RELATIVE AGES OF MARTIAN VOLCANIC CENTERS, CHANNELS, PLATEAU DEPOSITS AND UPLAND TERRAINS BASED ON VIKING DATA AND COMPARISON UNDER CRATER COUNTS ON THE MOON AND MERCURY. Harold Masursky, A. L. Dial and M. E. Strobe11, U. S. Geological Survey, Flagstaff, Az. 86001 . Recent high resolution Viking pictures have allowed us to make more detailed crater counts of Martian volcanic edifices and flows, channels and up1 and terrain units from which relative age determinations for individual surfaces can be inferred. We discuss relative ages, based on curves derived from crater densities; we have not converted the relative ages to ages based on crater flux curves for Mars (1) (2). Volcanic flow units vary from ancient intensely cratered surfaces that show an abundance of large craters to those that display such a sparse density of small craters that the surfaces are almost undatable by the crater counting technique (Fig. l).Volcanic activity appears to have occurred over a long period of time; relative ages, based on crater density curves, range from that shown by the curve for old cratered plains-"PC Mc"22 (Fig. 1) to the curve for the caldera of Olympus Mons (Fig. 2). Relative ages for channel information appear to be more restricted in time; that is the spread in crater densities is less than those volcanic units. However, statistics for the channel curves are less certain due to 1imitations set by channel area and crater numbers (3). It is not yet pos- sibl e to correlate fl uvial with volcanic episodes in order to determine whether a casual relationship exists between the two processes; that is, we cannot determine whether an episode of intense volcanism triggers a fluvial episode. Studies made of the ancient cratered terrain indicate that the inter- crater areas are comprised of lava flows extruded from several centers at several different times (Fig. 1). We have not yet located a Martian surface where the entire crater density curve indicates a relative age as old as that shown for the lunar highlands surface near Tsiolkovsky and Gagarin craters (Fig. 2). However, the large craters (not shown on the crater density curve) for an area in Terra Sabaea, a part of the Martian uplands northwest of Hellas Planitia in MC 20 (Fig. 2), falls on the lunar highlands curve. These large craters represent the ancient Martian surface protruding through the younger mantl ing lava flows. The lunar upland surface near Tsiol kovsky and Gagarin, and the ancient martian terrain that underlies the present surface at Terra Sabaea appear to be equivalent in age with similar crater densities. Crater density curves for Phobos and Deimos (Fig. 2) fall on the extension of the lunar highlands curve. The complete crater density curve for Terra Sabaea (Fig. 2) and other areas in the central uplands (Fig. 1) are displaced to the left of the lunar upland curve, showing the surfaces to be younger, because the counts were made on the mantling lava flows. Crater density curves for Mercury are also displaced from the lunar curve; relative displacement is the same as that shown by the curve for heavily cratered highland material on Mars. Mercury is also thought to show evidence of a mantling episode that has covered parts of its original surface (4) Radar altimetric, and photogrammetric measurements of layered deposits exposed in mesas on the floors of Candor and Hebes Chasmata indicate these deposits are about 1 km thick and the deposits in the canyon walls are as much as 7 km in thickness. Relative ages for the layered deposits that form the mesas and canyon walls cannot be obtained by the crater count technique because few, if any, craters are preserved in these canyon areas. However,

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H. Masursky et al. these rocks must be intermediate in age; they lie between the ancient crater- ed terrain material exposed to the southeast and the young lavas that comprise the Tharsis volcanoes that overlie the plateau deposits. These layered rocks may preserve the sequential record for a considerable portion of martian history, like that preserved in the terrestrial Grand Canyon sequence. Radiometric dating of material from these 1 ayered deposits, from possible weathered zones lying between the layers, and from volcanic deposits obtained by a Mars sample return mission, could be correlated with relative ages obtained from crater density curves. The weathered zones may record fluviatile episodes when channels formed in other areas and would also preserve a record of variations in solar activity, References: (1) Soderblom, L.A., West, R.A., Herman, B.M., Kreidler, T. J. and Condit, C.D. (1974) Icarus, v. 22, no. 3, p. 239-263. (2) Neukum, Gerhard and Wise, D.U. (1976))nce, v. 194, no. 4272, p. 1381-1387. (3) Masursky, Harold, Boyce, J.M., Dial, A. L., Schaber, G.G., and Strobell, M.E. (1977) Journal of Geophysical Research, v. 82, no. 28, p. 4016-4038. (4) Trask, N.J. and Guest, J.E. (1975) Journal of Geophysical Research, v. 80, p. 2461-2477.

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