Major Facies Changes Research Articles

Carbonate platform crisis in the Carnian (Late Triassic) of Hanwang (Sichuan Basin, South China): Insights from conodonts and stable isotope data

During the Carnian, the Hanwang area in the northwestern Sichuan Basin (South China) was characterized by shallow water carbonate sedimentation that underwent an abrupt demise associated to a sudden input of terrigenous sediments. This major facies change was considered to be the expression of the onset of the Carnian Humid Episode, a most remarkable environmental crisis in Late Triassic that is well recognized in northwestern Tethys margins and coincides with a major global perturbation of the carbon stable isotope record. However, the lack of detailed biostratigraphic constraints have so far prevented a precise dating of the carbonate platform demise in western Sichuan Basin.In this work, the Qingyan Gou (HWQ) section, cropping out in Hanwang, was investigated for its facies and microfacies, carbonate carbon and oxygen isotopes on brachiopods, microbial grains and bulk matrix. Facies analysis shows a marine transgression from inner ramp oolitic shoal to middle ramp siliceous sponge mound, then overlain by shale and calcareous siltstone with interbedded silty mudstones. Refined biostratigraphic data from HWQ and Guanyin Ya (HWG) sections imply that the demise of sponge mounds occur in the late Tuvalian or later.A negative carbon isotope perturbation was found in the bulk matrix immediately below the sponge mounds demise, but it was not found on the isotope record from brachiopods. This suggests that the negative shift in the bulk carbonate was probably related to diagenesis. Given the late Tuvalian (last substage of Carnian) age attributed to the demise of the sponge mounds and the absence of a carbon isotopic excursion, we infer that the carbonate platform crisis and strong terrigenous input in Hanwang cannot be related to the onset of the Carnian Humid Episode. These the demise of the carbonate platform and the facies deepening trend could be rather due to the interplay between accelerating subsidence rates, environmental chants and enhanced siliciclastic input related to the formation of a foreland basin during Indosinian orogenesis.

THE EFFECT OF BASEMENT FAULT REACTIVATION ON THE TRIASSIC – RECENT GEOLOGY OF KURDISTAN, NORTH IRAQ

The Zagros orogenic belt is underlain by a complex faulted Precambrian basement. Major fault trends originating in this basement have been invoked to explain large‐scale structural changes along the strike of the orogen, e.g. the development of the Kirkuk Embayment (Kurdistan, Iraq) and the Lurestan Salient (Iran). However, within the Kirkuk Embayment, these structural trends have not previously been considered as an interacting group of faults which are periodically reactivated. This contribution first presents a revised basement fault map for the Kirkuk Embayment, created from interpreted gravity data, existing fault maps and remote sensing lineament analyses. This map is then compared to surface structure maps, published facies maps and source rock maturity data using GIS techniques. The object is to define the relationship between the basement faults and surface structures, fades and source‐rock maturity through time.Surface anticline orientation and location, as well as a number of major facies changes within the cover sequence, and the maturity of Triassic source rocks, are constrained by the interaction of the Najd and Transverse fault trends. A third basement trend, the Nabitah trend, has a more subtle effect on Phanerozoic geology. The Kirkuk Embayment can be divided into a series of semi‐independent basement blocks, defined by these basement fault trends. The interaction of these semi‐independent blocks has created local but predictable differences in surface structures, sediment thickness and facies, and variations in the maturity of Triassic source units across the Kirkuk Embayment. An understanding of the location and behaviour of basement faults within this hydrocarbon province is therefore a valuable predictive tool in exploration.

Mesozoic radiolarites – accumulation as a function of sea surface fertility on Tethyan margins and in ocean basins

