Paleo-water Depth Research Articles

Geochemistry and depositional history of the Union Springs Member, Marcellus Formation in central Pennsylvania

Debate continues over paleoenvironmental conditions that prevail during deposition of organic-carbon (C)-rich marine source rocks in foreland basins and epicontinental seas. The focus of disagreement centers largely on paleowater depth and the prevalence of anoxia/euxinia. The issues of paleodepth and water column conditions are important for prediction of lateral variations in source quality within a basin because the viability of a hydrocarbon play depends on a thorough understanding of the distribution of source rock quality and depositional environments. We used inorganic geochemical data from the Middle Devonian Marcellus Shale in the Appalachian Basin to illustrate interpretive strategies that provided constraints on conditions during deposition. Source evaluation typically relies on the analysis and interpretation of organic geochemical indicators, potentially also providing evidence of the degree of thermal maturity and conditions of the preservation of the organic matter. The Marcellus Formation is thermally mature, making the evaluation of the organic-carbon fraction for geologic interpretation inadequate. Because most labile organic matter has largely been destroyed in the Marcellus Formation, analysis of inorganic elements may be used as an alternative interpretative technique. Several inorganic elements have been correlated to varying depositional settings, allowing for their use as proxies for understanding the paleodepositional environments of formations. A high-resolution geochemical data set has been constructed for the Union Springs Member along a transect of cores from proximal to distal in the Appalachian Basin in central Pennsylvania using major, minor, and trace elemental data. Our results suggested that during deposition, the sediment-water interface, and a portion of the water column, was anoxic to euxinic. As deposition continued, euxinia was periodically interrupted by dysoxia and even oxic conditions, and a greater influx of clastic material occurred. Such variations were likely related to fluctuations in water depth and progradation of deltaic complexes from the eastern margin of the Appalachian Basin.

Temporal and Spatial Variations of Accommodation and Sediment Accumulation during Transgressive to Highstand Stages as Reconstructed from a Latest Pleistocene to Holocene Sequence in the Intra-Arc Osaka Basin, Japan

Temporal and spatial variations in accommodation (i.e., paleo-water depth) and sediment accumulation (amount of deposition) in the intra-arc Osaka Basin, Japan, were reconstructed from the post-glacial transgression through the sea-level highstand, a total of 9000 years. At the beginning of the marine transgressive stage (about 11,000 cal y BP), paleo-water depths were shallow and the sediment accumulation was large. The area occupied by Osaka Bay gradually extended and sediment deposition decreased from 11,000 to 6000 cal y BP. During the period of maximum transgression (6000 - 5000 cal y BP), an inner bay, Kawachi Bay with a water depth of 5 - 10 m, was expanded in the inland eastern Osaka area, and paleo-water depths reached a maximum and depositional rates reached a minimum. During the subsequent highstand and small regression (about 5000 cal y BP to the present), however, deposition increased rapidly as a result of river delta and shoreline progradations. Regional differences were observed in accommodation and accumulation between the outer bay area and the inner bay area. During both the transgressive and regressive stages, deposition decreased in the inner bay area. In contrast, in the outer bay area and in the basin overall, deposition was high during the first part of the transgressive stage but it decreased during the maximum transgression, before reaching a maximum during the subsequent highstand and regression. During the regressive stage, fluvial delta progradation led to the formation of a thick sequence of delta body sediments. Sediment accumulation was 30% - 40% higher during the regressive stage than that during the transgressive stage.

The Cryogenian Balcanoona reef complexes of the Northern Flinders Ranges: Implications for Neoproterozoic ocean chemistry

