Dark halo craters in grooved terrain on Ganymede represent potential probes of the subsurface geology. These craters are surrounded by a broad, diffuse, low albedo annulus, or halo. Halo radius is linearly proportional to crater radius, indicating that the halo is related to the continuous ejecta deposit of the crater and is not removed by natural aging processes, such as regolith development and exposure to the space environment. The halos have orange/violet albedo ratios that are intermediate between ratios associated with grooved terrain and cratered terrain. We propose that halos result from the incorporation of cratered terrain material into crater ejecta. In Uruk Sulcus, only craters greater than ~12 km in diameter have halos, indicating that the cratered terrain material there forms a stratigraphic horizon, buried at depth in the grooved terrain, into which only larger craters can excavate. This depth can be calculated from the transition diameter by reconstructing the transient crater. We calculate a depth of excavation, corresponding to the depth of burial of cratered terrain, of 1.0 to 1.6 km. This shallow burial is inconsistent with deep subsidence and burial, stoping into the interior, or lateral spreading apart of ancient cratered terrain units, but is consistent with a filled graben model for grooved terrain, where relatively clean ice has been emplaced over downdropped blocks of cratered terrain.