Geology of the Apollo 16 Area, Central Lunar Highlands

133 Pages·2009·20.56 MB·English

Geology of the Apollo 16 Area,
Central Lunar Highlands
Edited by GEORGE E. ULRICH, CARROLL ANN HODGES, and WILLIAM R. MUEHLBERGER
G E O L O G I C A L S U R V E Y P R O F E S S I O N A L P A P E R 1 0 4 8
Prepared on behalf of the
National Aeronautics and
Space Administration
UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1981
Crystalline rock 6841.5 A. White angular boulder at Station 8, 3.4 km northeast of South Ray crater, source of samples 66415 and 66416.
Long-handled scoop casts shadow westward across bou.l der Hasselblad frame No. AS16-108-17697.,B Fresh broken surface of rock
66415 showing fine-grained crystalline texture with local vug-filling plagioclase ( a r r o w ) Lunar Receiving Laboratory No. S-73-39590.
C, Photomicrograph of typical texture in 68415 and 66416 showing twinned plagioclane (gray and w hite) an clinopyroxene (bright
colors). Cross-polarized light. Long side is 2.75 m. m D, Same as C in plane-polarized light showing subophitic texture of plagioclane and
darker high-relief pyroxene.
UNITED STATES DEPARTMENT OF THE INTERIOR
JAMES G. WATT, Secretary
GEOLOGICAL SURVEY
Doyle G. Frederick, Acting Director
Library of Congress Cataloging in Publication Data
Geology of the Apollo 16 area, central lunar highlands
Contributions to astrogeology. Geological Survey Professional Paper 1048
Bibliography; p. 534-539.
Supt. of Docs. No.: I 19.16:1048
1. Lunar geology. 2. Project Apollo. I. Ulrich, George E. II. Hodges,
Carroll Ann. III. Muehlberger, William R. IV. United States. National
Aeronautics and Space Administration. V. Series. VI. Series:
United States. Geological Survey. Professional Paper 1048.
QB592.G47 559.9'1 80-607170
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CONTENTS
Page
Preface
A. Summary of geologic results from Apollo 16, by William R. Muehlberger and George E. Ulrich 1
B. Apollo 16 regional geologic setting, by Carroll Ann Hodges 6
C. Apollo 16 traverse planning and field procedures, by William R. Muehlberger 10
Dl. Field geology of Apollo 16 central region, by Gerald G. Schaber 21
D2. Field geology of North Ray crater, by George E. Ulrich 45
D3. Field geology of areas near South Ray and Baby Ray craters, by V. Stephen Reed 82
D4. Field geology of Stone mountain, by Anthony G. Sanchez 106
E. Petrology and distribution of returned samples, Apollo 16, by Howard G. Wilshire, Desiree E. Stuart-Alexander, and
Elizabeth C. Schwarzman 127
F. Regolith of the Apollo 16 site, by Val L. Freeman 147
G. Ejecta distribution model, South Ray crater, by George E. Ulrich, Henry J. Moore, V. Stephen Reed, Edward W. Wolfe, and
Kathleen B. Larson 160
H. Optical properties at the Apollo 16 landing site, by Henry E. Holt 174
I. Morphology and origin of the landscape of the Descartes region, by John P. Schafer 185
J. Stratigraphic interpretations at the Apollo 16 site, by George E. Ulrich and V. Stephen Reed 197
K. Summary and critique of geologic hypotheses, by Carroll Ann Hodges and William R. Muehlberger 215
Ll. Documentation of Apollo 16 samples, by Robert L. Sutton 231
L2. Apollo 16 lunar surface photography, by Raymond M. Batson, Kathleen B. Larson, V. Stephen Reed, Robert L. Sutton. and
Richard L. Tyner 526
M. Impact geology of the Imbrium Basin, by Richard E. Eggleton 533
References cited 543
ILLUSTRATIONS
[Plates are in separate case]
FRONTISPIECE. Crystalline rock 68415.
