Facies and Facies Architecture of Paleocene Floodplain Deposits, Fort ...

Facies and Facies Architecture of Paleocene Floodplain Deposits, Fort Union Formation, Bighorn Basin, Wyoming1 MARY J. KRAUS~AND TINA M. I

Received August 29. 1998. Acapted Docember 23. 1998

2 Depsrmenl of Geolog~cal Sciences. Univers~lyof Colorado. Boulder. CO 80309.0399

.

3 Presenl Address BP Exploralion (Alaska) Inc PO Bax 196812. Anchorage. AK 99519-6612

ABSTRACT Floodplain deposits in the Fort Union Formation along Polecat Bench in the Bighorn Basin, Wyoming can be divided into two interstratified facies, one which is entirely fine-grained and one which is heterolithic consisting of sandstones and fine-grained rocks. Locally, the heterolithic facies is overlain and truncated by laterally extensive sheet sandstones interpreted as the trunk channels of the fluvial system. The fine-grained facies is distinguished by moderately developed paleosols with an organic-rich horizon, which also contains jarosite. The morphology and geochemical properties of the paleosols indicate that they formed in an environment that was waterlogged and strongly reducing. The degree of pedogenic modification indicates that the sediment on which the paleosols formed accumulated relatively slowly and episodically as a result of overbank flooding of the trunk channel. The heterolithic facies consists of ribbon and thin sheet sandstones that are enclosed by fine-grained deposits on which weakly developed paleosols formed. Those paleosols indicate that the heterolithic deposits accumulated rapidly and, thus, had a depositional history distinct from the floodplain deposits on which the moderately developed paleosols formed. The ribbon sandstones are generally less than 3 m thick and have widthlthickness ratios less than 10. Sheet siltstones or sandstones that are thin (el m thick) and commonly bioturbated laterally connect some ribbons. The connected ribbons show that networks of small channels once covered parts of the floodplain. The heterolithic deposits are interpreted as ancient avulsion complexes that formed in response to avulsion of the trunk channel. Although the heterolithic deposits also share similarities with crevasse-splay deposits, they differ from crevasse-splay deposits in their lateral extent and stratigraphic abundance. Their abundance further suggests that avulsion was an important process in constructing Fort Union floodplains. Moreover, avulsion deposits in the Fort Union Formation and in the overlying Willwood Formation both exhibit particular features that can be used to recognize avulsion deposits in other alluvial successions. INTRODUCTION Fine-grained floodplain deposits dominate many alluvial sequences. and a better understanding o f those deposits i s needed to reconstruct the depositional history o f alluvial basins. This paper is part o f a long-term study o f Paleogene floodplain deposits in the Bighorn Basin, Wyoming. Much o f that research has focuscd on characterizing the finc-grained deposits i n the lower Eocene Willwood Formation on the basis o f different kinds

n e Morrnlrarn Geologrsf. Vol 36. NO. 2 (April

o f floodplain paleosols that formed. In particular, lateral and vertical variations in the paleosols have been studled to reconstruct ancient fluvial landscapes and to document and interpret changes in the landscape over time (Bown and Kraus, 1987. 1993; Kraus 1987: Kraus and Aslan, 1993). This study expands upon those earlier studies by turning to floodplain deposits i n the Paleocene Fort Union Formation, which directly underlies the Willwood Formation in the Bighorn Basin. The major objectives o f this research are to describe and

1999).p. 57-70. The R a k y Mountrun Assoc~ationof Geologists © 2005 The Rocky Mountain Association of Geologists

