U.S. DEPARTMENT OF THE INTERIOR 

U.S. GEOLOGICAL SURVEY

LOCATIONS, DESCRIPTIONS, DENSITIES, AND MAGNETIC

SUSCEPTIBILTIES OF ROCK SAMPLES COLLECTED FOR

ANALYSES OF GRAVITY AND AEROMAGNETIC ANOMALIES IN

YELLOWSTONE NATIONAL PARK, WYOMING, IDAHO, AND

MONTANA

By

Howard W. Oliver1 and Robert L. Christiansen1 

!345 Middlefield Road, Menlo Park, California 94025

Open-file Report 98-247

This report is preliminary and has not been reviewed for conformity with the U.S., Geological 

Survey editorial standards or with the North American Stratigraphic Code. Any use of trade, 

product, or firm names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

1998
Locations, descriptions, densities, and magnetic susceptibilties

of rock samples collected for analyses of gravity and

aeromagnetic anomalies in Yellowstone National Park,

Wyoming, Idaho, and Montana

By 

Howard W. Oliver and Robert L. Christiansen

INTRODUCTION

Centered beneath Yellowstone National Park in northwestern Wyoming and 

adjacent parts of Montana and Idaho (Fig. 1; USGS, 1972) is one of the world's largest 

active magmatic systems (Christiansen and Blank, 1972; Christiansen, 1982; 1984; in 

press). The well known hydrothermal activity of the national park is a direct expression of 

a magmatic system that has sustained activity of the Yellowstone Plateau volcanic field 

(Fig. 4) through latest Pliocene and Quaternary time, producing three cycles of voluminous 

rhyolitic eruptions that each climaxed with an exceedingly large ash-flow eruption and 

concomitant formation of a large collapse caldera. The first caldera-forming eruption (the 

Huckleberry Ridge Tuff) occurred at 2.0 Ma, the second (the Mesa Falls Tuff) at 1.3 Ma 

and the third (the Lava Creek Tuff) at 0.6 Ma; this third cycle formed the Yellowstone 

caldera that now occupies an area about 80 by 30 km across within the park (Fig. 1). 

Together the three voluminous ash-flow tuffs form a stratigraphic unit designated the 

Yellowstone Group. Rhyolitic lavas preceded and followed each caldera-forming eruption 

and partly filled each caldera. Basalts have erupted repeatedly surrounding the Yellowstone 

caldera and have partially flooded areas within the two older calderas.

Stratigraphically beneath the rocks of the Yellowstone Plateau volcanic field and 

surrounding the plateau itself are rocks ranging in age from Precambrian to Eocene (Figs. 

2, 3). The regional basement consists principally of high-grade metamorphic rocks of 

Archaean age. That basement is overlain by predominantly marine sedimentary rocks of 

Paleozoic age that aggregate nearly 1,000 m in thickness and by Mesozoic strata nearly 

3,000 m thick (Fig. 2; Ruppel, 1972; Love and Keefer, 1975). The early Mesozoic strata 

include both marine and continental rocks; the Cretaceous section comprises more than 

2,500 m of sandstones and shales that are mostly of marine origin but include a continental 

volcanic-bearing orogenic-basin fill sequence at the top. The region was strongly deformed 

during the latest Cretaceous and earliest Tertiary Laramide orogeny. Predominantly 

intermediate volcanic and associated shallow-intrusive rocks were emplaced after the 

Laramide during the Eocene to form the Absaroka volcanic field (Fig. 3; Smedes and 

Prostka, 1972).

Most of the samples were collected in Yellowstone National Park, within the area of 

the gravity survey. Some samples of pre-Tertiary rocks, however, were collected just 

outside the park at Cinnabar Mountain (Fig. 1, numbers 11-26), where they are well 

exposed and easily accessible in a single measured section, and nearby at Yankee Jim 

Canyon (Fig. 1, number 27).

SAMPLES AND MEASUREMENTS

We collected a suite of 71 samples from 34 localities during the period July 23-26, 

1992. Rock densities and magnetic susceptibilities were measured in September, 1992 by 

Cameron Ainsworth and H. W. Oliver at the U.S. Geological Survey in Menlo Park, Calif. 

