GEOLOGIC MAP OF THE CENOZOIC DEPOSITS OF THE LOWER JEFFERSON VALLEY SOUTHWESTERN MONTANA MBMG Open File Report 537 2006 Susan M. Vuke This report has completed the review process for conformity with Montana Bureau of Mines and Geology’s technical and editorial standards Partial support has been provided by the STATEMAP component of the National Cooperative Geology Mapping Program of the U.S. Geological Survey under contract Number 05HQAG0093. LOWER JEFFERSON VALLEY TERTIARY STRATIGRAPHY Kuenzi and Fields (1971) established Tertiary lithostratigraphy for southwestern Montana partly as a modification of formal stratigraphy established in the Three Forks area (Robinson, 1963), and in the Toston area (Robinson, 1967). Modifications by Kuenzi and Fields (1971) included restricting the Sixmile Creek Formation of Robinson (1967) to Tertiary strata that overlie the mid-Tertiary (Hemingfordian) unconformity (Fig. 2), and defining a new stratigraphic unit, the Renova Formation, underlying the unconformity. Kuenzi and Fields (1971) described the Sixmile Creek Formation as typically coarse grained (defined as fine sand and coarser) and the underlying Renova Formation as fine grained (defined as greater than 70 percent terrigenous very fine sand and finer). Kuenzi and Fields (1971) divided the Renova Formation into three members west of the map area. Two of the members, the Dunbar Creek and Climbing Arrow, were previously formally established as Tertiary formations by Robinson (1963) in the Three Forks area. Kuenzi and Fields (1971) changed the stratigraphic rank of these two formations to member status in the upper Jefferson Valley. Subsequently, an informal late Arikareean unit, the Negro Hollow map unit north of Red Hill, was described by Lofgren (1985) and considered the upper part of the Renova Formation. Five sequences have been recognized in the Cenozoic deposits in the upper Jefferson Valley and other parts of southwestern Montana. These sequences are recognized principally from bounding unconformities rather than than lithology because of the lateral and vertical repetition of lithologies throughout the Tertiary section (Hanneman and Wideman, 1991). The sequence-bounding unconformities are recognized based on paleosols (primarily stacked calcic paleosols), erosion surfaces, and angular stratal relationships. Some paleosols have been traced from outcrop into the subsurface in the map area where they are recognized as seismic reflectors (Hanneman and others, 1994). The lithostratigraphic and sequence stratigraphic approaches to dividing Cenozoic deposits have both been greatly augmented by fossil data, especially vertebrate fossils, and both approaches recognize unconformity-bounded units. The International Subcommission on Stratigraphic Classification prefers the name synthem (Chang, 1975) for unconformity-bounded stratigraphic units (Salvador, 1987). The latest (1983) North American Stratigraphic Code introduced the category of allostratigraphic units that are comparable to synthems (Salvador, 1987). A synthem mapping approach was informally used for the Tertiary deposits in this report on the upper Jefferson Valley. Rather than lump all of the Tertiary deposits that overlie the mid-Tertiary (Hemingfordian) unconformity into the Sixmile Creek Formation or Sequence 4, for example, separate facies were mapped as informal units that comprise the Sixmile Creek synthem in the map area of this report, and other reports (Vuke, 2004; Vuke and others, 2004; Vuke, 2003). This approach takes into account intrabasinal and interbasinal facies changes within the unconformity-bounded sedimentary packages and also helps in recognizing faults that may juxtapose a younger part of the synthem against an older part. Additionally, only one horizon of stacked calcic paleosols was recognized in the area, in the Fairview map unit south of Three Forks, the only place a paleosol horizon could be used for mapping. No stacked calcic paleosols were recognized at the mid- Miocene (Hemingfordian) unconformity (Fig. 2) anywhere in the map area. In this report the Dunbar Creek and Climbing Arrow are treated as formations following their original designations by Robinson (1963) because the thick Climbing Arrow Formation is divided into many mappable units on the present map of the Lower Jefferson Valley. 