Applications of twin analysis to studying meteorite impact structures

tters 244 (2006) 530–540www.elsevier.com/locate/epsl

Earth and Planetary Science Le

Applications of twin analysis to studying meteorite impact structures

Andrew Schedl ⁎

Department of Geology, Indiana University, Purdue University at Indianapolis, Indianapolis, Indiana, 46202, USA

Received 26 September 2005; received in revised form 16 January 2006; accepted 10 February 2006Available online 24 March 2006

Editor: R.D. van der Hilst

Abstract

This paper describes a technique, twin analysis of dolomite and calcite, which was used to estimate the level of erosion of animpact crater. If the age of impact is known, the level of erosion gives the amount of sediments present at that time. The estimate ofthe level of erosion, and the presence or absence of tectonic strains in calcite constrains the age of impact. This technique mightalso be useful in identifying deeply eroded impact structures. Twin analysis gives the greatest shortening direction, ε1. Tiltcorrected ε1 directions converge at or above the present day surface, if a shallow gas explosion or meteorite impact explains thestructure, rather than a deep gas or cryptovolcanic explosion. The energy density recorded by twinning distinguishes an impactfrom a shallow gas explosion for structures ≤100 m diameter. At Serpent Mound, a well-established impact structure, ε1 directionsconverge at 1400±390 m (95% confidence interval of the mean) above the present day structure and the sample closest to thecenter of the structure records an energy density of 1.7×107 J/m3. These results are consistent with previous studies showing thatSerpent Mound is an impact (e.g., [R.W. Carlton, C. Koeberl, M.T. Baranoski, G.A. Schumacker, Discovery of microscopicevidence for shock metamorphism at the Serpent Mound structure, south–central Ohio: confirmation of an origin by impact, Earthand Planet. Sci. Lett. 162 (1998) 177–185]). The level of erosion and other data suggests that Serpent Mound formed between 290and 256 Ma and was originally 8.5 to 11 km in diameter. Dolomite twinning is not observed at Serpent Mound suggesting that thistechnique may only be useful for studying larger, more deeply eroded structures.© 2006 Elsevier B.V. All rights reserved.

Keywords: meteorite impact; calcite-twin analysis; level of erosion; cryptoexplosion

1. Introduction

As geoscientists' appreciation of the role of impactevents on the earth's history grows, controversies overthe origin of some near-surface structures have arisen.The only definitive low-pressure-shock-metamorphicfeatures for identifying impact structures are planardeformation features (PDFs) in quartz, which form atpressures ≥10 GPa; planar fractures (PFs), which form

⁎ Present Address: Department of Natural Sciences and Mathemat-ics, University of Charleston, Charleston, WV 25304, USA.

E-mail address: [email protected].

0012-821X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.epsl.2006.02.018

at ≥5 GPa; and shatter cones, which form at ≥2 GPa[2–4]. In simple craters, later sediments may coverrocks, which reached such pressures and in deeplyeroded complex craters, few rocks may be left whichexperienced these pressures. Upheaval Dome, Utah, is adeeply eroded structure and because of the absence ofPFs, PDFs in quartz and shatter cones both saltintrusion–withdrawal and meteorite impact are plausi-ble explanations for its origin [5,6]. (Work that is morerecent favors an impact origin [7].) Since carbonate-containing rocks (calcareous sandstone in the middleMoenkopi Formation and lenses of non-marine lime-stone in the Navajo Formation) are present at Upheaval

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531A. Schedl / Earth and Planetary Science Letters 244 (2006) 530–540

Dome and the shear stresses needed for calcite twinningare low, 10 MPa, twin analysis could test the impacthypothesis here.

In addition, of the 160 probable impact structures 60are in carbonate target rocks [8]. There are few methodsfor quantifying the impact induced deformation andshock metamorphism. Carbonate rocks begin to break-down and outgas at N40 GPa for limestone and N60 GPafor dolomite (see [9] and references therein). Fieldevidence for the breakdown reactions are back reactedphases such as larnite, pigeonite and augite; increases inδ13C and decreases in δ18O compared to unshockedequivalent carbonates. Field relations and thin sectiontextures indicate that carbonate rocks have also meltedat N50 GPa [10–13]. For low-pressure-shock metamor-phism, peak broadening of X-ray diffraction (XRD)patterns of calcite and electron spin resonance (ESR) ofcalcite have been tried [14,15]. These techniques arequalitative, since there is poor correlation between XRDand ESR estimates of shock pressure [16]. In this study,the inferred differential stresses and strain energies fromcalcite twinning may indicate low-pressure-shockmetamorphism, and knowing the height of the shockwave source is important in deformation studies.

