Journal articles on the topic 'Sequence stratigraphy. Geology, Stratigraphic Geology, Stratigraphic Geology Limestone Cyclostratigraphy'

In the northern San Juan Basin, the Niobrara Formation is represented by the upper half of the Mancos Shale (the Smoky Hill Member and Cortez Member). This section is generally equivalent to the Niobrara Formation along the Colorado Front Range. Although the Fort Hays Limestone is absent west of Pagosa Springs, the C Chalk and B Chalk are well-expressed as two resistant bench-forming calcareous units in the northern San Juan Basin. These two calcareous units have also been established as prospective hydrocarbon targets by operators in the area. Calcareous facies equivalent to the A Chalk were not deposited in the northern San Juan Basin due to siliciclastic dilution during the regressive latter half of the Niobrara marine cycle. The overall third-order Niobrara marine cycle includes these members of the Mancos Shale: the Juana Lopez, Montezuma Valley, Smoky Hill, and Cortez members. The Smoky Hill Member sits just above the basal Niobrara unconformity in most of the study area, and the entire section also has greater thickness and siliciclastic content than its equivalent farther east along the Front Range. Several extensive outcrop locations (in and around Pagosa Springs, Piedra, and Durango, CO) along with three new cores along the CO-NM border form the foundation for sequence stratigraphic interpretation of the Niobrara marine cycle in this study. All these locations and cores were tied back to the Mancos reference section at Mesa Verde National Park established by Leckie et al. (1997) which provides detailed description and biostratigraphy for the entire Mancos Shale. Establishing and applying a sequence stratigraphic framework to any section creates consistent reference standards for communication, research, and further correlation. Comparisons of chemostratigraphic data from equivalent strata between the northern San Juan Basin and Denver-Julesburg (DJ) Basin reveal significant differences in the timing and style of source-rock deposition (and associated low-oxygen conditions). The sequence stratigraphic framework also emphasizes tremendous lateral facies changes in the basal Niobrara section (i.e., Fort Hays Limestone to Tocito Sandstone). Once refined and applied, this stratigraphic framework can be used for predicting the distribution of reservoir properties, in addition to enhancing understanding of the Niobrara marine cycle and the Western Interior Seaway.

The Brodno section — a potential regional stratotype of the Jurassic/Cretaceous boundary (Western Carpathians) Compared to coeval successions from the Carpathians, the continuous Jurassic-Cretaceous (J/K) pelagic limestone succession of the Brodno section offers the best possibility to document the J/K passage in a wide area. This section comprises a complete calpionellid, and nannofossil stratigraphic record, that supports the older paleomagnetic data. Moreover, the sequence stratigraphy and stable isotope (δ18O, δ13C) data gave important results, too, enabling comparison with known key sections from the Mediterranean Tethys area.

Abstract This article is a first attempt of combining sedimentological analysis and geochemical systematics of the Alveolina Limestone Formation as a tool to identify the major stratigraphic surfaces, and to improve the sequence stratigraphy interpretation. This formation is Early Eocene in age and crops out in several well-exposed cliffs in the Serraduy – Roda de Isabena area (Graus-Tremp basin, NE Spain). Within this succession, nineteen carbonate and siliciclastic facies have been identified and grouped in environmental facies associations (based on their vertical stacking and lateral relationships): 1) coastal plain; 2) clastic deltaic complex; 3) shallow carbonate inner-ramp; 4) mid-ramp; 5) outer-ramp; 6) reefal facies. The depositional architectures studied in the Serraduy area can be directly assessed on the field, and a 3D reconstruction is proposed. This enables us to build a synthetic depositional model and to identify five small-scale T/R cycles, bounded by different kinds of sedimentary discontinuities : angular unconformity, firmground, erosional surface… In parallel, geochemical analyses (C and O isotopes, major, minor and trace elements) were carried out to help at hierarchizing the cycles and the boundaries previously identified. Four of them may be considered as major stratigraphic surfaces, corresponding either to regional-scale angular unconformities, or to exposure surfaces. The latter are characterized by a selective dissolution, a slight but sharp decrease in δ13CV-PDB and in Mg, Fe and Sr contents below the surface. The absence of typical sedimentary criteria of exposure (with the exception of these geochemical signatures) may be explained by short-term exposure, an arid to semi-arid climate, and a dominant low-magnesian calcite original mineralogy, precluding the development and the preservation of widespread vadose diagenetic products. A new sequence stratigraphy model for the Alveolina Limestone Fm is finally proposed and discussed.

