Florida's Plio-Pleistocene strata record episodes of sea-level highstands. The age of the strata is often ambiguous as there are no consistently reliable dating techniques that can be unequivocally applied to many of the units. The lack of preservation of continuous Plio-Pleistocene sedimentary sequences is a consequence of Pleistocene mean sea-levels not flooding peninsular Florida, the low volume of sedimentary supply, and the lack of new accommodation space. This study investigates a 6 m cyclothem-type set of six shallow-marine shell beds separated by five subaerially exposed packstone beds. These strata are part of the biostratigraphically-defined early Pleistocene (1.1 -- 1.6 Ma) Bermont formation and were likely deposited during a 160 kyr interval between 1.3 and 1.1 Ma. The shell beds are mollusk-rich and contain moderately well-sorted fine sands. The packstones contain sparry calcite cements and show evidence of subaerial weathering, such as an irregular upper solution surface, root molds, and sparry freshwater calcite cements. The upper surfaces of the packstones are unconformities that separate five episodes of highstand deposition. A grain-size analysis shows an upward-coarsening trend between depositional episodes, which most likely indicates a progressively decreasing water depth. The bivalve assemblages suggest a mean paleodepth during the deposition of the shell beds of approximately 7.5 m; alternatively, in situ Anodontia alba, which colonized these units after deposition, point to a depth of 1 m. A subsidence rate of 6 m/Ma is inferred from the thickness of deposits near the locality. Based on a comparison of the height of the strata with ex isting eustatic curves, the early Pleistocene age of the formation, and the 6 m/Ma subsidence rate, the most parsimonious duration for the interval between the cyclothems is 41 kyr, dominantly forced by obliquity orbital variability. Combining the data indicates that the early Pleistocene sea level was between 11.2 and 14.4 m above sea level (asl); previous estimates of early Pleistocene highstands have shown an elevation approximately 15 m asl. If the 1 m depth of Anodontia alba is used, the depth was likely 6.3 m asl.

Not available / text

The mixed carbonate–siliciclastic Abrigo Formation of middle and late Cambrian age, which crops out in southeastern Arizona, was deposited during the Sauk transgression in the craton interior, landward of the passive margin of Laurentia. The Abrigo Formation consists of ten basic rock types: claystone, siltstone, sandstone, lime mudstone, wackestone, bioclastic grainstone, packstone, oolitic packstone, oncolitic packstone, and intraclastic conglomerate. These comprise fifteen lithofacies, which are grouped into eight facies associations. They represent an array of shallow-marine environments that were dominated by wave and storm activity. The interpreted paleoenvironments include lower offshore, upper offshore, offshore transition, and lower, middle and upper shoreface. One hundred eighty-two collections, yielding 940 trilobite remains have been found in the Abrigo Formation. They represent 69 species and 42 genera. Eight of the species are new. The fossil age ranges from early Marjuman to late Steptoean. Eight trilobite biofacies are defined from the generic relative abundance data: Ehmaniella, Olenoides–Bolaspidella, Blairella, Eldoradia, Modocia–Paracedaria, Cedaria, Coosella–Coosina, and Camaraspis. Trilobites collected and identified in this study are assigned to five biostratigraphic zones: Bolaspidella, Cedaria, Crepicephalus, Aphelaspis, and Elvinia zones. In addition, two subzones had been defined. Cedaria eurycheilos Subzone recognized in the upper part of Cedaria Zone and Coosella helena Subzone recognized in the upper part of Crepicephalus Zone. The stratigraphic succession was divided into six distinct phases associated with large-scale relative sea-level fluctuations. An initial flooding over the Bolsa Quartzite forming the transgressive systems tract was terminated by maximum flooding, and a subsequent highstand systems tract developed during Bolaspidella Biozone time. The second sequence starts with another transgressive systems tract, and is overlain by a final highstand systems tract during the Cedaria and Crepicephalus biozones. The uppermost part of the second sequence represents a falling stage systems tract that developed during Aphelaspis Biozone time. The presence of Elvinia Biozone trilobites near the base of the highest sandstone unit suggests that delivery and deposition of these sands took place during the lowstand that followed the protracted and widespread Sauk II–Sauk III hiatus. Sedimentary dynamics were controlled by storm-induced wave action and offshore flows. There are two carbonate factories that operated simultaneously in this Cambrian inner shelf region. Dominance of carbonate versus siliciclastic strata in the offshore transition setting is interpreted to reflect periods when siliciclastic input was depleted, such that increasing accommodation and reduction of clay and possibly nutrients promoted carbonate production. Clay and silt bypassed the nearshore carbonate-depositing zone. Siliciclastic sediment input and dispersal were not only restricted to the falls in sea level, but appear to have dominated the transgressive systems tract and late phase of the highstand. Thus, carbonate sedimentation does not dominate the entire highstand systems tract as is commonly held but, rather, only during the late phase of the transgressive and early highstand phase. The comparison of this Cambrian model with younger mixed carbonate-siliciclastic units will help reveal the subtleties of the carbonate factory and how it operated in response to biotic evolution.

