Date of Award


Document Type


Degree Name

Master of Science (MS)



First Advisor

W.L. Moore


Precambrian rocks exposed in the northern Pathfinder Reservoir area, Natrona County, Wyoming, are partially exhumed crystalline rocks of the northwestern portion of the Granite Mountains. These rocks comprise three major types: (1) the oldest metamorphic rocks, which may represent a metamorphosed sedimentary-volcanic terrane, exposed on Black Rock Mountain and in other small areas south and west of Black Rock Mountain; (2) granites, which intrude the metamorphic sequence, and are exposed on the Sweetwater Rocks and areas marginal to Pathfinder Reservoir; and (3) diabase dikes with northeastern trends that intrude both the metamorphic rocks and granites. Exposures of the Precambrian rocks rise above flat-lying Tertiary rocks and sediments that fill intervening basinal areas.

Aeromagnetic data shows a nearly uniform magnetic field that is interrupted by a few isolated, but strong, anomalies, which are caused by magnetic contrasts within the basement complex. Studies of the magnetic characteristics of the exposed Precambrian rocks indicate that the magnetic susceptibilities of the amphibolites, pelitic schist, and diabase dikes are nearly twice that of the gneisses and granites. Partitioning of Fe and Ti oxides within the amphibolite-pelitic schist sequence, or depletion of magnetic minerals in the gneisses and granites may be responsible for some of the observed magnetic contrasts.

Comparison of the magnetic susceptibility and remanent magnetization measurements suggest that the main contribution to total magnetization is magnetic susceptibility. Although remanent magnetization does exist within the samples measured, modified Köinigsberger ratios (Q') indicate that it contributes a negligible component to total magnetization.

Strong magnetic anomalies northwest and south of Black Rock Mountain and interpreted to be caused by isolated bodies of “ironformation,” since the constraints provided by the modeling analysis of these bodies, and rock associations conform well with that of “iron-formations” found in other portions of the Granite Mountains and Precambrian terrane of the northern Rocky Mountains. A concealed rock mass, probably of mafic composition, is inferred to underlie one magnetic anomaly. Remaining portions of the study area, characterized by a nearly uniform magnetic field, are inferred to be underlain by the essentially non-magnetic gneisses and granites.

A fault is inferred on the observed irregularities in the magnetic contours associated with the amphibolite-pelitic schist sequence exposed on Black Rock Mountain.

Reliability of depth estimates varied with the nature of the anomalies used in the analysis. Broad anomalies produced more accurate estimates in comparison to narrow anomalies. Since the modeling technique used for depth estimates depended upon an accurate estimate of the half-maximum half-width parameter of the anomaly, narrow anomalies not only produced shallower than actual estimates, but were susceptible to greater error due to data spacing.

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