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OCR for page 159

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COULD POSSIBLE CHANGES IN Gf OBOE GROUNDWATER RESERVOIR CAUSE EUSTATIC SEA-LEVEL FLUCTUATIONS? ~ 63 SEDIMENTARY RESERVOIRS OF THE CONTINENTAL BLOCKS The sites of residence of sediments on the Earth's continental blocks can be grouped into three major catego- nes: cratonic, geosynclinal, and the coastal plains and continental shelves (Southam and Hay, 1981~. The sedi- mentary bodies at these sites cover approximately 80 per- cent of the land surface (Ronov, 1982) and have a total volume of over 790 x 106 km3 (Southern and Hay, 1981; Ronov and Yaroshevsky, 1977~. Their dimensions and characteristics are summarized in Table 9.2. Cratonic Sediments Cratonic sediments fringe the stable shield areas of the continental interior and the adjacent continental platforms. Because the central shield areas contain little or no sedi- ment cover, the majority of the cratonic sediments reside on the peripheral continental platforms (Southam and Hay, 19811. Using data compiled by Gilluly et al. (1970) and Ronov and Yaroshevsky (1977), Southam and Hay (1981) calculated the combined total area of the world's cratonic shields and cratonic platforms to be 96.3 x 106 km2, the cratonic shields occupying an area of 29.4 x 106 km2 and the platforms having an extent of 66.9 x 106 km2. How- ever, because most of the sedimentary cover resides on the peripheral areas of the platforms, they estimated that only 55 x 106 km2 of the total cratonic area contains appreciable sediment cover. They assumed the average global thick ness of the cratonic sediments to be 3 km so that the total volume is 165 x 106 km3. This accounts for almost 21 percent of the total sedimentary volume on the continental blocks (see Table 9.2~. Ronov (1982) calculated the percentages of the major rock types occurring on the continental blocks based on volume estimates of the different rock types of North America, Europe, and the Soviet Union (see Table 9.2~. He estimated that almost half of the cratonic platform sediments are clays and shales with carbonates and sand- stones almost 25 percent each. These three rock types make up nearly 93 percent of the total volume of cratonic platform sediments, the remainder being mostly volcanics and evaporites. Geosynclinal Sediments Geosynclinal sediments, occupying elongate regions of the Earth's continental crust, usually represent sites of thick sediment accumulation on passive or active conti- nental margins subsequently exposed by uplift and erosion as a result of plate tectonic processes. Geosynclines have been estimated by Ronov and Yaroshevsky (1977) to cover an area of 59 x 106 km2 and contain sediments to an average depth of 9 km. They represent over 67 percent of the sediment found on the continental block. Ronov (1982) compiled percentage estimates of the main rock types found in the Geosynclines as shown in Table 9.2. The relative proportions of 40.9 percent clays and shales, 19.2 percent carbonates, and 19.2 percent sands TABLE 9.2 Average Dimensions, Porosity, Pore Volumes, and Composition of the Major Continental Sedimentary Reservoirs (modified after data from Ronov and Yaroshevsky, 1977; Southam and Hay, 1981; Ronov, 1982) Sedimentary Reservoir Sediment Estimated Pore Volume and Percentage of Major Rock Types Reservoir Area Thickness Volumea Volumeb Porosity Volume Shale Sandstone Carbonate Volcanic Other Cratonic 55 3 165 157 20% 31.6 76.4 36.8 407.26 4.4 platforms 46.3% 22.3% 24.3%4.4% 2.7% Geosynclines 59 9 531 422 13% 54.9 217 102 102108.8 1.06 40.9% 19.2% 19.2%20.5%C 0.2% Passive margin, 31 3 95 91 shelves, and coastal plains Total 146 5.4 791 670 15.6% 20% 18.2 Note: Thickness is in km, areas are in 106 km2, and volumes are in 106 km3. 44 21 46.3% 22.3% 23 24.3% 4.2 2.5 4.4% 2.7% aIncludes volcanic rocks. bMinus volcanic fraction; Ronov (1982) assumed the passive margin shelves and coastal plains to have a composition equivalent to the cratonic sediment. CA compromise between 19.4% (Ronov, 1982) and 21.9% (Ronov and Yaroshevsky, 1977~.