Sedimentary column

The foregoing minimum pressure gradient assumes also that the sedimentary column pores are completely filled with saline water and that there is communication from pore to pore within the rock column from surface to depth. 

The Cenozoic portions of the Gulf Coast sedimentary basins are immature therefore, little cementing of the sediments has taken place. Poisson s ratio varies with depth for such sedimentary columns, reflecting the variation of properties through the column. At great depth (i.e., approaching 20,000 ft), Poisson s ratio approaches that of incompressible, plastic materials (i.e., 0.5) [35]. 

Figure 8-5 plots the carbonate parameters in the steady state as a function of depth between 0 and 400 centimeters. The figure shows that the saturation index, dissolution rate, and respiration rate all are very close to zero at 400 centimeters. The results for this simulation therefore do not depend on the total thickness of the sedimentary column, provided that this total thickness exceeds 400 centimeters, a limit that depends on the rate at which respiration decreases with increasing depth. 

The age-determination results by using 210Pb and 137Cs for the sedimentary columns of the alluvia beds along the trunk stream and main tributaries of the Anhui section of Yangtze River show that the variations of the contents of Cd and other heavy metals were basically corresponding closely to major historical events, say the establishment of PRC, the exercise of the policy of reformation and opening and the course of extensive industrialization, etc. 

Figure 8.4 Smearing of a burst in the input of an element i at the surface of the sediment by bioturbation over a layer of constant thickness L (from Ruddiman and Glover, 1972). With time, combined sedimentation at rate v and bioturbation, smears the concentration peak up the sedimentary column.

Particulate matter that reaches the seafloor becomes part of the blanket of sediments that lie atop the crust. If bottom currents are strong, some of these particles can become resuspended and transported laterally until the currents weaken and the particles settle back out onto the seafloor. The sedimentary blanket ranges in thickness from 500 m at the foot of the continental rise to 0 m at the top of the mid-ocean ridges and rises. Marine scientists refer to this blanket as the sedimentary column. Like the water column, the sediments contain vertical gradients in their physical and chemical characteristics. Similar to the vertical profile convention used in the water column, depth in the sediments is expressed as an increasing distance beneath the seafloor. 

Various fluxes and processes considered in the following discussion are represented schematically in Figure 9.20, adapted from Wollast and Mackenzie (1983). Steady-state conditions require that the mass balance for each element is fulfilled for the entire system and also separately for the water and sedimentary columns. To write these mass balances, we must consider the global rate of the reactions occurring in the two subsystems. Therefore, we define R and D as the annual amount of a given element transferred from the solid phase to the aqueous phase or vice versa. The fluxes are taken as positive if there is a net input to seawater, and negative if there is a net output. To maintain the concentration of an element constant in seawater, the net flux resulting from Rj + + Lj + Pj in... 

The Ordos Basin is located about 300 km to the north of Xi an, central China. With an area of 260,000 km it is the second largest sedimentary basin in China. Its central part is characterised by nearly horizontal strata some 4,000 m thick. The sedimentary column can be divided into three major units Lower Palaeozoic marine carbonates Upper Palaeozoic coal-bearing series and Mesozoic terrestrial sediments. Oil and gas fields occur throughout these sediments. The gas is believed to originate from over-matured Ordovician carbonates and coal beds, whilst the source of the oil is thought to be lacustrine sediments of Triassic age. The region is semi-arid and the soils are thin and poorly developed. 

Fluids can be released from solids by mineral dehydration thus adding fluids to the compaction-driven fluid flux in a sedimentary column undergoing burial. The relative contribution of such diagenetic processes to the compaction-driven flow, can be estimated. Dehydration of smectite may be important (Burst, 1969), but at greater depths illitization of kaolinite may be significant (Eq. (2)). Bjprlykke et al. (1986) and... 

Water is also part of the subducting sedimentary column and is present as pore water in sediments and as structurally bound water in metabasalts and gabbros. Estimates of the amount of structurally bound water vary from about 6 wt% at 20-30 km depth reducing to 2 wt% at 60 km and 1 wt% at 200 km (Poli Schmidt, 1995). Schmidt and Poli (1998) estimated that the water input into a subduction zone from oceanic lithosphere is 0.71-1.1 x 109 g/m2, although between 18 and 37% of this water will be released in the generation of arc magmas. 

A thick sedimentary column (up to 10 km in the depocentres) accumulated in the Sergipe-Alagoas basin (Fig. 4) from the Carboniferous to the Quaternary, recording the five phases of structural... 

The slow racemization of amino acids can take place not only in the water column but also in the bottom sediments, and this racemization has important geochemical implications. Most of the amino acids commonly found in the proteins of organisms have been found in small quantities in marine sediments (9, 35, 93, 94). The concentrations near the sediment-sea water interface are on the order of 0.1 to 2 mg per gram of dry sediment. The concentration decreases with increasing depth into the sedimentary column. 

These results suggest that the slow racemization of amino acids in the sedimentary column can be used to estimate the sedimentation rate, and in turn the age, of both marine and fresh water sediments. In sediments from areas in the deep ocean where sedimentation rates are very slow (i.e., a few mm per 1000 years), the amount of racemization of isoleucine would be the easiest to determine since the investigations can... 

Fig. 6.6. Vitrinite reflectance and temperature in the present sedimentary column of the Takhoukht...

The porosity p(Z,f) and the density ps(Z,t) of rocks within the sedimentary column and the weight of the column are computed for every time of basin evolution to determine the value of the average density in Eq. 6.14. 

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