Global Cycle Chart Research Articles

Biostratigraphic Study of the Calcareous Nannofossils of Well 02, Shallow Offshore, Niger Delta, Nigeria

A calcareous nannofossils biostratigraphic study has been carried out on the strata penetrated of well 02 located in the shallow offshore area of the Niger Delta, Nigeria. The study is aimed at establishing the age and nannofossils biozonations of the well. The samples were prepared using smear slide technique. Forty calcareous nannofossils species were identified and used to make biostratigraphic deductions of zonations and dating of the sequence.The distribution of the calcareous nannofossils enabled the establishment of three zones of Helicosphaera ampliaperta (NN4),Sphenolithus heteromorphus (NN5) and Catinaster coalitus (NN8) belonging to early Miocene - late Miocene following standard zonation schemes of previous workers. The zones were based on the first and last occurrences of marker species. Nannofossil abundance / diversity patterns calibrated with chronostratigraphical bioevents reveals four condensed sections when correlated to the Global Cycle Chart. This is associated with the 16.0 Ma, 14.8 Ma and 13.4 Ma and 10.89 Ma maximum flooding surfaces.

Recommendations for a uniform chronostratigraphic designation system for Phanerozoic depositional sequences

Many published sequence-stratigraphic frameworks lack a systematic and consistent designation for both depositional sequences and key surfaces, despite the original goal to provide a fully integrated stratigraphic architecture, including diagnostic age information. Based on a system in use for more than 10 yr at ExxonMobil, we recommend methodologies for a chronostratigraphic designation system (CDS) using more uniform and robust sequence-stratigraphic designations. After objectively defining important physical stratigraphic surfaces, biostratigraphic and other age-constraining information is used to designate surfaces and the packages of rocks they bound. This leads to the establishment of a sequence chronostratigraphic framework for a local area of investigation (outcrop section, field, region, or basin). Only after demonstrating clear well-documented ties to Phanerozoic global coastal onlaps or cycle charts are these sequences and associated surfaces considered as “global” entities and designated as such. Higher frequency sequences and surfaces are also accommodated in this CDS. Alternative designations for areas with limited or poor quality chronostratigraphic information are also discussed. The CDS has proven to have great use in all Phanerozoic strata, different tectonic settings, and depositional environments, especially when chronostratigraphic age constraints are robust. We have used this system at regional and basinal scales in many geographic locations to help reduce uncertainty in identifying and correlating reservoirs, sources, and seal rocks. Predicting the local distribution and quality of reservoirs as well as seals within a producing field and near field wildcats is also facilitated by this system. This system has demonstrated use in the correlation of outcrop sections within a basin or between basins. Rigorous use of the CDS proposed here will permit meaningful regional and/or interbasinal correlation that is difficult to carry out with the diverse systems currently in use. This uniform designation scheme will also facilitate communication within a company and between institutions, as well as among academic investigators.

Biostratigraphic Study of the Calcareous Nannofossil of well XH-1, Deep Offshore, Niger Delta, Nigeria.

A calcareous nannofossil biostratigraphic study has been carried out on sequences within the well XH-1 located in the deep offshore area of the Niger Delta, Nigeria. Forty-one (41) calcareous nannofossil species identified were used to make biostratigraphic deductions including zonation, dating and a tentative sequence stratigraphic framework for the intervals studied. Two major nannofossil zones (NN11 and NN10 / CN9 and CN8) belonging to the Late Miocene age were identified following the standard zonation schemes. Three major zones of Discoaster quinqueramus zone, Discoaster berggrenii zone, and Discoaster bollii zone, were erected for this well based on the index taxa and fossil assemblage recorded. Discoaster berggrenii zone was further subdivided into two subzones. Based on the lithostratigraphic studies, the sediments reveal hemipelagic shale with traces of sand at the lower half of the well. Nannofossil abundance/diversity patterns calibrated with chronostratigraphically vital bioevents reveals three (3) Condensed Sections as correlated to the Global Cycle Chart. This is associated with the 5.8 Ma, 7.0 Ma and 9.2 Ma Maximum Flooding Surfaces. Journal of Mining and Geology Vol. 43 (2) 2007: pp. 175-186

Regional to global correlation of lower Cretaceous (Hauterivian–Barremian) shallow-water carbonates of the southern Apennines (Italy) and Dinarides (Montenegro), southern Tethyan Margin

