BARRINGER IMPACT CRATER

by: Charles O’Dale

• Type: Simple
• Age (ma): 0.049 ± 0.003^a – HOLOCENE
• Diameter: 1.19 km
• Elevation: 1,719 m above sea level
• Location: Arizona, U.S.A. N 35° 2′ W 111° 1′
• Shock Metamorphism: Shatter cones & rare PDF in quartz grains.

^a Three different dating techniques, taking advantage of the natural decay of naturally radioactive elements, have been used to determine the age of the impact crater. All three techniques are in agreement:

• Sutton (1985) used thermoluminescence techniques to determine an age of 49000 +/- 3000 years;
• Phillips et al (1991) used cosmogenic 36-chlorine techniques to determine an age of 49700 +/- 850 years, and;
• Nishiizumi et al. (1991) used in situ production of 10-berilyum and 26-aluminum to determine a minimum age of 49200 +/- 1700 years.

Table of Contents

(I have added links to various chapters for ease of navigation)

1. Introduction

2. Geomorphology

3. Aerial Exploration

4. Ground Exploration – #1

5. Ground Exploration – #2 Breccia Lens

6. Ground Exploration – #3 Breccia

7. Side Notes & References

1. INTRODUCTION

Close to 50 thousand years ago, in the Pleistocene epoch, a 50 meter diameter 300,000 ton meteoroid, with a 15 km/sec relative velocity, impacted  the earth in the northern Arizona desert creating the Barringer crater. The Barringer Crater is located 60 km east of Flagstaff and 29 km west of Winslow in the United States‘ northern Arizona desert.

The rim of the crater rises nearly 50 m above the surrounding plain. Beyond the rim are low mounds of material thrown out by the impact. One of the crater’s interesting features is its jointed (squared-off) outline, which is thought to be caused by existing regional joints (cracks) at the impact site in the strata. (GEOLOGY PAGE).

The impacting meteorite was a coarse octahedrite containing siderophile elements (PGE, Ni, Au) identifying the impactor as a IA iron (Tangle, Hecht 2006). Meteorites from this fall are named “Canyon Diablo” after the canyon near where the first identified meteorite from this impact event was found.

The crater is referred to by scientists as Barringer Crater in honor of Daniel Barringer, who first claimed that it was created by meteorite impact. The crater is owned by the Barringer family privately through its Barringer Crater Company, which claims to be the “best preserved meteorite crater on Earth.” In November 1967, it was designated as a National Natural Landmark.

Before the Barringer Crater was identified as an impact crater it was called Coon Mountain or Coon Butte. The U.S. Geographic Names Board generally honors names of natural features from the nearest post office. Barringer, who first postulated that the feature was the result of a meteorite impact, had established a post office at the Sunshine flag stop on the nearby railroad. Thus the  crater’s original name was taken after this nearest (now defunct) post office, “Meteor”.

During the time of impact the local climate on the Colorado Plateau was much cooler and damper. The area was an open grassland dotted with woodlands inhabited by mammoths and giant ground sloths. The Neanderthals were still in Europe and Homo sapiens were just emerging and entering Europe. The North American Ice Age would not end for another 30,000 years!

 

2. GEOMORPHOLOGY

 

3. AERIAL EXPLORATION

 

General Area: This best known of all impact craters is 60 km ESE of Flagstaff, AZ on a flat plain south of the Little Colorado River. Although the crater is 1.19 km wide, it is very conspicuous because its bright rim contrasts with the darker plain of sedimentary rocks.

Specific Features: The Barringer Crater, approximately 150 km south east of the Grand Canyon, is classified as a simple meteorite crater (Kring 2007).

• The presence of millions of tons of finely pulverized silica, which could only have been created by enormous pressure.
• The large quantities of meteoritic iron, in the form of globular “shale balls”, scattered around the rim and surrounding plain.
• The random mixture of meteoritic material and ejected rocks.
• The fact that the different types of rocks in the rim and on the surrounding plain appeared to have been deposited in the opposite order from their order in the underlying rock beds.
• The absence of any naturally occurring volcanic rock in the vicinity of the crater.

