VOL. 76, NO. 23 JOURNAL OF GEOPHYSICAL RESEARCH
AUGUST 10, 1971
Pueblito de Allende Penetration Craters and Experimental Craters Formed by Free Fall
Donald P. Elston and G. Robert Scott U.S.
Geological Survey. Flagstaff, Arizona 86001
Five Pueblito de Allende craters, produced by the fall of 8- to 35-kg meteorites, were mapped in the northern, terminal part of the strewn field. The craters wore asymmetric in cross section; two to three walls characteristically were steep and locally undercut, and one wall was comparatively shallow. Asymmetric ejecta deposits lay adjacent, to the shallow walls, on the oast and north sides of the craters. One crater at the base of a bush was screened by an overhanging mantle of largely unbroken branches. From the screen and from considerations of crater and ejecta asymmetry, the stone is inferred to have arrived from the northwest at about 20°-25° from the vertical. The parent fireball is reported to have boon moving in a north-northeasterly direction at the time of its breakup. Experimental craters formed by the airdrop of 9- to 10-kg stones were produced to aid in a preliminary evaluation of the Allende craters. Low-velocity impacts produced by low-altitude drops (15 to 300 meters) produced craters with ejecta deposited down range, in the direction of projectile travel. Air drops from a 1.5-km height, with projectiles that fell at near-terminal velocity (about 100 m/sec) and encountered the surface at about 5° from the vertical, resulted in the preferential deposition of ejecta in the up — range direction. We tentatively conclude that some of the large stones in the northern half of the Allende strewn field arrived at the earth's surface at angles substantially from the vertical; curiously they appear to have been moving at the time of impact in directions generally opposite to the direction of travel of the parent fireball.
The Pueblito de Allende meteorite fall occurred at 1:09 A.M. CST, February 8, 1969, following the disintegration of a brilliant fireball about 25 km east of Hidalgo del Tamil, Chihuahua, Mexico Clark [ct al., 1909]. The trajectory is reported to have been from azimuth 217° [Clarke, 1970]. Earlier, McCrosky et al.  reported that the fireball approached from azimuth 210° at an elevation of 13° from the horizontal, and they suggested that fragmentation altitude was greater than 15 km. The strewn field trends northeast (approximately 029°), and it is about 42 km long and 9 km wide (Figure 1); the largest stones, up to about 35 kg, fell in the northern part of the field. Two of the five craters here investigated define the northern and eastern boundaries of the strewn field (El Morro and Rancho Blanco craters).
The Allende strewn field was visited by one of us (D.P.E.) shortly after the fall, and again three months later. Craters that had been degraded by persons searching for meteorite fragments were rejected for study. Two of the craters described here were examined during the first visit, and three additional craters that provided more complete coverage across the north end of the strewn field were examined later. The region is semiarid, and the craters examined during the second visit had been only slightly degraded by light rain and wind. Craters were sketch mapped in the field, and stereometric photographs were obtained. Near-vertical photographs served a.-, base maps for the compilation of field data.
This study was undertaken because ejecta patterns and crater shapes for the Allende craters seemed anomalous for projectiles that were expected to have been falling nearly vertically owing to atmospheric deceleration, or that might have fallen with a north-northeast component of motion inherited from the fireball. We hoped to see if the character of the low-velocity impact craters could be used to determine the fall directions of the stones and, indirectly, the initial trajectories following breakup of the parent body. Experimental data for evaluating ejecta patterns and crater shapes of the Allende craters were obtained from experimental craters produced by the impact of stones dropped from an airplane at various altitudes.
PUEBLITO DE ALLENDE CRATERS
Of the five Allende penetration craters mapped (Figure 2; Table 1), four were formed in nearly level alluvial surfaces; one crater (El Morro) was formed on a limestone hill. The Rancho Blanco and San Juan S craters were examined shortly after the fall. Stones that produced these two craters were recovered from their finders; the stones were fitted into their original impact holes to confirm that crater shapes largely reflected projectile shapes. The craters were subangular to angular in outline on two or three sides, and at the base.
The Rancho Blanco crater (Figure 2) was excavated by a tabular, subangular, 12-kg meteorite. A smooth, low-angle crater wall on the east marked the place of initial penetration, and a plane of penetration. This plane extended beneath the flat lower surface of the stone to the deepest point in the crater. The stone moved westerly and downward at an angle of about 35° during penetration. Impact striae, developed in fusion crust on the lower surface and leading edges of the meteorite, also support the interpretation of an east-west movement during penetration, A concave upper surface on the leading edge of the stone acted as a scoop, and a lobe of fine ejecta was thrown across the top of the meteorite and deposited to the northeast. The penetration path may have been due to the direction of. approach, to projectile shape, or to both factors.
