The Geologic Structures Observed in Drillhole DOE‑2 and Their ...

The Geologic Structures Observed in Drillhole DOE‑2 and Their ...

SANDIA REPORTSAND86-1495. UC-70Unlimited ReleasePrinted July 1987I RS-8232-21 t::i,zzoJThe Geologic Structures Observed inDrillhole DOE-2 and Their Possible Origins:Waste Isolation Pilot PlantD. J. BornsC-/111'!I'I~lll~~'I~~~1111111!1~~~!~~~ljll~II~11Prepared bySandia National LaboratoriesAlbuquerque, New Mexico 87185 and Livermore, California 94550for the United States Department of Energyunder Contract DE-AC04-76DP00789SF2900Q(S-S1 )

Issued by Sandia National Laboratories, operated for the United StatesDepartment of Energy by Sandia Corporation.NOTICE: This report was prepared as an account of work sponsored hy anagency of the United States Government. Neither the United States Governmentnor any agency thereof, nor any of their employees, nor any of theircontractors, subcontractors, or their employees, makes any warranty, expressor implied, or assumes any legal liability or responsibility for the accuracy,completeness, or usefulness of any information, apparatus, product, or processdisclosed, or represents that its use would not infringe privately ownedrights. Reference herein to any specific commercial product, process, orservice by trade name, trademark, manufacturer, or otherwise, does notnecessarily constitute or imply its endorsement, recommendation, or favoringby the United States Government, any agency thereof or any of theircontractors or subcontractors. The views and opinions expressed herein donot necessarily state or reflect those of the United States Government, anyagency thereof or any of their contractors or subcontractors.Printed in the United States of AmericaAvailable fromNational Technical Information ServiceU.S. Department of Commerce5285 Port Royal RoadSpringfield, VA 22161

SAND86-1495Unlimited ReleasePrinted July 1987DistributionCategory UC-70THE GEOLOGIC STRUCTURES OBSERVED IN DRILLHOLE DOE-2AND THEIR POSSIBLE ORIGINS: WASTE ISOLATION PILOT PLANTD. J. BornsDivision 6331Sandia National LaboratoriesAlbuquerque, NM 87185AbstractThe Department of Energy is developing the Waste Isolation Pilot Plant (WIPP)for underground storage of transuranic waste produced by defense-relatedprograms. The WIPP underground storage facility will be located approximately650 m below the surface within a sequence of evaporites over 1000 m thick. Thisevaporite sequence is divided into three major units (in descending order): the100-m-thick Rustler Formation; the 600-m-thick Salado Formation in which theunderground facility is placed; the 300-m-thick Castle Formation. After 10years of geological site-characterization, the major effort of this task isdrawing to a close. Still, such questions remain as the origins of evaporitedeformation within the Salado and Castile formations. Two miles north of theWIPP site, a stacked sequence of depressions, defined by marker beds in theSalado, was indicated by shallow boreholes. Such structures raise questionsregarding the role of dissolution and gravity tectonics at the WIPP site. Toinvestigate these structures, DOE decided to drill hole DOE-2 north of the WIPPsite.At DOE-2, the downward displacement of stratigraphic markers in the Saladoconfirmed the presence of stacked depressions, which were the primary target ofthe drilling program. The ha1itic units between the marker beds were thickenedcompared to the average section determined from basin-wide drilling. Ha1iticunits in the lower Salado show evidence of recrystallization during deformationand parasitic fold structures. Such deformation structures and thickenedha1itic units are inconsistent with dissolution within the Salado as the causeof deformation in the basal units of the Salado. The remaining question iswhether dissolution occurred in the underlying Castile and resulted in thedeformation of the Salado. At dril1ho1e DOE-2 and in nearby deep boreholes,complex structures in the Castile Formation are found. One hundred m or moreof halite is expected in an average section of the Castile Formation; however,the only Castile halite section penetrated by DOE-2 is less than 3 m thick. Incontrast, markers in the anhydrite units indicate recumbent structures andthickening of the anhydrite units by folding. As a consequence, the CastileFormation is nearly its average thickness, with the folded thickness ofanhydrite compensating for the missing halite. The nearby thickening of halitewithin the Castile, the absence of relic anhydrite laminae in the attenuatedhalite units, and the high strain fabric of the remaining halite suggest thatdissolution was not the dominant process in the Castile. The favoredhypothesis for the Castile structures is salt flow in response to gravityinversion of the anhydrite and halite units of the Castile.

