Pliocene-Pleistocene Sequence Research Articles

High‐resolution Pliocene–Pleistocene calcareous nannofossil distribution and palaeoenvironmental changes in the northwest Nile Delta, Egypt

The Pliocene–Pleistocene sequences in the Nile Delta are of immense economic significance due to their hydrocarbon potential. Hence, a better understanding of the time-calibrated basinal palaeoenvironmental changes is essential to improve the hydrocarbon recovery. A total of 182 sub-surface samples from the Sapphire-DC well in the offshore north-western part of the Nile Delta were analysed to reveal changes in calcareous nannofossil assemblage composition and palaeoenvironments during the late Miocene–Pleistocene (5.44–0.43 Ma). Sixteen marker bioevents and nine biozones are identified for the first time. The absence of Ceratolithus acutus suggests a hiatus at the Messinian–Zanclean (Miocene–Pliocene) boundary. The studied interval is marked by the dominance of five species; Pseudoemiliania lacunosa (24%), small Gephyrocapsa sp. (20%), Helicosphaera kamptneri (11%), Reticulofenestra minutula (10%) and Calcidiscus leptoporus (6%). Based on their distribution patterns, six intervals (Intervals 0 to 5) are identified. Throughout the study interval, cooler waters (except during Interval 0; Messinian Salinity Crisis, MSC, 5.44–5.26 Ma) and eutrophic conditions persisted within a shallow hemipelagic and near-continental setting, marked by moderate-to-high terrigenous input (more for Intervals 4 and 5; ˜2.73–0.42). Additionally, low species diversity and high species dominance are identified, suggesting a stressed environment. Species diversity shows a gradually declining trend, up section, whereas species dominance correspondingly increases. Interval 0 (MSC) is dominated by warm water species of Sphenolithus abies and Helicosphaera kamptneri. The cool water species, Reticulofenestra pseudoumbilica, and small Gephyrocapsa dominate in Interval 1; small Gephyrocapsa dominates in Interval 2. Interval 3 is marked by the deepening of the basin with a brief introduction of warm, somewhat oligotrophic and open-ocean waters. In the middle of this interval (at 3.73 Ma; the Early–Late Pliocene boundary), an abrupt decrease in species diversity with a corresponding increase in species dominance suggests deteriorating conditions. Interval 3 also encompasses the mid–late Pliocene warming period, which in the Nile Delta is marked by cooler waters. Intervals 3–5 are dominated by P. lacunosa, suggesting a continuation of cool waters and eutrophic conditions. Hence, throughout the study interval, cooler waters (except during the MSC) and eutrophic conditions within in a shallow hemipelagic and near-continental setting are inferred with moderate-to-high terrigenous input and low species diversity and higher species dominance, suggestive of stressed environmental conditions.

Provenance, weathering, and paleoclimatic records of the Pliocene-Pleistocene sequences of the Himalayan foreland basin, NW Himalaya

We present major oxides, trace element compositions, rare earth elements (REEs), clay mineral assemblages, and magnetic mineral parameters data for the siliciclastic fraction from the Upper Siwalik Subgroup (Parmandal, Nagrota, and Boulder Conglomerate formations) in the Tawi sub-basin of the Himalayan foreland basin, NW Himalaya in order to identify source rock compositions, continental weathering, and paleoclimate over the past 5.6 million years. The trace elemental ratios of Th/Co, Th/Sc, La/Sc, and Cr/Th suggest Upper Siwalik Subgroup sediments initially originated from a stable felsic source, which is possibly in the High Himalayan crystalline and Lesser Himalayan source regions. The REE patterns of the studied sediments with prominent negative Eu anomalies also suggest a felsic source for the Upper Siwalik Subgroup sediments deposited in the basin. The chemical index of alteration (CIA), plagioclase index of alteration (PIA), Rb/Sr ratio, and clay mineral assemblages as well as mineral magnetic parameters demonstrate the variable intensity of weathering and environmental conditions during the deposition of Parmandal, Nagrota, and Boulder Conglomerate formations between 5.6 and 0.6 Ma. The weathering proxy records of Parmandal (5.6 to 3.9 Ma) and Boulder Conglomerate (1.7 to 0.2 Ma) formations indicate relatively strong chemical weathering associated with the warm and wet climates in the source region. On the other hand, a decrease in chemical weathering intensity and change in environmental conditions during the formation of Nagrota formation (3.9–1.7 Ma) suggest a dry and cold climate in the source region.