PLATE 1. Geologic map of the Apollo 16 landing site and vicinity, by Carroll Ann Hodges.
2. Apollo 16, Descartes landing site.
3-11. Photographic panoramas taken on the lunar surface:
3. From within and near the lunar module.
4. The ALSEP area and partial panoramas of House and Outhouse rocks.
5. Stations 1 and 2 and a partial panorama of Buster crater.
6. Stations 4, 5, and 6 on Stone mountain.
7. Stations 8, 9, and 13, and partial panoramas ofShadowrock.
8. Station 11, North Ray crater, including sketch map.
9. Station 11, including telephoto mosaics.
10. Telephoto mosaics of Stone mountain taken from the lunar module and station 2 and of Smoky mountain taken
from station 11.
11. Telephoto mosaics of South Ray crater, Baby Ray crater, Stubby crater, and the central and northern parts of
the traverse area, taken from station 4.
12. Map of the impact geology of the Imbrium basin of the Moon, by Richard E. Eggleton.
VII
VIII CONTENTS
ABBREVIATIONS AND ACRONYMS
AET Apollo Elapsed Time, time after launch of LPM Lunar Portable Magnetometer
mission from Kennedy Space Center LRL Lunar Receiving Laboratory
AFGIT, ALGIT Apollo Field (Lunar) Geology Investigation LRV Lunar Roving Vehicle
Team LSM Lunar Surface Magnetometer
ALSEP Apollo Lunar Surface Experiment Package LSPET Lunar Sample Preliminary Examination
ANT Anorthosite-norite-troctolite rock suite Team
AP/C Analytical plotter, model C META-ANT Metamorphosed anorthosite-norite-troctolite
ASE Active Seismic Experiment MISC Miscellaneous
c c Capsule communicator at Mission Control in MPA Mortar Package Assembly
Houston, A. W. England N RAY CTR North Ray crater
CDR Commander, John W. Young PAN Photographic panorama, normally 360
C/S Central Station controlling the ALSEP PEN-2 Location of second penetrometer reading
CSM Command Service Module, spacecraft that or- PPAN Partial panorama
bited Moon during EVA’s. POIK Poikiloblastic or poikilitic
csvc Core Sample Vacuum Container PSE Passive Seismic Experiment
CSSD Contact Soil Sampling Device (Surface Sam- REE Rare-earth elements
pler) ROVER Lunar Roving Vehicle
CTR Crater RTG Radioisotopic Thermoelectric Generator
DC Dark-haloed crater SCB Sample collection bag
DMB Dark-matrix breccia SEQ Scientific equipment bay, in LM
DPS Descent Propulsion System on LM SPL Sample
DS Down-sun sampling, photograph S RAY CTR South Ray crater
DSB Down-sun before sampling, photograph SRC Sample return container
DT Drive tube, also core tube STA Station, sampling location on traverse
END CTR End crater STEREO Stereoscopic sequence or offset in photographs
EVA Extravehicular activity; astronaut activity STEREOPAIR Overlapping pair of photographs that give a
outside the LM three-dimensional view
EXP Experiment SURF SPLR Surface sampler (also CSSD)
FIIR Fine-grained intersertal igneous rock swc Solar Wind Composition device
HFE Heat-Flow Experiment USA Up-sun, after sampling, photograph
IR Interagency Report, U.S. Geological Survey USB Up-sun, before sampling, photograph
KREEP Lunar rock or soil with high concentrations of USD Up-sun, during sampling, photograph
potassium, rare-earth elements, and phos- UV CAMERA Far-ultraviolet camera, positioned in shade of
phorus the LM
LAC Lunar Aeronautical Chart XS, XSUN Cross-sun, sampling photograph
LM Lunar Module XSA Cross-sun, after sampling, photograph
LMB Light-matrix breccia XSB Cross-sun, before sampling, photograph
LMP Lunar Module Pilot, Charles M. Duke, Jr. XSD Cross-sun, during sampling, photograph
LOC Photograph of sample showing location with + , -Y FOOTPAD Front and rear footpads, respectively, of LM
respect to LRV or LM +,-Z FOOTPAD Left and right footpads, respectively, of LM
There you are, our mysterious and unknown Descartes highland
plains, Apollo 16 is going to change your image.”
