INTRODUCTION

In the southwestern United States, the sharpest pulse of the Laramide orogeny began during the middle Eocene. In northern New Mexico and Colorado, the northeastwardly moving Colorado Plateau impinged upon the basement buttresses of the Rocky Mountain foreland, producing significant tectonism. This formed a series of en-echelon, asymmetric downwarps, and tilted and subsided blocks that Chapin and Cather (1981) referred to as Echo Park-type basins, and Dickinson et al. (1988) referred to as axial basins.

One of these basins was the Galisteo-El Rito basin of north-central New Mexico. Its principal basin fill, the Galisteo Formation, is as much as 1300 m thick and consists of fluvial sandstone, mudstone, and conglomerate (Gorham and Ingersoll, 1979) (Fig. 1). Two Eocene fossil mammal local faunas (l.f.) are known from the Galisteo Formation. The lower part of the unit produces the Cerrillos local fauna of Wasatchian age, and the upper part produces the Tonque local fauna of Duchesnean age (Fig. 1; see also Lucas, 1982).