Pecos Structure, Pecos, New Mexico
Please proceed to revised paper at http://www.scribd.com/tmcelvain_1
In the early fall of 1998 after studying the Rochouart Impact Structure in France, I suspected that a bolide impact may have caused the complex geology in the vicinity of our home near Pecos New Mexico. I began collecting data on what I now believe is a deeply eroded, complex impact structure, approximately 15 kilometers in diameter, the center of which is located approximately 2.5 miles north of Pecos, NM.
The first clue I had of the possibility that there was an impact structure is a low level US Forest Service Aerial Photo of the area. See Aerial Photo below.
At first I thought that the round structure represented the transient crater of a simple impact structure. Further study of the photo caused me to ponder why the circle did not close at the top. Upon closer observation I noticed the outline of another circular feature or possible impact structure to the north. I believe the two impacts happened almost simultaneously, but I can not identify which came first. I then began to look at satellite images along with USGS DOQQ’s, and Topographic Maps, on which there seemed to be the possibility of a chain of impacts. Eventually I began to see evidence of a complex crater formation, with ring syncline, radial faults, accommodation faults and ridges, mega-breccia, stratagraphic evidence of a raised central uplift, and eventually I found a shatter cone to cap off my macro evidence. I also found another possible small impact structure within the trough or moat, which I named the Cajete Crater because it is located in what the natives call the Cajete, the Spanish name for an earthenware pot or bowl, which I thought appropriate.
As my field work progressed began to sketch a structural map of the greater crater, see Figure 4 below. This map is in the preliminary stages.
Figure 4. - Working Structural Map of the Pecos Impact Structure. Red lines outline either the central uplift conduit or a circular structural element in another impact structure, Blue lines are collapse accommodation structures, Black lines indicate monocline folds and faults.
Figure 5. – Satellite image of the same area as the map in Figure4.
The caption in her Figure 1.14, Road cut corresponding to picture, is the same road cut photographed in my Figure 13 illustrating the jumbled blocks of formation clasts in the megabreccia.
Figure 7. – Legend of formations units and symbols used in Sonja Heuscher’s geological illustrations and maps.
Figure 8. – This geologic map illustrates the rise of the central uplift or the breccia in the conduit, notice the small area of blue, Plp-3, chert bed, mapped right above the brown Qls in the southeast portion of the yellow Qls/Qc. This outcrop of Plp-3, which I would map as a large megabreccia clast, is mapped at a higher elevation than the Plp-3 outcropping on the canyon walls, suggesting that the land slid uphill. This relationship can also be seen in her Figure 1.14 but its elevation relative to the elevation of the Plp-3 outcropping in the canyon is not as well illustrated. The southward dipping resistant formations that the arrows point to also happen to be the contact of the “conduit” with the adjacent formations in both the primary Pecos Impact Structure and North Structure 1
The presence of other suspected impact structures probably coeval with the Pecos Structure inspired me to widen my search, which continued to expand into what I believe was a major impact event that in some way participated in the uplift of the Southern Rocky Mountains and Colorado Plateau. This concept is hard to swallow, but this conclusion has been forced upon me by the scope of the evidence I have found.
The morphology of the central uplift is immediately suspect, because it is a topographic low. The central uplift area is filled with a jumble of large blocks of broken formation sequences with bedding planes rotated and mixed into a matrix supported mega-breccia. I interpret the circular area in Figure 2 to be a conduit that acts like a pipe in the center of the central uplift. It is a conduit through which the clasts flow as they rise toward the surface. The formations in the moat area dip away from this conduit, which could be caused either by the drag of the material rising in the conduit or because of a general uplift of the formations in the central uplift. If the structure was not so deeply eroded the rise of the central uplift would appear to be bigger if one judged the size of the uplift by the transition of the dip from more or less level formations in the moat rising to form a dome. I have noticed this phenomenon in other impact structures resulting from this impact event.
I have anecdotal evidence that the Precambrian basement is about 480 meters higher within the central uplift than would be expected. A water well drilled near the southern edge of the central uplift encountered basement rock at about 50 meters. Comparing the thickness of the measured section at Dalton Bluff five kilometers north, to the elevation of the well head at the surface against the outcrop of the formations exposed in the moat at the contact of the conduit adjacent to the well, one would expect to encounter the Precambrian at a depth of about 480 meters, indicating a rise of 430meters. Mapping a chert marker bed in very large clasts or blocks of juxtaposed formation within the conduit against the elevation of the chert marker bed outside the central uplift, I have projected the formations rose about 200 meters within the conduit. The disparity in the different calculations of magnitude of the rise (230 meters) may be due to the collapse of the central uplift. I have found some quartz grains in sandstone with planar microstructures within the breccia, but the sandstone on the rim of the central uplift generally has a higher percentage of grains with planar microstructures. I have not found any evidence of brecciated basement rock within the central uplift.
