·~·· 

'1 

.-. 

Structural Geology of the 

Henbury Meteorite Craters 

Northern Territory, Australia 

By DANIEL J. MILTON 

CONTRIBUTIONS TO ASTROGEOLOGY 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 599-C 

Prepared on behalf of the National 

Aeronautics and Space Administration 

A group of small craters that 

exhibit an unusually wide 

variety of structural features 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1968 
UNITED STATES DEPARTMENT OF 'THE INTERIOR· 

STEWART L. UDALL, Secretary 

GEOLOGICAL SURVEY 

William T. Pecora, .Director 

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 

). 
CONTENTS 

Page Page 

Abstract __________________________________________ _ C 1 Structure of the craters---dontbiued 

Introduction--------------------------------------- 1 Structures related to a. single I:a.J;ge crater-Continued 

J>urpose and scope of report_ __________ · __________ _ 1 Crater wal,ls-Continued Previous work _________________________________ ._ 2 Folds with steeply dipping axial.planes ___ _ C7 

E'ield,vork ___ ------ ____________________________ _ 2 Folds with low-dipping axial planes ______ _ 9 

10 

11 

11 

11 

12 

12 

16 

16 

16 

1i 

Ackn9wledgmen ts.- ______ :.. _____________________ ~ _ 2 Thrust faults __________________________ _ 

Geographic setting------------------.---------------- 2 Combined structures in the Water Crater __ Location and access ___________________________ .:_ 2 

2 

2 

Crater rims _______________________________ _ 

Climate, flora, and fauna ___ ._ ___________________ _ Synclinal folds at the rim _______________ _ Geologic setting ___________________________________ _ Throwout and fallout _________ - _______ -_ 

Stimmary c;lescription of. t~e craters __________________ _ 3 Structures related to two craters _________________ _ 

Structure of the craters-------------------.----------- 5 · Character of deformation ___________________________ _ 

Structure of crater 10 __________________________ _ 5 Comparison with other impact craters __ . __________ _ 

Structures related to a single large crater _________ _ 7 Boxhole Crater--~---------------------------------- Crater walls ___________________ ~ ___________ _ 7 References cited _________________________ - _______ ---

PLATE 1. 

FIGUH.E 1. 

2. 

3. 

4-10. 

11. 

12-17. 

ILLUSTRATION·s 

Page 

Geologic map of the larg.er craters, Henbury, Northern Territory, Australia _____________________________ In pocket 

Outline map o' the Hen bury crater field ____________________________________________________ ---_-------_ C4 

Diagram illustrating terminology and measuremen~s of craters __________________________________________ -_ 5 

Geologic map of .crater 10. __________________________________________________________________ - __ ------- 6 

Photographs of crater structures: . 

4. Z-shap.ed fold in south wall of crater 10---------------------------------------------------------- 7 

5. Ae1;ial view of craters 6, 7, and 8 from the south__________________________________________________ 8 

6 .. View of Main Crater from the northw.est rim _________________________________________ ---_-_-----_ 9 

7. View of deformed beds in east bank of gully on the south wall of Main Crater________________________ 10 

8. Folds having low-dipping axial planes, south wall of Main ·cr~ter __ ~ ______________________________ - _ 10 

9. Overturned rim flap overlying pediment gravel, south rim of Main Crat~r __________________ - ___ ----- _ 11 

10. Southwest rim of Main Crater sho'wing thin ejecta layer, composed of overturned beds, overlying pediment gravel _______ ·- ___________________________________________________________________________ -- 12 

Diagram showing relationship of the larger craters ___________________ ..: ________________ :... ____________ - __ --_ 13 

Photographs of Main Crater: . 

12. Spur at.the interse.ction of craters 7a and 7b on the south. walL------------~------------------------ 13 

13. Underthrust in unit d near base of-north walL---------------------------------------------------- 14 

14. Thrust surface in unit din north walL--------------------------------·--------------------------- 14 

15. Rotated block along thrust surface shown i~ figure 14-----------------------------------"'--------- 14 

16. N appelike folded thrust slice in n·orth walL______________________________________________________ 15 

17. Axial region of fold shown in figure 16----------------------------------------------------------- 15 

Ill 
, 

CONTRIBUTIONS TO ASTROGEOLOGY 

STRUCTURAL. GEOLOGY OF THE HENBURY METEORITE CRATERS 

NORTHERN TERRITORY, AUSTRALIA 

By DANIEL J. MILTON 

ABSTRACT 

About 12 meteorite craters lie within a quarter sqt'lare mile 

near Henbury, Northern Territory, Australia. Nine craters, 

some of which are completely filled basins, range in diameter 

from about 20 to 210 feet. The largest craters form a close 

group comprising two complete craters about 230 and 300 feet 

in diameter and two craters about 390 and 480 feet in diameter 

that overlap to form an oval crater 600 feet long. The depth of 

the largest crater reaches 50 feet, and the rim is raised about 

20 feet above the surrounding surface. The bedrock, belonging 

to the Whmall Beds of late Precambrian age, consists of weakly 

indurated shale and siltstone and less abwldant moderately 

indurated sandstone. The bedrock dipped homoclinally about 

35° before the impact. The larger craters formed on a gently 

sloping surface covered by a thin layer of pediment gravel. 

Rock exposed in crater walls and rims has been displaced 

outward from the craters. A variety of mechanisms were involved in the displacement. Three types of folds were recognized in the larger era ters : 

1. Tangential folds having steeply dipping axial planes. These 

folds are commonly asymmetric, the limb away from the 

crater being vertical or overturned, and were formed by compressional stresses radial to the crater. Such folds are dominant in the lower walls. 

2. Folds whose axial planes dip toward the crater at low angles, 

approximately parallel to the crater wall. These folds were 

formed by shearing stresses parallel to those wWch produced 

the wall of the original crater itself. Such folds are shallow 

and are found where the original crater wall is least eroded. 

3. Punchlike folds, convex outward from the crater, having 

nearly horizontal axial planes. These folds were formed by 

outward stresses sharply localized within the crater wall. 

The craterward limbs of folds of the first type are commonly 

thrust over the crest; thrust faults parallel the axial planes 

of folds of the second type and form the boundaries of folds of 

the third type. Other overthrusts that have low and intermediate dips in the crater walls and on the rims are not closely 

associated with folds. Underthn1sts occur low in the crater 

walls. 

On the downdip side of the Main Crater, the beds steepen 

upward in the crater wall, becoming vertical and then overturned to form a flap of inverted strata lying on the precrater 

surface. The units in tws flap have been greatly thinned, which 

indicates that shearing along planes at a small angle to the 

bedding took place simultaneously with folding. Along part of 

the rim, the entire axial region of such a synclinal rim fold bas 

been thrust out over the precrater surface. The flanks of the 

rim farther from the cra.ter are covered by fragmental debris 

in which fragments from different bedrock units are in general 

unmixed. Scattered impact-mel·ted. glass fragments may represent a layer of fallout from the largest crater that has been 

nearly removed by erosion. 

Rock in the wall between two craters that are close together 

but do not intersect was thrown inrto a series of folds with 

approximately vertical axial planes parallel to the common 

wall. Where the centers of impact are closer, as in the two 

craters that form the Main Crater, the middle part of the intenening wall was eliminated, and rock in the outer parts was 

folded about axes approximately. normal to the commoo chord 

and thrust outward along the extension of the common chord. 

