GEOLOGIC MAP OF THE MICHAELANGELO (H-12) QUADRANGLE OF MERCURY By Paul D. Spudis and James G. Prosser 1984 Prepared for the National Aeronautics and Space Administration by U.S. Department of the Interior, U.S. Geological Survey (Published in hardcopy as USGS Miscellaneous Investigations Series Map IĐ1659, as part of the Atlas of Mercury, 1:5,000,000 Geologic Series. Hardcopy is available for sale from U.S. Geological Survey, Information Services, Box 25286, Federal Center, Denver, CO 80225) Please direct questions or comments about the digital version to: Richard Kozak U.S. Geological Survey 2255 N. Gemini Drive Flagstaff, AZ 86001 e-mail: rkozak@flagmail.wr.usgs.gov DESCRIPTION OF MAP UNITS PLAINS MATERIALS pvs VERY SMOOTH PLAINS MATERIALŃForms smooth, featureless plains on floors of c4 and c5 craters (for example, within BashO(o,Đ), Đ32Ą, 170Ą; FDS 0166846). Interpretation: Impact melt sheet and clastic fallback debris associated with host crater ps SMOOTH PLAINS MATERIALŃWidespread throughout map area in commonly contiguous patches May also occur as primary floor material in c4 and c3 craters and as crater-fill material in c3 to c1 craters. Surface planar to gently undulating. Appears sparsely cratered at Mariner 10 resolutions (FDS 0166837). Interpretation: Origin of regional deposits uncertain, but they most likely consist of volcanic flows analogous to those of lunar maria. Alternatively, could be associated with ballistic debris of multiring basins that had a large melt component. Primary crater-floor deposits probably consist of impact melt and clastic debris that are slightly more degraded than unit pvs due to greater age psi NTERMEDIATE PLAINS MATERIALŃForms planar to undulating surfaces that have higher crater density than smooth plains material, but are less heavily cratered than intercrater plains material. May locally fill floors of c3 to c1 craters. Gradational with smooth plains material in some places (as at Đ52Ą, 137Ą) and with intercrater plains material in others (Đ66Ą, 142Ą). Interpretation: Probably similar to smooth plains unit, but older; may consist of volcanic flows or distal ejecta of multiring basins or possibly a complex mixture of both pi INTERCRATER PLAINS MATERIALŃForms extensive, undulating to hummocky surfaces between areas of large, overlapping craters. In detail, displays complex topography of coalescing secondary craters. Appears to predate most map units, but locally may overlie c1 crater rims and numerous ghost craters. Interpretation: Complex unit including crater and basin deposits and possibly volcanic flows. Probably lithologically equivalent to lunar highlands megaregolith BASIN MATERIALS brl BEETHOVEN BASIN RIM MATERIALŃRadially lineated and grooved material outside rim of Beethoven (basin centered at Đ20Ą, 124Ą). Extensive to east and southeast of basin; in narrow band directly to west. Large crater chains (unit csu) aligned with radial texture. Interpretation: Ejecta produced by impact that formed Beethoven. Radial texture due to ballistic deposition of both melt and clastic debris. Some lineations may be structural in origin. Exact age uncertain; density of superposed primary impact craters (table 1) suggests a post-Caloris, late c3 age, but may be as old as early c2 due to large range of error in crater age estimate cvs CALORIS GROUP, VAN EYCK FORMATION, SECONDARY CRATER FACIESŃOverlapping craters larger than 20 km in diameter that appear to have formed simultaneously; in two clusters near west map border. Southern cluster (Đ49Ą, 182Ą) superposed on Dostoevskij Basin materials. Interpretation: Secondary impact craters from the Caloris Basin impact. Form, distribution, and resemblance both to lunar basin secondaries and to Caloris secondaries outside map area suggest similar origin trl TOLSTOJ BASIN RIM MATERIALŃRadially lineated and grooved material outside rim of Tolstoj (basin centered at Đ16Ą, 165Ą). Extensive south of basin, but appears embayed by intercrater plains material to southeast. Large crater chains (unit csu) aligned with radial texture. Interpretation: Ejecta produced by the impact that formed Tolstoj. Radial texture due to ballistic deposition of melt and clastic debris. Some lineations may be structural in origin. Density of superposed primary impact craters (table 1) suggests a pre-Caloris, c2 or c1 age drl DOSTOEVSKIJ BASIN RIM MATERIALŃRadially lineated and grooved material outside rim of Dostoevskij (basin centered at Đ44Ą, 176Ą). Extensive north and south of basin, but greatly restricted to east because of nondeposition or overlap by younger units. Extent of material to west cannot be determined due to terminator position just west of basin rim. Large crater chains and clusters (unit csu) aligned with radial texture. Interpretation: Ejecta produced by the impact that formed Dostoevskij. Radial texture due to ballistic deposition of melt and clastic debris. Some lineations may be structural in origin, particularly in the area at Đ40Ą, 174Ą, where the Dostoevskij rim intersects a ring of the preexisting Barma-Vincente Basin. Density of superposed primary impact craters (table 1) suggests a pre-Caloris, c1 age m MASSIF MATERIALŃForms generally isolated, equant to rectilinear massifs that protrude through adjacent units. Type area: Đ50Ą, 174Ą; FDS 0166843. Interpretation: Fragments of rings from nearly obliterated multiring basins Barma-Vincente, Hawthorne- Riemenschneider and Eitoku-Milton. Probably consist of brecciated autochthonous rocks and melt rocks produced by the basin impact. Used in conjunction with arcuate ridge segments and scarps to delineate basin rings CRATER MATERIALS CRATER MATERIALSŃCraters ł30 km in diameter. Interpretation: Impact crater materials of various degrees of degradation and infilling. Rate of degradation may be accelerated by adjacent impact events; therefore, stratigraphic significance of morphology is only approximate. Morphologic classification based on system of N. J. Trask (McCauley and others, 1981) c5 Material of craters with very fresh rim crest, terraces, radial texture, rays, and few or no superposed cratersŃFloor-wall contact very sharp. Floored by very smooth plains material (example: BashO(o,Đ), Đ32Ą, 170Ą) cp5 Central-peak materialŃMay consist of single peak, elongated peak, or peak dusters (a function of increasing crater size). Interpretation: Uplifted, brecciated autochthonous rocks formed contemporaneously with host crater cr5 Radially textured material and secondary crater field forming annulus outside some c5 craters cs5 Satellitic-crater materials forming secondary crater clusters and chainsŃHost crater identified; associated with c5 craters c4 Material of craters with continuous, slightly subdued rim crestŃFew superposed craters; generally no rays visible. Floor-wall contact sharp. Floor consists mostly of very smooth or smooth plains materials (example: Hawthorne, Đ51Ą,116Ą) cp4 Central-peak materialŃMay consist of single peak, elongated peak, peak clusters, or peak rings (a function of increasing crater size). Interpretation: Uplifted, brecciated autochthonous rocks formed contemporaneously with host crater cf4 Crater-floor materialŃHummocky, rough-textured material forming floor deposits in some c4 craters; usually embayed by plains material cr4 Radially textured material and secondary crater field forming annulus outside unit c4ŃShown only for craters >100 km in diameter cs4 Satellitic-crater materials forming secondary crater clusters and chainsŃHost crater identified; associated with c4 craters c3 Material of craters with rounded but continuous rimsŃFlat floors generally filled with smooth plains or intermediate plains materials. Central peaks uncommon except in basins such as Michelangelo. Moderate number of superposed craters (example: Barma (Yakovlev), Đ41Ą, 163Ą) cp3 Central-peak materialŃMay consist of single peak, elongated peak, peak clusters, or peak rings (a function of increasing crater size). Interpretation: Uplifted, brecciated autochthonous rocks formed contemporaneously with host crater cf3 Crater-floor materialŃHummocky, rough-textured material forming floor deposits in some c3 craters; usually embayed by plains material cr3 Radially textured material and secondary crater field forming annulus outside c3 cratersŃshown only for craters >100 km in diameter cs3 Satellitic-crater materials forming secondary crater clusters and chainsŃHost crater identified; associated with c3 craters c2 Material of craters with rounded, subdued rims and flat floorsŃFloor may be filled with either smooth plains or intermediate plains materials. No radial texture evident in exterior deposits. Many superposed craters (example: Sei, Đ64Ą, 90Ą) cp2 Central-peak materialŃVery rare single peak. Interpretation: Uplifted, brecciated autochthonous rocks formed contemporaneously with host crater c1 Material of