Late Miocene Sections Research Articles

Seismic characteristics and AVO response for non-uniform Miocene reservoirs in offshore eastern Mediterranean region, Egypt

The Eastern Mediterranean region, extending from the Offshore Nile Delta Cone of Egypt to the Levant Basin, is a confirmed hydrocarbon-rich territory with several giant gas discoveries. Numerous gas fields have been discovered in the Miocene reservoirs within the Nile Delta Cone, and the Levant Basin. The Miocene sedimentary sequences in this region are extremely heterogeneous, consisting mainly of turbiditic slope deposits, channels, and basin floor fans that were capped by evaporites formed during the Messinian Salinity Crisis. As a result, the seismic characteristics and interpreted properties of this heterogeneous section are ambiguous. The study area is located in the Offshore North Sinai Basin, where a thick Early Miocene section was deposited midway between the Nile Delta province, which includes the El-Fayrouz discovery, and the Levant Basin, which includes Tamar, Tanin, and several other discoveries. This study uses quantitative seismic interpretations methods, such as amplitude variations with offset and fluid replacement modeling, to assess the seismic acoustic impedance trend with depth. Also, determine the seismic amplitude response for the brine and gas sands reservoir of the Early and Late Miocene section to link the unexplored study area within the North Sinai Offshore Basin with the explored Nile Delta and Levant Basins. In addition to evaluate direct hydrocarbon indicator (DHI) of the dimming seismic amplitude that is compatible with the structure’s last closed contour of the Syrian Arc anticline of the Early Miocene reservoirs (EMT-1 prospect). Different vintages of 2D and 3D seismic data, six wells, and various published data were used in this study. The quantitative interpretation shows the pitfalls of the acoustic impedance trend and seismic response dependency on depth for gas and brine sand, which led to the drilling of the EMT-1 dry well. Also, the fluid replacement, P-wave velocity (Vp), and density (ρ) modeling confirmed that the seismic dimming amplitude was due to a seismic processing artifact, which was corrected by readjusting the overburden Messinian salt processing velocity model. This research concludes that the seismic quantitative interpretations are successfully used to assess the acoustic impedance versus depth and understand DHI pitfalls, as well as the processing workflow that could enhance the seismic image.

Marine influence in western Amazonia during the late Miocene

The dynamic environments of western Amazonia during the Neogene (23 to 2.58 million years ago [Ma]) included continental-scale wetlands that were episodically influenced by marine incursions originating from the Caribbean. The nature and frequency of such events is highly debated, with a general consensus on short events during the early (23 to 16) and middle (16 to 11.6 Ma) Miocene. However, scattered evidence for more events up until the late Miocene (11.6 to 5.3 Ma) exists. Based on a palynological analysis of borehole 1-AS-9-AM from the Solimões Formation (western Amazonia, Brazil), we provide further support for late Miocene marine influence in the region. Comparisons with other sites in the Solimões Basin indicate an age of earliest Tortonian (~11–10 Ma) for an assemblage with foraminiferal linings, acritarchs and dinocysts, totalling ~17.3% of the palynological count ( n = 374 palynomorphs) at 34.10 m of depth. This is the topmost sample with a significant amount of marine indicators and its age is consistent even when different biostratigraphic interpretations are appreciated. Our data and comparisons also show that a larger proportion of sediments of the Solimões Formation should be placed within the late middle to early late Miocene. Our reconstruction of the Solimões deposits in the early Tortonian indicates intermintent low salinity regimes and a persistent connection with the paleo-Orinoco drainage until the closure of this connection in the late Miocene to Pliocene times. • New pollen data is presented from a borehole (1-AS-9-AM) in western Amazonia. • Graphic correlation was applied with two nearby sites, ages found from middle to late Miocene. • A thicker late Miocene section than previously interpreted is suggested. • A marine assemblage (~17% of the palynological count) at 34 m is composed of foraminiferal linings, dinocysts and acritarchs. • Regional comparisons support an early Tortonian episode of marine influence in a wide geographical area in western Amazonia.

