ABSTRACT

Primary mineral facies, depositional facies, and diagenetic assemblage zones can be mapped in the Vieja Group (Eocene-Oligocene). Variations of primary mineralogy can be ascribed to both source and environmental effects. By recognizing those attributable to source effects, and by consideration of the depositional environments present, the sequence of eruption of volcanos and the nature of their activity can he deduced.

When they were deposited, volcanic-sedimentary rocks of the Vieja Group were composed mostly of three types of volcano-derived detrital constituents: glass shards and pumice fragments, volcanic rock fragments (VRF's), and monocrystalline mineral grains. Crystal fragments were most abundant in medium sandstones, glass fragments predominated in finer grained rocks and VRF's predominated in coarse sandstones and conglomerates. The actual abundance of any type of constituent was also a function of the abundance in which it was provided by the various volcanic sources and of the environment of deposition of the sediment. The volcano active during deposition of the Colmena formation provided abundant VRF's and plagioclase. One erupting during deposition of the Chambers formation produced de ris rich in sanidine and quartz. During deposition of the Capote Mountain formation, the Chinati Mountains-Pinto Canyon center produced abundant glass.

The rocks were deposited on a bajada-like apron by streams or mudflows that flowed radially away from volcanic eruptive centers, or in valleys where streams flowed between eruptive centers to the north and south. The streams entered the Southern Rim Rock Country from the west and continued eastward across the Trans-Pecos volcanic field. Stream channel and overbank deposits, with common paleosoil features, make up the valley facies. Medium sandstones in the valley facies are enriched in crystals relative to those deposited on aprons, either because of sorting by depositional processes or selective abrasion of glass fragments during transportation. Non-volcanic constituents are present in minor amounts in most valley facies rocks. They are the predominant grain type only in the valley f cies part of the Colmena Formation. Detrital clay is present in valley facies rocks and in the mudflow deposits of the Colmena Formation. Some rocks are the result of deposition by aeolian processes.