Stratigraphic Cycles Research Articles

Oxfordian magnetostratigraphy of Poland and its correlation to Sub-Mediterranean ammonite zones and marine magnetic anomalies

A nearly continuous magnetostratigraphic polarity pattern was compiled from several ammonite-zoned carbonate successions of southern Poland and from a composite magnetostratigraphy from the Iberian Range of Spain. The array of sections spans the middle two-thirds of the Oxfordian within the Sub-Mediterranean Province (Cordatum through Bifurcatus ammonite zones). The average paleopole calculated from eight of these Polish sections is at 78.5°N, 184.9°E ( δp = 2.6°, δm = 3.5°). The Sub-Mediterranean polarity pattern is consistent with an independent polarity pattern derived from the Boreal-realm sections of the British Isles, and improves the inter-correlation between these faunal realms. Cycle stratigraphy published for these ammonite subzones from southern France enabled temporal scaling of the polarity pattern, thereby facilitating correlation to marine magnetic anomalies M28 through M33 as modeled from deep-tow magnetometer surveys in the Western Pacific. The bases of the Middle and Upper Oxfordian substages as defined in the Sub-Mediterranean zonation in Poland correspond approximately to chrons M33 and M29 of that Pacific M-sequence model.

Sequence stratigraphic analysis of the first layer, upper second submember, Shahejie formation in Pucheng oilfield

In view of the high accuracy and predictability, high-resolution sequence stratigraphy had been extensively applied to oil exploration and gotten prominent practicable results. This article takes the first layer, upper second submember, Shahejie (沙河街) Formation from Pucheng (濮城) oilfield as an example to analyze the application of high-resolution sequence stratigraphy in reservoir study on the basis of a comprehensive study of core log data. Firstly, facies analysis of this area reveals the corresponding terminal fan system occurring where sediment-laden streams decrease in size and vanish as a result of evaporation and transmission losses. The model includes a tripartite zonation of terminal fan into feeder, distributary, and basinal zones. Secondly, electrofacies were made by well-log analysis and then matched with sedimentary facies defined by core analysis. Four electrofacies characterizing the main sedimentary facies association and depositional environments within target area are defined (channel, lag deposit, lake or flood-plain, and overflow deposits). Thirdly, related correlations based on high-resolution sequence stratigraphy were established. By observing the stacking arrangement of genetic sequences, different scales of stratigraphic cycle can be identified. Within scale and duration, the stratigraphic cycles are termed as genetic sequences, genetic sequence sets, and minor cycles.

Braided rivers within an arid alluvial plain (example from the Lower Triassic, western German Basin): recognition criteria and expression of stratigraphic cycles

AbstractBased on a detailed sedimentological analysis of Lower Triassic continental deposits in the western Germanic sag Basin (i.e. the eastern part of the present‐day Paris Basin: the ‘Conglomérat basal’, ‘Grès vosgien’ and ‘Conglomérat principal’ Formations), three main depositional environments were identified: (i) braided rivers in an arid alluvial plain with some preserved aeolian dunes and very few floodplain deposits; (ii) marginal erg (i.e. braided rivers, aeolian dunes and aeolian sand‐sheets); and (iii) playa lake (an ephemeral lake environment with fluvial and aeolian sediments). Most of the time, aeolian deposits in arid environments that are dominated by fluvial systems are poorly preserved and particular attention should be paid to any sedimentological marker of aridity, such as wind‐worn pebbles (ventifacts), sand‐drift surfaces and aeolian sand‐sheets. In such arid continental environments, stratigraphic surfaces of allocyclic origin correspond to bounding surfaces of regional extension. Elementary stratigraphic cycles, i.e. the genetic units, have been identified for the three main continental environments: the fluvial type, fluvial–aeolian type and fluvial/playa lake type. At the time scale of tens to hundreds of thousands of years, these high‐frequency cycles of climatic origin are controlled either by the groundwater level in the basin or by the fluvial siliciclastic sediment input supplied from the highland. Lower Triassic deposits from the Germanic Basin are preserved mostly in endoreic basins. The central part of the basin is arid but the rivers are supplied with water by precipitation falling on the remnants of the Hercynian (Variscan)–Appalachian Mountains. Consequently, a detailed study of alluvial plain facies provides indications of local climatic conditions in the place of deposition, whereas fluvial systems only reflect climatic conditions of the upstream erosional catchments.