Large rotaliid foraminifera as biostratigraphic and palaeoenvironmental indicators in northwest Borneo: An example from a late Miocene section in Brunei Darussalam

Palaeonvironmental interpretations and biostratigraphic zonations in northwest Borneo are still a matter of debate but are important for petroleum related applications. The outcrop of Ambug Hill, a late Miocene marine siliciclastic sequence in Brunei Darussalam, has been investigated for its foraminifera assemblage. From a 9.5 m clay-rich interval 50 taxa have been recognized and classified, whenever possible to species level. The studied fauna is characterized by mostly hyaline foraminifera, dominated by two rotaliid species: Cavarotalia annectens and Heterolepa dutemplei. Additionally, a number of miliolids, lageniids, bolivinids and also planktonic foraminifera are present but in much lower abundance. The dominance of the rotaliids points to a sea floor rich in organic carbon characterized by muddy substrates with very low light penetration and the possible growth of patchy sea grass or macroalgae in the system, as indicated by epiphytic foraminifera in the samples. The results obtained give additional insights on how the environmental conditions might have been during the deposition of the deltaic sequences very widespread in the region, and how the fauna coped with such settings. Throughout the investigated samples, the dominance of the assemblages clearly shows alternating stress conditions, typical of pro- deltaic environments.The large abundance of rotaliid foraminifera, points toward possible biostratigraphic applications as several taxa within this group have short biozones in the region even though have strong facies dependency. If their role as biostratigraphic indicators could be finally proven, they might be used to date the large depositional sequences occurring in the region, since planktonic index fossils are scarce due to the very proximal settings of the sediments.

Morphological Description of Calcareous Nannofossils Assemblage of a Middle-Miocene to Late-Miocene Section in the Niger-Delta, Nigeria

Forty four (44) different forms of nannofossils recovered from the middle-Miocene to late-Miocene sediments of a section ((8900 - 11,000) ft (2713 - 3353 m)) of a well in the prolific oil producing Niger delta, Nigeria were described and grouped into twelve, though not on phylogenetic relationship, for documentation purpose. The standard strewn-slide technique was used to recover the fossils. The aim is to present the distinguishing morphological features of each of the nannofossils group. Of the recovered forms Genus, Discoaster, was found prominent and divisible into six sub-groups: 6-rayed discoaster with bifurcations, 6-rayed discoasters without bifurcation, 5-rayed discoasters, 5-rayed discoaster with acute bifurcations, 5-rayed nannolith and the indeterminate forms. Genus Sphenolithus recovered is also divisible into two: species without prominent apical spines and species with prominent, monocrystalline apical spines. Other genera found and described are Catinaster, Cyclicargolithus, Coccolithu, Minylitha, Syracosphaera, Helicosphaera, Pontosphaera, Calcidiscus and Reticulofenestra. All the nannofossils fall within NN11, NN10, NN9, NN8 and NN5 zones belonging to the Middle-Late Miocene age.

Geology, mineralogy, and geochemistry of late Miocene paleosol and calcrete in the western part of the Central Anatolian Volcanic Province (CAVP), Turkey

The late Miocene was characterized by high global temperatures and partly to nearly complete desiccation of the Mediterranean Sea throughout the latter part of the Messinian age (the Messinian salinity crisis). The non-marine paleoenvironmental and paleoclimatic records from the Mediterranean area are highly limited in their spatial range. Late Miocene river flood-plain sediments in the Central Anatolian Volcanic Province (CAVP) of central Turkey consist of siliciclastic and pyroclastic sediments with paleosols and calcrete. Five lithofacies were identified as (A) massive conglomerate, (B) trough-cross-bedded sandstone, (C) massive mudstone, (D) disrupted mud, (E) and disrupted matrix-supported conglomerate. The paleosols were composed predominantly of smectite and illite with smaller amounts of feldspar, quartz, opal-A, pyroxene and amphibole, and the calcretes were composed mainly of calcite, with smaller amounts of feldspar, quartz and opal-A, and minor amounts of smectite, chlorite and illite. Authigenic clay minerals and micritic calcite coating on the detrital grains and soil peds and pendant cement in the calcrete and paleosol horizons were evidence of a vadose zone. In contrast, the large equant spar drusy mosaics were formed in phreatic environments. The weathering of ignimbrite and marble led to the depletion of SiO2, CaO, Al2O3, Fe2O3, TiO2, and K2O through the precipitation of smectite and illite in the paleosols, and CaO in the form of micrite and calcite in the calcrete horizons. Field observations, mineralogy, geochemistry and the results of δ18O and δ13C isotopic analysis reveal that alternating wet and dry periods resulted in an upward increase in precipitation of authigenic smectite and illite within the late Miocene sections, under the influence of geochemical and pH fluctuations in phreatic or vadose water within the river flood-plain paleoenvironment.