Some Q·f the slices were thrust over the precrater surface for as 

much a~ 80 feet. The outer edges of some slices a·re turned under 

where the main mass has ridden over them. Cross sections 

through the pile of imbricate slices suggest an Alpine nappe 

structure. Details of fault surfaces indicate that the structural 

blocks separated by faults were not in contact during deformation; rather, each block was displaced outward and rotated to 

a large degree independently of its neighbors. This indicates a 

momentary. dilation that probably resul'ted. Largely from the 

interaction of stresses from the two craters, although there is 

evidence that some dilation occurred in the walls where only a 

single cra,;tei" was involved. 

INTRODUCTION 

PURPOSE AND SCOPE OF REPORT 

Geologic study of the meteorite craters at Henbury, 

Northern Territory, Australia, was undertaken in 1963 

by the U.S. Geological Survey as part of a program of 

investigations of impact features conducted on behalf 

of the National Aeronautics and Space Administration .. 

This report briefly describes the crater field as a whole 

but is primarily concerned with the structural geology 

of the wall and rimcrest areas of the three largest craters. An earlier report (Milton and Michel, 1965) deals 

01 
C2 CONTRIBUTIONS TO ASTROGEOLOGY 

with crater 3, a smaller crater notable for the ray pattern shown by its ejecta. A companion study by E. C. T. 

Chao of the impact metamorphism associated with the 

craters is in progress. Certain phases that should be 

part of a complete study of the craters could not be carried out. during the ~ingle field season; among these are . 

more detailed mapping of the outer parts of the ejecta 

blankets and a study of the distribution of meteorl.tic 

fragments. 

PREVIOUS ·woRK 

Most of the previously available information on the 

craters derives from the original reconnaissance by Alderman (1932), who was not able to make more than a 

cursory study of the geologic structure of the craters. 

Soon after Alderman's expedition, R. Bedford and 

others collected meteoritic material and excavated some 

of the smaller craters (Spencer, 1933a). Rayner (1939) 

made a magnetic survey of the craters. 'l'he meteoritic 

material has been described by Alderman (1932) and 

Spencer (1933a). A study of the cosmogenic carbon-14 

content of the meteo_ritic · iron by Kohman and Goel 

( 1963) indicated a maximum terrestrial age of less than 

4,700 years. Impact-melted material from the largest 

crater has been described by Spencer (1933a), Taylor 

and Kolbe (1964, 1965), and Taylor (1967). An atlas of 

excellent photographs and a brief summary of the history of investigation of the craters has been published 

by Hodge (1965), who also has in progress a study of 

the distribution of microscopic meteoritic· material 

around the craters; Recent speculations on the mecha~ics 

of impact (Baldwin, 1963; Krinov, 1963) are based on 

the papers of Alderman and- Spencer. 

FIELDWORK 

Fieldwork was begun by F. C. Michel and the author 

on July 12, 1963. During July-planetable geologic -maps 

of_craters 3 and 10 and a planetable topographic map of 

the area of the three largest craters at a scale 'of 1. : 360 

were made. From Augu·st 1 to September 28, the writer, 

working alone, plotted the ge_~logy· on the topographic 

base. A marker was placed at the point where each attitude shown on plate 1 wa$ measured, and each. point 

was located by tape and compass with respect to reference points that had been surveyed by planetable. The 

top?graphic base was concurrently revis~d; as a result, 

contours in the areas of detailed· geology are somewhat 

more accurate than in other parts of the map. 

ACKNOWLEDGMENTS 

L. C. Ranford and P. J. Cook, geologists of the Henbury areal mapping party of the Bureau of Mineral Resources of Australia, helped in ma.ny ways, including 

relating the geology at the craters to the regional geology. I am deeply grateful to Reg Smith, manager of 

Henbury Station, and Mrs. Smith for their unfailing 

friendliness and help. 

The interest and help of many other Australian geologists and residents of the area are also appreciated. 

W. A. Cassidy, of Lamont Geological Observatory, 

kindly loaned aerial photographs of the craters and his 

notes from an E}arlier visit. 

GEOGRAPHIC SE'ITING 

LOCATIO~ AND ACC~SS 

The Henbury craters lie within a quarter square mile 

area near long. 133°09' E. and lat. 24°35' S. in the 

Northern Territory, Australia. They are about 7 miles 

west southwest of Henbury Homestead, 80 miles south· 

west of Alice Springs, and can be -reached by a track 

from the unpaved highway linking Alice Springs with 

South Australia. Although they lie wi!thin the confines 

of the Henbury C~ttle Station, the craters are protected 

by the Government as a Territorial Reserve. 

CLIMATE, FLORA, AND FAUNA 

The geography of central Australia has been described by R. A. Perry and associates (1962); the 

Chandlers land system of their classification describes 

the vicinity of the craters well. Annual rainfall is about 

8 inches, mostly in summer storms. Mulga (Actwia 

.aneura) grows along wet-weather water-courses, but the 

plains around the craters and the cra~er walls have only 

-a sparse growth o"f needlebush ( H akea leucoptera) and 

other shrubs. The floor of the Main Crater is sparsely 

covered by saltbush (Atriplero spp.). The wall of -the 

Water Crater ( cr~ter 6) has been breached, and the 

crater has captured a preimpact drainage system, so 

that water ·stands in the crater floor after rains. Con· 

sequenqy, the largest trees-in ·the vicinity grow in this 

crater, among which whitewood (Atalaya hemiglauca) 

is the dominant species. 

KangarooS -and dingoes, the only large members of 

the native fa~na; are oQcasionally seen at the craters; 

Cattle browse over the area and enter the craters especially to water aft(}r rains, wearing trails down . the 

walls. The band of pedimen,t gravel that crops out beneath ejected bedrock in the upper crater walls is particularly favored by rabbits for burrowing. The activities of human visitors and the fauna they have 

introduced probably makes the present rate of destruction of the craters many times what it was in the preEuropean era. 

GEOLOGIC SETTING 

The Henbury craters lie at the foot of the Bacon 

Range, a ridge that rises steeply from just south of the 

crater field to a crest several hundred feet higher and 

·;-., 
.A, 

STRUCTURAL GEOLOGY, HENBURY METEORITE CRATERS, NORTHERN TERRITORY, AUSTRALIA C3 

about 600 feet distant from the nearest craters. Bedrock at the craters, and up to the crest of the Bacon 

Range, is part of the Winnall Beds of late Proterozoic 

age (Ran ford and others, .1967). The predominantly 

sandstone Winnall Beds are the lateral equivalent of the 

predominantly shaly Pertatataka Formation to the 

north. The boundary has arbitrarily been placed several 

miles north of the craters. Although the beds at the 

south edge of the crater field are of Winnall type, the 

beds at the larger craters actually show typical Pertatataka lithology (P. J. Cook, oral commun., 1965). 

Lithologic characterization of units in this report is 

primarily for the purpose of identification, and field 

terms are retained. Petrographic descriptions and 

chemical analyses by Taylor and l(olbe (1965) 

indicate that the shale and siltstone of this report might more precisely be called subgraywacke. A 

fault near the crest of the Bacon Range brings the older 

Inindia Beds to the surface. The range is capped by the 

so-called grey billy, a dense chert formed by nearsurface silicification during 1

the Tertiary Period. 

