Summary of Lunar Stratigraphy- Telescopic Observations

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LIBRARY.

.1. Summary of Lunar Stratigraphy-

1 Telescopic Observations

--.· By DON E. W~LHELMS

., CONTRIBUTIONS TO ASTROGEOLOGY

GEOLOGICAL SURVEY PROFESSIONAL PAPER 599-F

Prepared on behalf of the National Aeronautics and Space A dminz"stration

\

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1970 UNITED STATES DEPARTMENT OF THE INTERIOR WALTER J. HICKEL, Secretar:v Y;'I GEOLOGICAL SURVEY William T. Pecora, Director

Library of Congress catalog-card No. 77-608123

For sale by the Superintendent of Documents, U.S. Government Printing . , .. ~Office, .Washington, D.C.. 20402- Price 60 cents (paper cover) ). CONTENTS

Page Page F1 ~bstract ------Lunar stratigraphic column-Continued Introduction ------­ 2 Imbrian System-Continued Previous work------3 Terra units of the Theophilus quadrangle F29 ~cknowledgments ------6 nfare material (formerly Procellarum Group) 29 ~pplication of stratigraphic principles to the moon __ 6 Nomenclature ------­ 30 Properties and sequence of lunar material units-­ 6 nfare materials with topographic relief _ 32 Lunar time-stratigraphic and geologic-time units _ 9 ~penninian and ~rchimedian Series ------32 Separation of observation and interpretation ___ _ 9 Imbrian or Eratosthenian Systems ------­ 33 Types of geologic units ------­ 11 Dark terra-mantling units ------­ 33 nfap conventions ------11 . Units with intrinsic relief ------33 Lunar stratigraphic column ------12 Eratosthenian and Copernican Systems ------­ 36 13 General observations ------­ Crater materials ------­ 36 15 Pre-Imbrian units ------­ nfantling materials ------­ 37 Pre-Imbrian and (or) Imbrian units ------22 Plains-forming materials ------­ 37 ..,.. 23 Imbrian System ------Units with intrinsic relief ------37 Definition and general features ------­ 23 39 Fra nfauro Formation ------­ 23 Slope material ------39 Definition ------25 nfiscellaneous units ------­ 39 nfembers ------25 ~bandoned names ------­ 40 Interpretation ------27 Crater material subunits ------Cayley and ~pennine Bench ~ormations ___ _ 27 Summary and-· conclusions ..,..,., __ _,,__,..; __ ..:...:_~...;.:. __ .:... __ .:.. __ _ 42 45 ··( Crater materials ------­ 28 References cited ------­ Materials near the Orientale basin ------28 Preliminary lunar geologic maps ------47

ILLUSTRATIONS

Page FIGURE 1. Index map showing status of mapping at the 1:1,000,000 scale as of late 1967------F2 2. Photomosaic of part of the lunar near-side showing areas covered by illustrations in this report______4 3. Photograph of the craters Copernicus and Eratosthenes and vicinity under low-sun illumination ______8 4. Photograph and geologic sketch map of the vicinity of the crater ~rchimedes ______: ______10 5. Rectified photograph of nfare Imbrium and its multiringed basin ______14 6. Photograph of nfare IIumorum ______~------19 7. Lunar ~stronautical Chart of IIyginus rille------20 8-14. Photographs of: 8. Central part of moon showing "Imbrian sculpture" ------~------21 9. The ~pennine nfountains and terra peninsula extending southward ______24 10. The type area of. the Fra Mauro Formation ______26 11. Flow lobes in Mare Imbrium------31 12. The craters Copernicus and Eratosthenes and vicinity under near-full-moon illumination______34 13. The ~ristarchus plateau and IIarbinger Mountains ------35 14. Part of the crater Theophilus and the Theophilus Formation ------38 15. Example of map explanation box for crater material subunits ------41

TABLES

Page TABLE 1. Composite lunar stratigraphic columnar section showing most units discussed in text ------F16 2. Formally defined lunar stratigraphic names ------39 3. Stratigraphic names that have appeared in Geological Survey publications but have not been defined formally 40

iii CONTRIBUTIONS TO ASTROGEOLOGY

SUMMARY OF LUNAR STRATIGRAPHY-TELESCOPIC OBSERVATIONS

By DON E. WILHELMS

ABSTRACT pre-Imbrian basins, but greater degradation hinders their The geology of much of the moon has been mapped from recognition. the earth by means of telescopic photographs and observa­ Sufficient time elapsed between the formation of each mar~ tions. This mapping is based on the premise that the lunar basin and the completion of its flooding by the dark smooth crust is composed of material units that can be defined by material called mare material for extensive units to form. physical properties and ranked in order of relative age by Those units in or near the Imbrium and Orientale basins are application of the principles of stratigraphy. Units are of Imbrian age, whereas those in the pre-Imbrian basins may ranked locally by their superposition and transection relations be either pre-Imbrian or Imbrian in age. The most common and both locally and regionally by subordinate criteria such units are crater materials and light plains-forming materials. as density of superposed craters and degree of topographic Crater materials include those of Archimedes in the Imbrium y.· freshness. basin and of Gassendi in the Humorum basin. Light plains­ A moonwide stratigraphic column of material units has forming materials of Imbrian age include the Apennine been divided into approximate and tentative time-strati­ Bench Formation on the shelf south of Archimedes and the graphic ·units, whose type areas are near the craters Coper­ Cayley Formation in troughs surrounding the Imbrium basin. nicus, Eratosthenes, and Archimedes. The oldest time-strati­ Most light plains-forming materials are confined to depres­ graphic unit is the pre-lmbrian (not a formal system), sions or low plateaus. Other light materials of probable followed by the lmbrian, Eratosthenian, and Copernican Imbrian age form undulating or rolling topography and Systems. . elevated plateaus. Pre-Imbrian materials have a subdued topography and are The top of the mare material between the craters Eratos­ highly cratered. Most are associated with craters and circular thenes and Archimedes has been used to define the top of the multiringed mare basins. Many pre-Imbrian craters originally Imbrian System. Most mare material is Imbrian in age but may have resembled the young crater Copernicus but are now some is post-Imbrian. The youngest mare materials ar~ the modified. The formation of each multiringed mare basin was darkest and least cratered; the oldest are almost as light and a major event which obliterated the earlier geologic record as heavily cratered as the light plains-forming materials. over a large area and started a new stratigraphic sequence. Indeed, mare and light plains materials may be composition­ On the basis of density of superposed craters and general ally similar, though different in age. Mare materials were freshness of appearance, the sequence of formation of some once called the Procellarian System and when this name was pre-Imbrian multiringed basins is believed to be, from oldest dropped, the Procellarum Group. The latter name as well as to youngest: Fecunditatis, Serenitatis, Nectaris, Humorum, the two series of the Imbrian System, the Apenninian and Crisium (or Crisium-Humorum). Other less distinct basins Archimedian, should be dropped. are also pre-Imbrian in age. Materials whose age relative to Imbrian mare material is Deposits called the Fra Mauro Formation that partly doubtful have been designated "Imbrian or Eratosthenian." surround another fresher appearing mare basin, the Imbrium They include crater materials, dark terra-mantling materials, basin, are used to define the base of the Imbrian System. The and materials that form domes, cones, sinuous rilles, and Fra Mauro apparently forms a blanket that grades in texture prominent stringy ridges. The dark mantling units occur and thickness away from the basin, and otherwise resembles mainly at the margins of maria and may be partly inter­ the ejecta of craters such as Copernicus that are believed to bedded with mare materials. have been produced by impact. Accordingly the Fra Mauro The two youngest lunar time-stratigraphic systems the Eratosthenian and Copernican, have been defined on the basis is believed to be ejecta from the impact that formed the of the physical properties of crater materials. Materials of Imbrium basin. Structures radial to and concentric with the rayless craters that are superposed on Imbrian mare material basin also apparently were formed at the same time as the are assigned an Eratosthenian age; some superposed on terra basin. The Fra Mauro and the structures divide the materials are also doubtless Eratosthenian but are not telescopically in a vast area of the earthward hemisphere into pre-Imbrian distinguishable from Imbriari crater materials. Materials of and younger units. A deposit like· the Fra Mauro Formation craters that have rays or dark halos are considered Coper­ covers a similarly vast area around the younger Orientale nican in age; they also appear fresh and have a high thermal basin on the west limb of the moon. Similar material units anomaly at eclipse. Some noncrater materials are also as- . may also have surrounded Humorum and the other circular signed Eratosthenian and Copernican ages. They include

Fl F2 CONTRIBUTIONS TO ASTROGEOLOGY some mare materials, a dark terra-mantling deposit, a possible major lunar features-especially the maria, mare light mantling deposit, a light plains-forming deposit, and basins, and large craters-and demonstrated their several kinds of probable constructional deposits. Bright slope diversity. Speculation about origins has been delib­ material apparently indicates fresh exposures of underlying units and is considered Copernican in age. Many other young erately held in check, however, by means of the deposits of ·various relief forms and albedo are known. mapping methods and conventions; so the maps Similar deposits probably were formed in earlier periods but should retain their validity even though concepts have been obscured by mass wasting, meteorite impact, and concerning formative processes change. The maps mantling by younger materials. will serve as a framework in which later, more re­ The ultimate goal of lunar geologic mapping, after mate­ rial units are recognized, is genetic classification. Most of the fined data about origin, composition, and absolute near-surface lunar crust is probably composed of complexly age can be evaluated. interfingering deposits of volcanic and impact origin, modified Lunar geologic mapping is based on the concept by mass wasting. Most mare and some light plains-forming that the crust of the moon, like that of the earth, is materials are believed to be volcanic flows because they neither homogeneous nor randomly heterogeneous terminate against higher topographic forms and because some mare materials have lobelike scarps resembling flow fronts. but is composed of material units, each having a The dark mantling materials and some light ones are prob­ limited range of properties and a limited lateral and ably free-fall tuff or ash-flow tuff. Materials of some craters, vertical extent. The relative uniformity of a unit such as those alined in chains, are almost certainly of vol­ canic origin. Craters that have extensive ejecta blankets, fields of satellitic craters, and extensive ray patterns are N probably of impact origin. The circular mare basins are also believed to be of impact origin because of their resemblance to the large probable impact craters and because of the great areal extent of their related structures.

INTRODUCTION The U.S. Geological Survey, on behalf of the Na­ .. tional Aeronautics and Space Administration, is en­ gaged in a program of systematic lunar geologic mapping based on extension of terrestrial strati­ graphic methods and principles. This paper summa­ rizes the stratigraphy that was developed while lunar geology was being mapped from telescopic ob­ servations, before extensive study of photographs taken by spacecraft began. The period covered is from 1960, when the program was begun by E. M. Shoemaker, to late 1967, when 36 quadrangle maps at a scale of 1:1,000,000 had been completed (fig. 1). The term "geology," whose roots refer to the study of land or ground as well as of the planet s earth (Ronca, 1965), is retained for this study of the moon to emphasize the similarity in basic ap­ FIGURE 1.-Index map showing the status of mapping at proach with terrestrial geology and to avoid prolif­ 1:1,000,000 scale as of late 1967. Maps of the area in heavy eration of terms. outline are either published or in press, and the stratigra­ phy portrayed is discussed in detail in this report. Pre­ The principal objectives of the telescopic recon­ liminary maps of the area in medium-weight outline were naissance mapping were to delineate the major completed and released in the open files. Number below elements of lunar stratigraphy and structure on the quadrangle name is the map number in the Geologic Atlas of the Moon, published by the U.S. Geological Sur­ near side of the moon, to develop principles and vey. Number above quadrangle name is the chart number techniques for remote geologic mapping, and to pro­ in the Lunar Astronautical Chart (LAC) series prepared vide the type of geologic framework essential for by the U.S. Air Force Aeronautical Chart and Informa­ effective targeting· of Ranger, Surveyor, and Lunar tion Center (ACIC), St. Louis, Missouri. The LAC charts (part of one is shown in fig. 7) serve as bases for the Orbiter spacecraft. The telescopic work also helped geologic maps. Quadrangles were called regions in the to narrow the range of possible origins for the first 10 geologic maps published. SUMMARY OF LUNAR STRATIGRAPHY-TELESCOPIC OBSERVATIONS F3

suggests that it was formed under a limited set of rounding the craters Copernicus, Eratosthenes, and conditions and within a limited time. The age rela­ Archimedes. McCauley (1967b) discussed the geol­ tions of these units can be determined from their ogy of a larger region and reviewed changes. in spatial relations because the principles of sequence stratigraphic thinking that had occurred since the ,j. are as valid on the moon as on the earth: in a depo­ early work, especially the clarification of rock­ sitional sequence that has not been overturned, stratigraphic and time-stratigraphic concepts. An­

I_ younger rocks overlie older rocks; intrusive rocks other regional study appeared in the form of a are younger than the rocks they intrude; rocks cut 1: 5,000,000-scale map which summarized the geol­ by faults are older than the faults. Because these ogy shown in preliminary 1: 1,000,000-scale maps of relations are commonly revealed in the pattern of the equatorial belt (lat 32° N.-32° S., long 70° the units and their contacts at the surface, they can E.-70° W.) (Wilhelms and Trask, 1965a; Wilhelms, be observed on photographs of the surface and Trask and Keith, 1965) .1 mapped. Photographs may show the overlap or em­ Before the present program began, the Survey bayment of an older unit by a younger unit, or the prepared thr.ee synoptic full earthside maps at .a contact of a younger unit transecting the contact scale of 1:3,800,000 for the U.S. Army Engineers between two older units. This concept of discrete (Hackman and Mason, 1961; Mason and Hackman, mappable units occupying specific stratigraphic po­ 1962). In addition to delineating the major lunar sitions reduces the enormous complexity of a plane­ physiographic provinces and discussing. the inferred tary crust to comprehensible proportions and allows engineering characteristics of the surface, Hackman much to be learned through remote means about the and Mason presented a generalized photogeologic , r crust's structure, history, and formative processes. map showing three stratigraphic units. The first section of this paper discusses the gen­ The only other systematic lunar geologic mapping eral principles by which lunar stratigraphic· units known to the author is that of the Soviet scientists are recognized, ranked in order of relative age lo­ Khabakov (1962, p. 275-303). anlunar craters on the basis of theory or in figures 3-14. comparison with terrestrial analogs. 'I • PREVIOUS WORK Two early Survey geologists who studied the This summary of lunar stratigraphic work by the moon were Gilbert (1893) and Shaler (1903). Gil­ U.S. Geological Survey is a sequel to several shorter bert argued for an impact origin for most craters summaries. Shoemaker (1962) and Shoemaker and and for the moon's largest feature, the basin of Hackman (1962) established the stratigraphic sys­ Mare Imbrium, but did not exclude a volcanic origin tem upon which the present system is based. Their 1 A new map at a scale of 1: 5,000,000, based principally on Lunar Orbiter · IV photographs and covering a larger area, has been prepared (Wilh~lms pioneering studies concentrated on the region sur- and McCauley, 1969. ·

.. F4 CONTRIBUTIONS TO ASTROGEOLOGY

FIGURE 2.-Photomosaic of the lunar near-side showing areas covered by illustrations in this r~ort. Information SUMMARY OF LUNAR STRATIGRAPHY-TELESCOPIC OBSERVATIONS F5

N 0 100 200 400 MILES

0 200 400 KILOMETERS

APPROXIMATE SCALE, IN CENTER OF DISK

....