Abstract In the Big Snowy Mountains of central Montana, USA, late Visean to Bashkirian strata preserve a nearly complete, but poorly documented, paleotropical stratigraphic succession that straddles the range of current estimates of the onset of the Late Paleozoic Ice Age (LPIA). Sedimentologic and stratigraphic investigation of the Otter (late Visean to Serpukhovian) and Heath (Serpukhovian) formations, with secondary focus on the overlying Tyler (late Serpukhovian to Bashkirian) and Alaska Bench (Bashkirian) formations, facilitated an appraisal of paleotropical environmental change preserved in this succession. Three facies associations reminiscent of environments currently forming in Shark Bay, Australia, were identified in the Otter Formation: shallow semi-restricted littoral platform, intertidal platform, and supratidal plain. Five facies associations broadly comparable to modern environments present in the Sunda Shelf and southern coast of the Persian Gulf were identified in the Heath Formation: offshore outer ramp, mid- to outer ramp, inner ramp, coastal plain, and sabkha. Facies associations preserved in the Heath Formation are here explained in the context of a protected, homoclinal carbonate ramp situated in a partially silled epicontinental embayment. A shift from low-magnitude relative sea-level oscillations preserved in the Otter Formation to a cyclothemic stratigraphic pattern entailing ≥ 6 fourth-order, high-frequency and high-magnitude relative sea-level fluctuations in the Heath Formation is here interpreted to record the main eustatic signal of the LPIA in central Montana. Current published biostratigraphic constraints for the observed stratigraphy estimate the main eustatic signal of the LPIA to have occurred approximately between 331 (base Serpukhovian) and 327 Ma in central Montana. A distinct upward transition from coal and paleosol-bearing depositional sequences in the lower Heath to evaporite and limestone-bearing depositional sequences in the upper Heath preserves a broad humid to arid paleoclimate shift during deposition of this unit, which influenced hydrographic circulation patterns and the resultant distribution of anoxic environments in the Big Snowy Trough during this time interval. Improved depositional and sequence stratigraphic models of the Heath Formation proposed in this study permit new insight into the theoretical distribution of, and water depth necessary to preserve, black, organic-rich claystone and shale in partially silled intracratonic basins, in addition to new temporal constraints on LPIA onset in paleotropical western Laurentia.

The first two calcarenite units at the base of the Urgonian limestones on the southern edge of the platform bear different depositional geometries depending on place (Cirque d’Archiane to Montagnette and Rocher de Combau). The lower calcarenite unit (Bi5 of Arnaud H. 1981. De la plate-forme urgonienne au bassin vocontien. Le Barrémo-Bédoulien des Alpes occidentales entre Isère et Buëch (Vercors méridional, Diois oriental et Dévoluy). Géologie Alpine, Grenoble, Mémoire 12: 3. Disponible sur https://tel.archives-ouvertes.fr/tel-00662966/document), is up to 200 m thick and shows three different patterns, in terms of accommodation space, from the western Archiane Cirque to the Montagnette to the east. On the western side of the Cirque, the unit begins on slope fine-grained limestone with thin sigmoïdal offlap geometry, suggesting little available space after a relative sea level fall. It is overlain by thick progradational/aggradational, then purely aggradational calcarenite capped by a coral and rudist-bearing bed. This bed is, therefore, interpreted as a maximum (although moderate) flooding facies. The depositional geometry is different on the eastern side of the Cirque, where a progradational pattern in the lower part of the unit is interrupted by a rotational movement affecting the depositional profile. The deformation promoted aggradation updip and retrogradation downdip as a result of starvation. The inferred growth fault updip (thought to be responsible for the change) began to function earlier at the Montagnette, explaining the huge calcarenite clinoforms found there, filling a deeper saddle created in the depositional profile. The same fault probably was reactivated later during the deposition of the overlying, thinner Bi6-1 unit, which appears at Rocher de Combau with an uncommon tidal facies at the base. A rotational bulge, created by the inferred growth fault, would have protected a small area behind it to spare the local calcarenite deposition from the waves for a while. These two examples show that sequence stratigraphic interpretation may differ from one place to the other, and even show opposite trends due to this kind of disturbance.

