Numerous studies of the returned lunar samples as well as geologic and remote-sensing investigations have emphasized the importance of impact melts on the surface of the Moon. Information concerning the distribution and relative volumes is important for (1) an improved understanding of cratering processes, (2) kinetic energy estimates and energy partitioning studies, (3) the proper interpretation of melt-bearing lunar samples, and (4) comparative planetology studies. The identification of major flows of fluidized material associated with impact craters on the surface of Venus has increased interest in impact melt flows on the other terrestrial planets. For a number of years, we have been investigating the distribution, modes of occurrence, and relative and absolute amounts of impact melt associated with lunar craters as well as the manner in which melt volumes vary as a function of crater size, morphology, and target characteristics. The results of this effort are presented

Hawke, B. Ray

Head, J. W.

English

NASA Technical Reports Server

LPI Contribution No. 790 37
TABLE 1. Location! of silei where samples were collected and • brief description
of the materials collected.
Site Name Location Rocks Reported
Ptmom Krom (Quarry)
(8 samples)
Phnom Basel (Quarry)
(2 samples)
Phnom Chetares
(1 sample)
Cbealea Village Group
f7 samples)
Phnom Batheay
(2 samples)
Tmat Pong Group (Quarry)
(2 samples)
Phnom Chiso (Quarry)
(3 samples)
Took Ban Massif
(1 sample)
lOkmSSW
of Siem Reap
(Angkor Wat)
22kmWNW
of Phnom Penh
30kmNW
of Phnom Penh
38kmN
of Phnom Penh
46kmN
of Phnom Penh
26 km W
of Phnom Penh
45kmS
of Phnom Penh
33kmSSW
of Phnom Penh
Compact anentei, ferrugenous scoria,
pink rtiyolitic lavas, dark cinerous sandstones,
and various volcanic conglomerates
Granite, fine-grained, leucocratic,
white quartz vein
Jointed polychrome jasperites,
lubvertic*! schists
Devitrified pyromeride, rhyolile, white fluidal
rhyolite, rhyolitic breccias, siliceous dark dacite
Upper Indosinias sandstone, vein of
acidic myolitk rock
Rhyolitic, weathered whitish, sometimes
fluidal
Massive crystalline sandstone, intensely
eroded black schist
Granite, fine-to-medium grained, pseudo-
vortical biotite flames, black slaty schists
Cambodian equivalents of the U.S. Departments of Commerce,
Interior, State, and Defense. For support in achieving this "diplo-
matic" objective we are indebted to Thach Xoval Say, Vice-
Director, Dept. of Mines and Geology, Sov Chivkun, Director,
Dept. of Geology and Mines, and Ith Praing, Vice Minister, Ministry
of Industry. We were restricted from visiting sites that were "off
limits" due either to unknown locations of land mines or known
locations of military bases.
Our scientific objectives were to find impact or shock metamor-
phosed rocks unambiguously related to the Tonle Sap basin, to
collect samples of rocks that may represent those melted to produce
Australasian tektites. and to learn as much as possible about
Cambodian geology. Using l:200,000-scale geologic maps with
fairly detailed descriptions of die rock units shown [2], we selected
a number of acceptable "phnoms" (hills that rise abruptly out of the
surrounding plain) that may contain rocks affected by the postulated
Tonle Sap impact A map of central Cambodia is shown in Fig. 1,
and the locations of sites where samples were collected are indi-
cated. A list of those sites, together with a description of the rocks
reported to be present at each site [2], is given in Table 1. No
obviously shock-metamorphosed or suevite-like rocks were ob-
served. Recent alluvium surrounding Tonle Sap is judged to be lake
sediment deposited when the lake surface was at a higher elevation.
Acknowledgment: Support provided by the Barringer Crater
Company is gratefully acknowledged.
References: [1] Hartung J.B. (1990) Meteoritics. 25,369-370.
[2] Dottin O. (1972) Carte Geologique de Reconnaissance a II
200.000. Republique Khmere, Phnom Penh, 41 pp., and Siemreap,
16 pp.. Editions du Bureau de Recherches Geologiques Minieres,
Paris. [3] Rasmussen W. C. and Bradford G. M. (1977) Ground-
Water Resources of Cambodia. Geological Survey Water-Supply
Paper 1608-P, 122 pp., 3 plates.
5\3* -
THE DISTRIBUTION AND MODES OF OCCURRENCE OF
IMPACT MELT AT LUNAR CRATERS. B. Ray Hawke' and
J. W. Head2, 'Planetary Geoscienccs.SOEST, University of Hawaii,
Honolulu HI 96822, USA, 2Department of Geological Sciences,
Brown University. Providence RI02912. USA. f-/ /'
Introduction: Nurnerousstudiesoftheretumedlunarsamples
[1-4] as well as geologic and remote-sensing investigations [5,6]
have emphasized the importance of impact melts on the surface of
