Sequence Of Carbonate Rocks Research Articles

Tertiary Stratigraphic Units of Western Mojave Desert, California

Scattered throughout the western Mojave Desert, mostly in the vicinities of Rosamond, Mojave, and Boron, are exposures of non-marine sedimentary and volcanic rocks of Tertiary age. These rocks rest on a deeply eroded surface of pre-Tertiary granitic and metamorphic rocks that form the crystalline basement complex of this region, and are overlain unconformably by alluvial sediments of Quaternary age. Where exposed the Tertiary rocks are moderately deformed and much eroded. With the exception of exposures in the foothills northwest of Mojave, the rocks of Tertiary age are mapped as a group, named the Tropico group, with a maximum exposed thickness of about 2,800 feet. This group is divided into several lithologic units of local extent. In most sections the lower unit is a pyroclastic formation, with associated rhyolitic intrusions and flow-breccias, and the upper unit is either fanglomerate or a sequence of carbonate rocks, clays, and sandstones of lacustrine and fluviatile origin. Several basalt flows occur locally in the group, mostly in the middle part. The group is non-fossiliferous except at one locality, where diatom remains in the upper unit suggest early Pliocene age. The lower unit is tentatively correlated with lithologically similar formations of known Miocene age in areas west and northwest. Formations in the Tropico group that are locally distinct are named the Gem Hill formation, Fiss fanglomerate, Bissell formation, Saddleback basalt, and Red Buttes quartz basalt. In the foothills northwest of Mojave the granitic basement is overlain by about 1,000 feet of terrestrial sandstones and clays named the Horned Toad formation. This formation yielded a mammalian fauna indicating middle or early Pliocene age, and is either younger than or correlative with the upper part of the Tropico group.

Use of Thermoluminescence of Limestones in Subsurface Stratigraphy

The thermoluminescence of the limestones of the Meramecian and Chesteran series (middle and upper Mississippian) of the Illinois basin was studied in an attempt to develop the thermoluminescence of limestones into a useful research tool in subsurface geology. A powder method of sample preparation is described which makes accurate quantitative measurement of thermoluminescence possible. The apparatus used in measuring and recording thermoluminescence is described briefly. The analysis of patterns of variation in intensity of thermoluminescence and in glow-curve shapes vertically through a formation appears to have some usefulness in subsurface stratigraphy in (a) identifying and characterizing a formation, (b) assisting in recognizing tops and bottoms of limestone formations where lithologic breaks are not present, (c) splitting thick carbonate rock sequences into small units useful in correlation, and (d) recognizing erosion or non-deposition of zones by the absence of parts of the typical pattern of variation. Within certain limitations, the variations in radioactivity of the limestones appear to be the dominant factor in the cause of variations in thermoluminescence.

Spectrochemical Sample Logging of Limestones

Visual examination and insoluble-residue analysis have not yielded uniformly satisfactory data for detailed differentiation and correlation of thick and unbroken sequences of carbonate rocks. The method here presented is an attempt to utilize quantitative spectrographic analysis for this purpose. An external standard method which gave results reproducible within inherent sample error was used. Ca, Mg, Sr, Ba, Al, Ti, Fe, and Si were present in all samples, and Mn and V in some. For practical reasons S, P, Na, and K, undoubtedly present, could not be determined. The most useful elements appear to be Mg, Fe, Al, and Sr. When analyses of both surface samples and well cuttings are plotted against stratigraphic interval or well depth, the curves may be interpreted in terms of stratigraphic correlation and differentiation. Standardization of the method is easy and the results are not dependent on the personal equation. Apart from initial expenditure for equipment, costs and time-consumption should be less than those required for foraminiferal and insoluble-residue analysis.