The Cryogenian Balcanoona reef complexes of South Australia consist of several dolomite platforms with well-defined backreef, reef margin and slope/basinal facies. The mature platforms have prograded large distances into the basin (>15km) and have high relief margins (over 1km of relief at the platform margin). Backreef facies are dominantly ooidal–peloidal grainstones with abundant tepees and marine cemented sheet cavities. The reef margin facies consists of upper stromatolitic boundstones and, where present, deeper water non-stromatolitic chambered and clotted boundstones. The deepwater chambered and clotted boundstones grew on near vertical escarpments below 150m paleo-water depths and consist of a complex array of marine cemented cavernous and calcimicrobial material. The strongly prograding platforms range from accretionary low-angle stromatolitic boundstone margins, through to more mature escarpment margins. The mature escarpment margins (>200m high escarpment) are characterized by the development of both stromatolitic and non-stromatolitic boundstones. The slope/basin facies of the mature escarpment margins consists of coarse debris deposits (talus, debrites and allochthonous blocks) proximal to the platforms and calcareous/dolomitic mudrocks distal to the platforms. The development of an accretionary escarpment margin, together with the large proportion of deepwater boundstone suggests that these Cryogenian reefs were fundamentally different from Phanerozoic reefal systems. The enormous volume of deepwater boundstone in these reefs may be due to a severely anoxic water column that prevailed during this period of the Cryogenian. Anoxia led to greater degrees of carbonate saturation at depth that enabled the development of deepwater boundstones with escarpment margins. A community of anoxic-adapted non-photosynthetic organisms that were perhaps similar in their ecology to Phanerozoic coelobiontic reefal communities may have constructed the enigmatic deepwater boundstone.Just prior to the final termination of the reef, the system was subject to large-scale erosion and margin collapse, producing a major regional unconformity. Following this, the reefs briefly re-grew, before final and complete cessation of the reef complexes. The large-scale margin collapse event may have been triggered by sea level fluctuation, and/or changes in ocean chemistry associated with Late Cryogenian oxygenation.

Discovery of Bioclastic Dolostone in the Second Member of Shahejie Formation in Bozhong Depression

Reservoir quality is the key for the petroleum exploration for middle-deep formation in the Bozhong Depression because the middle-deep reservoir is with the characteristics of big burial depth and poor physical property which is difficult to predict, but the physical property of dolostone reservoir (includes bioclastic dolostone and terrigenous clastic dolostone) is significant better than that of the clastic reservoir in the same depth in the second member of Shahejie Formation of QHD36-3 oil field, which could be one of the frontier for high production reservoir exploration. The paper concludes that the development of dolostone is mainly controlled by paleogeomorphology and paleo water depth as well as the distance from the source area. Building up the related sedimentary pattern is good for the exploration aiming at bioclasti dolostone and terrigenous clastic dolostone reservoir.

Effect of Some Input Parameters on 3D Basin and Petroleum Systems Modelling: A Case Study of the Norwegian Section of the Northern North Sea

The objective of this study is to test the influence of some key input parameters in basin modelling and to evaluate the resultant effect of varying these parameters on the model. 3D basin modelling and petroleum system analysis of the northern North Sea has been carried out using the PETROMOD software. The model was calibrated using well 34/8-7 in the Visund field. Different input parameters such as heat flow, source rock properties, fault properties, paleo water depth, source rock kinetics, migration methods and different erosion scenarios have been varied and their effects on the model assessed. The effect of the various input parameters has been assessed in terms of hydrocarbon volumes in the Kvitebjorn and Visund fields, source rock maturity, transformation ratios, hydrocarbon saturations and the time hydrocarbon generation began in the Draupne and Heather Formation source rocks. Increase in heat flow increases source rock maturity, the start of hydrocarbon generation, transformation ratios and results in the generation of a lot more gas than oil. Hydrocarbon generation starts at shallower depths with higher heat flow. Increasing Total Organic Content (TOC) and Hydrogen Index (HI) generally results in increase in the volume of hydrocarbons generated. The increase in HI, however, results in the generation of a lot more oil than gas. High TOC and HI also increase the hydrocarbon saturations in the source rock. Increasing Paleo Water Depth (PWD) has a marginal effect on the model. It increases the volume of oil and decreases the volume of gas marginally. Varying the PWD has no significant effect on source rock maturity, transformation ratios and hydrocarbon saturations. Opening the fault planes resulted in an increase in the volume of hydrocarbons generated. The increase was more evident in the volume of oil than gas. This increase in volumes is a consequence of additional migration pathways created by the faults. Varying the erosion thickness of the Draupne Formation did not have any effect on the model.

