Pre-Cambrian rocks

Tyler S. A. and Barghoorn E. S. (1954) Occurrence of structurally preserved plants in pre-Cambrian rocks of the... 

It should be noted, however, that deposits of vegetable matter are not limited to any particular era or period, but while these deposits occur even in pre-Cambrian rocks, the plants (i.e., terrestrial plants) that were eventually to become coal were not sufficiently abundant until the Devonian Period and it appears that such deposits really became significant during the Carboniferous Period. 

Mvumorphic Ores. These ores include sedimentary iron-ore deposits, which have been metamorphosed, as well as ores associated with mctamorphic rocks, m which the origin of the ore is obscured by recrystallizaiion. Essentially all of the Pre-Cambrian sedimentary iron formations are of this type. 

One conclusion drawn from radioactive measurements is that the pre-Cambrian history of the earth s crust extends beyond 2,700 million years, The pegmatites that have been found to be this old are located in North America and Australia, and they probably exist oil all the continents. The oldest rocks in the United States that have been measured are on the south rim of the ftridger Mountains near the Wind River Canyon in Wyoming. These ancient pegmatites intrude geologic formations of sedimentary and volcanic rocks that, themselves, are the result of even more ancient processes than those in which they were formed. Thus, a period of the order of 3,000 million years or more is available for geologic processes that have formed the crust seen today. 

Let us briefly describe some of the main areas of development of Pre-cambrian BIF of different formational types. Main attention will be paid to the general regularities of distribution of the iron-formations and to their age, spatial, and genetic relationships, rather than to the details of the geology of the individual areas. In so doing we will retain the terminology and nomenclature of the rocks used by the various authors. 

The framework grain composition and palaeo-current analyses indicate that the source rocks for the Serraria Formation were mainly granitic-gneissic rocks, schists and quartzites of the Pre-Cambrian Complex bordering the northern sector of the Afro-Brazilian depression (Garcia, 1992) (Fig. 3). Other less important source rocks include a volcanic suite. 

Alkanes in Fossil Fuels. Sedimentary rocks, which range in age from Recent years) to the pre-Cambrian period ( 3 x 10 ... 

Vein-like deposits in metamorphic rocks also occur in Pre-cambrian Shield areas, but they differ from unconformity-related uranium deposits in that they are not associated with major regional unconformities, the geometries of orebodies are different and they extend to greater depths. Vein-like deposits are closely associated with steeply dipping, brecciated major fault systems. Uranium minerals (pitchblende with some coffinite and brannerite) occur as open fracture fillings and as fine disseminations adjacent to the fractures in Proterozoic meta-igneous andmetasedimentary rocks. Common associated minerals are chlorite, hematite and pyrite. 

Further south, Ratter et al. (1988) postulated that progressive leaching of nutrients under the more humid climate would have led to acidification of the poorer soils associated with much of the pre-Cambrian shield, favouring expansion of cerrado (upland) savannas at the expense of dry forests. According to this hypothesis, dry forests would be expected to persist continuously since the LGM only in those areas (i.e. the contemporary SDTF refugia) where base-rich soils could be continuously replenished by weathering of parent calcareous rocks (Ratter et al., 1988). 

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