Lower Miocene Sediments Research Articles

Mud volcanoes in the Alboran Sea: evidence from micropaleontological and geophysical data

During the BASACALB-TTR9 cruise of the R/V Professor Logachev (1999), two mud volcanoes (called Marrakech and Granada) were discovered in the southern sector of the mud diapir province in the West Alboran Basin (WAB). This paper presents micropaleontological and geophysical data on these mud volcanoes from gravity core samples, sidescan sonar (OKEAN) images and high-resolution seismic lines. Mud breccia recovered from the Granada mud volcano is matrix-supported with well-consolidated clasts of limestone, marlstone, claystone, siltstone, sandstone and mudstone, whereas mud breccia from the Marrakech mud volcano contains unconsolidated clasts. The mud breccia matrix contains abundant Miocene calcareous nannofossils (CN), together with Pliocene–Pleistocene species and reworked late Cretaceous and Paleocene–Eocene species. CN dating indicates that clasts in the mud breccia derive from late Cretaceous, Paleocene, Eocene, and probable Miocene sediments. These data suggest that the mud diapirs and mud volcanoes in the WAB can be derived from the olistostromes of Unit VI, the basal stratigraphic sequence in the Alboran Sea basin. Unit VI consists of lower Miocene sediments that incorporated late Cretaceous and Paleocene–Eocene materials and basement-derived rock fragments. The mud volcanic deposits are covered by a thin drape of pelagic marls, suggesting that these two volcanoes are currently inactive. Structures determined on high-resolution seismic profiles across mud volcanoes and surrounding diapirs correspond to the late-stage, Pliocene-to-Quaternary diapir development. This stage is thought to have developed during a compressional tectonic setting that produced folding and wrench tectonics throughout the basin. Mud ascent at that time resulted in active diapirism and mud volcanoes on the seafloor.

Nypa megafossils from the Tertiary sediments of Northeast India

Nypa megafossils dominated by fruits are described from Oligocene and Lower Miocene sediments of Assam and Mizoram, respectively. The origin, past distribution and migration of Nypa are discussed in the light of recent findings. This study indicates that India might be the place of its origin. Its presence, along with some other coastal elements, reinforces the concept that the Bay of Bengal was extending considerably northward during these epochs than its present day boundary.

Fohn lamproite and a possible Late Eocene — pre‐Miocene diatreme field, Northern Bonaparte Basin, Timor Sea

Seismic profiles in the Northern Bonaparte Basin, Timor Sea, northern Australia, disclose an east‐northeast‐trending 120 × 25 km swathe of over 40 circular to subcircular features excavated in the pre‐Miocene erosional surface and buried by Lower Miocene sediments. The larger structures, typified by the Fohn Structure, include central structural highs overlying narrow vertical corridors of upward‐bulging seismic horizons (bulge‐forms). Associated with these structural highs are troughs that overlie broader vertical zones of slower seismic velocity (crater‐form features). Smaller circular features (Dc <2.0 km) include both crater‐form and bulge‐form structures. The first type shows zones of seismic velocity crater‐form structures under craters excavated below the base‐Miocene horizon. The second type exhibits vertical seismic bulge‐form features directly beneath palaeotopographic highs developed at the same horizon. Identification in drill cuttings of fragments of olivine‐leucite lamproite at Fohn 1, at depths of 690–880 m, suggests that this structure is a lamproite diatreme consisting of a massive volcanic plug ringed by pyroclastics, analogous to the classic champagne‐glass structure of alkaline volcanic diatremes. The seismic morphometry of several of the larger structures of the field is analogous to Fohn, suggesting that they too may be diatremes. The smaller bulge‐form features may represent deeply eroded diatreme feeder necks. Crater‐like structures underlain by seismic velocity crater‐forms are interpreted as relatively little‐eroded maars with low‐velocity infill, probably consisting of volcanic and country rock breccia as at Fohn 1, as well as unconsolidated clastics of the transgressive Lower Miocene Oliver Sandstone Member. Lamproite fragments at Fohn 1 are dominated by apatite‐rich assemblages consisting of saponite‐altered olivine, analcime and nontronite‐altered zoned leucite, high‐Ti K‐bearing diopside, Ti‐rich richterite, Ti‐rich phlogopite, Mg‐rich ilmenite, Cr‐bearing priderite, Ba and Sr‐rich fluorapatite and rare‐earth element‐rich apatite. Textural relations suggest early crystallisation of olivine, leucite and K‐bearing diopside followed by crystallisation of alkali amphibole, priderite, ilmenite and apatite in alkali‐Tindash;Fe–rich groundmass. Fohn 1 lamproites have very high whole‐rock P levels, high Na,Sr and Y and low K,Ba,Zr and Nb levels compared to Early Miocene West Kimberley lamproites. The mineral paragenesis suggests crystallisation of olivine‐leucite assemblages at temperatures >1100°C and pressures <0.1 GPa, followed by crystallisation of amphibole and phlogopite under water‐saturated conditions at temperatures below ∼1020°C.

