Gravity Survey in Part of the Snake River Plain, Idaho

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Gravity Survey in Part of the Snake River Plain, Idaho

by

Harry L, Baldwin, Jr.

ProQuest Number: 10795555

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A Thesis submitted, t^o the Faculty and the Board of Trustees of the Colorado School of Mines in partial fulfillment of the requirements for tho degree of Master of Science in Geophysical Engineering.

Signed: Student Goldent Colorado Date;

XZ

/760

Approved ; Thesis Advisor

Head of Department Golden » Colorado

Abstract

During the early summer of 1959, a total qf 1,187 gravity stations was occupied on the western part of the Snake River Plain ip Idaho. An area including 2,000 square miles of the plain extending from Glenns Ferny, Idaho, to Caldwell, Idaho, was covered with a station density of one station par two square miles.

An additional 1,200 square miles of

surrounding area, mainly from Caldwell, Idaho, to the Gregon-Idaho state line, was covered with a density of one station per seven square miles. The mean reproducibility of the observed gravities of these stations was 0.05 ragal, with a maximum discrepancy of Q.2 mgal.

Gravity data

were reduced to simple Bouguer values using a combined free-air and Bouguer correction of 0,06 mgal per ft. The only anomalies found with closure in excess of 10 mgal are two elongated highs, orientated northwest-southeast, with the northwestern high offset to the northeast by 10 miles.

The smaller of thes© highs

extends from Meridian, Idaho, to Nyssa, Oregon; the larger extends from Swan Falls, Idaho, to Glenns Ferry, Idaho.

The maximum value recorded

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is a simple Bouguer value of -66.5 rogal with respeqt to the International $p.lipeoid.

(^adienfs pn the sides of these highs are largest on the

npr^heast sides, reaching 6 mgal per mile in places.

Graticule inter-r

pretatipns of a profile across the southeastern high using a density 3 contrast of 0.3 per cm indicate an accuD>ulation of lava reaching a thickness of at least 14,000 feet.

iv

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Contents page Introduction , . . . . . . . „ . . . .

...

Purpose and organisation

T ....

The Sn^-ke Rivqr plain

............

Field observations and data reduction Accuracy . ..

1 . f . . .

T ,, , r . . . .

r ......

f . . . . . . . . . .

t .

^

Appendix 2:

16

.

16

.

2Q

....

28

...

30

Recommendations for further study . . . . . . . . . Appendix 1:

»

1?

Graticule analyses pf profile Ar-A' .. t Structural relationships in the Snake River Piain

1

. 8

.........

Minor features

.

3

Description of anomalie^ and qualitative interpretation Ma^or features . . . . .

1

2

. . . . . . . . . .

? T .. f . . . . . . . . .

.

Location descriptions and observed gravities of selected TqA^e stations , . . .

. 32

Gravity data for the Snake River Project

. 35

References efted . . . . . . . . . . . . .

v

.

.............

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Illustrations

Figure 1. Differences in repeat and tie measurements . „ « 2. Simple Bouguer anomalies . . . . . . . . . . . . . i(D

e

e

e

e

o

o

o

u

o

o

v

o

o

o

^

v

o

10 map

o

4. Assumed regional anomaly along line A-=A6 and its extensions » , „ . . . . . . . . .

21

5, Residual anomaly along line A-A* . . . . . . . .

21

6« A permissible interpretation of profile A-A"„ Basalt extends to surface

24

7» A permissible interpretation of profile A-A*= Basalt 4,000 ft deep „ „ „ . . . . . . . .

25

8. A permissible interpretation of profile A-A". Basalt 8,000 ft deep . . . . . . . . . . .

25

9* Two permissible interpretations of the assumed regional anomaly . . . . . . . . .

27

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Introduction

Purpose and Organization The Snake River Project was established by the United States Geological Survey for the purpose of searching out, defining, and interpreting gravity anomalies present on the western part of the Snake River Plain in Idaho.

In particular, the purpose was to define

gradients associated with the gravity high shown by the regional work of Bonini and Lavin (1957)» The Snake River Project was undertaken as part of a project on Vbleanism and Crustal Deformation, supervised by L« G. Pakiser of the Geological Survey.

Professor Rodgers of the Geophysics Department of

the Colorado School of Mines acted as advisor to the Snake River Project.

Field work was done by Harry L. Baldwin, Jr., and David P.

Hill, with Mrs, Baldwin acting as a valuable unofficial member of the field party. A preliminary report on the results of the Snake River Project has been released as a United States Geological Survey open-file report (Baldwin and Hill, i960).

1

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The Snake River Plain The part of the Snake River Plain concerned in this study is a large, relatively flat, basalt-capped plateau, of width varying from 40 to 100 miles, extending both north and south of the Snake River„ The area covered by the survey extends from lat 42°305 from long 115 15 ’ W. to 117 001 W„

to h4°00°

and

The Snake River Plain in this region

is bounded on the northeast and southwest by highlands of granite and silicic volcanic rocks. The Shake River and its main tributary in the region, the Bruneau River, have cut canyons in the basalt which reach depths of 700 feet in places.

Altitudes in the region range from about

2,200 feet above mean sea level near the Snake River to over 4,000 feet above mean sea level near the borders » The exposed rocks of the area are predominantly Tertiary and Quaternary basaltic and elastic rocks. Quaternary alluvium hides the lava.

In places, a thin cover of

In general, soil is thin or absent.

In the northwest part of the plain, clastic deposits form a substantial part of the geologic section.

Exposures in the Snake River canyon show

that interbedded layers of sediments and basalt extend down at least 700 feet below the top of the plateau.

Information from isolated wells

drilled in the area indicates that basalt is scarce to a depth of at least 1,100 feet below sea level between Bruneau, Idaho, and Glenns Ferry, Idaho, and to fL depth of at least 1,800 feet below sea level at Ontario, Oregon (Youngquist and Kiilsgaard, 1951)°

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yield Observations and Data Reduction

The field work, done between May 25 and July 31, 19599 included an occupation pf 1,187 stations, Of these, about 200 were occupied more than once in order to keep check on the accuracy of the survey.

The

2,000 square-mile area extending from Glenns Ferry, Idaho, to Caldwell 9 Idaho, was covered with a station density of one station per two square miles.

Another 1,200 square miles of surrounding area, mainly from

Caidwelt, Idaho, to the Oregon-Idaho state line, was covered with a density of one station per seven square miles. Horizontal and vertical control was obtained mainly from U. S. Geol. Survey quadrangle maps.

Most of the survey was done over terrain

covered by maps of scale 1:24,000; however, the control for about 80 stations was taken from 1:62,500 maps.

Locations and altitudes of 17

stations were obtained from A.M.S. charts with a scale of 1:250,000. The entire survey was done with Worden gravity meter No = 388 having a scale constant of 0.5328(1) mgal per dial division. be read to the nearest l/l0 dial division.