10'

....

s Base modified from LEM-lA (2d ed., Nov. 1962), published by Air Force Aeronautical Chart and Center, F6 CONTRIBUTIONS TO ASTROGEOLOGY for some features. Shaler and the non-Survey geolo­ uniform and nonlayered. Therefore, properties for gists Spurr (1944, 1945, 1948, 1949) and von Bulow lunar units are sought that ( 1) represent units of (1957), among others, assumed an internal origin finite thickness, rather than a surficial skin, and for nearly all lunar features and in their elaborate that (2) reflect lithology, rather than the effects of works gave scant attention to the alternatives. Per­ postformational processes. Most importantly, every haps the clearest early statement of stratigraphic effort must be made to recognize those units which principles applied to the moon was by the stratigra­ were formed in a discrete time interval relative to pher Barrell (1927), but he seems to have done no adjacent units ..Where true units cannot be recog­ systematic work. Spurr devised a highly subjective nized, as in complex, poorly photographed or heav­ time scale. The geologist Dietz (1946), the astro­ ily modified terrain, two-dimensional units ·are physicist Baldwin (1949, 1963), and the photogeolo­ mapped which at least do not impose premature in­ gist Hackman (1961), all proponents of the impact terpretations. hypothesis, pointed out that lunar craters exhibit Of all the properties by which lunar material varying states of apparent preservation which per­ units can presently be defined, topography is the mit an assessment of relative age. Numerous pa­ one that most directly reflects meaningful proper­ pers deal with relative ages based on crater popula­ ties of three-dimensional rock bodies underlying the tion studies (for example, Dodd and others, 1963; surface. Topography is best revealed by shadows Baldwin, 1964; Hartmann, 1967). Among the im­ cast by a low sun-that is, near the terminator­ portant recent works of wide scope are books by and disappears near full moon, when no shadows ... Baldwin (1949, 1963) and the astronomer-geologist are cast (compare figs. 3 and 12). In general, sim­ Fielder (1961, 1965) and a paper by the astronomer ple 6verall geometric form and lateral uniformity or Kuiper (1959). regular gradation of texture sugg,est uniform lithol­ ogy, although they may also reflect a common struc­ ACKNOWLEDGMENTS tural or erosional history. True units can be confi­ The work described here is the product of a dently identified if they have a topographic texture collective effort and is by no means solely that of that appears to be a primary depositional pattern. the writer; all lunar geologists of the Geological Such patterns include flow lobes, flow lineations, Survey have contributed to our knowledge of lunar and hummocks that coarsen toward a likely source, stratigraphy through their maps, reports, and oral such a crater. Additionally, the thickness of some discussions. In particular, E. M. Shoemaker, J. F. units can be estimated from the extent to which McCauley, R. E. Eggleton, Harold Masursky, and they bury craters and valleys (Marshall, 1961; Eg­ the writer have been active in systematizing the gleton, 1963). If is important to stress that in lunar stratigraphy. The following colleagues read the geologic mapping as in all geologic mapping, ma­ manuscript critically and contributed many valua­ terials are the things actually mapped and not topo­ ble and appreciated suggestions: J. F. McCauley, G. graphic forms-crater materials, not craters; plains­ W. Colton, H. G. Wilshire, M. H. Carr, and J. E~ forming materials, not plains. Carlson of the Geological Survey and T. A. Mutch Properties of the uppermost surface materials and R. S. Saunders of Brown University. may also reflect underlying lithology, just as the APPLICATION OF STRATIGRAPHIC properties of many terrestrial soils are related to PRINCIPLES TO THE MOON the composition of the underlying bedrock. The cor­ PROPERTIES AND SEQUENCE OF relation may be only indirect, as with many soils, LUNAR MATERIAL UNITS because external processes such as impact cratering, Material units of the lunar crust display various mass wasting, and radiation may have altered the topographic and surficial properties that can be original properties. Some distinctive properties can studied by visual observation or by means of instru­ : be used to characterize units even when the proper­ ments, through the telescope and on photographs. ties are poorly understood. The property most com­ These units are mapped by outlining areas that monly used is reflectivity of incident light (bright­ have a simple combination of such properties. Prop­ ness) in the visual spectrum, expressed either in erties can be combined in various ways to define relative terms or as a numerical value of albedo, units, and geologic judgment is necessary to select the reflectivity at full moon. Brightness differences units that are stratigraphically significant. The are enhanced while topographic differences disap­ most meaningful units on earth are three-dimen­ pear as the sun angle increases and fewer shadows sional bodies that are_ either tabular and layered or are cast. Albedo is commonly highest on steepest l SUMMARY OF LUNAR STRATIGRAPHY-TELESCOPIC OBSERVATIONS F7

slopes, so that the brightest units .at full moon ap­ that the formation of rim material is related to the pear roughest at low sun. However, there are also formation of the crater and that the material prob­ rugged dark units, as well as both light and dark ably consists of ejecta from the crater which may plains units.2 be of either impact or volcanic origin. The rim Another informative and thoroughly mapped material of an impact crater consists of debris property is radiation in the thermal region of the ejected while the crater is being formed; it prob­ infrared at eclipse (Shorthill and Saari, 1966; ably covers upthrust rock. The ejecta from a Saari, Shorthill, and Deaton, 1966). Eclipse in­ caldera consists of volcanic material erupted before frared values have been interpreted in different and during collapse. Ejecta from a maar, also vol­ ways but appear to correlate mainly with freshness canic, is, erupted sporadically over a period of time. of exposure (U.S. Geological Survey, 1967, p. The age of the rim deposits relative to other mate­ A132). Radar reflectivity may have similar meaning rials can be deduced from the map pattern. Where but has not been compared systematically with younger than other materials that surround the cra­ lunar geologic units. Reflectivity in other nonvisual ter, the rim material forms an unbroken roughly •· wavelengths, color (Whitaker, 1.966, p. 79-85), and circular pattern with a fringed or lobate boundary, polarization (Wilhelms and Trask, 1965b) are use­ and the concentric morphologic belts or facies are 1 ful for delineating some units. uninterrupted. Where older, it forms a circular map Two examples will show how lunar material units pattern interrupted by the younger materials, and believed to be true three-dimensional rock units are parts of one or more of the rim facies may be miss­ 1•• recognized and how superposition and transection ing at the surface. The pattern of the Archimedes w relations between them are deduced. First, the units rim material illustrates both relations (fig. 4). The which make up the maria have a flat surface and part of the rim material on the Apennine bench low albedo, which, taken together, distinguish the shows a continuous pattern, which indicates that it maria from other lunar· surface areas~· Surfaces is superposed, on the betich. ~n'd is ..-· younger~ How­ with these characteristics extend laterally for great ever, parts of the rim are covered by mare material, distances on the moon's near side. In many places and Archimedes is therefore older than the mare ...... these flat, dark surfaces terminate abruptly against surface material. This age relation is further con­ the rugged, generally lighter terra and continue firmed by the presence of mare material inside the into the intervening valleys (fig. 3). The most rea­ crater. sonable interpretation of this pattern is that the Additional evidence for the ages of crater mate­ mare surfaces are formed by layers of material em­ rials relative to adjacent material is provided by sat­ placed in a fluidlike state and that the mare mate­ ellitic craters-small elongate or composite craters rial is younger than the terra it embays. which surround a large circular crater and probably Crater rim materials, another important class of were formed nearly simultaneously with it. Most of lunar materials, are the second example. Rim mate­ these large craters surrounded by satellitic craters rial is defined as all material with positive relief seem to have formed by impact, and the satellitic from the rim crest outward. In young craters like craters seem to have formed by secondary im­ Copernicus (fig. 3) the texture of rim material sug­ pact of material ejected from them (Shoemaker, gests that at least the surface material was depos­ 1962). However, the stratigraphic meaning of the ited in a layer. The rim material deposits form gen­ superposition relations is the same under any inter­ erally uniform belts surrounding the crater and are pretation, provided that the satellitic craters were regularly gradational outward. Near the rim crest formed at about the same time as the parent cra­ is a high ring whose surface is randomly hummocky ters. The satellitic craters of Copernicus are clearly or has a pattern of branching convex-outward arcs; superposed on the surrounding mare surface and farther from the rim crest (starting at one-quarter thus are younger than the mare material; those of to one-third crater diameters), the rim material has Archimedes are present on and are therefore less relief and is characterized by radial ridges and younger than the bench material, but are obliter­ grooves. This radially symmetric pattern indicates ated by and are older than the mare material. Stratigraphic columns can be constructed locally t The tenn "mare" (plural; maria) is used to describe the dark plains; the antonym "terra" (plural, terrae) is used to describe light plains and by the methods just described with little recourse to both light and dark rugged terrain, but not individual craters. These genetic interpretations. A fairly detailed sequence terms are descriptive not positional, and mare material sometimes occurs as small patches in predominantly terra regions, and vice versa. has been worked out in the Archimedes area (fig. '::rj 00

8 z ;d tl:l -c:::..., z-0 ...,UJ 0

UJ> ~ ~ 0 t" 0 C) ....::

FIGURE 3.-Craters Copernicus (center), 90 km in diameter, and Eratos­ at top and east at right in this and all ensuing photographs, analogous thenes (upper right), 60 km in diameter; Carpathian Mountains (Montes to the orientation of terrestrial maps, the so-called astronautical conven­ Carpatus) above Copernicus and Apennine Mountains (Montes Apenninus) tion. Unpublished photograph 1196 taken with the 61-inch reflector at above Eratosthenes. Both mountain ranges, part of a high terra ring sur­ Catalina Observatory, University of Arizona Lunar and Planetary Labo­ rounding Mare Imbrium (see figs. 2, 5), are embayed by the younger mare ratory. Photographs taken with the Catalina telescope were unpublished at material. Rim and ray materials of Copernicus exhibit radial symmetry this writing, but many are now available in an atlas (Kuiper and others, and are younger than the mare material. Geology of this area was mapped 1967). by Schmitt, Trask, and Shoemaker (1967) and by Carr (1965b). North is

.. :t 'Y I .r. • .. .> l SUMMARY OF LUNAR STRATIGRAPHY-TELESCOPIC OBSERVATIONS F9

4) (Hackman, 1964, 1966; Shoemaker, 1964). The p. 344-351; Shoemaker and Hackman, 1962). Fol­ oldest recognizable unit is the rugged and strati­ lowing terrestrial convention, the major time-strati­ graphically complex terrain called "undivided mate­ graphic units are designated systems and their sub­ rial" of pre-Imbrian age in and northwest of the divisions, series. The corre8ponding geologic-time Apennine Mountains ("Apennines" in fig. 2). This units are periods and epochs, .. respectively. The undivided material is partly covered by the Fra three systems recognized at present, from oldest to Mauro Formation, a distinctively textured unit youngest, are the Imbrian, Eratosthenian, and Co­ (p. 23-27). Next is the smooth fiat light-colored pernican. (One system fewer than proposed by Shoe­ material of the bench (Apennine Bench Forma­ maker and Hackman, who included a Procellarian tion), which embays the rugged terrain as does System.) Materials older than the Imbrian System mare material. Following the light plains material are not yet assigned to systems and are designated are the crater material of Archimedes and the dark simply as pre-Imbrian. The Imbrian System has been mare material. Youngest are materials of two rayed divided into two series, the Apenninian and Archi­ craters (Aristillus and Autolycus, of Copernican median (Shoemaker, Hackman, Eggleton, and Mar­ age) which, like the materials of Copernicus, clearly shall, 1962). These series have been used on some overlie the mare material. maps but have never been formally proposed; the Secondary criteria are employed to supplement author recommends that they be abandoned (p. superposition and transection relations in determin­ 32-33). .,.. ing relative ages. These criteria include the density The systems are defined in type areas in the re­ of superposed craters and the apparent freshness of gions where lunar stratigraphy was fir~t worked units. Older units generally have more and larger out, near the craters Copernicus, Eratosthenes, and superposed craters than younger units of the same Archimedes. (Type area is a necessary lunar substi­ gross morphology and composition, except where tute for the terrestrial type section.) The base of superposed craters are locally and anomalously con­ the Imbrian System is the base. of the .Fra Mauro centrated. If the morphology of craters of similar Formation, the unit at the surface in much of the size is assumed to have been originally the same, Apennine Mountains, Carpathian Mountains, and the materials of craters with subdued rims are older the highland between the craters Copernicus and than those of craters with sharp rims in the same Fra Mauro. At the top of the Imbrian System is size category. mare material, and although no specific type area The value of a stratigraphic analysis is well illus­ has been formally defined, the area of mare mate­ trated by the preceding example. Archimedes is in rial between Eratosthenes and Archimedes is gener­ the Imbrium basin, the largest mare basin, and ally considered a type area (p. 23). Materials of ..... much of the history of this basin was deduced from rayless craters such as Eratosthenes, which are su­ the stratigraphic relations in and around Ar­ perposed on the mare material are assigned to the chimedes. These relations show, for example, that Eratosthenian System. Materials of rayed craters the mare material filling the Imbrium basin is (and many dark-halo craters) are assigned to the younger than the basin itself-sufficiently younger Copernican System, because the rays of Copernicus for the Apennine Bench Formation and materials of clearly overlie materials .of the nearly rayless crater Archimedes to have been formed before emplace­ Eratosthenes (figs. 3, 12). Although most rayed ment of the mare material. craters are younger than rayless ones, exceptions are known. Lunar time-stratigraphic units there­ LUNAR TIME-STRATIGRAPHIC AND GEOLOGIC-TIME UNITS fore express only approximate correlations and the Local stratigraphic sequences of lunar material approximate position of separated units in lunar units such as those of the Archimedes area are cor­ geologic history as a whole. They will be retained related by reference to widespread units or by for this purpose on most maps but will remain ten­ applying less certain assumptions, for example, that tative until moonwide correlations are made from the density of superposed craters is related to the spacecraft photographs. age of the underlying bedrock. A lunar strati­ SEPARATION OF OBSERVATION graphic column has been built up of lunar material AND INTERPRETATION units and, like the terrestrial column, has been di­ The foregoing discussion needs several qualifica­ vided into time-stratigraphic units for convenience tions which are partly common to geology in gen­ in summarizing geologic history (Shoemaker, 1962, eral and partly specific to the fledgling science of FlO CONTRIBUTIONS TO ASTROGEOLOGY

EXPLANATION 0 50 \ [£[] KI LOMETERS I ·i ' 00;~' I .;,./··-.,_. \ Crater material \ ( Ari~tillus ). · .\ I t­ ):__,,· .. · .Cc. ·. · ·. · J Crater material VI / z / / ...... :... . __...... , __,.. ~'-----' <>: / I . . ,.- /"""·-:--\ . ., / r::~~b::J / / A~tolycus . aJ"' / Apennine Bench Formation ::; / . "/ / / . "'-:_:...· . / / !Del / /' Fra Mauro Formation Im z 1 :rpr~: 1 Undivided material May be covered w i th thin lay­ er of Fra Mauro Formation H"-

Contact Dashed where approxi­ mately located ( =) Crater-rim crest

Approximate boundary be­ tween Imbrium inner bas­ in (upper left) and shelf SUMMARY OF LUNAR STRATIGRAPHY~TELESCOPIC OBSERVATIONS F11

lunar geology, which at this writing was still based finite three-dimensional extent that is known to solely on remote data. To control premature specu­ some degree, whereas the third dimension of lunar lation, the three operations discussed above must be material units is generally inferred. Second, rock­ clearly separated: objective delineation of units, in­ ·stratigraphic units are defined mainly by lithology, terpretation of these in terms of underlying mate­ whereas many distinguishing properties of lunar rials, and correlation of separated units. The means material uni.ts may prove to be secondary; certain for distinguishing fact from inference while con­ of their features may have been superposed by veying both are (1) the concurrent use of sev­ structural, erosional, impact, · or other processes. eral conceptual types of geologic units and (2) the Third, lunar material units are probably more com­ conventions, explanation, and text on the geologic plex than most terrestrial rock-stratigraphic units; map. even relatively simple lunar material units may con­ sist of an overlapping sequence of related rock units TYPES OF GEOLOGIC UNITS that on earth would be called a complex, or ~ven of A continuing problem in terrestrial stratigraphy unrelated rocks of similar visible properties. Such a has been premature inferences about the ages of complex lunar material unit may be a tectonic unit, material units. To achieve greater objectivity, the analogous to the Colorado Plateau for example. Stratigraphic Code (American Commi~sion Strati­ These differences resulting from uncertainties graphic Nomenclature, 1961) has distinguished be­ about lunar materials require that the definition of tween rock-stratigraphic units, material subdivi­ ..,. lunar material units be free of inferences not only sions of the crust that are distinguished solely on about age and origin but also about intrinsic lithol­ the basis of lithologic properties, and time-strati­ ogy and subsurface form. This is not to say that graphic units, material units which include all rocks inferences should not be made about lunar units, formed in a specific interval of time. Rock-strati­ only that they be kept separate from definitions. graphic units are the practical mapping units and Criteria selected to define units, regardless of the are the basis for defining time-stratigraphic .units. degree of interpretation that contributed to their A third unit is a nonmaterial subdivision-the geo­ selection, must be objective and recognizable by logic-time unit, which is defined in terms of time­ other geologists. All lunar material units are sub­ stratigraphic units. An effort to clarify the distinc­ ject to revision. The simpler the combination of tion among these three types of units for lunar defining properties-where topographic and surfi­ stratigraphy was undertaken by lunar geologists of cial properties coincide-the greater the confidence the Geological Survey in late 1963, and a strati­ that a valid unit has been recognized. However, as graphic classification parallel to the terrestrial one noted above, the units are probably more complex .... is now in use. (See McCauley, 1967b.) than presently depicted, and their components may The lunar parallel to the terrestrial rock-strati­ be recognized in the future. Properly defined units graphic unit is the lunar material unit. It is here will serve as a springboard for new work, whereas defined as a subdivision of the materials in the premature time-stratigraphic or other inferences moon's crust exposed or expressed at the surface will retard recognition of true units. and distinguished and delimited on the basis of physical characteristics. Although lunar material MAP CONVENTIONS units and rock-stratigraphic units are thus parallel Conventions and explanations of lunar geologic in definition and purpose by being objective practi­ maps are parallel to those of terrestrial maps but cal mapping units independent of inferred geologic are specially treated to separate interpretation from history, they differ in enough ways to make the spe­ observation. In the explanation for each unit, one cial lunar term desirable. First, the Stratigraphic paragraph labeled "characteristics" defines and Code implies that rock-stratigraphic units have a describes the unit, while a second labeled "interpre-