This paper illustrates how twin analysis can be usedto test the impact hypothesis at Serpent Mound, a well-established impact structure [1]. In addition, twinanalysis is used to estimate the amount of erosion.This estimate is evaluated as to whether it is consistentwith macroscopic, mesoscopic and microscopic obser-vations. Finally, the estimate of the amount of erosionand the absence of twinning strains associated with theAppalachian–Ouachita deformation is used to constrainthe age of impact.

2. Geologic setting

Serpent Mound lies at the edge of the AppalachianBasin, a foreland basin formed during the Alleghenianorogeny, a part of Appalachian–Ouachita deformation.Serpent Mound is nearly circular in plan view with adiameter of 7.3 to 8.0 km (Fig. 1). It is elongate in anorth–south direction [18]. This feature has threestructural units, a central uplift, a ring graben and atransitional area in between. In this study, the rocks atSerpent Mound are divided into upper, middle and lowerunits and quaternary alluvium. The lowermost unit isUpper Ordovician to Lower Silurian limestone and shale(Drakes (?) Formation, Brassfield Formation, NolandFormation, Dayton Formation and Estill Shale). Themiddle unit is Silurian in age and consists of dolomiteand shale (Bisher Formation, Lilley Formation, Peebles

Dolomite, Greenfield Dolomite and Tymochtee Dolo-mite). The upper unit consists of Upper Devonianthrough Lower Mississippian sandstone and shale(Hillsboro Sandstone, Olentangy Shale, Ohio Shale,Bedford Shale, Berea Sandstone, Sunbury Shale andCuyahoga Formation) [19].

One consequence of the Serpent Mound structurelying near the edge of a foreland basin is that the rocksdip to the east. The base of the Bisher Formation is 45 mlower on the eastside of Serpent Mound than the west[20,21]. Rocks adjacent to Serpent Mound are alsolikely to record twinning strains in calcite producedduring the Alleghenian orogeny. Based on a comparisonof twin, cleat and joint data most proximal to SerpentMound, calcite twinning should record a beddingparallel, NW to NNW greatest shortening direction(ε1) [22–24]. This shortening direction is consistentwith “Main Phase” Alleghenian deformation and is pre-to syn-folding in the Valley and Ridge of theAppalachians [25].

3. Twin analysis

Many minerals, calcite, dolomite, quartz, etc.,undergo mechanical twinning. Twinning occurs whenshear stresses acting on specific crystallographic planesof a mineral exceed a critical value. The twin itself is theresult of simple shear motion of atoms and hence thedeformation associated with twinning has a strain androtational component. Calcite is the most widely usedmineral in twinning studies and its use is well-established in structural geology. The calcite-twinningtechniques used in this paper are Turner's method [26],Groshong's calcite strain-gauge [27,28] and Jamisonand Spang's [29] differential stress piezometer. Al-though these techniques were developed for calcite, theycan be applied to other minerals.

Turner's [26] method determines the maximumcompressive and maximum tensile stress directions.This method makes assumptions about the angularrelationship between the crystallographic C-axis and etwin-pole, and the maximum compressive (C-axis) andtensile stress (T-axis) directions. This method isqualitative. Preferred grain orientations, the magnitudeof differential stresses and other deformation events(twinning during burial) cause the contoured maximumof measured C-axes and T-axes to deviate from their truedirection. Groshong's [27] calcite strain-gauge uses theC-axis and e pole orientations, the number of twins permillimeter, and the thickness of the twins in microns, toinfer the strain tensor associated with mechanical twin-ning. After “cleaning” [30], (which will be described in