Leaf compressions from the Late Miocene sections of NW Italy: research on an efficient, easy and quick consolidation treatment

Compressions of fossil leaves on marl blocks soaked with water are tricky palaeobotanic findings to deal with. In fact, this peculiarity makes the findings exposed to serious degradation: the evaporation of water causes shrinkage of the matrix and the leaf leading to fragmentation, delamination and exfoliation of the fossil, until its final disappearance. The aim of this research was to identify a consolidation method which satisfies museum needs and can be utilised for all conditions of hydration of the object. Following a survey of several natural science museums in Northern Italy, Paraloid™ B72 and PEG4000 along with other resins never previously used on this kind of fossil were tested. At the end of the research, two products gave adequate or good results, preserving the leaf from further degradation whatever the hydration condition of the object: the acrylic resin Paraloid™ B72 and a polyurethane resin.

Evidence for a marine incursion along the lower Colorado River corridor

Foraminiferal assemblages in the stratigraphically lower part of the Bouse Formation in the Blythe basin (lower Colorado River corridor, western USA) indicate marine conditions, whereas assemblages in the upper part of the Bouse Formation indicate lacustrine conditions and suggest the presence of a saline lake. Benthic foraminiferal assemblages in the lower part of the Bouse Formation are similar to lagoonal and inner neritic biofacies of the modern Gulf of California. Evidence suggesting a change from marine to lacustrine conditions includes the highest occurrence of planktic foraminifers at an elevation of 123 m above sea level (asl), the change from low diversity to monospecific foraminiferal assemblages composed only of Ammonia beccarii (between 110 and 126 m asl), an increase in abundance of A. beccarii specimens (above ∼110 m asl), increased number of deformed tests (above ∼123 m asl), first appearance of Chara (at ∼85 m asl), lowest occurrence of reworked Cretaceous coccoliths (at ∼110 m), a decrease in strontium isotopic values (between 70 and 120 m), and δ18O and δ13C values similar to seawater (between 70 and 100 m asl). Planktic foraminifers indicate a late Miocene age between 8.1 and 5.3 Ma for the oldest part of the Bouse Formation in the southern part of the Blythe basin. Benthic and planktic foraminifers correlate with other late Miocene sections in the proto–Gulf of California and suggest that the basal Bouse Formation in the Blythe basin was deposited at the northern end of this proto-gulf. After the marine connection was restricted or eliminated, the Colorado River flowed into the Blythe basin, forming a saline lake. This lake supported a monospecific foraminiferal assemblage of A. beccarii until the lake spilled into the Salton Trough and the Colorado River became a through-flowing river.

Cinnabar mineralization in fossil small mammal remains as a consequence of diagenetic processes

The peculiar red colour of fossil small mammal remains from a late Miocene section in southern Spain suggests an unusual diagenetic alteration. These remains have been studied by means of environmental scanning electron microscope equipped with different detectors, energy dispersive X-ray spectroscopy and an inductively coupled plasma optical emission spectrometer. The red colour is caused by the presence of cinnabar (HgS) in the pores of the fossil bones and teeth, filling the dentinal and bone tubules and other cavities that at life were filled by organic matter. A nearby cinnabar outcrop in the metamorphic materials of Sierra Nevada is interpreted as the source of mercury. This element was mobilized by meteoric diagenesis and incorporated as cinnabar in a palustrine environment, occupying pores and cavities of mammal remains shortly after the degradation of the organic matter (post-mortem enrichment), during the early diagenesis.

Pedogenic carbonate stable isotope record of environmental change during the Neogene in the southern Great Plains, southwest Kansas, USA: Carbon isotopes and the evolution of C4-dominated grasslands