FIGURE 4.-Upper: Vicinity of the crater Archimedes. Photo­ than the materials of the Apennine bench (including graph (L-35) taken with the 120-in. reflector, Lick Ob­ rugged materials and the plains-forming Apennine Bench servatory. Part of the excellent series taken by G. H. Formation) but older than the mare materials of Mare Herbig and one of the best telescopic photographs ever Imbrium and Palus Putredinus. The Apennine Mountains made. Features as small as 130 meters in diameter are (Montes Apenninus) are part of the high arc surrounding detectable, and the shape of features as small as 200 meters the Imbrium basin, and the Apennine bench is par~ of the can be discerned. Lower: Geologic sketch map of area in shelf. upper view (after ltackman, 1966). Archimedes is younger F12 CONTRIBUTIONS TO ASTROGEOLOGY tation" suggests a lithoJogy, form at depth, history, sion, not at the inferred or projected limits. Some and an origin that seem consistent with the charac­ units are designed to show both the underlying and teristics. Interpreted relative age is shown by the overlying layers. (See section entitled "Terra units order of units in the explanation. Geologic sections of the Theophilus quadrangle".) combine the interpretations of relative age and A lunar map unit commonly represents seyeral form at depth. A text summary accompanies the occurrences of like materials, such as crater mate­ most recent maps. rials or ponds of plains material, each of which may ,,'· Stratigraphic nomenclature and map conventions have formed at a different time but within a gj.ven reflect the state of knowledge about lunar units. interval of time. Each individual occurrence of-such Units interpreted as true three-dimensional rock material is regarded as a formation, as it would be units are given either formal names derived from on earth, but for convenience all are given the same geographic place names (for example, Apennine name. Accordingly, two occurrences of the same Bench Formation) or informal but specific descrip­ unit may be separated by a contact (for example, tions followed by the word "material (s)" (for where materials of one Copernican crater are example, mare material). Formal names may also superposed on those of another) ; this is not done on be terms of convenience for units not confidently terrestrial maps. The symbol for a lunar map unit, interpreted as rock units. If individual rock units like its terrestrial counterpart, consists of an abbre­ which contribute to the observed character of a viation of the system to which it is assigned (capi­ lunar material unit are not recognized, the unit is tal letter) and an abbreviation of the formal or noncommittally designated by a terrain description informal name (lowercase letters). Units that may followed by the word "material (s) ," such as "hum­ belong to either of two systems are given symbols mocky material," "regional pitted material," or with two capital letters, representing both systems "ridge material." All names are objective, not (youngest first). If the age of a unit is unknown or interpretive---"crater rim material," not "impact only approximately known, capital letters are omit­ ejecta" or "volcanic rocks." ted. Two or more superposed lunar material units are Detailed discussion of structural symbols is commonly recognized. The unit that contributes the beyond the scope of this paper; suffice it to say that surficial properties, such as albedo, may not appear they too follow terrestrial precedent with allow­ to be the unit that contributes the dominant topogra­ ances for lunar uncertainties. Mapped lunar fea­ phy, because boundaries between albedo and topo­ tures that resemble terrestrial features include graphic properties do not coincide. Alternatively, faults (some quite definite and some inferred), two superposed units, such as materials of two cra­ lineaments (indistinct linear depressions), probable ters, may both contribute to the surface topography. slump blocks', and circular and irregular depres­ If two superposed units are present, the one sions. Peculiar lunar features ·include riverlike sin­ regarded as most significant is shown in color. This uous rilles and mare ridges, scarps, and troughs. unit ordinarily is the youngest unit that contributes (Sinuous rilles and some linear rilles have been noticeable topographic expression, but it may be an shown as geologic units instead .of as structures on albedo unit (a dark or light patch) whose topo­ some maps.) The buried craters not shown by con~ graphic expression is that of an underlying unit. cealed contacts are shown by a rim-crest symbol. Opinions differ about which unit should be stressed. Lunar geologic sections contain additional interpre­ The choice of surface unit depends largely on the tive symbols for breccia lenses and igneous intru­ scale of mapping. For example, at telescopic resolu­ sions. tion, the rim material of an old crater in the south­ ern highlands is prominent, and on maps of LUNAR STRATIGRAPHIC COLUMN 1:1,000,000 scale it is the unit mapped. But if space­ Most of the remainder of this paper is a system­ craft photographs show that the rim has a mantled atic review of lunar material units which have appearance, then various mantling materials may appeared on U.S. Geological Survey 1: 1,000,000- be the units mapped on larger scale maps based on scale maps. The units are grouped into time-strati­ these photographs. Two superposed units. commonly graphic units as they are on the maps. The oldest are shown by dotted contracts and sympols in pa­ units, of pre-Imbrian and Imbrian age, are dis­ rentheses for the underlying unit or by an overprint cussed first. These include the major lunar strati­ pattern for the overlying unit. Buried contacts are graphic datum planes, areally extensive units asso­ drawn.. at ~he .li:wit ,.. of.,ql;>~~rved t~pographic expres- ciated with mare basins. Less space is devoted to SUMMARY OF LUNAR STRATIGRAPHY-TELESCOPIC OBSERVATIONS F13

units of Eratosthenian and Copernican age, which Oceanus Procellarum as a v.Thole is localized in a are mainly of local extent. These units, which are giant multiringed basin. younger than the main mare basin filling, have The stratigraphic sequence in each circular mul­ greater morphologic variation .than the older units. tiringed · basin is grossly similar to the sequence Miscellaneous units, which are also mostly young established in the Archimedes area for the Imbrium and varied but which are usually not assigned ages, basin, although the units vary in age from basin to are described in a separate section. Crater mate­ basin (table 1). At the base of each local section are rials, the dominant lunar materials, are found in all the materials formed before the basin. These mate­ lunar time-stratigraphic units and are discussed rials are exposed in the raised rings surroun.ding repeatedly. A special section is devoted to crater the basin that probably were uplifted wheri the material subunits, which account for a large per­ basin was formed (figs. 3, 4, 5, 6). The individual centage of the units on Survey lunar geologic maps. preuplift units in these rings are now unrecogniza­ GENERAL OBSERVATIONS ble, but presumably they were much like units of Many basins that contain mare material are large that terra in the southern highlands not affected by complex circular structures hundreds of kilometers basin formation .. in diameter consisting of an i:nner basin and several Partly covering these rings are materials believed outer concentric troughs separated by raised rings to be deposits of debris ejected. from the basin 8 during its formation. Materials interpreted as (Hartmann and Kuiper, 1962; Baldwin, 1963) • ejecta are seen around three basins-Imbrium Ori- These circular multiringed mare basins are the ' largest features on the moon and dominate its stra­ entale, and Humorum (fig. 6)-and patchy hum- tigraphy and structure. The formation of one is a mocky materials near other basins may also consist major event which largely obliterates the earlier of ejecta. stratigraphic record in its vicinity and starts a new Two kinds of units that are younger than the stratigraphic succession. The· time· of formation of basin-related materials are ·light plains-forming· the basins relative to one another has been esti­ materials and crater materials. The light plains­ mated by the freshness of structures surrounding forming materials fill troughs between the raised the basins and by the density of craters superposed arcs and cover the shelves of the basins (figs. 4, 5, on these structures. Tentatively, the sequence of 6) . In the Archimedes area of the Imbrium basin basins, from oldest to youngest, appears to be: (fig. 4), these materials are older than the crater Fecunditatis (the northern, larger part), Serenita­ Archimedes, but in most areas there is a complex tis, Nectaris, Humorum, Crisium, Imbrium, Orien­ overlapping sequence of plains-forming and crater tale; or Crisium may have preceded Humorum. The materials. first five are pre-Imbrian in age, and the last two· Next younger in all basins, and covering part of are Imbrian; the Imbrian Period began with the the lighter plains-forming materials in the troughs formation of the Imbrium basin (fig. 5). Imbrium and shelves and filling the inner basin, is the wide­ is a large basin whose associated stratigraphic units spread dark plains-forming matel-ial called mare and structures dominate a huge area of the moon's material. Mare material (formerly designated the earthward side and to which the rest of the earth­ Procellarum Group) was ·once thought to be approxi­ side stratigraphy is referenced. The deposits of the matel.Y the same age in all basins, but more and younger Orientale basin form another important more exceptions to this are being found. Lastly, datum plane on the west limb, an area less fully crater materials and other materials of the Eratos­ studied by telescope because it is only partly visible thenian and Copernican Systems overlie or are from earth. Other basins whose circular multi­ interbedded with the mare material in all basins. ringed form is less clear, such as Nubium and Tran­ Craters-young and old, large and small-are the moon's dominant features. Many topographic fea­ \ quillitatis, may each consist of a circular ringed basin or of two or more coalescing but separately tures are recognized as parts of craters after allow­ formed basins. Oceanus Procellarum is an irregular ance is made for faulting and partial burial. generally depresse

FIGURE 5-(Explanation on opposite page) SUMMARY OF LUNAR STRATIGRAPHY-TELESCOPIC. OBSERVATIONS F15· radial rim-material facies and extensive fields of The Imbrium basin, like all large circular mare satellitic craters. Subdued craters have low rounded basins, is surrounded by several concentric belts of rims, shallow broad floors, and only a narrow ring alternating ridges and troughs (fig. 5). The most of rim material. Crater materials dated only by prominent topographic ring is composed of rugged freshness are placed in three broad overlapping cat­ arcuate mountain ranges named in different areas egories: pre-Imbrian, pre-Imbrian or Imbrian, and th~ Carpathians (fig. 3), Apennines (figs. 3, 4), and Imbrian or Eratosthenian. The overlap of the three Caucasus. The subsurface material of the rugged designations assures that a crater unit cannot be . ring is by definition pre-Imbrian, because the sur­ seriously misassigned. face material, except for a few apparent pre­ Crater materials of several other types, whose Imbrian islands, is the Fra Mauro Formation, differences in appearance apparently are not due which defines the base of the Imbrian System. This entirely to differences in age, are not assigned to pre-Fra Mauro material is no doubt a complex systems. These include materials of irregular high­ assemblage of rocks and where exposed has been rimmed craters, low-rimmed round or slightly elon­ mapped by most workers as "undivided (undiffer­ gate craters, and craters alined in chains or along entiated) material" of pre-Imbrian age (table 1). rilles, such as Hyginus rille (fig. 7). Although the materials in the ring surrounding Round or irregular depressions are commonly the Imbrium basin are pre-Imbrian, the ring proba­ mapped by depression symbols or sometimes by the bly owes its present form to structural deformation interpretive convention of arcuate faults and not as in the Imbrian Period. Most of this deformation material units. probably occurred at the beginning of the Imbrian Period wh~m the basin was formed. Under the PRE·IMBRIAN UNITS impact hypothesis the basin was formed nearly instantaneously, and the ring was uplifted by shock Rocks older than the Fra Mauro Formation, the waves from the impact and. i:rnmedi~tety;.-.coyered by basal unit of the Imbriah System, are complex and the Fra . Mauro Formation, wh1ch consists of the have been mostly left undivided; only a few distinc­ ejecta from the impact. Renewed movement and tive units have been mapped. The main pre-Imbrian degradation have modified the ring to an unknown units are undivided materials in raised arcs concen­ degree. tric with mare basins, complex units in interbasin Much terrain near the Imbrium basin is tran­ areas, and crater materials. Near the Imbrium sected by a system of scarps, ridges, and troughs basin, rocks are recognized directly as pre-Imbrian radial to the center of the basin. These features are in age where they appear to be buried by the Fra collectively termed "Imbrian sculpture" (figs. ·5, 8) Mauro Formation, or indirectly, where they are cut (Gilbert, 1893; Hartmann, 1963). Gilbert and sev­ by elements of Imbrian sculpture (see fig. 8), which eral others believed that the troughs and ridges were are believed to be mostly contemporaneous with the sculptured by flying debris from the Imbrium im­ Fra Mauro. Farther from the Imbrium basin, units pact, but the morphology of the so-called sculpture are dated and correlated with units that are near can more convincingly be explained as a series of the Imbrium basin chiefly by comparison of crater fault-bounded horsts and grabens. "Sculpture" re­ densities; units whose density of superposed craters mains a convenient term for the set of features is greater than that of the Fra Mauro are inferred radial to the Imbrium basin. Similar sculpture is 11 to be pre-Imbrian. Additionally, pre-Imbrian Crater present around other basins (Hartmann, 1964). material" is distinguished from Imbrian crater ma­ The assumption that much of the Imbrian sculp­ terial on the basis of its more subdued topography; ture originated when the basin was formed has pre-Imbrian craters have broad shallow floors and been used to date the transected units. Under any crater-pocked low narrow rims. hypothesis of origin, the close geometric relation

FIGURE 6.-Mare Imbrium and its multiringed basin. Three related to the basin, notably the long concentric trough concentric rings are outlined by the circles: an island ring· north of the basin (Mare Frigoris) . Note the radial approximately 670 km in diameter around the inner basin, troughs and ridges composing the "1m brian sculpture." (See a middle ring approximately 950 km in diameter including fig. 8). Rectified photograph and concentric circles by the Alpes and the rugged terra on the Apennine bench Hartmann and Kuiper (1962), courtesy of Lunar and south of Archimedes, and an outer ring, 1,340 km in Planetary Laboratory, University of Arizona. (Other diameter, whose highest part is the Apennine Mountains rectified photographs are available in an atlas by Whitaker ("Apennines" in fig. 2). Mare materials occupy_ depressions and others, 1963.) F16 CONTRIBUTIONS TO ASTROGEOLOGY

TABLE !.-Composite lunatr stratigraphic columnar [Symbols are those that have been used on· published maps. Subunits and satellitic crater materials are not shown. Units are arranged according to the mare to have been formed contemporaneously with each basin are shown in narrow horizontal boxes (for example, the Fra Mauro Formation); the relative positions

LUNAR MATERIAL UNITS SYSTEMS Orientale Humorum Imbrium

ca Cavalerius CEv Vallis rille Cc Fm. Reirier Cc CC ~::~tdark Sin~o~~material Schri:iteri COPERNICAN Cre er material Gam~a Crater material ater material Ch Formation Formation . v' . Cobra Head CEm ~ Formation Mare lpmd? ~aterial Ec lpmd? Eld Ec .Eis Ec Em ERATOSTHENIAN Mare Doppel- Sulpicius Crater material Marius Mare Crater material material, mayer Crater material Gallus Group material dark Formation Formation dark A------~----~ Elh / \ Harbinger / Formation Ipm v Ipm Mare material Mare· material Ide Mar!pm~terial Bo~!~ch Diophantus Formation \ Formation Ic lea 1M BRIAN Crater material Cayley Formation lc It Crater m·aterial Terra lh material lab Hevelius Formation Apennine lplp Bench I pig Formation I pic Iplu Gassendi Group ·Piai ns-forming 1------'-----'-----'------t Crater material Undivided material If Fra Mauro Formation

pip vv Plateau material plv Vitello Formation

PRE-1M BRIAN pic pic plu plrh Crater material Crater material Undivided ·- Material of plu material Undivided material pic smooth ridges and hills Crater material plu Undivided material I .. SUMMARY OF LUNAR STRATIGRAPHY-TELESCOPIC OBSERVATIONS Fl7

section showing most units discussed in text basin they are nearest, but many units are not genetically related to the basins; units not near one of the basins named are listed under terra. Units believed of these boxes therefore indicate the estimated relative time of formation of the basins]

LUNAR MATERIAL UNITS -continued

Serenitatis Nectaris Fecunditatis Crisium terra

,lope 1toriol Cc Cc ~he~~ilus~Forma- Cc 1 Cc • tion 7, Crater material ater material 1rater material rater material

Ipm lpm lpm lpm Mare material Mare material Mare material Mare material

I pic I pip Crater material Plains-forming material Complex units I pic I pic I pip Crater material Crater materiaiJ----. I pic Plains­ ':'rater material forming material I pip I pic I pip Plains-forming Crater material Plains-forming material material Q) c

-;;; ~ ·~ .t:~ c.~ ..... tiet------'------1 § pi he ::r Hummocky ~-----~------4 material. coarse

pic plu pihf Hummocky Crater material Undivided material, material line pic pic plu Crater material Crater material Undivided pic plu material Undivided Crater material material pic plu Crater material Undivided material

(Unit page references on next page) F18 CONTRIBUTIONS TO ASTROGEOLOGY

PAGE REFERENCES TO PRINCIPAL TEXT DESCRIPTIONS OF UNITS Ic Crater material (including that of Archimedes) (28) Crater units younger than Imbrian mare material: Iplc Crater material (22) Cc Crater material (light-rayed) (36) Iplg Gassendi Group (23) Dark-halo crater material (Ccd) (36) has same stratigraphic position Plains-forming and other noncrater· units younger than the Ec Crater material (36) basins but older than the mare material: It Terra material (28) Miscellaneous units younger than Imbrian mare material: Irregular terra (29) -f Cs Slope material ( 39) lea Cayley Formation (27) Cca Cavalerius Formation (37) lab Apennie Bench Formation (27) Cre Reiner Gamma Formation (37) Ik Material of Kant Plateau (29) Cmd Mare material, dark (32, 37) Iplp Plains-forming material (22) Csr Sinuous rille material (38) (not yet shown on published maps near Ch Cobra Head Formation ( 38) Nectaris, Crisium, and Fecunditatis basins) Ct Theophilus Formation (37) ( 1 ) Plains-forming unit of Imbrian age present but CEv Vallis Schroteri Formation (38) not yet named on published maps CErn Mare material (32, 37) Em Marius Group (37) Units contemporaneous with formation of basins: Emd Mare material, dark (32, 37) Ih Hevelius Formation ( 28) Et Tacquet Formation (37) If Fra Mauro Formation (23) plv Vitello Formation (22) Units partly contemporaneous with Imbrian mare material: ('2) Basin-contemporaneous units not yet named on Eic Crater material (33) published maps Eid Doppelmayer Formation (33) Eis Sulpicius Gallus Formation (33) Units older than the basins: Eih Harbinger Formation (35) Iplc Crater material (22) Eib Boscovich Formation (35) Iplu Undivided material (19) Ide Diophantus Formation (28) pic Crater material ( 15) Ipmd? Mare material, dark (30); may be younger than pip Plateau material (18) Imbrian mare material plu Undivided material (15, 22) plrh Material of smooth ridges and hills (22) Mare material of Imbrian age (formerly called Procellarum Group): Other units: lpm (29) Iplhf}plhf Hummocky material,. fine (22) Crater units younger than the basins but older than the mare material: plhc Hummocky material, coarse (22) between the radial sculpture pattern and circular common on terrain northwest of Ptolemaeus and basin implies a genetic relation. This relation is west of Herschel (fig. 8) than in much of the south­ easily explained by the impact hypothesis, which ern highlands. The missing pre-Imbrian craters additionally implies a temporal relation. Most of the may have been buried by thick deposits, mapped as faults were probably produced by the shock ·proceed­ "plateau material," before the terrain was faulted ing outward from the basin-forming impact (Shoe­ by 1m brian sculpture (Howard and Masursky, maker, 1962, p. 349).; however, some faults probably 1968). were preexisting fractures and were only empha­ Extensive pre-Imbrian materials beyond the sized by the impact, and others may have been limits of the Fra Mauro Formation and lmbrian rejuvenated since the impact (Masursky, 1964, p. sculpture occur in arcs concentric to other basins. 128-129; Howard and Masursky, 1968). Therefore, Examples are the Altai Scarp (Rupes Altai) and a crater or other feature that is close to the several lower arcs around the N ectaris basin, and Imbrium basin and is cut by many basin-radial two .prominent arcs bounded by steep jagged scarps fractures is likely to be pre-Imbrian, even though around the Crisium basin (Hartmann and Kuiper, one or two such fractures do not prove pre-Imbrian 1962; Wilhelms, Masursky, Binder, and Ryan, 1965, age. This assumption is particularly helpful because summarized in U.S. Geological Survey, 1966, p. the Imbrian sculpture pattern can be traced farther A124). Arcs around three other circular basins­ from the basin than the Fra Mauro Formation. Serenitatis, Fecunditatis, and Humorum (fig. 6)­ One sculptured pre-lmbrian unit of possible depo­ are less well defined. The Fra Mauro Formation has sitional origin has been described on published not been identified on any of these arcs; so there is maps. Pre-Imbrian crater remnants are less ·no direct means of estimating their age. However, SUMMARY OF LUNAR STRATIGRAPHY-TELESCOPIC OBSERVATIONS F19 the number of large craters superposed on the arcs lmbrian structures, only the Altai Scrap and the is so much greater than on the surface of the Fra arcs of Crisium approach the Apennines in eleva­ Mauro that, unless the formation of craters was tion and ruggedness. 4 Like the Apennines and other localized along the arcs, the arcs must be pre­ elements of the main Imbrium ring (see fig. 5), the

Imbrian. Further evidence for the age of the arcs, '1 The structures associated with the Orientale basin a re fresher than although still more indirect, is provided by their those of the Imbrium basin; this and other evidence suggests that the Orientale basin is lmbrian in age. Undiv ided ring materials near Orien­ relatively subdued topography. Among the pre- tale must therefore be call ed "pre-Imbrian or Imbria n."

