Oligocene Sands Research Articles

The effect of geology and agricultural development on the groundwater, West Minia, Egypt

Fifty groundwater samples were collected from the Oligocene sand and Middle Eocene limestone aquifers and analyzed for major ions and stable isotopes. Aquifer sediment samples were examined to determine hydraulic properties, and sixty-six well logs were investigated to understand the subsurface geology. The study reveals that lithofacies, mineral composition, and geologic structures significantly influence groundwater quality and behavior. Karst features in the Middle Eocene limestone aquifer create subsurface caves, leading to challenges such as abrupt salinization and well failure. In the Oligocene aquifer, groundwater is predominantly of the sodium chloride type. In the Middle Eocene aquifer, 80 % of samples are sodium chloride, with variations of sodium bicarbonate (10 %) and sodium sulfate (10 %) types. Remote sensing from 2013 to 2021 shows significant agricultural expansion, with vegetation increasing by over 227 km². This growth, driven by large agricultural projects, has led to substantial groundwater consumption, resulting in a 5-meter decline in water levels within just one year (from 2020 to 2021). The over-extraction poses a risk to the aquifers, threatening long-term sustainability. Understanding the region's geology is crucial for mitigating these issues and ensuring sustainable groundwater management.

Petrophysical and seismo-stratigraphic analysis of the tertiary formations of the South onshore of the Rio Del Rey Basin (Cameroon margin, Gulf of Guinea) from well logs and seismic data

The present study used well log and seismic data to determine the main lithology types, characterize and evaluate the reservoirs properties and discuss about the hydrocarbon quality and productivity in the onshore south of the Rio Del Rey Basin (RDRB). The main results show seven lithologies inside the Isongo, Diongo and Nguti including clays, sandstones and clayey sandstones, the most dominant, and limestones, dolomites, sandstone limestones and clayey limestones, the less dominant. Seventeen reservoir zones with fifteen gas-bearing and two oil-bearing were identified in the Miocene sands of the Isongo, Diongo and Nguti Formations and very few in the Upper Oligocene sands of Akata Formation. The average values of petrophysical parameters obtained are Vshale (13.26–31.3%), NTG (64.48–87.07%), ∅eff (18.54–26.60%), ∅avg (23.27–33.59%), Sw (4.84–11.10%), Shc (88.90–95.16%), and K (1258.96–2187.26 mD). It shows that the identified reservoirs are of the good quality and can be commercially viable for the gas. Knowing that the RDRB and the neighbouring ones (Niger Delta, Rio Muni and Gabon basins which are OPEC members) have good petrophysical parameters, findings from this investigation show that the key difference between RDRB and others lay on the low continuity and vertical extension of these reservoirs, as well as the lower sedimentary thickness (6–7 km), the tectono-stratigraphic architecture and the production essentially focused on the Miocene horizons.

Provenance study of potential Oligocene reservoirs in Nile Delta and offshore Mediterranean

This study aims to investigate the provenance of the Oligocene sandstones exposed at Gebel Qatrani Formation at the Fayum Province and analogues subsurface Oligocene sandstone intervals cored from 2.7 to 5.12 km at the Nile Delta. The analysis and quantification of heavy mineral assemblages revealed that the two sites maintain similar heavy mineral suites but with variable concentrations and different patterns. The provenance reliable heavy mineral indices indicate that the exposed Oligocene sandstones at Gebel Qatrani Formation are much more mature than those of the Oligocene sandstone intervals at the Nile Delta. The lower and the upper members of Qatrani Formation display heavy mineral contents indicating more maturation than the middle member which reflects an influx of less mature sandstone. The middle member may reflect a tectonic reconfiguration of the hinterland during its deposition. The reliable heavy mineral indicators of the Oligocene sandstones at the Nile delta maintain their higher values at different depths along the entire wells indicating a significant rejuvenation of the source area and hence sand supply from a source(s) that was not far distant. In contrast, Habbar-1 well sand samples (cored from 3.8 to 4.5 km) display the highest pyroxene values along with considerable contents of amphibole, mica, sillimanite, and kyanite indicating transport from a shorter distant source including a metamorphic one. Thirty seismic cross sections are interpreted and used to construct isopach maps which revealed the diagenetic effect of the heavy mineral suits with depth. That will subsequently affect the reserving characteristics of Oligocene sand intervals. Moreover, according to the estimated petrophysical parameters, the Oligocene intervals in Habbar-1 and Port Fouad Marine Deep-1 wells are very promising.