M.J. Kraus and TM. Wells In the Rocky Mountain region commonly contains coals and I~gnites(e.g Flores and Hanley, 1984; Cherven and Jacob, 1985; Fastovsky and McSweeney, 1987), they are absent along Polecat Bench. Organlc-rich shales are present, and these dark gray to black beds are the most striking strata in the study area. Depos~t~on of the Fort Un~onFormation accompanied structural development of the Bighorn Basin and rise of the Bighorn and Beartooth Mountains during the latest Cretaceous-Early Tertiary Laram~deorogeny (Brown, 1993). The northern Blghorn Basin subslded in response to flexural load~ngalong the Beartooth uplift to the west, w ~ t hmaxlmum uplift during Paleocene time (Bown, 1980). The Absaroka Mountains on the southwest side of the basin resulted from volcan~cactivity that post-dated deposition of the Fort Union Format~on (Bown. 1980) Petrographic analysis shows that sandstones In the study areas are 1 1 t h arenites. ~ Chert and sedimentary rock fragments are common, averaging 20% of the framework gram. Granitic fragments are present but not common, averaging less than 3% of the framework grains. Heavy mineral assemblages also show that sedimentary rocks were the major sed~mentsources (Stow, 1952). These data suggest that the mountains surrounding the basin were the major sedlment source; however, older stratigraph~cunits on the mountan flanks prov~dedmuch of the sediment. and the crystalline cores of the ranges had not been s~gnificantlyexposed by Paleocene tlme. T h ~ study s focused on two stratigraphicintervals, each approximately 50 m thick, which are separated by about 300 m of vertlcal section (Fig. 2). Although both study intervals are thin, both are representative of Fort Union strata along Polecat Bench. The older stratigraphic interval 1s exposed towards the east end of Polecat Bench (Study Area I). and the younger interval is exposed at Study Area 11 (Fig. 1). Each stratigraphic interval was described and measured across an area of approximately 1.5 km', which is the maxlmum extent of the exposures. Fort Union rocks In the Bighorn Basin can be subdiv~dedInto b~ostratigraph~c zones on the bas~sof diagnostic mammal fossils (e g., Gmgench, 1983). The older stratigraphic interval has been ass~gnedto the Tiffanian 3 (Ti3) b~ostratigraphx zone and the younger section to the Tiffanian 5 (Ti5) b~ostrat~graphic zone (e.g., Gingerich, 1983; Gingerich, pers. comm., 1996) (Fig. 2). The paleomagnetostratigraph~csection of Butler et al (1981) shows that the older study interval falls into C26R, which totals about 265 m thick in the study area and spanned -3.0 m.y. (Cande and Kent, 1995). The younger stratigraph~cinterval at Study Area I1 falls Into C25R, which 1s 330 m thick (Butler et a1 , 1981) and spanned -1.2 m.y. (Cande and Kent, 1995). From these values, the accumulation rate for the older Fort Union section was sllghtly less than 0.1 m d y r and, for the younger Fort Union section, -0.3 mdyr. Paleobotanical study Indicates that the Tiffanian had a mean annual temperature (MAT) of lOoC and an annual temperature range of 25°C (Hickey. 1980). Mean annual precipitat~onhas not been estimated for the Bighorn Basin during Tiffanian time; however, the organic-rich character of the Fort Union sed~ments is believed to reflect a perennially high water table (Wing et al..

interpret two kinds of floodpiam depos~tsthat can be distinguished in the Fort Union Formation on the basis of differences In lithology and paleosol morphology. A fine-grained fac~eshas moderately developed paleosols whose morphology indicates that they are a kind of hydromorphic paleosol, termed a potentlal a c ~ dsulfate paleosol. This kmd of paleosol is unusual because it has rarely been descnbed from the ancient record. The paleosols provide information about the Paleocene chmate and serve to distinguish the fine-grained facies from the heterolithic fac~eswith whxh they are mterbedded. The heterolithic facles consists of both ribbon sandstones and fine-gramed deposits. We Interpret this faciec to represent avulsion deposits that formed as the trunk channel underwent avulslon. This paper describes the properties of the avulsion deposits, focusmg on the characteristics of the ribbon sandstones and the areal distribut~onand lateral relationsh~psamong the ribbon sandstones within individual heterolithic intervals. Avulsion deposits have also been recognized In the W~llwoodFormation (Kraus and Aslan, 1993; Kraus, 1996, Kraus and Gwmn, 1997). Because the Fort Union and Willwood format~onsformed under different climatic and tectonic condit~ons,we compare and contrast their avulsion deposits to determme what features they have in common that can be used to recognize avulsion deposits in other alluvial successions.

