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 The Caloris basin is surrounded by an extensive ejecta blanket that has been de- scribed by Trask and Guest (1975), Strom et al. (1975), McCauley (1977), and Guest and O'Donnell (1977). The Caloris basin and its associated ejecta dominate much (about 20%) of the outgoing Mariner 10 mo- saic of Mercury. Almost half of the basin is visible; its pictorial setting is very similar to that of the Orientale basin in the Lunar Orbiter IV photographs. The Caloris basin is surrounded by a belt of blocky terrain 1 to 2 km high (estimated from shadow lengths) and 100 to 150 km wide called Caloris Montes. The inner diameter of this 0019-1035/81/080184-19502.00/0 Copyright © 1981 by Academic Press, Inc. All rights of reproduction in any form reserved. encompassing ring structure is about 1300 km. A discontinuous, weakly expressed scarp surrounds the blocky terrain beyond which lineated terrain, first identified as Caloris ejecta by Trask and Guest (1975), extends as far as the large crater Shake- speare, 1000 km northeast of the inner edge of Caloris Montes. The Caloris ejecta is covered around much of the basin by an enormous annulus of non lineated smooth plains material that extends outward from Caloris Montes more than 1400 km. As on the Moon, where ejecta from the better-preserved impact basins such as Im- brium (Shoemaker and Hackman, 1962), Orientale (McCauley, 1967), and Nectaris 184CALORIS BASIN, MERCURY 185 (Stuart-Alexander and Wilhelms, 1975) were used to establish a regional strati- graphy, the ejecta from Caloris also can be used as a marker horizon. We here propose a formal rock-stratigraphic nomenclature for the distinctive deposits that encompass the Caloris basin and are clearly related to it. This nomenclature was developed during l:5,000,000-scale geologic mapping of the Tolstoj (H-8) (Schaber and McCauley, 1980) and adjacent Shakespeare (H-3) (Guest and Greeley, 1981) quadrangles, and is a refinement of the earlier photogeologic work of the Mariner 10 Experiment Team. The stratigraphy described is patterned, with some modification, after that used in geologic mapping of the Imbrium basin by Wilhelms and McCauley (1971) and the Orientale basin by Scott et al. (1977). Cer- tain morphologically similar assemblages of materials around Caloris are recognized which have mappable distribution patterns and can be treated as discrete rock-strati- graphic units. Figure 1 summarizes the cur- rent stratigraphic nomenclature for Orien- tale, Imbrium, and the nomenclature proposed for Caloris. Morphologically sim- ilar units around each of the basins are aligned vertically. Unlike the original lunar stratigraphy of Shoemaker and Hackman (1962), that given here is rock-stratigraphic rather than time- stratigraphic, i.e., no systems and accom- panying periods of time such as the Imbr- ian, Eratosthenian, and Copernican are defined. A time-stratigraphy for the more geologically complex surface of Mars has 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) Maunder Formation Odentale Group ,0- ,- I i '.bt ¢ ,oh. I 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 Apennin¢ Bench Formation IMBRIUM (From Wilhelms and McCauley, 1971) Materisi of Terra Aloes Fra Mauro Hilly Materials 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. Summary of rock stratigraphy for Orientale and Imbrium and 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 l:5,000,000-scale geologic mapping pro- gram. A formal time-stratigraphy for Mer- cury would be premature at this time be- cause of the limited Mariner l0 data. The geologic history of Mercury has been discussed by various investigators includ- ing Davies et al. (1978) and Strom (1978). Five epochs in the geologic history of Mer- cury are generally recognized ranging from the condensation of the solar nebula, through the culmination of the period of heavy meteorite bombardment, the Caloris event, a time of plains formation, and finally to the long period of relative inactiv- ity and scattered crater formation that reaches to the present. These sequences of geologic events do not represent a formal time-stratigraphy because they are not tied to actual rock sequences. We here propose the establishment of a single rock-stratigraphic group consisting of a number of individual Caloris-related formations. According to stratigraphic practice, groups are recognized for the pur-