Deposition Of Sedimentary Basins Research Articles

Applications of Natural Radiation-Induced Paramagnetic Defects in Quartz to Exploration in Sedimentary Basins

ABSTRACT Quartz grains in contact with uranium-bearing minerals or fluids are characterized by natural radiation-induced paramagnetic defects (e.g., oxygen vacancy centers, silicon vacancy centers, and peroxy radicals), which are amenable to study by electron paramagnetic resonance (EPR) spectroscopy. These natural radiation-induced paramagnetic defects, except for the oxygen vacancy centers, in quartz are concentrated in narrow bands penetrated by α particles: (1) in halos around U- and Th-bearing mineral inclusions and (2) in outer rims or along fractures. The second type of occurrence provides information about uranium mineralization or remobilization (i.e., sources of uranium, timing of mineralization or remobilization, pathways of uranium-bearing fluids). It can also be used to evaluate sedimentary basins for potential of uranium mineralization. In particular, the peroxy radicals are stable up to 800 °C and, therefore, are useful for evaluating metasedimentary rocks (e.g., Paleoproterozolc metasedimentary sequences in the central zone of the North China craton). EPR study of the Changcheng Series can focus on quartz from the sediment-basement unconformity and faults to determine the presence and types of natural radiation-induced paramagnetic defects, with which to identify and prioritize uranium anomalies. Other potential applications of natural radiation-induced paramagnetic defects in quartz include uranium-bearing hydrocarbon deposits in sedimentary basins. For example, the Junggar, Ordos, and Tarim basins in northwestern China all contain important oil and natural gas fields and are well known for elevated uranium concentrations, including economic sandstone-hosted uranium deposits. Therefore, systematic studies on the distribution of natural radiation-induced paramagnetic defects in quartz from host sedimentary sequences are expected to provide information about the migration of oil and natural gas in those basins.

A simple model of flow patterns in overpressured sedimentary basins with heat transport and fracturing

Overpressure zones in sedimentary basins are defined as sediments where the fluid pore pressure is substantially higher than hydrostatic. Chief among the causes for the formation of overpressure is the rapid deposition of low‐permeablity sediments so that compaction and dewatering are inhibited. This results in the support of the overlying material in part by the fluid, rather than by grain to grain contact. Many overpressure zones also have associated geothermal anomalies whereby the geotherms are hotter than normal. In this paper, we present a one‐dimensional model of overpressure generation and dissipation, with hydraulic fracture. The fluid flow is coupled to the fluid temperature field and to the motion of the solid sediment matrix. Compacted coordinates are introduced to decouple the fluid velocity and temperature from the solid matrix velocity. Using a maximum sedimentation rate of 1 km Myr−1 and a hydraulic conductivity at the top of the layer equal to 5×10−12 m s−1, 3 km of partially compacted sediment are deposited in 5 Myr, generating an overpressure of 0.81 of the lithostatic pressure at the bottom of the sediment layer. An undercompaction of 19% relative to normally compacted sediment is developed at a final depth of 3.66 km by the end of sedimentation. Dissipation of the overpressure takes place on a timescale of the order of 40 Myr. The equilibrium temperature generates thermal gradients in excess of 38°C km−1 at shallow depths. Fast sedimentation rate and high permeability in the fractured state favor oscillating behavior and propagation of fracture waves. Overpressure zones may constitute a proximal source of hydrothermal fluids for the genesis of ore deposits in sedimentary basins.

Three‐dimensional numerical modeling of coarse‐grained clastic deposition in sedimentary basins

A three‐dimensional numerical model of clastic deposition in sedimentary basins is used to investigate the development of coarse‐grained deltas in response to relative sea level changes. The model incorporates fully three‐dimensional sediment delivery and deposition, together with incised channel formation and slope failure. A combined random‐walk/steepest‐descent algorithm is used to simulate sediment delivery from a drainage basin outlet to the depositional shoreline, together with a nonlinear three‐dimensional diffusion equation for modeling slope‐dependent sediment erosion, transport, and deposition. When sediment channels (transport pathways) are out of a specified grade, advective erosion occurs, thereby increasing the total sediment supply delivered to the basin. Multiple sediment sources into a basin can be simulated, together with tectonic subsidence and eustatic sea level changes. The model results presented display many of the key features observed in both modern and ancient coarse‐grained deltas. During periods of slowly changing and high relative sea level (highstands), a broad delta front progrades approximately radially away from the sediment source. Conversely, incised channels are developed during rapid falls of relative sea level, leading to localized deposition of deltaic lobes at the mouths of these channels. The incised channels initiate at the exposed delta front and propagate backward toward the sediment source. The rate of growth of incised channels is shown to be an important control on delta morphology, sediment distribution, and lowstand lobe size and location. Lobes are particularly well developed during periods of slowly changing low relative sea level (lowstands). Periods of rapid rise of relative sea level lead to flooding and infilling of incised channel systems and a rapid landward shift in deposition (transgressions). Incised channels and delta lobes are best developed in a model where tectonic subsidence is similar to that expected for hanging‐wall‐sourced deltas in extensional half‐graben settings. In contrast, both channels and lobes are less well developed in a model where tectonic subsidence is similar to that expected for footwall‐sourced deltas in such settings. Many of the features developed in the models are, by their nature, three dimensional, and two‐dimensional analysis of the model results can lead to erroneous interpretations of the causes of along‐strike variability. These aspects of sequence variability have important implications for the application of the sequence stratigraphic methodology to many sedimentary basins.