FIGURE 6.-Mare Humorum. Large crater at top is occurs in patches. Dark material at bottom is Gassendi (G), 110 km in diameter. Gassendi tran­ Doppelmayer Formation (Eid); right-hand (east) sects one of the concentric structures of the Humorum occurrence of this formation overlies part of the basin and so is younger than the basin. It is embayed, rim and interior of the crater Doppelmayer. Note however, by mare material and thus is older than ridges (r) and mare-filled trough (arrow) concen­ the mare. Type area of the Vitello Formation (plv) tric with the basin. Inner basin probably lies between is in the lower right corner, southeast of the sharp southernmost part of Gassendi rim and northern­ 40-km diameter crater Vitello (V). Rugged terrain most part of Doppelmayer rim. Geology of this area to left of Gassendi may also be the Vitello Forma­ was mapped by Titley (1967). (Photograph 1607, tion. Premare light plains-forming material (lplp) Catalina Observatory.) "%j 1:'.:) 0

Q z0 ~ td c:: .....~ z0 Ul

~ 0

Ul> ~ :>;l 0 ~ 0 1:'"' 0 ~

FIGURE 7.-Hyginus rille (Rima Hyginus) and its chain of regularly telescopic observations; detail is shown better than on any single spaced uniform-sized craters. Part of lunar chart LAC 59, typical earth-based photograph and about as well as can be seen visually of the base charts produced at a scale of 1:1,000,000 by the Air with a large telescope under good condition~ . Each grid square is Force Aeronautical Chart and Information Center (ACIC). Com­ 2°, or about 60 km on a side. piled from many photographs with critical detail added from visual

• w. 0 is: is: ~ ><: 0 "':: t"' ' 0 ' z > ~ w. ~ t ~ ~t'.. 1 >-3 >~ ...... >-3 ~ > ::!1 "'><: I >-3 t:l:j t"' t:l:j w. 0 0 ...... 0"' 0 w.td t:l:j ~ > ...... >-3 0 w.z

FIGURE 8.-Central part of moon showing "Imbrian sculpture" the rim of Herschel are not cut. The flat floor material is assigned trending N. 30° W., radial to the Imbrium basin. Large flat-floored to the Cayley Formation of Imbrian age; Herschel is still younger. crater at bottom of picture is Ptolemaeus, 145 km in diameter; Sculptured terrain west of Herschel and northwest of Ptolemaeus fresh crater to right of center is Herschel (H), 40 km in diameter. where few old craters are present may be formed by a discrete The rims of Ptolemaeus and of many other flat-floored craters are pre-Imbrian plateau unit. Geologic map of this area by Howard cut by the Imbrian sculpture, but the floors of these craters and and Masursky (1968) . (Photograph 1907, Catalina Observatory.) >%j ....~ F22 CONTRIBUTIONS TO ASTROGEOLOGY

pre-Imbrian basin rings must have received their lar to the relation of Imbrian units to the Imbrium topographic configuration at the time of basin for­ basin. mation. These materials are designated mostly as The blanket designation "pre-Imbrian or Imbrian" uundivided" (or undifferentiated); those around the has come into wide use in lunar geologic mapping Humorum basin are designated "material of smooth because the base of the Imbrian System is marked ridges and hills" (Trask and Titley, 1966; Titley, over less area of the moon than the top of the sys­ 1967). tem. Outside the limits of the Fra Mauro Formation The surface materials on these rings differ from and. the Imbrian sculpture, the age of material . basin to' basin and are interpreted with varying cannot be confidently determined relative to the degrees of confidence. Hummocky material partly base of the Imbrian System. The approximate top resembling the Fra :Mauro Formation and also ten­ of the system is marked by the widespread mare tatively interpreted as ejected debris is present in material, much or possibly most of which is Imbrian. patches around the Humorum basin (fig. 6) (Titley Many units are in contact with the mare material and Eggleton, 1964, summarized in U.S. Geological or can be dated relative to it by density of super­ Survey, 1965, p. A131-132). This material was posed craters. Some units that cannot be dated this named the Vitello Formation (Trask and Titley, way are designated "pre-Imbrian ·or Imbrian" be­ 1966; Titley, 1967). Terrain around the basins of cause they do not seem as fresh as known postmare Crisium, Serenitatis, Nectaris, Fecunditatis, and units. (for example, terra dome _material and slope

other maria is complex, and the recognition of sig­ material (Trask and Titley, 1966). \ nificant units must await interpretation of Orbiter "Crater material" is a common "pre-Imbrian or photography. A few patches of hummocky material Imbrian" unit. This broad age designation may seem have been mapped and assigned pre-Imbrian or nearly meaningless but it is useful for those craters noncommittal pre-Imbrian or Imbrian ages. Exam­ which are much less fresh than Eratosthenes and ples that have appeared on published maps are other Eratosthenian craters and which have more uhummocky material, fine· (pre-Imbrian) and uhum­ superposed small craters, but are not quite as cra­ mocky material, coarse" (pre-Imbrian), near Mare tered, subdued, and degraded as craters that are Nubiu:m but not clearly relatep to any multiringed definitely pre-Imbrian. mare basin (Trask and Titley, 1966), and uhum­ Another common unit of upre-Imbrian or Imbrian" mocky material, fine" (pre-Imbrian or Imbrian) age, "plains-forming material," forms light flat near the Serenitatis basis (Carr, 1966a). If exten­ smooth surfaces and resembles the Apennine Bench sive hummocky material like the Fra Mauro· ever and Cayley Formations of Imbrian age. (Seep. 27.) surrounded the pre-Imbrian multiringed basins, it lt.occurs over much of the lunar surface, mostly in depressions and commonly in the troughs surround­ is either buried or its characteristic hummocky tex­ ing each pre-Imbrian multiringed mare basin and ture has been obliterated (by mass wasting, meteor­ on the shelf between the inner basin and the first ite bombardment, and (or) a cover of presently high ring. Light plains embay· the rugged terrain unrecognized·volcanic materials). of the circumbasin structures and are ordinarily not PRE-IMBRIAN AND (OR) IMBRIAN UNITS cut by the same faults which cut these structures; Many lunar units are designated upre-Imbrian or therefore the plains material is younger than the basins. Although this plains material near pre­ Imbrian," meaning that they are older than the Im.brian basins resembles the Cayley Formation, youngest units of the Imbrian System but that their it cannot be dated relative to the Fra Mauro Forma­ relation to units at the base of the Imbrian is tion or Imbrian sculpture and so cannot be ·correlated unknown. Other units are designated "pre-Imbrian with the Cayley. Because of this uncertaintly that and Imbrian," meaning also that they are older it i§ Imbrian in age, it is designated "pre-Imbrian than the youngest Imbrian units but that some indi­ or Irnbrian" (Carr, 1966a; Trask and Titley, 1966; vidual occurrences of the unit are older and some Titley; 1967). younger than the oldest Imbrian units. Most The slightly different designation "pre-Imbrian mapped pre-Inibrian and (or) Imbrian units are and Imbrian" has a more definite meaning. This age associated with pre-Imbrian multiringed mare designation refers to groups of crater materials basins and are younger than the basins but older which, like the pre-Imbrian or Imbrian plains-form­ than the mare material filling the basin. Their stra- ing unit, are younger than a pre-Imbrian multi­ ~tig·raphic· .. relations to the basins are therefore. simi- ringed basin but older than the mare material which SUMMARY OF LUNAR STRATIGRAPHY-TELESCOPIC OBSERVATIONS F23

fills the basin. The position of these crater materials tern. The Fra Mauro Formation was named and re­ in the local basin stratigraphy is thus analogous to mained as the base of the Imbrian System. The two that of Archimedes materials in the Imbrium basin series were retained ; the Fra Mauro and Apennine stratigraphy-younger than the basin but older Bench Formations belonged to the Apenninian than the basin-filling mare. The difference in desig­ Series and the Archimedes~type craters and the nation between the "pre-Imbrian or Imbrian" of the Procellarum Group belonged to the Archimedian plains unit and the crater unit is that while no indi­ Series. Because the base of the Imbrian System is ~·· vidual crater can yet be accurately dated, there is the Fra Mauro Formation and the top is mare mate­ reason to believe that some are pre-Imbrian and rial, the system includes essentially all the rocks some are Imbrian in age because craters are more formed between the time of formation of the Im­ numerous per unit area near the basins than brium basin and the time of completion of most of Imbrian-age Archimedes-type craters are near the its filling. Imbrium basin. Examples of their groups are the Confusion has persisted about the definition of craters Fracastorius in the Nectaris ·basin and the top of the Imbrian System. Neither the type Gassendi in the Humorum basin (fig. 6). Only the area of the former Procellarian System, presumably Gassendi Group has been formally named (Titley, in the Copernicus quadrangle, nor the type area of 1964, p. 67; U.S. Geological Survey, 1964, p. the Procellarum Group, informally established in the A141-142 (brief mention); Trask and Titley, 1966 Kepler quadrangle, was explicitly established as the (definition); Titley, 1967). (The section in this re­ type area of the top of the Imbrian System. (These port entitled "Crater material subunits" discusses type areas are further discussed in the section on the nomenclature and subdivision conventions of the nomenclature of the mare material.) This is fortu­ Gassendi Group.) nate because the mare material in the Copernicus Separate colors are used for those crater mate­ quadrangle and in all but the western part of the rials designated "pre-Imbrian or Imbrian" and Kepler qpadrangle is nearly obscured by ray mate­ those designated "pre-Imbrian and I.mbrian." In rial and therefore is not a satisfactory type for the map explanations, the former are shown to the left system. Furthermore, the western part of the Kepler of the main column, and the latter are incorporated quadrangle is far from the Copernicus-Eratosthenes­ in the main column below the mare material and Archimedes region, where the general lunar strati­ above units formed contemporaneously with the graphy was first worked out. The mare material in multiringed basins. the latter region that is least obscured by rays is that between Eratosthenes and Archimedes; this region IMBRIAN SYSTEM DEFINITION AND GENERAL FEATURES between Eratosthenes and Archimedes appears to be the most satisfactory type area for the top of the The name "Imbrian System," derived from Mare Imbrian System. Units in this area can be confi­ Imbrium, has been applied in several different ways dently assigned to either the Imbrian or Eratos­ and has not been formally defined. The name was thenian Systems. Accordingly, many mappers have restricted first to the hummocky blanket surrounding used it as an informal type area, but it is not now Mare Imbrium (Shoemaker and Hackman, 1962, p. proposed as a formal type area because Orbiter pho­ 293-294), now called the Fra Mauro Formation. tographs may reveal unsuspected complications. Next, in an informal publication (Shoemaker, Hack­ man, Eggleton, and Marshall, 1962) the system was FRA MAURO FORMATION enlarged to include two series, the Apenninian (the The terra surrounding the Imbrium basin has a hummocky regional blanket) and Archimedian ( cra­ hummocky surface which is generally similar in ap­ ter .materials), a usage which persisted through the pearance in belts concentric with the basin but publication of two quadrangle maps (Kepler region, changes 'progressively with distance from the basin, Hackman, 1962; Letronne region, Marshall, 1963). from coarsely hummocky and complexly structured Following a meeting of lunar mappers in November to relatively smooth (fig. 9) (Eggleton and Mar­ 1963, rock-stratigraphic terms were separated from shall, 1962). Within these belts, the terrain underly­ time-stratigraphic terms. The materials of the Pro­ ing the surface appears heavily mantled close to the cellarian System, formerly the system next younger basin and progressively less mantled farther from than Imbrian, became the rock-stratigraphic (lunar it. This regularity suggests that the surface mate­ material) unit Procellarum Group and the top of rial is a true rock-stratigraphic formation; that is, this group was taken as the top of the Imbrian Sys- the hummocky morphology is intrinsic to the unit, F24 CONTRIBUTIONS TO ASTROGEOLOGY and the unit forms a laterally continuous bed of Along with some now-excluded smooth materials, rock, even where covered by younger rocks. the present Fra Mauro was originally called the This unit, after passing through a series of name Imbrian System, because of its relation to the Im­ changes, is now called the Fra Mauro Formation. brium basin (Shoemaker and Hackman, 1962, SUMMARY OF LUNAR STRATIGRAPHY-TELESCOPIC OBSERVATIONS F25

p. 293, 294); it was thus equated with a time-strati­ the lunar literature, refers to the presence of many graphic system. Later (Shoemaker, Hackman, Eg­ such hills which together give an impression of dis­ gleton, and Marshall, 1962; Hackman, 1962; Mar­ order.) This definition of the Fra Mauro Formation shall, 1963) it was called the "regional material of includes all material in the rectangle (except crater the Imbrian System" and was equated with the materials) because the uniformity of the hummocks time-stratigraphic unit, the Apenninian Series. This and intervening depressions suggests that they ·are older of two series of the Imbrian System was intrinsic to a single depositional unit; that is, they named after the Apennine Mountains, where the would develop even if the formation were deposited unit is well developed. But basic units, in strati­ on a smooth surface. A supplementary reference graphic practice, should not be time stratigraphic area is between lat 3 ° S. and 4lj2° S. and long 17° units (American Commission Stratigraphic Nomen.. W. and 18° W. The topography of this area is simi­ clature, 1961); therefore the Fra Mauro Formation lar to that of the type area, although slightly was later est~blished by Eggleton, in a preliminary rougher. The top of the Fra Mauro Formation is open-file report (1964, p. 51-55), with specific type near the present surface, but the formation is prob­ areas and more exact physical definitions. This defi­ ably overlain by a regolith of fragmental materials nition began the divorce of the Fra Mauro from the and crater materials derived from the Fra Mauro Apenninian Series. Eggleton, however, still re­ itself; locally the Fra Mauro may be covered by garded Apenninian Series as the basic designation presently unrecognized volcanic or other rocks. The for the circum-Imbrium blanket that contained the depth of the base in the vicinity of the type area Fra Mauro Formation and the probably younger was estimated by Eggleton (1963) to average 550 Apennine Bench Formation as subunits. meters; the depth varies because the subjacent ter­ rain (pre-Imbrian crater materials and undivided DEFINITION materials) has high relief. This rugged pre-Imbrian The Fra Mauro Formation is defined here because terrain protrudes through the surface in places out­ its first definition (Eggleton, 1964) was not printed ~· side the type and reference areas and may also do in a recognized scientific publication and its first so, unrecognized, within them. (Where the Fra published description lacked a definition (Eggleton, ' Mauro is believed to be thin, a lined pattern has 1965). In another publication (U.S. Geological Sur­ been overprinted on its map color.) Exposure of the vey, 1964, p. A141), the unit was referred to by formation is limited laterally, outside the type. and name but not described. The type area (fig. 10) is reference areas, by dark (mare) and. light (Cay­ here designated a rectangle bounded by lat 0° and ley?) plains, which em bay Fra Mauro along a sin­ 2° S. and long 16° W. to 17lf2° W. (LAC 76, 2d uous contact and probably bury parts of it. Expo­ ed.), north of the crater Fra Mauro for which the sures separated from the type area by younger formation is named. (Fra Mauro is a pre-Imbrian units cannot be correlated with certainty. How~ver, crater buried by the formation.) This area is the. the Apennine Mountains contain extensive expo­ eastern three-quarters of Eggleton's type area of sures partly resembling those in the type area, and the hummocky member. Eggleton defined two mem­ the Fra Mauro Formation is believed to extend all bers-one hummocky and the other smooth-and around the Imbrium basin. referred to them as facies. Characteristics in the type area (mostly in Eggleton's words) are: abun­ MEMBERS dant, close-spaced, low, rounded, subequidimensional or north-south elongated hummocks generally 2--4 The definition of the Fra Mauro presented here km across. (A hummock is a low somewhat irregu­ excludes many smooth materials that had earlier lar hill; the term "hummocky," commonly used in been included in the "Imbrian System," "regional

FIGURE 9.-The terra peninsula beginning at the Apennine gradation in texture away from the Apennines is compar­ Mountains (top) is surrounded by Sinus Aestuum (left), able to the gradation in texture of crater rim material Mare Vaporum (right), and Sinus Medii (bottom). Notice away from a crater. Note also the dark smooth terra east the gradational change in the surface of the terra from of Sinus Aestuum which is part of the Sulpicius Gallus coarsely hummocky in the Apennines to relatively smooth Formation. North-south dimension of photograph is ap­ near Sinus Medii. This terra material is the Fra Mauro proximately 500 km. (Photograph 2668, Catalina Observa­ Formation and the smooth material (in box) is the refer­ tory.) ence area for the smooth member of the Fra Mauro. The F26 CONTRIBUTIONS TO ASTROGEOLOGY