AbstractRadiolarites, commonly comprising chert‐shale couplets, feature prominently in the Mesozoic stratigraphy of the Neotethyan realm. The origin and significance of these deposits, especially with regard to understanding palaeoproductivity in that region, remains controversial. This study examines transport processes and accumulation rates of radiolarians/radiolarites, as well as the chronology of radiolarite sedimentation based on revised data from the Southern Alps and new data from the Balagne Nappes, Alpine Corsica. Results suggest that the spatio‐temporal distribution of carbonate and silica on Tethyan margins was governed by extensive lateral transport of radiolarian tests from topographic highs to basins. Palaeoclimatic, rather than palaeotectonic, changes seem to have triggered changes in surface fertility which, in turn, brought about major facies changes observed in Tethyan marginal basins: Regional platform demise – possibly due to eutrophication and onset of radiolarite deposition in the Early Bajocian – can be correlated with a positive shift in the carbon isotope curve, whereas platform recovery and the end of radiolarite accumulation correlates with a gradual decline in δ13C‐values. The existence of a vigorous trans‐Pangaean, equatorial current system that could have produced regional upwelling is questioned because of evidence for Middle Jurassic to Early Cretaceous oligotrophic conditions in the Proto‐Caribbean and the Central Atlantic. The ‘Caribbean River Plume Model’ is proposed as a possible alternative to upwelling in the Jurassic Western Tethys. Dissolved nutrients may have been dispersed in low salinity lids that originated from river plumes of rivers from tropical Africa and warm‐temperate Eurasia. Vast areas of the pre‐Late Cretaceous world ocean must have accumulated radiolarites at low rates (<4 g cm−2 10−3years) represented by the condensed, long‐lived (>50 Myr) radiolarite sections observed in many Circumpacific terranes. Radiolarian chert and claystone could be the normal pelagic sediment during Ordovician to Early Cretaceous time in far‐offshore, open oceanic settings, where dilution by terrigenous material and periplatform shallow‐water carbonates was absent.

A timeline for terrestrialization: consequences for the carbon cycle in the Palaeozoic.

The geochemical carbon cycle is strongly influenced by life on land, principally through the effects of carbon sequestration and the weathering of calcium and magnesium silicates in surface rocks and soils. Knowing the time of origin of land plants and animals and also of key organ systems (e.g. plant vasculature, roots, wood) is crucial to understand the development of the carbon cycle and its effects on other Earth systems. Here, we compare evidence from fossils with calibrated molecular phylogenetic trees (timetrees) of living plants and arthropods. We show that different perspectives conflict in terms of the relative timing of events, the organisms involved and the pattern of diversification of various groups. Focusing on the fossil record, we highlight a number of key biases that underpin some of these conflicts, the most pervasive and far-reaching being the extent and nature of major facies changes in the rock record. These effects probably mask an earlier origin of life on land than is evident from certain classes of fossil data. If correct, this would have major implications in understanding the carbon cycle during the Early Palaeozoic.

At what stratigraphical level is the mid Ludfordian (Ludlow, Silurian) positive carbon isotope excursion in the type Ludlow area, Shropshire, England?

The balance of evidence suggests that the mid Ludfordian positive carbon isotope excursion (CIE) commences in the Ludlow area, England in the uppermost Upper Whitcliffe Formation, with the excursion continuing into at least the Platyschisma Shale Member of the overlying Downton Castle Sandstone Formation. The Ludlow Bone Bed Member, at the base of the Downton Castle Sandstone Formation has previously been considered to be of PA™A­dolA­ age. Conodont and thelodont evidence, however, are consistent with the mid Ludfordian age proposed here. New δ13Corg data are presented from Weir Quarry, W of Ludlow, showing a pronounced positive excursion commencing in the uppermost Upper Whitcliffe Formation, in strata with a palynologically very strong marine influence. Elsewhere in the world, the mid Ludfordian positive CIE is associated with major facies changes indicated shallowing; the lithofacies evidence from the Ludlow area is consistent with this. There appears not to be a major stratigraphical break at the base of the Ludlow Bone Bed Member.

δ18O composition of conodont apatite indicates climatic cooling during the Middle Pridoli

Phosphatic microfossils, such as conodonts and fish microremains (dermal scales) from the upper Silurian (Pridoli) of Lithuania have been studied for their oxygen isotope composition. The conodont colour alteration index of the biogenic apatite did not exceed 1.5 reflecting only a minor thermal alteration. Conodont δ 18O apatite values range from 17.7 to 19.2‰ V-SMOW, with the average values around 18.3‰. Fossil apatite of fish exoskeleton microremains from the same samples have lower δ 18O apatite values ranging from 15.2 to 17.4‰ V-SMOW, with the average values around 16.3‰ V-SMOW. Palaeoseawater temperatures calculated from conodont apatite δ 18O range from 24.8–31.5 °C, those derived from fish δ 18O give approximately 10 °C higher temperatures ranging from 32.6 to 38.1 °C. The lower δ 18O apatite values of fish apatite and thus the unrealistic high palaeotemperatures are interpreted as a result of diagenetic alteration. A significant positive shift in conodont apatite δ 18O of + 1.1‰ at the boundary between the Vievis and Lapės formations is observed, coinciding with a major facies change. It is interpreted as a mid-Pridoli event within the O zarkodina eosteinhornensis Bizone, or the O zarkodina remscheidensis Biozone reflecting a major cooling event, which may have resulted in the formation of an ice sheet in high latitudes and a glacio-eustatic sea-level fall.