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GEOLOGICAL SETTING This study focused on Fort Union exposures in the northern Bighorn Bas~n,along Polecat Bench (Fig 1) Here, the entire Fort Union Formation 1s exposed and is approximately 1050 m th~ck.Along Polecat Bench, the Fort Un~onFormation 1s characterized by channel sandstones that consist of very fine to coarse sandstone, light ohve gray to olwe gray floodplain mudstones, and fresh-water carbonate lenses (Jepson, 1940: H~ckey,1980; Gingerich 1983). Although the Fort Union Format~onelsewhere

Figure 1. Map of the northern half of the Bighorn Basin, Wyomlng showlng major mountain ranges surrounding the basin and location of Study Areas I and I1 along Polecat Bench. Modified from Clyde, 1997 The Rocky Mountam Assoc~at~on of Geolog~sts

58 © 2005 The Rocky Mountain Association of Geologists

Fort Union Formation, Bighorn Basin, Wyoming

GPTS

North American Land Mammal Polecat Age Bench Wasatchian Clarkforkiar

0Fine-grained deposits in Heterolithic Interval Sandstone in Heterolithic Interval

Figure 2. Diagram showing Early Paleogene magnetostratigraphic correlations of the two study sections (intervals I and 11) in the Fort Union Formation to the Global Polarity Time Scale (GPTS) of Cande and Kent (1995). Correlations for the section on Polecat Bench are from Butler et al. (1981). North American Land Mammal Age (NALMA) correlations are also shown. The Fort Union Formation is overlain by the Willwood Formation, which is primarily of early Eocene age. 1995), indicating humid conditions. As discussed in a later section, features of the floodplain paleosols confirm a humid climate. In both study areas, the Fort Union Formation can be subdivided into three facies (Fig. 3). Thick and laterally extensive sheet sandstones make up about 20% of the succession and are enclosed by and locally truncate floodplain deposits. The floodplain deposits are of two kinds, which alternate vertically and are distinguished by their lithologies and pedogenic properties (Fig. 4). The heterolithic facies consists of sandstone ribbons and thin sheets that are surrounded by fine-grained deposits showing only weak pedogenic modification. The fine-grained facies consists entirely of fine-grained rocks and shows stronger pedogenic development than the heterolithic facies. The following sections describe these facies in greater detail, focusing on the two kinds of floodplain deposits and their depositional significance.

Figure 3. Schematic diagram of the younger stratigraphic interval at Study Area I1 showing the three facies that characterize the Fort Union Formation along Polecat Bench. Major sheet sandstones form the base and top of the interval and locally scour the fine-grained and heterolithic facies. The heterolithic facies consists of fine-grained deposits and ribbon sandstones. Heterolithic deposits are interbedded with fine-grained deposits on which moderately developed paleosols formed. the sheets commonly form ridge tops and have been partly eroded. The one sheet whose total thickness is preserved is 10 m thick. Widths (transverse to paleoflow) range from about 0.8 to 1.2 krn.These too are minimum values because of the nature of the exposures, which dip 1loto the southwest so that individual beds are eroded to the northeast and go under cover to the south-

MAJOR CHANNEL SANDSTONES Each study section has two major sheet sandstones, one at the top and one at the bottom of the section (Fig. 3). The sheets have thicknesses of at least 6 m, which are minimum values because 59

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M. J. Kraus and T.M. Wells

Figure 4. Field view of Study Area I showing the banded nature of the stratigraphic sequence caused by the lighter-colored heterolithic facies (H) alternating vertically with the darker-colored fine-grained facies (F). The fine-grained facies is darker because of higher organic content related to soil formation. Arrow points to standing person for scale. vals are more amenable to paleocurrent analysis than the lower sheets, both of which are strongly contorted. The major sheet in the older stratigraphic interval has large-scale trough cross stratification indicating flow to 348" (n = 33), while the major sheet in the younger interval shows flow to 300" (n= 24). Despite the fact that lateral accretion deposits are not present, or at least not obviously preserved, the major sheet sandstones are interpreted as the channel deposits of sinuous, mixed-load rivers. The abundant fine-grained deposits surrounding the sheet sandstones indicate that the channels were carrying a large suspended load, and sinuosity tends to increase with higher proportions of suspended load (Schumm and Khan, 1972). WIT ratios are greater than 50, which is characteristic for the sands or sandstone bodies produced by meandering systems as compared to anastomosed systems (e.g., Tornqvist, 1993). The preserved thickness of individual stories suggests that the rivers were at least 3 m deep.

