The Yampa Bed—A Regionally Extensive Tonstein in the Williams

The Yampa Bed—A Regionally Extensive Tonstein in the Williams Fork Formation, Northwestern Piceance Creek and Southern Sand Wash Basins, Colorado Contributions to Stratigraphy

Scientific Investigations Report 2008–5033

U.S. Department of the Interior U.S. Geological Survey Cover. Excavated exposure of the Yampa Bed at the type locality, south of Craig, Colo. Yampa Bed is about 38 in. thick. Note 24-in. shovel for scale. The Yampa Bed—A Regionally Extensive Tonstein in the Williams Fork Formation, Northwestern Piceance Creek and Southern Sand Wash Basins, Colorado

By Michael E. Brownfield and Edward A. Johnson

Contributions to Stratigraphy

Scientific Investigations Report 2008–5033

U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior DIRK KEMPTHORNE, Secretary U.S. Geological Survey Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2008

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Suggested citation: Brownfield, M.E., and Johnson, E.A., 2008, The Yampa Bed—A regionally extensive tonstein in the Williams Fork Formation, northwestern Piceance Creek and southern Sand Wash Basins: U.S. Geological Survey Scientific Investigations Report 2008–5033, 32 p. iii

Contents

Abstract...... 1 Introduction...... 1 Geologic Setting...... 4 Cretaceous Paleogeography...... 4 Stratigraphy of Upper Cretaceous Rocks in Northwestern Colorado...... 7 Mesaverde Group in the Yampa Coal Field...... 7 Detailed Stratigraphy of Coal-Bearing Upper Cretaceous Rocks...... 7 Iles Formation...... 7 Tow Creek Sandstone Member...... 12 Double-Ledge Sandstone...... 12 Trout Creek Sandstone Member...... 12 Williams Fork Formation...... 12 Sub-Twentymile Sandstone...... 12 Twentymile Sandstone Member...... 13 Big White Sandstone...... 13 Three White Sandstones...... 13 Depositional Setting of the Mesaverde Group...... 13 Origin, Description, and Distribution of the Yampa Bed...... 14 Yampa Coal Field...... 17 Eckman Park Measured Section...... 17 Fish Creek Canyon Measured Section...... 18 Ute Gulch Measured Section...... 19 Eagle Mine Measured Section...... 19 Lay, Colo., Measured Section...... 19 Danforth Hills Coal Field...... 19 Piceance and Sand Wash Basins...... 19 Geophysical Log Response...... 23 Yampa Bed as a Correlation Tool...... 26 Summary...... 30 Acknowledgments...... 30 References Cited...... 30

Plates

1. Subsurface Correlations of Coal and Related Rocks of the Upper Cretaceous Iles and Williams Fork Formations in the Yampa Coal Field, Northeastern Colorado A. Coal correlations of the middle coal group along cross section A-A’ B. Coal correlations of the middle coal group along cross section B-B’ C. Lithologic interpretation of drill hole H–36–H iv

2. Subsurface Correlations of Coal and Related Rocks of the Iles and Williams Fork Formations in the Danforth Hills Coal Field, Northwestern Colorado A. Location map showing public drill holes and line of section A-A’ in the Danforth Hills coal field B. Subsurface correlations of coal and related rocks in the Iles and Williams ForkFormations in the Danforth Hills coal field

Figures

1. Geologic map and major Upper Cretaceous coal fields of northwestern Colorado...... 2 2. Index map of Yampa coal field...... 3 3. Photograph of baked strata (clinker) above Trout Creek Sandstone Member of the Iles Formation...... 4 4. Stratigraphic column of Upper Cretaceous and Tertiary rocks, Yampa coal field...... 5 5. Paleogeographic map of central part of North America during the late Campanian...... 6 6. Geologic map of Yampa coal field...... 8 7. Cross section of Mesaverde Group in the Yampa coal field...... 9 8. Cross section of Upper Cretaceous and Tertiary rocks in the Lower White River, Danforth Hills, and Yampa coal fields...... 10 9. Stratigraphic section of Eckman Park area...... 11 10–15. Photographs showing Yampa Bed 10. Excavated outcrop at type location...... 15 11. Drill core from Eagle Mine...... 15 12. Closeup of excavated outcrop at type location...... 16 13. Upper contact at type location...... 17 14. Lower contact at type location...... 17 15. Outcrop northeast of Eckman Park...... 18 16. Stratigraphic section in Fish Creek Canyon...... 20 17–20. Photographs showing Yampa Bed 17. Outcrop in Fish Creek Canyon...... 21 18. Outcrop in Ute Gulch...... 21 19. Closeup in Ute Gulch...... 22 20. Outcrop south of Eagle Mine...... 22 21. Outcrop of Yampa Bed and Peacock coal bed south of Lay, Colo...... 23 22. Closeup of Yampa Bed and Peacock coal bed south of Lay, Colo...... 23 23. Map of Danforth Hills coal field, Moffat and Rio Blanco Counties, Colo...... 24 24. Stratigraphic column of Upper Cretaceous and Tertiary rocks, Danforth Hills coal field...... 25 25. Drill core showing Yampa Bed, Danforth Hills coal field...... 26 26. Examples of geophysical log responses...... 27 27. Correlation diagram of E coal bed...... 28 28. Correlation diagram of Trout Creek and Twentymile Sandstone Members...... 29 v

Conversions and Abbreviations

To convert Multiply by To obtain inch (in.) 2.54 centimeter (cm) foot (ft) 0.3048 meter (m) mile (mi) 1.609 kilometer (km) square mile (mi2) 2.59 square kilometer (km)

Abbreviations Used in This Report ft foot, feet in. inch, inches Ma Mega-annum (millions of years ago) mi mile, miles mi2 square mile, square miles T., R. township, range USGS United States Geological Survey

The Yampa Bed—A Regionally Extensive Tonstein in the Williams Fork Formation, Northwestern Piceance Creek and Southern Sand Wash Basins, Colorado

By Michael E. Brownfield and Edward A. Johnson

Abstract Introduction

The Upper Cretaceous Williams Fork Formation of the Northwestern Colorado contains nine major coal fields Mesaverde Group in northwestern Colorado consists of nonma- that host many mineable coal beds within the coal-bearing rine to marginal-marine lithofacies that were deposited along Mesaverde Formation (or Group) in the southeastern Sand the western edge of the Cretaceous Western Interior seaway. Wash and Piceance Basins (fig. 1). The coal-bearing rocks of Complex intertonguing and rapid lateral facies changes, which the Mesaverde Formation and (or Group) crop out more or are characteristic of the Mesaverde Group, commonly make less continuously around the Piceance Basin and the southern correlation of lithofacies difficult. However, an altered volcanic boundary of the Sand Wash Basin. ash bed (tonstein) has been identified within the Williams Fork The Yampa coal field (fig. 1), covering about 1,700 square Formation that greatly facilitates regional correlations within the miles (mi2), is located in northwestern Colorado in Routt, Piceance Creek and Sand Wash Basins, Colorado. This region- Moffat, and Rio Blanco Counties (figs. 1, 2). U.S. Highway ally persistent and distinctive unit of Upper Cretaceous age is 40 traverses the coal field on the north and connects the small here formally named the Yampa Bed of the Williams Fork For- communities of Milner and Lay, which roughly define the east mation for exposures in the Yampa, Danforth Hills, and Grand and west limits, respectively, of the coal field (fig. 2). Major Hogback coal fields, Moffat and Routt Counties, northwestern cities in the area are Hayden in the central part of the field Colorado; the name is derived from the Yampa River valley. and Craig in the western part (fig. 2). The coal field is also The type section was measured in the NE¼SW¼ sec. 6, T. 5 N., traversed by the west-flowing Yampa River (fig. 2). The Yampa R. 91 W., about 8 miles south of Craig, Colo., where the bed is coal field is characterized by southwest-facing cliffs separated 38 inches thick and lies within the C-D coal bed in the lower by steep canyons on the south boundary and by north-dipping part of the Williams Fork, about 165 feet above the Trout Creek slopes along the north boundary of the Williams Fork Moun- Sandstone Member of the Iles Formation. The Yampa Bed is tains. Elevations in the coal field range from about 6,000 feet dated at 72.2±0.1 mega-annum (Ma) using the K-Ar method. (ft) above sea level near Lay to just over 10,600 ft in the north- Regionally, the Yampa Bed is a 0.5- to 5-foot-thick, eastern part of the coal field. regionally persistent tonstein that can be readily identified in Coal-bearing rocks of the Upper Cretaceous Williams several different lithofacies in the lower part of the Williams Fork Formation of the Mesaverde Group consist mostly of Fork Formation. The unit is useful as a regional datum in the nonmarine to marginal-marine lithofacies that were deposited correlation of facies within the Williams Fork and in paleogeo- along the western shoreline of the Cretaceous Western Inte- graphic reconstructions for the lower part of the formation. It rior seaway. Complex intertonguing and rapid lateral facies is easily recognized on geophysical logs by its low resistivity changes, which are characteristic of the Mesaverde Group, response. X-ray diffraction analysis of selected outcrop and commonly make regional correlation of lithofacies difficult drill-hole samples indicates that the Yampa Bed consists when those correlations are based on time-transgressive principally of kaolinite and smectite in varying proportions. sandstone units or coal beds of limited areal extent. In addi- Evidence that the bed is a diagenetically altered airfall ash tion, even where good exposures are present, the numerous and consists of the following: (1) petrographic studies of thin complexly distributed coals in the Williams Fork Formation are sections reveal a relict vitroclastic texture that was produced commonly burned, forming baked strata (clinker) that makes by clay-mineral replacement of glass shards; (2) samples surface and subsurface correlation of coal beds extremely dif- characteristically contain phenocrysts of euhedral to subhedral ficult (fig. 3). Within the Western Interior of North America, ß-quartz (α-quartz after ß-quartz), biotite, apatite, andesine, several Cretaceous and Tertiary coal beds and associated fine- sanidine, and zircon; and (3) outcrop and subsurface studies grained strata contain numerous laterally continuous claystone indicate a widespread areal distribution. partings. Laboratory studies have shown that many of these 2 The Yampa Bed—A Regionally Extensive Tonstein, Colorado Generalized geologic map of

EXPLANATION City Undivided Quaternary, Tertiary, Undivided Quaternary, Mesozoic, and Paleozoic Extrusive and intrusive volcanic rocks Mesaverde Fm. and Group coal-bearing rocks County line Colorado Figure 1. northwestern Colorado showing main outcrop areas of the coal-bearing Mesaverde Formation and Group (green) names of major coal fields (blue labels) within and Piceance southeastern Sand Wash Basins. Modified after Brownfield, Hettinger, and Johnson (2000), Brownfield others (2000a,b), Hettinger and others (2000), Johnson and others (2000). Tertiary Upper Cretaceous Crested Butte Oak Creek ROUTT EAGLE 00' GUNNISON o Crested Butte 107 Carbondale Carbondale Glenwood Springs Hayden Somerset Yampa Craig Somerset Paonia RIO BLANCO Lay Grand Hogback Meeker Axial DELTA Grand Mesa 00' Grand Mesa o Parachute 108 Piceance Basin Southeastern Rio Blanco Sand Wash Basin Sand Wash Danforth Hills 30 MILES GARFIELD Palisade MOFFAT 0 MESA Book Cliffs Rangely Grand Junction Lower White River 00' o 10 Dinosaur 109 30' 00' 30' 00' o o o o 40 39 40 39 Introduction 3

T. 6 N. T. 7 N. T. 6 N. T. 5 N. T. 4 N. T. 3 N. T. 2 N. T. 9 N.

Yampa River Yampa 131 40

Elk River Elk R. 85 W. Yampa River Yampa Oak Creek Oak

7

Creek Park Trout Milner McGregor Eckman X R. 86 W. 1 6 Twentymile Park 5 MILES 5 Fish Creek Canyon Bear River X Mount Harris 2 R. 87 W. 4 0

RIO BLANCO RIO River 3 Hayden

R. 88 W. ROUTT MOFFAT Yampa Fork R. 89 W.

Williams Fork Mountains 13 Williams R. 90 W. Craig Mesaverde Group Mesaverde Ute Gulch X Trapper Mine Trapper 4 R. 91 W. 2 3 X 1 X 5 Eagle Mine River R. 92 W.