ACKNOWLEDGEMENTSJ. W. Mercer (SNL) , R. P. Snyder (USGS), and M. Wilson (F&S) performed the bulkof the work during the initial drilling and core logging at drillhole DOE- 2.R. P. Snyder provided invaluable assistance in the detailed description andinterpretation of the structures observed in the DOE-2 core. F. E. Hensleyhelped with core slabbing and photography on numerous occasions. W. Casey(SNL) is thanked for his thoughtful review of this report.ii

ContentsINTRODUCTION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 1Geologic Setting...................................................... 1Processes of Interest for Origin of Deformation 4Dissolution: Processes and Evidence in the NorthernDelaware Basin.................................................... . . .. 4Gravity-Driven Deformation... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 5Syndepositional Basins............................... . . . . . . . . . . . . . . . .. 6SALADO AND CASTILE FORMATION STRATIGRAPHY IN BOREHOLE DOE-2............... 6Salado Formation " 6Castile Formation..................................................... 7MESOSCOPIC STRUCTURES..................................................... 7Salado Formation...................................................... 7Castile Formation..................................................... 7Scale and Timing of Deformation 11DISCUSSION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11REFERENCES " 16Figures1. Stratigraphic Section of WIPP Facility LithologicUnits (adapted from Lambert, 1983).................................... 12. Location of the Delaware Basin and the WIPP Site 23. Location of Noted Drillholes at the WIPP Site 34. Fence Diagram Using DOE-2 and Adjacent Holes 65. P: 3082.4-3082.8, Tight Fold of Anhydrite Laminae 86. M: 3233.4-3233.9, Sharp Fold Hinge in Banded Anhydrite 87. Q: 3168.1-3168.6, Brecciated Dipping Laminae of Anhydrite 88. 0: 3169.6-3170.1, Brecciated Laminae of Anhydrite 89. G: 3798.8-3799.3, Polyharmonic Folds in Finely Laminated Anhydrite.... 910. K: 3227.6-3228.3, Near Horizontal Bands of Anhydrite withGlauberite Overgrowths..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 911. I: 3779.9-3780.4, Folded Dolomitic Laminae in Anhydrite 912. F: 3690.4-3690.8, Anhydrite Bands with Developmentof Pull-Apart Textures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 1013. N: 3081.7-3082.3, Folded Anhydrite Laminae 1014. B: 3231.6-3232.3, Banded Anhydrite with Secondary AnhydriteLaminae at High Angles to Original Banding 1015. E: 3800.8-3801.2, Upper Contact of Halite Zone in the Castile atDOE-2 , 1016. A: 3808.9-3809.6, Lower Contact of the Halite Zone in theCastile at DOE-2 , 1117. Geophysical Well Logs of the Upper Castile in BoreholesWIPP-12 and -13, Displaying the Set of Anhydrite StringersPresent in Halite Units. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 1318. Salt Pillows, Salt Anticlines, and Peripheral Sinks 15iii

INTRODUCTION ~URFICIAL DEPOSIT')--DEWEY LAKE RED .EDSSince the early site characterizationstudies (Powers et al., 1978)of the Waste Isolation Pilot Plant(WIPP), geologists have been awareof a structural depression 2 minorth of the WIPP site center,within the evaporite sequence atWIPP. This depress ion has beennamed informally after a shallowborehole, FC-92. During the springof 1985, DOE drilled hole DOE-2,largely to investigate the structuresassociated with the Fc-92depression. Mercer et al. (1986)document the drilling history andcore description of units encounteredin DOE-2. The objectives ofthis report are to describemesoscopic structures observed inthe drillcore and to relate thesestructures to their possible originsand to the FC-92 depression ingeneral.Geologic SettingThe Delaware Basin of southeasternNew Mexico contains layered evaporites(see Figure 1). An area inthe northern part of the basin hasbeen selected for the WIPP site (seeFigure 2). Rock units of interestin this report are Ochoan, with theexception of the underlying DelawareMountain Group (DMG). The oldestOchoan unit is the Castile Formation,which overlies the Bell CanyonFormation of the DMG. Locally, theCastile consists of three anhydriteunits separated by two halite units.Above the Castile stratigraphicallyis the Salado Formation, which consistsof halite, anhydritic and/orpolyhalitic halite, and argillaceoushalite. Overlying the Salado is theRustler Formation, which containsRUSTLER==============~FORM AT IONUPPERMIDDLELOWER

