Iron Axis Structures, St. George, Utah

 

 Please proceed to revised paper at http://www.scribd.com/tmcelvain_1 

 

The Iron Axis Laccoltihs are located in southwest Utah north of the city of St. George. I am proposing that the laccolths are the remnants of the the central uplift of several large Mid-Tertiary impact craters. I believe the large melt bodies referred to as laccoliths are the result of impact melt or possibly the melt generated by the collapse of the central uplift, and are sedimentary rock melt. I do not believe the laccoliths are rooted and do not consist of melted basement rocks.

IronAxisLocation

 

Figure 1.

 

My interest in the Iron Axis Laccolith Group was peaked when I attended the 2007 Geological Society of America Rocky Mountain Section Meeting. While I was waiting to give my presentation on the shatter cone discovery near Santa Fe, NM, I sat through an entire session devoted to extending the emplacement time of the Pine Valley Laccolith. Apparently the bulk of the geological evidence indicated that the very large melt body was emplaced very quickly and in one continuous flow, which does not fit the generally accepted model of laccolith formation. My topic was about large impact structures and it suddenly came to me that a very large bolide impact could leave a very large melt body almost instantaneously. Consequently I signed up for the field trip and on the field trip I collected samples of sandstone in the Cretaceous and older rocks to look for shocked quartz. I had thin sections made and found that several of the quartz grains had planar microstructures. I began to read the literature on the Iron Axis Laccoliths which also had interesting conclusions which could also be attributed to a large impact. In May of 2008 I returned to collect more samples, particularly of the allochthonous Tertiary Claron Formation gravity slide blocks, which I thought, would have been closer to the surface at the time of impact and therefore the quartz grains would be more severely shocked. I made grain mounts of these samples of Claron Formation sandstone, and found the highest percentage of quartz grains with planar microstructures in any sample I have looked yet.

 

The following publications to which I have posted a link give a good overview of the geology of the Iron Axis Laccoliths:

 

ASSOCIATED MIOCENE LACCOLITHS, GRAVITY SLIDES, AND

VOLCANIC ROCKS, PINE VALLEY MOUNTAINS AND IRON AXIS

REGION, SOUTHWESTERN UTAH

Geological Society of America 2002 Rocky Mountain Section Annual Meeting

Cedar City, Utah

May 10, 2002

David B. Hacker

Daniel K. Holm

Department of Geology, Kent State University

Kent, Ohio 44242

Peter D. Rowley

Geologic Mapping, Inc., P.O. Box 651

New Harmony, Utah 84757

H. Richard Blank

U.S. Geological Survey, 904 W. Riverside

Spokane, Washington 99201,

 

http://geopubs.wr.usgs.gov/open-file/of02-172/chapters/chap7.pdf

 

And

SHALLOW LEVEL EMPLACEMENT MECHANISMS OF

THE MIOCENE IRON AXIS LACCOLITH GROUP,

SOUTHWEST UTAH

2007 Geological Society of America Rocky Mountain Section Annual Meeting

St. George, Utah

May 10, 2007

David B. Hacker

Department of Geology, Kent State University, Kent, Ohio 44242

Michael S. Petronis

Department of Natural Sciences, New Mexico Highlands University, Las Vegas, NM 87701

Daniel K. Holm

Department of Geology, Kent State University, Kent, Ohio 44242

John W. Geissman

Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM

87131

http://www.utahgeology.org/UGA_Pub-35/Hacker,%20Petronis,%20Holm,%20and%20Geissman.pdf

 

From the first mentioned publication ASSOCIATED MIOCENE LACCOLITHS, GRAVITY SLIDES, AND VOLCANIC ROCKS, PINE VALLEY MOUNTAINS AND IRON AXIS REGION, SOUTHWESTERN UTAH, I cut and pasted the following excerpts that lead me to suspect that a bolide impact could explain the genesis of the laccoliths.

 

Within the study area are numerous large allochthonous masses (sheets or blocks) of fractured,brecciated, sheared, and attenuated Tertiary volcanic and sedimentary rocks resting on low-angle faults. Faulting placed both younger rocks over older and older rocks over younger.

