ICARUS 47, 184--202 (1981) Stratigraphy of the Caloris Basin, Mercury JOHN F. MCCAULEY U.S. Geological Survey Flagstaff, Arizona 86001 JOHN E. GUEST University of London Observatory, London, United Kingdom GERALD G. SCHABER U.S. Geological Survey, Flagstaff, Arizona 86001 NEWELL J. TRASK U.S. Geological Survey Reston, Virginia 22092 AND RONALD GREELEY Arizona State University, Tempe, Arizona 85281 Received May 5, 1980; revised July 1, 1981 The 1300-km-diameter Caloris impact basin is surrounded by well-defined ejecta units that can be recognized from more than 1000 km, radially outward from the basin edge. A formal rock stratigraphic nomenclature is proposed for the Caloris ejecta units, which are collectively called the Caloris Group. Each of the individual formations within the Group are described and compared to similar rock units associated with the lunar Imbrium and Orientale basins. A crater degradation chronology, linked the the Caloris event, is also proposed to assist in stratigraphic correlation on a Mercury-wide basis. INTRODUCTION encompassing ring structure is about 1300 The Caloris basin is surrounded by an km. A discontinuous, weakly expressed extensive ejecta blanket that has been de- scarp surrounds the blocky terrain beyond scribed by Trask and Guest (1975), Strom which lineated terrain, first identified as et al. (1975), McCauley (1977), and Guest Caloris ejecta by Trask and Guest (1975), and O'Donnell (1977). The Caloris basin extends as far as the large crater Shake- and its associated ejecta dominate much speare, 1000 km northeast of the inner edge (about 20%) of the outgoing Mariner 10 mo- of Caloris Montes. The Caloris ejecta is saic of Mercury. Almost half of the basin is covered around much of the basin by an visible; its pictorial setting is very similar to enormous annulus of non lineated smooth that of the Orientale basin in the Lunar plains material that extends outward from Orbiter IV photographs. The Caloris basin Caloris Montes more than 1400 km. is surrounded by a belt of blocky terrain 1 As on the Moon, where ejecta from the to 2 km high (estimated from shadow better-preserved impact basins such as Im- lengths) and 100 to 150 km wide called brium (Shoemaker and Hackman, 1962), Caloris Montes. The inner diameter of this Orientale (McCauley, 1967), and Nectaris 184 0019-1035/81/080184-19502.00/0 Copyright © 1981 by Academic Press, Inc. All rights of reproduction in any form reserved. CALORIS BASIN, MERCURY 185 (Stuart-Alexander and Wilhelms, 1975) tain morphologically similar assemblages of were used to establish a regional strati- materials around Caloris are recognized graphy, the ejecta from Caloris also can be which have mappable distribution patterns used as a marker horizon. We here propose and can be treated as discrete rock-strati- a formal rock-stratigraphic nomenclature graphic units. Figure 1 summarizes the cur- for the distinctive deposits that encompass rent stratigraphic nomenclature for Orien- the Caloris basin and are clearly related to tale, Imbrium, and the nomenclature it. This nomenclature was developed during proposed for Caloris. Morphologically sim- l:5,000,000-scale geologic mapping of the ilar units around each of the basins are Tolstoj (H-8) (Schaber and McCauley, aligned vertically. 1980) and adjacent Shakespeare (H-3) Unlike the original lunar stratigraphy of (Guest and Greeley, 1981) quadrangles, and Shoemaker and Hackman (1962), that given is a refinement of the earlier photogeologic here is rock-stratigraphic rather than time- work of the Mariner 10 Experiment Team. stratigraphic, i.e., no systems and accom- The stratigraphy described is patterned, panying periods of time such as the Imbr- with some modification, after that used in ian, Eratosthenian, and Copernican are geologic mapping of the Imbrium basin by defined. A time-stratigraphy for the more Wilhelms and McCauley (1971) and the geologically complex surface of Mars has Orientale basin by Scott et al. (1977). Cer- been developed by Scott and Carr (1979) in ROCK STRATIGRAPHIC NOMENCLATURE (Orientale, lmbrium and Calofis Basins) ORIENTALE From Scott and others, 1977, and McCauley, 1977) Odentale Group ,0- ,- I i '.bt ¢ ,oh. I Maunder Formation Montes Rook Formation Hevdlus Formation lorm- mas~f facies lohi- inner facies Iork- knobby facies Ioht- transver~ facies Iobo- outer facies Iohs- secondary crater facies IMBRIUM (From Wilhelms and McCauley, 1971) Apennin¢ Bench Materisi of Terra Aloes Fra Mauro Hilly Materials Formation Momes materials Fommdon Formation materials of crater Ap~nnJnus clusters and chains Pre-lmbrisn Pre-lmbrlan ruined material lineated material CALORIS Caloris Group Calofis Mecates Nero Odin Van Eyck Forrnatiolt Formation Formation Formation cvl- Ikleated facies cvs- ~¢,cond|t~crater facies Increasing Distance from Center of Basin FIG. 1. Summaryof rock stratigraphyfor Orientale and Imbriumand that proposed here for Caloris. Morphologically similar units around each basin are in approximate vertical alignment. Horizontal alignment reflects approximate relative distance of each unit from the inferred center of the basin. 