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Remote sensing and GIS analyses of the Strangwaysimpact structure, Northern TerritoryH. Zumsprekel & L. Bischoffa Geologisch-Palaeontologisches Institut der Westfaelischen Wilhelms-UniversitaetMuenster , Corrensstr. 24, D-48149, Münster, GermanyE-mail:Published online: 27 Sep 2011.

To cite this article: H. Zumsprekel & L. Bischoff (2005) Remote sensing and GIS analyses of the Strangways impactstructure, Northern Territory, Australian Journal of Earth Sciences: An International Geoscience Journal of the GeologicalSociety of Australia, 52:4-5, 621-630, DOI: 10.1080/08120090500181077

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Remote sensing and GIS analyses of the Strangwaysimpact structure, Northern Territory

H. ZUMSPREKEL* AND L. BISCHOFF

Geologisch-Palaeontologisches Institut der Westfaelischen Wilhelms-Universitaet Muenster, Corrensstr. 24,D-48149 Munster, Germany

Remote sensing and GIS techniques play a substantial role for the identification of possible terrestrialimpact structures, for mapping target-rock lithologies and deciphering the structural style of knowncraters. In this case study the lithological and structural characteristics of the highly erodedProterozoic Strangways impact crater in the Northern Territory have been analysed on the basis ofLandsat Enhanced Thematic Mapper satellite imagery, topographical data and airborne geophy-sical data. Regarding Landsat data, the calculation of basic statistical parameters and the optimumindex factor has been found useful for a pre-selection of informative band combinations. By means ofthe analysis of multisensoral data, the distribution of crystalline basement rocks, siliciclastic targetrocks of the Roper Group as well as post-impact deposits and deeper seated Proterozoic dykes canbe detected. The original crater dimensions of the Strangways structure are carefully estimated at26 – 29 km by combining the remote sensing data with the distribution of shatter cones localised in thefield. The remote sensing/GIS approach of a geological interpretation based on multisensoral sourcesand combined fieldwork data can be successfully applied to other impact structures on earth, aswell.

KEY WORDS: geological mapping, GIS, impact crater, multispectral data, remote sensing, Strangways.

INTRODUCTION

At present more than 165 meteorite craters (impact

structures) are known on Earth, with the rate of

discovery having increased substantially over the last

two decades (Grieve 1998; Earth Impact Database 2004;

French 2004). Though craters are geologically short-

lived phenomena subjected to erosion, sedimentation

with younger deposits or tectonic deformation, it is

likely that more impact structures are yet to be

discovered. For example, careful estimates of the

average terrestrial cratering rate of 5.6 (+ 2.8) x 107 15

km2/year for craters with a diameter 5 20 km (Grieve &

Shoemaker 1994) suggest that 13 – 37 Phanerozoic impact

structures should exist in an area of the size of

Australia. However, though research programmes for

impacts have been conducted intensively in Australia,

so far only four Phanerozoic craters 4 20 km (Acraman,

Gosses Bluff, Tookoonooka, Woodleigh) have been

clearly identified (Glikson 1996; Earth Impact Database

2004). Three further structures (Talunidilli, Gnargoo,

Camooweal) in Australia could represent Phanerozoic

impact craters 5 20 km (A. Y. Glikson pers. comm.

2004).

An impact event can only be confirmed unambigu-

ously by identifying unique shock-metamorphic effects

in the target rocks, e.g. shatter cones, microscopic

planar deformation features in minerals (PDFs) and/or

meteoritic geochemical and isotopic signatures. How-

ever, especially in Australia, remote sensing has

strongly contributed to the discovery of impact struc-

tures, as they often appear as distinct ring anomalies

in satellite images cutting the regional tectonic envir-

onment. Remote sensing analysis plays a substantial

role in order to: (i) map the distribution of target rocks

and impactites; (ii) investigate the structural style of

large-scale craters; and (iii) define the present and

original crater dimensions more precisely. Multispec-

tral data designed to display the spectral contrasts of

rock types have been used for geological mapping of

exposed impact structures in arid environments (Gar-

vin et al. 1992; Prinz 1996), but their use is restricted in

areas with a denser vegetation. Therefore, recent

approaches are based on multisensoral, complementary

datasets like optical satellite imagery, gamma-ray

surveys, geophysical data and digital elevation models

(Abels 2003).