Fig. 1. Generalized geologic map of Serpent Mound (modified from [17]) shows sample locations and the respective ε1 trajectories at these locations.The filled circles (●) are limestone sample locations and the filled arrowheads connect them to their respective stereonet. On an equal-area stereonet,lower hemisphere projection, principal stains (εi) are plotted, and C-axes are contoured using the Kamb method [46] and have contour intervals of 2σ.N = the number data points in the cleaned sample. (For sample 95SPM1 only the positive expected values are plotted.) The lines with double openarrowheads show the projection of ε1 into map view. The open circles (○) with the letters H and B by them are, respectively, the sample locations forHillsboro and Berea Sandstone. The star (☆) shows the location for the samples 95SPM9 and 95SPM10. Open and shaded triangles (▵, ▴) on themap show the locations of shatter cones. In the middle–upper part of the figure is a map of the U. S. A. The filled triangle shows the location ofSerpent Mound.

532 A. Schedl / Earth and Planetary Science Letters 244 (2006) 530–540


detail later) average deviations from true strain magni-tude are ±11% and strain orientations are ±8° forsamples deformed in the laboratory. Jamison and Spang[29] assuming that (σ2−σ3)≪ (σ1−σ3), infer themaximum differential stress in a rock by determiningthe percent of grains with 1, 2, or 3 sets of twins (Fig. 2).This technique is accurate to ±15% [29,31].

For calcite twinning, there are two properties that areuseful in identifying impact structures: 1) The shearstresses needed for twinning are low, 10 MPa, soinformation about a structures origin can be obtainedfrom relatively undeformed rock. 2) Deformation is strainhardening: as the rock is twinned, it becomes moredifficult to produce additional calcite twins as indicated byfield studies. Thus, calcite twinning tends to record theearliest deformation. Field studies of calcite twinning infolds show initial layer parallel shortening is recordedrather than the bending strains associated with laterbuckling [32]. Calcite-twin analysis of a thrust sheetmoving over a ramp showed layer parallel shortening inthe host rock related to initial thrust detachment. Onlycalcite in later forming tectonic veins recorded thebending and unbending strains of the thrust sheet as itpassed over the ramp [33]. Calcite twinning in the fracturefill of en echelon veins record the local strains associatedwith fracture propagation, rather than the large scaleshearing which produced the vein system [34].

For this study, the most important field example ofcalcite twinning recording the earliest increment ofstrain are the far field strains associated with orogeny inNorth America. Ordovician and Devonian limestone innortheastern Iowa and southeastern Minnesota record

Fig. 2. Shows calcite twins in a thin section of sample 95SPM8. TheHTwith a dark arrow marks a grain, which has a high twin density. TheT with the white arrows marks three different twin sets (horizontal,vertical and diagonal) within a single grain. Rotation of this thinsection on the horizontal and or vertical axes of a U-stage will makeindividual twin sets more distinct. The scale bar is 100 μm.

N–S to NW–SE bedding parallel shortening [35]. Thisshortening direction is associated with the Appala-chian–Ouachita deformation and is no younger than≈240 Ma [36]. Not far away in Western Minnesota theLower Cretaceous Greenhorn formation records theSevier–Laramide (40–140 Ma) E–W shortening. (Theage of shortening recorded by calcite twinning inMesozoic rocks of the western United States is afunction of location [36]). If there was no strainhardening, then the Paleozoic rocks of northeast Iowaand southeast, Minnesota should record the Sevier–Laramide shortening [37]. Thus, calcite twinning tendsto record the earliest increments of strain [33].

Following Baranonski et al. [19] circular structures,such as Serpent Mound, should have either an exogenicorigin, produced by a force external to the earth, orendogenic origin, produced by a force within the earth.Structures of exogenic origin would be meteoriteimpacts. Structures of endogenic origin would be agas explosion, cryptovolcanic explosion, and saltintrusion and withdrawal.

