FenceLake Formation (Tertiary), west-central New Mexico by MargueriteMcLellan, Laura Robinson,Laura Haschke,M. DeverauxCarter, and Antoinette Medlin, U S GeologicalSurvey, Denver, C0, andReston, VA Rincon Hondo A seriesof fluvial sandstonesand conglom- R t9 ?:-. .i- erates that unconformably overlies the Creta- fl .-:l':r:- ;-! =.::: :- -::ir o. l ceous rocks in the Moreno Hill, Rincon Hon- ,\\ do, FenceLake, and FenceLake SW 7%-min I quadrangles, as well as in Twentytwo Spring and Cantaralo Spring quadrangles to the west (Anderson, l98l) and The Dyke and Cerro Prieto quadrangles to the east (Campbell, l98l), all in the north part of the Salt Lake coal field of west-central New Mexico (figs. I I. N and 2), is here named the Fence Lake Forma- T. tion for the community of Fence Lake in 5 southwestCibola County, New Mexico. FICURE l-lNDEX MAp sHowrNc sruDy AREA,coAL FIELDS,AND SURROUNDINGCEOLOGIC FEATURES; modified from Trumbull (1960). ' These rocks were mapped tN. as the upper Ool I member of the Bidahochi Formation by Re- , penning and lrwin (1954, p. 1,821-1,826), MorenoHill FenceLoke SW who mapped all Tertiary rocks on the Zuni O | 2 3 4 j5km Plateau as the upper member of the Bidahochi F-r--rr----rT--r---T Formation. Reagan (1924, p. 366) originally O | 2 3mi l- had named the Bidahochi Formation for 35 ft of coarse, well-cemented conglomerate that @ Quolernoryolluvium Formofion boundory overlies the Cretaceous rocks in the Ganado, Arizona, region, northeast of Bidahochi, but Quolernoryboso lt Quodrongleboundory gave no other description of the rocks. ffi Repenning and lrwin described the Bida- hochi Formation in northeast Arizona as Tertiory FenceLoke Formofion Meosuredseclion locolion being composed of three members. The lower member is composed of banded gray, brown, Creloceousundivided -- Foult: bell on downthrown and pink flat-bedded mudstone and argilla- W side ceous fine-grained sandstone and a few beds of white rhyolitic ash. This member was con- JurossicZuni Sondslone sideredto be a lacustrine deposit, which is pre- @ sent only in the area near Bidahochi, Arizona, Triossic ChinleFormotion attaining its maximum thickness of 214 ft ap- tr proximately 15 mi east of there. A volcanic FIGURE 2-GrNerurLrzED cEol-ocrc MAp oF MoRENo HrLL. RrNcoN HoNDo, FENCELAKE, ANDFENCE LAKE middle member composed of lava flows, ex- SW quaonnncLEs sHowrNGAREAL EXTENT or FeNcr LexB FonuettoN (TERTIARY)AND LocATIoN oF TypE truded in the Bidahochi area. and of detrital sEcrroN(ntc. 3); basefrom U.S. Geological Survey 1:100,000topographic base, 1981. 13> New Mexico Ceology November1982 volcanic material derived from them was de- mafic minerals. In some places, thin beds of sondstone scribed as separating the lower member from I volcanic-pebble and -cobble conglomerate al- the upper member in the Bidahochi area. The ternate with the lowest sandstone beds. The middle volcanic member was reported to at- @] conqlomerole maximum thicknessmeasured was 50 ft in sec. tain a maximum thickness of 100 ft in places, 6, T. 5 N., R. l9 W. This sandstoneapparently E-? interbeddedshole ond the higher parts of but no locations or detailed descriptions of its u corbonoceousshole was not deposited on Santa thicker sections were shown. The one mea- Rita Mesa and the Zuni Plateau, and at this sured section published showed 5 ft of pale time it is considereda fine-grained lens of the yellowish-brown conglomerate composed of lower unit. 5-15 mm subangular to angular quartzite and On the easternpart of the Zuni Plateau and basalt pebblesin a matrix of fine- to medium- over most of Santa Rita Mesa, the lower unit lies unconformably on the upper member of grained subrounded to rounded clear quartz '":] grains. Repenning and Irwin (1954) described ou:o the Moreno Hill Formation (Cretaceous),bev- the upper member as mostly white to very pale eling it and filling joints within it (fig. 6). To Oo places near Draw, and brown, crossbedded, poorly cemented, !; the north, at Jaralosa medium- to fine-grained, argillaceous sand- (J: to the west on the Zuni Plateau. Cretaceous stone and a few beds of white rhyolitic ash. rocks were eroded even more deeply and the They stated that the member is as much as 450 O Fence Lake conglomerate lies on the lower ft thick in drill holes on the Navajo Indian member of the Moreno Hill. The lower Fence Reservation. Lake Formation is much more resistant to ero- The middle volcanic member of Bidahochi FIGURE 3-Tvps sEcrroN oF FENcE LAKE FoR- sion than the underlying Cretaceous sand- Formation, near White Cone Settlement in MArroN (TERrrenv), src I, T. 4 N., R. l8 W., stonesand shalesand overhangs on the Creta- CrgoleCouNry, NBw Arizona, has recently been dated by K-Ar Mrxrco ceous along much of its outcrop on the east methods at about 6.7 m.y. (Scarborough and side of Santa Rita Mesa (fig. 5). Where the ce- others,1914,p.472). ment