The Geology of the Pinyon Peak Area, East Tintic Mts., Utah. The Geology of the Upper Tick Canyon Area, Los Angeles County, California

Abstract

[Part 1] The Geology of the Pinyon Peak Area, East Tintic Mountains, Utah: In G. F. Loughlin's report of 1919 on the Geology of the Tintic mining district, (Lindgren, Taldemar, and Loughlin, G. F., Geology and Ore Deposits of the Tintic Mining District, Utah: U. S. Geological Survey Prof. Paper 107, 1919), a new formation, the Pinyon Peak limestone, was described and tentatively assigned to Upper (?) Devonian age. This outcrop of limestone on Pinyon Peak is the only known occurrence of Devonian rocks in the Tintic mining district. The limited presence of the Pinyon Peak limestone was offered by Loughlin as evidence for the dating of a Lower Mississippian unconformity in the district....to be correlated with a suspected Lower Mississippian unconformity in other parts of Utah. Loughlin mapped the Pinyon Peak limestone as occurring below the Gardner formation (L. Miss.) and below the Victoria quartzite (L. Miss.). Loughlin found little more than five or ten feet of the Victoria ouartzite exposed on Pinyon Peak. Recent field work in the Pinyon Peak locality has disclosed that a two-hundred-foot thickness of Victoria quartzite is present on Pinyon Peak and that the Pinyon Peak limestone occurs above this formation. Several days were spent in the field in search of fossil remains in the Pinyon Peak limestone. A few poorly preserved fragments were found but nothing of diagnostic value. Attempts to locate Loughlin's original fossils have been unsuccessful. The author suggests, on the basis of evidence set forth in this paper, that the Pinyon Peak limestone is a limestone equivalent to a dolomitized member of the Gardner formation; that the Pinyon Peak limestone should be relegated to the status of a member of the Gardner formation; and that its age is still much in doubt. [Part 2] The Geology of the Upper Tick Canyon Area, Los Angeles County, California: The upper Tick Canyon area is in the northwest quarter of Los Angeles County, California. Within this area at least 5000 feet of Oligocene (?) Vasquez sediments and interlayered basaltic lavas is faulted against an undetermined thickness of pre-Cretaceous gneisses and is overlain unconformably by Tick Canyon elastic sediments of probable late lower Miocene age. The Vasquez sediments exposed in the upper Tick Canyon area are composed of cobble and boulder conglomerates, sandstones, siltstones, shales, tuffs, borate-bearing beds, and volcanic ash beds. The nature of the sediments strongly suggests fluviatile and lacustrine deposition in a continental basin under semi-arid to arid climatic conditions. The presence of four distinct flows of lava totaling at least 2300 feet indicates that Vasquez time was a period of active volcanism in this part of Southern California. Structurally the mapped area consists of a west-plunging syncline of Vasquez rocks faulted against the up thrown block of pre-Cretaceous gneisses to the north. Beds on the north limb of this fold dip vertically. Two minor anticlinal flexures occur in the synclinal trough which is further complicated by several major oblique slip faults of northeast trend. Measurable horizontal components of movement along these faults range from a few hundred to at least 1200 feet. In interpreting the geologic history of the region, previous investigators have suggested that the Vasquez sediments and lavas probably accumulated during early or early middle Tertiary in an east trending, canoe-shaped basin that was defined by block faulting in Mocene or early Oligocene time. The soft and relatively fine-grained Vasquez sediments in the upper Tick Canyon area responded to the compressional stresses exerted at the end of Vasquez time by folding to a much greater degree than the predominately coarser beds elsewhere in the region. Yielding also occurred along faults with an oblique-slip movement. Uplift and erosion at the end of the post-Vasquez orogeny was followed by the deposition of Tick Canyon terrestrial sediments, chiefly; conglomerates, sandstones, and silts. The presence of several levels of terrace gravels suggests a number of periods of uplift during Quarternary time in the Tick Canyon Area. At present, the region is being eroded by rejuvenated streams, which have developed a local topographic relief of approximately 1000 feet.