Lomonosov-Fleming is a 620 km, pre-Nectarian impact basin centered at 19¿N, 105¿E. Clementine multispectral images and a variety of spacecraft photographs were used to investigate the composition and origin of geologic units in the region. Mare basalt deposits in the Lomonosov-Fleming region are small and exhibit varying degrees of highlands contamination. Four previously unrecognized dark mantle deposits of probable pyroclastic origin were identified and mapped. A major expanse of cryptomare was mapped in and around the Lomonosov-Fleming basin. Spectral and chemical data obtained for dark-haloed craters (DHC) established that these impact craters excavated mare basalt from beneath higher-albedo highlands-rich surface units. The buried basalts exposed by DHCs in the region exhibit a variety of compositions, which range from very low titanium basalt to intermediate TiO2 mare basalt. Nectarian-aged units with subjacent cryptomare are much more extensive than Imbrian-aged deposits with underlying basalt. At least some of the buried mare basalts in the region may have been emplaced during pre-Nectarian time. While both impact basins and craters played a role in cryptomare formation by transporting highlands material to mare surfaces, the evidence indicates that most of the cryptomare inside the basin are of the distal basin ejecta type. The enhanced FeO and TiO2 abundances exhibited by these cryptomare surfaces, usually light plains units, demonstrate that ballistic erosion and sedimentation played an important role in their formation.