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Stratigraphy and Sedimentology of the Upper Jurassic Morrison Formation, Dillon, Montana

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Stratigraphy and Sedimentology of the Upper Jurassic Morrison Formation, Dillon, Montana Foster, J.R. and Lucas, S.G., eds., 2006, Paleontology and Geology of the Upper Jurassic Morrison Formation. New Mexico Museum of Natural History and Science Bulletin 36. 1 STRATIGRAPHY AND SEDIMENTOLOGY OF THE UPPER JURASSIC MORRISON FORMATION, DILLON, MONTANA JON J. SMITH1, STEPHEN T. HASIOTIS1, 2, AND WILLIAM J. FRITZ3 1Department of Geology, University of Kansas, 120 Lindley Hall, Lawrence, KS, 66045-7613, [email protected] of Geology and Natural History Museum and Biodiversity Research Center, University of Kansas, 120 Lindley Hall, Lawrence, KS 66045-7613, [email protected] 3Department of Geology, Georgia State University, 340 Kell Hall, Atlanta, GA 30303, [email protected] Abstract—Red, purple and gray-green mudrocks near Dillon, Montana, have been mapped traditionally as the Up- per Jurassic Morrison Formation. There are very few studies of the Mesozoic strata in this area and questions exist as to whether this unit actually is the Morrison Formation, or whether it is similarly variegated mudrocks of other formations at nearly the same stratigraphic interval. This study compares the stratigraphy, sedimentology, and paleon- tology of rocks near Dillon to the Upper Jurassic Morrison Formation elsewhere in the northern portion of the West- ern Interior Basin. The average thickness of the unit in question is approximately 24 m. It is composed of four lithofacies: 1) interbedded gray-green sandy siltstone showing weak pedogenic modification; 2) red calcareous mud- stone on which moderately-to well-developed paleosols formed; 3) intraformational mudclast conglomerates; and 4) black claystone. These lithofacies are interpreted as distal floodplain deposits of a mud-dominated alluvial system. The paleosols are characterized by mottled coloration, carbonate nodules, clay slickensides, and abundant carbonate rhizoliths and rhizocretions. Dinosaur-bone fragments and the Late Jurassic gastropod Viviparus reesidei were also present. The stratigraphy, sedimentology and paleontology compare best with the Morrison Formation of northern Wyoming and south-central Montana. INTRODUCTION 1980; Peterson, 1994; Demko et al., 2004; Turner and Peterson, 2004). Sedimentologic, paleopedologic, and paleobotanic studies indicate This paper describes the stratigraphy, sedimentology, and clay min- a strongly seasonal to monsoonal paleoclimate during deposition (Demko eralogy of relatively thinly-bedded siltstones and mudstones near Dillon, and Parrish, 1998; Rees et al., 2004). Paleosols and trace fossils of soil Montana, that have been interpreted traditionally as the Upper Jurassic organisms are common throughout Morrison strata, indicating extended Morrison Formation. Questions exist, however, as to whether this unit is periods of long-term subaerial exposure, relatively slow rates of deposi- actually the Morrison Formation or whether it is a multicolored unit similar tion, and pedogenesis (Hasiotis, 1999; Demko et al., 2004; Hasiotis, 2004). in appearance and stratigraphic position to the Morrison. The strata in ques- Black mudstone, carbonaceous mudstone, and coal in central Montana tion have been mapped as Morrison Formation on published geologic maps suggest wetter and less seasonal conditions at or near the top of the forma- of the Dillon region (Brandon, 1985; Ruppel et al., 1993), though usually tion in this portion of the Western Interior Basin (Demko et al., 2004). followed by a question mark, or the unit has been mapped as undifferenti- The Morrison Formation on the Colorado Plateau can be divided ated from the Lower Triassic Dinwoody Formation. into lower and upper parts based on a significant change in the clay miner- Few studies have been conducted on the Mesozoic system near alogy (Turner and Fishman, 1991). Clay minerals in the lower part of the Dillon. Scholten et al. (1955) described 40- to 100-m-thick exposures of formation consist mostly of non-swelling types, whereas the upper part is the Morrison Formation in the southern Tendoy Range about 30 km south dominated by the presence of swelling smectitic clays, including thin ben- of Dillon, Montana. The Morrison Formation, however, thins rapidly to tonite beds derived from the alteration of volcanic tuffs. This vertical change the north and is absent in the northern part of the Tendoy Range and be- can be traced as far north as northern Wyoming, but it is not found in yond (Scholten et al., 1955). Suttner (1969) examined exposures of the Montana where non-swelling clays are present . Upper Jurassic Morrison and Lower Cretaceous Kootenai Formations in In the Dillon area, the Paleozoic and Mesozoic sedimentary units southwestern Montana. While one of