Late Permian to Middle Triassic correlations and palaeogeographical reconstructions in south-western European basins: New sedimentological data from Minorca (Balearic Islands, Spain)

Description of sedimentary sections and interpretations of sequence stratigraphic evolution of Permian–Triassic red beds of Minorca, allow a new proposal of palaeoenvironment reconstructions. The Permian sediments mainly correspond to floodplain and pond deposits with sheetflood sand-bed rivers (environment DE 1), within a warm and semi-arid climate, and the preserved succession forms one complete stratigraphic cycle. The retrogradational phase of this cycle is represented by alluvial/colluvial deposits grading upwards to environment DE 1 and to extensive floodplain and pond deposits. The progradational phase is characterized by increasing fluvial influence (with palaeocurrent mainly oriented to the SW), palaeosol horizons, and preservation of a Late Permian palynoflora. The uppermost part of this succession is truncated by a major erosional surface. The sediments above this unconformity are characterized by braided river deposits (environment DE 2) with palaeocurrent oriented to the SSW within the more basal conglomeratic facies and to the WNW in overlying units. The absence of extensive floodplain deposits, a paucity of evidence for biogenic activity and palaeosols, together with the presence of ventifacts indicate an arid climatic setting for the succession. This environment evolved from braided rivers (palaeocurrents mainly oriented to the SE, environment DE 3) with in situ palaeosols and reworked breccias to fluvial–coastal and shallow marine deposits (environment DE 4) dated as upper Anisian to Ladinian. This succession characterizes a retrogradational trend. Although the transition between DE 2 and DE 3 was not observed, a switch in palaeocurrent directions may indicate a change in source area. A comparison at a regional scale with other Peri-Tethyan basins allows the succession to be placed within a broader palaeoenvironmental and stratigraphic context during Late Permian to Middle Triassic. During the Late Permian, sedimentation occurred in alluvial, playa-lake or ponded environments where fluvial systems generally flowed southward, except in south-eastern France (oriented to the NE). Within these SW European basins, such as Minorca, the Late Permian succession shows a major retrogradational (evolution from fluvial or alluvial fan deposits to extensive lake, playa or floodplain deposits) and a progradational trend (fluvial or alluvial fan deposits). The Permian–Triassic transition corresponds to an unconformity overlain by braided river deposits with arid climate indicators (aeolian deposits: ventifacts and aeolian dune sedimentation). At the scale of western Europe, this arid episode is dated as Smithian and the Induan age sedimentation deposits seem to be preserved only in the central part of the Germanic Basin. As with all other Peri-Tethyan basins, environment DE 2 of Minorca (above the major erosional surface) is attributed to the Smithian. In the upper part of the studied succession, braided river deposits indicative of less arid climatic conditions are preserved. This succession contains the earliest Mesozoic palaeosols, dated as Anisian by palynomorphs, and expresses a vertical evolution from fluvial to open marine depositional environments attributed to the Muschelkalk transgression.

Description of Cretaceous Sedimentary Sequence of the Second and Third Member of the Qingshankou Formation Recovered by CCSD-SK-Is Borehole in Songliao Basin: Lithostratigraphy, Sedimentary Facies and Cyclic Stratigraphy

The second and third member of the Qingshankou Formation recovered by CCSD-SK-Is borehole (China Cretaceous Continental Scientific Drilling-SongkeI-the south borehole) is 415.61 m long and 100% of cores recovery. The age of the member corresponds approximately to the Coniacian. The sequence and process of lithology-lithofacies and cyclic stratigraphy were revealed by a detailed core description. 12 rock types and 2 kinds of sedimentary subfacies including semi-deep lake and shallow lake were recognized from the drilling core of the second and third member of the Qingshankou Formation. 10 sedimentary microfacies are present, including dolomite, marl, limestone, oil shale, semi-deep lake turbidite, volcanic ash, seismite, semi-deep lake mudstone, shallow lake mudstone, and shallow lake turbidite microfacies. The second and third member of the Qingshankou Formation represents 422 meter-scale cycles (sixth-order cycle), 130 fifth-order cycles, 21 fourth-order cycles, and one third-order cycles. The special lithologies, such as mudstone, seismite, dolomite, volcanic ash, and so on are important to researches on source rocks and lacustrine event sediments.

