Late Cretaceous: South Dakota

The late Cretaceous is fascinating for paleomagnetic study, for both space and time applications. Concerning space, several debates and hypotheses are unresolved and untested: the Baja-BC hypothesis that places British Columbia at low-latitudes akin to present-day Baja at least until ca. 75 Ma and the 84 Ma true polar wander (rapid rotation of solid Earth about liquid core in order to reinstate Earth’s rotational stability) hypothesis. Across both intervals paleomagnetic data from North America (especially critical to Baja-BC for relative comparison between terrane and craton) is insufficient to test these ideas. Me and Joe Kirschvink (Caltech) are collaborating to produce high-quality paleomagnetic data from South Dakota’s Pierre Shale in hopes of harking in on the above debates.

Precambrian-Cambrian: Australia

True polar wander, or the sliding of solid Earth (crust and mantle) about the liquid outer core, can occur rapidly and does so in order for Earth (a rotating object) to conserve angular momentum. Just like an ice skater who brings their arms in about their waist to spin faster, the Earth would bring positive mass anomalies towards the equator. One such anomaly could have been the Neoproterozoic supercontinent Rodinia. Testing the original hypothesis of Joe Kirschvink (Caltech) that rapid Cambrian continental motions may have been caused by true polar wander (Kirschvink, 1992), David Evans and I are analyzing Precambrian-Cambrian sedimentary strata from the Amadeus Basin and Flinders Range of Australia. Recently, Tim Raub and Joe Kirschvink (Caltech) and Dave Evans (Yale) compiled the most reliable Precambrian-Cambrian poles---these data from six continents form a great-circle distribution, as expected from true polar wander since rotations would be about an equatorial axis and common motion shared by all continents. Raub et al. (2007) cleverly use this long-lived rotation axis to reconstruct the supercontinent of Gondwanaland, providing a “proof of concept” for the true polar wander hypothesis. Other intriguing work (and fantastic fieldwork!) on testing the geological record for empirical evidence for true polar wander is being carried out by Nick Swanson-Hysell and Adam Maloof’s Earth history team at Princeton.

Paleoproterozoic: Canada (NW Territories) & Wyoming

When did plate tectonics begin to drive continents around the globe? Close scrutiny of the quality-filtered extant paleomagnetic database by David Evans and Sergei Pisarevsky reveals that we can confidently say plate tectonics existed 1.1 Ga (billion years ago). David Evans and I plan to double the extent of this test back beyond 2 Ga by sampling critical paleomagnetic field tests in the Slave craton of the NW Territories of Canada and the Wyoming craton of Wyoming and Montana. Inset from Hoffman and Hall (1993).

Concerning time, the late Cretaceous global polarity timescale (GPTS) requires further work, requiring both new studies and confirmations of old ones. Sam Bowring (MIT) is trying to date zircon crystals of several volcanic ash beds that can be used to date the geomagnetic reversals---these ages, then, can be correlated to sections around the globe that are devoid of radiometric constraints. Don Penman (Columbia) and Peter Ward (U. Washington) are stratigraphically collecting and morphometrically measuring ammonites of the Pierre Shale in hopes of discerning basin-wide and global-scale evolutionary cadence and character.