The Strangways crater in the Northern Territory of

Australia (15812’S, 133835’E) is located 75 km south of

Elsey Station, near Mataranka (Figure 1). In this case

study, Strangways offers a good opportunity to test

and evaluate various remote-sensing data and proces-

sing methods in combination with field observations

for the analysis of impact structures in semiarid

terrains, as the crater has been mapped in consider-

able detail by previous workers, high-quality

multisensoral satellite and airborne datasets are

*Corresponding author: zumsprekel@onlinehome.de

Australian Journal of Earth Sciences (2005) 52, (621 – 630)

ISSN 0812-0099 print/ISSN 1440-0952 online ª Geological Society of Australia

DOI: 10.1080/08120090500181077

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available and our own field investigations have been

conducted through the 1999 field season. Situated on

the southwestern edge of the Proterozoic McArthur

Basin, the crater was first mapped on a scale of

1:250 000 by Dunn (1963) who interpreted it as a

magmatic collapse feature. Guppy et al. (1971) identi-

fied the impact origin of the structure from the

occurrence of shatter cones and impact-melt rocks in

the central uplift. The inner part was later remapped

by Ferguson et al. (1978). The most detailed field

mapping has been published by Shoemaker and Shoe-

maker (1996) in their comprehensive review of

Proterozoic Australian impact structures (Figure 2).

GEOLOGICAL SETTING AND FIELDINVESTIGATIONS

The target rocks affected by the impact comprise a

1500 m-thick succession of mildly deformed Mesoproter-

ozoic to Neoproterozoic sedimentary rocks of the lower

Roper Group and the underlying crystalline basement.

The strata of the Roper Group are composed of an

alternating succession of greywacke, mica-rich siltstone

(Mainoru and Corcoran Formations) and mature quartz-

ite and sandstone (Limmen, Abner and Bessie Creek

Sandstones).

Strangways represents a complex crater with a

central uplift, in contrast to smaller, bowl-shaped

impact craters which are limited to a maximum

diameter of 2 – 4 km on Earth depending on the target

properties (Grieve 1998). As a result of uplift, the oldest

target rocks (granite, syenite and gneiss) occur in the

crater centre. Following the mapping of Ferguson et al.

(1978) and Shoemaker and Shoemaker (1996) (Figure 2)

the crystalline rocks are concentrically surrounded by

sedimentary rocks of the lower Roper Group. The

younging direction generally points away from the

crater centre, although not all formations can be

followed in the field for the whole circumference due

to the limited outcrop.

The morphologically prominent formations are the

Limmen, Abner and Bessie Creek Sandstones which

form ridges of 120 – 140 m asl (above sea-level). As a

result of the structural uplift of the complex impact

crater the beds dip away from the crater centre at

varying angles (20 – 858). In the Limmen and Abner

Sandstones the beds are locally overturned, which is

mainly recognisable by sedimentary marks like cross-

bedding and ripple marks. The Bessie Creek Sandstone

in the outer parts dips away from the crater centre at

shallow to moderate angles. At a distance of *15 km

north and northwest from the crater centre the bedding

conforms with the regionally developed broad syncline.

The more heterogeneous Crawford and Mainoru

Formations are poorly exposed. In some outcrops in

the northern part the fine-grained Corcoran Formation

exhibits small-scale, possibly impact-induced folding. In

the eastern part the Corcoran Formation is tectonically

removed leading to direct contacts between the Abner

and Bessie Creek Sandstones, while in the west the

formation is concealed beneath Phanerozoic cover

sediments. Sandstones in the northeast not assigned

by Shoemaker and Shoemaker (1996) probably belong to

the Proterozoic Abner Sandstone (Figure 2), as stroma-

tolites occur at several localities.