John Young could hardly have known the truth of his prediction
when he first set foot on the lunar surface at the Apollo 16 landing site.
His mission was the most surprising geologically and has generated the
most controversy of all six Apollo landings. The Descartes region of the
central lunar highlands, since its first serious consideration as a site for
manned exploration 4 years earlier, had been strongly supported as a
place to sample volcanic rocks much different from those of the maria
and the basin margins. Three days of field exploration ranging 4 to 5 km
from the lunar module failed to turn up a single recognizable volcanic
rock. Instead, a variety of breccias, complicated beyond belief, were
collected from every location. Crystalline rocks were found whose tex-
tures were clearly igneous (see frontispiece), but they were not volcanic.
And therein lies the heart of the geologic mystery of Descartes.
“Well it’s back to the drawing boards, or wherever geologists go”
(T. K. Mattingly, Apollo 16 Command Module Pilot from lunar orbit).
PREFACE
Thi s volume sniatnoc eht lanif results compile d by th e Apollo Field
ygoloeG Investigations Team for the Apollo 16 mission. Some of the
data presented here were reported in preliminary form shortly after
th e missio n (ALGIT, 1972a. ;b2791 ,TIGFA 1973; nostaB and others,
1972; regreblheuM dna?rehto . 1972), hut most of the discussion and
interpretations that follow are products of individual efforts which
hav e incorporated much of the large body of dat a elbaliava from
postmission studies of the rocks, the geophysical and geochemical
data, and the extensive collection of photographs take n by the Apollo
6I astronaut crew on the lunar surface and from orbit. The chapter A significant stimulus t o the exceptional performance on the Moon
format was chosen to permit individual author s to develop their ideas was provide d yb the outstanding backup crew, Fred Haise. Edgar
independently, and we trust this approach will serve to stimulate ,llehctiM dna Stuart Roosa, whose hig h cifitneics standards the prime
rather than confuse the reader. crew was continuously challenged to surpass. eW hope that the
ruO esoprup niiht s emulov si ot ezirammus ehtleif d noitavresbo s at monthly mission-oriented field exercises planned and execute d yb
the Apollo I6 site and to hring together the various interpretations members of the Fiel d Geology Teatn prior to the Apoll o 6I flight
placed upon these observations yb the astronauts and the Field Geol- provide d the variety of experience in field situations that enabled the
ogy Team . Much foht e extensiv e lacimehcoeg dna geophysical data crew to make the appropriate observations and geologic judgments
puhlishe d ecnis 1974 no ehtllopA o 6I etis sah ton neeb incorporated required during the mission.
ro derrefer ot .ereh ehT tnetni si ton ot edivorp a dnarg sisehtnystuh We received valuable assistance before, during. and after the mis-
rather to document eht lacolna d lanoiger geologic relations and to sion from the following associates of the U.S. Geological Survey who
summarize what inferences can b e dam e from them. Our expectation eraon t credited elsewhere hut who nonetheless mad e significant con-
si taht eht emulov lliw eb desu sa a ecnerefer rof srehcraesergnirised tributions directly or indirectly to the preparation of this volume : .N
tnor e telpmoc e information on the geologic context of the Apoll o 16 .G ,yeliaB .FE . .noseeB B . M. .yeldarB .V .JrehsiF , M. H . .tiaH .E.D
samples and on th e interpretations of those intimately involved with Jackson, R. H. Jahns, D . F. ,nosnhoJ. .JS . ,namoL R. S. Madden, R.
