THE EOCENE THROUGH EARLY MIOCENE SEDIMENTARY RECORD IN WESTERN MONTANA Susan M. Vuke Montana Bureau of Mines and Geology, Butte, Montana ABSTRACT The unconformity-bounded late early Eocene to early Miocene lower Bozeman Group (Renova Formation and equivalents) is primarily preserved in western Montana erosional and extensional terrestrial basins. Proxi- mal volcanism and initial core complex exhumation were coeval with the late early Eocene onset of extension, and deposition of the oldest lower Bozeman Group rocks. Subsequently, distal volcanism contributed abundant ash to the basins. Fine-grained deposits dominated, with subordinate coarse-grained fl uvial and basin-margin deposits. Fluvial clast composition documents exposure of Laramide uplift basement cores and specifi c batho- liths and plutons during lower Bozeman Group deposition. The lithologic variability across the basins led some to adopt a sequence stratigraphic approach to Bozeman Group stratigraphy. Generally, however, the Renova Formation is considered the dominant lithostratigraphic unit of the lower Bozeman Group, although its extent and lower contact have not been applied consistently. Renova Formation age equivalents occur in northwestern Montana, in bordering Alberta and Saskatchewan, and in southeastern Montana. The initiation of Basin and Range extension produced a widespread unconformity at the top of the lower Bozeman Group that separates the Renova Formation and equivalents from the overlying Sixmile Creek For- mation of the upper Bozeman Group in much of western Montana. Unconformities within the lower Bozeman Group may refl ect augmented relief or the infl uence of climate such as the global Middle Eocene Climatic Opti- mum. Locally, paleodrainages may have reorganized as sediment input changed, volcanic eruptions occurred, or structural changes disrupted drainage patterns, but in widespread areas paleodrainage patterns remained consis- tent. Ongoing basin analysis projects are evaluating several hypotheses regarding basin formation and develop- ment and are refi ning understanding of sedimentology, depositional environments, paleoclimate, paleotopogra- phy, and paleodrainage. Stratigraphic dating has evolved from primarily vertebrate fossil-based methods, sup- plemented with magnetostratigraphy, to emphasis on radiometric dating, primarily of volcanic ash and detrital zircons. This has led to better constraints on depositional ages of units, and more refi ned evaluation of prove- nance. INTRODUCTION depositional and tectonic settings have been inconsis- tent, but recent work on these deposits has added new The lower Bozeman Group deposits discussed sedimentologic, magnetostratigraphic, thermochro- in this chapter represent the transition between the nologic, detrital zircon, and radiometric data, leading cessation of Laramide-Sevier contraction (late Pa- to refi ned interpretations of stratigraphic correlation, leocene-earliest Eocene) and the onset of Basin and tectonism, landscape evolution, and climate. The var- Range extension in western Montana (early Miocene). ious approaches and interpretations for this part of the Despite its importance, this part of the stratigraphic stratigraphic section in western Montana are reviewed section received little attention until the late 1950s in this chapter. Coeval deposits that occur as isolated through the 1970s when most studies emphasized remnants in the southeastern part of Montana and in vertebrate paleontology. Formal stratigraphic names southern Alberta and Saskatchewan are also discussed. are sparse, whereas informal names that refl ect various approaches to stratigraphy abound. Interpretations of 1 MBMG Special Publication 122: Geology of Montana, vol. 1: Geologic History Most of the original age assignments for the lower distinct” sequences within the Bozeman Group. They Bozeman Group (fi g. 1) were based on vertebrate pa- applied lithostratigraphic terminology to the two leontology. Revisions to Paleogene geochronology and sequences, which are separated by a “mid-Tertiary biochronology (Prothero and Swisher, 1992; Wood- unconformity.” They designated the lower sequence burne and Swisher, 1995; Prothero and Emry, 2004; Renova Formation and the overlying sequence Sixmile Tedford and others, 2004) changed the position of the Creek Formation. Paleogene-Neogene and the Eocene-Oligocene bound- Although many workers make a fi ne-grained aries from their previous placement by Wood and vs. coarse-grained distinction between the Renova others (1941). This paper uses the updated chronostra- and Sixmile Creek Formations, the original defi ni- tigraphy even when citing literature that pre-dates the tions of these units were more detailed. Kuenzi and change. The paper is intended to serve as an overview