About This Item

Share This Item

The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 54 (1970)

Issue: 3. (March)

First Page: 556

Last Page: 556

Title: Statistical Analysis of Calcisiltites from Bird Spring Group, Mountain Springs, Nevada: ABSTRACT

Author(s): Donald S. McDougall, Robert H. Osborne

Article Type: Meeting abstract


Seventeen petrographic variables observed in duplicate thin sections for each of 62 calcarenites and calcisiltites collected from the Monte Cristo Limestone and the Bird Spring Group are the basis for this study. The samples are assigned to 6 microfacies, of which 31 and 17 belong to microfacies 0 and 1, respectively. R-mode cluster analysis grouped the variables into 7 clusters; 5 are composed of various organic and inorganic grains and associated alteration products, 1 is composed of acid insolubles, and 1 of sparite and micrite. Interclass and intraclass variation was tested by means of the Mahalanobis D2 and associated F statistics. The null hypothesis was accepted for all diagonal elements and rejected for all nondiagonal elements. Initial clustering of ra - and transformed-data sets by means of Q-mode factor analysis--first using 17 primary variables, and then using 2 derived variables--indicated that the 2-variable case provided the best separation between microfacies 0 and 1. Null hypotheses concerning group dispersion and the equality of sample centroids were rejected for both the raw and transformed 2-variable case; consequently, an appropriate form of the T2 statistic was employed to test for a significant difference between these two microfacies. Associated F values indicate a statistically significant difference between these microfacies for both raw- and transformed-data sets. Computed discriminant functions are approximately 98 and 79% efficient for the raw and transformed cases, respectively. The upper Monte Cristo Li estone was deposited under relatively stable environmental conditions that produced massive, low-energy micrites with very few biogenic grains. After an interval of erosion, the alternate quartzose sandstones and biomicrites of the lower Bird Spring Group were deposited under relatively unstable conditions. Environmental stability increased during the deposition of the upper Bird Spring Group, which consists of an almost unbroken sequence of micrites and biomicrites.

End_of_Article - Last_Page 556------------

Copyright 1997 American Association of Petroleum Geologists