Magnetostratigraphy of the Lebo and Tongue River Members of the Fort Union Formation (Paleocene) in the northeastern Powder River Basin, Montana

Abstract

We analyzed paleomagnetic samples and documented the stratigraphy from two sections near Miles City, Montana to determine the geomagnetic polarity stratigraphy and to constrain the age and duration of the Lebo and Tongue River Members of the Fort Union Formation in the northeastern Powder River Basin. The resulting polarity sequence can be correlated to subchrons C29n-C26r of the geomagnetic polarity time scale. By interpolating measured sediment accumulation rates from the base of C28r to the top of C27n, and then extrapolating to the top of the Tongue River Member and the bottom of the Lebo Member, we developed two age models to estimate the durations of the Lebo and Tongue River Members. Based on the first model, which uses different sedimentation rates for the Lebo and Tongue River Members, we estimate the duration of deposition of the Lebo to be between 1.30 and 1.74 million years and of the Tongue River to be between 1.42 and 1.61 million years. Using the second model, which uses the same sedimentation rate for the Lebo and Tongue River Members, we estimate the duration of deposition of the Lebo to be between 1.33 and 1.76 million years and of the Tongue River to be between 1.00 and 1.25 million years. Our results indicate a decrease in sediment accumulation rates in C27r, which is likely the result of a 0.26 to 0.62 million-year long depositional hiatus in the middle of C27r, represented by the Lebo-Tongue River contact. This unconformity occurs ϳ2 million years earlier than previously suggested and is likely contemporaneous with unconformities in the Williston Basin and in southwestern Alberta, suggesting that it may be regionally significant.

A paleoclimatic and paleoatmospheric record from peatlands accumulating during the Cretaceous-Paleogene boundary event, Western Interior Basin, Canada

“…This transition is characteristic of the late lowstand systems tracts (LST) to transgressive systems tracts (TST) of fluvial sedimentary sequences (Shanley and McCabe, 1994;Ethridge et al, 1998). This succession is unconformably overlain by the latest Danian to Thanetian fluvial Paskapoo Formation and time-equivalent strata (Lerbekmo and Sweet, 2008;Peppe et al, 2011). Thus, in the study area, the CretaceousPaleogene boundary occurs within the late LST to TST of an ~3-6-m.y.-duration (third order; cf.…”

“…3; Gradstein et al, 2012). Up to 60 m of paleotopography is recognized on this surface (Kupsch, 1957), and it is overlain by a late Maastrichtian to Danian succession (Lerbekmo and Sweet, 2008;Peppe et al, 2011) characterized by an upward transition from coarser, more amalgamated fluvial channel sandstones to finer-grained, better preserved floodplain clastics and coal (Fig. 3;Eberth and O'Connell, 1995;Murphy et al, 2002).…”

Novel Insect Leaf-Mining after the End-Cretaceous Extinction and the Demise of Cretaceous Leaf Miners, Great Plains, USA

“…The Mexican Hat fossil site is located approximately 15 m above the contact between the Tullock and Lebo Members (ES Belt, personal communication 2012). At Signal Butte, Montana, located 40 km west of Mexican Hat, the magnetic polarity reversal between chrons C29n and C28r occurs approximately 10 m above the Tullock-Lebo contact [30] . Because of potential differences in sedimentation rates and topography, the sediments at Mexican Hat could have been deposited either during C29n or C28r, whose reversal boundary is currently calibrated to 64.95 Ma [31] .…”

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

“…In all Hell Creek paleomagnetic studies, coercivity spectra reveal that the characteristic remanent direction is constrained between 20 and 80 mT [ Archibald et al ., ; Swisher et al ., ; LeCain et al ., ] (Table ). These results are similar to those observed for other KPg Laramide continental deposits in the San Juan Basin, NM, Big Horn Basin, WY and MT, Powder River Basin, WY and MT, and other parts of the Williston Basin (southeastern MT, southern Canada, and eastern and central North Dakota) [ Butler et al ., ; Lindsay et al ., ; Butler et al ., ; Peppe et al ., ; Lerbekmo and Coulter , ; Lerbekmo , ; Lund et al ., ]. No further rock magnetic analyses have been conducted on rocks from the Hell Creek region since Swisher et al .…”

“…Since Butler and Lindsay [1985], intermediate titanohematite has been identified using similar techniques in KPg age Laramide clastic deposits across central North America, including the San Juan Basin (northwest New Mexico) [Butler and Lindsay, 1985;Force et al, 2001], the Bighorn basin (south-central Montana, north-central Wyoming) [Butler et al, 1987;Force et al, 2001], and much of the Williston basin (Western and central North Dakota and Eastern Montana, USA and Southern Alberta, Saskatchewan, and Manitoba, Canada) [Lund et al, 2002;Swisher et al, 1993;Lerbekmo, 1999;Force et al, 2001] (see Figure 1). We speculate that intermediate titanohematite is present in additional KPg age Laramide deposits, but that it has been misidentified as intermediate titanomagnetite or titanomaghemite due to inadequate rock magnetic analysis [e.g., Peppe et al, 2009Peppe et al, , 2011. Geochemistry, Geophysics, Geosystems…”