Investigation of Shuttle Radar Topography Mission data of the possible impact structure at Serra da Cangalha, Brazil

available online at http://meteoritics.org 237 © The Meteoritical Society, 2006. Printed in USA. Investigation of Shuttle Radar Topography Mission data of the possible impact structure at Serra da Cangalha, Brazil Wolf U. REIMOLD1*, Gordon R. J. COOPER1, Rafael ROMANO2, Duncan R. COWAN3, and Christian KOEBERL4 1Humboldt-Universit‰t zu Berlin, Museum f ̧r Naturkunde, Institut für Mineralogie, D-10115 Berlin, Germany 2Department of Geology, Federal University of Ouro Preto, Ouro Preto, Brazil 3Cowan Geodata Services, 12 Edna Road, Dalkeith, Western Australia, Australia 4Department of Geological Sciences, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria *Corresponding author. E-mail: [email protected] (Received 24 February 2005; revision accepted 18 September 2005) Abstract–The Serra da Cangalha crater structure in northeast Brazil, ∼13 km in diameter, has long been widely considered to be a confirmed impact structure, based on reports of shatter cone findings. Only very limited field work has been carried out at this crater structure. Landsat Thematic Mapper (TM) and Shuttle Radar Topography Mission (SRTM) data sets for the region around this crater structure are compared here with regard to their suitability to determine first-order structural detail of impact crater structures. The SRTM data provide very detailed information regarding drainage patterns and topography. A pronounced central ring of up to 300 m elevation above the surrounding area, two comparatively subdued intermediate rings of 6 and 10.5 km diameter, respectively, and the broad, complex crater rim of up to >100 m elevation can be distinguished in the Serra da Cangalha data. The maximum cratering-related regional deformation (radial and concentric features) seems to be limited to a radial distance of 16–18 km from the center of the structure. A first comparison of macrostructural information from several impact structures with that from Serra da Cangalha does not yield firm trends, but the database is still very small at this stage. The varied nature of the target geology strongly influences the development of structural features in any impact event.The Serra da Cangalha crater structure in northeast Brazil, ∼13 km in diameter, has long been widely considered to be a confirmed impact structure, based on reports of shatter cone findings. Only very limited field work has been carried out at this crater structure. Landsat Thematic Mapper (TM) and Shuttle Radar Topography Mission (SRTM) data sets for the region around this crater structure are compared here with regard to their suitability to determine first-order structural detail of impact crater structures. The SRTM data provide very detailed information regarding drainage patterns and topography. A pronounced central ring of up to 300 m elevation above the surrounding area, two comparatively subdued intermediate rings of 6 and 10.5 km diameter, respectively, and the broad, complex crater rim of up to >100 m elevation can be distinguished in the Serra da Cangalha data. The maximum cratering-related regional deformation (radial and concentric features) seems to be limited to a radial distance of 16–18 km from the center of the structure. A first comparison of macrostructural information from several impact structures with that from Serra da Cangalha does not yield firm trends, but the database is still very small at this stage. The varied nature of the target geology strongly influences the development of structural features in any impact event. INTRODUCTION AND BACKGROUND The Serra da Cangalha structure is centered at 8°05′S and 46°52′W in the extreme northeast of Tocantins state in Brazil (Fig. 1). Based on findings of shatter cones, it is widely held that Serra da Cangalha is indeed a confirmed impact structure (Dietz and French 1973; McHone 1979; Santos and McHone 1979; CrÛsta 1987, 2004), although bona fide microscopic evidence of shock metamorphism has never been reported (CrÛsta 2004). Past authors assigned a diameter of 12–13 km to the structure, which is comprised of several ring features. Most prominent of these is a 3 km (5 km, according to De Cicco and Zucolotto 2002) inner circular ring of mountains 250–300 m high that lends this structure an appearance similar to that of the Gosses Bluff structure of Australia, which has a 4.5 km inner ring and a subdued outer rim feature at 24 km diameter (Milton et al. 1996). Similar to Gosses Bluff’s inner ring structure and that of the Libyan impact crater Oasis (Koeberl et al. 2005a), the inner ring at Serra da Cangalha is thought to represent the differentially eroded remnant of a lithologically diverse central uplift feature formed from a layered sequence of target rocks. The Serra da Cangalha structure was formed in the intracratonic ParnaÌba basin (formerly known as the Maranhão basin). The basin stratigraphy involves Upper Silurian to Cretaceous sedimentary rocks. The geology of the region and the structure was reviewed by CrÛsta (1982, 1987, and references therein). The strata within the structure include upper Permian sandstones of the Pedra de Fogo Formation, Permian/Carboniferous sandstones of the 323–290 Ma PiauÌ Formation and the 354–323 Ma Poti Formation, as well as dark shales of the Upper Devonian Long· Formation. The structure is surrounded by tabular outliers (mesas) of Triassic sandstones of the SambaÌba Formation. In the center of the structure, these strata are intensely deformed and display vertical dips (CrÛsta 1982) (presumably on bedding surfaces). On the basis of stratigraphic drilling in the region by the Geological Survey of Brazil, the original stratigraphic depth of the strata now exposed in the center of the impact structure is estimated between 100 and 1300 m below surface, which 238 W. U. Reimold et al. gives us an estimate on uplift gradient. According to Adepelumi et al. (2003a; GÛes et al. 1993), the sedimentary rocks in the region showed a preferred NE-SW depositional direction (which we interpret to mean that there is a regional NE-SW-directed fabric). Serra da Cangalha was first proposed as a possible impact structure by Dietz and French (1973) because of the circular shape recognized in Landsat imagery (Fig. 2), the absence of volcanic rocks in drill core from the central part of the structure, and because it appeared unlikely that diapirism could account for the geometry of the structure (no carbonate or salt layers had been recognized in the sedimentary country rock stratigraphy). Dietz and