Adjacent Miocene Research Articles

Thermal evolution in the exhumed basement of a stratovolcano: case study of the Miocene Mátra Volcano, Pannonian Basin

The thermal influence of a Miocene stratovolcano (Mátra Volcano) on its basement was studied by apatite and zircon (U–Th)/He thermochronometry. The pre-Miocene substratum of the volcano contains Mesozoic sedimentary units in addition to the nearby exhumed igneous Recsk Complex. The Oligocene emplacement age of the Recsk Complex is constrained by zircon U–Pb geochronology to be 29.6 Ma, which serves as a benchmark for the beginning of its thermal history. All measured apatite (U–Th)/He ages (19.9–5.9 Ma) and most of the zircon (U–Th)/He ages (26.2–17.7 Ma) are considerably younger than the emplacement age of the Oligocene Recsk Complex, implying thermal overprinting by the adjacent Miocene Mátra Volcano. The apatite and zircon He ages of the Oligocene complex increase from south to north, providing clear evidence of a northwards-weakening thermal overprint. The post-Oligocene thermal history of the basement was reconstructed via one-dimensional subsidence/thermal modelling. According to zircon He modelling, the thickness of the covering units above the Recsk Complex was estimated to be 1000–1500 m and the heat flux was c. 200 mW m −2 during the Miocene volcanism. Thermal modelling based on apatite He data suggests that the Miocene volcanism was followed by intensive erosion and the exposure of the Recsk Complex by the Late Miocene. Supplementary material: The locality, petrography and age yield of the dated samples of the Recsk Igneous Complex and details of the laser ablation inductively coupled plasma mass spectrometry dating of zircons from the Recsk Complex are available at: https://doi.org/10.6084/m9.figshare.c.4127444

Three-dimensional petroleum systems modeling of the Mensa and Thunder Horse intraslope basins, northern deep-water Gulf of Mexico: A case study

The petroleum systems of two adjacent Miocene intraslope minibasins in the northern deep-water Gulf of Mexico are modeled to investigate why one of them produces primarily gas but the other produces oil. Specifically, the Mensa field produces gas from a faulted four-way closure that overlies a turtle structure, whereas the adjacent Thunder Horse field produces from a turtle structure with four-way structural closure. To resolve this issue, a three-dimensional petroleum-system model was constructed, whose results indicate that the Lower Cretaceous source interval, comprising type II kerogen, matured significantly earlier in the Mensa basin; the oil window was reached between 11.4 and 9.0 Ma, and the thermogenic gaswindowwas reached between 6.2 and 0.0Ma. By contrast, within the Thunder Horse basin, the source interval reached the oil window by 10.75 to 9.4Ma and largely remains in the oilwindow. TheThunderHorse trap had formed by 13.05Ma, which was before the end of the oil window. The Mensa trap (9.0-8.2Ma) was not in placewhen the source rock passed though the oil window. The primary control on the timing ofmaturation and charge is related to the original thickness of allochthonous salt that created the accommodation for the thick Miocene deep-water sediments. Originally, the Mensa minibasin contained thicker Cretaceous allochthonous salt than the Thunder Horse minibasin. Consequently, as the salt was loaded with sediment and completely evacuated, the turtle structure (trap) formed earlierin Thunder Horse field than in Mensa. By contrast, the source rocks matured earlier in Mensa, prior to the deposition of reservoir sands and the formation of the trap. The results indicate that turtle structures with similar appearances can have subtle differences in the timing of their petroleum systems, which ultimately control whether the feature is charged and with what fluid. These features must be modeled carefully in evaluating their exploration potential. Copyright © 2017 The American Association of Petroleum Geologists. All rights reserved.