the planning , ,noitucexe and analysis of the geologic exploration. ,llorraC W’. .E ,relliM .RA . Mills, .JC . ,llattuN .D .L .kceP .R.F . Sabala,
John Young, Charles Duke, and Kennet h ylgnittaM evresed special ..1 .T ,revliS .R .B .rennikS .L .B ,srewoS .G .A ,nnawS .HF . Thomas.J.
tiderc rofht e quality of their performance while exploring this com- .W .rciviDnaV an d D. E . .smlehliW lesnemmI y helpful editin g yb
plex area on the surface, from luna r ,tibro na d retal nisruocsid e htiw semaJ Pinkerton in preparing the manuscripts for publication was a
eht lunar science community. Their continuin g tseretni ni eht-poleved monumental rask and is greatl y appreciated.
ing story of Descarte s nageb with an unwavering enthusiasm for .M .B ,ekuD ,rotaruC dna .RB . ,nohguaL Assistant Curator, Lunar
cigoloeg gniniart sesicrexe ni eht .dleif htiW eht elba pleh foynohtnA Receivin g .yrotarobaL Johnson Spac e ,retneC Houston. very kindly
England, mission scientist and communicator durin g th e EVA's. and made arrangements for mrmbcrs of the Field ygoloeG Team to study
Friedrich Horz, their geologic trainer in Houston, their competence as the Apoll o I6 dna I7 thin-section collections and to use their photo-
cifitneics investigators reached the hig h level show n by rieht ydaer graphic equipment for illustrating some of the discussions in th e dleif
adaptation to eht unexpected conditions encountered on the mission. ygoloeg chapters of this report.
V
A. SUMMARY OF GEOLOGIC RESULTS FROM APOLLO 16
By WILLIAM R. MUEHLBERGER and GEORGE E. ULRICH
INTRODUCTION
The Apollo 16 mission to the central lunar highlands their location is considered essential to the discussion
has provoked a variety of stimulating debates concern- for purposes of context or clarification.
ing the nature of the original lunar crust, the effects of A glossary of abbreviations and acronyms used in
impact processes on this crust, and the interpretation the texts, illustrations; photographic and sample
of lunar landforms from photographic evidence. Con- catalogs, and the photographic panoramas is appended
siderable disagreement remains about ultimate to the volume.
sources of the samples returned from the Cayley plains The paragraphs that follow in this chapter are essen-
and the Descartes mountains. Although the major tially abstracts of each of the succeeding separately
problems of origin and lunar processes may not be re- authored chapters. Thus this section serves as an over-
solved in this volume, it is hoped that subsequent re- view or extended abstract of the volume that incorpo-
search will take into account the facts of field relations rates the major conclusions reached in the independent
as recorded by the cameras and first-hand observations chapters in the order in which they occur, beginning
of the astronauts. with the regional geologic setting and ending with the
The arrangement of topics in this volume is partly summary of geologic hypotheses.
chronologic in that discussions of geologic setting and
mission planning are followed by sections on the field
Chapter A.-The Apollo 16 landing site permitted
geology of four geographic areas sampled by the as-
investigation of two geologic units that are widespread
tronauts: central Cayley plains, North Ray crater,
in the lunar highlands: light plains and mountainous
vicinity of South Ray and Baby Ray craters, and Stone
“hilly and furrowed” terra, both superposed on old cra-
mountain. These observation sections are followed by
tered terrain. Outside the landing area, they are em-
topical discussions on the petrology, regolith, South
bayed by, and are therefore older than, the maria. A
Ray ejecta distribution, optical properties, morphology,
volcanic origin for these units, generally accepted prior
and stratigraphy of the landing site. A summary dis-
to the mission, was not supported by the mission re-
cussion of the source materials for the Cayley plains
sults. Various hypotheses of impact-related origins
and Descartes mountains in the light of available data
have been proposed to explain the crudely stratified,
concludes the interpretive part of the volume. Supple-
impact-generated breccias found at the site.