FIGURE 10.-Type area of the Fra Mauro Formation (upper box) and reference area (lower box). Large crater is Fra Mauro, 95 km in diameter. Fra Mauro Formation grades from hummocky in the north (nearest Imbrium basin) to smooth in the south; some of southern smooth material could be a younger unit. (Photograph 1994, Catalina Observatory.) material of the Imbrian System," or the "Apen­ and Cayley Formations (Wilhelms, 1965, summa­ ninian Series" (Eggleton and Marshall, 1962), but rized in U.S. Geological Survey, 1965, p. 123). Eg­ that are now mapped mostly as the Apennine Bench gleton's type area of the smooth member is explic- SUMMARY OF LUNAR STRATIGRAPHY-TELESCOPIC OBSERVATIONS F27

itly excluded from the definition of the Fra Mauro few old craters are visible, probably because the given here; as Eggleton realized, the smooth mate­ Fra Mauro mantles pre-Imbrian craters progres­ rial may be younger, unrelated material, possibly of sively more deeply toward the basin. The crater Ju­ the Cayley Formation. However, some relatively lius Caesar is most heavily mantled by the Fra smooth material that is laterally continuous and Mauro on slopes facing the Imbrium basin, as it gradational with the hummocky material is still cor­ would be if debris traveling in low trajectories piled related with the Fra Mauro Formation and is up against obstacles (Morris, 1964; Morris and Wil­ mapped as the "smooth member." The surface of helms, 1967). The outer part of the Fra Mauro is this material has very low ripplelike hummocks or lineated radially to the basin, as· is the outer part of sinuous ridges and is not as smooth as the Cayley crater rim material (Wilhelms, 1965). Since large Formation. Most of this material is farther from craters are probably produced by impact, the Fra the basin than the hummocky material, but some is Mauro material is interpreted to be the ejecta from on high ground nearer to the basin. A reference a great impact which exqavated the Imbrium basin.' area for the probable smooth member is in the Mare Vaporum .quadrangle between lat 7° N. and CAYLEY AND APENNINE BENCH FORMATIONS 8lj2 ° N. and long 0° and 11f2° W. (fig. 9). This ma­ Light plains-forming material is abundant and terial is gradational with hummocky material in the widely distributed on the moon's surface. Much of Apennine Mountains that resembles the type Fra this material in the central and northern parts of Mauro. Some patches of smooth blanketing material the near side is believed to be of Imbrian age be­ r that are detached from exposures of hummocky ma­ cause it is younger than the· Fra Mauro Formation terial are also tentatively mapped as smooth Fra and Imbrian sculpture (fig. 8); however, it is ap­ Mauro (for example, in other parts of the Mare Va­ parently older than the Imbrian mare material. One porum quadrangle, Wilhelms, 1968). occurrence, mentioned previously ( p. 9) , is the Where members are mapped, the hummocky ma­ light plains-forming material on the Apennine terial is designated the "hummocky member." In bench near the crater Archimedes ·(fig. 4); this ma­ the Ptolemaeus quadrangle an additional "ridged terial is cailed the Apennine Bench Formation member" was mapped, and an Imbrian unit called (Hackman, 1964, 1966 (definition); U.S. Geological "smooth material" was tentatively correlated with Survey, 1964,- p. A141 (brief men:tion)). Another the smooth Fra Mauro Formation (Howard and occurrence of the material is in a circum-Imbrium Masursky, 1968). On two published maps (Eggle­ trough near the crater Cayley (lat 4 o N., long 15° ton, 1965; Hackman, 1966) and several preliminary E.) (Wilhelms, 1965; Morris and Wilhelms, 1967). maps, a dark hummocky facies is recognized, but The material there is unaffected by the Imbrian most mappers now regard the dark coloration as sculpture which has greatly modified the adjacent that of a superposed unit of probable volcanic ori­ terrain. The material appears to be embayed by the gin, the Sulpicius Gallus Formation (discussed mare material of Mare Tranquillitatis and has a under "1m brian or Eratosthenian Systems") . higher crater density than the mare. Except for the

INTERPRETATION material assigned to the Apennine Bench Forma­ The Fra Mauro Formation, like all lunar material tion, all lunar plains-forming materials in the cen­ units, can be delineated objectively on the basis of tral part of the near side that resemble this mate­ physical properties, and age relations can be deter­ rial and that are of demonstrable Imbrian age are mined objectively from superposition and transec­ designated the Cayley Formation (U.S. Geological tion relations. However, the thought process that Survey, 1966, p. A123 (brief mention); Morris and led to its recognition as a unit involved an interpre­ Wilhelms, 1967 (definition) ) . tation of its origin. The interpretation was based on Most of the material of the Cayley Formation, the observation that Mare Imbrium occup_ies a basin like other plains-forming material, is smooth and that has many similarities to large ·craters. This level and occurs in depressions. Contacts are sharp basin is circular and is surrounded by rings of in places, gradational elsewhere. Where contacts are rugged topography. The surface material of these sharp, the ·Cayley may consist mostly of flows; rings, the Fra Mauro, resembles rim material of where contacts are gradational, it may consist fresh young craters in that its inner part is largely of free-fall tuff interbedded with mass­ coarsely hummocky and its outer part smoother. It wasted debris and be modified by downslope move­ seems to be thicke~t close to the basin where very ment. F28 CONTRIBUTIONS TO ASTROGEOLOGY

Many occurrences of smooth, level Cayley Forma­ upper strata of the mare material. Another crite­ tion merge without a detectable discontinuity in rion for this postbasin premare age is the presence albedo into tracts of subdued topography. These of satellitic craters on the Apennine Bench Forma­ tracts may be covered with thin layers of material tion and their absence on the mare. The materhtls that is either identical with level Cayley or is a py­ of Archimedes and other craters younger than the roclastic facies of it. On the basis of this interpreta­ Fra Mauro Formation and older than Imbrian mare tion, soine tracts of gentle relief are designated the material were previously called the Archimedian Se­ "hilly member" of the Cayley Formation (Milton, ries (see section entitled "Apenninian and Archi­ 1968; Wilhelms, 1968). Alternatively, the gentle median Series") but now are identified as "lmbrian hilly terrain may be mapped as undivided Cayley crater materials" (see section on "Crater material Formation, and the units believed to be buried and subunits" for subdivision conventions). Such mate­ to produce the hilly character may be shown by dot­ rials may be contemporaneous with, younger than, ted contacts and symbols in parentheses. In one or older than the Cayley Formation. Some crater place, where the buried terrain consists of round materials believed to be contemporaneous with the hills, a special pattern was used, to depict this on a mare material (Procellarum Group) have been map (Howard and Masursky, 1968). named the Diophantus Formation (Moore, 1965). Tracts of moderately rugged as well as gentle re­ lief mapped as lmbrian "terra material" in the Ptol­ MATERIALS NEAR THE ORIENTALE BASIN emaeus quadrangle may also be covered by mate­ The surface surrounding the Orientale basin,· rial correlative with the Cayley (Howard and whose center lies on the west limb of the earthside Masursky, 1968). Similar terrain assigned to the disk, resembles the surface of the Fra Mauro For­

Imbrian System and mapped as "terra material, un­ mation (McCauley, 1964). It is coarsely hummocky I (' differentiated" in the Julius Caesar quadrangle close to the basin and smoother farther from it; (Morris and Wilhelms, 1967) is also believed to in­ available photographs show no hummocks beyond clude a mantling material, but not necessarily a fa­ about 900 km from the basin center (compared with cies of the Cayley Formation.5 about 1,000 km from the Imbrium basin center for The Cayley Formation very likely includes mate­ the Fra Mauro). Prebasin craters seem to be almost rials that differ in age and lithology. Much of the completely filled close to the basin and progressively material ·that is lumped as Cayley in telescopic map­ less filled farther from it (as seen on Zond 3 photo­ ping has been divided into numerous units on graphs of the averted hemisphere. See McCauley, maps made from Ranger photographs (Milton and 1967b.) 6 The density of craters near the Orientale Wilhelms, 1966; Carr, 1966b; McCauley, 1966). basin is much less than on the surrounding terra. Even at telescopic resolution, differences in albedo The material underlying this surface has been and crater density are apparent from area to area. Post-lmbrian materials may be present within some separated into two units that are believed to consist areas mapped as Cayley; at high telescopic resolu­ of blanketlike deposits. The inner hummocky unit is tion small areas are seen which are less densely cra­ not yet formally named. The smoother, probably lat­ tered than most mare surfaces. Accordingly, either erally continuous unit is the Hevelius Formation subdivision of the Cayley Formation or introduction (McCauley, 1967a). Preliminary crater counts sug­ of new units will be required on future large-scale gest that these units are younger than the Fra maps. Mauro (McCauley, 1967b). The Hevelius·is embayed by dark mare material of questionable Imbrian age CRATER MATERIALS and is therefore tentatively assigned an Imbrian The crater Archimedes (figs. 4, 5) is not overlain age. The inner hummocky material, and tentatively by Fra Mauro Formation nor is it cut by Imbrian the Hevelius Formation, are interpreted as debris sculpture, although it is in the Imbrium basin; it is ejected by ·the impact which formed the Orientale thus younger than the basin and the Fra Mauro. On basin. The Hevelius would, therefore, be genetically the other hand, Archimedes is embayed a·nd filled similar to the smooth member of the Fra Mauro with mare material, and thus is older than the Formation. (Like the Fra Mauro, it may consist lo­ cally of bedrock older than its source basin and of 11 Another mantled unit assigned to the Imbrian but called "rugged terra" and including only terrain resembling the more rugged parts of presently unrecognized younger volcanic units.) the units in the Ptolemaeus and Julius Caesar quadrangles, was mapped in the Theophilus quadrangle by Milton (1968). See the section "Terra 6 After this was written, Lunar Orbiter IV provided photographs of units of the Theophilus quadrangle." Orientale far superior to those of Zond 3. SUMMARY OF LUNAR STRATIGRAPHY-TELESCOPIC OBSERVATIONS F29

The relative youth of the u·nits is supported by various volcanic materials that tended less to flow the apparent youth of the Orientale basin; the into depressions or otherwise conform to ·preexist­ scarps concentric with the basin appear very fresh, ing topography than do those of the Cayley Forma­ and there is comparatively little mare fill in the tion and the "irregular terra" units. Alternatively, basin or along the base of the scarps. Indirect evi­ the Kant Plateau could be mainly or entirely a tec­ dence suggests that the units formed before Eratos­ tonic feature. thenian time. Sculpture or stringers of ejecta are A lower age limit of Imbrian is assigned to both absent on mare material of probable Imbrian age as irregular terra units and materials of Kant Plateau little as 1,000 km from the basin center, although because they apparently cover the Fra Mauro For­ they should be present if the basin were postmare mation. However, the upper limit is very uncertain, because at this distance from the center of the Im­ and the upper parts of the units may extend into brium basin the Fra Mauro Formation is abundant the Copernican System. The relative ages are also and lmbrian sculpture prominent. uncertain, and the units are arranged in the expla­ nation of the Theophilus map according to the degree TERRA UNITS OF THE THEOPHILUS QUADRANGLE to which they mask underlying topography: from The first published map on which an analysis was bottom to top, Fra Mauro Formation, irregular attempted of extensive terra tracts outside the im­ terra materials,· materials of Kant Plateau, and mediate influence of mare basins is the Theo.philus Cayley Formation. quadrangle (Milton,- ·1968). (See also--Milton, 1964, Alternative units are being recognized in the ter­ summarized in U.S. Geological Survey, 1965, p. rae of other areas, and this work may clarify the A131.) Besides the light and level or gently undu­ nature, ages, and origins of terra material. But the lating plains materials of the Cayley Formation and undistinguished appearance of much terra suggests a few patches of possible Fra Mauro Formation, that meaningful rock-strat~graphic units may never two main categories of terra units are recognized. be recognized at the telescopic scale. One category is called "irregular terra," a name which conveys an appropriately uncertain interpre­ MARE MATERIAL (FORMERLY PROCELLARUM GROUP) .l. tation. The units are "rugged terra," 7 ''rugged Areas that appear dark, flat, and smooth through terra of crater rims," and "furrowed terra." These the telescope and that locally have characteristic units differ. from most lunar material units in that ridges (fig. 6) and domes are underlain by mare they probably reflect more than one superposed rock material, a widespread unit mentioned often in pre­ unit. Rugged terra of crater rims, for example, is ceding sections. In this section, the properties that characterized by the gross form of a crater ; but the distinguish mare materials and its subdivisions and crater may be either pre-Imbrian or Imbrian in age, the history of its stratigraphic nomenclature are whereas the Imbrian age assigned to the terra unit . discussed. The author considers the name Procel­ larum Group, commonly applied to most mare mate­ applies to a light-colored surficial blanket. The cra­ rial but never formally proposed, abandoned. (See ter may be of impact origin; and the blanket, vol­ discussion under the section "Nomenclature".) canic. The blanket may be the same as that which Several physical characteristics set mare units covers other units. apart .from other smooth, flat material units, such The other category of terra units is "materials of as the Cayley Formation, and also lead to internal Kant Plateau," another noncommittal name. This subdivisions. The most obvious properties on tele­ name is applied to terrain having positive relief scopic photographs are crater density and albedo. that occurs in an arc concentric with the Mare N ec­ Commonly, these two properties are related; the taris basin.-~Units are-labeled "smooth," "·pitted," more craters, the higher the albedo ·(the lighter the urugged," and ...~!.rugged bright,.!'. Unlike the irregu­ surface). Crater density-and possibly albedo-are lar terra units, where subjacent relief is conspicu­ secondary characteristics that are partly a function ous, only the broad elements of subjacent topogra­ of age. Some materials which appear to embay oth­ phy are detectable. The positive relief and the ers or lie as pools within them are darker and less surface textures suggest that Kant Plateau mate­ cratered than those that they appear to overlie; for rials are depositional in origin; they may consist of example, the sparsely cratered material of Palus ' Imbrian terra material given the same map symbol·· as rugged terra Putredinis embays both the Apennine Bench For­ was mapped in two adjacent quadrangles, Julius Caesar (Morris and Wil­ mation and lighter, more cratered, mare material helms, 1967) and Ptolemaeus (Howard and M1,1sursky, 1968), but includes ·relatively featureless as well as rugged terrain. (See p. 28.) (fig. 4). F30 CONTRIBUTIONS TO ASTROGEOLOGY

The reasons for albedo variation are not well un­ signed time-rock status as the Procellarian System derstood, and the intrinsic albedo of lunar materials (Shoemaker and Hackman, 1962, p. 294; Shoe­ is unknown. Plains-forming materials may be dark maker, 1962, p. 346); the type area was unstated like mare materials when young, then become but presumably was in the Copernicus quadrangle, lighter through cratering, mass wasting, and other where the study concentrated. Subsequently, differ­ processes which expose light blocky materials. Proc­ ences of crater density and albedo became apparent, esses similar to this "aging" can also occur in stan­ and certain crater rim materials were found to be taneously when crater rays are formed by second­ superposed on mare material in one sector but over­ ary impact. (~ee section "Eratosthenian and lapped by it in another (for example, Manilius, lat Copernican Systems, Crater Materials.") Albedo 14°30' N., long 9° E.). To handle such complexities, may also be partly a function of chemical composi­ and to follow correct stratigraphic practice by tion. avoiding premature age assumptions, the Procel­ Mare materials are almost certainly bedded. Com­ larian System was replaced by rock-stratigraphic monly, the contact between surface units is lobate, (lunar material) units. At first, the only such unit and this fact suggests encroachment on one flat­ defined was the Procellarum Group. This name was lying layer by arcuate flow fronts of another. A few first mentioned in a preliminary open-file report such arcuate contacts are topographic scarps that (Hackman, 1964, p.4), and "that part of Oceanus convincingly .resemble terrestrial volcanic flow Procellarum included in the Kepler region" (now fronts (:fig. 11). Some of these scarps coincide with quadrangle) was designated the type area. The unit slight color changes, which can be enhanced by was not formally defined, although the name was compositing photographs taken through ultraviolet used thereafter and the redefinition of the system to and infrared filters (Kuiper, 1965, p. 29-33, includ­ group was referred to in a formal publication (U.S. ing map by R. G. Strom, fig. 18; Whitaker, 1966, p. Geological Survey, 1964, p. A141). 79-95). The Procellarum Group consisted of the units Most mare surfaces are flat, and they terminate "mare material" and "dome material" and of other abruptly at the slightest rise in topography. This materials with relief (discussed below), each of fact, along with the steepness of the scarps men­ which constituted an informal formation. "Mare tioned above,· has been taken as evidence that mare material" was· called "flat mare material" on some material is composed of lava flows. However, some maps and "smooth mare material" on others. In smooth dark material that overlies barely percepti­ many areas flat mare material was further divided ble craters (ghosts) and other features is appar­ into members on the basis of albedo differences. ently gradational to flat mare material and has been Commonly four members have been· mapped whose mapped as mare. This mare material may be com­ symbols bear numerical suffixes. Member 1 has the posed of free-fall pyroclastic material or of ash-flow highest albedo (lightest) ; member 4, the lowest al­ tuff, which is implaced as fluid flows (O'Keefe and bedo (darkest) . The type areas for these members Cameron, 1962, p. 281-284; O'Keefe, 1966). Both kinds of material may compact differentially upon are in Mare Serenitatis, where there is less ray . cooling; so the topography of the surface resembles cover to obscure the relations among members than the subjacent topography, but in more subdued in Oceanus Procellarum (Carr, 1966a) .. However, form. Free-fall tuff, ash-flow tuff, and lava may all distant units do not always match these types ex­ be mare materials. A lunar mare may well be as actly. Furthermore it is difficult to compare albedos complex stratigraphically as a terrestrial volcanic exactly. Many contacts are sharp, but some are .province. blurred and may not represent true edges of beds; these are shown by a special contact symbol (Carr, NOMENCLATURE 1966a). A simpler classification of "light" and As lunar geologic· studies have progressed, mare "dark" has been used in some areas (McCauley, material has been increasingly subdivided. At the 1967a; Moore, 1967; Howard and Masursky, 1968), outset of the mapping program, mare material was and it is more satisfactory in rayed areas than the thought to have approximately the same age every­ fourfold classification. In both of these classifica­ where, except in relatively young· Mare Crisium, on tions, the darker members were interpreted as the basis of crater counts from the poor-resolution younger than the lighter because at least some dark photographs then available (Shoemaker, Hackman, materials embay areas of light materials ·and have and Eggleton, 1962). The mare material was as- fewer superposed craters. Ulc ~ ~ ::>:;> -< 0 "%j t"' cz ::>:;> Ul

~>-'3

~'1:! ::t1 1 tr:l t"' tr:l Ul 0 0 ...... '1:! 0 0 1:!) Ul tr:l ~ > ...... >-'3 z0 Ul

FIGURE 11.-Flow lobes in Mare Imbrium. Largest crater is Carlini, approximately 11 km in diameter. (Photograph 2862, Catalina Observatory.)