Origin and preservation processes of amorphous organic matter in the Maykop Series (Oligocene-Lower Miocene) of Precaucasus and Azerbaijan

The clay sediments of the Maykop Series accumulated during the Oligocene-Lower Miocene in eastern Paratethys are considered as one of the major source rocks of Precaucasus and Azerbaijan. As numerous marine source rocks, petrographical observations showed that amorphous organic matter (AOM) is widely predominant in these sediments [Fadeeva, 1973; Saint-Germes, 1998]. However, the geochemical characteristics (TOC, HI) of the Maykop Series vary widely [Polster et al. 1960; Sidorenko, 1964; Larskaya, 1966; Saint-Germes et al., 2000a, b]. The integrated geochemical and petrographical investigations were carried out in order to establish the origin of AOM in the Maykopian sediments. Moreover, the organic matter preservation in these sediments deposited in the dysoxic-anoxic paleoenvironments may result from several processes as revealed recently [Sinninghe-Damste et al., 1989; Largeau et al., 1990; Derenne et al., 1991; Lee, 1992; Keil et al., 1994]. Analysis of the sulphur content in the kerogens and ultrastructural observations by transmission electron microscope (TEM) of AOM particles were performed to determine the involved preservation processes.

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.

Taxonomic and biostratigraphical re‐assessments of Subterraniphyllum Elliott (Corallinales, Rhodophyta)

A taxonomic and biostratigraphical re‐assessment of Subterraniphyllum Elliott (Corallinales, Rhodophyta) is presented. Results from studies of the type collection and of newly collected material from north‐eastern Italy and northern Slovenia have shown that this taxon is not a geniculate coralline red alga as originally suggested by Elliott and most subsequent authors. Vegetatively, Subterraniphyllum most closely resembles certain living members of the Corallinales; however, the phenetic and phylogenetic relationships of Subterraniphyllum to other Corallinales cannot be determined with greater certainty. The exclusion of Subterraniphyllum from any group of Corallinaceae with genicula is based on unequivocal evidence that branch formation does not involve the occurrence of genicula. Subterraniphyllum seems to be restricted to the Oligocene. Reports of occurrences in Upper Eocene and Lower Miocene sediments cannot be substantiated. Subterraniphyllum, however, cannot be considered a useful stratigraphical marker until further data on its occurrence in well‐dated carbonate sequences are acquired. This study illustrates the problems associated with placing fossil coralline algal specimens into geniculate genera without the preservation of relevant morphological characters. This is especially true in the absence of the careful assessment of fossil material with respect to current taxonomic concepts of geniculate coralline genera, all of which are based on studies of living species. According to the current concepts for geniculate coralline genera, the placing of fossil specimens into geniculate genera without appropriate evidence must be avoided by grouping all potentially geniculate fragments under the informal group ‘Geniculate sensu lato’. Furthermore, for all those many fossil specimens where unequivocal evidence is not present, it is possible to utilize ‘form genera’ based on characters that are normally preserved. This leads to creating a consistent, workable system of applying names to most fossil corallines so that they can be reliably used in relation to stratigraphical and palaeoecological studies.

Upper Cainozoic stratigraphy in the central North Sea (Ekofisk and Sleipner fields)