For each explosion processes, the shock wave shouldproduce an initial–incremental-greatest-shortening di-rection, ε1, approximately parallel to the direction theshock wave is traveling. The property of strainhardening suggests that twinning should preferentiallyrecord the initial passage of the shock wave through thesample. The later modification stage produces presentday strike and dips of units. As such, the convergence oftwin-recorded ε1 directions should improve when bedsare rotated to horizontal. For a cryptovolcanic, deep gasexplosion or salt intrusion and withdrawal, ε1 shouldconverge below the present-day-erosion surface. If thestructure is of impact origin or due to a shallow gasexplosion ε1 should converge at or above the surface.The product of ε1 and (σ1−σ3) inferred from twinninggives, as will be shown later, a lower bound estimate ofthe energy density (J/m3). (In this study, the signconventions used are compressive stresses and shorten-ing strains are positive, so the energy densities arepositive.) If this estimate from the center of the structureis greater than the maximum energy density for a gasexplosion, then the feature is of impact origin. (Otherarguments, as will be seen later, can also be used to ruleout a shallow gas explosion.) If the analysis describedabove does not confirm previous studies that, SerpentMound is of impact origin. Then calcite twinning is notuseful in studying meteorite impacts.

A source of uncertainty with the calcite strain-gaugeis heterogeneity of strain within the sample [27]. Strainheterogeneity is a consequence of sedimentary rocksbeing porous. During deformation, voids result in stress


concentrations, which locally increase and reorientstresses. In thin section these stress concentrationsmanifest themselves as pore collapse, regions of highlytwinned and fractured calcite. This heterogeneity isevaluated by first calculating an expected value, theamount of strain that would be expected in a particulargrain from the previously calculated bulk strain tensor.The difference between the expected value and themeasured twinning strain in this grain gives thedeviation. A negative deviation results from a negativeexpected value (NEV) (a grain of that orientation shouldnot twin) or a larger measured twinning strain. Based onsamples from the field and experiments, rocks experi-encing uniaxial deformation can have up to 20% NEVsshowing that strain is heterogeneous. This data is“cleaned” by throwing out 20% of the measurementswith the largest deviations; thus improving agreementbetween experimentally imposed strain and strainestimated from twinning [30].

Although calcite twinning tends to record the earliestincrement of shortening, because of strain hardening,there are circumstances where a second event will berecorded. Twinning can occur along three differentplanes in the calcite crystal. If one plane is twinned,further twinning can only occur on the other twoe {0112} planes at higher stresses if the stress is at≥45°from the initial stress orientation [38]. In experimentswhere two noncoaxial deformation events are ≥45°apart, the NEVs of the samples are N40%. If data aresplit in two, those with positive and those with negativeexpected values, and each is analyzed separately, thenthe two deformation events can be distinguished [38].

At Serpent Mound, the other mineral that mightmechanically twin is dolomite. In experiments, twinningoccurs in dolomite at differential stresses of 200 MPa attemperatures between 150 and 500 °C [39]. The lowerbound on the temperature for twinning is poorly

Table 1Results from calcite twinning study

Sampleidentification

Strike and dipof bedding

Trend and plungeof ε1 before tiltcorrection

Trend and plungeof ε1 after tiltcorrection

95SPM1 N74°E 14°S 210.4° 31.2° 205.4° 21.4°95SPM1(PEV) N74°E 14°S 211° 37.7° 204.8° 27.6°95SPM1(NEV) N74°E 14°S 93.6° 36.5° 102° 30.8°95SPM3 Horizontal 211.3° 21.7° 211.3° 21.7°95SPM4 Horizontal 151° 2.7° 151° 2.7°95SPM8 N20°W 74°W 58.5° 15.1° 229.4° 56.9°95SPM12 Horizontal 296.1° 12.4° 296.1° 12.4°

NA = not applicable; NEV = negative expected value; PEV = positive expe

constrained, since the other low temperature experi-ments are at 20 and 24 °C, where twinning is absent[39,40]. Field studies are consistent with the 150°Clower bound temperature for mechanical twinning ofdolomite [29,41–43]. However, this result might be anartifact of high differential stresses required for twinningand the change in the strength of the lithosphere withdepth [e.g., [44]].

Sphalerite within and adjacent to Serpent Moundcontain primary and pseudosecondary fluid inclusionswith homogenization temperatures between 85 and165 °C [45] and thus minimum temperatures for sphal-erite formation. Also faults at Serpent Mound crosscutsphalerite mineralization and are healed by sphalerite[18], suggesting that sphalerite formation brackets thetime of impact. Thus, temperatures at the time of impactmay have been favorable for dolomite twinning.

