Introduction. ______________________________________ First interpretation of the structure of southeastern Idaho. The Bannock overthrust____________________.________ Original interpretation_________________________ Mansfield's final interpretation.__________________ Extent..._______-_____________.__ Displacement. _____________________________ Folding__________________________________ Age __ __ ______ __ Extrapolation of the Bannock overthrust._________ The Bannock thrust zone: present interpretation______ Area east and north of Georgetown.______________ Left Fork of Twin Creek and Georgetown Canyon._______________________________ Slug Creek_________________________________ Syncline underneath Snowdrift Mountain.
Meaning and problems of the Brigham__ Previous usage. -______-____--____ Definition ___________________ Idaho usage. _________________ Utah usage. _________________ Modifications proposed for Brigham. Formation versus Quartzite _ Abandonment versus retention. Restricted versus inclusive_____ Regional relations. _______________ 1. Stratigraphic sections of the rocks beneath the Bloomington Formation in the Bancroft and Soda Springs quadrangles. 2. Stratigraphic sections of the upper part of the Brigham Quartzite measured southeastward along the Portneuf Range to the western part of the Bear River Range. 3. Described measured sections of the Brigham Quartzite and associated rocks in southeastern Idaho and northern Utah. 4-6. Described Stratigraphic sections between the Bloomington Formation and the Brigham Quartzite showing former and new rock-stratigraphic assignments: 4. Eastward from the Bannock Range, across the Portneuf Range to the northeastern Bear River Range. 5. Eastward from the Malad Range, across the southern part of Portneuf Range to the east side of the Bear River Range. 6. Eastward across the Wasatch Range in northern Utah. 7. Sections showing relation of rock-stratigraphic assignments in the Bancroft and Soda Springs quadrangles to recognized Cambrian faunal zones. Index map of southeastern Idaho and northern Utah, showing the locations of the Bancroft, Soda Springs, and Preston quadrangles and of the Stratigraphic sections discussed---____..______-__-_-______-_____---____ Relief map of southeastern Idaho and northern Utah, showing the major physiographic features mentioned and their relations to the Bancroft, Soda Springs, and Preston quadrangles-___--__-_-_____________-______..
Classical European Hercynian two-mica granites host or are genetically related to tin, tungsten and uranium deposits. Similar rocks elsewhere in the world are associated with similar ore deposits, and a knowledge of two-mica granites can be a useful exploration tool.Two-mica granites occur usually as small plutons late in a plutonic sequence that intrudes high-grade metamorphic rocks of an orogen. In plan they are elongate or eliptical and in three dimensions they are tabular or sheet-like. They are usually leucocratic, massive, hypidiomorphic granular granitoids that consist of about 31 volume percent quartz, 26 percent K-spar, 31 percent plagioclase and about 10 percent mica in which muscovite may exceed biotite. In some, muscovite is absent in which event total mica is about 5 percent, except for a few types that contain abundant biotite. In two-mica granites hornblende is absent, magnetite is sparse, and sphene is rare. Common accessory minerals are apatite, zircon, monazite, garnet, and tourmaline.Two-mica granitoids are more similar to S-type and ilmenite-series granitoids of Chappell , and White (1974) and Ishihara (1977), respectively, than to their I-type and magnetite-series granitoids. Modal plots of two-mica granitoids fall predominantly in the monzogranite subfield of Streckeisen's (1976) modal classification of granitoids. Modal plots, however, do not uniquely define two-mica granites.
The Bannock overthrust in southeastern Idaho and northcentral Utah was originally described by Richards and Mansfield (1912) as a single large thrust fault that formed at the close of the Laramide orogeny and was folded by renewed compression near the end of Pliocene time. Later Mansfield expanded and revised his interpretation of the Bannock overthrust so that at least the northern part of the overthrust was thought to be a thrust zone in which the individual faults originated in a folded sole thrust. Detailed mapping in areas critical to Richards and Mansfield's interpretations has shown that the faults thought by them to be parts of one large thrust are separate faults, and that, although some of the thrust surfaces are curved, they were not folded in Pliocene time but probably were folded during a late stage of the thrusting. Extensions of the Bannock thrust to the north, south, east, and west based upon extrapolation of a single large folded thrust surface are not warranted. The Bannock overthrust is reinterpreted as a westward-dipping imbricate thrust zone possibly several tens of miles wide extending at least from southwestern Montana to north-central Utah. It is recommended that the name "Bannock overthrust" no longer be used, and that this zone of imbricate thrusts in the southeast corner of Idaho be called the Bannock thrust zone. Theathrustsrange in agefrom Late Jurassic totpost-Early Cretaceous and are progressively younger from west to east; strong regional compressive forces do not appear to have been active in the area as late as Pliocene time. The upper plates of the thrusts moved to the northeast in response to an unknown force. Steep eastward-trending tear faults formed during thrusting probably in response to differential movement among the eastward-moving thrust plates. In Tertiary and Quaternary time block faulting was extensive; it formed the northward-trending graben valleys seen in the area today.
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