The Winnall Beds dip homoclinally to the south, and 

their differential resistance to erosion determines the 

topography. The stratigraphically highest exposed 

beds are hard sandstone which, with the grey billy capping, fo:r;m the Bacon Range. A second zone of thickbedded hard sandstone forms a low bare ridge on which 

craters 10, 11, and 12 lie. Near cra1

ters 1-8 the Winnall 

Beds are composed predominantly of weaker shale and 

siltstone containing a few thin sandstone beds. As a 

consequence, bedrock has been corraded to an alluviumcovered pediment that slopes gently northward from 

the Bacon Range. The rill system south of crater 6 

(fig. 1) is older than the craters and suggests that a 

very low ridge was formed by the sandstone beds intersected by craters 6 and 8. 

Exposed beds near the crater field (mostly on the 

sandstone ridge) consistently strike east-west and dip 

south about 35 degrees. Despite the major fault in the 

Bacon Range, no faults and only a few small folds 

were noted in the V\Tinnall Beds cropping out near the 

craters. Preimpact deformation may have been greater, 

however, in the less competent shales and siltstones and 

may have caused some of the unexplained stratigraphic 

and structural anomalies. Nevertheless, the structures 

mapped on plate 1 are assumed to have resulted, with, 

at most, minor local exceptions, from deformation of a 

simple homoclinal sequence by meteorite impact. 

A small hogback of sandstone shown at the east edge 

of plate 1 is the only nearby outcrop of the beds intersected by the large craters except those in the crater 

walls themselves. The stratigraphic sequence is therefore based entirely on exposure of rocks that have been 

severely disturbed by the impact, and so the sequence 

is not as well defined as could be desired. About 450 

feet of section is exposed in the larger craters, in which 

eight major units, designated units a through h, and 

as many subunits have been mapped (pl. 1). The sequence of units e through h resembles that of units a 

through d, but there are sufficient differences to rule out 

structural repetition of the section. Instead, a cyclic 

pattern of deposition in the Winnall Beds seems to be 

indicated. 

The shales and siltstones, which make up the bulk 

of the section in the area on plate 1, crumble into chips 

or fragments that are at most a few inches in size, and 

they can easily be excavated with a shovel. The beds 

. of sandier siltstone and sandstone are more indurated 

and break into larger plates. Only in the south wall of 

the Water Crater are sandstone beds sufficiently indurated to break into blocks several feet across. 

Bedrock on the pediment is covered by a Quaternary 

a.lluvial deposit consisting of cobbles and smaller subrounded fragments of sandstone and grey billy from the 

Bacon Range and a few rounded pebbles of more distant provenance in a red silty matrix. Coarse fragments are concentrated close to the surface, producing 

a stony gibber plain. The thickness of this pediment 

gravel depends on the underlying ma.terial-it is thin 

or even absent over the more resistant sandstone beds 

and may reach a thickness of 15 feet over shale. In 

places shale units are weathered as much as 15 feet below the pediment gravel and grade upward into a. reddish clayey soil. 

SUMMARY DESCRIPTION OF THE CRATERS 

Twelve of the 13 craters described and numbered py 

Alderman (1932) are clearly recognizable; only possible small ill-defined craters can be added to this list 

(fig. 1). Hodge's a.tlas (1965) can be consulted for photographs of each crater. 

Craters 1 and 93.-N o raised rims or closed depressions remain at craters 1 and 2. As recognized by Alder-: 

man (1932), however, clay pans free from pediment 

gravel and supporting a growth of mulgas indicate 

craters about 80 and 90 feet in diameter that have been 

nearly obliterated by erosion of their rims and by filling 

by alluvium. 

Crater 3.-Crater 3 has a diameter of 170-230 feet, a 

depth of 9-15 feet, and a maximum rim height of 4 feet 

(terminology defined in fig. 2). A detailed map and 

description of this crater, which is notable for the pattern of ray loops in which ejected fragments from sandstone beds are distributed, has been published previously 

(Milton and Michel, 1965). 
C4 CONTRIBUTIONS TO ASTROGEOLOGY 

N 

I 

! I 

I Alice I 

i Springsi 

l--\--1-, i Area of 1 

' report f------~-

1 

L C) ' ............... \. 

AUSTRALIA 0tJ 

0 500 

r----

\ 

5 

) 1000 FEET 

FIGURE 1.-0utline map of the Henbury crater field. Dashed line indicates approximate outer limit of ejecta from the larger craters. 

Based on aerial photographs in the Division of National Mapping, Australia, and on planetable survey by D. J. Milton and 

F. C. Michel. 

Crater .4,.-Crater 4 has a diameter of 190-220 feet, a 

depth of 12-20 feet, and a maximum rim height of 5 

feet. As a·t crater 3, ejected blocks from sandstone beds 

in the predominantly shale bedrock sequence lie along 

rays. One ray, composed of blocks from a 6-inch-thick 

sandstone bed,, sta~rts at the foot of the raised rim about 

50 feet beyond the crest on the west side of the crater and 

extends radially outward 230 feet. The outer 100 feet of 

the ray is marked only by a few blocks, which lie in a 

wet-weather wUJter course. The fact thUJt they have not 

been transported suggests that the craters formed relatively recently. 

Orater 5.-Crater 5, nearly destroyed by erosion, is a 

circular crater 55 feet in diameter and not over 3 feet 

deep. A 1-foot-high rim is preserved on the soi1th and 

west sides. 

Orater 6 (Water Orater) .-Crater 6 is 280~320 feet 

in diameter and about 20 feet deep ; the rim is generally 

,'-

·r 
..... 

STRUCTURAL GEOLOGY, HENBURY METEORITE CRATE1RS, NORTHERN TERRITORY, AUSTRALIA C5 

Height 

of rim 

Diameter of crater ,;im crest 

..j;._ Rim flank 

----!D~~::.~A~ ~Wall Rim 

Floor 

l!..,Ioum:; 2.-Diagrnm illustrating terminology and measurements 

of craters used in this repo:rt. 

5-10 feet high but is higher along the common wall with 

crater 7. The wall of this crater has been breached, and 

a rill system, which before the impact drained to the 

northeast, has been captured and now drains into the 

crater. 

Orate'r 7 (AI ain Crater) .-Crater 7 is an oval 600 feet 

long }'\,nd 35-50 feet deep and has a maximum rim height 

of 20 feet (fig. (i). Crater 7 actually consists of two 

coalescing craters, 'vhich are designated craters 7a and 

7b. The diameter of crater 7 a is about 480 feet, and that 

of crater 7b, about 390 feet. 

O'rate'r 8.-Crater 8 is 230 feet in diameter. The depth 

generally ranges from 5 to 12 feet, increasing to over 

20 feet at the common wall with crater 7. The maximum 

rim height, aside from the common walls with craters 

6 and 7, is about 5 feet. 

O'rate'r .9.-It is uncertain whrut feruture corresponds 

to the "illdefined and doubtful'; cra.ter noted by Alderman (1932) southeast of crater 8. It may be a depression along th~ beheaded drainage system, but it is probably not a crruter. 

O'rater 10.-Crater 10 is 80-100 feet in diameter arid 

3-7 feet deep and has a ma.ximmn rim height of 4 feet. 

This cra.ter is describ~d in detail in a later section of 

this report. 

0'1·ate'I·11.-Crater 11 has a diameter of about 45 feet. 

The. original form has been destroyed by excavation· 

. in search of meteoritic iron (apparently unrewarded), 

but it was very shallow· and low rimmed. 

