Confederate dead were buried in shallow trenches when the battlefield was held by Federal soldiers. There are images made in Virginia of Confederate dead being buried in this way.

In the case of Chickamauga and Franklin, Confederates occupied the field afterwards, so there was some attempt to mark burials. Merkers were simple wooden slats, but usually without engraving. These deteriorated quickly, and information was lost.

For dead soldiers of both sides, it was very difficult to locate, identify and transport a body home. Your ancestor is quite likely buried at Confederate Cemetery in Marietta as an unknown.