Development of HFU-based permeability prediction models using core data for characterisation of a heterogeneous Oligocene sand in the Nam Con Son basin

Core data by both routine and special core analysis are required to understand and predict reservoir petrophysical characteristics. In this research, a total number of 50 core plugs taken from an Oligocene sand (T30) in the Nam Con Son basin, offshore southern Vietnam, were tested in the core laboratory of the Vietnam Petroleum Institute (VPI).
 The results of routine core analysis (RCA) including porosity and permeability measurements were employed to divide the study reservoir into hydraulic flow units (HFUs) using the global hydraulic elements (GHEs) approach. Based on five classified HFUs, 16 samples were selected for special core analysis, i.e., mercury injection capillary pressure (MICP) and grain size analyses for establishing non-linear porosity-permeability model of each HFU based on Kozeny-Carman equation, which provides an improved prediction of permeability (R2 = 0.846) comparing to that by the empirical poro-perm relationship (R2 = 0.633).
 In addition, another permeability model, namely the Winland R35 method, was applied and gave very satisfactory results (R2 = 0.919). Finally, by integrating the results from MICP and grain size analyses, a good trendline of pore size distribution index (λ) and grain sorting was successfully obtained to help characterise the study reservoir. High λ came with poor sorting, and vice versa, the low λ corresponded to good sorting of grain size.

Assessment of Paleocene to lower Oligocene formations and basement to estimate the potential hydrocarbon reservoirs using seismic inversion: a case study in the Upper Assam Shelf, India

The upper Assam shelf is a self-slope basin in north-east India, filled with nearly 7 km of sedimentary rocks of tertiary period with the granite basement and various oil fields along the border of the Naga thrust. The major producing fields are structural and strati-structural. The study area is placed in between the Mikir hills and Naga thrust. The objective of the study is to identify potential hydrocarbon reservoir zones in the geologically complex south upper of the Assam shelf using estimates of acoustic impedance and porosity derived by 3D post-stack seismic inversion. Well data, such as sonic velocity and density logs, from two wells (namely, KA and TE) are used in the inversion and validation of results. Inversion results are used to build a geological model in the form of acoustic impedance from which we derive 3D porosity cube which are used for hydrocarbon potential in the Paleocene to lower Oligocene sands, and the Precambrian basement. Although the amplitude maps provide an indication of potential reservoirs, the extent of these zones are much better identified in the inverted impedance maps and the corresponding estimated high-porosity zones. The analysis predicted the potential reservoir rocks in the Sylhet, Kopili and Barail formations, in which the Sylhet and Kopili appear to have good potential zones. Near the vicinity of the Naga thrust belt, the proximity of potential reservoir is predicted in the Kopili, Sylhet formation and in the fractured basement, respectively.

Numerical Simulation of Low Salinity Water Flooding on Core Samples for an Oil Reservoir in the Nam Con Son Basin, Vietnam

Low-salinity water flooding (LSWF) is environment-friendly and operates similarly to conventional waterflooding without the need for synthetic chemical materials. The application of LSWF makes sense in Vietnam as HC production has steadily declined since 2002, and the majority of main oil fields have become near mature and mature fields. In the next years, Enhanced Oil Recovery (EOR) should be a top priority for Petro Vietnam to boost its oil production, for which the key issue is how to select a suitable EOR technology. In this study, LSWF of the Lower Miocene sand using low salinity water from Lower Oligocene sand was investigated. Previously at the Ruby field in the Cuu Long Basin, an LSWF feasibility study was carried out based on a conventional core flooding experiment, which is time-consuming and costly. This study targets the Chim Sao field in the Nam Con Son Basin, for which a cheaper and faster assessing method is required. As a result, a numerical code written in Matlab was developed and successfully validated with the core flooding experiment results obtained at the Ruby field. The LSWF simulation was conducted using the multiple ion-exchange mechanisms (MIE), and the results obtained showed an increase in the oil recovery factor by 2.19% for the Lower Miocene Sand. Another important outcome of this study is the innovative proposal and successful simulation to use the abundant low salinity water from the underlying Lower Oligocene sand as a natural LSW source to inject into the Lower Miocene oil reservoir that can be a decisive factor to help apply LSWF in practice on a wide scale not only for Chim Sao but also other similar oil fields in southern offshore Vietnam.

