Uplift,erosion, and burial ol Laramidefault blocks, Salado Mountains, SierraCounty, New Mexico byWiltiam R. Seager,New Mexico State University, Las Cruces, NM 88003, and Anton B. Mayer,Leggette, Brashears, and Graham, Inc , 423Sixth St., NW, Albuquerque,NM87102 Introduction In south-centralNew Mexico, uplifts and mentary basins,the Love Ranchand Potrillo In recent years Laramide deformation in basins trend north-no south-central New Mexico has been com- west, highly oblique pared to that of the central Rockies of Wy- of late Tertiary fault bl oming (Seagerand Mack, 1986;Seager and rift. Thus, the Larami< others, 1986).The structuralstyle in both areas and segmentedby the is distinguished by basement-cored block fragmentsof the Laran uplifts separatedby broad basins filled with revealedin crosssectio synorogenic to post-orogenic sedimentary Such fragments,expos rocks. Thrust or ieverse fiults border uplift- from the CaballoMbuntains south to the East probably indicate an extension of the uplift basin margins, at leaston one side, and these, Potrillo Mountains, constrain the location, appear at various places in the Black lange as well as associatedtight, overturnedfolds, trend, and geometry of the Laramide Rio area. For example,near Lake Valley (Fig. 1) indicatesubstantial crustal shortening. Grandeuplift (Fig.1) and its two comple- nTTff,:*:T il"il.t:t:.[:ij:#?",3; headwater region of North Percha Creek 107' Laramide erosion exposed Precambrian I granite.The most instructiveoutcrops, how- Cristobal Mts. ever, are in the SaladoMountains, located l j Alsoin thisissue Oil and gas discoverywells drilled in New Mexicoin 1987 p.54 Chargesfor naturalresources on StateLands as of 1988 p. 60 Ore-testingcapabilities at the NewMexico Bureau of Minesand Mineral Resources p. 61 ChicosaLake State Park p.62 Abstractsfrom NMGS 1988 springmeeting p. 64 Upcomingmeetings and fieldtrios p. 69 Service/News p.70 Statfnotes p.72 NEW MEXICO Gomingsoon Typeand referencesections of Lower Cretaceousstrata Kit CarsonState Park Geomorphicdevelopment of Cityof rocks,Grant County FIGURE l-Location map. Shaded area shows extent of Laramide Rio Grande uplift as interpreted by Seager et al. (1986). 15 km nearly due west of Truth or Conse- thrown to the west, the whole fault block is group. Because of erosion during or follow- quences (Fig. 1; Lamarre, 1974;Mayer, L987\. tilted eastward (Fig. 2). In the Salado Moun- ing the Laramide or faulting during the late In this area not only is the Laramide fault- tains the easterly dip ranges from approxi- Tertiary, parts of the El Paso, Montoya, and block structure revealed, but also the manner mately 15 to 25 degrees, and strata dip Abo sections either are not exposed or are in which the fault blocks were eroded and eastward beneath gravels of the Palomas missing altogether. In spite of this, a thick- eventually buried by younger volcanic rocks. Basin. ness of approximately 500 m of Paleozoic rock Rocks ranging in age from Precambrian to crops out (Mayer, 7987). General feafures and stratigraphy of the Quaternary crop out in the Salado Moun- Middle Tertiary volcanic and clastic rocks Salado Mountains tains (Fig. 3). Precambrian rocks include overlie Paleozoic strata, the contact being an The Salado Mountains are a relatively small granite, muscovite phyllite and schist, and angular unconformity of modest (10 to 15 (approximately 20 km'?) group of hills and metadiorite, all exposed in the core of a Lar- degrees) discordance. Middle Tertiary strata peaks located near the center of the eastern- amide fault-blocli uplift. Paleozoic strata have an uneven thickness because lower parts most, outlying fault block of the Black Range. nonconformably overlie the Precambrian and of the section are inset against or onlap Lar- To the north, the fault block is known as range in age from Cambrian-Ordovician to amide topography whereas somewhat Sierra Cuchillo, to the south it goes by the Permian. Largely marine limestone, dolo- younger rocks overlap completely and bury name Animas Mountains. Bordered on the mite, shale, and sandstone, these rocks form the older terrane. Among the oldest of the west by a late Tertiary normal fault, down- the highest hills and peaks in the mountain Tertiary rocks are post-orogenic boulder con- glomerate and breccia beds, only a few me- -n ters thick, derived from an adjacent Laramide 1 L1< Tb' uplift. Clasts are as much as several meters I ^( IT in diameter and include Paleozoic strata as l old as the Bliss Formation. Conglomerates probablv are correlative with the Love Ranch Formati,on of the CaballeSan Andres Moun- \'8 tains area (Kottlowski et al., 1956).The Love tP/ Ranch strata grade upward into or are inter- bedded with intermediate-composition flows and volcaniclastic strata of the Rubio Peak Formation (Palm Park, Spears, or Datil Group equivalent; Elston 1957; Kelley and Silver, 1952; Seager and Hawley, 1973; Cather et al., 1987). Much of the formation is laharic in orp, origin. As much as 1,075 m thick, the for- mation thins