Identification of impact cratering-related similarities based on airborne geophysical data and satellite images: the Cabeça de Sapo Structure, Parnaíba Basin, northeast Brazil

Abstract

This study was performed to identify and interpret airborne geophysical responses and landform features of the Cabeça de Sapo Structure (CSS), a 6 km-diameter circular structure located in northeastern Brazil, SW of the Maranhão State, suspected of having been formed by the impact of a solid body. The application of processing techniques addressed to enhance the images from the remotely sensed data (gamma-ray spectrometry, magnetic, Sentinel 2B and ALOS PALSAR) enabled the identification of two airborne geophysical responses (ring-shaped magnetic anomaly and radiometric patterns) and five landform features ("A", "B", "C","D" and "E"). The ring-shaped magnetic anomaly is related to a 5.4 km-diameter well-defined circular structure and the depth of its magnetic sources is estimated in 300 m. Those sources are unknown, but they probably occur due to layers of dolerite sills. The radiometric patterns identified seem to be related to the "A" and "D" landform features. Feature "A" is an elevated area at the center of CSS. The radiometric response (high eTh, eU, and low K) at the top of this area possibly reflects the weathering effect, while the radiometric response (high K and low eTh, eU) at the hillside is possibly from the lithotypes that have supported this elevation. Feature “D” is a 6 km – diameter circular elevated area that corresponds to the outer boundary of CSS. This boundary is characterized by increased eU counts, which may happen due to weathering-resistant mineral content (e.g. zircon) present in eolian sandstones. Features “C” and “E” are outside of CSS. The first is interpreted as plateau-shaped residual relief formed by differential erosion of basaltic rocks and sandstones and the second is interpreted as a flat-relief dominated by eolian sandstones. This flat-relief may represent the landscape that has not been affected by the formation process of CSS, while the affected internal region may be represented by feature “B”. The interpretations also have revealed that CSS shows some similarities with typical features of impact structures, namely: disruption of the magnetic field by a ring-shaped magnetic anomaly; central elevation surrounded by a raised outer rim; local control of drainage and; differences in the relief pattern of internal and external parts of the structure. Those similarities reinforce the possibility of an exogenous origin of CSS, but new studies are required to confirm or refuse such possibility.