mentary sections on the surface photography and the
documentation of samples collected by Apollo 16 are
updated revisions of U.S. Geological Survey Inter- Chapter B.-Apollo 16 was the only site within the
agency Reports, Astrogeology 48, 50, 51, 54, prepared central lunar highlands to be explored by astronauts
immediately after the mission. Twelve folded plates in on the surface. It is on the Cayley plains, which are
the separate case include nine plates of lunar surface relatively level as compared with the adjacent rugged
panoramas mosaicked from 70-mm photographs and Descartes mountains. The site is about 70 km west of
annotated with respect to geographic features and the Kant plateau, which marks part of the third ring of
geologic data, a premission photomosaic map of the the Nectaris basin, and about 600 km west of the cen-
landing site (scale 1:25,000), a postmission geologic ter of that basin. Other multiringed basins that prob-
map of the landing-site region (1:200,000), and a post- ably influenced the geology of the landing site are Im-
mission map of Imbrium-basin-related geology (1:5 brium, centered about 1,600 km to the northwest, and
million) for the near side of the Moon. Orientale, centered 3,500 km to the west-southwest.
Some geographic names not yet approved by the In- A geologic map of the landing site and vicinity (pl. 1)
ternational Astronomical Union are used informally in prepared after the mission illustrates a current in-
the text and figures where identification or reference to terpretation of the distribution of geologic materials.
1
2 GEOLOGY OF THE APOLLO 16 AREA, CENTRAL LUNAR HIGHLANDS
The geologic aspects of the Cayley plains and Descartes ejecta afrom more distant regions (specifically North
mountains can be summarized as follows: (1) The sur- and South Ray craters). The percentage of rock types
face units are Imbrian in age; the plains surface has a collected from each station was clearly affected by time
cratering age that is similar to, if not identical with, constraints and may therefore not be representative of
that of Orientale basin ejecta c;ratering ages of the the stratigraphic sequence. The most intensively sam-
Descartes materials are not so well defined because of pled area, LM/ALSEP, probably yielded the most rep-
their rugged topography, but they are at least as old as resentative collection of the Cayley plains materials.
Imbrian. (2) The site is within the “sphere of influence” The rock types are similar in all respects to those col-
of the Imbrium basin, as evidenced by the radial lected at other stations during the mission. They in-
sculpturing of highlands northwest of the site and by clude fine- to medium-grained, moderately homogene-
the ridgy morphologic aspect of the Descartes ous crystalline rocks rocks; composed primarily of
mountains that appears nearly continuous with the glass; and breccias, by far the dominant type. The vari-
Imbrium sculpture. Thus Imbrium ejecta a and local ety of rock types collected indicates that the Cayley
material disrupted by the ejecta aproduced both the plains breccias are heterogeneous and suggests that
mountains and the plains. (3) Because of proximity to they are composed of isolated pockets of both light and
the Nectaris sbasin, the Apollo 16 stratigraphic column dark breccias deposited by a turbulent process.
probably includes Nectaris sbasin ejecta aat depth, but Extensive sampling and photography on the rim
the basin is so old that these materials are no longer (station 11) and near the outer edge of the continuous
exposed, except perhaps in the lowest walls of the ejecta ablanket (station 13) of North Ray crater provide
largest craters. a basis for stratigraphic interpretations in the north-
ern part of Apollo 16 traverse area. Breccias on the rim
Chapter C.-The three lunar-surface traverses of the
and walls are of two main types, light matrix and dark
Apollo 16 mission were designed to insure maximum
matrix. The area1 distribution and petrographic rela-
return of useful data for a community of scientists and
tions of the boulders sampled or photographed suggest
engineers with widely varying objectives. Because the
a generalized stratigraphic sequence within the crater
time available for geologic investigations and other
and, by extrapolation, in the northern part of the land-
experiments was limited, an intricate system of
ing site. The light-matrix boulders are friable,
priorities was established for both station locations on
rounded, and heavily filleted. Their abundance on the
each traverse and tasks to be performed at each sta-
rim and upper-crater wall suggests that they were de-
tion. The astronaut crew, John Young and Charles
rived from the upper part of the section. The dark-
Duke, kept abreast of the planning and the constantly
matrix boulders are coherent and appear to be the
changing priorities, in addition to learning how to
latest ejecta ato fall on the crater rim. One of these,
travel to and from the Moon. Their terrestrial field
Outhouse rock, was the source of several igneous and
training for 18 months before the mission was designed
metaclastic fragments. Most of the dark-matrix brec-
to simulate the lunar traverses and to develop their
cia s may be derived from a deeper horizon near the
skills in identifying and describing significant geologic
present crater floor.