~ 1:1:) """' F32 CONTRIBUTIONS TO ASTROGEOLOGY

Unfortunately, and contrary to intentions, a MARE MATERIALS WITH TOPOGRAPHIC RELIEF time-stratigraphic significance is still attached to The mare materials discussed above form com­ the Procellarum Group, and it has been used as a pletely or nearly flat surfaces. A common material synchronous stratigraphic datum much as was the assigned to the Procellarum Group that does not Procellarian System. Nearly all maps show the age form a flat surface is "dome material" or "mare of the Procellarum Group as Imbrian. Most mare dome material." Domes are low and round or ellipti­ material may well be of this age-that is, as old or cal in plan and have convex-upward profiles; some older than the material between the craters Eratos­ have summit craters. Their surface is similar in fine thenes and Archimedes. (As discussed in the section texture and albedo to that of flat mare, and all "Imbrian System," no formal type area has been es­ known domes are in contact with flat-- mare. The tablished.) However, many authors are aware that contact between most domes and adjacent mare is some mare material is younger than Imbrian, al­ marked by a sharp topographic break. Such domes, though they have shown the age as Imbrian; Mc­ especially those with summit craters, resemble ter­ Cauley (1967a) and Moore (1967) considered a restrial shield volcanoes and are probably volcanoes Copernican age possible for some of the dark mare superposed on the mare surface. Other domes merge material. gradually into the adjacent mare and may overlie Where authors chose to show post-Imbrian ages, subsurface intrusions, probably laccoliths. Both they mapped other material unit~ and dropped or kinds of domes may be younger than the uppermost restricted the name Procellarum Group. The name strata of the flat mare material nearby, but unseen "mare material, dark" was used by Carr (1966a) for domes are probably buried by flat mare. Domes and extremely dark material at the east edge of Mare flat mare are doubtlessly related genetically as well Serenitatis which is assigned an Eratosthenian age as spatially and texturally. Some of the possible lac­ because it is younger than most mare material coliths are transitional to mare ridges, which may y~t older than Copernican-age crater rays. Moore overlie sills or dikes. (1967) used the same designation for dark material Certain other domes and one additional minor that covers part of the Copernican crater Lichten­ unit are mapped as pari of the Procellarum Group. berg and is therefore itself Copernican. Wilhelms Several domes having rough summits or discrete .~ (1968) recognized the same four mare material sub­ summit hills are known and have been mapped sep­ units in the Mare Vaporum quadrangle as in Mare arately- from other domes in one area, as "rough Serenitatis, but concluded that the two darker sub­ dome material" (Moore, 1965). Second, "hummocky units are post-Imbrian because they are very material'' (or variations on the name) has been smooth and because one occurrence embays the Era­ mapped tentatively as another formation of the tosthenian or Copernican crater Manilius. These Procellarum Group in some areas because of its low units were therefore mapped as "Eratosthenian or albedo (Carr, 1965b; Moore, 1965; Howard and Ma­ Copernican mare material." ·The two lighter, more sursky, 1968). Such material may be clusters of cratered (probably Imbrian) units were assigned to small subresolution domes. The ridges that are the Procellarum Group. characteristic of maria are mapped as structures, The name Procellarum Group has thus become but might appropriately by shown as geologic units equated, in practice, with ~mbrian mare material. on future maps. This usage is both incorrect and unnecessary. A lunar material unit of a certain albedo range cannot APENNINIAN AND ARCHIMEDIAN SERIES always be assigned a particular age because albedo In many reports and maps of the Geological Sur­ probably is not always related to age. Nothing vey, the Imbrian System has been divided into the would be gained by redefining the group to remove Apenninian and Archimedian (Archimedean) Se­ these objections, and the name Procellarum Group ries. At present the base of the Apenninian Series is considered dropped and all material traditionally is defined as the base of the Fra Mauro Formation called mare is simply considered as the lunar mate­ and the top as the top of the Apennine Bench For­ rial unit "mare material." Subdivision on the basis mation (Hackman, 1966) . The Archimedian Series of albedo or other physical properties may continue includes all post-Apenninian Imbrian materials. to be desirable. Where spacecraft photographs are These series were not explicitly defined by Hackman available, accurate relative age ranking and assign­ but he employed them this way on his map; his def­ ment to time-stratigraphic units possibly can be inition appears only in an open-file report (Hack­ achieved. man, 1964, p. 4). SUMMARY OF LUNAR STRATIGRAPHY-TELESCOPIC OBSERVATIONS F33

The original definitions of the series, also not DARK TERRA-MANTLING UNITS published formally, were somewhat different (Shoe­ An important type of "Imbrian or Eratos­ >-. maker, Hackman, Eggleton, and Marshall, 1962). thenian" material is as dark or darker than mare The Apenninian Series included only the "regional and occurs mainly on the terra near the margins of material of the Imbrian System," now called the mare basins. Material of this kind adjacent to Mare Fra Mauro Formation. The Archimedian Series Serenitatis was named the Sulpicius Gallus Forma­ comprised only the crater-rim materials that are su­ tion (Carr, 1966a). Large conspicuous areas of sim­ perimposed on the Apenninian Series and are over­ ilar materials adjacent to Mare Vaporum and Sinus lapped by the Procellarian System. Thus the two Aestuum (Wilhelms, 1968) and near Copernicus units defined as series were in fact lunar material (Schmitt and others, 1967) also have been mapped units. as Sulpicius Gallus (fig. 9, 12). Similar material ad­ As presently defined these series are practical jacent to Mare Humorum (fig. 6) is called the Dop­ units only near Archimedes and the Apennine bench pelmayer Formation (Titley, 1967). (fig. 4), because only there can material units be as­ Several patches resembling the Sulpicius Gallus sign~d to them; elsewhere, the age of a plains-form­ Formation were formerly regarded as a dark hum­ ing unit or crater 1naterials relative to the age of mocky facies of the Fra Mauro Formation (Hack­ the Apennine Bench Formation or the crater Ar­ man, 1966). However, the dark material is now chimedes cannot yet be determined. At best, all that believed to be a thin surficial covering, because its is known is that they belong to the Cayley Forma­ relief is either similar to or more gentle than that of tion or are Archimedes-type craters (that is, they adjacent materials; ridges, craters, and other topo­ resemble the type Cayley or the crater Archimedes graphic forms that pass under the contacts are only and are younger than the Fra Mauro Formation slightly subdued. This weak topographic expression and older than the youngest mare material). There­ suggests that the mantling material is largely pyro­ fore these series na1nes should be dropped. clastic. The type Sulpicius Gallus is thickest along An alternative is to return to the original defini­ rilles,· which are probably its source (Carr, 1965a). tion of the Apenninian Series by restricting it to The source for other exposures of the formation may be small craters and domes which are seen as the Fra Mauro Formation and to place all the re­ dark spots on very high resolution full-moon photo­ maining Imbrian material units in the Archimedian graphs (Wilhelms, 1968). Series. This classification, however, would serve no Embayment relations ·suggest that the rugged practical purpose and would divide the time-strati­ parts of the Sulpicius Gallus Formation are mainly graphic column too unevenly. There would be only older than the mare material. ·However, parts of the one formation in the one series-Fra Mauro-and Sulpicius Gallus and much of the Doppelmayer For­ all the light-plains 1naterials, Archimedes-type cra­ mation appear to be contemporaneous with or ters, and mare material in the other. Furthermore, younger than the mare, because flat material and if the Fra Mauro Formation is correctly interpreted adjacent rugged material have the same low albedo. as consisting of the ejecta produced by a single im­ This suggests that a mantle of Sulpicius Gallus or pact, the two series could represent vastly disparate Doppelmayer extends over both the rugged terrain time spans-minutes on the one hand, tens of mil­ and the flat mare. lions of years on the other. Such flat areas would not be distinguishable from mare if .they were not laterally gradational to IMBRIAN OR ERATOSTHENIAN SYSTEMS rugged areas of Sulpicius Gallus or Doppelmayer. Materials that cannot be dated relative to Mare material and dark mantling material may Imbrian mare material but are believed to be mod­ therefore be basically similar and differ only in pro­ erately young are designated "Imbrian or Eratos­ portions of constituent rock types; mare material, thenian." These include crater materials, dark terra­ which tends to accumulate in available depressions, mantling materials, and materials with probably may consist predominantly, but not entirely, of intrinsic positive relief. Materials of craters that flows, whereas the mantling type may consist pre­ resemble known Eratosthepian ·craters but are su­ dominantly of free-fall ash. Some dark mare units perposed on terra instead of mare are by conven­ may be old flows with only a young thin ash cover.

'\, tion mapped as "Imbrian or Eratosthenian crater UNITS WITH INTRINSIC RELIEF material"; such craters are topographically sharp Some units mapped as "Imbrian or Eratos- ""' and rayless. thenian" apparently have intrinsic relief. One is the "%j C¢ 11::-

0 0z

l;l:!~ c::: ...... >-3 0z UJ >-3 0 > UJ e;>-3 ~ 0 t"' 0 ~

FIGURE 12.-Craters Copernicus (left center, 90 km in diameter) and Eratos­ nicus about a crater diameter away (black and white arrow). Very dark thenes (its center marked by black and white arrow) under near-full-moon patches are exposures of the Sulpicius Gallus Formation (Eis). Very illumination. Copernicus has a bright, conspicuous ray system; Eratos­ bright crater walls and other slopes are Copernican slope material. Com­ thenes' rays are barely visible. The rays of Copernicus are clearly super­ pare photograph with the low-illumination photograph of figure 3, in which posed on Eratosthenes. Several dark-halo craters younger than Copernicus relief is much more conspicuous. (Unpublished photograph 5818, U.S. are present; a conspicuous large one is Copernicus H, southeast of Coper- Naval Observatory, Flagstaff, Ariz.)

_.,_ SUMMARY OF LUNAR STRATIGRAPHY-TELESCOPIC OBSERVATIONS F35

FIGURE 13.-Aristarchus plateau, large parallelogram-shaped cone (28 km long) with summit crater (arrow). Probable raised area in western (left) half of picture; Harbinger volcanic material of several kinds forms the plateau (Vallis Mountains (Montes Harbinger) are the smaller complex Schroteri Formation, Cobra Head Formation, sinuous rille in the eastern (right) half; large fresh crater is Aristar­ material) and Harbinger Mountains (mainly Harbinger chus, 40 km in diameter; largest of the many sinuous Formation). Geologic maps of this area are by Moore (1965, rilles is Schroter's Valley (Vallis Schroteri) . Note large 1967). (Photograph 369, Catalina Observatory.)

Harbinger Formation east of the Aristarchus pla­ tion is designated "Imbrian or Eratosthenian" be­ teau (fig. 13) (Moore, 1964, 1965). Some of this cause parts of it may interfinger with mare formation's relief is apparently formed by volcanic material, whereas other parts seem to embay mare; domes, cones, and craters, though some may be due material that flowed out of the sinuous rilles may to the underlying Fra Mauro Formation. The Har­ form part of the mare material. The Harbinger binger Formation is also characterized by numerous Formation is part of an extensive volcanic province sinuous rilles. What appear to be the high ends of near the crater Aristarchus (fig. 13; table 1) ; some these rilles terminate at craters within the forma­ formations of this province may be as young as tion; the low ends terminate on mare. The rilles Copernican (seep. 38). may have been cut principally by flowing volcanic Another unit having intrinsic relief is the Bos­ material; the direction of flow may have been con­ covich Formation in the Julius Caesar quadrangle trolled partly by structure. The Harbinger Forma- (Morris and Wilhelms, 1967). This unit forms long · F36 CONTRIBUTIONS TO ASTROGEOLOGY

stringy ridges approximately parallel with Imbrian rays an age criterion for craters is that rays appear sculpture; near it are terra domes of probable vol­ to be among the most recent materials on the moon canic origin. The Boscovich and the domes may con­ and are superimposed on most other materials; in sist of viscous volcanic materials extruded from the the type example, the rays of Copernicus overlie the sculpture fractures. materials of the nearly rayless crater Eratosthenes (fig. 12) (Shoemaker and Hackman, 1962, p. ERATOSTHENIAN AND COPERNICAN SYSTEMS 295-298; Carr 1964, p. 12-15). External processes The ne.xt youngest lunar time-stratigraphic sys­ such as micrometeorite mixing and solar radiation tem after the Imbrian is the Eratosthenian, and the may cause rays to darken and disappear with time youngest is the Copernican. Although most mappers . (Shoemaker, 1962, p. 345). Rays may be an expres­ have separated the Eratosthenian and Copernican sion of myriads of small secondary and tertiary im­ Systems, the two are discussed together here be­ pact craters, and they may fade when bright steep cause of doubt that they can be validly separated in slopes of these satellitic craters become subdued. many areas. (See also McCauley, 1967b, p. 436.) (Rays are shown with a stipple pattern on lunar Moreover many of the units discussed here resem­ geologic maps.) ble unit~ discussed above that are assigned The presence or absence of visible bright rays, "Imbrian or. Eratosthenian" ages, and some of these however, is not entirely a function of age. If rays may be as young as Copernican. The ages of most fade from mixing with darker materials, they terra-mantling, plains-forming, and positive-relief · would fade more readily around small craters than materials are based on their superposition on crater around large ones. Furthermore, there are rayless materials and thus are only as reliable as the esti- craters with circular dark halos-mapped as "Co­ mated ages of the craters. . pernican dark-halo crater material"--clearly super­ .Because time-stratigraphic systems adjoin with­ posed on the ray and rim materials of light-ray cra­ out gaps, the base of the Eratosthenian System co­ ters (fig. 12) (Shoemaker and Hackman, 1962, p. incides with the top of the Imbrian System. The 297-298; Carr, 1964, p. 16). Similar dark craters of common boundary is not defined, but it is infor­ Copernican age may well occur where there are no mally regarded as the top of the mar_e material be­ light Copernican rays by which to determine their tween the craters Eratosthenes and Archimedes. relative age, and these craters may have been (See introduction to the section "lmbrian System".) mapped erroneously as Eratosthenian. Any one or

CRATER MATERIALS all of the following factors may explain the differ­ ence in brightness: (1) the nature or composition of Chief among lunar material units of the Eratos­ thenian System are materials of fresh rayless cra­ the material in which the crater was formed-for example, there are more telescopically resolvable ters. Chief among Copernican units are materials of rayed craters. (See the section "Crater material bright-rayed craters on some light terrae than on maria; (2) the presence of two superposed source subunits" for subdivision conventions). Most Co­ layers-this may explain Dionysius (lat 3° N., long pernican craters have a high signal in the thermal 17° 30' E.), which has dark as well as bright rays region of the infrared during eclipse. The signal of (Smalley, 1965), and Tycho and many other craters most Eratosthenian craters is lower and commonly that have both bright and dark halos; (3) material indistinguishable from the background (U.S. Geo­ in which rays formed-a ray that is superposed on logical Survey, 1967,. p. A132). both light and dark materials is commonly brighter Crater materials are shown as "Eratosthenian or on the light materials; ( 4) differences in origin­ Copernican'' for several reasons: (1) if the craters some or all dark-halo craters may be of volcanic ori­ do not have telescopically resolvable bright rays but gin, and most light-rayed craters may be of impact have bright halos which may be formed of unre­ origin; (5) later events, such as burial by ash fall. solved rays; (2) if they have bright halos or s?me rays but have only a low thermal signal at eclipse Probably the most accurate statement is that the (Wilhelms, 1968) ; (3) if they are small and appar­ presence of rays indicates a young crater, but the ently rayless but do not contact rays of large Coper­ absence of rays does not necessarily indicate an old nican craters and cannot be dated directly other­ crater (Carr, 1964, p. 16). wise (Eggleton, 1965; Schmitt and others, 196!); There are serious reservations, therefore, about however, such craters are mapped as Eratosthen1~n the assignment of craters to time-stratigraphic sys­ by most mappers. tems solely on the basis of the presence or absence The reason for making the presence or absence of of rays. Eventually, the younger subdivisions of the SUMMARY OF LUNAR STRATIGRAPHY-TELESCOPIC OBSERVATIONS F37

