A Seismic Study of an Impact Feature in Cass County, Michigan

Western Michigan University ScholarWorks at WMU

Master's Theses Graduate College

8-1985

Mancheol Suh

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Recommended Citation Suh, Mancheol, "A Seismic Study of an Impact Feature in Cass County, Michigan" (1985). Master's Theses. 1444. https://scholarworks.wmich.edu/masters_theses/1444

This Masters Thesis-Open Access is brought to you for free and open access by the Graduate College at ScholarWorks at WMU. It has been accepted for inclusion in Master's Theses by an authorized administrator of ScholarWorks at WMU. For more information, please contact [email protected]. A SEISMIC STUDY OP AN IMPACT FEATURE IN CASS COUNTY, MICHIGAN

Mancheol Suh

A Thesis Submitted to the Faculty of The Graduate College in partial fulfillment of the requirements for the Degree of Master of Science Department of Geology

Western Michigan University Kalamazoo/Michigan August 1985

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. A SEISMIC STUDY OP AN IMPACT FEATURE IN CASS COUNTY, MICHIGAN

Mancheol Suh, M.S.

Western Michigan University, 1985

A geophysical investigation, including a seismic

study, of the subsurface structure of the Calvin-28 oil

field, Cass County, Michigan, indicates that the

structure's origin may be related to an impact event. The

structural closures in the time structure maps and

drilling results both show a central uplift. The fault

system in the Trempealeau Formation shows a structure

similar to other proven astroblemes. Undisturbed layers

just beneath the central uplift also provide evidence of

an impact structure. The result of mathematical modeling

of the central .uplift under the assumption that the

structure is an impact structure corresponds well with the

drilling results. Therefore it may reasonable to accept

the impact origin for the Cass County structure.

The impact event occurred on the Utica shale of the

Late Ordovician age. The diameter of the original crater

was at least 2 kilometers. The mass of the meteorite was

about 9 x 104 tons.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ACKNOWLEDGEMENTS

The author appreciates Dr. Gerry Clarkson of Western

Michigan University for his suggestions and support during

this research. He is also deeply grateful to Dr. W.

Thomas Straw and Dr. William Harrison of Western Michigan

University for their critiques of the manuscript during

its preparation.

He is also thankful to Mr. Robert Mannes and Mr.

Ward Kelner of Mannes Oil Corp. and Mr. Douglas Daniels

of the Michigan Department of Natural Resources for

providing geological and geophysical information and for

the permission to use their data in this thesis.

Special thanks are extended to Mr. Moon Kim, Mr.

Jongwook Lee, Miss Sandra K. Barnick, Mr. Saiful B.

Baharom, and Mr. Russell G. Leviska for their help in

field work and to Mr. Tom Thibeault for his help in field

work and correcting the manuscript.

Further thanks are extended to the Department of

Geology of Western Michigan University for providing the

GEOPRO 8012A seismograph, to the Academic Computer Center

of Western Michigan University for providing computer time

and to The Graduate College of Western Michigan University

for funding the research. ii

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Finally, he also would like to express his deepest

appreciation and thanks to his wife Kyong-sook for her

help in drawing the figures and taking good care of him

and his daughter Jee-Eun.

Mancheol Suh

iii

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Suh, Mancheol

A SEISMIC STUDY OF AN IMPACT FEATURE IN CASS COUNTY, MICHIGAN

Western Michigan University M.S. 1985

University Microfilms

International 300 N. Zeeb Road, Ann Arbor, Ml 48106

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Repr o t e * w9h permlssion of the copyr|gh, owner; pumer reproduction ^ ^ TABLE OP CONTENTS

ACKNOWLEDGEMENT ...... ii

LIST OF TABLES ...... vi

LIST OP FIGURES ...... vii

LIST OF PLATES ...... ix

CHAPTER

I. INTRODUCTION

Objective ...... 1

Location ...... 3

Approach ...... 4

II. GEOLOGY ...... 7

III. GEOPHYSICAL EXPLORATION

Gravity and magnetic survey ...... 11

Seismic survey ...... 14

IV. GENERAL ASPECTS OF AN IMPACT SITE

Impact sites ...... 22

Geophysical characteristics of astroblemes ...... 27

Mathematical consideration of meteorites ...... 28

V. INTERPRETATION

Geological results ...... 33

Gravity and magnetic results ...... 38

Seismic results ...... 39

Mathematical modeling for isostatic uplift ...... 47

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OF CONTENTS CONTINUED

CHAPTER

Consideration of the meteorite ...... 49

VI. MODEL FOR FORMATION OF THE CASS COUNTY STRUCTURE ___ 51

VII. CONCLUSIONS ...... 53

BIBLIOGRAPHY...... 56

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OP TABLES

1. Stratigraphic Succession of the Study Area ...... 9

2. Meteorite Energy and Mass for Six North America Meteorite Craters ...... 32

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OP FIGURES

1. Location Hap of the Study Area...... 5

2. Location Map of Drill Holes and Seismic Lines. ... 6

3. Basement Province Map of the Southern Peninsula of Michigan (from Kellogg, 1971)...... 8

4. Bouguer Gravity Map of South-central Cass County (from Ghatge, 1984)...... 12

5. Second Harmonic Residual Gravity Map of Southcentral Cass County, Michigan (from Ghatge, 1984) ...... 13

6. Comparison of the Energy Sources. (a) Seismic Wave with Ten Guage Blank Shell, (b) Seismic Wave with Hammer Blow...... 16

7. T2 - X2 Graph of the Seismic Lines S-l and S-2...... 20

8. Location Map of Impact Sites (Numbered impact sites cited from Dietz (1961), 1. Kelly Lake, 2. Frank Town, 3. St. Lawrence Arc. 4. Sault- aux-Cochons, 5. Menihek Lake, 6. Mecantina, 7. Labrador, 8. Ungava Bay)...... 23

9. Evolution of a Fossil Crater (From Sawatzky, 1975)...... 26

10. Relationships between Meteorite Energy and Srater Diameter (after Innes, 1961)...... 31

11. Geologic Cross Section of the Study Area...... 36

12. Structure Map of the Traverse Limestone...... 37

13. Time Structure Map of the Traverse Limestone 43

14. Time Structure Map of the Trempealeau Formation...... 44

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 15. Seismic Section along HAL-81-7...... 46

16. A Simple Model for the Calculation of the Amount of Central Uplift...... 48

17. Subsurface Geological Model of the Cass County Structure. .... 52

vilt

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF PLATES

I. Seismic Section along HAL-81-1

II. Seismic Section along Hal-81-2

III. Seismic Section along HAL-81-7

IV. Seismic Section along HAL-82-8

V. Seismic Section along HAL-82-9

VI. Seismic Section along HAL-82-10

VII. Seismic Section along MRH-82-1

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER I

INTRODUCTION

Objective

Since the 1930 discovery of a two - well Traverse

Limestone field in Cass County, Calvin Township has

emerged as a major oil field in southwestern Michigan.

The Calvin - 28 oil field, located in sections 27, 28, 29,

32, 33, and 34, T14W, R7S of Cass County, Michigan (Fig.

1), has a circular production pattern with a diameter of

1.5 miles. This circular production pattern is very

unusual when compared with typical oil fields. This

unusual feature has led to research such as gravity and

magnetic surveys and deep drilling in the center and on

the edge of the circular producing area.

