INVESTIGATION OF THE PANAMINT VALLEY CRATER

By

Lee E. Humiston, Sheldon D. Elliot Jr., J. Kenneth Pringle, and Pierre St. Amand

Michelson Laboratory, U. S. Naval Ordinance Test Station, China Lake, California

 

ABSTRACT

This paper presents results obtained to date from the study of a topographic feature resembling a meteorite crater, situated in Panamint Valley, Inyo County, California (Lat. 36°05'N, Long. 117°22'W).  In its present form, the crater located in an alluvial fan, is closely circular in shape, some 225 feet in diameter and 45 feet deep, and lacks a raised rim.

     Drilling and excavation have revealed that originally it was an open pit at least 135 feet deep, and has filled to its present depth chiefly by water deposition.  This indicates a considerable greater depth-to-diameter ratio [0.9 to 1] than would be expected for a meteorite crater of this size [6 to 1].  No meteoritic matter has been found, either in the fill material or in the neighborhood of the crater.  The presence of faulting and exposed limestone beds in the adjacent foothills suggest that the feature may well be a limestone sink.  It seems highly improbable that volcanic activity can have been responsible for its formation.

     Animal remains found in the crater fill material have been submitted for identification and radiocarbon dating.  Further exploration, using magnetometric, gravimetric and seismic methods, as well as further excavation, is intended.  Although the possibility of a cosmic origin is not as yet completely excluded, it is probable that further results will be of interest primarily to geologists, paleontologists, and climatologists.

 

INTRODUCTION

The existence of an isolated crater, possibly of meteoritic origin, in Panamint Valley, Inyo County, California was brought to our attention late in 1960 by Dr. R. S. Dietz, of the Naval Electronics Laboratory, San Diego, California, who had observed it in an aerial photograph of the region.  The crater is situated at Lat. 36°05'N., Long. 117°22'W., in an arid alluvial fan, approximately one mile from the foot of the Argus Mountains on the western side of Panamint Valley.  An aerial reconnaissance flight from the Naval Ordinance Test Station provided the series of photographs presented in Figs. 1 through 5.

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 DISCUSSION

The crater is nearly circular in outline, with relatively steep walls; the northeastern half of its floor is covered with a playa.  A small gully enters the crater on the southwest from the uphill slope of the fan.  The trails spiraling into the crater have apparently been made by wild burros in search of water.  The upraised rim usually associated with meteorite impact craters does not appear in this case, although gullies bypassing the crater on either side look as though they might have been displaced outward around it.

 

In January 1961, the site was visited by a party from the Naval Ordinance Test Station, who surveyed it and searched the crater and its vicinity for meteoritic material.  The survey (see map, Fig. 6) showed the crater to be approximately 225 feet in average diameter, and to have a depth of some 45 feet from the highest point on the rim to the floor.  These dimensions yield a close fit to the empirical log diameter - log depth curve obtained by Baldwin (1949) for explosion pits and terrestrial meteorite craters.

 

The floor is reasonably flat, except for a small fan at the lower end of the gully, and is about 80 feet in diameter.  The playa is of extremely fine water-deposited silt, while the floor material becomes progressively coarser towards the gully fan.  The slope of the walls is approximately 30°, and at several points below the rim, ledges of cemented alluvium are exposed.  The rim was not found to be raised appreciably above, nor depressed below, the original fan surface, nor is any tilting of the rather coarse alluvial bedding apparent.  The fan is composed of water-deposited conglomerate, ranging in size from silt to large boulders with no apparent graded beds.

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Aside from the apparent deflection of the neighboring gullies, which might be regarded as fortuitous, there is no evidence of any disturbance of the fan in the vicinity of the crater, nor does any material appear to have been thrown out over the surrounding terrain.  The fan surface has the same composition near the crater as at a distance, and no change in the coloration, degree of desert varnish, nor in the character of the lag surface (desert pavement) can be detected, either on the ground or from the air.  There is likewise no trace of volcanic activity in or about the crater, although some lava probably of early Pleistocene age is found about 1 mile west.