Description of Cretaceous Sedimentary Sequence of the First Member of the Qingshankou Formation Recovered by CCSD-SK-Is Borehole in Songliao Basin: Lithostratigraphy, Sedimentary Facies, and Cyclic Stratigraphy

The first member of the Qingshankou Formation recovered by CCSD-SK-Is borehole (China Cretaceous Continental Scientific Drilling-SongkeI-the south borehole) is 81.41 m long, and the recovery of cores is 100%. The age of the member is approximately Turonian. The sequence and process of lithology-lithofacies and cyclic stratigraphy were revealed by a detailed core description. 6 rock types (mudstone, dolomite, marl, limestone, ostracode limestone, and volcanic ash) comprise the sedimentary strata. Deposition occurred in a deep lake, including 6 sedimentary microfacies: mudstone, sandy turbidite, dolomite, marl, limestone, and volcanic ash microfacies. The first member of the Qingshankou Formation represents 81 meter-scale cycles (sixth-order cycle), 25 fifth-order cycles, 6 fourth-order cycles, and 2 third-order cycles. The occurrence and detailed description of mudstone, dolomite, and volcanic ash are of significance for the study of anoxic events, the source rock development, and the potential linkage between lake-marine depositional events.

Description of Cretaceous Sedimentary Sequence of the Yaojia Formation Recovered by CCSD-SK-Is Borehole in Songliao Basin: Lithostratigraphy, Sedimentary Facies and Cyclic Stratigraphy

The Yaojia Formation recovered by CCSD-SK-Is borehole (China Cretaceous Continental Scientific Drilling-SongkeI-the south borehole) is 157.67 m long and 99.96% of cores recovery. The age of the formation corresponds with a range from the Santonian to the early Campanian. The sequences and processes of lithology-lithofacies and cyclic stratigraphy are revealed by a detailed core description. Eleven rock types and three kinds of sedimentary subfacies, including shallow lake, deep lake, and delta front, are recognized from the drilling core. There are eleven sedimentary microfacies including dolostone, argillaceous limestone, shallow lake turbidite, deep lake turbidite, subaqueous mouth bar, distal bar, sheet sandstone, subaqueous distributary bay, slump deposits, shallow lake mudstone, and deep lake mudstone. The Yaojia Formation represents one hundred and fifty-one meter-scale cycles (sixth-order cycle), forty-five fifth-order cycles, nine fourth-order cycles, and three third-order cycles. Meticulous depiction (centimeter level) of the whole Formation provides an approach to study its sedimentary process with high precision. The red beds with a thickness of 43% in the Yaojia Formation, being one of fewer continental red beds depicted meticulously in the world, will become a standard of potential formulation section for the studies of Cretaceous global oxygen-enriched events.

Description of Cretaceous Sedimentary Sequence of the Quantou Formation Recovered by CCSD-SK-Is Borehole in Songliao Basin: Lithostratigraphy, Sedimentary Facies and Cyclic Stratigraphy

The Quantou Formation recovered by CCSD-SK-Is borehole (China Cretaceous Continental Scientific Drilling-SongkeI-the south borehole) is 132.07 m long and the recovery of cores is 100%. The sequence and process of lithology-lithofacies and cyclostratigraphy are revealed by a detailed core description. Nine rock types and three kinds of sedimentary facies including meandering river, shallow lake, and delta front are recognized from the drilling core, there are ten sedimentary microfacies, which are point bar, natural levee, crevasse splay, crevasse channel, floodplain, flood lake, distributary mouth bar, interdistributary bay, mudstone of still water, and turbidite. The Quantou Formation represents seventy-six meter-scale cycles (sixth-order cycle), twenty-five fifth-order cycles, eight fourth-order cycles, and two third-order cycles. Meticulously depicted (centimeter level) continental red beds of the third and the fourth members of Quantou Formation will be a potential formulation profile for the studies of Cretaceous global oxygen-enriched events. The reservoir sandstones of the third and the fourth members were developed basin-wide as an inevitable product before the Songliao Basin was depressed or sagged on a large scale. The period of the third and the fourth members of Quantou Formation is a stage between the development of the fault basin and the sag basin on a large scale.

Backward stacking of submarine channel–fan successions controlled by strike-slip faulting: The Izumi Group (Cretaceous), southwest Japan