About 1200 feet of the Late Ordovician sedimentary

rock are missing in the Hawkes-Adams 1-28 deep well

located near the center of the producing area. However,

in the Lawson 1-28 deep well, located 0.75 miles northeast

of the Hawkes Adams 1-28, a complete Ordovician section is

present. The sequence from the Utica shale to the St.

Peter Sandstone was not penetrated in the Hawkes Adams

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2

1-28 deep well and the Trempealeau Formation was at least

1000 feet structually higher in comparison with that in

the Lawson 1-28 well. Therefore, the Calvin township oil

field has been considered to be similiar to

cryptoexplosive structures, which is a non-committal term

about its origin. Similiar features have been found in

several identified astroblemes such as the Kentland

structure in Indiana (Gutschick, 1976), Jeptha Knob

(Seeger, 1960) and the Versailles structure (Seeger, 1972)

in Kentucky, the Glasford structure in Illinois (Buschbach

and Ryan, 1963), and the Serpent Mound structure in Ohio

(Bucher, 1968).

The debate concerning whether cryptoexplosive

structures are of terrestrial origin or extraterrestrial

origin began about one half century ago. In general,

features identified as being cryptoexplosive structures

are roughly circular structures, with a ring depression

surrounding a central uplift of disordered and brecciated

rocks (Wilshire and Howard, 1968). Several origins, such

as cryptovolcanic, meteorite impact, faulting, and

diapirism, have been suggested to explain this type of

structure. The gross structure, as well as the common

occurence of shatter cones and unusual mineral

deformation, indicates an origin by explosive release of

energy, either from impact or from volcanic gas (Wilshire

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3

and Howard, 1968).

In Cass County it is difficult to even identify the

cryptoexplosive structure itself, as it is covered by

thick Silurian and Devonian formations and about 300 feet

of glacial drift. Therefore, geophysical methods combined

with drilling data are the only way to study the

structure.

The main purpose of this thesis is to outline the

basic parameters which identify the subsurface geological

structure of the Calvin - 28 oil field as a fossil crater

using pertinent maps, cross sections, previous gravity and

magnetic results, and seismic record sections.

Location

The area under investigation includes sections 14

through 35 of Calvin Township and sections 2 through 11 of

Mason Township (Fig. 2). The study area is about 5.5

miles by 5 miles and it is covered by the Adamsville

Quadrangle, 7.5 minute series, Michigan-Indiana (USGS

Maps).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Approach

A geophysical study of the subsurface structure and

its origin was made using results from drilling, gravity

and magnetic, and seismic data. Most of the seismic data

used in this study were obtained from Mannes Oil Company,

Holland, MI and were processed by Hosking Geophysical

Company, Mt. Pleasant, MI. The interpretation of these

data is solely that of the author. For the area where

seismic sections were lacking, a seismic exploration

program was conducted using a GEOPRO 8012A 12 channel

seismograph. These data were used to outline the Cass

County structure and its origin.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 5

STUDY Penn AREA

Jefferson Calvin

Ontwa ^ M a s a n .

Location map of the study area.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission HAL-e !-•

30 HAL-81-2

i20

8-1 34 35

ITS T l*

i KM MRH-82

SO Shot point 0 4 OiMIng hot*numbir

Fig. 2. Location map of wells and seismic lines (The wells are; 1. Haines 2-28, 2. Warren Smith 3, 3. Smith-Keck 1, 4. Hawkes-Adams 1-28, 5. Lawson 1-28, 6. Burns-Growhoski 2-33, 7. Kirkdorfer 1-34A).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER II

GEOLOGY

The Calvin - 28 oil field is located in Calvin

Township of Cass County, in southwestern Michigan just

north of the Indiana border. Geologically, it is located

in the southwestern flank of the Michigan Basin and has

been assigned to the central basement province which

consists of granite, felsic and mafic gneisses,

extrusives, and metasediments (Fig. 3). Depth

determination to the basement rock carried out by Kellogg

(1971) on the basis of aeromagnetic data show that the

average elevation of the Precambrian surface in Cass County

ranges from 3495 feet to 3740 feet below sea level. The

nearest basement drilling in section 10, Berrien County

showed Precambrian metasediments at 3800 feet below sea

level.

The surface geology of this area consists mostly of

the Quternary glacial drift which has an average thickness

of about 300 feet. Bed rock is the Antrim shale or the

Ellsworth shale of early Mississippian age (Table 1). The

Calvin - 28 field produces gas and oil from Middle

Devonian Traverse limestone.

The investigated area has a subsurface geology

consisting of Precambrian basement, a complex sequence of 7

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 8

V0LCANIC3 BASEMENT PROVINCE MAP

EXTRUSIVES AND IN - METAVOLCANiCS, METASE&UENTS TRUSIVES 9 0 9 ANO GNEISSES 20 MILES + 4 9 *0 0 4 9 *0 0 V

EXTRU SIVES, / METASEDIMENTS, MAFIC IN UETAVOLCANICS TRUSIVES/ AND GNEISSES NEISSES ANO META-' / WMENTS * 4 4 * 0 0 ’ 4 4 * 0 0 '+

METAVOLCANICS ANO

MAFIC INTRUSIVES, | GNEISSES ANO GRANULITE

4 3 * 0 0 '+ ^LCK GRANITE, f e l s ic and INTRUSIVES MAFIC GNEISSES, EX AND EXTRU TRUSIVES ANO SIVES A M ) /. IETA5E0IMENTS \ MAFIC/-/ U^JNEISSES

+ 4 2 *0 0 ’ 4 2 * 0 0 '+

+ + ♦ + • 2 * 3 0 ’ 96*30* •4 *3 0 ’ as *so’ PREDOMINANT EZ) FENOXEAN (I.9-I.6B.T.) O KEWEENAWANUN (l.0 9 —I.I9BY.I / STRUCTURAL TREND E3 CENTRAL (1 2 —1.9 2.T) E J ONENVILLC(Ot-l.li.T.) /

Fig. 3. Basement province map of the Southern peninsula of Michigan (Kellogg, 1971).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 1

Stratigraphic succession of the study area.