O'rate?•12.-Crater 12 is 80-95 feet in diameter. As it 

lies on the south slope of a sandstone ridge, the depth 

measured at the north wall reaches 18 feet, and at the 

south a wall only a few inches high separrutes the crater 

floor from a watercourse about 1 foot lower. The west 

and southeast sides show a distinct raised rim reaching 

a height of 3 feet. Bedrock in the walls has been deformed so that the strikes are ta.ngential, and beds near 

the rim crest are overturned. 

O?·ate'l· 13.-Crater 13 is about 20 feet in diameter and 

3 feet deep. The rim height is at most a few inches. 

About 450 pounds of meteoritic iron was recovered at 

a depth of 7 feet in this crater (Spencer, 1933a). 

Othe1· JJOssible orate1·s.-A probable rimless crater 

about 25 feet in diameter lies about 200 feet west-south287-324 0-68-2 

west of cra.ter 13 in pediment gravel (fig. 1) . A possible 

crater was noted at the circular patch of alluvium just 

south of crater 8 (pl. 1), and another just southeast of 

crater 4, where the struotures exposed in the gully appear to indicate a center of impact outside crater 4. 

Hodge ( 1965) reported an additional crater (his crater 

15) about 25 feet in diameter about 40 feet northeast of 

the crest of crater 12. This was not noticed during my 

fieldwork, and it may be only an erosional hollow at 

the base of a sandstone hogback. 

STRUCTURE OF THE CRATERS 

Current knowledge of the structure of meteorite craters is based largely on detailed investigations of Meteor 

(Barringer) Crater, Ariz. (Shoemaker, 1963), and the 

Odessa, Tex., craters (Evans, 1961), which have been 

considered type examples of two structural varieties of 

meteorite craters (Shoemaker and Eggleton , 1961). 

These craters were formed in horizontal strata and, 

hence, have an essentially radial·symmetry. Because of 

the dip of the strata at Henbury, the mechanics of deformation varied from point to point around the crater 

walls. Moreover, the interaction of stresses originating 

at the points of impact of separate fragments of the 

meteorite in the centers of craters 6, 7 a, 7b, and 8 add 

to the complexity of the structure. Fortunately, the 

Henbury craters are considerably younger than Meteor 

Crater and the Odessa craters and have been less affected 

by erosion o:£ the walls and rim than Meteor Crater or 

by infilling than Odessa. Rather dissection of the larger 

craters has reached a nearly ideal point-the spurs between gullies have retreated only slig~tly from the original crater walls, and cross sections of the walls can 

be examined in the gullies. Much of the walls is covered 

by colluvitun, but exposures are well distributed so that 

structures updip, downdip, and along the strike from 

the centers of impact can be examined . 

STRUCTURE OF CRATER 10 

Crater 10 (fig. 3) exhibits many of the structural features of the larger craters even though it is smaller and 

was formed in more competent rocks. It is described here 

as an introduction to the more complex craters of the 

main group. The crater lies on the crest of the bare sandstone ridge. The crater walls and rim are a chaos of sandstone fragments, but the blocks of the more massive 

sandstone beds show consistent attitudes and indicate 

the patterns of deformation. 

The north, or updip, wall of the crater has a very 

gentle slope of about 15° and exposes apparently undisturbed beds that dip southward 30°-the approximate preimpact attitude. The lack of deformation suggests that, with the direction of radial stress lying so 
C6 CONTRIBUTIONS TO ASTROGEOLOGY 

0 10 FEET 

CONTOUR INTERVAL 1 FOOT 

DATUM IS APPROXIMATE FLOOR OF CRATER 

3 4--------

25 

6---------

) 

~PLANATION ( -A~A 

Outcrop of marker bed 

50 

....1.._ -+-- 70 

-dInclined Vertical Overturned 

Strike and dip of beds 

60 

--- Inclined Vertical 

Strike and dip of 

fracture zones 

Geology and topography by D. J. Milton and F. C. Michel, 1963 

FIGURE 3.-Geologic map of crater 10. 

near to the plane of bedding, little effective shear was 

exerted beneath tJhe original crater wall, which is the 

boundary surface above which material was thrown out 

of the crater. 

On the west side of the crater, a thick sandstone bed 

(fig. 3, bed A) curves from its normal east-'west strike 

outside the crater to a nearly tangential northwest 

strike· in the crater wall. Within the wall, the. dip of the. 

bed steepens upward from 15° near the base to pass 

through the vertical and become overturned at the rim 

crest. On the east wall, one sandstone bed (bed B) has 

been deformed into a broad open anticline with its axis 

radial to the crater. Elsewhere in the east wall, zones of 

rock that retain their preimpact attitudes with nearradial strikes alternate with zones in which strikes are 

tangential. The· zones are separated by steeply dipping 

I• 

( 

(j 
STRUCTURAL GEOLOGY, HENBURY METEORITE CRATERS, NORTHERN TERRITORY, AUSTRALIA C7 

radial fractures. Some fracture zones are several inches 

wide and contain sandstone fragments parallel to the 

fracture. 

In the south wall, the original tangential strikes were 

little changed, but beds generally steepen from their preimpact attitude at the base to become vertical and, 

finally, overturned at the rim crest. One thin-bedded 

sandstone unit shows a Z-shaped fold in which the upper 

and lower limbs retain approximately their original attitude and the middle limb is overturned (fig. 4). A 

block ·measm:ing 4 by 3% by 1% feet, which lies 37 feet 

beyond the crest of the south rim and probably ·was derived from a bed that struck through the central part of 

the crater, is the largest single ejected block at any of 

the craters. 

i!~rauRE 4.-South wall of crater 10. The thin sandstone beds are 

overturned in the central limb of a tight Z·shaped fold 1 foot 

to the right of the hammer head. 

Crater 10 is markedly rectangular, having its walls 

parallel to the east-west strike and the north-south regional joint system. This orientation contrasts with that 

of Meteor Crater, Ariz. (Shoemaker, 1963), which also 

has a square outline but has its diagonals along the rectangular regional joint grid. The bending to the south 

rather than to the north of bed B on the east rim suggests that the focus of energy release was somewhat 

north of the center of the crater. 

STRUCTURES RELATED TO A SINGLE LARGE CRATER 

Probably nearly all the rock exposed in the crater 

walls of the large craters has been displaced outward 1 

from its preimpact position. Such displacement has in 

general reoriented bedding so that the strike tends to be 

tangential to the crater, or parallel to the crater walls. 

1 The reference point for inward and outward is the center of the 

crater, such that beds may be said to dip outward into the crater wall. 

The smooth flexure of units e through h at the southeast 

end of the Main Crater presents an unusually simple 

example of this displacement. Eleswhere a variety of 

mechanisms determined by particular combinations of 

the stress environment and the preimpact rock geometry 

were involved in the outward displacement and produced structural features of characteristic types. 

The structure of the large craters (figs. 5 and 6) is 

described in the following pages and is illustrated by a 

series of cross sections through the crater walls and 

rims (pl. 1) . 

The structures in the walls are described in terms of 

folds and faults, but these are not quite the structures 

produced during ordinary tectonic deformation. Folds 

are not continuous like those formed by slow intragranular movement but are mosaics formed by the reorientation of fracture-bounded blocks that are themselves little deformed. The fractures are usually so 

closely spaced-commonly on the order of an inch or 

inches-that, except under close inspection, they appear 

as continuous smooth folds. Similarly, surfaces at 

which originally separate bodies of rock are juxtaposed 

are called faults, although, as discussed in a later section, mechanisms different from the ordinary slippage 

of blocks in contact with one another were involved. 

