Stratigraphy of the Cretaceous and Eocene rocks of the Santa Monica Mountains. Geochemical studies of the epithermal deposits at Goldfield, Nevada Minor Thesis

摘要

Stratigraphy of the Cretaceous and Eocene rocks of the Santa Monica Mountains:ududThe problem consisted of separating the Upper Cretaceous from the Eocene rocks in the eastern part of the Santa Monica mountains; determining the local stratigraphy of the Upper Cretaceous and Martinez; and attempting to correlate the Cretaceous section with other sections in California.ududResults:udud(a) The Upper Cretaceous stratigraphy in descending order is as follows: E. Upper arkose and conglomerate, 1500 feet. D. Fine grained, fossiliferous sandstone, 300 feet. C. Cobble conglomerate with some arkose in lower part, 3500 feet. B. Coarse arkose with some shale and conglomerate, 2500 feet. Unconformity. A. Lower soft red conglomerate.udud(b) The lower conglomerate lies unconformably on metamorphosed Santa Monica slates of Triassie (?) age. The Martinez overlaps the Cretaceous with a distinct erosional unconformity although no great angular unconformity has been found. In some places the Cretaceous is overlapped by the Modelo and faulted against Topanga sandstone in others.udud(c) The lower red conglomerate has similar lithology and occupies the same relative position as the Trabuco formation of the Santa Ana mountains, so that it may well represent the same formation.ududMember D is correlated with the Williams formation in the Santa Ana mountains and with the Metaplacenticeras beds of the Dayton canyon section in the Simi Hills on the basis of the fauna.udud(d) The Martinez section in the Eastern part of the Santa Monica mountains is about 3500 feet thick. The lower 2000' consists of brown shale with reefs of white algal limestone and the upper 1500 feet is more than half coarse white arkose and conglomerate, but contains considerable shale and some algal limestone.ududGeochemical studies of the epithermal deposits at Goldfield, Nevada:ududThe Applied Research Laboratories grating spectrograph and a technique of quantitative spectro-chemical analysis are described. This method of analysis may show twenty or more elements present in vein and wallrock samples in quantitatively measurable amounts.ududApproximately fifty samples of vein material and wallrock from the ore deposits of Goldfield, Nevada were analyzed quantitatively to determine whether the quantitative distribution of the elements would give some clue to the loci of mineralization. It has been established that three elements, bismuth, silver, and tin are genetically associated with the gold values in the deeper veins at Goldfield and that these elements are determinable even in very low grade ore. It is probable that a further study of their distribution would shed much light on the problem of determining the loci of mineralization.ududThe quantitative variations of some of the elements in the wallrock have been determined and these variations give some hope of an economic application in determining the distance of a given wallrock sample from a vein.ududA vertical zoning of tin is present at Goldfield and some suggestion has been found of a zoning of bismuth and perhaps of the whole type of mineralization.ududThe present work has shown that the spectrograph is very useful in the study of geochemical problems and several possible problems for study are suggested.