The First Results of U–Pb Dating of Detrital Zircons from the Oligocene of the Southeastern Part of the Voronezh Anteclise and Their Importance for Paleogeography

The first results of U–Pb (LA–ICP–MS) isotope dating of detrital zircons (dZr) from the Upper Oligocene sands of the Poltava Formation exposed on the southeastern slope of the Voronezh anteclise are presented, providing information on the provenance sources. It is established that the dataset of dZr ages (provenance signal) is characterized by a predominance of ages within the range of 1–2 Ga with a low number of Phanerozoic, Late Proterozoic (younger than 1 Ga), and Archean dates. It is revealed that the cumulative ages of the complexes of the Svecofennian and Sveconorwegian orogens and, to a lesser degree, the Cadomides of Central Europe provide entirely the provenance signal recorded in the sands of the Poltava Formation. The data obtained allow us to refine the paleogeography of the Paratethys northern margin during the Late Oligocene. It has been found that the Poltava Formation was accumulated due to water transport of sediments from intensively eroded structures of western Fennoscandia from the North Sea to the Scythian Sea.

Stable Isotope Ratios of Carbonate and Organic Carbon from Silicified Tree Trunks, Petrified Forest, New Cairo, Egypt – Possible Interpretations of Palaeoenvironment

The silicified tree trunks of the New Cairo Petrified Forest – evidence of fossilized remains of Paleogene angiosperms also abundant in other sites in Egypt – are found in horizontal positions on the surface and buried completely or partly by Oligocene sand and gravel underlain by late Eocene sediments. Differences in petrographic characteristics allowed for grouping these silicified woods into two types: (1) Type A composed of microgranular quartz ± some goethite ± minor moganite and (2) Type B composed of chalcedony or a mixture of chalcedony and cryptograined quartz ± some goethite ± minor gypsum. The two type groups contain traces of carbonate and organic matter: 0.004 to 0.35 wt % CaCO3 and 0.004 to 0.16 wt % organic carbon. The carbonate of each type has characteristic δ13C and δ18O values: –3.2 ± 5.7‰ VPDB and –0.88 ± 5‰ VPDB for silicified wood Type A and –10.9 ± 6.8‰ VPDB and –7.64 ± 3.3‰ VPDB for silicified wood Type B, respectively. Covariation between carbonate δ13C and δ18O values of both silicified wood Type A and B samples indicate a hydrologically closed system with different inorganic carbon and meteoric water signatures. These interpretations are supported by mineralogical and chemical data from the samples. Various factors likely influenced the carbonate δ13C and δ18O values of the silicified wood samples during the time they formed, including biogenic CO2, water residence effect, salinity, and evaporation. The δ13C values of bulk organic matter of silicified wood Type A and B samples ranging from –24.04 to –26.89‰ VPDB are within a characteristic range of C3-type biomass. The shift to a less negative bulk organic matter δ13C value in these samples (particularly those of the silicified wood Type B) has been attributed to post-depositional changes.