Sedimentary hydrodynamic study of sand bodies in the upper subsection of the 4th Member of the Paleogene Shahejie Formation the eastern Dongying Depression, China

Petroleum is produced from a beach-bar sand reservoir in the upper subsection of the 4th member of the Paleogene Shahejie Formation (Es4s) in the eastern Dongying Depression, penetrated by many wells in the Guangli-Qingnan area and this subsection still has further exploration potential. Using drilling and logging data, we analyzed the sand body types, emphasizing the sedimentary characteristics of the beach-bar sand bodies. Combining these data with the concepts of lacustrine and oceanic hydrodynamics, we explain the formation and distribution of the beach-bar sands in the eastern Dongying Depression. The connectivity between beach-bar sand bodies within each individual hydrodynamic zone is better than that between sand bodies in any two adjacent zones. The tempestite sand bodies developed in this area are characterized by typical storm deposits and are located at the bottoms of the beach-bar sand bodies. They grade upward to normal shore and shallow-lacustrine beaches and bars. We also propose a new simple method to estimate the paleo-water depth using the thicknesses of the bar sand bodies in parasequences combined with the principle of hydrodynamic zonation. Based on the distribution of the beach-bar sands in parasequence set 3, we infer that the paleo-wind direction was from the north with an average paleo-wind force of 6 when the major beach-bar sand bodies formed.

Variability in Cenozoic sedimentation and paleo-water depths of the Weddell Sea basin related to pre-glacial and glacial conditions of Antarctica

The Weddell Sea basin is of particular significance for understanding climate processes, including the generation of ocean water masses and their influence on ocean circulation as well as the dynamics of the Antarctic ice sheets. The sedimentary record, preserved below the basin floor, serves as an archive of the pre-glacial to glacial development of these processes, which were accompanied by tectonic processes in its early glacial phase. Three multichannel seismic reflection transects, in total nearly 5000km long, are used to interpret horizons and define a seismostratigraphic model for the basin. We expand this initial stratigraphic model to the greater Weddell Sea region through a network of more than 200 additional seismic lines. Information from few boreholes is used to constrain sediment ages in this stratigraphy, supported by magnetic anomalies indicating decreasing oceanic basement ages from southeast to northwest. Using these constraints, we calculate grids to depict the depths, thicknesses and sedimentation rates of pre-glacial (145–34Ma), transitional (34–15Ma) and full-glacial (15Ma to present) units. These grids allow us to compare the sedimentary regimes of the pre-glacially dominated and glacially dominated stages of Weddell Sea history. The pre-glacial deposition with thicknesses of up to 5km was controlled by the tectonic evolution and sea-floor spreading history interacting with terrigenous sediment supply. The transitional unit shows a relatively high sedimentation rate and has thicknesses of up to 3km, which may attribute to an early formation of the East Antarctic Ice Sheet, which was partly advanced to the coast or even inner shelf. The main deposition center of the full-glacial unit lies in front of the Filchner-Ronne Ice Shelf and has sedimentation rates of up to 140–200m/Myr, which infers that ice sheets grounded on the middle to the outer shelf and that bottom-water currents strongly impacted the sedimentation. We further calculate paleobathymetric grids at 15Ma, 34Ma, and 120Ma by using a backstripping technique. Our results provide constraints for an improved understanding of the paleo-ice sheet dynamics and paleoclimate conditions of the Weddell Sea region.

Sequence stratigraphy and depositional environments of the Shamokin (Union Springs) Member, Marcellus Formation, and associated strata in the middle Appalachian Basin

Organic-carbon–rich shales of the lower Marcellus Formation were deposited at the toe and basinward of a prograding clinothem associated with a Mahantango Formation delta complex centered near Harrisburg, Pennsylvania. Distribution of these organic-carbon–rich shales was influenced by shifts in the delta complex driven by changes in rates of accommodation creation and by a topographically high carbonate bank that formed along the Findlay-Algonquin arch during deposition of the Onondaga Formation. Specifically, we interpret the Union Springs member (Shamokin Member of the Marcellus Formation) and the Onondaga Formation as comprising a single third-order depositional sequence. The Onondaga Formation was deposited in the lowstand to transgressive systems tract, and the Union Springs member was deposited in the transgressive, highstand, and falling-stage systems tract. The regional extent of parasequences, systems tracts, and the interpreted depositional sequence suggest that base-level fluctuations were primarily caused by allogenic forcing—eustasy, climate, or regional thermal uplift or subsidence—instead of basement fault reactivation as argued by previous workers. Paleowater depths in the region of Marcellus Formation black mudrock accumulation were at least 330 ft (100 m) as estimated by differences in strata thickness between the northwestern carbonate bank and basinal facies to the southeast. Geochemical analysis indicates anoxic to euxinic bottom-water conditions. These conditions were supported by a deep, stratified basin with a lack of circulation.