3

This meter could

The survey was tied in with

4

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the absolute gravity by initially placing a series of base stations near Boise, Idaho, and obtaining their observed gravities relative to the value at the Bqiee airport as determined by Woollard (1958)« Later in the survey, otfrer bases were established relative to these secondary bases» Loops ranged in time duration from two to five hours, with the average loop lasting about four hours and including about 12 stations, At least one station within a loop would be repeated for the purpose of checking instrument drift, and, in addition, a station from an adjacent loop would be reoccupied to keep a check on the accuracy of the observed gravities. This procedure was followed in all but three or four loops where time or terrain hindered repeats. The following information was recorded at each station : 1,

Instrument reading

2 . Time 3»

Elevation

4.

Latitude and longitude

5°

Location description

6„ Remarks Baldwin and Hill each operated the instrument on alternate loops— the one not operating the instrument recorded the first three of the above­ ilsted items. Mrs. Baldwin, who accompanied the field party, recorded the last three of the listed items.

Latitudes to the nearest half-

second (except for stations located by the A.M.S, charts where the

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5

accuracy was within two seconds) and longitudes to the nearest three second^ were scaled from available maps.

The location description

included type of spot elevation (such as bench mark, section corner, road intersection, etc.), section, township, and range in which the elevation appears ? and a brief description of exact location so the station could be reoccupied if necessary.

Information that would aid

in terrain corrections or later interpretations was recorded under remarks. Instrument drift curves were prepared at the end of each day. Between base-station readings made at the beginning and completion of each loop, drift was considered linear wfth time, unless repeat readings in the loop indicated otherwise.

A drift curve was never altered without

supplementary information from repeated stations. The drift curve was divided into segments, each of which was assigned a base-station value to the nearest l/lQ dial division.

Each time a base station was occupied,

at least two readings were taken in order to insure an accurate reading. If the first two readings were not the same, the instrument was reread until a duplication was obtained, and this last reading was the value recorded. Instrument drift seldom exceeded 0.1 mgal per hour.

The usual

pattern was for the instrument to drift down at the rate of about 0.0? mgal per hour until the hottest time of day (about 4:00 p.m.) and then to level off or even to rise slightly until sunset. Drift was small or absent on overcast days.

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6

A combined free-air and simple Bouguer correction of 0„06 mgal per ft was applied to each observed gravity where a station elevation was 3 i/ available. This value corresponds to a density of 2.6? gm per cm . Datura elevation was mean sea level. Mo terrain corrections were applied to the data.

Most of the area

covered by the survey is relatively flat, and it was estimated that terrain corrections out to zone M of the Hammer chart would not amount to more than about two milligals.

In the mountainous regions, the

terrain corrections might amount to as much as five milligals, but even this amount is negligible compared with the size of the larger anomalies found. Closely spaced stations would have about the same terrain corrections ; hence * no distortion of the anomalies would result from neglecting the terrain effect. Latitude corrections were taken from graphs which plotted the correction against the latitude.

Information for constructing these

graphs was obtained from “Values of Theoretical Gravity on the Inter­ national Ellipsoid“ in Mettleton (19h0, p. 139-1^3)«

iJ

3 The fact that the density is given to the nearest 0.01 gm per cm does not mean that the writer claims this accurate a knowledge of the density. It means only that this density is the one which leads to the convenient value of 0.06 mgal per ft for the elevation factor. The density of an average crustal section above mean sea level, however, is close to 2.6? gm per cm^.

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7

All gravity values were carried to 0.01 mgal within the survey. After correction for elevation and latitude, the simple Bouguer values were rounded off to the nearest 0,1 mgal. Exact location descriptions and observed gravity values of selected base stations are compiled in appendix 1.

Station number, latitude and

longitude, elevation, observed gravity, and Bouguer gravity for all stations occupied during the survey are compiled in appendix 2,

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Accuracy

The accuracy of each simple Bouguer value depends on the accuracy of the following : 1.

Woollard1s value for the Boise airport station

1.

2.

Observed gravity

3.

Station elevation

4.

Latitude correction

Woollard *s value for the Boise airport station : The observed

gravities of the stations in Woollard1s loops are accurate to within about 0.3 mgal (Woollard, 1958). 2.

Observed gravity:

Checks on the accuracy of the observed

gravities of about two hundred stations were made by a system repeats and ties.

of

Repeats are defined as readings at stations that

were occupied twice and whose observed gravities were computed in reference to the same base station for both cases. Ties are readings at stations whose observed gravities were computed twice from two

8

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9

différent bases. To be classified as a repeat„ two values of the observed gravity at a station had to be determined in reference to the same base, although not necessarily during the same loop.

It follows

that if ihe observed gravities of a repeated station differ, they will do so by integral multiples of i/lO dial division (0.053 mgal). Since the gravity values within the survey were carried to 0.01 mgal, ties may differ by integral multiples of this figure. There would be a difference, in terras of accuracy, between a repeat reading within a loop and one from a previous loop, if the repeat reading within the loopwere used to adjust the drift curve.

If

this adjustment were made, the difference between the repeat readings would be zero by construction.

Because of the short duration of most

of the loops and the observed linearity of the drift, only one drift curve was adjusted ip this manner.

The loop for which the drift curve

was adjusted lasted until about 6:00 p.m,, and previous experience had shown that the normal downward drift of the 4:00 p.m.

instrument stopped at about

This repeat was not counted in the totals.

A total of i?l repeats and 108 ties was recorded during the survey. Graph A (fig. 1) shows the number of repeats that differ from the original reading at a station by integral multiples of l/lO scale constant. Graph G (fig, 1) shows the number of ties that differ from the original reading at a station by integral multiples of 0.01 mgal. Only absolute values of the differences have been recorded. If one assumes that the sign of the discrepancy between readings is as often

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Graph A of

Number 80 Repeats

74

70|

66 60 50-

40 JO 20-

Z0-

/

Difference />» Repeat R eadings ( D i a l Divisions)

Number1 ^ /

1 T 66

60

Graph B

50

A>-

37 30-

20-

8

n 20

Différence in Repeated Rfeasctraments (p.o/ n?po/J

O.OS

REPEAT

AND

T /E

M EASUREM ENTS

positive as negative, the distribution curve for repeats would likely be as shown on graph B (fig. 1).

A continuous distribution would

probably be shown by a curve such as is drawn in graph

where the

area beneath the curve is equal to the sum of the areas of the rectangles.

The mean precision (repeatibility) of the readings is the

value of the difference such that one half of all differences are greater tjian this value and one half are less, the signs of the differences being ignored.

If this value be designated by 6 , then

RfOdx ~CK> where f(x) is the continuous distribution curve.

