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Syndeformational Magnetization in the Ordovician Bigfork Chert at Black Knob Ridge, Western Ouachita Mountains, Southern Oklahoma: (Part I)
Paleomagnetic and rock magnetic results from unweathered samples of the Ordovician Bigfork Chert at Black Knob Ridge in southern Oklahoma indicate the presence of a pervasive magnetization that resides in magnetite. The magnetization has a steep, southeast direction. Small and medium-scale folds acquired magnetization during or after Carboniferous folding. The declination in sites along Black Knob Ridge also shows a north to south 28° counter-clockwise shift.
The Bigfork Chert at Black Knob Ridge was folded and thrusted along the Ti Valley fault system in the late Paleozoic. A pervasive late Paleozoic magnetization with a shallow inclination occurs in the laterally equivalent Viola Limestone in front of the thrust and in the Arbuckle Mountains. This direction is used as a reference for comparison with the Bigfork Chert Formation at Black Knob Ridge. The steep inclinations in the rocks at Black Knob Ridge are interpreted to be primarily the results of rotation around a horizontal axis as a result of thrusting. The declination change along the ridge is also interpreted to be the result of rotational movements during thrusting.
The pervasive magnetizaion at Black Knob Ridge is interpreted to have been acquired during the deformation. The results of fold tests and an incremental test correcting for both strike and dip suggest remanence acquisition during late stage folding and during an early stage of thrusting. The low thermal history of the Bigfork Chert suggests that the pervasive component is a chemical remanent magnetization (CRM) although a strain-related mechanism cannot be definitely ruled out. The proposed CRM may be related to fluid migration through fractures produced during folding and thrusting. Alternatively, an in situ chemical process (e.g. maturation of hydrocrabons) may have caused the remagnetization.
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