These conclusions were championed by geologist George P. Merrill. Merrill analyzed a new type of rock discovered by Barringer at the crater, which Barringer called “Variety B”. He concluded that it was a type of quartz glass which could only be produced by intense heat, similar to the heat generated by a lightning strike on sand. Merrill also pointed to the undisturbed rock beds below the crater, which proved that the force which created the crater did not come from below (Merrill 1908).

It was in 1920 that the structure was finally recognized to be an impact crater, the first feature on Earth to be so recognized.

GUIDEBOOKS

• 2017 (new) guidebook to Barringer Crater
• 2007 guidebook
• 1974 Shoemaker guidebook

Shoemaker’s Proof

In 1960, Eugene Shoemaker, Edward Chao and David Milton were responsible for the discovery of a new mineral at the Barringer crater. This mineral, a form of silica called “coesite^a”, had first been created in a laboratory in 1953 by chemist Loring Coes. Its formation requires extremely high pressures and temperatures, greater than any occurring naturally on earth. Coesite and a similar material called “stishovite^b” have since been identified at numerous other suspected impact sites, and are now accepted as indicators of impact origin.

Finally, in 1963, Eugene Shoemaker published his landmark paper analyzing the similarities between the Barringer crater and craters created by nuclear test explosions in Nevada. Carefully mapping the sequence of layers of the underlying rock, and the layers of the ejecta blanket, where those rocks were deposited in reverse order, he demonstrated that the nuclear craters and the Barringer crater were structurally similar in nearly all respects. His paper provided the clinching arguments in favor of an impact, finally convincing the last doubters. (2016 The Barringer Crater Company)

^a Coesite is a form (polymorph) of silicon dioxide SiO₂ that is formed when very high pressure (2–3 gigapascals), and moderately high temperature (700 °C or 1,300 °F), are applied to quartz. Coesite was first synthesized by Loring Coes Jr., a chemist at the Norton Company, in 1953.

^b Stishovite is an extremely rare mineral forming only from the impact of a meteorite through the metamorphism of Quartz at extremely high temperatures. It is interesting to note that Stishovite is scientifically classified as an oxide and not as a silicate, even though it is polymorphous with Quartz.

Ground Exploration

One of the striking features of the crater is that its cross section clearly matches the geology of walls of the nearby Grand Canyon.

The image of the crater wall illustrates, from bottom to top ;

• Coconino – 265 my – sandstone fossil sand dunes – Permian.
• Toroweap – 260 my – yellowish shoreline sandstone – Permian;
• Kaibab – 250 my – cream coloured dolomite and sandstone,
• Kaibab 10 million year unconformity (the contact between older rocks and younger sedimentary rocks in which at least some erosion has removed some of the older rocks before deposition of the younger),
• Moenkopi –  240 my – reddish brown  silt-stone coastal floodplain cap (over the unconformity).

