Geophysics and Petrophysics of the Popigai Impact Structure, Siberia

Abstract

The 100-km-diameter Popigai structure was formed 35.7 Ma ago in a target consisting of ~1–1.5 km of Proterozoic, Cambrian and Permian sedimentary rocks overlying Archean crystalline basement. The structure is characterized by a gravity anomaly low of −35 mGal amplitude, one of the largest magnitude gravity anomalies associated with a terrestrial impact structure. Superimposed on the gravity low is a concentric ring-shaped high, ~45 km in diameter, that coincides with uplifted Archean basement. Magnetic data indicate a −300 nT amplitude simple anomaly low over the structure. Both the gravity and magnetic signatures of Popigai differ from those of most other impact structures of comparable size, being distinguished by the lack of a central circular gravity high related to a central structural uplift or high-amplitude magnetic anomalies caused by thick melt/suevite deposits.

Two-dimensional forward modelling of an E-W profile through the crater is initialized using existing and recently-acquired petrophysical data and a structural cross-section based on geologic mapping and drill hole information. Petrophysical measurements indicate that lithologies making up the crater fill have susceptibilities nearly two orders of magnitude less than those determined for the basement. Crater fill densities are also considerably lower, with values up to 0.4 gcm⁻³ less than the surrounding target rocks. Gravity data modelling indicates the presence of low-density crater fill plus an extensive region of reduced-density fractured basement. The magnetic data also suggest a significant volume (down to ~5 km depth) of fractured basement below the true crater floor.