10$-100-EXPLANATION--- - _. Fault, type unspecified; dastled where inferred,dotted wherl concealed..-............ ........... ......... Thrust foul!; so. tellh an uplhrown side,dalMd where interred, dotted whlre concealedNormal fault; hachure$ on downlhrown side,\~'J"._-1 ~1-_' ~ --- ;\" \e _ >f Y ,y._ 0 I}___ .~I -"../(/~oJ,J-'1"+dashed where jn'erred, dotted where concealed.Elongate, clonly compre",ed anticline;suggesn height, stupn..", or size or foldAllis a' major anticlineA.xis of major syncline.Dome.Boundaries of major slructural elementsEAS1!ERN(From Sley"" 5, Oriel, et 01., 1967)5 H f L FI'V32· r- .. ~" f~::~~: r l " ",61 ~«L ~ f'... \\J'"Ouochita Tectonic B,ll". I \"'" ,,< ~YI -," \ ..... 'f.. ~ '1;, ..~ \ ~. ........ ,,-.~107"f--1-'"/ f&:....-''0' I 1·'0'~o ~O 100 ~KMo 25 ~ 75 IOOMISCALEMAJOR REGIONAL STRUCTURESt2'· .......lOS·104-..'Figure 2,Location of the Delaware Basin and the WIPP Site.

R33E,....-l.,---....I----......--...l.-r-__...l...__......, -'- l-__.,~I;:U>NI-.............. EII)~ I-U>-------1;:I-L.-...,.---r----/o+~er18'U"Reservo"J.---..-l~NEWM~EXICO~"00R25EI R26E R27E I R28E "'R'29~ I R30E TE~ASR31E/6 12IIU>:eI-R30ER31ET22SWIPP SITE..,-----------T23S32'30' ...-""""'-""P"-------------~---103'50'oI1I2 MILESI103"45'...Figure 3.Location of Noted Drillholes at the WIPP Site.3

Processes of Interest for Origin ofDeformationPrior to the drilling of DOE-2(Figure 3), several processes weresuggested as possible origins forthe FC-92 depression. These suggestedprocesses can be summarizeda~ follows:o Dissolution- -subsidence ofSalado markers due todissolution removal ofhalite at the base of theSalado or within the Castile(Anderson, 1978)o Gravity-driven deformation-­flow of salt related to thedevelopment of anti- andsynclines in response toha10kinesis or gravitysliding (Borns et a1., 1983)o Syndepositiona1 basins-­Salado infill basins formedby dissolution or erosion atthe top of the Castile(Borns et a1., 1983).In the sections below, I will discussthe background and history ofthese processes.Dissolution: Processes and Evidencein the Northern Delaware BasinAnderson (1978) concluded that 50%of the original salt in the DelawareBasin was removed by dissolution.He also stated that most of thisdissolution occurred within theSalado. Earlier workers in the areasuch as Bachman (1974) recognizeddissolution features in the region,but Anderson suggested that dissolutionis the major active process inthe area and the process with themost ramifications for the siting ofthe WIPP.Dissolution is inferred from severalsurface features such as the karstterrane along the east side of thePecos River (Bachman, 1983), theNash Draw collapse features(Lambert, 1983), and breccia pipes(Snyder and Gard, 1982). Othertraces of active dissolution are thesalinity gradients in the BellCanyon, which increase to thenorthwest within the basin (Woodet a1., 1983). The presence ofdissolution in the Delaware Basin isnot questioned, but the extent,timing, and process of dissolutionare a matter of debate.Anderson / s (1978) primary line ofevidence for dissolution was theinterpretation of well logs fromthe basin. From these, he believedhe could trace units into regions ofthe basin where major portions ofthe section were missing. Heattributed this removal to deepdissolution. Lambert (1983) andBorns and Shaffer (1985) discuss theambiguities of well log interpretationwith special reference to thedelineation of dissolution structures.Generally, Anderson/s structuresdo not have unique basin-wideinterpretations; e.g., he did notconsider original facies variation,for which there are core and minedata in the Ochoan sequence (Holtand Powers, 1986). Also, some ofAnderson/s structures such as thePoker Lake anticline and synclinepair are not as extensive asoriginally thought (Borns andShaffer, 1985). Still, a sequenceof structures that Andersondelineated suggests the partialremoval of the Salado after deposition.This sequence is the same asthe terrane along the east side ofthe Pecos described by Bachman(1983). Bachman also concludes thatthis terrane, which begins aroundPoker Sink and extends southwardalong the Pecos, represents karstand extensive dissolution by watersof the ancestral Pecos River. Thefluids penetrated the Salado fromthe surface, and the dissolution4