Facies and climate/environmental changes recorded on a carbonate ramp: A sedimentological and geochemical approach on Middle Jurassic carbonates (Paris Basin, France)

A detailed sedimentological, geochemical and mineralogical study is carried out on the Early Bajocian to Early Callovian (Middle Jurassic) limestones of the Paris Basin. Objectives are to document and explain the facies changes in the context of the climate/environmental evolution at the NW European scale. Deposits include 18 lithofacies which are stacked into 10 third-order depositional sequences. At a greater time scale, 4 biosedimentary packages are distinguished by their allochems associations. (1) An intracratonic carbonate environment with coral reefs and crinoid-rich facies is typical of the Early Bajocian. (2) A major facies change occurred at the Early/Late Bajocian transition with a shift from crinoid- and coral-rich facies to ooid-rich facies. During the Late Bajocian, a southward-dipping ooid ramp with successive progradational trends was emplaced. (3) A large lagoon protected by ooid shoals developed during the Bathonian in a muddy ramp setting. (4) At the Bathonian/Callovian boundary, a second major change occurred with the lagoonal facies being superseded by an ooid-bioclastic (crinoids, corals) ramp associated with a waning of carbonate productivity (retrogradational trend). Our geochemical study including a compilation of bibliographic data allows us to refine the existing sea surface palaeotemperature pattern. A cooling from the latest Early Bajocian to the Late Bajocian and a subsequent warming from the earliest Callovian to the Early/Middle Callovian transition are described using a proxy of sea surface temperatures. Together with δ 13C and mineralogical data, our new δ 18O values suggest a palaeo-climatic/palaeo-environmental control of facies in this shallow carbonate ramp environment.

Extinction-Recovery Pattern of Level-Bottom Faunas Across the Triassic-Jurassic Boundary in Tibet: Implications for Potential Killing Mechanisms

Scarcity of fossiliferous boundary sections makes the Triassic-Jurassic biotic crisis the most enigmatic of the five major Phanerozoic faunal turnovers. We report a bivalve-dominated level-bottom fauna from the Triassic-Jurassic transition in southern Tibet, which is unique for two reasons: (1) it documents the faunal turnover across the system boundary without major facies changes, and (2) it provides paleobiological data from the immediate postextinction interval. In the extinction pattern, selectivity against burrowing suspension feeders and taxa with completely aragonitic shells emerges, but no major clades or ecological groups disappeared. The earliest postextinction fauna differs in three significant ways from typical survival faunas of other mass extinction events: (1) it was surprisingly diverse, (2) it was heterogeneous (Simpson D = 0.1078) without dominance of disaster taxa or opportunists, and (3) there was a prevalence of highly specialized, morphologically complex forms reaching normal growth sizes. In spite of these unusual features, extinction of typical Triassic taxa occurred comparatively sharply within the underlying beds. These findings support scenarios of relatively short environmental disturbances triggered by the volcanic activity of the Central Atlantic Magmatic Province, which led to rapid extinction of taxa but did not impose ongoing environmental stress on the survival fauna. The nearly instantaneous recovery of level-bottom faunas is in sharp contrast to a prolonged reef eclipse, which probably indicates both higher extinction rates of reef-organisms and intrinsic limitations to the tempo of recovery due to their high level of co-evolution.