The discrepancy

between the value of £ for repeats (0.04 mgal) and the value of g for ties (0.05 mgal) is believed due to the small size of the statistical sample from which these values were derived. The mean error$ that is, the mean difference from the true value, of the readings is slightly less than the precision.

This fact is

intuitively evident when one considers that a large difference between two readings at the same station does not mean that one reading is in error by the amount of the difference.

It is more likely that one

reading was too high and the other too low.

In the same manner, if two

readings at a given station do not differ, it does not necessarily mean that both readings are accurate, but that they are either both accurate (to within the reading limit of the instrument) or that they are both in error by the same amount.

A third possibility is that the measured

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12

difference is caused by both readings being in error in the same direction but by different amounts e Station elevation : Elevations from the U» S» Geol. Survey quadrangle maps are generally accurate to within three or four feet. A few spot elevations determined photogrammetrieally by the Geological These elevations are believed accurate to within

Survey were used. six or eight feet.

Spot elevations marked on the Is250»000 maps are

accurate to within five feet. elevation was uncertain.

At times the exact location of a spot

In areas where the station density was not

too great, a few stations whose elevations were known to within 10 feet were used.

No elevation with a possible inaccuracy of over 10 feet was

ever used.

An error of 10 feet in elevation would correspond to 0.6

mgal error in elevation correction.

In general, the error in the

observed gravity caused by an error in station elevation is believed to be less than 0.2 mgal. 4.

Latitude correction:

The latitude corrections were taken from

the values of theoretical gravity on the International Ellipsoid as given in the table in Mettleton (1940).

These values were plotted on

a graph of such a scale that the latitudes could be read to the nearest half“second, and the corrections read to the nearest 0.02 ragal.

The

change of theoretical gravity with latitude in the vicinity of the survey is about 0.03 mgal per second.

Because latitudes were read with

a maximum error of 2 seconds, the maximum error in latitude correction is about 0.06 ragal.

The mean error is believed to be about 0.02 mgal.

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Taking into account all the factors that contribute to the simple Bouguer values and analyzing their accuracy, we have the following results : Probable error (mgal)

Maximum error (mgal)

Woollard1s Boise value

0.1 ?

0»3

Observed gravity

0.03

0.12

Elevation (times 0.06 mgal/ft)

0,2

0.6

Latitude correction

0.02

0.06

The largest possible error in simple Bouguer gravity from all sources of error in the measuring process is about one milligal. The preceedirig paragraphs in this section have been limited to a discussion of errors in the measuring process.

Another type of possible

error arises from the assumed density used in the elevation factor, that is, the density of the material above the datum.

This error is

inherently different from the errors discussed above, in that it is a part of the process of interpretation. Regardless of what density is chosen to be used in the elevation factor, the computed anomaly corre­ sponding to this density is a Bouguer anomaly.

The anomaly may be of

doubtful significance, however, if the true density is greatly different from the density used in the elevation factor. If the assumed density is larger than the true density, then the Bouguer correction will be too large.

Inasmuch as the Bouguer correction

is subtracted from the observed gravity, the result will be a final

14

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gravity value which is too low, if the station.is above the datum plane. If all stations are at the same elevation, however, the shape of the anomaly will not be changed by using the wrong density, because all Bouguer corrections will be in error by the same amount.

The greater

the topographic relief in the area of a gravity survey, the more important it becomes to choose a reasonable density for the elevation factor, if a significant anomaly is to be obtained. In the region covered by the Snake River Project, the topography is generally of low relief.

Adjacent stations never differ in elevation

by more than about two hundred feet, and in most of the survey area, the maximum relief is about fifteen hundred feet.

Stations whose

elevations differ by this amount are separated horizontally by many miles. The density used in the elevation factor in the Shake River Project 3 is 2.67 gm per era . This value is certain to be very nearly correct in the areas of granitic rocks to the northeast and to the southwest of the surveyed area.

In the lava plain, however, the true density of the

material above sea level might be considerably less, because the upper part of the geologic section is composed predominantly of clastic rocks.

Clastic rocks found in the few wells drilled in the Shake River

Plain are mainly silts and shales, which have densities on the order of 2.3 gm per cm . The basalt which is interbedded with the sediments raises the bulk density.

The writer believes that the bulk density of

3 the material above sea level is not less than 2=4 gm per cm =

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Evem. if the assumed density is in error by being as much as 0 =3 gm per cm3 too high, the maximum effect on the computed anomalies will be less than six milligals, a small amount compared with the amplitudes of the larger anomalies found. That these anomalies are maxima proves that the density cannot be in error by this amount, for a definite minimum would be recorded if a slab of material of density 2 37 gm per 3

cm

exists between granitic masses which have densities of 2 =6? gm per

cm3 . The writer believes that any Bouguer value computed using the correct density would differ from the value obtained by using a density of 2.6? gm per erP by less than ten milligals0 The relative difference between adjacent stations would not exceed one milligal»

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Description of Anomalies and Qualitative Interpretation

Major Features The most conspicuous features on the gravity contour map of the western part of the Snake River Plain (fig. 2) are two elongated posi­ tive anomalies trending northwest-southeast„ The axes of these features are not co-linear— the northwestern high is offset 10 miles northeast from the axis of the southeastern one. The southeastern high is the larger of the two anomalies in both magnitude and areal extent. Its axis can be traced from Glenns Ferry9 Idaho, westward to Hammett, Idaho » and thence northwest to a point about 10 miles south of Nampa, Idaho, a total distance of about 80 miles. Its peak gravity is a simple Bouguer value of -66.5 mgal with respect to the International Ellipsoid. the axis.

This maximum value occurs about midway along

The gradient is steepest along the northeast side, where the

gravity drops 50 mgal in 8 miles. On the southwest, the gradient averages about J?0 mgal in 12 miles.

The total drop in gravity from the

peak to the surrounding area is over 65 mgal.

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The axis of the northwestern high can be traced from a point 3 miles south of Meridian, Idaho, northwestward to about 3 miles east of Nyssa, Oregon, a distance of about 40 miles. Its peak recorded valueP with respect to the International Ellipsoid, is -79-2 mgal, which occurs at about the middle of the northern half of the axis»

The gradient on the

western part of the southern end is the steepest on the anomaly, reaching 30 mgal in 5 miles.

The total drop in gravity from the peak to the

surrounding area is over 50 mgal. The most plausible explanation for the existence of these two large anomalies is that they are caused by thick accumulations of basalt » A large part of the surveyed area is veneered with lava, and great thick­ nesses of the lava may be present at depths below sea level.

Large

amounts of granite, both to the northeast and to the southwest, present a ready basis for the density contrast necessary to produce such anomalies.

Minor Features Aside from the two highs described above, no features— either highs or lows— having gravity relief in excess of ten milligals were defined in the survey.