Ground squirrels (Rodentia, Sciuridae) of the late Cenozoic Meade Basin sequence: diversity and paleoecological implications

AbstractThe Meade Basin, SW Kansas, yields a rich vertebrate fossil record from the late Cenozoic. Here, we review fossil ground squirrels (Sciuridae) from the region as a contribution to the broader Meade Basin Rodent Project. We recognize 14 species in seven genera: two species of giant ground squirrels (PaenemarmotaHibbard and Schultz, 1948) from the early Pliocene, and at least 12 species in six extant genera (AmmospermophilusMerriam, 1892;OtospermophilusBrandt, 1844;IctidomysAllen, 1877;PoliocitellusHowell, 1938;UrocitellusObolenskij, 1927;CynomysRafinesque, 1817) from the Pliocene–Pleistocene sequence, including the first regional records ofAmmospermophilus. Based on dental morphology and the ecology of modern congeners, we interpret faunal change through the sequence as primarily reflecting a shift from a Pliocene assemblage of “southwestern” taxa with granivorous/omnivorous diets (relatively low-crowned, transversely narrow cheek teeth) in warm and at least occasionally dry shrub or shrub-steppe habitats (Ammospermophilus,Otospermophilus,Ictidomys meadensis[Hibbard, 1941a]), to a Pleistocene temperate assemblage of grazing taxa that either exhibited relatively high-crowned, transversely wider cheek teeth (Urocitellus,Cynomys,Ictidomys tridecemlineatus[Mitchill, 1821]) or were otherwise dependent on grassland habitats (Poliocitellus). The early Pleistocene Borchers assemblage was transitional in this regard, heralding a “revolution” observed as well with other clades in the Meade Basin rodent community. This interpretation is broadly congruent with evidence of Pliocene climatic change and the staged development of regional grasslands, with the modern proportion of C3/C4plants established in the Meade Basin during the early Pleistocene.

NEOGENE RADIOLARIAN BIOSTRATIGRAPHY FROM ODP LEG 119, SITE 745

Abundant, generally well preserved radiolarians were used in a stratigraphic analysis of the Neogene sequence of ODP Leg 119, Site 745. Detailed biostratigraphic analysis has been undertaken of radiolarian assemblages of the Pliocene-Pleistocene sequence. Seventy-seven radiolarian taxa were identified including three reworked forms. Twenty-one stratigraphically important species are described and illustrated from this 99.55 m long core, and allow the recognition of four radiolarian zones, i.e., Phi, Chi, Psi and Omega (from oldest to youngest) and the Plio-Pleistocene boundary. These are then correlated with the earlier established zones and boundary in the Southern Ocean.

Foraminiferal biostratigraphy of the Pliocene–Pleistocene deep marine offshore, north Nile Delta, Egypt

A high resolution biostratigraphic analysis of the Pliocene–Pleistocene sequence of the offshore Nile Delta region in West Delta Deep Marine Concession (WDDM), in which 29 planktonic foraminiferal species belonging to 8 genera and 51 benthic foraminiferal species belonging to 40 genera were identified, allowed the recognition of seven foraminiferal zones and two subzones. These are as follows: Sphaeroidinellopsis Acme Zone (MPl1), Globorotalia margaritae Zone (MPl2), Globorotalia puncticulata / Globorotalia margaritae Zone (MPl3), Sphaeroidinellopsis s.l. Zone (MPl4), Globorotalia puncticulata Subzone (MPl4a), Globorotalia planispira Subzone (MPl4b), Globigerinoides elongatus Zone (MPl5), Globorotalia inflata Zone (MPl6) and Hyalinea balthica Zone.

The late Neogene-early Quaternary small vertebrate succession from the Almenara-Casablanca karst complex (Castellón, Eastern Spain): Chronologic and paleoclimatic context

