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Conodont Color Alteration, an Organo-Mineral Metamorphic Index, and its Application to Appalachian Basin Geology

By
Anita G. Harris
Anita G. Harris
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Published:
January 01, 1979

Abstract

Conodonts are apatitic marine microfossils of Cambrian through Triassic age. During incipient metamorphism (50°-300° C) they change color from pale yellow to brown to black due to carbon-fixing within the trace amount of organic matter in their skeletons. As thermal metamorphism continues (300°-550° C), conodonts change from black to gray to white to crystal clear as a result of carbon loss, release of water of crystallization, and recrystallization. The conodont color alteration technique provides a unique link between mineral and organic indexing of thermal metamorphism and is best suited for carbonate rocks.

Conodont color alteration index (CAI) isograd maps for three stratigraphic intervals in the Appalachian basin show: (1) Conodont color alteration is directly related to the depth and duration of burial and the geothermal gradient. (2) Tectonics affect color alteration only where folding and faulting act to significantly increase depth of burial. (3) Isograds and overburden isopachs are conformable throughout most of the northern half and in the western part of the southern Appalachian basin; in these areas, isograd values gradually increase eastward except for a major disruption in the area of the Rome trough. (4) South of central Virginia, isograds are disrupted and irregular because late Paleozoic thrusting has severed and telescoped original burial metamorphism isograd patterns. (5) Basin restoration using conodont CAI isograds indicates a maximum shortening in northeast Tennessee of about 115 miles (185 km). (6) The CAI 2 isograd (=brown conodonts) for each stratigraphic interval lies near the eastern limit of oil production for that interval; this limit shifts eastward for each successively younger stratigraphic interval concomitant with decreasing overburden. (7) Gas production is less related to isograds and depends mostly on primary and (or) secondary porosity and permeability. The CAI 4 isograd (= brownish-black conodonts), however, approximates the eastern limit of gas production because the temperature (depth of burial) necessary to produce this high level of organic metamorphism concurrently produces mineral metamorphism that reduces porosity and permeability and the likelihood of commercial reservoirs.

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Contents

SEPM Special Publication

Aspects of Diagenesis

Peter A. Scholle
Peter A. Scholle
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Paul R. Schluger
Paul R. Schluger
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SEPM Society for Sedimentary Geology
Volume
26
ISBN electronic:
9781565761568
Publication date:
January 01, 1979

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