Typically, the formations exhibit pervasive internal fracturing and shattering but are well indurated. Some formations are brecciated and consist of pebble-to- boulder-sized, angular to subangular rock fragments with a crushed matrix of the same composition as the fragments. The brecciated zones are commonly matrix-poor, with the fragments commonly tightly packed in a jigsaw-puzzle mosaic separated by a cataclastically generated sand-to-granule-size matrix.

 

Speed of Emplacement: The velocity of the gravity slides is a matter of speculation, but has special implications in the origin of the intrusions. The speed is estimated to be rapid and most likely catastrophic. The absence of field evidence for continued thrusting and gouging and the internal structure (e.g., brecciation extensional faulting) suggest movement by a body force (i.e. gravity). Very rapid to catastrophic movement of the gravity slides seems necessary to explain the following observations:

(1) There are no sedimentary deposits immediately beneath the slide masses indicative of erosion along an elevated area (i.e., laccolithic domes in this case) prior to sliding.

(2) No erosion material exists between the slide and the overlying volcanic rocks that erupted from the same source area following sliding. This indicates that volcanism was synchronous with or immediately followed sliding. Most of the volcanic material overlying the slides consists of ash-flow tuffs erupted from the laccoliths. The tuffs indicate violent eruptions and catastrophic emplacement.

(3) Extremely thin but stratigraphically preserved rock units that traveled at least 12 km over the former land surface.

(4) Internal brecciation and shattering of rock units within slide masses precludes a push from the rear and requires a “one-shot” emplacement mechanism, especially for such thin layers of rock.

 

Speed of Emplacement: The velocity of the gravity slides is a matter of speculation, but has special implications in the origin of the intrusions. The speed is estimated to be rapid and most likely catastrophic. The absence of field evidence for continued thrusting and gouging and the internal structure (e.g., brecciation, extensional faulting) suggest movement by a body force (i.e. gravity). Very rapid to catastrophic movement of the gravity slides seems necessary to explain the following observations:

(1) There are no sedimentary deposits immediately beneath the slide masses indicative of erosion along an elevated area (i.e., laccolithic domes in this case) prior to sliding.

(2) No erosion material exists between the slide and the overlying volcanic rocks that erupted from the same source area following sliding. This indicates that volcanism was synchronous with or immediately followed sliding. Most of the volcanic material overlying the slides consists of ash-flow tuffs erupted from the laccoliths. The tuffs indicate violent eruptions and catastrophic emplacement.

 

 

The Iron Axis Laccoltihs are located in southwest Utah north of the city of St. George. I am proposing that the laccolths are the remnants of the the central uplift of several large Mid-Tertiary impact craters. I believe the large melt bodies referred to as laccoliths are the result of the melt generated by the collapse of the central uplift, which melted the sedimentary rock which comprised the central uplift. I do not believe the laccoliths are rooted and do not consist of melted basement rocks. A large bolide impact would create a large central uplift and a melt within minutes. The allochthonous gravity slides blocks have been observed in the Chesapeake Impact Structure. In the Chesapeake Structure the allochthonous slide block seem to have originated as breakaway blocks from the edge of the transient crater. The melted fine grained ejecta, melt and vaporized target rock could be confused with volcanic melt and ash. If it is proven that the Iron Axis Laccoliths are the result of a bolide impact, the most likely source for the allochthonous slide blocks is from the edge of the transient crater where they detached from the rim and slid toward the central uplift.

Iron Axis Impact Structure, St. George, Utah

 

Figure 2.

In Figure 2 above I have plotted the locations where I collected samples. In these samples I found the quartz grains with the planar microstructures that fit the scale of planar deformation structures, but have not been verified as PDF’s. The numbers of these plotted GPS Waypoints represent the following information:

The first two numbers represent the year - 07 is 2007

The second three numbers is the order number of the samples I collected that year - 003 is the third sample I collected that year.

The numbers on the following photomicrographs represent the following information:

First five numbers identify the Waypoint where the sample was collected.

The second numbers identify a specific ink circle on a slide.

The third series of numbers such as – 400X is the microscope magnification of the grain.

The fourth letters if present – XP indicates the photomicrograph was illuminated with cross polarized light.