The investigation is based on analyses of planktonic and benthonic foraminifera and Bolboforma carried out in the uppermost Palaeogene and Neogene of the production well 2/4-C-11 (56°33' N, 3°13' E) on the Ekofisk Field and the exploration well 15/12-3 (58°14' N, 1°53' E) on the Sleipner Field. The analyses are based primarily on ditch cuttings, but in well 2/4-C-11, 49 sidewall cores and six short conventional cores were also available. Strontium isotope and paleomagnetic analyses were also carried out in well 2/4-C-11. A 420 m-thick accumulation of Lower Miocene sediments is present in the Ekofisk area. The Middle Miocene comprises approximately 180 m, and the Upper Miocene is represented by an approximately 500 m-thick interval. A hiatus is present in the Middle Miocene, and the middle part of the Middle Miocene is missing. A thin interval (approximately 120 m) of Lower Pliocene sediments is found overlying the Upper Miocene. Uppermost, a thick accumulation of Upper Pliocene-Pleistocene sediments (approximately 1000 m) is in place. More than half of this was deposited during the Pleistocene. The entire Upper Pliocene is represented in this area. This contrasts to the northern North Sea, as well as areas further north, where sediments of this epoch are partially missing in a hiatus comprising the lowermost part of the Upper Pliocene. In the Sleipner area the Lower and Middle Miocene are approximately 160 m and 180 m thick, respectively. The Upper Miocene (approximately 100 m) and the Lower Pliocene (approximately 40 m) thin from the Ekofisk towards the Sleipner Field. Uppermost, an approximately 900 m-thick succession of Upper Pliocene-Pleistocene sediments is present. Nearly two-thirds of this accumulation are Pleistocene in age.

Implications of late Eocene to early Miocene clay mineral assemblages in McMurdo Sound (Ross Sea, Antarctica) on paleoclimate and ice dynamics

The clay mineral assemblages of upper Eocene to lower Miocene sediments recovered at the CIROS-1 and MSSTS-1 drill sites on the McMurdo Sound shelf, Antarctica, were analyzed in order to reconstruct the Cenozoic Antarctic paleoclimate and ice dynamics. The assemblages are dominated by smectite and illite, with minor amounts of chlorite and kaolinite. The highest smectite amounts and best smectite crystallinities occur in the upper Eocene part of CIROS-1, below 425–445 mbsf. They indicate that during their deposition, chemical weathering conditions prevailed on the nearby continent. Large parts of East Antarctica were probably ice-free at that time, but some glaciers reached the sea and contributed to the glaciomarine sedimentation. In contrast, only minor total amounts of smectite are present in Oligocene and younger sediments due to the shift to mainly physical weathering on an ice-covered Antarctic continent. However, relative smectite percentages rise to more than 60% during two late Oligocene intervals (ca. 27.5–26.2 and 25.0–24.5 Ma) and during one early Miocene interval starting at ca. 23.3 Ma. These intervals are characterized by ice masses coming probably from the south, where volcanic rocks acted as a source, as also indicated by the composition of the sand and gravel fractions. During the other intervals, the ice came from the west, where the physical erosion of basement rocks and sedimentary rocks of the Beacon Supergroup in the Transantarctic Mountains provided high illite concentrations. Because the two drill sites are only 4 km apart, their clay mineral records can be correlated. This led to a new interpretation of the Oligocene paleomagnetic data of the MSSTS-1 site and to a more detailed lithostratigraphic correlation of the Miocene parts of the cores.

Seismic Data in the Southern Maturin Sub-Basin in Eastern Venezuela Reveals the Structure and Strata I Patterns of the Lower Miocene Sediments: ABSTRACT

Seismic Data in the Southern Maturin Sub-Basin in Eastern Venezuela Reveals the Structure and Strata I Patterns of the Lower Miocene Sediments: ABSTRACT

Thermohaline Pore Water Trends of Southeastern Louisiana: Geologic Applications and Controls on Fluid Movement: ABSTRACT

ABSTRACT Previous research has suggested that dissolution of salt diapirs and the formation of dense, saline brines at shallow depths are concurrent with large scale fluid migration. A critical foundation of these studies is the determination of salinity from the spontaneous potential (SP) log and the ability to drive fluid vertically through the sediment. Derivation of salinity using the perfect shale membrane model and contouring iso-salinity values over intervals of Lower Miocene and Upper Oligocene sediments that contain thick, impermeable carbonate deposits cloud these findings. The calculation of salinity is based on water resistivity (Rw) variations and the geological constraints on derivation of this variable. Application of the imperfect shale membrane model to determine Rw from the SP log provided a closer approximation to Rw from produced water samples over St. Gabriel Field in Ascension and Iberville Parishes, Louisiana than past SP models. Further analyses of temperature, pressure, salinity, and freshwater hydraulic head trends of Lower Miocene and Upper Oligocene deposits over the field and surrounding area suggest that dissolution of salt occurred prior to hydrocarbon generation and large scale fluid migration is not dynamic at present. Important controls that should be used in future studies of thermohaline fluid movement are the identification of local structure, stratigraphic variation, shale membrane efficiency, and time of salt diapirism.