High-resolution carbon isotope changes in the Permian–Triassic boundary interval, Chongqing, South China; implications for control and growth of earliest Triassic microbialites

High-resolution δ 13C CARB analysis of the Permian–Triassic boundary (PTB) interval at the Laolongdong section, Beibei, near the city of Chongqing, south China, encompasses the latest Permian and earliest Triassic major facies changes in the South China Block (SCB). Microbialites form a distinctive unit in the lowermost 190 cm above the top of the Changhsing Formation (latest Permian) at Laolongdong, comparable to a range of earliest Triassic sites in low latitudes in the Tethyan area. The data show that declining values of δ 13C CARB, well-known globally, began at the base of the microbialite. High positive values (+3 to 4 ppt) of δ 13C CARB in the Late Permian are interpreted to indicate storage of 12C in the deep waters of a stratified ocean, that was released during ocean overturn in the earliest Triassic, contributing to the distinctive fall in isotope values; this interpretation has been stated by other authors and is followed here. The δ 13C CARB curve shows fluctuations within the microbialite unit, which are not reflected in the microbialite structure. Comparisons between microbialite branches and adjacent micritic sediment show little difference in δ 13C CARB, demonstrating that the microbialite grew in equilibrium with surrounding seawater. The Early Triassic microbialites are interpreted to be a response to upwelling of bicarbonate-rich poorly oxygenated water in low latitudes of Tethys Ocean, consistent with current ocean models for the PTB interval. However, the decline of δ 13C CARB may be due to a combination of processes, including productivity collapse resulting from mass extinction, return of deep water to ocean surface, oxidation of methane released from methane hydrate destabilisation, and atmospheric deterioration. Nevertheless, build-up of bicarbonate-rich anoxic deep waters may be expected as a result of the partial isolation of Tethys, due to continental geography; release of bicarbonate-rich deep water, by ocean upwelling, in the earliest Triassic may have been an inevitable consequence of this combination of circumstances.

Upper Dalan Member and Kangan Formation between the Zagros Mountains and offshore Fars, Iran: depositional system, biostratigraphy and stratigraphic architecture

Upper Dalan Member and Kangan Formation between the Zagros Mountains and offshore Fars, Iran: depositional system, biostratigraphy and stratigraphic architecture

Late Maastrichtian rudist and coral assemblages from the Central Inlier, Jamaica: towards an event stratigraphy for shallow-water Caribbean limestones

The lithological succession of the Guinea Corn Formation in the Slippery Rock River, central Jamaica, comprises 91 m of limestones and subsidiary mudstones. The biostratigraphic distribution of rudist bivalves and corals demonstrates that the succession of biostratigraphic markers is consistent with the previously documented standard Guinea Corn Formation succession in the Rio Minho between Grantham and Guinea Corn, central Jamaica. Additionally, the Slippery Rock River succession shows the boundary between the Chiapasella radiolitiformis and C. trechmanni zones that has not previously been documented. The marker horizons are also consistent with major facies changes within both sections, demonstrating that both lithological changes and biostratigraphic markers are synchronous within the limestone successions of central Jamaica. This may prove to be a valuable tool for stratigraphic correlation elsewhere in Jamaica and within the Antillean region.

Three orders of regional sea-level changes control facies and stacking patterns of shallow platform carbonates in the Maestrat Basin (Tithonian-Berriasian, NE Spain)

Middle Tithonian-Berriasian shallow platform carbonates of the Maestrat Basin (Salzedella and Montanejos sections, NE Spain) are stacked in sequence stratigraphic units of different orders. Higher-order sequences (parasequences and subunits) have a shallowing or deepening-shallowing evolution. They have been related to the short-term eccentricity and precession cycles. Major facies changes and stacking pattern of parasequences reveal the presence of two 2nd-order sequences. The Lower Sequence is middle Tithonian-mid early Berriasian in age. The Upper Sequence extends up to the mid-late Berriasian. It is suggested that local subsidence changes along with regional sea-level changes controlled the long-term evolution of accommodation in the Maestrat Basin. Facies evolution, stacking pattern and sharp lithological changes have allowed the definition of five 3rd-order sequences in the Lower Sequence in Montanejos. The transgressive deposits are characterised by the progressive absence of the restricted lagoon facies, and the presence of deepening-upward intervals in the parasequences. The highstand deposits display an increase in siliciclastics and a progressive predominance of restricted lagoon facies. Some of the 3rd-order sequence boundaries match the sequence boundaries identified in other European basins and may be related to sea-level falls (induced by the long-term eccentricity cycle) enhanced during periods of long-term loss of accommodation.

