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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.

STRATIGRAPHY AND SEDIMENTOLOGY OF THE UPPER JURASSIC MORRISONFORMATION, 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, jjsmith@ku.edu2Department of Geology and Natural History Museumand Biodiversity Research Center, University of Kansas, 120 Lindley Hall, Lawrence, KS 66045-7613, hasiotis@ku.edu 3Department of Geology, Georgia State

University, 340 Kell Hall, Atlanta, GA 30303, wfritz@gsu.edu

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 asto whether this unit actually is the Morrison Formation, or whether it is similarly variegated mudrocks of otherformations 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 fourlithofacies: 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 carbonaterhizoliths and rhizocretions. Dinosaur-bone fragments and the Late Jurassic gastropod Viviparus reesidei were alsopresent. The stratigraphy, sedimentology and paleontology compare best with the Morrison Formation of northernWyoming and south-central Montana.

INTRODUCTION

This paper describes the stratigraphy, sedimentology, and clay min-eralogy of relatively thinly-bedded siltstones and mudstones near Dillon,Montana, that have been interpreted traditionally as the Upper JurassicMorrison Formation. Questions exist, however, as to whether this unit isactually the Morrison Formation or whether it is a multicolored unit similarin appearance and stratigraphic position to the Morrison. The strata in ques-tion have been mapped as Morrison Formation on published geologic mapsof the Dillon region (Brandon, 1985; Ruppel et al., 1993), though usuallyfollowed by a question mark, or the unit has been mapped as undifferenti-ated from the Lower Triassic Dinwoody Formation.

Few studies have been conducted on the Mesozoic system nearDillon. Scholten et al. (1955) described 40- to 100-m-thick exposures ofthe Morrison Formation in the southern Tendoy Range about 30 km southof Dillon, Montana. The Morrison Formation, however, thins rapidly tothe north and is absent in the northern part of the Tendoy Range and be-yond (Scholten et al., 1955). Suttner (1969) examined exposures of theUpper Jurassic Morrison and Lower Cretaceous Kootenai Formations insouthwestern Montana. While one of Suttner’s (1969) nineteen study sec-tions was near Dillon (Birch Creek), he provided no detailed informationon individual sections.

Many university field camps visit Dillon each summer to map Pa-leozoic and Mesozoic rocks in the area. The Upper Jurassic Morrison For-mation is often mapped by students, but is rarely seen in the field becauseof poor exposures. This study focused on the few well-exposed sectionsand compares the rock units near Dillon with the Upper Jurassic MorrisonFormation elsewhere in the northern portion of the Western Interior Basin.

GEOLOGIC SETTING

The Morrison Formation is the most extensive continental unit inNorth America (Frazier and Schwimmer, 1987), it forms an alluvial wedgeover 300 m thick in some areas, and covers nearly the entire Western Inte-rior Basin. In the northern part of this basin, the formation is predomi-nantly composed of varicolored mudrocks that range from purple and redto green and gray, with local deposits of limestone and cross-bedded sand-stone. The Morrison represents a mosaic of predominantly terrestrial depo-sitional settings in a uniquely large system of alluvial plains (Dodson et al.,

1980; Peterson, 1994; Demko et al., 2004; Turner and Peterson, 2004).Sedimentologic, paleopedologic, and paleobotanic studies indicate

a strongly seasonal to monsoonal paleoclimate during deposition (Demkoand Parrish, 1998; Rees et al., 2004). Paleosols and trace fossils of soilorganisms are common throughout Morrison strata, indicating extendedperiods of long-term subaerial exposure, relatively slow rates of deposi-tion, and pedogenesis (Hasiotis, 1999; Demko et al., 2004; Hasiotis, 2004).Black mudstone, carbonaceous mudstone, and coal in central Montanasuggest wetter and less seasonal conditions at or near the top of the forma-tion in this portion of the Western Interior Basin (Demko et al., 2004).

The Morrison Formation on the Colorado Plateau can be dividedinto lower and upper parts based on a significant change in the clay miner-alogy (Turner and Fishman, 1991). Clay minerals in the lower part of theformation consist mostly of non-swelling types, whereas the upper part isdominated by the presence of swelling smectitic clays, including thin ben-tonite beds derived from the alteration of volcanic tuffs. This vertical changecan be traced as far north as northern Wyoming, but it is not found inMontana where non-swelling clays are present .

