Bates College

Astronomy/Geology 110


Lunar and Planetary Science

LUNAR STRATIGRAPHY AND PHOTOGEOLOGIC MAPPING


The writeup for this lab is based on Photogeologic Mapping of the Moon, Exercise Sixteen in "Activities in Planetary Geology for the Physical and Earth Sciences", edited by Ronald Greeley and Kelly Bender, Arizona State University; and Robert Pappalardo, Brown University, 1998. This is NASA EP-179, pp. 199-213. The text has been modified to describe the large "desk scale" photographs we have, but otherwise it is pretty much unchanged.

Purpose

Through observation and analysis of photographs of the moon you will become familiar with the techniques of constructing geologic maps of planetary surfaces.

Materials

Introduction

A geologic map is a graphic portrayal of the distribution and age of rock types, structural features such as folds and faults, and other geologic information. Such a map allows geologists to link observations made at different localities into a unified form and to represent those observations in a form that can be easily understood by others. One of the first tasks in preparing a geologic map is the identification of units. By definition, a unit is a three-dimensional body of rock of essentially uniform composition formed during a specified interval of time and that is large enough to be shown on a conventional map. Thus, the making of geologic maps involves subdividing surface and near-surface rocks into different units according to their type and age. On Earth, this involves a combination of field work, laboratory studies, and analyses of aerial photographs. In planetary geology, geologic mapping must be done primarily by remote sensing methods, commonly the interpretation of photographs. Units are identified on photographs by their surface appearance (morphology- smooth, rugged, hilly, etc.),their albedo (how they reflect sunlight - light to dark), their state of surface preservation (degree of erosion), and other properties. In some cases remote sensing of chemical compositions permits refinements of photogeologic units.

Three decades of planetary exploration have shown that the solid-surface planets and satellites have been subjected to the same basic geologic processes: volcanism, tectonism, gradation, and impact cratering. The relative importance of each process in shaping the surface differs from body to body, depending on the local environment (presence of an atmosphere, running water, etc.). All four of these processes have worked to shape the surface of the Moon and have produced landforms and rock units that can be recognized and mapped. An important part of preparing a geologic map, once the units are identified, is interpreting the geologic process(es) responsible for the formation of each map unit. When preparing a planetary photogeologic map, unit descriptions are divided into two parts: the observation (what you see) and the interpretation (how you believe it formed).

After identifying the units and interpreting their mode of formation, the next task in preparing a photogeologic map is to determine the stratigraphic (age) relation among all the units. Stratigraphic relations are determined using: (a) the Principle of Superposition, (b) the law of cross-cutting relations, (c) embayment, and (d) impact crater distributions. The Principle of Superposition states that rock units are laid down one on top of the other, with the oldest (first formed) on the bottom and the youngest on the top. The law of cross-cutting relations states that for a rock unit to be modified (impacted, faulted, eroded, etc.) it must first exist as a unit. In other words, for a rock unit that is faulted, the rock is older than the faulting event. Embayment states that a unit "flooding into" (embaying) another unit must be younger. On planetary surfaces, impact crater frequency is also used in determining stratigraphic relations. In general, older units show more craters, larger craters, and more degraded (eroded) craters than younger units.

Once the stratigraphic relations have been determined, the units are listed on the map in order from oldest (at the bottom) to youngest (at the top). This is called the stratigraphic column. The final task, and the final objective in preparing the photogeologic map, is to derive a general geologic history of the region being mapped. The geologic history synthesizes, in written format, the events that formed the surface seen in the photo - including interpretation of the processes in the formation of rock units and events that have modified the units - and is presented in chronological order from oldest to youngest.

Figure 1 shows a sample geologic map, including its unit descriptions and stratigraphic column. The relative ages were determined in the following manner: The cratered terrain has more (and larger) craters than the smooth plains unit - indicating that the cratered terrain unit is older. In addition, fault 1 cuts across the cratered terrain, but does not continue across the smooth plains. Faulting occurred after the formation of the smooth plains - indicating that the smooth plains unit is younger than the cratered terrain and fault 1. The crater and its ejecta unit occurs on top of the smooth plains unit, and thus is younger. Finally, fault 2 cuts across all the units, including the crater and its ejecta unit, and is thus the youngest event in the region. The geologic history that could be derived from this map would be similar to the following:

This region was cratered and then faulted by tectonic activity. After the tectonic activity, a plains unit was emplaced. Cratering continued after the emplacement of the smooth plains unit, as seen by the craters superposed on the smooth plains and the large, young crater mapped as its own unit. Finally, there has been a continuation (or reactivation) of tectonic activity, indicated by the major fault which postdates the young crater.

The geologic mapping principles listed above have been applied to the Moon as a whole and a generalized geologic time scale has been derived (figure 2). Two important units on the moon are the Fra Mauro Formation and the Janssen Formation, ejecta deposits from the Imbrium and Nectaris impact basins, respectively. These are widespread units that were formed in the hours following the gigantic impacts that excavated the basins, and hence are excellent datum planes. Rock samples returned from several localities on the Moon enable radiometric dates to be placed on the generalized time scale.