Upper Oligocene marine fishes from nearshore deposits of the Central Paratethys (Máriahalom, Hungary)

A rich and diverse ichthyofauna is described from the upper Oligocene (Egerian) sands of Mariahalom, Hungary. The site is dominated by brackish molluscs that are preserved together with rare marine and terrestrial vertebrates. Based on the isolated elasmobranch and bony fish remains, eight sharks, four rays and seven teleost taxa were identified from Mariahalom. The ichthyofauna represents a nearshore marine subtropical community dominated by odontaspidid and carcharhinid sharks and by euryhaline durophagous sparids and by sciaenid teleost fishes. Epibenthic feeders were common, whereas piscivorous taxa included barracudas, billfishes and numerous sharks, including the macropredatory Otodus angustidens. Palaeontological, sedimentological and stratigraphical data indicates a tide-influenced and fluvial-influenced nearshore palaeoenvironment with brackish lagoons and normal marine littoral habitats. The remains of terrestrial, brackish and nearshore marine invertebrates and vertebrates accumulated in a tidal channel. The systematics of the ichthyofauna is consistent with the palaeogeography inferred from invertebrates, i.e. a Central Paratethys connected to the Mediterranean area during the Egerian.

Halloysite occurrence at the karstified contact of Oligocene sands and Cretaceous calcarenites in Hinnisdael quarries, Vechmaal (NE of Belgium)

1. Introduction Halloysite is a dioctahedral 1:1 clay mineral of the kaolinite group frequently discussed in literature because of its potential for nanotechnological applications (Keeling, 2015; Yuan et al., 2015; Yuan et al., 2016). Its geological occurrence has been primarily linked to soil and weathering environments, by the weathering and alteration of volcanic rocks (Vaughan et al., 2002; Velde & Meunier, 2008), the alteration of clay minerals like montmorillonite or biotite (Hill, 2000; Papoulis et al., 2009) or weathering of feldspars (Sheets & Tettenhorst, 1997; Adamo et al., 2001). Halloysite is also a common mineral constituent in karst and paleokarst environments as a result of acid weathering (Polyak & Güven, 2000; Joussein et al., 2005). In Belgium, halloysite was reported in over 30 localities, almost all with a very similar geological setting, i.e. karstified carbonate substrates filled up by Cenozoic sand deposits (Buurman & Van der Plas, 1968; Dupuis & Ertus, 1995; Goemaere & Hanson, 1997; Nicaise, 1998; Kloprogge & Frost, 1999; De Putter et al. 2002; Bruyère, 2004). A similar geological setting is found inside the underground quarries of Hinnisdael, locally known as “mergelgrotten” (“marl caves”), located in Vechmaal, Limburg province, Belgium (Fig. 1). In two of the Hinnisdael underground quarries, dolines filled with marine sand were intersected and an irregular white clay layer occurs at the contact between the karstified top of the Cretaceous calcareni

Discussion on ‘Late Cenozoic geological evolution of the northern North Sea: development of a Miocene unconformity reshaped by large-scale Pleistocene sand intrusion’, Journal of the Geological Society , 170, 133–145

In their recent paper, Loseth et al . (2013) propose a new model for the late Cenozoic evolution of the northern North Sea. They propose that during the early Pleistocene ( sensu ICS 2013), large volumes of sand were ejected from the Paleocene through the Eocene–Oligocene Hordaland Group, and deposited both as extrusive sand on the Pleistocene seafloor and as intrusive sand within the Oligocene section, c . 180 m below the seafloor. The paper builds on two earlier studies: (1) that by Loseth et al . (2012) in which the Pleistocene extrusive sand is presented in more detail; (2) that by Rodrigues et al . (2009) in which an experimental model related to such sand deposition is presented. The model of Loseth et al . (2013) is illustrated by a seismic section in their figure 12, showing intrusive sands within the Oligocene and extrusive sands within the Pleistocene. Details of this model, such as feeder dykes and blowout fissures that create ditches, are shown in their figure 8 (Visund Field) and figure 11 (Snorre Field). The geological events are summarized in their figure 16. As illustrated on a stratigraphic scheme (their figs 2 and 15), Loseth et al . interpret the Oligocene sands as being injected from a Paleocene parent sand. They propose that the mounds at the top of the Hordaland Group resulted from forced folding over the sand injectites, as exemplified in their figure 11. Furthermore, Loseth et al . interpret escarpments on the Top Hordaland Group Unconformity as cliff sections similar to those of southeastern England today and claim that the northern North Sea was subaerially exposed during a 10 myr Miocene time span. Loseth et al . (2012, 2013) not only challenge our previous turbiditic interpretation of the Pleistocene sand (Eidvin & …

