Neritic Shelf Research Articles

Astronomical Dating of the Middle Miocene Hanjiang Formation in the Pearl River Mouth Basin, South China Sea

AbstractThe Hanjiang Formation of Langhian age (middle Miocene) in the Pearl River Mouth Basin (PRMB), South China Sea consists of deltaic siliciclastic and neritic shelf carbonate rhythmic alternations, which form one of the potential reservoirs of the basin. To improve stratigraphic resolutions for hydrocarbon prospecting and exploration in the basin, the present study undertakes spectral analysis of high‐resolution natural gamma‐ray (NGR) well‐logging record to determine the dominant frequency components and test whether Milankovitch orbital signals are recorded in rhythmic successions. Analytical results indicate the orbital cycles of precession (∼19 ka and ∼23 ka), obliquity (∼41 ka), and eccentricity (∼100 ka and ∼405 ka), which provide the strong evidence for astronomically driven climate changes in the rhythmic alternation successions. Within biochronological constraint, a high‐resolution astronomical timescale was constructed through the astronomical tuning of the NGR record to recent astronomically calculated variation of Earth's orbit. The astronomically tuned timescale can be applied to calculate astronomical ages for the geological events and bioevents recognized throughout the period. The first downhole occurrences of foraminifers Globorotalia peripheroronda and Globigerinoides sicanus are dated at 14.546 Ma and 14.919 Ma, respectively, which are slightly different from earlier estimates in the South China Sea. When compared with the global sea‐level change chart, the astronomical estimate for the sequences recognized based on microfossil distributions have the same end time but the different initiation time. This is probably due to the local or regional tectonic activities superimposed on eustatic rise which postponed the effect of global sea‐level rising. Astronomical timescale also resolves the depositional evolution history for the Langhian Stage (middle Miocene) with a variation that strongly resembles that of Earth's orbital eccentricity predicted from 13.65 Ma to 15.97 Ma. We infer that the main factor controlling the variability of the sedimentation rate in the Hanjiang Formation is related to the ∼405‐ka‐period eccentricity.

Early Burdigalian infill of the Puchkirchen Trough (North Alpine Foreland Basin, Central Paratethys): Facies development and sequence stratigraphy

To improve the prediction of gas and oil in strata along the tectonically imbricated southern margin of the Puchkirchen Trough (North Alpine Foreland Basin, Central Paratethys), a better understanding of facies distribution and stratigraphic control of the basin is essential. The present study provides bio- and chemofacies analyses and a biostratigraphic evaluation for the pelitic Hall Formation from the borehole Hochburg 1 in the central part of the Puchkirchen Trough. A statistical evaluation of benthic foraminiferal assemblages together with geochemical proxy records (TOC, sulphur, hydrogen index, δ13CTOC, δ15NTN) reveals a succession of early Burdigalian depositional environments.Following a major subaqueous erosional hiatus, conglomeratic sands at the base of the section, which contain reworked Chattian and Aquitanian foraminiferal assemblages, record the reactivation of a basin-axial channel system. During the early Burdigalian eustatic sea-level rise, the channel was cut off from its sediment sources on the shelf and a deepening bathyal environment was established. Agglutinated foraminiferal assemblages with abundant Bathysiphon filiformis developed that were adapted to an unstable environment with frequent deposition of turbidites. The middle part of the Hall Formation is characterized by prograding high-relief clinoforms, which are fed by the Wachtberg Delta of the Paleo-Inn river. High sedimentation rates are interpreted and increased input of terrestrial-derived organic matter is documented with a strong correlation of TOC and TOC/S and low HI values, as well as frequent occurrences of Ammodiscus spp. and other opportunistic agglutinating foraminifers. A renewed transgression reestablished a eutrophic and suboxic bathyal environment, followed by the development of an oxygenated outer–middle neritic shelf environment heralding the ultimate infill of the deep-water basin.Based on a comparison of the revealed development to existing sequence stratigraphic models for the Puchkirchen Trough, three sequences and their corresponding systems tracts can be identified for the lower, middle and upper Hall Formation. Biostratigraphic evidence from benthic foraminifers and calcareous nannoplankton indicates that they correspond to the regional substages of the middle and upper Eggenburgian and lower Ottnangian, and to global 3rd-order sequences Bur 1–3. The observations suggest a primary control of eustatic sea-level on the Puchkirchen Trough rather than Alpine tectonics during the Burdigalian.

