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PALEOMAGNETISM AND ROCK MAGNETISM OF CARBONATE ROCKS FROM THE HELENA SALIENT, SOUTHWEST MONTANA

PALEOMAGNETISM AND ROCK MAGNETISM OF CARBONATE ROCKS FROM THE HELENA SALIENT, SOUTHWEST MONTANA. Ben Baugh . Introduction. North American cordilleran fold and thrust belt Helena salient Wyoming salient Vertical-axis rotation: paleomagnetism

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PALEOMAGNETISM AND ROCK MAGNETISM OF CARBONATE ROCKS FROM THE HELENA SALIENT, SOUTHWEST MONTANA

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  1. PALEOMAGNETISM AND ROCK MAGNETISM OF CARBONATE ROCKS FROM THE HELENA SALIENT, SOUTHWEST MONTANA Ben Baugh

  2. Introduction • North American cordilleran fold and thrust belt • Helena salient • Wyoming salient • Vertical-axis rotation: paleomagnetism • (Grubbs and Van der Voo, 1976; Eldredge and Van der Voo, 1988; Jolly and Sheriff, 1992) • * Sampling Mississippian carbonates, this study aims to investigate curvature of the Helena salient (Harlan et al., 2008) (Weil and Sussman, 2004)

  3. Geology of Western Montana • Fold and thrust belt propagation • 72 -56 Ma (Hoffman et al., 1976) • Pre-folding diorite sills • 77 Ma (Harlan et al. , 2008) • Montana transverse zone • Lewis and Clark shear zone • Boulder Batholith • 78-68 Ma (Tilling et al., 1968) • Helena Embayment • Pre-cambrian reentrant • (Harrison et al., 1974) (Harlan et al., 2008) (Eldredge and Van der Voo, 1988)

  4. Objectives 1.) Are carbonates of the Helena salient remagnetized? 2.) Quantify vertical-axis rotation, if any Establish age of magnetization Use reference direction to quantify vertical-axis rotation 10 µm (McCabe and Elmore, 1989) (Weil and Sussman, 2004)

  5. Field Area Eldredge and Van der Voo (1988)

  6. Sampling • Paleozoic carbonates • Madison Group carbonates • (20 sites) • Mission Canyon Limestone • Lodgepole Limestone • Cambrian Meagher limestone • (1 site) • Devonian Jefferson carbonates • (3 sites)

  7. Mission Canyon (Mississippian) LodgepoleLs Meagher Ls (Cambrian) 9 sites – Madison Group 1 site – Meagher Ls

  8. Mission Canyon (Mississippian) LodgepoleLs Jefferson Fm (Devonian) Pilgrim Fm (Cambrian) 5 sites – Madison Group 3 sites – Jefferson Fm

  9. Mission Canyon (Mississippian) LodgepoleLs 6 sites – Madison Group

  10. Methods: • Thermal Demagnetization (°C): • 144 specimens • NRM, 100, 200, 275, 350, 400, 440, 480, 520 • Magnetic Hysteresis • Ms, Mrs, Hc and Hcr • Aids in Characterizing magnetic grain size • Magnetic Susceptibility • Degree of magnetization induces by applied magnetic field

  11. Directional data analysis: Fold Test • Determines age of magnetization relative to folding • Pre-tilting • Post-tilting • Syn-tilting • McElhinny (1964) • Incremental fold test • Applied to all folds • Tauxe and Watson (1994) • Treats directions as Eigen vectors • Applied to Turner anticline (Weil and Sussman, 2004)

  12. Results: • Demagnetization • Stable, but weak magnetizations • 16/23 sites resulted in enough samples to generate site means • Devil’s Fence: 6/9 • Three Forks: 6/8 • Turner: 4/6

  13. Results: • Devil’s Fence and Turner anticlines reveal two apparent components of magnetization: • Lower hemisphere component, steep inclinations • Upper hemisphere component, shallow inclination Devil’s Fence

  14. Fold Tests: Devils Fence anticline • Two apparent components • Lower hemisphere component passes the fold test at 90-100% untilting • Pre-tilting • Grand mean direction: • D = 35°, I = 72°, α95 = 8°

  15. Fold Tests: Three Forks anticline • One apparent magnetization component • Two sites reversed • Sites 22 and 24 • Site 23 split into two components • passes the fold test at 100% untilting • Pre-tilting • Grand mean direction: • D = 37°, I = 70°, α95 = 23°

  16. Fold Tests: Turner anticline • Two apparent magnetization components • Lower hemisphere component passes the fold test at 100% untilting • Pre-tilting interpretation • Grand mean direction: • D = 224°, I = 69°, α95 = 29°

