Abstract

The U. S. Geological Survey is currently preparing a geologic map of the earthward hemisphere of the Moon at a scale of 1:1,000,000, using direct visual and photographic observations through large telescopes, and more recently, images transmitted by Ranger spacecraft. The purpose of the mapping is to develop and portray our understanding of the history of the Moon, to provide a basis for selection of favorable sites for lunar missions, and to serve as a framework for the much fuller geologic survey that will be carried out when manned landings are made. For this purpose a lunar stratigraphy is used that is closely analogous in principle to that used on Earth.

Both rock-stratigraphic and time-stratigraphic units are used. The fundamental rock-stratigraphic unit is the formation. In the absence of direct information on lithology, formations are characterized by homogeneity of physiographic expression and albedo. Some formations have been given formal binomial names; for others only the age designation and characterizing features are used. Related formations are assembled into groups, and facies within formations are distinguished, corresponding to members on Earth although not formally designated as such.

Although each formation has a distinctive composition and mode of origin, hypotheses concerning these can form no part of the definition of the formations. On the contrary, the stratigraphic assignments, based on observed appearances and relationships, should aid in formulating such hypotheses.

Time-stratigraphic classification is based on the laws of superposition and intersection. These may be applied to the formations directly, or they may be used to develop secondary criteria of age, as in the case of certain fresh-appearing craters which are surrounded by rays (radial streaks of bright material). Whenever a relationship of superposition between two craters or their ray systems can be determined, the crater with the brighter rays is younger than the crater with the fainter rays or with none at all. It thus appears that rays fade with time, eventually reaching the point of invisibility, so that a ray system indicates that its central crater is relatively young. Another useful index of age is the spatial frequency of small impact craters on a unit, which is higher the longer a surface has been exposed to meteorite bombardment.

The fundamental time-stratigraphic unit is the system, comprising the formations deposited during a period. Three systems are now recognized, one of which has been subdivided into two series. Older rocks have not yet been formally classified. In descending order the systems are:

     Copernican System
     Eratosthenian System
     Imbrian System
         Archimedian Series
         Apenninian Series.

The Copernican System includes the deposits associated with ray craters, such as the crater Copernicus itself. In addition, formations of volcanic or other origin that can be shown to be stratigraphically above a ray center are included in the Copernican System.

Erastosthenes, a fresh-appearing crater without bright rays, serves as the type locality for the Eratosthenian System. Most fresh appearing craters without rays are mapped as Eratosthenian, but since other factors than age, such as the albedo of the background, have some effect on the visibility of rays, a degree of uncertainty in the Eratosthenian-Copernican boundary is inevitable. Aside from the rays, Eratosthenian and Copernican craters are similar, although the effects of erosion (presumably by meteorite bombardment) are typically more developed in Eratosthenian craters. Crater formation was the dominant, but not exclusive, process active during the Eratosthenian Period, as in the Copernican Period.

The Imbrian System includes units associated with the formation and the filling of the Mare Imbrium basin, and correlative units elsewhere on the Moon. The base of the Imbrian System is defined by the base of a widespread blanket of material surrounding Mare Imbrium. The deposition of this blanket is believed to have resulted from a single catastrophic event (apparently impact) by which the basin occupied by Mare Imbrium formed (Shoemaker, 1964). A system of lineaments, probably faults, known as Imbrian sculpture, apparently developed at the time of this event. The presence of Imbrian sculpture in a rock unit serves as a criterion of pre-Imbrian age.

The Apenninian Series, named for the Apennine Mountains on the edge of Mare Imbrium, includes the blanket mentioned above and other widespread formations apparently symmetrically distributed around Mare Imbrium. It is uncertain whether the entire series is composed of material deposited in a brief period of time as a result of the Mare Imbrium event or whether it includes genetically unrelated materials deposited over a longer span of time.

The Archimedian Series begins with deposits from the oldest craters superimposed on Apenninian material, such as the crater Archimedes, and extends to the top of a widespread unit of extensive smooth dark mare materials, named the Procellarum Group after Oceanus Procellarum. When the first maps were prepared, it was thought that deposition of all the mare material took place in a brief period, which was distinguished as the Procellarian Period, between the Imbrian and Eratosthenian. Further mapping, however, has shown that deposition of mare material occurred over a lengthier span of time, simultaneously with continued crater formation. The time significance of the mare material is thus less and the Procellarum Group is now regarded as a rock unit of Archimedian age. The spatial frequency of small craters is approximately uniform over the surface of the Procellarum Group, suggesting that deposition terminated at roughly a single point in time. This surface is consequently used to define the Imbrian-Eratosthenian boundary.

Work is in progress on the division of pre-Imbrian time, primarily on the basis-of deposits and structures related to the formation of basins older than Mare Imbrium. Local stratigraphic columns have been developed, but as yet none are sufficiently certain for incorporation into a Moonwide stratigraphic system.

The absolute length of the periods can be estimated only crudely. An assumption that the majority of craters are of impact origin allows the spatial frequency of craters on surfaces of different ages to be compared with the flux in space of potential crater-forming objects and the spatial frequency of presumed impact structures on Earth. Such comparison suggests that, if the present rate of flux prevailed through the Copernican and Eratosthenes Periods, these periods occupied the greater part of time since the origin of the planets. If so, the pre-Imbrian and Imbrian must have been periods of short duration during which the rate of impact of crater-forming objects exceeded that in subsequent time.