Geophysical signature of the Tunnunik impact structure, Northwest Territories, Canada

Abstract

In 2011, the discovery of shatter cones confirmed the 28 km diameter Tunnunik complex impact structure, Northwest Territories, Canada. This study presents the first results of ground-based electromagnetic, gravimetric, and magnetic surveys over this impact structure. Its central area is characterized by a~10 km wide negative gravity anomaly of about 3 mGal amplitude, roughly corresponding to the area of shatter cones, and associated with a positive magnetic field anomaly of~120 nT amplitude and 3 km wavelength. The latter correlates well with the location of the deepest uplifted strata, an impact-tilted Proterozoic dolomite layer of the Shaler Supergroup exposed near the center of the structure and intruded by dolerite dykes. Locally, electromagnetic field data unveil a conductive superficial formation which corresponds to an 80-100 m thick sand layer covering the impact structure. Based on the measurements of magnetic properties of rock samples, we model the source of the magnetic anomaly as the magnetic sediments of the Shaler Supergroup combined with a core of uplifted crystalline basement with enhanced magnetization. More classically, the low gravity signature is attributed to a reduction in density measured on the brecciated target rocks and to the isolated sand formations. However, the present-day fractured zone does not extend deeper than~1 km in our model, indicating a possible 1.5 km of erosion since the time of impact, about 430 Ma ago. PLAIN LANGUAGE SUMMARYThis study reveals the geophysical signature of the buried structure of an eroded impact crater, Tunnunik, located in Northwest Territories, Canada. A positive magnetic anomaly was detected at the center, showing the uplift of some deep geological formations and the possible presence of strongly magnetized basement. A negative gravimetric anomaly is also observed, mostly corresponding to the fracturing/brecciation of the impacted rocks inside the crater. Using numerical models constrained by laboratory measurements on rock samples, the physical properties and geometry of the buried geological formations are estimated. An important implication of this study is the link between the geophysical remains of an impact crater and the postimpact erosion. KEY POINTSA 3 mGal negative gravity anomaly and a 120 nT positive magnetic anomaly were measured over the center of the Tunnunik impact structure. A numerical model constrained by laboratory measurements on rock samples suggests an uplifted magnetic crystalline basement. The fracturing/brecciation extends down to 1 km in depth.

Field gamma ray spectrometry as a diagnostic tool for the assessment of hypothetical melt rocks from the Maâdna structure (Talemzane, Algeria): Implications for the crater formation process

“…Regardless of the origin of the circular depressions of potential impact origin, there also is a distinct increase in the application of existing geophysical methods to the study of known impact structures. These studies succeeded to provide an improved in-depth structural feature identification and characterization of the deep-seated anomalous sources (e.g., Gilder et al, 2018;Grieve & Pilkington, 1996;Morgan & Rebolledo-Vieyra, 2013;Pilkington & Grieve, 1992;Quesnel et al, 2013Quesnel et al, , 2020Zylberman et al, 2017). In some cases, these investigations were used in the evaluation of the potential economic mineral deposits (e.g., Reimold et al [2005] and references therein).…”