4. Procedures

Twelve oriented limestone and dolomite sampleswere collected from outcrops within and around SerpentMound for twin analysis (Fig. 1). From these samples,two mutually perpendicular thin sections were made fortwin analysis. Crystallographic C-axes and poles to twinplanes, twin densities and twin thickness were measuredusing aU-stage. Strain and C-axes were calculated usingthe program TWIN [27].

PDFs in quartz are an important criterion foridentifying meteorite impacts. To this end, sandstonesamples were collected. Samples came from outcrops ofBerea Sandstone (Lower Mississippi). Six samples arefrom a monomict breccia composed of Hillsboro San-dstone (Upper Devonian). These samples are from theclosest mapped occurrences of sandstone to the center ofthe structure. Thin sections of these samples were madeand examined.

Projectedelevation beforetilt correction(m)

Projectedelevation aftertilt correction(m)

Valueof ε1

PercentNEVs(%)

Differentialstress(MPa)

2120 1370 1.2 42 1152750 1820 3.5 NA NANA NA 1.6 NA NA1610 1610 1.3 30 145NA NA 1.6 17 70−230 1350 12.6 14 260900 900 2.2 18 90

cted value.

Fig. 3. Shows fracturing in a thin section of Hillsboro Sandstone, amonomict breccia. The F with the dark arrowhead marks conchoidalfractures. The dark mineral cementing the rock and staining fractures islimonite. The scale bar is 100 μm.


Detrital quartz is present in the carbonate rocks.Samples 95SPM8, 95SPM9 and 95SPM10 wereexamined in detail for the presence of PDFs and PFs(planar fractures). Samples 95SPM9 and 95SPM10 aredolomites from opposite limbs of an outcrop scale fold.Two different sampling methods were used to findquartz PDFs and PFs. First, the thin sections of sample95SPM9 were examined on a U-stage at 375X. A seriesof parallel linear traverses were made using the U-stageSchmidt guide. When a low birefringence grain wasencountered, the grain was examined for planarstructures (fluid inclusion planes or more-or-less planarfractures) by tilting through the complete range ofangles on both the N–S and E–W axes. If a planar

Fig. 4. Plot of the orientation of planar structures in quartz. Thirty-two grainplanar structures are present in these grains. Note that the structures are random

structure was encountered then the pole to the structureand the grain's C-axis were measured. For samples,95SPM10 and 95SPM8, thin sections were examinedalong a series of parallel-linear traverses using a pointcounting stage at 25X. If a grain was thought to be agood candidate for PF or PDFs, then the grain wascircled. These grains were examined and measuredusing a U-stage.

5. Results

Twinning is absent in all dolomite samples. Table 1and Fig. 1 show the calcite twinning data. Fig. 2 showsthe appearance of calcite twins in thin section. In theHillsboro Sandstone, fractures are conchoidal (Fig. 3).Limonite cements fracture zones and coats grain scalefractures. PDFs are absent. In Berea Sandstone thinsections, deformation–lamellae and undulatory extinc-tion are the dominant deformation features. Only a fewfractures cut these sections.

In the carbonate samples detrital grains of quartz andfeldspar are rare, ≈1%, and range in size from 10 to60 μm. In samples 95SPM8 and 95SPM10 systematictraverses at 25X resulted in six grains being selected forfurther U-stage analysis. These grains are small(≤20 μm,), biaxial, (2V=41° to 94°), and the fine,closely spaced features are twins. These grains are allfeldspars. For sample 95SPM9, 202 grains wereexamined for ‘planar structures’. Thirty-two of thesegrains have these structures. These ‘planar structures’occur in isolation rather than as parallel features spaceda few microns apart. There is no strong preferred

s out of 202 grains have planar structures. Thirty-seven orientations ofly oriented. Of the PF and PDFs orientations c,ω and π are not present.


orientation of these structures (Fig. 4). Of the five mostcommon orientations for PDFs, three orientations, c, ωand π, are not presented in the data at all [4]. Of PForientations c is not present and r is weakly developed.These data suggest that PF and PDFs are rare or absentin surface samples from Serpent Mound. Reidel et al.[18] also looked for such features and did not findthem.