Abstract: Delineation of Subsurface Proterozoic Chuar Group / Unkar Group (Grand Canyon Supergroup) Sedimentary Basin Deposits in Northern Arizona Utilizing Gravity and Magnetic Geophysical Techniques 

Abstract: Delineation of Subsurface Proterozoic Chuar Group / Unkar Group (Grand Canyon Supergroup) Sedimentary Basin Deposits in Northern Arizona Utilizing Gravity and Magnetic Geophysical Techniques 

The crustal structure of the southern Caspian region

The south Caspian basin appears to behave as a relatively rigid aseismic block within the otherwise deforming Alpine–Himalayan orogenic belt. This anomalous character suggests that there is a fundamental compositional difference between the crust of the south Caspian basin and that of the surrounding region. Five new estimates of crustal velocity structure beneath the southern Caspian region are determined by the teleseismic receiver function method. These models show that the crust in Turkmenia along the trend of the Apshceron–Balkhan Sill–Kopet Dag Mountains is 50 km thick. In the southwestern part of the Caspian basin the crust is 33 km thick and consists of a 13 km thick sedimentary section lying on a high-velocity (Vp∼7.1 km s−1) lower-crustal section. In the southeastern part of the basin the crust is 30 km thick and consists of a 10 km thick sedimentary section overlying a 20 km thick low-velocity (Vp∼5.8 km s−1) crystalline crust. The receiver function models are combined with velocity models from previous Russian Deep Seismic Sounding studies into a ∼1800 km long ESE–WNW-trending crustal cross-section across the Kura Depression, the south Caspian basin and the Kopet Dag Mountains. The most significant features of this crustal model are the 20 km variation in thickness of Cenozoic sedimentary basin deposits, the absence of a ‘granitic’ (Vp∼5.8–6.5 km s−1) crustal layer in the central part of the south Caspian basin, and 20 km of crustal thinning beneath the central part of the basin. The Moho beneath the south Caspian basin has a broad arch-like structure whose western boundary is a relatively narrow zone across which the crust thins rapidly (∼20 km thinning over a 100 km zone) and whose eastern boundary has a more gradual change in crustal thickness (∼20 km thinning over a 400 km zone). The velocity–depth profiles derived from the receiver functions are compared to laboratory velocity estimates made at pressures and temperatures appropriate for the lower crust in Turkmenia, but the seismic P-wave data alone do not permit differentiation between the various possible rock types. The crustal model developed for the south Caspian basin is consistent with the hypothesis that the crystalline crust of the basin is a section of oceanic crust that is being overthrust by the continental crust around much of its margins. However, we cannot rule out the possibility that the crystalline crust beneath the basin is a section of lower continental crust whose upper crustal section has been removed by erosion or faulting.

Stable cratonic sequences and a standard for Silurian eutasy : Johnson M.E., ed B.J. Witzke et al., (Geological Society of America; Special Paper, 306), Paleozoic sequence stratigraphy: views from North American craton, 1996, (203–211)

Accurate evaluation of uranium sources and identification of their spatiotemporal correlations with uranium deposits (or sinks) in sedimentary basins are important for exploring sandstone-type uranium deposits. This study examines the source-to-sink system of Mesozoic–Cenozoic sandstone-type uranium deposits in the Qaidam Basin (QB) through a literature review in conjunction with logging and seismic data and yields the following findings. (1) The uranium-supplying capacity of a source rock in the uranium source area depends on primarily its type and to a lesser extent, the tectonic zone where it is located and the episode during which it formed. (2) The initial uranium content, U0, is the core parameter for classifying and evaluating the uranium-supplying capacity of a source rock. At U0 > 1.5 ppm, uranium begins to migrate from a source rock. The uranium variation coefficient, ΔU, and the amount of uranium migration, ΔUt, decrease continuously to –55 % and –5 ppm respectively as U0 increases up to 9.5 ppm, which suggests that at most 5 ppm of uranium in the source rocks can migrate away. As U0 increases beyond 9.5 ppm, ΔU and ΔUt fluctuate around –55 % and –7 ppm respectively, which suggests that approximately 7 ppm of the uranium in the source rocks can migrate away. (3) Of the different types of rocks, silicic igneous rocks (including both intrusive and volcanic igneous rocks) and granitic gneisses have the highest uranium-supplying capacity; intermediate igneous rocks and gneisses come second; mafic–ultramafic igneous rocks and the other metamorphic rocks (except granitic gneiss and gneiss) come third. Of the sub-source areas (tectonic zones), the western segment of the Eastern Kunlun and the Qiman Tagh have the highest uranium-supplying capacity; the Central and Southern Altyn Tagh come second; the others come third. (4) Uranium sources are the fundamental prerequisite for the formation of uranium sinks. Other geological factors control the specific formation process of uranium sinks as well as their spatiotemporal distribution patterns. For QB, the most economic and prospective uranium sink, Neogene formations of Gy anticline, was fed by the source areas which had the highest uranium-supplying capacity. This suggests that the Neogene of WEK anticline belt, Quaternary of Kumukol Basin and Qarqan River Basin are the most noteworthy metallogenic prospective areas. The uranium-source evaluation scheme and “source-to-sink” ore exploration approach introduced in this study have practical value for advancing the exploration of uranium deposits in sedimentary basins in northern China and other parts of the world.