Cenozoic tectonic subsidence in deepwater sags in the Pearl River Mouth Basin, northern South China Sea

The Cenozoic tectonic subsidence of the deepwater area in the Pearl River Mouth Basin, northern South China Sea is studied by subsidence analysis via backstripping calculations based on data of newly interpreted sequence boundaries. In the subsidence analysis local porosity–depth parameters were estimated based on well data, the paleo-water depth parameters and lithology were estimated for all grid nodes based on well data and sedimentary system maps, and ages of sequence boundaries were adjusted according to the International Chronostratigraphic Chart v2013/01. Sensitivity analysis shows that these are essential to ensure the quality of the subsidence analysis in the slope areas. Maps of tectonic subsidence rates of 18 Cenozoic sequences were constructed and spatial–temporal variations of the tectonic subsidence were discussed. The subsidence was restricted in the Baiyun and Liwan Sags before 30 Ma and extended to uplifts after 30 Ma. This indicates that the 30 Ma unconformity is the breakup unconformity that separated the syn-rift and post-rift sequences. Four substages of post-rift subsidence were identified by cyclic changes in the rate and pattern of tectonic subsidence at boundaries of 23.3 Ma, 17.2 Ma and 11.9 Ma, respectively. Anomalous post-rifting tectonic subsidence was significant and totaled to ~ 1200 m at the center of the Baiyun Sag, and punctuated by rapid subsidence and uplift events. The Baiyun Event was manifested in the study area by a short-lived uplift event in 23.9–23.3 Ma followed by a short-lived rapid subsidence in 23.3–19.8 Ma in the central study area. This localized rapid subsidence caused northward jump and anti-clockwise rotation of the shelf break. Two-order teeterboard (seesaw)-like subsidence was observed in the study area in the post-rifting stage, suggesting elastic deformations of the crust and lithosphere. • Tectonic subsidence of deepwater sags estimated via backstripping on newest data • Maps and histograms of subsidence rate for 18 Cenozoic sequences are constructed. • Results indicate that the 30 Ma unconformity is the breakup unconformity. • Cyclic anomalous post-rift subsidence of ~ 1200 m confirmed for central Baiyun Sag • Baiyun event near Oligoc./Mioc. boundary is evidenced by rapid local subsidence.

Abyssal benthic foraminifera in the eastern equatorial Pacific (IODP EXP 320) during the middle Eocene

We report on the faunal transition of benthic foraminifera during the middle Eocene at Site U1333 (4862 m water depth, 3,560–3,720 m paleo-water depth) of Integrated Ocean Drilling Program Expedition 320 in the eastern equatorial Pacific Ocean. During the period ∼41.5–40.7 Ma, which includes carbonate accumulation event 3 (CAE-3), the benthic foraminiferal accumulation rate (BFAR) increased gradually and then it declined rapidly. In contrast, BFAR was considerably lower during ∼40.7–39.4 Ma, corresponding to the middle Eocene climatic optimum (MECO), and then it increased during ∼39.3–38.4 Ma, including CAE-4. Diversity (E [S200]) was slightly lower in the upper part of the study interval than in the lower part. The most common benthic foraminifera were Nuttallides truempyi, Oridorsalis umbonatus, and Gyroidinoides spp. in association with Globocassidulina globosa and Cibicidoides grimsdalei during the period studied. Quadrimorphina profunda occurred abundantly with N. truempyi, O. umbonatus, and G. globosa during ∼39.4–38.4 Ma, including CAE-4, although this species was also relatively common in the lower part of the study interval. Virgulinopsis navarroanus and Fursenkoina sp. A, morphologically infaunal taxa, were common during ∼38.8–38.4 Ma, corresponding to the late stage of CAE-4. Based on Q-mode cluster analysis, four sample clusters were recognized and their stratigraphic distributions were generally discriminated in the lower and upper parts of the study interval. Thus, there was only a small faunal transition in the abyssal eastern equatorial Pacific during the middle to late-middle Eocene. The faunal transition recognized in this study may be related to recovery processes following intense carbonate corrosiveness in the eastern equatorial Pacific during MECO.

