HEMATITE IN THE TIME OF GEOLOGY: LESSONS FROM SEDIMENTARY ROCKS OF THE NORTH AMERICAN MIDCONTINENT RIFT

The ubiquity of hematite on Earth's oxidized surface environment makes it a valuable archive of Earth's past geomagnetic field behavior and paleogeography. It is an essential (antiferro)magnet in the time of geology. Hematite poses long-standing challenges as it is present in sedimentary rocks as both a primary detrital and a secondary authigenic mineral. Distinguishing between these two is essential when using hematite for chronostratigraphy. Additionally, detrital hematite directions are flattened through deposition and compaction complicating efforts to reconstruct paleolatitude. We present a trio of insights into hematite remanence gained through coupled paleomagnetism, petrography, rock magnetism, and sedimentology of Midcontinent Rift sedimentary rocks. The first insight comes from siltstone rip-up clasts that were reoriented in fluvial sands within the depositional environment. The rip-up clasts constrain coarse-grained hematite to hold a primary detrital remanence that reoriented along with the clasts. Finer-grained pigmentary hematite within the intraclasts formed following burial with a distinct direction further along Laurentia's apparent polar wander path. Very high resolution thermal demagnetization is necessary to differentiate between the pigmentary and detrital hematite in these siltstones. The second insight comes from interflow fine-grained sandstones where detrital inclination-shallowing can be empirically quantified via comparison to the well-constrained paleomagnetic direction of the basaltic lavas. These data reveal pigmentary hematite in the interflow sediments to have formed very soon after deposition — exactly reproducing the direction of intercalated volcanics. The third insight comes from comparative study of fluvial and lacustrine sediments where the preservation of the magnetic mineral assemblage systematically varies as a function of oxic, suboxic, and anoxic water chemistry. In suboxic lacustrine fine-grained siliciclastics deposited at the end of Midcontinent Rift magmatism, detrital magnetite and hematite are preserved holding the same direction enabling the development of a new high-quality paleomagnetic pole. Overall, these data provide new constraints of the paleogeography, chronostratigraphy, and paleoenvironments of interior Laurentia during the assembly of Rodinia in the latest Mesoproterozoic.