Miocene to Quaternary paleoenvironments and uplift history on the mid Norwegian shelf

Based on benthic and planktic foraminifera, Bolboforma, oxygen isotope measurements and seismic data, major changes in Miocene, Pliocene and Pleistocene paleoenvironments on the mid Norwegian shelf are discussed and a possible scenario of the late Cenozoic uplift history is given. The dating of the Neogene sequence has been done using foraminifera and Bolboforma. Four main assemblage zones have been identified with nine distinct subzones. Most of the Miocene sequence is preserved. The lower Miocene sediments contain only siliceous microfossils. A period of high fertility and upwelling in the study area prevailed. The early Miocene-early mid Miocene (15 Ma?) change from a siliceous to a calcareous rich microfauna, dominated by Nonion barleeanum, can be related to increased surface-water circulation due to overflow across the Iceland-Faeroe ridge. During the Miocene the temperature decreased in the study area. Evidence of increased amounts of coarser sediments may suggest that an uplift of the mainland areas occurred during the mid-late Miocene. Lower Pliocene sediments contain a foraminiferal fauna that seems to occur in slightly colder conditions than the late Miocene fauna suggesting a further cooling. Possibly, Arctic waters entered the study area in the early Pliocene. A very marked change in lithology (from compacted claystone to unconsolidated diamicton), fauna (from deep dwelling to shallow dwelling species) and seismic signature (from flat lying reflectors to prograding clinoforms) occurs during the mid?-late Pliocene. A two step cooling trend is indicated by the microfauna of these prograding wedges. 1. (1) The first wedge buildups might have been associated with an uplift of the mainland during the early late Pliocene (mid Pliocene, ca. 4 Ma). However, the age determination is somewhat uncertain and may very well be of late Pliocene age. 2. (2) The second step of wedge buildup is associated with a glacial phase where the dominating microfauna exists of arctic species. Large continental ice sheets might have occurred at this time reaching coastal areas and that possibly many of the geomorphological features such as the strandflat were made during this episode. The Pleistocene epoch is represented by an increased percentage of boreal foraminifera intermingled with high arctic species which indicates that interglacial-glacial cycles prevailed and the dynamics of the glacier system changed.

Neogene Planktonic Foraminiferal Biostratigraphy of the Northeastern Gulf of Mexico

We have evaluated the planktonic foraminiferal biostratigraphy of several Gulf of Mexico Eureka boreholes, emphasizing the Miocene depositional history of the northeastern region (De Soto Canyon). The E68-136 borehole (De Soto Canyon; 1826', 557m present depth) provides the most continuous Miocene record, although deposition was punctuated by several short (<2.0 m.y.) hiatuses. The E66-73 borehole (De Soto Canyon; 2802', 854m present depth) provides an uppermost lower to upper Miocene and upper Pleistocene record. E68-151 (north of the Florida Escarpment; 4340', 1323m present depth) provides a discontinuous record of lower, upper middle, and upper upper Miocene sediments. By integrating planktonic foraminiferal biostratigraphy with nannoplankton biostratigraphy and Sr-isotope stratigraphy, we were able to estimate the ages of the hiatuses associated with six distinct unconformities at E68-136 (23.7-23.2 Ma; 18.0-16.5 Ma; 16.3-15.2 Ma; 14.1-13.6 Ma; -10.4-9.5 Ma, 5.0-1.9 Ma), three at E66-73 (-11.5-10.8 Ma, 10.3-9.2 Ma, 6.5-1.7 Ma), and two at E68-151A (19.0-12.6 Ma, 12.5-6.9 Ma). We correlate these unconformities with previously reported unconformities from other deep-sea locations, although their causes (i.e., whether related to sea-level or deep-water changes) remain uncertain. We relate a distinct Pliocene unconformity to intensification of the Loop Current associated with the closure of the Isthmus of Panama.