Formation 1 Age

GLACIAL DRIFT I PLEISTOCENE

COLDWATER SHALE I MISSISSIPPIAN

ANTRIM SHALE --- TRAVERSE FORMATION TRAVERSE LIMESTONE DEVONIAN DUNDEE LIMESTONE DETROIT SYLVANIA ---

C UNIT --- NIAGARAN SERIES CLINTON SILURIAN CABOT HEAD SHALE MANITOULIN DOLOMITE ---

CINCINNATIAN SERIES UTICA SHALE TRENTON FORMATION BLACK RIVER GROUP ORDOVICIAN GLENWOOD SHALE ST. PETER SANDSTONE PRAIRE DU CHIEN ---

TREMPEALEAU FORMATION --- FRANCONIA SANDSTONE DRESBACH SANDSTONE CAMBRIAN EAU CLAIRE MT. SIMON

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Paleozoic sedimentary rocks, and Quaternary glacial drift

(Table 1). The Paleozoic sequence overlying the

Precambrian basement consists of the Mt. Simon Sandstone,

Eau Claire, Dresbach, Franconian and Trempealeau

Formations of Cambrian age; the Praire du Chien Group,

St. Peter Sandstone, Black River Group, Trenton Group and

Cincinnatian series of Ordovician age; the Cataract Group

and Clinton Formation, Niagrian Series, Salina Series and

Bass Island Groups of Silurian age; and the Sylvania

Sandstone, Detroit River Group, Dundee Limestone, Traverse

Limestone and Traverse Formation of Devonian age. The

Antrim Shale and Ellsworth Shales of Early Mississippian

age directly underlie the Quaternary glacial drift (Table

1 >- Structurally, the area is cut by several reverse

faults with a NNW-SSE orientation below the glacial drift

and a dramatic upward displacement in the Ordovician

formations. Ordovician rocks from the St. Peter

Sandstone to the Utica shale are missing in the Hawkes -

Adams 1-28 deep well.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER III

GEOPHYSICAL EXPLORATION

Gravity and Magnetic Survey

A gravity and magnetic survey was carried out over

portions of the Penn, Porter, Jefferson, Mason, and Ontawa

Townships in Cass County of southwestern Michigan (Fig.

1) by Ghatge (1984). The Bouguer gravity anomalies were

analyzed using double Fourier series to obtain residual

gravity values of the area.

The Bouguer gravity anomaly for south-central Cass

County (Fig. 4) shows mainly a regional source. It does

not show any local anomalous closures. The contours of

anomalies show a roughly semi-circular feature for the

entire survey area. The magnitude of the Bouguer anomaly

ranges from -21 milligals at the north part of the area to

-31 milligals at the south center. The second harmonic

residual anomaly map with wavelength of 28 miles (Fig. 5)

shows a positive anomaly of 0.75 milligal around the

Calvin - 28 oil field.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. BOUGUER GRAVITY ANOMALY MAP,

SOUTH-CENTRAL CASS COUNTY, MICHIGAN. tMW |-ONitd €) Uflf M w t tea Immm *mt l*»* IIP WS0 ItMlIfWwf D«U < • ’** •»*« IMP M Aft N + L K « « M Q o m tUti

h*K kii/ i«

Fig. 4. Bouguer gravity anomaly map of the South-central Cass County (Ghatge/ 1984).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. SECOND HARMONIC RESIDUAL GRAVITY MAP, WAVELENGTH: 20

MILESt SOUTH-CENTRAL CASS COUNTY, MICHIGAN. Ur*m *v«40-i9 9 Util l e e Ih w uJTif®**11" Imr OU» 1*0 to MMI U«M UflM '3

ii'SWSS ;

Pig. 5. Second harmonic residual gravity map of the South-central Cass County (Ghatge, 1984).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 14

Seismic Survey

Field Procedure

A seismic exploration program including both

refraction and reflection surveys was conducted using a

GEOPRO 8012A 12 channel seismograph. At first, a 10-lb

sledge hammer and 10-gauge blank shells were compared to

determine which source was better in the research area.

Seismic waves were recorded with each three shots of both

the 10-lb sledge hammer and the ten guage blank shells

using a band pass filter of 75-440 Hz. The assignment of

gain numbers for each seismic trace was exactly same for

both energy sources. Figure 6 shows the sledge hammer to

be a better energy source than the ten gauge blank shells

for this area. This may result from the difference in

the amount of energy transfered into the ground. In hammer

blows, most of the energy is transfered into the ground

through the steel plate, while in shot gun blows, much of

the energy may be absorbed by the air and soft soil.

For the refraction survey, the geophone spread was

offset from the shotpoint location by distances of 20

feet, 260 feet, and 500 feet while the shot point remained

at the same location. This gave refraction data over

distances of 20 - 720 feet. But the last geophone array

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 15

in which the offset was 500 feet, gave poor data because

the arrival energy was too weak. For this problem, some

energy source larger than the hammer is recommended. Four

or five hammer blows were stacked for each shot to enhance

the seismic data. A reverse shot was also done using the

same method.

For the reflection survey, the geophone spread was

220 feet long with 12 geophones spaced at 20 foot

intervals. This geophone array was shot with five or six

stacked hammer blows at a distance of 100 feet from the

first geophone. The spread was then moved to the next

position in the seismic line. The geophones were buried

in the ground to reduce severe noise. A 10-lb sledge

hammer was used as the energy source. Several band pass

filters such as 35 BE (BESSEL) - 200 BE, 75 BE - 440 BE,

and 70 BU (BUTTERWORTH) - 1000 BU were tested to see which

gave the best seismic data. With the 35 BE - 200 BE band

pass filter there seemed to be much low frequency noise

and ground roll. With some field experiments, it was

found that a low pass filter of 440 Hz and a high pass

filter of 75 Hz gave the best seismic data in the area.

Data Processing

In the refraction data interpretation, layer

velocities and intercept times were determined from a T-X

plot. The thickness of each layer was determined using

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission...... y : ! : : IE \ IE V **jj. ‘ .1 .1 -i :i A. • r, *•,•»;• i »•*';' ...... - - . i ^ i * • » • • j • . . • • : j : : 1 1 : • cl

! i | ! :'! 'I i ■; i : ; | : , E i ~E ->• Wvj*; *~-

%!•*•/“ 4*. j- -•* ----—•- E : ! : : : ' I : : ,: . . T-C —j -4-V ,|-Vk*:

-JLU^>M 4 i X* i • j • V V V |\* • v V k '*, ‘ j, :j : '

. i !: i .!'!!:! j . .L^:. •-.,» ...*. .t 4*4 i»,\ v 4 i m; a- # a," *.*>.*>.[ A 4 * 11 • 4 < 4 :. ; L, ^ 7 PIT~f . •'V y ? v'r^ V* :*-r*; t; v*: : | f: << * %•; -.r*’ -. » j • ’ • • } • * t ! s I ! ^ P v v . . } ;.; ;v. t. , . •(»!»,: %.■» ,•* 7 .«*\/■* j ••; 2 4 .1,.j*5.> a *E, . • '•* * v* J/ » '• ; v 'I * M : : I a e !, Lj j-L ^ ’iE I**! ... *’ ■* • v ‘ :•.* ' s *•» V»j l .:, L -J 1‘il^i. ki.y v.r-r-S1 Vvi;. ;<\v^ y A A*: ■ K 1/ ^ 1

'L_ • '.< * )nwij ,.v . ; ! i f i i 'I i i i h . :::!::::! :•%>' :i *’■■■] ; ;•» v- iw.t yif.s* msec t f S -nii•msec CHANNEL P GAIN rHP53iS!f-lii:!3;l CHHlUIEt P GAIN KITKnr^nrS-is! ] •j 2« % E 1 + * E 2 I i :f E 3 + l\ i E * E 4 + * E 5 + f I + * E + n . v + I E 9 + * E 19 ii* E 10 i ♦ Ell * E i 1 + I E 12 I E 12 + m m i m (a) (b)

Fig. 6 . Comparison of the energy sources; (a) Seismic wave with 10-guage blank shell,- (b) Seismic wave with hammer blow. All the other conditions are the same.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 17

the calculated intercept time and velocity with the

equation

Tin_i = 2 (Zm/Vm )*[l-(Vm/Vn)2] V 2 -- (1)

(Telford, Geldart, Sheriff, and Keys). With

the refraction study for the geophone spread

S - 2 - 1 (Fig. 2) three horizontal near

surface layers were detected with velocities and

thicknesses of 1250 ft/sec, 2500 ft/sec, 5000 ft/sec and

20 feet, 60 feet respectively. The first layer appears to

be a weathered zone above the water table, the second a

saturated weathered zone and the third unweathered glacial

fill. These results were used for weathering and

elevation corrections of the reflection data.

