McNair Scholars Journal

Volume 1 | Issue 1 Article 6

1-1-1997

Geothermometry Constraints on the Architectureof the Ouachita Mountains in ArkansasCurran KimpGrand Valley State University

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Recommended CitationKimp, Curran (1997) "Geothermometry Constraints on the Architecture of the Ouachita Mountains in Arkansas," McNair ScholarsJournal: Vol. 1: Iss. 1, Article 6.Available at: http://scholarworks.gvsu.edu/mcnair/vol1/iss1/6

Geothermometry Constraints on the Architecture of the OuachitaMountains in Arkansas

Student: Curran Kemp

Mentor: John Weber, Ph.D.

26

AbstractA calcite and quartz microstructurestudy was done to constrain the bur­ial and exhumation history of thePa leozoic ro cks in the OuachitaMountains, Arkansas. Thin sectionswe re pet rographi cally ana lyzed andpa leo temperature est imates derivedfrom them were plotted on a map.These indicate tha t the most thermal­ly mature roc ks crop out in the coreof the Benton uplift. These rockshave experienced grea ter exhumationthan those on the surroun ding flanksof the uplift. Also , the western por­tion of the core area has experiencedgrea ter ex humation than the easternport ion.

IntroductionThe Ouachita Mountains of Arkansasand Oklahoma consis t of deforme dlate Paleozoic roc ks forme d by theco llision be tween the proto-North

Figure 1. Ouachita Mountainstratigraphic column

Note. Age increases upward.

American and proto-Sou th Americancontine nts. The burial, exhumation,and up lift history of these low-grademetamorphic roc ks is largely un ­known, mainly because they lack theindex minerals needed to track subtlevar iations in internal metamorphicgrade (temperature and pressure con­ditions during their formation) . Thepurpose of this study is to determinethe var iations in internal met amorphicgrade of this mountain ran ge usingcalcite and quartz microstru ctures,bo th of which form during deforma­tion and vary systematically with peakdeformation pa leotemperatures.

~icrostructures

Calcite microstructures have bee nstudied systematically by Groshong( 988) , Ferrill ( 991), and Burkhard( 993). Groshong (988) provides anexcellent , general review of low-tem­perature deformation mechanisms forcalc ite and other min erals, wh ichincludes crystal plasticity, twin lamel­lae straining, and straining by latticebending and breaki ng , resulting inundulatory extinction , subgrain forma­tion , and recrystallization. Ferrill (991)studied calcite twin wid ths and intensi­ties, and their relation to peak meta­morphism , strain , and temperature. Hefound that the thickness of calcitetwins depends on peak deformationtemperature in naturally strained , low­grade metamorphic rocks . Burkhard's(993) study provides an exce llentreview of these calcite microstructurestudies , and the relation between thewidths of calcite twins and the temper­ature of deformation . Also, a recentstudy by Webe r et al., (996) puts fur­ther constraints on this temperaturedependence. These papers form thebasis for the calcite microstructure por­tion of this study.

Calci te microstructures can beused to determine thermal historiesof the rocks which contain them.They permanently record the high­es t temperat ur es of deformation

Ouachita Mountains Geo thermometry Constraints

obtaine d by the host rocks. Calcitegrains deform by twinning. Four dif­ferent twin typ es can be distinguish edpetrographically, and essent ia llyrecord temperatures of de formation.Typ e I tw ins are thin (less than 1micrometer thick) and form at 1700 to2000 C. Type II tw ins are thicker, usu­ally abo ut 5 microns thi ck. Type IIcalcite tw ins form during deformationat 1500 to 3000 C. All twin types ca nbe clearly distin gui shed using anoptical mic roscope . Both Typ e I andII ca lcite tw ins are straight with nocurvature at the ir ends. Typ e III twinshave curved ends and are substant ial­ly thicker (» 5 urn) tha n e ithe r Typ e Ior Typ e II twins , and form at > 2000

.

C. Typ e IV twins are recrys tallizedtwins which form at > 2500 C.

In a similar manner, quartz mi­crostruc tures have been divided intothree tempe rature-depe ndent regim esby Hirth and Tu llis ( 992) in their in­depth ex pe rime ntal study of quartzmicrostructures and how they varyw ith temperature and strain rate . Thequa rtz microstruc tu re analy sis in thisstudy follows the distinguishingguidelines outlined in Hirth and Tullis( 992) and discussed below.