Density measurements were made by weighing the rocks both in air and under water with a 

very accurate balance (Sartorius 1264 MP); densities were calculated by dividing the weight 

in air by the weight loss in water. Magnetic susceptibilities were measured with a 

susceptibility bridge (Scintrex SM05). This work was undertaken in order to help interpret 

the isostatic gravity map of Yellowstone National Park (Carle and others, 1990) and the 

aeromagnetic map (U.S. Geological Survey, 1973). Sample locations are shown by
corresponding numbers in Figure 1, and approximate stratigraphic positions are indicated 

similarly in Figures 2-4. For convenient reference, the sample locations are also listed in 

Table 1, both by UTM coordinates and by latitude and longitude.

Table 1. Locations of samples collected for density and susceptibility measurements.

Locality

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

UTM 

(meters north)

4892850

4894430

4898900

4928590

4926490

4923630

4928230

4973980

4975080

4973750

4992520

4992400

4992350

4992340

4992220

4992200

4992200

4992200

4991970

4991790

4992850

4992900

4993000

4993000

4993060

4993190

5001760

4951170

4945500

4963510

4982360

4981010

4961080

4942050

UTM 

(meters east)

527530

527220

528160

557840

563840

571690

560160

555500

572000

570290

516490

516330

516290

516230

516180

516140

516080

516020

516560

516760

516480

516520

516410

516520

516430

516660

510510

527680

520400

493390

493900

494000

491520

510770

Latitude 

(degrees north)

44.1903

44.2045

44.2447

44.5103

44.4909

44.4644

44.5068

44.9190

44.9275

44.9157

45.0879

45.0868

45.0864

45.0863

45.0852

45.0850

45.0850

45.0850

45.0830

45.0813

45.0909

45.0913

45.0922

45.0922

45.0928

45.0939

45.1712

44.7153

44.6645

44.8269

44.9966

44.9845

44.8050

44.6337

Longitude 

(degrees east)

-110.6555

-110.6593

-110.6473

-110.2723

-110.1971

-110.0988

-110.2432

-110.2968

-110.0876

-110.1095

-110.7905

-110.7925

-110.7930

-110.7938

-110.7944

-110.7949

-110.7957

-110.7964

-110.7896

-110.7871

-110.7906

-110.7901

-110.7915

-110.7901

-110.7912

-110.7883

-110.8663

-110.6505

-110.7427

-111.0836

-111.0774

-111.0761

-111.1072

-110.8642
SAMPLE DESCRIPTIONS AND LOCATIONS MAG.

DENSITY SUSC.

(g/cm3) (10-3 SI) 

1. Lewis Canyon Rhyolite (Pleistocene).

Lat. 44.1903°, Long. -110.6555°

Collected from the southernmost roadside overlook of Lewis 

Canyon (Figs 1,4). This rhyolite, here a single lava flow about 

930,000 years old, partly fills the Snake River segment of the firstcycle caldera of the Yellowstone Plateau volcanic field 

(Christiansen and Blank, 1972; USGS, 1972). The flow is at least 

100 m thick, but, as its base is not seen, it could be much thicker.

1A. Crystallized rhyolitic lava with prominent 2.28 5.04

phenocrysts of plagioclase and quartz. Typical of 85-90%

of the outcrop.

IB. SimilartolA. 2.42 7.27

1C. Similar to 1A and IB but with abundant 2.33 7.71

lithophysae (subspherical cavities lined with vapor-phase

minerals). Represents the lithology of about 10-15% of

the outcrop.

2. Lava Creek Tuff, Yellowstone Group (Pleistocene).

Lat. 44.2045°, Long. -110.6593°

Collected from member B of the Lava Creek Tuff about midway 

from south to north along the west rim of Lewis Canyon (Figs. 1, 

4). This densely welded tuff is the typical extracaldera lithology of 

this widespread 600,000-year-old caldera-forming unit of the third 

volcanic cycle of the Yellowstone Plateau volcanic field 

(Christiansen and Blank, 1972; Christiansen, 1984). Here the unit 

is about 60 m thick, but elsewhere it exceeds 550 m in thickness.

2A. Contains only sparse welded pumice and 2.36 5.80

represents about 60% of the local lithology of the unit.

2B. Similar to 2A but with abundant welded pumice 2.32 4.32

and lithophysal crystallization in secondary open spaces,

representing about 40% of the local lithology of the unit.

3. Central Plateau Member of Plateau Rhyolite 

(Pleistocene); Pitchstone Plateau flow.

