Cambrian sediments

This further emphasizes the importance of sedimentary processes in the development of such fractionated Rb/Sr and Eu/Sr ratios. Second, in the context of the LFB, the Ordovician turbidites have progressively lower Eu/Sr ratios than the feldspathic Cambrian sediments from which they were partly derived. Thus, it also appears that the displacement to higher Eu/Sr is linked to the number of weathering and erosion cycles involved in the formation of particular sedimentary rock suites. In the examples in Figure 20, the reduction in strontium content for the most evolved Ordovician sediments is nearly 10-fold, and this clearly results in an associated increase in Rb/Sr, and hence with time in strontium isotope ratios. 

Logan G. A., Summons R. E., and Hayes J. M. (1997) An isotopic biogeochemical study of Neoproterozoic and early Cambrian sediments from the Centralian Superbasin, Australia. Geochim. Cosmochim. Acta 61, 5391-5409. 

In general, the calceous-dolomitic rocks from the Cambrian age are affected by their upper beds, by sulphide mineralization of lead, zinc and iron contemporaneous with sedimentation. The oxide lead and zinc minerals are disseminated through dolomitic limestone. As a consequence of the action of the descending process, these formations may assume different types of mineralization. According to the intensity of the oxidation process, which is associated with the different characteristics of the country rock, this country rock may be (a) principally calceous, (b) calceous with dolomitized zones and (c) primarily dolomitized. 

The Period-averaged mass ratio of calcite to dolomite (Figure 10.29) is relatively high for Cambrian, Permian, and Tertiary System rocks, whereas this ratio is low for Ordovician through Carboniferous age sediments and rises in value from the Triassic through the Recent. The generalized sea level curve of Vail et al. 

Most organisms that survive the destructive processes of death and burial to become fossils have some kind of hard parts shell, bones, seeds, or teeth. These strong materials resist decay and may not be easily crushed by covering sediment. Organisms having only soft tissue are rarely preserved as fossils. Organisms with hard parts first appeared about 600 million years ago, in the Cambrian era. 

On the basis of the review of present ideas on the relationship of the processes of volcanism and sedimentation in the formation of the Pre-cambrian iron cherts, several general conclusions can be drawn ... 

Figure 20 Plots of (a) Rb/Sr versus Eu/Sr and (b) Eu/Stn versus Eu/Sr for the reference igneous and sedimentary suites referred to in this chapter, compared to estimates of the continental crust (UC, upper crust BC, bulk crust LC, lower crust, all connected by tie lines). The contrasting trends produced by igneous and sedimentary differentiation processes in (b) are arrowed. Data sources are as for previous figures, with the LFB granites and Cambrian Delamerian Orogen sediments (southeastern Australia) from Kemp (2003), unpublished data), and the granitic and sedimentary rocks of the Damara Orogen from McDermott and Hawkesworth (1990) and...

As stated above, carbonate rocks comprise —30% of the mass of Phanerozoic sediments. Given and Wilkinson (1987) reevaluated aU the existing data on Phanerozoic carbonate rocks, their masses, and their relative calcite and dolomite contents (Figure 38). It can be seen that, as with the total sedimentary mass (Garrels and Mackenzie, 1971a,b), the mass of carbonate rock preserved is pushed toward the front of geologic time. The Tertiary, Carboniferous, and Cambrian periods are times of significant carbonate preservation, whereas the preservation of Silurian and Triassic carbonates is minimal. 

Outstanding concentrations of n-Ci, n-Cn, and n-Ci in early Paleozoic rocks Gloeocapsomorpha prisca, marine phytoplankton of uncertain affinity, probably an alga identified in Cambrian-Devonian sediments but most prominent in Ordovician. Estonian kukersite is a typical source. Blokker et al. (2001) and Powler (1992)... 

Fossils called conodonts (including neurodontiforms) occur from the lower Cambrian to the top of Triassic sediments. The earliest mineralogical designation of the substance composing these denticular plates was... 

Barnaby, R.J. Read, J.F. (1992) Dolomitization of a carbonate platform during late burial Lower to middle Cambrian Shady Dolomite, Virginia, Appalachians. J. sediment. Petrol., 62, 1023-1043. 

Chafetz, H.S. (1979) Petrology of carbonate nodules from a Cambrian tidal inlet accumulation. Central Texas. J. sediment. Petrol., 49, 215-222. 

Gregg, J.M. Shelton, K.L. (1990) Dolomitization and dolomite neomorphism in the back reef of the Bonneterre and Davis Formations (Cambrian), southeastern Missouri. J. sediment. Petrol., 60, 549-562. 

In the study region the paleogeographic history may be subdivided into two stages, viz. the Lower Cambrian characterized by predominantly continental environments and the Middle Cambrian with transitional and marine sedimentation. The Lower Cambrian is represented by alluvial and proluvial deposits, conglomerates, pebble beds and coarse angular and poorly sorted sandstones. 

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