CRATER W .A.LLS 

FOLDS WITH STEEPLY DIPPING AXIAL PLANES 

Although folding and faulting are closely associated 

in the crater walls, folding dominates in the north wall 

of crater 7a, where bedding before impact dipped inward at an angle slightly greater than the slope of the 

present crater wall. The dominant structure produced 

by the impact is an anticline that has a nearly horizontal axis extending along much of the lower slope of 

the north wall of the crater. Part of its trace is marked 

by the outcrop of unit a, which, as a projection downdip from the outcrop beneath the pediment gravel in 

the gully at the northwest of the crater indicates, would 

normally lie at an elevation below that of the crater 

floor. The anticline is asymmetric--the craterward limb 

retains nearly the original dip of the beds, and the short 

outer limb has been rotated through more than 120° 

and is now vertical or locally overturned. Along much 

of the anticline (as in section B-B') the inner limb is 

thrust over the crest. 

In addition, the north wall shows tight small-amplitude folds that ha,·e axial planes roughly parallel tto 

that of the main anticline, and open folds that have 

axes radial to the crater. The outlying patch of unit b 

west of the line of section B-B' is preserved in an open 

radially plunging syncline. Such cross folds are char-
FIGURE 5.-Aerial view of craters 6, 7, and 8 from the south. Reproduced from Hodge (1965) through the courtesy of the author. 

() 

00 
STRUCTURAL GEOLOGY, HENBURY METEORITE CRATERS, NORTHERN TERRITORY, AUSTRALIA C9 

FIGURE 6.-View of Main Crater from the northwest rim. Ejected bedrock in foreground with pediment gravel behind. Photograph 

by Douglass Eaglin. 

acteristic of the inner limb of the main anticline, 

though tangential folds predominate on the outer limb 

and higher on the crater wall. 

The expression of the main anticline as a ridge persists because units a and b have slightly greater resistance to erosion than unit c. The ridge is probably not, 

however, a product entirely of differential erosion but 

an original feature of the immediate postimpact topography. If so, the original crater wall would not have 

been a single conical or concave surface, as the simplest 

mechanical theory of cratering would suggest. Some 

craters produced by hypervelocity impact of projectiles 

into pyrex retain a doughnut-shaped ring of material 

at the intersection of the steep, upper wall and the flatter floor (H. J. Moore, oral commun., 1964). Although 

folding is not involved in these, the similarity of crater 

profile may indicate a somewhat analogous pattern of 

stresses. Many large craters on the moon show irregular 

concentric structures on the inner walls. These have 

usually been interpreted as the result of slumping into 

the crater, but some may be folds that were produced 

at the time of impact. 

FOLDS WITH LOW-DIPPING AXIAL PLANES 

The major structure exposed in the lower part of the 

south, or downdip, wall of crater 7a (section E-E') is, 

as in the updip, north wall, an anticline whose axial 

plane is approximately vertical. Higher in the wall the 

outward dip steepens to become vertical, and the beds 

in the upper half of the slope are overturned. Superimposed on this broad structure are a series of small 

folds that have axial planes dipping at low angles into 

the crater and at least one minor thrust fault parallel to 

the axial planes of these folds (figs. 7, 8) .' These folds 

are very shallow-they are seen on the upper bank of 

the gully but almost die out near its floor. These folds 

and minor thrusts apparently formed as the result of 

shearing stresses that Yaried irregularly in intensity 

on different planes parallel to the original crater wall 

(the surface along which shear was sufficient to displace rock over the crater rim). These shear folds are 

thus fundamentally different from the compressional 

folds that occur, for example, in the lower part of this 

wall or in the opposite north wall. Folds of this type 
QIO CONTRIBUTIONS TO ASTROGEOLOGY 

FIGURE 7.-View of the east bank of gully on the south wall of 

the Main Crater near the line of section E-E', showing increase in dip in the upper wall and shallow folds whose axial 

planes are approximately parallel to the crater wall. Map 

units e and f are bedrock uni!JS; Qcl is ·postcrater colluviun1. 

Photograph by M. R. Dence. 

would be destroyed rapidly by erosion and perhaps 

were more abundant in the freshly formed crater. 

A somewhat different type of fold having a nearly 

horizontal axial plane is best illustrUJted by section 

D-D'. Bedding in this part of the wall of the Main 

Crater appears little deformed except within a small 

area in the middle slope. Here the rock is displaced outward in a. fold concave toward the crater. In the upper 

part of this broad fold is a smaller but much sharper 

fold with the same sense. The base of the main fold is 

in part a thrust surface on which the folded rocks 

moved outward over the unfolded rocks. The axial plane 

of •the upper fold dips inward at about 10°, a.nd the 

basal surface of the main fold has a low dip also. The 

entire structure appears to have been produced by a 

punchlike stress whose principal axis lay at an angle 

lower than that of the present crUJter wall. 

THRUST FAULTS 

Thrust faults have been mentioned above in association with each type of fold in the crater walls. In parts 

of the crater wa.lls, thrust faulting is more prominent 

than folding. In the west \vall of the Main Crater, the 

shock waves at impact were propagated nearly along 

the original planes of bedding. Reorientation of bedding 

into the present tangential attitudes, with the strike at 

nearly right angles to the preimpact strike, was apparently accomplished largely by folding, but thrust faulting also occurred. The broad structural features along 

the line of section .A-.A' are an anticline whose axis is in 

the midslope and a syncline in the lower ·wall that brings 

unit b to the surface. The more obvious, although perl!~IGURE 8.-Closeup of folds having low-dipping axial planes at 

center in figure 7. A single bed of hard sandstone has been 

emphasized. 

haps more superficial, feUJtures are an imbricate series 

of -thrust plates in the lower wall. The lowest discontinuity is a sharply defined surface dipping 30° inwa.rd, 

along which vertical beds are thrust outward over gently 

dipping beds. The thrust fault may be considered as 

occupying the axial plane of the syncline. The gentle 

inward dip of the underlying plate increases outward, 

particularly at a sharp synclinal bend, to a second discontinuity which consists of a disturbed zone about a 

foot wide (shown as a thrust faul•t on the map) in ·w·hich 

both faults and folds occur. The next underlying plate 

repeats the pattern of gentle inward dips steepening outward and is terminaked at the outer edge by a. small 

tight anticline. Beyond this anticline the gentle inward 

dip prevails again to the axis of the major anticline. 

These shallow structures represent localized outward 
STRUCTURAL GEOLOGY, HENBURY METEORITE CRATERS, NORTHERN TERRITORY, AUSTRALIA Cll 

displacements superimposed on the broader folds. 

Thrusting was dominant nearest the crater, both thrusting and folding occurred in the middle, and folding 

was dontinant farthest out. 

The ·wall at the ''est end of the Main Crater about 60 

feet south of the line of section A-A' shows features 

resemb-ling those of both sections A-A' and B-B'. As in 

section B-B', an asymmetric anticline has the inner 

limb thrust over-the crest. Above the gently dipping and 

locally horizonta.l inner limb is another thrust fault. 

. The plate above this thrust consists of steeply clipping 

beds and may be a continuation of the inner pla:te to the 

north along the line of section A-A'. 

COMBINED STRUCTURES IN THE WATER CRATER 

The walls of the Water Cra•ter show all the structures 

described above. On the south wall, sandstone beds are 

folded about gently dipping a.xial planes and are displaced along at least one shallow thrust fault and an apparent tear fault (sec-tion H-Il'). The sandstone (subunit hs) on the rim flank consists mostly of broken ejecta 

from the beds exposed in the wall. To the east, the tops 

of the shales of unit h and the band of sandstone (bed 

hs3) on the crater rim are indeterminate, and these beds 

may form an inverted rim flap. 

