Annealing of fission tracks in minerals, like the generation and maturation of hydrocarbons, is a function of temperature and time, the temperature zone of fission track annealing in apatite (70-125°C, 158-257°F, for heating times of 10⁶-10⁸ years) coinciding closely with the oil-generation window. Patterns of fission-track age, track length, and length distribution can thus give an important record of the thermal evolution of a sedimentary basin.

The early Cretaceous Otway Formation in southeastern Australia is a 3-km (10,000-ft) thick sequence of volcanogenic sediments in the late Jurassic-Tertiary Otway basin. Fission-track ages on detrital apatite, sphene, and zircon in outcropping sandstones from this formation are concordant, indicating a contemporaneous volcanic source. This concordance also indicates that the outcrop samples have not been heated above about 70°C (158°F) since deposition.

Apparent apatite ages from deep wells in the subsurface section of the Otway Formation decrease with depth, reaching zero at subsurface temperatures of about 125°C (257°F). Wells in different parts of the basin show a variety of thermal histories. In some wells, the rocks are experiencing their maximum temperatures now, whereas others show evidence of higher temperatures in the past. The extreme example, at the eastern end of the basin, experienced maximum temperatures prior to a period of rapid uplift about 95 Ma. The apatite data show that hydrocarbon generation in this area occurred less than 30 m.y. after deposition and prior to this structuring event. The fission-track evidence also requires that the early Cretaceous geothermal gradient was substantially higher than at resent.

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