Rocks Of Cretaceous Age Research Articles

Implications of Organic Matter Input, Sedimentary Environmental Conditions, and Gas Generation Potential of the Organic-Rich Shale in the Onshore Jiza-Qamar Basin, Yemen.

The Jiza-Qamar Basin is one of the most important exploration sedimentary basins in Yemen. For over a decade, the exploration of hydrocarbons has been occurring in this basin. Late Cretaceous age rocks are the most occurring organic-rich sediments in this basin, including coals, coaly shales, and shales. The studied organic-rich shale beds are from the Late Cretaceous Mukalla Formation and associated with coal seams. These organic-rich shales can serve as source rocks for hydrocarbon generation potential. The current study investigates the geochemical characteristics, including assessing the organic matter (OM) input, sedimentary environmental conditions, and hydrocarbon generation potential of the organic-rich shale within the Mukalla Formation from three well locations in the onshore Jiza-Qamar Basin using organic geochemistry, biomarker, and carbon isotope measurements. The studied shale samples have high OM content with total organic carbon values between 0.74 and 19.48 wt %. Furthermore, they contain mainly hydrogen-poor Types III and IV kerogen, indicating the presence of the gas-prone source rock. The presence of these types of kerogen indicates the abundance of vitrinite and inertinite macerals, as established by microscopic investigation. However, the studied organic-rich shales had biomarker features, including high Ph/Ph ratio between 3.82 and 7.46, high Tm/Ts ratio of more than 7, and high C29 regular steranes compared to C27 and C28 regular steranes. Apart from the biomarker results, the studied Mukalla shales are characterized by the abundance of land-derived OM that deposited in fluvial to fluvial deltaic environments under highly oxic conditions. The finding of the considerable concentration of terrigenous OM is probably confirmed by the bulk carbon isotope and maceral composition data. The maturity indicators show that the examined organic-rich shale samples in the studied wells exhibit low VR values of up to 0.71%, and thereby, they have not been yet reached the high maturity for gas generation. This low maturity level in the studied wells is probably attributed to shallow burial depth, exhibiting depth of up to 2835 m. Therefore, the substantial gas exploration operations from the organic-rich shale source rock system of the Late Cretaceous Mukalla Formation can be recommended in the deeper stratigraphic succession in the offshore Jiza-Qamar Basin.

Petrography and geochemistry of the Sanaga river sediments, central Cameroon: Constraints on weathering, provenance, and tectonic setting

The Sanaga River (SR), central Cameroon, flows predominantly over metasediments (2.1 Ga), Paleoproterozoic amphibolites and orthogneisses, granitoids from the Adamawa-Yade batholith (AYB), sedimentary clastic rocks of Cretaceous age, and Cenozoic rocks of the Cameroon Volcanic Line. To infer the origin of alluviums and tectonic setting of source rocks, the texture, heavy mineral, major, trace, and rare earth element concentrations of stream sediments were investigated. The very high SiO2/Al2O3 ratios and the ZTR (Zircon-Tourmaline-Rutile) index of SR sediments suggested a high maturity. This interpretation was additionally confirmed by the Th/Sc vs Zr/Sc plot. The Plagioclase- and Chemical Index of Alteration (PIA and CIA) values, the (A-K)-C-N and A–CN–K triangular plots show an intense degree of weathering in the source area. Numerous discrimination diagrams, based on trace elements, indicate that sediments were derived from felsic source rocks mainly granodiorites, granites, and biotite granitoids exposed in the SR watershed. This shows that the metamorphic and volcanic rocks do not significantly contribute to the stream sediments. Paleotectonic investigations using multi-element discrimination plots indicate passive and rift settings for the SR drainage basin. Rift settings were also inferred from downstream sediments of the SR. They are probably related to Proterozoic events including opening of large oceanic basins. The passive and rifted margin settings are supported by the occurrence of rare ophiolitic rocks, which indicate an old oceanic crust.