front remained perched on theaquitard formed by the upper Castileanhydrite. Thus, dissolution isenvisioned in a portion of the basinwithin the Salado and is also seenin the advance of Nash Draw withinthe Rustler and upper Salado to thewest of the WIPP site. Borns andShaffer (1985) stated that dissolutionwas not evident within 10 to15 km of the WIPP site, referencedto the location of the structuresalong the Pecos River and NashDraw. However, Davies (1983) hasinterpreted a series of depressionscentered on Drillhole FC-92, 2 minorth of the WIPP site (seeFigure 3). The geometry of thesedepressions of marker beds in theupper Salado is comparable tonumerical models for dissolutionthat Davies produced. He thereforeattributedd the stacked depressionsto dissolution in the Salado, wellwithin the area of interest to theWIPP site.If dissolution is present in thebasin, the important considerationbecomes the rate at which a dissolutionfront can advance verticallyand laterally. Anderson (1978)assumed that essentially all of hissalt removal (50%) occurred in thelast 4-6 million years (rna), whichhe believes corresponds with themajor stage of uplift for thebasin. Bachman (1983) and Lambert(1983) have pointed out that morethan one period of dissolution hasoccurred since deposition of theevapor i te units in the basin.Hence, dissolution may be integratedover a 250·-ma period. For example,Bachman (1983) reported episodes ofdissolution as follows:o Syndepositional1y with theCastile-Salado transition.o At the close of Salado depositionand beginning ofthe Rustler deposition, thewest edge of the sequenceowas uplifted and truncated.This process left a seriesof solution valleys that areinfi11ed with Rustlersediments.In the Triassic, troughs andkarst terrane formed inresponse to the ancestralPecos River.o During the Tertiary, troughsup to 400 m deep formed inTexas.The complexity of deciphering whichdissolution process was active ina given rock unit is apparent.Anderson's mas s removal of salt inthe last several million years givesan upper bound on the rate ofadvance. But if Bachman's sequenceof troughs is related to theancestral Pecos, the process andrate are not applicable to the restof the basin. In terms of the WIPPsite, the rate of advance for NashDraw becomes the concern.Gravity-Driven DeformationIn other salt basins, such as eastTexas and the Gulf Coast, gravitydrivendeformation is manifest,ranging from salt pillows to diapirs(Seni and Jackson, 1983). Bornset al. (1983) demonstrated in anumerical model that gravitationalinstability would arise within theevaporite stratigraphy of the WIPPsite. Many observations have beenmade of flow fabrics on both themeso- and microscopic scales (Borns,1983a; Borns et al., 1983).However, such flow textures may alsobe associated with dissolutionstructures, especially at slowerdissolution rates (Davies, 1983).The distribution of structures bothareal1y and vertically, (i.e.,whether a structure is an anticlineor a syncline) may distinguishbetween the dissolution and gravity-5

driven mechanisms. Lambert (1983)pointed out that anomalous structuresin the northern Delaware Basinare distributed as both highs andlows. In a dissolution model, theassociated anomalies are expected tobe lows.The timing of gravity-driven deformationis not precisely known. Jones(1981) and Borns et al. (1983)suggested that such deformation hasoccurred over the past 70 rna and maybe active at present. Evidence forsuch time brackets are the deformationof Gatuna sediments atDrillhole ERDA-6 (Jones, 1981) andcurrent gravitational instability ofthe Castile density stratification(Borns et al., 1983). The age isalso inferred from the initiation ofbasin uplift over the past 70 rnawith a resurgence in the last 6 rna.Syndepositional BasinsSnyder in Borns et al.(1983) observed that thehalite units of the lowerSalado thinned overstructural highs (e.g.,Borehole WIPP-11) andthickened over structurallows (e.g., BoreholeB-IOJR) in the Castile.However, Snyder noted exceptionssuch as BoreholeERDA-6, where the Saladowas not thinned over aCastile high. Still,Snyder put forth the conceptthat the Salado wasdeposited during the deformationof the Castileor closely thereafter. Inthis hypothesis, the lowerSalado structures resultedfrom accumulation in formingbasins, which hadbeen filled in or ceasedto be active by the timethat the middle Salado wasdeposited.-500SALADO AND CASTILE FORMATIONSTRATIGRAPHY IN BOREHOLE DOE-2Salado FormationThe presence of the structuraldepression, as discussed in theobj ectives of the borehole, wasconfirmed by the drilling project.The distinctive marker beds of theSalado are lower relative to depthsin adj acent boreholes WIPP-ll, -12,and -13 (see Figure 4). The amountof relative displacement in depthfor these marker beds in DOE-2increases with depth compared toadjacent holes. Between thedownwarped marker beds within theSalado, the halitic units thickenwith depth relative to the samesection in adj acent holes. Hence,the borehole penetrated a structurein the Salado that results in amarked downwarp of marker beds,which is accompanied by thickeningof the interlayered halitic sectionof the Salado Formation.EROA-9WIPP-12DOE-235003408 3422....-- 5160 ----..,-..--- 5830 -------......--- 3890 _-1000WIPP-11,,,.Figure 4. Fence Diagram Using DOE-2 andAdj acent Holes.-90'T.D.6

