Stratigraphy,paleontology, anddepositional systems ofthe EoceneGub Mountain Formation, Lincoln County, New Mexico- a preliminaryreport by SpencerG. Lucas,New Mexico Museum of NaturalHistory, P. 0. Box7010, Albuquerque, New Mexico 87194; StevenM. CatherNewMexico Bureau of Minesand Mineral Resources, Socorro, New Mexico 87801; PaulSealey New Mexico Museum ol NaturalHistory, P. 0. Box7010, Albuquerque, New Mexico 87194; andJ.Howard Hutchison Museum of Paleontology,University of Calilornia, Berkeley, California 94720 Introduction Bodine (1956) first used the name Cub Mountain Formation to refer to "Laramide" shata that lie above the coal-bearingMesa- verde Group and below the volcaniclasticand volcanic rocks of the Sierra Blanca basin in --l' western Lincoln County, New Mexico (Fig. o v '-srERRA 1). Although it was then clear that the age ( of the Cub Mountain Formation must be Late Bhtf"'o Cretaceousand/or early Tertiary, no precise .4" age data were available.This resulted in var- ied age assignments and differing interpre- tations of the stratigraphic relationships of the Cub Mountain Formation to nearby "Lar- amide" units. Furthermore, no precise in- formation on the depositionalenvironments, provenance, and evolution of the Cub Mountain depositional basin has been pub- Iished. In this paper we present a prelimi- nary effort to determine the stratigraphy,age, depositional environments, and provenance of the Cub Mountain Formation near its type section. o4 .l',.,.:. Intrusivestocks, plugs,and Previous studies sills (dikes not depicted) Ptior to L956,strata of the Cub Mountain Outcroos of Formation either were included in the Upper Cub MountainFormation CretaceousMesaverde Group or apparently were not recognized(e.9., CampbelI, L907; 20 miles Wegemann, 1914; Sidwell, 1946;Allen and fones, 1951).Bodine (1956,p.8) introduced 20 km the term Cub Mountain Formation and mapped its distribution in the vicinity of Capitan. Although he indicated that the for- mation name was for Cub Mountain south of. Carraozo, Bodine did not designate or describea type section for the Cub Mountain FIGURE 1-Outcrops of the Cub Mountain Formation and intrusive igneous rocks in lty of Sierra Formation. (A footnote inadvertently omit- "icit (modified from Dane and Bachman, 1965). ted from Bodine's article would have cred- Blanca basin ited the concept of the Cub Mountain Formation to R. H. Weberand explained that proper definition of the unit was forthcom- Tertiary" age. Lochman-Balk Q96a, p. 58) quartzite, silicic volcanic rocks, chert, gran- ing in an article by Weber.) Nevertheless, listed a "latest Upper Cretaceous(?)(sic) - ite, and petrified wood. He also noted vol- Bodine (L956,p.9) presentedevidence that Eocene(?)"age for the Cub Mountain For- canic debris in the upper part of the Cub at least 100 m of strata are missing between mation, and Kelley and Thompson (19(4, p. Mountain Formation and suggestedit might the Cub Mountain Formation and underly- 120)equated the Cub Mountain and McRae provide a basis for dividing the Cub Moun- ing MesaverdeGroup. Although he noted formations (also see Thompson, l9U, 1966, tain into two members, a lower, nonvolcanic that the Cub Mountain Formation could be 1972) and assigned them a "Laramian and member and an upper, volcanic member. as old as Late Cretaceousor as young as Paleocene"age. Weber (1954,p.106) very tentativelycorre- Miocene,Bodine (1955, p. 10)concluded that Weber Q96a, p. 105)designated and de- lated the Cub Mountain with the EoceneBaca "apparently, the Cub Mountain formation is scribed a type section of the Cub Mountain Formation. another Tertiary intermountain deposit, sim- Formation in SandersCanyon between Cub In the two decadesthat followed Weber's ilar to the Bacaformation of SocorroCounty, and Chaves Mountains (Fig. 2), from the (1964)article, very little work was done on the Galisteo formation of north-central New SW1/6W1/a,sec. L6 to the SW1/+SW1/+,sec. the Cub Mountain Formation. Thompson Mexico, and possibly the McRae formation 24, T9S, R10E. According to Weber 09e), (1966,p.17-19) argaed on a lithologic basis of south-centralNew Mexico."Griswold(1959, the Cub Mountain type section is about 730 for identity of the Cub Mountain Formation p. 12) noted the similarity of the Cub Moun- m of light-colored arkosic sandstone inter- and the McRae Formation in the Elephant tain Formation to the BacaFormation of west- bedded with dominantly red mudrock and Butte area of Sierra County. He also -unpf9 central New Metco and assignedit an "early minor conglomerateconsisting of pebblesof New Metico Ceology February 7989 RlOE FIGURE 2-Location of fossil localities, measured stratigraphic section and cub Mountain-Mesaverde Group contact in the Chavez Canyon-Little cub contact bolwoen Moaav6rd€ Gaoup (lo N) dnd YBU'"u Cub Mountaln .m€aaurod of the "McRae Formation" Formillon (to S) \ .gctlon the distribution '\'1- 4 J in the Sierra Blanca-Cub Mountain area NM local r 344 T Kottlowski (1981,p. 22) indicated a Creta- o J ceous-Paleoceneage. Smith et al. (1985)as- S signed a middle Eocene-earlyOligocene age M€!av€rd€ Grouo to the Cub Mountain Formation. // (o w, .nd / Cub Mounlaln \ Formailon(to E) I StratigraPhYand dePositional \)\ environments i\ 16 of \----j,1 We examined two stratigraphic sections (7,\-' the Cub Mountain Formation,one partial and one complete,during fune, L987.The,partial section is located on the northern flank of Little Cub Mountain (Fig. 3A); the complete ,K: FIGURE 3-Some outcrop photographs of the Cub Mountain Formation' A, / MOUNTATN Overview of section on-norther-n flank of Little Cub Mountain' B, Basal / (-\ conglomerate of Cub Mountain Formation near Little Cub Mountain (Unit 5 of rieasured section in Figure 4). C, Interbedded sandstone and mudstone at the stratigraphic level of vertebrate-fossil occurrences at NM localities 1384, N 1385 and 1586 (sandstone outcroP is about 5 m thick). D, Root traces and I burrows in red mudstone exposed in Chaves Canyon. February 1989 New Merico Ceology section is exposedalong Chavesand Sanders Canyons (Fig. 2). At both sections,the Cub Mountain Formation disconformably over- lies strata of the Upper Cretaceous Mesa- verde Group. Although no angular unconformity is apparent locally, Kelley and Thompson (1964)and Kelley (1971)noted that the Cub Mountain Formation overstepspro- gressivelyolder Cretaceousunits to the east. The basal contact is exposedin Chaves Can- yon in the SW'/4,sec. 16, T9S, R10E and north of Little Cub Mountain in the NEl/+,sec. 11, T9S, R10E (Fig. 2). A basal conglomeratic sandstone (Fig. 38) marks the upsection changefrom drabmudstone, sandstone, and Eocene fossils,NM rare pebbly sandstone of the Mesaverde localities 1384. 1385, and 1386 Group to the coarsersandstone, variegated red mudstone,and conqlomerateof the Cub Mountain Formation(Fig. a). No precise lithologic definition of the Cub Mountain Formation has been published. Perhaps because of ambiguities in its defi- nition, Arkell (1983,1986) extended the term Cub Mountain to include beds mapped as MesaverdeGroup by Weber (1964)and Thompson (1966). Our work supports as- signment of these beds to the Mesaverde Group becauseof their lithologic similarity to Mesaverdeexposures to the west and their dissimilarity to the coarser grained, more brightly hued beds that overliethem, In our view, Arkell's (1983)basal unit and lower part of the main body of the Cub Mountain For- mation are correlative with the Mesaverde Group and are separatedfrom superjacent Tertiary strata by an unconformity. The Cub Mountain Formationis about 730 m thick (Weber, 1964), although the upper part of the sectionin SandersCanyon may be repeated by faulting. Estimated averaBe .g 6c conglomerate:sandstone:mudstoneratio is :.9 about 5:70:25in exposuresnear Cub Moun- =P tain, and the unit becomesfiner grainedup- 6E section. Sandstoneis the dominant lithology >E T ^o I 5 meters and mainly occurs as gray to pink tabular :rr t units that contain abundant horizontal and o low-angle stratification and minor trough crossbedding(Fig. 3C). Conglomerateis largely restricted to the lower one-third of conolomerate/ the formation and commonly occurs in scours conllomeratic ss in the basalportions of sandstoneunits (Fig. trough-crossbedded 38). Pebbles are dominantly chert and sandstone quartzite with subordinant felsic volcanic, planar-crossbedded sandstone, siltstone, Iimestone and silicified sandstone wood clasts.Maximum clastsize is about 10 cm. Mudstone units are tabular in shapeand massive sandstone typically display variegated red coloration. Evidence of bioturbation in mudstones is bioturbated common (Fig. 3D), and calcareousnodules sandstone of probablepedogenic origin were occasion- unconformity ally noted. shale/mudstone Stratification styles in the sandstone lith- osomeare similar to thosedescribed by McKee ironstone et al. (1975) for modern flood deposits of Mesaverde concretions ephemeral Bijou Creek, Colorado. Inter- Group bedded mudstonespresumably accumulated siltstone in vegetatedflood basinsand pondsadjacent to aggrading channel complexes.Overall, fa- ciescharacteristics are quite similar to those FIGURE 4-Measured stratigraphic section of the lower part of the Cub Mountain Formation on the of distal-fan deposits of the EoceneBaca For- northern slope of Little Cub Mountain. SeeFigure 2 for location of section and Table 1 for description mation (Cather and Johnson, 7984,1986), aI- of lithologic units. 13 New Mexico Geology February 7989 though the Cub Mountain Formationcontains which only produced