ABSTRACT

Fission-track ages of apatite from six samples of crystalline rock from a near-vertical topographic profile within the mass of the Beartooth Mountain Block and from eight samples collected from surface outcrops of detrital sandstones within the Bighorn Basin document a thermal event at 52 + 3 m.y. ago that annealed fission tracks in all the basin samples and in all but two of the samples from the Beartooth Block. This annealing is interpreted to have been accomplished by burial under 3km of superjacent rock in which a normal geothermal gradient was maintained, or by elevated temperatures briefly maintained under a thermal blanket of Absaroka volcaniclastic rock that was areally more extensive than other field evidence suggests.

The Beartooth Block was not a topographically significant component of an Ancestral Rockies mountain system.

The observed structural relief between the Beartooth Block and the Bighorn Basin was fully developed by early Eocene time.