Castile FormationThe Castile section as observed inDOE-2 departs distinctly from thestratigraphy exhibited in adjacentboreholes WIPP-II, -12, and -13.The upper anhydrite that wasencountered in DOE-2 was 1.5 to 2times thicker than in neighboringboreholes (see Figure 4). Incontrast, only one halitic unit wasobserved within the Castile atDOE-2. This unit was less than 3 mthick and, therefore, significantlythinner than in adj acent holes.This halite unit was markedly clear,with no observable relics of theanhydrite stringers that are observedin Halite II and I in otherholes. Below the halite unit, thelower anhdrite unit of the Castileappears to be a normal section ofAnhydrite I relative to the adj acentholes.MESOSCOPIC STRUCTURESSalado FormationOther than the correlated downwarpof marker beds and the thickening ofthe hal i t: ic section, mesoscopicstructures such as small-scale(second-order) folds and extensionfrac tures are not evident in theupper and middle portions of theSalado Formation. In the lowerSalado, extension fractures of nearvertical orientation are observed inclay and anhydrite interbeds. Suchfractures are infilled with crossfiberhalite. We have been unableto determine whether this halite isoriginal or a pseudomorph of gypsum.The lowermost subunit of the Saladois the Infra-Cowden Halite, whichthickens significantly in BoreholeDOE- 2. Laminar markers in thishalitic unit can be traced throughoverturned fold hinges with subhorizontal axes. These laminarmarkers display a range of dips from90 to 00. Smalleranhydrite stringersexhibit pull-apartptygmatic folding.Castile Formation«1 em wide)in the halitestructures andThe upper 3 m of Castile anhydritepenetrated by Borehole DOE-2 arecharacterized by several zones ofnear-vertical laminated anhydrite.The steeply dipping laminae changeabruptly to a horizontal orientation.Tight folds or sharp changesin orientation of banding can beseen in Figures 5 and 6 (3082 and3233 ft, respectively). As mentionedby Borns (1983a) in the state-lineoutcrops of deformed Castile Formation(Kirkland and Anderson, 1970),several generations of structuresmay be present, including productsof syndepositional deformation. Theapparent fold structures of thesetwo plateswith their evidence oftruncation of fold limbs and absenceof similar structures in the intervalimmediately below suggest suchsyndepositiona1 soft-sedimentde formation. The occurrence ofbrecciated laminae (Figures 7 and 8,3168 and 3169 ft) may also be evidenceof interstradal deformationduring deposition of the unit.Basically, these observations do notprove a syndepositional origin forthese structures, but do suggestthat the structures are not allhalokinetic or tectonic in origin.Howeve r , the re exis t meso - andmicrostructures, as observed in thecore of DOE-2, that are characteristicof deformation consideredhalokinetic in neighboring holes(e.g., WIPP-11 and WIPP-13) (Bornset al., 1983). The polyharmonicfolds of finely laminated anhydrite(Figure 9, 3798 ft) are typical ofsuch structures. With folding,pull-apart structures develop andare infilled with secondaryanhydrite, which forms subparallel7

Figure 5. P: 3082.4-3082.8. TightFold of Anhydrite Laminae (secondaryanhydrite aligns perpendicular tothe original banding; circulardarker grey spots are glauberiteovergrowths) .Figure 7. Q: 3168.1-3168.6, BrecciatedDipping Laminae of Anhydrite.Figure 6. M: 3233.4-3233.9, SharpFold Hinge in Banded Anhydrite (dipof fold limbs goes from vertical tohorizontal at bottom of the coreslab; large patches of glauberite[darker grey] replaced anhydrite).Figure 8. 0: 3169.6-3170.1, BrecciatedLaminae of Anhydrite (matrixinfilled with secondary anhydriteand possible glauberite).8