Field surveys and numerical simulations were conducted to examine lithostratigraphic cyclicity in strike-slip basins, which is still poorly understood due to its complexity. The basin-filling processes in strike-slip basins are closely associated with regional tectonics represented by configuration of faults and spatial/temporal variations in the slip rate. We attempted to bridge the gap between qualitative sedimentary facies analyses and quantitative numerical models in order to better understand the formation of these sedimentary successions. This paper focuses on the Izumi Group (Upper Cretaceous), southwest Japan, which was deposited in an elongate basin (300 km long by 10–20 km wide) along the Median Tectonic Line, which at the time of deposition was a sinistral strike-slip fault related to oblique subduction along a forearc margin. The depositional environments of the group were deduced from five lithofacies associations (LAs): submarine channel-fill facies (LA I), proximal facies of lobes or frontal splays (LA II), distal facies of lobes or frontal splays (LA III), slope-apron facies (LA IV), and basin floor facies (LA V). LAs I–III represent point-sourced submarine channel–fan successions in the axial facies, with unidirectional paleocurrent directions from ENE to WSW, and LAs IV–V constitute the marginal facies, the paleoslope of which dipped to the SSW. Two units of submarine channel–fan successions are stacked with ~10 km of eastward (backward) shift. Each unit shows a cyclic lithostratigraphy of rapid upward coarsening and thickening in the lower part (~350 m thick) and gradual upward fining and thinning in the upper part (1–3.5 km thick). It is estimated to have taken 5–7 × 10 5 yr for 10 km offset on each stratigraphic unit based on the depositional ages. Although many processes can control the stratigraphic architecture, such as global and local sea level, climate, and tectonics, the stratigraphic cyclicity observed in the study area is closely related to the depocenter migration, suggesting that fault movement was the primary control on the stratigraphy. On the assumption that the formation and filling processes of the Izumi sedimentary basin were basically controlled by strike-slip faults, a numerical simulation suggests that episodic changes in fault-slip rate or sediment-supply rate could control the stratigraphic cyclicity. In this paper, we propose a model where cyclic stratigraphy is ascribed to temporal variations of fault activity controlling accommodation generation, sediment supply, and relative sea level, which could generate cyclic stratigraphy associated with depocenter migration in strike-slip basins.

Architecture and stratigraphy of alluvial deposits, Morrison Formation (Upper Jurassic), Utah

This article presents a unique cross section of a 13.5-km (8.3-mi)-long by 150-m (492-ft)-thick stratigraphic interval containing braided stream and associated flood-plain deposits. The cross section is oriented approximately parallel to depositional strike. This cross section is a resource for geoscientists and engineers interested in the measurements of stratigraphic architectural elements, such as dimensions and continuity of facies tracts and facies associations, stratigraphic and geographic changes in sandstone/mudstone proportions (net to gross), and frequency and cause of vertical fluid communication between superposed reservoirs.In addition to presenting this rich data resource, observed and documented stratigraphic relationships add to our conceptual understanding of certain attributes of the stratigraphic process-response system. For example, we show that, like meandering rivers, aggradational braided river systems also build levees and alluvial ridges, providing the supra-elevation above the adjacent floodplain to create extensive crevasse splay and channel complexes. We show that superposed channel sandstone reservoirs may be brought into physical contact not only by the erosion of an upper sand into a lower sand but also by the vertical aggradation of the lower sandstone, producing a pyramid on which a younger sandstone is deposited.Four stratigraphic cycles of increasing and decreasing accommodation/sediment supply (A/S) regimes are identified. These cycles are recognized from systematic vertical changes in stratigraphic and sedimentologic attributes. At the largest scale, there is a progressive downhill shift in facies tracts recording a basinward stepping of the four stratigraphic cycles. At the intermediate scale, there is a systematic change in channel types, from laterally amalgamated braided channels, to vertically building braided channels, to steer-head distributary channels encased within lacustrine-dominated fines. At the smallest scale, systematic and repeated vertical successions of facies occur within the three types of channel belts. These systematic changes are related to progressive changes in the A/S regime that occur during superimposed stratigraphic base-level cycles of three different scales.

Multiscale stratigraphic analysis of a structurally confined submarine fan: Carboniferous Ross Sandstone, Ireland

