Lunar and Planetary Science XXX 1180.pdf

THE MARIUS HILLS VOLCANIC COMPLEX: A STRATIGRAPHIC STUDY S. K. Dunkin and D. J. Heather, Department of Physics & Astronomy, University College London, Gower Street, London WC1E 6BT, UK ([email protected] and [email protected])

Introduction: Marius Hills is a volcanic complex est FeO contents on the Moon. Volcanic features are covering an area of 35,000km2 in the south-central seen on both high- and low-Ti mare units with some, region of Oceanus Procellarum. It is host to a variety such as sinuous rilles, cutting through several flows of of volcanic landforms including sinuous rilles, low more than one unit. As noted by [5] and [7] there ap- domes, steep-sided domes and cones [1]. Previous pears to be no correlation between mare colour and work on this region include both photogeological volcanic features. Mare flows formed both before and studies [1,2,3] and, more recently, multispectral work after the sinuous rilles, as evidenced by the fact that from the Galileo [4] and Clementine missions [5,6,7]. some mare flows overlie part of Rille A (see below) Using the Clementine dataset, we have constructed a making it indistinguishable from the surrounding multispectral mosaic and FeO and TiO2 maps of the mare on the multispectral image. When seen on a Lu- Marius Hills region from 9-16N and 300-312E, in nar Orbiter photograph however, the presence of the order to determine the mare stratigraphy in the region. rille can still be seen at the point where the flow cov- Data Reduction: The Clementine data for this re- ers the rille, which suggests that the flow itself is thin gion was processed using the ISIS software and meth- relative to the depth of the rille. Since this flow is ods developed by the US Geological Survey (T. thin, it would not be unreasonable to assume that per- Becker, pers. comm.). Please contact the authors if haps other flows in the area are also thin. This being you wish to know the photometric coefficients used in the case, it should be possible to determine the stratig- this work. A multispectral mosaic processed at a raphy of the uppermost layers by consideration of resolution of 100m/pixel was constructed where the compositions revealed by relatively small (~1km) im- 750/415nm ratio controlled the red channel, the pact craters on particular units, and by looking at the 750/950nm ratio controlled the green band and the composition revealed within the walls of the sinuous 415/750nm ratio the blue band. Using this combina- rilles. tion, mature mare soils will appear red (low titanium) Sinuous rilles: Three rilles have so far been stud- or blue (higher titanium) with fresh basaltic material ied here, Rima Marius (16.5N, 311.1E) and two showing as yellow (low titanium) or green (high tita- smaller rilles (Rille A, 13.7N 304.3E, and Rille B, nium). Other variations in colours will be explained as 14.2N 304.3E) studied in [2]. The largest, Rima they are met in this abstract. In addition to the mul- Marius, traverses through different mare units, with tispectral mosaic, use was made of the B (750nm) its crater-like source cutting into a dark red flow, in- filter to look at albedo variations, and maps of FeO terpreted here to be composed of a (possibly thin) low- and TiO2 were constructed using the latest algorithms Ti mare unit overlying a high-Ti layer, giving it a of [8]. Together with analysis of Lunar Orbiter V darker appearance in the multispectral image than a frames 215-M and 210-M these give a complete pic- pure low-Ti mare. Further from the source, the rille ture of the Marius Hills area. cuts through both red and blue flows, and the transi- Mare Units: As noted in [5], the mare units in tion between units is mimicked along the walls of the Marius Hills are a patchwork of red and blue in the rille (i.e. as it passes into a red mare, the walls of the multispectral image, corresponding to titanium abun- rille reflect a low-Ti fresh basalt and when in a blue dance differences rather than maturity variations [7]. mare unit the walls reflect a higher-Ti fresh basalt). This is supported by an excellent correlation between This suggests that the mare deposits surrounding variations in the TiO2 map and mare colours in the Rima Marius are thicker than the depth of the rille multispectral frame. The FeO map of the region indi- itself. By deriving the depth of the rille through vari- cates that the Marius Hills has an exceptionally high ous units, it will be possible to obtain a lower limit to FeO content in both high- and low-Ti mare units. At the thickness of the mare in that region. The walls of the present time we are unable to give absolute values the rille in the high-Ti unit also show an extensive for the FeO content as our FeO maps appear to give cyan colour along the length of the rille. While this consistently high estimates of the FeO content when has previously been interpreted as representing a compared to known values such as at the Apollo sites. highland material [5,6], we find from five-point spec- However, even allowing for this, it would appear that tra that a strong 1mm band is present, suggestive of Marius could be home to soils with some of the high- these materials having a basaltic origin. To confirm Lunar and Planetary Science XXX 1180.pdf

