Abstract

The nature and history of volcanic activity in the Trans-Pecos volcanic field of Texas controlled depositional timing, environmental distribution, and lithologic character of associated sediments. Tuffaceous and epiclastic sedimentary rocks of the Tascotal and Fresno Formations and the Perdiz Conglomerate are part of the intermediate-source facies of the field. Rhyolitic and trachytic activity in major calderas produced abundant pyroclastic debris that has been reworked into sandy alluvial fans adjacent to the caldera complexes, whereas post-volcanic alluvial-fan sediments are tuffaceous or epiclastic conglomerates.

The intermediate-source facies consists of four components: active apron, inactive (or epiclastic) apron, eolian sand sheet, and valley facies. The arrangement and timing of development of these facies depend upon the eruptive history of the volcanic centers. The Tascotal, Fresno, and Perdiz contain five depositional units. (1) The Chinati apron unit recorded rhyolitic and trachytic activity in the Chinati Caldera during the 105–106 yr after its collapse. (2) The southern apron unit formed around the San Carlos–Sierra Rica eruptive center in Mexico before eruption of the San Carlos Ignimbrite. (3) The conglomerate and limestone unit formed as epiclastic fans around the Solitario uplift and as braided-stream and lake deposits between aprons and the Solitario. (4) The eolian unit of the Tascotal formed as drainage off the Chinati eruptive center adjusted from carrying fine pyroclasts to coarser epiclasts. (5) The Perdiz Conglomerate and equivalent epiclast-rich, conglomeratic parts of the Tascotal and Fresno are a bajada-like accumulation of fluvial epiclastic conglomerate from non-volcanic sources and the Chinati eruptive center after its activity ceased.

Framework mineral composition is a function of grain size; very fine and fine tuffaceous sandstones consist mostly of reworked glass shards, and rocks that have textures coarser than medium sand are mostly epiclastic VRF's. Active apron rocks formed by deposition from sheet-floods and poorly defined channels in a bajada-like accumulation that resembles sandy distal alluvial-fan deposits because the volcanos provided so much sand-sized debris, but little clay.

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