A fossil sperm whale (Cetacea, Physeteroidea) from the Pleistocene of Nauru, equatorial southwest Pacific

Several Recent cetacean species have a near-cosmopolitan dis tribution in the open-ocean, with some taxa rarely venturing near shore (Rice, 1998; Jefferson et al, 2008). In contrast, the fossil record of Cet cea is primarily derived from continental shelf sequences (Fordyce and Muizon, 2001). Rare exceptions to this bias include fragmentary fossils dredged from abyssal plains (Turner, 1880; Eastman, 1903; Roux and Geistdoerfer, 1988), and, rarer still, those from oceanic islands (Estevens and vila, 2007). These sources of fossil Cet cea are important because they have the potential to (1) sample fossil cetacean diversity from otherwise poorly represented oceanic paleoenvironments; and (2) add insights into the past zoogeography and ecology of cetaceans that may not be apparent from neritic shelf assem blages. A significant limitation on the last point is the gener ally poor stratigraphic control associated with dredged cetacean fossils (Bianucci et al., 2007). With such uncertain stratigraphic contexts, the broader significance of open-ocean fossil Cet cea is problematic. Although fossil cetaceans from oceanic islands may be associated with more evidence on geologic age, such de posits have hitherto proved extremely rare: late Miocene-early Pliocene fossils on the Azores in the mid-North Atlantic repre sent the sole pre-Holocene record of Cet cea from oceanic is lands (Estevens and Avila, 2007). The Pacific has been the largest ocean throughout cetacean history and its Rim has produced an excellent fossil record (Fordyce, 2008). Yet until now, none of its ~20,000 oceanic islands (i.e., on oceanic crust) have yielded pre Holocene Cet cea. This note reports the first discovery of a pre-Holocene cetacean from a Pacific oceanic island-a stem physeteroid from the Pleistocene of Nauru, represented by an isolated tooth (NMV P204274) (Figs. 1-2). This new fossil evidence adds to knowl edge of both the spatial and temporal distribution of stem Phy seteroidea.

Foraging plasticity in a pelagic seabird species along a marine productivity gradient

The foraging plasticity in a pelagic seabird species, Cory's shearwater Calonectris diomedea, was compared during incubation and chick-rearing among 7 different breeding sites in the North Atlantic. These sites, with contrasting ecological conditions, were situated in the Azores (Corvo, Faial, Graciosa and Sta. Maria), Berlengas (Berlenga), Madeira (Desertas) and Selvagens (Selvagem Grande) archipelagos. Behavioural data were obtained by intensively deploying compass- temperature and temperature-depth data loggers, monitoring 302 and 22 foraging excursions, respectively. A principal component analysis revealed 2 main groups, one foraging along a neritic shelf (Berlengas population) and the other foraging in pelagic oceanic areas (populations from Azores, Madeira and Selvagens). There was a significant positive correlation (rS = 0.55) between the percentage of short foraging trips in the different breeding sites and the concentration of chlorophyll-a in the waters surrounding those sites. Birds exploiting neritic areas used shallower depths (4.9 ± 2.3 m) and shorter dives (17.0 ± 2.2 s) than birds feeding in oceanic waters (9.8 ± 2.4 m and 32.0 ± 5.8 s respectively). This indicates that birds adapted their modes of predation to cope with the diverse dis- tribution of their prey items. Overall, the behavioural response of the different populations was mainly dictated by the heterogeneity of their habitat, which was driven by 2 productivity gradients present in the north Atlantic. (1) Productivity is expected to increase from the subtropical warmer waters close to Selvagens to northern colder waters north of the Azores archipelago. (2) Oceanic environments are expected to be less profitable than coastal areas (upwelling areas), such as the Portuguese and African coasts.

