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The Bannock Thrust Zone Southeastern Idaho

Description: Abstract: 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. The thrusts range in age from Late Jurassic to post- 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 ...
Date: 1963
Creator: Armstrong, Frank C. & Cressman, Earle Rupert
Partner: UNT Libraries Government Documents Department

Miocene Plants from Idaho

Description: Abstract: The author describes 75 species of plants from the Miocene of about 30 localities in Idaho. These plants represent 40 genera in 28 families and 17 orders, and the most common types are species of Acer, Quercus, Populus, Betula, and Laurus. There are 2 ferns, 3 monocotyledons, and 70 dicotyledons, 18 of which are no longer present in the northwestern United States. There are some xerophytic types, but the majority are mesophytic, possibly indicating a mixture from different altitudes. The beds are correlated with the Latah formation and considered to be of upper Miocene age.
Date: 1934
Creator: Berry, Edward Wilber
Partner: UNT Libraries Government Documents Department