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

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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 paleontology 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 mudstone 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 predominantly 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 are high rates of sedimentation. Rare carbonate rhizocretions and a paucity composed of four major elements: 1) gray-green sandy siltstone, 2) reddish of primary sedimentary structures throughout, however, suggest incipi- brown, calcareous mudstone on which moderately to well-developed ent soil formation in these deposits. paleosols formed, 3) mudclast conglomerate, and 4) thinly bedded, black claystone (Table 1). Although the stratigraphic position of the lithologies Calcareous Mudstone varies from site to site, such characteristics as color, sedimentary struc- Description tures, pedogenic features, and fossil content remain relatively consistent. The second most common lithology in the study areas is brown- Sandy Siltstone ish red to purple calcareous mudstone (Fig. 3b). Lenticular beds of this Description lithology are locally present within gray-green siltstone beds. The mudstone is mottled grey-green and contains carbonate nodules as well as Sheet-like beds of sandy siltstone comprise most of the formation in clay slickensides. Mottles are abundant locally in association with the study area. Sandy siltstones are predominantly gray-green and gener- rhizoliths and spherical to elongate carbonate nodules. Fine-grained ally lack well-defined sedimentary structures. The sand-sized fraction is quartz and chert, as well as round, medium-grained globules of clay composed of angular to subrounded quartz, chert, and lithic fragment grains. often show hematite-stained halos in thin section. The less than 0.2 µm Mudstone clasts, or mudstone-clast conglomerate beds, are locally present clay fraction of the mudstone is composed of illite (50%), kaolinite at the base of this unit where it overlies calcareous mudstone. The less than (34%), and chlorite (16%). Quartz is also present in very minor amounts 0.2 µm clay fraction of the sandy siltstone is composed of illite (86%), in the less than 0.2 µm fraction but smectite was not detected in any of kaolinite (10%), chlorite (3%) and quartz (1%). Trace amounts of smectite the samples analyzed. were detected as mixed-layered illite-smectite based on intensity ratio cal- culations (Srodon, 1984). Paleontology Sets of parallel to wavy laminae, up to 1 mm thick, are locally present Cylindrical, branching, and downward tapering tubules, inter- at the base of siltstone beds (Fig. 3a). Laminae are also present adjacent to preted as rhizoliths, are common throughout the mudstone facies. The lenticular beds of mudclast conglomerate. Black siliceous concretions are 3

Table 1: Summarized characteristics of the four lithofacies in the study area near Dillon, Montana.

Lithofacies Colors Lithology Sedimentary Structures Fossils Interpretation

Siltstone Gray-green to Siltstone to sandy Lacks well-defined Rare carbonate Poorly drained floodplain brown-green siltstone macro-bedding; fine rhizocretions; rare deposits showing weak wavy laminae; black Gastropoda and fossil pedgogenic development, concretions bone fragments crevasse-splays

Mudstone Red, brownish Calcareous mudstone Some lenticular beds, Carbonate rhizoliths and Moderately- to well- red, purple to sandy mudstone but otherwise lacks rhizocretions; gastro- developed paleosols well-defined macro- pods and burrows; few forming on distal bedding; mottles, bone fragments floodplain deposits carbonate nodules, clay slickensides

Conglomerate Brownish gray, Mudclast conglomer- Lenticular, matrix Few dinosaur bone Channeled high-energy green, pinkish ate in a sandy supported, unstratified fragments flood and erosional brown to red siltstone matrix and ungraded thin overbank scour deposits beds; clast supported, stratified and normally graded thicker beds

