Third-order Sea-level Changes Research Articles

Astronomical pacing of third-order sea-level sequences during the middle miocene in the northern south China sea

The Miocene Hanjiang Formation of the Pearl River Mouth Basin (PRMB) forms one of the most productive hydrocarbon reservoirs in China. Sparse biostratigraphic age control and limited dating hinder a thorough understanding of sea-level change and its sedimentary expression through this formation. To establish the relationship between third-order sea-level changes, their expression in the lithological record and astronomical forcing, a sequence stratigraphic and cyclostratigraphic analysis has been conducted through the Hanjiang Formation in the LF14 borehole. One second-order sequence and six third-order sequences are recognized based on gamma ray (GR) data, vertical lithofacies changes and sedimentary stacking patterns. Time series analysis of the GR data and subsequent interpretation allows recognition of approximately eighteen 405 kyr long eccentricity cycles. Based on this cyclostratigraphy, we constructed a ∼7.22-Myr floating astronomical time scale (ATS) for the Hanjiang Formation. We anchored this timescale to globally significant biostratigraphic events preserved in the basin to build an absolute ATS spanning 17.423 ± 0.4 to 10.221 ± 0.4 Ma. A prominent ∼1.2-Myr periodicity is identified in the amplitude modulation (AM) envelope curves of ∼41-kyr obliquity cycles preserved in the record. This ∼1.2-Myr periodicity shows good agreement with our sequence stratigraphic analysis and global eustatic curves. Our results suggest that ∼1.2-Myr obliquity AM was the main driver of third-order sea-level change and sequence development in the PRMB. Combined application of cyclostratigraphic and sequence stratigraphic analysis (utilizing INPEFA and wavelet transform methods) are demonstrated to be useful for understanding the drivers and subdivisions of third-order sequence evolution on a regional scale.

Astronomical cycles calibrated the sea-level sequence durations of Late Miocene to Pliocene in Qiongdongnan Basin, south China sea

Several research works have demonstrated that climate variations are consistent with quasi-periodic orbital parameters, particularly ∼1.2-Myr obliquity amplitude modulation cycle has been demonstrated as a controlling factor of glacio-eustasy sequence formation in icehouse periods. In the current research, we have studied the glacio-eustasy third-order sequences of Late Miocene to Pliocene age in Qiongdongnan Basin (QDNB), South China Sea and identified a prominent ∼1.2-Myr periodicity in a 41kyr-tuned gamma ray series of Well LS33-1-1. In the current research, we developed an astronomical time scale (ATS) for Late Miocene to Pliocene in QDNB and conducted cyclostratigraphic analyses on Huangliu and Yinggehai Formations in Well LS33-1-1 using high-resolution natural gamma-ray (GR) data. Time-series analyses on GR data revealed rich Milankovitch frequency band series and ∼5.6, 3.8 and 6.0 m cycles were respectively filtered for three segments in depth domain which presented direct correlation with 41-kyr obliquity cycles in La2004 solution. In this work, foraminiferal and calcareous nannofossil biozones in South China Sea were selected as age markers for tuning. Correlation coefficient analysis was applied to quantitatively measure the fitting of witnessed sedimentary cycles to astronomical periods and provide a possible range for sedimentation rate. Also, we developed a sedimentary noise model to detect high-resolution sea-level variations under the control of orbital forcing which were supported by previous third-order eustasy variations. In addition, six third-order sequences were identified in the range of Late Miocene to Pliocene representing astronomical forcing through both ∼1.2-Myr filtering of DYNOT model and ∼1.2-Myr modulation of obliquity amplitude based on sedimentary noise modeling. Finally, it was concluded that the ∼1.2-Myr cycle was the most important driving force for sequence development and third-order sea-level changes.

New insights into organic matter accumulation from high-resolution geochemical analysis of a black shale: Middle and Upper Devonian Horn River Group, Canada

Abstract Organic matter (OM) accumulation in organic matter-rich mudstones, or black shales, is generally recognized to be controlled by combinations of bioproductivity, preservation, and dilution. However, specific triggers of OM deposition in these formations are commonly difficult to identify with geochemical proxies, in part because of feedbacks that cause geochemical proxies for these controls to vary synchronously. This apparent synchronicity is partly a function of sample spacing, commonly at decimeter to meter intervals, which may represent longer periods of time than is required for the development of feedbacks. Higher resolution data sets may be required to fully interpret OM accumulation. This study applies a novel combination of technologies to develop a high-resolution geochemical data set, integrating energy-dispersive X-ray fluorescence (EDXRF) and infrared imagery analyses, to record proxies for redox conditions, bioproductivity, and clastic and carbonate dilution in millimeter-resolution profiles of 133 core slabs from the Middle and Upper Devonian Horn River shale in the Western Canada Sedimentary Basin, which provides decadal-scale temporal resolution. A comparison to a more coarsely sampled data set from the same core results in substantially different interpretations of variations in bioproductivity, redox, and dilution proxies. Stratigraphic distributions of organic matter accumulation patterns (bioproductivity-control, siliciclastic/carbonate-dilution, and redox conditions-control) show that organic enrichment events were highly varied during deposition of the shale and were closely related to second- and third-order sea-level changes. High-resolution profiles indicate that bioproductivity was the predominant trigger for organic matter accumulation in a second-order highstand, particularly during deposition of third-order transgressive systems tracts. Organic matter accumulation was largely controlled by dilution from either carbonate or clastic sediments in a second-order lowstand. Bioproductivity-redox feedbacks developed on timescales of decades to centuries.