The magnetostratigraphy of the Chitarwata and lower Vihowa formations of the Dera Ghazi Khan area, Pakistan

Three sections of the Chitarwata and lower Vihowa formations were sampled along the Dalana River on the southeastern flank of the Zinda Pir Dome in the southern Sulaiman Range near Dera Ghazi Khan, Pakistan. Together they provide nearly 800 m of a continuous sedimentary record of the Miocene derived from the uplifted Himalayan highlands. Previous studies have examined the Middle and Upper Miocene sediments, the Siwalik Group, of the Potwar Plateau to the north. However, detailed investigations of earlier periods are impossible in that area due to the absence of Oligocene and Lower Miocene sediments caused by continued overthrusting associated with the Himalayan orogeny. Fortunately, the Sulaiman basin to the south, which was further removed from the tectonic activity, provides a record of the Early Miocene in the form of the Chitarwata Formation. The Dalana A, B, and C sections (DGA, DGB, and DGC) were examined and sampled for this study. A magnetostratigraphic analysis was carried out to correlate and date the Chitarwata and Vihowa formations exposed in this area. The samples were subjected to step-wise demagnetization to resolve the primary and secondary remanent magnetization components. Thermal demagnetization trajectories indicate that the majority of samples have sufficient internal consistency that their calculated polarities are reliable. Likewise, the majority of the 126 measured sites showed statistically significant agreement between the three measured samples per site. However, the bimodal data set does not pass the reversal test and so is deemed unsuitable for tectonic interpretations. The individual sections were initially correlated using lithologic and stratigraphic methods, and the relationship was reinforced by the magnetostratigraphy. The composite DG section was tentatively correlated with the standard magnetic polarity time scale placing the disconformable base of the Chitarwata in the DGA section at just older than 22 Ma, the Chitarwata/Vihowa contact at 18.6 Ma, and the top of the Vihowa in the DGC section at about 16 Ma. The assignment of these dates to the Chitarwata will aid in future biostratigraphic and lithostratigraphic correlation of Early Miocene sediments, effectively extending the well-established Siwalik faunal sequence back by four million years.

Early Miocene flora of the Manuherikia Group, New Zealand. 1. Ferns

Abstract Three ferns are described from lower Miocene sediments of the Manuherikia Group, New Zealand. One of these is compared with a species in the extant New Zealand Blechnum “procerum/capense/kiokio” complex, B. cf. procerum. Pneumatopteris sp. (distinct from the extant New Zealand P. pennigera (Forst, f.) Holttum, Thelypteridaceae) is recorded. A further form is of unknown affinity.

Coalification, shale diagenesis and thermal modelling in the Alpine Foreland basin: the Western Molasse basin (Switzerland/France)

The analysis of coalification and shale diagenesis in Oligocene and Lower Miocene sediments in the western part of the Molasse basin (Western Switzerland and Haute-Savoie, France) shows a close relationship between the very low-temperature metamorphism in the foreland and the tectonic history in the orogenic belt. The degree of thermal maturation generally increases towards the alpine front. Isomaturity lines, however, run slightly oblique to today's alpine front. This is probably due to the tectonic burying under the alpine nappes in Early Miocene time. Second order variations of the coalification pattern are explained as resulting from a regional discharge of warm fluids controlled by tectonic structures.

Braarudosphaera-Rich Sediments on the Exmouth Plateau, Eastern Indian Ocean

Calcareous nannofossil assemblages enriched in Braarudosphaera occur in lower Oligocene to lower Miocene sediments at Ocean Drilling Program Sites 762 and 763 on the central Exmouth Plateau. Although beds of Braarudosphaera-rich sediments are well known in the Oligocene of the South Atlantic (the Braarudosphaera Chalk), this is the first published report of this phenomenon in the easter Indian Ocean. Braarudosphaerids appear rather abruptly in the lower Oligocene (in Zone NP 21) on the Exmouth Plateau. They reach their greatest numbers in the lower Oligocene (in Zones NP 22 and NP 23), where they comprise up to 10% of some samples. Braarudosphaera bigelowii is the overwhelmingly dominant species, occurring together with rare specimens of B. discula and Micrantholithus pinguis. The holococcoliths Peritrachelina joidesa and Lanternithus minutus are also associated with the Braarudosphaera enrichment. There are two populations of B. bigelowii: one of normal size (10-14 {mu}m) and one of large specimens (20-22 {mu}m). The larger braarudosphaerids are more common than the smaller forms. Braarudosphaera-rich sediments are absent at the nearby Wombat Plateau sites during the same time interval. The authors attribute this to latitudinal control, since the Wombat sites are about 4{degree} north of the Central Exmout Plateau sites. The occurrencemore » of braarudosphaerids on the Exmouth Plateau is related to an Oligocene to early Miocene oceanographic event. Stable-isotope and trace-element analyses suggest that upwelling of cool, low-salinity, nutrient-rich water along a Divergent Zone created the Braarudosphaera-rich sediments in the Indian Ocean.« less