Fossiliferous strata in the Meade Basin (southwest Kansas) preserve numerous superposed mammalian faunas and calcareous paleosols that range in age from the Clarendonian North American Land Mammal Age (NALMA; 12.0–9.0 Ma, early late Miocene) to the early Irvingtonian NALMA (ca. 2.5–ca. 1.0 Ma, early Pleistocene). Faunas from these sections document the evolution of the small mammal community of the modern grassland ecosystem of the region, and the stable isotope composition of paleosol carbonates provides a means by which the environmental context of the evolution of the modern ecosystem may be documented. We used the stable carbon isotope composition (δ 13 C relative to Vienna Peedee belemnite [VPDB]) of 194 pedogenic carbonates from 19 measured sections to reconstruct the history of C 4 grass abundance in the Meade Basin. Paleosol carbonate δ 13 C values reflect the proportion of C 3 (trees, shrubs, cool-climate grasses) and C 4 (warm-climate grasses) plants that grew in an ancient soil and provide a means with which to reconstruct past mammalian habitats. Paleosol carbonate δ 13 C values record a three-phase increase in the abundance of C 4 biomass during the Neogene in the Meade Basin. Late Miocene sections have mean δ 13 C values of −7.6‰ ± 0.90‰ (Clarendonian) and −6.5‰ ± 0.31‰ (Hemphillian NALMA, 9.0–4.9 Ma), consistent with 17% and 26% C 4 biomass, respectively. Miocene δ 13 C values from Meade are statistically identical to published δ 13 C values for Miocene paleosol carbonates elsewhere in the southern Great Plains, supporting the widespread presence of ∼20% C 4 biomass on average in the region throughout the Miocene. The abundance of C 4 biomass increased between the end of the Hemphillian section and the beginning of the early Blancan NALMA (5.0–3.0 Ma). Early and middle Blancan (3.0–2.5 Ma) carbonates have statistically identical δ 13 C values (−4.9‰ ± 0.90‰ and −5.0‰ ± 1.10‰, respectively), suggesting a stable ecosystem during the early Pliocene, although high δ 13 C variability in densely sampled intervals suggests a high degree of landscape-scale variation in C 4 abundance. The final phase, geochronologically controlled by two well-characterized ashes (Huckleberry Ridge, 2.10 Ma; Cerro Toledo B, 1.47–1.23 Ma) and magnetostratigraphy, is a trend to higher δ 13 C values from the late Blancan to early Irvingtonian (ca. 2.5–ca 1.0 Ma) from −4‰ at the base of the section to ∼1‰ at the top, corresponding to an increase from almost 50% to 65% C 4 biomass. The abundance of C 4 biomass first reaches modern levels for the region (78% ± 10.9%) around the level of the Cerro Toledo B ash, indicating that a modern-like grassland ecosystem first appeared in the region ca. 1.3 Ma, although δ 13 C values do not remain consistently high through the rest of the section.

Cenozoic multiple-phase tectonic evolution of the northern Tibetan Plateau: Constraints from sedimentary records from Qaidam basin, Hexi Corridor, and Subei basin, northwest China

An integrated research of sedimentology, stratigraphy, and provenance analysis on eleven sedimentary sections from the Qaidam basin, Hexi Corridor, and Subei basin representing ∼36 km Cenozoic strata provides a detailed record of the northern Tibetan Plateau growth since the early Eocene. Sections are divided into three groups based on age, geological similarities, and geographical locations. Group One includes three early Eocene–late Miocene sections from the northern Qaidam basin; Group Two contains four sections along the Altyn Tagh fault (ATF) which preserve a complete stratigraphic record from the Oligocene to late Miocene; Group Three contains the four youngest sections investigated from the northeastern Qaidam basin and Hexi Corridor which preserve a record since the middle Miocene to Quaternary. Together, the sections reveal a multiple-phase tectonic history of the northern Tibetan Plateau. The punctuated history can be divided into four phases. (1) The Eocene Lulehe Formation from the northern Qaidam basin is interpreted as a synorogenic conglomerate deposited by high-gradient depositional systems. Strong unimodal paleocurrent towards the southwest, coarse lithology and distinct, recognizable clast types constrain the sediment source within the North Qaidam and South Qilian terranes, indicating activity on inferred thrust faults within the North Qaidam and South Qilian terranes in response to the initial India-Eurasia collision. The activity on these thrust faults continued through the late Eocene. (2) The early Oligocene conglomerate from Group Two formed in response to the sinistral transpression related to motion on the ATF, suggesting inception of substantial slip on the ATF in the early Oligocene in order to accommodate the continuing indentation of India into Eurasia. (3) Oligocene–early Miocene fine-grained fluvio-lacustrine sediments from Groups One and Two formed as the result of development of internal drainage systems in the Qaidam basin in response to the large-amplitude slip motion on the ATF. Paleocurrents collected from the Oligocene–early Miocene strata of Group One are northwest-directed, pointing towards the ATF, consistent with the pre-existing subsurface data showing the shift of depocenter from along North Qaidam and South Qilian terranes toward the ATF. (4) All eleven sections preserve a post-early Miocene upward-coarsening sequence, consistent with the extensive crustal shortening and topographic growth across the northern Tibetan Plateau.