Astronomical Cycles of the Late Permian Lopingian in South China and Their Implications for Third-Order Sea-Level Change

The late Permian (Lopingian) was a crucial climate transition period from the late Paleozoic Ice Age to the early Triassic of exceptionally high temperatures. However, the origins of the third-order sea-level changes during the Lopingian Epoch remain unclear. Here, we presented astronomically calibrated gamma-ray (GR) log and non-U GR (computed gamma ray or CGR) curves from the clastic and carbonate successions of well GFD-1 in the Pingle Depression of South China for studying the sea-level oscillations during the Lopingian. Spectral analyses of the 405 kyr-calibrated GR and CGR time data revealed periodicities close to about 405, about 100, about 44.2, about 35.1, about 21, and about 17.5kyr, supporting the existence of Milankovitch forcing in the sedimentary records. A high-resolution astronomical time scale and high-resolution sedimentation rate curve of the Lopingian from well GFD-1 were constructed by cyclostratigraphic analysis. The eccentricity and obliquity amplitude modulation cycles suggested long periodicities of about 2.4 and about 1.2 myr, respectively. In the Wuchiapingian greenhouse of the Lopingian, the about 2.4 myr eccentricity oscillation controlled’ weak’ glacio-eustasy and/or aquifer eustatic changes related to the global third-order sea-level changes and that a lowstand (W2) was initiated by an eccentricity oscillation minimum. In contrast, during the Changhsingian, which exhibited a cooling event, an about 1.2 myr obliquity cycle was probably strong, with the sea-level records highlighting the link between the ‘icehouse’ sea-level lowering (C2 and C1) and the obliquity nodes. Moreover, dynamic sedimentary noise model as an indicator of sea-level showed local third-order sea-level variations, the coevolution trends in the orbital power, global and local sea-level changes, and sedimentation rate had significant implications for establishing the global nature and synchronicity of these million-year-scale eustatic records and reconstructing the temporal depositional history at a regional scale. In addition, the volcanism and tectonism that continued into the early-middle Wuchiapingian probably led to a series of climate changes that drove the hydrological cycles not paced by the Milankovitch cycles.

Temporal changes in an epeiric paralic deposition during a third-order relative sea-level cycle (Late Pennsylvanian, western North China): Insights from integrated facies and sequence stratigraphic analysis

The upper Chinssu Member (Late Pennsylvanian) of the Taiyuan Formation in the Linxing block of western North China records a complex sedimentological and stratigraphic arrangement of sandstone, mudstone and coal attributed to paralic environments that developed in an epicontinental setting with a tropical rainforest climate. Facies analysis of core TB01 through the upper Chinssu Member from the Linxing block is used to distinguish seven facies associations, reflecting successive depositions in the tide-dominated estuary, transgressive vegetation-dominated coastal plain, back-barrier tide- and wave-influenced lagoon, storm-wave-dominated inner shelf, tide-dominated open-coast tidal flat, and regressive vegetation-dominated coastal plain. Sequence stratigraphic analysis of core TB01 and nearby boreholes and outcrops is used to construct a framework of transgressive and highstand systems tracts and their bounding surfaces. Within the framework, nine types of facies contacts are identified to distill the internal makeup of systems tracts. The high-resolution framework enables the deconvolution of various controls on temporal changes in the epeiric paralic deposition during a third-order relative sea-level cycle. The high-amplitude third-order sea-level change is the dominant control on the stratal stacking patterns. The channel migration, peat compaction, shoreline autoretreat, and bedrock inheritance modified the depositional response to overall rise in relative sea level through time. This study contributes to a better understanding and prediction of the changes in depositional processes in the cratonic interior in response to relative sea-level changes.

