INTRODUCTION

The Horned Toad Hills are located along the northwestern margin of the Mojave Desert, California, where 430 m of nonmarine sediments are exposed in low hills just south of the Garlock fault, northwest of the town of Mojave, Kern County, California. The definition of the Horned Toad Formation is extended to include the original three members described by Dibblee (1958) and two new members. The late Miocene to early Pliocene sediments of the Horned Toad Formation were deposited in fluvial and lacustrine environments on an erosional terrain of Cretaceous granitic and Miocene volcanic rocks. Based on analyses of clast composition, the sediments of the Horned Toad Formation were derived locally. A volcanic tuff in the Horned Toad Formation has been geochemically identified as the Lawlor Tuff that has been dated elsewhere at 4.83 ± 0.04 Ma (McLaughlin et al. 2005; Sarna-Wojcicki et al. 2005b).

The Horned Toad Formation is deformed into open folds associated with basement-involved reverse faults. Major structural features trend northeast-southwest, roughly parallel to the trace of the Garlock fault that is located just north of the Horned Toad Hills. Structural deformation of the granitic basement and overlying sediments within the Horned Toad Hills may be related to post-5 Ma movement on the Garlock fault or to regional north-south compressional deformation in the Mojave Desert (Bartley et al. 1990). The deformed Horned Toad Formation is unconformably overlain by alluvium of Pleistocene (?) to Quaternary age.

Fossil mammals from the Horned Toad Formation include the following taxa: Cryptotis sp., cf. Scapanus, Hypolagus vetus, Hypolagus edensis, ?Spermophilus sp., Prothomomys warrenensis n. gen., n. sp., Perognathus sp., Repomys gustelyi, Postcopemys valensis, Peromyscus sp. A, Peromyscus sp. B, Jacobsomys dailyi n. sp., Borophagus cf. B. secundus, cf. Agriotherium, Machairodus sp. cf. M. coloradensis, Rhynchotherium sp. cf. R. edensis, Pliomastodon vexillarius, Dinohippus edensis, Teleoceras sp. cf. T. fossiger, cf. Prosthennops, Megatylopus sp. cf. M. matthewi, Hemiauchenia vera, Camelidae gen. et. sp. indet., and cf. Sphenophalos. The majority of fossil localities are confined to a 20 m thick stratigraphic interval within Member Two of the Horned Toad Formation, although limited fossils have also been collected from higher in the stratigraphic sequence in Members Three and Four.

Fossil mammals from below the Lawlor Tuff are placed in the Warren Local Fauna. Based on taxonomic composition and stage of evolution, the Warren Local Fauna is latest Hemphillian in age (Hh4; Tedford et al. 2004) and shows affinities with faunas from California, the southern Great Basin, and Mexico. Tephrochronology and magnetic polarity stratigraphy suggest correlation of the Warren Local Fauna with Chron 3n.3r or 4.896 – 4.997 Ma following the time scale of Lourens et al. (2004). Fossil mammals from Members Three and Four of the Horned Toad Formation are from a younger reversed polarity magnetozone that is correlated with Chron 3n.2r or 4.631 – 4.799 Ma. The stratigraphically lowest normal polarity site in the intervening normal polarity magnetozone is from the Lawlor Tuff (4.83 ± 0.04 Ma). Fossils found above the Lawlor Tuff include Jacobsomys dailyi n. sp. and Dinohippus edensis, and may be early Blancan in age, although no definitive Blancan taxa have yet been identified from this stratigraphic interval. The interpreted age of the Warren Local Fauna (4.85 – 5.0 Ma) is consistent with an age for the Hemphillian-Blancan land mammal age boundary of approximately 4.8 – 4.9 Ma (Lindsay et al. 2002).