Geophysical Evidence Of A Possible Impact Structure At The K-T Boundary Of The Solimões Basin, Brazil

Abstract

The Tefe River possible impact structure (4o57’’44’’S, 66o03’17’’W) was formed in the Late Cretaceous of Solimoes Basin, Brazil. The target rocks are Cretaceous continental sandstones underlain by Paleozoic siliciclastic, carbonate and evaporitic rocks, which were intruded by thick diabase sills in the Mesozoic. The overburden is a 350 m-thick Tertiary (Miocene to Pliocene) sequence. A 2D reflection seismic lines dataset allowed the identification of a complex-type impact structure with a 2 km in diameter central uplift which consists of a chaotic arrangement of thrust faults, moderate- to steep-dipping beds and with slump-slided borders. A 5 km in diameter ring depression forms a syncline or graben around the central high, which was probably produced by normal listric faulting and by rotational block movement associated to the uplift of the central high. The outer rim is eroded by the Tertiary lower sequence boundary and its original diameter could reach more than 15 km. The infill of the crater is probably composed of slumped and fluidized Cretaceous sandstones, suggesting the presence of high water level and unconsolidated sediments, thus showing a very contrasting mechanical behavior in relation to Paleozoic rocks and diabase sills, which collapsed at steeper fault angles. Modelling of ground gravimetry and magnetometry measurements supports the presented structural interpretation, both leading to the hypothesis of a low-angle northeast-to-southwest bolide trajectory. As the structure remains undrilled, impact origin confirmation by means of shock-metamorphic features identification is to be done, as well accurate dating. Available dating based mostly on palinomorphs indicate a tens-of-million-years hiatus between Cretaceous and Tertiary sequences.