6. Discussion

When the tilt corrected trends and plunges of ε1(Table 1) are projected into map view, all except95SPM4 converge at the center of the structure (Fig. 1).The orientation of ε1 in this sample is 151° and 2.7°,consistent with Alleghenian bedding parallel, NW toNNW shortening [25].

Excluding 95SPM4, one can calculate the height (H)of the source of the shockwave,H=DtanP, whereD is thedistance from the sample site to the point where theprojection of ε1 passes closest to the center of the structureand P is the plunge of ε1. At Serpent Mound if the heightestimate is not corrected for tilt of bedding, elevations forthe explosion source range from −230 to 2750 m(Table 1). After correcting for strike and dip of bedding,the source height ranges from 900 to 1820 m above theSilurian/Ordovician limestone. The improvement inconvergence after tilt correction is consistent with thehypothesis that twinning mainly occurred as the initialshock wave passed through the sample.

Sample 95SPM1 has 42% NEV, so data sets withpositive and negative expected values were separatedand reanalyzed. The tilt corrected positive expectedvalues yield a ε1 orientation of 204.8° and 27.6° similarto the complete data set, 205.4° and 21.4° (Table 1). Thenegative expected values yield a bedding corrected ε1orientation of 102° and 30.8°. This orientation is notsub-parallel to bedding and it projects away from thecrater center, so it is not due to Alleghenian shortening.It is easily distinguished from the strain generated by theinitial shock wave, since it is smaller 1.6% versus 3.5%(Table 1).

Since ε1's convergence above the structure, it is notvolcanic. As noted earlier, one distinguishes a shallowgas explosion from an impact by comparing the energydensity of a gas explosion with the calcite-twin-recordedenergy density. From the definition that the change inwork equals the mean force times the change indistance, and following the approach of Means [47],the energy density is:

DW0 ¼ ð1=2Þriei ði ¼ 1; 2; 3Þ ð1Þ

Where σi and εi are, respectively, principal stresses andstrains.

The derivation of Eq. (1) assumes that stress-and-strain are linearly related. Since calcite twinning is strainhardening, (1) gives too high an estimate. Strains in thisequation are taken to be infinitesimal (b1%), whereasthe calcite recorded strains may be 10% to 15%. Theerror introduced by this assumption is probably b15%.In addition, the energy contributions of, the kineticenergy imparted to the rock during impact, the change ingravitational potential energy due to change in height,heating during deformation, energy release duringfracture and grain rotation during mechanical twinning,are ignored. Taken together Eq. (1) is a lower boundestimate of energy density. This equation also assumesthat the respective principal strains and stresses areparallel to one another. Fig. 1 shows that C-axesmaximum and ε1 direction for sample 95SPM8 lie closeto one another, so the assumption is reasonable. Shattercones in nearby limestone imply that the Hugoniotelastic limit was exceeded, so σ2≈σ3 [48] so Eq. (1)becomes,

DW0cð1=2Þr1e1 þ 1=2r3ðe2 þ e3Þ ð2Þ

Twinning of calcite conserves volume, so −ε1= (ε2+ε3)[28] and thus

DW0cð1=2Þðr1−r3Þe1 ð3Þ

For 95SPM8 σ1−σ3=260 MPa and ε1=0.13, so theenergy density is 1.7×107 J/m3 (Table 1). [Also, abovethe Hugoniot elastic limit differential stresses areconstant [48]. Based on experiments with carbonaterocks this would imply differential stresses in sample95SPM8 were between 340 and 1200 MPa suggestingdifferential stresses inferred from twinning are low[48]].

The chemical reaction describing a gas explosion isCH4+2O2=NCO2+2H2O, and the energy (heat) re-leased can be calculated from the enthalpy of reaction.The critical factor in determining the energy release isthe source of the oxygen. In silicate minerals and buriedorganic compounds oxygen is so tightly bound to otherelements that one must use unstable, man-madefluorinated halogens to extract oxygen gas [49]. Thus,the source of oxygen in a gas explosion is theatmosphere.

