marks a distinct horizon and may be traced Reinterpretation0ftype section 0f over an immense area in New Mexico." How- ever,Rankin (1944,p. 12, 19,20) was the first to formally name the unit and identified it JuanaLopez Member (often incorrectly) in a number of measured sectionsin northern New Mexico and southern Colorado. Rankin (9a4, p. 12, 19, 20) ap- of MancosShale plied the name "Juana Lopez sandstonemem- ber of the Carlile shale" to l0 ft (3 m) of very calcareoussandstone containing Prionocyclus byStephenC. Hook, Paleontologist, NewMexicoBureauof Minesand Mineral Resources, Socorro, NM, and WilliamA. Cobban, Paleontologist, U.S.Geological Survey, Denver, C0 wyomingensis near the top of the Carlile Shale. The type section for this new unit was given as sec.32, T. 15 N., R. 7 8., The Juana Lopez Member (Upper Creta- face (Dane, 1960;Molenaar,1973i Campbell, on the Mesita Juana Lopez Grant, six miles ceous)of the MancosShale (San Juan Basin), 1979). Microfossil studiesby Lamb (1968) and northwestof Cerrillos, SantaFe County, New Carlile Shale(Denver and Raton Basins)or Thompson (1972), however, indicate that this Mexico" (Rankin, 1944,p. l9). BentonShale (North Park Basin)is one of the surface may be younger toward the northern Kauffman (in Dane and others, 1966, p. most distinctiveand laterallypersistent litho- edge of the San Juan Basin. Outcrop sections H6, H7, Hl3, H14) examinedthe type section stratigraphicunits in New Mexico,Colorado, we visited indicate that the top of the Juana of the Juana Lopez Member in 1963and con- Utah, and southwesternKansas. The member Lopez Member lies in a lower faunal zone cluded that l) the type section of the member containsan abundantlate Turonian (middle along the south side of the San Juan Basin wasactually in sec.33,T. 15N., R.7 E.;2) Cretaceous)invertebrate fauna whose ele- than it does along the east side. ln the the thicknessof the type sectionwas only 3 ft mentshave restrictedvertical ranges and are southernSan Juan Basin near Hosta Butte, 32 l0 inches. the remainder of Rankin's esti- widely distributed in time-equivalentrocks mi (52 km) east of Gallup, the top of the mated l0 ft being composed of baked shale of throughout the Western Interior of North Juana Lopez Member lies in the Prionocyclus Niobrara age; 3) the member at the type sec- America(Dane and others, 1966). macombi Zone. whereas on the east side near tion was composed predominantly of calcare- On the east side of the San Juan Basin. Cuba, the top lies two zones higher in the nite and limestone(hence, the term sandstone where Dane and others (1966)established a Pr ionocyc lus n ovi mexi canus Zone. was dropped from the member name); 4) the 107-ft-thick(32.6 m) well-exposedreference The distinctive lithologic and faunal nature member at the type section was abnormally section (fig. l), the Juana Lopez Member of the Juana Lopez was first noted by New- thin and containedan unusuallysmall percent- rangesin thicknessfrom 90 to 135ft (27.4to berry (1861)near Las Vegas,New Mexico. In age of shale compared with sectionsmore than 4l.l m). Here, and at other localities,the a later publication, Newberry (1876, p. 33) 100 ft (30 m) thick in the San Juan Basin; 5) memberconsists of thin bedsof rusty-brown- presented a detailed measured section of the member at the type section was succeeded weathering,highly fossiliferouscalcarenire in- Cretaceous rocks along the Canadian River disconformably by rocks equivalent to the terbeddedwith dark-graynoncalcareous clay south of Taylor Springs, New Mexico; it in- basal part of the Niobrara Formation; and 6) shale. The calcarenitesoccur as both con- cluded the Juana Lopez as a distinct lithology the Juana Lopez at the type section was the tinuous bedsand discontinuouslenses, range characterized as ". ferruginous, lami- lithologic and time equivalentof only the up- from a fractionof an inch to 2 ft (0.6m) thick, nated, sandy limestone . [that is] a great per part of the member at the reference sec- are fine to coarsegrained, and are composed store-houseof fossils." Newberry (1876, p. tion. of bioclasticdebris. This debrisgenerally con- 107) also noted that "this ferruginous stratum In connectionwith work on the Juana Lo- sistsof the fragmentedprismatic layer of In- pez Member of the Carlile Shale in the Taylor oceramusshells. Fragmented remains of fish Springsarea of northeastNew Mexico, 109mi teeth,bones, and scalesare commonand led (175 km) northeastof the type section(Hook early workersto describethese beds as "fish- Rro and Cobban, 1980,fig. 6), we visited the type tooth conglomerates"or "fish- bone beds." -l section of the Juana Lopez. After remeasuring Gray septarianlimestone concretions are com- ..