German

Contact: Heiko Zumsprekel, Geologisch Paläontologisches Institut, Corrensstr. 24, D-48149, Tel.: +49-(0)251-83-33908,  zumspre@uni-muenster.de

In Australia numerous impact structures of probable or known Precambrian age have been recognized by remote sensing and following field investigations during the last years. The good preservation of these ancient meteorite craters on the Australian continent is mainly due to two factors: the excellent exposition of moderately deformed and largely unmetamorphosed Proterozoic sedimentary rocks, and the fairly structural stability of the Australian craton throughout Earth's history. Besides, the identification of impact structures is favored by (semi)arid climate conditions in most parts of inner Australia.

Within the scope of this Phd project the Australian impact structures Strangways (N.T.) and Shoemaker (W.A.) were investigated using complementary remote sensing data, field data acquired between July and September 1999, and previous geological mapping data conducted by the regional geological surveys and E.M. and C.S. Shoemaker, respectively. With the construction of a comprehensive GIS database for both meteorite craters, the study focuses on a direct comparison of the impact signatures on a micro- (shock metamorphism), meso- (shatter cones, impactites) and macro-scale (satellite and airborne imagery). Both craters are comparable regarding (i) target material consisting of siliciclastic sediments on top of crystalline basement rocks (ii) their present diameter of about 20 km (Strangways) and 31 km (Shoemaker) and (iii) their probably Proterozoic age.
Strangways (646 +/- 42 Ma) consists of sedimentary rocks of the Proterozoic Roper Group which form a collar around a core of granites and gneisses. During the field work shatter cones have been found in the granitoid basement and the lower part of the Roper Group (Limmen Sandstone). Remnants of melt-bearing impactites are limited to a few outcrops in the inner part of the structure. Impact breccias dominated either by granitoid or sedimentary fragments are distributed in the N‘ and E‘ part of the core. Further features diagnostic for an impact event are PDFs found in quartz and feldspar of the granitoid basement. In the Shoemaker Structure (1630 Ma ?) impactites are totally removed as the structure is eroded below its previous crater floor. The granitoid core is surrounded by sedimentary rocks of the Proterozoic Earaheedy Group which are dropped down in a ring syncline. Indistinct shatter cones are developed in granitoid rocks and the lower stratigraphic units of the Earaheedy Group (Yelma and Frere Formation), PDFs occur in granites and syenites in the N‘ core. Outcrops of upper parts of the Earaheedy Group and dipping directions indicate that the level of erosion decreases in NE‘ direction.

ASTER image of the Shoemaker Impact Structure. The crater is easily detectable by the circular arrangement of Proterozoic granular iron formations which appear dark in the image. (Acquisition date 04Nov2000, bands: 3, 2, 1(rgb), coverage ca. 63.5 x 61 km).	ERS 2 radar image of Strangways combined with DEM (violet: low topography to yellow: high topography). The crater can be traced by the circular arrangement of Proterozoic sandstones. The yellow elevated plateaus in the center consist of post-impact Cretaceous sandstones (coverage ca. 40 x 40 km.).

After the GIS integration of topographic and geological maps, multispectral Landsat 5 TM data have been processed in order to obtain highly decorrelated RGB-colour-composites for visual interpretation and computer-supported classification. Among the statistical analysis of original TM bands the use of standard image processing methods like ratioing and principal components transformation has been evaluated in order to enhance the discrimination between target materials, impactites (Strangways) and post-impact lithologies. Especially principal components of higher order lead to a better differentiation of Proterozoic sedimentary rocks of the Roper and Earaheedy Group, respectively. Because of the limited spatial (30 x 30 m) and spectral resolution and interferences between the spectral characteristics of rock types, weathering products and vegetation types, the results of computer-supported unsupervised and supervised classifications of Landsat TM data of the Shoemaker Structure are not satisfactory. The object-oriented classification method represents a more flexible way and allows the integration of spatial and textural decision rules in the classification process which can lead to a slight improvement of the classification results compared with pixel-based methods.
Complementary ERS-SAR radar data have been processed which allow the analysis of the crater morphologies. The data can be used to add detailed textural information to digital elevation models by merging methods. Aeroradiometric data represent powerful additional information which are less influenced by morphological parameters, surficial deposits and vegetation effects. As the processing of aeromagnetic data differs significantly from optical remote sensing data, pre-processed data of the craters supplied by the Australian Geological Survey Organisation have been used in this case. The aeromagnetic data allow careful temporal correlations between the intrusion of dykes and the impact events which correlate with geochronological dating of the impact structures.
Rock distributions and lineations have been mapped on the basis of a visual interpretation scheme considering all remote sensing datasets and have been compared with field observations. In order to reconstruct original crater diameters, the spatial distribution of shatter cones has been used leading to an original crater diameter of 21 to 29 km in case of Strangways and < 42 km in case of the Shoemaker Structure. Among causal connection between the datasets, the GIS database allows the spatial integration of field observations and an adequate 2,5-D-visualisation of the crater morphologies.
Regarding further investigations on the Strangways and Shoemaker Structure the integration of hyperspectral and additional seismic data is suggested. As the two study cases show, a GIS approach with its central organisation of impact data can also be successfully applied for the investigation of other terrestrial impact structures.

Download the full thesis (in German) as PDF (16 MByte). Send an email for obtaining a CD-Rom with the GIS data in ArcView/ArcExplorer.