Genesis and health risk implications of an unusual occurrence of fibrous NaFe3+-amphibole

Fibrous NaFe3+-amphiboles (winchite, richterite, and magnesioriebeckite) form primarily by alkali metasomatism from magmatic fluids expelled from carbonatite or peralkaline silicate magmas, and have been implicated in high rates of death and disease at Libby, Montana (USA). Fibrous NaFe3+-amphiboles, principally winchite and magnesioriebeckite, are found as fracture-fill veins and as replacement of magmatic hornblende in faulted margins of the dominantly subalkaline, metaluminous Miocene Wilson Ridge pluton, Mohave County, Arizona (USA). Here, the fibrous NaFe3+-amphiboles formed from hypersodic, high-![Graphic][1] hydrothermal fluids, which circulated through active faults as the pluton cooled through subsolidus temperatures. Halite deposits in adjacent Miocene sedimentary basins are the likely source of Na in the hydrothermal fluid. Amphibole fibers are <1 µm in diameter (typically <0.5 µm), vary from tens to hundreds of microns in length with length-to-width aspect ratios of 20:1 to over 100:1, are capable of dust transport and human inhalation, and should be considered hazardous. Transport and deposition of sediment eroded from primary pluton sources significantly increase the areal distribution of the fibrous amphiboles. Mitigation strategies require an understanding of the geologic settings where hazardous geologic materials are found. Our results suggest that fibrous NaFe3+-amphibole may be present in areas not previously considered at risk for naturally occurring asbestos. [1]: /embed/inline-graphic-1.gif

Provenance, diagenesis, tectonic setting and reservoir quality of the sandstones of the Kareem Formation, Gulf of Suez, Egypt

The Middle Miocene Kareem sandstones are important oil reservoirs in the southwestern part of the Gulf of Suez basin, Egypt. However, their diagenesis and provenance and their impact on reservoir quality, are virtually unknown. Samples from the Zeit Bay Oil Field, and the East Zeit Oil Field represent the Lower Kareem (Rahmi Member) and the Upper Kareem (Shagar Member), were studied using a combination of petrographic, mineralogical and geochemical techniques. The Lower Rahmi sandstones have an average framework composition of Q95F3.4R1.6, and 90% of the quartz grains are monocrystalline. By contrast, the Upper Shagar sandstones are only slightly less quartzose with an average framework composition of Q76F21R3 and 82% of the quartz grains are monocrystalline. The Kareem sandstones are mostly quartzarenite with subordinate subarkose and arkose. Petrographical and geochemical data of sandstones indicate that they were derived from granitic and metamorphic terrains as the main source rock with a subordinate quartzose recycled sedimentary rocks and deposited in a passive continental margin of a syn rift basin. The sandstones of the Kareem Formation show upward decrease in maturity. Petrographic study revealed that dolomite is the dominant cement and generally occurs as fine to medium rhombs pore occluding phase and locally as a grain replacive phase. Authigenic quartz occurs as small euhedral crystals, locally as large pyramidal crystals in the primary pores. Authigenic anhydrites typically occur as poikilotopic rhombs or elongate laths infilling pores but also as vein filling cement. The kaolinite is a by-product of feldspar leaching in the presence of acidic fluid produced during the maturation of organic matter in the adjacent Miocene rocks.Diagenetic features include compaction; dolomite, silica and anhydrite cementation with minor iron-oxide, illite, kaolinite and pyrite cements; dissolution of feldspars, rock fragments. Silica dissolution, grain replacement and carbonate dissolution greatly enhance the petrophysical properties of many sandstone samples.

Quantitative climate reconstructions of the late Miocene Xiaolongtan megaflora from Yunnan, southwest China