west. Although exposures are three-dimensional, beds cannot be traced more than approximately 1 km in a direction either perpendicular to paleoflow or parallel to paleoflow. Widthlthickness (WIT) ratios calculated for these sands range from 75 to 200. Basal contacts are irregular, erosive surfaces overlain by pebble-sized to cobble-sized gray mudclasts and organic-rich mudclasts, both derived from the surrounding floodplain deposits. The sheets are multistoried with two to three stories separated by bounding surfaces that are also delimited by gray mudclasts. Maximum preserved story thickness is about 3 m. Grain size ranges from very fine to coarse sand, but most is very fine to medium sand. Internally, sheet sandstones are dominated by massive bedding and medium (10-20 cm) to large-scale (21- 100 cm) trough cross-stratified sandstone. The massive bedding appears to be the result of intense soft sediment deformation. In fact, the basal sheet sandstone in Study Area I1 underwent such extensive soft sediment deformation that determining paleoflow was difficult. Parallel stratification with primary current lineation and, less commonly, small-scale cross stratification are present toward the tops of the sandstone bodies. The sheets are locally casehardened, which is a surficial weathering feature of sandstones (Conca and Rossman, 1982). Casehardened parts of the sheet often exhibit trough cross-stratification and horizontal stratification. Lateral accretion sets were not evident in the major sheets; however, the abundant massive and/or contorted bedding may obscure or have destroyed them. The upper sheets in the stratigraphic interThe Rocky Mountain Association of Geologists

FINE-GRAINED FLOODPLAIN DEPOSITS AND MODERATELY DEVELOPED PALEOSOLS Description Quantitative grain size analyses show that the fine-grained facies consists of a nearly even mix of mudstones (clay content of 33-67%) and claystones (> 67% clay). XRD analysis shows that 60

© 2005 The Rocky Mountain Association of Geologists

Fort Uniotr Fortnariotr, Bighortr Basin, Wvornirig illite and smectlte, lncludlng Interstratified U S . domlnate the clay mlneralogy. The mudstones and claystones show color bandlng, abundant roots and slickensldes, and Intense mottling. whlch indlcate that these are moderately developed paleosols. The degree of pedogenic development suggests that the paleosols formed on overbank deposlts sufficiently far from the active channel and that aggradation was gradual. Two basic kinds of carbonaceous paleosols were recognized on the bass of total organic carbon (TOC) values in the profiles. Type I paleosols commonly have a single TOC maximum in the middle of the profile (Fig. 5). In all of the paleosols analyzed, the

TOC wt.% IS at least 2% and can be as high as 1 1 %. At least one dark brown or black bed in the middle interval contains woody material (roots and stems) and leaves along bedding planes. whlch Imparts a laminated appearance to the beds. Yellow (5 Y 716) mottles and powdery nodules, which X-ray microprobe analysls shows to be jarosite, are associated w~ththe organlc material (Fig. 6). The middle part of a profile is darker colored (values of 2 to 4) and has higher clay content than either the upper or lower parts of the profile. The upper and lower parts of the profile generally consist of mudstones with hght colors (values of 5 to 7) that range from yellowish gray and pale olive to brown.

Clay Wt%

Clay Wt% TOC Wt% 40

0

90

TOC Wt%

20 60 100

50 1

5 P

6

4I

100..

1

120.. b

&

-

180-

200..

Mudrock Colors & Sandstones

-

Olive grays (5Y312.4H) Lt. olive gray & grayish olive (5Y512 & 611, 1OY412) Yellowish gray & pale olive (5Y 712, 1OY 612) Pale brown (5 YR 512) & pale yellowish brown (10 YR 612) Browns (5YR411; 5YR 312; 10YR412)

... . . ..

Sandstones

SVMsoLs

4 roots

5 slickensides

@ yellow-brown mottles J jarosite G gypsum

*

fossil leaves on bedding planes

Figure 5. Examples of the type 1 and type 2 paleosol profiles showlng their morphology. clay content. and total organlc content Bg = gleyed B horizon: Gr = permanently reduced horizon: 2C = C honzon developed on different parent matenal than higher In the profile. 61 © 2005 The Rocky Mountain Association of Geologists