On the oldest Jurassic ammonites of Europe (Northern Calcareous Alps, Austria) and their global significance

The earliest ammonite species of Jurassic type, Psiloceras spelae GUEX, was unknown till 1998. This species bridges the long gap between the latest Triassic and the earliest Jurassic ammonoids. It was discovered in North America, then in South America and finally in Europe. The species is proposed as boundary marker for the base of the Jurassic system. It is known in two forms which are regarded as subspecies. The European subspecies is here introduced as P. spelae tirolicum n. ssp. This taxon occurs in the Northern Calcareous Alps of Tyrol as the oldest European Jurassic type ammonite. Both subspecies characterize the first Jurassic ammonite biohorizon and are closely related. Their slight differences may be due to biogeographic separation. The preservation admits a complete diagnosis of the new subspecies. Besides psiloceratid characters still phylloceratid characters are present indicating the origin of the psiloceratids. Additionally, slightly younger psiloceratids are described which were known only from South and North America thus far. They are older than the oldest psiloceratids hitherto known from Europe. All these forms are elements of the first biochronological ammonite unit of the Jurassic, the Tilmanni Chronozone which is situated below the Planorbis Chronozone. It is assumed that during the endTriassic biotic crisis only those ammonites survived which lived in the open parts of the oceans. A re-population of neritic shelf areas took place from the oceanic realms in line with the gradual Hettangian transgression. This may explain the global similarity of the first Jurassic ammonites and their later biogeographic diversification. A time calculation based on the sedimentation rate of the P. spelae tirolicum n. ssp. bearing sections leads to the conclusion that the earlier Triassic-Jurassic ammonite gap of about 250 to 300 ka now can be reduced to approximately 100 ka.

Vertical biological zoning of the continental slope depending on the sedimentation conditions

Of particular interest in the vertical biological zoning of the continental slope in seas and oceans are the substantial differences in the composition, diversity, and abundance (biomass and production) of benthic faunal communities populating the upper and lower subzones of the bathyal zone. The upper bathyal subzone (down to depths of 1500 ± 500 m) resembles the neritic (shelf) one, while its lower subzone is characterized by both reduced diversity and biomass, being similar in its biological parameters to the near-slope abyssal zone (>3000 m). The study of the bottom contour currents and related sediments (contourites) made the geological prerequisites for such differences understandable. Based on a comparative analysis of the bionomical boundaries and core depths of the contour currents in the North Atlantic and Gulf of Alaska, a changing of trophic zones, as well as quantitative and production characteristics of benthic communities, are established. It is shown that the differences between the biological parameters of the upper and lower bathyal subzones (the benthic biomass, the feeding mode of invertebrates, the growth rate, and the maximal sizes of adult specimens) are related to geological agents such as roiling and redeposition of the sediments.

A tsunami deposit at the Cretaceous/Paleogene boundary in the Neuquén Basin of Argentina

A coarse-grained sandstone bed of Cretaceous/Paleogene (K/Pg) boundary age occurs in a homogeneous neritic shelf mudstone sequence (Jagüel Formation) in the Neuquén Basin of Argentina. This bed, 15–25 cm thick, contains abundant plagioclase, broken shells and sharks' teeth. Sedimentological features include an erosive base, abundant rip-up clasts, normal grading and hummocky cross-bedding. The K/Pg boundary age of the bed was confirmed by calcareous nannofossils. Similar to other sections in the Gulf Coast region and the Danish Basin, a “dead zone” significantly depleted in macrofossils is evident in the basal 1 m above the clastic layer. In combination, these features suggest that the clastic layer represents a tsunami deposit that was related to the Chicxulub impact event in Yucatan/Mexico. Mechanisms of tsunami wave amplification in this extremely distal and somewhat protected setting are poorly understood but the funnel-shape of the basin may have promoted the unusually strong sedimentological response.