The late Miocene Xiaolongtan megaflora from Kaiyuan in southeast Yunnan (23°48′45″N, 103°11′52″E, 1050 m a.s.l.) was chosen for palaeoclimatic reconstruction using three quantitative techniques, i.e. the Coexistence Approach (CA), Leaf Margin Analysis (LMA), and the Climate–Leaf Analysis Multivariate Program (CLAMP). The reconstructed climatic parameters are also compared with those of the two adjacent Miocene floras currently available in Yunnan, i.e. the early to middle Miocene carpological Mangdan flora (24°24′N, 97°49′E, 1620 m a.s.l.) and the late Miocene Lühe palynoflora (25°10′N, 101°22′E, 1930 m a.s.l.). Quantitative analyses of the Xiaolongtan flora supports the previous qualitative results of a southern, humid subtropical climate, being more humid and having a slightly higher mean annual temperature (MAT) than today. The MATs calculated by CA, LMA, and CLAMP overlap (16.7–19.2 °C, 22.3 ± 2.05 °C, 18.1 ± 1.2 °C, respectively) and are close to the present day value (19.7 °C). The overlapping of temperatures derived using the three techniques is unusual and probably related to the low latitude of the Xiaolongtan area and the southern subtropical nature of its vegetation. Both the mean temperatures of the warmest month (WMT) and of the coldest month (CMT) reconstructed by CA (WMT = 25.4–26.0 °C, CMT = 7.7–8.7 °C) and CLAMP (WMT = 25.9 ± 1.6 °C, CMT = 10.8 ± 1.9 °C) are similar to those of today (WMT = 24.3 °C, CMT = 12.8 °C), but great changes appear in the mean annual precipitation (MAP). The CLAMP results suggest a higher precipitation (1964 ± 335.9 mm) than CA (1215–1639 mm), but they are much higher than the present MAP (820.5 mm). This is consistent with results from the Lühe palynoflora, which also developed under a warmer subtropical climate with higher precipitation (803.6–1254.7 mm) than that of today (815.9 mm). In contrast, the Mangdan flora, situated in a more complicated topographic region to the west of Xiaolongtan and near the Sino-Myanmar border, implies a slightly lower precipitation (1170–1300 mm) than that of today (1300–1400 mm). Overall, the wetter climate during the late Miocene around the Xiaolongtan area suggests that the Himalayas had not yet uplifted to its present altitudes at that time.

Diagenesis and reservoir quality of Bhuban sandstones (Neogene), Titas Gas Field, Bengal Basin, Bangladesh

This study deals with the diagenesis and reservoir quality of sandstones of the Bhuban Formation located at the Titas Gas Field of Bengal Basin. Petrographic study including XRD, CL, SEM and BSE image analysis and quantitative determination of reservoir properties were carried out for this study. The sandstones are fine to medium-grained, moderately well to well sorted subfeldspathic arenites with subordinate feldspathic and lithic arenites. The diagenetic processes include clay infiltration, compaction and cementation (quartz overgrowth, chlorite, kaolinite, calcite and minor amount of pyrite, dolomite and K-feldspar overgrowth). Quartz is the dominant pore occluding cement and generally occurred as small euhedral crystals, locally as large pyramidal crystals in the primary pores. Pressure solution derived from grain contact is the main contributor of quartz overgrowths. Chlorite occurs as pore-lining and pore filling cement. In some cases, chlorite helps to retain porosity by preventing quartz overgrowth. In some restricted depth interval, pore-occlusion by calcite cement is very much intense. Kaolinite locally developed as vermiform and accelerated the minor porosity loss due to pore-occlusion. Kaolinite/chlorite enhances ineffective microporosity. Kaolinite is a by-product of feldspar leaching in the presence of acidic fluid produced during the maturation of organic matter in the adjacent Miocene or deeper Oligocene source rocks. The relation between diagenesis and reservoir quality is as follows: the initial porosity was decreased by compaction and cementation and then increased by leaching of the metastable grains and dissolution of cement. Good quality reservoir rocks were deposited in fluvial environment and hence quality of reservoir rocks is also environment selective. Porosity and permeability data exhibit good inverse correlation with cement. However, some data points indicate multiple controls on permeability. Reservoir quality is thus controlled by pore occluding cement, textural parameters (grain size, pore size and sorting) and depositional environment. The reservoir finally resumed partly its pre-cementation quality after development of secondary porosity.

