Structural significance of Tertiary volcanic rocks in southwestern Utah

Abstract

Most of the silicic volcanic rocks of the Great Basin are ignimbrites rather than lava flows. The more common types were probably formed by nuees ardentes which spread laterally as density currents of very high fluidity, but there are aberrant types that may be in some manner transitional between ignimbrites and lava flows, and the whole problem of mechanism of origin is as yet little understood. Uncertainty on this score is of no concern here; mapping of many of the units indicates that, whatever their origin, they assumed a distribution approaching that of an equal volume of water, filling valleys to a common level and forming sheets of substantially uniform thickness where the relief of the pre-existing surface was low. Some of the individual ignimbrites are many hundreds of feet thick and have an areal extent of as much as 10,000 sq. mi. Because each of these extensive and initially flat sheets was formed everywhere at the same instant of time, they are very nearly ideal stratigraphic units. They occur in most parts of the Great Basin and range in age through most of the Tertiary. Eleven ignimbrites which are widespread in southeastern Utah are described and given formal stratigraphic names. The fact that the oldest of them lies unconformably across the beveled edges of thrusts and folds involving late Cretaceous strata indicates that the beginning of volcanic activity postdates the Laramide orogeny. As planar units which provide a record of Tertiary crustal movements, the ignimbrites confirm the Gilbert theory, based originally on physiographic evidence, that block faulting has been the characteristic type of post-Laramide deformation in the Great Basin. The stratigraphic-structural approach makes it possible 1) to work out the geometry of the block faulting with a precision not obtainable by use of displaced erosion surfaces; 2) to deal with episodes of block faulting that occurred during the early Tertiary and are not expressed by the present topography; and 3) to crossdate these and other geologic events on a regional scale. On the basis of detailed work in southwestern Utah, and reconnaissance elsewhere, it is stated as a deliberately provocative working hypothesis that block faulting has been the only type of regional tectonism in the Great Basin in postorogenic time. The first requirement in testing this hypothesis is the recognition, as such, of flexures and thrust faults developed 1) by emplacement of hypabyssal intrusions, and 2) by gravity sliding from primary relief features raised by intrusion and block faulting. These 2 classes of structures are of much interest in their own right; their significance for present purposes is merely that they may be readily mistaken as evidences of regional tectonism. Equally critical is the need for distinguishing postorogenic deformational effects from those produced during the orogeny. Examples are given of use of the ignimbrites in making these distinctions.