We propose a high-resolution regional to global scale correlation of two Hauterivian/Barremian shallow water carbonate platform sections belonging to different palaeogeographic domains of the Southern margin of the Tethys. These successions are now located more than 500 km apart and are represented by 137 m of core drilled at Monte Raggeto (southern Apennines, Italy) and by 80 m of well stratified rocks cropping out in the Podgorica Mountains (Dinarides, Montenegro). They display three superimposed orders of stratigraphic cyclicity defined by elementary cycles grouped into bundles and superbundles and attributed to orbitally induced composite sea-level fluctuations. Elementary cycles are interpreted as the precession and the obliquity periodicities (≅20–40 ky), while bundles and superbundles correspond to the short (≅100 ky) and long (≅400 ky) eccentricity cycles, respectively. As both the ratios between the elementary cycles and the bundles and between the bundles and the superbundles do not always reflect the classical 5:1 and 4:1 relationship, the sedimentary record of both the studied sections is likely to include omission of bundles or their portion. The general evolution of depositional and diagenetic facies, the vertical change in thickness of the cyclic units, and the interpretation of superbundles as small-scale depositional sequences, suggest that elementary cycles and their groups are superimposed on lower-frequency environmental oscillations. These display an upward Transgressive/Regressive Trend (T/RT) of the facies and testify to larger-scale variations of accommodation potential on the shelf. T/RTs, sequence stratigraphic interpretation of the superbundles as well as the common occurrence of the biostratigraphic dated Hauterivian/Barremian boundary in the studied sections, support the long-distance regional correlation between the successions to be carried out on the 100 ky scale. Moreover, by assuming the maximum flooding intervals of time-equivalent superbundles as virtually isochronous intervals, we have detected stratigraphic gaps of 1200 and 600 ky in the Monte Raggeto and in the Podgorica Mountains sections, respectively. Both sections are formed of 10 complete superbundles each of them testifying to the 400-ky-long eccentricity. Hence, the Monte Raggeto and Podgorica Mountains successions represent a duration of at least 4.0 My in terms of elapsed time, regardless of gaps within the stratigraphic record. Finally, a correlation between the orbitally induced compared sections and both the 3rd order Tethyan stratigraphic cycles and the global cycle chart is suggested. Our study confirms that cyclically organized, shallow marine carbonate platform deposits controlled by orbitally induced sea-level fluctuations represent a powerful tool for high-precision regional and global correlations. This can be of fundamental importance for quantification of sedimentary, diagenetic, and evolutionary processes as well as for the understanding of autocyclic versus allocyclic control on sequence development.

Sequence stratigraphy as a scientific enterprise: the evolution and persistence of conflicting paradigms

In the 1970s, seismic stratigraphy represented a new paradigm in geological thought. The development of new techniques for analyzing seismic-reflection data constituted a “crisis,” as conceptualized by T.S. Kuhn, and stimulated a revolution in stratigraphy. We analyze here a specific subset of the new ideas, that pertaining to the concept of global-eustasy and the global cycle chart published by Vail et al. [Vail, P.R., Mitchum, R.M., Jr., Todd, R.G., Widmier, J.M., Thompson, S., III, Sangree, J.B., Bubb, J.N., Hatlelid, W.G., 1977. Seismic stratigraphy and global changes of sea-level. In: Payton, C.E. (Ed.), Seismic Stratigraphy—Applications to Hydrocarbon Exploration, Am. Assoc. Pet. Geol. Mem. 26, pp. 49–212.] The global-eustasy model posed two challenges to the “normal science” of stratigraphy then underway: (1) that sequence stratigraphy, as exemplified by the global cycle chart, constitutes a superior standard of geologic time to that assembled from conventional chronostratigraphic evidence, and (2) that stratigraphic processes are dominated by the effects of eustasy, to the exclusion of other allogenic mechanisms, including tectonism. While many stratigraphers now doubt the universal validity of the model of global-eustasy, what we term the global-eustasy paradigm, a group of sequence researchers led by Vail still adheres to it, and the two conceptual approaches have evolved into two conflicting paradigms. Those who assert that there are multiple processes generating stratigraphic sequences (possibly including eustatic processes) are adherents of what we term the complexity paradigm. Followers of this paradigm argue that tests of the global cycle chart amount to little more than circular reasoning. A new body of work documenting the European sequence record was published in 1998 by de Graciansky et al. These workers largely follow the global-eustasy paradigm. Citation and textual analysis of this work indicates that they have not responded to any of the scientific problems identified by the opposing group. These researchers have developed their own descriptive and interpretive language that is largely self-referential. Through the use of philosophical and sociological assumptions about the nature of human activity, and in particular the work of Thomas Kuhn, we have attempted to illustrate (1) how the preconceptions of geologists shape their observations in nature; (2) how the working environment can contribute to the consensus that develops around a theoretical approach with a concomitant disregard for anomalous data that may arise; (3) how a theoretical argument can be accepted by the geological community in the absence of “proofs” such as documentation and primary data; (4) how the definition of a situation and the use or non-use of geological language “texts” can direct geological interpretive processes in one direction or another; and (5) how citation patterns and clusters of interrelated “invisible colleges” of geologists can extend or thwart the advancement of geological knowledge.

