Tactite Rocks of the Iron Mountain District, Sierra and Socorro Counties, New Mexico. Stratigraphy of the Easternmost Ventura Basin, California, with a Description of a New Lower Miocene Mammalian Fauna from the Tick Canyon Formation

Tactite rocks of the Iron Mountain district, Sierra and Socorro Counties, New Mexico: The pyrometasomatic deposits at Iron Mountain, near the northern end of the Sierra Cuchillo in Sierra and Socorro Counties, New Mexico, have been formed through replacement of calcareous beds of Paleozoic age, generally at or near contacts with intrusive masses of rhyolite, rhyolite porphyry, aplite, and fine-grained granite. The metamorphism is probably mid-Tertiary in age. Its two chief products are (a) light-colored, dense, fine-grained granulites rich in diopside, clinozoisite, bytownite, and other iron-poor silicate minerals, and (b) coarser-grained, dark-colored, iron-rich rocks, called tactites. The spatial and temporal relations of these iron-poor and iron-rich contact rocks, not only to each other but to adjacent igneous bodies and to relatively unmetamorphosed beds, appear to have been determined in a regular and definite manner; this is discussed and illustrated in detail by several examples. The iron-rich pyrometasomatic deposits contain a large number of unusual minerals. Most remarkable are helvite and at least three other beryllium-bearing silicate minerals, which are known to occur in noteworthy concentrations in only one type of rock, a peculiar rhythmically layered variety of tactite to which the name "ribbon rock" has been given. The structure of such tactite is very conspicuous, and appears in section as thin, finely crenulated bands of magnetite alternating with similar bands of silicate minerals and finely crystalline fluorite. Concentric banding about fluorite-rich pods is common. Bodies of "ribbon rock" vary in size from inch-thick lenses to large masses amounting to thousands of tons; most appear to have formed along contacts between recrystallized limestone and massive magnetite-andradite tactite, chiefly by replacing fluids penetrating the limestone from fractures. The layered structure is interpreted as a diffusion effect. The formation of massive and "ribbon rock" tactites can be traced through a range of falling temperatures from a stage characterized by deposition from iron-rich vapors to a stage in which hydrothermal solutions were dominant. Both vapors and liquids appear to have been acid, and reducing conditions undoubtedly existed during the latter part of the hydrothermal stage. The occurrence of beryllium in "ribbon rock", but not in typical massive tactite may signify that its compounds in pyrometasomatic deposits are confined to rocks of hydrothermal origin. The occurrence of "ribbon rock" itself is suggested as a potentially useful clue for recognition of beryllium-bearing contact deposits elsewhere; at least two other occurrences of what apparently is "ribbon rock" have been described in the literature. Stratigraphy of the easternmost Ventura Basin, California, with a description of a new lower Miocene mammalian fauna from the Tick Canyon Formation: The great thickness of dominantly nonmarine Tertiary strata in the eastern part of the Ventura Basin has impressed geologists since the time of the earliest surveys of the region. The younger of these strata, comprising the so-called Mint Canyon formation, have been eroded to form a great basinlike area in which numerous badland exposures occur (see fig. 1). Erosion has been due principally to the poor consolidation of the sediments and in part also to the rather scant cover of typical Sonoran vegetation. Although several earlier investigators examined these badlands, the first recorded discovery of vertebrate remains came as a result of reconnaissance mapping by Kew in 1919. In the publication that followed, a provisional list of the forms found here was furnished by Stock, although no detailed study of the fauna was made.