AbstractA carbonate carbon isotope curve from the Aalenian–Bathonian interval is presented from the Óbánya valley, of the Mecsek Mountains, Hungary. This interval is certainly less well constrained and studied than other Jurassic time slices. The Óbánya valley lies in the eastern part of the Mecsek Mountains, between Óbánya and Kisújbánya and provides exposures of an Aalenian to Lower Cretaceous sequence. It is not strongly affected by tectonics, as compared to other sections of eastern Mecsek of the same age. In parts, a rich fossil assemblage has been collected, with Bathonian ammonites being especially valuable at this locality. The pelagic Middle Jurassic is represented by the Komló Calcareous Marl Formation and thin-bedded limestones of the Óbánya Limestone Formation. These are overlain by Upper Jurassic siliceous limestones and radiolarites of the Fonyászó Limestone Formation. Our new data indicate a series of carbon isotope anomalies within the late Aalenian and early-middle Bajocian. In particular, analysis of the Komló Calcareous Marl Formation reveals a negative carbon isotope excursion followed by positive values that occurs near the base of the section (across the Aalenian–Bajocian boundary). The origin of this carbon-isotope anomaly is interpreted to lie in significant changes to carbon fluxes potentially stemming from reduced run off, lowering the fertility of surface waters which in turn leads to lessened primary production and a negative δ13C shift. These data are comparable with carbonate carbon isotope records from other Tethyan margin sediments. Our integrated biostratigraphy and carbon isotope stratigraphy enable us to improve stratigraphic correlation and age determination of the examined strata. Therefore, this study of the Komló Calcareous Marl Formation confirms that the existing carbon isotope curves serve as a global standard for Aalenian–Bathonian δ13C variation.

The Pleistocene deposits of Denmark are largely composed of two major facies that interfinger with each other: 1) tills, and 2) waterlaid outwash material. The two facies are occasionally interbedded with inter­glacial or interstadial deposits. By applying lithostratigraphy combined with structural analysis in open exposures, a glacial stratigraphy of the Pleistocene has been established, and the glacial history interpreted. Only till units are classified lithostratifically because of their high regional consistency compared to waterlaid outwash deposits. Cor­relation of till units is based on three basic properties: lithology, stratigraphic position and associated gla­ciotectonic boundaries. Lithic characteristics, some of which have local value whilst others indicate regional properties, include data on local glacier flow directions from fabric analyses and the contents of provenance dependent com­positional features, the latter may provide information on the long range path of glaciers. Fine gravel ana­lyses and stone counts demonstrate, that distinct stratigraphical relationships exist between tills rich and poor in quartz as well as between tills of Fennoscandian and Baltic provenance primarily indicated by the quantities of Paleozoic limestone clasts. The contests of re-deposited Quaternary foraminifera primarily serve as lithic characteristica and secondly as a guide line to estimate the possible age of a till unit. Bounding relations between till units are studied with special emphasis on glaciotectonic unconfor­maties. They not only serve as supreme marker horizons on a regional scale, but they also supply high rank information on the local direction of glacier movement. Analysis of glaciotectonic structures often constitutes the implement by which the stratigraphy of a dislocated glacial sequence can be recognized. Using a combined set of glacial-stratigraphic methods, about 200 selected localities in the central part of Denmark have been investigated over the past ten years. Studies at eight principal localities and two key localities, some of which are classical exposures, provide the foundation for a litho-stratigraphic model for the till units. The Elsterian glaciaiton is represented by three till units (S0NDER VISSING TILL, PALSGARD TILL, SNOGH0J TILL) of Norwegian, Middle Swedish and Baltic provenance respectively. The Saalian comprise three till units (TRELDE NIES TILL, ASHOVED TILL, LILLEBJELT TILL), the two former are of Fennoscandian provenance and the latter of Baltic provenance; deposited in the above mentioned order by icesheets from the north, the north east and from the east respectively. The Weichselian is build up of six till units, the oldest of which (RISTINGE KLINT TILL) was most probably deposited later than 36800 BP. This Baltic till is followed by three tills of Fennoscandian provenance (KATTEGAT TIL:i.,, MID DANISH TILL, NORTH SJ,ELLAND TILL) deposited respectively from the north, the northeast, and the east. The two youngest Weichselian till units are of Baltic provenance (EAST JYLLAND TILL, BJELTHAV TILL) and they were deposited from south-easterly directions before 13500 BP. Till beds are erected into formal lithostratigraphic units of Formation rank and they are extended to cover extraregional till units, correlation is based on stratigraphic position, lithology and boundaries. The following glacial history can be outlined. During the Elsterian the S0nder Vissing till, the Palsgard till and the Snoghl'lj till were deposited by three glacial advances probably in the chronological order as mentioned above. During the Holsteinian in­terglacial, marl and diatomite were deposited in lake basins in the central part of the region. The Saalian glaciation was initiated with the deposition of outwash material by southerly to southwest­erly directed meltwater streams and succeeded by an ice-stream from southern Norway, which deposited the Trelde Nres Till. After deposition of this till, interstadial conditions prevailed, and outwash material was deposited by westward flowing meltwater streams. This interstadial phase was followed by the second Saalian glacial advance during which the Ashoved Till was laid down by an ice-sheet from Middle Sweden. Prior to the last Saalian glacial phase, outwash material was deposited by generally westward flowing melt­water streams, that probably emerged from the Palaeobaltic ice. This latter deposited the Lillebrelt Till and invaded the country from the western part of the Baltic. During the Eemian, lake sediments were formed in kettle bogs on the surface of the Lillebrelt Till, while at the same time, marine sediments were deposits in the southern and northern part of the examined re­gion. Tundra vegetation developed in a dry polar climate characterize the Early- and the larger part of the Middle Weichselian. Interstadial deposits with an age of about 36800 BP were formed on Sejerl'l - prob­ably prior to the Old Baltic ice advance, which in the southern and eastern part of the region deposited he Ristinge Klint Till from the Baltic. The Old Baltic advance was succeeded by the Norwegian advance which came from southern Norway crossing the Kattegat depression and deposited the Kattegat TIU in the northern part of the region. After deposition of the previously mentioned two till units, interstadial condi• tions re-occurred and meltwater streams transported outwash material northward into the Kattegat basin. A change in meltwater palaeocurrent direction towards the west indicates the approach of the Main Weichselian advance. This advance crossed Middle Sweden and deposited the Mid Danish Till, and it probably reached its maximum extension along the Main Stationary Line at around 20000 BP. A read• vance during the general retreat of the Main Weichselian icestream crossed Fyn from the NE, and outwash streams were generally directed towards the north until a younger re-advance from easterly directions de­posited the North Sjrelland Till in the eastern part of Denmark. This re-advance may have occupied the northeastern part of Denmark and Kattegat before giving way to the transgression of the Younger Yoldia Sea, which was initiated about 15000 BP in Vendsyssel. Whilst this transgression was progressing the Main Weichselian ice sheet retreated to a probable position along the Swedish west coast. Northward flowing meltwater streams prevailed, at this time, in the central Danish region, until the Young Baltic icesheet ad• vanced from the Baltic depression and deposited the East Jylland Till. This advance formed the East Jyl• land ice border line, probably around 14000 BP. During the East Jylland advance, northward directed drainage patterns prevailed in the northern part of the region, whereas outwash material was deposited by westward flowing streams in southern Jylland. The East Jylland ice sheet retreated to a position south-east of the examined region before the Brelthav re-advance deposited the Brelthav Till. Still supplied from the Baltic this readvance reached the classical Odsherred ice marginal zone of Northwest Sjrelland and gave rise to the final molding of the ice marginal hills of North Sams0 and the archshaped shoals in Storebrelt. During the Weichselian, the Saalian, and probably also the Elsterian, the direction of the majority of ice sheet advance that invaded Denmark changed in a "clockwise" manner. During each glacial, an initial ad­vance from the north was succeeded by one from northeast ending up with one from the east and south· east. The change in direction of advance is accompanied by a corresponding change in the provena􀁁ce de­pendent elements of the till units. Hence, the "clockwise" pattern established for the Danish region can reflect a fundamental trend in the dynamic evolution of consecutive Scandinavian ice sheets. The present study therefore, may provide important guide lines for future attempts to establish more detailed the• oretical glaciological models for the dynamics of these former Scandinavian ice sheets.