Application of Sequence Stratigraphy to Exploration and Development in Eastern Pearl River Mouth Basin, South China Sea: ABSTRACT

Some appropriate conditions for applying sequence stratigraphy in PRMB exist: (1) The PRMB is a Cenozoic extension basin with weak tectogenesis after rifting; (2) The late Tertiary successive sediments deposited over the area from coast to deep marine basin are well developed and their seismic reflection is good in quality; (3) High resolution quantitative biostratigraphy has provided more precise data of eustasy. Twenty-two third order sequences since 30Ma were identified and are correlative to global cycle chart, but TB2.1 can be further divided into two third-order sequences. The 22 sequences from a [open quote]retrograding stacking pattern[close quote] which resulted in no large constructive delta or delta-related rollover anticlines. However, widely deposited seal rock possibly sealed the incised-valley, basin floor fan (BPF), and transgressive sandstone to form valid stratigraphic traps. Stratigraphic traps are particularly important in future exploration because of poor anticline trap types in the basin. Some ER stratigraphic traps can be predicted after studying the distribution of systems ER tracts in each sequence as a basic exploration unit. The BPF of TB2.1 which is closed to the source kitchen area could form a large subtle trap. Using the parasequence as a basic unit in reservoir scale will provide amore » new method to discover new reservoirs. For example, new oil reserves within the predicted stratigraphic trap related to K22, which can be divided into several parasequences in C.A. 16/08, were obtained after drilling. Sequence stratigraphy has become a valid high resolution tool for chronostratigraphic correlation and fine-division and exploration of stratigraphic traps.« less

Biostratigraphic and magnetostratigraphic intercalibration of latest Cretaceous and Paleocene depositional sequences from the deep-water Basque basin, western Pyrenees, Spain

Latest Cretaceous (Late Maastrichtian) and Paleocene depositional sequences from the deep-water Basque basin have been calibrated with published [1,2] and newly acquired magnetostratigraphic data. Sequences mostly consist of hemipelagic marls and limestones, and their relative ages are well constrained with planktic foraminifera. They accumulated during a phase of tectonic tranquillity and reduced clastic input into the basin, and therefore they can be attributed to eustatic sea-level changes with reasonable confidence. On the basis of the planktic foraminifera zonation, a good match has been observed previously between the depositional sequences of the Basque basin and specific sea-level cycles of the 1988 version of the Exxon Global Cycle Chart (GCC) [3]. Here, a new attempt at correlation using their respective magnetostratigraphic data has failed to confirm such a match. The disagreements observed may indicate that, for the studied interval, (1) the current planktic foraminifera biostratigraphy lacks the necessary level of resolution to ensure synchrony between sequences of different basins, and/or (2) the magnetostratigraphy of the GCC needs to be revised. Whatever the case, the new findings lend support for some criticism on the use of the GCC for interregional correlations.

Some tests of the Exxon global cycle chart

The Exxon global cycle chart is now widely used as a stratigraphic correlation tool, although it remains a controversial one. It requires continual testing against new data, but tests are difficult at present because the assumptions behind the chart have not so far been precisely specified. Some reasonable assumptions are made here, and a few simple tests are then proposed on the basis of these. Some of these tests are appropriate to specific situations in which the lest data are estimated sequence boundary ages: others are for situations in which sets of undated sequence boundaries are available that are only known to lie within particular parts of the Exxon chart. No general test for the Exxon chart has yet been developed, and the specifications for such a test are discussed.

