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Magnetostratigraphy of a Core from Raton Basin, New Mexico

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Magnetostratigraphy of a Core from Raton Basin, New Mexico Magnetostratigraphyof a core lrom Raton Basin, NewMexico-implications forsynchroneity of CretaceouslTertiary boundary euents by MichaelA. Payne,En<onProduction ResearchCo., Houston, TX, and DonaldL. Wolberg,Paleontologist, and Ailrian Hunf, ResearchAssistant, New Mexico Bureau of Mines and Mineral Resources,Socorro, NM The magneticpolarity stratigraphyof a The Raton Formation may be up to 610 m various proposed Cretaceous-Tertiary 30.5-m(100.l-fQ-long core from the Raton (2,001ft) thick (Ash and Tidwell, L976) and boundaries have proven inconclusive. This Basinin New Mexicohas been obtained. contains the Cretaceous-Tertiary (K-T) situation is unfortunate given the extant pa- The core includesthe palynologicallyde- boundary. Fossilsare poorly representedwith leomagnetic/paleontologic documentation terminedCretaceous-Tertiarv boundarv and the exception of paleobotanicmaterials. Leaf available. However, Orth provided accessto aniridium abundanceanomily. The bound- (100.1-ft) ary is shownto lie in a normalpolarity zone, floras are known from the Trinidad Sand- the 30.5-m Raton core, drilled by asverified by hand samplescollected from stone, the Vermejo Formation, and the Raton Los Alamos Laboratorv, with a demon- the basin,thus placingin gravedoubt the Formation (Brown, 1943).Brown (7943,1962) strated iridium anomaly-at the palynologic validity of a synchronousworldwide ex- consideredthe lower 15m (49ft) of the Raton CretaceourTertiary boundary. Addition- tinction event. Formation, containing the problematic plant ally, samples were taken from surface ex- f.ossl Paleoasterinquirenda Knowlton, to be posures near the town of Raton at the Introduction Cretaceousin age and the upper part Paleo- Cretaceous-Tertiary boundary. Fig. 3 is a cene. Tschudy (1973)located the palynolog- generalized representation of the 30.5-m The Raton Basinis a large sedimentary and ical Cretaceous-Tertiarv boundarv at 81 m (100.1-ft)core showing dominant lithologies structural basin in northeast New Mexico and (266 ft) and 88 m (289 it; above the base of and paleomagnetic sampling intervals. southeastColorado 10,000 km, (3,861 mir) in the Raton Formation on the basis of core area (Fig. 1). More than 3,700 m (12,138ft) samples.Tschudy (in Orth and others, 1981) Experimental procedure of sedimentary rocks of Late Cretaceous- reestablishedthe palynological Cretaceous- Eocene age are present in the Raton Basin Cylindrical plug samples were obtained at Tertiary boundary in a newly drilled core and Wood, 1956;Baltz, 1965).The approximately 30-cm intervals from the Los flohnson "within a L m interval between a coal bed Upper Cretaceousand Paleocenerock units, Alamos core (Fig. 3). The well-consolidated centered at a depth of 255.7 m and a car- in ascending order, are the Trinidad Sand- siltstones were cored and sliced wet to 2.5 bonaceous shale at 256.7 m." This palyno- stone, the Vermejo Formation, the Raton cm (1 inch) cylinders. The friable shaleswere Iogical boundary occurs at the same Formation, and the Poison Canyon Forma- sliceddry to fit inside 1.7 x 2 x 2 mm plas- stratigraphic position as an iridium abun- tion; these rock units record the iinal regres- tic containers. Three additional oriented dance anomaly with concentrations of up to sion of the Cretaceousepeiric seaway from sampleswere collected in Raton Park on the 5,000ppt, and it is the first record of an iri- the Westem Interior of North America. west side of Raton, New Mexico. These sam- dium anomaly in continental sediments. ples were between 15 cm below and 57 cm We found it apparent that 1) anomalously above the palynological Cretaceous-Tertiary high concentrations of Ir at the Cretaceous- boundary and iridium abundancezone. Three Tertiary boundary, when combined with specimenswere taken from each sample. Ex- magnetic polarities, might have a far-reach- treme care was taken to maintain vertical up ing value for stratigraphic correlation of geo- for all specimens. graphically widely separated rock units and that 2) the concentrationsmight provide use- ]F ful data for resolving shatigraphic argu- ffi ments regarding the stratigraphy of the San A.F ol @lcrop ol uprrmGl Alsoin this issue: r€loc€ous ond lorer ond m'ddre Juan Basin at or adjacent to the Cretaceous- p. Tertiary Uraniumindustry in NewMexico 45 boundary. Paleontologic/magneto- Bryozoanand crustaceanfrom stratigraphic data were interpreted to indi- p.52 cate that the FruitlandFormation Cretaceous-Tertiary boundary, Redefinitionof Zuni ".nit*€ based on the highest occurrence of din6- p,56 saurs/ occurs within the normal polarity zone Sandstone CoyoteCreek State Park p, 60 correlative with anomaly 29 (Butler and others, p.62 1977;Lindsay and others, 