Atmospheric gas at depth at depths of 1–2 km islikely to be hydraulically connected to the surfacebecause its viscosity and density are many orders ofmagnitude less than natural waters [50,51]. This is why


free oxygen gas has never been found trapped below thesurface during oil and gas exploration [52]. Ifatmospheric gas at depth (1–2 km) is hydraulicallyconnected to the surface, air pressure will be ≈0.1 MPa.Thus, one can use the enthalpies of formation for CO2,H2O, CH4 and O2 at 1 atmosphere and 25 °C to calculatethe energy release when all oxygen in 1 m3 of air isconsumed [50]. This calculation gives 9.4×105 J/m3

versus 1.7×107 J/m3 recorded by twinning. Thus,yielding results, which are consistent with earlier workshowing that Serpent Mound is of impact origin. Theenergy density estimate for a gas explosion is an upperbound. It assumes 100% porosity, 100% conversion ofthermal energy into mechanical energy, and that thegases are well mixed in the correct proportions. [Astructure as large as Serpent Mound requires a fewhundred megatons to form, which is too large an energyfor a gas explosion [48]. The above calculation mayonly be useful for a structure ≤100 m in diameter.]

6.1. Independent indicators of erosion

The range of elevations for the shock waves source,900 to 1820 m, implies Serpent Mound has experiencedsubstantial erosion (Table 1). Abels et al. [53] note thatSerpent Mound is one of the few unequivocal impactstructures which are eroded below the true crater floor.In this study, more quantitative macroscopic, meso-scopic and microscopic indicators of erosion areexamined. There are expressions describing the pre-erosion relationship between crater diameter, and thesize and distribution of crater features, e.g., the diameterof the central uplift. Discrepancy between observedcrater diameter and the diameter predicted by theserelations is an independent indicator of erosion.

The smallest circle encompassing all exposures ofOrdovician–Silurian limestone within the crater definesthe diameter of the central uplift (Dcp) which is 3.65 km.From the relationship, Dcp=0.31D

1.02 the predicteddiameter (D) of the crater is 11.2 km [54]. Around theedge of complex craters, there is a rim syncline. Becauseof the eastward regional dip, the radius of the rimsyncline (Rs) was measured in an N–S direction and hasa value of 3.2 km. The radius (R) of the crater is given byR=Rs / 0.6 to 0.75 implying a crater diameter of 8.5 to10.7 km [55]. Taken together these estimates give a pre-erosion diameter for Serpent Mound of 8.5 to 11 km,consistent with extensive erosion.

The absence of PFs and PDFs in rocks from the surfaceat Serpent Mound (this study and [18]) and the presenceof shatter cones, suggest that exposed rocks experiencedpressures of 2 to 5 GPa [4]. The PDFs Carlton et al. [1]

found are from polymict breccias, and thus do not reflectconditions at the present level of erosion. Generally,shatter cones are found as float in a small region withinless than half a kilometer of the center of Serpent Mound[17,56,57] (Fig. 1). This and the low maximum pressurehere are consistent with substantial erosion.

Most of the calcite twin samples are from outside theregion where shatter cones occur. Thus showing thattwin analysis can test the impact hypothesis, even whenthe usual indicators of impact, shatter cones, PFs, PDFs,etc., are absent.

The height of the shock wave source gives a lowerbound on the amount of erosion. To estimate the heightof the source one needs to know the nature of the error.In map view the maximum deviation of the anglemeasured by the line defined by ε1, and the line drawnfrom the sample location to the presumed center ofSerpent Mound is ≤8° (Fig. 1). In experiments onaverage, there is an 8° difference between the true ε1direction and the ε1 direction determined by twinanalysis [30]. The errors in our field study and inexperiments are comparable. Thus, the 95% confidenceinterval of the mean is x̄Fza=2r=

ffiffiffi

np

, where x̄ is themean, zα/2 is the critical value of the standard normaldistribution for a particular confidence level, σ is onestandard deviation and n is the number of samples [58].Using this formula, the source of the shock wave was at1400±390 m at the 95% confidence interval.