Although discontinuous because of their karst nature, the Almenara-Casablanca complex records a very complete small vertebrate succession (amphibians, squamates, insectivores, bats, rodents and lagomorphs), characterized by the richness of their sites, a richness which cannot be expected in sites of fluvio-lacustrine origin. This late Neogene-early Quaternary succession includes the sites of Almenara-Casablanca M (Miocene-Pliocene boundary), Almenara-Casablanca 4 (late Pliocene), Almenara-Casablanca 1 (earliest Pleistocene) and Almenara-Casablanca 3 (late early Pleistocene). In this way, the Almenara-Casablanca succession complements the data from other well know terrestrial Pliocene-Pleistocene sequences in the Iberian Peninsula, most notably that of the Guadix-Baza Basin. The complementation between the almost continuous Pliocene-Pleistocene sequence of the Guadix-Baza Basin and the discontinuous but very rich succession from the Almenara-Casablanca complex enables accurate dating of some of the faunal events that punctuate the Pliocene-early Pleistocene time interval. Furthermore, the assemblages from the Almenara-Casablanca complex contain the evidence of the earliest occurrences of several taxa as well as the persistence of other that had already disappeared from other European regions. Therefore, it provides new evidence for the particular role which the Iberian Peninsula played during the late Neogene and early Quaternary, acting at certain moments as a bridge for the entry of African taxa or, alternatively, as a refuge for other European taxa. This paper provides an updated review of the small vertebrate succession from this complex.

Caratterizzazione idrogeochimica delle acque sotterranee abruzzesi e relative anomalie

The geological and stratigraphical sequence of the Abruzzo region (Central Italy, fig. 1) is dominated by the Jurassic to Oligocene succession, constituted mainly by limestone, marley limestones, calcareous sandstones, calcareous marls, marls and clayey marls along the Apennines, hilly areas and shoreline. The Miocene sequence, outcropping along the Apennines front, is constituted of limestones, marley limestones, calcareous marls and marls. The top of the succession, in the central and northern areas, is constituted by Messinian deposits of the marley-arenaceous turbidite formation, while, in the southern area delimited by Pescara river at north, by the evaporitic terrigeneous succession. The Pliocene-Pleistocene sequence represents the greater part of the hill domain, mainly constituted by marley clays and clayey marls surrounding arenaceous and conglomerate bodies. A gravity flow (Oligocene-Miocene Molisan sequence) originated from the Apennines is located at south of Maiella within the Pliocene-Pleistocene sequence; it is mainly constituted by clays and marls, with smaller proportions of limestones, calcareous sandstones, calcareous marls, Cretaceous and Miocene reworked marls and chalks, with maximum thicknesses up to approximately 2000 m. Lacustrian deposits are found in the intra-mountain basins, while Quaternary alluvial deposits lay on top of the marine deposits along main rivers and streams. The alluvial domain is mainly constituted by continental terraced deposits in the fluvial plains, while intramountain basins by fluvial-lacustrian deposits, continental head deposits and in a lesser extent by Pliocene-Pleistocene sandstones and conglomerates (fig. 1). Aquifers, mainly recharged by rainfall river contribution; groundwater flow is significantly influenced by paleo-river bodies and bedrock geometries (Desiderio et alii , 1999). These mostly non-confined aquifers, mainly constituted by gravelly-sandy deposits, are locally multilayered and semi-confined, especially in the intra-mountain and coastal zones. Exchange between surface waters and groundwater can be significant. The Abruzzo main aquifers are located in the Appennine carbonate domain (Celico, 1983; Boni et alii , 1986; Conese et alii , 2001; Petitta & Tallini, 2002; Nanni & Rusi, 2003; Desiderio et alii , 2002). The major springs (Q > 50 l/s) are located at the border of the carbonate hydro-structures along the contact with less permeable deposits that constitute the aquiclude (Miocene, Pliocene and Pleistocene terrigenous deposits) or aquitards (continental Pliocene and Pleistocene deposits). Minor springs (Q < 20 l/s) are controlled by local factors as head deposits, impermeable layers within the lithological sequence and tectonic discontinuities. Perennial springs are seldom found in the Appennines fore-areas and hilly areas, constituted by calcareous sandstones, marls, clays, sandstones and chalks originated from the alloctonous units, Miocene turbidite and evaporitic successions, including clayey marls and clays from Pliocene and Pleistocene pelagic successions. The few springs are related to aquifers within the more permeable deposits, mainly recharged by rainfall. Surface and buried bituminous and evaporitic sediments sometimes increase the spring waters salinity (Desiderio & Rusi, 2004). Groundwater classification of the four hydrogeological domains has been accomplished within this study on the basis of 690 chemical analysies of 325 monitoring points. Piper diagram heighlights that spring waters from the Apennine carbonate domain are classified as calcium bicarbonate facies (fig. 2); groundwaters from the intra-mountain domain are slightly shifted (fig. 5), while those from springs and wells in the hilly domain (fig. 6), thus originating from terrigenous deposits, show a greater dispersion with a slight prevalence of alkaline earth-bicarbonate and sulphate-chloride-alkaline earth waters. Alluvial domain groundwaters (fig. 7) are even more disperse with a prevalence of calcium bicarbonate facies. The mother groundwaters in the Apennine carbonate domain undergo a first mixing within the intra-mountain basins with waters flowing in the alkali rich mainly lacustrian continental deposits. More significant mixing occurs when the Apennine groundwater flow into the alluvial domain. Greater residence times and alluvial lithological heterogeneity imply greater groundwater salinity with a significant increase in alkali elements, sulphates and chlorides. Saline intrusion in coastal areas are related to groundwater withdrawals causing significant variations of original facies. Groundwater flowing within the terrigenous domain do not undergo mixing due to aquifer geometries and small discharges. The hydrochemical facies dispersion is related to the extreme lithological heterogeneity of this domain. A modest circulation of mineralised waters originates from groundwater flow within the Adriatic Pliocene-Pleistocene foredeep, allochthonous deposits and Miocene evaporites (figs. 9, 10 and 12). The hydrochemical facies are sodium-chloride, sodium-chloride-sulphate, sodium-bicarbonate, sodium-sulphate and without dominant ions; these groundwaters undergo different degrees of mixing with those of the main domains, thus modifying their characteristics (fig. 13).

