Paleocene) and five biozones (P9-P13 ) in Jaddala Formation (Early-Middle Eocene), where the contact between the two formations is unconformable that represents an interval of about 4 My, recorded by disappearance of three biozones P6, P7 and P8. Sequence stratigraphy analysis, as calibrated by sedimentary facies and biostratigraphy, delineated two sequences in Paleocene deposits that together interpret as 3rd order sequence, and 2nd order sequence in Eocene deposits. The two sequences shows good correlation of the northern Arabian Plate with other part of the platform during Paleocene and Eocene.

The role of slump scars in slope channel initiation: A case study from the Miocene Jatiluhur Formation in the Bogor Trough, West Java

This paper investigates slope channel initiation by seabed irregularities that were initially formed by slump scars in the lower to middle Jatiluhur Formation, part of the middle- to late Miocene successions in the Bogor Trough, West Java. This Miocene succession is up to 1000m thick in the study area, and is interpreted as a prograding slope–shelf system that formed during a period of falling- and lowstand stages in relative sea level. The lower part of the formation is a siltstone-dominated siliciclastic succession, containing slump deposits, slump-scar-fill deposits, and minor channel-fill deposits, which formed in slope and shelf-margin environments. In contrast, the middle part, which gradationally overlies the lower part, is characterized by shallow-marine carbonates.The slump-scars-fill deposits have an overall lenticular geometry, and are 140–480m wide and 0.4–1.6m thick. Some have distinct erosional bases, which cut into the underlying siltstones, in association with medium- to coarse-grained sandstones with lateral-accretion surfaces and tractional structures common in channel-fill deposits. The incident link of slump-scar-fill deposits and channel-fill deposits in the prograding slope–shelf succession of the lower to middle Jatiluhur Formation suggests that some slump scars formed incipient seabed irregularities that may have played an important role in the development of slope channels. The present study provides one example of the various potential mechanisms that can result in channel formation in a slope setting.

Encrusting micro-organisms and microbial structures in Upper Jurassic limestones from the Southern Carpathians (Romania)

Late Jurassic–Early Cretaceous Stramberk-type reef limestones are known from some parts of the Southern Carpathians in Romania. The Upper Jurassic deposits mainly consist of massif reef limestones including a variety of microbialites associated with micro-encrusters. They played an important role in the formation and evolution of the reef frameworks and thus are of significant importance for deciphering the depositional environments. For our study, the most important encrusting organisms are Crescentiella morronensis, Koskinobullina socialis, Lithocodium aggregatum, Bacinella-type structures, Radiomura cautica, Perturbatacrusta leini, Coscinophragma sp., and crust-forming coralline sponges such as Calcistella. Based on microscopic observations, microbial contribution to reef construction is documented by the abundance of dense micrite, laminate structures, clotted, thrombolithic or peloidal microfabrics, constructive micritic cortices, biogenic encrustations and cement crusts, as well as by other types of microbial structures and crusts. Most of the investigated carbonate deposits can be classified as “coral-microbial-microencruster boundstones” which are characteristic for the Intra-Tethyan domain. Their paleogeographical significance is indicated by the presence of many features comparable with carbonate deposits of rimmed platform systems from the Northern Calcareous Alps or Central Apennines. Based on the distribution of the facies and facies associations within the carbonate sequences under study we can distinguish slope and external shelf margin environments. The microbial crusts, the encrusting micro-organisms, and in some cases the syndepositional cements have stabilized and bound the carbonates of the slope facies types. Subsequently, the stable substrate favored the installation of coral-microbial bioconstruction levels.

Paleoenvironments and origin of the sedimentary phosphorites of the Napo Formation (Late Cretaceous, Oriente Basin, Ecuador)

The Napo phosphorites were deposited at the edge of a stable marine shelf during the Upper Cretaceous (Coniacian) oceanic anoxic event (OAE 3) at the transition from bioclastic limestone to organic-rich shale facies. Phosphogenesis was triggered in the shelf margin environment by a number of factors including strong upwelling currents, high biological activity, plankton blooms, and large amounts of organic matter production and subsequent accumulation. Dissolved phosphate levels increased in the sediment from a combination of anoxic conditions and microbial activity. Once dissolved phosphate concentrations were high enough, apatite began to form around nucleic sites including mineral grains, shells, wood fragments, and foraminifera tests forming peloidal fluorine rich carbonate fluoroapatite (francolite). As the peloids formed, sedimentation continued and dissolved phosphate concentrations diminished. A period of minor winnowing ensued, and as dissolved phosphate concentrations remained low, shale layers were deposited separating the various phosphate layers.