flat-floored craters with low, discontinuous rim crestsŃ No radially textured ejecta; no preservation of secondaries. Floor is filled with smooth plains or intermediate plains materials and is filled locally by ejecta blankets from adjacent craters. Large number of superposed craters (example: Milton, Đ26Ą,175Ą) csu Satellitic crater material, undividedŃOccurs as crater pairs, clusters, or chains that are satellitic to a larger host crater or basin. Location of host feature mostly uncertain. Interpretation: Secondary impact craters formed by ballistic erosion of subjacent terrain CONTACTŃQueried where doubtful; dotted where buried SCARPŃProbably a fault. Bar and ball on downthrown side RIDGEŃInterpreted as mare-type wrinkle ridge within smooth plains and intermediate plains materials; probably of compressive tectonic origin. Symbol on ridge crest RIDGE SCARPŃAssociated with rupes structures; probably of compressive tectonic origin. Line marks base of slope; barb points downslope DEPRESSION OF PROBABLE STRUCTURAL ORIGIN, LINEAR TO ARCUATEŃBarbs point downslope CRATER RIM CREST CRATER RIM CREST, SUBDUEDŃEither degraded by age (c1 and c2 craters) or buried by later unit IRREGULAR DEPRESSIONŃPossibly a collapse crater BASIN RING CRESTŃInterpreted as part of basin ring of structural origin MULTIRING BASIN RINGŃSubdued ring of large multiring basins Barma-Vincente, Bartok-Ives, Hawthorne-Riemenschneider, and Eitoku-Milton. Solid line indicates mappable structural or topographic element; dots indicate inferred ring position AREA OF BRIGHT CRATER-RAY MATERIALŃInterpreted as fresh crater ejecta AREA OF ABNORMALLY LOW ALBEDO INTRODUCTION The Michelangelo quadrangle is in the southern hemisphere of Mercury, where the imaged part is heavily cratered terrain that has been strongly influenced by the presence of multiring basins. At least four such basins, now nearly obliterated, have largely controlled the distribution of plains materials and structural trends in the map area. Many craters, interpreted to be of impact origin, display a spectrum of modification styles and degradation states. The interaction between basins, craters, and plains in this quadrangle provides important clues to geologic processes that have formed the morphology of the mercurian surface. Several low-albedo features are evident in Earth-based views of the Michelangelo quadrangle (Davies and others, 1978, p. 15), but these features do not appear to correlate directly with any mapped terrain unit. Solitudo Promethei may correspond to a deposit of plains materials centered at Đ58Ą, 135Ą, and Solitudo Martis may correspond to similar materials at Đ30Ą to Đ40Ą, 90Ą to 100Ą. The color data (orange/ultraviolet) presented in Hapke and others (1980) likewise show no particular correlation with mapped terrain types. The ŇyellowÓ region (moderately high orange/ultraviolet) centered at Đ33Ą, 155Ą appears to correspond to a smooth plains deposit, but the region overlaps into adjacent cratered terrain. Mariner l0 data include complete photographic coverage of the quadrangle at a resolution of about 2 km. In addition, twelve stereopairs cover scattered areas in the quadrangle (Davies and others, 1978, p. 114Đ115); these photographs were used to supplement the geologic interpretation. About 10Ą of longitude of the H-13 quadrangle (Solitudo Persephones Province) adjacent to the west is included in the map area because not enough Mariner 10 data were acquired of this quadrangle to justify the production of another map. STRATIGRAPHY ANCIENT BASIN MATERIALS Systematic mapping of the Michelangelo quadrangle has revealed the presence of four nearly obliterated multiring basins These basins are here named for unrelated superposed, named craters, as was done for highly degraded lunar basins (Wilhelms and El-Baz, 1977). From oldest to youngest, the basins are: Barma-Vincente, centered at Đ52Ą, 162Ą; Bartok-Ives, centered at Đ33Ą, 115Ą; Hawthorne-Riemenschneider, centered at Đ56Ą, 105Ą; and Eitoku-Milton, centered at Đ23Ą, 170Ą. Diameters of the basin rings are given in table 1. The presence of these basins is indicated by three criteria: (1) isolated massifs (unit m) that appear to protrude through superposed materials; (2) arcuate segments of ridges (rupes) aligned with massif material; and (3) arcuate scarps aligned with both massifs and ridges. Because none of the four basins has ejecta deposits that are preserved, the basins are