The Ordovician System is extensively represented in the Precordillera of San Juan Province, Argentina. At the Cerro La Chilca in the Jáchal area, the limestone of the San Juan Formation is paraconformably overlain by interbedded limestone and shale of the Gualcamayo Formation. The present contribution reports new data on the conodont fauna and biostratigraphy of these darriwilian units, revising local and regional chronostratigraphic relationships. New information on the composition of conodont and graptolite associations through the stratigraphic sequence is presented. The presence of Paroistodus horridus horridus, Yangtzeplacognathus crassus, and Histiodella sinuosa constrain the uppermost strata of the San Juan Formation to the lower part of the Y. crassus Zone, according to the Baltoscandian scheme, and to the H. sinuosa Subzone of the Periodon macrodentatus Zone of the North American scheme. In the overlying Gualcamayo Formation the co-occurrence of Y. crassus with Histiodella holodentata enable the recognition of the Y. crassus Zone and the H. holodentata Subzone of the P. macrodentatus Zone. The identification of these zones allows for precise global and regional correlation. A graptolite assemblage that belongs to the epipelagic and deep-water biotopes with some components restricted to low paleolatitudes is recognized. This diverse assemblage is characteristic of the pelagic biofacies. The important diversity of graptolites in this section suggests a favorable environment for their development. Local changes in the taxonomic composition are recognized through the Gualcamayo Formation. When comparing this fauna with that of different study localities from the Central Precordillera (Cerro Potrerillo, Oculta Creek, Cerro Viejo de Huaco and Las Aguaditas Creek) slight differences in the generic composition are observed. Taxonomic differences support the preference of certain associations for particular environments; though, graptolites are more diverse in black shales facies, which represent deeper environments (the Los Azules Formation), in relation to the calcareous-shale facies of the Gualcamayo Formation from Cerro La Chilca and correlative unit at Las Aguaditas Creek.

Summary The Wafra field is located in the partitioned neutral zone between Kuwait and Saudi Arabia. The field produces oil from the Ratawi oolite reservoir, which has been under production since 1956. Barriers to fluid movement have severely restricted aquifer support to the overlying carbonate grainstone reservoir, leading to production-induced pressure depletion and low recovery rates. Creative integration of three-dimensional seismic aspects, well log stratigraphy, and engineering analysis revealed an unexploited reservoir extension that is more open to aquifer pressure support. Wells drilled along this extension are expected to yield higher initial production rates and longer sustained production. The model was used to drill two successful step-out wells that have increased field production by over 12,000 BOPD. Eight of ten additional wells have now been drilled as a follow-up to this success. This paper reviews the case history with a focus on the multidisciplinary integration that led to opportunity identification and exploitation. Introduction The Wafra field, jointly operated by Saudi Arabian Texaco and the Kuwait Oil Company, is located in the partitioned neutral zone (PNZ) between Kuwait and Saudi Arabia (Fig. 1). The field has produced from the Lower Cretaceous Ratawi reservoir since 1956. Liquid withdrawal over the years has depleted reservoir pressures in some parts of the field, leading to a decline in production. Based on prior reservoir characterization and simulation studies in 1995,1 a peripheral water injection and an extension development program have been undertaken in order to arrest the decline and increase the production by over 40,000 BOPD.2 In 1996, a 104 sq mile three-dimensional (3D) seismic survey was acquired to help design and implement these programs. Structural and Stratigraphic Framework The Wafra field is a large anticline approximately 6 by 10 miles in dimension (Fig. 2). The field is composed of a main NW-SE trending structural feature called the Main area, and a lower amplitude extension area east of the Main area called East Wafra. The Ratawi oolite reservoir is found at a depth of 6,135 ft subsea at the structural crest and had an original oil-water contact at about 6,520 ft subsea. Most of the structuring occurred in Middle Cretaceous time as sediment draped over deep-seated horst blocks. Oil migration and accumulation are thought to have occurred primarily in Early Eocene time. The Ratawi formation consists of a marine transgressive sequence of carbonate rocks deposited during Early Cretaceous time. The formation is composed of three distinct intervals: the lower-most Ratawi oolite reservoir, and the overlying Ratawi Limestone and Ratawi Shale cap rocks (Fig. 3). The Ratawi oolite reservoir was formed by a prograding carbonate sand shoaling sequence deposited on a low-angle carbonate ramp or detached platform. The commercially productive reservoir interval is composed primarily of porous grainstones and packstones. Less porous packstone, mudstone, and wackestone facies resulted from a more-restricted lagoonal environment in the central part of the field, and deeper marine shelf facies on the platform boundaries. Stratigraphic analysis of well log data provides an understanding of the depositional framework and serves as a basis for modeling facies distribution within the reservoir. Fig. 4a is a well