On the west wall (section G-G'), steep- and shallowdipping folds are not distinctly separable as in the Main 

Craker, and the chara.cteristics of both types are shown 

in a single anticline that has a curved axial surface. The 

trace of the axial surface of this fold is intersected twice 

by the line of section G-0'; in the upper outcrop band 

of bed hs1 it dips at a steep angle, and some 6 feet up 

the wall it is again intersected as an overturned fold 

that has a low dip. The entire folded bedrock sequence 

has been thrust outward. The pediment gravel exposed 

low in the wall may be continuous with that on the rim, 

but it more likely is preserved in a syncline below the 

thrust, as indica.ted in the cross section. Beds in the 

isolated patch of units h and hs on the rim crest do not 

match the Ji.thology of any of the beds in nearby parts 

of the wall, and the preimpact position from. which they 

were thrust (or perhaps thrown out, as there is no evidence that they are not upside down) is unknown. 

CJRATER RIMS 

The distinction between debris and disturbed bedrock 

in crater rims at Henbury is much less clearcut than at 

Meteor Crater (Shoemaker, 1933), and the two have 

not been explicitly distinguished on the map. Attitudes 

were, however, recorded wherever consistent orientations could be found, so that areas of bedrock units on 

the rims without attitude symbols on the map may be 

assumed to consist of small fragments with little or no 

common orientation (this is not necessarily true of areas 

on the walls, where colluvium hinders the measurement 

of attitudes). In these patches, however, the fragments 

are entirely or predominantly from a single bedrock 

unit. The mixed ejecta map unit consists for the most 

part of thoroughly mixed fragments of several bedrock 

units and pediment gravel. In some areas, particularly 

on the southeast rim crest of the Main Crater, this unit 

includes discrete patches or bands of monolithologic 

ejecta that can be distinguished but are too small and 

discontinuous to be mapped. 

SYNCLINAL FOLDS AT THE RIM 

The structural style of coherent ejecta on the rim is 

related to the style in the walls below. In section E-E' 

the dips in the upper part of the wall of the Main Crater 

progressively steepen through vertical to an overturned 

attitude. This synclinal structure is continued on the 

rim, where the three bedrock units e, f, and ·g form a flap 

lying in inverted sequence upon pediment gravel. The 

exposed band o£ pediment gravel between bedrock in 

the upper crater wall and the rim flap is in places as 

narrow as 1 foot (fig. 9). The consistent attitudes of 

bedding within the flap indicate much greater structural coherence than would be found in throwout piled 

in inverted sequence by ejection along ballistic trajectories. On the other hand, the greatly reduced thickness 

of each of the three units in the flap in comparison with 

the thicknesses exposed in the crater wall and the pinchout of unit f near the outer edge of •the flap show that the 

flap cannot simply be the overturned limb of a fold. The 

flap must actually consist of thin slices that were formed 

by strong shearing, which took place simultaneously 

FIGURE 9.-0verturned rim flap overlying pediment gravel, south 

rim of Main Crater slightly west of line of section E-E'. From 

bottom to top are bedrock units in place, pediment gravel, 

illnd bedrock units in the overturned flap. Marp units e, f. and 

g are bedrock; Qpg is pediment gravel; Qcl ~s ·pootcrater colluvium. Weighted line is a fault. 
C12 CONTRIBUTIONS TO ASTROGEOLOGY 

with the outward rotation of the whole pile, along planes 

lying at small angles to the bedding. The minor structures in this part of the flap, such as the two plunging 

inverted· synclines, probably reflect the topography on 

which the flap fell. 

The extension of the flap far down the wall of the 

Water Crater suggests that the \Vater Crater is fractionally older than the Main Crater. If the shock wave 

propagated outward from the centers of impact at a 

. speed of 640 meters per second-the average shock speed 

measured in alluvium at the Sedan nuclear cratering 

event (Nordyke and Williamson, 1965)~displacement 

of rock near the rim of craters 6 and 8 began at about 

80 milliseconds and 120 milliseconds after impact. If 

the shock propagation was slower-in accordance with 

the slower arrival times recorded at the Scooter high 

explosive cratering event (Hess and Nordyke, 1961)-

corresponding values are 120 and 190 milliseconds. 

The time intervals betwee.n the impacts in the larger 

cra:ters are probably negligible in comparison. Farther 

southwest, ejecta from the Main and ·water Craters appear to have interpenetrated and intermingled, but the 

pattern is not at all clear. 

Northeast of the line of section E-E', the rim flap 

shows a somewhat different structure. Bedding in the 

material immediately above pediment gravel at the crater lip is right side up, as indicated by primary sedimentary features, but witJhin a few feet outward it is folded 

through nearly 180° to form a thin overturned flap (fig. 

10). The axis of the fold, which has at least one radial 

offset, can be traced for about 60 feet along the rim. The 

same type of synclinal rim folding as that shown in secFIGURE 10.-Southwest rim of Main Crater showing thin ejecta 

layer, composed mostly of overturned beds of unit e, overlying 

pediment graveL Tops of trees growing in the Water Crater at 

left, sandstone ridge in middle background, and main ridge 

of Bacon Range at right background. Units d, e, e,, e,, and ea 

are bedrock; Qpg, pediment gravel; Qej, mixed ejecta; Qcl, 

postcrater colluvium. Weighted line is a fault. 

tion E-E' apparently occurred, but in addition, a major 

outward thrust developed along a surface that intersected the lower limb of the syncline, so that the axial 

region is preserved, thrust over the pediment gravel. 

THROWOUT AND FALL OUT 

'Dhrowout debris crops out in a broad belt surrounding the large craters except where it has been removed 

by erosion, as south and east of crater 8, or buried beneath later alluvium, as east of the Main Crater. The 

absence of exposed throwout debris on the steeper parts 

of the rim flank, as northwest of the Main Crater, is 

probably the result of erosion or burial, although much 

of the mapped pediment gravel could be throwout debris 

from pediment gravel that lay within the limits of the 

crater. 

At its inner limit throwout debris from bedrock units 

cannot be clearly differentiated from coherent ejecta. 

Within the area on the northwest rim of the Main 

Crater shown on plate 1, fragments of units b, c, and d 

lie in zones in inverted sequence on pediment gravel 

and were probably ejected along ballistic trajectories. 

An approximate outer limit of throwout from the 

larger craters is shown in figure 1. In the outer parts 

of the zone, west and north of the Main Crater, throwout debris is generally a few inches thick, but in some 

broad mounds it is nearly 2 feet thick. Some mixing of 

debris from different units occurs, but exposures are 

commonly monolithologic. Fragments from unit d apparently dominate in the outer part of the throwout 

west and northwest of the Main Crater. 

No exposures of material corresponding to the fallout 

debris of Meteor Crater (Shoemaker, 1963) were 

found. Probably, however, fragments of impact glass 

are remnants of a fallout layer from crater 7a that 

has been destroyed by erosion. Such glass is most abundant in approximately the area of throwout on the 

north rim of crater 7a and decreases in abundance outward. Scattered fragments were also found more than 

2,000 feet east of the craters. According to Spencer 

(1933b), tear-shaped drops and threads were found 

along a narrow strip of ground extending eastward a 

mile from the crater. This pattern probably indicates an original asymmetric distribution of fallout, 

although possibly the smaller fragments were 

transported by wind. 

