MARE EMPLACEMENT IN THE ORIENTALE BASIN. R. Greeley, Univ. Santa Clara - NASA Ames Research Center, M/S 245-5, Moffett Field, CA 94035. A model has been proposed for the emplacement of mare basalts (I), based on the identification of surface features and comparisons with terrestrial analogs. At least two styles of volcanism are interpreted for the Moon: 1) flood basalt eruptions, involving high rates of eruption that produce relatively thick, ponded flows typically lacking lava tubes and channels (sinuous rilles), and 2) plains-basalt eruptions, involving lower rates of eruption that produce thin flows and flow units which are typically less than %30-m-thick and have tubes and channels. Based on this interpretation, most of Mare Smythii, M. Crisium, and M. Serenitatis were emplaced with basalts of flood- type eruptions, whereas parts of Mare Imbrium and Oceanus Procellarum were filled with lavas from plains-basalt eruptions. Although the flood- type basalts generally are early in age and the plains basalts late-- suggesting a possible evolutionary sequence-- each basin must be examined individually. Unfortunately, much of the early mare-filling history for the nearside impact basins is obscured. The Orientale Basin, however, is gener- ally considered a lltype" example for large impact basins, and perhaps affords the opportunity to observe a representative early filling stage. Several different mare units occur in the Orientale basin, four ofwhich are considered here : (1) Mare Orientale (restricted to the center of the basin), 2) Lacus Veris (between the basin center and Rook Mts. ) , 3) Lacus Autumni (between the Rook Mts. and the Cordillera Mts .), and 4) a high albedo mare- like unit that occurs predominantly on the margin of Mare Orientale (in part, the light plains unit of McCauley (2) and others). Suspected lava tubes and channels (sinuous rilles) have been mapped and crater-frequency distributions have been determined (3) for all craters 150 m in diameter and larger to prc- vide a basis of relative dating for each unit. The high albedo mare-like unit is the oldest of the four and has a crater frequency nearly the same as the crackled floor unit of McCauley (2), which may be impact melt (2, 4, 5, 6, 7). In some places the light mare-like unit appears to be gradational with the crackled floor unit and has one sinuous rille whose source appears to be within the crackled floor unit. A genetic relationship of the two units is tentatively suggested to explain the early filling of the basin: Impact melt and fallback formed the crackled floor unit (2, 3), melt drained from beneath crusted parts of the unit, leading to its cracked, draped and wrinkled appear- ance, and filled low-lying areas to produce the high albedo mare-like unit. In one case, drainage occurred through a lava tube-channel. An anomalously high frequency of craters %1 km in diameter in the crackled floor unit might be explained by the presence of collapse craters resulting from withdrawal of support as drainage took place. This stage of filling, which is psuedo- volcanic and has no known terrestrial counterpart, was followed by the emplacement of dark mare units (Mare Orientale) in the center of the basin. This unit is characterized by its mare ridges, irregular f lat-floored depres- sions, and general lack of fractures, obvious vents, and sinuous rilles. It is considered to be analogous to a flood basalt and is interpreted to have been emplaced rapidly by lavas from fissure eruptions that produced ponded 0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System MARE EMPLACEMENT IN ORIENTALE R. Greeley lava flows. blare ridges approximately follow the "shore line" of the basin and are interpreted as deformations of lava lake crust (8). Concurrent with (or slightly after) the filling of the center of the basin, basalts were erupted to produce Lacus Veris. This unit has the highest frequency of sinuous rilles, most of which originate in the Rook Mountains. The presence of sinuous rilles (lava tubes-channels) and lack of collapse depressions suggests that Lacus Veris was not "pondedl', but rather, is analogous to plains basalts and was emplaced by relatively thin flows that were tube fed. This interpretation is supported by the presence of numerous small (%lo to 15 km-diameter) shield volcanoes, typical of plains basalts. Lacus Auturnni has the lowest crater frequency of the four mare units and is considered youngest. Presence of sinuous rilles and small volcanic constructs suggests that, like Lacus Veris, it was emplaced with plains basalts. In some regions, small, closely-spaced knobs of pre-mare material form fields of kipukas, suggesting that the mare unit is very thin in contrast to Mare Orientale where the kipukas are widely separated, and where collapse depres- sions are more than 500 m deep. In conclusion, the Orientale basin has mare units representing three modes of emplacement: 1) an early impact-melt source, 2) flood basalt erupt- ions that filled the center of the basin, and 3) two stages of plains-type eruptions, one of which was nearly contemporaneous with the flood basalts. The apparent sources for the fttube-fed'fmare lavas typically are on the out- ward margins of the depressions that they fill, an arrangement that could reflect the large-scale basin fracture system. In addition, the location of the outer mare units and the arrangement of the vents could be related to activity in Oceanus Procellarum. Future work will involve comparisons of other basins with relations of rilles, types of eruptions, and ages, as observed for Mare Orientale. References Cited Greeley, R., 1975. Lunar Science VI, Pt. 1, p. 309. McCauley, J. F., 1968. Astrogeology 7, U. S. Geol. Survey. Greeley, R. and D. Gault, in preparatFon. Saunders, R. S., 1968. Astrogeology 7, U. S. Geol. Survey. Howard, K. A., 1974. Rev. Geophys. space Phys. -12, 309. Head, J. W., 1974. Moon 12, 299. Scott, D. H. and J. F. ~cGuley,in press. Geological Map of the West Limb Region of the Moon, U. S. Geol. Survey. Schultz, P., 1972. Ph.D. Thesis, Univ. Texas. 0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System .