1 USGS 7.5' quadrangles Doherty Negro Milligan Three Kalispell MONTANA Mountain Hollow Canyon Forks 15 Great Falls 90 Missoula Helena Three Jefferson Sapping- Willow 94 Forks Butte Island ton Creek Bozeman Billings SE 90 90 15 Location of Lower Willow Pony Harrison Creek Norris Jefferson Valley map Reservoir NE 112°00' R3W R2W R1W R1E 111°30' 46°00' R2E 46°00' Big Mtn 287 T3N T3N N e g r ro H o Shoddy llll ow Springs Bull Mtn Bull Mtn M u r d e JEFFERSON CO v S i CO BROADWATER p rri R in T2N T2N Sheep g G Rock u r llc Red e h d Hill l u M o Miil 90 B lllii ga 287 Doherty Mtn an n 69 illig Cre M ek CC Cottonwood aa Three Canyon nn T yy r Forks imberimber o e 90 Canyon o v n i n R n Round o Fairview Spring rs 2 e Cemetery La Hood Park Parker Homestead ff Cardwell e State Park J Je JJ ff Jefferson ee er Lewis & Clark ff s ff o Island ee nCaverns S.P. rr ss T1N T1N o Riv n er Willow Creek C yo an on 2 New London Stateler LONDON HILLSPlacer Monument Sappington r e Shaw v k i Basin e Basin R e r k r e C e e v Dog i t o Cr R w n S er w ew o l l r i e W d l n u GALLATIN CO o o s e i T1S B r 287 T1S p d C 359 MADISON CO lo a e h t e M t n p o u A l o e S t n k A e re e l C Willow Cr t t i Reservoir L Harrison N w lo il W k e T2S T2S re Pony C n ia eg w r o 0 5 Miles N 287 45°37'30" 45°37'30" 112°00' R3W R2W R1W R1E R2E 111°30' Figure 1. Location of the Lower Jefferson Valley map and index of 7.5' quadrangles. 2 NORTH AMERICAN EPOCHS Million years before present LAND MAMMAL AGE 2.02 PLIOCENE Blancan 5.03 Madison Plateau map unit Hemphillian Carey Ranch m.u. 8.41 SIXMILE Clarendonian Harrison m.u. CREEK 11.5 SYNTHEM Madison Valley formation Barstovian Cottonwood Canyon m.u. MIOCENE 15.9 Hemingfordian mid-Miocene unconformity 19.0 late Arikareen: Negro Hollow m.u. Arikareean Arikareen: Dunbar Creek Formation OLIGOCENE 30.0 Whitneyan 31.7 RENOVA Orellan SYNTHEM 33.6 Dunbar Creek Formation Chadronian 37.7 Climbing Arrow Formation Duchesnean Milligan Creek Formation Red Hill m.u. 42.4 EOCENE Uintan 46.4 basalt and rhyolite Bridgerian 50.9 Sequence 1 Sequence 2 Sequence 3 Sequence 4 Wasatchian no record 55.7 Figure 2. Lower Jefferson Valley correlation chart, modified from Rasmussen (2003), showing Tertiary sequences of Hanneman and Wideman (1991). 3 Southwest Montana Transverse Zone and Jefferson Canyon Fault The east-striking Southwest Montana Transverse Zone is a significant tectonic feature that transects the map area along the Jefferson Canyon Fault (Schmidt and O’Neill, 1982; Schmidt and others, 1987). The Jefferson Canyon fault not only separates Proterozoic Belt Supergroup LaHood Formation to the north from Archean metamorphic rocks to the south, but also marks the southern margin of a major eastward bulge in the Montana part of the fold and thrust belt (Schmidt and O’Neill, 1982) (Fig. 3). The Archean crystalline rocks south of the fault have a pronounced northwest-striking structural grain (Schmidt and Garihan 1983). Many of the northwest-striking faults are associated with northwest- plunging hanging-wall anticlines cored with Archean metamorphic rocks (Schmidt and O’Neill, 1982). The Jefferson Canyon Fault and the northwest-striking faults to the south all experienced significant episodes of movement during the Proterozoic, Late Cretaceous, and late Cenozoic. Many studies have pieced together the geologic history of this area, especially during Late Cretaceous and early Paleocene compressional tectonics (Garihan and others, 1983; O’Neill and others, 1990; Schmidt, 1975; Schmidt, 1976; Schmidt, 1979; Schmidt and Garihan, 1983, Schmidt and Garihan, 1986a, Schmidt and Garihan, 1986b; Schmidt and Geiger, 1985; Schmidt, and others, 1989; Schmidt, and Hendrix, 1981; Schmidt, and O’Neill, 1982; Schmidt and others,1981; Schmidt and others, 1987; Schmidt, and others, 1988; Schmidt and others, 1990, Schmidt and others, 1991), but also during post-Laramide extensional tectonics (Schmidt and Garihan, 1986b; Schmidt and others, 1987). Latest Cretaceous, Paleogene, and early Miocene Near the end of the Cretaceous, about 70 million years ago, 10,000 or more feet of Elkhorn Mountains Volcanic rocks overlay older rocks in the area (Klepper and others, 1957). A tremendous volume of the volcanic rocks was eroded from the area during the late Maastrichtian and Paleocene and deposited to the east in the Upper Cretaceous Livingston Group (Lageson and others, 2001) and lower Paleocene Fort Union Formation. Basalt and rhyolite on Bull Mountain and in the southeastern part of the map area, mark the start of Eocene extension. The Red Hill map unit may be the oldest preserved Tertiary sedimentary unit deposited during the Eocene in the map area. It is exposed in limited areas throughout the map area, always on bedrock and usually overlain by younger Tertiary units. The unit may represent colluvium, mass flow deposits, and alluvium associated with red, kaolinitic siltstone and mudstone that may have a pedogenic origin during a humid climate (Richard, 1966; Kuenzi, 1966). At Red Hill, Timber Canyon, and the east side of Doherty Mountain, the Red Hill map unit may be associated with Tertiary thrust faults, and may have a tectonic origin. If that is the case, the unit may be as young as Miocene.