The smaller anomalies are confined mainly to parts of

the area where the surface rocks are distributed irregularly and the terrain is uneven.

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In the Montour 15 * quadrangle (lat ^3°^51 N» to 44°00” N0„ long ll6 151 W* to 116 301 W= ), many minor features were found.

A general

drop in gravity readings was found from the west to the east in the Montour quadrangle. The -120-mgal contour gives the approximate shape of this change.

The direction of this contour changes from southeast

to southwest as the contour passes southward through the quadrangle. A possible reason for the presence of this gradient is the wedging out of the Columbia River basalts as the Idaho Batholith granites are approached. On either side of this gradient are found lows of about six milligals each, whose locations correspond closely to outcrops of sedimentary deposits. A small feature of interest appears in a valley north of Boise. This feature is a high with closure slightly in excess of 6 mgal and with gradients on its sides as steep as 6 mgal per mile.

Outcrops of

basalt are found along the valley sides » and this rock probably is present below the alluvium in the valley floor. A possible explanation for this anomaly is an accumulation of basalt in the valley in contrast to granitic cores of the mountains bordering the valley. Another type of minor feature was found repeatedly.

Scattered

over the plain between Boise and Mountain Home are numerous small hills* locally called "buttes," varying in height from 50 to about 300 feet. Readings taken on the tops of these hills„ after corrections » invariably differed from the trend in the region by as much as -4 milligals.

The lows located at lat 43°15*

, long 116°12’ W.9 and

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lat 43

19

’H», long 116°01’ W. (fig, 2) are examples of this type of

minor anomaly.

These minima are quite restricted in area for readings

near the bottoms of the hills fit into the regional trend quite well. Only when the stations are right on the tops of the hills do these anomalies appear.

The writer originally thought that terrain corrections

would account for the difference, but detailed terrain corrections for two of these hilltop stations show that terrain effects cannot account for more than about one milligal.

No relationship seems to exist between

height of hill and size of minimum; two of the largest of these minima (both over four milligals) are from stations on hills 300 and 75 feet high, respectively. vents.

These hills may be surface indications of lava

The upper parts of these vents may be filled with vesicular

lava giving rise to an absence of mass, which, in turn, would result in low gravity readings; however, the lava on the tops of these hills did not differ noticeably from the lava found elsewhere in the region.

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Graticule Analyses of Profile A-A’

Profile A-fA1 is along a line 32 miles long trending No 34° E u The southwest end of the line is at lat 42°531 N. and long 116°018 W.

The

axis of the southeastern gravity high is intersected at right angles by the profile. In profile the gravity anomaly cut by A~A? appears as a symmetrical, bell-shaped curve, with an amplitude of 55 mgal (fig. 3) ° The gradient of the northeast side is 5-2 xrigai per mile, and the gradient of the southwest side is 4.6 mgal per mile.

The peak of the anomaly occurs

7 l/2 miles northeast of the Snake River as measured along the profile. No gravity readings were obtained along extensions of line A-A8. Information can be deduced about the extension of the profile in the southwestern direction, however, by noting the trend in gravity values along the line of stations that extends south from lat 42°h8' N. along o the 115 531 W* meridian. The Bouguer values become successively less in a southerly direction, and the writer believes that the gradient would be about the same along the southwestern extension of profile A-A8.

20

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21

/O

zs

fS F /6 U ft£

-3

—*

PftôFILE.

3Û

A " 'A *

— fro-f>‘ /e A}4

m

Atsunte*/

rtcgicno/

X

/6 F ig u r e

4

—

A ssum ed

80

70

ÆO

R e g io n a l A n o m a ly A lo n g L /n e

A-A* a n d

its

E x te n s io n s

so 40

30

. zo

io

/O

30

25

/es

F !gone 5 —

R e s /dual

An o m a l y

along

U

ne

A-A‘

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According to Bonini and Lavin (1957) » the Bouguer gravity near the center of the Idaho Batholith is =-235 mgal, in reference to the Inter­ national Ellipsoid»

Thus, the gradient of the northeastern extension of

profile A-A * would be about the same as the gradient of the southwestern extension. The writer therefore assumed that the regional gravity along extensions of profile A-A' is as shown in figure 4. The residual anomaly (fig. 5) is obtained by subtracting the assumed regional gravity from the measured anomaly along line A-A '» The removal of the regional anomaly has reduced the amplitude of the anomaly along line A-r-A' to 43 mgal. The method described by Hubbert (19*48) was used in the interpretation of the residual anomaly.

The accuracy of this method depends on two

major assumptions ; (1) that the length of the body being analyzed is so great in comparison with its width and depth that it can be considered to extend infinitely along its axis, and (2) that a reasonable density contrast has been chosen.

Reference to the map (fig. 2) will show that

the anomaly cut by line A-A1 is greatly elongated and, hence, that the first assumption is valid. Considerable latitude exists in choosing a density contrast.

Presumably, rocks cut by a section along line A-A’

are of three general types : granitic, basaltic, and sedimentary. A 3 density of 2.7 gm per cm was assigned to both the sedimentary rocks and to the granitic mountains » and the writer assumed that the granite at depth is of the same density. 3-0 gm per cm^.

The mean density of basalt is about

This figure was adopted for the density of the basalt

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23

in the Snake River Plain „ The density contrast obtained using these 3 values is 0.3 gm per cm . The densities of the main types of rocks perhaps increase with depth, but the writer assumed that the density contrast remains constant with depth. Information from wells drilled in the Snake River Plain indicates basalts may be rare down to a depth of at least 4,000 ft (Youngquist and Kiilsgaard, 1951» Littleton and Crosthwaite, 1957)° In consideration of this knowledge, three permissible interpretations have been developed. The first was made assuming that the basalt extends to the surface of the plain.

The second and third interpretations were made assuming that

the top of the basalt is at depths of 4,000 ft and 8,000 ft, respectively, and that the material above the basalt has no density contrast with the granitic rocks. Cross sections of the three possible anomaly-causing bodies, along with the theoretical and measured residual anomalies, are shown in figures 6, 7» and 8 . In the ease where the top of the basalt is assumed to be at a depth of 8,000 ft, the regional anomaly was taken to be 4 mgal less than in the other two cases.

This was done to compensate for the

greater vertical component of attraction at stations at the ends of line A-A1. The outline of each of these bodies is only intended to give an idea of the general shape of the feature. Even with the relatively high density contrast chosen, a thick accumulation of basalt is required to produce the anomaly.

A total

thickness of 14,000 ft of basalt is required if the top of the basalt

T910

24

reaches t-he surfape of tfye p^Lain; a thickness of 22,000 ft is require^ if the top of the basal^ is at a çtepth of 8,000 ft.