The Middle Jurassic Entrada Sandstone of the Four Corners region, USA, is composed predominantly of very fine-grained, red, silty sandstone with poorly defined sedimentary structures. The origin of this facies is enigmatic, even though it is common both on the Colorado Plateau and globally, and is spatially situated between deposits recording unambiguous marine and aeolian environments. Eleven sections were measured along an 85 km transect from the Blanding Basin in southeastern Utah to the San Juan Basin in northwestern New Mexico. Outcrop and laboratory analyses distinguish eight facies: (1) silty shale, (2) shallow subaqueous reworked, fine- to medium-grained sandstone, (3) brecciated, very fine-grained sandstone, (4) crinkly laminated, very fine-grained sandstone with preserved wind ripples and abundant silty laminae, (5) weakly laminated, fine-grained sandstone with occasional silty laminae, (6) planar-laminated, fine-grained, wind-rippled sandstone, (7) cross-stratified, fine- to medium-grained aeolian cross-stratified sandstone, and (8) micritic limestone. Lateral and vertical relationships of these facies show a proximal to distal transition from cross-bedded wind-lain facies to loess-dominated sabkha facies with increasing abundance of water-lain facies basinward. The well known Todilto Limestone (facies 8) is situated stratigraphically below loess-dominated sabkha facies (facies 4 and 5) within the Entrada Sandstone, reinforcing previous interpretations that the unit represents a catastrophic flooding event and not a local groundwater flux. / text

The study area is situated along the obliquely converging boundary of the Caribbean and South American plates offshore eastern offshore Trinidad. Major structural elements in the shelf break and deep-water slope regions include normal and counter-normal faults to the south and large transpressional fault zones to the north. Well logs and biostratigraphic information were analyzed for twenty-four wells in the study area to refine previous depositional environment interpretations. For purposes of this net sand distribution analysis it was decided to consider the deltaic portion of the shelf transit cycle, against the marine portion of the shelf transit cycle and were named T and R cycles, respectively. T and R cycles were interpreted based on well log patterns and depositional facies shifts. Six T/R cycles were interpreted within the Pliocene to recent stratigraphic succession and shelf edge trajectories were also mapped for each of these cycles based on earlier stratigraphic correlations. Net-to-gross (NTG) ratios were calculated for each component of the T/R cycles and plotted against total thicknesses and net sand values. In addition, NTG trends were mapped for each interval and analyzed based on their proximity to the corresponding shelf edge. Mapping of the shelf edge trajectories (SET) revealed that (1) SET migrate northeasterly across the Columbus Basin through time and (2) shelf edge orientations are parallel to the strike of growth faults in the south but deflect to the northeast near the Darien Ridge indicating a strong underlying structural control. The NTG plots and maps also revealed that (1) For T cycles, NTG values never exceed 60% and are inversely proportional to total thickness, (2) For R cycles, NTG values are highly variably ranging from 35% to 90%, (3) NTG values increase as the shelf break is approached and (4) The distribution of NTG ratios is also controlled by accommodation space created by local structures. The Guiana current is believed to play an important role in the redistribution and reworking of sand in the Columbus Basin. Aggradation and progradation distances were computed for each interval and the results suggest that the younger Sequences C2 (T-R cycle E) and C3 (T-R cycle F) show a stronger progradational trend than the older C4, C5 and C6. This strong progradational trend might indicate delivery of sand basinwards, while for the older intervals; the aggradational trend suggests an increase in sediment storage. In long-term scale (1-2 m.y.) the Orinoco Delta seems to behave as an aggradational delta that increases sediment storage due to growth fault and high subsidence rates. However, in the short-term scale, the Orinoco delta seems to behave as a rapid progradational delta, for the younger sequences C2 and C3, where sediment bypass is more likely to occur; and as a rapid aggradational (slow prograding) margin for the older intervals C4, C5 and C6. / text