lunar time scale can be redefined on the basis of (Moore, 1967) apparently issued from part of a crater freshness or the dating of noncrater post­ very long linear midmare fracture. Rays of Lichten­ Imbrian rock units. berg and Copernicus are obscured along this frac­ ture for hundreds of kilometers (Moore, 1967; U.S. MANTLING MATERIALS Geol. Survey, 1967, p. A131). Some extensive units morphologically (and proba­ Other, lighter materials also form plains. An ex­ bly lithologically) like the Sulpicius Gallus and Dop­ ample is the flat smooth-surfaced material which oc­ pelmayer Formations that apparently form a thin curs in depressions in the rim material of the Co­ dark cover over subj~cent terrain and add little re­ pernican crater Theophilus (fig. 14). This material, lief of their own are assigned Eratosthenian and obviously younger than the crater and therefore Copernican ages. The Cavalerius Formation of Co- also Copernican in age, is the '.'smooth member" of .pernican age (McCauley, 1967a) is superposed on the Theophilus Formation (Milton, 1964, p. 22; rim material of the postmare crater Cavalerius, on 1968). The topography of the crater rim material is the adjacent mare surface, and on Copernican rays. subdued around the smooth plains, probably by (The Cavalerius Formation is the unit on which the thinner material ("hilly member") related to the Soviet probe Luna 9 probably landed.) Several . plains-forming material. other units of probable Copernican age are so thin that subjacent topography is not modified at all. In UNITS WITH INTRINSIC RELIEF order not to obscure the more important subjacent Certain materials younger than the local mare units on the map, the dark covering materials are material have much intrinsic relief. One is the Tac­ shown with an overprint pattern, and the subjacent quet Formation along the southern edge of Mare units are shown in color (Milton, 1968; Howard and Serenitatis (Carr, 1966a). This unit, assigned an Masursky, 1968). Darkening by other causes or the Eratosthenian age, forms an elongate low bulbous absence of bright covering may be .shown similarly dark ridge ("dark member") surrounded by lighter, (Schmitt and others, 1967). and probably older, material with lower relief At least one thin unit of possible· covering mate­ ("light member"). Rilles, some with raised rims rial, the Reiner Gamma Formation (lat 7° N., long which grade into the rest of the formation, run the 59° W., west of the crater Reiner and south of the length of the dark ridge and were probably its Marius Hills) (also Copernican), is brighter than source. Low ridges in the light member oriented the adjacent material (McCauley, 1967a). This pe­ normal to the rilles may be flow lobes, and low culiar formation possibly consists of ash-flow tuff or scarps bounding the formation may be flow fronts. an irregular area of surface alteration. Attention is This unit is part of the young marginal belt of now being given to other areas of light covering Mare Serenitatis, as are the two dark members of materials not associated with craters, but mapping typical mare material of Imbrian age, the still-dark­ conventions have not yet been established. er mare material of Eratosthenian age, and the PLAINS-FORMING MATERIALS Sulpicius Gallus Formation. In the northwest quadrant of the moon there are As discussed above (p. 32), some post-Imbrian two complexes of domes and other materials that dark materials are probably mare materials; that superposition relations suggest are younger than is, they are dark and appear smooth and flat at t~le­ the local mare material. One, assigned an Eratos­ scopic resolution. Most of these young materials thenian age, is the Marius Group west of the crater occur at the edges of mare basins and in smaller Marius (McCauley, 1965, 1967a; summarized in depressions of the terra. This is true both of "Coper­ nican or Eratosthenian mare material" mapped in U.S. Geological Survey, 1966, p. A124.). This unit the Mare Vaporum quadrangle (Wilhelms, 1968) consists of undulating plateau-forming material and and of other materials that either have been as­ at least two kinds of domes: one low and convex in signed a possible Imbrian age (see p. 32) or are as profile like mare domes, and the other with steeper yet unreported in the published literature. The dark­ slopes that are concave in profile. By analogy with est material yet recognized, mapped as "Eratos­ terrestrial landforms the domes are interpreted to thenian mare material, dark" {Carr, 1966a), occurs be volcanic in origin. Some steep domes are super­ along the east margin of Mare Sernitatis. On the posed on low domes, as in certain terrestrial vol­ other hand, the dark material mapped as "Coper­ canic provinces, where the cause may be a change nican mare n1aterial, dark" that embays the rayed in either the bulk composition of the magma or in crater Lichtenberg in the Seleucus quadrangle its volatile components. F38 CONTRIBUTIONS TO ASTROGEOLOGY

' .

FIGURE 14.-Part of crater Theophilus (at bottom of picture). The crater's rim material extends over most of remainder of picture. In the center is the main occurrence of the Theophilus Formation (Milton, 1964, 1968), a plains unit lying on the rim material. Picture covers an area approximately 90 km east-west. (Photograph ECD 37 taken with 120-in. reflector, Lick Observatory.)

The other volcanic complex in the northwest lis Schroteri, the largest of all sinuous lunar rilles. quadrant of the moon, assigned "Eratosthenian or The topographically higher end or head of the rille Copernican" age, is the Vallis Schroteri Formation is a crater called the Cobra Head that is in a very in the Aristarchus plateau (Moore, 1965, 1967), an large dome; the material of the dome is the Cobra area also known as Wood's spot (fig. 13). In addi­ Head Formation (Moore, 1965). This formation and tion to containing thin blanketing material that the material in the floor of the rille (mapped as does not visibly subdue the underlying topography, "sinuous rille material" by Moore 1965, 1967) are the formation includes plains-forming materials, assigned a Copernican age because of their appar­ domes, low-rimmed craters, and cratered cones; one ent topographic freshness. Like the other sinuous elongate cone is 13 by 28 km. As in the Harbinger rilles, Vallis Schroteri may have been formed in Formation, sinuous rilles head in the Vallis Schro­ part by erosion as fluid material flowed from the teri Formation and terminate in the adjacent mare. crater at the head (Cobra Head) to the low end of The two formations are similar and probably repre­ the rille. Since the low end is on the mare, the mare sent continued volcanism of the same type. material (mapped as the Procellarum Group) may The Vallis Schroteri Formation is transected by include materials of Copernican age that flowed the feature for which the formation is named, Val- from the rille. SUMMARY OF LUNAR STRATIGRAPHY-TELESCOPIC OBSERVATIONS F39

SLOPE MATERIAL TABLE 2.-Form.ally defined lunar stratigraphic names The material on the walls of craters and on other Page in Name Publication th.is report Age steep slopes is commonly brighter under full-moon Apennine Bench Hackman, 1966 -- 27 1m brian. illumination than the adjacent materials (fig. 12), Formation. and some has the highest albedo of all lunar mate­ Boscovich Morris and 35 .lmbrian or rials. Steepest and youngest slopes-where down­ Formation. Wilhelms, 1967. Eratosthenian. slope movement of rock is most likely to occur-are Cavalerius McCauley, 1967a _ 37 Copernican. Formation. commonly the brightest, sugges~ing that albedo is a Cayley Morris and 27 1m brian. function of the amount of fresh material exposed. Formation. Wilhelms, 1967. This bright material is, therefore, assigned a Coper­ Cobra Head M~ore, 1965 38 Copernican. nican age, even whe.re it occurs in craters or on Formation. other features of pre-Copernican age. The bright Diophantus Moore, 1965 28 1mbrian. slope material may consist mostly of mass-wasted Formation. debris. However, since it may also include freshly ~· Doppelmayer Titley, 1967 33 lmbrian or exposed bedrock, "Copernican slope material" may Formation. Eratosthenian. not be everywhere a rock unit by itself but rather Fra Mauro Eggleton, 1964, 23 1m brian. Formation. 1965; Wilhelms, an indicator of the freshest exposures of other rock this report. units. Gassendi Group Trask and Titley, 23 Pre-Imbrian and Trask and Titley (1966) mapped two additional 1966; Titley, 1m brian. slope units. One, assigned an Eratosthenian or 1967. Imbrian age, is brighter than the adjacent mare Harbinger Moore, 1965 ----- 35 lmbrian or material, although not as bright as the Copernican Formation. Eratosthenian. slope material. The other, assigned an 1mbrian or Hevelius McCauley, 1967a __ 28 1m brian. Formation. pre-Imbrian age, is not bright and was mapped to emphasize the slope of a long ridge. Marius Group. McCauley, 1967a _ 37 Eratosthenian. Procellarum U.S. Geol. Survey, 29 1m brian. 1 MISCELLANEOUS UNITS Group. 1964. Up to this point, the units discussed have been Reiner Gamma McCauley, 1967a _ 37 Copernican. Formation. assigned ages and given either formal (table 2) or informal names. All maps show a variety of addi­ Sulpicius Gallus Carr, 1966a 33 Imbrian or Formation. · Eratosthenian. tional small material units of distinctive morph­ Tacquet Carr, 1966a 37 Eratosthenian. ology that normally are not assigned ages. These in­ Formation. clude materials of chain· craters; materials of other Theophilus Milton, 1968 _____ 37 Copernican. types of craters discussed earlier that are not part Formation. of the .main sequence of unclustered round craters; Vallis Schroteri Moore, 1965, 1967 38 Eratosthenian or dark-halo crater material; pitted material; hum­ Formation. Copernican. mocky material; ridge material; material of cones Vitello Trask and Titley, 22 Pre-Imbrian. with summit craters; and terra dome material, Formation. 1966; Titley 1967. which is sometimes divided into light and dark. In 1 Abandoned. map explanations, most of these are grouped with the structural symbols, apart from the main box ex­ ABANDONED NAMES planation. This arrangement is followed because This section lists stratigraphic names that have ages are commonly known only to within three geo­ been introduced casually but never defined formally logic-time periods-lmbrian to Copernican or pre­ nor used on published maps of the· Geological Sur­ Imbrian to Eratosthenian. Superposition relations, vey series discussed in this paper (table 3). The obvious on the map, show what is known about the names in table 3 were introduced by Survey geolo­ ages of individual occurrences. Some large units, gists in preliminary open-file reports to the Na­ however, are assigned to specific systems. Some tional Aeronautics and Space Administration, and morphologic forms shown by structural symbols on were then published as part of short informal stra­ most recent inaps were shown on earlier maps as tigraphic summaries (U.S. Geological Survey, 1964, geologic units, such as sinuous rille material and 1965, 1966). These names are now considered aban­ linear rille material. A common inclusive unit on doned. Additional abandoned names, not listed here, early maps is rille and chain crater material. appeared only i~ the preliminary reports and maps. F40 CONTRIBUTIONS TO ASTROGEOLOGY

TABLE a.-Stratigraphic name.~ that have appeared in Geological Survey publications but have not been defined formally

RefMence: U.S. Geol. Survey Research Current published Reason for Name Author of the year indicated designation abandoning Bond H. A. Pohn ------1966 ------Unpublished (probably will Not formally defined; no Formation. be called plains-forming need for name. material). Censorinus D. P. Elston ______1964 (see also Elston, 1964) _Unpublished Not formally defined. Formation.

Cordillera J. F. McCauley ______1964, 1965 ~------Includes Hevelius Forma- To be replaced by Hevelius Group. tion; part unpublished. Formation and possibly another formation. Cruger J. F. McCauley ______1964 ------Unpublished (probably will Not formally defined; no Group. be called crater material). need for name. Humorum S. R. Titley ______1964, 1965 ______Assigned to the Vitello For- Distinctiveness of the Group. mation, material of smooth three units. ridges and hills, and plains- .4. forming material. Pyrenees D. P. Elston ______1964 (see also Elston, 1964)_Unpublished ------Not formally defined. Formation.

The only stratigraphic names known to the au­ rial generally meet at a sharp break in slope. Where thor that have been proposed outside this mapping there is no. break in slope, as in conical or bowl­ program and not used in it are those of Spurr shaped craters, the eqtire interior is mapped as wall (1944-49)-highly interpretive time units-:-Khaba­ material. Subunits are not mapped in small craters, kov (1962, p. 288); Sukhanov, Trifonov, and Flo­ and the contact between crater materials and adja­ renskiy (1967); and Dodd, Salisbury, and Smalley cent materials is drawn at the outer most limit of ( 1963) . In the last report, the names Ptolemaic and positive relief, not at the rim crest. Subunits of Highlandian Series were proposed for units pre­ large craters may not be mapped separately if they viously recognized by the Geological Survey. are indistinct, as in many old craters. All materials of a single crater are collectively re­ CRATER MATERIAL SUBUNITS garded as an informal formation, and each of the A crater is a topographic form-a depression­ subunits discussed above as an informal member but its rim, walls, floor, and other features are com­ (fig. 15). These members may be divided into sub­ posed of material units. Materials of large craters members, the more common of which are shown in are extensively subdivided on lunar geologic maps figure 15. Additional members and submembers of 1:1,000,000 scale, and the subunits make up from may be distinguished as the need arises. An outer one quarter to more than half the number of the rim facies in which some positive relief and many map units. Material with positive relief outside the satellitic craters occur together has been called rim crest is mapped as "crater rim material"; the "cratered rim material" and mapped as a separate 8 inclined parts inside, as "crater wall material"; the geologic unit (Trask and Titley, 1966), or it has generally level parts in the center, as "crater floor been delimited by a special line symbol and given material"; and the single or multiple peaks at or the color of the underlying unit (Schmitt and oth­ near the center, as "crater peak material." Interior ers, 1967). Examples of other special units are material similar in texture to rim material is some­ "ring material" forming circular ridges within two times mapped as rim material instead of wall mate­ craters in the Pitatus quadrangle (Trask and rial, especially in craters whoSe rim crests are in­ Titley, 1966) and "ridge material" of the centra.l distinct. The level surfaces of arcuate terraces in ridge in the crater Alphonsus (Howard and Masur­ the largest craters are also usually mappeq as rim sky, 1968). Units can also be divided on the basis of material because the terraces are interpreted to be albedo where this property is believed significant: slumped parts of the rim. (The original crater di­ "rim material, dark"; "floor material, dark." ameter was therefore less than the present rim Crater materials are subdivided in order to call crest-to-rim crest diameter.) Wall and floor mate- attention to differences in physical properties, rela­ tive ages, and probable origins among the subunits 8 On two maps (Carr, 1965b; Hackman, 1966) this material was des­ ignated "slope material." On all maps including these, bright matet·ial of a single crater and also among different craters. on crater walls and other slopes is called slope material and assigned a Copernican age. (See section on "Slope ~aterial.") · The near-surface rim material is interpreted as SUMMARY OF LUNAR STRATIGRAPHY-TELESCOPIC OBSERVATIONS F41

displayed by Copernicus, for example-are probably cw cfs characteristics of young impact craters (Shoe­ ~- maker, 1962). In the absence of radial rim mate­ < rials and satellitic craters, a subdued circular or poly­ < cf c <~ gonal ring of indistinctively textured material cr crh ~ err cfh cp > may represent either a crater that did not form by .;> ~ impact, or an old impact crater modified by mass wasting, superposed materials, and isostatic re­ FIGURE 15.-Example of map explanation box showing bound (Masursky, 1964; Danes, 1965). (Rim mate­ approximately stratigrapric relation among crater material rial of all craters, especially seemingly old ones, is subunits. All materials of a single crater are regarded as an informal formation; each formation may be divided gradational with the surroundings, and the contact into members, and each member may be divided into may be difficult to map.) submembers. Common submembers are illustrated above. Wall material is usually interpreted as fragments The "c" represents the formation "crater material"; of rim and precrater rock which have moved. down­ ~here it stands alone it represents undivided material · slope, although the unit as mapped may also include (shown on left). The second lowercase letter stands for a member and the third for a submember, as follows: outcrops of these rocks. Downslope movement may continue for some time, after the formation of a cr, rim material crater, and the explanation boxes of many maps crh, hummocky 1 Laterally gradational facies of show wall material as partly younger than the rim rim material recognized in some material, although assigned to the same system (fig. large craters. err, radial 15). Material on crater walls is generally somewhat cw, wall material (Includes floor material and peak brighter than the rim material and where especially rna terial in some small eraters ; bright may be substantially younger than the cra­ may also include rim material extending inside some craters.) ter. This brighter material is usually mapped as cf, floor material (Includes peak material in some "Copernican slope material," rather than as wall small craters.) material (see p. 39). cfs, smooth Floor material may have formed with the crater cfh, hummocky or afterwards. It is interpreted, alternatively, as the cp, peak material Boxes in lower row refer to materials formed contempo­ exposed original brecciated crater floor, as impact raneously with the crater. Wall material (·cw) and fallback, as material transported from the wall, as smooth floor material (cfs), particularly, may continue penecontemporaneous or substantially younger vol­ to form after cratering. Peak material and hummocky canic materials, or as a combination of these. floor material may do so also and are so shown on some Smooth floor material, in particular, may consist of maps. The "c" may be replaced by letters representing the volcanic materials emplaced some time after the descriptive designation of craters (such as "sc" for satel­ crater formed and is shown as partly younger on lite crater material and "ch" for chain-crater material). many maps. Such material may not be related to The "c" may also be replaced by letters representing in­ the primary crater-forming event. Where smooth formal but specific names (such as "a" for materials for material in crater floors can be correlated with the crater Alpetragius) or formal names (such as "g" mare materials or other plains-forming units, it is for Gassendi Group). (Even though the letter refers to a group, not to a formation, the materials of each individual mapped as these units and not as floor material. crater are still regarded as an informal formation; these Crater peak material may be either material from formations together compose the formal group.) On a depth that rebounded immediately after impact or map, a capital letter representing an age assignment volcanic material emplaced later. (C, CE, E, EI, I, lpl, pi) normally precedes the lower­ The conventions described above are different case letters. (On early maps, crater subunit boxes were separate, not joined as in this figure, and did not show from those used on the first two maps published at stratigraphic relations among subunits.) 1:1,000,000 scale (Hackman, 1962; Marshall, 1963), where only three main subunits were employed: ejecta of either impact or volcanic origin (p. 7). "rim material," "floor material," and "Copernican The surface texture and apparent areal extent ( rel­ slope material." In addition to material outside the ative to rim-crest diameter) of rim materials are rim crest, "rim material" included parts of the indicators of crater age and origin. Extensive rings upper walls, the entire wall when the slopes were of distinctively textured hummocky and radial rim not bright, and the entire crater when no subunits facies and large fields of satellitic craters-all well were mapped. Rim material of the large crater Kep- F42 CONTRIBUTIONS TO ASTROGEOLOGY