The Zalew Balaton Quarry geosite in Trzebinia, southern Poland: an update of geological data

A development project of the Zalew Balaton area is being presently carried out. One of the project assumptions is to create a geological path across the object. However, detailed data about the geological structure of this place is not available. For this reason, the author has researched and updated the information about the rocks occurring in the walls of the old quarry. Primarily, terrain (textural and structural analysis, layers strike and dip measurements, geological boundaries delimitation) and laboratory analysis (X-Ray Powder Diffraction, microscopic rock samples analysis) were conducted. Collected data was afterwards compared with available literature. Within the steep slopes of excavation, strong-karsted, faulted Upper Jurassic (Oxfordian) carbonate sediments were revealed. These are massive, platy and crumpled limestone. These rocks could be deposited in changing conditions of sedimentation, when calm carbonate sedimentation was disturbed by underwater gravity flows, as demonstrated by their detrital character. One of the karst funnel on the northern slope was fi lled with Oligocene sand. As a result, groundwater fl uctuations created a sarsen structure, popularly called the “Wielbląd”. In relation to the planned geological path across the Zalew Balaton area, previous knowledge should be supplemented with new data, respectively processed and included on information panes, located within object. Plans of the path should pay attention to potential geotouristic safety, as well as to the geological values of the protected object, during its future modernization.

The origin of ‘tauw’, an enigmatic building stone of the Mergelland: a case study of the Hesbaye region, southwest of Maastricht (Belgium)

AbstractThe origin of a peculiar type of silicified limestone with nodular flints used in parish churches at Sluizen and Vreren, south of Tongeren (province of Limburg, Belgium), has now been elucidated by the discovery of the same rock type in its natural setting, namely the silicified top of the Cretaceous which underlies Clay-with-flints (‘flint eluvium’) and Oligocene sands in a disused quarry at Elst, municipality of Riemst (Limburg, Belgium). Co-operation between professional geologists and amateur palaeontologists has allowed to characterise this rock type, here referred to as ‘Elst tauw’, both petrographically and palaeontologically. The rich echinoid fauna has also been assessed, on the basis of a comparison with assemblages from the Clay-with-flints at Halembaye (Haccourt/Lixhe, province of Liège, Belgium) and at Zichen-Eben Emael in the Hesbaye region. P.J. Felder's ecozones, based on bioclast assemblages, substantiated by analyses of petrographical biofacies features of the original calcarenite, has allowed lithostratigraphic correlation of the ‘Elst tauw’ with the ‘Roosburg block’, which is a variety of ‘Maastricht stone’. Both methods indicate that the ‘Elst tauw’ developed in beds that can be assigned to the condensed Valkenburg-Schiepersberg interval of the lower Maastricht Formation. Petrographical analysis has shown the ‘Elst tauw’ to be quite distinctive; in addition, its natural occurrence at the Elst quarry matches the building stone records in rock type. The latter stem from the same small area, situated southwest of Maastricht. The major steps in its diagenetic history could be reconstructed, starting with pervasive pyritisation of the calcareous allochems, followed by silicification of the grains and pore spaces (different silica cements that became partially recrystallised), completed by oxidation of pyrite with transformation into limonite and, finally, dissolution of the non-pyritised or partially pyritised skeletal allochems creating a mouldic porosity. Silicification probably was achieved prior to the Oligocene. This particular mode of formation has generated a discussion on the proper use of the vernacular term ‘tauw’, a term used in a different sense by the industry, stratigraphers and students of building stones.

Hazard impact and genetic development of sand dunes west of Samalut, Egypt

A southeast mobile sand dune field extends west of the Nile Valley and encroaches on the cultivated land. These dunes have been investigated by geological and geomorphological mapping using Landsat ETM images and field work. Fifty sand samples had been collected from sand dunes and five samples from substratum. The laboratory investigations involve granulometric parameters using binocular microscope and carrying out the collective diagrams and granulometric indices. Morphoscopic studies using Scanning Electron Microscope (SEM) elucidate the surface process affected on sand grains. These dunes have their source from Oligocene sand and gravel deposits to the north of the study area and from the substratum. While the sands are shiny and more rounded mat grains in the northern part of these dunes due to fluvial processes, the south sands become more markedly “aeolinized” by including less rounded and striated sand grains. They move and increase in size in the down wind direction. The initiation of linear dunes explanted here west of the Nile Valley where the eastern tails of the lee side of the barchan dunes are longer than the western ones and sometimes these tails are connected with each other forming longitudinal sand dunes. These dunes are in mature stage and the southern dune may be in the last modification stage.