Detailed Analysis of the Turonian to Campanian Part of the Global Cycle Chart: Evidence from the Western Interior of the United States: ABSTRACT

Detailed Analysis of the Turonian to Campanian Part of the Global Cycle Chart: Evidence from the Western Interior of the United States: ABSTRACT

Sequence Stratigraphy of Jurassic Strata in the Eastern Gulf Coastal Plain: Comparison to the Global Cycle Chart: ABSTRACT

Sequence Stratigraphy of Jurassic Strata in the Eastern Gulf Coastal Plain: Comparison to the Global Cycle Chart: ABSTRACT

Sequence stratigraphy in the upper cretaceous of the Basco-Cantabrian Basin (northern Spain)

The sedimentary cycles of the Cenomanian to Maastrichtian were investigated in the Basco-Cantabrian Basin (BCB) in northern Spain (Provinces of Alava, Vizcaya and Burgos). The depositional area was a distally steepened carbonate ramp which extended from Catalonia northwestwards to the Basque country. The investigated sediments range from calciturbidites and pelagic marls to marl-limestone alternations deposited on a distal carbonate ramp. Shallow marine limestones, marls and intertidal clastics and carbonates were deposited on the proximal part of the carbonate ramp. The establishment of a regional sequence analysis is based on the investigation of seismic profiles, well logs and outcrop sections. Examples of outcrop sections are interpreted in terms of sequence stratigraphy (unconformities of third- and second-order cycles, depositional geometries, systems tracts). The sequence stratigraphic interpretation of outcrop sections is based on facies analysis, interpretation of observed depositional geometries and correlation of unconformities and marine flooding surfaces through the basin. A biostratigraphic framework is established based on ammonites, inoceramids, planktonic and benthic foraminifera. As a result, a regional sequence stratigraphic cycle chart is presented and compared with published global cycle charts. The correlation of the regional cycle chart with published cycle charts is good. In the Cenomanian and Turonian, several sequence boundaries in the BCB are shifted by up to one biozone compared with the global chart. Some type 1 boundaries of the standard chart are only type 2 in the BCB. Important type 1 boundaries in the BCB are: top Geslinianum Zone with a 100 m lowstand wedge at the basis of the sequence (sequence boundary 92.2) ; base Petrocoriense Zone with a 250 m shallowing-upwards lowstand wedge at the basis (sequence boundary 89.2); and within the Syrtale Zone (sequence boundary 85.0).

Depositional environments, sea-level history and palaeobiogeography of the late Campanian-Maastrichtian on the Calabar flank, SE Nigeria

Campanian-Maastrichtian palaeoenvironments and seal-level history on the Calabar Flank are interpreted by recognising palynofacies based on statistical and morphological variations in organic-walled microplankton, mainly dinoflagellates and miospores. A holomarine palynofacies defining an inner-neritic depositional environment is characterized by > 80% dinoflagellates, mainly spiniferate cysts with short, simple to complexly hooked or barbed processes. Fluvio-marine palynofacies corresponding to a littoral setting shows close abundance percentages of dinoflagellates and miospore elements. Both spiniferate and peridinioid cysts with short, long and simple processes occur abundantly and the miospore elements are dominated by mangrove-type species. A non-marine palynofacies representing a fresh-water environment is dominated by miospores of hinterland affinities. Few peridinioid cysts with long processes are present. Sedimentation and palynofacies disposition was mainly controlled by eustatic sea level fluctuations. Resolution of palynofacies, in relation to outcrop data, presents two depositional cycles, each exhibiting trangressive, highstand and lowstand systems tract deposits. The boundary between the two cycles is marked by an abrupt change from non-marine to holomarine palynofacies. These cycles, dated as late Campanian-mid Maastrichtian and late Maastrichtian, relate favourably with the 75 Ma and 71 Ma depositional sequences recorded on the Global Cycle Chart.