Paleobathymetry and sequence stratigraphic interpretations from benthic foraminifera: Insights on New Jersey shelf architecture, IODP Expedition 313

Integrated Ocean Drilling Program (IODP) Expedition 313 drilled three holes (Sites M27, M28, and M29; 34–36 m present water depth) across a series of prograding clinothems from the inner continental shelf of the New Jersey (USA) margin, a region that is sensitive to sea-level change. We examined 702 late Eocene to Miocene samples for benthic foraminiferal assemblages and planktonic foraminiferal abundances. We integrate our results with lithofacies to reconstruct paleobathymetry. Biofacies at all three sites indicate a long-term shallowing-upward trend as clinothems built seaward and sediment filled accommodation space. Patterns in biofacies and lithofacies indicate shallowing- and deepening-upward successions within individual sequences, providing the basis to recognize systems tracts, and therefore sequence stratigraphic relationships in early to early-middle Miocene sequences (ca. 23–13 Ma). The clinothem bottomsets and the lower portions of the foresets, which contain the thickest parts of clinothems, yield the deepest water biofacies. Shallower bio facies characterize the sequences in the upper portions of the clinothem foresets and on the topsets. Topsets are characterized by transgressive (TST) and highstand systems tracts (HST). Foresets contain lowstand systems tracts (LST), TSTs, and HSTs. Flooding surfaces mark parasequence boundaries within LSTs, TSTs, and HSTs. Superimposed on the long-term trends, short-term variations in paleowater depth are likely linked to global sea-level changes indicated by global oxygen isotopic variations.

Mesozoic Syn- and Postrifting Evolution of the Central Apennines, Italy: The Role of Triassic Evaporites

AbstractJurassic–Cretaceous syn- and postrift successions from the central Apennines were backstripped to gain information on the Mesozoic evolution of the passive margin of the Adriatic Plate. Early Jurassic rifting led to the development of a horst-and-graben paleogeography (the Latium-Abruzzi Carbonate Platform and the Sabina-Umbria-Marche Pelagic Basin). Subsidence curves were built for both carbonate platform and pelagic-basin domains from original and literature stratigraphic data. The paleodepositional depths of the deepwater sediments were reconstructed from field geology data, including new paleontological data. It is proposed that after the deposition of lower Hettangian shallow-water carbonates, an abrupt increase in paleowater depth, to 600–1000 m, occurred during the late Hettangian–Sinemurian synrift stage. The postrift stage was characterized by basin filling, with decreasing paleowater depths during the Jurassic, and by a new deepening during the Cretaceous. Our backstripping curves show, ...

Impact-Induced Sediment Deposition On An Offshore, Mud-Substrate Continental Shelf, Cretaceous-Paleogene Boundary, Brazos River, Texas, U.S.A

Abstract Data from twenty Brazos River stratigraphic sections of the K–Pg event deposit are compiled to provide a record of changing depositional processes on the Texas continental shelf that left a record of disturbance triggered by the Chicxulub impact. Foraminiferal paleobathymetric depth control of enclosing sediments indicates paleo–water depths of 75–125 m with gradual deepening above the K–Pg boundary. Outcrop-scale observations indicate that the most complete deposits have a basal unit of mudstone clast-bearing mass flows indicative of movement by noncohesive laminar flow. These mudflows were mobilized by impact-generated seismic shaking of seafloor sediment. The material of the mass flows consists of reworked shell-rich mud matrix containing outsize clasts of consolidated mudstone and cemented concretions. The upper parts of the K–Pg event deposit consist of units of high-energy upward-fining sandstones, each with a basal lag layer of spherule-rich coarse sandstone lag overlain by poorly or moderately sorted hummocky cross-bedded sandstone. The last deposited sandstone is capped with a silt-mud suspension settling layer that as much as 10 cm thick where cemented with calcite, but is much thinner elsewhere. The provenance of these K–Pg Brazos River event deposit sediments is offshore marine muds (lower units) and shallow-water sands (higher units) with admixed impact spherules and no identifiable land-derived component. Deposition was the result of secondary processes activated by energy from seismic disturbance or by atmospheric disturbance, with no deposits directly attributable to tsunami wave control.