In the north deeply weathered volcanic rocks occur,

which have been mapped as Lower Cambrian Antrim

Plateau Volcanics by Dunn (1963). Shoemaker and

Shoemaker (1996) reported a direct overlap of unde-

formed volcanics on deformed Bessie Creek Sandstone.

This means that the extrusion of volcanics occurred

after the impact event. Though we could not prove a

direct contact between the volcanics and the target

material, we found fragments of tightly folded shales of

the Corcoran Formation in a basal conglomerate under-

lying the volcanics. Assuming that the folding in the

Corcoran Formation is caused by the impact, these

Figure 1 Location of the Strangways crater in the Northern

Territory.

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fragments indicate that the volcanics are post-impact

and Strangways is of Neoproterozoic age.

In the crater centre Shoemaker and Shoemaker (1996)

found a poorly exposed sequence of chert and red

sandstone of unknown age stratigraphically situated

between the crystalline basement and post-impact

Cretaceous sandstone (Mullamen beds) which they

interpreted as possible remnants of crater-fill sediments.

We found no outcrop of these rocks, except for some

boulders of chert in the crater centre. However, as these

boulders occur near petrographically similar Cretac-

eous sandstones, they could also represent erosional

remnants of these sandstones. The undeformed Mulla-

men beds form prominent morphological plateaus up to

190 m asl covering much of the crater centre and outer

zones in the south.

Unambiguous shock metamorphic features comprise:

(i) impact breccias; (ii) impact melt-bearing breccias;

(iii) PDFs (Figure 3d) and other microscopic shock

indicators developed in the impactites and crystalline

rocks; and (iv) shatter cones occurring in granite and

the Limmen Sandstone (Figure 3e). Impact breccias with

predominantly granitoid clasts are preserved in the

eastern and northeastern part of the crater centre

(Figure 3a), and are clearly distinguishable from poly-

mict breccias with mainly sedimentary fragments

which crop out near Limmen Sandstone ridges in the

north and east of Strangways (Figure 3b). The Limmen

Sandstone itself sometimes exhibits monomict breccia-

tion.

At some localities in the western part of the crater

remnants of impactites with melt content are pre-

served building up isolated ridges up to 20 m high.

These extremely fine-grained, dark-brown rocks are

underlain by coarse impact breccias with granitoid

fragments (Figure 3c) grading downwards to coherent

granite and overlain by breccias containing mostly,

but not exclusively, sedimentary clasts. In thin-

sections the melt matrix is recrystallised to opaque

phases and needle-formed feldspars known as whisker

crystals. Further shock features displayed are clasts

with PDFs (Figure 3d) and diaplectic glasses. Morgan

et al. (1981) determined an enrichment of siderophile

elements (Ir, Ni) in the impact melt-bearing rocks

that indicates contamination by the meteoritic pro-

jectile.

Bottomley et al. (1990) suggested an age of ca 470 Ma

on the basis of the youngest argon-release ages. How-

ever, in view of the pre-Antrim Plateau Volcanics age of

the impact, this may represent a resetting age. Shoe-

maker and Shoemaker (1996) were the first to postulate a

Precambrian age for Strangways, mainly based on their

observed discordant contact between Cambrian Antrim

Plateau Volcanics and Roper Group sediments affected

Figure 2 Geological sketch map of Strangways crater (simplified after Shoemaker & Shoemaker 1996). Granite and syenite in

the core are surrounded by a rim of mainly siliciclastic Roper Group sediments. The grid spacing of the UTM coordinates is

5 km.

Strangways impact crater 623

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by the impact in the northern part. This is confirmed by

more recent laser probe 40Ar/39Ar dating of impact

rocks from the centre, which leads to an age of 646+ 42

Ma for Strangways (Spray et al. 1999).

