U.S. Department of the Interior U.S. Geological Survey Miscellaneous Investigations Series Map 1-2567 Pamphlet Geologic Map of P

U.S. DEPARTMENT OF THE INTERIOR MISCELLANEOUS INVESTIGATIONS SERIES U.S. GEOLOGICAL SURVEY MAP 1-2567 PAMPHLET GEOLOGIC MAP OF PRECAMBRIAN ROCKS ALONG PART OF THE HARTVILLE UPLIFT, GUERNSEY AND CASEBIER HILL QUADRANGLES, PLATTE AND GOSHEN COUNTIES, WYOMING By P.K. Sims, W.C. Day, G.L. Snyder, and A.B. Wilson, with a section on Geochronology of post-tectonic intrusive rocks by Zell E. Peterman 1997 INTRODUCTION represented by scattered horizontal folds in the southernmost part of the uplift, is believed to The Hartville uplift is a north-northeast have resulted from vertical compression caused trending Laramide anticline on the southeast by crustal thickening attributed to collision to margin of the Archean Wyoming craton. It the south along the 1. 7 6-Ga Cheyenne belt consists of a Precambrian core (fig. 1) overlain (Premo and Van Schmus, 1989). by partly eroded gently outward-dipping Paleo In this report, we use informal lithologic zoic strata and, locally, by flat-lying Tertiary names to describe the supracrustal rocks of the and Quaternary strata. The uplift is the east Whalen Group (Smith, 1903) in the map area. ernmost Laramide structure on the Wyoming For the intrusions, granite of Haystack Range craton. of Snyder (1980) and diorite of Twin Hills of The uplift exposes deformed Late Archean Snyder (1980), we herein propose the formal granite (2.58 Ga) and Late Archean or Early names Haystack Range Granite and Twin Hills Proterozoic supracrustal rocks that were de Diorite. The type area of the Haystack Range formed during three, and possibly four, major Granite is in T. 27 N., R. 65 W. (this report); compressional events in Early Proterozoic time, the type area of the Twin Hills Diorite is Twin extending from approximately 2.1 to 1. 76 Ga. Hills, in the Moore Spring quadrangle, immedi These events are recorded in the Guernsey and ately north of the Casebier Hill quadrangle. Casebier Hill quadrangles by (1) inversion of A geologic map of the Good Fortune mine supracrustal succession (D1) as a result of a area by Ebbett (1956) was invaluable to us probable fold nappe; (2) folding of rocks at least inasmuch as the mine area has been rehabili in the southern part of uplift on east-west axes tated subsequent to his work and exposures of {Dz ); (3) west-vergent folding on northeast the ore body and immediately adjacent rocks trending axes and associated thick-skinned have been covered by dirt. The thesis map was thrusting with concomitant prograde metamor made available to us by Robert S. Houston, phism; and (4) scattered horizontal folds and University of Wyoming. associated structures in the extreme southern part of the area. STRATIGRAPHY The overturned succession probably con stitutes the lower limb of a fold nappe (D1), Three principal lithologic rock units, from which could have formed as an early phase of oldest to youngest: aluminous and ferruginous deformation Dz . The Dz folds apparently were schist, metadolomite, and metavolcanic rocks, caused by north-vergent collision {approximate compose the Whalen Group in the map area. ly 2.1 Ga) along the southern margin of the The succession is known to be inverted in the Wyoming craton. The main, most widespread map area, probably as the result of a fold nappe deformation in the uplift (D3; approximately developed before or during the early stages of 1.82 Ga) constitutes a west-vergent fold-thrust deformation D2 . Evidence for an overturned belt on the western, convergent continental succession is given mainly by stromatolite margin of the Trans-Hudson orogen (Lewry bearing carbonate rocks (XWd) in the vicinity and Stauffer, 1990); the continent-arc suture is of Hartville. On the north limb of the Graves not exposed but is inferred to lie a few kilome Ranch anticline, in SE1/4 sec. 7, T. 27 N., R. ters to the east of the uplift. The youngest 65 W., overturned beds dip moderately (40°- recognized significant deformation (D 4), 500) north, and on the south limb of the 1 anticline, steeply dipping beds face north. The ate rocks in the Whalen Group are permissive anticline is a downward-facing, asymmetrical, with an Early Proterozoic rather than a Late east-plunging anticline. Other downward-facing Archean age (Zell E. Peterman, written com beds in dolomite occur in the area between mun., 1994). An Early Proterozoic age is also Hartville and Sunrise, and in the area immedi supported by the presence in the Whalen Group ately northeast of the Chicago mine. of Lake Superior-type banded iron-formation The quartzofeldspathic (aluminous) schist (XWfi) (Gross, 1965; James, 1983), which in unit (XWs) west of the Hartville fault consists the northern hemisphere characteristically is mainly, of biotite-quartz-plagioclase schist con Early Proterozoic in age (Gross, 1980, p. 218). taining sporadic garnet. This unit grades lat erally into ferruginous schist (XWf) in the area STRUCTURE east of Hartville. The ferruginous schist con tains layers and