Iles Mountain Iles Yampa Milk Creek Milk City, town City, 40 Measured section Measured

X 1. Eckman Park Park Eckman 1. Canyon Cr. Fish 2. Mine Trapper 3. Gulch Ute 4. Mine Eagle 5. bed Peacock Lay, 6. 13

Duffy Mountain R. 93 W. EXPLANATION COLORADO

6

X River Lay Location of Yampa coal field and approximate locations of mines measured sections, Moffat, Rio Blanco, Routt Counties, Colo. Outcrops Mesaverde Location of Yampa Lay, Peacock bed

Coal mine Coal R. 94 W. 1. Eagle Mine, abandoned Mine, Eagle 1. Mine Trapper 2. Mine II-W Seneca 3. Mine Yoast 4. Mine II Seneca 5. Mine Creek Foidel 6. abandoned Mine, Edna 7. Yampa Figure 2. Group (green) delineate boundaries of coal field. 4 The Yampa Bed—A Regionally Extensive Tonstein, Colorado regionally extensive units are altered airfall ash (Bohor and Colorado (fig. 1) (for example, see Brownfield and Johnson, others, 1976; Bohor and others, 1979; Ryer and others, 1980; 1986; Johnson and Brownfield, 1988; Brownfield and others, Triplehorn and Bohor, 1981; Brownfield and Johnson, 1986; 2000a,b; Johnson and others, 2000), we introduce the term Johnson and Brownfield, 1986). For many years, European “Yampa Bed” as a formal name for this unit. The designated geologists have successfully used kaolinitic claystone partings type section is in the NE¼SE¼ sec. 6, T. 5 N., R. 91 W., which called “tonsteins” for correlation of Carboniferous coal-bearing is located in the Yampa coal field about 8 miles (mi) southwest strata. Bohor and Triplehorn (1993) presented an excellent of Craig, Colo. (fig. 2, measured section 3); the bed there is discussion on the characterization and usage of tonsteins in 38 inches (in.) thick, and lies within the C-D coal bed of the nonmarine rocks. Although altered ash beds are common in Williams Fork Formation, about 165 ft above the Trout Creek Western Interior Cretaceous rocks they have not been widely Sandstone Member of the Iles Formation (fig. 4). The name is used in regional correlation of nonmarine strata because they derived from the Yampa River valley. Local and regional strati- were not recognize as being of volcanic origin. Bentonites, graphic relations are discussed in a later section. which are marine equivalents of the nonmarine altered ash beds, however, have been recognized for many years as a valuable tool for correlating marine strata. A thick, regionally extensive, tonstein bed has been Geologic Setting identified in the lower part of the Williams Fork Formation in northwestern Colorado (figs. 1, 2). The tonstein bed was first Cretaceous Paleogeography recognized in the summer of 1977 during a U.S. Geological Survey (USGS) coal exploration drilling program. Because it During the Cretaceous Period, a large, north-trending epicon- has since been demonstrated that this ash bed provides a means tinental seaway, the Western Interior Seaway, occupied what for making precise regional stratigraphic correlations within is now central North America (fig. 5). The seaway extended the Piceance Creek and Sand Wash Basins of northwestern north-south from Mexico to Alaska and east-west from

Figure 3. Burned or baked strata (clinker) above Trout Creek Sandstone Member of the Iles Formation, Mesaverde Group, on Iles Mountain (see fig. 2), northwestern Colorado. Geologic Setting 5

Figure 4. Upper Cretaceous and Tertiary Age Ammonite rocks in Yampa coal field, showing depositional Unit zone environments, coal groups, and ammonite zones (Brownfield and others, 1999). Yampa Bed and C-D coal bed not drawn to scale. Modified after Bass and others (1955). Ammonite zones from Wasatch Eocene Izett and others (1971). Formation Formation Fort Union Paleocene Lance Formation

Fox Hill Sandstone

Baculites clinolobatus Lewis Shale Baculites eliasi EXPLANATION Upper coal group Alluvial, coastal plain, and mire

Twentymile Sandstone Member Upper Cretaceous Baculites reesidei Upper shoreface and estuarine Middle coal group Williams Fork Formation Lower shoreface and Yampa Bed offshore marine Trout Creek Sandstone Member Exiteloceras jenneyi Yampa Bed—Not to scale

C-D coal bed—Not to Lower coal group scale Iles Formation

Tow Creek Sandstone Member Baculites

1,000 FEET perplexus Shale Mancos 6 The Yampa Bed—A Regionally Extensive Tonstein, Colorado

y

wa a

e

S

r

o

i r

45° e

t n

I Yampa

coal field

Sevier Highlands

n

r

e

t

s e W

30°

EXPLANATION

30° Paleolatitude Alluvial plain Coastal Peat Western Interior and highlands plain swamp Seaway Thrust fault—Sawteeth on upper plate Figure 5. Paleogeographic map of central part of North America during late Campanian Stage of the Cretaceous Period. Yampa coal field shown in relation to western shoreline, coastal plain, and peat swamps associated with the Western Interior seaway. Modified from Roberts and Kirschbaum (1995). Detailed Stratigraphy of Coal-Bearing Upper Cretaceous Rocks 7 central Utah to eastern Nebraska. A stable cratonic platform with the Mancos Shale; it contains Baculites perplexus in its bordered the seaway on the east, and the tectonically active upper part (fig. 4), and it underlies the Lewis Shale, which Sevier orogenic belt bordered the seaway on the west. Sedi- contains Baculites eliasi and Baculites clinolobatus (Izett ments transported eastward from the Sevier orogenic high- and others, 1971). To the east, the Mesaverde grades into lands (fig. 5) were deposited along the fluctuating western marine rocks of the Pierre Shale, but evidence of this transi- shoreline, resulting in a complex package of intertonguing tion was removed by erosion that resulted from uplift of the marine and nonmarine rocks. Park Range during the Tertiary. To the west, the Mesaverde becomes increasingly fluvial and, in central Utah just east of the Sevier orogenic belt, equivalent strata are composed Stratigraphy of Upper Cretaceous Rocks in almost exclusively of conglomerate. Northwestern Colorado The Cretaceous sedimentary rocks exposed in the study area range in age from Early Cretaceous (Albian) to Late Detailed Stratigraphy of Coal-Bearing Cretaceous (Maastrichtian) and include the Mancos Shale, Mesaverde Group (Iles and Williams Fork Formations), Upper Cretaceous Rocks Lewis Shale, Fox Hills Sandstone, and Lance Formation (fig. 4). The Cretaceous rocks are unconformably overlain Iles Formation by Tertiary rocks. The main coal-bearing rocks in the Yampa coal field are the Upper Cretaceous Iles and Williams Fork The Iles Formation (fig. 4) of the Mesaverde Group was Formations of the Mesaverde Group (fig. 6). These units named by Hancock (1925) for coal-bearing strata exposed are exposed from Oak Creek to Lay, Colo., southwest to the at Iles Mountain in the western part of the Yampa coal field Danforth Hills and Lower White River coal fields, and south (fig. 2). The formation is conformable with the Mancos Shale to the Grand Hogback coal field (fig. 1). below and the Williams Fork Formation above and consists of sandstone interbedded with siltstone, mudstone, shale, Mesaverde Group in the Yampa Coal Field claystone, carbonaceous shale, and coal. It ranges in thick- ness from about 1,300 ft in the western part of the coal field Holmes (1877) first applied the name Mesaverde Group (Hancock, 1925) to 1,500 ft in the eastern and central parts to a sequence of sandstone, siltstone, mudstone, shale, (Bass and others, 1955). claystone, and coal in southwestern Colorado. Fenneman The lower two-thirds of the Iles Formation consists and Gale (1906a,b), noting a similar sequence of rocks in of massive ledge-forming sandstone beds interbedded with the Yampa coal field, extended the name Mesaverde into mudrock, carbonaceous shale, and coal that form steep cliffs northwestern Colorado; they did not subdivide the sequence rising above the broad lowland formed on the Mancos Shale into formations but described two regional sandstones, which along the south and west boundaries of the Yampa coal field. they named the Trout Creek and the Twentymile Sand- Most of the coal beds within the lower coal group (fig. 5) are stone Members. In addition, they subdivided the coal in the in the upper part of the Iles Formation about 400 ft above the Mesaverde into lower, middle, and upper coal groups. The base (Bass and others, 1955). Three principal coal beds or lower group contains all the coal below the Trout Creek, zones were recognized by Fenneman and Gale (1906a,b) in the middle group contains all the coal from the Trout Creek the lower coal group at Oak Creek; these beds were referred through the Twentymile, and the upper group contains all of to as No. 1, No. 2, and No. 3 coal zones (fig. 9) by Bass and the coal above the Twentymile. others (1955). The uppermost part of the formation consists Hancock (1925) later subdivided the Mesaverde Group into of a mudrock sequence capped by cliff-forming sandstone. the Iles Formation and overlying Williams Fork Formation, and The mudrock sequence is a transgressive marine tongue he defined the Trout Creek Sandstone as a member at the top (figs. 7, 8) that ranges in thickness from about 400 ft just of the Iles and the Twentymile as a member more or less in the west of Oak Creek to about 100 ft in the western part of the middle of the Williams Fork (figs. 4, 6). Bass and others (1955) coal field and pinches out in the Danforth Hills coal field. It later extended these names into the eastern part of the coal field. is capped by the regressive Trout Creek Sandstone Member Masters (1966) introduced the name Mount Harris member for (figs. 7, 8). the lower part of those rocks between the Trout Creek and the Three persistent sandstone units in the Iles Formation Twentymile and he reintroduced an earlier name, Holderness deserve special mention as guides for correlation within member, for those rocks in the Williams Fork above the Twen- the eastern part of the coal field (fig. 9). They are the Tow tymile; neither of these names is commonly used today. Creek Sandstone Member at the base, the informally named In northwestern Colorado, the Mesaverde (figs. 7, 8) “Double-ledge sandstone” unit about 400 ft above the base, comprises an eastward thinning, wedge-shaped package of and the Trout Creek Sandstone Member at the top of the marine and nonmarine rocks that overlies and intertongues formation. 8 The Yampa Bed—A Regionally Extensive Tonstein, Colorado T. 9 N. T. 8 N. T. 7 N. T. 6 N. T. 5 N. T. 3 N. T. 2 N. T. 4 N. Oak Creek Oak Milner R. 86 W. R. 85 W. 7 5 MILES 8 0 R. 87 W. 6 Mount Harris Mount 5 R. 88 W. Hayden R. 89 W. 11 10 4 Eastern Sand Wash Basin Eastern Sand Wash R. 90 W. 9 Craig R. 91 W. 2 3 Buck Peak anticline Peak Buck R. 92 W. 1 12. Axial Basin anticline Basin Axial 12. 1. Round Bottom syncline Bottom Round 1. anticline Fork Williams 2. syncline Bottom Big 3. or Mountain Breeze 4. anticline Creek Sage 5. anticline Creek Fish 6. 7. Twentymile Park syncline Park Twentymile 7. anticline Creek Tow 8. syncline Hart 9. anticline Creek Beaver 10. anticline Seely 11.