Facies dependence of spore assemblages and new data on sedimentary influence on spore taphonomy

Palynological studies have been undertaken on Lower Carboniferous (Mississippian) rocks encountered in boreholes from the North Sea to the Baltic Sea (Rügen) in Northern Germany and from an outcrop at the northern border of the Rhenish Slate Mountains in western central Germany. The studied sections belong to different facies realms within the neritic shelf facies. They begin with very near shore coastal clastic facies in the North Sea and onshore Northern Germany, ranging into carbonate platform facies at Rügen, and then into platform slope facies at the northern margin of the Rhenish Slate Mountains. The focus of this study is the analysis of the relationship between sedimentary facies and spore assemblages; therefore, spores were considered only as sedimentary particles. Thus, it was necessary to create a new spore classification scheme based only on relevant sedimentary features. The new scheme distinguishes three classes of miospore based on shape, with each class subdivided into three subclasses based on features of ornament. The diversity of the spore assemblages and the total palynofacies have also been studied. Correlating palynological results from sections representing different sedimentary facies has led to a better understanding of the influence of sedimentary facies on spore assemblages. Correlations have been made at two stratigraphical levels. Regional correlation is undertaken on the Upper Viséan from the coastal clastic facies (North Sea, Northern Germany), the carbonate platform facies (Rügen) and the marginal platform slope facies (Velbert). Local correlation within the carbonate platform facies has been attempted using samples of Mid-Viséan TS zone from four boreholes on Rügen. Certain important differences have been observed. The local correlation at Rügen shows a decrease in spore frequency and spore assemblage diversity related to increasing resedimentation within subsided blocks of the platform. The composition of spore assemblages, strongly dominated by round-shaped spores, is nearly the same in all sections. The regional correlation shows clear proximal–distal trends for the total number of spores and also the diversity and composition of spore assemblages. A strong decrease in triangular and transitional spores from proximal to distal facies was observed. In conclusion, spore assemblages seem to be strongly influenced by sedimentary facies as well as by different sedimentary processes within the facies. Facies dependence of spore assemblages is shown to be an important factor for stratigraphical use of spores and for establishing vegetational and climatic trends.

Late Oligocene to early Miocene dinoflagellate cysts from the Kuohsing area, central Taiwan

Two dinoflagellate assemblages, first defined from the Pearl River Mouth Basin of the South China Sea, occur in the Oligocene–lower Miocene sediments of the Kuohsing area, Taiwan. The older Homotryblium plectilum– Cordosphaeridium gracile Assemblage (I), characterized by the last occurrence of Chiropteridium lobospinosum, Cordosphaeridium gracile, Homotryblium abbreviatum, Homotryblium tenuispinosum, and Membranophoridium aspinatum, is late Oligocene; the younger Polysphaeridium zoharyi– Lingulodinium machaerophorum Assemblage (II), characterized by the first occurrence of Hystrichosphaeropsis obscura, Melitasphaeridium choanophorum and Tuberculodinium vancampoae, is early Miocene. The dominance/abundance of Polysphaeridium zoharyi in some of the Kuohsing samples suggests that the sediments of this area were deposited on a tropical–subtropical, inner neritic shelf. Slight sea-level fluctuations during the period of deposition reflect two shifts from an outer inner neritic to an inner inner neritic shelf, and ultimately to an inner outer neritic shelf environment at the time of highest sea-level. The present study strengthens previous findings that P. zoharyi is a warm-water indicator restricted to low latitudes, whereas Bitectatodinium tepikiense and Deflandrea are temperate- to cold-water indicators restricted to middle and high latitudes; and, that Nematosphaeropsis labyrinthus and Impagidinium prefer outer neritic/oceanic environments, whereas P. zoharyi prefers a shallow, inner neritic environment.

Latest marine horizon north of Qomolangma (Mt Everest): implications for closure of Tethys seaway and collision tectonics

The newly discovered marine horizon at Zhepure Syncline, north of Qomolangma, revises timing of the closure of the eastern Neo‐Tethys seaway in the central Himalayas. The marine Pengqu Formation conformably overlies a Lutetian shallow‐water carbonate platform and comprises shales interbedded with sandstones deposited in a neritic shelf environment. The strata are dated by nannofossils and foraminifera as late early Lutetian to late Priabonian age (NP15–NP20, deposited ≈ 47–34 Ma), indicating that the final closure of the Tethys seaway in this region occurred at ∼ 34 Ma. The newly discovered strata provide evidence about emplacement of a major thrust sheet south of southern Tibet prior to ∼ 37 Ma, which affected the regional climate.