Geology and tectonics of the Ryukyu Islands

The Ryukyu Islands are divided morphologically and geologically into three island groups: north, central and south Ryukyus. North-central Ryukyu represents the geological continuation of the Outer Belt of southwest Japan, composed of Mesozoic-Eocene sedimentary sequences, whereas south Ryukyu is characterized by high-pressure metamorphic rocks, Eocene volcanics and limestone, and lower Miocene sediments. The geological and structural contrasts between north-central and south Ryukyus are conspicuous before the late Miocene transgression covered the whole area. The Ryukyu Islands have been established since then. The successive developments of the Goto-Tunghai-Senkaku basins since the Paleogene, of the Okinawa Trough since the Miocene, and of the grabens near the Island group since the Pliocene, signify a southeastward shift of the basins in relation to the activity in the granite diapir zones accompanied by volcanism.

Miocene Carbonate Reservoirs Related to Tectonic and Thermal Evolution of Southeast Asian Marginal Basins: ABSTRACT

The early Miocene global sea level rise and oceanic warming period allowed the tropical reef growth belt to expand and fostered the development of major carbonate buildups throughout southeast Asia. A regional paleogeographic reconstruction for 18 m.y. places reefal, shelf, and basinal facies in a tectonic setting of island arcs, subduction zones, and marginal basins. For typical basins, such as the Sulu, Celebes, and South China Sea basins, basin formation and sedimentation models have been developed based on ages inferred from identified marine magnetic anomalies and heat flow data. These basins have many of the attributes needed for hydrocarbon development and maturation. They accumulated sediment from pelagic sources and surrounding island arcs and landmasses fringed by reefs. During the early Miocene, limited water circulation in restricted basins, such as the Sulu and Celebes basins, may have induced dysaerobic conditions that enhanced organic carbon preservation. Models of marginal basin formation provide the basis for studying the time-dependent thermal histories of their sediment sequences. The authors models show that, for example, lower Miocene sediments deposited at a rate of 100 m/m.y. on 20-m.y.-old crust in a typical basin have just entered the oil-generation window. Lower sedimentation rates require deposition on younger crustmore » in order for the sediments to reach an equivalent maturation stage. Estimates of the hydrocarbon potential of such marginal basins should be based on a sequential time-slice analysis of each basin in terms of sediment type, sedimentation rate, sea floor age and thermal regime, and the presence of reservoirs.« less

Paleoenvironmental Data as Exploration Tool in Lower Miocene of Offshore Texas: ABSTRACT

Most paleontological reports based on foraminiferal assemblages in well cuttings include an interpretation of the environment of deposition of the sediments penetrated. For an individual well, these data may be summarized as a paleoenvironmental curve. Data on a group of wells can be synthesized into paleoenvironmental maps and cross sections--useful tools for predicting sand distribution. These maps, used independently or in conjunction with net sand maps, can indicate the locations of ancient delta systems, hence sand sources. Paleoenvironmental cross sections graphically display transgressions and regressions. The lower Miocene sediments in the Mustang Island and Matagorda Island areas of offshore Texas were deposited in a wide range of shelf and upper slope paleoenvironments. Paleoenvironmental maps, based on about 50 wells, suggest that a number of major delta systems developed in the Mustang Island and Matagorda Island areas during the early Miocene. Electric-log data show that thick pods of sand are associated with each of these ancient delta systems. Paleoenvironmental cross sections indicate that, although the section just above Siphonina davisi in southern Mustang Island is strongly regressive, the equivalent section in eastern Matagorda Island is transgressive. Determination of such transgressive/regressive trends is vital to predicting the dip position of potential reservoir sands. A cross section through Matagorda block 622 illustrates that a considerable thickness of deep-water sediments can overlie an older, shallower water, sandy interval. Therefore, the penetration of a thick sequence of deep-water shales does not necessarily indicate that underlying prospective sections will not be encountered. End_of_Article - Last_Page 242------------

Acid volcanic glass and pumice in lower Miocene sediments of the Northern Pacific SSE of Hawaii

Acid volcanic glass shards and pumice with SiO 2 contents of about 72 ± 2% were found in early Miocene pure radiolarian ooze at Site VA 13 1 SSE of Hawaii between Clarion and Clipperton fracture zone. An origin from northern Central America is suggested.