REMOTE SENSING ANALYSIS

Multispectral data

The multispectral analysis was based on Landsat

Enhanced Thematic Mapper (ETM) data acquired at

the beginning of the dry season (April 2001) in order to

minimise vegetation effects. The Landsat ETM sensor

works in wavelength regions of the visible (VIS), very

near infrared (VNIR) and short wave infrared (SWIR)

with six spectral bands (Table 1) at a spatial resolution

of 30 m. Three bands in the VIS collect data between 0.45

and 0.69 mm. Bands 4, 5 and 7 are important sources of

information about natural materials, as spectral char-

acteristics of vegetation and distinct absorption features

of rock-forming minerals like iron oxides, OH-bearing

silicates and carbonates occur in the VNIR and SWIR

region. The panchromatic (PAN) band 8 covers a broad

range of the VIS with a higher spatial resolution of 15 m.

In addition, two thermal bands collect data in the

Figure 3 Impact breccia with

predominantly (a) granitoid

clasts (347583mE, 8323055mN)

and (b) sedimentary clasts

(350022mE, 832449mN) (scale

given by spinifex leaves at top

right). (c) Coarse brecciated

granite overlain by choco-

late-brown impact melt-bear-

ing breccia (343259mE,

8321581mN). (d) PDFs in quartz

clast of impact breccia [thin-

section from sample from out-

crop in (c)]. (e) Shatter cones

developed in Limmen Sand-

stone (348490mE, 8324821mN).

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thermal infrared region (TIR) at 60 m resolution. Due to

their moderate spatial (TIR bands) and spectral resolu-

tion (PAN band) these bands were excluded from further

processing. As the full Landsat dataset covers an area of

185 x 185 km, a smaller dataset 43 x 48 km centred on the

Strangways structure was used in the analysis.

The further decision process as to which multi-

spectral bands are the most informative for a

geological interpretation often depends on the back-

ground knowledge and subjective impression of the

investigator—a fact which has often been criticised in

remote-sensing geology. One selection method is a

visual selection by testing various band combinations

in a RGB (red – green – blue) colour space, especially

those which have been found useful in previous remote

sensing investigations: e.g. a combination of the Landsat

bands 7 (red), 4 (green) and 1 (blue) often represents a

promising colour composite for geological investiga-

tions. Other workers compare basic statistics of single

bands in order to improve objectivity in the process of

data selection and to minimise the time-consuming

process of visual selection. Following this approach,

the evaluation of spectral information in single bands of

the Strangways structure was based on their mean

values (Gmean) representative for brightness, and stan-

dard deviations (s) representative for the contrast in one

band. Except for ETM band 5 (Gmean = 96.72, s= 12.170),

the data show a general low to moderate reflection

intensity with Gmean values between 56.854 and 72.531,

and poor image contrast with standard deviations

between 3.417 and 9.880 (Table 1).

In order to compare the spectral information of two

bands, their correlation coefficients r were computed.

The coefficient can have values ranging from 7 1 to 1. A

high positive or negative coefficient points to high

correlation of the two bands, whereas a coefficient near

0 indicates that the spectral information in the two

bands is less redundant. Regarding Landsat ETM bands,

strong to moderate correlations exist between bands in

the VIS (ETM-1 to -3) and in the SWIR range (ETM-5 and

-7), but ETM-4 is less correlated with bands of the VIS as

well as SWIR band 7 (Table 2).

Taking into consideration that three single bands can

be combined to form a colour composition in the RGB

space, Chavez et al. (1982) developed the optimum index

factor (OIF). With the OIF the most important statistical

parameters standard deviation (s) and correlation

coefficient (r) are combined by dividing their sums for

all possible three band combinations:

OIF ¼Sn

i¼1si

Sn

i¼1rij j

where n ¼ no: of bands ðn ¼ 3Þ

Examples of OIF computations for MSS, TM and ASTER

data are given by Bischoff and Prinz (1994), Zumsprekel

and Prinz (2000) and Ren and Abdelsalam (2001). Band

combinations with highest OIF values are given in

Table 3. Due to its low correlation with other bands,

ETM band 4 is contained in all five combinations,

together with bands of the VIS/VNIR and SWIR region.