3D and 4D controls on carbonate depositional systems: sedimentological and sequence stratigraphic analysis of an attached carbonate platform and atoll (Miocene, Níjar Basin, SE Spain)

AbstractThis study investigates the controls on three‐dimensional stratigraphic geometries and facies of shallow‐water carbonate depositional sequences. A 15 km2 area of well‐exposed Mid to Late Miocene carbonates on the margin of the Níjar Basin of SE Spain was mapped in detail. An attached carbonate platform and atoll developed from a steeply sloping basin margin over a basal topographic unconformity and an offshore dacite dome (Late Miocene). The older strata comprise prograding bioclastic (mollusc and coralline algae) dominated sediments and later Messinian Porites reefs form prograding and downstepping geometries (falling stage systems tract). Seven depositional sequences, their systems tracts and facies have been mapped and dated (using Sr isotopes) to define their morphology, stratigraphic geometries, and palaeo‐environments. A relative sea‐level curve and isochore maps were constructed for the three Messinian depositional sequences that precede the late Messinian evaporative drawdown of the Mediterranean. The main 3D controls on these depositional sequences are interpreted as being: (i) local, tectonically driven relative sea‐level changes; (ii) the morphology of the underlying sequence boundary; (iii) the type of carbonate producers [bioclastic coralline algal and mollusc‐dominated sequences accumulated in lows and on slopes of &lt; 14° whereas the Porites reef‐dominated sequence accumulated on steep slopes (up to 25°) and shallow‐water highs]. Further controls were: (iv) the inherited palaeo‐valleys and point‐sourced clastics; (v) the amount of clastic sediments; and (vi) erosion during the following sequence boundary development. The stratigraphy is compared with that of adjacent Miocene basins in the western Mediterranean to differentiate local (tectonics, clastic supply, erosion history, carbonate‐producing communities) versus regional (climatic, tectonic, palaeogeographic, sea‐level) controls.

Structure and age of the Cerro de Pasco Cu-Zn-Pb-Ag deposit, Peru

The world-famous Cu-Zn-Pb-Ag deposit at Cerro de Pasco, Peru, consists of texturally massive pyrite, texturally massive sphalerite-galena-pyrite, and veins containing pyrite and enargite. Historically the deposit has been considered to be the hydrothermal product of the adjacent Miocene volcanic and intrusive complex (locally known as the “Vent”). However, both the texturally massive sulfides of the deposit and the pre-Miocene strata are cut by the Longitudinal fault, one of the largest faults in the district, but the Vent is not. Imbrication by the Longitudinal fault zone (duplex structures) has thickened the deposit so that it is amenable to open-pit mining. Dikes and pyrite-enargite veins pass from the Vent into the massive sulfides; fragments of massive pyrite occur in the Vent. Thus, no matter what their origin, the texturally massive sulfides are older and, therefore, genetically unrelated to the Vent.

Diagenetic controls on the reservoir properties of gas bearing Neogene Surma Group sandstones in the Bengal Basin, Bangladesh

The diagenetic history of Neogene reservoir sandstones of the Surma Group is described based on petrological analyses of over fifty core samples from four onshore boreholes in the Bengal Basin of Bangladesh. Diagenesis in the sandstones principally involves the development of cements in the following order: chlorite, siderite, illite, quartz and Fe-calcite. There is also a late dissolution stage creating secondary porosity via the dissolution of feldspars and carbonate cements which was followed by the generation of a kaolinite cementing phase. These latter processes require acidic pore waters which are considered to have been produced during the maturation of organic matter in the adjacent Miocene source rocks. In some of the sandstones an early poikilotopic Fe-calcite cement formed and effectively halted further diagenesis after the formation of the chlorite and siderite. The carbonate and quartz overgrowth cements have caused the most significant decreases in the porosity values with the clays having less effect and may even have helped preserve porosity. Secondary porosity occurs at depth, but its effect is reduced by the generation of the late phase kaolinite cement in the secondary pore spaces.