Using THEMIS, MOC and MOLA data, we have found 51 craters in the diameter range 6--11.8 km within southwestern Utopia Planitia and Isidis Planitia that are significantly deeper than typical fresh craters in the northern lowlands of Mars. The restricted geographic distribution of these craters, their simple morphology, and data from impact and explosion crater studies suggest that unusually strong target materials (as much as a factor of 2 greater than average materials in the Martian lowlands) are the cause of the excessive crater depth. We propose that the greater target material strength acts to delay gravity-dominated collapse to larger crater sizes. Furthermore, we suggest that a regional, olivine-rich mafic to untramafic rock unit identified by TES and THEMIS is a reasonable candidate for these strong materials. The unit is exposed on the southern edge of Isidis basin and in crater ejecta within the basin, and forms layers that dip toward the Isidis Basin center.