A NEW GEOLOGIC INTERPRETATION OF MONTURAQUI METEORITE IMPACT CRATER, CHILE

Monturaqui Crater is a small (~350 m diameter), simple meteorite impact crater located in the Atacama Desert of northern Chile. The target rock is Ordovician granite overlain by discontinuous Pliocene ignimbrite which is cut by numerous mafic dikes. The ejecta deposits are primarily granite and ignimbrite, with lesser amounts of dark impact melt and rare tektites and iron shale. The impact restructured existing drainage systems in the area that have subsequently eroded through the ejecta deposits. New mapping and field observations at Monturaqui highlight the distribution of ejecta around the crater. In particular, the new map distinguishes between ejecta types and target rock in much greater detail than previously observed. The crater rim is significantly eroded, with as much as 5 m of erosion over the past 663 kyr since the crater’s formation, much higher than erosion rates reported elsewhere in the Atacama Desert. Strike and dip measurements of planar features in the ignimbrite show evidence of structural overturning on the rim and suggest the fold hinge of the overturned rim sequence, located in the northwest quadrant of the crater, is preserved but currently not exposed. New samples collected for spatial and geochemical analysis include granite and ignimbrite target rock, and all four types of ejecta. Tektites are reported at Monturaqui for the first time. Dark impact melt is most heavily concentrated on the southeast side of the crater, but was also encountered for the first time on the southwest and northwest sides of the rim. The new observations show that Monturaqui is less well-preserved than satellite images would suggest, despite its relatively young age and arid location. In addition, the ejecta deposits are more heterogeneous than previous work suggested. This is likely a result of erosion over the past 663 kyr. A southeasterly impact direction was proposed based on previous finds of dark impact melt; however the new larger distribution of impact melt and iron shale may warrant a re-evaluation of the projectile’s impact direction to a more south-southeasterly direction.