Late early Oligocene deep-sea benthic foraminifera and their faunal response to paleoceanographic changes in the eastern Equatorial Pacific

Abstract We documented new data on the late early Oligocene benthic foraminifera and sediment geochemistry in the eastern Equatorial Pacific Ocean (EEP) from Integrated Ocean Drilling Program (Exp. 320) Site U1334 (paleo-water depth: ~ 3650 to ~ 3850 m) with some published data on those from Ocean Drilling Program (Leg 199) Sites 1218 (~ 3850 to ~ 4000 m) and 1219 (~ 4270 to ~ 4370 m) in order to understand the response of abyssal benthic foraminifera to paleoceanographic conditions during the interval ~ 32–28 Ma. The relative abundances of Nuttallides umbonifer were distinctly different between Sites 1218 and 1219 during 31.0 to 29.9 Ma, although other major taxa (e.g., Globocassidulina subglobosa , Cibicidoides spp. and Oridorsalis umbonatus ) showed no marked changes. The characteristics (e.g., Shannon–Wiener (H′)) of the benthic foraminiferal fauna are affected by the abundance of a few dominant species rather than by the change in the number of species in the EEP. Based on multi-dimensional scaling (MDS), two MDS axes were recognized: MDS axis 1 (characteristic taxa: N. umbonifer and Cibicidoides sp. A) and MDS axis 2 (characteristic taxa: G. subglobosa and Bolivina huneri ). The tendency of both MDS axes (1 and 2) showed a distinct depth gradient among the three sites after 28.6 Ma, whereas the tendency was different from 31.0 to 28.6 Ma, especially at Site 1218. Increased abundance of N. umbonifer in the EEP after ~ 29 Ma was similar to that in the Atlantic Ocean after the expanded distribution of this species to shallower depths was finished. The faunal change at 28.6 Ma might be related to the change of deep water circulation, possibly with the influences of the Southern Component Water (SCW) and/or Northern Component Water in the South Atlantic Ocean. In contrast, the faunal change at 31.0 Ma might be related to the formation of the SCW and/or meridional overturning circulation in the Southern Ocean. In particular, MDS axis 1 exhibited the opposite tendency between Sites 1218 and 1219 around the Oi-2 event, whereas the depth gradient was clearly observed among the sites around the Oi-2* event. The opposite tendency of MDS axis 1 around the Oi-2 event might be explained by the varying influences of the SCW on different depths of the EEP, especially in terms of carbonate undersaturation. The influence of the SCW on benthic foraminifera in the EEP was different among the depths of the three sites in the Oi-2 event, however, the depth gradient of the SCW influence was gradually intensified in the Oi-2* event and the 28.6–28.0 Ma interval.

Progradation of the paleo-Danube shelf margin across the Pannonian Basin during the Late Miocene and Early Pliocene

The basin of giant Lake Pannon in Central Europe was filled by forward accretion of sediment packages during the Late Miocene and Early Pliocene. Successive positions of the shelf-margin are represented by a series of clinoforms in seismic profiles. The height of the clinoforms (and thus the inferred paleo water depth) is 200–600m in the successions; the width of the slope, measured from the shelf-break down to the toe of slope, varies between 5 and 15km. Geographical position of successive shelf-margin slopes indicates that about 2/3 of the basin area was filled by sediment transport systems supplying sediments from the NW, from the Alps and Western Carpathians. The first shelf-margin slope was built by the paleo-Danube in the Kisalföld/Danube sub-basin about 10Ma ago, and during the subsequent 6Ma it prograded ca. 400km to the SE across the Pannonian Basin, with an average of 67km/Ma slope advance. The most significant agent of this shelf growth was the sediment dispersal system of the paleo-Danube, hence we designate this northwestern shelf the paleo-Danube shelf. The northeastern part of Lake Pannon was filled by the paleo-Tisza system, supplying sediments from the Northeastern and Eastern Carpathians. Additional local systems carried sediments from E to W along the eastern margin and S to N along the southern margin of the Pannonian Basin, respectively. The deep-water environment disappeared from the Pannonian Basin and the endemic, brackish biota of Lake Pannon went extinct probably 4Ma ago, when the paleo-Danube shelf margin and a (yet unidentified) shelf margin prograding in the opposite direction met in the southeastern corner of the Pannonian Basin.

