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

(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)

Geology of Western Montana

(Eldredge and Van der Voo, 1988)

(Harlan et al., 2008)

• 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)

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

(Weil and Sussman, 2004)

Objectives

(McCabe and Elmore, 1989)

Field Area

Eldredge and Van der Voo (1988)

Sampling• Paleozoic carbonates

• Madison Group carbonates (20 sites)

• Mission Canyon Limestone

• Lodgepole Limestone

• Cambrian Meagher limestone

(1 site)

• Devonian Jefferson carbonates

(3 sites)

Mission Canyon (Mississippian)Lodgepole Ls

Meagher Ls(Cambrian)

9 sites – Madison Group

1 site – Meagher Ls

Mission Canyon (Mississippian)Lodgepole Ls

Jefferson Fm(Devonian)

Pilgrim Fm(Cambrian)

5 sites – Madison Group

3 sites – Jefferson Fm

Mission Canyon (Mississippian)Lodgepole Ls

6 sites – Madison Group

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

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)

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

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

Results:

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°

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°

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°

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

Expected directions:

• Calculated using paleopole location, and location of study area

• Angular distance:p = cos -1 [(sinp sins + cosp coss cos(Øp-Øs)]

• DeclinationDx = cos -1 (sinp - sins cosp / coss sinp)

• InclinationIx = tan-1(2cot p)

Geologic Period Paleopole Expected Dec.

Expected Inc.

ΔDx ΔIx

Late Cretaceous 72.3°N/194.8°E

335.8° 70.1° 6.3° 3.6°

Mississippian 29.9°N/130.1°E

310° 8.2° 4.7° 2°

Mississippian

Late Cretaceous

Paleopoles from McFadden and McElhinny (1995)

Vertical-axis rotations: K-group• Devil’s Fence:

• 59° ± 25 clockwise

• Three Forks:• 62° ± >60° clockwise

• Turner:• 111° ± >60° counter-

clockwise

K-group K-group

Vertical-axis rotations: M-group

• Restoration of K-group to Cretaceous expected direction

• 22 ± 18° - 59 ± 14° clockwise rotation

• Timing difficult to constrain

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

• 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)

Camparison with previous studies outside the Helena salient

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)

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

Conclusions

• Remagnetization

• Remagnetized prior to deformation Late Jurassic- Late Cretaceous

• Some areas retain primary Mississippian magnetizations

• No remagnetization trends observed

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

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

Questions?

Remagnetized carbonates in Canadian Rockies

(Enkin et al., 2000)

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

Remagnetized sedimentary rocks in Appalachia

(Stamatakos et al., 1996)

Post-tilting

Syn-tilting

Pre-tilting

HINTERLAND

FORELAND

• 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

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

Methods:Magnetic Hysteresis

• Plotting Mrs/Ms vs Hcr/Hc helps 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)

Directional data analysis:Getting a site mean

Interpret ChRM

Direction on stereoplot

Multiple samples from site plotted

Generate site mean

• Grand-mean direction: Mean generated for a cluster of site-means

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