In Regime I, deformed quart zgrains sho w only sweeping to undu­latory extinctio n, and subgrains makeup no more than 15 percent of a sa m­ple . In Regime II, subgrains make upmor e that 15 percent of a sample , andmost quartz grains have sweep ingundulatory ex tinc tion. The onset ofneoblast fo rmation , (new, strain-freequartz grains with 1/20th the diarne­tel' of subgra ins), ma rks the tran sitionfrom Regim e I to Regim e II , with thelarger qua rtz grains being progres­sively converted into these sma llernew grains. Gra ins in Regime II havethe most interesting optical cha racte r­istics . They are commonly elongated,cigar-shaped , and boudinaged.Regime III mark s a high-tempe ratureenviro nment, w here neoblasts obtainlarger sizes than subgrains and hav eno undulator y extinction becausethey formed by dynamic recry stalliza­tion to relieve strain at high tempera­tures. Exce pt for the study of Weberet al., ( 996), the temperatures oftransitions of quartz microstructura l

reg imes for naturally deformed rocksis largely undocumented. The data ofWeber et al., (996) constrains thatthe Regime I-Regime II tran sitiontemperature is >2000 C, and be tween230 0 - 330 0 C in nature (w ith theupper bound poorly co nstraine d) .

MethodologyGe lleralDuring the first week of March, 1996,the research team of Dr. John \XTeber,Jill Ralston , and Norm Mannikko, ofGrand Valley State University, collec t­ed sp ecimens from 17 different loca­tions in the Ouac hita Mountain s forth is ca lcite and qu artz survey .

The formation nam e and geo log icage was determined for the roc ks co l­lected at each site from the Arka nsasgeolog ic map. The number of sam­ples for this study is small; it is meantto be only a reconnaissan ce study . Amore detailed survey should followbecause this reconnaissance studyshows that applying the se new tech ­niq ues in the Ouac hita orogenic beltyie lds so me very useful informationand new geolog ic insights.

Field samples were oriented usinga compass. Oriented samples caneas ily be studied geometri cally underthe microscope to reveal the rockfabric , microstructu res, and ot herlarge- scale stru ctures of the rock . Fo rthis study the samples were cut into2 em x 4 cm x 1 em blocks. Theywer e shipped to an outside firmwhich mad e pe tro log ic thin se ctions(30-micron-thick slices of rock gluedto glass slides) . The thin sectio nswere then viewed an d studied undera pet rologic microscope , w ith a mag­nification ran ge of 20x to 400x, unde rboth plain and polarized light tocharacterize the calcite and quartzmicrostructures . Forty-eig ht th in sec­tions were mad e from the samplesan d studied.

Ca lcite MicrostructuralGeotbermomet ry

Calcite microstructures were char­acterized by determining the specifictyp e of twinning in each thin section.Th e ca lcite twin thi ckness wasobtained qu alita tively by visual ins­pection , comparing their thickness

with that of the microsco pe's crosshairs, which are « 1 urn .

Quartz Microstructura lGeotb er mometry

The qua rtz microstructures ob­se rved in the specimens werecategorized into the thre e subdivi­s ions (Regime I, Regime II, RegimeII I) of Hir th and Tullis (1 992) .Regim e I qua rtz has large and smallqua rtz grains w ith sweeping undula­to ry extin ction , bu t w ith only raresubgrains occurring w ith in singlegrains. In Regime II, abunda nt quartzsubgra ins oc cur within larger quartzgra ins , and sma ll, strain-free, recrys­tallized quart z grains ca lled neoblastare also present along grain and sub ­grain boundaries . The presence orabsence of suc h neoblast categorizesa rock 's qu art z microstru ctures asbeing eithe r Regim e I or Regime II .Regime III rocks cons ist entirely ofver y large recryst alli zed quartzneoblasts that are co mplete ly free ofstrain .

ResultsThe dat a revealed that only Type Iand II calcite tw ins an d only Regime Iand II qua rtz microstructures are pre­se nt in the sp ecimens. This ind icatesthat rocks in the Ouachita Mountainswere not deformed at very hig h tem­peratures , relat ively spea king.

Type I calcite is found only alongthe outer rim of the O uachitaMountai ns. one is found in the co reof Benton uplift , where the oldest andpresumably most deformed and meta­morphosed roc ks cro p out. Typ e Icalc ite fo rms at temperatures less than2000 C (Burkha rd, 1993). The co re ofthe Benton uplift co nta ins mainl yType II calcite, which forms at tem­peratures of 1500-3000 C, slight lyhigh er than that of the Type I tw ins.Similar results ex ist for the qu artzmicrostructures. The co re of the rangecontains Regime II qu art z microstru c­tures , while on the flank s of the co rearea both Regime I and Regime IIquartz microstru ctures are located.