features while photographically documenting and
Several types of evidence other than the fresh-rayed
sampling the rocks and soils representing these fea-
appearance argue for the youthfulness of North Ray
tures.
crater. Spallation exposure ages of 27 to 51 m.y. have
As a result, all primary geologic objectives were es-
been reported for five North Ray rocks. Within that
sentially achieved. Well-documented samples were re-
time interval, a very thin regolith (approximately a
turned from Cayley plains, North and South Ray crater
centimeter thick) formed locally; it thickens to 15 cm or
ejecta a,nd Stone mountain materials that may be rep-
more where it forms fillets around the friable light-
resentative of the Descartes mountains in this part of
matrix boulders.
the lunar highlands. Photographic coverage of all sam-
South Ray and Baby Ray craters are fresh blocky
pling areas and the entire traverse route and telephoto
craters in the southwestern part of the Apollo 16 land-
views of all important points remote from the traverse
ing site. Rays from South Ray can be traced as far as 10
area were obtained.
km from the crater to the vicinity of North Ray crater.
Chapter D1-D4 .-The central region of the Apollo 16 Although South Ray crater itself was not actually
landing site was investigated at three locations, LM/ visited by the astronauts, Cayley plains materials ejec-
ALSEP, station 1, and station 2. The samples ted from it probably are present at most stations. Sta-
documented probably represent materials of the under- tion 8 was purposely located on a bright ray from the
lying Cayley plains down to depths of 70 m or more and crater to insure collection of South Ray materials;
SUMMARY OF GEOLOGIC RESULTS
dark-matrix breccias and light-gray igneous rocks Five breccia types have been derived by comminu-
were the two main rock types sampled. They appear to sion of a first-cycle breccia that consisted of anorthosi-
represent two lithologic units in South Ray crater, tic clasts in a fine-grained matrix ranging from melt
dark-matrix breccias being the upper unit. texture to metamorphic texture. The first-cycle breccia
The South Ray event, if correctly dated by the 1- to is considered to be multiring-basin ejecta because it
4-m.y. exposure ages in the boulders, apparently depos- contains clasts of plutonic rock whose origin appears to
ited ejecta recognizable only in the coarse debris at be deep in the lunar crust. These breccias have been
station 8, about five crater diameters away. Associated modified to varying degrees by subsequent smaller im-
soils are reported to give much older ages. No ejecta pacts.
from the younger Baby Ray crater were recognized in Rocks representative of first-cycle breccias are suffi-
the sample suite, although such materials may be ciently abundant in the Apollo 16 collection that
present in small amounts. least-metamorphosed samples may be identified. From
Three sampling localities were established on Stone some such samples displaying minimum modification,
mountain at the south limit of the traverse area with it should be possible to date the crystallization of the
the objective of collecting materials representative of original crustal rocks, the preexcavation metamorph-
the Descartes mountains. The two highest stations (4 ism of these rocks, and the time of excavation. A review
and 5) appeared on premission photographs to be out- of age data shows that most samples selected for
side ray patterns related to South Ray crater, but con- isotopic measurement are so severely modified by sub-
tamination by South Ray ejecta appears likely at Sta- sequent impact that the ages are ambiguous. The sam-
tion 4. The location of station 4a on the edge of ejecta ples petrologically most favorable for dating significant
from Cinco a crater suggests that samples collected and identifiable events in the histories of the rocks are
might contain local material from a depth of 15 m on tabulated with the hope that they will help in obtain-
Stone mountain. Sampling at station 5, on the wall of a ing unambiguous ages, because such data from Apollo
small crater shadowed from South Ray and void of 16 rocks are now so scarce that basin chronologies are
visible blocky ray material, would be expected to in- only speculation.