Production of Heavy Crude Oil: Topside Experiences on Grane

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper OTC 18240, "Production of Heavy Crude Oil - Topside Experiences on Grane," by S. Husoy and S. Hatlem, Norsk Hydro A/S, prepared for the 2006 Offshore Technology Conference, Houston, 1–4 May. Grane field production started in September 2003. Oil is exported to the onshore terminal at Sture in a 28-in. 212-km-long pipeline. Gas for injection is imported in an 18-in. 50-km-long pipeline from the Heimdal field center. Grane is the first oil field on the Norwegian continental shelf producing heavy crude. Close cooperation between Norsk Hydro's operations department, project-development team, contractors, and partners forms the basis for completing the Grane facilities on schedule, below budget, and without any serious incidents. Introduction Grane is the tenth field put into production by Norsk Hydro and is the first heavy-crude-oil (19°API) field on the Norwegian Continental Shelf. The full-length paper presents some of the challenges and experiences that Norsk Hydro as an operator has faced during field development and the production ramp-up period. Most of the challenges were well known beforehand, and the facility and organization were prepared accordingly. However, some of these challenges were underestimated. The production facility, including the organization, has been able to use the flexibility present to adjust directly or prepare temporary solutions while permanent solutions were developed and implemented. Environmental Focus The platform was designed to meet the zero-discharge philosophy of the operator. A disposal well was drilled as part of the predrilling campaign. This well was completed with 13 3/8-in. casing to an Oligocene sand reservoir 500 m true vertical depth above the oil reservoir. A drill-cutting slurrification unit in the drilling module grinds all drill cuttings and creates a seawater-based slurry. This slurry is injected into the disposal well. A disposal well for produced water was drilled as the first platform-drilled well and delivers produced water to the same formation as the drill cuttings. No produced water is discharged to the sea under normal operation. Water treatment includes degassing and hydrocyclones. Grane is allowed to direct 10% of yearly produced water to sea; however, the water-injection system uptime was more than 95% in 2005. The energy requirement for heating the oil is met by use of a waste-heat recovery unit that recovers energy from the exhaust of the turbines. This energy is transferred into the separation process by use of compact heaters. Heat transfer also is achieved from produced water before injection and from the crude before it enters the export pumps. The wellstream has a 70°C temperature entering the first-stage separator. Because sufficient separation of the crude is not achieved at this low temperature, heat is supplied upstream of the second-stage separator where inlet temperature is increased to 90 to 110°C.

A new species of Patriocetus (Mammalia, Cetacea) from the Late Oligocene of Kazakhstan

ABSTRACT For many years the primitive extinct toothed whale genus Patriocetus has been known only from the holotype partial skull of P. ehrlichi (van Beneden, 1865), and a more nearly complete paratype skull, both from uppermost Oligocene sands near Linz, Austria, in the western Paratethys region. A virtually complete skull found in Late Oligocene beds on the Mangyshlak Peninsula of the Republic of Kazakhstan in the 1960s is the first specimen of Patriocetus known from the eastern Paratethys region, extending the range of this genus approximately 4,000 km eastward from the type locality of the genus at Linz. Herein named Patriocetus kazakhstanicus, this new species of Patriocetus differs from P. ehrlichi in size, morphology, and dental count. Direct comparison of the holotype skull of P. kazakhstanicus to that of P. ehrlichi has provided a more accurate definition of the arrangement of the bones in the skull roof of P. ehrlichi, long a source of uncertainty because sediment cemented to the surface of both the holotype and the paratype of the latter species partially obscures the sutures, making it difficult to determine the boundaries of the frontals, parietals, and supraoccipital. The skull of P. kazakhstanicus also shares well-defined generic characters with the partial skull of an undescribed species of Patriocetus from Germany (Rothausen and Sanders, in preparation), for which there are casts of the complete skull. The rostra of those two specimens are more nearly complete than those of the holotype and the paratype of P. ehrlichi, each of which is missing the anterior end of the rostrum. All four specimens share similarities in dentition and rostral characters that now confirm Rothausen's (1968) conclusions that members of the genus Patriocetus were longirostrine forms.