San Juan S, the largest of the five craters, was excavated by a comparatively small stone (8 kg). Smooth, low-angle walls wore formed on the north, and steep walls were formed on the south. The projectile broke on impact and the upper part, about 3 kg, was deposited on a lobe of ejecta that extended asymmetrically to the north. Moreover, a clod of soil in the ejecta lobe was related to its source area in the steep southwest crater wall. The smooth, concave, low-angle crater wall on the north occupied an area much larger than the cross-sectional area of the projectile, and thus could not strictly be considered a plane of penetration. If low-angle crater walls and ejecta asymmetry occur in an up-range direction for craters produced by laterally moving projectiles falling near terminal velocity, penetration was from north to south at some angle greater than 35 from the horizontal.
San Juan N crater was excavated beneath the overhanging branches of a l5-meter-high bush, a few centimeters directly west of its base. The bush formed a dense screen on the east. Twigs about 25-30 cm directly above the crater were broken; a mantle of higher branches directly above and to the north and south of the crater was unbroken. The lower part of the south wall of the crater was undercut, and a small possible plane of penetration occurred at the northwest corner of the crater. An asymmetric lobe of ejecta lay to the north. The condition of the bush aided in determining the approach path of the projectile. To miss the unbroken overhanging branches, the angle of approach had to be about 70° or less from the horizontal. Penetration of the projectile along the inferred penetration plane to-produce the undercut wall on the south would suggest that the direction of flight was towards 120°, and the angle of approach was not less than about 06° to the horizontal, the dip of the crater wall on the northwest.
Santa Ana de Prado crater was excavated in extremely compact pebbly pediment material, which probably accounts for its shallowness. Its low-angle crater wall and asymmetric ejecta deposit occurred on the east, as was the case for the Rancho Blanco crater. Direction and angle of penetration may have been similar to Rancho Blanco crater.
El Morro crater, although produced by the fall of the largest stone reported, was shallow and broad because it impacted into a thickness of no more than 25-30 cm of unconsolidated soil and rock fragments (colluvium) overlying limestone bedrock. The projectile struck on the northern slope (5°-10°) of a hill and shattered on impact. The finder reported that he collected about 33.5 kg of fragments, and that the largest, single piece, about 3.5 kg, came to rest in a reentrant excavated in colluvium in the south wall of the crater. Southerly movement of the fragment may have been due, in part, to a local southerly dip of the bedrock-colluvium surface. At the time of our investigation, we recovered numerous projectile fragments in a broad arc north of the crater, but none to the south, although the original finder reported that he found one fragment to the south.
In summary, ejecta were deposited asymmetrically around four of the craters; deposits extended to the north for the San Juan craters, and to the east for the Santa Ana and Rancho Blanco craters. Northerly asymmetric ejection of projectile fragments occurred at El Morro. Mean direction of ejecta asymmetry (035°) is about 5° from the flight direction of the fireball (030°), and 175° from its retrograde flight direction (Table 1; Fiuure 1). The shallowest crater walls occurred adjacent to the lobes of ejecta; the steepest, locally undercut walls commonly occurred on the opposite sides. Shapes of the craters in their lower parts reflected the shapes of the projectiles. The projectiles appear to have moved laterally during penetration; for two of the craters, ejecta were deposited generally opposite to directions of penetration. The San Juan N stone appears to have approached from the northwest at an inclination of about 00° to 70° from the horizontal, and the Rancho Blanco stone moved westerly during penetration.
Experimental airdrops at low to comparatively high altitudes (15 to 1500 meters) were carried out to obtain information on the effects of projectile shape, lateral motion, and fall velocity on crater and ejecta morphology. Drops were made from an airplane moving at an air speed of 100 km/hr (about 45 m/sec), on a hearing of about 125°. Drops carried out at altitudes of 15 to 300 meters produced craters whose ejecta were deposited preferentially down range. Drops from an altitude of 1.5 km produced craters whose ejecta were deposited preferentially up range (Figure 3). The projectiles, observed from the air and the ground, showed no tendency to tumble during fall. Streamers attached to the stones for the highest drops remained stable during flight. Thus crater shape and ejecta distribution were due mainly to some combination of vertical and lateral velocities, and projectile shape.
Directions of crater elongation and ejecta asymmetry for the 15-meter to 1500-meter airdrop experiments are summarized in Figure 4. Most craters were elongate in the direction of projectile travel for all drops. The greatest departures from a down-range direction of elongation occurred for two angular, tabular blocks dropped from 1.5 km. For these, projectile shape became a dominating factor. Low-altitude drops resulted in preferential distribution of ejecta in the down-range direction. One drop from about 900 meters produced a crater with ejecta that was fairly uniformly distributed around the crater. Asymmetric deposits of ejecta that lay in a generally up-range direction characterized the 1.5-km drops.
For the 1.5-km drops, observed times of fall were approximately 24 seconds; observed angles of fall during the last 50 meters or so were approximately 85° from the horizontal; velocities are estimated to have been about 100 m/sec. For comparison, impact velocities of meteorites producing typical impact pits in soft ground are estimated to be of the order 100-200 m/sec [Mason, 1962, pp. 15-16].