On a thermoconvective mechanism for oscillatory vertical crustal movements

The sedimentary record testifies that one of the main peculiarities of vertical crustal movements is their rhythmic behavior which consists of a periodic change in the sign of movements. The long history of deposition in sedimentary basins is inevitably interrupted by episodes of erosion that indicates elevation. The vertical crustal movement in sedimentary basins can be considered as a slow subsidence on which a small-amplitude oscillation (amplitude of the order of 100 m) is superimposed. In the present work oscillatory crustal movements are related to thermoconvective oscillation of the upper thermal boundary layer formed by large-scale convection in the Earth's mantle. The instability of the boundary layer was analyzed by many researchers who used a Newtonian fluid rheological model or a power law non-Newtonian fluid model for the Earth's mantle. However, a real material has a memory, in contrast to Newtonian and power law fluids. The author proposed recently a non-linear integral model consistent with general creep theory and laboratory studies of rock creep to describe the Earth's mantle rheology. To investigate the boundary layer instability this rheological model, having a memory, is used for the mantle and the crust but a thin uppermost layer which may be considered to be purely elastic at geological timescales is distinguishable. The upper boundary of the layer is considered as a surface loaded by sediments, the thickness of which is changed with time. It is shown that the boundary layer instability is oscillatory and the period of thermoconvective oscillation is of the order of 100 Ma.

Condensation processes in shallow platforms

Sequence stratigraphy allows elucidation of the time-space relationships of deposits in sedimentary basins. However, the relative distribution of unconformities and condensed sections can neither be used as diagnostic criteria for particular palaeogeographic settings, nor as indicators of maximum regional transgressions. Condensation processes also occur in the shallowest portions of platforms and during maximum regression episodes. Three separate processes of condensation are here distinguished: stratigraphic, sedimentary and taphonomic. These three categories of condensation can be used to analyze patterns of sediment distribution, and also to estimate lateral and vertical variations in sedimentation rates. Contrary to the usual interpretation, the maximum values of rate of sedimentation and degree of accommodation of sediments, as well as the highest relative sea level, occur in the lowest levels of the shallowing-upward sequences. The distinction between these condensation processes allows one to predict that the degree of sedimentary and stratigraphic condensation will be higher towards the distal portions of the platforms, whereas the stratigraphic condensation processes without sedimentary condensation will show the maximum intensity and frequency in the shallowest portions of the platforms.

The role of geopressure zones in the formation of hydrothermal Pb-Zn Mississippi Valley type mineralization in sedimentary basins

Abstract Geopressure zones immediately subjacent to platform carbonate rocks can be modelled to serve as proximal sources of hydrothermal fluids for epigenetic Pb-Zn deposits in sedimentary basins. In some geopressure zones of the Gulf of Mexico, geothermal gradients can be as high as 10°C 100 m −1 . This arises because the water-saturated geopressured shale masses act as thermal insulators. Geopressure zones may have sufficient fluid pressure to rupture overlying strata, providing a vertical conduit for hot mineralized brine to migrate directly into carbonate host rocks.

Maturation-based geodynamic/geochemical classification of sedimentary basins

Geodynamic principles and processes are combined with continental margin type basin position on a plate, and crustal type to classify sedimentary basins in a tectonic mode. Tectonic basin classifications, however, have not improved the rate of discovery of hydrocarbons or mineral deposits in sedimentary basins. To improve prediction of commercial hydrocarbon occurrence and sedimentary-hosted ore deposits, it is recommended that the proposed geodynamic basin classification could be combined with organic (petroleum) or inorganic (metals) geochemical classification systems. Organic geochemical basin classification discriminates basin types using three criteria: charge factors, migration drainage factors and impedance factors (Demaison and Huizinga, 1989a, b). A similar approach is applicable to mineral deposits. An inorganic geochemical classification is proposed that recognizes three criteria of inorganic maturation: metal solubility factor, migration drainage factor, and environmental potential for mineral precipitation. This combined geodynamic and geochemical maturation approach to basin classification should enhance resource exploration efficiency.