Classification of the Late Cambrian (Steptoean) trilobite genera Cheilocephalus Berkey, 1898 and Oligometopus Resser, 1936 from Laurentia

All recent work has assigned the genera Cheilocephalus Berkey, 1898 and Oligometopus Resser, 1936 to the family Cheilocephalidae Shaw, 1956, a small group of largely Laurentian trilobites from the Late Cambrian Steptoean Stage. New data from Nevada and Newfoundland indicate that the Cheilocephalidae is polyphyletic and that Cheilocephalus and Oligometopus are allied with different superfamilies. The Cheilocephalidae, which includes Cheilocephalus and Pseudokingstonia Palmer, 1965, is assigned to the Dameselloidea Kobayashi, 1935, and a pygidium from the Cow Head Group of Newfoundland indicates that Cheilocephalus first appears in the late Marjuman. The Dameselloidea disappeared on other continents at an extinction interval at the end of the Marjuman, but Cheilocephalus survived and underwent modest diversification in Laurentia in the succeeding Steptoean Stage. Oligometopus is transferred to the leiostegioidean family Leiostegiidae Bradley, 1925. The genus shows closest affinities with such Gondwanan genera as Idamea Whitehouse, 1939, and its appearance in mid-Steptoean strata records immigration of the Leiostegoidea into shelf-margin environments of Laurentia.

Facies de patch reefs en la Formación Picún Leufú (límite Jurásico/Cretácico) en la región de Zapala, Cuenca Neuquina

Patch reefs facies in the Picun Leufu Formation (Jurassic/Cretaceous boundary) in the Zapala region, Neuquen Basin, Argentina. In the locality of Cerrito Caracoles, situated in the nearby of Zapala city, in the western central region of the Neuquen province, Argentina, the basal part of the Picun Leufu Formation (Jurassic/Cretaceous boundary) crops out. The facies and microfacies association analysis points out to an inner shelf margin environment, with development of well preserved patch reefs. The description of these bodies (coral and algae bafflestone, bryozoan bafflestone and algal boundstone), and of a low relief mound reef (skeletal algal packstone) are presented in this paper. The associated facies are represented by low energy deposits (mudstone), bar complexes (sandy skeletal packstone and skeletal packstone) and channels (skeletal intraclast floatstone and intraclast skeletal ooidal floatstone).

Trilobite diversity patterns in an Upper Cambrian stage

Recovery of trilobite faunas following a mass extinction during the Upper Cambrian Sunwaptan Stage (equal to the “Ptychaspid Biomere”) involved increases in both within-habitat (alpha) diversity and in levels of biofacies differentiation. This pattern parallels, at a lower hierarchical level, the global increase in Phanerozoic marine invertebrate species richness (Sepkoski et al. 1981), which was influenced by changes in provinciality and in alpha diversity. Diversification slowed markedly in subtidal shelf carbonate habitats in the upper half of the stage, but continued in carbonate shelf margin environments, possibly due to higher rates of immigration from off-shelf sites. Trilobite clades appearing early in the stage have low centers of gravity, reflecting high immigration rates during and shortly after an interval of mass extinction. Expansion of shallow-water carbonate deposition in the upper half of the stage was accompanied by the spread of carbonate bank biofacies. Families occurring in these biofacies gradually increased in species richness and tended to have high centers of gravity.

Regional Lithostratigraphy of Permian Phosphoria Formation, Western Overthrust Belt: ABSTRACT

A regional synthesis of the lithostratigraphic relationships for the Permian Phosphoria Formation illustrates the cyclic lithofacies types, carbonate reservoir, and potential organic facies for the western Overthrust belt of Wyoming, Idaho, and Utah. Palinspastically restored subsurface control sections (70) and measured outcrop sections (35) indicate a complex, intertonguing, transgressive-regressive sequence of shallow-water carbonates, clastics, and highly organic phosphatic mudstone deposited in a shallow marine shelf-margin environment. Distribution of facies types was affected by paleostructural growth of late Paleozoic structural elements including the Sublette basin of Utah and Bannock highs of western Wyoming-southeastern Idaho. These highs helped define the Wyom ng shelf-margin slope. Identification of paleostructural growth is based on interpretation of depositional environments and thickness patterns of the Permian deposits. The Phosphoria Formation and the equivalent beds of the Park City Formation are divisible into two main depositional cycles--the Meade Peak-Franson Members and the Retort-Ervay Members--based on the regional correlation network generated for the Overthrust belt. Carbonate deposition and reservoir development occurred during maximum transgressive stages, whereas deposition of phosphorite and organic shales occurred during maximum regressive stages. The reservoir facies is closely associated with early diagenetic dolomitization of carbonate shelf deposits concentrated along paleostructural high areas of the shelf-margin environment. End_of_Article - Last_Page 952------------