Well-Log Seismic Sequence Stratigraphy of Aruba and its Application to the Hydrocarbon Exploration in the Caribbean Area

Several wells have recently been drilled in offshore Aruba approximately 12 degrees NOrth Latitude and 70 degrees West Longitude, just north of the Westernpart of Venezuela, South Central Caribbean. One of the wells, the Oxy Chuchubi No. 1 well penetrated a largely carbonate section that ranged in age from early Pliocene to early Oligocene to late Eocene at its total depth of 9,210 feet. High resolution biostratigraphy of benthic and planktonic foraminifers and calcareous nannofossils provided the abundance and diversity histograms necessary to recognized a series of maximum flooding surface condensed sections (MSF). Fourteen maximum flooding surfaces within the MFS condensed sections were identified on the well log and dated using the planktonic foraminifers and calcareous nannofossils. These maximum flooding surfaces range in age from 4.0 Ma or possibly 36.5 Ma. The paleobathymetry varies from middle to outer neritic in the upper portion of the well to lower bathyal to abyssal in the lower portion of the well. Fourteen third order sequences and their systems tracts were identified and annotated on a well log. These sequences were correlated with the Global Cycle Chart of Hag, 1987. The sequence stratigraphy provides a means of chronostratigraphic correlation to Venezuela and provides amore » direct tie to the geological history and hydrocarbon potential of the area.« less

Exxon global cycle chart: An event for every occasion?: Comments and Reply

Research Article| March 01, 1993 Exxon global cycle chart: An event for every occasion?: Comments and Reply William R. Dickinson; William R. Dickinson 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar Walter S. Snyder; Walter S. Snyder 2Permian Research Institute, Department of Geosciences, Boise State University, Boise, Idaho 83725 Search for other works by this author on: GSW Google Scholar Claude Spinosa; Claude Spinosa 2Permian Research Institute, Department of Geosciences, Boise State University, Boise, Idaho 83725 Search for other works by this author on: GSW Google Scholar Andrew D. Miall Andrew D. Miall 3Department of Geology, University of Toronto, Toronto MSS 3B1, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information William R. Dickinson 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Walter S. Snyder 2Permian Research Institute, Department of Geosciences, Boise State University, Boise, Idaho 83725 Claude Spinosa 2Permian Research Institute, Department of Geosciences, Boise State University, Boise, Idaho 83725 Andrew D. Miall 3Department of Geology, University of Toronto, Toronto MSS 3B1, Canada Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1993) 21 (3): 282–285. https://doi.org/10.1130/0091-7613(1993)021<0282:EGCCAE>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation William R. Dickinson, Walter S. Snyder, Claude Spinosa, Andrew D. Miall; Exxon global cycle chart: An event for every occasion?: Comments and Reply. Geology 1993;; 21 (3): 282–285. doi: https://doi.org/10.1130/0091-7613(1993)021<0282:EGCCAE>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract No abstract available This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Integrated sequence stratigraphy of Neogene deposits, New Jersey continental shelf and slope: Comparison with the Exxon model

We have evaluated the Neogene stratigraphic succession beneath the continental shelf off New Jersey using high-quality seismic and well data to (1) construct geologic cross sections that relate seismic geometries to lithofacies in a prograding, tectonically quiescent setting; (2) apply a sequence-stratigraphic approach to subdivide the stratigraphic section and relate the characteristics of the deposition observed sequences to the published Exxon model; and (3) determine the resolution of the available biostratigraphy in the correlation of the offshore New Jersey depositional sequences to the published eustatic cycle chart. In the offshore New Jersey area, the well-developed progradational geometries in the Neogene section, the high-quality seismic data, and the relatively abundant lithologic information from wells allow us to use the results of this study to predict lithofacies distribution in similar, but less geologically constrained, settings. Depositional sequences are characterized by similar overall stacking patterns within component systems tracts; however, the New Jersey sequences are different in several ways. These differences include the lack of type-2 unconformities, an absence of leveed channel deposits within lowstand wedges, very poorly developed transgressive systems tracts, and sandstone-rich, high regressive highstand systems tracts. These differences are explained in terms of the limited Neogene accommodation on the Newmore » Jersey shelf that resulted from slow subsidence rates and a second-order Miocene eustatic fall. Paleontologic data bracket the ages of the individual sequences but are not detailed enough at present to correlate precisely to the time scale and inferred global sea-level records. Based on the available biostratigraphy, however, Neogene depositional sequences from offshore New Jersey have a cyclicity consistent with interpreted third-order eustatic fluctuations on the global cycle chart. 18 refs., 6 figs., 1 tab.« less

Palaeogeographical significance of the Triassic-Jurassic unconformity in the north Iberian basin (Sierra del Moncayo, Spain)