An integrated, multiscale analysis of the Ross Sandstone documents important evolutionary patterns of a structurally confined submarine fan. Through time, the following are evident: (1) depocenters of high-frequency stratigraphic cycles stack vertically, (2) the depositional area increased, (3) regional paleocurrent patterns define a fanlike pattern, (4) the Ross Sandstone correlates in the landward direction to a bypass surface, (5) the Ross Sandstone inherited the basin physiography from the preceding Visean carbonates, and (6) the Ross Sandstone is ponded in the Carboniferous Shannon Basin. These observations demonstrate that the submarine fan filled an actively subsiding, structurally confined basin in an aggradational pattern. A high-resolution geologic map and a cross section of the Ross Sandstone at Loop Head Peninsula depict the location and stratigraphic architecture of every exposure of the Ross Sandstone. These data reveal that (1) lobes are the dominant architectural element (by area) at all stratigraphic positions in the Ross Sandstone; (2) although channels are the most commonly studied features in the Ross Sandstone, they occupy 7% or less of the total cross sectional area of the formation; (3) channels increase upward at the expense of lobes; (4) slumps increase upward; and (5) percent sandstone decreases upward mostly because of an increase in slumps. These trends define an increasing architectural diversity upward within the Ross Sandstone. Observations in the Ross Sandstone coupled with regional patterns for younger formations in the shallowing-upward, basin-fill succession are used to introduce a unifying model for the stratigraphic evolution of the Namurian strata in the basin. The model proposes that each formation in the shallowing-upward succession reflects its own depositional system related to an evolving landscape, and each was sourced from a different direction. This study quantifies vertical trends in stratigraphic features and analyzes them in a statistical manner to test the significance of external controls on local stacking patterns. The statistical analysis reveals that temporal changes in regional conditions, including sediment supply, source area, and basin shape, are statistically and empirically related to local changes in stratigraphy, including architectural element associations and percent sandstone. These relationships suggest that fans with a larger area have more architectural diversity and heterogeneity than their smaller counterparts. Regardless of fan area, lobes are the dominant architectural element. Also, source area is related to the type of architectural element present and percent sandstone. Lobes and their genetically related channels were sourced from the south, whereas slumps and heterolithic strata are mostly sourced from the west. Physical similarities, including climate, eustasy, size and shape of the basin, thickness of stratigraphic cycles, regional stacking patterns, subsidence, percent sandstone, and architectural elements between the Ross Sandstone and the ponded phase of the northern Gulf of Mexico minibasin-fill successions, imply that the Ross Sandstone is an excellent outcrop analog for these successions.

Quantitative tests for stratigraphic cyclicity

AbstractPeriodic Milankovitch (M‐) orbital forcing provides an explanation for subjectively recognized short‐term repetition of lithofacies‐‘cycles’‐in the stratigraphic record. Tests of this explanation often find no order in the lithofacies and/or no regularity in the recurrence of lithofacies. This does notdisprovethe influence of M‐forcing, but a sedimentary response in terms ofirregularM‐forced ‘cycles’ is indistinguishable from one in which repetition of facies is not M‐forced. Use of such cycles in time calibration is correspondingly suspect. Stricter,dimensionalcyclicity invokes Sander's Rule, which suggests periodicity in sedimentation, for which M‐forcing provides an obvious explanation. Time calibration on the basis of strict cyclicity thus appears more dependable. Objective tests for regular M‐forced stratigraphic cyclicity commonly depend upon spectral analyses. Such tests are not unambiguous. Bilogarithmic thickness/frequency plots derived from objective layer thickness inventories (LTI) provide an alternative. Commonly, such plots show power‐law relationships that preclude dimensional M‐cyclicities. By contrast, a model data series that perfectly encodes the M‐cyclic fluctuations in terrestrial insolation generates a strongly inflected, non‐power‐law LTI plot. Power‐law plots result where the model data series is decimated by random hiatuses, with numbers and durations tuned to M‐cycle frequencies. It seems improbable that natural data series record such tuning. The general absence of strict cyclicity in the M‐frequency range is more likely to reflect the nonlinear response of sedimentary systems to cyclic M‐forcing of insolation. Interestingly, when applied to the classically cyclic lacustrine Triassic sediments of the Newark Basin, USA, the LTI test suggests a decimated record, preserving some evidence of M‐cyclicity. Copyright © 2008 John Wiley & Sons, Ltd.

Carboniferous continental margin deposits in southern Tuscany, Italy: results from geological mapping of the geotopes Farma Valley and San Antonio Mine Area

AbstractRelics of the Carboniferous shelf, slope and basin margin section occur as intensely tectonized slices within the middle Tertiary Monticiano‐Roccastrada metamorphic core complex in southern Tuscany. These sedimentary successions are interpreted as predominantly siliciclastic second‐order sequence stratigraphic cycles, deposited in a successor basin (Farma Basin) established near the Southern Convergence Front of the westmediterranean Hercynides. The units developed as highstand systems tracts above a downlap surface and buried pelagites of a starved, moderately deep‐water basin (Risanguigno Basin). The Risanguigno Basin was characterized by dys‐ and anoxic conditions and underwent partial tectonic inversion during the early Carboniferous. Detailed mapping has revealed that Carboniferous turbidites deposited on the slope, base of slope and Farma Basin margin are present in the eastern part of the Farma Valley and in the San Antonio Mine Area. Coeval lithologies—storm influenced deposits of the inner shelf and lower shoreface—crop out in the western part of the Farma Valley. Both units, considered to be related to storm‐triggered offshore bottom currents bypassing the outer shelf, are covered by offlapping shallow marine and littoral clastics probably of Permian age. Subsequent tectonic uplift and partial inversion of the Farma Basin resulted in subaerial exposure, erosion of the upper part of the basin fill and deposition of the terrestrial Verrucano Group (middle Triassic), which sealed these early Carboniferous to late Permian sequences considered to represent a late Palaeozoic preligurian failed rift. Copyright © 2008 John Wiley & Sons, Ltd.