Evaluation of the ASL and Hawara formations using seismic- and log-derived properties, October Oil Field, Gulf of Suez, Egypt

October Field is one of the most prolific offshore oil fields in the Gulf of Suez of Egypt. It consists of a number of marine platforms and produces oil from different reservoirs of different geological ages from the Lower Cretaceous to the Miocene. The aim of this study was to enhance a seismic-log evaluational procedure to evaluate the Miocene-aged Asl and Hawara Formations which encounter the main hydrocarbon and source rock potentials in the area North of October Oil Field. The well logging data, the borehole seismic data, and the velocity surveys are all used in this study. A number of synthetic seismograms are constructed and interpreted together with the deduced seismic impedance and reflection coefficient data for many wells in the study area. In addition, a comprehensive velocity analysis is performed using the seismic times and the computed average and interval velocities. These seismic-derived parameters are primarily used in recognizing and locating precisely the marl and sand sections of Asl Formation which show low to middle amplitudes. Furthermore, a quantitative well logging analysis is carried out over Asl and Hawara Formations to shed light over their hydrocarbon potentiality. Good oil saturation is exhibited by the Asl sand section which reaches to more than 90% in the southern parts of the study area. The petrophysical characters of this sand are very good in terms of good effective porosity (9% to13%), low shale volume (Vsh 85%). The sections of the Asl marl and the Hawara shale on the other hand are considered completely wet. An integrated model making use of the seismic- and log-derived properties is applied over the two studied formations for better understanding the reservoir of interest. Many relations are constructed between velocity, seismic impedance, and the rock pore spaces on one hand and between the velocity, lithology, and fluid content on the other hand. This study revealed that the sand section attains very good oil-bearing potentiality in the study area and proved that the application of an integrated model of the log- and seismic-derived properties led to an enhanced evaluation of the Asl and Hawara Formations, good discrimination between their lithological components besides precise differentiation from the overlying Middle to Late Miocene sections.

Intraspecific morphological variation in late Miocene Bolboforma, and implications for their classification, ecology, and biostratigraphic utility

A preliminary morphological study of large, well preserved bolboform populations from the late Miocene section of DSDP Site 593, in the southern Tasman Sea, reveals a relationship between test size and intraspecific morphological variation interpreted as a gradual transformation in test shape and surface ornamentation. The succession begins with small, smooth, thin-walled subspheroidal tests. As test size increases, rudimentary ornaments become more common and the shape more globose. With further size increase, the walls thicken and ornaments become more structured, but specimens still retain a rounded globose shape. In still larger tests, the ornament becomes more prominent and tests develop a squatter shape with flatter aboral surfaces. The transformation generally ends with large, thick-walled, highly ornate forms with squat globose tests, robust ornaments, and relatively flat aboral surfaces. Although the size of the test increases notably through the transformation, there appears to be no corresponding change in the internal chamber size from its initial value, and so the transformation is largely due to the addition of calcite to the outer surface of the test wall. This appears to be an inherent biological feature and does not involve calcite dissolution/reprecipitation or other retrograde processes. The detail of the transformation varies between species and is presented here for Bolboforma subfragoris s.l., B. gruetzmacheri n. sp., and B. metzmacheri s.s., which exhibit different types of ornament — spinose, irregularly ridged, and cancellate, respectively. When different morphological forms of a single species occur in the same sample, they have the potential to be interpreted as different species. Taxonomic confusion is further compounded by the morphological similarity of many small, smooth-walled, and weakly ornamented forms of different species. These features are common in bolboform populations and have contributed to the misinterpretation of some species. Differences in test shape and ornament are most notable in large ornate forms and taxonomic interpretations should be based wherever possible on such forms. Once the taxonomy is clear, the biostratigraphic utility of the late Miocene bolboforms from Site 593 has been in the recognition of single species (or at most two) in a given sample. This provides an excellent biostratigraphic framework for a period of time that lacks significant alternative microfossils for age control in New Zealand sediments. The distribution of bolboforms in (mainly) southern hemisphere marine sediments appears to track the palaeocirculation of Southern Component Intermediate Water (SCIW). Hence addition of calcite to the outer surface of the test wall probably occurred while the bolboforms were entrained in subsurface flows of SCIW. In this respect, the increased complexity of ornamentation may have been a morphological adaptation to counter the loss of buoyancy as calcite was added to the outer surface of the test wall. This would have presumably contributed to the entrainment and dispersal of bolboforms in the flow of subsurface water masses, and it suggests that the tests of some bolboforms at least may represent a dispersal phase in the life cycle of this enigmatic group of microfossils.