Geologic mapping of impact-crater related deposits requires some knowledge of the impact process. When one planetary object such as a meteoroid strikes another, there is a transfer of energy that causes the crater to form by having material excavated from the "target" surface. Most of the incoming object is destroyed by fragmentation, melting, and vaporization. Figure 3 is a diagram showing typical impact crater deposits. Extending about one crater diameter outward from the rim is a zone of continuous ejecta deposits consisting of material thrown out from the crater (called ejecta) and local material churned up by the ejecta. Extending farther outward is a zone of discontinuous ejecta deposits; unlike the zone of continuous ejecta deposits, these are surfaces that have been affected only locally by the impact. Bright, wispy rays extend beyond the zone of discontinuous ejecta deposits. Distinctive secondary craters formed by blocks of ejecta occur in singlets, doublets, triplets, chains, and clusters. They often form a "herringbone" ridge pattern, the apex of which points toward the primary or parent crater.

On the Moon and Mercury, geologic mapping involves distinguishing various deposits related to impact craters. In addition, most of the terrestrial planets have experienced volcanism that produced vast basaltic lava flows. Samples returned by the Apollo astronauts show that the dark, smooth areas of the Moon, named maria, are basalt flows. Some of these basalt flows were generated as enormous "floods" of lava, similar to the Columbia River Plateau of the northwest United States; others were produced as thin sheets that were fed by rivers of lava, visible today as sinuous rilles (Figure 4).

Procedure and Questions

The area you will be mapping is the Euler (pronounced 'oiler') crater region on the Moon. Euler is an impact crater, 28 km in diameter, located at 23 degrees 20'N, 29 degrees 10'W, placing it on the rim of the Imbrium basin on the near side of the moon (see figure 5). It is about 450 km northwest of the 93 km diameter Copernicus impact crater. The photograph was obtained with a mapping camera on board the Apollo 17 service module from an altitude of about 117 km. The photograph is 180 km on a side; the sun elevation angle at its center is about 6.5 degrees.

To establish age relations and interpret the mode of formation of the rock units in the area it is best to examine the area in detail by studying the large "desk top" photograph.

Part A

1. Study the southwest quadrant of the photograph in detail and list observations and evidence which might establish the relative age of the rugged clusters of hills and the intervening smooth regions.






2. List possible reasons why craters are more readily apparent on the smooth regions (vs. the hills).






3. Indicate your conclusion about the relative age of the two terrains (which is older).






4. List the characteristics of the smooth unit which might bear on its mode of formation; suggest a possible origin(s) for this unit.






5. List the characteristics of the rugged hills and suggest possible origins. Interpretations should be preliminary pending examination of the rest of the photograph.






6. Study the northwest quadrant of the photograph. List any additional characteristics associated with the smooth region which might bear on its mode of origin.






7. Briefly describe the several clusters of craters visible in the northwest quadrant of the photograph. What is their age in relation to the smooth region?






8. Propose a tentative mode of origin for these crater clusters, including any possible directional information.






9. Study the northeast quadrant of the photograph. Briefly describe the various characteristics of the topography surrounding and associated with the crater Euler.






10. Propose an origin for the topography of the material at crater Euler.






11. Study the outer boundary of the unit which includes Euler and its associated crater materials (recall Figure 3). Using the photograph, measure and record the distance from the rim crest of Euler to the outer boundary of the crater materials in a NW, NE, SE, and SW direction.






Asymmetry in deposits surrounding a crater can result from several factors, including: (1) oblique impact of projectile, (2) fractures in bedrock causing asymmetric ejection of material, (3) strong prevailing winds (on Earth and Mars), (4) later events modifying parts of the deposits, (5) topographic effects on flow of material from craters (on Venus), (6) a combination of the above.

12. Study the stratigraphic relations around the Euler deposit and list evidence to account for the observed deposit asymmetry.






13. Study the southeast quadrant of the photograph together with the rest of the picture. What is the age relation of the deposit surrounding Euler and the smooth unit? Present evidence for your conclusions.






14. Describe the large crater clusters in the northeast quadrant of the photograph. What is their age in relation to Euler? What is their age in relation to the smooth unit?






15. Describe the mode of origin of these clusters and include any directional information concerning their source.






Part B

Examine the photograph in detail and classify the terrain into geologic units based on surface morphology, albedo, crater frequency, and other characteristics. There are at least 3 major geologic units in the region. Tape the tracing paper over your small copy of the photo. Put tape at the top only so that the tracing paper can be flipped up to see the photograph. Make reference marks in the four corners, in case the tracing paper shifts while you are working on it, and also to help in overlaying with other maps for comparison. Draw preliminary contacts around the units. DO NOT WRITE ON THE PHOTO. Label the units by writing the name, or letters symbolizing the name, within each unit. Areas of a unit need not be laterally continuous on the surface, but may exist as isolated patches. Use symbols for features such as faults, grabens, fractures, and crater rims. Tabulate the units and describe their main characteristics. Names are of your choice, such as "mountain unit", "smooth plains". Names should be based on observations, not interpretations of possible mode of origin ("smooth plains" rather than "volcanic plains"). Using colored pencils, color your map, using a different color for each unit.

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Using the stratigraphic relations and interpretations developed in Part A, compile a stratigraphic column and geologic history for the Euler region. Based on your observations, determine the stratigraphy of the units (the relative order of units from youngest to oldest). List the units in the column "Geologic Unit" in order from youngest at the top to oldest at the bottom. Place any structural information in the column "Structural Events". Use the lunar time scale to determine the age (e.g., Eratosthenian) in which the units formed and note this information on your stratigraphic column and in the geologic history.