-/-\ the type section of the member in a 1965 rail- tLosl- mon in the lower and middle parts of the Alomos road cut of the Atchison, Topeka, and Santa member.Although shaleis the dominant li- saNraFE Fe Railway Company in NW%SE% sec. 33, thology, the hard, platy-weatheringcalcare- f s)o/ sANoovAt\o ; SonroFe T. l5 N., R. 7 E., we suggestmodification of nitesare more conspicuousin outcrop, often z oo,/ l.--.type all but the first of Kauffman's conclusions. littering the slopeswith debris,and generally d ;;xi section This paper presentsour measuredtype section stand in .O/ l; :ocerr,tlos topographicrelief above the softer +/ of the Juana Lopez Member (table l) and shales(Dane and others, 1966). ( t Mesrtode Jlono compares this newly rerheasuredsection with Lopez In the subsurfacethe calcarenitesproduce a _ _ l_ _ _ / | Gtonl | the reference section establishedby Dane and (resistive \ :8ER ALtL Lo ^ ,^ r^h, distinctive "kick" deflection) on NC|A\., | "--1,:____:- others (1966) on the east side of the San Juan (Lilly, \ electriclogs 1952,fig. 2; Bozanic,1955, tQ t" to6" Basin. pls.l-3; Lamb,1968, fig. 5; Molenaar,1913, p. 94). The top of this resistivezone (referred FIGURE l-LocnrroNs oF rHE TypEAND REFERENcE Lithology and thickness to by oil-companygeologists sECTtoNSop tHE Junne Lopez MeN4genop rHr as Sanosteeor MnNcos Ssele, Snrrl Fe eNo SnNoovnLCoun. A 1965 railroad cut of the Atchison, SanasteeMember of the Mancos Shale)has rrEs, New Mexrco (modified from Dane and Topeka, and Santa Fe Railway Company ex- long beenconsidered a time-synchronoussur- others,1966, fig. l). poses not only the entire type section of the Juana Lopez Member of the Mancos Shale, but also severalhundred feet of Mancos Shale both above and below this member (fig. 2). Two points are readily apparent from a cur- sory examination of this cut. First, the ledge- forming calcarenites in the upper part of the member (fig. 3) are approximately l0 ft (3 m) thick and therefore closely match Rankin's Q9aa, p.20) original estimate.In fact, these upper beds are probably all Rankin intended to include in his definition, although he shows the Juana Lopez to be 50 ft thick on his graphic section no. 6 (Rankin, 1944, fig. 5). Second, the basal calcarenites, although thin and rather inconspicuous on the south side of the cut (fig 2), are well exposed on the north side (fig. 4) and can be traced laterally (to the south) in outcrop. These basal calcarenites also carry the Lopha lugubris-Prionocyclus FIGURE 3-Clost-up vtEw oF Flc 2 sHowtNc upprR LEDGE-FoRMTNGCALCARENITE uNtr oF JueNe Loprz macombi fauna characteristic of the lower MrvsEn Upper contact of Juana Lopez Member shown as dashed line; 5-ft-long (l.5 m) Jacob's staff Juana Lopez in the San Juan Basin and are for scale. separatedfrom the upper, ledge-forming beds The Juana Lopez Member at its type section by approximately 90 ft (27 m) of shale. Hook can be subdivided into three informal units measured the Juana Lopez in this railroad cut similar in lithology and thickness to those at on July 16-18, 1979 (table l) and obtained a the reference section: a lower calcarenite unit ALSOIN THISISSUE: thickness of 106 ft (32.3 m). This measured with approximately l0 ft interbedded shales, ChinleFormation p.22 thickness is almost identical to that obtained (3 with scattered m) thick; a middle shale unit Proterozoictectonic setting p 27 by Dane and others (1966, p. HlO-Hl2) for lenses calcarenite that hosts septarian of Service/News p.30 the referencesection (fig. 5). The Juana Lopez limestone concretions, approximately 8l ft was measured at the type locality by the (24.7 m) thick; and an upper calcarenite unit Jacob's staff method (Kottlowski, 1965, p. with interbedded shales, approximately l5 ft COMINGSOON: 6l-75). The thickness obtained is thought to (4.5 m) thick. These same informal subdivi- . Red Rocktalc deposits be accurate to within l0 percent of true sions at the reference section are approxi- . Geologyand paleontology,Tortugas Mtn. thickness. Several factors other than those mately ll ft (3.3 m), 83 ft (25.3 m), and 12 ft . Kirtlandcrocodile listed by Kottlowski (1965, p. 122) may con- (3.7 m) thick (fie. 5). Dakota-Mancos,Zuni Basin tribute some error. First, a reliable dip and The calcarenites are hard, brittle, thin- strike measurementcould be obtained only on bedded, platy-weathering rocks that range the uppermost calcarenites (fig. 3); this mea- from a fraction of an inch to 9 inches (0.2 m) surement was then used to measure the thick- thick. Bedding surfaces are generally irregular ness of the entire member. Second, several and highly burrowed and bioturbated. The minor faults with displacementsof lessthan 5 calcarenites are generally fine grained, with ft (1.5 m) cut the member; in unit 19 (table 1) most grains being bioclastic debris derived it was impossible to offset on a key bed to from Inoceramus and Lopha shells; com- New AAexfic@ avoid one of these faults.