Our civilization, and presumably any other industrial civilization on earth, utilizes huge quantities of steel reinforced concrete. While the structures may only last decades, the material would be recognizably unnatural for hundreds of millions of years. There is no natural process which would produce limestone with veins of high concentration iron ore running through, nonetheless produce it everywhere on earth in a huge variety of conditions. Further, all the deposits of this weird mineral combination would form at the same time, and this period would be a blink of an eye geologically speaking. While there may be theories other than industrial civilization put forward, but it would be obvious to everyone that something incredibly weird happened at the time that civilization existed, and detailed investigation would certainly find other technosignatures once they knew where to look.