The most plausible

of the three interpretations seems to be the one which assumes that the top of the basalt is at a depth of 4,000 ft. a basait thickness of 16,000 ft.

This assumption results in

This interpretation is favored because

it is consistent with the avaiiable geologic information and yet it does not require as extreme an accumulation of basait as is necessary when the top of the hasa-^t is a,sspme>'es

hsft/th 60

F/GUfi£

9 —

Two FteftMisstBtÆ /v w p /w ra r/a y f

of

rtte Assumbo R&homal Avomalt

T910

Stmctwal Relationships in the Snake River Plain

The Tectonic Map of the United States (1944) shows the Snake River Plain as a downwarp*

This structural interpretation is consistent with

the three gravity interpretations of profile A-A’ discussed above. However, in the interpretations where the top of the basalt is assumed to be at depth, the contact between the basalt and the granite would be too steep to make the possibility of a downwarp likely9 unless the downwarp is associated with extensive faulting. Faults are in evidence on the surface of the plain, and the trend of these faults closely parallels the direction of the contours along the northeastern slope of the southeastern gravity high (Malde, 1959) The extrusion of any large amount of basalt, such as the amount indicated by the gravity highs on the western part of the Snake River Plain, would have two effects s 1) an overloading of the crust in the area of the accumulation of the basalt, and 2) a decrease in pressure at the source of the basalt. Either of these effects would tend to cause subsidence in a region; if the source of the basalt were located

28

T910

29

beneath the area of extrusion9 then the combination of increased pressure pn the surface and decreased pressure at the source of the basalt would make subsidence even more likely« The processes of extrusion and subsidence must necessarily be nearly contemporaneous, for the resulting conditions of instability would almost certainly cause relatively rapid adjustments in the crust. Therefore, great topographic relief would not be needed for the accumulation of large thicknesses of basalt.

T910

Reoomrriendatlong for Further Study

If further gravity investigations are carried out in the Snake River Plain, the writer recommends that the following projects be considered : 1.

An extension of the surveyed area to the northeast and

to the southwest of the southeastern high, in order to define the relationship between the high and the regional gravity. 2.

A detailed gravity survey (station density about 200

stations per square mile) of the area near one of the "hill-top lows" which were discussed under "Minor Features „" Two men could survey and obtain the gravity readings for this project in about eight days. 3»

A detailed topographic survey in the vicinity of Swan

Falls, Idaho, where the Snake River has cut through ?00 feet of basalt and sediments.

Such a survey would enable one to carry out accurate

terrain corrections for stations at the top and the bottom of the canyon and to make a bulk density determination. If readings were taken at both the top and the bottom of the canyon at stations with little

30

T910

31

horizontal separation8 any difference in the readings would be due, after terrain corrections, only to the difference in elevation between the stations and to the effect of nearby anomaly-causing bodies« If the local anomaly is caused by layers of basalt located at least 3,000 feet below the bottom of the canyon, then the local anomaly would be changed very little by a 700-foot change in elevation.

Therefore, any

difference in readings could be eliminated by applying the proper elevation correction. One could then compute the density to be used in the elevation correction which would result in identical corrected gravities at the two stations.

This density would be the average, or

"bulk, " density of the material above the level of the bottom of the canyon,

T910

Appendix 1 Location Descriptions and Observed Gravities of Selected Base Stations

Tabulated below are descriptions of selected base stations occupied during the course of the field work for the Snake River Project»

The

base stations were selected for inclusion in this list pn the basis of ioqation and ease of description. The base numbers are the numbers used in the Snake piver Project»

All of these base stations are marked

as spot elevations on U. S» Geol. Survey 7 l/2-minute quadrangle maps. All gravity readings were taken at ground level unless otherwise indicated. With the exception of Woollard1s Boise airport station (base 1-0), the observed gravities of all bases were established by averaging the observed gravities obtained from three readings made on three different loops. The observed gravities are believed accurate to within 0.2 mgal.

32

T910

Base

1-0

33

Lat No

Long ¥,

43*34'06"

116*13'12"

Elevation (ft above mean sea level) — —

Observed gravity (mgal minus 979»000 mgal) 1208.2

"la foyer of entrance to terminal lobby from field 01 8 (Woollard, 1958, p. 533) ^3°3 5'25 "

13.6° 2 3 '3 ^ "

2618

1 2 5 3 .3

4 ft N of stopsign, SW corner of intersection, sections 13, 18, 19, 24, T3H, R1W, HIE. 4-0

43°40,53"

116°18'29"

2585

1232.8

6 ft E of # in section 14, T4N, HIE. 6-0

43°50 122 "

H 6 ° 2 8 ,55"

2474

12 58.5

2 ft E of EH in center of section 20, T6N» Rl¥„ 9-0

43°2 8 ' 2 6 "

rL 6 °0 6 '3 3 "

3414

1169.3

1 ft E of south marker of YAWL in section 28, T2N, R3E. 12-0

43°13'02"

U 5 049’26"

3189

H76.3

L ft S of ÇM in section 26, T2S, R5E. 16-0

43°20'54" L f(, W of m

18^0

43°18'47"

116°06'30"

3133

1179.4

in section 9, T1S, R3E.

116°01'19"

3149

2 ft S of BM in section 30, T1S, R4E.

1170.2

T910

34

Base

Lat H»

24-0

43°14’20” %

28^0

Long W„ 116O22,01m

Elevation

Observed gravity

3018

1204.1

top of BM âj> section 20» T2S, R1E. o 43 2? ' 3 0 "

o

116 34*19"

2723

1233-6

1 ft W of EM in section 3, TIN, R2W. 30-0

43°39148 "

ll6°36’44"

2426

1277.9

SE corner of intersection » sections 19» 20» 29, 30, T4N, R2W,

32-0

At edge of pavipg.

43032'56i"

116°47'28"

226?

l/l6 marker between sections 34 and 35» T3N* R4W.

1257.8 On

top of brass plate in center of road36-0

42O57'30i"

12.5°16'53"

2542

1199.8

4 ft E of EM in section 28, T5S, R10E. 37-0

42°54'384"

115°55'40 "

2622

1 ft E of BM in section 12, T6S, B4E.

1164.4

T910

A-ppendlx 2 Ckravltv Data for the Snake River Project

The station^numbering system that was used in the Snake River Project can best be explained by an example. Station number 3-8 refers to the eighth station occupied in loops run from base three - The observed gravity of station 3-8 was 8 therefore, determined relative to base three « The station number for base three is 3-0» If a station was occupied twice in loops run from the same base, the original number of the station was retained „ If a station was occupied a second time in a loop run from another base, then this station was given a second number.

However, only the first value of

the observed gravity is listed in the data below. numbers are missing in the list of stations.

