Mathematics – Logic
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.509g&link_type=abstract
Meteoritics, vol. 30, no. 5, page 509
Mathematics
Logic
1
Breccia, Craters, Impact, Impacts, Remote Sensing, Zhamanshin
Scientific paper
On Earth the impact record is preserved in the form of ~140 landforms [1], although current cratering flux estimates [2] suggest that hundreds of structures remain undiscovered on the terrestrial continents. A primary focus of our ongoing research efforts in this area has necessarily emphasized the geologically most recent impact events, especially those which formed in the last few million years. For example, we have comprehensively examined the orbital remote sensing characteristics of the Zhamanshin impact feature of Kazakhstan, a ~ 14 km diameter complex crater which apparently formed only ~870,000 years ago in a mixed sedimentary target [3]. In this case, we have been most fortunate to have available TM, SPOT Panchromatic (i.e, 10 m spatial resolution), SRL-1 and SRL-2 multiparameter SAR, and a ~ 90 m horizontal resolution DEM, along with excellent field data. The orbital multispectral data (TM) allowed us to discriminate the larger deposits of allogenic breccias at this youthful feature from erosionally emplaced surficial units, and a subtle signature of those areas covered with lag deposits of impact-related glass (zhamanshinites) was also identified [3,4]. As part of an ongoing collaboration with SRL scientists R. Greeley and D. Blumberg, we have also observed that L-band orbital SAR data clearly reveals the subtleties of the drainage networks that developed as a consequence of the cratering event, and which are apparently controlled by crater-related structures and deposit porosities [5]. When the geomorphically subtle Zhamanshin feature is compared against the Bosumtwi crater of Ghana, which apparently formed in crystalline shield rocks at around the same time (~ 1 million years ago), it appears that target rock properties have strongly influenced the level of preservation of these craters. Indeed, SPOT XS remote sensing data for Bosumtwi reveals a relatively pristine "lunar-like" complex crater with a raised rim, a quasi-polygonal outline, and a deep interior basin with inner rim wall terraces [3,4]. There are suggestions that geobotanical anomalies revealed by the multispectral data may be associated with surficial ejecta deposits. It is our opinion that quantitative remote sensing of the most recent complex cratering events in Earth history will help define "pathfinder algorithms" which could then be used as search criteria for as yet undiscovered impact landforms in such regions as Africa, South America, and even parts of Siberia. For instance, the recently proven Aorounga multiringed impact feature (13 km diameter, located in Chad, with a potential Cenozoic age [2]) was observed in TM data by several investigators (including us) and considered as a possible impact feature in spite of its close proximity to a major volcanic province (Tibesti) and its appearance similar to an eroded ring dyke. Table I highlights the datasets available, with future opportunities for exhaustive remote sensing analyses of additional features possible from RADARSAT SAR, EOS (MODIS, Aster, etc.), Landsat 7, and SPOT 4,5. The most significant lesson learned from our investigation is that terrestrial impact craters require more precise and comprehensive remote sensing analyses than those of nearby planets due to the efficacy of erosion on Earth, which serves to rapidly obscure or degrade "thin-skinned" landforms such as impact features. It is our observation that the combination of orbital SAR with multispectral imaging best defines impact-related deposits and structures. Furthermore, topographic data generated by means of orbital SAR interferometry (SRL, ERS-1, and Radarsat) and perhaps from orbital laser altimetry (e.g., Shuttle Laser Altimeter on STS-72 in 1995) will be of great value when attempting to reconstruct pre-erosional crater dimensions [3]. {We acknowledge the NASA Goddard Director's Discretionary Fund for initial support of this work, and Drs. G. Soffen and Klineberg}. References: [1] Grieve R. A. F. (1991) Meteoritics, 26, 175-194. [2] Grieve R. A. F. and E. M. Shoemaker (1994) in Hazards Due to Comets and Asteroids (T. Gehrels, ed.), 417-462, Univ. of Arizona, Tucson. [3] Garvin J. B. and C. C. Schnetzer (1994) GSA Spec. Pap. 293, 249-258. [4] Garvin J. B. et al. (1992) Tectonophys., 216, 45-62. [5] Greeley R. et al. (1995) Eos Trans. AGU, 76, S197. Table I shows the summary of satellite remote sensing data of impact structures.
Garvin James B.
Grieve Richard A. F.
Schnetzler Charles C.
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