Ejecta Facies at Lonar Crater, India: Inventory of Expected Impactites

Large Meteorite Impacts and Planetary Evolution IV (2008) 3099.pdf

EJECTA FACIES AT LONAR CRATER, INDIA: INVENTORY OF EXPECTED IMPACTITES, THERMAL INFRARED SPECTROSCOPY, AND IMPLICATIONS FOR MARS Shawn Wright, School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-6305, [email protected]

Introduction: While a common goal of the study beads, and dumbbells [1,2] reportedly not found in the of terrestrial impact sites and their impactites is for lithic breccia unit. insight into the impact process, another goal is the ap- This DLE structure described for Lonar Crater is plication of these data and lessons learned to the un- similar, if not identical, to the “throw out” and “fall derstanding of the surfaces of other planetary bodies. out” layers observed at Meteor Crater [8] or the Bunte Here, the ejecta structure of one unique terrestrial Breccia and Suevite at Ries Crater [9], and suggests ejecta blanket is described in detail along with descrip- that two processes are responsible [10,11] for the bal- tions of impactite sample analyses and how these data listic emplacement of the lithic breccia moments be- can be applied current and future rovers of Mars and fore the “falling out” of the fall-out suevite layer. perhaps the Moon. Shoemaker [8] identified a suevite layer within Meteor Lonar Crater ejecta structure: Whereas field Crater, then later surmised that this unit likely used to data of any preserved terrestrial ejecta blanket is rele- be thicker in near-rim regions of the CEB, but has vant to understanding the impact process and products, eroded, as the fine matrix of this unit is easily trans- studies of Lonar Crater, India have implications for ported by the SW winds of the Colorado Plateau to Mars [1-4]. The target Deccan basalt provides an op- leave behind the clasts of the suevite layer as a lag [8]. portunity to examine terrestrial shocked basalt similar Whereas the lower, more weathered basalt flows to shergottites from Mars [4-5]. Further, Deccan ba- have not been identified as protoliths for Classes 2 salt has been labeled as an excellent analog for Surface through 5 in the suevite breccia, heavily fractured ba- Type 1, a thermal infrared spectral type identified from salt corresponding to these basalt flows is observed orbital and Rover observations [6]. Field geology at that are either unshocked or Class 1, implying that Lonar Crater (diameter = 1.8 km) reveals a DLE struc- these deeper strata are incorporated into the suevite ture with two distinct layers of ejecta [1,7] (Figure 1). layer. At Meteor Crater, the Coconino Sandstone The lower unit is lithic breccia extending to the limits serves as a lithologic tracer, as highly shocked lechat- of the continuous ejecta blanket (CEB), or 1.4 km (~1 lerite is distributed in the Meteor Crater fall-out layer ½ crater radii) from the crater rim and measuring ~8 m [8], suggesting material deep in the target sequence is at maximum thickness. The clasts in the lithic breccia incorporated into this unit. This provides data to are angular, highly fractured, and either unshocked “ground truth” modeling of Lonar Crater, Meteor Cra- (mineralogically) or Class 1 shocked basalt (0-20 GPa) ter, and similar small (1-2 km diameter) craters [12]. [1], which consists of fractured grains but no melting or mineral phase changes [1]. As shock pressures are typically 1-2 GPa near the crater rim, no intense shock Table 1. Inventory of Lonar ejecta layers and materials metamorphism has occurred. From comparisons to basalt flows exposed in the crater walls, the clasts Airfall fines? originated from both the oldest flows that have more (not found at Lonar or Meteor Crater but theorized secondary mineralization of groundmass and from the based on nuclear tests) youngest flows that lack this feature. This is attributed to the level of the pre-impact water table of the ~65 Suevite breccia Ma Deccan basalts [7]. The matrix consists of finely clasts: target rock subjected to all degrees of shock pulverized basalt. In theory, this unit grades into what metamorphism, including Class 2 through Class 5 would be overturned or inverted strata near the crater rim, but the crater has degraded since its initial diame- matrix: pulverized fines + melt spherules, beads ter (1.7 km, based on gravity surveys [2]), meaning that ~50 m of the original crater rim has eroded to con- melts: clasts of Class 5 plus spherules, beads in matrix tribute to the post-crater fill. The upper ejecta unit is a suevite breccia containing clasts shocked to all degrees Lithic Breccia: of shock pressure from unshocked up to Class 5 (> 80 clasts: unshocked + some Class 1 target rock subjected GPa) of Kieffer et al. [1]. The suevite layer measures to low (<< 10 GPa) pressures ~1 m in thickness and extends to ~0.5 km (~½ crater radii) from the rim. The matrix is finely pulverized matrix: pulverized fines basalt but with the addition of local glass spherules, melts: none Large Meteorite Impacts and Planetary Evolution IV (2008) 3099.pdf

Figure 1 (right). Structure of the Lonar Crater, India ejecta blanket. The figure represent a stratigraphic cross section viewed as a slice through the Lonar ejecta. The number on the clasts represents the class of shocked basalt [1], and thus the shock level, with “un” representing unshocked basalt.

Spectroscopy: The uniqueness of the Lonar target [5] Wright (2007) 7th Int. Conf. Mars, #3399 [6] Bandfield et rocks provides ideal samples to acquire the TIR spectra al. (2000) Science 287, 1626-1630 [7] Wright and Newsom of various degrees of shocked basalt. Due to detection (2008) GRL, submitted [8] Shoemaker (1963) Moon, Meteor- limit (~10%) of TIR spectroscopy and the overall scar- ites, & Comets 4, 301-335 [9] Hörz et al. (1983) Rev. Geophys. city of materials shocked to intermediate (~20 GPa) to Space Phys. 21, 1667-1725 [10] Barlow et al. (2000) J. high pressures (~1%) [13], these data are better applied Geophys. Res. 105, 26733-26738 [11] Komatsu et al. (2007) J. to current and future rover (field) data of individual Geophys. Res. 112, 10.1029/2006JE002787 [12] Wright rocks rather than the large pixels associated with orbital (2007), Bridg. Gap. II, #8061 [13] French (1998) Traces of TIR remote sensing. However, regardless of remote Catastrophe [14] Wright (2008) LPSC XXXIX, #2330 sensing, the amount of impactites found in the martian soil (either by sample return or in-situ analysis) has implications for the impact and geologic history of that regolith. TIR spectroscopy for each member of the list of impactites (Table 1) is shown as Figure 2. The more predominant type of impactite is pulverized basalt fines that make up the bulk of both ejecta units. Details on the spectroscopy of Class 2 (20-40 GPa) [14] and Class 5 impact melts [4] are described in previous conference proceedings. SEM and petrographic data will be shown. Acknowledgements: H.E. Newsom is thanked for discussions. P.R. Christensen provided financial support and guid- ance for early Lonar efforts. The Barringer Family Fund provided funding for a future trip to Lonar for more sample col- lection. A. Dube, K. Louzada, A.C. Maloof, J.R. Michalski, S. Misra, S.T. Stewart-Mukhopadhyay, and B.P. Weiss are

thanked for field discussions at Lonar. References: [1] Kieffer et al. (1976) Lun. Plan Sci. Conf. VII, Figure 3 (above). TIR spectra of various Lonar impactites 1391-1412 [2] Fudali et al. (1980) Moon & Planets 23, 493- Surface Type 1, an orbital TIR end-member and the labora- 515 [3] Stewart et al. (2005) Rol. Vol. Atm. Martian Impact tory spectrum of the Los Angeles shergottite are shown for Crater., #3045 [4] Wright et al. (2006) LPSC XXXVII, #1786 comparison. Spectra are offset for clarity.