The Late Jurassic Arab D Formation in Saudi Arabia hosts the some of the world’s largest hydrocarbon reservoirs including Ghawar, the world’s largest oil field, and Khurais, the world’s largest supergiant to come into production in the last 5 years. Despite the vast oil reserves within the Arab D, and the central role of this reservoir at Ghawar in making up short-falls in global production, our understanding of the much fundamental characterization work both in terms of modern sequence stratigraphic reservoir frameworks and linked structural/fracture characterization. This study of Arab D reservoir at Qatif, immediately to the north of Ghawar, provides one of the first looks at a modern sequence analysis of this producing interval and illustrates that porosity zonations, and ultimately flow unit architecture may be substantially different than currently in use. The Arab D of the Arabian Plate is a carbonate ramp system of exceedingly low angle (<1o) developed during a low-eustatic-amplitude greenhouse Milankovitch setting. Combined macroscopic and petrographic data analysis allowed recognition of nine depositional facies: 1) spiculitic wackestone, 2) Planolites-burrowed wackestone, 3) bioturbated skeletal-peloidal packstone, 4) pelletal packstone, 5) peloidal-skeletal grain dominated packstone, 6) peloidal-skeletal grainstone, 7) skeletal-ooids grainstone, 8) cryptalgal laminites and 9) anhydrite. The depositional facies defined are used to interpret three facies tracts from deep to shallow across the ramp profile: 1) low energy sub-storm wave base (SWB) dominated facies that may illustrate disaerobic tendencies, 2) high energy within-fair-weather-wave-base ramp-crest or mid-ramp facies including foreshore and upper shoreface oolitic and skeletal grainstones that define one of the key reservoir pay zones and 3) back-barrier tidal flats consisting of cryptalgal laminites, sabkha-type anhydrites, and salina-type anhydrites. Three high frequency sequences are defined: QSEQ 1 is asymmetrical, dominated by subtidal lithofacies; and QSEQ 2 and QSEQ 3 are symmetrical and record a complex history of the fill on an intrashelf basin. Detailed cycle-scale correlations using core-based cycles and wireline log patterns allowed a cycle-scale correlation framework to be established that illustrates a north to south progradation of the Arab D reservoir strata, building landward from the Rimthan Arch. Diagenetic features observed in the Arab D reservoir include fitted fabric (chemical compaction), dolomitization, and cementation. These features play a major role altering reservoir quality properties as they essentially control fluid flow pathways which ultimately alter primary porosity and permeability. / text

I have compiled digital seismic and well data over a region of approximately 700,000 km² to better improve the correlation of the Mexican sector of the Gulf of Mexico (MGOM) with the better studied and more explored U.S. sector. I have ~25,000 km of regional 2D lines that were collected by the University of Texas in the 1970's. I have digitized data from published PEMEX data from the MGOM using SEG-Y converter software and incorporated these data into my seismic grid. Using these data, I interpreted and correlated 20 surfaces that range in age from Late Jurassic to Recent. The combined shelf-slope-basin dataset from the MGOM allows for correlation of units from the deepwater MGOM, across into the Mexican Ridges passive margin foldbelt, and onto the Mexican shelf. I have also incorporated seismic data from the offshore Chicxulub crater and correlated units in the Yucatan platform area with the deepwater MGOM. This regional data set indicates that normal, growth faulting linked with downdip toe thrusts and folds of the Mexican Ridges initiated in post-Middle Miocene time and are therefore unrelated to the earlier Paleogene Laramide uplift deformation phase. Shelf-slope-deep basin seismic facies of Eocene and Oligocene units show an influx of clastic materials linked with regional uplift and volcanic events affecting central Mexico during this period. I propose that the deepwater folds of the Mexican Ridges accompanied shelf-edge gravity sliding and normal faulting activated during accelerated Oligo-Miocene uplift, regional volcanic activity, and erosion of the Mexican landmass. Downdip sliding occurred on the seaward-dipping top Cretaceous carbonate unit (7° to 13°) along with overlying horizons that range in dip from 1° to 2°. Shelf-slope-deep basin seismic facies of the Paleocene units around the Yucatan peninsula suggest a sediment-starved and slide-free carbonate margin with a current basinward dip of approximately 12° and significantly greater than those dips observed along the present-day eastern Gulf of Mexico. Based on the seismic interpretations and plate reconstructions, I propose four major tectonosequences fill the Gulf of Mexico basin: 1) A Late Jurassic to Late Cretaceous passive margin phase; 2) a Late Cretaceous to Late Eocene Laramide deformational phase; 3) a Late Eocene to Middle Miocene passive margin phase; and 4) a Late Miocene to Recent Neogene deformational phase. / text