R. 93 W. EXPLANATION Bar on Bar Lay Fault— Williams Fork Williams Formation Formation Iles side downthrown Syncline Anticline City, town City, R. 94 W. 12 Figure 6. coal field, showing distribution of Iles and Williams Fork Formations Upper Cretaceous Mesaverde Group major faults Generalized geologic map of Yampa (1976). folds. Modified after Tweto Detailed Stratigraphy of Coal-Bearing Upper Cretaceous Rocks 9 SE Trout Creek Sandstone Trout Nonmarine deposit Nearshore marine deposit Lower shoreface and offshore marine deposit EXPLANATION Oak Creek sandstone Twentymile Sandstone Twentymile Double-ledge sandstone

NET SHORELINE TRANSGRESSION

Sub-Twentymile sandstone Sub-Twentymile NET SHORELINE REGRESSION SHORELINE NET Tow Creek Sandstone Tow Member Big White sandstone Rim Rock sandstone Three White sandstones MANCOS SHALE LEWIS SHALE Generalized northwest-southeast cross section of Mesaverde Group in the Yampa coal field, showing stratigraphic positions of major sandstone units and Generalized northwest-southeast cross section of Mesaverde Group in the Yampa

MESAVERDE GROUP MESAVERDE NW Figure 7. related major regressions and transgressions. Modified after Siepman (1985).

10 The Yampa Bed—A Regionally Extensive Tonstein, Colorado

Upper Cretaceous Upper Eocene Paleocene

Formation Formation

Lance Formation Lance Williams Fork Formation Fork Williams Formation Iles Mancos Shale Mancos Lewis Shale Lewis

Wasatch Wasatch East Union Fort Marine shale Marine shale Lower coal group Fox Hills Sandstone Upper coal group Middle coal group Tow Creek Sandstone Tow Member bed C-D coal Western Yampa coal field Yampa Western Lewis Shale Trout Creek Sandstone Trout Member Yampa Bed Yampa Twentymile Sandstone Twentymile Member Wasatch Formation Fort Union Formation Mancos Shale

Lower coal group Barren interval Lion Canyon coal group Goff coal group Fairfield coal group Rimrock sandstone

Black Diamond coal group

Formation

Williams Fork Formation Fork Williams

Iles Formation Iles

Fort Union Fort Wasatch Formation Wasatch Danforth Hills coal field Lion Canyon Sandstone

? ?

? ? Castlegate Sandstone Member Lower White River coal field

Coal unit (part)

Lower unit Upper unit Sego Ss. Mbr.

Wasatch Fm. Wasatch (part)

Fort Union Fm. Group Mesaverde

Ohio Creek Fm. ? Mancos Shale Mancos

Paleocene-Eocene Upper Cretaceous Upper West

Yampa Yampa 1,000 FEET 1,000 Hills Danforth Index Map Line of section Upper Cretaceous Alluvial, coastal Alluvial, mire and plain, shoreface Upper estuarine and and shoreface Lower marine offshore Bed, not Yampa to scale C-D coal bed, not to scale Unconformity River

EXPLANATION Lower White Figure 8. rocks in the and Tertiary Danforth Lower White River, coal fields, Hills, and Yampa northwestern Colorado, showing interpreted depositional environments. Compiled from USGS field Bed and C-D studies. Yampa coal bed not drawn to scale. Member; Fm., Formation; Mbr., Ss., Sandstone. Detailed Stratigraphy of Coal-Bearing Upper Cretaceous Rocks 11 Shale Lewis

Fish Creek coal bed Fish Creek sandstone

Twentymile Sandstone Upper coal group Member

EXPLANATION Williams Fork Formation Marsh or swamp Lennox coal bed Wadge coal bed

Shoreface Wolf Creek coal bed Yampa Bed Trout Creek Sandstone Middle coal group Alluvial and Member coastal plain

Lower shoreface and offshore marine Oak Creek sandstone Yampa Bed No. 3 coal No. 2 coal No. 1 coal Lower coal group Iles Formation Double-ledge sandstone

Tow Creek Sandstone 500 FEET Member Shale Mancos

Figure 9. Composite stratigraphic section of Eckman Park area (see fig. 2, measured section 1) showing Yampa Bed, interpreted depositional environments, and major coal beds in eastern part of Yampa coal field (Brownfield and others, 1999). Compiled from U.S. Geological Survey coal- exploration drill holes and field studies. Coal bed names from Bass and others (1955). 12 The Yampa Bed—A Regionally Extensive Tonstein, Colorado