STRUCTURES RELATED TO TWO CRATERS 

Figure 11 shows the approximate outlines of the 

craters that would have been produced around each 

center of impact had there been no mutual interference. 

The distances between craters 7 a and 6 and between 

craters 7b and 8 are great enough that little interaction 
STRUCTURAL GEOLOGY, HENBURY METEORITE CRATE'RS, NORTHERN TERRITORY, AUSTRALIA 013 

7a:· 

. . 

. 

. 

. 

. . . . 

. . . 

FIGURE 11.-Diagram showing relationship of the larger craters. 

Heavy lines indicate rim crests, light lines base of walls; 

dotted lines complete craters 7a and 7b as either would be had 

the other not formed. 

would be expected, and no evidence of it was found, 

although exposures of bedrock in the critical areas are 

not good. 

The two pairs of craters 7b and 6 and 6 and 8 are 

closer. The common walls in both pairs have a folded 

internal structure. The outcrop and attitude pattern of 

sandstone beds of subunit hs in the wall between craters 

6 and 8 indicates a broad syncline with a pair of tight 

folds in the axial region. The wall between craters 7b 

and 6 shows a series of folds (section F -F'). A syncline 

near the crest is best shown by a sandstone bed somewhat east of the line of section. The principal fold must 

be anticlinal because the bedrock at the crest lies at 

least 20 feet higher than its preimpact elevation. The 

plunging anticline slightly south of the crest may be the 

main fold (the intricate folding on its south limb is 

probably shallow shear folding related only to crater 6). 

The structure at depth is unknown; a series of decollements along small thrusts seems likely. 

The centers of impact in 7a and 7b were so close that 

the intervening wall did not survive, at least above the 

level of the present alluvial crater floor, except as short 

spurs at either end projecting from the common wall of 

the Main Crater. The spur on the south shows bedrock 

that strikes northeast-southwest overall and dips southeast but is folded into a series of folds whose axial 

planes trend at right angles to the general strike (fig. 

12) . The inner two folds have curved axial surfaces and 

are tight folds with an overturned limb between them; 

the outer two are much more open. The pattern suggests 

that the bedding was bowed into a strike tangential 

with crater 7a and compressed radially from crater 7b, 

probably accompanied by a buttressing effect from the 

northward movement of the wall of crater 6. 

On the northern wall of the Main Crater, the prominent spur at the intersection of craters 7a and 7b has 

diverted drainage to form deep gullies on either side. 

The complexities of structure in this area (the most 

complex found at Hen bury) in part result from displacement of separate masses of rock outward from the 

two centers of impact and in part result from significant displacement only by stresses from one center but 

facilitated by dilation of the rock caused by interaction 

of the two stress fields. 

The structure on the face of the spur at the intersection of craters 7a and 7b mirrors that of the corresponding spur on the opposite side of the Main Crater. 

The general trend of strike is at right angles to the line 

joining the centers of craters 7 a and 7b (although no 

attitudes could be measured in subunits e 1 and e3 , the 

ground surface apparently is close to a dip slope near 

the southern nose. Cross folds, such as are shown at the 

southeast end of section D-D', were produced by outward compression. The entire moderately dipping sequence is thrust outward. Bedding in the outer edge of 

the thrust plate has been folded up into a synclinal 

bend. Below this plate is another thrust slice that is also 

internally folded. The line of section D-D' intersects 

the outcrop of this slice in the outer limb of an anticline 

FIGURE 12.-Spur at the intersection of craters 7a and 7b on 

the south wall of Main Crater. Folds are indicated by the 

outcrop pattern of two outlined sandstone beds in unit e. 
C14 CONTRIBUTIONS TO ASTROGEOLOGY 

whose inner limb is best exposed southeast of the gully 

in the outcrop area of subunit e2 • 

On the opposite side of the spur, a series of thrust 

slices can also be seen. Along the line of section 0-0' 

the outcrops nea.rest the crater belong -to a plate of rock 

that retains approximately its preimpact attitude. The 

rock below and outside this plaote has been rotated more 

than 90° through an arc concave upward, so that bedding is overturned and dips north (fig. 13). The lower 

zone, which is itself part of a thrust slice, has been displaced outward relative to the upper plate, so that the 

break between them is at least locally a surface of underthrusting. Exposures farther out in the gully continue 

to show steep northward dips and probably represent 

the outward continuation of the lower zone. Indica1tors 

of bedding tops are lacking in these beds, however, and 

the possibility of isoclinal folding cannot be eliminated. 

The surface of the discontinuity between the upper 

and lower zones as exposed on the south side of the 

gully is not smooth but irregularly stepped, so that 

motion of the upper and lower plates would have been 

impossible while they were in contact. Figure 14 shows 

a small block along this surface in which bedding is 

oriented nearly at right angles to the bedding below and 

above. The block could have acquired its orientation 

only while out of contact with the adjacent blocks. Such 

features indicate that deformation in the wall occurred 

during an instant of dilation, which allowed structural 

blocks in the wall to deform and rotate independently 

of each other, after which rthe whole mass settled into 

its present position. Perhaps it is going too far to suggest that the divergent beds at the top of the lower zone 

in figure 15 were pushed out in opposite directions when 

FIGURE 13.-Underthrust in unit d near base of north wall of 

Main Crater along the line of section 0-0'. Flat faces at left 

are the top surfaces of beds in the upper plate dipping toward 

the crater at front and left. In the lower plate at right beds 

are overturned and tops face toward the left. Map unit Qcl is 

post-crater colluvium. 

the upper plate fell on them. The best evidence of dilation was found in the space between craters 7a and 7b, 

which suggests that dila.tion resulted from the interFIGURE 14.-Thrust in unit d exposed in south wall of gully 

south of the line of section 0-0' in north wall of Main Crater. 

The thrust surface follows a zig-zag trace from lower right 

to upper left. 

FIGURE 15.-Detail along same thrust surface as shown in figure 

14. The block in center must have been rotated to its vertically 

dipping attitude during a moment of greater separation of the 

adjacent blocks. Qcl is post-crater colluvium; bx breccia. 
STRUCTURAL GEOLOGY, HENBURY METEORITE CRATERS, NORTHERN TERRITORY, AUSTRALIA C15 

action of stress from craters 7a and 7b. Nowhere in any 

of the Henbury craters, however, were slickensides or 

gouge related to the impaot found, and even fine-grained 

breccia (except as ejecta) is rare. This suggests that 

dilation, or at least. light conta.ct of structural blocks, 

during deformation of the crater walls was the rule even 

where only one center of impact was involved. 

Between the lines of sections 0-0' and D-D', the 

series of thrust slices may be followed continuously 

from the crater wall onto the rim. The highest slices in 

the pile tend to have low craterward dips, corresponding 

to the inner plateS along the two lines of section. At the 

head of the gully northwest of the spur, bedding is seen 

to turn down aJ1d under in an overturned anticline with 

an overthrust of small slip along the axial surface (figs. 

16 and 17). Beneath the inverted limb is a complex 

thrust zone of broken bedrock, and below that is pedi-

:ment gravel. Apparently the main overthrust mass 

rolled over its own outer edge like a caterpillar tread, 

producing a. feature resembling on a small scale an 

Alpine nappe. The inverted limb of the fold is in a 

somewhat analogous structural position to the ·outer 

limb of the fold in •the middle slice in section D-D', except that the inverted limb has been thrust farther out 

and lies beyond the outcrop of the pediment gravel. 