Therefore, some

Any station that was

occupied at least three times during three different loops became a base station and was given a base number. Station numbers 5-^3 through 3-29 are missing.

The readings

corresponding to these stations were discarded, because a tie reading was more than 0.2 mgal in error.

Later, the stations were reoccupied.

35

T91Q

the observed gravities were computed relative to another base, and the stations were given new numbers,

36

T910

Station number

3?

Latitude north

west

Elevation (feet above mean sea level)

Observed gravity (mgal minus 979,000 mgal)

Bouguer grav­ ity (mgal in reference to International Ellipsoid)

43°34*06” U 6 013'12“

— —

1208.2

43Q34l20lti 116°14'34"

2804

1213.4

-118.9

1-2

43°33,28tt 116°14'55"

2797

1216.1

-U5.3

1—h

43^31'55"

14.6013'56"1

2873

1211.1

-II3.5

1—5

43°30,38$l H6°13,36 i,‘

3139

1192.7

-114.0

1-6

43°31,02m

116°13'36"

3050

1198.0

-114.1

1«?7

43*33'16"

116°13'48"

2817

1212.5

-117.4

1-8

43*33'05"

116012'45"

2854

1208.3

-119.2

1-9

43°31'54&" 116012'45"

2917

1206.9

-II5.I

1—10

43°31'54&" U.6011'33'*

2946

1203.0

-117,2

1—11

43°31'54&” U6°10'21"

2998

1199.8

-117.2

1-12

43*31*02” U6°X0 '21 "

3027

1199.1

-114.9

1-13

43*31*02"'

U6 °11’32"

3016

1199.4

-II5.3

1-14

43*30*10"' 116°11'34-

3163

1189.3

-115.2

1-15

43*32 *4611 ll6o10'55"

2921

1202.8

-120.2

1-16

43*32*29n H6°09'17"

3003

1197.0

-120.6

1-17

43*33'39"

116o10'03"

2873

1204.5

-122.7

1-18

43*3313911 U 6 o 07'51“

2827

1208.1

-121.8

1-19

43°34'42i" 116°10,22"1

2744

1212.7

-123.8

1-20

43035'22i'' ll6°ll,32n

2723

1214.5

-124.3

1-21

^3°36'33l" H6°ll'10“

2716

1216.1

-124.9

1-0

T91Q

Station

38

Latitude

Longitude

Elevation

Observed gravity Bouguer gravity

1-22

43o36l03i" 116°10'02"

2731

1214.7

-124.6

1-23

43o35'20'i

U6°09»4 o "

2730

1213.5

-124.8

1-24

43°34'2li" 116o 07'40"

2765

1212.6

-122.1

2-0

43035'24"

H 6 o15,09"

2776

1214.2

-121.4

2-1

43°35'26"

116°17,35"

2733

1223.4

-114.9

2-2

43°35,26"

H6°18'48"

2698

1230.1

-IIO.3

2-3

43°35,26i" 116°21'13"

2658

1241.9

-100.9

2-4

43034'33in 1l6°22'24"

2648

1245.7

™ 96.3

2-5

43°34*33i" ll6o20'00"

2697

1235-1

-104.0

2-6

43°34'33i" 116018'48"

2724

1229.5

-108,0

2-7

43°34'33"

U 6 016'23"

2745

1220.8

-II5 .5

2-8

43°33,4l"

H 6 017'36"

2772

1 2 2 3 .6

-IO9 .7

2-9

43°33,4l"

116°20'00"

2711

1234.0

-102.9

2-10

43°33'41"

U 6 ° 2 2 ,24"

2675

1242.3

- 96.8

2-11

43°37'io|" 116°22'24"

2607

1247.9

-100.5

2-12

43°37'io|" H6°21'13"

2624

1241.8

-IO5 .6

2-13

43°36'18"

n 602 o'oo“

2652

1236.3

-108.2

2—14

43°37'10"

Il6018'48"

2662

1228.3

-116.9

2-15

43°36'24§" 116°17'35"

2686

1 2 2 3 .6

-118.9

2-16

43032'4d|" 116018'48"

2729

1229.9

-104.7

2 -1 7

43°32'48%" 116°21*13"

2727

1232.5

-102.2

2-18

43°31'35"

116022,24«

2735

1230.3

-102.6

2-19

43°30,ll"

U6° 2 2 ,24"

2707

1227.7

-104.2

T91G

Station

39

Latitude

Longitude

Elevation Observed gravity Bouguer gravity

2-20

43°3o,lo|M H6°20'43"

2753

1223.3

-IO5.9

2-21

43^31'03"

116o 20*37"

2744

1225.4

-105.6

2-22

43031'55*" 116°20*02"

2733

1228.8

-104.2

2-24

43°30'45i" 116°16'33"

3053

1202.6

-IO9.4

2-25

43q 31'04"

H 6 017'37"

2982

1207.6

-IO9.2

2-26

43°31'03i” 116018«48“

2782

1222.1

-106.6

2-27

43031,56'' H6°18f48"

2801

1223.0

-IO5.9

2-31

43o36'07i" U6°14'33"

2729

1216.1

-I23.5

2-32

43°35,50"

H 6°13'22"

2745

1214.1

-124.0

2-34

43°37,24|" 116o13'03"

2677

1218.9

-I25.7

2-35

43°37,39"

U6°14'43"

2656

1219.7

-126.5

2-37

43033,58"

13.6°06'52"

2793

123,2.4

-120*0

2-38

43°32'17*

H6°05'28“

■=»•=*«=>=•

1207.3

2-39

43°3J.,35"

U6°03'58"

2841

1208.2

-117.8

2-40

43°34«29"

116O00*54"

3522

U 63.2

-126.3

2-41

43033,37" 116o 03'13"

3707

1157.1

*=.120«0

2-42

43°30'08"

116°04'24"

3625

1156.7

'=120«1

2-43

43031'53"

116°05'38“

3138

1189.4

-119.2

2-44

43°31'53"

116°06'48"

3142

II90.8

-117.6

2-45

43031'52"

116°08*00"

3097

U92.3

-118.7

2-51

43029'18"

ll6o13'50"

2985

1202.5

-111.4

2-53

43025'29i" 116°14'18"

2907

1202.7

-110.2

2-54

43^25'18"

116°13'U"

2930

1199.7

-111.5

T910

Station

40

Latitude

Longitude

Elevation

Observed gravity

Bouguer gravity

2-55

^3°25'06|" 116*12'03"

2963

II96.3

-112.7

2-56

43025'16"

116*08'03"

3138

1181.3

JJ.7.4

2-57

4-3°2?'05" 116*0?'44"

3242

1177.3

-II7.9

2-58

43°27'32"

116*09'40"

3168

1184.1

-U6.2

2-59

43027'321' 116*11'35"

3082

1191.6

-II3.9

2-60

43028'25" U6°14'00"

3014

1198.4

-II2.5

3-0

43°35125" 116*23'34"

2618

1253.3

- 91.9

3-3

43°34'32" 116*23'34"

2656

1249.2

- 92.4

3-5

43°32'47"

116*23'35"

2732

1238.5

- 95-8

3^7

43*30'11" 116*24'45"

2684

1232.6

-IOO.7

3-8

43*31'04"

116*25 '57"

2668

1238.3

- 97-3

3-9

43*31'56"

116*24'46"

2689

1237.0

- 98.7

3-10

43*32'51"

116*25'58"

2645

1243.1

- 96.6

3-11

43*33'41 « 116*24*46"

2692

1246.6

- 91.5

3-12

43°35'25f" 116*24'46"

2604

1257.6

- 88.4

3-13

43°34,351, 116*25’58"

2696

1248.4

- 90.8

3-14

43*33"41"

116*27*09"

2630

1247.0

- 94.9

3-15

43*31'5?"