clude rocks of the Descartes mountains. Station 6, on a The distribution of the various sample types shows
bench at the base of Stone mountain very near a ray, no significant differences between Cayley and De-
may be a mixture of fragments from the Cayley plains scartes materials. Statistical and compositional data
and materials of the Descartes mountains. on soils support the view that the Cayley Formation
and materials of the Descartes mountains are facies of
Chapter E.-Apollo 16 rocks are classified by a de- the same ejecta deposit. The Cayley Formation may
scriptive scheme into three groups: crystalline rocks, contain a somewhat higher proportion of matrix con-
subdivided as igneous (C,) or metamorphic (C2); glass sisting of melt and powdered rock.
(G); and breccias (B,-B,), subdivided on the basis of
clast and matrix colors and proportions. These rock- Chapter F.-The appearance of the regolith is gen-
type symbols are used throughout this volume. erally that of a rocky gray soil. Rays from young cra-
The crystalline igneous rocks consist of 1 certain and ters in hard substrata are distinguishable mainly as
1 possible anorthosite, 11 fine-grained ophitic to inter- local concentrations of blocky fragments. The bright-
sertal rocks of troctolitic to anorthositic composition, ness of a ray appears to result from a combination of
and 1 troctolite enclosed in fine-grained meltrock of the the density and the angularity of fragments, both of
same composition. Derivation of the fine-grained igne- which are higher for South Ray than for North Ray
ous rocks by impact melting of feldspathic plutonic crater.
source rocks is indicated by the common occurrence of The regolith thickness on the plains has a median
unmelted relics derived from coarse-grained plutonic value of between 6 and 10 m based on photogrammetric
rocks and a bulk compositional range like that of the measurements of concentric craters. The thickness of
plutonic rocks with essentially the same compositions. regolith on Stone mountain ranges from a minimum of
Metamorphic crystalline rocks studied consist of 1 5 to 10 m to more than 20 m and may vary greatly
medium-grained granoblastic rock considered to be a owing to the accumulation of mass-wasted debris on a
product of metamorphism in a plutonic environment softer, weaker bedrock that may underlie much of the
prior to excavation and 10 poikiloblastic rocks. Grada- Descartes mountains.
tion from poikiloblastic to unequivocally igneous tex- Regolith compositions for most of the site are chemi-
tures in these rocks is taken as evidence of metamor- cally similar except for North Ray soils, which are
phic origin with minor melting. significantly enriched in alumina and depleted in iron,
4 GEOLOGY OF THE APOLLO 16 AREA, CENTRAL LUNAR HIGHLANDS
titania, and nickel by comparison with the remaining Imbrium impact event. The main highlands mass
stations. Soils from station 4 tend to be intermediate in probably is a tongue of Imbrium basin ejecta. The
titania and nickel content with respect to soils from the fourth highland terrain type is represented by isolated
plains and North Ray crater. As a group, the soil sam- mountains inferred to be older Nectarian massifs pro-
ples are a homogenized mixture of the bulk rock jecting through the mantle of Imbrium ejecta.
analyses from the entire site. The mountain terrain can be traced beneath the
Cayley plains. The plains materials are thin enough
Chapter G.-South Ray crater ejecta totaling 5 to 10 along some margins to reveal a subdued reflection of
million m3 are scattered over the Apollo 16 landing the buried mountain terrain but thick enough in cen-
site in an irregular pattern that reflects a nonuniform tral parts to conceal the mountainous unit. The grada-
mantle of debris. The ejecta thin rapidly from about tional character of the morphologic contact between
10-15 m at the crater rim to an estimated 1 cm or less plains and mountains does not indicate intergradation
of equivalent uniform thickness at the southern sam- between the units but rather the overlapping of Cayley
ple localities (stations 4, 5, 6, 8, and 9) and to less than fill on the edge of slightly older mountain terrain.