log cross section traversing the Main area and East Wafra along the path A-B in Fig. 2. The gamma ray (GR) log curves are flattened on the base of the Ratawi limestone (cap rock) and span the interval of the Ratawi oolite reservoir. The GR curves indicate a remarkable character similarity from well to well that is almost exclusively related to the presence of uranium minerals.1 This determination is supported by x-ray analysis of core data that found an absence of clay. Additional evidence is found in comparisons of the GR (uranium, potassium and thorium) with the computed GR (potassium and thorium) from spectrometry gamma ray logs. The computed GR data show a largely diminished log character, implying that the GR log character is largely a function of uranium content. Hence, the correlative nature of the GR curves indicates that the uranium was present at the time of deposition—probably due to regional-scale climatic or environmental influences such as atmospheric fall-out from volcanic activity. This explains the consistent levels of uranium, independent of lithology and porosity, and allows detailed chronostratigraphic correlations to be made. Fig. 4b is an east-west stratigraphic cross section through the reservoir along the same path as in Fig. 4a, showing porosity logs with GR depositional time lines superimposed. The thick solid lines mark lithostratigraphic boundaries between an interval consisting primarily of porous grainstone, which for purposes of this paper will be referred to as the "upper reservoir," a tight interval of predominantly mudstone and packstone, referred to as the "Basal barrier," and a porous grainstone interval called the "lower reservoir." Almost all of the Ratawi oil production is from the upper reservoir grainstones. The chronostratigraphic facies heterogeneity evident in Fig. 4b owes its origin to a transgressive sequence of prograding grain shoals deposited in relatively shallow water.3 During the early stages of transgression, as the shoals prograded over the Wafra paleo-high, muds and finer grain carbonates were deposited in intershoal lagoons. As the sea level rose, carbonate sediment productivity and accumulation surpassed the rise in sea level, resulting in an overall shallowing with time. Evidence of this can be seen in the general coarsening upward character of the porosity logs. With progressively shallower water depths and associated higher depositional energy, the grain shoals became spatially more extensive while the lagoonal areas retreated, ending in fairly expansive grain shoals in the later stages of reservoir development. At the end of Ratawi oolite time, a rapid increase in relative sea level drowned the shoaling sequence, and deposited the deeper marine Ratawi limestone and shale members.

The Permian reef complexes of West Texas and New Mexico are among the classic carbonate sequences in the world and have significantly influenced geologic thinking for over half a century. Study of the reefs can be subdivided into 6 broad periods. The first period involved early exploration of the region, establishment of regional stratigraphic relationships and attempts at dating stratigraphic units. The Guadalupian Fauna typifies this early period. The second period, during the 1920-30's, was a time of early petroleum exploration in the region, following on discovery of the Kendrick Field in Winkler County, Texas, and resulted in attempts to explain the complicated subsurface stratigraphy. Development of a marginal reef model and research on facies relationships between the basin and shelf resulted in refinement of stratigraphic nomenclature.The third period, here termed the King period, was a time of more intense study of the outcrops and their subsurface extensions. It was a time when facies became more clearly differentiated and when the great diversity and abundance of fossils in the region became appreciated. This period ended when World War II curtailed research in the region. The fourth period began after the war, with heightened interest in reefs and paleoecology. It was a time when carbonate petrology and paleoecology rose as major fields of interest. It was also a time of mega-paleontology. Tens of tons of fossiliferous limestones were processed at the U.S. National Museum and the American Museum of Natural History and collections of literally millions of fossils were assembled. The earlier publication of Geology of the Southern Guadalupe Mountains, Texas and the later publication of The Permian Reef Complex of the Guadalupe Mountains Region, Texas and New Mexico characterize the period.The fifth period is marked by the return of industry investigators to study the reefs and associated rocks, perhaps spurred as much by Dunham's "Vadose pisolites in the Capitan reef" as by any single paper. The period was one of concern about origins of the distinctive pisolites of the complex, nature of the massive Capitan Limestone, diagenesis of carbonates and by concern for understanding the economically significant rocks of the backreef sequence. The sixth period, termed the Wisconsin phase, continued research along lines of the fifth period but was a time when faculty and students of the University of Wisconsin, and their associates, re-examined all facies of the Guadalupe Mountain reef complexes as a major effort, while industry became less broadly involved. Those efforts, and those now initiated by faculty and students of the University of Nebraska and Rice University, bring us essentially to date, but much still remains to be discovered and understood about the reef complexes.