Age and recharge pattern of water in the Oligocene of the Mazovian basin (Poland) as indicated by environmental tracers

Controversies exist about the recharge pattern, age and renewal rate of water in the Oligocene sands of the Mazovian basin, Poland. Hydrodynamic modelling yields ages of about 1.5ka whereas earlier estimates based on environmental tracer methods suggested from 30ka to more than 200ka. This age discrepancy mainly resulted from apparent inconsistency of environmental isotope data and partly from unidentified flow paths, sparse data, and poor knowledge about the hydrodynamic parameters of the overlying Pliocene sediments. However, in spite of δ18O and δD values only slightly lighter than those typical for local Holocene waters, low inferred noble gas temperatures obtained within this study (0.3–4.9°C in comparison with the present air temperature of 7.5°C) and low 14C contents (0–7pmc), indicate a dominant role of glacial recharge. The isotope shifts to heavier values and to the right of W.M.W.L., which decrease the difference between the Holocene and glacial waters, are caused by evaporation prior to recharge via numerous paleolakes, and by admixture of ascending saline water. Though several earlier 36Cl analyses suggest ages of the order of 200ka, such great ages are excluded by 40Ar/36Ar ratios close to the atmospheric equilibrium and relatively low 4He excess values obtained within the present study. A good correlation of 4He excess with Cl− content indicates its origin to be related mainly to local ascension of older waters, and to be little depended on the radiogenic production within the aquifer, or to the accumulation of crustal flux along the flow paths. Therefore, the noble gas data have appeared to be indispensable for the identification of the glacial age of water far from erosion windows, in spite of weak δ18O and δD climatic signal, and for confirmation of a conceptual model invoking significant vertical recharge by leakage through Pliocene sediments.

Stratigraphic Framework and Heavy Minerals of the Continental Shelf of Onslow and Long Bays, North Carolina

One hundred fourteen vibracores from the Atlantic continental shelf offshore of southeastern North Carolina were opened, described, and processed over several contract years (years 6-9) of the Minerals Management Service Association of American State Geologists Continental Margins program. Reports for years 9 and 10 of the program compiled the results of the work and assembled the data for release as an interactive CD-ROM report, respectively. The continental shelf of Onslow and Long Bays consists predominantly of outcropping Cretaceous through late Tertiary geologic units. Nearshore these units are covered and incised by late Tertiary and Quaternary units. From oldest to youngest, formally recognized geologic units mapped as part of this study are the Late Cretaceous Peedee Formation a muddy, fine-to medium-grained quartz sand with trace amounts of glauconite and phosphate; the Paleocene Beaufort Forma tion a muddy, fine-to medium-grained glauconitic quartz sand with locally occurring turritelid-mold biosparrudite; the middle Eocene Castle Hayne Forma tion a sandy bryozoan biomicrudite and biosparrudite; the Oligocene River Bend Formation a sandy molluscan-mold biosparrudite; and the Miocene Pungo River Formation a medium-grained, poorly sorted slightly shelly phosphatic sand. Infor mal units include a very widespread, unnamed fine-to very fine grained, well-sorted, dolomitic muddy quartz sand that is biostratigraphically equivalent to the Oligocene River Bend Formation; several large valley-fill lithosomes composed of biomicrudite, biomicrite, and biosparrudite of Plio Pleistocene age; muddy, shelly sands and silty clays of Pliocene, Pleistocene, or mixed Plio Pleistocene age; and loose, slightly shelly, medium- to coarse-grained sands assigned a Holocene age. Heavy minerals (SG>2.96) comprise an average of 0.54 wt % (on a bulk-sam ple basis) of the sediments in 306 samples derived from the 114 vibracores. Heavy-mineral content ranges from <0.01 to 3.69 wt %. The economic heavy mineral content (EHM ilmenite zircon rutile aluminosilicates leucoxene [altered ilmenite] monazite) of the bulk samples averages 0.26 wt % in a range of <0.01-1.70 wt %. As a percentage of the heavy-mineral concentrate, the average EHM value is 45.78 % in a range of 0.27-68.60 %. The distribution of heavy minerals offshore of southeastern North Carolina is controlled by the lithostratigraphic framework. The unnamed Oligocene sand unit has the highest heavy-mineral content, averaging 0.86 wt % on a bulk-sample basis. The remaining geologic units and their heavy-mineral content (in decreasing order of abundance) are Beaufort (0.64 %), Holocene sand (0.60 %), Plio-Pleistocene muddy sand and silty clay (0.59 %), Peedee (0.42 %), River Bend (0.34 %), Plio-Pleistocene carbonate (0.12 %), and Castle Hayne (0.08 %). The heavy-mineral assemblage is fairly consistent throughout the different units. Significantly smaller percentages of heavy minerals correlate with increased amounts of CaCO3 in the sediments. The sediments analyzed in this study have significantly lower overall heavymineral content, as well as lower EHM content than sediments that are known to host commercially important heavy-mineral deposits in the southeastern United States. The potential for economic deposits of heavy minerals in the area of this study, therefore, appears to be limited.

