Chapter 6

Calculation of Overburden Above the 

Mahogany Zone in the Piceance Basin, 

Colorado

By Tracey J. Mercier

Chapter 6 of 7

Oil Shale and Nahcolite Resources of the Piceance 

Basin, Colorado

By U.S. Geological Survey Oil Shale Assessment Team

U.S. Geological Survey Digital Data Series DDS–69–Y

U.S. Department of the Interior

U.S. Geological Survey

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U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2010

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Suggested citation:

Mercier, T.J., 2010, Calculation of overburden above the Mahogany zone in the Piceance Basin, Colorado: U.S. 

Geological Survey Digital Data Series DDS–69–Y, chp. 6, 6 p.

ISBN: 1–4113–2668–2
iii

Contents

Introduction.....................................................................................................................................................1

References Cited............................................................................................................................................1

Figures

1–5. Maps showing:

1. Mahogany zone structure contour lines drawn on the Digital Elevation 

Model (DEM) clipped to the Piceance Basin...........................................................................2

2. Triangular Irregular Network (TIN) model of the top of the Mahogany 

zone subsurface structure ..........................................................................................................3

3. Overburden above the Mahogany zone in the Piceance Basin ...........................................4

4. Overburden above the Mahogany zone in the Piceance Basin, 

hillshade effect applied ...............................................................................................................5

5. Cubic miles (mi3) of overburden per section above the Mahogany zone 

in the Piceance Basin ..................................................................................................................6
Introduction

As part of the National Oil Shale Assessment, the volume 

of overburden material was calculated above the Mahogany 

zone in the Piceance Basin. A common Geographic Information System (GIS) method was used to calculate the amount of 

overburden by subtracting a model of the Mahogany surface 

from a Digital Elevation Model (DEM). To maintain consistency with other models in the assessment, all overburden 

calculations and resultant files were analyzed using a one-acre 

(208.7 ft/63.615 m per side) raster-model cell size with an 

effective area of 43,560ft2

/4,047 m2. 

The first step was to digitize the Mahogany ledge outcrop, Parachute Creek Member outcrop, and detailed structure 

lines (fig. 1) of the top of the Mahogany zone from two published 1:100,000-scale geologic maps (Hail and Smith, 1994; 

1997). The structure contour lines were converted to a point 

file containing elevation values for the top of the Mahogany 

zone. For the Mahogany outcrop, line surface elevation values, 

in ft, were extracted from the DEM to obtain elevation values 

for the Mahogany ledge. The two point files were merged to 

generate a surface model in ArcGIS (ESRI, 2006) by using the 

GeoStatistical Analyst’s Radial Basis Function (RBF) contouring method. The RBF surface model was then converted to an 

ESRI GRID as well as a Triangulated Irregular Network (TIN) 

model (fig. 2) to use in ArcGIS’s 3D Analyst extension. 

The Mahogany surface GRID was then subtracted from 

the DEM GRID using the ArcGIS Spatial Analyst Raster 

Calculator. The resultant GRID (fig. 3) contains values of total 

overburden, in ft, above the Mahogany zone for each one-acre 

cell. The overburden GRID was converted to a TIN. Figure 4

shows the effect of applying hillshade to the overburden TIN 

Calculation of Overburden Above the Mahogany Zone in 

the Piceance Basin, Colorado

By Tracey J. Mercier

to aid in visual analysis. In order to calculate the volume of 

overburden material contained in specific polygonal areas, we 

used ArcToolbox’s TIN Polygon Volume function to calculate 

the total volume of overburden, in mi3

, found within the basin, 

as well as the amount of overburden found within each Public 

Land Survey System (PLSS) section (fig. 5).

This analysis found the total volume of overburden above 

the Mahogany zone in the Piceance Basin to be approximately 192 mi3

 of rock and other material. For comparison, 

that amount is approximately equal to 22 times the volume of 

water in Lake Mead; 320,000,000 Olympic-size swimming 

pools; 286 times the amount of rock removed during the Mt. 

St. Helens eruption; one-fifth the volume needed to fill the 

Grand Canyon; and over seven billion hopper cars at 4,000 ft3

capacity.

References Cited

Environmental Systems Research Institute, Inc. (ESRI), 2006, 

ArcMap, ver. 9.2.

Hail, W.J., and Smith, M.C., 1994, Geologic map of the northern part of the Piceance Creek basin, northwestern Colorado: U.S. Geological Survey Miscellaneous Investigations 

Series Map I-2400, scale 1:100,000, http://pubs.er.usgs.gov/

usgspubs/i/i2400.