View looking east of the southern contact of beds within the central uplift and the moat. I have indicated the fault zone (white) between the relatively flat lying beds of the moat and the enechelon faults as the formations rise within the central uplift. Center of the uplift is toward the left. Yellow line marks the top of the chert marker bed near the top of the La Posada member of the Pennsylvanian Madera Formation. I mapped the chert marker, exposed in large clasts of intact formation, rising 200 meters within the central uplift relative to the adjacent beds in the moat. As suggested in Figure 5 above, there was more vertical movement in the middle of the central uplift than at the edge, probably due to friction at the contact of the central uplift and the more level formations of the moat.
Figure 10. – Sketch of the central uplift illustrating the conduit that I am proposing.
The following figures are photographs of some of the clasts within the central uplift mega breccia:
Figure 11. – Enechelon faulted and rotated blocks of a limestone member of the La Posada Member of the Pennsylvanian Madera Formation as the beds rise toward the center of the central uplift toward the right.
Figure 12 – Photo megabreccia in a road cut within the central uplift illustrating the shattered and juxtaposed blocks of the La Posada formation (same road cut mapped in Sonja Heuscher’s map Figure 6 above).
One can see the geomorphic indication of a syncline on my structural map superimposed on the satellite image, which is born out with field work. The central uplift is roughly concentric with the syncline. The diameter of the syncline seems a little too big for a central uplift of this size. However, I theorize that the present outcrop of the impact structure was covered with up to 700 meters of sediment at the time of impact, under those conditions the ring syncline could be of appropriate size. I am dating the impact event mid-Tertiary which would account for the 700 meters of overburden. Dating this impact event mid-Tertiary is a stretch, but I am assuming it was coeval with overall impact event that I am proposing; however, no Tertiary formations are exposed and the only evidence supporting my date is the freshness of the broken formations and the spatial relationship to other impact structures in the area that disturb early to mid-Tertiary formations.
The monoclines at both edges of the central uplift have steep dips and have been mapped with accompanying ring faults. The cross section Figure 10 below is copied and pasted from the Draft USGS Geologic Quadrangle Map of the Pecos Quadrangle. The details are hard to see, however, one can see the ring faults accompanying the monoclines, and the red arrow points to the ring fault at the contact between the central uplift and the formations in the moat.
Figure 15 – Cross Section of the ring syncline cut and pasted from the Preliminary USGS Geologic Map of the Pecos Quadrangle.
Accommodation Folds and Faults
I have found folds, faults and crushed formation, or pressure ridges. These features are described in the following paper that I have cut and pasted appropriate sections from by Thomas Kenkmann and Ilka von Dalwick:
The tree cover is too heavy to get a good photograph of these structures, but they are very easy to map in the field, and conform favorably to the illustrations in the Kenkmann et. al. paper.
In the jointed siliceous shale intercalated with the chert marker bed, I found joints, within the central uplift, exhibit very complex flame or hachured structure. I found one jointed block that I believe is a shatter cone; however, it may not be as well developed as it could be (see Figure 12 below). It has some characteristics of shatter cones in that it is a convex surface with horsetail striations and evidence of several layers of these striated surfaces, a cone within a cone. There needs to be more excavation of this bed to determine how many shatter cone like structures there are.
Figure 15 – Possible shatter cone found within the central uplift.
In some of the hard limestone beds I have found that the limestone was fractured and injected with very fine grained, angular quartz and some feldspar clasts cemented with chlorite. These structures may be soft sediment deformation, but the nature of the injected material leads me to believe that a shock wave caused by a bolide impact is the best explanation for the source of these very fine shattered grains.
Figure 16 – Photo of fractured hard Pennsylvanian limestone injected with cataclasticly shattered sand grains,
Figure 17 – Close-up of the clastic material injected into the limestone.
Figure 18 - Photomicrograph of the injected clastic material, field of view 2.5mm.
Figure 19 – Higher magnification of the photomicrograph in Figure 16 above reveals at least one quartz grain with a set of Planar Microstructures, field of view 1.00mm.
I have found planar structures in some grains of Pennsylvanian sandstone. Most of the quartz grains have one set of planar structures but I have found up to three sets in some grains. The grains with planar micro-structures are found in porous and permeable Pennsylvanian sandstone beds.
Figure 20 – Quartz sand grain with 1 set of PM’s, field of view 0.25 mm.
Figure 21 – Same grain illustrated in Figure 17 above illuminated with cross polarized light.
Figure 22 – Quartz sand grain with 1 primary set of PM’s and the hint of another.
The following histogram of the frequency of angle of the pole of planar microstructures to the c-axis resembles similar histograms from known impact craters if one accounts for the shift of higher angles probably caused by the concentration of the shock wave in porous sandstone formations. This is my first attempt at measuring and indexing these angles, and I am not completely comfortable with the results.
The final verdict on whether the Pecos Structure is an impact structure or not awaits the verification that the planar microstructures (PM’s) found in the crater are actually planar deformation features (PDF’s).