Lat. 44.2447°, Long. -110.6473°

Collected from near the east end of the Pitchstone Plateau Trail 

(Figs. 1,4). This rhyolite flow is about 70,000 years old and was 

erupted within the 600-ka Yellowstone caldera where it overlaps 

the 2.0-Ma first-cycle caldera (Christiansen and Blank, 1972; 

USGS, 1972). The flow is as much as 350 m thick and is but one 

of a sequence of similar flows of unknown total thickness within 

the Yellowstone caldera.

3A. Crystallized rhyolite with prominent phenocrysts 2.33 3.30

of quartz and sanidine and having the lowest porosity of

the 3 collected here.

3B. Similar to 3A but contains lithophysae. 2.30 4.08

3C. Similar to 3B. 2.13 1.64
4. Langford Formation, Absaroka Volcanic Supergroup

(Eocene).

Lat. 44.5103°, Long. -110.2723°

Collected from the east end of the parking area at Lake Butte (Figs.

1,3). This unit, mainly volcanic breccia, underlies much of

southeastern Yellowstone National Park and is as much as 600 m

thick (Smedes and Prostka, 1972; USGS, 1972).

4A. Fragments of pyroxene andesite and dacite(?) in a 2.41 18.6

silt-sized matrix of andesitic composition. Sample appears

to have been affected slightly by contact metamorphism

from nearby intrusion of diorite or hypabyssal andesite

and, thus, may be slightly denser than is typical for the

unit.

4B. Similar to 4A, collected from about 15m away. 2.51 8.5

5. Stock intrusive into Langford Formation, Absaroka

Volcanic Supergroup (Eocene).

Lat. 44.4909°, Long. -110.1971°

Collected from the westernmost road cut above Clear Creek (Figs.

1, 3; Smedes and Prostka, 1972; USGS, 1972). This intrusion

probably is the source of a gravity high of +34 mGal east of

Yellowstone Lake.

5A. Fine-grained medium-gray hornblende-biotite 2.69 42.6

diorite with sparse phenocrysts of hornblende up to 4-5

mm. Represents the most abundant lithology at this

locality.

5 B. Leucocratic hornblende diorite (color index about 2.56 27.1

half that of 5A).

5C. Hornblende diorite porphyry with aphanitic 2.26 36.1

groundmass and abundant large phenocrysts of

hornblende to 1 cm and some plagioclase to 2 mm; sparse

olivine to 4 mm. Rock contains numerous inclusions of

varied rock fragments of 5-30 mm. Probably represents

<5% of this outcrop.

6. Trout Peak Trachyandesite, Absaroka Volcanic 

Supergroup (Eocene).

Lat. 44.4644°, Long. -110.0988°

Collected along the roadside about 5 km east of Sylvan Pass (Figs. 

1, 3). Consists of lava flows and breccias about 300 m thick 

(Smedes and Prostka, 1972; USGS, 1972).

6A. Dark lava with phenocrysts of augite up to 6 mm, 2.73 12.3

plagioclase to 3 mm, (commonly saussuritized), and

olivine to 3 mm (iddingsitized). This is the most

common, characteristic rock type of the outcrop (about

70%).

6B. Similar to 6A but with weathered surfaces; 2.70 12.5

represents <2% of the outcrop.

6C. Similar to 6A but vesicular; represents about 10 % 2.58 33.7

of the outcrop. Two separate pieces of the sample were 2.56 34.1

measured.
7. Langford Formation, Absaroka Volcanic Supergroup 

(Eocene).

Lat. 44.5068°, Long. -110.2432°

Collected 1 km east of Lake Butte (Figs. 1,3). This andesitic

mudflow breccia is the predominant lithology of the Absaroka

Range in southeastern Yellowstone National Park, where it is as

much as 600 m thick (Smedes and Prostka, 1972; USGS, 1972).

The rock here is more typical of the unit than that at locality 4,

which is in an intrusive-contact aureole.

7A. Andesitic lava fragments in a silt-sized matrix. 2.32 8.00 

Two separate pieces of the sample were measured. 2.33 8.74 

7B. Similar to 7A. 2.41 9.15

8. Granitic gneiss (Precambrian).

Lat. 44.9190°, Long. -110.2968°

Collected in Lamar Canyon near its west end (Figs. 1, 2; Ruppel, 

1972). A similar basement lithology is widely exposed in the 

Beartooth and Absaroka Ranges to the north and northeast.

8A. Biotite granitic gneiss with almandine-rich garnet 2.73 25.3

and epidote.