Hail, W.J., Jr. and Smith, M.C., 1997, Geologic map of the 

southern part of the Piceance Creek basin, northwestern 

Colorado: U.S. Geological Survey Miscellaneous Investigations Series Map I-2529, scale 1:100,000, http://pubs.

er.usgs.gov/usgspubs/i/i2529.
2 Calculation of Overburden Above the Mahogany Zone in the Piceance Basin, Colorado

R 96 W R 95 W

R 101 W R 100 W R 99 W R 98 W R 97 W R 94 W

T

8

S

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R 93 W

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R 94 W

R 101 W R 100 W R 99 W R 98 W R 97 W

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108°30' 108°00'

40°00'

39°30'

0 3 6 12 MILES

COLORADO

0 5 10 2.5 KILOMETERS

EXPLANATION

Mahogany ledge outcrop

Base of Parachute Creek Member

Faults

Structure on top of Mahogany ledge, in ft 

> 9,000

8,001–9,000

7,001–8,000

6,001–7,000

5,001–6,000

< 5,000

Piceance

Basin

Figure 1. Mahogany zone structure contour lines (Hail and Smith, 1994; 1997) drawn on the Digital Elevation Model 

(DEM) clipped to the Piceance Basin.
References Cited 3

R 96 W R 95 W

R 101 W R 100 W R 99 W R 98 W R 97 W R 94 W

T

8

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R 93 W

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R 101 W R 100 W R 99 W R 98 W R 97 W

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R 96 W R 95 W

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108°30' 108°00'

40°00'

39°30'

0 3 6 12 MILES

COLORADO

0 5 10 2.5 KILOMETERS

Mahogany ledge outcrop

Base of Parachute Creek Member > 9,000

8,001–9,000

7,001–8,000

6,001–7,000

5,001–6,000

≤ 5,000

Mahogany structure (TIN), elevation in ft

EXPLANATION

Piceance

Basin

Figure 2. Map showing the Triangular Irregular Network (TIN) model of the top of the Mahogany zone subsurface 

structure.
4 Calculation of Overburden Above the Mahogany Zone in the Piceance Basin, Colorado

R 96 W R 95 W

R 101 W R 100 W R 99 W R 98 W R 97 W R 94 W

T

8

S

T

7

S

R 93 W

T

6

S

T

5

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4

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3

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9

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R 94 W

R 101 W R 100 W R 99 W R 98 W R 97 W

T

2

S

T

1

S

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1

N

R 96 W R 95 W

T

2

N

108°30' 108°00'

40°00'

39°30'

0 3 6 12 MILES

COLORADO

0 5 10 2.5 KILOMETERS

EXPLANATION

Mahogany ledge outcrop

Base of Parachute Creek Member

Mahogany overburden (GRID), in ft

> 2,000

1,501–2,000

1,001–1,500

501–1,000

≤ 500

Piceance

Basin

Figure 3. Map showing the overburden above the Mahogany zone in the Piceance Basin.
References Cited 5

R 96 W R 95 W

R 101 W R 100 W R 99 W R 98 W R 97 W R 94 W

T

8

S

T

7

S

R 93 W

T

6

S

T

5

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4

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T

3

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T

9

S

R 94 W

R 101 W R 100 W R 99 W R 98 W R 97 W

T

2

S

T

1

S

T

1

N

R 96 W R 95 W

T

2

N

108°30' 108°00'

40°00'

39°30'

0 3 6 12 MILES

COLORADO

0 5 10 2.5 KILOMETERS

EXPLANATION

Mahogany ledge outcrop

Base of Parachute Creek Member

Mahogany overburden, in ft

> 2,000 

1,751–2,000

1,501–1,750

1,251–1,500

1,001–1,250

751–1,000

501–750

251–500

≤ 250

Piceance

Basin

Figure 4. Map showing the overburden above the Mahogany zone in the Piceance Basin, hillshade effect applied.
6 Calculation of Overburden Above the Mahogany Zone in the Piceance Basin, Colorado

R 96 W R 95 W

R 101 W R 100 W R 99 W R 98 W R 97 W R 94 W

T

8

S

T

7

S

R 93 W

T

6

S

T

5

S

T

4

S

T

3

S

T

9

S

R 94 W

R 101 W R 100 W R 99 W R 98 W R 97 W

T

2

S

T

1

S

T

1

N

R 96 W R 95 W

T

2

N

108°30' 108°00'

40°00'

39°30'

0 3 6 12 MILES

COLORADO

0 5 10 2.5 KILOMETERS

EXPLANATION

Mahogany ledge outcrop

Base of Parachute Creek Member

Mahogany overburden, in mi3

0.31–0.40

0.21–0.30

0.11–0.20

< 0.10

Piceance

Basin

Figure 5. Map showing cubic miles (mi3) of overburden per section above the Mahogany zone in the Piceance 

Basin.
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