Oscillations of global sea-level elevation during the Paleogene correspond to 1.2-Myr amplitude modulation of orbital obliquity cycles

The mechanisms involved in the formation of 1- to 2-Myr-long third-order stratigraphic sequences through the Phanerozoic have been extensively debated, but the main underlying forcings remain uncertain. In this study, we investigated third-order sequences of Paleocene-Eocene age in the East China Sea Shelf Basin, identifying a prominent ∼1.2-Myr periodicity in the 405-kyr-tuned gamma ray profile of the WP-1 borehole. We infer that this ∼1.2-Myr cycle, which corresponds to long-period amplitude modulation of obliquity, was the main driver of third-order sea-level changes and sequence development. This modulation entails variation in the range of obliquity amplitude between a maximum of 2.4° (i.e. 24.5°–22.1°) and a minimum of 0.3° (i.e. 23.3°–23.0°) as a result of interactions among the three primary parameters of Earth's orbit (i.e., eccentricity, obliquity, and precession). We anchored our floating astronomical time scale (ATS) to the T42 seismic horizon, a major sequence boundary between the Lin Feng and Mingyuefeng formations that corresponds to the top of Nannofossil Zone NP8 (reported age of 57.7 Ma). This yields a fixed ATS for the late Paleocene-to-middle Eocene interval spanning 60.84 to 48.71 Ma. Finally, we evaluated our findings relative to the third-order sequence stratigraphy of the East China Sea Shelf Basin and to records of early Cenozoic eustasy and global events.

The lower Upper Cretaceous of the south-eastern Münsterland Cretaceous Basin, Germany: facies, integrated stratigraphy and inter-basinal correlation

Integrated stratigraphic (litho-, bio-, event, chemo-, gamma ray, and sequence stratigraphy) and sedimentologic analyses of two new core sections greatly improved the understanding of facies development, sea-level changes and correlation of the lower Upper Cretaceous in the south-eastern Munsterland Cretaceous Basin, Germany. A large-scale second-order sea-level cycle is mirrored by the increasing importance of offshore facies and thicknesses of depositional sequences, reflecting the rise of accommodation during the Cenomanian to Early Turonian. In the Middle Turonian, this trend started to become reversed and the cycle ends with a major unconformity at the base of the Soest Grunsand Member in the mid-Upper Turonian. Condensation of the mid- and uppermost Turonian reflects the lack of accommodation during a phase of second-order lowstand, followed by a retrogradational trend during the Early Coniacian that marks the transgressive part of a new second-order cycle. Sedimentary unconformities in the Cenomanian–Turonian successions provide evidence for third-order sea-level changes superimposed onto the first early Late Cretaceous second-order cycle. They correspond to sequence boundaries SB Ce 1–5 and SB Tu 1–4 that have been identified in Central European basins and elsewhere, supporting their eustatic origin. The sea-level fall expressed by Upper Turonian unconformity SB Tu 4 is of major magnitude. The overlying Soest Grunsand Member is the only level of greensands in the Upper Turonian of the south-eastern Munsterland: the Alme Grunsand, introduced for another, allegedly uppermost Turonian greensand level, does not exist. Carbon stable isotopes from the mid-Upper Cenomanian to Lower Coniacian allowed calibrating the successions on intra- and interbasinal scales. A conspicuous mid-Middle Turonian positive isotope event has been newly named, i.e., the Niederntudorf Event. Sequence boundaries, marker beds (marl layers) and bentonites turned out to be isochronous within the chemostratigraphic framework. The identification of Turonian bentonites greatly improved the understanding of the stratigraphic relationships, especially in the Upper Turonian while natural gamma radiation logs turned out as a valuable method for intrabasinal correlation. In conclusion, the new sections provide a high-quality standard succession for the lower Upper Cretaceous in the south-eastern Munsterland Cretaceous Basin.

Depositional Model, Pebble Provenance and Possible Reservoir Potential of Cretaceous Conglomerates: Example from the Southern Slope of Medvednica Mt. (Northern Croatia)

Upper Cretaceous deposits in Medvednica Mt. are composed of coarse-grained conglomerates, sandstones, shales and the pelagic Scaglia Limestones. Such deposits in the wider region possess reservoir potential, not previously studied in Northern Croatia. Modal composition of conglomerates, size and distribution of clasts, porosity and permeability were studied from one new exposure in Medvednica Mt., and the results were compared with previously published data from neighboring successions. Conglomerates are polymictic, clast- to matrix-supported, with clasts and matrix entirely composed of local bedrocks. Porosity varies between 4.98 to 10.89% and permeability from 1.13 to 43.3 mD. Overlying pelagic Scaglia Limestones contain pelagic foraminifera of the latest Santonian to Early Campanian age (83 to 85 Ma). Clasts were eroded from the local hinterland, probably transported to the beach by short-term torrents and deposited along the shelves of the proto-Medvednica Island. Previously presumed alluvial transport is not likely. Deposition took place in a Gosau-type basin during the subsidence phase, additionally controlled by a third-order sea-level change at the Santonian-Campanian boundary. According to this study, Upper Cretaceous clastites possess possible reservoir potential, and deserve more attention in future hydrocarbon research in Croatia.