8B. Similar to 8A, from another part of the same 1.5-m 2.69 25.3

block.

9. Lamar River Formation, Absaroka Volcanic 

Supergroup (Eocene).

Lat. 44.9275°, Long. -110.0876°

Collected in Icebox Canyon (Figs. 1, 3). This andesitic mudflow 

breccia underlies much of the Absaroka Range in northeastern 

Yellowstone National Park, where it commonly is 300-700 m thick 

(Smedes and Prostka, 1972; USGS, 1972).

9A. Fragments of sparsely porphyritic andesite in an 2.45 7.46

andesitic mudstone matrix. The largest blocks are up to 5

m, but many fragments are in the range 1-5 cm.

9B. Similar to 9A. 2.49 7.25

9C. Similar to 9A and 9B. 2.43 5.80

10. Madison Group (Mississippian).

Lat. 44.9157°, Long. -110.1095°

Collected at Round Prairie (Figs. 1, 2). This mainly calcitic

limestone is an important regional aquifer and at about 360 m is

one of the thickest limestones of the Yellowstone Paleozoic section

(Ruppel, 1972; Love and Keefer, 1975).

10A. Aphanitic, nearly massive limestone. 2.69 0.01 

10B. Similar to 10A. 2.66 0.02 

10C. Similar to 10A and 10B. 2.68 0.03

Samples 11-26 are from the Cinnabar Mountain 

stratigraphic section, located about 8 km northnorthwest of Gardiner, Montana (Figs. 1, 2) ; most of 

the section was measured by Fraser and others (1969, 

p. 89-97).
11. Kootenai Formation (Lower Cretaceous) 2.57 0.04 

Lat. 45.0879°, Long. -110.7905°

Following the nomenclature of Ruppel (1972). Designated as the 

lower sandstone and conglomerate member of the Cleverly 

Formation by Fraser and others (1969). The sample is a finegrained quartzitic sandstone. Thickness 10 m.

12. Lower sandstone member of Thermopolis Shale 2.57 0.05 

(Lower Cretaceous)

Lat. 45.0868°, Long. -110.7925°

Following the nomenclature of Ruppel (1972). Designated as the 

upper sandstone member of the Cleverly Formation by Fraser and 

others (1969). The sample is a medium-grained cross-laminated 

quartzitic sandstone. Thickness 27 m.

13. Middle shale member of Thermopolis Shale (Lower 

Cretaceous)

Lat. 45.0864°, Long. -110.7930°

Following the nomenclature of Ruppel (1972). Designated as the 

shale member of the Thermopolis Shale of Fraser and others 

(1969). Contains fossil tracks. Thickness of the unit 85 m, of 

which about 25% consists of siltstones and sandstones similar to 

these samples.

13 A. Thin-bedded gray pyritic siltstone intercalated in 2.59 0.08

the predominant shales of the member.

13B. Similar to 13A, from the same outcrop. 2.57 0.07

13C. Similar to 13A and 13B, from the same outcrop. 2.61 0.08

14. Middle shale member of Thermopolis Shale (Lower 2.48 0.18 

Cretaceous)

Lat. 45.0863°, Long. -110.7938°

Following the nomenclature of Ruppel (1972). Designated as the 

shale member of the Thermopolis Shale by Fraser and others 

(1969). Conchoidally fractured black shale or mudstone that is the 

predominant lithology of the member. Thickness of the unit 85 m, 

of which about 75% consists of shales similar to the sample.

15. Upper sandstone member of Thermopolis Shale 2.42 0.05 

(Lower Cretaceous)

Lat. 45.0852°, Long. -110.7944°

Following the nomenclature of Ruppel (1972). Designated as the 

Muddy Sandstone Member of the Thermopolis Shale by Fraser and 

others (1969). Medium- to coarse-grained lithic arkosic sandstone 

with about 20% of dark rock and mineral grains that give the 

lithology a distinct "salt and pepper" appearance. Calcareous 

cement. Thickness 35 m.

16. Mowry Shale (Lower Cretaceous) 2.57 0.12

Lat. 45.0850°, Long. -110.7949°

Following the nomenclature of Ruppel (1972) and Fraser and

others (1969). Black thin-bedded siliceous shale with blocky

fracture. Thickness 99 m.
17. Frontier Sandstone (Upper Cretaceous) 2.52 0.07

Lat. 45.0850°, Long. -110.7957°

Following the nomenclature of Ruppel (1972). Designated as the

lower sandstone Member of the Frontier Formation by Fraser and

others (1969). Sample, from near the stratigraphic top of the

formation, is medium-grained, thick-bedded calcareous sandstone

of lithic "salt and pepper" type. Thickness 17 m.