French (1973) and McHone (1979) referred to shatter cones occurring on quartzite boulders of a conglomerate from the base of the Poti Formation observed in the inner ring structure. These authors also described intricate fracturing of quartz, as well as microspherules occurring in microscopic fractures; however, neither of these constitutes proof for the existence of an impact structure. CrÛsta (1987) referred to occurrence of shock metamorphic features in the form of “shock lamellae” and “breccia,” but did not provide further detail that could be used to confirm that these features represent bona fide shock deformation. The same author reported in 2004 that, to date, no definitive evidence of shock metamorphism has been reported for this structure (CrÛsta 2004). Consequently, with exception of the early reference to shatter cones, no evidence for impact has been reported from Serra da Cangalha. Consequently, we consider Serra da Cangalha as only a possible impact structure, still to be confirmed. No firm constraints for the age of the Serra da Cangalha structure have been obtained yet, either. Based only on stratigraphic considerations, a maximum age of 250 Ma can be estimated for the formation of the crater structure (i.e., the structure was formed in strata of Triassic or younger age). Some geophysical analysis of the structure and modeling of the data has been performed in recent years (Adepelumi et al. 2003a, 2003b, 2004, 2005a, 2005b). Interpretation of aeromagnetic data (Adepelumi et al. 2003b, 2004) indicated a diameter of 12.7 km for the Serra da Cangalha structure, and the magnetotelluric investigation by Adepelumi et al. (2004) resulted in the hypothesis that the likely impact-induced structural deformation below and in the environs of the crater did not exceed a depth of 2 km. A 2-D resistivity model (Adepelumi et al. 2005a, 2005b) suggested a 4-layer model for the structure: a thin resistive layer underlain by a conductive layer, weathered basement and, ultimately, resistive crystalline basement. The depth to the basement is estimated at 1.1 km. Three-dimensional forward modeling significantly reduced the basement resistivity, the effect of which was related by the authors to impact-induced brecciation, fracturing, alteration of the shock-deformed zone, and the presence of new low-magnetic materials and fluids (see also Abraham et al. 2004). Here, we use Shuttle Radar Topographic Mission (SRTM) data for the area of the Serra da Cangalha structure to Fig. 1. Location of the Serra da Cangalha crater structure in the Tocantins state of north-central Brazil. Investigation of Shuttle Radar Topography Mission data 239 investigate the morphology and geometry of the structure, as well as the regional influence exerted by this possible impact event. We also test the recently developed circular sunshading method (Cooper 2003) with this application. Finally, we set out to investigate whether or not the occurrence of and spacing between specific morphological features in impact structures follow a definite relationship. Several images generated from the SRTM data over the Serra da Cangalha area were recently shown by Almeido-Filho et al. (2005), though without any geological interpretation. DATA AND PROCESSING METHODOLOGY Shuttle Radar Topography Mission (SRTM) data for the region around the Serra da Cangalha structure were available for this study. Global SRTM single pass radar interferometry data (Farr and Kobrick 2000) were obtained by the STS-99 space shuttle mission between 11 and 22 February 2000. SRTM digital elevation model data have a horizontal resolution of 1 arc second (equivalent to 30 m at the equator) and a vertical resolution of 10 m, for the C-band radar. The United States Geological Survey (USGS) is the responsible data archiving agency; data were made available by NASA/ Jet Propulsion Laboratory. Global 3 arc second data have been released, whereas 1 arc second data are only available for North America. Initial comparison between 3 arc second SRTM and older GTOPO DEM (Global Topography 30 arc second Digital Elevation Model) of the USGS (Cowan and Cooper 2003) showed that the resolution of SRTM DEM is a significant improvement, and will be particularly valuable in areas for which limited topographic data are available. The Thematic Mapper (TM) Landsat data available for the Serra da Cangalha region (Fig. 2) provide a means for comparison with the SRTM data. In addition, several enhancement methods were applied in this study. Sunshading techniques, such as fractional order sunshading (Cooper and Cowan 2003a, 2003b) and a technique that enhances circular anomalies (Cooper 2003) were also employed. A standard filter used to enhance linear features in images is sunshading. It determines the reflectance from the data of a light source located at infinity. Linear features that lie parallel to the azimuth of the light source (the “sun”) are Fig. 2. Landsat image (bands 7-4-2 as RGB) over the Serra da Cangalha impact structure. The scene limits are 47°01′05′′W to 46°41′50′′W, and 12′21′′S to 7°57′19′′. In this and the following images, the top of the image is north. 240 W. U. Reimold et al. attenuated while those that lie orthogonal to it are enhanced (Horn 1982). Because sunshading is a form of high-pass filtering, it enhances both detail and noise in an image. The sunshading filter uses the first horizontal derivatives of the data, but if noise is a problem then lower-order derivatives may be used instead. These derivatives are of non-integer order and are best computed in the frequency domain (Cooper and Cowan 2003a). The results from three different sunshading operations, each of which uses derivatives of different order, may be combined to form an RGB image. High-frequency features then appear blue, while lower frequency features appear red (Cooper and Cowan 2003b). Impact structures are mostly near-circular; the sunshading filter was therefore modified to enhance features that lie either on or orthogonal to radial vectors that pass through a chosen origin position. This was achieved by making the sun azimuth a variable over the image, rather than a constant (Cooper 2003). As shown below, the circularity of this impact structure is strongly enhanced by this method. However, in the application of the circular sunshading technique to the search for further impact structures the user should be well aware that there are a range of other geological features that may have similar geometry, for example, ring structures caused by differential erosion above an intrusion, kimberlite pipes, or other volcanic features.