Biostratigraphy and depositional environment of the sediments in Borno Basin, North Eastern Nigeria

A biostratigraphic study was carried out on 83 drill cutting samples retrieved from Kemar-1 well, Borno Basin, northeastern Nigeria. They were subjected to palynological and foraminiferal analyses to determine their biozonation, age and depositional paleoenvironments. The studied intervals ranged between 600 and 1745 metres and were sampled at 30- and 20-metre intervals for the palynological and foraminiferal analyses respectively. Lithologically, the drill cutting samples are made up mainly of predominantly grey to dark grey shale, sandy mudstone and silty mudstone. An igneous rock with argillaceous sandstone and argillaceous siltstone intercalations was also observed. Three major palynological assemblage zones were recognised in the study area. They are Cretacaeiporites scabratus/mulleri Assemblage Zone, which correlates with the basal sequence, Droseridites senonicus Assemblage Zone, and Proteacidites dehaani/Dinogymnium euclaense Assemblage Zone of the well section’s upper part. Also, two foraminiferal biosequences were established for the studied interval. The environment of deposition of the well was generally shallow. It ranges from Coastal Deltaic in the upper part through Fluviomarine to Shallow Inner Neritic in the lower part of the Fika ‘Shale’ Formation. The age of the fossiliferous interval of the studied section of the well ranges from Early Turonian at the base to Early Campanian at the top. Three unconformities were observed and mapped within the studied section of the well at 790m, 1300m and 1700m. The unconformities were marked by noticeable faunal discontinuities, associated with reduction in paleowater depth and abrupt shift in the environment of deposition. Biostratigraphy is a valuable tool in oil and gas exploration when it is integrated with other geological and geophysical data sets, such as outcrops, seismic lines or well logs. This paper constitutes a significant contribution to the understanding of the biostratigraphy of Borno Basin, which is currently the focus of intensive hydrocarbon exploration activities. Keywords: Biostratigraphy, Depositional environments, Bornu Basin, Fika “Shale” Formation, Turonian

Biostratigraphy and Depositional Environment of the Sediments in Borno Basin, North Eastern Nigeria

A biostratigraphic study was carried out on 83 drill cutting samples retrieved from Kemar-1 well, Borno Basin, northeastern Nigeria. They were subjected to palynological and foraminiferal analyses to determine their biozonation, age and depositional paleoenvironments. The studied intervals ranged between 600 and 1745 metres and were sampled at 30- and 20-metre intervals for the palynological and foraminiferal analyses respectively. Lithologically, the drill cutting samples are made up mainly of predominantly grey to dark grey shale, sandy mudstone and silty mudstone. An igneous rock with argillaceous sandstone and argillaceous siltstone intercalations was also observed. Three major palynological assemblage zones were recognised in the study area. They are Cretacaeiporites scabratus/mulleri Assemblage Zone, which correlates with the basal sequence, Droseridites senonicus Assemblage Zone, and Proteacidites dehaani/Dinogymnium euclaense Assemblage Zone of the well section's upper part. Also, two foraminiferal biosequences were established for the studied interval. The environment of deposition of the well was generally shallow. It ranges from Coastal Deltaic in the upper part through Fluviomarine to Shallow Inner Neritic in the lower part of the Fika 'Shale' Formation. The age of the fossiliferous interval of the studied section of the well ranges from Early Turonian at the base to Early Campanian at the top. Three unconformities were observed and mapped within the studied section of the well at 790m, 1300m and 1700m. The unconformities were marked by noticeable faunal discontinuities, associated with reduction in paleowater depth and abrupt shift in the environment of deposition. Biostratigraphy is a valuable tool in oil and gas exploration when it is integrated with other geological and geophysical data sets, such as outcrops, seismic lines or well logs. This paper constitutes a significant contribution to the understanding of the biostratigraphy of Borno Basin, which is currently the focus of intensive hydrocarbon exploration activities.