The Jinshajiang–Ailaoshan Suture Zone, China: tectonostratigraphy, age and evolution

The Jinshajiang Suture Zone is important for enhancing our understanding of the evolution of the Paleo-Tethys and its age, tectonic setting and relationship to the Ailaoshan Suture Zone have long been controversial. Based on integrated tectonic, biostratigraphic, chemostratigraphic and isotope geochronological studies, four tectono-stratigraphic units can be recognized in the Jinshajiang Suture Zone: the Eaqing Complex, the Jinshajiang Ophiolitic Melange, the Gajinxueshan “Group” and the Zhongxinrong “Group”. Isotope geochronology indicates that the redefined Eaqing Complex, composed of high-grade-metamorphic rocks, might represent the metamorphic basement of the Jinshajiang area or a remnant micro-continental fragment. Eaqing Complex protolith rocks are pre-Devonian and probably of Early–Middle Proterozoic age and are correlated with those of the Ailaoshan Complex. Two zircon U–Pb ages of 340±3 and 294±3 Ma, separately dated from the Shusong and Xuitui plagiogranites within the ophiolitic assemblage, indicate that the Jinshajiang oceanic lithosphere formed in latest Devonian to earliest Carboniferous times. The oceanic lithosphere was formed in association with the opening and spreading of the Jinshajiang oceanic basin, and was contiguous and equivalent to the Ailaoshan oceanic lithosphere preserved in the Shuanggou Ophiolitic Melange in the Ailaoshan Suture Zone; the latter yielded a U–Pb age of 362±41 Ma from plagiogranite. The re-defined Gajinxueshan and Zhongxinrong “groups” are dated as Carboniferous to Permian, and latest Permian to Middle Triassic respectively, on the basis of fossils and U–Pb dating of basic volcanic interbeds. The Gajinxueshan “Group” formed in bathyal slope to neritic shelf environments, and the Zhongxinrong “Group” as bathyal to abyssal turbidites in the Jinshajiang–Ailaoshan back-arc basin. Latest Permian–earliest Middle Triassic synorogenic granitoids, with ages of 238±18 and 227±5–255±8 Ma, respectively, and an Upper Triassic overlap molasse sequence, indicate a Middle Triassic age for the Jinshajiang–Ailaoshan Suture, formed by collision of the Changdu-Simao Block with South China.

Facies variation, depositional setting and hydrocarbon potential of the Upper Cretaceous rocks in the United Arab Emirates

The Upper Cretaceous (Aruma Group) of the United Arab Emirates (UAE) was deposited over nearly 23 Ma. This was after a period of major erosional emergence in the Turonian. The variable thicknesses and lithofacies of the sediments of this group were controlled by regional subsidence induced by plate collision and partial subduction, eustatic sea-level fluctuations and differential sedimentation rates. The Upper Cretaceous in the southern UAE (Abu Dhabi region) consists of two major transgressive-regressive cycles involving the sediments of four formations, the Laffan, Halul, Fiqa and Simsima. In the central UAE (Dubai region) the group consists of the Laffan, Ilam, and Fiqa Formations, while in the northern UAE it consists of the Muti, Juweiza, Qahlah and Simsima Formations. The earliest Late Cretaceous sedimentation accompanied a marine transgression that probably began in the late Turonian, with the sea spreading to cover most of the UAE by Coniacian times. This marine event was marked by the deposition of basinal muds (not shales) and neritic marls. Later, during Santonian, neritic shelf carbonates were deposited. Subsequent subsidence in the early Campanian resulted in the accumulation of deep-water marls, argillaceous limestones and muds (shales). During the Maastrichtian, a widespread regression occurred, and neritic shelf bioclastic limestones and dolomites containing rudists and orbitoids were deposited on the shelf margin. Toward the centre of the basin, in the northern UAE, thicker, basinal, slope and shelf edge sediments were deposited resulting in a succession that consists mostly of marls, argillaceous limestones, cherts, conglomerates, sandstones and shales. Commercial hydrocarbons in the Upper Cretaceous are confined to the Simsima and Ilam Formations, which are the best potential exploration targets, particularly in the offshore of the northeastern Emirates in a belt parallel to the Oman Mountains. The basal part of the Upper Cretaceous is a major seal over middle Cretaceous reservoirs of the UAE.

