Seismic Investigations at Barringer Crater, Arizona

Introduction: We have investigated the structurally complex and highly fractured near-surface of the Barringer (aka Meteor) Crater, Arizona. The Barringer Crater is situated near Winslow, Arizona, and is one of the best preserved impact craters in the world. The crater was excavated some 49,000 years ago by the impact of an iron-nickel meteorite which hit the preexisting Colorado Plateau with a velocity of 11-20 km/s. Detailed structural geological studies had been performed at this unique site for decades. Though, several questions still remain unanswered about the structure: its asymmetry, depths and orientation of fractures, thickness of the ejecta blanket (a sheet of debris thrown out of the crater during the impact), and near-surface high-resolution seismic velocity structure (especially Swave). To unravel some of these mysteries, the University of Houston, the University of Texas (Austin), and the LPI led a joint expedition at the crater site in May, 2010. Different geophysical measurements performed during this expedition included seismic, gravity, magnetic, ground penetrating radar (GPR) and ultrasonic measurements. In this paper, we provide results related to seismic experiments along with some ultrasonic measurements. The primary goals of this paper are : a) to image the ejecta blanket thickness, and b) to obtain the near-surface seismic velocity structure. Geological Settings and Physical Properties: The present day stratigraphy at the Barringer Crater consists of the following sequences – white Coconino sandstone formation overlain by the very thin Toroweap (sandstone) formation, followed by the yellowish Kaibab (dolomite and minor sandstone) formation, and then the red Moenkopi (calcareous siltstone with ironrich matrix) formation. The impact overturned and inverted the layers at the crater rim and the overturned sequences extended to a distance of one to two kilometers outward from the crater's edge. So, the entire sequence from the top to bottom is now – the overturned sequence (Coconino-Kaibab-Moenkopi) underlain by bed-rock Moenkopi-Kaibab-Coconino. The whole overturned sequence package is termed as the ‘ejecta blanket’. The ejecta blanket consists of the overturned sequence along with the alluvium derived from those overturened sequences and fragments of meteorites [2]. Some early studies ([5], [1], [4]) were performed to characterize the physical properties of the Barringer Crater units. Early studies showed that the ejecta blanket tapers away from the crater rim and it has the highest thickness in the southern flank of the crater. The near-surface of the crater is unconsolidated and of lower dry bulk density. Hence, low near-surface velocity is expected. The thickness of the ejecta blanket varies from 0-26 m and the dry bulk densities vary between 1.87-2.17 gm/cc. The average thickness of the underlying bed-rock Moenkopi is 12 m with dry bulk densities ranging between 2.19-2.48 gm/cc. Seismic Surveys: One set of seismic experiments (Radial and Cross lines in Figure 1) were performed using a 10 lb sledgehammer as the source and another set of experiments were performed using a truckmounted Accelerated Weight Drop (AWD line in Figure 1) as the source. We have used and analyzed seismic data sets related to both types of sources, but only vertical geophones as receivers. The hammer-seismic lines are of smaller lengths (66 m) and the AWD line is of longer length (645 m).