Timing and origin of orogenic remagnetizations in Mississippian carbonates, Sawtooth Range, Montana V.J. O'Brien a, ⁎ , R.D. Elmore a,1 , M.H. Engel a,1 , M.A. Evans b a School of Geology and Geophysics, The University of Oklahoma, Norman, OK 73019, USA b Department of Physics and Earth Sciences, Central Connecticut State University, New Britain, CT 06050, USA Received 15 August 2005; accepted 16 November 2005 Available online 13 March 2006 Abstract Paleomagnetic data and rock magnetic results suggest that a widespread orogenic remagnetization caused a pervasive chemical remanent magnetization (CRM) that resides in magnetite in the Mississippian Madison Group in the Sawtooth Range, Montana, during the Late Cretaceous to Early Tertiary. The CRM is similar to a CRM reported by workers from equivalent units in the southern Canadian Cordillera. The CRM is interpreted to be related to alteration by fluids, and there are two likely fluids: hydrocarbons which migrated into the unit and externally derived radiogenic fluids. © 2006 Elsevier B.V. All rights reserved. Keywords: Chemical remagnetization; Paleomagnetism; Thrust fault; Orogenic fluids 1. Introduction Dating diagenetic events can be difficult, and paleomagnetic studies of secondary magnetizations can provide dates for fluid migration and burial diagenetic events in some cases. Orogenic remagnetiza- tions (e.g., burial or fluid migration) are common (Enkin et al., 2000), but their origins are not completely understood. The primary objective of this study is to investigate the origins and timing of widespread orogenic remagnetizations in the Madison Group carbonates that form the thrust sheets in the Sawtooth Range, Montana. A secondary objective is to sample perpendicular to the mountain front to test for remagnetization trends similar to those reported by other workers (Stamatakos et al., 1996; Enkin et al., 2000). These trends may further our understanding of how thrusting events may be related to remagnetization. 2. Geologic setting In northwestern Montana, shortening related to Laramide deformation resulted in eastward propagating thrusts that juxtaposed Mississippian Madison Group carbonates atop Cretaceous shales and sandstones, termed the Montana disturbed belt. Mudge (1972) subdivided the region from east to west into subbelts I through IV. The Sawtooth Range lies in subbelt III, a succession of north–south trending imbricate thrust faults. Journal of Geochemical Exploration 89 (2006) 297 – 301 www.elsevier.com/locate/jgeoexp ⁎ Corresponding author. Fax: +1 405 325 3140. E-mail addresses: obrienvj@ou.edu (V.J. O'Brien), delmore@ou.edu (R.D. Elmore), ab1635@ou.edu (M.H. Engel), evansmaa@ccsu.edu (M.A. Evans). 1 Fax: +1 405 325 3140. 0375-6742/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.gexplo.2005.11.089