Petrographic provinces and provenance of the Upper Triassic Luning Formation, west-central Nevada: Summary

Abstract

The Upper Triassic Luning Formation (Muller and Ferguson, 1939) provides a key to the understanding of the Late Triassic paleogeography of west-central Nevada. Investigation of the Luning Formation, part of an ongoing re-evaluation of the structure and stratigraphy of pre-Tertiary rocks in west-central Nevada (Oldow, 1976, 1978a, 1978b; Speed, 1977a, 1977b, 1978), is particularly fruitful because of the formation9s widespread distribution. Although predominantly a shallow-marine carbonate assemblage, the Luning does contain siliciclastic units. The mineralogic composition of siliciclastic rocks within the Luning Formation is reported here. The sandstone units are characterized using the detrital grain parameters of Dickinson (1970). The characterizations provide the basis for establishing petrographic provinces—compositionally distinctive bodies of rock that form natural units in terms of age, origin, and distribution (see Suttner, 1974). Vertical and lateral changes in sandstone composition delimit the spatial and temporal distribution of petrographic provinces within the Luning. Furthermore, aspects of the mineralogic composition of the sandstone can be attributed to specific source rocks rather than to weathering, transportation, depositional environment, or diagenesis. Determination of source rock type and demarcation of petrographic provinces permit improve understanding of the paleogeographic setting and tectonic environment in which the Luning Formation was deposited. Structurally, Mesozoic rocks of the Mina region (Fig. 1) make up two discrete terranes, an allochthon and autochthon, separated by a regionally extensive thrust fault, the Luning thrust (Oldow, 1975, 1978b; Speed, 1977b). The allochthon consists of numerous, far-traveled thrust nappes, most of which are composed of rocks of single formations; it is deformed internally in polyphase folds. The subjacent autochthon also contains thrust faults, thought to be of small displacement (Oldow, 1978b), but data currently on hand indicate that thrusts within the autochthon are structurally discrete with respect to thrusts in the allochthon.