Tectonic and climate driven fluctuations in the stratigraphic base level of a Cenozoic continental coal basin, northwestern Andes

Changes in the sedimentologic and stratigraphic characteristics of the coal-bearing middle Oligocene–late Miocene siliciclastic Amagá Formation, northwestern Colombia, reflect major fluctuations in the stratigraphic base level within the Amagá Basin, which paralleled three major stages of evolution of the middle Cenozoic Andean Orogeny. These stages, which are also traceable by the changes in the compositional modes of sandstones, controlled the occurrence of important coal deposits. The initial stage of evolution of the Amagá Basin was related to the initial uplift of the Central Cordillera of Colombia around 25 Ma, which promoted moderate subsidence rates and high rates of sediment supply into the basin. This allowed the development of aggradational braided rivers and widespread channel amalgamation resulting in poor preservation of both, low energy facies and geomorphic elements. The presence of poorly preserved Alfisols within the scarce flood plains and the absence of swamp deposits suggest arid climate during this stage. The compositional modes of sandstones suggest sediment supply from uplifted basement-cored blocks. The second stage of evolution was related to the late Oligocene eastward migration of the Pre-Andean tholeitic magmatic arc from the Western Cordillera towards the Cauca depression. This generated extensional movements along the Amagá Basin, enhancing the subsidence and increasing the accommodation space along the basin. As a result of the enhanced subsidence rates, meandering rivers developed, allowing the formation of extensive swamps deposits (currently coal beds). The excellent preservation of Entisols and Alfisols within the flood plain deposits suggests rapid channels migration and a humid climate during deposition. Moderate to highly mature channel sandstones support this contention, and point out the Central Cordillera of Colombia as the main source of sediment. Enhanced subsidence during this stage also prevented channels amalgamation and promoted both, high preservation of geomorphic elements and high diversity of sedimentary facies. This resulted in the most symmetric stratigraphic cycles of the entire Amagá Formation. The final stage of evolution of the Amagá Basin was related to the early stage of development of the late Miocene northwestern Andes tholeitic volcanism (from ∼10 to ∼8 Ma). The extensive thrusting and folding associated to this volcanism reduced the subsidence rates along the basin and thus the accommodation space. This permitted the development of highly aggradational braided rivers and promoted channels amalgamation. Little preservation of low energy facies, poor preservation of the geomorphic elements and a complete obliteration of important swamp deposits (coal beds) within the basin are reflected by the most asymmetric stratigraphic cycles of the whole formation. The presence of greenish/reddish flood plain deposits and Alfisols suggests a dry climate during this depositional stage. The presence of channel sandstones with high contents of volcanic rock fragments supports a dry climate, and suggests an incipient phase of the Combia tholeiitic magmatism present during deposition of the Amagá Formation. The subsequent eastward migration of the NW Andes magmatic arc (after ∼8 Ma) may have produced basin inversion and suppressed deposition along the Amagá Basin.

On the duration of the Paleocene‐Eocene thermal maximum (PETM)

The Paleocene‐Eocene thermal maximum (PETM) is one of the best known examples of a transient climate perturbation, associated with a brief, but intense, interval of global warming and a massive perturbation of the global carbon cycle from injection of isotopically light carbon into the ocean‐atmosphere system. One key to quantifying the mass of carbon released, identifying the source(s), and understanding the ultimate fate of this carbon is to develop high‐resolution age models. Two independent strategies have been employed, cycle stratigraphy and analysis of extraterrestrial helium (HeET), both of which were first tested on Ocean Drilling Program (ODP) Site 690. These two methods are in agreement for the onset of the PETM and initial recovery, or the clay layer (“main body”), but seem to differ in the final recovery phase of the event above the clay layer, where the carbonate contents rise and carbon isotope values return toward background values. Here we present a state‐of‐the‐art age model for the PETM derived from a new orbital chronology developed with cycle stratigraphic records from sites drilled during ODP Leg 208 (Walvis Ridge, Southeastern Atlantic) integrated with published records from Site 690 (Weddell Sea, Southern Ocean, ODP Leg 113). During Leg 208, five Paleocene‐Eocene (P‐E) boundary sections (Sites 1262 to 1267) were recovered in multiple holes over a depth transect of more than 2200 m at the Walvis Ridge, yielding the first stratigraphically complete P‐E deep‐sea sequence with moderate to relatively high sedimentation rates (1 to 3 cm/ka, where “a” is years). A detailed chronology was developed with nondestructive X‐ray fluorescence (XRF) core scanning records on the scale of precession cycles, with a total duration of the PETM now estimated to be ∼170 ka. The revised cycle stratigraphic record confirms original estimates for the duration of the onset and initial recovery but suggests a new duration for the final recovery that is intermediate to the previous estimates by cycle stratigraphy and HeET.