The Llanos foreland basin of Colombia is located along the eastern margin of the northern Andes. The Llanos basin is bounded to the north by the Mérida Andes, to the east by the Guiana shield, to the south by the Serrania de la Macarena, and to the west by the frontal foothills thrust system of the Andes (the Cordillera Oriental). The Llanos foreland basin originated in the Maastrichtian, after a post-rift period during the Mesozoic, and recorded an abrupt pulse of middle Miocene subsidence possibly in response to subduction and collision events along the Pacific margin of northwestern South America. Regional east-west shortening, driven in part by collision of the Panama arc along the Pacific margin of Colombia, has built the widest part of the northern Andes. This wide area (~600 km) includes a prominent arcuate thrust salient, the Cordillera Oriental, which overthrusts the Llanos foreland along a broad V-shaped salient that projects 40 km over the northern Llanos foreland basin. In this study, I interpret 1200 km of 2D seismic data tied to 18 wells and regional potential fields (gravity and magnetic) data. Interpreted seismic data are organized into four regional (300 to 400-km-long) transects spanning the thrust salient area of the northern Llanos basin. I performed 2D flexural modeling on the four transects in order to understand the relative contributions of flexural subsidence due to tectonic and sedimentary loading. Sedimentary backstripping was applied to the observed structure maps of six Eocene to Pleistocene interpreted horizons in the foreland basin in order to remove the effects of sedimentary and water loading. Regional subsidence curves show an increase in the rate of tectonic subsidence in the thrust salient sector of the foreland basin during the middle to late Miocene. The flexural models predict changes in the middle Miocene to recent position of the eastern limit of foreland basin sediments as well as the changing location and vertical relief of the flexurally controlled forebulge. Production areas of light oil in the thrust belt and foreland basin are located either south of the thrust salient (Cusiana, Castilla, Rubiales oilfields) or north of the salient (Guafita-Caño Limon, Arauca oilfields) but not directly adjacent to the salient apex where subsidence, source rock thicknesses, and fracturing were predicted by a previous study to be most favorable for hydrocarbons. There are no reported light oil accumulations focused on the predicted present or past positions of the forebulge, but detailed comparisons of seismic reflection data with model predictions may reveal stratigraphic onlap and/or wedging relationships that could provide possible traps for hydrocarbons. / text

The uplift history of the Tibetan Plateau is poorly constrained in part due to its complex and extended tectonic history. This study uses basin analysis, stable isotope analysis, magnetostratigraphy, detrital zircon U-Pb dating, and paleoaltimetry, and frequency analysis to reconstruct the tectonic, spatial, and environmental evolution of the Zhada basin in southwestern Tibet since the late Miocene. The Zhada Formation, which occupies the Zhada basin and consists of ~ 850 m of fluvial, alluvial fan, eolian, and lacustrine sediments, is undeformed and lies in angular unconformity above Tethyan sedimentary sequence strata. The most negative Miocene δ¹⁸Opsw (paleo-surface water) values reconstructed from aquatic gastropods are significantly more negative than the most negative modern δ¹⁸O(sw) (surface water) values. In the absence of any known climate change which would have produced this difference, we interpret it as indicating a decrease in elevation in the catchment between the late Miocene and the present. Basin analysis indicates that the decrease in elevation was accomplished by two low-angle detachment faults which root beneath the Zhada basin and exhume mid-crustal rocks. This exhumation results from ongoing arc-parallel extension and provides accommodation for Zhada basin fill. Sequence stratigraphy shows that the basin evolved from an overfilled to an underfilled basin but that further evolution was truncated by an abrupt return to overfilled, incising conditions. This evolution is linked to progressive damming of the paleo-Sutlej River. During the underfilled portion of basin evolution, depositional environments were strongly influenced by Milancovitch cyclicity: particularly at the precession and eccentricity frequencies.