18. Cody Shale (Upper Cretaceous) 2.47 0.13

Lat. 45.0850°, Long. -110.7964°

Following the nomenclature of Ruppel (1972). The rocks of this

locality were included as the middle shale member of the Frontier

Formation by Fraser and others (1969). Collected near base of

unit. Crumbly dark gray to black fissile shale. Thickness of the

formation about 470 m.

19. Telegraph Creek Formation (Upper Cretaceous)

Lat. 45.0830°, Long. -110.7896°

Following the nomenclature of Ruppel (1972) and Fraser and

others (1969). Thickness about 90 m.

19A. Well sorted arkosic siltstone 2.64 0.09 

19B. Similar to 19A. 2.63 0.10 

19C. Similar to 19A and 19B. 2.60 0.11

20. Eagle Sandstone (Upper Cretaceous) 2.58 1.00

Lat. 45.0813°, Long. -110.7871°

Following the nomenclature of Ruppel (1972) and Fraser and

others (1969). Medium-grained biotitic sandstone, ranging from

medium- to thick-bedded. Thickness about 240 m.

21. Swift Formation, Ellis Group (Jurassic) 2.66 0.08

Lat. 45.0909°, Long. -110.7906°

Following the nomenclature of Ruppel (1972) and Fraser and

others (1969). Sandy limestone. Some beds, including that from

which the sample was taken, are highly fossiliferous, with

conspicuous pectins. Thickness 20 m.

22. Augite-diorite porphyry dike (Eocene?). 2.64 7.38

Lat. 45.0913°, Long. -110.7901°

This intrusion is only about 12m thick but stands out from the

surrounding sedimentary rocks with about 35 m of relief. This is

either a dike of Eocene age or a sill of latest Cretaceous age.

23. Woodside Formation (Triassic) 2.70 0.19

Lat. 45.0992°, Long. -110.7915°

Following the nomenclature of Ruppel (1972). Discussed as the

lower part of the Chugwater Formation by Fraser and others

(1969). This unit forms the conspicuous red stripe of Devils Slide

on the east side of Cinnabar Mountain. It consists predominantly

of red calcareous siltstone, very thin-bedded and laminated, with a

few very thin limestone interbeds. Thickness about 12m.
24. Shedhorn Sandstone (Permian) 2.67 0.06

Lat. 45.0922°, Long. -110.7901°

Following the nomenclature of Ruppel (1972). Designated as the

Park City Formation by Fraser and others (1969). Highly

fractured dark crystalline limestone in beds 40-50 cm thick,

weathering to light gray. Elsewhere this stratigraphic interval is

occupied by varied sandstones, cherts, and phosphorites.

Thickness 29 m.

25. Quadrant Sandstone (Pennsylvanian) 2.55 0.04

Lat. 45.0928°, Long. -110.7912°

Following the nomenclature of Ruppel (1972). Designated as the

Quadrant Quartzite by Fraser and others (1969). Fine-grained

thick-bedded light- to medium-gray quartzite. Thickness 54 m.

26. Mission Canyon Limestone, Madison Group 2.42 0.01

(Mississippian) 2.32 0.02

Lat. 45.0939°, Long. -110.7883°

Following the nomenclature of Ruppel (1972). Discussed as the

Madison Limestone by Fraser and others (1969). Collected from

the top 15 m of the unit. Massive light-gray limestone. Thickness

about 250 m. Two separate pieces of the sample were measured.

Sample 26 is the last of the samples collected from the 

Cinnabar Mountain section.

27. Gneiss (Precambrian).

Lat. 45.1712°, Long. -110.8663°

Collected at the head of Yankee Jim Canyon, about 18 km north of 

Gardiner, Montana (Figs. 1, 2). This is a common lithology in the 

area of a surrounding gravity high.

27A. Well foliated biotite-microcline-plagioclase-quartz 2.72 0.22

gneiss.

27B. Similar to 27A. 2.70 0.20

27C. Similar to 27A and 27B. 2.72 0.16

28. Lava Creek Tuff, Yellowstone Group (Pleistocene).

Lat. 44.7153°, Long. -110.6505°

Collected along the Virginia Cascade loop road, about 5 km east of 

Norris Junction (Figs. 1, 4). This is the caldera-forming ash-flow 

tuff of 600,000-year age (Christiansen and Blank, 1972), which 

locally is >100 m thick. The samples were collected from the 

bottom 10 m of member B, near the middle of the local section.