Figure 9. G: 3798.8-3799.3, PolyharmonicFolds in Finely LaminatedAnhydrite (pull-aparts of bandingformed coeval with folding; anhydriteinfills the separation thatdeveloped; these infillings crossthe laminae at high angles; suchtextures are typical of deformedCastile Formation units in neighboringdrillholes WIPP-1l and WIPP-l3).vein networks. These pull-apartstructures are characteristic ofthis style of deformation in theCastile. Figures 10 and 11 (3228and 3780 ft , respectively) showstyles of pull-apart in whichdolomitic laminae are extendedeither in a fold or along a plane.Other pul1- apart structures(Figure 12, 3690 ft) open a separationthat is infilled with halite.Nearly all these structures suggestsome fluid migration during deformation,accompanied by recrystallizationor replacement of the mineralassemblage. Such textures are thereplacement of anhydrite byglauberite and the development ofsecondary anhydrite in pull-apartstructures. These textures includethe development of secondaryanhydrite in a parallel fabric athigh angle to the primary banding orlamination (Figures 13 and 14, 3082and 3232 ft, respectively). Thissecond orientation is observed toparallel local fold axes and mayFigure 10. K: 3227.6-3228.3, NearHorizontal Bands of Anhydrite withGlauberite Overgrowths (inter1ayeredwith the anhydrite bands are darkerdolomitic laminae that show pullaparttextures).Figure 11. I: 3779.9-3780.4,Folded Dolomitic Laminae inAnhydrite (dolomitic laminae exhibitpull-apart texture; pull-aparts areinfi1led with secondary anhydrite).represent a form of crenulationcleavage. With depth, the upperanhydrite of the Castile at DOE-2becomes more massive, except forrare do1omitic laminae. Hence,folds or horizons that were oncehalite (if either existed) aremasked by the now massive anhydriteunit. The contac t between this9

Figure 12. F: 3690.4-3690.8, AnhydriteBands with Development ofPull-apart Textures (halite infillsthe separation; some brecciation ofthe bands can be observed).Figure 14. B: 3231.6-3232.3, BandedAnhydrite with Secondary AnhydriteLaminae at High Angles to OriginalBanding.Figure 13. N: 3081.7-3082.3, FoldedAnhydrite Laminae (recrystallizedanhydrite in laminae aligns parallelto axial plane of folds; recrystallizedanhydrite overgrows primarylaminae on one fold limb).anhydrite and the halite below dips20-30 0 • Laminae in the anhydriteare conformable to this contact(Figure 15, 3801 ft). Halite at thecontact is uniform in grain-size andforms a highly lineated shape fabricthat dips parallel to the contact.Below this contact, the halite isthe only halitic unit encountered inthe Castile. The unit is only 3 mFigure 15. E: 3800.8 - 3801. 2, UpperContact of Halite Zone in theCastile at DOE-2 (contact is conformableto fine laminae in theanhydrite above; halite at the contactis uniform in grain size andforms a lineated shape fabricparallel to the dip of the contact).or less thick and represents amajor attenuation of halitic unitsin the Castile. The halite isgenerally clear and displays adistinct shape fabric as above. Thelower contact of the halite dipsparallel to the contact above,20-30 0 • The laminae in the underlyinganhydrite and lineation of10

the halite above are conformable tothe contact (Figure 16, 3809 ft).The anhydrite below the contactcorrelates to Anhydrite I, completewith laminations with minor crenulationsinternal to the unit.Figure 16. A: 3808.9-3809.6, LowerContact of ·the Halite Zone in theCastile at DOE-2 (contact is againconformable to the fine laminae inthe anhydrite below [probablyAnhydrite I]; lineated shape fabricagain has developed in the halite).Scale and Timing of DeformationAn estimate of the amount of straindue to the formation of the DOE-2structures can be made by tracingmarker beds from adj acent boreholes.The strain can be approximatedby calculating the change inradius of a bent beam while assuminga c yc1 i ndrical fold shape (Price,1966). Calculations made for theSalado Castile contact and the topof Anhydrite II at DOE-2 indicatedstrains of 0.73 and 0.95, respectively.These strains are on thesame order as strains determined atERDA-6 (Anderson and Powers, 1978),but are greater than strains calculatedat WIPP-12 (Borns et al.,1983). Borns (1983b) and Jacksonand Talbot (1986) have calculatedthe strain rates for salt pillowgrowtg af being between 10- 14 and10- 1 s·. At such rates, thedevelopment of the strains in theDOE-2 structures would require 2 to30 maoThe upper anhydrite unit of theCastile exhibits complex fold shapesand associated structures (e. g. ,sharp truncation of fold limbs andsharp changes in orientation at thefold hinge associated with becciadevelopment) . These observationsraise the question of whethersyndepositional processes wereactive during portions of therecorded deformation. This wouldfollow the hypothesis of Snyder asstated in Borns et al. (1983). Incontrast, the development of pullapartstructures with cross-fiberinfillings within the Salado andCastile units at DOE-2 is evidenceof a postdepositional deformation.Borns (1983a) discusses textures inportions of the Castile that areevidence of both syndepositional andpostdepositional deformation withinthe same outcrop. Hence, withseveral stages of deformationevident, we must be careful not toplace too much emphasis on singlestructures. Generally, the onset ofdeformation of the evaporite sectionin the Delaware Basin is assumed tobe related to uplift of the Basin inthe last 30 ma (Anderson, 1978;Borns et al., 1983). Borns (1985)pointed out that while the 30-mauplift is a major event, othersignificant uplifts occurred duringthe Mesozoic and at the Cretaceous­Tertiary boundary. Each uplift mayhave triggered an episode ofdeformation.DISCUSSIONThe stratigraphy of unitsintersected by Borehole DOE-2 andthe structures that developed withinthem confirm the structuraldepress ion indicated by earlier11

