Sediment clastic

Loess is a well-sorted, usually calcareous, non-stratified, yellowish-grey, aeolian clastic sediment. It consists predominantly of silt-sized particles (2-50 mm), and contains normally less than 20 percent clay and less than 15 percent sand. It covers the land surface as a blanket, which is less than 8 meters thick in the Netherlands (exceptionally 17 meters) but can reach up to 40 meters in Eastern Europe and 330 meters in China. 

Let us first introduce some important definitions with the help of some simple mathematical concepts. Critical aspects of the evolution of a geological system, e.g., the mantle, the ocean, the Phanerozoic clastic sediments,..., can often be adequately described with a limited set of geochemical variables. These variables, which are typically concentrations, concentration ratios and isotope compositions, evolve in response to change in some parameters, such as the volume of continental crust or the release of carbon dioxide in the atmosphere. We assume that one such variable, which we label/ is a function of time and other geochemical parameters. The rate of change in / per unit time can be written... 

The first modem publication of Li isotope data from sediments came from Chan et al. (1994a), who reported the isotopic compositions of unaltered turbidites from DSDP Hole 477 and All A, from the Gulf of California. These samples have been reanalyzed (LH Chan, written communication 2003), yielding lighter values. Similarly, Chan and Frey (2003) suggested a range for marine clastic sediments (5 Li = -1 to +5.6) based on new data and re-analysis of samples previously considered to be isotopicaUy heavy. 

Clastic sediments are reservoirs of information about weathering processes, but are sufficiently complex that no study has yet to realize their potential. Despite a number of initial reports of relatively isotopically heavy samples, the majority of data for clastic sedimentary rocks have an average 8 Li 0, equivalent to the estimated average isotopic composition of the continental crust. 

Figure 15. Diagram showing the major components of the global calcium cycle with b Ca values (denoted as 5). The modem residence time of Ca in the oceans is about 1 million years (Holland 1978 1984). Abbreviations used are SW = seawater, Sed = sedimentation, clastic = clastic sediments, carb = marine carbonate sediments, hydrol = mid-ocean ridge hydrothermal systems, lith = continental lithosphere.

Clastic sediments from oxic environments (loess, turbidites, grey shales) 1 ... 

Clastic sediments from anoxic environments (black shales) Il L. 

Hydrothermal (low-T) Igneous (basalt granite) Molybdenites (MoSj) Clastic sediments ... 

Clastic and pelagic sediments. Only a handful of clastic and pelagic samples have been analyzed for 5 Mo (Siebert et al. 2003). Clastic sediments (two samples) are indistinguishable from igneous rocks in their Mo isotope compositions. Pelagic clays (two samples) show enrichment in Mo (3.4 and 185 ppm), and also a shift toward lighter d Mo similar to ferromanganese sediments. 

McLennan, S.M., Hemming, S., McDaniel, D.K., Hanson, G. N. 1993. Geochemical approaches to sedimentation, provenance, and tectonics. In Johnson, M.J. Basu, A. (eds.). Processes Controlling the Composition of Clastic Sediments. Geological Society of America. Special Paper, 284, 21-40. 

Sackett WM (1978) Carbon and hydrogen isotope effects during the thermocatalytic production of hydrocarbons in laboratory simulation experiments. Geochim Cosmochim Acta 42 571-580 Sackett WM, Thompson RR (1963) Isotopic organic carbon composition of recent continental derived clastic sediments of Eastern Gulf Coast, Gulf of Mexico. Bull Am Ass Petrol Geol 47 ... 

Hm hematite, Gt goethite, Ch chamosite, Sd siderite, Im ilmenite, Fh ferrihydrite in which volcanism is associated with clastic sedimentation... 

Fig. 15.6 Schematic representation of iron oxide formation at a textural (= Eh) discontinuity in unconsolidated clastic sediments.

In these clastic sediments the dominant form of sulfur is pyritic, while organic sulfur is usually present only in trace amounts. For this reason, much work on sulfur in these sediments focuses on pyrite formation and its crystallization has been studied in detail by Berner (IT), Sweeney and Kaplan (12). Rickard (13). Rickard (14) and others. Under saline and hypersaline conditions precipitation of monosulfides may be the initial step. Sulfur is then added to these precipitates, converting them to pyrite. Laboratory studies indicate that if griegite is present in the original precipitate, sulfurization may produce framboidal aggregates (12). Conversion may depend on chemical factors such as H2S concentrations (9). In contrast, in conditions that are undersaturated with respect to monosulfides, but supersaturated with respect to pyrite, pyrite may form directly and rapidly from... 

Experimental study of the porosity and permeability of clastic sediments. J. Geol., 43 910-1010. 

The third type is characterized by the association of thin and rapidly-pinching-out iron cherts with clastic sediments, carbonate rocks and carbonaceous shales. These deposits belong to the remote jaspilite formation, developed either at the same time as the jaspilite formation of Krivoy Rog type or after it (analog of the rocks of the upper suite of the Saksagan district). 

Chernov et al. (1970), who studied the cherty iron-formations of Karelia, concluded that they are related to volcanism not only of basic, but mainly also of acid composition. In turn, on the basis of the composition of the parent lavas, they distinguished spilite-diabase and leptite-porphyry formations among those of eugeosynclinal type, formed simultaneously but in different paleotectonic conditions. A large part of the formations of the spilite-diabase series of the Baltic shield is confined to the junctions between geosynclinal depressions and central massifs. The leptite-porphyry series of geosynclinal formations is characterized by a close association of acid and basic volcanics with iron cherts and less often with limestones and clastic sediments (Fig. 9). 

After deposition of the BIF, dolomite was deposited in a neutral environment and gradually came down to the present level. The actual picture undoubtedly was complicated by oxidation-reduction reactions and periods of clastic sedimentation, which will be considered in more detail in formulating the general model of the genesis of BIF. 

Deposition of the iron cherts was controlled by the laws of chemical differentiation of matter, and that of the clastic sediments by the laws of mechanical differentiation. 

Thus secondary differentiation among the purely chemogenic iron sediments was possible, as a result of which the maximum of hydroxide accumulation was shifted to the deeper-water part of the basin and the maximum accumulation of Fe-Mg carbonates was shifted to the shallow-water parts, where finely dispersed clay sediments were being deposited. The maximum accumulation of organic matter was shifted still more into the region of deposition of clastic sediments. 

---