Limestone-associated clays

Attempts were made to collect samples of the limestone-associated clays in the hill country. Samples were obtained from the el-Jib area, from near Qastel, at Beit Ummar, and at Arub. 

This is widespread usually as a gangue (impurity) mineral in hydrothermal veins associated with metallic ores some also occurs in limestone or clay. Its crystals are usually tabular and often diamond shaped, but fairly symmetrical. It has two features of commercial interest. Its specific gravity, 4.5, is the highest of the non-metallic minerals and it is chemically inert. It has a Mohs hardness of 3.5, and a refractive index 1.64-1.65. 

Reg soils are closely associated with desertic regions. They have developed on stable surfaces where coarse, gravelly desert alluvium is exposed, and are characterized by a well-developed desert pavement and exhibit some well-defined soil horizons. They occur mostly on depositional surfaces where stones and gravels have been deposited since Neogene times. The surfaces commonly consist of stony, unconsolidated sedimentary deposits in which limestone, dolomite, chalk, flint and marl predominate, together with some fines (silt and clay). Sandstone and granite debris have also been reported to contribute to Reg formation. Less frequently, they form on sedimentary bedrock (Fig. 1.5). 

In Gubbio, Italy, a 1 cm layer of clay between extensive limestone formations marks the boundary between the Cretaceous and Tertiary Periods. This clay layer was known to have been deposited about 65 million years ago when many life forms became extinct, but the length of time associated with the deposition was not known. In an attempt to measure this time with normally deposited meteoritic material as a clock, extensive measurements of iridium abundances (and those of many other elements) were made on the Gubbio rocks. Neutron activation analysis was the principal tool used in these studies. About 50 elements were searched for in materials like the earth s crust, about 40 were detected and about 30 were measured with useful precision [26-28]2. 

These clays occur in limestones, dolomites, evaporites, shales, siltstones, and hydrothermal deposits. All the sedimentary material appears to have a diagenetic origin. Although the physical environments vary, the chemical environments should be similar. Saline or even super-saline conditions are implied by the presence of evaporite minerals associated with some of the deposits. In the other deposits it is possible that temporary evaporitic conditions (e.g., tidal flats) existed long enough for brucite to precipitate between the layers of expanded-layer minerals. It appears plausible that the parent material was a montmorillonite-like mineral (probably detrital in most cases). 

Fig. 7. Schematic north—south cross-section through San Antonio and the Muil Field (see Fig. 1). Jurassic and Lower Cretaceous sediments (the aquifer ) are shown by the limestone Pattern since most are carbonates. They are overlain by Upper Cretaceous clay and chalk. The northern-most outcrops are associated with the Balcones Fault zone near San Antonio. The shallow part of the section is underlain by Paleozoic basement , or by Triassic rocks further south. Jurassic salt is shown by solid hatchures. Darkened areas at A, B and C depict the three producing zones, the Stuart City Reef Trend (the carbonate platform margin), the Karnes Trough and the Atascosa Trough, respectively. Note the faulting associated with them. The temperature axis is speculative below 5 km.

The acidity of the soil is an important factor that can influence, in particular, the uptake of many trace elements by plants. Both manganese and cobalt are poorly absorbed by plants from calcareous soils, whereas low molybdenum levels of herbage are usually associated with acid soils. The induced copper deficiency known as teart (see p. 123), associated with high herbage molybdenum levels, generally occurs on pasture grown on soils derived from Lower Lias clay or limestone. 

Gypsum is a widely distributed mineral and the commonest sulfate mineral, frequently associated with haHte and anhydrite in beds, inter-stratified with limestone, shale and clay. 

Figure 32.6 The ranges of selected brachiopod taxa through the Estonian Ordovician successions preliminary brachiopod associations have been established for the northern belt. I - limestone, 2 - argillaceous limestone, 3 - micritic (aphanitic) limestones, 4 - mainly marls, 5 - clay, 6 - black shale, 7 - sand, 8 - red colouration, 9 - glauconitic ooids, 10 - kukersite above the K-bentonite.

Colorado, is hosted in folded and brecciated Mississippian dolomite, shale, sandstone and coal, which is in fault contact with Precambrian gneiss. The Precambrian gneiss is anomalously enriched in uranium, and a Mesozoic or Caenozoic age of mineralization in the Mississippian host rocks, formed by downward percolation of uraniferous groundwater from the adjacent Precambrian terrain, is inferred. Mineralization in the Pryor Mountains region, Montana, is hosted in karsts developed with the Mississippian Madison limestone and consists of uraninite-tyuyamunite grading up to 7% UsOg associated with clay minerals and silicified collapse breccias. A Caenozoic age of mineralization, under conditions similar to those of Pitch mine, is favoured. 

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