Cette étude vise a mettre en évidence l'influence des variations eustatiques sur la distribution de matière organique (MO) particulaire. Dans ce but, le contenu organique des dépôts berriasiens des Bassins Vocontien et Ultrahelvétique (domaine téthysien) et du Bassin du Yorkshire (domaine boréal) est étudié ici. Le Bassin Vocontien (sud-est de la France) : Trois coupes de sédiments carbonatés profonds, d'âge Tithonique supérieur - Berriasien, dont le stratotype du Berriasien à Berrias, ont été étudiées. Malgré un faciès monotone, elles ont été choisies pour leur contenu faunistique (ammonites et calpionelles) permettant un découpage biostratigraphique précis. Si l'étude de la MO particulaire (palynofaciés) a démontré des résultats probants dans les sédiments sil ici clastiques, les connaissances actuelles sont, en revanche, très restreintes dans les carbonates. L'origine (marine ou continentale), les propriétés sédimentologiques (taille, tri, habitus) et le degré de biodégradation des constituants organiques ainsi que le contenu en microfossiles organiques (abondance, diversité, paléoécologie) permettent d'identifier des intervalles régressifs et des intervalles transgressifs. Combinée aux données sédimentologiques et biostratigraphiques, l'étude de la MO permet de définir huit séquences (au sens de la stratigraphie séquentielle) dans les dépôt berriasiens du sud-est de la France. Les surfaces de discontinuités sont caractérisées par les tendances suivantes du palynofaciès: limites de séquence - apparition de débris ligneux de grande taille, fréquemment dégradés, et diminution abrupte de la proportion de plancton (abondance et diversité des kystes de dinoflagellés), surfaces supérieures du bas niveau - brusque augmentation de plancton (abondance et diversité des kystes de dinoflagellés) et de débris ligneux aciculaires, surfaces d'inondation maximale - pic de plancton (abondance et diversité des kystes de dinoflagellés) et de débris ligneux aciculaires. En raison de divergences entre deux outils biostratigraphiques (kystes de dinoflagellés et calpionelles), un doute subsiste sur la corrélation des séquences du Berriasien supérieur. Le Bassin ultrarahelvétique (Suisse): La coupe de Villarbeney, constituée d'alternances marno-calcaires, datée du Berriasien moyen à supérieur a été étudiée. La concordance des arguments de terrain et des résultats de l'étude du palynofaciès (basée sur la même approche que dans le sud-est de la France) permet d'établir un découpage séquentiel, corrélable avec le Basin Vocontien. Cela confirme aussi les tendances dans la distribution de la MO observées dans le Bassin Vocontien. Le Bassin du Yorkshire (Angleterre): Une discontinuité majeure caractérise la limite Jurassique - Crétacé dans le Bassin du Yorkshire (discontinuité tardi-cimmérienne) et seule la partie supérieure du Berriasien affleure. Deux coupes ont été étudiées dans la Formation de la "Speeton Clay". Elles consistent en des argilites sombres, riches en MO. L'interprétation séquentielle proposée est dérivée des résultats d'analyses optiques (le palynofaciès) et physico-chimiques (la pyrolyse "Rock-Eval" qui permet de définir, entre autres, la quantité et le type de MO: du type l, dominé par du matériel algaire, au type III, riche en phytoclastes). La corrélation séquentielle avec le Bassin Vocontien est seulement suggérée, l'absence de contrôle biostratigraphique ne permettant pas de la confirmer. Cette étude démontre le potentiel de l'analyse de la MO, comme outil complémentaire aux méthodes de terrain et de laboratoires classiques, en stratigraphie séquentielle.