28A. Porphyritic welded rhyolitic tuff, with phenocrysts 2.40 4.61

of quartz, sanidine, and sodic oligoclase.

28B. Similar to 28A. 2.27 5.31

28C. Similar to 28A and 28B. 2.36 4.50
29. Central Plateau Member, Plateau Rhyolite 

(Pleistocene); Gibbon River flow.

Lat. 44.6645°, Long. -110.7427°

Collected on the east side of Gibbon Canyon about 3 km south of

Beryl Spring (Figs. 1, 4; Christiansen and Blank, 1972; USGS,

1972). This flow, of about 90 ka, is the southernmost in a line of

rhyolitic lava flows and domes erupted between the Norris Geyser

Basin area and Mammoth Hot Springs (Fig. 1). It is about 120 m

thick.

29A. Rock from crystalline interior of the flow. 2.33 3.65

Contains abundant limophysae. Two separate pieces of 2.33 3.60

the sample were measured.

29B. Similar in composition to 29A but a dense black 2.35 3.10

vitrophyre from the exterior of the flow.

30. Schist (Precambrian).

Lat. 44.8269°, Long. -111.0836°

Collected on the west side of Grayling Creek, along U.S.

Highway 191, about 6 km north of the 191-287 intersection (Figs.

1, 2; see Witkind, 1969). This is the basement lithology of the

northwestern part of Yellowstone National Park, here underlying

the basal contact of the Huckleberry Ridge Tuff.

30A. Quartz-biotite schist. 2.70 0.25 

30B. Similar to 30A. 2.73 0.33 

30C. Similar to 30A and 30B but somewhat more felsic 2.73 0.26 

and less schistose.

30D. Finer-grained, darker, and denser than 30A-30C. 3.00 0.70 

Color index about 80.

31. Dioritic stock or laccolith (Eocene)

Lat. 44.9966°, Long. -111.0774°

Intruded into limestone of the Madison Group. Collected along the 

upper Gallatin River about 2 km upstream from the intersection of 

Specimen Creek (Figs. 1, 3). This stock is near the center of a 40- 

mGal gravity high. Mapped as the Gallatin River laccolith by 

Witkind (1969).

31 A. Hornblende-diorite porphyry with abundant 2.76 42.2

hornblende and plagioclase phenocrysts in an aphanitic

matrix. Hornblende phenocrysts are up to 3 cm. The

diorite contains abundant metamorphic inclusions,

including hornblende-rich inclusions up to 6 cm. Epidote

is disseminated throughout.

3 IB. Similar to 31 A. 2.72 32.4

31C. Similar to 31A and 3IB. 2.74 37.8

32. Lodgepole Limestone, Madison Group

(Mississippian).

Lat. 44.9845°, Long. -111.0761°

Collected from the east side of the Gallatin River Valley across

from Snowslide Creek (Figs. 1, 2; Ruppel, 1972; Witkind, 1969).

Thickness about 120 m.

32A. Dark-gray limestone with quartz-siltstone laminae 2.67 0.02

constituting about 5% of the rock.
32B. Brecciated limestone with recrystallized coarse 2.71 0.04

calcite cement.

32C. Aphanitic limestone. 2.72 0.07

33. Huckleberry Ridge Tuff, Yellowstone Group 

(Pliocene).

Lat. 44.8050°, Long. -111.1072°

Collected along the west side of Grayling Creek about 3 km north 

of the junction of U.S. Highways 191 and 287 (Figs. 1,4). This 

is the 2.0-Ma caldera-forming rhyolitic ash-flow tuff of the first 

volcanic cycle (Christiansen and Blank, 1972). Thickness of the 

Huckleberry Ridge Tuff at this locality is about 150 m, but 

elsewhere it is >300 m.

33 A. Welded rhyolitic ash-flow tuff with moderately 2.31 4.39

abundant (about 25%) phenocrysts of quartz, plagioclase,

and sanidine, generally about 1-2 mm. Contains some

lenses of white welded pumice to 4 cm and lithic

inclusions, mainly of Precambrian metamorphic rocks.

Limonitic staining is pervasive.