The Traverse Limestone reflection was picked using

the anticipated reflection time which was calculated using

sonic logging data compiled by Mannes Oil Company,

Holland, MI and drilling data stored at the Department of

Natural Resources, Plainwell, MI. The picked times were

static corrected using a datum level of 800 feet A.S.L.

The weathering correction value (Tw) and the elevation

correction value (Te) were calculated using the

following two equations;

Tw = (l/Vi)*(Zsi+Zgl) + (l/V2)*(Zs2+Zg2) ---- (2)

Te=[Es+Eg-(Zsl+Zs2+Zgl+Zg2)-2D]/V3 (3)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 18

where, Vi; the velocity of the unsaturated

weathered zone

V 2 ? the velocity of saturated weathered

layer

V3 ; the velocity of glacial fill

Es ; the elevation of the shot point

Eg ; the elevation of each geophone location

Zgi; the thickness of the unsaturated

weathered layer at shot point

ZS2 ,* the thickness of the saturated

weathered layer at shot point

zgl' thickness of the unsaturated

weathered layer at each geophone place

Zg2 ; the thickness of the saturated

weathered layer at each geophone place

D; datum level

Both correction values were subtracted from the

picked reflection time and the corrected reflection time

was used to make a T2 - X2 graph (Pig. 7).

From the T2 - X2 graph of seismic lines S-l and

S-2 (Fig. 7), two-way reflection times at the shot point

from the top of Traverse Limestone were calculated using

the equation

T2=T02+(1/Vrms2) * X2 — - (4)

where, T; the reflection time at the distance X

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 19

T0 ; the reflection time at the shot point

X; a distance between shot point and the

first geophone

vrms' root mean square velocity

(Telford et al., 1976).

Then the reflection time at each shot point was

plotted on the time structure map of the Traverse

Limestone.

As the amplitude of the arrival wave was too small

around the anticipated reflection time, no reflection

event from the top of the Trempealeau Formation could be

found. This may be due to the great depth to the top of

the Trempealeau Formation and a strong absorption of the

seismic energy.

For the seismic lines HAL-81-1, HAL-81-2, HAL-81-7,

HAL-82-8, HAL-82-9, HAL-82-10, and MRH-82-1 (Fig. 2), the

data were recorded for nine seconds using the DFS V type

48 channel seismograph with a Vibroseis for energy source

and a band pass filter of 18 - 128 Hz. The geophone group

interval was 110 feet and the offset of the geophone

spread was 220 feet. Data processing included a static

correction using a datum of 800 feet A.S.L. and

correction velocity of 5000 ft/sec, normal moveout

correction, deconvolution, and automatic statics by

Hosking Geophysical Company, Mt. Pleasant, MI. Then, the

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 20

corrected data were stacked from 12 fold to 24 fold and

the stacked data for the time interval 0-2 seconds were

printed. The printed seismic sections with their

interpretations are shown in plates 1 - 7. Using these

seismic sections the reflection times for the Traverse

Limestone and the Trempealeau Formation were picked along

each seismic line. Faults were also identified from these

seismic sections.

Xltf MSEC2 60-

40-

30. X 10 F E E t

T2 - X2 graph of the seismic lines S-l and S - 2

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 21

Data Plotting

The two way reflection times from the Traverse

Limestone and the Trempealeau Formation at given locations

were plotted on the California Computer Products(CALCOMP)

906 Drum Plotter using the FORTRAN program NEWMAP.FOR

written by Mr. Kent E. Meisel, Department of Geology,

Western Michigan University.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER IV

GENERAL ASPECTS OF AN IMPACT SITE

Impact Sites

From modern studies of lunar craters, it seems that

the major meteorite bombardment of the moon - and so of

the earth - must have taken place three billion years or

more ago, during the first half of the life span of the

earth - moon system (Dietz, 1961). Since this time, the

earth's tectonic and meteorological processes have

obliterated the scars of this early bombardment, still so

much in evidence on the moon.

The earth should retain a geological record of young

impact events comparable to the youngest of the lunar

craters, that is those lunar craters which have ray

systems (Dietz, 1961). The near side of the moon displays

about 130 impact sites of this kind in an area roughly

equivalent to that of North America (Dietz, 1961).

However there are only 36 impact sites that have been

identified in North America according to Dietz (1961) and

Sawatzky (1975) (Fig. 8 ). So, more impact sites can be

expected to be found on the earth's surface or in the

earth's crust.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 23

METEORITE IMPAC

Greenland

C A N AT) A NICHOLSON £ STCCN ft P,k0T *- * , ^QUEBEC • * CARSWELL ^ 6 COUTURE, ° O' 0V»JSTASTIN L- DEEP BAY TAPOK_— ^ ^**yCLEIJftWArEft LK HARTNEV— , ^ \ a MANICOUAOAlflj VIEWFIELD-j J LK. ST. MARTIN ^ } 4 • W H4WK l . charlevo Pacific RED WING Clt-i •S.JCN-SU06URY Atlantic

Ocean HOLLEFQRD U.S. A KENkentland ^ SERPESERPENT MOUNDA BARRINGER CROOKED CK.A .n Y M *rJ, WELLS CK. A 4f LYNN LESENO s j O (Ma,ternor3 0»tw f^— — jlOESSA AS M Itir Cam Silt \A siERRA MADERA • p«Mil craUr -- Mexico \ DCwitc cr»ter

fig. 8 . Location map of impact sites in North America (after Sawatzky, 1975) (Numbered impact sites cited from Dietz (1961); 1. Kelly Lake, 2. Frank Town, 3. St. Lawrence Arc., 4. Sault-Aux-Cochons, 5. Menihek Lake, 6 . Mecantina, 7. Labrador, 8 . Ungava Bay).

Impact sites in the earth's crust have been

classified as being of three types. The first, and the

most obvious, type is impact sites with visible craters;

the second is sites with well - preserved fossil craters;

and the third type is sites in which only the deep radial

fracture patterns from the crater remain. In the last

type all other evidence has been removed by long exposure

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 24

to the terrestrial environment. The origin of anomalous

structures of the third type are likely to be very

difficult to determine, particulary if subsequent

structures such as a solution - collapse features are

developed (Sawatzky, 1975). The highly altered impact

structures found today belong to the third type of impact

sites and are called astroblemes (Dietz, 1961).

As an astrobleme has few features characteristic of

an impact site, it is not easy to determine its origin.

In general, two features, shatter cones and coesite, have

been recognized as indicators of the shock waves

associated with an impact event. However it is not easy

to find these features because the sites are so severely

altered by various geological phenomena such as erosion,

crustal movement, faulting, and folding.