The influences of sea-level changes on the quality of bank reservoirs of the Lower Cambrian Longwangmiao Formation, in the Gaoshiti-Moxi area, Sichuan Province, China

Reservoir properties of banks of the Longwangmiao Formation show strong lateral and vertical variations in the Gaoshiti-Moxi area, central Sichuan. In this study, we utilized outcrop, core data, thin sections, wireline logs, and seismic data to investigate the influences of sea-level changes on the development of banks and penecontemporaneous dissolution which controlled the formation of high-quality bank reservoirs. Our results revealed that the Longwangmiao Formation corresponds to a third-order sea-level change cycle, which consisted of a rapid sea-level rise and a slower sea-level fall; we further identify four fourth-order sea-level cycles. Bank reservoirs are dominated by grain dolostone, silt to fine sand-sized crystalline dolostone, and mottled silt to fine sand-sized crystalline dolostone. The main reservoir spaces were pores and vugs, with an average porosity of 4.81% and average permeability of 4.75 × 10−3 μm2. Bank deposition and dissolution during the third-order sea-level fall developed better than during the rising period, and the late fall stage was better than the early fall stage. Under the influence of fourth-order sea-level changes within the third-order sea-level change, the development of banks shows strong variations between different areas, which are evidenced by the fact that banks in the Moxi block were better developed than those in the Gaoshiti block. Bank dissolution in the Moxi block developed during the early and the late stage within the third-order sea-level fall, whereas the Gaoshiti block was developed only during the late stage. Therefore, we have concluded that sea-level changes played a significant role in controlling the quality of bank reservoirs in the Gaoshiti-Moxi area.

Upper Jurassic–Lower Cretaceous depositional environments and evolution of the Bilecik (Sakarya Zone) and Tauride carbonate platforms, Turkey

Three Late Jurassic–Early Cretaceous carbonate platform stratigraphic sections have been studied in central western Turkey (Bilecik carbonate platform at Bilecik and Sarıcakaya) and in the Taurides in southern Turkey (Tauride carbonate platform at Beysehir) by using sedimentological and cyclostratigraphical methods. One section was analyzed by detailed magnetic susceptibility measurements. Peritidal carbonates on both platforms display cyclic patterns throughout the sections and do not contain any siliciclastic material. This cyclicity is generally expressed as shallowing upward cycles and interpreted as records of short-term sea-level changes.Magnetic susceptibility (MS) analysis revealed that small scale changes in the MS values can be followed by cyclic alternation of facies and may indicate that varying detrital input during wet and dry climate alternation can be one of the controlling factors for short term sea level changes. The large-scale MS evolution records an increase just around the Jurassic–Cretaceous boundary interval, and a decrease above the boundary. This curve pattern can be correlated with some European MS sections and seems to indicate a consistent MS pattern around this critical interval. Consequently, this study implies that sea-level variations on studied shallow carbonate platforms were on the order of few meters. Pelagic equivalents of these sea-level records can be traced along the margins. Short term climatic control can be seen as one of the controlling factors for small-scale sea-level changes. The large-scale changes can be associated with oceanographic/climatic, third-order sea-level changes and the influence of regional tectonics.

Lithofacies palaeogeography and biostratigraphy of the lowermost horizons of the Middle Triassic Hallstatt Limestones (Argolis Peninsula, Greece)

Condensed ammonoid beds of the Hallstatt facies (Anisian–Ladinian) are widespread around the Ancient Theatre of Epidaurus, in the locality Theokafta of the Argolis Peninsula (eastern Peloponnesus). The Hallstatt Formation in Argolis appears, generally, in the form of lensoid bodies of variable sizes, inclination and direction and is always found overlying a formation consisting of keratophyric tuffs. In fact, the contact of the keratophyric tuffs with the overlying limestones, specifically evidenced by an in situ brecciated zone, is stratigraphic and constitutes the base of the Hallstatt Limestones. The contact of the Hallstatt Limestones with the overlying radiolarites is stratigraphic as well.Lithofacies and biostratigraphic research has focused on the lowermost horizons of the Hallstatt Limestones of Anisian age (average thickness about 1.30m), where a dense sampling has been performed, followed by detailed facies analysis. The lowermost horizons of the Hallstatt Limestones of Theokafta represent typical hiatus beds/concretions sensuWetzel and Allia (2000), characterized by discontinuous sedimentation and erosion. They consist of red ammonoid-bearing hemipelagic limestones with calcium carbonate nodules floating in an enriched Fe-oxides matrix with dispersed lensoid/prismatic calcium carbonate crystals. This part of the section is characterized by condensed sedimentation, due to significant lowering of the rate of sedimentation and includes omission surfaces, firmgrounds and hardgrounds along certain horizons. Nine lithostratigraphic units have been distinguished in the lowermost horizons of the Hallstatt Limestones, including radiolarian packstones, volcaniclastic facies, packstones/floatstones with ammonoids and lag deposits.Tselepidis (2007) defined nine distinct ammonoid biozones from the Anisian to Ladinian, documenting deposition of the Hallstatt facies during a low depositional rate over nearly 5 million years (using the timescale of Gradstein et al., 2004). The biozones: Japonites/Paracrochordiceras, Hollandites, Procladiscites/Leiophyllites, zoldianus, trinidosus, Reitziites/Parakellnerites and the Nevadites (Anisian) and the biozone curionii (Lower Ladinian). Although sedimentation was very condensed, it didn’t reach the level of mixing fauna.Synsedimentary and early burrowing processes differentiated the primary texture characteristics of the deposited sediments. Multiphase diagenesis occurred not very deep below the sediment surface and includes boring and/or encrustation, burial and cementation. The deposition of the studied Hallstatt Limestones is considered to be due to anaerobic oxidation of organic matter, which provided excess alkalinity, inducing carbonate precipitation. Sedimentation took place on differentially-subsided deep swells. After drowning, the swells were covered by pelagic carbonate deposits. Further slight rotation of blocks, along listric faults, may have led to additional differential subsidence of the blocks. Shelf bathymetry and third-order sea-level changes played a significant role in the formation of the Hallstatt beds. In terms of sequence stratigraphy, the studied hiatus concretions and beds are considered genetically linked to rising or high sea-level, formed at the initiation of transgressions, as well as during the time of maximum rate of transgression, in areas where the sediment input was strongly reduced (“condensed section”).Taking into consideration the present location of the Hallstatt Formation, in the context of the Hellenides, an area suitable for the deposition of the Hallstatt Limestones, should be located between the sub-Pelagonian (western part of the Pelagonian zone) and Pindos geotectonic zones, which during the Triassic corresponded to a platform slope and a deep ocean, respectively. The widespread Middle Triassic Han Bulog Limestones (ammonoid-bearing pelagic limestones) from Triassic successions of the Eastern Alps (Dinarides, Hellenides) may have formed partly in similar slope environments.