In the Dillon area, the Paleozoic and Mesozoic sedimentary unitsare folded against the Pioneer Batholith in a series of north-northeast-trend-ing, plunging anticlines and synclines produced during the Laramide orog-eny (Suttner et al., 1981; Sears et al., 1989). The mudrocks interpreted asthe Morrison Formation are bounded unconformably below by the LowerTriassic Dinwoody Formation and above by the Lower Cretaceous KootenaiFormation. The Dinwoody Formation is a marine, mixed carbonate-siliciclastic shelf-margin sequence 100 to 250 m thick in southwesternMontana (Ruppel et al., 1993; Boyer et al., 2004). The Lower CretaceousKootenai Formation is a 200- to 400-m-thick continental sequence com-posed of mudstone, siltstone, and sandstone (Ruppel et al., 1993). TheKootenai Formation is equivalent to the Cloverly Formation of northernWyoming and south-central Montana, and both contain a basal quartziteand red-chert-cobble conglomerate that unconformably overlies theMorrison Formation (De Celles, 1986).

METHODS

Five field sites in the Dillon area were selected for this study(Fig. 1), and detailed measured sections were produced from those ar-eas with the best exposures. These included the Birch Creek, Dutch-

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men Spring, and Ziegler Anticline field sites (Fig. 2). Lithologic de-scriptions consist of unit thickness, grain size, color, sedimentary struc-tures, pedogenic features, and body and trace fossils. Specimens werecollected from the measured sections at 1-m intervals for thin sectionand X-ray diffraction (XRD) analyses. Thin sections were examinedusing a transmitted light microscope.

The clay mineralogy of siltstone and mudstone samples was deter-mined with powder XRD using sample preparation techniques describedby Moore and Reynolds (1997). The less than 0.2 ì m clay fraction wasseparated by centrifuge and mounted on glass slides. Three oriented slideswere produced per rock sample; one was heated to 550°C for one hour,one was placed overnight in a closed desiccator over an ethylene glycolbath, and one was left untreated. Each slide was then analyzed using thePhillips Model 12045 X-ray diffractometer equipped with a MDI Databoxat Georgia State University, Atlanta. Clay-mineral abundances were esti-mated from ratios of various peaks on diffractograms, using a semi-quanti-tative method developed by Biscaye (1965).

SEDIMENTOLOGY

Rocks interpreted as the Morrison Formation in the Dillon area arecomposed of four major elements: 1) gray-green sandy siltstone, 2) reddishbrown, calcareous mudstone on which moderately to well-developedpaleosols formed, 3) mudclast conglomerate, and 4) thinly bedded, blackclaystone (Table 1). Although the stratigraphic position of the lithologiesvaries from site to site, such characteristics as color, sedimentary struc-tures, pedogenic features, and fossil content remain relatively consistent.

Sandy Siltstone

Description

Sheet-like beds of sandy siltstone comprise most of the formation inthe study area. Sandy siltstones are predominantly gray-green and gener-ally lack well-defined sedimentary structures. The sand-sized fraction iscomposed of angular to subrounded quartz, chert, and lithic fragment grains.Mudstone clasts, or mudstone-clast conglomerate beds, are locally presentat the base of this unit where it overlies calcareous mudstone. The less than0.2 µm clay fraction of the sandy siltstone is composed of illite (86%),kaolinite (10%), chlorite (3%) and quartz (1%). Trace amounts of smectitewere detected as mixed-layered illite-smectite based on intensity ratio cal-culations (Srodon, 1984).

Sets of parallel to wavy laminae, up to 1 mm thick, are locally presentat the base of siltstone beds (Fig. 3a). Laminae are also present adjacent tolenticular beds of mudclast conglomerate. Black siliceous concretions are

common in siltstone beds as individuals or in clusters oriented along thebedding plane. Most concretions are oblate and less than 2 cm in diameter,although some are elongate and up to 6 cm long. Carbonate rhizocretionsare present in a green siltstone unit at one site, but are much more commonin red calcareous mudstone.

Facies Interpretation

The siltstone facies is interpreted as overbank-flood deposits,with some crevasse-splay deposits, based on the sheet-like geometry,relatively coarse-grain sizes, wavy laminae, and general lack of pe-dogenic features. Mudrock clasts at the base of siltstone beds are inter-preted as rip-up clasts produced by scouring of the floodplain surface.Low chroma matrix colors suggest generally reducing conditions eitherfrom high water tables, poorly drained conditions on the floodplain, ora high organic content of the original units (Kraus, 1996; Vepraskas,1999). The absence of mature pedogenic features implies relativelyhigh rates of sedimentation. Rare carbonate rhizocretions and a paucityof primary sedimentary structures throughout, however, suggest incipi-ent soil formation in these deposits.