Tectonic Evolution of the Eastern Mediterranean Basin and its Significance for the Hydrocarbon Prospectivity of the Nile Delta Deepwater Area

ABSTRACT The deepwater area of the Nile Delta is within the eastern Mediterranean basin on the Nile Delta Cone between the Herodotus abyssal plain to the west and the Levant basin to the east. The complex evolution and interaction of the African, Eurasian and Arabian plates have shaped the Late Miocene to Recent Nile Cone and its substratum. The tectono-stratigraphic framework is controlled by deep-seated basement structures with distinct gravity and magnetic expressions, and by the interaction of the NW-trending Misfaq-Bardawil (Temsah) and NE-trending Qattara-Eratosthenes (Rosetta) fault zones. In addition, significant salt-induced deformation of a Messinian evaporitic sequence up to 4,000 m thick has occurred, together with large-scale rotational block movement. The deformational pattern is largely the result of multiphase tectonic movements along pre-existing basement faults on the continental margin of the Neo-Tethys ocean. The Nile Cone consists of late Paleogene to Late Miocene sediments that pre-date the Messinian evaporites, and Pliocene-Pleistocene sequences. In the east, the pre-salt deposits (as much as 3,000 m thick) are primarily deepwater sediments with local condensed sequences over syndepositional intrabasinal highs. Shale occurs westward across the Rosetta trend. The Messinian evaporitic sequence exhibits three distinct seismic facies suggesting cyclic deposition with the occurrence of interbedded anhydrite, salt and clastic sequences and pure halite deposition. During the Messinian salinity crisis, large-scale canyons were excavated that resulted in multiphase cut-and-fill clastic systems. The Pliocene-Pleistocene sequences were deposited in a slope to basin-floor setting. Exploration targets are the Pliocene-Pleistocene deepwater channel and basin-floor turbidite sands in a variety of structural settings. Water depths range from 800 to 2,800 m. The Upper Miocene sequence offers additional exploration objectives in the form of fluvial and/or turbidite sands. The focus of pre-salt exploration is the delineation of distal turbidities within the Serravallian to Tortonian sequence and the identification of new reservoir sequences deposited on pre-existing intrabasinal highs. Hydrocarbon charge has yet to be proven by drilling, but seismic amplitude anomalies and the occurrence of natural surface slicks suggest both gas and liquid charges from pre-salt source rocks through faults and salt-withdrawal windows.

Stonewall anticline: An active fold on the Oregon continental shelf

Stonewall Bank is the site of a growing west-verging anticline striking north-northwest on the continental shelf at 44.5 ° N, southwest of Newport, Oregon. To the east are Pliocene-Pleistocene strata of the Newport syncline, onlapping eastward against gently west-dipping late Miocene and older rocks of the Oregon Coast Range. Folding of Stonewall anticline results in fine-grained middle Miocene strata being exposed at the sea floor along the anticlinal crest; rocks as old as Eocene are encountered in the Unocal P-0093-1 Grebe well, drilled at the anticlinal crest. Rates of folding are based on deformation of an unconformity between Pliocene and Miocene strata (PM unconformity) and of a stream channel that crossed Stonewall Bank during the last glacial maximum. The bed length of the unconformity is shortened across the Stonewall anticline and adjacent folds by about 400 m between structures west of Stonewall Bank and the Newport syncline. The PM unconformity has a vertical separation of about 1000 m between the anticline and the first syncline to the west. The horizontal shortening and vertical separation imply that Stonewall anticline is underlain by a blind reverse fault. Retrodeforming the PM unconformity shows that this fault dips 65°‐70° E. A vertical separation of 1000 m on a fault with this dip yields a slip of 1070‐1080 m along the fault. If folding of the PM unconformity is assumed to have begun 2‐3 Ma, this would give a long-term slip rate of 0.4‐0.6 mm/yr. If most folding began after deposition of the entire Pliocene-Pleistocene sequence, the slip rate would be 1.0‐1.1