the Moon. Information concerning the distribution and relative
volumes is important for (1) an improved understanding of cratering
processes, (2) kinetic energy estimates and energy partitioning
studies, (3) the proper interpretation of melt-bearing lunar
samples.and (4) comparative planetology studies. The identifica-
tion of major flows of fluidized material associated with impact
craters on the surface of Venus has increased interest in impact melt
flows on the other terrestrial planets. For a number of years, we have
been investigating the distribution, modes of occurrence, and rela-
tive and absolute amounts of impact melt associated with lunar
craters as well as the manner in which melt volumes vary as a
function of crater size, morphology, and target characteristics. The
purpose of this paper is to present the results of this effort
Method: Impact melt deposits were identified using the crite-
ria established by Howard and Wilshire [S] and Hawke and Head
[6-8]. Qualitative estimates were made and trends were established
utilizing a population of over 100 fresh impact craters that was
characterized in a previous paper [6], plus additional lunar craters
for which adequate Lunar Orbiter and Apollo photography exists.
Quantitative determinations of impact melt volumes were made for
those craters for which high-quality topographic data are available
from Lunar Topographic Orthophotomaps.
https://ntrs.nasa.gov/search.jsp?R=19930000960 2020-03-17T10:05:00+00:00Z
38 7992 Sudbury Conference
Results and Discussion:
Melt occurrence as a function of crater size and morphology.
Impact melt is more common at fresh simple craters (D < -IS km)
than has previously been thought. The smallest extensively studied
crater with interior melt is 750 m in diameter, but we have noted the
occurrence of even smaller melt-containing craters. At very small
craters (D < 2 km), impact melts typically occur as narrow ponds of
low-albedo material on crater floors, less common dark streaks on
walls, and very thin discontinuous veneers around the rim crests.
The melt deposits associated with slightly larger simple craters (D =
2-7 km) are similar but more abundant than those at smaller craters.
Shallow ponds often occur among the small floor hummocks, and
hard-rock veneers cover much of the crater floors. It appears that
some of the melt flowed onto the floor from the lower portions of the
crater walls and embayed clastic debris emplaced by mass wasting
from the crater walls. Though some minor wall failure has occurred
at craters in this size range, the positions of these craters on depth-
diameter plots indicate that there has been very little, if any,
reduction in depth [8-10].
Interior melt volumes are quite variable in fresh craters from 7
to 12 km in diameter. These deposits range from unobserved or
present in only trace amounts to quite abundant. Extensive deposits
of exterior melt are first observed around craters near the upper limit
of this size range [6,8].
Numerous workers have documented the changes in lunar crater
morphology and morphometry, which start at a diameter of about
IS km as smaller, simple craters undergo a transition to larger,
complex craters that exhibit central peaks and wall terraces [e.g.,
9,10]. It appears that the crater modification processes operative at
craters between IS and 25 km in diameter influence melt deposit
morphologies and abundances. While most fresh primary craters in
this diameter range for which adequate photography exists do
contain at least some melt, the amounts are extremely variable.
Da wes (D = 17 km) is typical of craters in this size range. Significant
accumulations of impact melt are restricted to a small area imme-
diately east of the central peak [7]. Additional melt was probably
present initially but was buried by scallop material slumped onto the
crater floor during the modification stage of the impact cratering
event. Fresh craters in this size range that exhibit little or no interior
melt are generally characterized by the presence of extensively
scalloped walls and/or swirl-textured floors, features indicative of
pervasive wall failure [7,8]. The results of our analysis of the
interior morphologies of these craters indicate that much of the
interior melt was totally buried by scallop material. We conclude
that the variable amounts of interior melt associated with craters in
this size range can best be explained by differences in the degree and
style of wall failure.
Fresh impact craters over 25-30 km in diameter are extensively
modified and exhibit terraced walls, central peaks, and flat floors
