Published onlineDownloaded September from gsabulletin.gsapubs.org 25, 2009; doi:10.1130/B26474.1 on 26 September 2009 Geology of Lonar Crater, India Adam C. Maloof1, Sarah T. Stewart2, Benjamin P. Weiss3, Samuel A. Soule4, Nicholas L. Swanson-Hysell1, Karin L. Louzada2, Ian Garrick-Bethell3, and Pascale M. Poussart1 1Department of Geosciences, Princeton University, Guyot Hall, Washington Road, Princeton, New Jersey 08544, USA 2Department of Earth and Planetary Sciences, 20 Oxford Street, Cambridge, Massachusetts 02138, USA 3Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA 4Woods Hole Oceanographic Institution, Geology and Geophysics, Woods Hole, Massachusetts 02543, USA ABSTRACT of the ejecta. The ejecta profi le is thickened tion. Around large (gravity-dominated) craters, at the distal edge and similar to fl uidized the shock deformation signifi cantly weakens Lonar Crater, India, is one of the young- ejecta structures observed on Mars. the rock mass, and the fl uid-like collapse of the est and best preserved impact structures on transient crater leads to signifi cant widening and Earth. The 1.88-km-diameter simple crater INTRODUCTION shallowing of the cavity and formation of cen- formed entirely within the Deccan traps, tral peak structures (Melosh, 1989; Melosh and making it a useful analogue for small craters Motivation Ivanov, 1999; Kenkmann, 2002). on the basaltic surfaces of the other terres- The details of how rocks respond to the high trial planets and the Moon. In this study, we Impact cratering is a dominant surface modi- stresses and strain-rates associated with impact present a meter-scale–resolution digital ele- fi cation process in the solar system, yet aspects cratering are still poorly understood (Herrick vation model, geological map of Lonar Crater of cratering mechanics remain poorly under- and Pierazzo, 2003). Laboratory-based stud- and the surrounding area, and radiocarbon stood. Information about high strain-rate rock ies have provided useful insights into the prin- ages for histosols beneath the distal ejecta. deformation and ejecta emplacement processes cipal styles of fracturing beneath and around Impact-related deformation of the target are recorded in the geology of impact structures. small (several-cm–scale) crater cavities (e.g., rock consists of upturned basalt fl ows in the However, due to the high erosion rates on Earth, Polanskey and Ahrens, 1990; Ai and Ahrens, upper crater walls and recumbent folding few craters have retained a complete record of 2006), and seismic studies have estimated the around rim concentric, subhorizontal, non- the cratering process. Lonar Crater, India, is a depths of fracture zones (summarized in Ahrens cylindrical fold axes at the crater rim. The young, well-preserved simple crater formed in et al., 2002). Three sets of fractures (conical, rim-fold hinge is preserved around 10%– the Deccan trap basalts, making it a rare ana- radial, and concentric) have been observed 15% of the crater. Although tearing in the log for impact structures observed on the ba- around strength-dominated craters in the lab- rim-fold is inferred from fi eld and paleomag- saltic surfaces of other terrestrial planets and ora tory and in the fi eld. Structural deformation netic observations, no tear faults are identi- the Moon. The present study focuses on geo- around simple impact craters is characterized by fi ed, indicating that large displacements logic mapping of Lonar Crater (Fig. 1), includ- (1) brecciation (of ejected and displaced mate- in the crater walls are not characteristic of ing the structural deformation around the rim rials), (2) conical and radial fractures, (3) fold- small craters in basalt. One signifi cant nor- and the physical properties of the ejecta blanket. ing and tearing in the crater rim, (4) uplifted mal fault structure is observed in the crater When a bolide impacts a planetary surface at strata, and (5) listric faulting and slumping of wall that offsets slightly older layer-parallel hypervelocities, a shock wave propagates both the crater wall (e.g., Shoemaker, 1960; Brandt slip faults. There is little fl uvial erosion of down into the surface and up into the projectile, and Reimold, 1995; Kumar, 2005; Kumar and the continuous ejecta blanket. Portions of the compressing both materials and slowing the Kring, 2008). However, target lithologies and ejecta blanket are overlain by aerodynami- projectile. A rarefaction wave from the rear of preexisting structural features infl uence the cally and rotationally sculpted glassy impact the projectile and the surrounding free surface generation and activation of fractures during spherules, in particular in the eastern and overtakes the shock wave leading to a decaying impact cratering (Kumar and Kring, 2008), western rim, as well as in the depression hemispherical shock pulse. The decaying shock which complicates the generalization of impact- north of the crater known as Little Lonar. pulse generates an excavation fl ow fi eld that induced deformation processes from the limited The