Traction structures in deep-marine, bottom-current-reworked sands in the Pliocene and Pleistocene, Gulf of Mexico

Research Article| October 01, 1993 Traction structures in deep-marine, bottom-current-reworked sands in the Pliocene and Pleistocene, Gulf of Mexico G. Shanmugam; G. Shanmugam 1Mobil Research and Development Corporation, P.O. Box 650232, Dallas, Texas 75265-0232 Search for other works by this author on: GSW Google Scholar T. D. Spalding; T. D. Spalding 2Mobil New Business Development, Europe/Africa/Russia, P.O. Box 650232, Dallas, Texas 75265-0232 Search for other works by this author on: GSW Google Scholar D. H. Rofheart D. H. Rofheart 2Mobil New Business Development, Europe/Africa/Russia, P.O. Box 650232, Dallas, Texas 75265-0232 Search for other works by this author on: GSW Google Scholar Geology (1993) 21 (10): 929–932. https://doi.org/10.1130/0091-7613(1993)021<0929:TSIDMB>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation G. Shanmugam, T. D. Spalding, D. H. Rofheart; Traction structures in deep-marine, bottom-current-reworked sands in the Pliocene and Pleistocene, Gulf of Mexico. Geology 1993;; 21 (10): 929–932. doi: https://doi.org/10.1130/0091-7613(1993)021<0929:TSIDMB>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The Pliocene-Pleistocene sequence cored in the Ewing Bank and Garden Banks areas in the Gulf of Mexico provides an example of deep-marine sand reworked by bottom currents. A distinctive attribute of these sands is their traction structures, which include cross-stratification, current ripples, and horizontal lamination. These sands also exhibit sharp upper contacts, inverse size grading, mud offshoots, and flaser bedding. An important aspect of these structures is their occurrence in discrete units, but not as part of a vertical sequence of structures, such as the Bouma sequence. Presumably, the Loop Current, a strong, wind-driven surface current in the Gulf of Mexico, impinged on the sea bottom, as it does today, and resulted in bottom-current-reworked sands. A depositional model is proposed in which the bottom-current-reworked sands tend to develop a facies in interchannel slope areas that is distinctly different from channel-levee turbidite facies. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Process Sedimentology and Reservoir Quality of Deep-Marine Bottom-Current Reworked Sands (Sandy Contourites): An Example from the Gulf of Mexico

Deep-marine bottom-current reworked sands (sandy contourites) have been recognized in hydrocarbon-bearing sands of the Gulf of Mexico. A distinctive attribute of these sands is their traction bed forms, which occur in discrete units. Common sedimentary features of traction currents include cross-bedding, current ripples, horizontal lamination, sharp upper contacts, and inverse size grading. These sands also exhibit internal erosional surfaces and mud offshoots, indicating oscillating current energy conditions. The Pliocene-Pleistocene sequence cored in the Ewing Bank Block 826 field in the Gulf of Mexico provides an example of sand distribution and reservoir quality of deep-marine bottom-current reworked sands. Presumably, the Loop Current, a strong wind-driven surface current in the Gulf of Mexico, impinged on the sea bottom, as it does today, and resulted in bottom-current reworked sands. A depositional model based on the integration of well (core and log) and three-dimensional seismic data suggests that the reworked sediment package may be thick and continuous, but individual sand layers within the package may be thin and discontinuous. This unconventional model, which depicts the distribution of bottom-current reworked sands in interchannel slope areas as a distinctly different facies f om channel-levee facies, has the potential for general application to other slope plays outside the study area. In the Ewing Bank Block 826 field, the type 1 (L-1) reservoir with 80% sand exhibits higher permeability values (100-1800 md) than the type 2 (N-1) reservoir with 26% sand (50-800 md). The increased permeability in the type 1 sand has been attributed to high sand content, vigorous reworking, and microfractures. The clean, porous, and well-sorted type 1 sands with good communication between sand layers have produced at higher rates and recovery efficiencies than the type 2 sands with numerous interbedded mud layers.