assumed to be the oldest features in the map area; moreover, they are embayed or buried by all other units. The relative ages of the basins are given in table 1, based on the density of superposed primary impact craters and stratigraphic relations. These results are uncertain, as the crater density of heavily cratered terrain on Mercury ranges from 11.2 to 17.4 x 10-5 km-2 for craters of diameters 20 km or greater (Guest and Gault, 1976). The results obtained are consistent with a qualitative assignment of relative age that is based on position and size of these ancient basins. The basins have largely controlled subsequent geologic processes in the map area. Large concentrations of smooth plains deposits are found within the basin boundaries and at the intersections of rings of different basins. Moreover, the trends of scarp segments, interpreted by some workers to be expressions of thrust faults associated with global compression (Strom and others, 1975; Dzurisin, 1978), are deflected into basin-concentric patterns at their intersection with basin rings. These relations have also been noted for ancient basins on both the Moon (Schultz, 1976) and Mars (Schultz and others, 1982; Chicarro and others, 1983). In addition to the four multiring basins, an ancient two-ring basin, Surikov, is also evident at Đ37Ą, 125Ą. It is unique among the two-ring basins in the map area because, although the inner ring is well preserved and similar in morphology to peak rings of fresh basins such as Bach, the outer ring is almost totally obliterated. This morphology is similar to that of the lunar basin Grimaldi and is suggestive of an extended period of structural rejuvenation along the margins of the inner ring. Crater density on this basin suggests that it is one of the oldest in the map area (table 1). OLDER PLAINS MATERIALS The oldest recognizable plains unit in the map area is the intercrater plains material (unit pi), originally described by Trask and Guest (1975). This material is generally undulating to hummocky and appears to underlie tracts of cratered terrain, as evidenced by the superposition of many coalescing secondaries from adjacent large craters. In some areas, the intercrater plains material appears to embay c1 craters, and it is found in all of the degraded basins described above. The origin of mercurian intercrater plains material remains unknown. Both volcanic (Strom and others, 1975; Trask and Guest, 1975; Strom, 1977) and impact-debris models (Wilhelms, 1976a; Oberbeck and others, 1977) have been proposed. The material is most likely polygenetic, including both crater and basin debris and possibly ancient volcanic flows. Physically and lithologically it resembles the lunar highlands megaregolith. YOUNGER BASIN MATERIALS At least seven basins in or partly in the Michelangelo quadrangle postdate or are contemporaneous with the last stages of deposition of intercrater plains material. Dostoevskij (Đ44Ą, 176Ą) displays only one ring; presumably the inner peak ring is buried by plaint material. The ejecta from this basin (unit drl) may be mapped as far as 450 km from the rim; several secondary crater chains occur southeast of the rim. Although Dostoevskij was considered a type example of a c3 large crater (McCauley and others, 1981), crater counts indicate that it is much older (table 1). The Dostoevskij impact probably occurred in cl time. The Tolstoj Basin is centered in the Tolstoj quadrangle at Đ16Ą,165Ą (Schaber and McCauley, 1980). It consists of three discontinuous rings (table 1); ejecta (unit trl) may be mapped as far as 350 km from the outermost ring. The density of superposed craters suggests an age older than the Caloris Basin, either late c1 or early c2. A small. unnamed basin at Đ48Ą, 136Ą may also have formed in this time interval (table 1), but its age is uncertain due to its partial burial by ejecta from crater Delacroix (Đ44Ą, 129Ą). The effects of the Caloris impact on the map area are not immediately apparent. No Caloris ejecta are evident, and most structural trends appear to be unrelated to this impact. However, near the west border of the map are two groups of large, overlapping craters centered at Đ31Ą, 183Ą and Đ49Ą, 182Ą. These groups appear to have formed simultaneously, as no specific stratigraphic sequence is evident. On the basis of crater clusters of similar appearance in the lunar highlands, which have been interpreted as Imbrium and