DISTRIBUTION AND VOLUMES OF LAVA PONDS IN THE ORIENTALE REGION OF THE . Lisa R. Gaddis and James W. Head, Dept, of Geological Sciences, Brown University, Providence, R. I, 02912 Stratigraphic and volumetric studies of mare deposits on the Moon have generally been restricted to areas which display several stages of mare fil- ling (1-3). With the notable exception of the Imbrium flows (4), little work has been done on the volumes and extent of individual flows and small volcanic deposits isolated by nap-mare units. Informati on cn these small mare deposits or lava ponds could lead to a better understanding of minimum eruption volumes, the relati onship between these volumes and associated vent characteristics , and their eruption conditions. In addition, the distribution and mode of occurrence of small lava ponds may be useful in determining the nature of the early stages of volcanism in the highland crust. One approach to this problem is LO examine individual lava ponds in high- land areas adjacent to the major lunar maria. The purpose of this study is to present an analysis of such an area, between and southwestern Oceanus Procell arum, in order to further characterize erupti on vol umes and the relation of volcanism to the lunar crustal environment. Distribution of Lava Ponds - Lava ponds chosen for this study are con- centrated in an arc between Mare Orientale and southwestern (Figure 1). Of the thirty-three ponds analyzed, eleven are located within the mu1 ti-ringed basin of Orientale (nos. 1 - ll), ten occur predominantly in crater floors (nos. 12 - 21 ) , and the remaining twelve are inter-crater ponds (nos. 22 - 33). Soviet Zond 8 and Lunar Orbiter IV photos, and the U .S.G.S. map of the lunar west side (5) were used to identify the lava ponds. Lava ponds were distinguished on the basis of their relatively low albedo and surface smoothness. All ponds in the study area were included except those few which were specifically associated with young impact craters and could be identified as impact melt deposits (e.g. - Olbers A (6)). Volumes of the 33 lava flows were determined using planimetric measure- ments obtained from prepared maps of their areal extents and thickness esti- mates based on one or more of the following: 1) shadow measurements of flow scarp heights, supplemented by data on flow front thicknesses (7); 2) estimates of the thickness of lava surrounding and within partially buried craters (2), based on rim height data (8); 3) estimates of the relief of localized pre-existing topography; and 4) estimates of the roughness of the Orientale ejecta blanket using the topography of the Fra Mauro deposit as an analog. Locations and average volumes for each flow are presented in Table 1; volume ranges and details of deposits are presented elsewhere (9), --Discussion - Volumes - The range of volumes (Table 1) includes values comparable to small lunar maria such as Mare Spumens (4000 km3) and Undarum (7000 km3) (10) and strongly suggest that multiple flow sequences may be involved in the larger pond deposits, Largest volumes are found in Orientale basin ring ponds and on the larger crater floors. Smallest volumes occur predominantly in intercrater ponds and on smaller crater floors. All volumes are smaller than those associated with terrestri a1 plateau basalt accumul a- tions (120,000 - 700,000 km3) (15). Some fall in the range of the Iceland Laki 1783 flow (%I2 km3) (15), and the vast majority are less than the Roza member of the Columbia River plateau basalts (1500 - 2000 km3) (16). The preferential occurrence of large-vol ume ponds a1 ong basin ring structures and in large crater floors may be related to larger basin and crater-associated fracture systems, Large fissure widths strongly favor high effusion rates (1 1 ) and significant lava accumulations.

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Associated Volcanic Features - Sinuous rilles are the most prominent feature associated with the lava ponds and they occur predominantly with the most voluminous deposits. The majority of the Orientale basin ring ponds contain one or more sinuous rilles (12), some of which originate in tiearby uplands. The association of these rilles with large volume deposits may also be related to large fissure widths and high effusion rates (11). Geographic Distribution - Lava pond distribution near the western shores of Oceanus Procellarum and is distinctly parallel to this shoreline. Lava ponds fall off in density with increasing distance from this area and become concentrated in the northeastern quadrant of the Orientale basin (Fig. 1). No lava ponds have been observed westward of these deposits. This very distinctive distribution may be related to variation in lithospheric thickness from the Procellarum-Grimaldi region of anomalously thin 1 i thosphere (13,14) to the thicker lithosphere related to the Orientale region.

Table 1

Are? AV!;Q{. Area Av Vol (Km L Pond --- Locat,?.. ..(.w . Tim3) Orienta!e Bas!!! Ring. !'q?_ds 1) N of Orlentale 10n5, 97OW 1740 870 7) SE of 2) NW of Lacus Vnris---N 9"5, 9Z0W 165 20 8) Lacus Autunni---N 3 W 10"s. 91"W 1640 820 9) S 4 E llnS, R8"W 2560 1280 10) S of ---W 5) Lacus Veris 15"5, 86"W 11510 5755 11) E 6) SW of Lacus Veris 2I0S, 85"H

Crater Floor-P-onds 12) Schluter 55, 83"W 730 130 17) Grimaldi 13 S of Vasco Da Gama---NE llnN, 84"W 70 10 18) A 141 SW gnN, 84nW 155 20 19) Criiger 15) 4"N, 76"W 565 70 20) Schirkard---NW 16) 2"s. 74OW In60 '325 71) SE

lnlercrater Ponds

22) 159, 68"W 23) NE of Cruger 14"s. 64"W 24) E of Cruger 169, 64"W 25) SW of Byrgius---W 27'S, 67"W 26 E 27%, 67"W 27) 5E 27"s. 66"W

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in the bientale~ ~- ~ eaion of~ the.- Moon. ~. 101, DeHon.-. ~ R. A. and 6askbn.'~.-119761. PLSC 7. 2729. 11) Wilson'. L. and tlead, J. W. (1951) J.G.R., in press. 12) Gkeeley, R. (1976) ~LS_C1.. 274;. i5j

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