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0.6 +/- 0.6

3.8 +/- 1.2

1.0 +/- 0.3

0.9 +/- 0.3

1.3 +/- 0.3

0.8 +/- 0.7

0.8 +/- 0.4 1.1 +/- 0.4

1.0 +/- 0.4

1.5 +/- 0.51.0 +/- 0.4

2.2 +/- 0.82.9 +/- 0.9

2.637 +/- 0.166 3.518 +/- 0.1665.067 +/- 0.288

0.013 +/- 0.002

0.117 +/- 0.007

0.019 +/- 0.001

0.024 +/- 0.001

0.652 +/- 0.026

0.287 +/- 0.023

0.155 +/- 0.0090.110 +/- 0.009

0.152 +/- 0.012

0.603 +/- 0.028

0.013 +/- 0.001

0.981 +/- 0.0390.105 +/- 0.005

1.073 +/- 0.086

0.056 +/- 0.004

0.095 +/- 0.029 0.108 +/- 0.005

0.205 +/- 0.009

0.024 +/- 0.010

0.042 +/- 0.002 0.183 +/- 0.008

0.031 +/- 0.006

0.018 +/- 0.004

0.018 +/- 0.012

0.013 +/- 0.010

0.225 +/- 0.028

0.096 +/- 0.004

0.077 +/- 0.017

0.016 +/- 0.0970.012 +/- 0.017

0.239 +/- 0.015

0.270 +/- 0.021

0.127 +/- 0.020

1.113 +/- 0.040

0.237 +/- 0.017

0.227 +/- 0.013

0.326 +/- 0.016

0.217 +/- 0.1270.515 +/- 0.019

0.120 +/- 0.006

0.014 +/- 0.0020.308 +/- 0.025

0.089 +/- 0.013

0.014 +/- 0.005

0.211 +/- 0.012

0.188 +/- 0.0110.191 +/- 0.008

0.114 +/- 0.0050.133 +/- 0.0060.105 +/- 0.005

3.226 +/- 0.2582.840 +/- 0.1612.747 +/- 0.220

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T50N

T50N

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R80WR85W

T40N

T40N

T35N

T35N

T55N

T30N

T55N

T50N

T45N

T40N

R65W

R70WR75WR80WR85W R65W

R70WR75WR80WR85W R65W

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T30N

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R50E R55E

107°30'0"W

107°0'0"W

107°0'0"W

106°30'0"W

106°30'0"W

106°0'0"W

106°0'0"W

105°30'0"W

105°30'0"W

105°0'0"W

105°0'0"W 104°30'0"W

46°30'0"N

46°0'0"N

46°0'0"N

45°30'0"N

45°30'0"N

45°0'0"N

45°0'0"N

44°30'0"N

44°30'0"N

44°0'0"N

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43°30'0"N

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Block diagram of clinker-dominated landscape (from Heffern and Coates, 1997)

Schematic cross section B–B', Tongue River Valley, near Ashland, MT

(Cross sections from Heffern and others, 2007)

Schematic cross section A–A', Rochelle Hills, east of Wright, WY

Cross section

Ma = millions of years

0

0 5 km

5 mi

0 4 miles2

clinker of Knobloch coal zone

clinker of Anderson coal zone

clinker of other coal beds

0.227+/- 0.013

4921 ft

4593 ft

3937 ft

3281 ft

The Rochelle Hills escarpment capped by the clinker formed by the burning of the Wyodak coalbed, at Keyton Canyon east of Wright, Wyoming.

Ed Heffern below the base of the clinker formed by the burning of the Knobloch coalbed in the Tongue River valley north of Ashland, Montana. Collapsed blocks of reddish baked sandstone overlie the basal ash zone that marks the original base of the coalbed, underneath which is unburned sandstone.

A 25-foot high chimney of hard, welded breccia and paralava formed by the natural burning of the Wyodak coalbed in the Rochelle Hills, Wyoming. Note the person (white oval) standing on the right side of the chimney.

Mark Engle of the U.S. Geological Survey sampling gases coming off a coalbed fire along the banks of the Tongue River north of Sheridan, Wyoming.

View of collapse along burn front of the same active coalbed fire shown in picture to left, north of Sheridan, Wyoming. The reddish color in parts of the collapsed area marks areas where clinker has formed.

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0.5 +/- 0.30.8 +/- 0.5

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0.5 +/- 0.40.3 +/- 0.3

0.3 +/- 0.2

0.4 +/- 0.2

0.5 +/- 0.30.4 +/- 0.2

0.7 +/- 0.3

0.198 +/- 0.011 0.482 +/- 0.027

0.127 +/- 0.010

0.536 +/- 0.025

0.205 +/- 0.012

0.502 +/- 0.028

0.550 +/- 0.0310.615 +/- 0.035

0.216 +/- 0.012

0.120 +/- 0.008

0.010 +/- 0.001

0.115 +/- 0.008

0.219 +/- 0.009

0.211 +/- 0.012

0.213 +/- 0.012

0.096 +/- 0.0060.249 +/- 0.023

0.007 +/- 0.006

0.016 +/- 0.006

Inset Map 2R70W

T40N

R69W R68W R67W

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0.352 +/- 0.027

0.301 +/- 0.045

0.396 +/- 0.032

1.009 +/- 0.073

0.202 +/- 0.030

0.924 +/- 0.244

0.578 +/- 0.080

0.498 +/- 0.051

0.802 +/- 0.029

0.565

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0.242 +/- 0.0690.130 +/- 0.022