In comparing these thermal "matu­rity" results wi th the stratigraphic (seeFigure 1) and structural position of thesamples, the yo unges t unit in the

GVSU McNa il'Sc!Jo /a rs J o l/ r ll a / VOLUME 1.1997 27

Figure 2. Map showing the location of the Ouachita Mountains.

and co ntain Regime I and Regime IIqu artz microstruc tures, respectively.Sample 96-Sb-16-3 co ntains Regime IIquartz microstructures.

The Polk Creek Shale, BigforkChe rt, and the Womble Sha le makeup the "middle-aged" rocks in theco re area. One samp le (OWN-96)was co llected from the Womble,w hich co ntains Regime II quartzmicrost ructur es. The Crystal Moun­tain Sandstone is slightly olde r, and inthe central part of the core area . Thetwo samp les co llec te d fro m th eCrystal Mountain sandstone , 96-0CM­8 and 96-cv , bo th co ntain Regime IIquartz microstruc tures .

The oldest formation ex posed inthe Ouac hita Moun ta ins of Arkansasis the Collier Limestone , which cro psout in the heart of the core area . Eightdifferent thin sec tions we re cut fromCollier samples and divided intono rt he rn and western gro up s. Thetwo sam ples from the northe rn gro upbo th have Regime I and II quartzmicro-stru ctur es and Type I and IIcalcite twins . These indi cate that thepeak temperatures these specime nsex perienced was 150°-300° C, proba­bly abo ut 200° C. In the west, sample96-oC-5 co ntains so lid Regime IIqu artz microstructures and Typ e IIcalc ite twin. This is the most the rma l­ly mature speci me n, which was cer­tainly heated to > 200° C.

Discussio n and ConclusionsThe pa ttern of paleotemperaturesobta ined from calci te and quartzmicrostructures as determined in thisstudy are consistent with the interpre­tation that the most deeply buriedrocks have been exhumed, and arenow exposed, in the core of theBenton uplift. The calcite twins werefou nd to be either Type I or Type IItwins . Type III and Type IV twins arenot present because the burial depthof Ouac hita rocks were insufficient forthem to have formed. It is my predic­tion that Type III calcite twins cou ldbe found just be neath the surface inthe core area of the Benton uplift.

Arne 's (992) apatite fission trackstudy indicates no thermal differencebetween Paleozoic samples close toCretaceous intru sives (see Figure 2)and such samples distant from bod­ies . The sma ll size and qui ck intru­sion rate might explain the lack ofa regional effect. For example, theMagn et Cove pluton is very smallin area , and cooled very qui cklybecause of its sma ll size. Based onthis evide nce, the the rmal grade westudied across the Paleozoic rocks ofthe Ouachita Mou nta ins refle ct theeffects of burial alone, not plutonism.So without great buria l depths ,Type III and Typ e IV calcite couldnot have developed in these rocks.

Regime I and II quartz microstru c­tures are both present in the study.The lack of Regime III quartzmicrostructures in these rocks is con­sistent with the interpretation above .Only mo de rate temperatures wereatta ined by our samples . Rocks of theOuachita Mountains hav e never beendeeply buried. Because the heating ofthe samples is not due to plu tonism,the burial tha t these rocks must haveexperienced sho uld have been of suf­ficient len gth in time tha t quartzwould ha ve transformed fromRegime II to III , and Type IV calci tetwins would have formed, if highenough temp era tures we re reached .In summary, they never attained"super-deep" metamorphi c burial.

The coexistence of Type II calcitetw ins and Regime II quartzmicrostruc tures shows that the pa le-

35 '

PR EFLY SCHA rkan ••• No va cu l it ea n d old. r ro ck a

'"

LOWER PALEOZO IC

.. '

UPPER PA LEOZ OIC

1 0 2 0 3 0 40 50, , I , ,

.. '

CENOZ OI C andME SO ZOIC

study area came from the Jackfork for­mation. On ly one samp le was collect­ed from the Jackfork formatio n. Thissamp le location is the northernmostone , well out of the core area . Thissample con tains late Regime I to earlyRegime II quartz microstructures.

From the Stanley sands tone , thenext oldest un it, three samples we recollected from different locations.The maturity of these var ies . Sam ple96- Ms-18 and 96-Ms-15-2 both haveRegime I qua rtz microstru ctures andType II calcite indicating a thermalmatu rity of <200° C. Stanley samplesfrom the so uth part of the study area ,however, contain no calcite and onlyRegime I quartz microstructures. Boththe Jackfork Sandstone and StanleySha le crop out on the flan ks of thecentral core of the Benton uplift.