Fields (1971) defi ned the Renova Formation as rock of various observations and interpretations. containing greater than 70 percent terrigenous, very fi ne-grained sand, and fi ner sediment, and/or carbon- STRATIGRAPHY ate rock; and less than 30% coarse-grained sediment The lower Bozeman Group (fi g. 1) discussed in with conglomerate generally as a relatively minor this chapter refers to a part of the early Eocene to early component. They defi ned the overlying Sixmile Creek Miocene section in western Montana that is bounded Formation as typically containing fi ne-grained sand by two signifi cant, regional unconformities, and is and coarser sediment, characteristically including con- generally referred to as the Renova Formation and glomerate. The names Renova and Sixmile Creek For- equivalents (fi g. 1), or as Sequences 1, 2, and 3 (fi g. mations have been widely used, but are not universally 2). Robinson (1963) was the fi rst to apply the formal accepted as the best approach for inter- and intra-val- stratigraphic name Bozeman Group to Paleogene and ley correlation (Hanneman and Wideman, 2016). Neogene post-Laramide deposits of western Montana. In northwestern Montana, the Kishenehn Forma- Fields and others (1985) originally placed the oldest tion (Daly, 1912) correlates stratigraphically with the Bozeman Group deposits as coeval with early Eocene Renova Formation (fi gs. 1, 3). Informal names (in- extensional volcanism. However, an addendum at formal unit designations start with lowercase letter) the end of the report (Fields and others, 1985) modi- were applied to other strata of this interval in parts of fi ed the basal Bozeman Group to a younger position western Montana such as Medicine Lodge beds, Sage that excluded deposits genetically related to the ini- Creek formation (Scholten and others, 1955), Fort tial early Eocene extensional volcanism. Hanneman Logan formation (Douglass, 1903; Koerner, 1940), and Wideman (1991) indicated that early Eocene and Blacktail Deer Creek formation (Douglass, 1901; extensional volcanic rock (e.g., Lowland Creek and Hibbard and Keenmon, 1950). Fields and others Challis; see Mosolf and others, 2020) and associated (1985) suggested that the designation Renova Forma- sedimentary deposits should be considered part of the tion should apply to such informal units. The Ren- Bozeman Group, as originally designated. Based on ova Formation was not recognized in an interpreted Robinson’s (1963) designation of the Bozeman Group synsedimentary rift zone in southwestern-most Mon- as “post-Laramide,” Rasmussen (2003) agreed that tana (Janecke, 1994; Janecke and Blankenau, 2003), it should include volcanic rock from the initial early and in the Deer Lodge, Flint Creek, and Divide Basins Eocene extension and noted that it should also include (Stroup and others, 2008). However, others have ap- the Absaroka Supergroup volcanic rocks. Fritz and plied the name Renova Formation in these areas (e.g., others (2007), on the other hand, indicated that the Dunlap, 1982; Rasmussen, 1989; Barnosky and others, area where Renova Formation occurs is geographical- 2007; Retallack, 2009; Elliott, 2017; Harris and others, ly bordered by the Eocene Challis, Lowland Creek, 2017). Renova Formation has been applied by some and Absaroka volcanic fi elds, and thus does not in- workers as far west as the Lemhi Basin in southeast- clude that volcanic rock, except where it interfi ngers ern Idaho (Harris and others, 2017) and the Bitterroot with basin sediment. Valley of western Montana (DesOrmeau and others, 2009), and as far northwest as the Ninemile Valley Lithostratigraphy (Hendrix and others, 2014; fi g. 3). Kuenzi and Fields (1971) recognized two uncon- formity-bounded, “lithologically and homotaxially 2 MAGNETIC AGE POLARITY AGE AGE BASINS PERIOD EPOCH AGE NALMA (Ma) (Ma) (Ma) Horse Prairie HIST. ANOM. CHRON. Flint Creek Deer Lodge Divide Grasshopper Cr. Lemhi Muddy Creek 12 .5 5A SERRAVALLIAN UPPER C5A 13.82 BOZE- Sixmile Creek Barstovian MAN Barnes Creek Formation LANGHIAN Sixmile 15 5B 15 GROUP beds (part) C5B Creek Sixmile Creek Fm Sixmile Creek Fm X 15.97 16.3 (part) Fm Flint Creek X 5C C5C beds Sixmile Creek Not 5D C5D Hemingfordian X Formation Preserved 5E C5E BURDIGALIAN 18.5 (part) 6 C6 Unconformity 5 Bannock Pass 20 MIOCENE 20.44 20 beds 6A C6A hiatus X 6B C6B AQUITANIAN RENOVA FM X 6C C6C 23.03 Cabbage Patch Everson 7 C7 Arikareean beds 25 7A C7A 25 Cabbage Cabbage Creek 8 CHATTIAN C8 Patch Patch beds LATE MIDDLE X 9 C9 beds beds X 27.82 10 C10 30 11 C11 29.5 30 12 RUPELIAN Whitneyan Uncon- formity 4 hiatus C12 31.8 ? RENOVA FORMATION ? FORMATION RENOVA ? OLIGOCENE Orellan 13 C13 33.9 33.9 Douglas Medicine Lodge 15 C15 FORMATION RENOVA Creek Climbing 35 35 FORMATION RENOVA Chadronian beds or C16 beds ? FORMATION