Geologic time periods

Epoch Usually, the smallest division of geologic time. Periods are divided into epochs. The Holocene epoch ranges from the end of the last glaciation about 11 500 years ago to the present. 

Quartzite A metamorphosed quartz-rich rock. Sandstones may metamorphose into quartzites. Quaternary A geologic time period that ranges from 1.75 million years ago to the present. 

Permian Period The last geologic time period of the Paleozoic Era, noted for the greatest mass extinction in earth history, when nearly 96% of species died out. 

Silurian Period The geological time period of the Paleozoic Era following the Ordovician, between 435 and 395 million years ago, when plants colonized the land. 

Radiogenic isotopes have proved a most powerful tool for understanding mantle processes. By studying mantle peridotites and mantle-derived melts from a variety of different geological time periods it is possible to define evolutionary curves for the different isotopic systems within the mantle. These curves, when plotted on isotope ratio versus time diagrams, can be used to characterize the chemical evolution of the mantle over time. Deviations from the chondritic trend are used to identify chemical fractionation events in the mantle during Earth history. Of particular... 

The rocks in which large volumes of petroleum are able to accumulate are termed reservoir rocks. They require suitable porosity (typically 10-25%) and permeability (typically 1-1000mD 1 mD or milliDarcy = c.10 9m2), with reasonably sized pores and an impermeable cap rock or seal to prevent escape of petroleum over geological time periods. They must also be in place before the onset of oil generation. Sandstones often provide suitable reservoir characteristics. More than 60% of all oil occurrences are in clastic rocks, while carbonate reservoirs account for c.30%. The smaller molecules present in gases can escape through narrower pores than oil components, and seals are often slightly leaky with respect to gas. 

Light hydrocarbons, particularly methane, can diffuse relatively rapidly through imperfect seals. Because reservoir seals are usually not perfect the gases are likely to become depleted over geological time periods. Because diffusion is related to molecular mass, methane containing the 12C isotope diffuses more rapidly than that containing 13C, so there is an isotopic fractionation effect, with the residual gas becoming isotopically heavier (see Section 5.8.6b). 

Pyrolytic PAH distributions in ancient sediments sometimes differ from those typical of Recent sediments in exhibiting enhanced levels of the more highly peri-condensed structures, especially benzofe]pyrene, benzo[gfe]perylene and coronene (Fig. 7.4 Killops Massoud 1992). The reasons are as yet unknown but may reflect the effects of either different formation conditions or varying geochemical processes over geological time periods. 

Fluctuations that have been measured in the 5 G values of sedimentary organic matter over the Earth s history (e g. Schidlowski 1988) can thus be interpreted in terms of the productivity in the water coltrrrm and the availability of DIG in a particular geological time period. In a study of sediments from the central equatorial Pacific Ocean spanning the last 255,000 years it has been demonstrated that the carbon isotopic composition of fossil organic matter depends on the exchange between atmospheric and oceanic GO. Ghanges with time can then be used to estimate past atmospheric carbon dioxide concentratiorrs (Jasper et al. 1994). 

The role of the benthic interface is not to be defined exclusively by one transport direction. On the one hand, deposition and burial of marine sediments remove elements from the marine cycles over geological time periods. Yet, as the example of the carbon cycle shown above already demonstrated, an immense proportion of accumulated particles are subject to dissolution or microbial decomposition in the course of early diagenesis (cf. Chapter 9). Marine sediments therefore also act as a secondary source of remineralized dissolved components. The coexistence of these two fundamental, but oppositely directed mass movements constitutes one of the most essential phenomena at the seafloor. Next to studies on particle fluxes through the water column and element-specific accumulation rates, the quantification of benthic flux rates across the sediment/ water boundary represents the third pre-condition for obtaining a complete balance of the marine material cycles. 

Fig. 11. The inadequacy of diffusive mixing, even after geological time periods, (a) Very poor mixing in a thin reservoir because the diffusive length scale L is small relative to the lateral extent of the reservoir. (Deff> effective diffusion coefficient, m /s t, time period, s). (b) Moderate diffusive mixing in a thick gross reservoir. The small inset figures show generalized API gravity and/or GOR, and bubble point and reservoir pressure depth trends...

The natural analogue sites are areas in which uranium ores have be preset for geologic time periods. In most cases, these sites have not been afiected by human activities, so the record of geologic, long term effects are well preserved. A number of such sites are being studied around the world studies fi om a few sites of different characteristics are reviewed here. 

Large, deeply buried potash deposits are mainly associated with marine e aporite sequences and less commonly with non-marine evaporites throughout the world. Potash deposits occur on every inhabited continent and have been identified in most geologic time periods from the Cambrian to the present. The most abundant potash mineral in commercial potash deposits is sydvite Sylvite and halite (NaCl) form the common potash ore called sylvinite. In most occurrences, fairly pure sylvinite... 

Prom geology we know that the composition of the subsurface depends on its geological history, and may roughly be separated into units created during different geological time periods, and in different depositional environments. Hence, there exists a time (geological periods) and space (depositional envi-... 

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