A central uplift of the crater bottom is most

characteristic of impact sites. The bottom of the crater

is pushed up by isostatic processes. With a central

uplift, some astroblemes that are scarcely discernible on

the ground, can appear as a faint circular features in

aerial photographs and in the geological maps of surface

or subsurface formations. In many cases, older formations

outcrop in the center of the circular feature and are

encircled by younger formations. In the Versailles

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 25

astrobleme, the oldest formation (undifferentiated

Cynthiana Formation and Lexington Limestone of middle

Ordovician age) in the area exists in the center of the

structure (Seeger, 1972). In the Hartney structure

(Manitoba, Canada), granitic basement rock is located in

the center of the structure (Sawatzky, 1975). In the

Kentland structure, there is a large central uplift of

about 2000 - 3000 feet and the uplifted part consists of

Ordovician and Silurian formations while the surrounding

area is consists of Devonian rocks (Gutschick, 1976). In

the Sierra Madera structure, there is an outcrop of

Permian rocks in the central uplift zone while Cretaceous

rocks are present outside the central uplift area. This

type of structure is explained easily by Sawatzky's (1975)

model of the evolution of a fossil crater with the process

of central uplift (Fig. 9).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 26

EVOLUTION OF A LARGE FOSSIL CRATER

V-V

Fig. 9. Evolution of a fossil crater (Sawatzky, 1975).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 27

Geophysical Characteristics of Astroblemes

Frequently gravity studies of impact sites show a

circular negative Bouguer anomaly approximately centered

on the impact point. This centrosymmetrical negative

gravity anomaly may be due to a density deficiency between

the country rock and the rock debris which was produced by

the impact event and by later sedimentation. If a central

uplift is present, a central positive gravity anomaly with

a surrounding negative anomaly generally exists.

A circular negative anomaly has been reported in most

cases of identified astroblemes such as Lake Wanapitei of

Canada (-15 mgal), Nicholson lake of Canada (-7.5 mgal),

Deep Bay Crater of Canada (-20 mgal), Gosses Bluff of

Australia (-50 mgal), and the Ries structure of Germany

(-18 mgal) (Ghatge, 1984). In the Kentland structure of

northwestern Indiana, there is a positive anomaly of 3.5 -

4.0 milligals which indicates that a central uplift is

present (Gutschick, 1976). For the other astroblemes such

as the Brent, Holleford, Clearwater Lake, and Carswell

structures of Canada, Steinheim Basin of Germany, and

Popigay Crater of U.S.S.R., circular negative anomalies of

unknown magnitude were reported (Ghatge, 1984).

Some results of magnetic surveys show small

variations in intensity over the central portion of the

impact structure in a few astroblemes such as the Deep Bay

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. crater (+100 gammas), the Gosses Bluff structure of

Austrailia ( -75 gammas), the Ries structure of Germany

(-100 gammas), and the Steinheim Basin of Germany (+ 2

gammas). However, the results of magnetic surveys are

generally not as conspicuous as those of gravitational

surveys.

Mathematical Consideration of Meteorites

From an analytical investigation of the quantitative

aspects of a crater formation by Innes (1960), it is

possible to estimate the volume of brecciated country rock

in a meteorite crater. Assuming that both the crater

floor and the lower boundary of the breccia zone have the

shape of a paraboloid of revolution, it is possible to

derive equations which permit predictions of the volumes

associated with crater formation. The total volume of the

country rock ruptured by the exploding meteorite can be

expressed as a function of the crater diameter as follows;

V = ?V24 x D3 ---(5)

(Innes, 1961). The total volume of the ruptured rock

increases simply as the cube of the crater diameter. This

relationship was confirmed by gravitational exploration

conducted by Innes (1961).

Thus it is possible to estimate with some confidence

the minimum energies required for crater formation by

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 29

direct calculation of the amount of work required to crush

the rock. From the results of controlled underground

nuclear experiments, the energy consumed in crushing was

estimated to be about 47 % of the total energy released,

equivalent to about 6.4 x 107 erg/g (Johnson et al., in

Innes, 1961). This value, combined with the volume of

fragmented rock as estimated from the equation (5), should

provide an estimate of the minimum energies required to

form a crater. This relation leads to the following

general equation for the energy consumed in brecciation

during the cratering process;

^crushing = 2.39 x 10^^ x ^ ergs (6 )

for the diameter (D) given in feet, or

^•crushing = 8*40 x 1 0 ^ x ^ ergs (7)

for the crater diameter (D) given in meters (Innes, 1961),

where Pc is the mean density of the country rock.

The solid curve in Fig. 10 illustrates equation (6 )

for the general case, assuming a crustal density of 2.67

q / c m ? . The crushing energies estimated for six North

American craters of probable meteoric origin are listed in

column 5 of table 2 (Innes, 1961). In a large crater,

there are some differences between the crushing energy

from equation (6 ) (solid line, Fig. 10) and that from the

gravity data (dashed line, Fig. 10). But for the craters

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 30

less than 8000 feet in diameter/ the two results agree

well. Continued geophysical and structural studies with

drilling programs will provide important information about

the effects of impact events for the higher energy ranges.

The estimates of total energy are based on the

assumption that the proportions of the total available

energy that go into heating, crushing, fracturing, and

elastic effects in meteorite crater explosions are the

same as in underground nuclear explosions and are given by

^meteorite = 5.08 x 10^*^ D2 ergs ——— (8 ).

for the diameter (D) given in feet (Innes, 1961). The

corresponding meteorite mass can be estimated using the

equation = 1/2 MV2 and the impact velocity of 20

km/sec (Innes, 1961) along with the equation (8 ). The

total energy and corresponding mass of six meteorites are

shown in Table 2. If the meteorite is spherical, then

such meteorites would have diameters that are 1/40 or 1/56

of the diameter of their resulting craters, depending on

whether the meteorites are of stone or of iron (Innes,

1961).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. IQ ir -a ENERGY OF CRUSHING ENERGY OF CRUSHING (GRAVITY DATA) ------TOTAL METEORITE ENERGY ij 0 NUCLEAR EXPLOSIONS 2 „ <-> 2 o til IF ...tf ID

r - 10*

10 14

in 10 «

• TOTAL METEORITE ENERGY ESTIMATEO =-10** BY : 1 OPIK (1958) 10 *0- 2 MACPHAIL (1950) 3 HILL B GILVARRY (1956) — I0-* 4 SHOEMAKER (1959) 5 RINEHART (1950) 10 '» 6 WYLIE (1943) 7 BALDWIN (1949) =-IO*4

10 14 i i ' r n ' i | 1------1— i i i‘ rTTj- 10* I04 01 AM ETER(ll)

Fig. 10. Relationship between meteorite energy and crater diameter (after Innes, 1961).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 2

The energies and masses of six North America meteorite craters (Innes, 1961).