Tow Creek Sandstone Member with the Lewis Shale. The formation consists of interbedded mudrock, sandstone, and lesser amounts of carbonaceous shale The Tow Creek Sandstone Member of the Iles Forma- and coal, which are especially well exposed in the Williams tion (figs. 4, 7, 8, 9) is the basal unit in the Iles in the eastern Fork Mountains (fig. 2). Approximate thicknesses have been part of the Yampa coal field. Crawford and others (1920) reported as follows: (1) 1,600 ft in the western part of the named the Tow Creek for exposures between Milner and Yampa coal field (Hancock, 1925) (2) 1,800 ft thick in the Bear River (fig. 2). The Tow Creek consists of light gray to Round Bottom quadrangle southwest of Craig (Johnson, 1987) white sandstone ranging in thickness from 35 to 125 ft (Bass and (3) 1,600 ft thick near Mount Harris in the eastern part of and others, 1955). This unit is prominent west of Fish Creek the Yampa coal field (fig. 2) to nearly 2,000 ft thick farther to canyon (fig. 2) in the south-central part of the coal field. the west (Bass and others, 1955). Four USGS coal exploration drill holes located in the western part of the coal field show an average thickness of 1,915 ft for the Williams Fork. Double-Ledge Sandstone The lower two-thirds of the Williams Fork Formation The “Double-ledge sandstone” (figs. 7, 9), informally consists of thick ledge-forming light-gray and white sand- named by Bass and others (1955) and consists of 1 to 3 beds stones interbedded with mudrock, carbonaceous shale, and of light-gray and white cliff-forming sandstone and interbed- coal of the middle coal group (figs. 4, 8). Throughout the ded siltstone about 400 to 460 ft above the base of the Iles eastern part of the coal field the principal coal beds are, in Formation. The unit ranges in thickness from 130 to 250 ft and ascending order, the Wolf Creek, Wadge, and Lennox (Bass forms prominent ledges visible along the steep south-facing and others, 1955). These beds are in the lower 400 ft of the cliffs of the Williams Fork Mountains in the eastern and south- Williams Fork, immediately above the Trout Creek Sandstone central parts of the Yampa coal field. Member of the Iles Formation. Data compiled by the authors indicate that the number of coal beds in the middle coal group increases westward in the Yampa coal field (Johnson and Trout Creek Sandstone Member Brownfield, 1988; Johnson and others, 2000). The upper part of the middle coal group is characterized by mudrock capped Fenneman and Gale (1906b) named the Trout Creek by cliff-forming sandstone. The upper third of the Williams Sandstone Member of the Iles Formation (fig. 4) for exposures Fork Formation contains the upper coal group and consists of in the eastern part of the Yampa coal field. The Trout Creek sandstone, mudrock, carbonaceous shale, and coal. is a cliff-forming unit of regional extent that transitionally A regional cross section by Roehler (1987) extend- overlies a sequence of marine shale (figs. 4, 7, 8, 9) containing ing from the vicinity of Mount Harris, Colo., northwest to Exiteloceras jenneyi (Izett and others, 1971) and is overlain Rock Springs, Wyo., shows that the Williams Fork is roughly by coal-bearing rocks. Fenneman and Gale (1906b) and equivalent to the upper part of the Ericson Sandstone and the Bass and others (1955) reported the Trout Creek to be about Almond Formation. Southwest of the Yampa coal field, the 100 ft thick in the eastern part of the coal field, and Johnson name Williams Fork Formation is also used in the Danforth (1987) reported the unit as ranging from 67 to 79 ft thick in Hills (figs. 1, 8) coal field (Hancock, 1925, Hancock and Eby, the western part. Siepman (1985), in his regional study of the 1930) unit, determined that it ranges from 140 ft thick near Mount Five persistent stratigraphic units recognized in the Harris to 31 ft thick just west of Oak Creek (fig. 2). The unit Williams Fork Formation are valuable guides for correlations has an average thickness of about 67 ft. Toward the northwest, within the Yampa coal field; they are, in ascending order, (1) the Trout Creek pinches out in the subsurface of the Sand the Yampa Bed, (2) the informally named “sub-Twentymile Wash Basin (Siepman, 1985; Roehler, 1987). Regionally, sandstone” in the middle coal group, (3) the Twentymile the Trout Creek (fig. 8) is well exposed in the Danforth Hills Sandstone Member, (4) the informally named “Big White (Hancock and Eby, 1930) and to the west pinches out just east sandstone” in the upper coal group, and (5) the informally of the Lower White River coal field (Hail, 1974; Barnum and named “Three White sandstones” at the top of the formation Garrigues, 1980). To the south, the Trout Creek Sandstone (fig. 7). Member is equivalent to the Rollins Sandstone Member of the Mesaverde Formation (or Mount Garfield Formation) in the southern Piceance Basin of west-central Colorado (Collins, Sub-Twentymile Sandstone 1976). The name “sub-Twentymile sandstone” was informally introduced by Kitely (1983) for a sandstone unit that lies about Williams Fork Formation 150 ft below the base of the Twentymile Sandstone Member in the east-central part of the Yampa coal field (fig. 7). Most of Hancock (1925) named the Williams Fork Formation for what is known about this unit comes from the subsurface, but it coal-bearing strata in the upper part of the Mesaverde Group. probably has many physical characteristics in common with the It conformably overlies the Trout Creek Sandstone Member Trout Creek and Twentymile Sandstone Members of the Iles of the Iles Formation (figs. 7, 8, 9) and is also conformable Formation and Williams Fork Formation, respectively. The unit Depositional Setting of the Mesaverde 13 averages about 30 ft thick and contains one to three sandstone Three White Sandstones bodies. Masters (1966) defined a sandstone unit, the Hayden Gulch sandstone, at this same stratigraphic level that is likely The term “Three White sandstones” was informally equivalent to the sub-Twentymile. Siepman (1985) recognized applied by Brownfield and others (1999) and Johnson and the sub-Twentymile and included it at the top of his sub- others (2000) in the western part of the coal field, just south Twentymile unit. The sub-Twentymile grades into Mancos of Craig (fig. 2), to three vertically stacked, 50- to 60-ft-thick Shale to the east, and to the west it loses its distinctiveness by sandstone units in the upper part of the Williams Fork Forma- splitting into a number of smaller sandstone bodies. tion (fig. 7). Lithologically, they resemble the Twentymile Sandstone Member. In addition, each sandstone unit overlies a thin sequence of marine rocks and is overlain by nonmarine Twentymile Sandstone Member rocks. Where best exposed along the Yampa River south- The Twentymile Sandstone Member of the Mesaverde west of Craig, the base of the lower sandstone lies about 310 Formation was named by Fenneman and Gale (1906b) for ft above the top of Big White sandstone. The stratigraphic exposures in Twentymile Park (fig. 2) in the eastern part of distance between the base of the lower sandstone and the base the Yampa coal field and was later revised and assigned to the of the Lewis Shale is about 435 ft. To the west, the lower and Williams Fork Formation by Hancock (1925). The strati- middle sandstones grade into nonmarine rocks, but the upper graphic thickness from the top of the Trout Creek Sandstone sandstone continues to the west edge of the coal field. To the Member and the base of the Twentymile Sandstone Member east, the three sandstones extend for several miles east of the ranges from about 900 to 1,100 ft (Bass and others, 1955). The Yampa River before grading into Mancos Shale. marine shale unit below the Twentymile Sandstone Member in the Fish Creek canyon area (fig. 2) contains Baculites reesidei (Izett and others, 1971). The Twentymile typically forms dis- tinctive cliffs of yellowish-gray to white sandstone and, unlike Depositional Setting of the Mesaverde the Trout Creek, generally consists of two to three sandstone Group units separated by finer grained strata. Bass and others (1955) reported that the Twentymile is about 100 to 200 ft thick in the During the Late Cretaceous, the west edge of the Western eastern part of the coal field. Siepman (1985) reported that the Interior Seaway was continually being modified by sediment unit ranges from 184 ft thick in Fish Creek Canyon in the east- influx from the tectonically active Sevier orogenic belt to the ern part to 28 ft thick at Duffy Mountain in the western part. west (fig. 5). According to Haun and Weimer (1960), as much The Twentymile Sandstone Member begins to lose its identity as 11,000 feet of sediment was deposited in the seaway during on the western edge of the coal field (fig. 8), and it grades into this time. Along the west margin of the seaway, Mesaverde nonmarine rocks toward the northwest in the subsurface of the deposition is characterized by a series of westward transgres- Sand Wash Basin (Siepman, 1985) and to the southwest short sions and eastward regressions of the shoreline that resulted of the Danforth Hills (Brownfield and others, 2000b). in the cyclic deposition of marine and nonmarine lithofacies (Weimer, 1960; Zapp and Cobban, 1960). Big White Sandstone The sediments deposited in northwestern Colorado during this time include the Mancos Shale and the Iles and Williams The term “Big White sandstone” was informally intro- Fork Formations of the Mesaverde Group (fig. 4). Variation in duced by geologists of the W.R. Grace and Company while the sedimentation rates, drainage patterns, amounts of basin they were conducting coal-exploration drilling in the late subsidence, and eustatic sea level changes contributed to the 1970s; the term refers to a sandstone unit that crops out about complex cyclic deposition of Upper Cretaceous lithofacies. 200 ft above the Twentymile Sandstone Member in the central Masters (1966) recognized three large-scale regressive cycles part of the Yampa coal field (fig. 7). In the western part of the in the Mesaverde: one from the base of the Iles to the base coal field, a discontinuous sandstone exists at the same strati- of the marine shale underlying the Trout Creek Sandstone graphic level as the Big White sandstone that is referred to Member, a second from the base of the Trout Creek Sand- informally by USGS geologists as the “Fuhr Gulch sandstone” stone Member to the base of the marine shale underlying the of the Williams Fork Formation (Johnson, 1987). Johnson Twentymile Sandstone Member, and a third from the base of (1987) described this unit at the mouth of Fuhr Gulch as a the Twentymile Sandstone Member to the top of the Williams 40-ft-thick light gray, very fine grained to fine-grained sand- Fork (fig. 9). In each cycle, the top of the coal-bearing pack- stone. In the eastern part of the coal field, Campbell (1923) age and the base of the overlying marine shale are separated informally named a sandstone unit at the same stratigraphic by a disconformity. In general, the Iles represents net shoreline level as Big White (in Eckman Park, figs. 2, 9) the “Fish Creek regression, and the Williams Fork represents a net shoreline sandstone.” On the basis of regional correlations, all three transgression. However, in detail the Williams Fork shows sandstone units are stratigraphically equivalent and represent a evidence of several shoreline fluctuations, as indicated by the poorly exposed, regional sandstone unit (Brownfield and oth- vertical juxtaposition of the formation’s three major deposi- ers, 1999; Johnson and others, 2000). tional settings: offshore marine, nearshore marine, and fluvial. 14 The Yampa Bed—A Regionally Extensive Tonstein, Colorado

The westward-thinning tongues of marine shale that 1986). Results of X-ray diffraction analysis of selected outcrop directly underlie the Trout Creek and Twentymile Sandstone and drill-hole samples show that the Yampa Bed consists Members were deposited on an open-marine shelf, as indi- primarily of kaolinite and smectitic clays in varying propor- cated by the presence of marine fossils and deep-marine trace tions. Diffractograms showing kaolinite display sharp, narrow fossils. Most likely, this same environment is represented in basal peaks (00l) and well-defined prism reflections indicat- the strata that underlie the sub-Twentymile sandstone in the ing an authigenic origin in samples collected in the upper and central part of the coal field. A marginal-marine environment lower parts of the bed (Brownfield and Johnson, 1986). X-ray is interpreted for the strata that underlie the Three White diffraction analysis of samples collected from the central part sandstones on the basis of shallow-marine trace fossils in the displayed greater amounts of smectitic clays and relict volcanic western part of the coal field. minerals where the unit is greater than 12 in. thick. Heavy- The thick sandstones, such as the Trout Creek and liquid separations reveal that the nonclay minerals compose a Twenty-mile Sandstone Members and the sub-Twentymile, relatively small percentage of the total sample by weight. The Big White, and Three White sandstones, were deposited in lightweight fraction of the nonclay minerals consists of angular a progradational shoreface environment (Boyles and others, grains of euhedral to subhedral quartz, sanidine, andesine, and 1981). This conclusion is supported by an upward increase biotite partly altered to chlorite. The quartz component of this in grain size and a corresponding upward decrease in bed fraction includes euhedral grains of ß-quartz-form phenocrysts thickness, by hummocky cross stratification followed upward (α-quartz after ß-quartz). The heavier fraction is minor and by trough cross stratification, and by the presence of shallow- consists mainly of euhedral apatite and zircon crystals. marine trace fossils. Relatively complete stratigraphic sections Where exposed on the surface or observed in drill core, of the Trout Creek and Twentymile Sandstone Members the Yampa Bed is a white to grayish-white massive claystone reveal that the units generally consist of a basal transitional (figs. 10, 11). In the subsurface, it becomes an important part that was deposited below wave base and an overlying regional marker bed that is easily identified on drill-hole geo- shoreface part deposited above wave base; rarely foreshore physical logs and ranges in thickness from less than 1 ft to 6 ft. and backshore deposits are also preserved. Many workers (for In the central and western parts of the coal field, the unit lies example, Siepman, 1985) believe that these sandstone units about 100 and 260 ft, respectively, above the top of the Trout were deposited along wave-dominated deltaic, strand plain, Creek Sandstone Member. However, in the eastern part of the and barrier island systems, in a microtidal setting. coal field the stratigraphic separation is less than 20 ft, and on The strata that overlie the nearshore marine sandstones some geophysical logs the Yampa Bed appears to rest directly display sedimentological characteristics indicating fluvial, on the Trout Creek. lagoonal, salt marsh, and freshwater swamp environments on The type locality is located in the NE¼SW¼ sec. 6, T. 5 a coastal plain that sloped gently seaward with little topo- N., R. 91 W., about 8 mi south of Craig, Colo. (fig. 2, mea- graphic relief. The fluvial lithofacies is composed of channel sured section 3). The Yampa Bed is located in the informally sandstones and associated overbank deposits of sandstone, named C-D coal bed (Johnson and Brownfield, 1988) about siltstone, shale, and lenticular coal beds. The lagoonal and 165 ft above the Trout Creek Sandstone Member of the Iles bay environments are represented by low-energy deposits of Formation (figs. 4, 7). shale, siltstone, and minor amounts of fine-grained sandstone. At the type locality (fig. 10), the Yampa Bed of the The lagoonal sandstones include washover fans, flood-tidal Williams Fork Formation is 38 in. thick. Where observed, deltas, and crevasse splay deposits. The salt marsh lithofacies it typically weathers white to cream or, less commonly, to contain evenly laminated, fine-grained deposits consisting of light gray, yellow gray, or light tan. The bed appears to lack carbonaceous shale, siltstone, very fine grained sandstone, and stratification, is highly cohesive when moist, and displays a impure coal. The freshwater swamp lithofacies are represented distinct blocky fracture (fig. 10). The upper 1 to 2 in. displays by coal. rooting. The upper and lower contacts are generally sharp where the Yampa Bed is interbedded with coal (figs. 10, 12), and the contacts display sedimentary structures indicating Origin, Description, and Distribution of minor transport or ponding during deposition. At the type locality, the Yampa Bed (fig. 13) is overlain by a 4-in.-thick the Yampa Bed carbonaceous mudstone to siltstone unit. The lower part of the units is massive and the upper part is laminated. It is underlain The Yampa Bed has been identified in lagoonal, fresh- (fig. 14) by a 2-in. thick layer of hard carbonaceous mudstone water swamp, and fluvial environments throughout the study and siltstone that is strongly laminated. The mudstone contains area in the lower part of the Williams Fork Formation. The plant material, and jarosite was observed. Where associated bed, regionally 0.5–5 ft thick, is generally poorly exposed in with lagoonal or fluvial rocks, the upper and lower contacts of outcrop and only nine locations have been studied in detail. the Yampa Bed contain rip-up clasts that again indicate minor Thin section analyses of several outcrop samples reveal transport. Where the altered ash is associated with clinkered coarse crystals of authigenic kaolinite, giving the material rocks it has been further altered to a highly resistant white to the appearance of a sandy siltstone (Brownfield and Johnson, cream-colored porcellanite. Origin, Description, and Distribution of the Yampa Bed 15