Storm and fair-weather wave base: A relevant distinction?

Surface waves are an important mechanism for the redistribution of sediment on shallow marine shelves, and are commonly interpreted as comprising two distinct populations: fairweather waves and storm waves, the latter of which are generally thought to penetrate to greater water depths. Here we used >2.3 ! 10 6 spectral density estimates for the surface ocean collected between 1996 and 2008 from 32 buoys in the Caribbean, the Gulf of Mexico, and the western Atlantic to test the hypothesis that surface waves in the modern ocean comprise two size modes. Although distinct wave size classes occur in some individual measurements and over the time scales of some individual storms, time-averaged frequency distributions of wave size are unimodal. Thus, there is no empirical basis for presupposing a distinct bimodal separation in the size of fair-weather and storm waves, or in the manifestation of such differences in stratigraphic successions. Instead, there is a continuously increasing probability that a wave will reach the bottom with decreasing water depth and a separate probability that describes the hydrodynamic state of the sediment-water interface. Wave size does, however, exhibit signifi cant geographic bimodality. Locations in the relatively protected Gulf of Mexico and Caribbean regions have modal wavelengths that are ~50 m less than waves at locations along the western Atlantic. Time-integrated estimates of the depth of wave penetration provide empirical constraints on the paleo-water depths of ancient sedimentary deposits and highlight differences between sheltered shelf environments, such as those that characterized many ancient epeiric seas, and open-ocean‐facing, narrow continental shelves.

Seismic interpretation of mass-transport deposits within the upper Oligocene Frio Formation, south Texas Gulf Coast

We integrated well logs and three-dimensional seismic data to describe a wedge of deformed shallow Frio rocks lying above a major bed-parallel decollement within the upper Oligocene Frio Formation located between the Houston and Norias deltas on the south Texas Gulf Coast. Our analyses show that the identified deformed shallow Frio rocks can be divided into proximal clay-rich and low-permeability sandstones characterized by discontinuous, mounded, and chaotic seismic events; near-proximal, clay-poor, and high-permeability sandstones characterized by parallel to subparallel bedded seismic events; and distal sand-, silt-, and mudstones composed of a mix of proximal and near-proximal rocks. All of the deformed rocks are composed of acoustic-impedance materials that are lower than those of the undeformed shallow Frio and are underlain by low-velocity, overpressured, shale-rich rocks. The mechanism that triggered the collapse of the shallow Frio and subsequent development of mass-transport deposits is attributed to an uprising, overpressured, shale-rich high and the development of a shelf-edge listric fault. Proprietary biostratigraphic data show that the collapse of the shallow Frio in areas between the Houston and Norias deltas occurred between 27.5 and 25.3 Ma—approximately the same time as the Hackberry collapse in the Mississippi delta. In the proximal area, interpreted paleowater depths from biostratigraphic data based on benthic foraminifers range from 60 to 120 ft (20–40 m) in a shallow neritic environment. In contrast, the distal area lies in paleowater depths interpreted to be between 120 and 300 ft (40–90 m) in a middle neritic environment.

Assessing subsidence rates and paleo water-depths for Tahiti reefs using U–Th chronology of altered corals

We present uranium–thorium chronology for a 102m core through a Pleistocene reef at Tahiti (French Polynesia) sampled during IODP Expedition 310 “Tahiti Sea Level”. We employ total and partial dissolution procedures on the older coral samples to investigate the diagenetic overprint of the uranium–thorium system. Although alteration of the U–Th system cannot be robustly corrected, diagenetic trends in the U–Th data, combined with sea level and subsidence constraints for the growth of the corals enables the age of critical samples to be constrained to marine isotope stage 9. We use the ages of the corals, together with δ18O based sea-level histories, to provide maximum constraints on possible paleo water-depths. These depth constraints are then compared to independent depth estimates based on algal and foraminiferal assemblages, microbioerosion patterns, and sedimentary facies, confirming the accuracy of these paleo water-depth estimates. We also use the fact that corals could not have grown above sea level to place a maximum constraint on the subsidence rate of Tahiti to be 0.39mka−1, with the most likely rate being close to the existing minimum estimate of 0.25mka−1.