Not simply volcanoes – The Geoheritage of the Cretaceous System in the Land of the Extinct Volcanoes Geopark, West Sudetes (SW Poland)

Volcanic geoheritage is emphasized as the main asset and distinctive characteristic of the Land of Extinct Volcanoes Geopark in the West Sudetes (SW Poland). However, the geoheritage values of the region are not limited to the legacy of ancient volcanism but include various other elements. This paper explores the contribution of geosites that expose sedimentary rocks of Cretaceous age and landforms developed upon these rocks. Six localities from the Geopark area, included in the comprehensive regional inventory of geosites, are presented. They represent natural and man-made sandstone outcrops and show, among others, lithological variations, small- and large-scale post-sedimentary deformation structures, landforms arising from differential weathering (rock shelters, honeycombs), boulder fields and a sandstone xenolith in volcanic rocks. Next, five localities from outside the Geopark, but still within the Pogórze Kaczawskie region, are described. Qualitative and quantitative evaluation of both groups is attempted, and the results show that, in general, geosites within the Geopark rank higher. However at least two from the other group also have significant geotourist potential. Finally, a brief comparative analysis with other parts of the Sudetes, where Cretaceous sedimentary rocks occur, is offered.

Geology and Petrographic Studies of Rocks in Anka Sheet 52, Northwestern, Nigeria

Over the years, models of evolution of the schist belts of Nigeria was proposed and classified under ensialic and ensimatic processes of formation. The study area (Anka Sheet 52) lies between latitudes 12 o 00′N and 12°30′N and longitudes 5°30′ E and 6°00′E and covers an area of about 3,086.76 km² (Anka Sheet 52). Field mapping and laboratory studies was considered for this study by taking every detail in the field necessary such as geologic boundaries, structures, physical appearances of rocks and orientation noted in field notebook. Structural characteristics on outcrop was also recorded with the help of photographs and sketches along with text description. Representative rock samples mostly granitoids were prepared into thin for microstructural observations to reconstruct the deformation features and mineralogical assemblages under polarized light microscope. The study area comprises of gneiss of sedimentary protolith and is the oldest rock unit forming a massive sheared and elongated ridge trending north-south for about 1800m with a porphyroblastic texture of gradual lineation of quartz and feldspar in places. Metasediments comprises of quartzite, quartz schist, pellitic schist, phyllite, hornfels and metaconglomerate accounting for more than 35 percent of the study area. Pan-African granites intruded both the gneiss and metasediments and comprises of coarse grained granites, porphyritic granites, diorite and rhyolite to accounts for about 20 percent of the study area. Field and petrological evidences indicates that the Pan-African reworking may have led to the recrystallization of the protolith rocks to form gneiss (paragneiss) resulting from partial melting signifying that metamorphism possibly reach lower amphibolite facies in part of the study area. These rocks were overlain by sedimentary rocks of Cretaceous age. Keywords: Granitoids, Protolith, Metamorphism, Porphyroblastic, Paragneiss DOI : 10.7176/JEES/10-1-06 Publication date: January 31 st 2020

Hanna of Wyoming—The Rockies’ Deepest Basin

The Hanna Basin is one of the world’s deeper intracratonic depressions. It contains exceptionally thick sequences of mature, hydrocarbon-rich Paleozoic through Eocene rocks and has the requisite structural and depositional history to be a significant petroleum province. The Tertiary Hanna and Ferris formations consist of up to 20,000 ft of organic-rich lacustrine shale, shaly mudstone, coal, and fluvial sandstone. The Upper Cretaceous Medicine Bow, Lewis, and Mesaverde formations consist of up to 10,000 ft of marine and nonmarine organic-rich shale enclosing multiple stacked beds of hydrocarbon-bearing sandstone. Significant shows of oil and gas in Upper Cretaceous and Paleocene rocks occur in the basin. Structural prospecting should be most fruitful around the edges where Laramide flank structures were created by out-of-the-basin thrust faults resulting from deformation of the basin’s unique 50-mile wide by 9-mile deep sediment package. Strata along the northern margin of the basin were compressed into conventional anticlinal folds by southward forces emanating from Emigrant Trail-Granite Mountains overthrusting. Oil and gas from Pennsylvanian to Upper Cretaceous aged rocks have been found in such structures near the Hanna Basin. Only seven wells have successfully probed the deeper part of the Hanna Basin (not including Anadarko’s #172 Durante lost hole, Sec. 17, T22N, R82W, lost in 2004, hopelessly stuck at 19,700 ft, unlogged and untested). Two of these wells tested gas at commercial rates from Upper Cretaceous rocks at depths of 10,000 to 12,000 ft. Sparse drilling along the Hanna Basin’s flanks has also revealed structures from 3,000 to 7,000 feet deep which yielded significant shows of oil and gas.