Four penetration craters produced by 1.5-km drops of basalt and sandstone projectiles were investigated (Figure 3; Table 1). B-l was a basalt volcanic bomb, ovoidal in shape; B-2 was a subangular basalt block that in general shape and appearance was similar to Pueblito de Allende stones; S-l and S-2 were angular, tabular blocks of well-cemented sandstone. The target was a level area of agriculturally developed alluvium. The basalt blocks impacted a bare field that had been plowed several months earlier; the sandstone blocks impacted an un- plowed field that had once been planted in a rye type of crop.
Crater B-l, produced by the subrounded, sub-equant volcanic bomb, was fairly symmetrical in plan and cross section. A small lobe of ejecta was deposited in a direction nearly opposite to the direction of flight.
Crater B-2 generally reflected, in plan view and cross section, the subangular shape of the projectile. The southern part of the crater was excavated in a subdued east-west trending ridge, which resulted in a higher south rim and wall. An irregular edge of the block was responsible for the locally undercut wall on the northwest; the projectile apparently rotated slightly during penetration, possibly owing to the ridge. The main ejecta blanket was rather symmetrically distributed around the crater. A lobe of ejecta was deposited on the north, apparently coinciding with the orientation of a smooth flat surface of the projectile that penetrated at a high angle.
Crater S-l was broad and shallow (Table 1). The angular projectile struck on one of its broad flat surfaces and shattered. One of the main fragments that remained in the crater was inverted, as inferred from an Upward-facing surface that exhibited pronounced impact striae. A central, linearly fractured (north-south), mound of alluvium in the crater floor marked the site of impact of the flat surface. The mound was a feature produced by elastic re-bound during fragmentation of the projectile. Some projectile fragments, and ejecta, were deposited preferentially to the west.
Crater S-2 was produced by the pointed, edgewise penetration of a tabular, angular sand-stone block. The crater was more circular in cross section in its upper parts than was the projectile but closely conformed to the projectile shape at the bottom of the crater. Flat projectile surfaces on the east, west, and north coincided with lobes of ejecta. The large down-range projectile surface produced an undercut wall along the entire south side of the crater, and no ejecta occurred in this direction.
In summary, crater shape generally reflected projectile shape at free-fall, near-terminal velocities. Flat surfaces of projectiles were responsible for some of the asymmetric deposition of ejecta. Moreover, ejecta were asymmetrically deposited in an up-range direction; the mean direction of ejecta asymmetry (318°) is near the retrograde flight direction (305°). A pronounced undercut crater wall occurred in the down-range direction in one crater owing to down-range lateral motion of the tabular projectile during penetration; in another crater, a locally undercut crater wall occurred in the up-range direction possibly owing to projectile rotation during penetration.
The Allende and experimental crater data lead to the conclusion that arrival direction, projectile shape, and fall velocity influence penetration direction, crater shape, and the direction of ejection of unconsolidated material. Experiments indicate ejecta patterns for craters produced by projectiles falling at near-terminal velocity tend to be asymmetric in directions generally opposite to the direction of flight, even though fall angles arc only about 5° from the vertical. We further tentatively conclude that:
1. Some of the large Allende stones in the northern part of the strewn field fell with southerly or westerly components of motion, generally opposite to the direction of travel of the fireball.
2. Some angles of fall deviated significantly from the vertical.
Note added in proof.
A very large, fragmented stone (110 kg) was found near Rancho El Cairo, about 4 km northeast of the El Morro crater, eight months after the meteorite fall (see Hoy 8. Clarke, Jr., et al., The Allende, Mexico, Meteorite Shover, Smithsonian Contributions to the Earth Sciences, Number 5, 1970, issued by the Smithsonian Institution, Washington, D. C, in February 1971).
We thank Mr. Charles G. Overton of Hidalgo del Parral for his generous and most effective assistance, which not only resulted in the location of the craters described herein, but which also left fond memories of a pleasant people and a pleasant country. Publication has been authorized by the Director, US. Geological Survey, and was supported, in part, by NASA contract NSR-09-051-001.
Clarke, R. 8., Jr. The Allende, Mexico, meteorite: Strewnfield and specimen morphology (abstract). Meteorit. Soc., 5, 189. 1970.
Clarke, R. S., Jr., E. Jarosewich, B. Mason, and J. Nelen. The Allende meteorite (abstract), Eos Trans. AGU , 50, 39, 1969.
Mason, R. Meteorites, 274 pp., John Wiley, New York, 1962.
MeCrosky, R. E., A. Posen, G. Schwartz, and C. A. Tongas, Preliminary comments on the trajectory, orbit, and initial mass of the Allende meteorite (abstract), Eos Tram. AGU, 50, 39, 1969.
(Received January 7, 1971; revised March 8, 1971.)