Orientale basin secondaries (Schultz, 1976; Wilhelms, 1976b; Eggleton, 1981), these crater groups are interpreted to be Caloris Basin secondaries. Following the terminology developed by McCauley and others (1981) we have assigned them to the Van Eyck Formation, Secondary-Crater Facies (unit cvs). These secondaries overlie Dostoevskij ejecta and thus confirm that basin as pre-Caloris. We determined a reference crater density for Caloris in the Shakespeare quadrangle (table l) in order to correlate basin ages to that stratigraphic datum. The Beethoven Basin (Đ20Ą,124Ą), partly exposed in the Michelangelo quadrangle, consists of one ring 660 km in diameter. The exact age of Beethoven is uncertain; the density of superposed primary impact craters (table 1) suggests a post-Caloris, late c3 age, but it may be as old as early c2 age due to the large range of error in the crater age estimate. The ejecta from Beethoven (unit brl) are very extensive east and southeast of the basin rim and are mappable as far as 600 km downrange from the rim. However, ejecta appear to be almost absent on the west side of the basin. The reason for this asymmetry is unclear; possibly Beethoven is the result of an oblique impact that produced an asymmetric ejecta distribution (Gault and Wedekind, 1978), or possibly basin radial texture in the western rim area has been obliterated by ejecta from VO(a,Đ)lmiki. The other basins in the quadrangle are Michelangelo, VO(a,Đ)lmiki, and Bach (table 1). All contain two rings and appear to be transitional between large craters and multiring basins. All postdate the Caloris event. YOUNGER PLAINS MATERIALS The oldest of the three younger plains units is intermediate plains material (unit psi). It forms planar to gently undulating surfaces and both embays tracts of cratered terrain and fills crater floors. Both upper and lower contacts with other plains units are gradational. These gradations suggest that the assignment of age to plains deposits on Mercury is partly dependent on the relative abundance of superposed secondary craters, whose densities vary widely as a function of nearby source craters. The smooth plains unit (ps) forms both widespread regional deposits and crater floor material. The regional deposits are significantly less cratered than those of other plains units, although they typically display crater densities comparable to older lunar maria (Murray and others, 1974). The unit characteristically contains mare-type ridges, although no flow fronts have been observed in the map area. The origin of the younger plains materials is critical to mercurian geologic history. They are thought to be either volcanic (Strom and others, 1975; Trask and Strom, 1976) or a facies of ballistic ejecta (Wilhelms, 1976a; Oberbeck and others, 1977). The interpretation favored here is that large parts of these smooth plains are of volcanic origin, because (1) they are distributed regionally and have no obvious source for ballistic deposition; (2) large tracts are confined within basin depositional environments, analogous to the lunar maria; (3) indirect evidence elsewhere on Mercury exists for volcanic modification of impact craters (Schultz, 1977); and (4) possible volcanic collapse craters are associated with plains-filled craters (Đ61Ą, 161Ą and Đ57Ą, 102Ą). Parts of smooth plains deposits may be a complex mix of overlapping crater ejecta. A very smooth plains unit (unit pvs) occurs only as floor material in younger c4 and c5 craters. The material is interpreted to be crater impact melt and associated clastic debris. CRATER MATERIALS Crater deposits are mapped stratigraphically according to a morphologic degradation sequence devised by N. J. Trask (McCauley and others, 1981). This method assumes that (1) all craters of a given size range initially resemble fresh craters and (2) degrees of impact erosion are constant for all craters within a morphologically defined sequence. Although these conditions hold generally, degradation may be accelerated locally by adjacent impact events and flooding by plains materials and, rarely, may be decelerated by structural rejuvenation of topographic elements of craters. Thus, the stratigraphic significance of crater morphology is only approximate. By analogy with lunar materials, all mapped crater materials are thought to be of impact origin. Only craters larger than 30 km in diameter are mapped. The large basins of the Michelangelo