CHRONOLOGICAL FRAMEWORK FOR THE LOWER CRETACEOUS SECTION IN SOUTHWESTERN MONTANA BASED ON DETRITAL ZIRCON U-PB GEOCHRONOLOGY FROM MULTIPLE LITHOLOGIES

Lower Cretaceous foreland basin fluvial, marine, and lacustrine strata in southwestern Montana were deposited during an important time in the evolution of the western North American Cordillera, recording initial orogenesis, incursion of the Western Interior Seaway, and possible early dynamic subsidence. Paramount to using these strata to constrain these events is establishing the timing of deposition, which has been a topic of debate for decades. Most previous biostratigraphic analyses indicate Aptian to early Albian deposition, whereas palynological analyses of interbedded mudstone suggest Berrasian or Barremian to early Albian onset of deposition. We present maximum depositional ages (MDAs) based on U-Pb geochronology of detrital zircons from the Lower Cretaceous section acquired primarily via laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS), supplemented with some chemical abrasion-thermal ionization mass spectrometry (CA-TIMS) dates, from fluvial sandstone, floodplain mudstone, and lacustrine carbonate. We prefer MDAs based on the youngest probability peak (YPP), weighted mean age (WMA), and youngest statistical population (YSP) for this dataset. In many samples the YSP and WMA are identical. MDAs determined from LA-ICP-MS data are <1 m.y. younger than the CA-TIMS MDAs, and the YSP MDAs overlap at 2σ with the CA-TIMS MDAs for the limestone samples but not for the sandstone sample in between. Most of our MDAs across multiple methods are consistent with other chronologic constraints and young up-section, indicating that MDAs can be interpreted to represent true depositional ages. MDAs for the lowermost Kootenai Formation range from ~140-125 Ma, whereas those from the upper part cluster between ~115-107 Ma. The data potentially suggest an older Kootenai Formation depositional center to the northwest within the study area with progressively younger deposition of the formation to the south and east. Deposition of the overlying Blackleaf Formation initiated ~112-109 Ma in the northwest and appears to young to the south and east where deposition began ~106-105 Ma. Overall, we find that floodplain and lacustrine lithologies yield a higher percentage of depositional age grains relative to fluvial sandstone, especially during periods of subdued volcanic activity.