Field work carried out on the Lower Jurassic (i.e., Hettangian-lower Pliensbachian) of the north Iberian basin (Sierra del Moncayo, Spain), including the mapping of unconformities and sedimentary units as well as the facies analysis, has allowed us to recognize two episodes in the sedimentary evolution of the basin. A lower one, Hettangian in age, includes sediments deposited during the period of extensional tectonic activity, which involved the formation of half-graben basins by normal listric faults. Sedimentary breccias and rudites, located mainly at the edges of the half-grabens are derived from the erosion of blocks uplifted during the Triassic. In the depocenter of the half-grabens which show most subsidence these breccias pass into thick dolomitic and evaporitic successions, deposited in sabkha environments, under negligible marine influence. The later episode, Sinemurian and lower Pliensbachian in age, consists of shallow marine carbonate units deposited in a low-angle ramp setting. Evaporite accumulation took place during phases of relative lowstand of sea level (i.e., lowstand systems tract), when the basin was cut ooff from open marine conditions; whereas the carbonate sediments mainly represent transgressive and highstand systems tracts. In the Iberian basin, the lowstand episode during the Hettangian was mainly induced by the breakup of the Upper Triassic ramp, which favoured the isolation from open marine conditions of several subsident sub-basins. In addition, it should be noted that the Hettangian is considered as a lowstand sea level episode in some of the proposed global eustatic cycle charts, so that a eustatic component may also be postulated for the isolation of the basin at this time. The infilling of the Hettangian half-grabens left an even topography which was flooded during the Sinemurian sea level rise, leaving shallow carbonate deposits. The results presented here give new information on the paleogeography of northeast Iberia at the Triassic-Jurassic boundary in the context of the opening of the western margin of the Tethys.

Exxon global cycle chart: An event for every occasion?

The basic premise of the recent Exxon cycle chart, that there exists a globally correlatable suite of third-order eustatic cycles, remains unproven. Many of the tests of this premise are based on circular reasoning. The implied precision of the Exxon global cycle chart is not supportable, because it is greater than that of the best available chronostratigraphic techniques, such as those used to construct the global standard time scale. Correlations of new stratigraphic sections with the Exxon chart will almost always succeed, because there are so many Exxon sequence-boundary events from which to choose. This is demonstrated by the use of four synthetic sections constructed from tables of random numbers. A minimum of 77% successful correlations of random events with the Exxon chart was achieved. The existing cycle chart represents an amalgam of regional and local tectonic events and probably also includes unrecognized miscorrelations. It is of questionable value as an independent standard of geologic time.

Controls on Late Jurassic seismic sequences, Inner Moray Firth, UK North Sea: a critical test of a key segment of Exxon's original global cycle chart

ABSTRACTA new interpretation of a comprehensive seismic‐ and well‐database has resulted in the subdivision of the Mesozoic into four, basin‐wide, seismo‐stratigraphic depositional megasequences in the Inner Moray Firth (IMF) basin. Regional mapping of the megasequences has led to the construction of a new model for Mesozoic‐Recent basin development in the IMF. It now appears that extensional tectonics was the main control on the basin's evolution during the Mesozoic. Structural geometries suggest that both the Triassic (Tr) and Rhaetian‐mid Oxfordian (Jl) megasequences were controlled by regional broad‐based subsidence associated with local extensional fault activity prior to the onset of renewed rifting in the IMF. In contrast, the late Oxfordian‐Ryazanian (Berriasian; J2) megasequence developed in response to active extension characterised by half‐graben development. Subsequent Early Cretaceous (Kl) deposition appears to have occurred during a further period of broad regional (thermal) subsidence. It is evident that strike‐slip movement on the Great Glen Fault played a negligible role in Mesozoic basin development and it appears only to have had a local control on structural styles during its reactivation in the Tertiary as it accommodated regional uplift and basin inversion.Further subdivision of the J2 megasequence was possible using biostratigraphically‐controlled seismic reflector terminations and led to the definition of five regional seismo‐stratigraphic sequences (J2.1–2.5). Their geometric, thickness and sedimentary facies variations imply that the onlap‐defined sequence boundaries within the late Oxfordian‐Ryazanian (Berriasian; J2) megasequence were caused by syn‐sedimentary extensional tectonism in a fully marine domain, rather than by fluctuations in global sea‐level in a basin that was relatively quiescent tectonically. The new interpretation has particular significance in view of the fact that the Late Jurassic of the IMF was used by Exxon workers to construct part of their chart demonstrating relative changes of coastal onlap and global eustatic sea levels. As they considered that data from the area showed ‘no evidence that tectonics caused the unconformities’, the new interpretation casts doubt on the global applicability of the Late Jurassic section of Exxon's original sea‐level chart. Furthermore, the study demonstrates that reflector terminations within both tectonically active and/or fully marine sequences should be treated with extreme caution and not be used to define either periods of apparent low‐stand or coastal onlap. Indeed, their appearance may sometimes only represent relatively local, auto‐ and allo‐cyclic sedimentary processes such as submarine fan avulsion or channel switching, unrelated to changes in sea‐level. Finally, the work shows that care must be taken in the selection of seismic lines used to establish and illustrate the nature of depositional sequences and their geometries if pitfalls are to be avoided.