Two Permian-Triassic boundary (PTB) successions, Lung Cam in Vietnam, and Lukač in Slovenia, have been sampled for high-resolution magnetic susceptibility, stable isotope and elemental chemistry, and biostratigraphic analyses. These successions are located on the eastern (Lung Cam section) and western margins (Lukač section) of the Paleo-Tethys Ocean during PTB time. Lung Cam, lying along the eastern margin of the Paleo-Tethys Ocean provides an excellent proxy for correlation back to the GSSP and out to other Paleo-Tethyan successions. This proxy is tested herein by correlating the Lung Cam section in Vietnam to the Lukač section in Slovenia, which was deposited along the western margin of the Paleo-Tethys Ocean during the PTB interval. It is shown herein that both the Lung Cam and Lukač sections can be correlated and exhibit similar characteristics through the PTB interval. 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Mesozoic sedimentary rocks, of both deep- and shallow-water character, are present throughout Italy and well exposed in mountains and river valleys. Detailed studies of these sections by Italian geologists, beginning in the nineteenth century and continuing to this day, have produced a high-resolution biostratigraphy that allows identification of increments of geological time of less than a million years. Early work relied largely on ammonites to define biostratigraphy but was applicable primarily to sediments of Jurassic age. Study of deep-marine pelagic limestones and shales of Cretaceous age were subsequently, in the twentieth century, investigated using planktonic microfossils, the size of a sand grain, and even smaller nannofossils of micron scale. Pioneering work on magnetostratigraphy and cyclostratigraphy, undertaken primarily on Cretaceous sediments cropping out in Marche-Umbria, added further refinement to the measuring of small intervals of time in rock. With this stratigraphic background, distinct lithological and chemical signals, discovered first in Italian sequences, could be recognized world-wide and proven to be of global significance. In particular, the involvement of carbon isotopes has underscored the utility of chemostratigraphy, not only as a further aid to correlation, but also as a testimonial to major environmental change. Most significant in this context are the Cretaceous oceanic anoxic events, whose sedimentary record was first documented from Italy. These events were characterized by the development of organic-rich black shales deposited in waters largely lacking in oxygen during times of extraordinarily high temperatures known as hyperthermals. Hyperthermals, likely driven by supply of carbon dioxide from large-scale volcanic eruptions, illustrate the environmental impact on a planet affected by extreme global warming.

Abstract Quarries between Old Radnor and Presteigne, Welsh Borderlands, expose a Silurian nearshore succession, which onlaps a rocky palaeotopography of the Neoproterozoic basement that had been uplifted along the Church Stretton Fault Zone. The succession documents the Aeronian to Sheinwoodian transgression of an island or islands, with the following sequence of events: deposition of shallow marine sandstones (Folly Sandstone Formation), regional uplift, preservation of a rocky shoreline and associated deposits (Dolyhir Rudite Member), deposition of limestones characterized by a profusion of coralline algae and the abundant remains of reefs (Dolyhir and Nash Scar Limestone Formation), and finally deposition of trilobitic silty mudstones (basal Coalbrookdale Formation). Facies analysis, carbon isotope (δ13Ccarb) values, sequence stratigraphy, and collections of bryozoans, conodonts, thelodonts, and trilobites have been used here as a means of refining our stratigraphic understanding of this unique succession. The revised stratigraphy demonstrates many similarities with the adjoining Midland Platform and the wider Silurian world. Notable features include the globally recognized early Sheinwoodian carbon isotope excursion and sea-level changes of regional and global extent. As one of the best examples of its kind, the palaeoshoreline and nearshore succession of Old Radnor and Presteigne acts as a depositional model for ancient rocky shores worldwide.