A prominent ridge extends northeast from the intersection of craters 7 a and 7b to well beyond the area 

mapped on plate 1. This ridge ma.y be compared to 

ridges at right angles to the line joining the centers of 

intersecting craters formed by the simultaneous detonation of separate equal-sized buried explosive charges 

(Vortman, 1965). At Henbury the axis of the ridge 

FIGURE 16.-Nappelike folded thrust slice in south bank of gully 

in north vvall of Main Crater soUJtb of line of section 0-0'. The 

fold axis (dashed line) is in part a thrust surface. Above are 

gently dipping beds of unit e; below are overturned beds grading downward into breccia ( bx) ; and at bottom is pediment 

gravel (Qpg) over which the entire na ppe bas ridden. 

Qcl is postcrater colluvium. Photograph by M. R. Deuce. 

FIGURE 17.-Axial region of nappelike folded thrust slice in 

north wall. Area shown is at extreme upper left in figure 16. 

Beds above hammer are right side up ; beds below are overturned; and beds at lower left are brecciated. 

appears to bend so that, although the inner portion is 

at right angles to the line of centers, the outer portion 

is more nea.rly radial to the larger crater. 

The diminishing .frequency of attitude symbols on 

plate 1 outward along the ridge reflects a steadily 

increasing degree of brecciation but indicates that 

structurally coherent thrust slices can be followed for 

80 feet over the precrater ground surface. Farther out, 

ejecta from different bedrock units becomes increasingly 

mixed, and the contact ·between the ejecta from specific units and the mixed ejecta is drawn somewhat 

arbitrarily. The stratigraphic relation of the mixed 

and unmixed ejecta is uncertain-it could be a lateral 

transition or an overlap with either unit on top. The 

sides of the ridge are cut by shallow gullies at right 

angles to its length (just visible in fig. 5). Slight differ-
C16 CONTRIBUTIONS TO ASTROGEOLOGY 

ences repeated from gully to gully in the composition 

of the ejecta on the inner and outer walls suggest that 

erosion was controlled by an original corrugated pattern 

of throwout. 

CHARACTER OF DEFORMATION 

The variety of structural features in the walls of the 

craters at Henbury in large part reflects the range of 

orientation of the deforming stress. Stress in nearly 

horizontal planes, which tends to occur low in the 

craters, usually produced tangential compressional 

folds that have approximately vertical axial planes. 

Underthrusting accompanied such folding principally 

where dilation from simultaneous impacts loosened the 

wallrock. Sharply localized outward stress within the 

walls produced relatively uncommon folds that are 

concave toward the crater and have flat-lying axial 

planes. 

Overthrusts were produced by stress at higher angles 

of elevation. Thrust faulting was most extensive where 

stresses from two centers of impact were active simultaneously b~t also occurred in simple craters. 

Folds having axial planes parallel to the crater wall 

indicate stress at a still higher angle of elevation. Such 

folds are particularly susceptible to erosion and were 

perhaps more common in the fresh crater than in the 

existing exposures. 

Stress at an even higher angle produced overturned 

rim flaps. The occurrence of such rim flaps on the south 

rim of crater 10 as well as at crater 7a and perhaps the 

Water Crater suggests that they developed particularly 

where the bedding dips away from the crater. Finally, 

debris in inverted stratigraphic sequence was ejected 

along ballistic trajectories by stress acting at the highest 

angles of elevation. 

COMPARISON WITH OTHER IMPACT CRATERS 

Shoemaker and Eggleton ( 1961) have suggested that 

impact craters are of two structural types. In the Barringer (l\ieteor Crater) type, upward folding of planes 

that were horizontal before impact increases from the 

unaffected terrain outside the crater inward to the crater 

wall and from below up to the ground surface and 

culminates in an overturned synclinal flap at the rim. 

The Odessa type is characterized by an anticline whose 

axial surface crops out in the upper crater wall; the upturning in the rim is not carried so far as to form a rim 

flap. Outward-dipping underthrust faults occur in the 

walls of the Barringer type, and inward-dipping overthrusts in the walls of the Odessa type. Largely by 

analogy with craters produced by buried nuclear devices, the differences were attributed to a larger scaled 

depth of penetration of the meteorite in the Barringer 

than in the Odessa type. The 24-kilometer diameter Ries 

crater in south Germany was suggested as a variant of 

the Odessa type, characterized by imbricate slices on the 

rim (the "Schollen" and "Schuppen" of Bentz, 1927), 

which were attributed to the greater relative importance 

of gravity in the mechanics of so large a crater. 

The characteristics of all three structural types can 

be found in the Henbury craters. In the Main Crater, 

anticlinal folds dominate in the lower wall and synclinal folds nearer the rim, as in some experimental 

hypervelocity impact craters.2 A shallower depth of 

burst would tend to produce anticlinal folding nearer 

the rim, as in the Odessa craters. The absence of anticlinal folding in the lower walls at Barringer Crater 

may be due to the greater competence of the rock rather 

than the stress pattern. The factors controlling overthrusting (aside from dilation by simultaneous impacts) 

are not clear, but certainly large size of the crater is not 

essential. The structural features at each crater certainly provide information on the mechanics of crater 

formation; yet their variety, even within a single crater 

field such as Henbury, suggests that many more impact 

craters must be mapped before a comprehensive synthesis of impact mechanics can be made safely. 

BOXHOLE CRATER 

One day was spent at the Boxhole Crater at Dneiper 

(formerly Boxhole) Station, 185 miles northeast of the 

Henbury craters. The similarity of chemical composition and general appearance (particularly the prevalence of shrapnellike twisted fragments) of meteoritic 

iron at Henbury and Boxhole suggests that the two may 

be paired falls. 

In the time spent, little additional information could 

be obtained to supplement the original description of 

Madigan (1937). A crude plari.etable sketch confirmed 

Madigan's statement that the crater is nearly circular 

and has a diameter of about 575 feet. The crater lies on 

the south side of a ridge of schist and gneiss containing 

many silicified zones and quartz veins (shown simply 

as quartz reef on the small scale geologic map of Smith, 

1963). There is little mappable structure in the bedrock, 

so that only the crudest determination of the pattern 

of deformation seems possible. 

Most of the ejecta from the crater appears white and 

contrasts with the red-brown color of undisturbed nearsurface materials, apparently because the bulk of the 

exposed ejecta comes from below a weathered zone in 

which quartz has developed a heavy red stain. Weathered bedrock crops out at many places along the crater 

wall. The contact between the ejecta and bedrock is ex2 This is best shown by craters in ductile material, such as wax 

(Frasier and Kar.pov, 1962). 

,. 
IJ 

STRUCTURAL GEOLOGY, HENBURY METEORITE CRATERS, NORTHERN TERRITORY, AUSTRALIA C17 

posed around a bench on the southeast side of the crater 

and slopes gently outward. 

l\1ttdigan believed that the less steep slopes at the Boxhole Crater indicate an age greater than that of the 

I-Ienbury craters. But it is the south wall of the Main 

Crater that gives the impression of steepness at Henbury, and its steepness is probably an original feature 

caused by interaction with craters 6 and 8. If only the 

simple parts of the walls of craters 7a and 7b are considered, the average slope is, if anything, slightly less 

than at Boxhole Crater. 

Qualitative comparison of the degree of erosional 

modification and of the very weakly developed weathering profiles on the ejecta at both the Boxhole and the 

I-Ienbury craters suggests that the craters could be the 

same age. 

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0 

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