Calcareous Mudstone

Description

The second most common lithology in the study areas is brown-ish red to purple calcareous mudstone (Fig. 3b). Lenticular beds of thislithology are locally present within gray-green siltstone beds. The mud-stone is mottled grey-green and contains carbonate nodules as well asclay slickensides. Mottles are abundant locally in association withrhizoliths and spherical to elongate carbonate nodules. Fine-grainedquartz and chert, as well as round, medium-grained globules of clayoften show hematite-stained halos in thin section. The less than 0.2 µmclay fraction of the mudstone is composed of illite (50%), kaolinite(34%), and chlorite (16%). Quartz is also present in very minor amountsin the less than 0.2 µm fraction but smectite was not detected in any ofthe samples analyzed.

Paleontology

Cylindrical, branching, and downward tapering tubules, inter-preted as rhizoliths, are common throughout the mudstone facies. The

FIGURE 1. Map with locations of the five field sites in the study area: 1) FryingPan Gulch, 2) Dutchman Spring, 3) Birch Creek, 4) Sandy Hollow, and 5) ZieglerAnticline.

FIGURE 2. Measured sections produced from the three best exposures of MorrisonFormation in the study area and keyed to localities in Figure 1: 2) Dutchman Spring,3) Birch Creek, and 5) Ziegler Anticline.

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rhizoliths are cast in powdery to solid carbonate and range from lessthan 1 mm to 3 mm in diameter. Fine calcite-filled rhizoliths are abun-dant in thin sections of this unit (Fig. 3c).

Stacked, spherical carbonate-nodules, successively smaller in di-ameter with depth, are interpreted as rhizocretions (Fig. 3d). Therhizocretions are concentrated at the tops of a series of red and purplemudstone beds at the Dutchman Spring field site (Fig. 3b). Nodularsegments of the rhizocretions range from 10 mm to as much as 55 mmin diameter. The rhizocretions are up to 20 cm long and branching ispresent in some specimens. Carbonate nodules, common to the mud-stone, may be disarticulated segments of rhizocretions.

In thin section, faint and indistinct clay-filled tubules from 0.5mm to 1 mm in diameter are interpreted as invertebrate burrows (Fig.3e). Most occur as diffuse to sharp-edged, circular to elongate mottleswith no distinct internal or surficial morphology. A few contain backfillthat is slightly coarser grained than the surrounding matrix.

Fossils of gastropods and bivalves are common in several mudstoneintervals (Fig. 3f). The gastropods are identified as Viviparus reesidei basedon the morphology of known Jurassic species from the Colorado Plateau(Yen, 1950; Evanoff et al., 1998). Extant members of this group are proso-branchs or gill-breathing snails that live in well-oxygenated, quiet waterhabitats of lacustrine and fluvial environments (Evanoff et al., 1998). Frag-mentary bivalve fossils are less common, but could not be assigned to aspecific taxon.

The mudstone contains well-rounded to rounded, polished, 2- to3-cm-diameter quartzite clasts. The quartzite clasts were present mostly intalus on the weathered surface of the mudstone, though a few were ob-served in situ, indicating that they originate from this unit. Such polished

clasts are referred to generally as gastroliths or stomach stones if found inclose association with dinosaur fossils, particularly the ribcage of an articu-lated skeleton (e.g., Gillette, 1991). While this explanation seems plausible,it is currently not possible to differentiate gastroliths from other roundedclasts. Bone fragments are also present in the mudstone, though rare, butcould not be assigned to specific taxa.

Facies Interpretation

The mudstone facies is interpreted as fine-grained, distal overbankdeposits on which moderately to well-developed soils formed. Evidencefor paleopedogenesis includes red and purple matrix colors, gray-greenmottles, carbonate nodules, clay slickensides, abundant carbonate rhizolithsand rhizocretions, and burrows.