Tentative Revision of the Global Pliocene-Pleistocene Sequences Based on the Sequence Stratigraphy in the Gulf of Mexico

The Pliocene-Pleistocene sequence chronostratigraphy presented in this paper is based on a vast amount of data obtained from more than 100 wells drilled over the past eight years in the south additions and deep-water areas in offshore Texas and Louisiana, Gulf of Mexico. This high-resolution biostratigraphic data base consists of individual checklist with the abundance, diversity, occurrences, and ranges of planktonic and benthic foraminifers and calcareous nannofossils plotted against depth. The benthic foraminifers have been interpreted largely for their paleobathymetric significance and result in a water-depth curve for each well studied. These wells have been further calibrated by having a portion of the study wells tied to sequence stratigraphic interpretations of seismic record sections through a two-way-time log or synthetic seismogram. The Pliocene and Pleistocene is tentatively divided into 14 fourth-order sequences from 3.0 to 0 Ma and three third-order cycles from 5.5 to 3.0 Ma. Each of the cycles is bounded by a sequence boundary and has an age-dateable maximum flooding surface. Depending on location, each sequence may have lowstand, transgressive, and highstand systems tracts within the basin. The age (Ma) of the sequence boundaries are third-order, 5.5, 4.2, 3.8, fourth-order, 3.0, 2.6, 2.4, 1.86, 1.4., 1.0, 0.82, 0.72,more » 0.62, 0.52, 0.42., 0.32, 0.22, 0.12, and 0.02. Within these sequence boundaries are the 5.0, 4.0, 3.4, 2.7, 2.45, 2.0, 1.47, 1.3, 0.92, 0.76, 0.66, 0.56, 0.46, 0.36, 0.26, 0.16, and 0.06 Ma maximum flooding surfaces, respectively. All of the condensed sections associated with the maximum flooding surfaces, systems tract boundaries, and sequences boundaries in the Pliocene-Pleistocene can be recognized and traced on well logs and seismic record sections in the offshore Texas and Louisiana areas.« less

近畿地方の第四紀テフラ研究

Pliocene-Pleistocene sequences deposited in lacustrine and bay environments are widely distributed in the Kinki District, and many water-based volcanic ash layers are intercalated in those sediments, though active volcanoes of Quaternary age are not located. The main interest in the study of Kinki district tephras is related to tephra identification for use as chronostratigraphic horizons.The history of research on tephras is divided into three stages. During the first stage, fundamental tephra discrimination was carried out on the basis of thickness, color, grain size, and succession in the field. In the second stage, petrographic characters (heavy mineral composition, glass shape, refractive index, magnetic characteristics, etc.) were analyzed for tephra identification. Recently, we have carried out tephra identification on the basis of the single grain method using EDX chemical analysis and refractive index data obtained by the thermal immersion method (RIMS 86), adding petrographic data.One of the recent topics of tephra study in the Kinki District is obtaining continuous and sequential sections in Lake Biwa and Osaka Bay. In Osaka Bay, the volcanic glass stratigraphy labeled V1-V17 (in descending order) was constructed using 65 drilling core samples. Two of them are correlated with the widespread tephra, and V17 is correlated with the Azuki volcanic ash layer that is the most valuable key bed in the Osaka Group. In Lake Biwa, two drilling operations (at 200m depth and 1, 400m depth) were carried out. The uppermost part (T bed) is composed mainly of massive clay and reaches a thickness of 250m. It is thought that this continuous clay includes most of the tephra that has fallen in the Kinki District during about 400, 000 years. Thirty-eight volcanic ash layers are intercalated in the T bed. Some of them (especially in the upper part) are correlated with some of the widespread tephras. The Lake Biwa and Osaka Bay sequences are very valuable in the identification of a tephrostratigraphy for the Kinki District.