STRATIGRAPHY OF THE MARIUS HILLS: S. K. Dunkin and D. J. Heather

that these signatures are basaltic and not highland appear form the strength of the red colour in the mul- gabbros, further comparisons of spectra need to be tispectral image that the thickness of the flow is not made. uniform throughout and is greater in some places. The Rille A is 48km long [2] and reveals predomi- lack of suitable craters in the area prevents us from nantly high-Ti green and cyan basalts in its walls. determining this for sure. This is in contrast to the unit it cuts through, which is A similar analysis is underway for the whole of the a darker red unit, similar to that in which the source Marius Hills area. So far, we have measured 270 cra- of Rima Marius is found. This supports our interpre- ters on the multispectral image and correlated them tation of the darker red unit as a thin, low-Ti flow with craters on Lunar Orbiter V frames 210-M and overlying a higher-Ti unit, since the rille appears to 215-M. From these we will build up a stratigraphic have cut through the low-Ti surface to expose the picture of the area, and extend our analysis to other higher-Ti unit below. According to [2], in its upper Lunar Orbiter frames. From initial analysis of the portion (where the high-Ti walls are dominant), the multispectral image alone, we see that the mare thick- rille has an average depth of 56m, and so the thick- nesses to the north-west and south-west are greater ness of the red surface flow must be less than this. and their flows more extensive than those which occur Midway along the rille is a 2.8km diameter crater closer to the volcanic centres. which reveals a high-Ti content in its cyan colour. Acknowledgments: We acknowledge the use of Assuming an excavation depth:crater diameter ratio of the STARLINK computing facilites, funded by the UK 1:10 [9], this crater will have excavated material from Particle Physics & Astronomy Research Council approximately 280m below the surface and therefore (PPARC). This work was carried out while SKD was a suggests that the high-Ti unit(s) beneath the rille ex- PPARC Research Fellow. We would like to thank tend at least to that depth. Tammy Becker and Jim Torson at the USGS and Jeff Rille B is just 36km long [2], and lies within a Gillis for their assistance in data reduction, and Ian single red unit. The upper portion of Rille B has an Crawford and John Guest for their comments on an average depth of 95m [2] and its yellow walls reflect earlier version of this script. the low-Ti content of the surface mare, so we must References: [1] Whitford-Stark J. L. and Head J. conclude that the mare unit here extends to a greater W., (1977), PLPSC, 8, 2705-2724, [2] Greeley R., depth than that around Rille A. (1973), The Moon, 3, 289-314, [3] Guest J. E., (1971) Although there are several other sinuous rilles in in Geology and Geophysics of the Moon, ed. Fielder the Marius Hills region, not all of them are seen in the G., 41-53, [4] Sunshine J. M., Pieters C. M., Head J. multispectral image due either to the possibility that W., (1992), LPSC, 23, 1387-1388, [5] Gillis J. J. and they are of the same composition as their surrounding Spudis P. D., (1995), LPSC, 26, 459-460, [6] Bussey mare, or are beyond the resolution of the Clementine D. B. J. and Spudis P. D., (1996), LPSC, 27, 183-184 data. [7] Weitz C. M. and Head J. W., (1998), LPSC29, [8] Craters: Impact craters are a useful window into Lucey P. G., Blewett D. T., Hawke B. R., (1998), JGR, the underlying stratigraphy of a region. While large 103, 3679-3699 [9] Melosh H.J., (1989), Impact Cra- craters are required to look deep into the layering of tering: A geologic process, Oxford University Press the regional crust, we have seen here that there are some quite thin mare units at the Marius Hills. Small craters may therefore help to identify these thin flows. We have already seen that part of Rille A cuts through a thin (<56m) low-Ti layer. A crater within this flow, just to the north of the rille, appears to have excavated high-Ti basalt. The crater (measured from a Lunar Orbiter frame) is approximately 90m in diameter (al- though the diameter itself is below the resolution of the Clementine image, the ejecta sheet we are meas- uring extends much further). So the low-Ti mare unit may be as little as 9m thick here. The same value is obtained by looking at a crater to the east of the rille, also in the red unit, which appears to have cut through the boundary between the low-Ti unit on the surface and the subsurface high-Ti unit. However, it would