 

None of the planar microstructures in the following photomicrographs have been verified as diagnostic of the intensity of shock metamorphism that is only found in a impact structure. The sandstone samples were collected in are porous and permeable, which is evidence enough to rule out tectonic pressures forming Bohm lamellae The quartz grains with planar microstructures come from different formations separated in time by tens of millions of years making it extremely unlikely that the grains were transported from an another terrain containing shocked quartz or quartz with Bohm lamellae and deposited in this location. In my opinion the planar microstructures were shock generated and more investigation and tests will eventually confirm some of the grains in these formations have quartz with PDF’s.

 

The curvature of some of the planar microstructures in grain mounts 08002 – 08005 may be due to the distortion of the transmitted light caused by the spherical nature of the grain. I have found that when I mount these grains on a spindle stage and rotate the grain so the plane of the planar microstructure is perpendicular to the stage of the microscope the curviplanar microstructure image becomes planar. I have measured the angle of the plane of the PM to the c-axis and made histograms of the indexed angles of some 250 quartz grains in my study of the overall impact event: however, I have not made any measurements of the angles on this structure. Even though the histograms are similar to histograms of known craters I have not had any success convincing the skeptics that they are PDF’s. Until I can get a qualified expert to verify my results, it is a waste of time for me to continue making these time consuming measurements.

 

Table 1 below is a copy of my field notes and gives the coordinates of the Waypoints where I collected samples.

Waypoint

UTM

UPS

Thin Section

Hand Sample

Location

07002

12S 0299595.9

4121535

 

Limestone breccia in the Virgin Anticline east of Hurricane, UT

St George Utah Area

07003

12S 0286219.4

4129788

 

Cretaceous Iron Springs Formation at the eastern base of Pine Creek Laccolith in Leeds Canyon, in Leeds Canyon, UT

St George Utah Area

07004

12S 0286406.9

4129380

 

Cretaceous Iron Springs formation.

UT

07005

12S 0286592.9

4129169

 

Oak Grove below the Pine Valley Laccolith

Utah Field trip stop 1, St George Utah Area

07006

12S 0306731.1

4178500

 

STOP-3 - Collected a sample of sandstone 17a - probably the sandstone on top of the Temple Cap Limestone, sample of ore and Temple Peak Formation 17b. WP 17b is at the edge of an old iron mine along a fault with slickensides. Sample of ore - hematite and magnetite the Temple Cap Formation is the host rock.

Utah Field trip stop 3, St George Utah Area

07007

12S 0281093.1

4154007

 

Near a volcanic vent collected a cobble of very hard and tuff - that weathered out of an ash flow tuff. The ash flow tuff also contains lithic fragments and clasts from igneous (basement?) to Tertiary.

Utah Field trip, St George Utah Area

08001

12S 0267472.4

4144177

08001.1 Grain mounts of the sand from this sample seems to be mainly toasted and cooked, very few quartz grains, a few PM's, and lots of either chert or devitrified diaplectic glass.

Clarion Formation collected just east of the main intersection of Central Utah and Hwy 18

Pine Valley Laccolith Area, N of St George UT

08002

12S 0277765.5

4157426

A grain mount reveled that probably 50% of the quartz grains have PM's. This sample has the highest percentage of shocked grains that I have ever seen.

Clarion Formation collected Pinto, Utah

Pine Valley Laccolith Area, N of St George UT

08003

12S 0285365.1

4160152

08003.1 Grain mount reveals shocked material and PM's, but the percentage of shocked grains is much less than 08002 above. One explanation may be, because the Carrion in 08002 is target material that has slid off the central uplift (allochthonous) or more probably the transient crater rim, and the Iron Springs Formation (Cretaceous) in this sample is in place and further from the surface and point of impact. There are several of the melted and quenched grains with remnant PDF's that I have found in other thin sections of shocked sandstone (La Sal and Abajo among others).

Iron Springs Formation outcrop in a little knoll in Richie Flat, Utah.

Pine Valley Laccolith Area, N of St George UT

08004

12S 0289061.1

4162557

08004.1 There are grains with PM's, but not nearly in as high of a % as 08002.

Clarion Formation collected NE of Page Ranch

Pine Valley Laccolith Area, N of St George UT

08005

12S 0291933.2

4164721

08005.1 Grain mount reveals a rather high percentage of well developed PM's in the grains.

Clarion Formation collected at the Junction of Stoddard Canyon and HWY 56 UT

Pine Valley Laccolith Area, N of St George UT

 

Table 1.