0.562 +/- 0.0340.275 +/- 0.120

0.334 +/- 0.132

0.456 +/- 0.075

Inset Map 1

1 Mile

1 Km

MTWY

R84W R83WR85W

T56N

T57N

T9S

R36E R37E

T9S

T10S

A

A'

B

B'

Forsyth

Miles City

Colstrip

Lame Deer Ashland

Busby

Crow Agency

Broadus

Ekalaka

Hulett

Gillette

Wright

Douglas

Casper

Kaycee

Buffalo

Sheridan

Lodge Grass

Pumpkin Buttes

Rochelle

Hills

Bear LodgeMountains

See Inset Map 1

see Inset Map 2

MontanaWyoming

Custer CountyPowder River County

Fallon CountyCarter County

Big Horn CountyTreasure County

Yellowstone County

Ros

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Campbell CountyConverse County

Johnson CountyNatrona County

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Crook CountyWeston County

Big Horn CountyWashakie County

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Antelope Mine

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Bigh

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Coal CreekMine

Cordero-RojoMine

Caballo MineBelle AyrMine

WyodakMine

Dry Fork Mine

Eagle ButteMine

RawhideMine

BuckskinMine

East DeckerMine

SpringCreekMine

Dave Johnston

Mine

RosebudMine

AbsalokaMine

#I

West DeckerMine

Clinker Distribution and Age in the Powder River Structural Basin

ByEdward L. Heffern, U.S. Bureau of Land Management (Retired)

Peter W. Reiners, University of Arizona, Tucson, AZCatherine A. Riihimaki, Drew University, Madison, NJ

2013

Age of ClinkerHow long ago did coal at a given location burn to form clinker? Clinker can be dated by several means, including uranium-thorium / helium ratios and fission-track counts in zircon grains from within baked sandstone, as well as paleomagnetic dating of magnetite, hematite, and goethite in paralava. These dates, in turn, help us to better understand how the landscape evolved over geologic time as major river systems cut down into the sediment of the Powder River Basin, and the rate at which coalbeds were exposed, burned, and released combustion gases into the atmosphere.

Uranium-Thorium/Helium (ZHe) ages of clinker, in millions of years (Ma)

&-

&-

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0–0.0490.050–0.1490.150–0.2490.250–0.499

&-

&-

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0.500–0.9991.000–1.9992.000 and older

All ZHe analyses are from Reiners and others (2011), Riihimaki and others (2009), and Heffern and others (2007).

Zircon fission-track (ZFT) ages of clinker, in millions of years (Ma)

Paleomagnetic samples of clinker #*

#*

Normal polarity

Reversed polarity

")

")

")

0–0.1490.150–0.2490.250–0.499

")

")

")

0.500–0.9991.000–1.9992.000 and older

All ZFt analyses are from Heffern and others (2007) and Coates and Naeser (1984).

All paleomagnetic analyses are from Jones and others (1984).Normal polarity indicates the coal burned more recently than the Matuyama-Brunhes magnetic reversal at about 0.78 Ma.

The authors thank Stefan Nicolescu, Victor Valencia, and George Gehrels for analytical assistance on dating the ZHe samples; Jason Whiteman, William Reiners, Paul Gore, Zoe Ruge, Camille Jones, Jessica Scheick, and Kaia Davis for field assistance in sample collection; Don Luse for access to the Padlock Ranch; Donald Coates, Charles Naeser, and Roger Colton of the U.S. Geological Survey (retired), for their geological advice and expertise; and Larry Neasloney of the U.S. Bureau of Land Management for Geographic Information System advice.

Layout by Susan Smith, MBMG. Edited by Susan Barth, MBMG.

Base map created in ArcMap for a poster presentation at AAPG Rocky Mountain Section Meeting, Cheyenne, Wyoming June 2011.

No warranty is made by the Bureau of Land Management as to the accuracy, reliability, or completeness of these data for individual use or aggregate use with other data.

Transverse Mercator ProjectionUTM Zone 13N, NAD 1983

Clinker

Clinker outcrops

Clinker is rock that has been baked, sintered, or melted by burning of underlying coalbeds. Clinker forms reddish layers that cap hills and escarpments in the landscape. In the southeast corner of this map (and Inset Map 2), the Rochelle Hills form an east-facing escarpment capped by clinker of the Wyodak coal zone. The coal is being mined just west of its outcrop/burn line. Nearly all of the coal in the map area occurs in formations of Tertiary age. The clinker in the map area results from natural burning in prehistoric times of coalbeds in the Paleocene Fort Union and Eocene Wasatch Formations. Additional clinker may be present in the subsurface beneath younger alluvium, but not exposed at the surface. Clinker is also common in landslides and talus deposits downslope from clinker-capped ridges.