quadrangle have been dated relatively by counting the cumulative density of superposed primary impact craters that have diameters greater than 20 km (table 1). This technique has proven to be of great value in dating lunar basins (Wilhelms, in press), where obvious superposition relations do not exist. Results of these crater counts indicate that Dostoevskij, presumed to be of c3 age (McCauley and others, 1981), is actually one of the oldest basins in the map area (early c1). Thus, strict morphological determination of stratigraphic age may be significantly in error. Throughout the map area are crater clusters and chains (unit csu) that are satellitic to both craters and basins, but the host crater may not be identifiable everywhere. This material is interpreted to be from secondary impact craters of a wide variety of ages. Many mercurian secondaries are well preserved and have sharp, unrounded rims. This morphology is probably a consequence of the stronger mercurian gravity, relative to the Moon. that produces higher impact velocities for crater ejecta on the mercurian surface (Gault and others, 1975; Scott, 1977). STRUCTURE The rings associated with the four ancient basins (table 1) are the oldest structures within the mapped area and have to some degree controlled the structural trends of subsequent tectonism. Several of the lobate ridges described by Strom (1979) follow arcuate patterns along rings of the Barma-Vincente Basin; Hero Rupes is an example. These lobate ridges appear to be of compressive tectonic origin and, although global in distribution, may be deflected locally by the presence of preexisting, basin-related structure. Additional effects of these ancient basin rings may be seen where the rim of Dostoevskij intersects the Barma-Vincente rings (for example, the horst at Đ40Ą, 174Ą); parts of the Dostoevskij rim appear to have been structurally accented by this intersection. These relations are similar to those associated with highly degraded, ancient basins on Mars (Chicarro and others, 1983). The smooth plains material (unit ps) displays numerous ridges that generally resemble lunar mare ridges and also are considered to be of tectonic origin. The mercurian ridges are probably related to minor compressive stresses that postdate smooth plains emplacement. Numerous lineaments are associated with basin rim material (units drl, trl, brl), but most of these lineaments are probably related to ejecta deposition. A few may be faults, particularly where they occur close to preexisting basin rings. GEOLOGIC HISTORY The interpretable geologic history of the Michelangelo quadrangle begins with the formation of the four ancient, multiring basins. From oldest to youngest, they are: Barma-Vincente, Bartok-Ives, Hawthorne-Riemenschneider, and Eitoku-Milton. These basins presumably formed during the period of heavy bombardment inferred from lunar history (Wilhelms, in press). Contemporaneous with their formation and shortly afterward, was the deposition of the intercrater plains material. This unit has a complex history of deposition; it was reworked in place and probably includes brecciated plutonic rocks and possibly ancient volcanic flows. Deposition of the intercrater plains material was waning as the next oldest basins (Dostoevskij, Tolstoj) were formed. Partly overlapping their formation was the deposition of the intermediate plains material, probably emplaced partly as distal basin ejecta and partly as volcanic flows. Regional deformation of these plains units by compressive tectonics, forming scarps, was contemporaneous with there deposition. The Caloris impact occurred during the time of formation of the intermediate plains material. In the map area, Caloris ejecta may be present at depth or may have been reworked locally by adjacent impacts. Two groups of Caloris secondary craters (unit cvs) are evident. Shortly after the Caloris impact, extensive smooth plains material, probably of volcanic origin, was deposited. During this period of deposition occurred the impacts of the last of the major basins (Beethoven, Michelangelo, VO(a,Đ)lmiki, and Bach). Minor tectonic activity continued as scarps and lunar mare-type wrinkle ridges developed within the smooth plains materials. The cratering rate declined rapidly as the c3, c4 and c5 craters were produced. Regolith production continues to the present day on all units. If the geologic history of the Moon is