Claystone Black, dark Claystone Lenticular and None observed Backswamp to ephemeral brown, tan to massively bedded; pond deposits gray iron-segregation features common rhizoliths are cast in powdery to solid carbonate and range from less clasts are referred to generally as gastroliths or stomach stones if found in than 1 mm to 3 mm in diameter. Fine calcite-filled rhizoliths are abun- close association with dinosaur fossils, particularly the ribcage of an articu- dant in thin sections of this unit (Fig. 3c). lated skeleton (e.g., Gillette, 1991). While this explanation seems plausible, Stacked, spherical carbonate-nodules, successively smaller in di- it is currently not possible to differentiate gastroliths from other rounded ameter with depth, are interpreted as rhizocretions (Fig. 3d). The clasts. Bone fragments are also present in the mudstone, though rare, but rhizocretions are concentrated at the tops of a series of red and purple could not be assigned to specific taxa. mudstone beds at the Dutchman Spring field site (Fig. 3b). Nodular Facies Interpretation segments of the rhizocretions range from 10 mm to as much as 55 mm in diameter. The rhizocretions are up to 20 cm long and branching is The mudstone facies is interpreted as fine-grained, distal overbank present in some specimens. Carbonate nodules, common to the mud- deposits on which moderately to well-developed soils formed. Evidence stone, may be disarticulated segments of rhizocretions. for paleopedogenesis includes red and purple matrix colors, gray-green In thin section, faint and indistinct clay-filled tubules from 0.5 mottles, carbonate nodules, clay slickensides, abundant carbonate rhizoliths mm to 1 mm in diameter are interpreted as invertebrate burrows (Fig. and rhizocretions, and burrows. 3e). Most occur as diffuse to sharp-edged, circular to elongate mottles Red, purple or gray colors indicate different concentrations of Fe with no distinct internal or surficial morphology. A few contain backfill and Mn oxide. These differences are likely due to varying redoximorphic that is slightly coarser grained than the surrounding matrix. conditions and local organic matter concentrations in the original soil pro- Fossils of gastropods and bivalves are common in several mudstone files (e.g., Bigham et al., 1978; Torrent et al., 1980; Schwertmann, 1993; intervals (Fig. 3f). The gastropods are identified as Viviparus reesidei based Vepraskas, 1999). Red colors result from a mix of hematite (Fe2O3) and on the morphology of known Jurassic species from the Colorado Plateau goethite (FeO(OH)), and indicate moderately well-drained and oxidizing (Yen, 1950; Evanoff et al., 1998). Extant members of this group are proso- soil conditions (e.g., Schwertmann and Taylor, 1977; Cornell and branchs or gill-breathing snails that live in well-oxygenated, quiet water Schwertmann, 1996; Scheinost and Schwertmann, 1999). Purple colors habitats of lacustrine and fluvial environments (Evanoff et al., 1998). Frag- are characterized by more widely dispersed hematite and suggest less well- mentary bivalve fossils are less common, but could not be assigned to a drained conditions (e.g., McBride, 1974; Blodgett, 1988; Wright et al., specific taxon. 2000). Gray colors generally indicate the absence of Fe-oxide minerals (e.g., The mudstone contains well-rounded to rounded, polished, 2- to PiPujol and Buurman, 1994; Vepraskas, 1999). Most of the gray-green 3-cm-diameter quartzite clasts. The quartzite clasts were present mostly in mottles are probably rhizoliths and burrows that underwent preferential talus on the weathered surface of the mudstone, though a few were ob- gleying—local reduction and oxidation of Fe and Mn—due to the presence served in situ, indicating that they originate from this unit. Such polished of organic matter in these structures (Schwertmann, 1993; Kraus and 4