Application of correspondence analysis to palaeobathymetric reconstruction of Cenomanian and Turonian (Cretaceous) rocks of Eastern Algeria

The vertical succession of facies is used to characterize the hierarchy of stratigraphic cycles, depositional sequences, and systems tracts in Upper Cretaceous rocks of the Aurès Mountains (Eastern Algeria). Correspondence analysis was used to identify the main palaeontological associations and the systems tracts linked to them, resulting in a tentative reconstruction of the changing palaeobathymetry through the succession. The results were corroborated by comparison with a eustatic curve and subsidence data. Comparisons of bathymetric and eustatic curves can help calibrate and scale regional bathymetric curves and verify if the water depths derived by other palaeoecological methods agree with those determined from correspondence analysis.

Multi-carbonate component reconstruction of mid-carboniferous (Chesterian) seawater δ13C

This study presents the first stratigraphically continuous δ 13C carb record for the Chesterian interval developed from Antler foreland basin platform successions. The bulk carbonate-derived δ 13C curve was constructed using ∼ 700 biostratigraphically constrained samples providing a 10 5-yr resolution. A multi-component study of δ 13C and δ 18O variability in petrographically and geochemically screened abiotic and biogenic calcite components delineates the degree of preservation and δ 13C heterogeneity in these ancient epicontinental carbonates. Furthermore, it constrains the influence of kinetic and vital effects, and local-to-regional-scale C cycling on their δ 13C carb values. The δ 13C and δ 18O values of micrites and bulk carbonates overlap regardless of lithofacies and show minimal intrasample variability. Overall, biogenic calcite δ 13C and δ 18O values are higher than the average isotope compositions of bulk carbonates and micrites, although δ 13C values in contemporaneous samples are only minimally (0.3‰; 2 std err of ± 0.3‰) different relative to much larger differences in δ 18O values. Although brachiopod δ 13C and δ 18O values are higher than average bulk carbonate and micrite values, their δ 13C values exhibit considerable isotopic variability by taxa and ultrastructure. Notably, the δ 13C values of brachiopod secondary shell layers overlap with bulk carbonate, micrite and marine cement δ 13C values. Conversely, δ 13C values of brachiopod tertiary prismatic layers, including Composita, are on average 2 to 5‰ higher than the δ 13C values of contemporaneous bulk carbonates, micrites and brachiopod secondary shell layers. An observed average ∼ 2‰ decrease in the δ 13C values of secondary cross-bladed laminar layers in productids relative to contemporaneous bulk carbonate, micrite and marine cement values suggests a previously unrecognized kinetic and/or vital effect on Carboniferous productid δ 13C values given that they have the highest δ 18O values of all analyzed components. The observed higher δ 13C values in these tertiary prismatic layers are comparable to greater than the 13C-enrichment (∼ 1‰) previously documented in Pennsylvanian Composita. The overlap in δ 13C values of bulk carbonates with diagenetically screened, microdrilled matrix, marine cements and brachiopod secondary shell layers indicates that the Chesterian bulk carbonate-derived δ 13C values may reliably record, to within 1‰, the evolution of ambient seawater δ 13C in the outer carbonate ramp of the Antler basin. Comparison of the Chesterian δ 13C record to the bulk-carbonate C isotope curve derived from the Arrow Canyon, NV carbonate succession, further suggests that these bulk carbonate-derived δ 13C records are reliable proxies of the composition and trends in seawater δ 13C regionally along the equatorial eastern Panthalassan margin. The newly resolved Chesterian stratigraphy documents multiple δ 13C excursions, including previously unrecognized significant C isotope shifts. A shift to higher frequency and amplitude shorter-term δ 13C fluctuations in the upper third of the Chesterian Stage occurred simultaneously with a change in stratigraphic cyclicity from dominantly 3rd-order to 4th-order carbonate cycles suggesting a mechanistic linkage between ambient seawater δ 13C and sea-level fluctuations in the Antler basin. This change in the character of the shorter-term δ 13C fluctuations coupled with a subsequent significant (∼ 4‰) positive C isotope excursion across the mid-Carboniferous boundary, previously recognized in all Carboniferous curves, is interpreted to record the onset of late Paleozoic glaciation and glacio-eustasy.