Crater |Diameter c Total Mass Crushing Meteor 1te Energy Meteor!te ruptured, Energy, mass,Tons ft g/cm or ergs ergs Mt Vs20km/sec

Odessa 550 2.6 1.60x1012 1.03x1020 2.19x1020 5x10”3 120

Barringer 4150 2.6 6.89x10 *4 4.44X1022 |9.44x1022 2.3 5.2x104

Ho 11eford 7709 2.79 4.74XI015 3.05X1023 6.49X1023 3.6x10s (2.47x1023) 16 New Quebec 1 1290 2.67 1.42x1016 9.14x1023 I 1.95x1024 47 1.0x10®

Brent 11500 2.67| 1.50x10,8I 9.66x1023 I 2.06x1024 50 1. 1x10s j(8.69x1023)I

Deep Bay 40000 2.671 6.34x10 *7 j 4.08X102® 1B.69X1025 210 4.9x107 (2.01x10 )I

NotesEnergy values in parentheses(co). 5) are based on gravity analyses.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER V

INTERPRETATION

Geological Results

Many wells have been drilled in the Calvin - 28 oil

field, but most of them are shallow wells for oil

production, and are not useful for studying the deeper

subsurface geological structure of the area. Six holes

drilled along the periphery of the production area and one

deep hole at the center of the field were selected to

determine the subsurface geological structure of this area

(Pig. 2). An 8th well (William Gemberling 1), which is

located 2.5 miles southeast of the Well 7, was selected as

a reference point.

In the well descriptions of the Calvin - 28 oil field

there is no evidence of an impact event such as shatter

cones and coesite. Even if a crater is due to an impact

event, however, it is difficult to find these features

after central uplift and later erosion. Moreover, no

circular feature is apparent in the surface geology

because of later deposition and the thick glacial drift.

Because post Ordovician sedimentary rocks and glacial

drift mask the structure it is impossible to define the

whole structure without geophysical methods. However

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 34

information from wells drilled for oil reveal some aspects

of this structure.

Three wells (Wells 4, 5, and 8 in Pig.2) reveal rough

aspects of the geological structure in the Paleozoic rocks

of the area and the five remaining wells define the

circular area of oil production (Pig. 11). The data

indicate a domal structure whose apex is centered on Well

4. The Traverse Limestone lies only 700 to 800 feet below

the surface and the resulting feature is a roughly

circular anticline about 2 miles in diameter (Fig. 12).

Structural closure is at least 100 feet above regional and

represents the greatest structural relief in the Cass

County area. This kind of structure appears to have been

caused by uplift during Late Ordovician time.

The entire Paleozoic section has been uplifted at

Well 4 in comparison to Well 5 and Well 8 . The Ordovician

units from the St. Peter Sandstone to the Utica shale are

absent in Well 4. This section (St. Peter Sandstone to

Utica Shale) has apparently been faulted out, and the

Trempealeau Formation is structurally higher by about 1000

feet. In contrast, rocks above the Utica Shale are only

about 100 feet higher than regional. It seems likely that

faulting occured during deposition of the Utica Shale.

The upthrown block was elevated into and above the surf

zone of the Late Ordovician sea and was subsequently

eroded. Erosion removed rocks from the top of the

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 35

Trempealeau Formation to the Utica Shale. This hypothesis

is substantiated in part by the presence of a lense of

white sandstone in the Utica Shale adjacent to the central

uplift (Straw, Personal Communication, 1985). These facts

seem to indicate some geological event such as an impact

which caused an uplift in the Later Ordovician.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 36

3 1000- •\ .*• * P . ‘

• o • : • o ’. O • • 773 500-

TRVF aSX] 5 5 2 k TRVP_ TRVL T *VL 3I2E jUE HE -1&£E- T>vir TRVt JAVi. 1RVL DOS* DOS TRVL 0 - DOS C-N GLACIAL ORI FT|PLEISTOCENE till C-N I COLOWATER SH. M1SS1SSIPPIAN I ANTRIM SH. — IRVF TRAVERSE FM. CLTN -500 CLTN U r v l It r a v e r s e LS. C-M :: DUNDEE LS. 1 X J Z lODSi DETROIT POC C-M 1 ■ — ’SVLVANIA — 32E \ \ |p _"iTl C UNIT ..... -— i\ UT LlZILJ NIAGARAN SR. CCLB ?.\ -1000- CCS |CLTN | CLINTON l\\ I CABOT HEAD SH. 1\\ EDIm a n i t o u l i n DL.- BLKR i\\ ED CINCINNATIAN Sfl— , * \' NTS • \ * TRNT |u T | UTICA SH. -1500- GLWO BLKR |THNT|TRENTON SPS GLWO |b LKr |b laCK RIVER GR. TPL

I ORS |DRESBACH SS.

{ECLRjEAU CLAIRE

Fig. 11. Geologic cross section o£ the area

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. J7S

2 7

3 3 34

• OiL Well hJ. Dry Hole 115 — Elevation C Feet) P OH Well # Gas Well 1/1 M ile

Fig. 12. Structure of the Traverse Limestone of the Calvin-28 oil field.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 38

Gravity and Magnetic Results

According to the results of the gravity and magnetic

surveys reported by Ghatge (1984), there is no gravity

anomaly around the Calvin - 28 oil field in the Bouguer

gravity map of the south-central Cass County, Michigan

(Pig. 4). Nor did a magnetic survey show an anomaly in

this area (Ghatge, 1984). A central positive anomaly

representing the central uplift and linear trending

features representing faults are present on the second

harmonic residual gravity map with a fundmental wavelength

of 28 miles (Fig. 5). However this positive anomaly

closure is not associated with the anomalous Hawkes-Adams

1-28 deep test but is about 2 miles to the west of the

well. Therefore, the interpretation of the genesis of the

central uplift and the absence of the Ordovician sediments

is inconclusive from the gravity and magnetic survey

(Ghatge, 1984).

Most astroblemes that have a circular negative

gravity anomaly are near-surface features or have not been

deeply eroded. Structures like the Plynn Creek Crater in

Tennessee do not have any gravity anomaly closures

associated with them, even at the level of 1 milligal;

this is similar to the Bouguer gravity map for the Cass

County structure (Ghatge, 1984). Therefore, despite the

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 39

lack of evidence in the gravitational and magnetic

studies, it is possible that the central uplift could be

related to an impact event, especially if the site was

deeply eroded after the impact event. A more detailed

gravity survey might show a negative anomaly around the

central uplift.

Seismic Results

Time Structure Map

Time structure maps of Traverse Limestone and

Trempealeau Formation were made using an interval of ten

milliseconds and were combined with some identified faults

in each seismic section. As faults are present in this

area, most contours were cut. Figure 13 shows that there

is a main dome structure in the center of the investigated

area as shown in the structure map of the Traverse Lime

(Fig.12). The reflection time increases gradually from the

center to the edge of the survey area in all directions-.

The reflection time ranges from 230 milliseconds at left

middle part of the map to 190 milliseconds at the center

The time-structure map of Trempealeau Formation also

shows a dome structure in the center of the area. This

structure appears to have a smaller area than that of

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 40

Traverse Limestone (Fig. 14). The contour lines appears

to have a linearity in the east - west direction in the

northern part of the area. The reflection time ranges

from 460 milliseconds at the northern margin of the area

to 350 milliseconds at the center of the circular

structure (Fig. 14). The difference in reflection time

for the Trempealeau Formation between the center of the

circle and the background is larger than that for the

Traverse Limestone. In fact, in the central uplift the

Trempealeau Formation has been elevated about 1500 feet

higher than regional whereas the Traverse Limestone is

only about 100 feet higher (Fig. 11).