Geological-Engineering Evaluation: Castilla and Chichimene Fields, Cubarral Association Contract, Colombia

A multidisciplinary geological and engineering evaluation project was undertaken for fields within the Chevron Colombia operated Cubarral Association Contract. The goal of the project was to combine geological and engineering evaluations to form a strategy for maximizing the efficiency of ongoing field development, recompletion, and production procedures. The Association Contract is located in the Llanos basin about 100 km southeast of Bogota. The fields: Castilla, Chichimene NE and Chichimene SW contain approximately 2200, 100, and 65 MMBO in place, respectively. The Guadalupe sandstone produces 14-21[degrees] API oil. The geological evaluation consisted of three phases: (1) revision of the structural interpretations, (2) detailed stratigraphic analyses, and (3) delineation of tilted fluid contacts and determination of original-oil-in-place. Structural interpretations show that reservoir properties and continuity are strongly affected by depositional environment which varies between the fields. A major facies change between massive, braided-stream sandstones at Castilla and deltaic to shallow marine sandstones at Chichimene northeast is reflected in production characteristics. The fields show strongly tilted oil-water contacts due to regional hydrodynamic gradients. Geometrical and hydrodynamic calculations were used to delineate the direction and magnitude of the tilts to assess original oil-in-place.

Ramp sedimentation in the Dinantian limestones of the Shannon Trough, Co. Limerick, Ireland

During the late Chadian and Arundian (Lower Carboniferous), an extensive carbonate ramp (Limerick Ramp) developed over County Limerick, southwest Ireland, dipping northwestwards. Three distinct facies can be recognised corresponding to position on this ramp: inner, mid- and outer ramp. The inner ramp facies of oolitic and crinoidal grainstones (Herbertstown Limestone Formation) in east Limerick formed a major shoal behind which peritidal limestones were deposited. The mid-ramp facies of muddy bioclastic limestones and shales (Cooperhill facies) in north Limerick formed between fairweather and storm wave bases. The outer ramp (basinal) facies of mudstones and thin graded resedimented limestones (Rathkeale Beds) in west Limerick developed below storm wave base when fine terrigenous input was high. Later in the Arundian there was progradation of the nearshore oolitic and crinoidal grainstones over the mid-ramp facies. By the Holkerian, the deep-water basinal facies in west Limerick was buried beneath mid-ramp facies (Durnish Limestone). The initiation of the Limerick Ramp is closely related to the formation of the Shannon Trough. In the late Courceyan, accelerated subsidence in the Shannon area during deposition of Waulsortian facies marked the onset of a sag phase. Following a quiescent period in early Chadian, subsidence was renewed in the late Chadian and Arundian, when major facies changes occurred on the ramp. Comparison of the Shannon Trough with the Dublin Basin shows that in the latter, tectonic events in the Chadian and Arundian, particularly syn-sedimentary faulting, created a sharp division between platform and basinal sedimentation. Such tectonic influence is not recognised in the Shannon Trough. Here differential subsidence and eustatic sea-level changes led to more permanent ramp existence, modified only by westwards progradation.

Tetrapod extinctions during the Late Triassic: evidence from the western United States

Most recent workers have identified a major extinction of nonmarine tetrapods during the Late Triassic at the Carnian-Norian boundary. However, this identification is based principally on the stratigraphic distribution of tetrapods where there either is a major facies change at the Carnian-Norian boundary (Germany) or a significant unconformity that spans much of the early Norian (Argentina). Instead, we have examined the stratigraphic record of tetrapods in the western United States (Chinle Group) where there is a relatively continuous, nonmarine depositional and fossil record across the Carnian-Norian boundary that does not encompass major facies changes. The Chinle Group, because of its great area of exposure, more than one century of extensive collection and relatively complete publication, contains the world's best record of tetrapods across the Carnian-Norian boundary.The tetrapod record from the Chinle Group does not record a significant tetrapod extinction at the Carnian-Norian boundary. Instead, it documents a staggered extinction of some taxa during the late Carnian and the persistence of most taxa across the Carnian-Norian boundary. Thus, rhynchosaurs become extinct at the end of the early-late Carnian, whereas dicynodonts become extinct somewhat later, during the latest Carnian. Among metoposaurids, only the large taxa (Buettneria) disappear at the end of the Carnian. Other groups - phytosaurs, aetosaurs, rauisuchians, trilophosaurs, etc. - only exhibit some generic turnover at the Carnian-Norian boundary. The first appearance of dinosaurs, linked by some to a Carnian-Norian tetrapod extinction, is well in advance of the end of the Carnian. Thus, we conclude that the highest resolution tetrapod record of the late Carnian and early Norian does not document a significant tetrapod extinction at the end of the Carnian.