with abundant deposits of impact melt. Wall failure has been more
extensive and deep-seated at the larger terraced-walled craters, and
little melt appears to have been buried during the modification
stage. The results of detailed mapping of interior and exterior melt
distributions indicate that ponded material becomes relatively more
abundant on the floors and rims of these larger craters [8].
Exterior meltvolumes as a functionof crater diameter. Previous
work has emphasized the role of oblique impact and preexisting
topography in controlling the distribution and amounts of exterior
melts [5,6,8]. While it is clear that these factors do cause variable
amounts of melt to be emplaced on crater rims, a variety of evidence
indicates that relatively greater quantities of melt are present on the
rims of larger craters: (l)the dominance of large exterior melt ponds
over flows and hard-rock veneer at craters over SO km in diameter
[6,8]; (2) the tendency for melts to occur at greater distances from
the parent craters as a function of crater size; (3) the observation that
exterior melt ponds are larger and more widespread at larger craters;
and (4) quantitative estimates of melt volumes, which indicate that
relatively more melt is present on the rims of larger structures. Even
so, this may not imply that a greater percentage of the total melt has
been ejected since the total amount of melt generated was also
relatively greater at larger structures [8].
Interior melt volumes as a function of crater diameter. There
also appears to be a systematic variation in the amounts of molten
material in crater interiors. Since die extent and thicknesses of the
ponded material on crater floors tend to increase as a function of
crater size, more melt may be present in the interiors of larger
craters. Support is provided by quantitative estimates of interior
melt volumes for specific craters where detailed topographic data
exist [1 1,12]. A similar trend has been noted for the impact melt
volumes associated with terrestrial impact structures [13,14].
Influence of substrate onmelt generation. Numerous cratering
studies have demonstrated the importance of target characteristics
in determining the morphology of lunar craters [e.g., 1 5 , 1 6] . There-
fore, we made an attempt to determine the influence of substrate on
the relative amounts of impact melt associated with craters in
highland vs. mare terrains. A comparison of the mapped interior
melt deposits in similar-sized craters (D < SO km) suggests that
highl and craters contain melts in amounts either equal to or less than
the amounts present in mare craters. This observation does not
necessarily indicate that more melt was generated by impact into
mare targets. The observation could be explained by one or more of
the following: (1) for a given impact energy, larger craters may be
formed in the highlands relative to the mare; (2) the style and degree
of wall failure is known to be dependent on terrain, topography, and
substrate [IS]; and (3) a limited amount of evidence suggests that
more melt was ejected from highland craters.
References: [1] Simonds C. et al. (1976) Am. Mineral., 61,
S69. [2] Simonds C.etal.(1977)/Voc.LSC«dk, 1869. [3]TaylorS.
R. (1982) Planetary Science: A Lunar Perspective, 481 pp. [4]
Spudis P. et al. (1991) Proc. LPSC. Vol. 21, 151 . [5] Howard K. and
Wilshire H. (1975) J. Res. US. Geol. Survey. 3. 237. [6] Hawke B.
and Head J. (l9n)ImpactandExplosionCratering,9l5. [7] Hawke
B. and Head J. (1977) LSC VIII. 415. [8] Hawke B. and Head J.
(1979) LPSC X, 5 10. [9] Wood C. and Andersson L. (1978) Proc.
LPSC9tk, 3669. [10] Pike R. (1977) Impact and ExplosionCratering.
489. [11] Lange M. and Hawke B. (1979) Conf. Lunar Highlands
Crust, 99. [12] Lange M. and Hawke B. (1980) Proc LPSC 10th.
599. [13] DenceM. et al. (\911) Impact and Explosion Cratering.
247. [14] Grieve R. et al. (1977) Impact and Explosion Cratering.
791. [15] CintalaM.etal. (19T7) Proc. LSC 8th. 3409. [16] Head J.
(1976) Proc. LSC 7th, 2913.
N 9 3^*04
LARGEIMPACTS IN THE BALTIC SHIELD WITH SPECIAL
ATTENTION TO THE UPPLAND STRUCTURE. H. Henkel
and R. Lilljequist, Institute for Fotogrammetry, KTH, S- 10044
Stockholm and Department of Geology, University of Stockholm,
S-106 91, Stockholm. Sweden.
Within the Baltic Shield several very large structures have been
identified and are suspected to be of meteorite impact origin (Fig. 1
and Table 1 ). Some of these deeply eroded circular features will be


The distribution and modes of occurrence of impact melt at lunar craters

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server