Deep-Seated Salt Sheet: Eastern Calcasieu Parish, Louisiana

ABSTRACT Interpretation of recent seismic and well data in the Gillis-English Bayou and West Manchester area of Calcasieu Parish, Louisiana, supports a complex structural history involving allochthonous salt, erosion and faulting. Since its discovery in 1934, Gillis-English Bayou has produced over 195 BCFG and 18 NMBL from Oligocene and Miocene age sands. Unlike the aerially small, shallow salt dome fields along trend, Gillis-English Bayou covers approximately eighteen square miles, by area the largest field in the parish. West Manchester, to the south, has produced over 120 BCFG and 2 MMBL from Oligocene Lower Hackberry sands. The associated salt sill underlies at least thirty square miles at depths of 15,000 to 20,000 and can be indirectly related to hydrocarbon accumulation in the area. Associated with this vertical and lateral movement of salt is a significant Vicksburg erosional event underlying West Manchester Field. In this area of thick Vicksburg sediments, over 1,200 of section is absent.

Relative Contributions of Precambrian Metamorphic Rocks and Cretaceous-Tertiary Igneous Rocks to Oligocene and Holocene Fluvial Sands and the Unroofing of a Magmatic Arc

Oligocene and Holocene fluvial sands were deposited in small extensional basins in a magmatic arc in southwestern Montana under relatively humid and semi-arid conditions, respectively. The source rocks are roof-pendants and thrust-slices of Precambrian metamorphic rocks (PCM) and Cretaceous-Tertiary igneous rocks (KTI) that make up the arc. The authors have surveyed 143,607 heavy mineral grains (HMGs) in polished thin sections of 55 samples collected from adjacent but discrete geomorphologic units. In the Holocene sands, of 5440 HMGs 519 are garnets and of 97,667 HMGs 395 are zircons. In the Oligocene sandstones, of 6397 HMGs 998 are garnets, and of 45,940 HMGs 331 are zircons. Garnets are absent in the igneous rocks and zircons are extremely rare in the metamorphic rocks. Garnets ar estimated to be about 100 times as abundant in the metamorphic rocks as the zircons are in the igneous rocks. Mass balance calculations show that the proportion of PCM/(PCM+KTI) ranges from 0 to 21% in Oligocene sandstones, and from 3 to 76% in Holocene sands in different local units. However, the overall PCM/(PCM+KTI) proportions in the Holocene and the Oligocene sands in southwestern Montana are 19% and 18%, respectively. This suggests that the roof pendants, thrust slices, and more » magmatic arc rocks have been unroofed in constant proportions since the Oligocene although locally the proportions have been different. « less