dramatically both because its lower beds pinch out by onlap onto Lar- New AAexnc@ Tba GEOLOGV -t o o Scienceand Seryice g) c\ /Ts Volume 1O, No. 3, August 1988 \-nl4 Editor:Carol A \ L'\ Hjellming \J/ Drafting assistaflce:Rebecca Titus Published quarterly by q) QTp New Mexico Bureau of Mines and Mineral Resources to"R a diviskrn of Nry Mqi(! Ir6titute of Mining & firhdotJt BOARD OF REGENTS Ex Officio Gany Camthes, Gmmor of Nru Mexico Alan Morgan, Surytintend.entof Publiclnstruction x Appointed (t Irnton Malry, Pra ,1985-1991,Ahuqurque Robert O Anderuon, Se llrus , 1987-1993, RruII Gilbert L Cano, 19&5-1%9,Afuunque Donald W Monis, 1983-19'119,Ins Alamq Steve Tores, 1967-1991, S6oto M-'€F New Mexico lnstitute of Mining & Tahnology Tba't5 F. Pr$iddl laurene H littman New Mexico Bureau of Mines & Mineral Resurces T3 I Dietor Frank E Kottlowski F' i\ Deputy Director fames M Robertsn 4*a Subrriptions: Issued quarterly, February, o Mat Autust, November; subsiption price $6 00/calendaryear o Editqial mttq: Articles submitted for publication should 9) be in the editor's hands a minimum of five (5) months (D 'o before date of publietion (February,May, August, or f"o. November) and should be no longer than 20 typewriF ten, double-spacedpages A-Urientific papere will be reviewed by at leasttwo people in the appropriate field km of study Addressinquiries to CarolA Hiellming, Editor ol Nru Metiu Geology,New Mexico Bwau of Mines & 2-Generalized geologic IfCURf map of the SaladoMountains, modified after Mayer (lggn. pe, Mineral Rercuces, Saorc, NM 87801 l'recambrianrocks; Pl, lower Paleozoicrocks; DM, Devonianand Mississippianrocks; P, pennsylvanian Publishedre public domin, thoefore reprdrcible uithout Wr- rocks; Pa, PermianAbo Formation;Tlr, Love Ranch Formation;Trp,'ft.ubio peai Formatidn; Ttur, mission, Sourceqedit rcqrcted unnamed lithic ash-flow tuff; Ttu, unnamed crystal ash-flow tuff; iba, basalticandesite flows; Ts, Cirillation:1,ffi Fe Group;,Tb, basalt.flows; palomas P/iflra: University of New Mexico Printint Plant l:1"^' ?ylt erp, Formation;Tai, biotite andesitedike; Tiu, lgneouslntruslons/ undifferentiated. August 1988 New MexicoCeology amide topography and becauseits upper beds tains and buried on the east by post-Lar- thrown on the south side. Precambrianand are truncated by an unconformity. Radio- amide volcanic and sedimentary rocks. In lowest Paleozoicrocks in the uplifted block metric dates from elsewhere in the Black between, the Laramide geology is well ex- are juxtaposed against Ordovician to Penn- Rangeand south-centralNew Mexico region posed, particularly in the Chavez Canyon sylvanian strata in the down-faulted block to indicate the Rubio Peak Formation is late area (Fig. 2). the north. Stratigraphicseparation is as much Eoceneto earliestOligocene in age(e.g. Kott- The central Laramide structure is the as 400 m. Unfortunately, the dip of the fault lowski et al., 1969;Marvin and Cole, 1978; Chavez Canyon fault (Fig. 2), movement on is not known precisely because the fault is Clemons, 1"982;Loring and Loring, 1980). which raised a Laramide fault block of mod- intruded throughout its length by a 50-m- Unconformably above the Rubio Peak For- est relief. The fault trends N85'W and is up- thick biotite andesite dike. The trace of the mation, two ash-flow tuffs of probable mid- Thickness dle Oligocene age crop out along the eastern tIo riloGene \ o ! o v c -v fl and southern flanks mountains. The t c 0 -10m of the Helstocene Palomas Fm. 0-, ,_r Oco r tl. lower tuff, lithic rich and 100 to 200m thick, Pliocene bagalt ,.{' 35i-7Om has not been correlated with other tuffs of the region. The upper tuff, 30 to 60 m thick, !ower Santa is crystal rich and also has not been corre- Mlocene Fe GP. 150+m lated with other tuffs. Scatteredremnants of basaltic andesite flows, inferred to be cor- relative with BearSprings basalt (Elston, 1957) basaltlc endeslte 15i0+m or basalticandesite of PovertyCreek (Elston unnamed et al., 1973),unconformably overlie the tuffs crystal tulf m or Rubio Peak Formation. RecentK-Ar dat- Oligocen e 0-200 ing indicates these units are approximately unnamed 11; 28 Ma old (Seageret al., 1984). tlthlc tulf i,\?".'ii' Unconformably overlying various parts of o l,:, tr ^ Ara the middle Tertiary volcanicsection is a se- o : _!l..-_i' -- I quence of "earIy rift" fanglomerate, sand- o o :>' stone, and siltstone assignedto the lower ot 3ao-..fva l, SantaFe Group. A thousand meters or more >. o >/ ' tu >^'1 thick in along Salado the _.'- rjv \ outcrops Creek, \._/ \ .\.l formation was derived primarily from mid- .E --: _ ,),o Lo' dle Tertiary volcanic rocks. It may be correl- b --- Q' Rubio Peak Fm. \.ct L g 0- 1075m ative with all or part of the Miocene Hayner I , -\.-- Ranch and Rincon Valley Formations of the o '-:-./ '\t ' ,.',.n' E o /' CaballoMountains-Hatch Valley area(Seager --l ^t I v-qY- I o ^o rt and Hawley, 1973) and with the Popotosa o \.--: 'r 6! e ut Formation of the Socorroarea (Denny, 1940; ^ :^ 9A-':.^Y/<1 Bruning, 1973;Chapin and Seager,1975).