Eocene oil shales from Jordan - their petrography, carbon and oxygen stable isotopes

ABSTRACT Sediment petrographic studies combined with carbon and oxygen stable-isotope analyses are considered to be powerful tools in deciphering the depositional and diagenetic history of carbonate rocks. These studies have been found valuable for understanding the environmental controls and global changes of climate and oceanography. Two cores of Eocene bituminous marls (OS-22 and OS-23) from central Jordan with thicknesses of 222.4 m and 256.3 m, respectively, were logged. A total of 103 thin sections were analyzed for sediment petrographic investigations. Another 295 bulk rock samples from Core OS-23 were selected for stable-isotope analysis (δ13C, δ18O). The two cores show similar lithologic variations that correspond to third-order sea-level changes. The sediment petrographic study reveals the presence of six major microfacies that range from mudstones to grainstones. The distribution of these six types of microfacies in the two cores indicates an overall shallow-marine environment in an interior carbonate-platform setting. Within this environment, conditions fluctuated between open and restricted marine. The stratigraphic variability of the carbon-isotope data of Core OS-23 reflects a highly dynamic depositional system that exhibits a variable rate of organic matter accumulation in the sediments, which can be directly linked to the interaction between primary organic-matter burial and oxidative weathering of reworked organic matter. Within the overall diagenetically controlled δ18Ocarb profile an excursion is observed for a specifically enriched organic matter interval. This excursion is believed to coincide with the Middle Eocene Climatic Optimum (MECO), suggesting that next to seawater, porewaters were influenced by this event and making it the first record of this event in the region.

Black shale deposition during Toarcian super-greenhouse driven by sea level

Abstract. One of the most elusive aspects of the Toarcian oceanic anoxic event (T-OAE) is the paradox between carbon isotopes that indicate intense global primary productivity and organic carbon burial at a global scale, and the delayed expression of anoxia in Europe. During the earliest Toarcian, no black shales were deposited in the European epicontinental seaways, and most organic carbon enrichment of the sediments postdated the end of the overarching positive trend in the carbon isotopes that characterises the T-OAE. In the present study, we have attempted to establish a sequence stratigraphic framework for Early Toarcian deposits recovered from a core drilled in the Paris Basin using a combination of mineralogical (quartz and clay relative abundance) and geochemical (Si, Zr, Ti and Al) measurements. Combined with the evolution in redox sensitive elements (Fe, V and Mo), the data suggest that expression of anoxia was hampered in European epicontinental seas during most of the T-OAE (defined by the positive carbon isotope trend) due to insufficient water depth that prevented stratification of the water column. Only the first stratigraphic occurrence of black shales in Europe corresponds to the "global" event. This interval is characterised by >10% Total Organic Carbon (TOC) content that contains relatively low concentration of molybdenum compared to subsequent black shale horizons. Additionally, this first black shale occurrence is coeval with the record of the major negative Carbon Isotope Excursion (CIE), likely corresponding to a period of transient greenhouse intensification likely due to massive injection of carbon into the atmosphere–ocean system. As a response to enhanced weathering and riverine run-off, increased fresh water supply to the basin may have promoted the development of full anoxic conditions through haline stratification of the water column. In contrast, post T-OAE black shales during the serpentinum and bifrons Zones were restricted to epicontinental seas (higher Mo to TOC ratios) during a period of relative high sea level, and carbon isotopes returning to pre-T-OAE values. Comparing palaeoredox proxies with the inferred sequence stratigraphy for Sancerre suggests that episodes of short-term organic carbon enrichment were primarily driven by third-order sea level changes. These black shales exhibit remarkably well-expressed higher-frequency cyclicities in the oxygen availability in the water column whose nature has still to be determined through cyclostratigraphic analysis.