 

The possibility of an artificial origin could at once be completely discounted.  There were no signs of excavation, and counting of growth rings in sections taken from shrubs growing on the crater floor showed the feature to antedate by far any ordinance activity in the area.

 

The search for meteoritic material was considerably hampered by the large amount of natural magnetite and garnet present in the fan, and by the dark desert varnish covering exposed pebbles and rocks.  A thorough canvas of the crater and its vicinity was conducted, both visually and with the aid of a large AlNiCo magnetic rake, and samples were studied in the laboratory, but no recognizable meteoritic material was found.

 

It appeared from this preliminary survey that further efforts to determine the origin of the feature would be desirable.  A truck-mounted auger was lowered into the crater, and, during the month of February, a series of holes was drilled in a radial pattern over the floor, with the objective of determining the original profile of the crater and locating, if possible, any meteoritic mass which might be present.  Drilling was continued in each hole until material either too coarse or too hard for further penetration was encountered.  Around the periphery of the floor coarse gravel and rocks limited hole depths to a few feet; nearer the center the holes reached several tens of feet, the fill material evidently consisting of more or less a sandy silt with occasional layers of gravel and a few larger rocks.  It appears that the pattern of playa and gully outflow observed at the surface must persist to considerable depths.  With a few localized exceptions the material brought up by the drilling was extremely dry, indicating an efficient subsurface drainage mechanism.

 

Of the sixty holes thus drilled all of those penetrating below thirty feet were concentrated in a relatively small area about twenty feet north of the apparent center of the crater.  One of these holes reached the limit of the available drill string, going to 73 feet without obstruction.

 

For comparison, an attempt was made to drill several holes in the fan outside the crater.  The material encountered here proved considerably denser and more compact than anywhere within the crater; none of the holes penetrated more than a few feet.

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Some fifty samples of material brought up by the drill, as well as, a dozen or so obtained from the surface in and around the crater, were taken to the laboratory for detailed study.  Standard specimens of the drill cuttings were weighed out and washed free of silt, and their magnetic fractions were extracted and examined microscopically (see Table 1).  While a small number of magnetic spherules of presumably meteoritic origin were found, these were no more abundant than in typical surface samples from other locations.  No other recognizably meteoritic particles were found, and none of the questionable fragments tested yielded any indication of the presence of nickel.  Similar results were obtained with the surface samples studied.

 

As for the next stage of the investigation, a contract was let for a five-by-six foot timbered shaft to be sunk to depth of 100 feet below the crater floor, in the vicinity of the deepest drill holes.  Down to approximately 87 feet the fill was found to consist of fine-grained, well-bedded layers of water-deposited silt, sand, and fine-pebble gravel, essentially similar to the material at the surface.  Locally, larger boulders were encountered which presumably had tumbled in from the crater wall.  It would appear, therefore, that the crater had at one time the form of an open pit some 135 feet deep, and rather less than its present 225 feet in diameter, a conclusion reinforced by the large numbers of bones of small, medium-sized, and relatively large animals encountered with increasing frequency as the excavation progressed.  If the deposition of this portion of the fill material can be assumed to have proceeded at a rate comparable to that prevailing in the recent times, which the appearance of the strata traversed by the excavation would suggest to be the case, the filling of the crater to its present depth must have occupied a considerable span of time, probably several thousand years.  Samples of the animal remains found have been submitted to experts for identification and radiocarbon dating, with the objective of establishing an absolute time sequence.

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At 87 feet there is a well-defined transition to a completely chaotic zone of extremely loosely-packed boulders, cobbles, pebble-sized gravel, sand, and silt with numerous voids, which continues to the bottom of the shaft.  This material is quite different in appearance from that in the undisturbed parts of the fan, although the same type of rocks re represented; it would appear to have tumbled into place in an earlier, more cataclysmic phase of the filling process.  Like the stratified layers above, the material in this zone was found to be completely dry.  Examination of samples from these lower depths for meteorite fragments, again yielded negative results.  (see Table 1.)