116°27'09"

2617

1243.1

- 96.9

3-16

43*30'11"

116*27’09"

2646

1240.1

- 95-5

3-17

43*30’11"

116*29’32"

2622

1243.7

- 93.3

3-18

43*31'04"

U 6 028'22"

2611

1244.0

95-0

3-20

43*32'49"

116*28'21"

2573

1248.2

- 95-7

3-21

43*36’18"

116*25'57"

2566

1264.3

- 85-2

T910

Statlqn

41

Latitude

Longitude .Elevation

Observed gravity

Bouguer grs

43036,1?"

%16°23'34"

8609

X254.0

T-

3-23

43°37'10"

116° 24'46"

2576

X259-5

- 90.8

3-25

43°3?'11"

U 6 029'33"

25x3

X272.3

t. 81.8

3-26

43934'341

1J.6°29 '34 "

2570

X253.O

- 91.0

3-2?

43°33'41"

116°29'34"

2555

X253.X

- 93-2

3-28

43°34'33"

116°28'21»

2629

X25X.4

— 91.8

3-29

43035'26"

116°2Ç'21"

2654

1255.2

r- 87.8

3-30

43036'35i" 116928'22"

2524

1271.8

- 80.7

4rQ

43°40'53"

U 6 018'29"

2585

1232.8

-122,5

4^1

4394Q'39"

116°16'42"

2008

1229.8

-7123.8

4-2

43°39'34"

ll6°17 '05'*

20X2

1229.3

rl22.4

4r3

43°35'55"

116°18'48"

2653

1228.2

-120.1

4-=4

43°38'55"

^16°21'13"

2621

1238.4

T - lll. S

4-5

43°38i02'' 116°22'25‘!

2608

1245.9

-IO3.7

43°38 *02f11 ll6°2O'0O"

264X

1233-2

-114.5

93-0

4-8

43q 37,09"

116°16'26"

2699

1219.5

r l

4—9

43p 38,33"

116°X6'24"

2684

1221.4

-124.5

4-10

43°4l'41"

H 6 918'47b

2640

1228.3

-124.9

4-11

43°43'15»

Il6919'11"

2662

1226.7

-127-6

4-12

43^44'25" 116°18'05"

273X

1227.7

-124.2

4-13

43°43 '4#" U6°l6'05"

2807

1223.4

-I23.O

4-14

43p42120"

U6°15'39"

3X88

1191.5

-

4-15

43O40'00"

116°15'09"

2667

1224,4

-124,6

23.4

129.8

T910

Station

42

Latitude

Longitude

Elevation

Observed gravity

Bouguer gr
32,25"

353^

1097.8

—II8.7

36-37

42°45,52iM 115°31'36"

3492

1098.7

-119.6

36-38

42°45,54|ri U 5 O30'22"

3504

IO97.5

-120.2

36-39

42°56,18tl 115°21'08"

2560

I203.6

- 86.3

36-40

42°53'59i" U5°22 '34"

3122

1159.5

- 93-2

36-41

42°33'09i" 115°26’52"

2886

1182.6

- 86.0

36-42

42°53'44i" 115°26'22"

3110

1161.4

- 91.7

36-43

42052f55'1 115°24'37“

3136

1155.5

- 94.7

36-44

42°52‘53«

H5°26'46-

2893

H75.8

- 88.9

36-45

42°52»o 6|,î 115°26'33"

2891

1170.8

- 93-0

Station

Elévation

Observed gravity Bouguer gr:

T910

Latitude

Longitude

Elévation

Observed gravity

Bouguer gr,

42O50*5Xi" 115°25'05"

2839

1164.4

-100.4

36-47

42O5!*05i" U5°26'46"

2853

1165.4

- 99-0

36-48

42O48,07§-u U5°26'40"

2885

1148.1

-110.0

36-49

42°45l52iM 115°28'00"

3219

1114.9

-119.8

36-50

42045»23i-» 115°27'00"

3258

1110.8

-120.8

36-51

42°46,38i-tl U5°24'53"

3088

1127.9

-II5.9

36-52

42046,19iM H 5 O24'01"

3089

1127.8

—115*4

36-53

42045»57t» H 5 o23'01"

3081

1126.6

-II6.5

36-54

42°45*ll«

U 5 ° 2 2 140 "

3098

1121.0

-119.9

36-55

U5°21'23"

3173

1115-4

-120.7

36-56

42°45105 " 115O20'18"

3162

1115.4

-121.5

36-57

42°45 '40"

115°19'37"

2997

1127.9

-119.8

36-58

42°46'13&" 115°20'47"

3154

1120.5

-118.6

36-59

42°4713o|-» 115021'15"

3186

1121.6

-II7.5

36-60

42O50*35t 11 115°23'53"

2854

3159.4

-104.3

36-61

42°5X,33m

H5°24'l6"

3126

1147.8

-101.0

36-62

42052,46n 115°22'36"

3140

1152.7

- 97.1

36-63

42°58t02|n 115032'46"

2859

1191.8

- 82.8

36-64

42058,56n H5°34'21"

3006

1182.1

- 85-0

36-66

42°58'55i" 115°35'52"

3023

1179.8

- 86.2

36-67

42°58'04§4' U 5 o37’01"

2933

1188.2

- 81.9

36-68

42°57?1XH 115035'52"

2889

1189-3

- 82.2

36-69

42°58'OOi" 115O35'03"

2862

1194.0

- 80.3

0

36-46

t

Station

85

Station

Latitude

Longitude

Elévation

Observed gravity

Bouguer gravity

36-.70

42°59,23"

115032'51"

2976

1185-2

*■» 84.4

36-71

42058'54ÿ" U 5 o30'58"

2878

1199.8

—

36-72

42°58'25"

115°29'02"

2554

1213.7

79-7

36-73

42°57l14|" 115°28’56"