The Arabian Gulf: A Modern Carbonate-evaporite factory; a review

The Arabian Gulf is a foreland basin lying between the Arabian Shield lo 11w southwest and Ihe Zagros bid beil lo Ihe noriheasi. It is a shallow marine basin with depths generally < 100 m. It has a marked asymmetry witb the deepest water close to the northeast Iranian shore at the foot of the Zagros fold tnountains and the depths decrease to the southwest towards the Arabian Shield. The area is a marginal sea of the Indian Ocean. High temperatures and low rainfali have produced a marine environment with a low influx of fresh water and higb temperatures and salinity. It is colonised by an abundant but low diversity Indian Ocean flora and fauna. Biological production of skeletal material, together with chemical precipitation, have produced a wide variety of carbonate sediments. Evaporation in the adjacent coastal plains of Arabia has caused precipitation of evaporite sediments and reaction between biologically produced carbonate and salme ground waters has resulted in dolomitization. The supply of terrigenous allochthonous sediment by aeolian and fluvial processes is markedly asymmetric and is restricted to the northwestern and northeastern sides of the Gulf and is virtually absent along the southwestern Arabian shoreline. tbe modern jocies pattern reflects the geological setting, asymmetric supply of terrigenous material and the climate. A series ofjbciesbelts parallel the Gulf and the causative tectonic trend and are, from northeast to southwest: neritie basinal maris; Arabian neritic shelf carbonates; coastal barrier-lagoonal carbonates; and coastal plain evaporites. The Iatter pass southwestwards into the aeolian dune sands and alluvial-fan gravels of Arabia. In thc northwest, filling the Tite Arabian Cui» A Modern Carbonate-evaporite fauory a review 63 basin longitudinally and parallel to the tectonic trend is the TigrisEuphrates delta. Ihe pattern of sedimentation of the modern Arabian Gulf has proved to be a useful model for the elucidation of carbonateevaporite sedimentation in various parts of the geologic column, but particularly for the Permo-Triassic deposits of northwestern Europe.

Biogeography of marine armoured dinoflagellates and dinocysts in the NE Atlantic and North Sea

The distribution patterns of 250 species of armoured planktonic dinoflagellate, 28 species of armoured planktonic and potentially cyst-forming dinoflagellate and 22 types of dinocyst have been analysed by multivariate clustering and ordination techniques. The area of the study consisted of the NE Atlantic, bounded by 20°S and 70°N latitude and 25°W longitude, and the larger part of the North Sea. The overall distribution patterns were examined by cluster analysis ( Twinspan) which gave a separation into four biogeographic zones clearly relating to the prevailing oceanographic conditions. The zones defined are: (1) Sub-boreal, which consists of the area north of 60°N; (2) NW European Shelf, which comprises the North Sea and the shelf area around the British Isles and France; (3) Temperate Oceanic, the area of the Atlantic between 40 and 60°N which is bounded on the east by zone 2; (4) Warm Temperate, the southern portion of the area sampled which is mainly oceanic but within which is situated the localised area of the NW African upwelling which relates more closely with zone 2. The Twinspan analyses of cyst-forming dinoflagellates and the dinocysts clearly show that the NW European neritic shelf sea is a quite distinct biogeographic zone from the more oceanic areas where relatively few dinocyst types apart from those of Impagidinium are found. Since they are the most important groups for the formation of cysts the 21 gonyaulacoid species and the 50 protoperidinioid species of thecate dinoflagellates were separately analysed. The gonyaulacoids showed a clear influence of temperature on distribution, and the cyst-forming species all seem to share similar requirements since they form a tight cluster in the ordination. The protoperidinioids formed a very strong cluster which included most of the cyst-forming forming species but excluded the rare warm-water species. Analysis of the 22 dinocyst types revealed no clear difference between the majority of the gonyaulacoid or peridinioid cysts. However, Nematosphaeropsis labyrinthus, one of the five cysts in the analysis linked to the thecate dinoflagellate Gonyaulax spinifera, and the five Impagidinium species occupied a distinct position on both analyses.