Seismic Stratigraphy in Low-Energy Carbonate Depositional Environment: ABSTRACT

Seismic stratigraphic mapping in carbonate rocks has been historically concentrated on the shelf margin environment where unit geometry and abrupt facies-velocity changes are often easily discernible with the seismic tool. In contrast, low-energy, more subtle shelf interior stratigraphic features have attracted less geophysical attention. This report describes the study of seismic data from five United States oil fields which produce from reservoirs formed from shelf interior deposits. Three Smackover fields, a Permian basin producer, and a Williston basin example are compared. Review of the depositional model indicates the difficulty of seismic mapping in non-reef carbonates. If sedimentary accretion is nearly vertical, porosity traps are predicted to exhibit only lateral change in unit velocity. In regressive or transgressive sequences, the geologic model indicates that angular discordance due to sediment buildup in combination with a velocity contrast generates an angular feature that has a different velocity pattern. These five fields provide examples of both trap types. Jay field (Florida) and Walker Creek field (Arkansas) both produce from the Smackover Formation and are (seismic-defined) stratigraphic traps which illustrate angular discordance and velocity contrast owing to regressive buildup. The trapping mechanism at Big Escambia Creek (Alabama), also productive from the Smackover, is from a transgressive pulse devoid of vertical buildup. Hence, the porosity trap is indicated only by a lateral velocity change without angular discordance. Subsurface geologic sections of San Andres production End_Page 798------------------------------ at Cato San Andres field (New Mexico) and Mission Canyon pay at Haas field (North Dakota) reveal no diachronous buildup. As expected these fields demonstrate only a lateral change in velocity within the formation. Application of stratigraphic-seismic techniques to low-energy carbonate facies can fall into two depositionally controlled divisions. When sediment accretes vertically, traps are mapped solely by lateral changes in velocity. Sedimentary buildup in transgressive or regressive sequences provides angularity, which combined with velocity contrast facilitates seismic detection of the trap. End_of_Article - Last_Page 799------------

Environments of Deposition and Diagenesis of Stuart City Formation (Cretaceous), Speary Gas Field, Karnes County, Texas: ABSTRACT

Carbonate rocks of the Stuart City Formation accumulated at the Lower Cretaceous shelf margin from late Trinitian through middle Washitan. Paleobathymetry controlled facies patterns in and around reef buildups. Following uplift of the San Marcos arch during middle Washitan and consequent exposure of some reefs, fresh water diagenesis produced secondary porosity and permeability in part of the upper Stuart City. Facies present in cores from the Speary gas field reflect deposition in backreef lagoon and open shelf-margin environments of the Comanche platform. Miliolid wackestones accumulated in protected areas behind the rudist reefs which were fringed by wave and tide-washed grainstones of rudist debris. Stromatoporoid boundstones reflect patch reef deposition close behind the main reef complex. Adjacent shoal areas were characterized by packstones of rudist and associated reef debris coated with abundant algal encrustations. Mollusk wackestones were deposited as a transitional facies between the shelf-margin facies and the lagoonal facies. As Stuart City deposition ended, shelf-margin facies were offlapped by shelf-lagoon facies. Early marine phreatic diagenesis lithified much sediment but did relatively little to alter the original rock texture. Micritized grain rims and bladed calcite after acicular aragonite rim cement, characteristic of marine diagenesis, are ubiquitous. Dissolution of aragonitic skeletal grains followed, and was then followed by bladed or blocky cement infilling of the grain molds. During a later stage of cementation a coarse, calcite mosaic was precipitated in remaining interparticle pore spaces and grain molds. Interparticle porosity was rarely preserved. The preserved intraparticle porosity is, however, present in all facies and notably in body cavities of large rudist fragments. Moldic porosity occurs only in grainstones and only in those cores in which the Stuart City was deposited on a bathymetric high. The widespread distribution of the coarse, calcite mosaic in the cores indicates that fresh water diagenesis affected all of the rocks. However, diagenesis was more extreme on bathymetric prominences as indicated by the isolated occurrences of moldic porosity. Grainstones containing moldic porosity have the highest porosity and permeability in the cores, 16% and 14 md, respectively. End_of_Article - Last_Page 1552------------