2548

1211.5

“ 80.5

36-74

42°58'54|" U5°18'10"

2610

1197.2

— 93.6

36-75

42059'15*" 115°I9'15"

2697

1191.6

- 94.5

36-76

42°5810211 U5°19'10"

2623

1199,8

— 88,9

36-77

42°58ll6u 115°21'02"

3005

1171.2

—

36-78

42°59'20"

U 5 ° 2 1 ,37"

3129

1162.2

— 98.1

36-79

42059'46"

115°22'57"

3072

1166.2

— 98.2

36-80

42058'53"

U 5 023'59"

3076

H 68.3

■= 94.5

36-81

42°58'48"

n.5o26'09"

3051

1174.6

—

36-82

42°59'24i" U 5 ° 2 8 ,14"

3081

1175.3

36-83

42°59'2li" 115°28'47"

3097

1174.4

-

36-84

42059'X8"

X15°27'34"

3083

1173.9

— 89.2

36-85

42°58'19&" 1X5°27’18"

2758

1197.6

“

36-86

42°57'38"

115°27'37"

2603

1209-3

80.0

36-87

42°56'19"

U5°27'38"

2536

1212.1

— 79.3

36-88

42°56i45"

115°25'17"

2546

1211.2

-

80.2

36-89

42057'57"

115°25'24"

258I

1208.1

—

83.0

36-90

42°57'34"

115°21,48"

2563

I203.8

36-92

42°58'49"

115°15'48"

2519

1197.9

-

98.2

36-93

42°55'24|" 1X5018'51"

2765

1183.8

-

92,5

74.9

94.9

89,6

— 88.0

87.8

83.4

87.9

T910

Station

8?

Latitude

Longitude

Elevation

Observed gravity

Bouguer gravity

36-94

42°53 48"

115*18 08"

3H7

1152.3

-100.4

36-95

42*53 02i" 115*18 04»

3106

H 50.8

-101.4

36-96

42*51 32» 115*18 30"

2996

1149.2

-IO7.3

36-97

42*51 14*" 115*17 54»

3034

II38.6

,-113.8

36-98

42*48 52"

115°18 00"

3177

1122.6

-II9.2

36-99

42°46 53*" 115*17 53"

3191

1116.7

-121.2

36-100

42*46 O4^" 115*17 53"

3153

1116.7

-122.2

36-101

42*45 10"

115*18 03"

3225

H 09.9

-I23.4

36-102

42*45 42"

U5°15 28»

3318

U O 5.9

-122.6

36-103 42*47 2?*" 115*16 02»

3201

1118.0

-120.2

36-104

42*48 41»

115*16 40"

3140

1125.6

-118.0

36-105

42*51 18"

115*17 35"

3012

1146.0

-IO9.2

36-106 42°51 32» 115*16 08»

3127

1138.6

-110.1

36-107 42*52 15" 115*18 25"

3012

1152.4

-104.3

115*15 49"

2676

1185.4

- 93.6

36-109 42*57 35" 115*15 50"

2601

1193.4

- 95.9

37-0

42*54 38*" 115*55 40»

2622

1164.4

-II9.3

37-1

42*53 43"

115*53 56"

2620

H59.7

-I22.7

37-2

42*53 22"

115*55 24»

2715

1151.3

-124.9

37-3

42*53 15"

115*56 38"

2652

1153.5

-126.3

37-4

42*54 19i" 115*58 04"

2606

II59.3

-124.9

37-5

42°52 48&" 115*57 22»

2716

H49.1

-I26.2

37-6

42*53 38"

115*51 46 »

2524

H 69.O

-II9.I

36-108

42*53 41»

T910

88

Latitude

Longitpde

Elevation

37-7

b2Q52’36"

115°50'29"

2580

1162.9

-120.2

37-9

42°53'56" H 5 ° 4 9 ,24"

2509

1177.3

-112.1

37-10

42°54'59|" 115°49,34"

2599

1176.3

-109,2

37-11

42°56'0l|" 115o50'47"

2674

1175.6

-IO7 .O

37-12

42°56'19" 1X5°52,02»

2658

1176.9

-IO7.2

37-13

42°^6'37"

115053'15n

2641

3178.5

-107,0

37t-14

42056'22"

115O55,02"

2658

1170.4

-II3.7

37-15

42°56’01"

U5°55,02"

2657

II68.3

-II5.3

37-16

42°57'15" U5°55,02"

2628

1177.6

-109.6

37-17

42°58,36"

115°55'08"

2811

1173.2

-IO5 .O

37-18

42°57'25i" 1X5°51'52"

2665

1183.4

-101.9

37-19

42°56,53"

U 5 ° 5 0 ,04"

2708

1180.8

-101.1

37-20

42°55'29"

1X5°55'01"

2715

1161.3

-118.1

37-21

42054'59|« H5°55'0l"

2654

1163.9

-118.4

38-0

43°31'56i" 116°29'32"

2573

1246.4

- 96.3

39-0

43031'52i" U 6 ° 4 5 ,23"

2324

1253.8

-103.6

gtation

Observed gravity Bouguer gravity

T910

References Cited

Baldwin, H. L,, Jr., and Hill, D. P., i960, Gravity survey in part of the Snake River Plain, Idaho— a preliminary report t U. S, Geol. open-file report, 68 p,, 3 figs» Bonini, W. E., and Lavin, P. M*, 1957» Gravity anomalies in southern Idaho and southwestern Montana (jabs»] : Geol.

Soc. America Bull.,

v. 68, no. 12, pt. 2, p. 1702. Rubbert, M. K., 1948, Line integral method of computing gravity: Geophysics, v. 13» p. 215-225» Littleton, R. T,, and Crosthwaite, E. G., 1957» Ground-water geology of the Bruneau-Grand View area, Owyhee County, Idaho:

ü. S. Geol.

Survey Water-Supply Paper 1460-D, p. 147-198. Mal.de, H. E., 1959, Fault zone along northern boundary of northern Snake River plain:

Science, v. 130, no. 3370, p. 272.

Mettleton, L. L., 1940, Geophysical prospecting for oil: McGraw-Hill, 444 p.

89

Mew York,

90

T910

Tectonic map of the United States, 19^» prepared under the direction of the Committee on Tectonics, Division of Geology and Geography, National Research Council-

published by the Am, Assoc. Petroleum

Geologists. Woollard, G. P. , 1958, Results for a gravity control network at airports in the United States : Geophysics, v. 23, no. 3, p. 520535. Youngquist, W a lte r, and K iils g a a rd , T , H., 1951» Recent te s t d r illin g , Snake R iv e r p la in s , southwestern Idaho : G eologists B u ll., v . 35» no. 1 , p . 90-^96.

Am, Assoc, Petroleum

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