The Early Cretaceous Barents Sea Sill Complex: Distribution, 40Ar/39Ar geochronology, and implications for carbon gas formation

Mafic igneous rocks of Cretaceous age (80–130Ma) scattered around the Arctic Ocean are commonly referred to as the High Arctic Large Igneous Province (HALIP). We have mapped out the distribution of HALIP igneous rocks in the Barents Sea region over the past decade based on integrated seismic–gravity–magnetic interpretation, field work, review of publications, and analyses of new and vintage borehole and field samples. The mapping reveals abundant igneous rocks in the northern and eastern Barents Sea covering an area of ~900,000km2 with a conservative volume estimate of 100,000 to 200,000km3 of intrusions. The igneous province is dominated by sheet intrusions injected into Triassic and Permian sedimentary rocks. Hydrothermal vent complexes are rare, and only two potential vent complexes have been identified on seismic data in the eastern Barents Sea. We have further done extensive radiometric dating of the igneous samples in the Barents Sea region. New 40Ar/39Ar dating of thirteen samples from Svalbard reveal ages of crystallization and alteration. The large age span (60–140Ma for the raw ages) is likely due to partial or complete overprint of the K/Ar system in plagioclase, and the age of the magma emplacement is better represented by U/Pb TIMS ages. Only one of our 40Ar/39Ar analyses of plagioclase yielded a statistically valid age that is in line with the recently published U/Pb TIMS ages of 122–125Ma. The new data clearly document that relying on published data from the K/Ar system can lead to erroneous conclusions on the age of crystallization in this province without a careful use of additional 40Ar/39Ar degassing data (i.e., K/Ca). We propose that the magmatism on Svalbard and Franz Josef Land represents a distinct magmatic event near the Barremian/Aptian boundary (125Ma) in the Barents Sea. This Early Cretaceous Barents Sea magmatism resulted in the formation of the BSSC (Barents Sea Sill Complex). BSSC age rocks are also present in Arctic Canada (Sverdrup Basin) and on Bennett Island (New Siberia Islands). The massive injection of hot magma into potentially organic-rich sediments in the eastern and northern Barents Basin caused rapid organic matter maturation and formation of thermogenic gas and oil in contact aureoles. We estimate that up to 20,000 Gt of carbon were potentially mobilized, corresponding to 175 trillion barrels of oil equivalent. The production rates and fate of the carbon gases are uncertain. However, we speculate that rapid release of aureole greenhouse gases (methane) may have triggered the Oceanic Anoxic Event 1a (OAE1a) and the associated negative δ13C excursion in the Early Aptian. Some of the methane may also be trapped in the vast hydrocarbon gas accumulations found in the east Barents Basin.

U–Pb and Re–Os geochronology of the Haolibao porphyry Mo–Cu deposit, NE China: implications for a Late Permian tectonic setting

AbstractNew geochronological data for the Haolibao porphyry Mo–Cu deposit, NE China, yield Permian crystallization zircon U–Pb ages of 278 ± 5 Ma for granite and 267 ± 10 Ma for the granite porphyry that hosts the Mo–Cu mineralization, and four Re–Os molybdenite ages yield an isochron age of 265 ± 3 Ma. These ages disagree with the previous K–Ar age determinations that suggest a correlation of intrusive rocks of the Haolibao area with the Yanshanian intrusive rocks of Cretaceous age. The mineralizations at the Haolibao area may be related to the tectonic–magmatic activity caused by collisional events between the North China Plate and Mongolian terranes during the Permian. The occurrence of the Haolibao plutonic rocks indicates that the Palaeo-Asian-Mongolian Ocean closed during the Permian along the Xilamulun River suture.

Protungulatum, Confirmed Cretaceous Occurrence of an Otherwise Paleocene Eutherian (Placental?) Mammal

Neither pre-Cenozoic crown eutherian mammals (placentals) nor archaic ungulates (“condylarths”) are known with certainty based on the fossil record. Herein we report a new species of the Paleocene archaic ungulate (“condylarth”) Protungulatum from undisputed Late Cretaceous aged rocks in Montana USA based on an isolated last upper premolar, indicating rare representatives of these common early Tertiary mammals appeared in North America a minimum of 300 k years before the extinction of non-avian dinosaurs. The other 1200 mammal specimens from the locality are characteristic Late Cretaceous taxa. This discovery overturns the current hypothesis that archaic ungulates did not appear in North America until after the Cretaceous/Tertiary (K/T) boundary and also suggests that other reports of North American Late Cretaceous archaic ungulates may be correct. Recent studies, including ours, cannot determine whether Protungulatum does or does not belong to the crown clade Placentalia.