Red, purple or gray colors indicate different concentrations of Feand Mn oxide. These differences are likely due to varying redoximorphicconditions and local organic matter concentrations in the original soil pro-files (e.g., Bigham et al., 1978; Torrent et al., 1980; Schwertmann, 1993;Vepraskas, 1999). Red colors result from a mix of hematite (Fe2O3) andgoethite (FeO(OH)), and indicate moderately well-drained and oxidizingsoil conditions (e.g., Schwertmann and Taylor, 1977; Cornell andSchwertmann, 1996; Scheinost and Schwertmann, 1999). Purple colorsare characterized by more widely dispersed hematite and suggest less well-drained conditions (e.g., McBride, 1974; Blodgett, 1988; Wright et al.,2000). Gray colors generally indicate the absence of Fe-oxide minerals (e.g.,PiPujol and Buurman, 1994; Vepraskas, 1999). Most of the gray-greenmottles are probably rhizoliths and burrows that underwent preferentialgleying—local reduction and oxidation of Fe and Mn—due to the presenceof organic matter in these structures (Schwertmann, 1993; Kraus and

Poorly drained floodplaindeposits showing weakpedgogenic development,crevasse-splays

Moderately- to well-developed paleosolsforming on distalfloodplain deposits

Channeled high-energyflood and erosionaloverbank scour deposits

Backswamp to ephemeralpond deposits

Siltstone

Mudstone

Conglomerate

Claystone

Gray-green tobrown-green

Red, brownishred, purple

Brownish gray,green, pinkishbrown to red

Black, darkbrown, tan togray

Siltstone to sandysiltstone

Calcareous mudstoneto sandy mudstone

Mudclast conglomer-ate in a sandysiltstone matrix

Claystone

Lacks well-definedmacro-bedding; finewavy laminae; blackconcretions

Some lenticular beds,but otherwise lackswell-defined macro-bedding; mottles,carbonate nodules,clay slickensides

Lenticular, matrixsupported, unstratifiedand ungraded thinbeds; clast supported,stratified and normallygraded thicker beds

Lenticular andmassively bedded;iron-segregationfeatures common

Rare carbonaterhizocretions; rareGastropoda and fossilbone fragments

Carbonate rhizoliths andrhizocretions; gastro-pods and burrows; fewbone fragments

Few dinosaur bonefragments

None observed

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

Lithofacies Colors Lithology Sedimentary Structures Fossils Interpretation

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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 andrmd2), 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 ofrmd2. c) Photomicrograph of the mudstone facies showing numerous fine, calcite-filled rhizoliths and hematite staining; plane-polarized light. d) Outcrop photograph ofcarbonate 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 sectionshowing numerous carbonate-filled rhizoliths and invertebrate burrows; plane-polarized light. White arrow indicates the most obvious burrow, but faint circular to burrowoutlines 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.

5Hasiotis, 2006). Modern soils with similar features as found in themudstone facies are waterlogged for several months of the year fol-lowed by longer periods of better drainage and lower water tables(Bigham et al., 1978; Torrent et al., 1980; Farrell, 1987; Vepraskas,1999). Calcite accumulations in and around the ancient plant roots alsosuggest seasonal fluctuations of soil moisture (Klappa, 1980; Hasiotis,2004; Kraus and Hasiotis, 2006).

Mudclast Conglomerates

Description

Thin to thick lenses of mudclast conglomerate are present in gray-green siltstone near the top of several sections. Thin, unstratified, ungradedconglomerate lenses are up to 0.6 m thick and generally less than 3 mwide. These consist of rounded mudstone clasts (95%) and angular chertclasts (4%) and lithic fragments (1%) in a gray-green, sandy siltstone ma-trix. The clasts are matrix supported and range from less than 1 mm to 10mm in diameter. In thin section, the mudclasts are texturally similar to thecalcareous mudstone, whereas the matrix is compositionally identical tothe surrounding gray-green siltstone (Fig. 4a).

A thick conglomerate lens with a distinctly concave base is locatednear the top of the Ziegler Anticline section. This unit is approximately 2 mat its thickest and about 14 m wide. The conglomerate is composed of astacked series of 15- to 30-cm-thick beds. Individual beds contain matrix-supported, cobble-size mudclasts in a siltstone matrix, and the beds fineupward to sandy siltstone (Fig. 4b). The clasts are rounded to subrounded,1 to 60 mm in diameter, and are composed of mudstone (90%) and car-bonate (10%). The conglomerate also contains lesser amounts of coarse-grained chert, lithic fragments, bone fragments, and polished quartzitepebbles similar to those found in calcareous mudstone units.