A combination of ETM bands 4, 5 and 7 is statistically

defined as the most informative colour composite

(Figure 4a). However, as spectral variations are not

only caused by bedrock or controlled by geological

factors, the OIF analysis does not free the investigator

from a visual evaluation of other band combinations

with a high OIF value as well.

Division of the digital number (DN) values of single

bands, known as ratioing, is an image-processing

technique often used to enhance the spectral features

of rock forming minerals and typical weathering

products in arid environments (Drury 1993; Sabins

1997). Compared with materials showing a similar

reflection intensity in two bands, these materials

exhibit a high reflection in one band and low reflection

in the other, and are therefore pronounced in the

resulting ratio image. Ratios of ETM bands 5/4, 5/3, 3/

2 and 3/1 may enhance iron oxides and hydroxides. ETM

ratio 5/7 is commonly used to emphasize OH-bearing

Table 1 Landsat sensor and scene properties of the Strangways

area.

Wavelength

(mm)

Spatial

resolu-

tion (m)

Mean

value

Gmean

Standard

deviation

s

Band 1 0.45 – 0.52 (VIS) 30 72.531 3.417

Band 2 0.52 – 0.60 (VIS) 30 60.769 4.567

Band 3 0.63 – 0.69 (VIS) 30 61.055 8.123

Band 4 0.76 – 0.90 (VNIR) 30 61.745 5.789

Band 5 1.550 – 1.750 (SWIR) 30 96.720 12.170

Band 7 2.080 – 2.350 (SWIR) 30 56.8544 9.880

Acquisition date: 2001/04/12. Subset extension: 1477 rows x 1599

cols (ca. 43 x 48 km)

Note that the thermal bands have been excluded from the

statistical analysis due to moderate spatial resolution.

Table 2 Matrix of correlation coefficients, which allows the

estimation of data redundancy in two Landsat ETM bands.

Band ETM-1 ETM-2 ETM-3 ETM-4 ETM-5 ETM-7

ETM-1 1 0.789 0.761 0.358 0.655 0.684

ETM-2 0.789 1 0.842 0.572 0.777 0.752

ETM-3 0.761 0.842 1 0.372 0.810 0.864

ETM-4 0.358 0.572 0.372 1 0.503 0.299

ETM-5 0.655 0.777 0.810 0.503 1 0.906

ETM-7 0.684 0.752 0.864 0.299 0.906 1

Table 3 Highest Optimum Index Factor (OIF) rankings of ETM

band combinations (no RGB colors are designated).

Ranking Combination OIF

1 [457] 16.299

2 [347] 15.500

3 [345] 15.479

4 [147] 14.233

5 [145] 14.100

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phyllosilicates. Like original ETM bands, ratios can be

combined to produce an RGB colour image.

In Figure 4 the Landsat ETM bands 457 (RGB) and

ETM ratios 5/7, 5/4 and 3/1 (RGB) are juxtaposed for a

visual comparison. Figure 4a allows a general differ-

entiation between bedrock lithologies and alluvial

sediments which possess a smooth texture and cover

most of the southwestern part of the investigation area.

Massive sandstone units of the Roper Group are easily

detectable due to their homogeneous blue spectral

colour in Figure 4a. They form morphologically positive

features with a circular arrangement in the crater area.

Specifically the Abner Sandstone can be traced for

almost the whole circumference of the central uplift (1

in Figure 4a), whereas the older Limmen Sandstone

builds up a sequence of disrupted ridges in the north,

east and south of the crater centre (2 in Figure 4a).

