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The Rb-Sr isotopic dating method can be applied to sedimentary rocks to determine the time of formation of diagenetic illite. In the Pleasant Bayou 1 geothermal test well in Brazoria County on the Texas Gulf Coast, the interval from 9,300 to 16,500 ft (2,800 to 5,000 m) consists mainly of overpressured shale and sand of the upper Oligocene Frio Formation. Rb-Sr isotopic analyses of the less than 0.06 µ fraction indicate that clays within the zone of "hard" geopressure, which extends downward from 11,000 ft (3,400 m), formed in equilibrium with pore water and record an age of diagenesis at 23.6 ± 0.8 m.y. This sharply defined age is in contrast to the result that would be expected if burial diagenesis had been a gradual, continuing process, in which clays at diff rent depths would have accumulated various amount of Rb at different times in the past. If this were true, the ages of diagenesis would have varied continuously from older in the deeply buried part of the stratigraphic section to younger in the upper part of the section.
Instead, the data favor a "punctuated diagenesis" in which clay transformation occurred in a relatively brief episode early in the history of the sediment. Once formed, the clay particles persisted as closed isotopic systems. Rapid clay diagenesis in a thick package of sediment could release an enormous amount of interlayer water, and this release would have been a potential mechanism for transporting cement constituents as well as petroleum from the deeper shale beds.
Oxygen isotope data from these clays can be interpreted as supporting a smectite-to-illite conversion at a much lower temperature than those prevailing at depth today. Low-temperature early diagenesis could reduce permeability through precipitation of released silica. Further burial would then have created an optimal situation for the formation of geopressure by aquathermal pressuring.
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