The Organic Petrology and Thermal Maturity of Lower Carboniferous and Upper Devonian Source Rocks in the Liard Basin, at Jackfish Gap-Yohin Ridge and North Beaver River, Northern Canada: Implications for Hydrocarbon Exploration

Organic petrology and geochemical studies (Rock-Eval/TOC) were carried out on shales from the Besa River (Upper Devonian-Lower Carboniferous), Yohin, Clausen, Prophet, Golata, and Mattson Formations (Lower Carboniferous) in exposed sequences at Jackfish Gap-Yohin Ridge and in a subsurface section in the North Beaver River area of the Liard Basin. Basinal shales of the Besa River Fm. have TOC values ranging from 1 to 4% and contain abundant type II, dominantly amorphous, kerogen of marine origin. Shales in the Yohin Fm. (slope to neritic shelf), Clausen Fm. (basin and slope deposits), Prophet Fm. (carbonate slope), and Golata Formations are of mixed marine and terrestrial origins and yield TOC values of 1 to 3%

Hackberry Oil and Gas Fields in Southeast Texas: Channel/Fan Depositional Systems and Structural Controls: ABSTRACT

Deep-water sandstones of the Hackberry Formation (Oligocene) host significant quantities of oil and gas. They remain one of the most important deep exploration targets in southeasternmost Texas; new fields producing from the Hackberry have been discovered at a steady rate from 1946 to the present. The Hackberry contains two hydrocarbon plays. The updip play is relatively shallow, oil-rich, and lies near the updip limit of deep-water deposition. Some of the fields in this play actually produce from shallow-water Frio sandstones of Hackberry age rather than from Hackberry sandstones. The downdip play is gas rich and generally geopressured. The reservoirs lie either within or on the flanks of major channel systems and are often bounded updip by small growth faults. The discontinuous distribution and complex lithofacies of these channel and fan sands demand a careful understanding of the component depositional environments in order to discover and efficiently produce hydrocarbons. The Hackberry Formation is a wedge of sand and shale with bathyal fauna that separates upper Frio sandstone and shale from middle and lower Frio shale and sand. The main sandstone lies atop a channeled unconformity at the base of the formation; some sandstones are also found locally within the shale wedge. Sandstones in a typical sand-rich channel evolve upward from a basal channel-fill sand to more widespread valley-fill deposits of interbedded sand and shale. Topmost are proximal to medial fan deposits with slightly meandering channels and overbank turbidites. This sequence suggests that the Hackberry sands were laid down by an aggrading, onlapping submarine canyon-fan complex that eroded headward into the contemporaneous Frio barrier bar-strand plain. Regional mapping and seismic i terpretation outlines a network of partly sand-filled channels extending from the strand plain toward the southeast. The downdip limits of lower Hackberry sand are not defined by available well data. The early structural history of the area is obscure, but Vicksburg-age faulting associated with continental slope sedimentation is possible. Small growth faults displace the Hackberry section less than 500 ft (150 m) and extend upward into the Miocene strata. Isopach and isolith maps indicate that the Orange, Port Neches, and Fannett salt domes were active uplifts during Frio and Anahuac deposition. Near Spindletop dome, however, only a north-south trending salt-cored ridge was present. The Hackberry channels are somewhat influenced by salt activity, but major channel axes extend across the uplifts. The genesis of the deep-water Hackberry embayment is obscure. Middle Frio strata underlying the Hackberry are neritic shelf deposits to the west but may include deeper water shales in the central and eastern parts of the area. The embayment may have formed by subsidence of a large part of the Frio-Vicksburg continental shelf with consequent canyon erosion. Alternatively, the Hackberry canyons may be analogous to canyons currently forming on the flanks of the Niger delta in an entirely deep-water regime. End_of_Article - Last_Page 458------------

Upper Eocene-Lower Oligocene Foraminiferal Biostratigraphy and Paleoecology in Eastern Gulf Coast Region: ABSTRACT