The Rocky Mountam Assocmon of Geolog~srs

M. J. Kraus and T M . Wells dries, the mobilized iron can re-precipitate in more oxidized areas as redox concentrations. Although this process, termed gleying, can selectively remove iron from one soil horizon and concentrate the iron in a different soil horizon, redox depletions and concentrations commonly co-exist within a single soil horizon. Water can move more easily along root traces and the organic material promotes reduction and iron removal. A few mm away from the root, that iron may precipitate because conditions are more oxidizing. Jarosite, which is present in some beds, generally forms as an oxidation product of pyrite; its presence indicates that the soils originally contained pyrite (e.g., van Breemen, 1973, 1982; Miedema et al., 1974). Pyrite, which precipitates under saturated conditions in the presence of organic matter, and the other redoximorphic features are consistent with very poorly drained soil conditions. Pyrite is typical of modem "potential acid sulfate soils," so called because, if the soils are later drained, the pyrite oxidizes and produces acidic conditions in which acid sulfate soils can then form (Brinkman and Pons, 1973; Dent, 1986). Acid sulfate soils are characterized by yellow mottles of jarosite, which forms from the pyrite. The presence of jarosite in the Fort Union paleosols suggests that they originally formed as potential acid sulfate soils, and that, at some time in their development, the pyrite was oxidized to jarosite. In Soil Taxonomy (Soil Survey Staff, 1975), clayey soils with jarosite are considered Sulfaquepts, and clayey potential acid sulfate soils like those in the Fort Union Formation are approximately equivalent to Sulfic Haplaquents. As for individual soil profiles, the dark claystones with jarosite were probably permanently reduced, and they are interpreted as Gr horizons, where the G indicates a gleyed horizon and the subscript indicates permanently reduced conditions (e.g., Dent, 1986). Gr horizons form in modern hydromorphic or waterlogged soils and generally have abundant and only partly decomposed organic matter because of the reduced conditions. Furthermore, pyrite commonly precipitates in Gr horizons. The overlying, lighter-colored mudstone lacks jarosite and is interpreted as an ancient Bg (gleyed B) horizon or A/Bg sequence (Fig. 5). Although indirect, the evidence suggests that acidification and jarosite formation may be relatively recent processes. First, for jarosite to form in the Paleocene, the sulphidic Gr horizons must have been drained. Because the Fort Union Formation was actively aggrading, it is more probable that the soils were buried by more sediment and that the waterlogged soils were pushed even farther below the groundwater table. With Late Cenozoic erosion to form the Bighorn Basin, the Fort Union Formation was exposed, and it was probably then that the pyrite was oxidized and j arosite formed. This hypothesis is strengthened by studies of lignitic parts of the Eocene Wilcox Group in Texas (Dixon et al., 1982) and soils forming on pyrite-bearing marine shales (e.g., Carson et al., 1982; Ross et al., 1982). These studies show that acidification occurs in the modern weathering zone because the pyrite becomes exposed and oxidized. Moreover, Dixon et al. found that, in the Wilcox Group, jarosite formation due to modern weathering can extend as deep as 8 to 20 m

Figure 6. Photomicrograph of a jarosite nodule (J) associated with shiny, black organic matter in mudstone matrix. Gr horizon of a Type 1 paleosol. Frame length is 2.5 rnrn; reflected light. Type 2 paleosols show two TOC maxima, one in the upper part of a profile and a second in the middle part of the profile, and those TOC values are generally only 0.5 to 1.0% (Fig. 5). Consistent with the TOC maximum, the upper part of the profile has dark colors. Like Type 1 paleosols, the middle part of Type 2 paleosols has a bed or beds with organic material along bedding planes and jarosite associated with the organic matter. As in Type 1 paleosols, the clay content decreases and colors become lighter in the lower part of the profile, which represents the C horizon. Redoximorphic features, which result from alternating oxidizing and reducing soil conditions (Vepraskas, 1994) characterize both kinds of paleosols in the field and in thin section. The redoximorphic features include the low chromas (generally 1 or 2) of the matrix colors, iron depletions, and iron concentrations. Iron depletions are commonly present as gray areas in the olive and brown matrix. They are irregular in shape to elongate and branching. In thin section, the boundaries are diffuse, and some gray areas contain small, black organic fragments. Many, especially the branching and/or elongate examples, are probably root channels. Because of the presence of organic material, such features are especially prone to the reduction and removal of iron oxides, which causes the gray color (e.g. Schwertmann, 1993). Yellowbrown halos around gray root channels show that iron was removed from the gray areas and then concentrated in better-oxidized areas farther from the root. In some thin sections, these pore linings are heavily impregnated with goethite. A second kind of iron concentration is yellow-brown (goethite) mottles, which are relatively common throughout the matrix of the paleosols (Fig. 5).