Lithostratigraphy, sedimentology, and cyclicity of the Duwi Formation (late Cretaceous) at Abu Tartur plateau, Western Desert of Egypt: evidences for reworking and redeposition

The Duwi Formation (late Cretaceous) represents the first onset of fully marine conditions and comprises sediments that were confined by a preexisting depression enclosed by the Dakhla uplift in the west and Kharga uplift in the east. The Duwi Formation overlies unconformably the Qusseir Formation and underlies conformably the Dakhla Formation. The Duwi Formation could be subdivided vertically into three remarkable informal lithostratigraphic units. The Duwi Formation comprises a heterogeneous suite of shallow marine rocks (phosphorite, shale, mudstone, marlstone, glauconite, and dolomite). A model has been presented for the mode of phosphorite accumulation. This model includes aspects from both the traditional upwelling phosphogenic model and key elements from the non-upwelling model. The role played by high-energy events (e.g., internal waves) in phosphorite accumulation was achieved by the interplay between the physicochemical and hydrodynamical processes. Internal waves are thought to be the triggering mechanism for the density flows. Internal waves and induced bottom currents are sporadic but strong enough to bring sediments into suspension. Accumulation of phosphorites by high-energy waves is evidenced by the predominance of many types of bedform sequences. Such type of bedforms are represented by fining upward, firm grounds, erosive pockets and pot holes, planar lags, imbrication, sorting, and fragmentation. The Duwi Formation has been divided into several meter-scale cycles of three types (gradual, non-gradual, and truncated). This represents the higher frequency sea-level fluctuations forming shallowing–upward cycles that were arranged into the larger regressive megasequence during a third-order sea level change.

Sea level changes and rapid crustal movements in cratonic areas in the Late Paleozoic

In the Phanerozoic, the sea depth in epeiric sedimentary basins showed considerable variations, often accompanied by regression. In periods of regression and erosion, the subaerially exposed shelf and the adjacent parts of the marine basins gave rise to numerous nonstructural (stratigraphic) hydrocarbon traps. Sea depth variations with a magnitude of up to 100–200 m and 1–3 myr long (third-order cycles) are usually attributed to the eustatic fluctuations of the sea level. To estimate their possible range, a model is proposed which describes the water depth variations as a function of eustatic fluctuations in tectonically subsiding carbonate platforms. We take into account the crustal isostatic response to the changing water load and the finite time necessary for soil and karst formation in the exposed shelf or its upper part. This model allowed analyzing data on the reference sections of the North Timan shallow-water sediments. According to the analysis, the third-order sea level changes in the Middle Carboniferous, Late Carboniferous, and Early Permian did not exceed several tens of meters. During the same period, shorter fluctuations (∼100 kyr) occurred owing to the waxing and waning of large Gondwanan ice sheets. In the first half of the Bashkirian Age (Early Pennsylvanian), regression took place in the East European and North American cratons and then shallow-water sedimentation resumed. This regression is usually attributed to a considerable sea level fall. In some other areas, slow shallow-water sedimentation continued throughout the Bashkirian. This suggests that the Bashkirian regression was due to the crustal uplifts. Short-term uplifts can be explained by ascending convective currents beneath the asthenosphere. In southern North America, they brought an active fluid into the lithosphere. This caused rapid eclogitization-related crustal subsidence in the Arkoma and Anadarko basins as well as intense lithospheric weakening and shortening in the Ouachita Fold Belt.

Cross-Basin Variations in Magnetic Susceptibility Influenced by Changing Sea Level, Paleogeography, and Paleoclimate: Upper Devonian, Western Canada Sedimentary Basin