lenses of banded iron Four episodes of compressional deforma formation; lenses of specularite, which have tion, designated D1 through D4, are recorded in been mined at the Good Fortune, Sunrise, and the supracrustal rocks and their basement in Chicago mines; and discontinuous layers of the map area, as follows: D1, overturning of quartzite. The ferruginous schist unit predomi the Early Proterozoic or Late Archean strati nantly contains chemical sediments, in contrast graphic succession; Dz, folding on east to the more clastic facies of the quartzofeld trending axes, as exemplified in the southern spathic schist. East of the Hartville fault, the part (Guernsey domain) of the map area; D3, quartzofeldspathic schist (XWb) is in the am folding on northeast-trending axes, coeval with phibolite metamorphic facies and contains gar west-directed, thick-skinned thrusting; and D4, net, sillimanite, and, locally, andalusite. In the scattered, small-scale recumbent folds and as eastern area, the schist is intruded and domed sociated structures in the southern part by the 1.72-Ga Haystack Range Granite (Xh). (Guernsey domain) of the map area. Compres Structural analysis in the map area sug sional deformation (D1 through D4 episodes) gests the existence of only one carbonate unit was followed by doming of the supracrustal in the Whalen Group (XWd), as shown on rocks around the 1. 72-Ga Haystack Range cross section A-A'. Previously, Snyder (1980, 1989, 1993) assigned the carbonate rocks to Granite in the area east of the Hartville fault. Compressional deformation is thought to have two informal formations, an upper "Wildcat Hills Formation" and a lower "Muskrat Canyon spanned the approximate interval 2.1-1.76 Ga. Formation." Metabasalt of the uppermost Deformations D1 and Dz were not recognized stratigraphic unit (XW m) was previously infor in the area previously. mally called "Mother Featherlegs basalt" To facilitate discussion of the structure, (Snyder, 1980). It appears to form a good the map area is divided into four domains (fig. 2) stratigraphic marker throughout the area. from south to north: (1) Guernsey domain, (2) The age of rocks in the Whalen Group is Graves Ranch anticline and Sunrise syncline equivocal because internal isotopic ages are not domain, (3) transition zone domain, and (4) available, and in this report Whalen Group rocks Frederick Ranch domain. Because the doming are designated as either Late Archean or Early by the 1. 72-Ga Haystack Range Granite dis Proterozoic. Snyder (1980, 1993) considered turbed and strongly modified the older deforma the Whalen Group to be Late Archean because tional structures east of the Hartville fault, the he interpreted the rocks within it to be intruded structural domains are delineated only in the by the Late Archean granite of Rawhide area west of the Hartville fault. Buttes. Recent structural studies (Day and The type area of Dz deformation in the others, 1994), however, indicate that the major uplift is the Guernsey domain (fig. 3). Dz struc granite-supracrustal rock contacts are ductile tures are well preserved in this area because it shear zones, and the relationship between the was virtually unaffected by younger D3 defor granite and the supracrustal rocks is uncertain. mation, and D4 structures are not regionally The Whalen Group is known to be older significant. Northward, Dz structures are also than the 1. 98-Ga granite of Flattop Butte (sec. the dominant ones in the Graves Ranch anti 35, T. 31 N., R. 64 W.) and probably entirely cline and Sunrise syncline domain. In the tran older than the approximately 2-Ga metadiabase sition zone domain, D3 structures are super dikes (Xmd) (Snyder, 1993), for scattered dikes posed on Dz structures, and in the Frederick cut rocks of the Whalen Group. Strontium iso Ranch domain, D3 structures have largely oblit tope compositions of metamorphosed carbon- erated Dz structures through reorientation. 2 D1 AND D2 DEFORMATIONS to the south of the Hartville uplift. Houston and others (1992) recognized that the north Deposition of the Whalen Group on a vergent Reservoir Lake fault separates more in southeast-facing continental margin was fol tensely deformed rocks (Deep Lake Group} to lowed by development of a fold nappe (D1). The the north from less-deformed Early Proterozoic extent of the overturned limb of the nappe is rocks to the south, but they apparently consi not fully known, but reconnaissance mapping in dered the deformation as local in extent. They the adjoining Hell Gap quadrangle, immediately assigned this deformation of rocks north of the north and west of the Frederick Ranch, indi Reservoir Lake fault to their Fz (Karlstrom and cates that stromatolitic beds in the dolomite others, 1981, fig. 1. 9); it preceded intrusion of unit are upright. Accordingly, the northern gabbroic sills in the Deep Lake Group, whose limit of overturning in this southern part of the intrusive age is 2,092±4 Ma (Houston and uplift coincides approximately with, but probably others, 1992, p.

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