Pore pressure patterns in Tertiary successions and hydrodynamic implications, Beaufort-Mackenzie Basin, Canada

Abstract Mud weight and pore pressure (drill stem test, repeat formation tester) data from 250 exploration wells were used to investigate the fluid pressure regime of the Beaufort-Mackenzie Basin. Four patterns of pore pressure variation with depth are recognized, each of which is associated with a specific tectono-stratigraphic domain and reflects the interplay between compaction and tectonics, or other geological factors causing or redistributing the overpressure. Overpressure in the southwest Beaufort Sea is likely to be a result of a combination of compaction and northeast-southwest contractional tectonics. In the north, shale diapirism may create fracture systems on top of anticlines, causing migration of overpressured fluid to a shallower depth. In the center of the Mackenzie Delta, where listric faulting prevails, compaction is the major controlling factor and the depth of overpressure depends on lithology and rate of deposition. Overpressure is mainly confined to Tertiary sedimentary successions but it may exist in pre-Tertiary strata (as inferred from mud weight data in a few wells) along the southeast basin margin, possibly in association with gas generating source rocks of Cretaceous age. The spatial variations of pore pressure at given depths, as well as along specific stratigraphic sequences, indicate that basin-scale flow is primarily driven by the upward expulsion of overpressured fluids. Analysis of pore pressure patterns suggests that regional fault zones can be both a barrier and a preferred flow path network to deep fluid fluxes. In general, fault zones are more likely to be regional barriers to lateral flow in an aquifer, but are the preferred flow networks for vertical fluid migration.

Migration pathways in the Central North Slope foreland basin, Alaska USA: solute and thermal constraints on fluid flow simulations

AbstractThe North Slope foreland basin, Alaska, USA is an east–west asymmetrical trough‐shaped basin adjacent to the Brooks Range fold‐thrust mountain belt. Lower Cretaceous age rocks make up much of the sediment fill, including flysch‐like marine turbidites and shales of the Torok and Fortress Mountain formations and marine and sandstones, shales and conglomerates of the overlying Nanushuk group. Lower Cretaceous age rocks were deposited on top of a Palaeozoic and Mesozoic age passive margin sequence. We have conducted numerical simulations of fluid flow driven by topographic recharge in the Central North Slope foreland basin. These simulations are constrained by salinity estimates from well logs, location of oil and gas fields, vitrinite reflectance and heat flow measurements. Our model results indicate that there are two south to north pathways for fluid migration. The primary pathway for fluid movement is downward through the Fortress Mountain formation, then upwards along the interface between the Fortress Mountain and Torok Formation and finally northward through the permeable Nanushuk group. A smaller mass of groundwater moves along sands below the Torok formation and into offshore sediments north of Alaska. Very little meteoric water enters the underlying Palaeozoic rocks in our simulations, which could explain the presence of deep saline pore waters. Our results also show that permafrost is a primary control on the pathway and rate of fluid flow by controlling the distribution of surface recharge and discharge. For example, areas of high heat flow and low saline waters along the arctic coast may represent upward groundwater discharge because of the absence of permafrost. As surface temperatures were warmer in the Miocene, the absence of permafrost would produce a more local fluid circulation pattern and less transfer of heat energy from south to north.

Epithermal gold mineralization in an old volcanic arc; the Jacinto Deposit, Camagueey District, Cuba