Abstract Upper Devonian rocks in western Canada were deposited as carbonate-platform and surrounding slope and basinal rocks during a second-order sea-level cycle, characterized by long-term climatic warming, that ended with cooling associated with the Frasnian–Famennian mass-extinction event. In this contribution, we assess the hypothesis that high-resolution magnetic susceptibility (MS) data records cross-basin variations that are intimately tied to sea level and associated climatic change, the depositional environment, and the paleogeographic location of siliciclastic sediment sources. Four main trends are apparent in the MS data: (1) the magnitude of the MS signature increases from west to east across western Canada; (2) platform facies generally have low MS signatures compared with off-platform facies; (3) the overall MS signature increases and then decreases throughout the stratigraphic profile, largely in response to changes in depositional setting (i.e., item 2); (4) MS is generally high during third- and fourth-order sea-level lowstand and early transgression and lower during late transgression and highstand. During the late Middle (Givetian) to early Late (Frasnian) Devonian, siliciclastic sediments (silt and fine sand size) sourced from the west Alberta arch and Peace River arch, contributed to relatively high MS signatures in facies deposited proximal to these source areas. Other important detrital sediment sources were the Ellesmerian orogenic belt in northern Canada and the Laurussian continental landmass to the east. These latter sources supplied fine-grained sediments that were dispersed basinwide and resulted in a nearly order-of-magnitude increase in MS values to the east. This trend and fine grain size implies that some detrital sediment was delivered from the east as eolian dust from the “Old Red Continent.” By the early Frasnian, the west Alberta arch was submerged, and by middle to late Frasnian time the Peace River arch was largely covered. During the late middle Frasnian a pronounced rise in MS signatures can be tied directly to deposition of the clay- and locally silt-rich Mount Hawk Formation. During the late middle to late Frasnian, carbonate-ramp facies prograded from east to west, resulting in seaward migration of the shoreline and increased siliciclastic input to the ramp slope that influenced the pronounced increase in MS signatures to the east. By the late Frasnian, cross-basin MS profiles record uniformly low susceptibilities indicating that high MS siliciclastic detritus was either bypassed seaward, was swamped by carbonate input, or was dominated by diamagnetic quartz. Individual MS events can be accurately correlated across much of the basin at a resolution that is higher than single conodont zones despite the local influences on variability of the signature. Variations in the MS signature can thus be explained partly by depositional environment (platforms with low MS, basins with higher MS) and partly by third- and fourth-order sea-level and associated climatic changes. Fourth-order changes in sea level are routinely interpreted to result from variations in paleoclimate in the Milankovitch band and recent work indicates that third-order sea-level changes may be linked to longer-term modulation of Milankovitch-band orbital variations. The increase in MS during prolonged Frasnian warming thus appears to be directly linked to paleoclimatic change, and comparison of the MS signature with more direct measures, like oxygen isotopes, would serve as a test of its utility as a paleoclimate proxy.

From Cycles to Sequences: Sequence Stratigraphy and Relative Sea Level Change for the Late Cambrian of the North China Platform

Abstract In the Late Cambrian, the North China Platform was a typical carbonate ramp platform. The Upper Cambrian of the northern part of the North China Platform is famous for the development of bioherm limestones and storm calcirudites and can be divided from bottom to top into the Gushan, Changshan and Fengshan formations. In this set of strata, the deep-ramp mudstone and marls and the shallow-ramp packstones and grainstones constitute many carbonate meter-scale cycles of subtidal type. More tidal-flat dolomites are developed in the Upper Cambrian of the southern margin of the North China platform, in which limestone and dolomite beds also constitute many carbonate meter-scale cycles of the peritidal type. These cycles are marked by a variety of litho-facies successions. There are regularly vertical stacking patterns of meter-scale cycles in long-term third-order sequences, which is the key to discerning such sequences. Third-order sequence is marked by a particular sedimentary-facies succession that is the result of the environment-changing process of deepening and shoaling, which is genetically related to third-order sea level changes. Furthermore, four third-order sequences can be grouped in the Upper Cambrian of the North China Platform. The main features of these four third-order sequences in the northern part of the platform can be summarized as follows: firstly, sequence-boundaries are characterized by drowning unconformities; secondly, the sedimentary-facies succession is generally constituted by one from deep-ramp facies to shallow-ramp facies; thirdly, a succession of “CS (?)+HST” (i.e., “condensed section and high-stand system”) forms these four third-order sequences. The chief features for the third-order sequences in the southern part of the North China Platform comprises: more dolomites are developed in the HSTs of third-order sequences and also developed more carbonate meter-scale cycles of peritidal types; the sedimentary-facies succession of the third-order sequences is marked by “shallow ramp-tidal flat”; the sequence boundaries are characterized by exposure punctuated surfaces. According to the changes for the third-order sequences from the north to the south, a regular sequence-strati graphic framework can be established. From cycles to sequences, the study of sequence stratigraphy from litho-facies successions to sedimentary-facies successions exposes that as follows: meter-scale cycles that are used as the basic working unit actually are litho-facies successions formed by the mechanism of a punctuated aggradational cycle, and third-order sequences that are constituted by regularly vertical stacking patterns of meter-scale cycles are marked by sedimentary-facies successions. On the basis of the changing curve of water depth at each section, the curve of the relative third-order sea level changes in the late Cambrian of the North China Platform can be integrated qualitatively from changing curve of water depth. The correlation of Late Cambrian long-term sea level changes between North China and North America demonstrates that there are not only similarities but also differences, reflecting control of long-term sea level changes both by global eustacy and by regional factors.

Drowning of the Upper Marble Falls carbonate platform (Pennsylvanian), central Texas: A case of conflicting “signals?”