Figure 10. Excavated exposure of the Yampa Bed at the type locality, south of Craig, Colo. (see fig. 2, measured section 3). The Yampa Bed is about 165 ft above Trout Creek Sandstone Member of the Iles Formation and is a parting in the 12-ft-thick C-D coal bed. Upper contact rooted and burrowed. Upper and lower contacts contain bedding structures indicative of reworking in the Figure 11. Drill core of Yampa Bed in the C-D coal bed (Johnson mire. Note 24-in. shovel for scale. and Brownfield, 1988) from the Eagle Mine (also known as the Empire Mine, now abandoned). See fig. 2, mine location 5. The Yampa Bed is about 38 in. thick. Lower contact displays bedding structures indicative of reworking, whereas upper contact displays rooting, possible burrows, and bedding structures. Evidence that the Yampa Bed is a diagenetically altered airfall ash includes the following: (1) petrographic studies of thin sections reveal a relict vitroclastic texture that was pre- Four dating methods were considered to determine the sumably preserved by the clay-mineral replacement of the age of the Yampa Bed: (1) fission track dating of zircons, but original glass shards; (2) samples characteristically contain results from other similar claystones reportedly gave dates euhedral to subhedral phenocrysts of ß-quartz-form, biotite, that were known to be too young; (2) K-Ar dating of biotite, sanidine, andesine, and zircon, all of which are common but most of the biotite was altered to chlorite; (3) K-Ar dating in volcanic rocks; and (3) outcrop and subsurface studies using sanidine, but properly cleaning the sanidine was difficult; indicate a widespread areal distribution. Moreover, detailed and (4) K-Ar dating using plagioclase (andesine), which has a laboratory studies of similar diagenetically altered volcanic higher analytical error owing to the lower potassium content in ash in other study areas have shown that they originated as the plagioclase. However, using K-Ar methods on andesine, the airfall ash (Bohor and others, 1979; Ryer and others, 1980; unit was dated at 72.5±5.1 mega-annum (Ma) (Richard Marvin, Brownfield and others, 1987; Bohor and Triplehorn, 1993). U.S. Geological Survey, written commun., 1983). 16 The Yampa Bed—A Regionally Extensive Tonstein, Colorado

Figure 12. Closeup of excavated exposure of Yampa Bed at type locality (see fig. 2, measured section 3). The bed is 38 in. thick and displays a blocky texture. Upper and lower contacts are sharp with the C-D coal bed (Johnson and Brownfield, 1988). Upper contact is rooted and burrowed, and upper and lower contacts contain bedding structures resulting from reworking in the mire. Note 24-in. shovel and penny (white dot, upper left) for scale.

The Yampa Bed has been observed in several outcrops A possible source was the Late Cretaceous eruption of and on many geophysical logs throughout the southeastern the Elkhorn Mountains in southwestern Montana (Smedes, Sand Wash Basin and northeastern Piceance Creek Basin 1966) that produced a wide variety of volcanic rocks bracket- (fig. 1; pls. 1, 2). In particular, the Yampa Bed has been ing the ages of the Williams Fork Formation ammonite zones identified throughout the Yampa and Danforth Hills coal (Obradovich, 1993; Roberts and Kirschbaum, 1995); it has the fields (fig. 1), encompassing an area of more than 2,000 composition of the relict volcanic mineralogy of the Yampa mi2 (Brownfield and Johnson, 1986; Brownfield and others, Bed (Smedes, 1966). This source seems to provide the best 2000a,b; Johnson and others, 2000); all locations discussed correlation but the actual source remains highly uncertain. below are within the Yampa coal field (fig. 2). Origin, Description, and Distribution of the Yampa Bed 17

Figure 13. Excavated exposure of upper contact of Yampa Bed at Trapper Mine, south of Craig, Colo. (see fig. 2, measured section Figure 14. Excavated exposure of lower contact of the Yampa 3). Upper contact is rooted and contains sedimentary structures Bed (top of photograph) at Trapper Mine, south of Craig, Colo. (see indicating reworking. The 4-in.-thick carbonaceous shale on top of fig. 2, measured section 3). Lower contact contains sedimentary bed indicates reestablishment of the coal mire and marks the start structures indicating reworking. Carbonaceous shale at bottom of of deposition of the D coal bed (Johnson and Brownfield, 1988). Yampa Bed marks the termination of peat deposition of the C coal Note penny for scale. bed (Johnson and Brownfield, 1988). Note penny for scale.

Yampa Coal Field north and west sides of the park, the Twenty-mile Sandstone Member of the Williams Fork Formation forms massive, white Eckman Park Measured Section sandstone cliffs. The next younger sequence, the upper coal group, is about 190 ft thick and contains the 1- to 6-ft-thick Eckman Park, about 7 mi northwest of Oak Creek (fig. 2, Fish Creek coal bed (fig. 9) that was mined by Energy Fuels measured section 1), contains gently northward dipping strata Corporation in the 1970s. The Lewis Shale conformably over- of the middle coal group of the Williams Fork Formation, lies the Williams Fork. from which the Wadge coal bed (fig. 9) is mined at the Foidel The Yampa Bed crops out from 0 to about 15 ft above Creek (longwall) Mine (coal mine 6, fig. 2). In this area, the the Trout Creek Sandstone in Eckman Park (fig. 2) and ranges Trout Creek Sandstone Member of the Iles Formation is con- from 0 to less than 1 ft thick (figs. 9, 15). The bed lies about formably overlain by a 330-ft-thick section of the middle coal 5 ft above the Trout Creek Sandstone Member northeast of group (fig. 9). This section, in turn, is overlain by a 600-ft- Eckman Park (fig. 15) but was not preserved in the Edna thick sequence of marine strata containing late Campanian Mine area (fig. 2), where it was deposited and subsequently Baculites reesidei (Izett and others, 1971) above which, on the reworked in an estuary or salt marsh environment. 18 The Yampa Bed—A Regionally Extensive Tonstein, Colorado

Figure 15. Excavated exposure of Yampa Bed (white), northeast of Eckman Park (see fig. 2, measured section 1). Yampa Bed is approximately 5 ft above the Trout Creek Sandstone Member of the Iles Formation. Note 12-in. hammer above the Yampa Bed for scale.

Fish Creek Canyon Measured Section sandstone about 340 ft above the Tow Creek. Kucera (1959) applied the name “Oak Creek sandstone” to a 100-ft-thick Fish Creek Canyon contains one of the best stratigraphic cliff-forming sandstone that crops out about 3 mi north of sections (fig. 16) of the uppermost Mancos Shale and the Oak Creek (fig. 2). This unit was mapped by the authors from Mesaverde Group in the eastern part of the Yampa coal field that location through Trout Creek and Eckman Park to the (fig. 2, measured section 2). This section, previously mea- Fish Creek Canyon (fig. 2), where it is about 85 ft thick (fig. sured by Bass and others (1955), was remeasured and the 16). A strike valley above the Oak Creek sandstone is formed depositional environments were interpreted. On the west end in the marine strata that underlie the Trout Creek Sandstone. of Fish Creek Canyon there are excellent exposures of marine The Trout Creek, about 110 ft thick, is well exposed in this sandstone units in the upper part of the Mancos Shale, which section and the marine shale below the Trout Creek contains also contains shale units with Baculites perplexus (Izett and Exiteloceras jenneyi (Izett and others, 1971). Directly above others, 1971). The eastward dip of the canyon section is about the Trout Creek is carbonaceous shale that we interpret as 40° on the western end and 55° at the eastern end. The Iles a salt marsh; this shale, in turn, is overlain by rippled sand- Formation within the canyon contains four progradational stone and siltstone interpreted as a washover fan deposit. The shoreface sandstone units: they are, in ascending order, the Yampa Bed (Brownfield and Johnson, 1986), about 20 in. Tow Creek Sandstone Member, the informally named “Dou- thick, is exposed above the washover fan in lagoonal or mire ble-ledge sandstone” (Bass and others, 1955), the informally deposits (fig. 17) and is both underlain and overlain by carbo- named “Oak Creek sandstone” (Kucera, 1959), and the Trout naceous shale. The Wolf Creek coal bed is burned (clinkered) Creek Sandstone Member. The Tow Creek Sandstone Mem- in the canyon but the Wadge coal bed (about 11.8 ft thick) ber is the basal unit of the Iles Formation and is about 70 ft was mined for a short period. The Lennox coal bed was not thick. Bass and others (1955) informally named the “Double- observed. The Twentymile Sandstone Member, interpreted ledge sandstone” for a 250-ft-thick, massive ledge-forming to be a shoreface sandstone, forms a massive white cliff and Origin, Description, and Distribution of the Yampa Bed 19 dip slope on the eastern end of the canyon and is about 185 ft Danforth Hills Coal Field thick. The Twentymile and Trout Creek are much thicker than modern shoreface deposits, and Siepman (1985) believed that The Yampa Bed is in the lower part of the Fairfield these thicknesses resulted from vertical stacking of shoreface coal group of the Williams Fork Formation (Brownfield sand units. The overlying strata of the Williams Fork Forma- and others, 2000b) in the Danforth Hills coal field (figs. 1, tion represent lagoon and tidal inlet deposits that are, in turn, 23, 24). This coal group, which was named for the Fair- overlain by Lewis Shale. field Mine near the town of Meeker (fig. 23), is at the base of the Williams Fork and averages 1,300 ft in thickness (Hancock and Eby, 1930), but the complete interval was Ute Gulch Measured Section rarely penetrated in the drill holes studied. The Fairfield is equivalent to the Cameo-Wheeler coal zone in the southern The Yampa Bed crops out in Ute Gulch, south of Craig, part of the Piceance Basin and to the Wheeler-Fairfield Colo., (fig. 18) in the NW¼SW¼ sec. 9, T. 5 N., R. 91 W., coal zone along the southern part of the Grand Hogback approximately 170 ft above the Trout Creek Sandstone (Hettinger and others, 2000). It is correlated with the Member of the Iles Formation (see fig. 2, measured section middle coal group of the William Fork Formation (fig. 8) in 4). The unit there is about 4.3 ft thick (fig. 19) and is in the the Yampa coal field (Brownfield and others, 2000b). The C-D coal bed (figs. 4, 8) (Johnson and Brownfield, 1988). Fairfield coal group is predominately terrestrial in origin The upper and lower contacts are generally sharp with and consists of thin to thick beds of sandstone interbedded carbonaceous shale units both above and below. Even where with mudstone, carbonaceous shale, and coal deposited in a good exposures are present, the numerous and complexly coastal plain environment. distributed coals are burned and form baked strata or clinker Where exposed on the surface or observed in drill core (for example, see fig. 3), which makes surface mapping of (fig. 25), the Yampa Bed is a white to grayish-white structure- the coal beds and projection of coal beds into the subsurface less claystone with a sharp base and a rooted upper contact. In extremely difficult. the subsurface, it serves as an important regional marker bed that is easily identified on geophysical logs (Brownfield and Eagle Mine Measured Section others, 2000b). In the Danforth Hills coal field, the Yampa Bed ranges in thickness from less than 1 to more than 3 ft The Yampa Bed crops out just south of the Eagle Mine and lies from 100 and 300 ft above the top of the Trout Creek (also known as the Empire Mine, now abandoned; see fig. 2, Sandstone Member. mine location 1), in SE¼NE¼ sec. 36, T. 6 N., R. 92 W. (fig. 20). The bed there is approximately 170 ft above the Trout Creek Sandstone Member of the Mesaverde Group; it is Piceance and Sand Wash Basins about 38 in. thick and lies within the C-D coal bed (figs. 5, 8) An ash bed was excavated near the expected (Johnson and Brownfield, 1988). The upper and lower con- stratigraphic position of the Yampa Bed in Upper Cretaceous tacts are generally sharp and display sedimentary structures Mesaverde Group rocks in the northern part of the Piceance probably resulting from minor transport or ponding. The Creek Basin, in the western part of the Lower White River upper contact is rooted and overlain by carbonaceous mud- coal field (figs. 1, 8). Another ash bed, ranging from less stone or siltstone; the lower part of the carbonaceous unit than 1 to about 3 ft thick and lying above the Rollins is massive and the upper part is laminated. The unit below Sandstone Member of the Mesaverde Formation, has been consists of carbonaceous mudstone or siltstone that displays observed in outcrop and recognized on geophysical logs in strong lamination. the southern part of the Piceance Creek Basin and the Grand Mesa coal field (Eager, 1978; geophysical logs discussed Lay, Colo., Measured Section below). C.R. Dunrud (U.S. Geological Survey, retired, oral commun., 2007) reported a 3-ft-thick ash bed in the B About 1.5 mi south of Lay, Colo. (fig. 2, location 6), coal bed, 75 ft above the Rollins Sandstone Member of the the Yampa Bed crops out in the SE¼SW¼ sec. 31, T. 7 N., Mesaverde Formation, near Paonia (fig. 1) in the southern- R. 93 W., within the Peacock coal bed zone (Brownfield and most part of the Piceance Basin. In a measured section near Anderson, 1988; Brownfield and Prost, 1979; Johnson and Palisade, Colo., (fig. 1) Cole and Cumella (2003) reported Brownfield, 1988). At that location, the bed is about 25 in. an ash bed above the Rollins Sandstone Member of the thick and lies about 210 ft above the Trout Creek Sandstone Mesaverde Formation in the Book Cliffs coal field (fig. 1). Member of the Iles Formation (figs. 21, 22). The upper and That ash bed is 1 ft thick in the “ash coal” and lies about lower contacts are generally sharp and sedimentary structures 102 ft above the Rollins Sandstone. If one uses the projected indicate minor transport; the uppermost part is rooted, indi- Rollins Sandstone Member or Trout Creek Sandstone cating the reestablishment of the mire following deposition Member strandlines of Zapp and Cobban (1960), that ash bed of the ash bed. Units above and below the ash consist of is in the same stratigraphic position as the Yampa Bed in the carbonaceous mudstone and siltstone. Danforth Hills and Yampa coal fields. 20 The Yampa Bed—A Regionally Extensive Tonstein, Colorado Shale Lewis