The difference in the size of the uplifted area

between the time structure map of Traverse Limestone and

that of Trempealeau Formation may come from the fact that

Traverse Limestone was deposited after the large central

uplift occured and Trempealeau Formation was eroded. In

this explanation, the Trempealeau Formation represents the

section below the Late Ordovician unconformity and the

Traverse Limestone represents the section above that

unconformity.

The Trempealeau Formation is cut by more minor faults

around the uplifted area than is the Traverse limestone

(Fig. 13 and Fig. 14). Most of the minor faults near

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. the structural closures on the Trempealeau Formation are

upthrown toward the center of the circle whereas they are

dipping inward, thus they are reverse faults. This

feature is predictable at the last stage of central uplift

formation because the original formations move inward and

upward on a shortened perimeter from their original

flat-lying position (Wilshire and Howard, 1968). Several

normal faults are present around the central closure and

they also form a circular pattern that has a diameter of

about 2 kilometers. It is common that several normal

faults exist around the rim of an impact site which has a

central uplift. So, the circular normal fault zone of

this area appears to be a rim area of the original crater.

Therefore, the central part is the horst and the

surrounding part is the graben which is bounded by faults

trending NNW-SSE, NW-SE, E-W, and NE-SW as indicated on

the time structure map of Trempealeau Formation (Fig.

14). Such a horst-and-graben type structure has been

observed in astroblemes such as the Wells Creek structure

in Tennessee (Sterns efc al., 1968).

Ghatge (1984) suggested two hypotheses to explain the

cause of the central uplift and faulting, these are (1 )

the uplift may have been a result of isostatic adjustment

with older faults being rejuvenated by the impact

producing the horst-and-graben structure and the general

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 42

rectilinear fault-block pattern; and (2 ) the area could

have been a focal point of regional stress with the

presence of parallel left lateral strike-slip faults

causing shearing within the central block. The second

hypothesis can explain the general rectilinear fault

blocks, but it can not explain the local circular

structural closures as shown in time structure maps of

Traverse Limestone and Trempealeau Formation. The

different faulting system and the different amount of the

central uplift in both formations also is not easily

explained with the second hypothesis. The different

faulting system between Trempealeau Formation and Traverse

Limestone may mean that there was an abrupt event during

the period between the deposition of these formations.

The major faults running NW-SE could be a result of

regional stress. However, the different fault systems,

the structural closures and the different amounts of

movement on the central uplift in the two formations can

be explained well with the impact hypothesis.

Seismic Section along HAL-81-7

The seismic section along HAL-81-7 was selected to

compare with Sawatzky's (1975) structural model because it

goes through the center of the Calvin - 28 structure. In

this seismic section four layers, the Traverse

Limestone and the Clinton, Glenwood, and Trempealeau

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 43

200

T7S T8S

'SO

20 0 '

I MILE .210-

Fig. 13. Time structure map of the Traverse Limestone

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 44

410' 410

■410'

T7S TBS

420.

I MILE

Fig. 14. Time structure map of the Trempealeau Formation

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 45

formations are shown (Fig. 15). In general, the Traverse

Limestone and Clinton Formation show very good continuity

through the entire section and show a broad uplift

centered around shot point 50. On the other hand, the

Glenwood Formation and Trempealeau Formation show severe

disturbance around shot point 50, and they are cut by

several reverse faults. The Glenwood Formation does not

exist around shot point 50 and the Trempealeau Formation

shows much uplift. Therefore, the structure may be

interpreted as a central uplift of the formations below

the Cincinatian Series with erosion prior to deposition of

the Late Ordovician rocks.

Several horizontal layers are present beneath the

uplifted section which indicates that there is no

geological disturbance in these layers. This fact

indicates that the origin that the local central uplift

was generated from above, not from below and is futher

evidence for an impact event.

The seismic structure looks very similiar to the last

model of Figure 9. There is a severely disturbed central

section, a surrounding synclinal depression, and a rim

area in the seismic section. This structure is very

similiar to the generalized cross section of the Sierra

Madera structure made by Wilshire and Howard (1968) using

drilling data. From this seismic section and the general

model of a fossil crater, the diameter of the original

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Fig. Fig. 15. Seismic section along HAL-81-7

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. crater can be determined. According to Sawatzky's model

of a fossil crater, the point of discontinuity in the

formation produced by a inward dipping normal fault at

both sides of the central uplift could be regarded as a

part of the rim of the fossil crater. This kind of

discontinuity exists beneath shot points 27 and 6 8 .

Therefore, the diameter of the original crater may be at

least 2 kilometers.

Mathematical Modeling of Isostatic Uplift

According to isostatic principles the pressure of the

bottom of the ruptured zone should be the same as the

pressure at the same depth outside the ruptured zone.

Thus, since the ruptured rock has a lower density than the

original country rock, isostatic uplift will tend to occur

in the ruptured zone (Fig. 16). This relation can be

expressed with the following equation

X = D/3 - (fa/friz (9)

where, X ; the amount of central uplift

D ; diameter of the original crater

1° c? density of country rock

(°r; density of ruptured rock Z ; the thickness of ruptured zone

center of the crater.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Parameters such as the density contrast (fir/fc)

and the thickness of ruptured rock can be determined from

the other craters which are similar to the Cass County

structure but have no central uplift.

^bet±o'*n S owgJflfti crater' VaD - f ft £ r r< x 'oo^ ' O •" of rvptuwd 20"® i ✓

Fig. 16. A simple model for the calculation of the amount of central uplift.

Using equation (9), the anticipated amount of a

central uplift at the Cass County structure can be

calculated. The density contrast and the thickness of

ruptured zone for the Cass County structure were estimated

from the Holleford impact structure which has a diameter

(7708 feet) similar to that of the Cass County structure.

The density contrast and thickness of the ruptured zone

are 0.77 and 0.24D respectively, Where D is the diameter

of the original crater.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 49

The calculated amount of central uplift for the Cass

County structure is about 1000 feet. In fact, the Praire

Du Chien is about 1200 feet higher in Hawkes - Adams 1-28

than in well 8 . Considering the isostatic uplift due to

subsequent erosion after a uplift, the anticipated value

of uplift calculated frofl the equation (9) is very

reasonable. Thus the uplift in the Cass County structure

can be explained by an impact event.

Consideration of the Meteorite

From the interpretation the the seismic data, the

original crater diameter w*s at least 2000 meters. Using

Baldwin's hypothesis that the depth of broken rock for a

meteorite impact is very nearly equal to one third of the

crater diameter (Baldwin in lnnes,1961) and the results of

controlled underground nuclear experiments (Johnson et al

in Innes, 1961), the minimuifl energy of a meteorite which

makes a crater of diameter D in feet is expressed by

equation (4). The corresponding meteorite mass can be

estimated using the equation f°r kinetic energy, an impact

velocity of 20 km/sec (Inneh/ 1961), and equation (4). As

the impact is considered have occurred on the upper

part of the Utica shale, tfhich has a density of 2.7

g/cm^ (from density legs, Mannes Oil Company,

Unpublished), the total energy °f the meteorite can be

calculated to be about 1 .0x1 0 ^ ergs and with a mass of

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 50

9xl04 tons. The diameter of the meteorite would have

been about 36-50 meters depending on whether the meteorite

was composed of iron or stone. By comparison with the

data from six North American meteorite craters (Table 2),

the original size of the meteorite and the Cass County

crater seems to be slightly larger than the Barringer

Crater of Arizona (Fig. 10).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER VI

MODEL FOR FORMATION OF THE CASS COUNTY STRUCTURE

A history of the subsurface geological structure of

Calvin - 28 oil field can can be developed using

Sawatzky's model (Sawatzky, 1975), the drill hole data and

the results of the seismic study. The initial impact

occurred on the Utica Shale of upper Ordovician age and

several underlying formations were also effected by the

compressional shockwave. As there was some mass

deficiency owing to dispersion of rock debris and

rupturing of country rock, the bottom of the crater

started to uplift as an isostatic process (Stage 3 of Fig.