Prior to this study and a recent paleomagnetic study[59], the age of impact at Serpent Mound was poorlyconstrained. The youngest deformed unit is theCuyahoga Formation (330 Ma) and the structure islocally overlain by Illinoisan glacial material (0 Ma), sothe age of Serpent Mound is between 330 and 0 Ma.Watts [59] showed that the magnetization of the lowerSilurian limestone (Brassfield) was acquired afterSerpent Mound formed and the magnetization is256+15/−12 Ma. Thus, sediments and paleomagnetismlimit the age of Serpent Mound to 330 and 256 Ma.

The shock wave source is one or two projectilediameters below the surface, which implies that theoriginal ground surface was N1400±390 m aboveBrassfield. Comparing this estimate to stratigraphiccolumns elsewhere in the Appalachian Basin andthermal maturities near the study area, the impact isLower Permian or younger (290 Ma) [60–63]. Burialhistories from a well near the Ohio and West Virginiaborder show the structure is older than 30 Ma [64].

If twinning associated with the Appalachian–Oua-chita orogeny occurred prior to impact, then as sample95SPM4 shows, a ε1 of ≈1% oriented bedding paralleland at ≈150° should be present throughout the Serpent


Mound structure (Table 1). If the Appalachian–Ouachitaevent is present, then samples 95SPM1, 95SPM3 and95SPM12 should record this event, since their strainsare ≈1%. 95SPM12, a rock with moderate porosity hasan NEV b20%, so the NEV can be explained by porecollapse. 95SPM1 has a 42% NEV indicating twoepisodes of deformation. The strain not associated withthe initial shock wave has an orientation of 102°, 31°.This orientation is not consistent with the Appalachian–Ouachita deformation. 95SPM3 has an NEVof 30% andcalculating the orientation of the NEVs gives an e1 of0.3% and oriented 117°, 20°, which again is notconsistent with Appalachian Ouachita deformation.The absence of this event implies Serpent Mound canbe no younger than 240 Ma [36]. Thus, the twin datasuggest that Serpent Mound is between 290 and 240 Maconsistent with the paleomagnetic age, 330 to 256 Ma.The age of impact for Serpent Mound is 290 to 256 Ma.

At other impact craters ages of impact are con-strained, but not the amount of sediment present at thattime. Jackson and van der Voo [65] dated the Kentlandstructure paleomagnetically as Late Cretaceous (100–65 Ma). The sediment thickness during the LateCretaceous in this part of the Midwest is unknown,since the closest Cretaceous sediments are ≈600 kmaway. Calcite twin analysis on the carbonate rocksexposed at Kentland could give this thickness.

Assuming the fluid inclusions of the sphaleriterepresent the conditions at the time of impact, themaximum pressure correction (2 km of burial athydrostatic conditions) is 15 °C [66]. As such, thepossible range of temperatures at Serpent Mound wouldbe 85 to 180 °C. The twins here provide a furtherconstraint on temperatures at the time of impact [67].Sample 95SPM8F has the largest mean twin width of1.3±0.3 μm of all samples. Its average twin intensity is161 twins/mm and J2 is 11.2%. These numbers areconsistent with a temperature ofb170 °C [67]. Thetemperatures at Serpent Mound were likely between 85and b170 °C. If 150 °C is the minimum temperature atwhich dolomite twinning occurs, then the rocks atSerpent Mound might not have been warm enough torecord twinning. Thus, dolomite twinning may only beuseful in studying larger, more deeply eroded structuresthan Serpent Mound such as the Malmani subgroup atVredefort [68,69].

Acknowledgements

I am grateful to Donald Zenger of Pomona College forloaning a U-stage, a long working distance condenserand microscope objectives. Bob Hall of I. U. P. U. I. and

Matt Poslusny of University of Charleston providedaccess to microscopes. Dewey Sanders of MarshallUniversity is thanked for help taking the photomicro-graphs. In addition, I would like to thank George Sears ofThermoset Incorporated of Indianapolis for access totheir saws for precise cutting of oriented samples. R. A.F. Grieve reviewed an earlier version of this manuscriptmaking many helpful suggestions. Comments of twoanonymous greatly improved this manuscript. NSFGrant EAR-9496267 partially supported this study.

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Applications of twin analysis to studying meteorite impact structures

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