Twentymile Sandstone Member

EXPLANATION Williams Fork Formation

Marsh or swamp

Wadge coal bed

Shoreface Yampa Bed Trout Creek Sandstone Member Alluvial and coastal plain

Lower shoreface and offshore marine Oak Creek sandstone Yampa Bed Iles Formation Double-ledge sandstone

Tow Creek Sandstone

500 FEET Member Shale Mancos

Figure 16. Strata in Fish Creek Canyon (see fig. 2, measured section 2) showing interpreted depositional environments and position of Yampa bed (Brownfield and others, 1999). Interpreted depositional environments compiled from U.S. Geological Survey field studies. Major geologic units modified after Bass and others (1955). Origin, Description, and Distribution of the Yampa Bed 21

Figure 17. Excavated exposure of Yampa Bed in Fish Creek Canyon, west of Oak Creek, Colo. (see fig. 2, measured section 2). Yampa Bed lies above a washover fan in lagoonal or mire deposits and is about 20 in. thick. Directly above Trout Creek Sandstone Member of the Iles Formation (far left) is carbonaceous shale that is interpreted as a salt marsh deposit and is overlain by a washover fan consisting of rippled sandstone and siltstone. Note 16-in. shovel below ash and 12-in. hammer above ash for scale.

Figure 18. Outcrop of Yampa Bed in Ute Gulch (see fig. 2, measured section 4), northwestern Colorado. Yampa Bed is approximately 170 ft above Trout Creek Sandstone Member of the Iles Formation and is 4.3 ft thick in the C-D coal bed (Johnson and Brownfield, 1988). Much of the coal-bearing interval has been burned or baked forming baked strata or clinker. Area within white box shown in figure 19. 22 The Yampa Bed—A Regionally Extensive Tonstein, Colorado

Figure 19. Closeup of excavated exposure (see fig. 18) of Figure 20. Excavated exposure of the Yampa Bed south of the the Yampa Bed in Ute Gulch (see fig. 2, measured section 4), Eagle Mine (also known as the Empire Mine, now abandoned; northwestern Colorado. Yampa Bed is approximately 170 ft above see fig. 1, mine location 1), northwestern Colorado. Yampa Bed is the Trout Creek Sandstone Member of the Iles Formation and is 4.3 approximately 170 ft above Trout Creek Sandstone Member of the ft thick within the C-D coal bed (Johnson and Brownfield, 1988). Iles Formation and is 38 in. thick within the C-D coal bed (Johnson Note 16-in. shovel in center of photo for scale. and Brownfield, 1988). Note 12-in. hammer for scale.

The Yampa Bed has also been traced with geophysical harder to identify in the northwestern and westernmost parts logs from the central part of the Yampa coal field to near the of the study area owing to local reworking of the ash by fluvial Wyoming state line in the Sand Wash Basin. The unit becomes processes. Geophysical Log Response 23

Figure 21. Outcrop of Peacock coal bed (Brownfield and Prost, 1979; Brownfield and Anderson, 1988) and the Yampa Bed south of Lay, Colo. (see fig. 2, measured section 6). Yampa Bed is about 210 ft above Trout Creek Sandstone Member of the Iles Formation of the Mesaverde Group. Area within white box shown in figure 22.

Figure 22. Closeup of excavated exposure of the Peacock coal bed (fig. 21) (Brownfield and Prost, 1979; Brownfield and Anderson, 1988) and the Yampa Bed, south of Lay, Colo. (see fig. 2, measured section 6). Yampa Bed is about 210 ft above Trout Creek Sandstone Member of the Iles Formation of the Mesaverde Group and is 25 in. thick. Note 12-in. hammer for scale.

Geophysical Log Response

The Yampa Bed is easily recognized on geophysical logs—the Buckpeak State 11–30 and the USGS BRG–3 drill holes are excellent examples of the distinctive geophysical log signatures of the unit (fig. 26)—and therefore it is a valuable marker for both local and regional subsurface correlations (see next section). On oil and gas logs, the bed is characterized by a low resistivity response that can readily be identified by a corresponding sharp high-conductivity spike. On coal logs, it typically registers the lowest low single-point resistance value within the logged interval (figs. 26, 27). The natural gamma response is typically high. Yampa Bed density log responses, although higher than those for coal, are generally not useful for reliable identification (figs. 26, 27). Log responses can differ because of differences in the mineral composition of the altered ash, so the bed can display different combinations of low resistivity–high conductivity and high gamma responses (the low resistivity–high conductivity response is the more common). 24 The Yampa Bed—A Regionally Extensive Tonstein, Colorado T. 4 N. T. 3 N. T. 2 N. T. T. 1 N. T. R. 92 W. R. 92 W. COLORADO 13 Danforth Hills coal field Colowyo coal mine Axial R. 93 W. 13 T. 5 N. T. Meeker Fairfield Mine R. 94 W 13 64 COUNTY RIO BLANCO MOFFAT COUNTY MOFFAT R. 95 W. 5 MILES 0 R. 96 W. 1 Danforth Hills coal field (shades of green) in Moffat and Rio Blanco Counties, northwestern Colorado, showing general geology, outline Danforth Hills coal field (shades of green) in Moffat and Rio Blanco Counties, northwestern Colorado, showing general geology,

EXPLANATION Alluvial deposit (Quaternary), Browns Park, Green River, and Fms. (Tertiary), Wasatch Mancos Shale (Cretaceous) Williams Fork Formation Iles Formation Mancos Shale Fault— Bar and ball side on downthrown County line Colorado State highway Undivided Quaternary, Tertiary, Undivided Quaternary, Cretaceous Upper Cretaceous Figure 23. of Danforth Hills coal field drawn on the base Iles Formation, area Colowyo mine, and location abandoned Fair Mine. Origin, Description, and Distribution of the Yampa Bed 25

EXPLANATION Fm. Wasatch

Alluvial, coastal plain, and mire

Upper shoreface and estuarine Paleocene-Eocene Fort Union Fm. Lower shoreface and offshore marine Yampa Bed Unconformity Lion Canyon coal group FGG

Lion Canyon Ss. Mbr.-

FGF Goff coal group

FGE

Barren interval

FGD Williams Fork Formation of the Mesaverde Group

FGC 250 FEET

Yampa Bed Fairfield coal Upper Cretaceous FGB group

FGA Yampa Bed Trout Creek Ss. Mbr. Black Diamond coal group Iles Formation Lower coal group of the Mesaverde Group 1,000 FEET Mancos Sh.(part)

Figure 24. Generalized stratigraphic column showing depositional environments for a portion of the Upper Cretaceous and Tertiary rocks in the Danforth Hills coal field (fig. 1) about 25 mi south of Craig, Colo. (see fig. 1). Shown are major divisions of the Fairfield coal group (FGA– FGG) (Brownfield and others, 2000b) and stratigraphic position of Yampa Bed within the Williams Fork Formation. Yampa Bed not drawn to scale. Fm., Formation; Mbr., Member; Sh., Shale; Ss., Sandstone. Modified after Hancock and Eby (1930) and Brownfield and others (2000b). 26 The Yampa Bed—A Regionally Extensive Tonstein, Colorado

Figure 25. Drill core from Danforth Hill coal field, south of Colowyo Mine (pl. 2, part A) about 25 mi south of Craig, Colo. (fig. 1). Yampa Bed here is 21.5 in. thick and lies about 170 ft above Trout Creek Sandstone Member of the Iles Formation.