Paleosol Classification Redoximorphic features are characteristic of soils that are episodically waterlogged (e.g., Duchaufour, 1982; Vepraskas, 1994). When the soil is wet, iron is reduced and mobilized to produce local redox depletions. As water table levels fall and the soil The Rocky Mountain Association of Geologists

62 © 2005 The Rocky Mountain Association of Geologists

especially its coarser grain size, indicate that it 1s probably geneucally related to the heterolithic facies depos~tedabove the soil.

Environmental Significance The incomplete deconipositlon of plant litter. Iron reduction. and pyrite precipitation indicate that the Fort Unlon paleosols were waterlogged most of the year. Plant remans that could be ~dentifiedfroni organic debris in the paleosols are from conifers. primarily Glyprosrmbus. Glyptosrrobus,a swamp cypress, grows today on wet soils that experience frequent flooding (Henry and McIntyre. 1926). The dominance of gray, jarosite-beanng paleosols in the study sections, and throughout the Fort Union Formation along Polecat Bench, and the paleobotanical Information indicate that gleying was primarily the result of a wet climate rather than merely local drainage conditions. In addition to their climatic significance, the paleosols provide information on the chemistry of groundwaters during Paleocene time. The presence of acid sulfate soils in the Fort Union Formation is unusual because most modern solls of this klnd form in coastal areas, where sulfate is available. The Fort Union Formation formed in a freshwater environment, and the presence of latest Cretaceous fluvial rocks below the Fort Union Formation indicates that the Cretaceous Seaway had retreated from the basin long before the Tiffanm part of the Fort Unlon Forniat~onwas deposited (Bown, 1980). The few examples of modern sulphidic soils that have been described from inland areas developed because they drain sedimentary rocks contain~ngsulfide minerals (Chenery, 1954; Poelman, 1973; Dent, 1986). The Mesozoic strat~graphlcsuccession is a conceivable source of sulfate in the Paleocene groundwaters. First, thm-section study of the sandstones suggests that Mesozoic sedimentary rocks were a source for the Fort Union Formation. Glauconite grams are present in all samples, and they are probably derived from the Jurassic Sundance Formation, which 1s nch in glauconite (Downs, 1952).The abundant chert grains (averaging 8.5% of the framework grains) could have been derived from the lowermost Cretaceous Cloverly Formation (Moberly, 1962), although other potentla1 sources include the Paleozoic Madison L~mestoneand Amsden Formation (Sando, 1975). Second, the Mesozoic interval contains Cretaceous marine shales including thick intervals of organic-rich gray to black shales (Eicher, 1962; Keefer, 1972).

HETEROLITHIC FLOODPLAIN DEPOSITS Thickness The thickness of individual heterolithlc deposits can only be approximated because pedogenic modification has obscured the contacts with the moderately developed paleosols that overlie and underhe the heterolithic mtervals (Fig. 7). Furthennore, the bases of the heterollthic intervals show signs of erosion only where rlbbon sandstones locally scour into underlying nioderately developed paleosols. Estimated thicknesses for heterolithic intervals range from I to 5 m (n = 8) in the older study section and froni 1 to 4 ni (n = I I) in the younger study section. Individual intervals can be traced across the extent of the exposure. or approximately I km both perpendicular and puuallel to the paleoslope, before being lost to cover or erosion. Fine-Grained Deposits and Weakly Developed Paleosols

In contrast to the dark colors typical of some horizons in the potential acid sulfate paleosols described above, the fine-grained deposlts in the heterollthic intervals are ~nvariablyllght colored, commonly yellow gray (5Y 611 and 712) and pale ohve (IOY 612) (Fig. 7). Each bed may or may not have rellct stratification. Many beds show evidence for soil modification, including mottles, root traces and burrows, and sparse sl~ckensides(Fig. 7); however, such features are weakly developed. The incipient nature of these features and the presence of relict bedding show that soil profiles were weakly expressed. Horizon development is poor; in fact, recognizing individual profiles is diflicult. Where they can be delimited, profiles are thin (