 

A magnetic survey of an area approximately 1500 feet square, centered on the crater, was conducted with the aid of a Varian M-49 proton precession magnetometer.  This survey was complicated considerably by topographic effects, by local anomalies of as much as a few hundreds of gammas, due evidently to boulders, both buried and exposed, of material differing significantly in magnetic susceptibility from the general run of the alluvium in the fan, and by the presence of iron stakes and cables associated with the excavations.  In general, however, the picture is one of an increase in the magnetic intensity at a rate of, roughly, 100 gammas per 1000 feet downslope along the fan, upon which is superimposed an anomaly amounting to a few tens of gammas, negative to the south and positive to the north (magnetic) of the crater.  It appears, from comparison with data obtained in traversing nearby washes and gullies, that this anomaly may well be purely topographic, the crater thus representing a void in the moderately magnetic alluvium, while at the same time the alluvium increases in depth over a basement of lower magnetic intensity for the region (ca. 51,500 gammas) is very close to the value projected for 1961 from the magnetic charts of the U.S> Coast and Geodetic Survey.  No satisfactory values could be obtained within the shaft, due to the presence of ferrous materials in the timbering.

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A geological reconnaissance of the crater, the fan, and the neighboring mountains yielded evidence suggesting that the crater may lie over a fault in a limestone basement, lying at no great depth below the fan surface.  This opens the possibility that the crater may have originated as a limestone sink [sinkhole].  Some one-half million cubic feet of [alluvial] material is missing from the crater in its present form, while if its original depth was on the order of 135 feet, and the water-deposited portion of the fill has come from elsewhere, at least twice this volume must be accounted for.  In the absence of a raised rim, or of evidence that the missing material was distributed over the surrounding terrain, removal from below offers a reasonable solution to this problem.

 

There remains the possibility that this relatively deep crater was caused by a penetrative, rather than explosive, impact of a meteoritic body of large mass and high density, moving at a relatively low velocity.  In this event, the missing material would have to be accounted for by compaction of the relatively porous alluvium, which might be taken to explain the apparent deflection of the adjacent gullies.  To have avoided detection thus far, the impacting body, if intact, must either be of abnormally low magnetic susceptibility, or else be buried to a considerable depth.  If it is presumed to have disintegrated upon impact, it must have been composed of materials not readily distinguishable from those comprising the alluvium already present.

 

Plans are underway to conduct additional magnetic, gravimetric, and seismic studies of the crater and its vicinity.  It is also hoped that the present excavations can be extended, both downward through the chaotic portion of the fill, and horizontally along the discontinuity between this and the material lying above.  Apart from the fundamental problem of its origin, the crater’s subsequent history appears to offer considerable opportunity to geologists, climatologists, and paleontologists for future studies.

 

We would like to express our appreciation to Drs. R. E. von Huene, W. R. Haseltine, and R. L. Engel for their invaluable assistance in these studies, and to Mr. J. L. Eisel for his aid in taking various measurements.

 

 

REFERENCES:

Baldwin, R. B., (1949), The Face of the Moon, Univ. of Chicago, p.132.

 

APPENDIX:

FIGURE 1. - Aerial Photo of Crater and Vicinity

FIGURE 2. - Aerial Photo of Crater and Vicinity

FIGURE 3. - Aerial Photo of Crater and Vicinity

FIGURE 4. - Aerial Photo of Crater and Vicinity

FIGURE 5. - Aerial Photo of Crater and Vicinity

 

FIGURE 6. - Topographic Map of Crater and Vicinity

 

TABLE 1. - Magnetic Separation Data

TABLE 1. - Magnetic Separation Data (Cont'd.)