Preservational, paleoecological, and evolutionary patterns in the Wyoming-Montana Paleogene and Siwalik Neogene records

The Paleocene to early Eocene sequence of Wyoming-Montana and the Miocene to Pleistocene Siwalik record of Pakistan are exceptionally long, continental sequences, each containing a rich and well documented fossil record, especially of mammals. The two sequences are broadly similar in tectonic setting and sedimentary environment, in duration and facies changes, and in diversity of fossils. Each contains a paleoclimatic record in stable isotopes and, in the Paleogene, floras. Comparison of these two sequences has focused our attention on the interaction of tectonic, climatic, sedimentologic, and taphonomic factors that produce a particular fossil record and on the co-occurring ecological and evolutionary changes that result in a historical series of biotas, each the product of local and global events.In the Paleogene record, the geographic scope of the record encompasses much of the floodbasin, and the spatial distribution of paleoenvironments formed fairly straightforward gradients from channel to distal floodplain. The Siwalik record has a smaller window onto a larger, heterogeneous fluvial system often with multiple, contemporaneous river systems that differ in magnitude. The spatial distribution of paleoenvironments was a mosaic without long proximal to distal gradients. In both records, major facies changes are correlated with striking changes in fossil productivity.The overall pattern of fossil preservation by depositional environment differs substantially in the two areas. The Siwalik sequence has a greater variety of depositional environments that produce fossils throughout the section. The primary productive environment in the older part of the Paleocene record declined in productivity upsection, while a previously unproductive facies became the major source of fossils. Much of the record represents attritional accumulation in each area, but a significant portion is transported. The taphonomic processes that created fossil concentrations led to better taxonomic resolution for most Paleogene localities than in most Siwalik localities.In each record, both aquatic and terrestrial components of the vertebrate faunas are correlated with facies. Since facies varied in productivity over time, some changes in faunal composition may have resulted from change in the prevalence or productivity of particular facies. At least one faunal turnover coincided with major facies changes in each sequence.For mammals in each record, immigration rather than speciation in situ was the primary means of appearance of new species. Episodes of immigration were not closely followed by extinctions of resident species. Mean species longevity appears to have been more than twice as great in the Neogene than in the Paleogene record. Changes in faunal composition and species richness occurred during times of global climatic change; causal connections are still being explored. Changes in species richness did not track changes in relative abundance of taxa or changes in size within lineages or faunas. In terms of guild structure, the herbivore guild had high relative generic diversity through most of both sequences. The Paleogene record had a more even distribution of taxa in the five principal guilds, while the Siwalik record was heavily dominated by the herbivore guild. Size distributions differed substantially, reflecting the early and late windows into the mammalian radiation, rather than sampling bias.

Late Triassic to Middle Jurassic stratigraphy, palaeogeography and tectonics west of the British Isles

Abstract The Upper Triassic to Middle Jurassic stratigraphy of the Atlantic margin basins west of the British Isles (including Celtic Sea, Porcupine, Slyne, Erris, Hebrides, and west of Shetland) is summarized in terms of six major genetic sequences, defined as packages of conformable strata separated by surfaces of abrupt major facies change. They indicate common tectonic histories and are interpreted as representing episodes of evolution comprising extensional phases with laterally variable sequences (1A Late Triassic to Early Sinemurian; 2A latest Toarcian to Late Bajocian), thermal and loading sag phases (1B mid-Sinemurian to earliest Toarcian; 2B Late Bajocian to Bathonian) and stabilization phases with sequences of reduced thicknesses and hiatuses (1C Toarcian; 2C Late Bathonian to Callovian). The basins west of Shetlands, north of the Wyville-Thomson Ridge, have a different history, especially in the Early Jurassic. Similarly, basins south of an extension of the Clare Lineament (south of Celtic Sea and Porcupine Basins) differ in having limited Middle Jurassic subsidence. These transverse elements divide the Atlantic basin system into local subsystems, each of which has a different tectonic history.