Paleoenvironmental evolution of the Guercif basin (south Rifain corridor, northern morocco). Implications for the Messinian salinity crisis

Abstract Foraminiferal biostratigraphic and paleoenvironmental results from the MDS1 borehole of the Guercif basin reveal the detailed history of its paleogeographic evolution during the late Neogene. Four biostratigraphic events are recognised based on the study of planktonic foraminifera. In addition to the placement of the Tortonian-Messinian boundary at 798 m depth, they also allow correlation of the MSD1 borehole with other late Miocene sections in the Mediterranean area. The diversity of benthic foraminifera is significant in the studied borehole. 111 species assigned to 55 genera, were identified. Three successive assemblages of benthic foraminifera were recognised, each one conveying a different paleoenvironmental information. They thus recorded a number of fundamental changes in the environmental and water mass characteristics of the Guercif basin. Following the oldest upper Tortonian transgressive marine deposits found at the bottom of the borehole, marls accumulated at lower epibathyal to upper mesobathyal depths (~400 to ~800 m). They recorded sea-floor dysoxia and associated stratification of the water column for the time interval between ~7.5 and ~7.3 Ma. During the latest Tortonian-early Messinian the basin experienced a rapid shallowing, first from the previously bathyal environments to deep neritic (~100 to ~200 m), and to shallow neritic afterwards (0 to ~30 m). The total absence of any foraminifera in the conglomerates situated at the top of the study borehole suggests that these sediments were deposited in subaerial environments. At this time the connection between the Atlantic Ocean and the Mediterranean Sea through the South Rifain Corridor was stopped. This local event contributed to the establishment of the particular conditions of the Messinian salinity crisis.

Relative geomagnetic field intensity and reversals from Upper Miocene sections in Crete

Two Late Miocene sections at Potamida and Kotsiana in western Crete have been resampled at about 10 cm intervals (288 and 130 horizons, respectively). The two sections record the same two reversals bounding a normal polarity zone just below the Tortonian-Messinian boundary. Mineral-magnetic analysis identifies pseudo single-domain magnetite as the carrier of the main fraction of magnetization in both sections. Normalizations with ARM, SIRM and χ, on different specimens of the same cores, yield very similar results. After checking that it is not correlated to bulk magnetic parameters, the ARM-normalized record was used as a proxy for geomagnetic field intensity changes. In the lower part of the reverse polarity zone, sampled at Potamida, the field intensity appears to increase slightly then to decrease progressively to low values, prior to the R/N polarity transition. There is no evidence for a fast recovery of field intensity across the two transitions: the intensity remains very low after the R/N reversal and only progressively builds to a maximum in the middle of the following normal chron. It then progressively decreases until the N/R transition. No sudden recovery of the field intensity is observed across this reversal either. These results are thus inconsistent with the ‘saw-tooth’ record obtained by Valet and Meynadier for the last 4 million years. Either the behaviour of the field is different for different intervals of time or the hypothesis of sedimentary artefacts in the original saw-tooth record should be considered.

Sequence Stratigraphic Applications to Deep-Water Exploration in the Makassar Strait, Offshore East Kalimantan, Indonesia: ABSTRACT

A sequence stratigraphic study was conducted to help evaluate the exploration potential of the Makassar PSC, offshore East Kalimantan, Indonesia. The PSC is on the present-day slope in water depths of 500-3000 ft and borders the large oil and gas fields of the Mahakam delta. The study provided important insights on reservoir distribution, trapping style, and seismic hydrocarbon indicators. Lowstand deposition on a slope modified by growth faulting and shale diapirism controlled reservoir distribution within the prospective late Miocene section. Three major lowstand intervals can be seismically defined and tied to deep-water sands in nearby wells where log character and biostratigraphic data support the seismic system tract interpretation. The three intervals appear to correlate with third-order global lowstand events and are consistent with existing sequence stratigraphic schemes for the shelf and upper slope in the Makassar area. Seismic mapping delineated lowstand features, including incised valleys and intraslope to basin-floor thicks. Regional information on positions of middle-late Miocene delta lobes and shelf edges, helped complete the picture for sand sources, transport routes, and depocenters.