oreholes such as FC-92. In theSalado Formation, this depression ischaracterized by the downwarp ofdistinctive marker beds. Thickeningof halitic units between marker bedsincreases downward, accompanying thedownwarp. Within the CastileFormation, the stratigraphic andstructural relationship are notdistinct. The halite units of theCastile are markedly attenuatedrelative to other holes in theregion. Anhydrite I appears to beintact, but the boundary between theupper anhydrites is indistinct. Theremnants of Anhydrites III and IIare thickened by tight folding andbrought into direct contact witheach other by deformation.As mentioned in the introduction,dissolution and gravity-driven deformationare two possible causes ofthe structural depression at DOE- 2.If it occurred at DOE-2 in the lastmillion years, dissolution would besignificant relative to the adj acent(2 mi) WIPP site. However, ourresults indicate it unlikely thatthe depression was formed by dissolutionin the Salado. Dissolution ismarked by the removal of salt, butthe salt section of the Salado atDOE-2 is thickened. Near absence ofhalite in the Castile suggests thatthe dissolution hypothesis be examined.In Halite I and especiallyHalite II in nearby holes, there arenumerous anhydrite stringers 1 cm to1 m wide (Figure 17). These stringersoccur at consistent stratigraphicpositions within the halites.If dissolution of Halite II and Ioccurred, these stringers shouldremain within the horizon, but notnecessarily intact. Examination ofthe core from DOE-2 has not shownrelic stringers or remnants withinthe Castile.The only evidence for dissolution isthe thinned halite section, butthinning of a halite unit need notbe due to dissolution. Halite canalso be thinned by deformation. Thecross-section of Salado and Castilestratigraphy (see Figure 4) fromDOE-2 and adjacent holes shows thatthe Castile salt is anomalouslythickened at Borehole WIPP-ll, 1 minorth of DOE-2. Within this portionof the Delaware Basin, the CastileFormation deforms into a series ofanti- and synclines (Borns et al.,1983). Salt flow accompanies suchdeformation. Thickening of salt atWIPP-1l will be accompanied bythinning elsewhere. In some cases,the area of removal is called asalt-removal basin or a peripheralsink (Seni and Jackson, 1983). Thestructure at DOE-2 may represent asalt removal basin (Figure 18).Salt flowage is consistent with thestrongly lineated fabric exhibitedby halite remaining at DOE - 2 . Theelongation of the halite grains mayrepresent the direction of flow.In summary, we conclude that thestructural depression intersected atDOE-2 formed in response to gravitydrivensalt flow, as suggested bydistribution of salt structureswithin adjacent holes and thedistribution of structures withinthe Castile. Dissolution is notfavored, due to the thickening ofsalt in the Salado and the apparentabsence of residues or relics ofinsoluble portions of the Castileand Salado Formations within thethinned units.12

II_"_1Pl,AIIIIA ."'1'10 ....I 'coiI'OROSITY lULl( DENSITY D.ILLIItK TillEI .... 1/#'.'" K &._'~CClIPI...,-n.~~... ""/tc -",.r.. ... : .. ~ 0 '0"'0 .~ .. to ..[it£IIARaS:>l t.....-.. iF(woe 't.CIXI'•..,. '00.3--:-"'T~.~~_"m10: .. >I i) :~""N'")~~ ? I(err~If=o.~•'0:::§~II--••",t--_..~2 =~.....t;>s.. IX ftj

! ..,_n 1.11. In =!! i : ""..nlin IllIInI ..LlIli "I.I i. _M .. .... .E = I r ...".,......"'.n........." - ...," .,,,1---4;--I;---tff,-'===-'-III:r.-'-_-'---~"'~rt-t-.~= rsf..-:---::..:L.._-..:'L-.·=~r==i~...=====~..=====::::;i,.~. :1=~'";:;"i:::i1tIII""'. . It I 1----1I!ol. ;I-__ Ir:l,.rt=~-+'4-l'~ ..-i5;,­I--f--,'; r.ilIo-;......+-1..._.