Sea-level events during the Early Cretaceous in Western Europe

Lower Cretaceous sedimentary successions in NW Europe are discussed in the light of relative changes in sea-level, by comparing the boreal marine deposits of the North Sea and Germany, the paralic and shelf deposits of southern England, the Paris Basin and Atlantic margin basins, and the Tethyan carbonate shelf deposits of southern France. Examination of the major facies changes and unconformities allows comparison with Exxon models of sequence stratigraphy. In addition, broad changes in palaeoceanographic and palaeoclimatic influence are suggested. Relative sea-levels during the Early Cretaceous can be compared to published models of European and global sea-level change.

Biostratigraphic, Chronostratigraphic, and Stratigraphic Sequence Analysis of Lower Tertiary Marine Sediments of Alabama for Indicators of Sea-Level Change

Early Eocene to late Oligocene marine sedimentary units in southwestern Alabama were sampled at closely spaced intervals to derive a precise time-stratigraphic framework and to determine the paleoecological and mineralogical responses to fluctuations in sea level. Paleontologic control consisted of planktonic, smaller and larger benthonic foraminifera, calcareous nannofossils, dinoflagellates, and megafossils. Paleomagnetic reversals were delineated in two boreholes which, when supplemented by strontium isotope dates and the biostratigraphic control, provided a robust in situ chronostratigraphy for the Gulf Coast lower Tertiary. Paleoecologic trends in regression and transgression can be clearly correlated across major regional facies changes. Using the chronostratigraphy developed here, the second-, third-, and fourth-orders of Vail's global sea-level cycles can be recognized and demonstrate the influence of sea-level change on sedimentation. Stratigraphic systems tracts (SSTs) and bounding surfaces in outcrop were determined by lithologic variations and paleoecologic trends, and additionally by gamma logs in the cores. The lower sequence boundary occurs at a contact where an older, relatively fine-grained, deep-water, fossiliferous unit was abruptly succeeded by a coarse-grained, shallow-water, poorly fossiliferous unit. The transgressive surface occurs at the base of a fining- and deepening-upwards unit that was commonly glauconitic and very fossiliferous. Transgression culminated with a pulse ofmore » planktonic microfossils in a bed having reduced clastic sedimentation; on the log the surface of maximum starvation was marked by a gamma spike.« less

Geology and Seismic Modeling of an Aneth-Type Desert Creek Mound Trend, Paradox Basin, Southwest Colorado: ABSTRACT

In the Paradox basin of southwest Colorado, a major facies change from thick, porous algal dolomite to thin, tight anhydrite occurs within the Desert Creek zone of the Pennsylvanian Paradox Formation. This northwest-trending algal mound belt is subparallel to and separated from the ancestral Uncompaghre mountain front by lagoonal anhydrite and nearshore arkosic fanglomerates. In the belt, carbonate rocks attain a gross thickness of 200 ft, which can include 100 ft of reservoir quality dolomite. These rocks have an updip and abrupt eastward facies change to thin evaporites that form a regional master trap for potentially significant hydrocarbon accumulations. Reservoir parameters and trapping mechanisms are similar to Utah's Aneth field, where oil production exceeds 340 million bbl. The subsurface stratigraphic changes have been seismically modeled with a microcomputer for the purpose of defining the extent of each facies. The modeling is based on sonic logs from two wells that exhibit the end-member facies of thick, porous dolomite versus thin, tight evaporite. An interpolation routine creates intermediate logs between these end-members, thereby defining the geometry of the model. The depth model is convolved with a zero-phase wavelet of 10/20-40-60 hz, and the result is an extra seismic cycle where reservoir thicknessmore » exceeds 40 ft. Updip loss of the cycle defines the transition from reservoir dolomite to trap anhydrite. The modeling demonstrates that this major facies change can be detected at frequencies reasonably attainable by modern seismic methods. Regional subsurface stratigraphic analysis and modern seismic data provide an integrated approach to hydrocarbon prospecting in the Pennsylvanian Desert Creek zone.« less