Shock metamorphism of the Coconino Sandstone at Meteor Crater, Arizona
Susan Werner Kieffer
ABSTRACT
A study of the shocked Coconino sandstone from Meteor Crater, Arizona, was undertaken to examine the role of porosity in the compression of rocks and in the formation of highpressure phases. A suite of shocked Coconino specimens collected at the crater is divided into five classes, arranged in order of decreasing quartz content. The amounts of coesite, stishovite (measured by quantitative X-ray diffraction), and glass vary systematically with decreasing quartz content. Coesite may comprise 1/3 by weight of some rocks, whereas the stishovite content does not exceed 1%. The five classes of rocks have distinct petrographic properties, correlated with the presence of regions containing coesite, stishovite, or fused silica. Very few occurrences of diaplectic glass are observed. In the lowest stages of shock metamorphism (class 1), the quartz grains are fractured, and the voids in the rock are filled with myriads of small chips derived from neighboring grains. The fracture patterns in the individual quartz grains are controlled by the details of the initial morphology of the colliding grains. In one weakly shocked rock, it was possible to map the general direction of shock passage by recording the apparent direction of collision of individual grains. The principal mechanism of energy deposition by a shock wave in a porous material is the reverberation of shock and rarefaction waves through grains due to collisions with other grains. A one-dimensional model of the impact process can predict the average pressure, volume, and temperature of the rock if no phase changes occur but cannot predict the observed nonuniformity of energy deposition. In all rocks shocked to higher pressure than was necessary to close the voids, high-pressure and/or high-temperature phases are present. Locally high pressures enduring for microseconds and high temperatures enduring for milliseconds controlled the phases of SiO₂ that formed in the rock. Collapsing pore walls became local hot spots into which initial deposition of energy was focused. Microcrystalline coesite in class 2 rocks occurs in symplektic regions on quartz grain boundaries that were regions of initial stress and energy concentration, or in sheared zones within the grains. The occurrence and morphology of the coesite-rich regions can be explained only if the transformation from quartz to coesite proceeds slowly in the shock wave. In class 3 rocks, microcrystalline coesite occurs in opaque regions that surround nearly isotropic cores of cryptocrystalline coesite. The cores are interpreted to be the products of the inversion of stishovite (or a glass with Si in sixfold coordination) that initially formed in the shock front in regions of grains shocked to pressures near 300 kb. Stishovite is preserved only in the opaque regions, which are believed to have been cooler than the cores. In class 4 rocks vesicular glass occurs in core regions surrounded by opaque regions containing coesite. The relation of the glass to the coesite and quartz suggests that the glass was formed by inversion of stishovite formed above 350 kb on release to lower pressure. Class 5 rocks are composed almost entirely of glass, with vesicles uniformly distributed in the glass. These vesicles were probably formed by exsolution of water that had been dissolved in melted SiO₂ during passage of the shock.

Journal of Geophysical Research 10 August 1971

Over seventy drill holes have been completed in the rim of the crater as part of a continuing research program of impact craters by the U.S. geological Survey. The drilling has shown that the overturned flap with its inverted stratigraphy is generally continuous out to about 400 m beyond the rim crest. The overturned flap is thickest on the southern side of the crater. Correlations between drill holes show rim uplift of at least 16 m at a distance of 30 m from the crater walls. The interpretation of the drill data suggests that relatively little erosion has occurred since the formation of the crater.

The rim of the 1.19 kilometre diameter Barringer Crater is still well defined, even after approximately 49 thousand years of erosion. It has been estimated that the first two stages of the cratering process (time from initial contact of the impactor until the end of the excavation stage) here at Barringer took approximately 6 seconds! Almost 63 million cubic metres were evacuated from this area in that time to form the crater. The height of the rim over the surrounding plain is 36 – 61 metres. Investigations around this rim confirmed an “overturned rim sequence”.

The Barringer Crater is one of the youngest impact sites on this planet and the effects of the impact still remain in situ. On the rim of the crater I noted fracturing of this country rock by the impact shock wave. Note that the country rock at this point was uplifted approximately 45 metres from its original position over the surrounding plain. Erosion has not yet exposed the fractured rocks buried outside of the crater. An overturned rim sequence is also present at the rim of the Barringer Crater and is now recognized as one of the hallmarks of an impact crater.

Aerial Exploration

On our planet, erosion will quickly remove an ejecta blanket and destroy any surviving meteorite fragments with the result that crater ejecta remains in only the youngest and best-preserved impact crater. This ASTER Infra Red image documents the pattern of the ejecta blanket around the relatively young Barringer Crater. The pattern of the ejecta blanket, although it has been modified by 49K years of erosion, implies an impact from the south west. The majority of the ejecta blanket forms in the north east, downrange, direction.

Some of the Barringer Crater ejecta curtain can been seen here in visible light as illustrated in this image. I took this image of the crater from about 1000′ above the ground. The “lighter” coloured sand is the remnants of the ejecta curtain. The pattern of the ejecta implies that the direction of the impactor was from the upper left of this image. If I was flying here at the time of impact, 49 thousand years ago, I would not know what hit me!!

We did a complete orbit of the crater in order to document it from each direction. Note the “square” shape of the crater, technically called a jointed crater. This shape is determined by the faults in the geology of country rock at the impact site.