9). With the uplift, some inward-dipping normal faults

were produced around the rim area. Faults were also

produced by inward and upward movement of rock formations

due to the central uplift. Layers such as the Utica

Shale, Trenton Limestone, Black River Limestone, Glenwood

Shale, and St. Peter Sandstone of the uplifted section

were eroded and this eroded material was deposited in the

ring synclinal depression area which surrounds the central

uplift zone (Stage 4 of Fig. 9). Later formations such

as the Cincinnatian Series and several Silurian and

Devonian formations were deposited across the uplifted

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 52

topography (Fig. 17). Therefore, the Praire Du Chien

Group of early Ordovician age contacts with the

Cincinnatian Series of Late Ordovician age at the

Hawkes-Adams 1-28 deep hole section (Fig. 11). A

subsequent dome structure was developed in the Silurian

and Devonian formations. The Traverse Limestone, which is

the pay zone of the Calvin - 28 oil field, is also

included in the subsequent structural closure. The

structural closure of the Silurian and Devonian may also

be the result of a delayed isostatic adjustment.

GLACIAL DRIFT

traverse lime DEVONIAN

SILURIAN

U.T V eclr sps MTS

ECLR MTS MTS.

Pig. 17. Subsurface geological model of the Cass County structure

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER VII

CONCLUSIONS

In general, it is not easy to determine the genesis

of an astrobleme because of severe disturbances. As no

conclusive geological evidence such as shatter cones and

coesite that would support the impact theory have been

found, the structure in Cass County has been called a

crypto-explosive structure.

Several lines of evidence support the concept of a

central uplift in the Cass County structure: (1) the

results of drilling data, (2 ) structural closures in the

time structure maps of Trempealeau Formation and Traverse

Limestone, and (3) and discontinuities in the Glenwood

Shale and Trempealeau Formation in the seismic section of

HAL-81-7. The structure of the central uplift appears to

be a horst-and-graben feature surrounded by several

reverse faults. Such structures are very similar to those

in proven astroblemes.

The coincidence between the amount of tha central

uplift observed at the Hawkes - Adams 1-28 well and the

calculated isostatic uplift associated with an impact also

support the idea that the structure is the result of a

meteorite impact.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 54

The section from the Utica shale to the St. Peter

sandstone is missing in the Hawkes-Adams 1-28 deep well

and there are some differences between the upper section

and the lower section of the omitted part. The first is

the difference of the amount of uplift and the second is

the difference in the faulting systems. These differences

can mean, with a certainty, that the central uplifts in

both sections are not the result of the same process or at

least that they did not occur at the same time. These

differences can be readily explained by the impact

hypothesis. No disturbance in the rock beneath the

Trempealeau Formation in the seismic section of HAL-81-7

also supports an impact origin of the Cass County

structure.

Although there is no direct geological evidence of a

meteorite impact, it is reasonable to acept the impact

origin of the Cass County structure on the basis of

several lines of indirect evidence such as a central

uplift, a horst-graben structure surrounded by several

reverse faults, and the results of this seismic study.

Then, a geological history of the structure can be

developed with Sawatzky's model (Sawatzky, 1975) using

drilling and seismic data. A meteorite fell on the Utica

Shale and the bottom of the resulting crater was uplifted

by a isostatic forces. Then the top of central uplifted

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 55

section was eroded and the later formations were deposited

on the top of the eroded section. So the Early Ordovician

Praire Du Chien Group is in direct contract with the Late

Ordovician Cincinnatian Series in the Hawkes-Adams 1 - 2 8

well. The Devonian and Silurian formations also have

structural closures which may be a result of a delayed

isostatic adjustment or of a draping over the uplifted

section. Therefore, the Traverse Limestone makes a

circular oil field.

By a comparision of the seismic section of the

HAL-81-7 with the Sawatzky's model of the evolution of a

fossil crater, the original diameter of the impact crater

was at least 2000 meters. The mass of the meteorite can

be calcuated as being 9x10^ tons and the diameter was

about 36-50 meters depending on wheather the meteorite was

composed of stone or iron.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. BIBLIOGRAPHY

Bucher, W. H., 1968, Cryptoexplosion Structures caused from within or without the earth (Astroblemes or Geoblemes?), American J. Sci., Vol. 261, pp 567-649.

Buschbach, T. C. and Ryan, Robert, 1963, Ordovician explosion structure at Glasford, Illinois, A.A.P.G. Bulletin, Vol. 47(12), pp 2015-2022.

Dietz, R. Robert, 1961, Astroblemes, Scientific American, Vol. 205, pp 50-58.

Dietz, R. Robert, 1963, Cryptoexplosion structures : A Discussion, American Jour. of Sci., Vol. 261, pp 650-664.

Ghatge, Suhas L., 1984, A geophysical investigation of a possible astrobleme in Southwestern Michigan, M.S. thesis of Western Michigan University.

Gutschick, Raymond C., 1976, Geology of the Kentland structural anomaly Nortwestern Indiana, Field Guide 10th Annual Meeting, North Central Section, G.S.A., pp 59.

Innes, M. J. S., 1961, The use of gravity method to study the underground structure and impact energy of meteorite craters, J. of Geophysical Research, Vol. 6 6 , No. 7, pp 2225-2239.

Kellogg, R. L., 1971, An aeromagnetic investigation of the Southern peninsula of Michigan, Ph.D. Dissertation, Michigan State University.

Sawatzky, H. B., 1975, Astroblemes in Williston basin, A.A.P.G. Bulletin, Vol. 59, No. 4, pp 694-710.

Seeger, C. R., 1960, Origin of the Jeptha Knob structure, Amer. Jr. of Sci., Vol. 266, pp 630-660.

------, 1972, Geophysical investigation of the Versailles, Kentucky, Astrobleme, G.S.A. Bulletin Vol. 83, pp 3515-3518. 56

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Stearns, R.G., Wilson, Jr. C. W., Tiedemann, H. A., Wilcox, J. T., and Marsh , P. S., The Wells Creek structure, Tennessee, Shock Metamorphism of Natural Materials, Baltimore, MD: Mono Book Corp., pp. 291.

Straw, W. Thomas, 1985, Personal communication.

Telford, W.M., Geldart, L. P., Sheriff, R. E., and Keys, D. A., 1976, Applied Geophysics, Cambridge, England: Cambridge University Press.

Wilshire, H. G. and Howard, K. A., 1968, Structural pattern in central uplft of cryptoexplosion structures as typified by Sierra Madera, Science, Vol. 162, pp 258-261.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. PLATE I

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