Clinker outcrops shown on this map of the Powder River Basin cover a total of 1,472 sq. mi. (3,812 sq. km.). Of this total, 1,094 sq. mi. (2,833 sq. km.) of clinker are in Montana and 378 sq. mi. (979 sq. km.) are in Wyoming. These clinker outcrops represent the natural burning of tens of billions of tons of coal. Much additional clinker, perhaps an order of magnitude more, has been eroded away and carried downstream into the tributaries of the Missouri River. The record of this eroded clinker is present as clinker cobbles in Quaternary and late Tertiary gravel terraces. Natural coal fires have been a significant source of greenhouse gas emissions in prehistoric times, although the rate of natural burning is much less than the current rate of coal mining.

The clinker outcrops shown were mapped by E.L. Heffern, the lead author, from the following public sources: published geologic maps and digital data files from the U.S. Geological Survey, Montana Bureau of Mines and Geology, and Wyoming State Geological Survey; data interpreted by the author from color and color infrared aerial photography and National Agriculture Imagery Program color and color infrared orthophotos; and data from coal mine permit maps on public file with the State of Wyoming Land Quality Division. The full reference list is available on the Montana Bureau of Mines and Geology website at http://www.mbmg.mtech.edu/.

The Tertiary bedrock of the Fort Union and Wasatch Formations is mostly poorly to moderately consolidated sandstone, siltstone, and shale interfingered with coalbeds. When a subaerially exposed coalbed catches fire, the fire front burns back beneath the sandstone, siltstone, and shale overburden of these sediments. It ‘fires’ the overlying rock in a manner similar to ‘firing’ bricks. The result is the hardened, fractured rock called clinker. Most clinker is red, but some contains shades of yellow, orange, and black. In general, baked sandstone retains its original grain structure. The grains of fine-grained sediments are sintered—heated and welded together to form a coherent mass without melting—resulting in a rock similar to porcelain. A small amount of rock—primarily in fissures that served as chimneys— has been melted to hard black paralava. Virtually all of this clinker is harder than the parent rock. At the base of the clinker lies a thin layer of light tan ash mixed with greenish glass. In addition, clinker is highly fractured and allows much water to drain through rather than run off. The greater hardness, combined with fracture permeability, makes the clinker more resistant to erosion than the unfired bedrock strata above and below. Consequently, regional downwasting leaves the clinker forming the tops of scarps and ridges. See block diagram and cross sections.

Area underlain by Tertiary sediments

Early Quaternary / Late Tertiary gravel terraces (only those in Montana shown)

!

4301#I

Land and Resource Data

Cities and towns

State boundary

County boundary

Major paved highways

Summit elevations in feet above mean sea level

Interstate highways

Rivers and streams

Mined areas through 2010

Coal Mines

The 16 active coal mines in the Powder River Basin—12 in Wyoming and 4 in Montana—include the largest coal mines in North America. These mines produced 462.6 million short tons, 42.2% of total U.S. production, in 2011 (U.S. Energy Information Adminis-tration). Of this total, 426.1 million short tons came from Wyoming and 36.5 million came from Montana. The approximate areas where coal has been mined are shown in dark gray on the map. These areas are based on mine plans and annual reports of coal mine permits on public file with State mine regulatory agencies. Total areas mined through the year 2010 cover about 164 sq. mi. (425 sq. km.). Each mine has a program to quickly extinguish coal fires in mining pits, highwalls, and spoil piles.

The Powder River Basin, with its dry climate, low-rank coalbeds rich in volatile matter, and commonly occurring range fires, provides ideal conditions for coalbed fires. Near-surface and exposed coalbeds above the water table have ignited due to causes as diverse as lightning strikes, wildfires burning trees and bushes rooted in coalbeds, and spontaneous combustion in coal mines and outcrops. The 81 coal fires shown were compiled by the lead author from personal contacts and observations, published maps and articles, U.S. Bureau of Mines fire control project reports, and U.S. Office of Surface Mining, Wyoming Abandoned Mine Land Division, and U.S. Bureau of Land Management records. The recent vast range fires in the northern Powder River Basin during the summer of 2012 may have ignited additional coalbed fires.

Coalbed Fires

42

Active and historical coal fires, identification number indicated. Refer to coal-fire spreadsheet for details, available on the Montana Bureau of Mines and Geology website at http://www.mbmg.mtech.edu/.

$

$$

Mine fires

Spoil fires

Outcrop fires4147

This map prepared in cooperation with the U.S. Bureau of Land Management.

10 0 10 20 30 40 miles

10 0 10 20 30 40 50 kilometers

MONTANA BUREAU OF MINES AND GEOLOGYA Department of Montana Tech of The University of Montana

MBMG Geologic Map 64; Plate 1 of 1Clinker Distribution and Age in the Powder River Structural Basin, 2013

Published by theMontana Bureau of Mines and Geology

MBMG

Maps may be obtained from: Publications Office

Montana Bureau of Mines and Geology1300 West Park Street

Butte, Montana 59701-8997Phone: (406) 496-4167 Fax: (406) 496-4451

http://www.mbmg.mtech.edu