Samp les we re collected from theArkansas Novaculite in the westernportion of the range. Regime I qu artzmicrostruc tures were observe d insamp le MDA-1-CG. Located strati­graphica lly beneath th e ArkansasNovaculite is the Missouri MountainSlate and the Blaylock Sands tone ,which make up some of the youngestformations exposed in the core area .Three samp les we re collected fromthe Blaylock Sandstone : 96-0B-12,96-Sb-16-3, and 96-0B-12-A. Samples96-0B-12 and 96-0B-12-A are fromthe eastern portion of the study area ,

28 Ouachita Mount ains Geo thermo me try Cons train ts

otern pe rature estima tes made , usingtwo se pa rate techniq ues , a re inagreeme nt. No te th at th e flyschdeposits flanking the co re area co n­tain Regime I qu artz microstructuresand no calc ite .

An apa tite fission-track study ofArne ( 992) , alth ough co nd uc tedge nerally no rth of this study area,shows that all of his Paleozoic sa m­ples have been hea ted to tempera­tures above 1100 C. The ages of hissamp les show that these rocks cooledth rough 1100 C during the LateCretaceo us or Early Tertiary per iod .

We also noted that so me possiblesystematic differen ces in peak pa le­otempe ratures ex ist within the coreof the Benton upl ift. Samples fromwestern o utcrops of th e Sta nleySandstone indica te high er tempera­tures than those from the eas t. Also,microstructures from the Blaylocksa ndstone sho w that it is slightlycoole r in the east than the west. But

microstructures fro m the Co llie rLimestone samples seem to show auniform buri al depth, regardless ofeast-to-west position . This may sim­ply mean that the Collie r Limestonewas exhumed from a grea t eno ughdepth and high eno ugh temperaturethat all of its microstru ctures are thesame . These observations aga in showthat the yo unger strata have neverbeen buried to the grea t depths thatthe olde r strata we re.

A ma jor limitation in using calc iteand qua rtz microstru ctu res to deter­mine pa leotemperatures is that mostoften they give only relative , notabsolute , resu lts. Further, the relativepercentages of qu art z and calcite ina sa mple may affec t paleotempera ­ture estimates. To correc t for this ,poi nt counts should be done of thethin sec tions .

AcknowledgementsI would like to thank Dr. John Weber forall his help in deciphering the use of cal­cite and qu artz microstructures and hismentorship throughout the pro ject; HazelCochran and the M CI air Fellowship forfundi ng this researc h; and Dr. RichardLefebvre for constructive editing of thispaper. Also , Collin Plank helped with mystructural interpret ati ons , and SarahTourre and Dr. Patricia Videctich helpedwith my thin-section an alysis. I alsowould like to thank Jill Ralston for hercontribution and discussio ns of fission­track analysis, and I look forward to theresu lts of her study.

ReferencesArne , D.C. (1992). Evidenc e from apatite fission-track analysis for reg ional Cretaceous coo ling in

the Ouachita Mount ain fo ld belt and Arkom a Basin of Arkansas. The Amer ican Association ofPetroleum Geologist Bulletin 76 (3) , pp. 392-402.

Burkhard , M. (1993). Calcite twins, their geom etry , appea rance, and significance as stress-strainmarkers and indica tors of tect onic regime : A review. Iournal of Structura l Geo logy 15 0-5),pp . 351-368.

Ferrill , D.A. (1991). Calcite twin width s and intensities as metamorphic ind icators in natural low­temperature deformation of limeston e . Iournal of Structu ral Geology 13 (6), pp . 667-675.

Grosho ng, R.H., Jr. (1988). Low-temperature deformation mechanisms and their interpretation .Geo logical Societv of Amer ica Bulletin 100 pp . 1329-1360.

Hirth, G., and Tullis, j. (1992). Dislocation cree p regimes in quartz aggregates. Iournal ofStructura l Geol ogv, 14, (2). pp . 145-159.

Lowe, D.R. (1988) . Stratigraphy, sed imentology, and deposition se tting of pre-orogenic rocksof the Ouachita Mountains, Arkansas and Oklahoma. Geolog ical Societv of Americapp . 575-591.

Weber, j.c. , Ferrill, D.A., and Rod en -Tice , M.K. (1996). Calcite and qu artz microstructuregeothermom entry of low-grad e metased imentary rocks of the Northe rn Range of Trinidad .Journal of Structural Geologv. In prep aration.

GVS McNai,. Sc h o lars fournat VOLUME 1. 1997 29