emplacement of the continuous ejecta moves material fi rst downward, then up and out- terrestrial data. More detailed geologic studies blanket can be likened to a radial ground- ward, creating a hemispherical transient crater. of terrestrial craters are needed to improve our hugging debris fl ow, based on the preserved The motion is accommodated by brecciation and understanding of cratering mechanics. thickness distribution of the ejecta, the effi - deep fracturing in the target rocks and folding Much of our knowledge of impact ejection cient exchange of clasts between the ejecta in the crater rim. In small (strength-dominated) processes is derived from laboratory experi- fl ow and the underlying histosol, and the craters, the excavation fl ow is impeded by the ments and explosion craters (e.g., Roddy et al., lack of sorting and stratifi cation in the bulk strength of the crater wall rock, and the transient 1977) and observations of lunar craters (e.g., cavity is widened and shallowed by slumping of Pike, 1976). The principal aspects of the forma- †E-mail: [email protected] the crater walls in the end stage of crater forma- tion of continuous ejecta structures are captured GSA Bulletin; January/February 2010; v. 122; no. 1/2; p. 109–126; doi: 10.1130/B26474.1; 12 fi gures; Data Repository item 2009131. For permission to copy, contact [email protected] 109 © 2009 Geological Society of America Downloaded from gsabulletin.gsapubs.org on 26 September 2009 Maloof et al. 68 72 76 80 24 A AURANGABAD 20 Telecom Pits Little Lonar N. Quarry S. MUMBAI LONAR CRATER B 16 Lonar 12 North Durga Devi Dhar DECCAN TRAP Fault FLOOD BASALTS Hotel 8 m Lonar Lake 0 1000 5000 –6000 –3000 Kalapani Lake Figure 1. (A) EToPo2 (http://www.ngdc.noaa.gov/mgg/fl iers/01mgg04.html) topography of India (color scale is in meters, coordinates in latitude and longitude), showing the extent of the Deccan Plateau (Deshmukh, 1988; Bondre et al., 2004) and the location of Lonar Crater; (B) Four-band, pan-sharpened, true-color Quickbird satellite image of Lonar Crater, draped on the digital elevation model of Figure 2. The locations of measured stratigraphic sections from Figure 3 are marked with yellow arrows. The crater rim diameter is 1.88 km. by the ballistic erosion and sedimentation model dynamics that are still poorly understood. At the 2006). In addition, distinct ejecta layers (e.g., a (Oberbeck, 1975). In this model, an inclined cur- 1.2-km-diameter Barringer Crater (a.k.a. Coon ballistic layer overlain by a suevitic layer) have tain of ballistic ejecta impacts the surrounding Butte and Meteor crater), Arizona, the continu- been identifi ed at Ries (Osinski et al., 2004) and terrain with increasing velocity from the crater ous ejecta blanket contains distal lobes (Grant Chicxulub (Wittmann et al., 2007). rim outward. The thickness of the ballistic ejecta and Schultz, 1993) suggesting that a ground- The record of ejecta processes is even more decreases with distance by about a –3 power law hugging fl ow modifi ed an original power-law diverse on Mars. Martian ejecta morphologies (McGetchin et al., 1973). Around small craters, profi le around a small simple crater. The ejecta have been described as layered or fl uidized the fi nal ejecta blanket closely resembles the blankets around the larger Ries (24 km) and (Barlow et al., 2000; Barlow, 2005). Fluidized ballistic ejecta distribution. However, around Chicxulub (~170 km) impact structures record ejecta are found around all fresh, and many larger craters, the ballistic ejecta impact with even larger ground-hugging fl ows. At Ries, the older, craters larger than a few km on Mars suffi cient velocity to generate secondary craters continuous ejecta deposit is characterized by a (Barlow, 2005). They are characterized by that excavate the surrounding surface materials. large volume fraction of secondary materials lobate profi les distinct from lunar power-law The surface materials are mixed with the pri- and ground-hugging debris surge (Hörz et al., ejecta thickness profi les, longer runout distances mary ejecta with enough outward momentum 1983) in agreement with the ballistic sedimen- than lunar craters, and sinuous and continuous to generate a radial ground-hugging debris fl ow tation model. The extent of the debris surge is terminal ramparts (Carr et al., 1977; Mouginis- (of unpredicted travel distance). Hence, the fi nal surprising, however, with clasts found as far as Mark, 1978; Barlow and Bradley, 1990). These continuous deposits around large craters are 10 km from their original location. At Ries, the observations suggest that similar processes may composed of an increasing volume fraction of large runout of ejecta fl ows has been attributed affect ejecta blanket emplacement on Earth and secondary materials with radial distance and a to decoupling of near-surface target material fol- Mars, which are distinct from the ballistic sedi- fi nal thickness profi le that is shallower than the lowed by dragging of the ejecta curtain along mentation model for the Moon and Mercury.