High-frequency sequences and their stacking patterns: sequence-stratigraphic evidence of high-frequency eustatic cycles

A hierarchy of interpreted eustatic cyclicity in siliciclastic sedimentary rocks has a pattern of superposed cycles with frequencies in the ranges of 9–10 m.y., 1–2 m.y., 0.1–0.2 m.y., and 0.01–0.02 m.y. (second- through fifth-order cyclicity, respectively). Stratigraphic units displaying this cyclicity include composite sequences, sequences, and parasequences. On the Exxon global cycle chart, fundamental third-order cycles (1–2 m.y. average duration) stack into related groups (second-order cycles: 9–10 m.y. duration). A much larger pattern (about 200 m.y.) is interpreted as tectonically controlled eustasy probably related to sea-floor spreading rates.One and probably two higher orders of cyclicity (fourth-order: 0.1–0.2 m.y.; and fifth-order: 0.01–0.02 m.y.) are now observed in work with well logs, cores, and outcrops in areas of very rapid deposition. These frequencies are in the range of Milankovitch cycles, and may represent part of the Milankovitch hierarchy which has been widely interpreted for cyclical units in carbonate rocks.High-frequency (fourth-order) sequences, which form at a 0.1–0.2 m.y. cyclicity, have all the stratal attributes of conventional sequences, including constituent parasequences and systems tracts, and play a dominant role controling reservoir, source, and sealing rock distribution. A consistent hierarchy of stratigraphy is observed. Parasequences (probable fifth-order cyclicity) stack into sets to form systems tracts in fourth-order sequences. Groups (sets) of fourth-order sequences are deposited between major third-order boundaries within third-order composite sequences. Sequences in these sets stack in prograding and backstepping patterns to form third-order lowstand, transgressive, and highstand sequence sets.Third-order sequence boundaries are marked by greater basinward shifts in facies, by larger more widespread incised valleys, and by more extensive onlap than are fourth-order sequence boundaries. Third-order condensed sections commonly are widespread, faunally rich, and widely correlated biozone and mapping markers. Fourth-order sequence analysis helps to understand reservoir, source, and seal distribution at the play and prospect scale. An example from the Gulf of Mexico is discussed.

Major erosion surfaces in the basal Wealden Beds, Lower Cretaceous, south Dorset

The transition from evaporitic and lagoonal Purbeck Beds into the fluviatile basal Wealden Beds (Ryazanian-Valanginian) has in the past been interpreted as a continuous regressive succession. Mupe Bay and Bacon Hole, in south Dorset, are the only localities in the Wessex Basin of southern England at which the transition strata are well exposed and not extensively faulted. On the basis of facies analysis the lowermost Wealden Beds are interpreted as a regressive-transgressive-regressive package. Environments fluctuated between lagoon, inner-lagoonal shoreline and fluvial flood plain. The overlying succession is remarkable for the presence of two major erosion surfaces which, based on field geometries and facies relationships, are interpreted as unconformities. The lower unconfor­mity is overlain at Mupe Bay by sediments of lagoonal or lacustrine origin. The upper unconformity is expressed as the celebrated Mupe Bay palaeo-oilseep, an oil-cemented conglomeratic sand, overlain by a thick succession of fluvial sediments. A level near the basal sand and the two erosion surfaces are treated as candidate sequence boundaries (WBl-WB3), with the thin intervening ?lagoonal deposits representing peaks of transgression. The pattern of one minor (conformable) sequence boundary followed by two major (unconformable) sequence boundaries bears a strong resemblance to the Early Cretaceous sequence stratigraphy suggested in recent ‘global cycle charts’. However, biostratigraphical calibration of the sections is currently inadequate to allow close correlation with candidate sequence boundaries in other areas.