The Jacinto vein system in the Camaguey district of central Cuba is a low-sulfidation, adularia-sericite epithermal gold deposit. It is hosted by volcanic and volcaniclastic rocks of Cretaceous age along the margin of the Camaguey batholith. Jacinto is unusually old and much more closely associated with a large batholith than would be expected for an epithermal vein deposit in an island arc. This study was undertaken to determine how Jacinto could have been preserved for so long. The Jacinto system includes at least five veins that range in length up to 1 km, have maximum widths of 60 m, and extend to depths of at least 150 m. The veins consist almost entirely of quartz, with minor amounts of adularia, calcite (commonly bladed), pyrite, and gold, which form at least 20 growth zones that can be divided into five main stages and can be correlated from one vein to another. Gold ore shoots are restricted vertically and are more continuous horizontally. Their present configuration suggests that the entire system has been tilted about 5 degrees to the northeast. Wall-rock alteration is limited and consists of minor K feldspar and sericite. Fluid inclusions in vein quartz homogenize at temperatures of 200 degrees to 320 degrees C and freeze at temperatures characteristic of salinities of about 0 to 1 wt percent NaCl equiv. The presence of bladed calcite and silica pseudomorphs after bladed calcite suggests that the hydrothermal fluids boiled. Stable isotope analyses of quartz, calcite, and inclusion fluids indicate that the veins were deposited by waters with delta 18 O and delta D values that range from values just above those of magmatic water to those typical of Cretaceous meteoric water. 40 Ar- 39 Ar analyses of adularia in the Jacinto veins indicate an age of about 72 Ma for mineralization, and geologic relationships suggest that the vein system is related to La Sierra rhyolite flows and domes of that approximate age. La Sierra rhyolite was extruded onto an erosion surface that exposed the Camaguey batholith and related volcanic rocks, indicating that erosion rates were relatively rapid immediately prior to formation of the deposit. Shortly after the deposit formed, however, it was tilted and then buried beneath Late Cretaceous to Eocene sediments of the Maraguan basin, which are being eroded to expose the system today. This fortuitous sequence of volcanism, batholith emplacement, rapid erosion to deep batholith levels, extrusion of rhyolite, and local sedimentation appears to be the result of collision between the Cuban arc and the Bahamas platform. Similar collisions between arcs and continental masses could have formed other epithermal deposits.

The Overthrusted Zaza Terrane of Middle Cretaceous Over the North American Continental Carbonate Rocks of Upper Jurassic-Lower Cretaceous Age--Relationships to Oil Generation: ABSTRACT

The Zaza Terrane is part of the Caribbean plate thrust over the southern edge of the North American basinal and platform carbonate rocks of upper Jurassic-Lower Cretaceous age. Zaza Terrane are volcanic and ophiolitic rocks of Cretaceous age. The ophiolites are mostly serpentines which behave as reservoirs and seals. All Cuban oil fields are either within Zaza Terrane or basinal carbonates underneath, or not far away to the north of the thrust contacts. It appears that the overthrusting of the Zaza Terrane caused the generation of oil in the basinal carbonate source rocks underneath, due to the increase of rock thickness which lowered the oil window to a deeper position and increased the geothermal gradient. Oil generation was after thrusting, during post-orogenic. API gravity of oil is light toward the south and heavy to very heavy to the north. Source rocks to the south are probably of terrigenous origin.

Site amplification from coda waves: Validation and application to S-wave site response

Abstract The subject of this article is to see if the response to S waves at a specific site can be determined from coda waves. We used aftershock records of the 1992 Landers and Big Bear earthquakes observed at four TERRAscope stations (PAS, PFO, SVD, and GSC) in southern California. The wide dynamic range of the TERRAscope instrument provided a broadband seismogram for S waves, and, at the same time, coda waves over a long lapse-time window. We analyzed a total of 708 three-component waveforms from 65 events whose magnitudes ranged from 3.3 to 5.4. The station SVD is located on Quaternary alluvium, and the remaining stations are on rocks of Cretaceous age. We first evaluated the coda site-amplification factor using a spectral ratio method (May-eda et al. , 1991; Koyanagi et al. , 1992) for frequency bands centered at 0.75, 1.0, 1.5, 2.0, 3.0, 4.0, and 6.0 Hz for lapse-time windows after 70 sec from the origin time. We found that the site-amplification factor for the radial component is similar to that for the transverse component. This similarity was expected because codas consist of backscattered waves coming from all directions into the station, resulting in a directionally averaged site response. We then evaluated the S -wave site-amplification factor using an inversion method (Andrews, 1986) on the direct S -wave portion of the same data set. We found that the coda site response agrees with the S -wave site response within a factor of about 1.5, supporting the earlier findings by Tsujiura (1978) and justifying the use of coda waves in determining site-specific amplification for S waves. This agreement between S -wave site response and coda site response is understandable if we assume that coda waves are dominated by S waves, as theoretically predicted by Aki (1992) and Zeng (1993).