1 mm at the northern localities (stations 11 and 13). The smooth to gently undulating surface of the
The power function best describing this thinning has a Cayley plains indicates high mobility of the plains-
slope of approximately -3.0. The fragment population forming materials at the time of their deposition. Of
on the lunar surface (for sizes larger than 2 cm) can the hypotheses currently offered, the concept that the
account for most of the total volume of ejecta, although plains represent fluidized ejecta from one or more
an equal amount of finer grained material can be ac- multiring basins is most consistent with the
commodated by the model. morphologic evidence.
Ray material from South Ray Crater can be deter-
mined best by the combined evidence of computer- Chapte r J.-The ejecta deposits from craters that
enhanced orbital photographs and the density of fresh penetrated materials beneath the Apollo 16 landing
rock fragments observed on the lunar surface. Station site, together with the morphologic characteristics of
8 has the highest potential for materials from South the craters themselves, provide the best clues for a
Ray; next most likely are stations 9, 6, 4, and 5. The stratigraphic interpretation of the region. The Cayley
probability of identifying South Ray ejecta from field plains, whatever their source, consist of three textural
data for areas farther away than these stations (3.5-4 rock units: light-matrix breccias, dark-matrix breccias,
km from the crater) is remote. Possible exceptions are and nearly holocrystalline rocks. These materials are
station 2 samples taken within a bright ray patch in locally mixed but form a gradational assemblage com-
the central part of the landing site. patible with a crudely layered sequence of rocks whose
chemical composition is grossly homogeneous.
Chapter H.-An investigation of the photometric At the north end of the traverse area, samples from
properties of the Apollo 16 landing site indicates that the ejecta of North Ray crater reveal a population dom-
albedo values of several areas, including the rim of inated by friable light-matrix breccias. These rocks,
South Ray crater, are 50 to 55 percent, the highest easily eroded, account for the convex upper slopes of
measured at any Apollo site. Measurements for the the crater wall and the rounded and deeply filleted
sampled areas range from 15 percent at the central boulders on the rim and ejecta blanket. The lowermost
area, 20 percent in the Stone mountain and station 8 materials of the crater’s floor mound are most likely
areas, to 24 percent at North Ray crater. represented by coherent glass-rich dark-matrix rocks
The polarimetric properties of the north and east found as sparse unfilleted blocks on the rim. The third
wall of North Ray crater reveal that very little, if any, main lithologic type is coherent light-gray igneous-
crystalline material is present in that area and that, textured rock that occurs interstitially in light-matrix
most of the rocks are more highly shocked than the Fra breccias and as inclusions within dark-matrix breccias.
Mauro breccias at Cone crater. This type, the holocrystalline rocks, reaches sizes of 50
cm at station 8 and occurs as smaller angular rocks in
Chapte r I.-Four highland terrain types have been the central part of the landing site.
morphologically defined in the Descartes mountains in The relative abundances of the holocrystalline rock
and adjacent to the Apollo 16 landing site. Lineated and the dark-matrix breccias at stations 8 and 9 and
patterns of crater chains, ridges and scarps, and the photographic evidence for layering within South
crosslineations represent three of these. These features Ray and Baby Ray craters suggest that the crystalline
exhibit both erosional and depositional characteristics rocks occur as large lenslike masses underlying and
whose orientations show that they were formed by the grading upward into melt-rich to melt-poor breccias