Revision of the Plio-Pleistocene Cycles and Their Application to Sequence Stratigraphy and Shelf and Slope Sediments in the Gulf of Mexico

ABSTRACT The ages, cycles and correlation of the Pliocene and Pleistocene have been discussed and argued for decades. The revision of the Pliocene-Pleistocene sequence chronostratigraphy presented in this paper is based on data obtained from more than 100 wells in the south additions and deepwater areas, offshore Louisiana and Texas, Gulf of Mexico. The high resolution biostratigraphic data, combined with well-log sequence stratigraphic interpretations and correlations of the seismic record sections through a two-way-time log or synthetic seismogram permits the Pliocene and Pleistocene series to be divided into 14 fourth order sequences and seven third order sequences. We have discussed and illustrated the most important events that have led up to this detailed revision of the Plio-Pleistocene. This documentation is very important because most of these papers have been published in journals that have not been readily available to most geologists and geophysicists.

Depositional, Tectonic, and Eustatic Controls on Hydrocarbon Distribution in Devergent Margin Basins--Gulf of Mexico Case History: ABSTRACT

The origin and distribution of hydrocarbon reservoirs on divergent margins, like that of depositional sequences, reflects an interplay among sediment supply and depositional process, eustatic sea level change, and tectonic history. Cenozoic sequences of the northwestern Gulf Coast basin contain prolific hydrocarbon fairways defined by each of these endmember processes. The Oligocene Frio sequence records moderate rates of sediment supply, locally rapid load or salt-induced subsidence, and infrequent, relatively low-amplitude eustatic change. Accommodation space was created by growth-fault extension or salt evacuation along with progradation into the deep-water Gulf. Shore-zone deposits are sand rich because limited eustatic shore line shift promoted effective reworking and storage of coastal facies. Progradational depocenters were characterized by large, wave-dominated shelf-edge deltas; sands were stored in close proximity to the delta fronts. Prolonged intervals of aggradation and retrogradation preserved thick interdeltaic facies sequences deposited by barrier/lagoon systems. More than 18 billion BOE of produced Frio hydrocarbons accumulated primarily in structurally and depositionally defined plays. In contrast, Pliocene-Pleistocene sequences are the products of frequent, high-amplitude eustatic cycles, rapid rates of subsidence, and locally high rates of sedimentation. Accommodation space was created by salt evacuation and relative rises in sea level. Shorezone deposits are heterolithic because rapidmore » subsidence and frequent eustatic cycles prevented extensive reworking. Progradational depocenters were characterized by small, mud-rich, river-dominated shelf-edge deltas. Rapid lowering of sea level promoted deposition of highly elongate, sand-rich submarine fans extending more than 120 km from the paleoshelf margin. Structurally defined Pliocene-Pleistocene plays contain nearly 1.5 billion BOE.« less

The Interpretation of Seismic Reflection Data from the Western Platform Region of Taranaki Basin

The western-most region of Taranaki Basin, the Western Platform, has a stratigraphy which permits subdivision into major seismic units. The reflectors separating these units are easily identifiable. Each of the units and reflectors has typical reflection characteristics which are often correlatable with the lithology of the unit. Lateral velocity variations, caused by lateral deposition and compaction variations in prograding sequences, area major problem in developing depth conversion models for this region. Analysis of travel time data from wells shows that velocity variations in both the Oligocene-Miocene and Pliocene-Pleistocene sequences are predictable from the thickness variations of the units (and hence from interval travel times). The imerval velocity variations of the Paleocene-Eocene transgressive sequence are dependent on the overburden history and lithology of the unit.

K-Ar and 40Ar/39Ar dating of the hominid-bearing Pliocene-Pleistocene sequence at Koobi Fora, Lake Turkana, northern Kenya

In the Koobi Fora region, east of Lake Turkana, northern Kenya, there occurs a sequence ∼500 m thick of lacustrine, fluvial, and deltaic sediments that contains abundant vertebrate fossils, including hominids, as well as stone tools. Rhyolitic tuffs within the sedimentary sequence have facilitated stratigraphic mapping. Some of the tuffs contain pumice clasts, from which anorthoclase phenocrysts have been separated, providing ideal material for K-Ar and 40 Ar/ 39 Ar dating. Seven tuffs have been dated, generally yielding concordant ages on multiple samples from each tuff. Results are consistent with the stratigraphic sequence. The 40 Ar/ 39 Ar age spectra give nearly ideal flat patterns, indicating that the feldspars have remained undisturbed since crystallization and cooling, with no evidence of thermal overprinting. As the pumice clasts are considered to have been deposited very soon after their eruption, the measured ages provide a close approximation to that of deposition of the tuffs. The Moiti Tuff, ∼30 m above the base of the sequence, has a maximum age of 4.10 ± 0.07 Ma. The Toroto Tuff, some 70 m higher in the sequence, is very securely dated at 3.32 ± 0.02 Ma. Stratigraphically higher tuffs and their ages include the Ninikaa Tuff, 3.06 ± 0.03 Ma; the KBS Tuff, 1.88 ± 0.02 Ma; the Malbe Tuff, 1.86 ± 0.02 Ma; the Chari Tuff, 1.39 ± 0.02 Ma, and the Silbo Tuff, 0.74 ± 0.01 Ma, near the top of the sequence. The geochronology and geological data indicate at least three hiatuses in the sequence, each on the order of 0.5 to 0.7 Ma duration. Conventional K-Ar age measurements on basalts from the basin margin suggest that deposition of the Koobi Fora Formation began no earlier than ∼4.3 Ma ago in the early Pliocene. These results provide a well-documented numerical time framework for the sedimentary sequence in the Koobi Fora region and for the hominids and other fossils contained therein. Most of the hominid fossils, including forms assigned to the Homo lineage and a coexisting Australopithecus lineage, occur within sediments deposited in the basin over the interval from ∼2.0 to 1.4 Ma ago in the late Pliocene and early Pleistocene.

