PETROLOGY OF UPPER DEVONIAN PELLETOIDAL LIMESTONES, ARROW CANYON RANGE, CLARK COUNTY, NEVADA

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SUMMARY

Approximately 1,200 ft. of Devonian limestone, 900 ft. belonging to the Arrow Canyon Formation and 280 ft. to the Crystal Pass Limestone, were sampled at 2 ft. intervals.

A thin-section was prepared for each sample and examined petrographically. Polished specimens were examined also and a selected group of these slabs were investigated by X-ray diffraction analysis to determine their gross mineralogy.

The petrographic examination involved the measurement of the particle size and determination of the frequency of occurrence of all the inorganic particles such as pellets and quartz grains. The organic elements such as Amphipora, stroniatopo- roids, calcispheres, bryozoans, ostracods and crinoids were also counted. On the basis of these measurements the samples were divided into ten microfacies, which are calcilutite, fossiliferous calcilutite, calcilutite with pocket-like structures, breccia- conglomerate, poorly sorted calcarenite, arenaceous poorly sorted calcarenite, well sorted calcarenite, arenaceous well sorted calcarenite, sandstone and bioconstructed Amphipora-stromatoporoid limestone.

The raw data were subjected to a multiple correlation analysis in order to determine the inter-relationships of the microfacies. This information also made it possible to order the microfacies in such a way that the microfacies from the highest environmental energy was one end-member, and the microfacies formed in the lowest environmental energy was the other end-member.

These two end-members were deposited in the deepest and the shallowest parts respectively, of the carbonate environment.

The variation in relative depth was plotted against the lithological column to give the bathymetric curve. Tangents drawn to the maxima and minima of this curve defined the degree of Juctunfion of the environmental energy. The mean of these maxima and minima values was defined as the energy index. This was also plotted against the lithological column in order to trace the evolution of the environmental energy.

Near the base of the measured section the degree of fluctuation is large and the energy index also has a great range in intensity. As sedimentation progressed, the degree of fluctuation lessened and the environmental energy attained a uniformly high value. The periodicity, that is apparent in the lower portion of the section for both the degree of fluctuation and the energy index, becomes less pronounced and then disappears completely in the Crystal Pass Limestone. The increasing environ- mental energy and the decreasing fluctuation in the bathymetry have been interpreted as being due to the carbonate basin being slowly infilled with sediments. The deposits were subjected to prolonged periods of reworking and occasional violent disturbances such as severe storms.

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