iron axis structures

8/4/2019 Iron Axis Structures

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Iron Axis Structures, St. George, Utah

Tim McElvan

The Iron Axis Laccoliths are located in southwest Utah north of the city of

St. George. I am proposing that the laccolith are the remnants of 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.


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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 generallyaccepted 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


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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 attenuatedTertiary volcanic and sedimentary rocks resting on low-angle faults.Faulting placed both younger rocks over older and older rocks overyounger.

Typically, the formations exhibit pervasive internal fracturing andshattering but are well indurated. Some formations are brecciatedand consist of pebble-to- boulder-sized, angular to subangular rockfragments with a crushed matrix of the same composition as thefragments. The brecciated zones are commonly matrix-poor, with thefragments commonly tightly packed in a jigsaw-puzzle mosaicseparated by a cataclysmically generated sand-to-granule-sizematrix.

Speed of Emplacement: The velocity of the gravity slides is a matterof speculation, but has special implications in the origin of theintrusions. The speed is estimated to be rapid and most likelycatastrophic. The absence of field evidence for continued thrustingand gouging and the internal structure (e.g., brecciation extensional

faulting) suggest movement by a body force (i.e. gravity). Very rapidto catastrophic movement of the gravity slides seems necessary toexplain the following observations:

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

(2) No erosion material exists between the slide and the overlyingvolcanic rocks that erupted from the same source area following

sliding. This indicates that volcanism was synchronous with orimmediately followed sliding. Most of the volcanic material overlyingthe slides consists of ash-flow tuffs erupted from the laccoliths. Thetuffs indicate violent eruptions and catastrophic emplacement.

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


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(4) Internal brecciation and shattering of rock units within slidemasses precludes a push from the rear and requires a one-shotemplacement 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 theintrusions. The speed is estimated to be rapid and most likelycatastrophic. The absence of field evidence for continued thrustingand gouging and the internal structure (e.g., brecciation, extensionalfaulting) suggest movement by a body force (i.e. gravity). Very rapidto catastrophic movement of the gravity slides seems necessary toexplain the following observations:

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

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

The Iron Axis Laccoltihs are located in southwest Utah northof the city of St. George. I am proposing that the laccolthsare the remnants of the the central uplift of several largeMid-Tertiary impact craters. I believe the large melt bodiesreferred to as laccoliths are the result of the melt generatedby the collapse of the central uplift, which melted thesedimentary rock which comprised the central uplift. I do notbelieve the laccoliths are rooted and do not consist of melted

basement rocks. A large bolide impact would create a largecentral uplift and a melt within minutes. The allochthonousgravity slides blocks have been observed in the ChesapeakeImpact Structure. In the Chesapeake Structure theallochthonous slide block seem to have originated asbreakaway blocks from the edge of the transient crater. Themelted fine grained ejecta, melt and vaporized target rock


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could be confused with volcanic melt and ash. If it is proventhat the Iron Axis Laccoliths are the result of a bolideimpact, the most likely source for the allochthonous slideblocks is from the edge of the transient crater where they

detached from the rim and slid toward the central uplift.


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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

PDFs. The numbers of these plotted GPS Waypoints represent thefollowing 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 havebeen 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 willeventually confirm some of the grains in these formations have quartz with

PDFs.

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


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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 PDFs. 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 Loca

07002 12S0299595.9

4121535 Limestone breccia inthe Virgin Anticlineeast of Hurricane, UT

St GUtah

07003 12S0286219.4

4129788 Cretaceous IronSprings Formation atthe eastern base ofPine Creek Laccolith

in Leeds Canyon, inLeeds Canyon, UT

St GUtah

07004 12S0286406.9

4129380 Cretaceous IronSprings formation.

UT

07005 12S0286592.9

4129169 Oak Grove below thePine Valley Laccolith

Utahtrip sSt GUtah

07006 12S0306731.1

4178500 STOP-3 - Collected asample of sandstone17a - probably thesandstone on top ofthe Temple Cap

Utahtrip sSt GUtah


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Limestone, sample ofore and Temple PeakFormation 17b. WP17b is at the edge ofan old iron mine alonga fault withslickensides. Sampleof ore - hematite andmagnetite the TempleCap Formation is thehost rock.

07007 12S

0281093.1

4154007 Near a volcanic vent

collected a cobble ofvery hard and tuff -that weathered out ofan ash flow tuff. Theash flow tuff alsocontains lithicfragments and clastsfrom igneous(basement?) toTertiary.

Utah

trip, GeoUtah

08001 12S0267472.4

414417708001.1 Grainmounts of thesand from thissample seems tobe mainly toastedand cooked, veryfew quartz grains,a few PM's, and

lots of eitherchert ordevitrifieddiaplectic glass.

Clarion Formationcollected just east ofthe main intersectionof Central Utah andHwy 18

PineValleLaccAreaSt GUT

08002 12S0277765.5

4157426A grain mountreveled that

Clarion Formationcollected Pinto, Utah

PineValle


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probably 50% ofthe quartz grainshave PM's. Thissample has thehighestpercentage ofshocked grainsthat I have everseen.

LaccAreaSt GUT

08003 12S0285365.1

416015208003.1 Grainmount revealsshocked material

and PM's, but thepercentage ofshocked grains ismuch less than08002 above.One explanationmay be, becausethe Carrion in08002 is targetmaterial that hasslid off the centraluplift(allochthonous)or more probablythe transientcrater rim, andthe Iron SpringsFormation(Cretaceous) in

this sample is inplace and furtherfrom the surfaceand point ofimpact. There areseveral of themelted and

Iron SpringsFormation outcrop ina little knoll in Richie

Flat, Utah.

PineValleLacc

AreaSt GUT


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quenched grainswith remnantPDF's that I havefound in otherthin sections ofshockedsandstone (LaSal and Abajoamong others).

08004 12S0289061.1

416255708004.1 Thereare grains withPM's, but not

nearly in as highof a % as 08002.

Clarion Formationcollected NE of PageRanch

PineValleLacc

AreaSt GUT

08005 12S0291933.2

416472108005.1 Grainmount reveals arather highpercentage ofwell developedPM's in the

grains.

Clarion Formationcollected at theJunction of StoddardCanyon and HWY 56UT

PineValleLaccAreaSt GUT

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 aremost likely shock induced Planar Deformation Features.


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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.


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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.


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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

PDFs but they are slightly curved.

The following photomicrographs of quartz grains in samples fromWaypoints 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.


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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 SpringsFormation. I am not sure if the Iron Springs Formation atthis location is an allochthonous block or in place, but thepercentage of shocked quartz grains in this sample is muchless the samples from the Claron Formation. This grain08003.1-4 has one primary set of planar microstructuresand one set of intersecting planar microstructures that donot appear to reach across the grain in this view.


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Figure 8. 08002-1-400X

The quartz grain from the Claron Formation in Figure 7 above has two sets

of intersecting planar microstructures.


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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.


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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.


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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.


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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.


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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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iron axis structures

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