Importance of titanohematite in detrital remanent magnetizations of strata spanning the Cretaceous‐Paleogene boundary, Hell Creek region, Montana

Abstract

Intermediate composition titanohematite, Fe 2-y Ti y O 3 with 0.5 y 0.7, is seldom the focus of paleomagnetic study and is commonly believed to be rare in nature. While largely overlooked in magnetostratigraphic studies, intermediate titanohematite has been identified as the dominant ferrimagnetic mineral in an array of Late Mesozoic and early Cenozoic Laramide clastic deposits throughout the central United States. Intermediate titanohematite is ferrimagnetic and has similar magnetic properties to titanomagnetite, except its unique self-reversing property. Due to these similarities, and with detrital remanent magnetizations masking its self-reversing nature, intermediate titanohematite is often misidentified in sedimentary deposits. Past studies relied upon nonmagnetic techniques including X-ray diffraction and electron microprobe analysis. While these techniques can identify the presence of intermediate titanohematite, they fail to test whether the mineral is the primary recorder. To facilitate the identification of intermediate titanohematite in sedimentary deposits, we characterize this mineral using low-temperature magnetometry and high-temperature susceptibility experiments, and present a new identification technique based on titanohematite's self-reversing property, for sediments that span the Cretaceous-Paleogene boundary (Hell Creek region, Montana). Results from the self-reversal test indicate that the majority of remanence is held by minerals that become magnetized parallel to an applied field, but that intermediate, selfreversing titanohematite (y 5 0.53-0.63) is an important ancillary carrier of remanence. While earlier literature suggests that intermediate titanohematite is rare in nature, reanalysis using specialized rock magnetic techniques may reveal that it is more abundant in the rock record, particularly within depositional basins adjacent to calc-alkaline volcanics, than previously thought.

Early Paleocene Magnetostratigraphy and Revised Biostratigraphy of the Ojo Alamo Sandstone and Lower Nacimiento Formation, San Juan Basin, New Mexico, USA

“…We identified the occurrence of intermediate titanohematite in all samples were analyzed for rock magnetism (Fig. 7), which is similar to what has been found in previous work in the San Juan Basin and contemporaneous Laramide basins (Butler and Lindsay, 1985;Force et al, 2001;Sprain et al, 2016). In addition, we documented the presence of (titano)magnetite, hematite, and possibly greigite in some samples.…”

“…suggesting intermediate titanohematite (Sprain et al, 2016). In some samples, the remanence held from 100 -300 mT dropped beginning at 100-200 °C and was completely demagnetized by 400 °C, indicating the presence of only intermediate titanohematite (Sprain et al, 2016) (sample P16BT06, Fig. 6D).…”

“…In all of these samples, the Triaxial IRM Lowrie tests indicated that intermediate titanohematite was the major remanence carrier. Titanohematite is easily reset, and as a result, frequently overprinted (Dunlop and Ozdemir, 1997;Sprain et al, 2016). In addition to the presence of titanohematite, all samples from Mesa de Cuba, had evidence for the occurrence of (titano)magnetite as well.…”

Conceptual models for short‐eccentricity‐scale climate control on peat formation in a lower Palaeocene fluvial system, north‐eastern Montana (USA)

“…The latter mineral is particularly known for its ability to show a self-reversal that could bias palaeomagnetic interpretations (e.g. Nagata et al, 1951;Sprain et al, 2016). Based on those earlier results, it was decided to use both TH and AF demagnetization.…”

Long‐eccentricity regulated climate control on fluvial incision and aggradation in the Palaeocene of north‐eastern Montana (USA)

“…Previous rock magnetic analyses in the lower Fort Union Formation showed that the dominant magnetic remanence carriers are magnetite and maghemite (Swisher et al ., ; Sprain et al ., ). Intermediate‐composition titanohematite could also be present and, if abundant, this mineral might complicate palaeomagnetic interpretations because of its ability to self‐reverse (Sprain et al ., ).…”