Experimental evidence shows that blocking temperature-relaxation time theory for single-domain magnetite can grossly underestimate laboratory thermal demagnetization temperatures of a present day viscous remanence in Devonian limestones of New York State. Thermoviscous remagnetization at moderately elevated burial temperatures during the late Paleozoic cannot therefore be readily excluded for the origin of the stable secondary component in these rocks. More generally, these results emphasize that single domain thermal activation theory cannot be assumed a priori to correctly predict temperature of thermoviscous remanence acquisition in all magnetite-bearing rocks. The Prob leto

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Thermoviscous remagnetization in some Appalachian limestones

Black shale is a dark-colored mudrock containing organic matter that may have generated hy- drocarbons in the subsurface or that may yield hydrocarbons by pyrolysis. Many black shale units are enriched in metals severalfold above expected amounts in ordinary shale. Some black shale units have served as host rocks for syngenetic metal deposits. Black shales have formed throughout the Earth's history and in all parts of the world. This suggests that geologic processes and not geologic settings are the controlling factors in the accumulation of black shale. Geologic processes are those of deposition by which the raw materials of black shale are accumulated and those of diagenesis in response to increasing depth of burial. Depositional processes involve a range of relationships among such factors as organic productivity, clas- tic sedimentation rate, and the intensity of oxidation by which organic matter is destroyed. If enough or- ganic material is present to exhaust the oxygen in the environment, black shale results. Diagenetic processes involve chemical reactions controlled by the nature of the components and by the pressure and temperature regimens that continuing burial imposes. For a thickness of a few meters beneath the surface, sulfate is reduced and sulfide minerals may be deposited. Fermentation reactions in the next several hundred meters result in biogenic methane, followed successively at greater depths by decarbox- ylation reactions and thermal maturation that form additional hydrocarbons. Suites of newly formed min- erals are characteristic for each of the zones of diagenesis.

Black shale - Its deposition and diagenesis.

Optical thermal maturity parameters and organic geochemical alteration at low grade diagenesis to anchimetamorphism: A review

Based on collections from 110 localities, the temperature-induced color change associated with low-grade metamorphism in conodonts is used for the first regional assessment of the degree of heating...

Conodonts as paleotemperature tools in Ordovician rocks of the Caledonides and adjacent areas in Scandinavia and the British Isles

Evidence for Late-Paleozoic brine migration in Cambrian carbonate rocks of the central and southern Appalachians: Implications for Mississippi Valley-type sulfide mineralization

Maps are presented which show indices of organic diagenesis, and form part of a data base which includes previously published stratigraphic and structural data for assessing hydrocarbon potential in the Appalachian and structural data for assessing hydrocarbon potential in the Appalachian basin (de Witt, 1975; de Witt and others, 1975; Harris, 1975; Miller, 1975). The potential for oil and gas production in any basin depends on the presence of source beds, favorable hydrocarbon channelways, and structural and stratigraphic traps. Crucial to these factors is the level of organic diagenesis or thermal maturity within the basin. Numererous studies have shown that depth and duration of burial and geothermal gradient (time and temperature) are the chief elements producing organic diagenesis.

Oil and gas data from Paleozoic rocks in the Appalachian Basin; maps for assessing hydrocarbon potential and thermal maturity (conodont color alteration isograds and overburden isopachs)

Conodont color alteration index (CAI) isograd maps and overburden isopach maps are provided for 3 stratigraphic intervals in the Appalachian basin-Ordovician, Silurian through Middle Devonian, and Upper Devonian through Mississippian. Isograds follow isopach trends throughout the basin; both increase eastward. The CAI 2 isograd lies near the eastern limit (cutoff) of oil production for each stratigraphic interval. Gas production shows less relation to isograds because gas generation occurs over a very broad thermal range and its production depends mostly on primary and (or) secondary porosity and permeability. However, the CAI 4 isograd approximates the eastern limit of gas production over large areas of the basin because the temperature necessary to produce this high level of organic diagenesis concurrently produces mineral metamorphism that reduces porosity and permeability.

J.B. Epstein

Papers

Oil and gas data from Paleozoic rocks in the Appalachian Basin; maps for assessing hydrocarbon potential and thermal maturity (conodont color alteration isograds and overburden isopachs)

Oil and gas data from Paleozoic rocks in the Appalachian basin: maps for assessing hydrocarbon potential and thermal maturity