in the Piceance and Sand Wash Basins and, in particular, the Yampa Bed as a Correlation Tool Yampa and Danforth Hills coal fields. These cross sections are The Mesaverde Group is characterized by complex as follows: intertonguing that generally makes regional correlation of Figure 27. Correlation of the E coal bed (Johnson and lithofacies difficult; in early stratigraphic studies in north- Brownfield, 1988) and associated lithofacies in the lower part western Colorado and adjacent areas, this difficulty led to of the Williams Fork Formation in the western part of the errors in regional correlations of coal beds and associated Yampa coal field. lithofacies. Since its importance as a precise regional time Figure 28. Correlation of the Trout Creek Sandstone line was recognized many years ago, however, the Yampa Member of the Iles Formation and the Twentymile Sandstone Bed has been used extensively as a datum in subsurface Member of the Williams Fork Formation across the western research. The Yampa Bed has been used to correlate coal part of the Yampa coal field. beds in the Yampa and Danforth Hills coal fields and in Plate 1, cross sections A-A’ and B-B’—Correlation of regional lithofacies correlations of the Mesaverde Group units and distribution of coal zones within the middle coal throughout northwestern Colorado (Affolter and Brownfield, group and the Twentymile Sandstone Member of the Williams 1987; Brownfield and Affolter, 1988; Johnson and Brown- Fork Formation and correlation of the Trout Creek Sandstone field, 1988; Brownfield and others, 2000b; Johnson and Member of the Iles Formation across the Yampa coal field. others, 2000). Plate 2, cross section A-A’—Correlation of units and Five cross sections that use drill-hole geophysical logs distribution of coal zones within the Fairfield coal group of the are presented to illustrate the importance and usefulness of the Williams Fork Formation and Trout Creek Sandstone Member Yampa Bed as a regional datum in stratigraphic correlations of the Iles Formation across the Danforth Hill coal field. Yampa Bed as a Correlation Tool 27

Oil and Gas Exploration Coal Exploration

Buckpeak State 11-30 BRG-31 T. 6 N., R. 89 W., Sec. 30 T. 6 N., R. 92 W., Sec. 26

Spontaneous Resistivity Conductivity Natural Density Single-point potential gamma resistance

1,000'

1,200'

1,400' 1,100' Yampa Bed

1,600'

1,200'

1,800' A B 1 Johnson and Hook (1985) EXPLANATION

Yampa Bed

Trout Creek Sandstone Member of the Iles Formation

Figure 26. Examples of geophysical log responses of the Yampa Bed. A, oil and gas exploration geophysical log showing spontaneous potential, resistivity, and conductivity logs; B, coal exploration geophysical log showing natural gamma, density, and single- point resistance logs. 28 The Yampa Bed—A Regionally Extensive Tonstein, Colorado 100' 50' 0 R D 4 NG

750'

d

e

b

l

a

o

c

5.3 MILES E Yampa Bed Yampa D R 3 SP EXPLANATION NG R–8–HG; sec. 25, T. 6 N., R. 93 W. (Meyer, 1978) (Meyer, 6 N., R. 93 W. R–8–HG; sec. 25, T. (Jonhson and Brown, 1979) 5 N., R. 92 W. E–21–RdB; sec. 10, T. (Johnson, 1978) 5 N., R. 91 W. E–1–RdB; sec. 5, T. (Johnson and Hook, 1985) 5 N., R. 90 W. CG–8; sec, 8, T. D Gamma-gamma density NG Natural gamma R Single-point resistance SP Spontaneous potential

1 2 3 4

d

e b

1,000'

l

a

o

c

E 4.5 MILES Yampa Bed Yampa D Yampa Bed Yampa E coal bed R 2 SP NG T. 5 N. T.

T. 6 N. T. T. 7 N. T.

1,050'

d e

b

l

4

a

o

c

R. 90 W. E Yampa Bed Yampa 5 MILES 6 MILES 3 R. 91 W. D 0 Index Map R 1 2 R. 92 W. Subsurface correlation of E coal bed in Williams Fork Formation, western part of the Yampa coal field, using Yampa Bed as datum. Based on Johnson (1978) and coal field, using Yampa Subsurface correlation of E coal bed in Williams Fork Formation, western part the Yampa SP

NG 1 R. 93 W. Figure 27. Johnson and Brownfield (1988). 1,150' Yampa Bed as a Correlation Tool 29 C R

4 2,000' 1,000' 1,500' SP C Sandstone Bed, altered volcanic Yampa ash Exploration well 5.5 MILES R Resistivity C Conductivity SP Spontaneous potential R

3 3,000' 3,500' 4,000'

SP EXPLANATION 6.5 MILES C R

2

1,500' 500' 1,000' sec. 11, T. 6. N., R. 92 W. sec. 11, T. 6 N., R. 88 W. 0–28–6–88W sec. 28, T. sec. 15, T. 6 N., R. 90 W. sec. 15, T. 6 N., R. 89 W. sec. 15, T. SP Enstar Petroleum Inc., Silver 9–11; Exploration, Leukulich No. 1; Trend Exploration, State No. 1–16; Trend Fuelco Dry Creek, Creek Unit 1 2 3 4 T. 5 N. T. T. 6 N. T. 4 R. 88 W. Yampa Bed Yampa 6 MILES 3 10.5 MILES R. 89 W. Trout Creek Sandstone Member Trout Twentymile Sandstone Member Twentymile 0 2 C R. 90 W. Index Map R. 91 W. Subsurface correlation of Trout Creek Sandstone Member of the Iles Formation and Twentymile Sandstone Member of the Williams Fork Formation in the Yampa Sandstone Member of the Williams Fork Formation in Yampa Creek Sandstone Member of the Iles Formation and Twentymile Subsurface correlation of Trout R

1

1 2,000' 1,500' 1,000' R. 92 W. SP Figure 28. Bed as datum. Based on Johnson and Brownfield (1988). coal field, using Yampa 30 The Yampa Bed—A Regionally Extensive Tonstein, Colorado

Summary Bass, N.W., Eby, J.B., and Campbell, M.R., 1955, Geology and mineral fuels of parts of Routt and Moffat Counties, Colo- rado: U.S. Geological Survey Bulletin 1027–D, p. 143–250. The Yampa Bed is a formal name given to a regionally persistent tonstein bed that was deposited in the Late Creta- Bohor, B.F., Hatch, J.R., and Hill, D.J., 1976, Altered volcanic ceous as an airfall ash that settled in several different coeval ash partings as stratigraphic marker beds in coals of Rocky environments during the deposition of the Williams Fork Mountain Region [abs.]: American Association of Petro- Formation. The name is derived from the Yampa River valley leum Geologists Bulletin, v. 60, p. 651. in northwestern Colorado. The diagenetically altered ash Bohor, B.F., Phillips, R.E., and Pollastro, R.M., 1979, Altered bed consists primarily of kaolinite and smectitic clays with a volcanic ash partings in Wasatch Formation coal beds of the minor nonclay mineral fraction that includes ß-quartz-form northern Powder River Basin—Composition and geologic phenocrysts, biotite, sanidine, andesine, and zircon. Outcrop applications: U.S. Geological Survey Open-File Report and subsurface studies indicate that the unit has a wide areal 79–1203, 38 p. distribution in the Yampa, Danforth Hills, and Grand Hog- back coal fields, Moffat and Routt Counties, northwestern Bohor, B.F., and Triplehorn, D.M., 1993, Tonsteins—Altered Colorado. The Yampa bed was dated at 72.5±5.1 Ma using volcanic-ash layers in coal-bearing sequences: Geological K-Ar methods on andesine. The persistent tonstein bed is Society of America Special Paper 285, 44 p. here named the Yampa Bed of the Williams Fork Formation. The Yampa Bed was named for the Yampa River valley. Boyles, M.J., Kauffman, E.G., Kiteley, L.W., and Scott, A.J., Evidence that the Yampa Bed is a diagenetically altered 1981, Depositional systems of Upper Cretaceous Mancos airfall ash consists of the following: (1) petrographic studies Shale and Mesaverde Group, northwestern Colorado: Soci- of thin sections reveal a relict vitroclastic texture that was ety of Economic Paleontologists and Mineralogists, Rocky presumably preserved by the clay-mineral replacement of the Mountain Section field trip guidebook, 146 p. original glass shards; (2) samples characteristically contain Brownfield, M.E., and Affolter, R.H., 1988, Characterization grains of euhedral to subhedral ß-quartz-form phenocrysts of coals in the lower part of the Williams Fork Formation, and biotite, sanidine, andesine, and zircon, all of which are Twentymile Part district, eastern Yampa coal field, Routt common in volcanic rocks; and (3) outcrop and subsurface County, Colorado, in Carter, L.M.H., ed., USGS research studies indicate a widespread areal distribution. on energy resources, 1988 Program and Abstracts: U.S. The Yampa Bed is extremely useful as a regional datum Geological Survey Circular 1025, p. 64–65. in correlations of facies and paleogeographic reconstruc- tions within the Upper Cretaceous coal-bearing rocks in the Brownfield, M.E., Affolter, R.H., and Stricker, G.D., 1987, Piceance Creek and southeastern Sand Wash Basins. Crandallite group minerals in the Caps and Q coal beds, Tyonek Formation, Beluga energy resource area, upper Cook Inlet, south-central Alaska, in Rao, P.D., ed., Focus on Alaska’s Coal ’86, Proceedings: Fairbanks, University of Acknowledgments Alaska MIRL Report 72, p. 142–149.