ler was mapped on the basis of albedo, not topog­ is youngest and includes materials of rayed and raphy. It was divided into a dark inner unit and a dark-halo craters and materials contemporaneous bright outer one, and the outer limit was placed at with them. the limit of the continuous bright halo, which ex­ Circular multiringed mare basins are the largest tends beyond the continuous positive relief of the lunar features, and the formation of such a basin is radial rim material. Floor material included mate­ a major event that obliterates the earlier record rial of the central peak, as well as of the relatively over a large area and starts a new stratigraphic se­ flat interior. quence. Basins consist of an inner basin and several Crater material subunits are now mapped differ­ concentric alternately raised and depressed arcuate ently, therefore, in the following respects: (1) More structures composed of deformed prebasin mate­ emphasis is placed on topography and less on al­ rials. The original form of these materials is unrec­ bedo, (2) a hierarchical subdivision is established ognizable, but they presumably resembled interba­ whose basic formational unit is "crater material," sin materials like those of the mainly pre-Imbrian and (3) a wall unit is introduced to emphasize the southern highlands. These arcs were probably differences between the interior and exterior crater formed when the basin was formed but may have materials and to deemphasize the albedo differences been later modified by renewed movement. They among interior materials. A cartographic dividend may be partly covered by basin ejecta. In pre­ of distinguishing between rim and wall materials is Imbrian time several mare basins were formed that it aids portrayal of intersection and superposi­ whose materials are now subdued, heavily cratered, tion relations between the crater materials and faulted, and partly buried by younger materials. other units, for example, where a unit younger than The estimated order of formation of some conspicu­ the crater covers rim material but does not en­ ous pre-Imbrian basins from oldest to youngest is croach on the wall material inside. Fecunditatis, Serenitatis, N ectaris, Humorum, and Crisium (or Crisium-Humorum). The largest and SUMMARY AND CONCLUSIONS youngest basin on the near side is the Imbrium The geology of muGh of the moon's earthside sur­ basin, whose formation marks the beginning of the face was mapped from earth-based telescopic data Imbrian Period and whose extensive etjecta ( ?) de­ (~ before spacecraft photography became available. posits (Fra Mauro Formation) and radial fracture Lunar geologic mapping is based on the concept system (Imbrian sculpture) are used to separate that the lunar crust, like the terrestrial crust, is the materials of a vast area into pre-Imbrian and composed of discrete material units. Relative uni­ younger ages. Orientale is a still younger basin of formity of topographic and surficial properties is Imbrian age, which similarly dominates a vast area the basis for recognizing and mapping a unit and centered on the west limb. implies that most units are three-dimensional bodies The principal deposits that were emplaced after having a limited range of lithology and age. The the formation of each basin are light plains mate­ order in which such units were formed is deter­ rial, dark plains (mare) material, and crater mate­ mined by their superposition and transection rela­ rials, all of which interfinger complexly. Both kinds tions and by secondary characteristics such as cra­ of plains-forming materials fill depressions such as ter density and degree of topographic freshness. A the basins themselves and their peripheral concen­ composite column of such lunar material units, con­ tric and radial depressions, whereas crater mate­ ceptually equivalent to terrestrial rock-stratigraphic rials may be formed anywhere. Most light plains units, has been divided into four approximate time­ materials are older than most dark plains materials stratigraphic units. The oldest unit is the pre­ and may have evolved from originally dark plains Imbrian (not a formal system); all materials · through cratering and other processes. Light plains formed before the formation of the Imbrium basin materials in or near the Imbrian basin (Apennine are conveniently assigned to it. Next youngest is the Bench and Cayley Formations) must be Imbrian in Imbrian System, which includes all materials age, whereas those in the pre-Imbrian basins and formed during and after the formation of the Im­ on the terra between basins may be of either pre­ brium basin but before the deposition of most of the Imbrian or Imbrian age. Many mare materials can mare material in. the· basin. The Eratosthenian Sys­ be correlated with those mare materials that infor­ tem follows; it tentatively includes materials of ray­ mally define the top of the Imbrian System and less craters and some other materials superposed on are therefore of Imbrian age. Hence mare material the Imbrian mare material. The Copernican System is an approximate stratigraphic datum common to SUMMARY OF LUNAR STRATIGRAPHY-TELESCOPIC OBSERVATIONS F43

all basins; however, mare materials do differ in (Theophilus Formation). Finally, bright slope mate­ age, and a substantial but undetermined percentage rial indicates fresh exposures of other units. of dark mare material is younger than Imbrian. In general, young lunar material units have more Much dark young mare material as well as young textural detail than old ones. Much relatively old mantling and positive-relief materials are concen­ light-colored lunar terrain appears subdued, and trated near basin margins. Filling of a basin by this fact suggests that is has been eroded or blan­ mare material only ends long after the basin was keted by light-colored stratigraphic units (such as formed. the hilly member of the Cayley Formation and var­ Craters and their deposits have formed through­ ious Imbrian terra units). The generally old terrain out lunar history. Craters older than basins are all of the southern highlands is doubtless complex in­ pre-Imbrian except some near Orientale. Pre­ ternally, but little detail is seen at telescopic reso­ Imbrian craters are very numerous in areas such as lution and this terrain is still poorly understood. A the southern highlands where· basins have not pre-Imbrian plateau-forming material and a few formed. Materials of craters such as Gassendi other distinct units, in addition to crater-material which are younger than the pre-Imbrian basins but units, have, however, been recognized in the high­ older than the mare material that fills the basins lands. are virtually all pre-Imbrian or Imbrian in age. Lunar stratigraphic studies were aimed, in their early stages, at recognizing material units, mapping Those craters such as Archimedes which 'a'te simi­ their spatial relations, and establishing their se­ larly related to the Imbrium and Orientale basins l, quence of formation. The ultimate goal, however, and their mare filling are Imbrian in age. Among includes unraveling the origin of lunar materials. relatively round craters with concave-upward rim­ Evidence of the operation of general processes such flank profiles, there seems to be a progression in as emplacement of flows or ejection of debris is pro­ morphology and age from subdued, probably old vided by characteristics of topography visible at tele­ craters like many in the southern highlands to scopic resolution. Details of processes and compo­ fresh, probably young craters like Copernicus. Ray­ sition must, however, be inferred by drawing -) less fairly fresh-appearing craters not in contact comparisons. with terrestrial analogs of lunar fea­ with the mare material may be either Imbrian or tures and from laboratory experiments and theory. Eratosthenian in age; most rayless craters super­ The inferences made earlier in this paper will be 0 posed on mare surface are assigned an Eratos­ summarized here. thenian age. Rayed craters everywhere are assigned · Some craters form by volcanism and some by im­ a Copernican age. All evidence points to the rela­ pact, and many impact craters have volcanic mate­ tive youth of rayed craters, but age designations of rials superposed on them. Chain craters that are most rayless craters are only tentative. Rays seem uniformly sized and regularly spaced along a gra­ to fade with time, but at varying rates, and they ben and low-rimmed, commonly elongate craters may never have been present around some craters. exactly centered at the tops of shieldlike mare domes Indeed, some rayless and dark-halo craters are cer­ must be volcanic. On the other hand, large craters tainly very young, as indicated by superposition re­ such as Copernicus and Tyc)lo are probably formed lations or pronounced thermal anomalies. Several by impact. The pattern of secondary craters and the classes of irregular, elongate, or low-rimmed craters very long rays around such craters are explained in and craters alined in chains (except those satellitic detail by ejection of fragments in a single sudden to large craters) are not assigned ages. release of explosive energy from a point or line Additional types of material, mostly relatively source in the parent crater. An explosion of suffi­ young and of local extent, include dark blanketing cient energy could easily be generated by the impact materials (mapped as the Sulpicius Gallus, Doppel­ of a relatively small object from space, and many mayer, and Cavalerius Formations); materials of such objects are known to exist in the Solar System various domes, cones, plateaus, and other appar­ and must have existed in the past. Not even the ently constructional features, (materials of Kant largest terrestrial volcanic explosion, however, has Plateau; Harbinger, Boscovich, Tacquet, Vallis released sufficient energy suddenly enough to pro­ Schroteri, and Cobra Head Formations; Marius duce features of the scale observed on the moon. Group) ; and sinuous rille materials. One moderately In several respects mare basins are morphologi­ light plains-forming unit is Copernican in age cally similar to large impact craters, and some ba- F44 CONTRIBUTIONS TO ASTROGEOLOGY sins are surrounded by deposits that resemble impact tonic origin; (9) materials of volcanic domes that crater eJecta; for example, the Imbrium basin is have gentle slopes; (10) materials of volcanic domes surrounded by the hummocky, laterally gradational, and ridges that have intermediate and steep slopes; and apparently blankletlike Fra Mauro Formation. ( 11) slope materials transported by mass-wasting. These geometric similarities, in addition to the All these materials were probably formed great areal extent of radial and concentric struc­ throughout lunar history; however, they are subject tures that were presumably formed when the basin to degradation and change, and some have been rec­ was created, suggest that the basins (not their fill­ ognized o"nly in recent systems. Only mare-basin ing) formed by impact. materials, crater materials, plateau-forming mate­ Volcanic materials appear to be common on the rials, and possibly light plains materials are now moon. Many domes and other constructional fea­ known to have formed in pre-Imbrian time. The tures are clearly of volcanic origin, and their vari­ same materials also formed in the Imbrian Period, ety of form suggests different magma compositions as did dark plains-forming (mare) and terra-man­ or modes of eruption. Sinuous rilles suggest the tling materials. The light materials, especially those flowage of fluid material of some sort. The fact that that form plains, may have evolved from the dark many light and dark plains-forming materials have ones. Constructional volcanic features are known from lobelike contacts and terminate abruptly against the Imbrian and become common in the Eratos­ higher topographic forms suggests that they were thenian and Copernican Systems; no doubt many emplaced as flows with the properties of a fluid. old ones are also present but unrecognized because Basalt lava comes to mind because of the plateau of degradation and burial. Crater materials also basalts on earth, which are highly fluid, laterally ·were formed in the Eratosthenian and Copernican extensive, and dark. The dark and possibly the light Periods, and the features that reveal their origin mantling materials, on the other hand, are· more .are best preserved in these young craters; origins of likely to be deposits of free-fall tuff. They grade to many older craters may never be known. Materials plains-forming materials in many places; thus, have doubtless moved down slope in all periods, but free-fall tuff· may be interbedded with the plains only those active in the Copernican can be sepa­ flows. An alternative to these interpretations is that rated from the materials from which they were some or all plains and mantling materials are ash­ formed. The more thoroughly the Moon's surface is flow tuff. Like plateau basalt, this material is exten­ examined, the more small areas are found which sive terrestrially and on the regional scale does not can be distinguished from their surroundings by appear significantly different from basalt. Like differences in texture, albedo, and crater density. free-fall tuff, ash-flow tuff compacts differentially Materials in most of these smail areas are of post­ upon cooling, and so the buried topography remains Imbrian age, and the stratigraphy of .the Eratos­ expressed, but in subdued form. thenian and Copernican Systems doubtless will be­ The role of erosional processes in the degradation come increasingly complex as studies continue. of lunar landforms is not yet known, but the preva­ Geologic mapping has shown, therefore, that a lence of subdued topography suggests considerable variety of formative and destructive processes and erosion. The light terra-mantling deposits may well a complex· evolutionary history have produced a het­ be mostly mass-wasted debris. Meteorite impact . i presumably is an important erosional agent and erogeneous lunar crust. Contrary to a widespread probably produces a fragmental layer on exposed presumption, no single origin~neither impact nor surfaces. volcanism-adequately accounts for all craters and In brief, lunar materials are currently believed to other features. Throughout its history the crust fall into the following major genetic classes: (1) probably was repeatedly affected by volcanic activ- · impact ejecta deposits from craters; (2) impact ity and was constantly bombarded and deformed by ejecta blankets from mare basins; (3) volcanic ma­ the impact of bodies from space and of fragments terials erupted from craters; ( 4) dark plains-form­ of its own crust secondarily ejected from the cra­ ing (mare) materials composed chiefly of volcanic ters thus formed. All surfaces are degraded by a flows; ( 5) dark terra-mantling materials composed process analogous to terrestrial erosion. This heter­ chiefly of pyroclastics; (6) light plains-forming ma­ ogeneity means that during future exploration, terial composed at least partly of volcanic flows; ground visits must be made to many representative (7) light terra-mantling materials of mixed origin; points. We cannot expect one place to reveal the (8) plateau-forming materials of volcanic or tee- complete composition of tlie -·crust and the complete SUMMARY OF LUNAR STRATIGRAPHY-TELESCOPIC OBSERVATIONS F45 history of the moon. Synoptic spacecraft photogra­ of the Moon: U.S. Geol. Survey Misc. Geol. lnv. Map phy will be necessary to study regional differences 1-458. and to extend the lunar stratigraphic framework Eggleton, R. E., and Marshall, C. H., 1962, Notes on the Apenninian Series and pre-Imbrian stratigraphy in the which owes its beginning to earth-based telescopic vicinity of Mare Humorum and Mare Nubium, in Astro­ studies. The simple but powerful methods of stra­ geol. Studies Semiann. Prog. Rept., February 1961- tigraphy applied to photographs will continue to be August 1961, pt. A: U.S. Geol. Survey open-file report, the most economical means of deciphering the struc­ p. 132-137. ture, history, and formative processes of the moon. Elston, D. P., 1964, Pre-lmbrian stratigraphy of the Colombo quadrangle, in Astrogeol. Studies Ann. Prog. Rept., REFERENCES CITED August 1962-July 1963, pt. A: U.S. Geol. Survey open­ file report, p. 99-109. American Commission on Stratigraphic Nomenclature, 1961, Fielder, Gilbert, 1961, Structure of the Moon's surface: New Code of stratigraphic nomenclature: Am. Assoc. Petro­ York, Pergamon Press, 266 p. leum Geologists Bull., v. 45, no. 5, p. 645-665. --- 1965, Lunar geology: London, Lutterworth Press, Baldwin, R. B., 1949, The face of the Moon: Chicago, Ill., 184 p. U niv. Chicago Press, 239 p. Gilbert, G. K., 1893, The Moon's face, a study of the origin of --- 1963, The measure of the Moon: Chicago, Ill., Univ. its features: Philos. Soc. 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Titley, S. R., and Eggleton, R. E., 1964, Description of an Ann. Prog. Rept., July 1964-July 1965, pt. A: U.S. Geol. extensive hummocky deposit around the Humorum basin, Survey open-file report, p. 29-34! in Astrogeol. Studies Ann. Prog; ·Rept., July 1963-July --- 1965b, Polarization properties of some lunar geologic 1964, pt. A: U.S. Geol. Survey open-file report, p. 85-89. units, in Astrogeol. Studies Ann. Prog. Rept., July 1964-­ Trask, N. J., and Titley, S. R., 1966, Geologic map of the July 1965, pt. A: U.S. Geol. Survey open-file report, Pitatus region of the Moon: U.S. Geol. Survey Misc. p. 63--80. Geol. lnv. Map 1-485. Wilhelms, D. E., Trask, N. J., and Keith, J. A., 1965, Pre­ U.S. Geological Survey, 1964, Lunar geologic mapping, in liminary geologic map of the equatorial belt of the Moon, Geological Survey research, 1964: U.S. Geol. Survey in Astrogeol. Studies Ann. Prog. Rept., July 1964--July Prof. Paper 501-A, p. A140-A142. 1965, map. supp.: U.S·. Geol. Survey open-file report. --- 1966, Earth-based lunar investigations, in Geological Survey research, 1965: U.S. Geol. Survey Prof. Paper PRELIMINARY LUNAR GEOLOGIC MAPS 626-A, p. A130-A133. Following are preliminary open-fUe geologic maps --·- 1966, Earth-based lunar investigations, in Geological Survey research, 1966: U.S. Geol. Survey Prof. Paper at a scale of 1: 1,000,000 that have not yet been su­ 650-A, p. A122-A125. perseded by a published map. Dates shown refer to --- 1967, Earth-based lunar investigations, in Geological the Astrogeologic Studies Annual Progress report Survey research, 1967: U.S. Geol. Survey Prof. Paper to the National Aeronautics and Space Administra­ 57~-A, p. A130-A132. tion of which the maps were a part. Whitaker, E. A., 1966, The surface of the Moon, in Hess, W. N., Menzel, D. H., and O'Keefe, J. A., eds., The· July 1963-July 1964 nature of the lunar surface-proceedings of the 1965 IAU-NASA symposium: Baltimore, Johns Hopkins Grimaldi quadrangle, by John F. McCauley (with text) Press, p. 79-98. July 1961,.-July 1965 Whitaker, E. A., Kuiper, G. P., Hartmann, W. K., and Spradley, L. H., eds., 1963, Rectified lunar atlas-supp. Byrgius quadrangle, by N.J. Trask (with text) 2 to the Photographic Lunar Atlas: Tucson, Univ. Ari­ Cleomedes quadrangle, by A. B. Binder zona Press, SO p. Colombo quadrangle, by Donald P. Elston (text in July 1963- Wilhelms, D. E., 1965, Fra Mauro and Cayley Formations in July 1964) the Mare Vaporum and Julius Caesar quadrangles, in Fracastorius quadrangle, by Donald P. Elston Astrogeol. Studies Ann. Prog. Rept., July 1964-July Langrenus quadrangle, by J.D. Ryan and D. E. Wilhelms 1965, pt. A: U.S. Geol. Survey open-file report, p. 13-28. Macrobius quadrangle, by H. A. Pohn (with text) --- 1968, Geologic map of the Mare Vaporum quadrangle Mare Undarum quadrangle, by Harold Masursky of the Moon: U.S. Geol. Survey Misc. Geol. lnv. Map Petavius quadrangle, by D. E. Wilhelms 1-548. Purbach quadrangle, by H. E. Holt Wilhelms, D. E., and McCauley, J. F., 1969, Preliminary Rupes Altai quadrangle, by L. C. Rowan geologic map of the near side of the Moon: U.S. Geol. Taruntius quadrangle, by D. E. Wilhelms Survey open-file report. July 1965-July 1966 Wilhelms, D. E., Masursky, Harold, Binder, A. B., and Ryan, Cassini quadrangle, by N. J Page J. D., 1965, Preliminary geologic mapping of the eastern­ J. Herschel quadrangle, by G. E. Ulrich most part of the lunar equatorial belt, in As.trogeol. Maurolycus quadrangle, by N. D. Cozad and S. R. Titley Studies Ann. Prog. Rept., July 1964-July 1965, pt. A: Plato quadrangle, by J. W. "M'Gonigle and D. L. Schleicher U.S. Geol. Survey open-file report, p. 45-53. Rheita quadrangle, by·· Desiree E. Stuart-Alexander Wilhelms, D. E., and Trask, N. J., 1965a, Compilation of Riimker quadrangle, by R. E. Eggleton and E. I. Smith geology in the lunar equatorial belt, in Astrogeol. Studies Sinus lridum quadrangle, by G. G. Schaber

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