33B. Similar to 33A. 2.35 3.43

33C. Similar to 33A and 33B. 2.31 5.00

34. Central Plateau Member, Plateau Rhyolite 

(Pleistocene); Nez Perce Creek flow.

Lat. 44.6337°, Long. -110.8642°

Collected on the east side of Firehole Canyon about 3 km south of 

Madison Junction (Figs. 1,4; see Christiansen and Blank, 1972; 

USGS, 1972). This is one flow, about 90 m thick, in the much 

thicker sequence that fills much of the Yellowstone caldera; the 

flow is lithologically typical of those lavas in general.

34A. Crystallized interior of rhyolite flow, with some 2.26 4.16

lithophysae. Moderately abundant phenocrysts of quartz

and sanidine. Together, samples 34A and 34B probably

represent about 80% of the volume of the flow.

34B. Similar to 34A but darker and with pervasive 2.34 3.49

irregular fracture surfaces.

34C. Welded glassy flow breccia with black glass 2.21 1.05

lenses in a reddish-brown glass matrix. Composition like

that of the crystallized samples. Together, samples 34C34E probably represent about 20% of the volume of the

flow.

34D. Vitrophyre with spherulites and lithophysae. Two 2.27 5.90

separate pieces of the sample were measured. 2.26 6.24

34E. Partly hydrated vitrophyre of composition similar 2.02 1.18

to samples 34A-C. Two separate pieces of the sample 2.24 0.70

were measured.

SUMMARY

On the basis of the foregoing measurements, average densities were estimated for 

several packages of rock units that might be useful in modeling the gravity field of the 

Yellowstone Plateau region. For this purpose, the following units were considered: 

Precambrian metamorphic basement rocks, Paleozoic sedimentary rocks, Mesozoic 

sedimentary rocks, early Tertiary volcanic and volcaniclastic rocks, early Tertiary intrusive

10
rocks, ash-flow tuffs and interlayered rhyolites and basalts of the Yellowstone Plateau 

volcanic field, and postcollapse volcanic rocks and sediments within the Yellowstone 

caldera. Average densities were estimated for each of these gross units, weighting them 

according to our estimated percentages of various lithologies represented in each gross unit. 

On that basis, we estimate the following average densities :

Density

Aggregate unit (g/cm3) 

Pleistocene postcollapse rocks and deposits in the Yellowstone caldera 2.28 

Pleistocene and Pliocene ash-flow tuffs and interlayered volcanic rocks 2.32 

Tertiary volcanic and volcaniclastic rocks 2.49 

Tertiary intrusive rocks 2.62 

Mesozoic sedimentary rocks 2.54 

Paleozoic sedimentary rocks 2.65 

Precambrian metamorphic rocks 2.72

REFERENCES CITED

Carle, S. R, Glen, J. M., Langenheim, V. E., Smith, R. B., and Oliver, H. W., 1990,

Isostatic gravity map and principal facts for 694 gravity stations in Yellowstone

National Park and vicinity, Wyoming, Idaho, and Montana: U.S. Geological Survey

Open-File Report 90-649, 31 p. 

Christiansen, R. L., 1982, Late Cenozoic volcanism of the Island Park area, eastern Idaho,

in Bonnichsen, B., and Breckenridge, R. M., eds., Cenozoic geology of Idaho: Idaho

Bureau of Mines and Geology Bulletin 26, p. 345-368. 

Christiansen, R. L., 1984, Yellowstone magmatic evolution: Its bearing on understanding

large-volume explosive volcanism, in Explosive volcanism: Inception, evolution, and

hazards: Washington, D.C., National Academy of Sciences, p. 84-95. 

Christiansen, in press, The Quaternary and Pliocene Yellowstone Plateau volcanic field of

Wyoming, Idaho, and Montana: U.S. Geological Survey Professional Paper 729-G. 

Christiansen, R. L., and Blank, H. R., Jr., 1972, Volcanic stratigraphy of the Quaternary

rhyolite plateau in Yellowstone National Park: U. S. Geological Survey Professional

Paper 729-B, 18 p. 

Eraser, G. D., Waldrop, H. A., and Hyden, H. J., 1969, Geology of the Gardiner area,

Park County, Montana: U.S. Geological Survey Bulletin 1277,118 p. 

Keefer, W. R., 1971, The geologic story of Yellowstone National Park: U.S. Geological

Survey Bulletin 1347, 92 p. 