A new calibration point for the Late Miocene section of the geomagnetic polarity time scale: 40Ar/39Ar dating of lava flows from Akaroa Volcano, New Zealand

Recent dating of geomagnetic field reversals over the past 2 Ma, by both the radiometric and astrochronologic techniques, indicates that the K‐Ar derived ages for these reversals are too young. We report herein on 40Ar/39Ar incremental heating study of two geomagnetic field reversals trapped in lavas on Akaroa Volcano, New Zealand, that demonstrate that the K‐Ar derived ages for field reversals at 10–9 Ma are too young by about 7%. The new age for the termination of chron 5, a critical tie point on the geomagnetic polarity time‐scale for late Miocene time, is 9.67±0.11 Ma.

Magnetobiostratigraphy of a Late Miocene section from the Moroccan Atlantic margin

Paleomagnetic study of a Late Miocene section from Atlantic Morocco correlated with detailed biostratigraphic data suggests an assignment of the observed polarity sequence to the uppermost part of chron 7, to entire chron 6 and to the beginning of chron 5. Comparison with time equivalent sections in the Southern Hemisphere does not suggest noticeable delay in long faunal migration (between the Northern and Southern Hemisphere) during the late Miocene. Thus the postulation of a delay in arrival of species in the Eastern Mediterranean at the base of the Messinnian has to be explained by the presence of selective barriers which could have been located either between the Mediterranean and Atlantic or between the Western and Eastern Mediterranean in the basins delineated during the final evolution of the Tethys. This study also indicates a possible application of biomagnetostratigraphy to the reconstruction of basin interconnections.

The Magnetic Epoch-6 carbon shift: A change in the ocean's 13C/ 12C ratio 6.2 million years ago

Tropical Indian Ocean planktonic and benthic foraminifera have 13C/ 12C ratios which change abruptly within Magnetic Epoch 6 about 6.2 million years ago. All species analyzed in the Late Miocene section of DSDP Site 238 show a shift towards lighter values of δ 13C by about 0.8‰. The oxygen isotope signal indicates that the pre-shift period is climatically quiet while the post-shift period has strong fluctuations. We suggest that the shift reflects a sudden increase in the rate of supply of organic carbon from coastal lowlands and from shelves exposed by regression, as well as a change in deep circulation patterns and ocean fertility. The event marks the transition of the ocean—atmosphere system from a quiet Early and Middle Neogene climatic regime toward a Late Neogene regime characterized by climatic amplifying mechanisms (albedo feedback, bottom water production) located around the northern North Atlantic. The beginning of this regime may have been strongly influenced by the isolation of the Mediterranean basin.

Preliminary Revaluation of Late Miocene Biostratigraphy of California: ABSTRACT

Preliminary studies of the type Mohnian and Delmontian Stages and regional biostratigraphic studies of other late Miocene sections of California by the use of benthonic Foraminifera and fish indicate the following. 1. The Delmontian Stage of Kleinpell is correlative with his Mohnian Stage on the basis of the occurrence of (a) Epistominella gyroidinaformis (Cushman and Goudkhoff) in the lower part of the type Delmontian (this species does not range stratigraphically above early Mohnian in California); (b) Bolivina hughesi Cushman (=Bolivina sinuata alisoensis Cushman and Adams of others), Bulimina delreyensis Cushman and Galliher, and Etringus scintillans Jordan in the upper part of the type Mohnian in California; and (c) Late Miocene megafossils in the Santa Margarita Sandstone, which overlies with apparent conformity beds that are near, and correlate with, the upper part of the type Delmontian. 2. Kleinpell's Bolivina obliqua Concurrent-range Zone (Delmontian) overlaps the range of species characteristic of his subjacent Bolivina hughesi Zone in the type are of the Mohnian Stage and all other Mohnian zones at other localities in California. 3. Many of the species characteristic of his late Mohnian Bolivina hughesi Zone overlap both his subjacent Bulimina uvigerinaformis Zone and his superjacent Bolivina obliqua Zone. 4. Many of the species characteristic of his Bulimina uvigerinaformis Zone overlap both his subjacent Bolivina modeloensis Zone and his superjacent Bolivina hughesi Zone. 5. Bulimina uvigerinaformis Cushman and Kleinpell and Bulimina delreyensis Cushman and Galliher have the same stratigraphic range and appear not to range above middle Mohnian in California. Therefore, all of Kleinpell's late Miocene concurrent-range zones are in need of revision or redefinition. End_of_Article - Last_Page 559------------