 

The following photomicrographs are of quartz grains collected from sandstone outcrops within the inner ring of the Iron Axis Impact Structure. The planar microstructures fit the scale of Planar Deformation Structures, most of them has several sets of planar microstructures, and to my mind are most likely shock induced Planar Deformation Features.

07003-11-400X

 

Figure 4. 07003-11-400X

The grain in the center of Figure 4 is a quartz grain with 4 or 5 sets of planar features in a sample of the Cretaceous Iron Springs Formation. The formation at this location would probably be underneath the Pine Valley Laccolith before erosion as are the grains in Figures 5 and 6 below.

07004-7-400X

 

 

Figure 5. 07004-7-400X

The quartz grain in Figure 5 from the Cretaceous Iron Springs Formation has one set of rather faint planar microstructures and a hint of a second set of even fainter planar microstructures.

07006-1-200X

 

Figure 6. 07006-1-200X

The sample in Figure 6 above was collected from Stop 3 on the 2007 field trip from a sandstone stringer on top of the Jurassic Temple Cap Limestone. The quartz grain in the center of the photomicrograph has two sets of planar microstructures. The scale of the planar microstructures fit the scale of PDF’s but they are slightly curved.

 

The following photomicrographs of quartz grains in samples from Waypoints 08002 through 08005 were collected from allochthonous detachment blocks of Tertiary Claron Formation, and in the case of sample 08003 from the Iron Springs Formation probably also a detachment block.

08003-1-4-200X

 

08003-1-4-200X-XP

 

08003-1-3-200X

 

08003-1-3-200X-XP

 

Figure 7 Sample 08003.1Two grains with 2 sets of planar microstructure illiuminated in both plane and polarized light.

Sample 08003 was collected from the Iron Springs Formation. I am not sure if the Iron Springs Formation at this location is an allochthonous block or in place, but the percentage of shocked quartz grains in this sample is much less the samples from the Claron Formation. This grain 08003.1-4 has one primary set of planar microstructures and one set of intersecting planar microstructures that do not appear to reach across the grain in this view.

 

08002-1-400X

 

Figure 8. 08002-1-400X

The quartz grain from the Claron Formation in Figure 7 above has two sets of intersecting planar microstructures.

08002-1-400X-XP

Figure 9. 08002-1-400X-XP

The quartz grain if Figure 8 above is the same grain in Figure 8 illuminated with cross polarized light.

08002-6-200X

 

The quartz grain in Figure 10 above is from the Claron Formation and has one primary set of planar microstructures, but there is a hint of two or more sets.

08005-1-5-200X

 

Figure 11. 08005.1-5-200X

The quartz grain in Figure 11 is from the Claron Formation and has two intersecting sets of planar microstructures.

08005-1-5-200X-XP

 

Figure 12. 08005.1-5-200X-XP

Quartz grain in Figure 12 is the same grain as Figure 11 above illuminated with cross polarized light.

Iron Axis Impact Structure, St. George, Utah

If the planar microstructures, PM's, are proven to be planar deformation features, PDF's, in the quartz grain illustrated in Figure 13 above would be the most convincing photomicrograph confirming the Iron Axis Laccoliths are Impact Structures. In this photomicrograph one can see Planar Microstructures extending into the overgrowth in a sand grain in the Tertiary Claron Formation, which would indicate that the grain was shocked in place and was not a shocked grain transported from another terrain and deposited in the Claron Formation.

 

There is strong evidence both microscopic and macroscopic that the Iron Axis Laccoliths are the result of large bolide impacts. I can see circular features that would probably represent the central uplift or some of the inner rings, and there is a faint outer ring around the Pine Valley Laccolith approximately 60 miles in diameter; however, the ring has been destroyed to the south and east.

 

 

 

 

 

 
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Sample 08003 was collected from the Iron Springs Formation. I am not sure if the Iron Springs Formation at this location is an allochthonous block or in place, but the percentage of shocked quartz grains in this sample is much less the samples from the Claron Formation. This grain has one primary set of planar microstructures and one set of intersecting planar microstructures that do not appear to reach across the grain in this view.