Here, we report a previously unrecognized impactite from the Steen River impact structure in Alberta, Canada, which was intersected by continuous diamond drill core into the allochthonous proximal deposits of this buried 25-km-diameter complex crater. A suite of high-temperature minerals defines the matrix, formed by grain growth in a solid state by static recrystallization of an originally clastic matrix, deposited at temperatures ≥800 °C. This rock type is predominantly a result of the recrystallization of target material driven by the acceleration of hot gasses from volatilized sedimentary cover mixed with variably shocked crystalline basement. Approximately one-third of terrestrial impact structures occur in mixed target rocks; therefore, this type of impactite may be more common than previously realized. Contact metamorphism between entrained sedimentary target rocks and the juxtaposed hot matrix resulted in carbonate decomposition to form a rare spinel-group mineral, magnesioferrite. In crater environments, magnesioferrite has been found in the distal Chicxulub (Mexico) ejecta and may prove a novel indicator mineral for impact into carbonate-bearing target rocks.