a guide, most of the events discussed were essentially complete within the first 1.5 to 2.0 billion years of MercuryŐs history (Murray and others, 1975). A summary of global mercurian geology may be found in Guest and OŐDonnell (1977) and Strom (1979). REFERENCES CITED Chicarro, Augustin, Schultz, P. H., and Masson, Philippe, 1983, Basin control of ridge patterns on Mars: Abstracts of papers submitted to the Lunar and Planetary Science Conference, 14th, Houston, 1983, p. 105Đ106. Davies, M. E., Dwornik, S. E., Gault, D. E., and Strom, R. G., 1978, Atlas of Mercury: National Aeronautics and Space Administration Special Publication SP-423, 128 p. Dzurisin, Daniel, 1978, The tectonic and volcanic history of Mercury as inferred from studies of scarps, ridges, troughs, and other lineaments: Journal of Geophysical Research, v. 83, no. B10, p. 4883Đ4906. Eggleton, R. E., 1981, Map of the impact geology of the Imbrium basin of the Moon, in Geology of the Apollo 16 areaŃCentral Lunar Highlands: U.S. Geological Survey Professional Paper 1048, pl. 12. Gault, D. E., Guest, J. E., Murray, J. B., Dzurisin, Daniel, and Malin, M. C., 1975, Some comparisons of impact craters on Mercury and the Moon: Journal of Geophysical Research, v. 80, no. 17, p. 2444Đ2460. Gault, D. E., and Wedekind, J. A., 1978, Experimental studies of oblique impact: Lunar and Planetary Science Conference, 9th, Houston, 1978, Proceedings, v. 3, p. 3843Đ3875. Guest, J. E., and Gault, D. E., 1976, Crater populations in the early history of Mercury: Geophysical Research Letters, v. 3, no. 3, p. 121Đ123. Guest, J. E., and OŐDonnell, W. P., 1977, Surface history of Mercury: A review: Vistas in Astronomy, v. 20, p. 273Đ300. Hapke, Bruce, Christman, Craig, Rava, Barry, and Mosher, Joel, 1980, A color-ratio map of Mercury: Lunar and Planetary Science Conference, 11th, Houston, 1980, Proceedings, v. 1, p. 817Đ821. McCauley, J. F., Guest, J. E., Schaber, G. G., Trask, N. J., and Greeley, Ronald, 1981, Stratigraphy of the Caloris Basin, Mercury: Icarus, v. 47, no. 2, p. 184Đ202. Murray, B. C., Belton, M. J. S., Danielson, G. E., Davies, M. E., Gault, D. E., Hapke, Bruce, OŐLeary, Brian, Strom, R. G., Suomi, Verner, and Trask, N. J., 1974, MercuryŐs surface: Preliminary description and interpretation from Mariner 10 pictures: Science, v. 185, no. 4146, p. 169Đ179. Murray, B. C., Strom, R. G., Trask, N. J., and Gault, D. E., 1975, Surface history of Mercury: Implications for terrestrial planets: Journal of Geophysical Research, v. 80, no. 17, p. 2508Đ2514. Oberbeck, V. R., Quaide, W. L., Arvidson, R. E., and Aggarwal, H. R., 1977, Comparative studies of lunar, martian, and mercurian craters and plains: Journal of Geophysical Research, v. 82, no. 11, p. 1681Đ1698. Schaber, G. G., and McCauley, J.F., 1980, Geologic map of the Tolstoj quadrangle of Mercury: U.S. Geological Survey Miscellaneous Investigations Series Map I-1199, scale 1:5,000,000. Schultz, P. H., 1976, Moon morphology: Austin, Tex., University of Texas Press, 626 p. ______1977, Endogenic modification of impact craters on Mercury: Physics of the Earth and Planetary Interiors, v. 15, nos. 2Đ3, p. 202Đ219. Schultz, P. H, Schultz, R. A., and Rogers, John, 1982, The structure and evolution of ancient impact basins on Mars: Journal of Geophysical Research, v. 87, no. 12, p. 9803Đ9820. Scott, D. H., 1977, Moon-Mercury: Relative preservation states of secondary craters: Physics of the Earth and Planetary Interiors, v. 15, nos. 2Đ3, p. 173Đ178. Strom, R. G., 1977, Origin and relative age of lunar and mercurian intercrater plains: Physics of the Earth and Planetary Interiors, v. 15, nos. 2Đ3, p. 156Đ172. ______1979, Mercury: A post-Mariner 10 assessment: Space Science Reviews, v. 24, p. 3Đ70. Strom, R. G., Trask, N. J., and Guest, J. E., 1975, Tectonism and volcanism on Mercury: Journal of Geophysical Research, v. 80, no. 17, p. 2478Đ2507. Trask, N. J., and Guest, J. E., 1975, Preliminary geologic terrain map of Mercury: Journal of Geophysical Research, v. 80, no. 17, p. 2461Đ2477. Trask, N. J., and Strom, R. G., 1976, Additional evidence of mercurian volcanism: Icarus, v. 28, no. 4, p. 559Đ563. Wilhelms, D. E., 1976a, Mercurian volcanism questioned: Icarus, v. 28, no. 4, p. 551Đ558. ______1976b, Secondary impact craters of lunar basins: Lunar Science Conference, 7th, Houston, 1976, Proceedings, v. 3, p. 2883Đ2901. ______The geologic history of the Moon: U.S Geological