Ridges of the Bessie Creek Sandstone are traceable in

the eastern part of Strangways (3 in Figure 4a) and north

of the central uplift area (4 in Figure 4a). However, the

spectral signatures of the Roper Group are too similar to

discriminate sandstone units from each other. Inter-

calated sediments of the Mainoru and Crawford

Formations display a low morphological relief and are

spectrally heterogeneous. Therefore, they are easy to

mistake for young alluvials. A contiguous area of

Antrim Plateau Volcanics in the north exhibits a

reddish colour with a smooth texture (5 in Figure 4a).

Post-impact Cretaceous sediments are readily detectable

in the image as dark plateaus with dendritic margins in

the southeastern crater centre (6 in Figure 4a). A

prominent feature is the Strangways Fault (7 in Figure

4a) northeast of the crater where bedrocks of the Roper

Group crop out over longer distances and build sharp

ridges. The fault is related to a system of normal, north-

trending faults associated with the opening of the

Proterozoic McArthur Basin pre-dating the impact

event.

In Figure 4b the decorrelation by ratioing leads to a

colour composite which is richer in contrast than

original ETM bands, but spectral signatures of Roper

Group sandstones remain too similar to be resolved in

more detail. The advantage of this image is that it is

easier to recognise the spatial distribution of impact

lithologies and other rock units as ratios are morpho-

logically subdued and spectral features become more

homogeneous as they are less influenced by sun

illumination.

As a net result, the multispectral Landsat ETM data

provide a useful basis to map the occurrence of impact-

related rocks of Strangways. However, a visual inter-

pretation of the crater lithologies is not possible without

geological background information derived from field

mapping, as their spectral characteristics in Landsat

ETM colour composites extracted on the basis of

statistical parameters strongly overlap.

Topographic data

For an improved visualisation of the present crater

morphology, a digital elevation model (DEM) was

derived from digitisation of the 1:100 000 topographic

maps (Gorrie map sheet 5567 and Mais map sheet 5667).

The DEM processing comprised the following steps: (i)

digitisation of height and drainage information in

separate layers; (ii) calculation of a continuous raster

dataset (GRID) of 10 x 10 m2 pixel size based on a method

after Hutchison (1993); (ii) due to the low relief the image

was exaggerated by a factor of 10 and artificially

illuminated from the eastern direction: and (iv) in order

to add more topographic detail the image was combined

with an ERS-2-SAR radar scene with a sensor illumina-

tion from eastern direction (corresponding to the

Figure 4 (a) RGB image of Landsat ETM single bands 457 (RGB) (b) Landsat ETM ratios 5/7, 5/4, 3/1 (RGB) of the Strangways

crater. 1. Abner Sandstone; 2, Limmen Sandstone; 3, 4, Bessie Creek Sandstone; 5, Antrim Plateau Volcanics; 6, Cretaceous

Mullamen beds; 7, Strangways Fault.

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artificial illumination of the DEM). The two datasets

were merged by a RGB-IHS-transformation of the DEM

and replacement of the intensity component with the

ERS-SAR image (Figure 5).

The most prominent morphological feature corre-

sponds to the occurrence of the Cretaceous Mullamen

beds forming a plateau in the southwestern centre of

Strangways with heights at about 180 m asl (1 in Figure

5). The oldest unit of the Roper Group (Limmen

Sandstone) is traceable as an arched ridge adjoining

the plateau to the east (2 in Figure 5). The Abner

Sandstone is the most distinct unit of the target

material, and builds up a coherent ring with heights

up to 140 m asl (3 in Figure 5). In the northern part the

ring ridge shows an arcuate course possibly indicating a

gentle folding of the sandstone (4 in Figure 5). A distinct

feature in outer parts of the crater is a major lineation

cutting through Abner Sandstone which might be a

remnant of the terraced rim area of the crater (5 in

Figure 5).

Airborne radiometrics (aeroradiometrics)

Radiometric (gamma ray) and magnetic data of the

investigation area were acquired within the area of an

airborne survey of the northern part of the Urapunga

SD53-10 1:250 000 map sheet in 1994. The survey was

flown along lines 500 m apart in the east – west direction

and 100 m above ground surface. In order to check

measurements, additional 5000 m tie lines in the north –

south direction were flown.