Revisiting Our Predictions of Petroleum Reservoir Properties, U.S. East Coast Mesozoic Offshore: ABSTRACT

In the early 1980`s the major international oil corporations were focusing on potential giant oil fields in the U.S. frontiers. The offshore U.S. East coast Mesozoic shelf edge was one such frontier play area. The oil industry was preoccupied by an apparent continuous Mesozoic shelf edge build-up which appeared in seismic records. At the time there were no wells that penetrated the seismic build-up. Additionally, the shelf edge prospects were in water depths exceeding 1500 meters making difficult drilling technological problems and associated extremely high expenses. What made the U.S. East Coast so attractive was that if carbonate reservoirs did exist the oil potential would be in the ten`s of billions of barrels range. In the June 1983 issue (volume 67/6) of the A.A.P.G. Bulletin, Gerald Friedman and the author published an article applying modern carbonate diagenetic findings in conjunction with the global seismic stratigraphy of P.R. Our article predicted porous carbonate build-ups formed during low sea-level stand in the Middle Jurassic, Late Jurassic, and Lower Cretaceous Age rocks. In the mid 1980`s, Shell Oil Corporation drilled seaward off the continental shelf edge. The wells encountered Jurassic and Cretaceous carbonate shelf edge oolitic grainstones, coral stromatoporoid grainstones and boundstones. Core,more » and log analysis reveal that reservoir quality carbonates do exist with the Jurassic and Cretaceous Age carbonate rocks. He suggested the porosity was formed during various subaerial exposures caused by low sea level stands. In summary, our integration of global seismic stratigraphy and modern carbonate diagenetic models help predict reservoir quality in a totally undrilled area of continental proporations. Unfortunately, thus far the tested prospect was water saturated (wet). In the foreseeable future with the price of crude oil at its current level it is unlikely any further drilling will take place.« less

Calcretes, Palycretes and Silcretes in the Paleogene Detrital Sediments of the Duero and Tajo Basins, Central Spain

AbstractThree duricrust types (calcretes, palycretes and silcretes) have been distinguished in the Lower Paleogene arkosic materials from the margins of the Duero and Tajo basins (central Spain). In the calcretes the cements are composed of calcite plus palygorskite, whereas palygorskite is the only cement in the palycretes. In the silcretes, the cement consists chiefly of opal and chalcedony, with minor quartz. The important duricrust development in the Paleogene detrital materials can be correlated with a drastic climatic change towards colder and more arid conditions during this time, as inferred from the sedimentological features and the clay mineral evolution. The origin of calcretes and palycretes is related to the movement of alkaline phreatic waters generated from dolomitic rocks of Cretaceous age cropping out in the near surroundings. Silcretes originated from the vertical transfer of silica from supersaturated groundwaters, from the Hercynian granitic rocks, up to the surface. Silcretes are developed either on unconsolidated arkosic materials or replace previous calcrete or palycrete levels.

Controls on the Distribution of Cretaceous Source Rocks in South America

More than thirty South American basins, exhibiting a variety of structural styles, contain petroleum source rocks of Cretaceous age. However, the presence of truly [open quote]world-class[close quote] source rocks, capable of supplying multi-billion barrel oil provinces, is restricted to relatively few basins and appears to be primarily a function of large scale Cretaceous tectonic setting. In Early Cretaceous times the best source rocks were preserved in both a southern ocean and in the rift between South America and Africa. By the Late Cretaceous, these southern and eastern continental limits had become narrow passive margins. In contrast, on the northern continental margin a wide shelf to a restricted tropical sea was developing at this time. Periodic upwelling enhanced surface productivity on this shelf, which led to development of some of the world's richest source rocks. On the tectonically active western margin moderate quality source rocks were forming in a series of back-arc basins, whilst further west, in the Pacific fore-arc, organic-rich intervals were rarely deposited. This article documents what is known about each of the explored basins (including the volume and character of discovered petroleums), it investigates the geologic factors which governed the richness and quality of petroleum source rocks andmore » it assesses how continued tectonic activity has modified or even destroyed primary source quality. Finally it predicts which of the as yet underexplored basins should contain good quality source rocks and could become prolific petroleum provinces of the future.« less

El valor científico de una colección mineralógica en la reconstitución de tres antiguos distritos argentíferos: Arqueros, Algodones y Rodaito (Norte de Chile)

Arqueros, Algodones and Rodaito are silver districts from northern Chile that were of major economic importance during the XIX century. Since the deposits were extensively mined during the last century, at present, the main mineralogical record is kept at the «Coleccion Ignacio Domeyko», a major collection belonging to the Mineralogical Museum of La Serena (Chile). The study of the collection together with a revision of the local geology of the districts led to the following conclusions: 1) the districts are formed by Comstock/Creede-type epithermal silver-base metals deposits, which are hosted by volcanic and sedimentary rocks of Cretaceous age: 2) the studied parageneses can be ascribed to the Cu-Pb-Zn-Ag association (e.g. Creede, Colorado, USA; Zacatecas, Mexico); 3) the presence of skutterudite and erythrite suggests, however, a transitional character towards other mineralogical associations: Ag-Bi-Co-Ni-As (e.g. Cobalt, Ontario, Canada) and Pb-Zn-Ag (Sb-As) (Coeur d'Alene, Idaho, USA).