FIGURE 3. a) Photograph of sandy siltstone specimen with wavy sets of laminae. b) Dashed lines in this outcrop photograph mark the contacts of distinct red (rmd1 and rmd2), purple (pmd), and gray-green (gslt) paleosols with abundant carbonate rhizocretions. The outcrop is approximately 7 m thick from the base of the pmd to the top of rmd2. c) Photomicrograph of the mudstone facies showing numerous fine, calcite-filled rhizoliths and hematite staining; plane-polarized light. d) Outcrop photograph of carbonate rhizocretions in a red mudstone bed. Rhizocretions taper with depth and some branch. Hammer is 30.5 cm long. e) Photomicrograph of red mudstone thin section showing numerous carbonate-filled rhizoliths and invertebrate burrows; plane-polarized light. White arrow indicates the most obvious burrow, but faint circular to burrow outlines are abundant in mudstone thin sections. White rectilinear to curvilinear lines are interpreted as mottles associated with fractures and rhizoliths, respectively. f) Photograph of fossil gastropod Viviparus reesidei collected from the red mudstone facies. 5 Hasiotis, 2006). Modern soils with similar features as found in the Thin beds of dark gray to black siliceous claystone up to 2 m thick mudstone facies are waterlogged for several months of the year fol- are present at the Ziegler Anticline and Frying Pan Gulch sites. In out- lowed by longer periods of better drainage and lower water tables crop the claystone appears massive and devoid of sedimentary struc- (Bigham et al., 1978; Torrent et al., 1980; Farrell, 1987; Vepraskas, tures. In thin section, the claystone contains a faint blocky texture and 1999). Calcite accumulations in and around the ancient plant roots also gray-green reduction halos around dispersed, very fine grains of quartz, suggest seasonal fluctuations of soil moisture (Klappa, 1980; Hasiotis, chert, and hematite. The less than 0.2 µm clay fraction is composed of 2004; Kraus and Hasiotis, 2006). illite (87%), kaolinite (9%), and chlorite (4%). Quartz is also present Mudclast Conglomerates in very minor amounts in the less than 0.2 µm fraction, but smectite was not detected in any of the samples analyzed. Description Facies Interpretation Thin to thick lenses of mudclast conglomerate are present in gray- green siltstone near the top of several sections. Thin, unstratified, ungraded The claystone is a minor component of the study area and is conglomerate lenses are up to 0.6 m thick and generally less than 3 m interpreted as poorly drained back-swamp or shallow pond deposits. wide. These consist of rounded mudstone clasts (95%) and angular chert The blocky texture observed in thin section may indicate incipient clasts (4%) and lithic fragments (1%) in a gray-green, sandy siltstone ma- paleopedogenesis in these deposits, though other evidence of soil de- trix. The clasts are matrix supported and range from less than 1 mm to 10 velopment was not observed. mm in diameter. In thin section, the mudclasts are texturally similar to the calcareous mudstone, whereas the matrix is compositionally identical to DISCUSSION the surrounding gray-green siltstone (Fig. 4a). The mudrocks near Dillon are interpreted as the product of dis- A thick conglomerate lens with a distinctly concave base is located tal floodplain deposition in a mud-dominated fluvial system. The dis- near the top of the Ziegler Anticline section. This unit is approximately 2 m tal interpretation is suggested by the absence of well-defined, sand- at its thickest and about 14 m wide. The conglomerate is composed of a stone-filled channel deposits. Mud-dominated deposits in modern flu- stacked series of 15- to 30-cm-thick beds. Individual beds contain matrix- vial systems are typically associated with low-gradient, slow flowing, supported, cobble-size mudclasts in a siltstone matrix, and the beds fine suspended-load meandering rivers (Miall, 1992). Cooley and Schmitt upward to sandy siltstone (Fig. 4b). The clasts are rounded to subrounded, (1998) proposed that an anastomosing fluvial system deposited muds 1 to 60 mm in diameter, and are composed of mudstone (90%) and car- of the Morrison Formation in the Gallatin and Beartooth Mountain bonate (10%). The conglomerate also contains lesser amounts of coarse- ranges of south-central Montana east of our study area. How that flu- grained chert, lithic fragments, bone fragments, and polished quartzite vial system relates to the fluvial system in southwest Montana remains pebbles similar to those found in calcareous mudstone units. unclear. Facies Interpretation The stratigraphy, sedimentology and especially the paleontology of mudrocks in the study area suggest correlation with the Morrison Forma- The mudclast conglomerate is interpreted as rip-up clasts scoured tion farther east and southeast in northern Wyoming and central Montana. from the local floodplain surface and deposited in shallow fluvial channels. There, the Morrison Formation consists primarily of red, green or gray Intraformational erosion occurs when a strong current of water, as pro- mudrocks, and lesser occurrences of ribbon-type sandstone beds and thin duced by a local rainstorm, flows overland and erodes through a muddy limestone beds. These are interpreted as stacked overbank floodplain, substrate (Lucchi, 1995). Loose sediment on the surface, along with ripped paleosols, stream channels, and crevasse-splay deposits, as well as wetland up chunks and flakes of the underlying stiffer mud, are incorporated in the and sparse lacustrine carbonate deposits (Peterson, 1994; Demko et al., current and deposited as the flooding event subsides. 2004; Turner and Peterson, 2004). The dominance of non-swelling, illite- rich mudrocks clays in the study area is also consistent with the clay miner- Claystone alogy of the Morrison Formation in northern portions of the Western Inte- Description rior Basin (Turner and Peterson, 2004).

FIGURE 4. a) Photomicrograph of a thinly bedded mudclast conglomerate. Clasts are predominantly rounded mudstone whereas the matrix is texturally similar to the gray-green siltstone; plane-polarized light. b) Outcrop photograph of the thickly bedded and coarser-grained intraformational mudclast conglomerate at the Ziegler Anticline field site. Dashed lines denote the base of horizontally stratified beds exhibiting normal grading to a sandy siltstone. Hammer is 30.5 cm long. 6 The fossil gastropods, Viviparus reesidei, collected in the study tion. Somewhat similar lithologies exist between the study area and area are known only from the Brushy Basin Member of the Morrison the Morrison Formation on the Colorado Plateau. The units in question Formation in central Colorado, southern Wyoming and eastern Utah in the area of Dillon, Montana, however, are most similar to strata of (Evanoff et al., 1998). The occurrence of V. reesidei in southwestern the Morrison Formation in northern Wyoming and south-central Mon- Montana significantly extends the range of this taxon and supports the tana, and thus, should be referred to as the Morrison Formation. contention that the mudrocks near Dillon are Late Jurassic in age. ACKNOWLEDGMENTS CONCLUSION This work is a portion of a Masters Thesis by J. J. Smith con- In the area of Dillon, Montana, interbedded siltstone and calcar- ducted at Georgia State University. Funding was provided by the De- eous mudstone, with minor claystone and conglomerate, outcrop at nearly partment of Geology at GSU. J.J. Smith thanks T. E. La Tour and W. C. the same stratigraphic interval as the Morrison Formation. Illite is the Elliott for guidance and laboratory space and equipment. We thank dominant clay mineral in siltstone and mudstone with chlorite, kaolin- William C. Hood and Fred Peterson for helpful comments and sugges- ite and quartz also present. The formation most likely resulted from the tions that greatly improved the manuscript. We also thank the aggradation of sediments on the floodplains of numerous small stream IchnoBioGeoScience Research Group at the University of Kansas for channels. No evidence was found in this study that precludes these additional improvements to the manuscript. units from being designated as the Upper Jurassic Morrison Forma-

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