EXTRUSIVESALT DOMENAMAKIERsfI ....DETACHEDDIAPIR'~12-Figure 18. Salt Pillows, SaltJackson and Talbot,Anticlines,1986).and Peripheral Sinks (after15

ReferencesAnderson, R. Y. (1978). Deep Dissolution of Salt. Northern DelawareBasin, Report to Sandia National Laboratories.Anderson, R. Y. and D. W. Powers (1978). Salt anticlines in Castile­Salado sequence, northern Delaware Basin, Geology and MineralDeposits of Delaware Basin and Adjacent Areas, New Mexico Bureauof Mines and Mineral Resources, Circular 159, 79-84.Bachman, G. o. (1974). Geological Processes and Cenozoic History Relatedto Salt Dissolution in Southeastern. New Mexico, U. S. GeologicalSurvey Open-File Report, 74-194.Bachman, G. o. (1983). Regional Geology of Ochoan Evaporites. NorthernPart of the Delaware Basin. New Mexico Bureau of Mines andMineral Resources, Open-File Report 184.Borns, D. J. (1983a). Petrographic Study of Evaporite Deformation nearthe Waste Isolation Pilot Plant (WIPP), SAND83-0l66, SandiaNational Laboratories, Albuquerque.Borns, D. J. (1983b). Kinetics of evaporite deformation, GeologicalSociety of America, Abstracts with Programs, 15, 530.Borns, D. J. (1985) . !;JM~a.!:.rk~e=.r~....:!B~e:::.l:d~..i:lo::!.3.::..9.... ; ---!A~-=S=..l:t:..l::u~d~y---:o~f~...!:D~r~i~l~l~c~o~r..:;::e---,f!::.Jr!:..lo:!.!.m!!.-.s.aSystematic Array, SAND85-0023, Sandia National Laboratories,Albuquerque.Borns, D. J., L. J. Barrows, D. W. Powers, and R. P. Snyder (1983).Deformation of Evaporites near the Waste Isolation Pilot Plant(WI PP) Site, SAND82 -1069, Sandia National Laboratories,Albuquerque.Borns, D. J. and S. E. Shaffer (1985). Regional Well-Log Correlation inthe New Mexico Portion of the Delaware Basin, SAND83-l798, SandiaNational Laboratories, Albuquerque.Davies, P. B. (1983) . Assessing the potential for deep-seated saltdissolution and subsidence at the Waste Isolation Pilot Plant(WIPP), prepared for the State of New Mexico Environmental GroupConference, WIPP Site Suitability for Radioactive Waste Disposal,May 12 and 13, 1983, Carlsbad, New Mexico.Holt, R. and D. Powers (1986).Shaft, DOE-WIPP-86-008,Project.Geotechnical Activities in the ExhaustU. S. DOEjWaste Isolation Pilot PlantJackson, M. P. A. and C. J. Talbot (1986).and dynamics of salt structures,Bulletin, 97, 305-323.External shapes, strain rates,Geological Society of America16

Jones, C. L. (1981). Geologic Data for Borehole ERDA 6. Eddy County. NewMexico, Open-File Report 81-468, United States Geological Survey.Kirkland, D. W. and R. Y. Anderson (1970). Microfolding in the Castileand Todi1to evaporite, Texas and New Mexico, Geological Societyof America Bulletin, 81, 3259-3282.Lambert, S. J. (1983) . Dissolution of Evaporites in and around theDelaware Basin. Southeastern New Mexico and West Texas,SAND82-0461, Sandia National Laboratories, Albuquerque.Mercer, J. W., R. Beauheim, R. P. Snyder, and G. M. Fairer (1986). BasicData Report for Dril1ho1e DOE-2 Drilling and Hydrologic Testing,SAND86-0611, Sandia National Laboratories, Albuquerque.Powers, D. W., S. J. Lambert, S. E. Shaffer, L. R. Hill, and W. D. Weart,eds, Geological Characterization Report. Waste Isolation PilotPlant (WIPP) Site. Southeastern New Mexico, Vo1s 1 and 1,SAND78-1596, Sandia National Laboratories, Albuquerque.Price, N. J. (1966). Fault and Joint Development in Brittle and Semi­Brittle Rock, Pergamon Press, Oxford.Seni, S. J. and M. P. A. Jackson (1983). Evolution of salt structures,East Texas Diapir Province, part 2: patterns and rates ofha1okinesis, American Association of Petroleum GeologistsBulletin, Ql, 1245.Snyder, R. P. and L. M. Gard, Jr. (1982). Evaluation of Breccia Pipes inSoutheastern New Mexico and their Relation to the Waste IsolationPilot Plant (WIPP) Site (with a section on drill-stem tests,WIPP-31, by J. W. Mercer), U. S. Geological Survey Open-FileReport 82-968.Wood, B. G., R. E. Snow, D. J. Cos1er, and S. Huji-Djufari (1983).Delaware Mountain Group

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