The authors wish to thank Mary-Margaret Coates, Brownfield, M.E., and Anderson, K., 1988, Geologic map and Thomas Finn, Ronald Johnson, William Keefer, and Douglas coal sections of the Lay SE quadrangle, Moffat County, Nichols for their suggestions, comments, and editorial reviews, Colorado: U.S. Geological Survey Coal Investigation Map which greatly improved the manuscript. C–117, scale: 1:24,000. Brownfield, M.E., Hettinger, R.D., and Johnson, E.A., 2000, A summary of the stratigraphy, coal resources, and coal-bed methane potential of northwest Colorado, in Kirschbaum, References Cited M.A., Roberts, L.N.R., and Biewick, L.R.H., eds., Geo- logic assessment of coal in the Colorado Plateau—Arizona, Affolter, R.H., and Brownfield, M.E., 1987, Characterization Colorado, New Mexico, and Utah: U.S. Geological Survey of lower Williams Fork Formation coals from the eastern Professional Paper 1625–B, chap. I, 25 p., CD–ROM. Yampa coal field, Routt County, Colorado [abs.]: Geologi- cal Society of America Abstracts with Programs 1987, Brownfield, M.E., and Johnson, E.H., 1986, A regionally v. 19, no. 7, p. 567. extensive altered air-fall ash for use in correlation of lithofa- cies in the Upper Cretaceous Williams Fork Formation, Barnum, B.E., and Garriques, R.S., 1980, Geologic map northwest Piceance Creek and southern Sand Wash Basins, and coal sections of the Cactus Reservoir quadrangle, Rio Colorado, in Stone, D.S., ed., New interpretations of north- Blanco and Moffat Counties, Colorado: U.S. Geological west Colorado geology: Rocky Mountain Association of Survey Mineral Investigations Field Map MF–1179. Geologists 1986 symposium, p. 293–295. References Cited 31

Brownfield, M.E., Johnson, E.A., Affolter, R.H., and Barker, Hail, W.J., Jr., 1974, Geologic map of the Rough Gulch quad- C.E., 1999, Coal mining in the 21st century—Yampa coal rangle, Rio Blanco and Moffat Counties, Colorado: U.S. field, northwest Colorado, in Lageson, D.R., Lester, A.P., Geological Survey Geologic Quadrangle Map GQ–1195, and Trudgill, B.D., eds., Colorado and adjacent areas: Geo- scale 1:24,000. logical Society of America Field Guide 1, p. 115–133. Hancock, E.T., 1925, Geology and coal resources of the Axial Brownfield, M.E., and Prost, G.L., 1979, Geologic map and and Monument Butte quadrangles, Moffat County, Colo- coal sections of the Lay quadrangle, Moffat County, Colo- rado: U.S. Geological Survey Bulletin 757, 134 p. rado: U.S. Geological Survey Open-File Report 79–1679, scale: 1:24,000. Hancock, E.T., and Eby, J.B., 1930, Geology and coal resources of the Meeker quadrangle, Moffat and Rio Blanco Brownfield, M.E., Roberts, L.N.R., Johnson, E.A., and Mercier, Counties, Colorado: U.S. Geological Survey Bulletin T.J., 2000a, Assessment of the distribution and resources of 812–C, p. 191–242. coal in the Deserado coal area, lower White River coal field, northern Piceance Basin, Colorado, in Kirschbaum, M.A., Haun, J.D., and Weimer, R.J., 1960, Cretaceous stratigraphy Roberts, L.N.R., and Biewick, L.R.H., eds., Geologic assess- of Colorado, in Weimer, R.J., and Haun, J.D., eds., Guide ment of coal in the Colorado Plateau—Arizona, Colorado, to the geology of Colorado: Geological Society of America, New Mexico, and Utah: U.S. Geological Survey Digital Rocky Mountain Association of Geologists, and Colorado Professional Paper 1625–B, chap. N, 37 p., CD–ROM. Scientific Society guidebook, p. 58–65.

Brownfield, M.E., Roberts, L.N.R., Johnson, E.A., and Hettinger, R.D., Roberts, L.N.R., and Gognat, T.A., 2000, Mercier, T.J., 2000b, Assessment of the distribution and Investigations of the distribution and resources of coal resources of coal in the Fairfield coal group of the Williams in the southern part of the Piceance Basin, Colorado, Fork Formation, Danforth Hills coal field, northwest Colo- in Kirschbaum, M.A., Roberts, L.N.R., and Biewick, rado, in Kirschbaum, M.A., Roberts, L.N.R., and Biewick, L.R.H., eds., Geologic assessment of coal in the Colorado L.R.H., eds., Geologic assessment of coal in the Colorado Plateau—Arizona, Colorado, New Mexico, and Utah: U.S. Plateau—Arizona, Colorado, New Mexico, and Utah: U.S. Geological Survey Professional Paper 1625–B, chap. O, Geological Survey Professional Paper 1625–B, Chap. M, 106 p., CD–ROM. 28 p., CD–ROM. Campbell, M.R., 1923, The Twentymile Park district of the Holmes, W.H., 1877, Report (on the San Juan district, Yampa coal field, Routt County, Colorado: U.S. Geological Colorado): U.S. geological and geographical survey of the Survey Bulletin 748, 82 p. Territories, 9th annual report for 1875, p. 237–276. Cole, Rex, and Cumella, Steve, 2003, Stratigraphic architec- Izett, G.A., Cobban, W.A., and Gill, J.R., 1971, The Pierre ture and reservoir characteristics of the Mesaverde Group, Shale near Kremmling, Colorado, and its correlation to the southern Piceance Basin, Colorado, in Peterson, K.M., east and west: U.S. Geological Survey Professional Paper Olson, T.M., and Anderson, D.S., eds., Piceance Basin 684–A, 19 p. 2003, Rocky Mountain Association of Geologists field trip Johnson, E.A., 1978, Geophysical logs for 18 holes drilled guidebook, p. 385–442. during 1977 in the Round Bottom area, Yampa coal field, Collins, B.A., 1976, Coal deposits of the Carbondale, Grand Moffat County, Colorado: U.S. Geological Survey Open- Hogback, and southern Danforth Hills coal field, eastern File Report 78–229, 48 p. Piceance Basin, Colorado: Colorado School of Mines Quar- terly, v. 71, no. 1, 138 p. Johnson, E.A., 1987, Geologic map and coal sections of the Round Bottom quadrangle, Moffat County, Colorado: U.S. Crawford, R.D., Willson, K.W., and Perini, V.C., 1920, Some Geological Survey Coal Investigations Map C–108, scale anticlines of Routt County, Colorado: Colorado Geological 1:24,000. Survey Bulletin 23, 59 p. Johnson, E.A., and Brown, Robert, 1979, Geophysical logs Eager, G.P., 1978, Geophysical logs of coal test drill holes in the for six holes drilled during 1978 in the Round Bottom area, Grand Mesa coal field, Delta and Mesa Counties, Colorado: Yampa coal field, Moffat County, Colorado: U.S. Geologi- U.S. Geological Survey Open-File Report 78–540, 97 p. cal Survey Open-File Report 79–328, 28 p.

Fenneman, N.M., and Gale, H.S., 1906a, The Yampa coal Johnson, E.A., and Brownfield, M.E., 1986, A regionally field, Routt County, Colorado: U.S. Geological Survey Bul- extensive altered air-fall ash for use in correlation of lithofa- letin 285–F, p. 226–239. cies in the Upper Cretaceous Williams Fork Formation, Fenneman, N.M., and Gale, H.S., 1906b, The Yampa coal field, northeast Piceance Creek and southern Sand Wash Basins, Routt County, Colorado: U.S. Geological Survey Bulletin Colorado [abs.]: American Association of Petroleum Geolo- 297, 96 p. gists Bulletin, v. 70, no. 11, p. 1763. 32 The Yampa Bed—A Regionally Extensive Tonstein, Colorado

Johnson, E.A., and Brownfield, M.E., 1988, Regional cor- Roberts, L.N., and Kirschbaum, M.A., 1995, Paleogeography relations of the middle coal group of the Upper Cretaceous of the late Cretaceous of the Western Interior of Middle Mesaverde Group, Yampa coal field, Moffat and Routt North America—Coal distribution and sediment accumu- Counties, Colorado: U.S. Geological Survey Coal Series lation: U.S. Geological Survey Professional Paper 1561, Map C–123. 115 p.

Johnson, E.A., and Hook, L.H., 1985, Geophysical logs for 34 Roehler, H.W., 1987, Surface-subsurface correlations of the holes drilled during 1980 in the Yampa coal field, Moffat Mesaverde Group and associated Upper Cretaceous forma- and Routt Counties, Colorado: U.S. Geological Survey tions, Rock Springs, Wyoming, to Mount Harris, Colorado: Open-File Report 85–43, 125 p. U.S. Geological Survey Miscellaneous Field Studies map Johnson, E.A., Roberts, L.N.R., and Brownfield, M.E., 2000, MF–1937. Geology and resource assessment of the middle and upper Ryer, T.A., Phillips, R.E., Bohor, B.F., and Pollastro, R.M., coal groups in the Yampa coal field, northwestern Colo- 1980, Use of altered volcanic ash falls in stratigraphic stud- rado, in Kirschbaum, M.A., Roberts, L.N.R., and Biewick, ies of coal-bearing sequences—An example from the Upper L.R.H., eds., Geologic assessment of coal in the Colorado Cretaceous Ferron Sandstone Member of the Mancos Shale Plateau—Arizona, Colorado, New Mexico, and Utah: U.S. in central Utah: Geological Society of America Bulletin, Geological Survey Professional Paper 1625–B, chap. P, part I, v. 91, p. 579–586. CD–ROM, 81 p. Siepman, B.R., 1985, Stratigraphy and petroleum potential Kitely, L.W., 1983, Paleogeography and eustatic-tectonic model of Trout Creek and Twentymile Sandstones (Upper Creta- of late Campanian (Cretaceous) sedimentation, southwestern ceous), Sand Wash Basin, Colorado: Colorado School of Wyoming and northwestern Colorado, in Reynolds, M.W., Mines Quarterly, v. 80, no. 2, 59 p. and Dolly, E.D., eds., Mesozoic Paleogeography of the West- Central United States—Rocky Mountain Paleogeography Smedes, H.W., 1966, Geology and igneous petrology of the Symposium 2: Society of Economic Paleontologists and northern Elkhorn Mountains, Jefferson and Broadwater Mineralogists Rocky Mountain Section, p. 273–302. Counties, Montana: U.S. Geological Survey Professional Kucera, R.E., 1959, Cretaceous stratigraphy of the Yampa Paper 510, 116 p. district, northwestern Colorado, in Haun, J.D., and Weimer, Triplehorn, D.M., and Bohor, B.F., 1981, Altered volcanic R.J., eds., Cretaceous Rocks of Colorado and Adjacent ash parting in the Ferron Sandstone Member of the Mancos Areas,: Rocky Mountain Association of Geologists 11th Shale, Emery County, Utah: U.S. Geological Survey Open- annual field conference guidebook, p. 37–45. File Report 81–0775, 45 p. Masters, C.D., 1966, Sedimentology of the Mesaverde Group Tweto, Ogden, 1976, Geologic map of the Craig 1° × 2° and the upper part of the Mancos Formation, northwest quadrangle, northwestern Colorado: U.S. Geological Colorado: New Haven, Conn., Yale University, Ph.D. Survey Miscellaneous Investigations Series I–972, scale dissertation, 88 p. 1:250,000. Meyers, R.F., 1978, Geophysical logs of 20 holes drilled in 1977 in the Yampa coal field, Hamilton, Horse Gulch, and Weimer, R.J., 1960, Upper Cretaceous stratigraphy of the Pagoda quadrangles, Moffat County, Colorado: U.S. Geo- Rocky Mountain area: U.S. Geological Survey Bulletin, logical Survey Open-File Report 78–366, 51 p. v. 44, part 1, p. 1–20. Obradovich, J.D., 1993, A Cretaceous time scale, in Caldwell, Zapp, A.D., and Cobban, W.A., 1960, Some Late Cretaceous W.G.E., and Kauffman, E.G., eds., Evolution of the Western strand lines in northwestern Colorado and northeastern Interior Basin: Geological Association of Canada Special Utah: U.S. Geological Survey Professional Paper 400–B, Paper 39, p. 379–396. p. B246–B249.