Geophysics and Petrophysics of the Popigai Impact Structure, Siberia

Abstract

The 100-km-diameter Popigai structure was formed 35.7 Ma ago in a targetconsisting of similar to1 - 1.5 km of Proterozoic, Cambrian and Permiansedimentary rocks overlying Archean crystalline basement. The structureis characterized by a gravity anomaly low of -35 mGal amplitude, one ofthe largest magnitude gravity anomalies associated with a terrestrialimpact structure. Superimposed on the gravity low is a concentricring-shaped high, similar to45 km in diameter, that coincides withuplifted Archean basement. Magnetic data indicate a -300 nT amplitudesimple anomaly low over the structure. Both the gravity and magneticsignatures of Popigai differ from those of most other impact structuresof comparable size, being distinguished by the lack of a centralcircular gravity high related to a central structural uplift orhigh-amplitude magnetic anomalies caused by thick melt/suevite deposits.Two-dimensional forward modelling of an E-W profile through the crateris initialized using existing and recently-acquired petrophysical dataand a structural cross-section based on geologic mapping and drill holeinformation. Petrophysical measurements indicate that lithologies makingup the crater fill have susceptibilities nearly two orders of magnitudeless than those determined for the basement. Crater fill densities arealso considerably lower, with values up to 0.4 gcm(-3) less than thesurrounding target rocks. Gravity data modelling indicates the presenceof low-density crater fill plus an extensive region of reduced-densityfractured basement. The magnetic data also suggest a significant volume(down to similar to5 km depth) of fractured basement below the truecrater floor.