Using airborne radiometric spectrometers, the gam-

ma-ray emission of naturally occurring unstable

isotopes can be detected. Most diagnostically useful

gamma rays for geological mapping correspond to the

decay of 40K (1.46 MeV), 214Bi (1.765 MeV) from the 238U

decay series and 208Tl (2.614 MeV) from the 232Th decay

series. As the latter two indirect measurements of

daughter isotopes imply the presence of equilibrium

condition within the decay system, they are reported as

equivalent uranium (eU) and thorium (eTh), respec-

tively. Minty (1997) gave detailed information on

airborne gamma ray spectrometry, and Horsfall (1997)

summarised technological aspects of Australian air-

borne surveys. Because gamma rays can penetrate up to

30 cm of soil and rock, airborne radiometric data are

less affected by vegetation cover and weathering effects

of bedrocks.

Aeroradiometric point data of the investigation area

have been processed to a triangulated irregular network

dataset (TIN) with a spatial resolution of 90 x 90 m. The

data are displayed as a RGB image with K as red, eTh as

green and eU as the blue layer (Figure 6). In particular

the distribution of crystalline rocks in the centre of

Figure 5 Digital elevation mod-

el of the Strangways crater

with an exaggeration factor of

10 and an artificial illumina-

tion from the east. The digital

elevation model has been

merged with an ERS radar

scene. 1, Cretaceous Mullamen

beds; 2, Limmen Sandstone; 3,

Abner Sandstone; 4, arcuate

ridges of Abner sandstone pos-

sibly indicating gentle folding;

5, lineations cutting through

Abner sandstone (remnants of

terraced rim area).

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Strangways can be mapped due to their higher potas-

sium content (1 in Figure 6). Other potassium-rich units

include the Corcoran Formation with intercalations of

claystones (2 in Figure 6), post-impact Antrim Plateau

Volcanics north of the crater (3 in Figure 6) and a

dolerite sill east of Strangways (4 in Figure 6), which is

embedded in sandstone units of the Roper Group.

Massive sandstones (Abner, Limmen and Bessie Creek)

appear in blue or exhibit a low signal in all three bands

(5 in Figure 6). The Cretaceous Mullamen beds are

depleted in potassium and show a bright green to blue

radiometric signature (6 in Figure 6).

Airborne magnetics (aeromagnetics)

Magnetic anomalies related to impact events are

generally more variable than signatures from other

forms of remote sensing or gravity data. Shock mechan-

isms can either serve to reduce or remove existing

magnetisation of the target resulting in a ‘quiet

magnetic zone’ which can be discriminated from ‘noisy’

geological surroundings, or rocks may acquire a shock

remanent magnetisation. However, the dominant effect

of an impact event on magnetic properties is a magnetic

low or subdued zone, which does not necessarily reflect

a one-to-one correspondence with the present crater

morphology and structural style (Grieve & Pilkington

1996).

The aeromagnetic data of Strangways has been

processed by the Northern Territory Geological Survey.

Figure 7 shows the total magnetic intensity (TMI)

covering the investigation area. The most prominent

feature, a ring of high magnetisation, which can be

traced for more than 1808 round the circumference of the

crater on the north, east and southeast, corresponds to

the Abner Sandstone due to its higher content of

Figure 6 Aeroradiometric data of the Strangways crater. K,

eTh, and eU contents are displayed in red, green and blue,

respectively. 1, crystalline basement rocks; 2, Corcoran

Formation; 3, Antrim Plateau Volcanics; 4, dolerite sill near

the Strangways Fault; 5, Roper Group sediments with low

radiometric signal; 6, Cretaceous Mullamen beds.

Figure 7 Aeromagnetic data showing the total magnetic

intensity (TMI) of the Strangways area (image processing

by Northern Territory Geological Survey). 1, Abner Sand-

stone with high magnetisation; 2, northwest-striking ridges

of Abner Sandstone; 3, local maximum within the ‘quiet

magnetic zone’ of the crater centre; 4, north-trending

dolerite dykes cut by the impact structure; 5, Strangways

Fault bending to the southwest.

