Fission Tracks in Basin Analysis
NAESER, NANCY D., U.S. Geological Survey, Denver, CO
Over the last 12 years, fission-track analysis has emerged as a widely used method for tracing the depositional and thermal history of sedimentary basins. The fission tracks found in minerals are zones of intense damage formed by the spontaneous fission of 238U. Because spontaneous fission takes place at a constant rate, fission tracks can be used to date these minerals. In many studies, fission tracks have established geochronology and correlation in basins, particularly by dating volcanic ash beds in sedimentary sequences.
However, major interest in fission tracks in basin analysis centers on their ability to provide information about thermal history. Fission tracks remain relatively stable in most minerals at ambient surface temperatures, but when a mineral is heated (as during burial in a basin), the tracks become progressively shorter and ultimately disappear. The temperature range over which this "annealing" takes place depends on the mineral being analyzed and the duration of heating--the shorter the heating time (as in very young basins or geothermal systems)--the higher the temperatures required for annealing. Apatite is by far the most commonly used mineral in thermal history studies. One reason for its popularity is that more experimental and empirical data on annealing temperatures are availab e for apatite than for any other mineral, but what is more important for basin analysis is that apatite annealing temperatures coincide with the temperature range of hydrocarbon generation and a number of other low- to moderate-temperature processes in basins. The reduction in mean track length, change in track-length distribution, and reduction in fission-track age that results from annealing can be used to trace, within this critical temperature range, the timing and magnitude of heating and cooling (due, for example, to burial and subsequent uplift and erosion) of sedimentary sequences.
Apatite fission-track analysis has been used in basin studies throughout the world to reveal differential heating of rocks separated, for example, by the active White Wolf fault in the southern San Joaquin Valley, California; recent, rapid heating in the Los Angeles basin, California; Pliocene cooling in the northern Green River basin, Wyoming; and post-middle Miocene cooling of Cretaceous sandstones in the southwestern Powder River basin, Wyoming.
AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)