Survey Professional Paper 1348 (in press). Wilhelms, D. E., and El-Baz, Farouk, 1977, Geologic map of the east side of the Moon: U.S. Geological Survey Miscellaneous Investigations Series Map I-948, scale 1:5,000,000 NOTES ON BASE This map sheet is one of a series covering that part of the surface of Mercury that was illuminated during the Mariner 10 encounters (Davies and Batson, 1975). The source of map data was the Mariner 10 television experiment (Murray, 1975). ADOPTED FIGURE The map projections are based on a sphere with a radius of 2,439 km. PROJECTION The Lambert conformal conic projection is used for this sheet, with a scale of 1:4,623,000 at lat Đ22.5Ą. Latitudes are based on the assumption that the spin axis of Mercury is perpendicular to the plane of the orbit. Longitudes are positive westward in accordance with the usage of the International Astronomical Union (IAU, 1971). Meridians are numbered so that a reference crater named Hun Kal (lat Đ0.6Ą) is centered on long 20Ą (Murray and others, 1974; Davies and Batson, 1975 ). CONTROL Planimetric control is provided by photogrammetric triangulation using Mariner 10 pictures (Davies and Batson, 1975). Discrepancies between images in the base mosaic and computed control-point positions appear to be less than 5 km. The base mosaic was tied to a much later iteration than the base mosaics of other Mercury quadrangles. Discrepancies as large as 20 km were adjusted along the north edge to match the Tolstoj (H-8) and Beethoven (H-7) quadrangles. No attempt was made to join the Discovery (H-11) quadrangle to the east or the Bach (H-15) quadrangle to the south. Discrepancies as large as 40 km exist on these boundaries. MAPPING TECHNIQUES Mapping techniques are similar to those described by Batson (1973a, b). A mosaic was made with pictures that had been digitally transformed to the Lambert conformal conic projection. Shaded relief was copied from the mosaics and portrayed with uniform illumination with the sun to the west. Many Mariner 10 pictures besides those in the base mosaic were examined to improve the portrayal. The shading is not generalized and may be interpreted with nearly photographic reliability (Inge, 1972; Inge and Bridges, 1976). Shaded relief analysis and representation were made by P. M. Bridges. NOMENCLATURE All names on this sheet are approved by the International Astronomical Union (IAU, 1977, 1980). H-12: Abbreviation for Mercury (Hermes) sheet number 12. H 5M Đ45/135 G: Abbreviation for Mercury (Hermes) 1:5,000,000 series; center of sheet, lat Đ45Ą, long 135Ą; geologic map, G. A small part of the H-13 quadrangle is included on this sheet because insufficient data are available to justify preparation of a separate sheet. REFERENCES CITED Batson, R. M., 1973a, Cartographjc products from the Mariner 9 mission: Journal of Geophysical Research, v. 78, no. 20, p. 4424Đ4435. _____1973b, Television cartography: U.S. Geological Survey Open-File Report, Astrogeology 58, 35 p. Davies, M. E., and Batson, R. M., 1975, Surface coordinates and cartography of Mercury: Journal of Geophysical Research, v. 80, no. 17, p. 2417Đ2430. Inge, J. L., 1972, Principles of lunar illustration: Aeronautical Chart and Information Center Reference Publication RP-72-1, 60 p. Inge, J. L., and Bridges, P. M., 1976, Applied photointerpretation for airbrush cartography: Photogrammetric Engineering and Remote Sensing, v. 42, no. 6, p. 749Đ760. International Astronomical Union, 1971, Commission 16: Physical study of planets and satellites, in 14th General Assembly, Brighton, 1970, Proceedings: International Astronomical Union Transactions, v. 14B, p. 128. _____1977, Working Group for Planetary System Nomenclature, in 16th General Assembly, Grenoble, 1976, Proceedings: International Astronomical Union Transactions, v. 16 B, p. 330Đ333, 351Đ355. _____1980, Working Group for Planetary System Nomenclature, in 17th General Assembly, Montreal, 1979, Proceedings: International Astronomical Union Transactions, v. 17B, p. 291 . Murray, B. C., 1975, The Mariner 10 pictures of MercuryŃan overview: Journal of Geophysical Research, v. 80, no. 17, p. 2342Đ2344. Murray, B. C., Belton, M. J. S., Danielson, G. E., Davies, M. E., Gault, D. E., Hapke, Bruce OŐLeary, Brian, Strom, R. G., Soumi, Verner and Trask, Newell, 1974, MercuryŐs surface: Preliminary description and interpretation from Mariner 10 pictures: Science, v. 185, no. 4146, p. 169Đ179.