13C and 18O compositions of carbonates from a cyclic carbonate-evaporite rock sequence: Evidences for meteoric water input

Twenty-five carbonate rock samples from a core section composed of seven thick rhythmic carbonate-evaporite sequences (Lower McKnight Formation, Early Cretaceous, south Texas, U.S.A.), were analyzed for carbon and oxygen isotopic compositions. Results show the carbonate rocks to have δ 13 C = −0.38 to + 1.68‰ and δ 18 O = −4.60 to −2.48‰ vs. PDB, consistent with the reported world-wide values for carbonate rocks of Cretaceous age. Despite this low ∼2‰ overall variation, δ 18O shows more variability than δ 13C throughout the core section studied. Furthermore, δ 13C and δ 18O are covariant, becoming progressively lighter toward the overlying evaporites within each carbonate-evaporite sequence. The tendency toward lighter isotopic compositions together with the covariance between δ 13C and δ 18O in these carbonates is thus not reconcilable with a simple model of carbonate deposition in a marginal evaporitic basin upon continuous evaporation of seawater. Instead, the data imply contributions from isotopically light meteoric waters.

A comparative hydrocarbon soil-gas study of the Flathead Valley and Townsend Valley areas, Montana

Abstract Surface soil-gas geochemical data are used to compare the hydrocarbon source potential of Flathead and Townsend Valleys which are located in northcentral and central Montana, respectively. These valleys are of interest because both areas were previously considered frontier regions with no hydrocarbon source potential. Based upon the soil-gas data, Flathead Valley appears to have a good hydrocarbon source potential. The distribution of specific hydrocarbon gases and various gas ratio values have a strong resemblance to those of Cretaceous age rocks near Marias Pass, Montana, which are known to have a good to very good hydrocarbon source potential. The correlation between the Flathead Valley and Marias Pass soil-gas signatures suggests that the Precambrian strata in Flathead Valley might have been thrust over Mesozoic age source rocks in agreement with recent structural interpretation. Townsend Valley soil-gas data, in contrast, show extremely low magnitude dry-gas signatures. This observation supports a low assessment of hydrocarbon source potential, based on the geologic considerations. Only the extreme southeastern portion of the area shows an encouraging wet-gas signature. In both areas, regardless of the actual hydrocarbon potential, the higher magnitude data are preferentially associated with linear features mapped on satellite images and/or high altitude air photographs. This association is very important for the Flathead Valley area. The northeast-trending features, which probably resulted from differential movement along the Lewis thrust fault, are associated with the highest concentrations of hydrocarbons. The pronounced correlation between hydrocarbon soil-gas concentrations and these linear features suggests that they act as hydrocarbon migration pathways from proposed Cretaceous source-rock in a subthrust position, up through the Precambrian, Tertiary and Quaternary sections to the surface.

Geochemical character of the hydrothermal alteration zones around the Madenkoy-Siirt massive sulfide deposit and implications for geochemical exploration

The Madenkoy-Siir region lies in southeastern Anatolia, Turkey to the northeast of Siirt. The study area is in the southeastern Anatolian Thrust Belt, which forms the boundary between the Border Folds on the northern edge of the Arabian plate and the Taurids. In the region, limestones of the Midyat Group of Eocene-Miocene age and alternating marls and sandstones of the Lice Formation of Early-Middle Miocene age are the autochthonous units. Three thrust slices were emplaced over the autochthonous units during the Miocene Epoch. The lowest slice consists of interbedded sandstones, mudstones, marls, limestones, and conglomerates of the Sason flysch, the Madenkoy spilite, and the Toptepe conglomerate, all of Eocene age; the middle slice consists of the Guleman ultramafic rocks of Cretaceous age; and the uppermost slice consists of the Bitlis metamorphic rocks of Paleozoic age.