Sequence stratigraphy of the Precambrian

The method of sequence stratigraphy requires the application of the same workflow and principles irrespective of the age of strata under analysis. In that respect, its application to Precambrian successions is similar to the approach used for Phanerozoic case studies. Differences, however, are recorded in terms of the preservation potential and the amount of data available for analysis; the rates and intensities of the allogenic controls on sedimentation; the environmental conditions and related physical processes; and the evolution of competing groups of organisms and associated biogenic processes. The combined effect of these contrasting aspects accounts for differences in the architecture of depositional sequences, particularly with respect to the relative contributions of various systems tracts to the makeup of a sequence. The application of sequence stratigraphy to Precambrian basins has considerably enlarged the perspective on the fundamental principles governing the processes of sedimentary basin formation and the mechanisms controlling stratigraphic cyclicity in the rock record. These first-order principles are perhaps the most important contribution of Precambrian research to sequence stratigraphy. At the broader scale of Earth's geological history, the tectonic regimes governing the formation and evolution of sedimentary basins are shown to have been much more erratic in terms of nature and rates than originally inferred solely from the study of the Phanerozoic record. This provides important clues with respect to the criteria that should be involved in the hierarchy system of classification of stratigraphic sequences and bounding surfaces.

Sedimentology and high-frequency sequence stratigraphy of a forearc extensional basin: The Miocene Caleta Herradura Formation, Mejillones Peninsula, northern Chile

Facies and sequence stratigraphic interpretation for the 380-m-thick Caleta Herradura Formation (Miocene) are presented, based on detailed and comprehensive outcrop data from the Caleta Herradura half-graben, Mejillones Peninsula, northern Chile. The Caleta Herradura Formation contains an array of lithofacies comprising sandstones, sandy mudstones, diatomites and breccio-conglomerates that are interpreted as the products of inner-shelf to non-marine depositional settings. Complete exposure allows for recognition of a number of distinct and laterally persistent key stratal surfaces that permitted the identification of twenty-five high-frequency (meter to tens of meters-scale) sequences. Based on their internal organization, two main motifs of such unconformity-bounded depositional packages can be distinguished: deepening and deepening-to-shallowing upwards. Transgressive and highstand systems tracts were resolved based on such facies stacking patterns. The origin of sequences is difficult to decipher and remains somewhat uncertain. However, several lines of direct and indirect evidence constrain interpretations for both the sequence architecture and bounding unconformities, and help to discriminate between the various mechanisms that may have driven their development. In essence, these demonstrate that neither tectonically driven sea-level oscillations nor climatically induced changes in sediment supply can adequately explain the distinctive features of unconformities and the facies architecture of sediments they delimit. Instead, these high-frequency changes in relative sea level are best explained as a consequence of glacio-eustatic oscillations. The contemporaneous deep-ocean proxy records appear to support a causal link between the observed stratigraphic cyclicity and glacio-eustatic changes in sea level due to modulation of short-term Milankovitch-scale events by longer-period astronomical variations. However, the type of astronomical forcing remains elusive, hindered by lack of adequate age controls and a poor understanding of pacing mechanisms of high-frequency climate changes during the middle to late Miocene.

CARBONATE DEPOSITIONAL ENVIRONMENTS, SEQUENCE STRATIGRAPHY AND EXCEPTIONAL SKELETAL PRESERVATION IN THE MUCH WENLOCK LIMESTONE FORMATION (SILURIAN) OF DUDLEY, ENGLAND

Abstract: The Much Wenlock Limestone Formation of the Dudley inliers, West Midlands, contains one of the world's richest and most exquisitely preserved Silurian marine biotas. However, for most museum specimens, little is known of their exact provenance and mode of preservation. Detailed comparisons between outcrops and museum collections allow the identification of five faunal‐lithological associations and numerous horizons of exceptional skeletal preservation. The associations are interpreted as a series of transient carbonate mid‐platform environments extending from below storm wave‐base to above fair‐weather wave‐base. Erosive surfaces, condensed sections, flooding surfaces and the stacking patterns of genetically related bed‐sets (parasequences) have allowed the formation to be interpreted as a single third‐order sequence stratigraphic cycle of sea‐level change. The articulated preservation of taxa such as pelmatozoan echinoderms and trilobites can be attributed to either rapid burial by obrution deposits close to fair‐weather wave‐base or smothering by storm sequestered muds in slightly deeper‐water settings. Such intervals of exceptional preservation are commonly associated with flooding surfaces, presumably reflecting reduced likelihood of reworking once rapid burial had taken place.