Definition of the Eocene-Oligocene boundary in the eastern Gulf Coast region is complicated because the upper Eocene and lower Oligocene strata in Mississippi consist of clastic-dominated deposits whereas time-equivalent strata in south-central Alabama are comprised primarily of carbonates. These strata intertongue across southwestern Alabama. Based on vertical planktonic foraminiferal distribution in these strata, the Eocene-Oligocene boundary occurs at the top of the Shubuta Member of the Yazoo Clay in Mississippi and southwestern Alabama and at the top of the Crystal River Formation in south-central Alabama. The Pachuta and Shubuta Members of the Yazoo and Crystal River are assigned to the upper Priabonian Globorotalia cerroazulensis (s.1.) Interval zone. The Red Bluff Clay and Bumpnose Limestone are placed in the lower Rupelian Pseudohastigerina micra Interval zone. Vertical compositional and diversity changes are apparent in the foraminiferal populations and indicate a rise in sea level through the Priabonian with a decrease in water depth in the lower Rupelian. The decrease in water depth might be attributed to a drop in sea level and/or to progradation of the Forest Hill delta across Mississippi into Alabama. Lower Yazoo sediments accumulated in an inner to middle neritic shelf paleoenvironment, and the upper Yazoo and Crystal River marls are middle to outer shelf deposits. The Red Bluff and Bumpnose sediments were deposited under middle to inner shelf conditions. West to east foraminiferal compositional and diversity trends are apparent. During th Priabonian and lower Rupelian, water depths in Mississippi exceeded those of south-central Alabama. End_of_Article - Last_Page 567------------

Distribution and Depositional History of Neogene Phosphorites Along Pacific Coast of North America: ABSTRACT

Pelletal and nodular phosphorites occur commonly to abundantly in neritic (shelf) and bathyal (slope) deposits of uppermost Oligocene (25 Ma) to upper Miocene (10 to 7 Ma) age from 23°N on the peninsula of Baja California, Mexico, to 39°N near Point Arena, northern California, encompassing a belt of deposition about 1,430 mi (2,300 km) in length. In addition, pelletal phosphorite sands are commonly present within adjacent middle and lower bathyal deposits of similar age range representing redeposited material in conduits, feeding basins, and submarine fans. In some areas, pelletal phosphorites reach 200 ft (60 m) in thickness. Mining of Miocene phosphorites is now well under way in Baja California. The age of peak formation and accumulation of Pacific Coast phos horites appears to become younger from south to north implying variations in patterns of upwelling and/or eustatic, climatic, and tectonic control of shelf character and flux of terrigenous clastics to the margin. Later reworking of the Miocene deposits has allowed reconcentration of the phosphorites in adjacent Pliocene, Pleistocene, and Holocene neritic units. Paleontologic, isotopic, and sedimentologic evidence indicate that the widespread Neogene phosphorites formed under a special set of climatic, oceanographic, eustatic, and tectonic conditions associated or coincident with a major climatic threshold occurring in mid-Miocene (15 Ma) time and commencement of a glacial climatic state. Three key factors were apparently responsible for allowing the unusually prolific formation of Miocene phosphorites as well as simultaneous widespread deposition of diatomaceous sediments in this region including (1) vigorous upwelling of nutrient-rich water and accelerated productivity as a function of deteriorating Neogene climate, (2) associated development of intense oxygen minima impinging against the various slope and shelf areas creating appropriate b ogeochemical conditions for phosphorite precipitation, and (3) the absence or severe reduction in delivery of terrigenous clastics to sites of phosphorite precipitation as a function of Neogene eustatic, climatic, and tectonic events. End_of_Article - Last_Page 1691------------

Stratigraphic Significance of Some Oligosteginid Assemblages from Lurestan Province, Northwest Iran

Oligosteginid species are distributed in all of the nine biostratigraphic zones established for the Cretaceous Bangestan group (Albian-Campanian) in the Lurestan (Luristan) province, northwest Iran. The distributional pattern is similar to that found elsewhere in the Tethys province of Europe and Asia. The rich oligosteginid faunas of the Sarvak formation (Albian-Turonian) are apparently restricted to a narrow belt separating true neritic shelf faunas from fully pelagic faunas. This suggests that oligosteginids are probably benthonic organisms which, on death, were not transported far into adjacent biotopes. The new subspecies Calcisphaerula innominata lata is included among the forms systematically described.