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magnetite (1 in Figure 7). In the western region the

anomaly is not detectable, which might be a result of a

thicker alluvial cover. Northwest-striking areas with

higher magnetisation are caused by Abner Sandstone or

Bessie Creek Sandstone (2 in Figure 7). The magnetisa-

tion in the inner part of the Strangways structure is

quite homogeneous with a local maximum located

slightly southwest of the centre (3 in Figure 7). Further

features include linear structures north of the crater (4

in Figure 7) which cannot be correlated with the

outcropping geology, but due to their high magnetisa-

tion and north-trending course are interpreted as

dolerite dykes and sills similar to those close to the

Strangways fault. Dating of the dolerites by McDougall

et al. (1965) indicated ages of 1280 and 1370+ 31 Ma. This

indirectly corroborates the age determination for

Strangways of Spray et al. (1999), as the impact structure

clearly cross-cuts the dyke swarm. The north – south-

trending Strangways Fault exhibits a generally high

magnetisation due to the emplacement of dolerite sills.

South of Strangways the fault bends to the southwest

indicating that it might have been affected by the impact

(5 in Figure 7).

DISCUSSION

Integration of all datasets into a GIS environment (e.g.

remote sensing data and their results, pre-existing

geological maps and field observations) allow a further

analysis. For example, the distribution of shock-

generated shatter cones observed in the field can be

used to estimate the original crater diameter Dfinal of

Strangways, as its extent roughly coincides with the

diameter of the transient cavity Dtc (Deutsch et al.

1995). Shatter-cone localities have been found within a

radius of 14.5 km (Figure 8). A simple relationship of

Dtc = 0.5 – 0.7 Dfinal leads to an original crater diameter

of 21 – 29 km. As the low end of the range is more or

less the present diameter of the crater comprising the

central uplift up to possible remnants of the terraced

rim area in the west, an original diameter between

26 km (as suggested by Spray et al. 1999) and 29 km

seems more likely, although there is considerable

uncertainty in these estimates. This estimated dia-

meter also coincides with early estimations of Spray et

al. (1999) and Shoemaker and Shoemaker (1988),

whereas the latter’s later assumption (Shoemaker &

Shoemaker 1996) that Strangways could be up to 40 km

across is hard to reconcile with the shatter-cone

distribution. Assuming an original diameter of 26 –

29 km and a pre-impact age of the Strangways Fault,

this fault might have been affected by the impact, as

implied by the bend to the south in the aeromagnetic

data.

The degree of structural uplift (SU) in the central

peak can be estimated using the relationship: SU = 0.086

D1.03 (Grieve & Pilkington 1996). Using a diameter of 26 –

29 km, this would imply an uplift of about 2.4 – 2.7 km in

the central zone of Strangways.

The field observations and interpretation of aero-

magnetic data support the impact age determinations of

Spray et al. (1999) as Neoproterozoic. Although highly

eroded, the Strangways crater has been protected at

least twice from erosion by younger cover rocks (the

Lower Cambrian Antrim Plateau Volcanics and the

Cretaceous Mullamen beds) during long periods of the

Phanerozoic.

ACKNOWLEDGEMENTS

We are grateful to the Northern Territory Geological

Survey for field support and for freely supplying

airborne radiometric and magnetic data. John Creasey

and John Gorter are thanked for the many useful

comments they made in their formal reviews. Further-

more, we would like to thank Andrew Glikson for his

helpful comments and discussion, which improved this

manuscript substantially. This study has been sup-

ported by the Deutsche Forschungsgemeinschaft under

the Graduiertenkolleg ‘Origin and Evolution of the

Solar System’ at the Institute of Planetology (Muenster,

Germany).

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Received 26 June 2004; accepted 26 April 2005

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