Love, J. D., and Keefer, W. R., 1975, Geology of sedimentary rocks in southern

Yellowstone National Park, Wyoming: U.S. Geological Survey Professional Paper

729-D, 60 p. 

Ruppel, E. T., 1972, Geology of pre-Tertiary rocks in the northern part of Yellowstone

National Park, Wyoming: U.S. Geological Survey Professional Paper 729-A, 66 p. 

Smedes, H. W., and Prostka, H. J., 1972, Stratigraphic framework of the Absaroka

Volcanic Supergroup in the Yellowstone National Park region: U.S. Geological

Survey Professional Paper 729-C, 33 p. 

U.S Geological Survey, 1972, Geologic map of Yellowstone National Park: U.S.

Geological Survey Miscellaneous Investigations Series 1-711, scale 1:125,000. 

U.S Geological Survey, 1973, Aeromagnetic map of Yellowstone National Park and

vicinity, Wyoming-Idaho-Montana: U.S. Geological Survey Open-File Map, scale

1:125,000, 4 sheets. 

Witkind, I. J., 1969, Geology of the Tepee Creek quadrangle, Montana-Wyoming: U.S.

Geological Survey Professional Paper 609, 101 p.

11
South Entrance

EXPLANATION

Surficial deposits 

Basalt flows

Plateau Rhyolite

Ash-flow tuffs of Yellowstone Group 

and related lava flows

Absaroka volcanic rocks

Intrusive igneous rocks

Tertiary formations

Mesozoic formations

Paleozoic formations

Precambrian gneiss and schist

Contact

Caldera boundary 

8 27

Sample locality

FAULT AND FOLD SYMBOLS

Dotted where concealed beneath

younger unfaulted rocks

Reverse fault

Sawteeth on side

that moved up

Normal fault

Symbol on side that moved down

Reverse fault, along which there was later normal-fault movement

Anticlinal axis 

0 5 10 KM

, i _j

>- tx.<

o oo z.

Figure 1. Generalized geologic map of Yellowstone National Park. Numerals indicate approximate locations by sample number. More detailed locations are given in the text and Table 1. Yellowstone caldera located by red line. Modified from Keefer (1971, PL 1).
Figure 2. Columnar section of pre-Tertiary stratigraphic units of Yellowstone 

National Park and vicinity. Units shown are those for the northern part of the 

park, based upon data in Ruppel (1972). A slightly different stratigraphy in the 

southern part of the park has been described by Love and Keefer (1972), but the 

physical properties of major lithologic units are similar. Numerals indicate 

approximate stratigraphic positions of samples.

13
NORTHWEST NORTHEAST SOUTHEAST

Lower part Therefore Crtek Group interflng«r«d with Sunlight Group

EXPLANATION

Twi Wiggms Formation TireTo Two Ocean Formation Ts

Tl Langford Formation TsfTt Trout Peak Trachyandesite TsdTw Wapiti Formation TicTm Mount Wallace Formation TecTeh Crescent Hill Basalt Tsc

Lamar River and Cathedral Cliffs FormationSepulcher Formation, undivided

Fortress Mountain MemberDaly Creek Member

Lost Creek Tuff Member

Elk Creek Basalt MemberConglomerate Facies

Figure 3. Diagrammatic section of stratigraphic units in the Eocene Absaroka volcanic field of Yellowstone National Park and vicinity, modified after Smedes 

and Prostka (1972). Numerals indicate approximate stratigraphic positions of samples.
EXPLANATION

Third volcanic cycle

Second volcanic cycle

First volcanic cycle

Roaring Mountain Member |

Central Plateau Member , , .

Obsidian Creek Member > Plateau Rhyohte

Upper Basin Member

Mallard Lake Member J

Osprey Basalt

Madison River Basalt

Swan Lake Flat Basalt

Falls River Basalt

Basalt of Mariposa Lake

Lava Creek Tuff

Undine Falls Basalt

Mount Jackson Rhyolite

Mesa Falls Tuff

Sediments and basalts of The Narrows

Lewis Canyon Rhyolite

Huckleberry Ridge Tuff

Rhyolite of Broad Creek

Junction Butte Basalt

Figure 4. Diagrammatic section of stratigraphic units in the late Pliocene and Quaternary 

Yellowstone Plateau volcanic field of Yellowstone National Park and vicinity, modified after 

Christiansen and Blank (1972). Numerals indicate approximate stratigraphic positions of 

samples.

15