  17. Age of Magnetization: • Fold tests: pre-deformational • Upper limit 77 Ma (Harlan et al., 2008) • Steep, lower hemisphere component • K-group: Late Cretaceous remagnetization • Shallow, upper hemisphere component • M-group: Mississippian primary detrital magnetization

  18. Expected directions: • Calculated using paleopole location, and location of study area • Angular distance: • p = cos­-1 [(sinpsins + cospcoss cos(Øp-Øs)] • Declination • Dx= cos­-1 (sinp - sins cosp/ cosssinp) • Inclination • Ix = tan-1(2cot p) Mississippian Late Cretaceous Paleopoles from McFadden and McElhinny (1995)

  19. Vertical-axis rotations: K-group • Devil’s Fence: • 59° ± 25 clockwise • Three Forks: • 62° ± >60° clockwise • Turner: • 111° ± >60° counter-clockwise

  20. Vertical-axis rotations: M-group • Restoration of K-group to Cretaceous expected direction • 22 ± 18° - 59 ± 14° clockwise rotation • Timing difficult to constrain K-group K-group

  21. Combined with Eldredge and Van der Voo (1988) Results: • Clockwise rotation along southern margin • Counter-clockwise rotation along northern margin • Clockwise rotation within the Elkhorn plate

  22. Camparison with previous studies outside the Helena salient • Wolf Creek, MT • Late Cretaceous Two Medicine Formation • (Jolly and Sheriff, 1992) • Transverse Zone • Late Cretaceous Diorite sills • (Harlan et al., 2008) • Sawtooth Range • Mississippian Madison Group • (O’Brien et al., 2007)

  23. Hysteresis Results • Wide Hcr/Hc range, Narrow Mrs/Ms range • M-group and K-group Plot along SP+PSD, and SP+SD mixing lines • Consistent with remagnetized limestones (Suk et al., 1993; Xu et al., 1998; Channell and McCabe, 1994)

  24. Bulk Magnetic Susceptibility • Decreasing Trend • West-to-east • Remagnetization event affected rocks more intensely towards the foreland? • M-group most negatively susceptible • Least amount of ferromagnetic material

  25. Conclusions • Remagnetization • Remagnetized prior to deformation Late Jurassic- Late Cretaceous • Some areas retain primary Mississippian magnetizations • No remagnetization trends observed

  26. Conclusions • Vertical-axis rotation • Clockwise along southern margin • Counter-clockwise along northern margin • Clockwise rotation within the Elkhorn plate • Primary component may reveal a pre-remagnetization rotation • Rotation minimal beyond transverse zones

  27. Acknowledgements • Advisor: Bernie Housen • Committee: Liz Schermer and Chris Suzcek • Russ Burmester • Field assistant: Steve Shaw • Fellow graduate students • Funding: • GDL Foundation • Graduate School RSP grant • Geology Department

  28. Questions?

  29. Remagnetized carbonates in Canadian Rockies • Enkin et al. (2000) • Cambrian-Jurassic carbonates • Cretaceous age chemical remanent magnetization (CRM) • Front range: normal polarity • Foothills: Reverse polarity • Proposed mechanism for such a trend: ~100 Km (Enkin et al., 2000)

  30. Remagnetized sedimentary rocks in Appalachia • Early Paleozoic sedimentary rocks • Widespread Pennsylvanian-Permian remagnetization • Hinterland: Post-folding • Foreland: Pre-folding • Central belt: Syn-folding • Fluids migrated faster than fold and thrust belt propagation during a unique geochemical setting (Stamatakos et al., 1996) HINTERLAND FORELAND Post-tilting Syn-tilting Pre-tilting

  31. Remagnetized carbonates in the Sawtooth Range • O’brien et al. (2007) • Madison Formation • Castle Reef dolomite • Allan Mountain limestone • Late Jurassic – Early Tertiary remagnetization (CRM) • Pre-tilting • Two folds syn-tilting • Chemical and Petrographic analysis • Elevated 87Sr/86Sr values • Externally derived fluids • Hydrocarbon migration • No remagnetization trends • All reverse polarity • Vertical-axis rotation not evaluated

  32. Methods: Magnetic Hysteresis • Plotting Mrs/MsvsHcr/Hchelps characterize grain size of ferrimagnetic material • Primary magnetizations • Single-domain + multi-domain (SD-MD) mixing line • Remagnetized carbonates • Single domain + superparamagnetic (SD-SP) mixing line (Dunlop, 2002)

  33. Directional data analysis: Getting a site mean Direction on stereoplot Multiple samples from site plotted Generate site mean Interpret ChRM • Fold test criteria: • Devil’s Fence and Turner anticlines: α95 < 20° • Three Forks: α95 < 25° • Minimum of four samples per site (n≥4) • Two sites (Sites 16 and 23) split into a group of three and a group of two • Grand-mean direction: Mean generated for a cluster of site-means

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