Facies Interpretation

The mudclast conglomerate is interpreted as rip-up clasts scouredfrom the local floodplain surface and deposited in shallow fluvial channels.Intraformational erosion occurs when a strong current of water, as pro-duced by a local rainstorm, flows overland and erodes through a muddysubstrate (Lucchi, 1995). Loose sediment on the surface, along with rippedup chunks and flakes of the underlying stiffer mud, are incorporated in thecurrent and deposited as the flooding event subsides.

Claystone

Description

Thin beds of dark gray to black siliceous claystone up to 2 m thickare present at the Ziegler Anticline and Frying Pan Gulch sites. In out-crop the claystone appears massive and devoid of sedimentary struc-tures. In thin section, the claystone contains a faint blocky texture andgray-green reduction halos around dispersed, very fine grains of quartz,chert, and hematite. The less than 0.2 µm clay fraction is composed ofillite (87%), kaolinite (9%), and chlorite (4%). Quartz is also presentin very minor amounts in the less than 0.2 µm fraction, but smectitewas not detected in any of the samples analyzed.

Facies Interpretation

The claystone is a minor component of the study area and isinterpreted as poorly drained back-swamp or shallow pond deposits.The blocky texture observed in thin section may indicate incipientpaleopedogenesis in these deposits, though other evidence of soil de-velopment was not observed.

DISCUSSION

The mudrocks near Dillon are interpreted as the product of dis-tal floodplain deposition in a mud-dominated fluvial system. The dis-tal interpretation is suggested by the absence of well-defined, sand-stone-filled channel deposits. Mud-dominated deposits in modern flu-vial systems are typically associated with low-gradient, slow flowing,suspended-load meandering rivers (Miall, 1992). Cooley and Schmitt(1998) proposed that an anastomosing fluvial system deposited mudsof the Morrison Formation in the Gallatin and Beartooth Mountainranges of south-central Montana east of our study area. How that flu-vial system relates to the fluvial system in southwest Montana remainsunclear.

The stratigraphy, sedimentology and especially the paleontology ofmudrocks in the study area suggest correlation with the Morrison Forma-tion farther east and southeast in northern Wyoming and central Montana.There, the Morrison Formation consists primarily of red, green or graymudrocks, and lesser occurrences of ribbon-type sandstone beds and thinlimestone beds. These are interpreted as stacked overbank floodplain,paleosols, stream channels, and crevasse-splay deposits, as well as wetlandand sparse lacustrine carbonate deposits (Peterson, 1994; Demko et al.,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-alogy of the Morrison Formation in northern portions of the Western Inte-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 thegray-green siltstone; plane-polarized light. b) Outcrop photograph of the thickly bedded and coarser-grained intraformational mudclast conglomerate at the Ziegler Anticlinefield site. Dashed lines denote the base of horizontally stratified beds exhibiting normal grading to a sandy siltstone. Hammer is 30.5 cm long.

6The fossil gastropods, Viviparus reesidei, collected in the study

area are known only from the Brushy Basin Member of the MorrisonFormation in central Colorado, southern Wyoming and eastern Utah(Evanoff et al., 1998). The occurrence of V. reesidei in southwesternMontana significantly extends the range of this taxon and supports thecontention that the mudrocks near Dillon are Late Jurassic in age.

CONCLUSION

In the area of Dillon, Montana, interbedded siltstone and calcar-eous mudstone, with minor claystone and conglomerate, outcrop at nearlythe same stratigraphic interval as the Morrison Formation. Illite is thedominant clay mineral in siltstone and mudstone with chlorite, kaolin-ite and quartz also present. The formation most likely resulted from theaggradation of sediments on the floodplains of numerous small streamchannels. No evidence was found in this study that precludes theseunits from being designated as the Upper Jurassic Morrison Forma-

tion. Somewhat similar lithologies exist between the study area andthe Morrison Formation on the Colorado Plateau. The units in questionin the area of Dillon, Montana, however, are most similar to strata ofthe Morrison Formation in northern Wyoming and south-central Mon-tana, and thus, should be referred to as the Morrison Formation.

ACKNOWLEDGMENTS

This work is a portion of a Masters Thesis by J. J. Smith con-ducted at Georgia State University. Funding was provided by the De-partment of Geology at GSU. J.J. Smith thanks T. E. La Tour and W. C.Elliott for guidance and laboratory space and equipment. We thankWilliam C. Hood and Fred Peterson for helpful comments and sugges-tions that greatly improved the manuscript. We also thank theIchnoBioGeoScience Research Group at the University of Kansas foradditional improvements to the manuscript.

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