Some Depositional Patterns at Continental Margin of Southeastern Mediterranean Sea

upper Miocene to Holocene sedimentary strata in the continental margin of the southeastern Mediterranean Sea depict two depositional regimes. upper Miocene sequence is predominantly evaporitic and forms the southeastern portion of the upper Miocene evaporites present throughout the Mediterranean region. Pliocene-Quaternary sequence is predominantly detrital and its major source of sediments has been the Nile River. Interpretation of data derived from several multichannel seismic profiles suggested facial variations in the upper Miocene and the Pliocene-Pleistocene formations. Two depositional facies of the upper Miocene evaporites, indicating basinal and shelf depositional environments, were found. Statistical analyses show correlations of the thickness of the evaporites with their interval seismic velocity, their depth, and the present bathymetry, indicating the autochthonous characteristics of the sequence. basinal and the shelf depositional facies are separated by a transition zone that trends NNE-SSW and is associated with faulting. It is suggested that this zone, commonly known as The Pelusium Line, was the shelf-edge zone during the late Miocene. Facial analysis of the data pertainin to the Pliocene-Pleistocene sequence depicts its allochthonous characteristics. Statistical negative correlation was calculated between the distance from the continental shelf and the thickness of this sequence, indicating its detrital origin. Variations in thicknesses of both formations compared with the bathymetric depths suggest a post-Miocene subsidence of the southeast Mediterranean basin.

Marine Plio-Pleistocene of Iceland and problems of its correlation

The marine Pliocene-Pleistocene sequence of Iceland contains four main assemblages of mollusks: (1) Venerupis rhomboides et al., (2) Serripes groenlandicus et al., (3) Portlandia arctica et al., and (4) recent species. Analysis of the stratigraphic distribution of 100 species shows that the percentage of extinct species in the assemblages decreases from 40% in the oldest to 4–8% in the youngest. There is also a change from south boreal affinities in the oldest assemblage to north boreal in the youngest. The molluskan assemblages of Iceland can be correlated with Pliocene-Pleistocene assemblages of England and North Europe: (1) Coralline Crag, (2) Red Crag—lower part of the Icenian strata, (3) upper part of the Icenian-Cromerian strata. The upper boundaries of the Pliocene and Eopleistocene are defined by paleomagnetic data and radiological dating. The upper boundary of the Pliocene presumably should be established in the basalts lying between the Tjörnes and the Breidavik deposits by changes between the second and third mollusk assemblages and by the Gilsa paleomagnetic event.

Late Mesozoic and Early Cenozoic Sediment Cores from the Arctic Ocean

Only two sediment cores taken from ice island T-3 in the Arctic Basin have material older than Pliocene age. One of these cores includes a Late Cretaceous silicoflagellatc assemblage contained in tuffaceous sediment; the other is of Paleocene to Eocene age, containing abundant phytoplankton in tuffaceous sediment. The two cores represent displaced Cretaceous and early Cenozoic masses within a Pliocene-Pleistocene sequence. Both cores were taken from the Alpha Cordillera. These cores provide the first firm evidence that the Arctic Ocean existed at least by Late Cretaceous time, a time during which it had no ice cover and was a much warmer ocean than it is now. Warm water conditions continued at least into Eocene time. No Oligocene or Miocene material has been identified, but, by Pliocene time, the Arctic was ice covered, and this condition has existed until the present.