GEOLOGY OF THE COMPLEX IMPACT STRUCTURE SERRA DA CANGALHA, BRAZIL

Serra da Cangalha is a complex impact structure with a crater diameter of 13,700 m and a central uplift diameter of 5,800 m, situated at the border between Tocantins and Maranhao states of Brazil (centered at 8°05’S/46°52’W). New findings of shatter cones, planar fractures, feather features, and possible planar deformation features are presented. Several ring-like features that are visible on remote sensing imagery are caused by selective erosion of tilted strata. The target at Serra da Cangalha is composed of Devonian to Permian sedimentary rocks, mainly sandstones that are interlayered with siltstone and claystones. NNE-SSW and WNW-ESE striking joint sets were present prior to the impact and also overprinted the structure after its formation. As preferred zones of weakness, these joint sets partly controlled the shape of the outer perimeter of the structure and, in particular, affected the deformation within the central uplift. Joints in radial orientation to the impact center did not undergo a change in orientation during tilting of strata when the central uplift formed. These planes were used as major displacement zones. The asymmetry of the central uplift, with preferred overturning of strata in the northern to western sector, may suggest a moderately oblique impact from a southerly direction. Buckle folding of tilted strata, as well as strata overturning, indicates that the central uplift became gravitationally unstable at the end of crater formation. We roughly estimate ~500 m of erosion since the time of impact. This estimate is based on the preserved shock features in the central uplift that suggest peak shock pressures of not more than 10 GPa, and the apparent lack of allochthonous breccias fill in the crater moat. If the estimate of erosion is correct, the original crater diameter would have been 14-14.5 km.