186 MCCAULEY ET AL. a planetwide synthesis of the earlier mon with these lunar basins. Similarities l:5,000,000-scale geologic mapping pro- between Orientale and Caloris have been gram. A formal time-stratigraphy for Mer- pointed out by Strom et al. (1975) and by cury would be premature at this time be- McCauley (1977). cause of the limited Mariner l0 data. McCanley (1977) described the morpho- The geologic history of Mercury has been logic dissimilarities between the four rings discussed by various investigators includ- of Orientale and emphasized their general ing Davies et al. (1978) and Strom (1978). lack of circumferential continuity. Much Five epochs in the geologic history of Mer- attention has been given to the ring struc- cury are generally recognized ranging from tures in and around lunar basins, and there the condensation of the solar nebula, are many widely divergent opinions as to through the culmination of the period of how these features developed during the heavy meteorite bombardment, the Caloris cratering process (Head, 1974; Moore et event, a time of plains formation, and al., 1974; Hodges and Wilhelms, 1976; finally to the long period of relative inactiv- Wilhelms et al., 1977). Differences in the ity and scattered crater formation that nature of the target material, the projectile reaches to the present. These sequences of itself, the angle and speed of impact, and geologic events do not represent a formal the gravitational field are some of the varia- time-stratigraphy because they are not tied bles that affect the size and shape of the to actual rock sequences. final crater and the distribution of ejecta We here propose the establishment of a and secondary craters. Regardless of these single rock-stratigraphic group consisting complexities, impact cratering is an orderly of a number of individual Caloris-related process with characteristic types of de- formations. According to stratigraphic posits and structures formed in and around practice, groups are recognized for the pur- the final cavity of excavation (Shoemaker, pose of expressing the natural relations 1962, 1963; Gault et al., 1968; Oberbeck, between associated formations, and the 1975; Morrison and Oberbeck, 1975; Gault group consists of divisions that are formally et al., 1975; Roddy, 1976). named formations (Cohee, 1974, pp. 4,8). On the basis of morphology, McCauley The formation is, however, the fundamen- (1977) showed that the materials that make tal unit in rock-stratigraphic classification, up the Montes Rook at Orientale are similar and mappability is essential to its validity. to those of the Caloris Montes. Both arcu- We intend here to formally define four ate mountain ranges consist of jumbled, laterally gradational but mappable forma- roughly rectilinear blocks or massifs mostly tions on Mercury that are synchronous with about l0 to 50 km long and several kilome- the Caloris basin. These impact-produced ters high that form a distinct belt around the units are morphologically and also probably inner and lower parts of each basin. The lithologically, distinct from one another and Caloris Montes vary somewhat in morphol- are akin to the various basin-related rock- ogy circumferentially around the basin, but stratigraphic units recognized on the Moon. they are different from the terrain farther from the edge of the basin. A similar situa- tion prevails in the Montes Rook around LUNAR COMPARISONS AND GENERAL Orientale where the mountains on the north CONSIDERATIONS and south sides of the basin are less blocky Although it is the youngest and largest of and have stronger radial trends than else- the Mercurian basins seen so far, Caloris is where (Fig. 2). Over much of the extent of more degraded than the two youngest ba- the Montes Rook, the long axes of the sins on the Moon, Orientale and Imbrium. blocks are either radial to, or concentric Caloris does, however, have much in corn- with, the center of the basin, but locally FIG. 2. The Orientale basin. The small arrows point to the massif facies of the Montes Rook Formation. The easternmost small arrow points to a probable tear fault in Montes Rook. The broad upward-curving arrows point to the knobby facies of the Montes Rook Formation, the northernmost large arrow indicates where the discontinuous Cordillera scarp is overlain by knobby facies. Larger patches of knobby facies lie outside the Cordillera scarp on the southwest side