of the lower Fence Lake has dissolved, In 1958, Repenning, Lance, and Irwin ex- boulders from the conglomerate cover the cluded the highJevel gravel deposits near slope concealing the contact between the Cre- Fence Lake, New Mexico, from their discus- taceousand the FenceLake Formation. In sec. sion of the Bidahochi Formation in the Black l, T.4 N., R. 18 W., at the type section,the Mesa Basin (p. 123). However, they did in- conglomerate is 4l ft thick. It is 100 ft thick in clude older alluvium in adjacent parts of the sec.35, T. 5 N., R. 19 W., in the north-central valley of Carrizo Wash in the upper member part of Moreno Hill quadrangle, where it con- of Bidahochi Formation (Repenning and tains subrounded boulders as large as 2 ft in others, 1958,p. 126, fig.3). This poorly con- diameter; the conglomerate was reported by solidated sandstone-to-siltstone unit is inset below and clearly younger than the high-level conglomeratic unit near Fence Lake. Recent work by Anderson (1981, and personal com- munication, January 1982), as well as earlier work by Akers (1964), supports identification of the deposits in the valley of Carrizo Wash (and in the Twentytwo Spring and Cantaralo Spring quadrangles in the western part of the Zuni Plateau) as the upper Bidahochi mem- ber. Two main units comprise the Fence Lake Formation: a lower coarse conglomerate in a sandstone matrix with some sandstone strata FIGURE 4_FENCE LAKE FoRMATIoN (TERTIARY) and an upper sandstone with a few lenses of ovERLyrNc uppER MEMBER oF MoRENo HrLL FoR- conglomerate. The lower unit covers most of MATTON(CRETAcEous) rN AREA oF TypE sEcrroN, the Zuni Plateau and Santa Rita Mesa, but the sncl, T.4 N.,R. l8 W. FIGURE 5-LowER uNIr oF FBNce Lexr Fon- upper unit is present only on a few small MArroN,spc I, T. 4 N., R. 18 W.; note overhang Lake Moreno Hill Formation. mesas. A type section containing both units of Fence on was measuredin sec. 1, T.4 N., R. 18 W. (figs. 3 and 4). The age of Fence Lake Forma- to coarse-grained poorly sorted pinkish-gray tion is Miocene (John Hawley, personal com- (5 YR 8/l) or light-gray (N6-N8) calcareous munication, November l98l). The Fence Lake sandstone.On most exposedsurfaces the sand is older than the upper Bidahochi Formation matrix weathers readily, leaving a lag-gravel- of the Twentytwo Spring area, and clasts in it covered slope. Caliche also occurs as thin beds were derived from Oligocene-agebasaltic an- within or on top of the conglomerate. A few desites and rhyolites of the Datil-Mogollon thin lensesof sandstone,similar to the matrix, area (John Hawley, personal communication, are interbedded with the conglomerate in most January 1982). outcrops. The lower conglomerate unit (fig. 5) was re- Along the Zuni Plateau in the western parts ferred to informally as the "Fence Lake of Moreno Hill and Rincon Hondo quad- gravel" by R. J. Marr (1956). This unit is rangles, the lowest part of the lower unit is composed of subrounded to subangular gran- composed of sandstone. It is a subangular ules, pebbles, cobbles, and boulders of basalt fine- to coarse-grained,poorly sorted, weakly and rhyolite, as well as other volcanic rocks, cemented, calcareous, mostly light-gray (N6- FIGURE 6-8ASAL FENCE LAKE FoRMATIoN FlLLrNc chalcedony, petrified wood, sparseagate, and N8) sandstonecomposed mainly of quartz but FRACTUREIN UPPERMEMBER oF MoRENo HILL FoR. rare jasper and quartzite, in a matrix of fine- containing some grains of orthoclase and MArroN,sEc. I, T. 4 N., R. l8 W. November1982 New Mexico Geology Marr (1956)to be 120 ft thick in the SE% sec. the upper unit. Within the upper unit on Cerro 30, T. 5 N., R. 19 W. To the north and to the Blanco, lensesof gray, volcanic conglomerate GeologicHighway Map of west it seems to become thinner and less interfingering with the pink sandstone (fig. 7) coarse. Approximately 50-60 ft of the con- are much more numerous than at the type sec- NewMexico published glomerate overlies the 50-ft sandstone lens in tion, l0 mi west, and become increasingly The New Mexico Geological Society in coopera- sec.6, T. 5 N., R. 19W. common nearer the top of the section. Basaltic tion with the New Mexico Bureau of Mines and Min- publishing The upper unit, at the type section,is 180ft boulders within the conglomerate lenses are eral Resources is a completely revised Geologic Highway Map of New Mexico. Modeled thick and is composed mostly of poorly shown in fig. 8. To the east of Cerro Blanco after the 1975 Utah Ceological Highway Map and, sorted, very fine-grained to coarse-grained, the contact of Fence Lake Formation with the in part, after the seriesof Geological Highway Maps calcareous, grayish-pink (5 R 8/2) sandstone, Moreno Hill Formation is exposed in the ar- of the American Association of Petroleum Geolo- containing grains of multicolored quartz, feld- royo wall of FrenchesDraw.