Suttner’s (1969) nineteen study sec- are folded against the Pioneer Batholith in a series of north-northeast-trend- tions was near Dillon (Birch Creek), he provided no detailed information ing, plunging anticlines and synclines produced during the Laramide orog- on individual sections. eny (Suttner et al., 1981; Sears et al., 1989). The mudrocks interpreted as Many university field camps visit Dillon each summer to map Pa- the Morrison Formation are bounded unconformably below by the Lower leozoic and Mesozoic rocks in the area. The Upper Jurassic Morrison For- Triassic Dinwoody Formation and above by the Lower Cretaceous Kootenai mation is often mapped by students, but is rarely seen in the field because Formation. The Dinwoody Formation is a marine, mixed carbonate- of poor exposures. This study focused on the few well-exposed sections siliciclastic shelf-margin sequence 100 to 250 m thick in southwestern and compares the rock units near Dillon with the Upper Jurassic Morrison Montana (Ruppel et al., 1993; Boyer et al., 2004). The Lower Cretaceous Formation elsewhere in the northern portion of the Western Interior Basin. Kootenai Formation is a 200- to 400-m-thick continental sequence com- GEOLOGIC SETTING posed of mudstone, siltstone, and sandstone (Ruppel et al., 1993). The Kootenai Formation is equivalent to the Cloverly Formation of northern The Morrison Formation is the most extensive continental unit in Wyoming and south-central Montana, and both contain a basal quartzite North America (Frazier and Schwimmer, 1987), it forms an alluvial wedge and red-chert-cobble conglomerate that unconformably overlies the over 300 m thick in some areas, and covers nearly the entire Western Inte- Morrison Formation (De Celles, 1986). rior Basin. In the northern part of this basin, the formation is predomi- nantly composed of varicolored mudrocks that range from purple and red METHODS to green and gray, with local deposits of limestone and cross-bedded sand- Five field sites in the Dillon area were selected for this study stone. The Morrison represents a mosaic of predominantly terrestrial depo- (Fig. 1), and detailed measured sections were produced from those ar- sitional settings in a uniquely large system of alluvial plains (Dodson et al., eas with the best exposures. These included the Birch Creek, Dutch- 2 FIGURE 1. Map with locations of the five field sites in the study area: 1) Frying Pan Gulch, 2) Dutchman Spring, 3) Birch Creek, 4) Sandy Hollow, and 5) Ziegler Anticline. men Spring, and Ziegler Anticline field sites (Fig. 2). Lithologic de- FIGURE 2. Measured sections produced from the three best exposures of Morrison scriptions consist of unit thickness, grain size, color, sedimentary struc- Formation in the study area and keyed to localities in Figure 1: 2) Dutchman Spring, tures, pedogenic features, and body and trace fossils. Specimens were 3) Birch Creek, and 5) Ziegler Anticline. collected from the measured sections at 1-m intervals for thin section common in siltstone beds as individuals or in clusters oriented along the and X-ray diffraction (XRD) analyses. Thin sections were examined bedding plane. Most concretions are oblate and less than 2 cm in diameter, using a transmitted light microscope. although some are elongate and up to 6 cm long. Carbonate rhizocretions The clay mineralogy of siltstone and mudstone samples was deter- are present in a green siltstone unit at one site, but are much more common mined with powder XRD using sample preparation techniques described in red calcareous mudstone. by Moore and Reynolds (1997). The less than 0.2 ì m clay fraction was separated by centrifuge and mounted on glass slides. Three oriented slides Facies Interpretation were produced per rock sample; one was heated to 550°C for one hour, The siltstone facies is interpreted as overbank-flood deposits, one was placed overnight in a closed desiccator over an ethylene glycol with some crevasse-splay deposits, based on the sheet-like geometry, bath, and one was left untreated. Each slide was then analyzed using the relatively coarse-grain sizes, wavy laminae, and general lack of pe- Phillips Model 12045 X-ray diffractometer equipped with a MDI Databox dogenic features. Mudrock clasts at the base of siltstone beds are inter- at Georgia State University, Atlanta. Clay-mineral abundances were esti- preted as rip-up clasts produced by scouring of the floodplain surface. mated from ratios of various peaks on diffractograms, using a semi-quanti- Low chroma matrix colors suggest generally reducing conditions either tative method developed by Biscaye (1965). from high water tables, poorly drained conditions on the floodplain, or SEDIMENTOLOGY a high organic content of the original units (Kraus, 1996; Vepraskas, 1999). The absence of mature pedogenic features implies relatively Rocks interpreted as the Morrison Formation in the Dillon area
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