Marble Falls and Smithwick Formation carbonates and shales were deposited on the western margin of the slowly subsiding Fort Worth Basin of central Texas during Early Pennsylvanian time. The Upper Marble Falls Member carbonate platform back-stepped towards the west–southwest as Smithwick outer shelf–slope black shales drowned platform margin areas to the east. The shelf-to-basin profile observed from subsurface data from the Fort Worth Basin and outcrops from the Llano Uplift area records platform drowning; however, the platform began to shoal upwards as it back-stepped towards the west–southwest, due to the presence of the Llano Uplift paleobathymetric high. The geometry of these facies tracts was controlled primarily by a combination of subsidence and second-order sea-level change, while the development of sequences and unconformities was controlled by third-order sea-level changes and environmental factors, and not subsidence. The termination of platform deposition in the Fort Worth Basin was caused by the interplay of third-order sea-level changes, environmental deterioration in advance of siliciclastic burial, and a lack of accommodation space. The depositional history of the Upper Marble Falls carbonate platform does not fit “classic” examples of drowned platforms described from other areas, but may be typical of platforms in slowly subsiding basins.

Controls on Precambrian sea level change and sedimentary cyclicity

Although uniformitarianism applies in a general sense to the controls on relative and global sea level change, some influences thereon were more prominent in the Precambrian. Short-term base level change due to waves and tides may have been enhanced due to possibly more uniform circulation systems on wide, low gradient Precambrian shelves. The lack of evidence for global glacial events in the Precambrian record implies that intraplate stresses and cyclic changes to Earth's geoid were more likely explanations for third-order sea level change than glacio-eustasy. Higher heat flow in the earlier Precambrian may have led to more rapid tectonic plate formation, transport and destruction, along with an increased role for hot spots, aseismic ridges and mantle plumes (superplumes), all of which may have influenced cyclic sedimentation within the ocean basins. A weak cyclicity in the occurrence of plume events has an approximate duration comparable to that of first-order (supercontinental cycle) sea level change. Second-order cyclicity in the Precambrian largely reflects the influences of thermal epeirogeny, changes to mid-ocean ridge volume as well as to ridge growth and decay rates, and cratonic marginal downwarping concomitant with either sediment loading or extensional tectonism. Third-order cycles of sea level change in the Precambrian also reflected cyclic loading/unloading within flexural foreland basin settings, and filling/deflation of magma chambers associated with island arc evolution. The relatively limited number of studies of Precambrian sequence stratigraphy allows some preliminary conclusions to be drawn on duration of the first three orders of cyclicity. Archaean greenstone basins appear to have had first- and second-order cycle durations analogous to Phanerozoic equivalents, supporting steady state tectonics throughout Earth history. In direct contrast, however, preserved basin-fills from Neoarchaean–Palaeoproterozoic cratonic terranes have first- and second-order cycles of considerably longer duration than Phanerozoic examples, supporting less evolved tectonism affecting cratonic plates. It is possible that oceanic tectonic realms underwent more rapid and dynamic plate movements and arc generation, whereas early continental cratonic plates offered more stable platforms and may have been subject to slower migration rates. The wide range of controls on Precambrian sea level change, allied to their apparent variability (in rates and periodicity) through Precambrian time supports the conclusion that each order of cyclicity is relative and must be defined within the stratigraphic context of each individual case study. This underlines the importance of establishing a hierarchical order of cyclicity in sequence stratigraphic interpretations of Precambrian basins based on the relative importance of sequences rather than their temporal duration.

‘A 3D outcrop analogue model for Ypresian nummulitic carbonate reservoirs: Jebel Ousselat, northern Tunisia’ by E. Vennin et al .

Simon J. Beavington-Penney and Andrew J. Barnett write: Vennin et al . (2003) have provided an interesting study of the Ypresian (Eocene) El Garia Formation of Tunisia. This unit has been the subject of numerous studies, largely because it reservoirs hydrocarbons within the Gulf of Gabes (e.g. Loucks et al . 1998; Racey et al . 2001; Jorry et al . 2003). Because of the economic significance of this unit, its correct interpretation, especially in terms of the continuity and correlation of depositional sequences (and, therefore, reservoir layering) is crucial. Using the well-studied outcrops to the west of Kairouan in north-central Tunisia, Vennin et al . (2003) present a sequence stratigraphic model of the El Garia Fm. Their high-resolution model recognizes three orders of depositional sequences (third to fifth), with their ‘small-scale’ cycles occurring as individual decimetre- to metre-scale beds (which, they suggest, fall within the range of the duration of Milankovitch cycles) and groups of these beds which form metre- to decametre-scale, fifth-order packages. Vennin et al . (2003) claim that this hierarchy of cycles was produced by a nested hierarchy of eustatic sea-level change. We would like to suggest that these cycles (and the invoked causal sea-level change) are debatable, not least because they contain no unambiguous evidence of relative sea-level change (i.e. subtidal deposits capped directly by subaerial exposure surfaces). The El Garia Fm. was deposited over a period of approximately 2.8 Ma (Racey et al . 2001), during which time several third-order sea-level changes might be expected to have occurred. However, earlier studies from both outcrop and subsurface (offshore Tunisia borehole core) have not noted widespread surfaces that can be correlated within the El Garia Fm. (such as the Fe-rich hardgrounds which Vennin et al . (2003) record as bounding their fifth-order cycles) and have …