1978;Lindsay and Service/News others, 1981). Thus, dinosaur extinction in Gomingsoon the San ]uan Basin occurred later than ma- rine foraminiferal extinctions at Gubbio, It- BacaFormation glyptosaurine lizard aly, where extinctions occurred high in the Cephalopodaptychifrom Los Moyos FIGLJREl-Location map of study area showing Limestone strucfural boundaries, distribution of Cretaceous reversed magnetozone betwden anomalies and Tertiary rocks, and sampling localities for Ra- 29 and 30 (Alvarez and others, 1977). Five-yearcumulative index to Nenr ton core (LA-YCI) and exposures(RH-L,3, 4; aftet To date, our efforts to locate an iridium Mexico Geology fohnson and others, 1966). anomaly in the San |uan Basin at or near Results The samples from the Raton Basin core were generally very weakly magnetized. NRMs"were typically on the-ordeiof a few '). times 10-'O ^' (10-oA m Demagneti- zation to 20 mT usually decreasedthis by a factor of 2 to 5. In addition, the median de- structive field (MDF) for these samples was relatively low. MDFs were typically less than 20 mT with many of them being lessthan 10 mT. This lowness usually indicates that the primary magnetic carrier is magnetite, as- sumed to be detrital . Fig. 2 is a plot of the inclination of the magnetically cleaned spec- imens as a function of core depth; the mean inclination for eachsample is also plotted. A certain degree of scatter in the paleomag- netic data is apparent. Therefore, a set of criteria was developed after the one men- tioned by Lerbekmo and others (1979). Be- cause the inclination to be expected for a = Cretaceous-Paleocenesample collected from - ^-^ the Raton Basin is about 60', and the incli- a 5fu nations measured for normal-polarity San o |uan Basin sampleswere on the order of 30'- 50" (Lindsay, Butler, and others, 1981),the I range of possible inclinations was divided into three segments: - 90" to - 30' (re- versed), -30o to +30'(indeterminate), and + 30'to * 90" (normal).A samplewas deemed to be of reversed (R) polarity if one or more of its specimenswere reversedand none were normal; or if two were reversed and one were normal; or if there were one specimenin each of the three segments.A samplewas deemed to be indeterminate (I) if all three specimens were indeterminate; or if two were indeter- minate and one were normal; or if two were . lnlensity > lO-4Am-l o tntensify < lo-4Am-l New A4exnco x Meon of three specimens GEOLOGY o Scioncoand Sorvico Volume 5, No. 3, August 1983 published quarterly by -9O-8O -60 -40 -?O O +20 +4O +60 .80 +90 New Mexico Bureau ofMines and Mineral Resources a division of New Mexico Institute of Mining & Technology Incl inotto n FIGURE 2-Inclination of magnetization for specimens from Raton Basin core as a function of core BOARD OF REGENTS depth. The palynological Cretaceous-Tertiary boundary and iridium abundance zone occurs at 839 Ex Officio a 1; Toney Anaya, Covenor of Ne|| Mexico (Orth and others, 1981).Solid dots represent samples with intensities Breater than L0 A m open Leonard DeLayo, Szperintendenl of Public Inslruction dots represent samples with intensities less than 10 a Am 1. The mean of three specimens yielding Appointed sample inclination is represented by an x. Judy Royd, Prcs., l9?7-1987, Ia Ctuces Willim G. Abbon, Secty/Ireas., 196l-1985, Hobbs Donald W Moris, 1983-1989, /-or Al@os Roben Ir€ Silchez, 1983-1989, Albuqwrque Steve Tores, 1967-1985, Socorro A few of the cylindrical samples were in- microprocessorwas used to make the mag- itially measured on a Schonstedt SSM-I netization measurements.Demagnetiza- New Mexico lnstitute of Mining & Technology Pr5idql . ...... LaurenceH.Lattman spinner magnetometer interfaced with a tions were performed using a Schonstedt New Mexico Bureau ofMines & Mineral Resources North Star microcomputer. The specimens GSD-5 tumbling specimenAF demagnetizer. Director . .. FrankE.Kottlowski Geor8,e S. Austitr were demagnetized in 5-mT (milliteslas- At least one specimen from each sample was Deputt Direclot Subscriptiod: lssued quarterly, February, May, August' measurementof magnetic intensity) steps fully stepwise demagnetized in 10 mT steps November; subscription price $6.00/yr. using a home-built alternating-field demag- to at least 40 mT. The natural remanent mag- Editoriol motter Contributions of possible material for netizer.The weaknessof the magnetization netization (NRM) and the magnetization re- consideration in future issues of NMC are welcome. Articles submitted fot publication should be in the ed- precluded demagnetizing beyond 15 mT or, maining after at least one demagnetization itor's hands a minimum of five (5) months before date in some cases,20 mT. Most of the measure- (usually 20 mT) were measured for each of of pub[cation (Februaty, May, August, or Novenber). Address inquiries to Marla D. Adkins-Heljeson, editor of ments and further demagnetizations of the the remaining specimens.Fisher statistics New Mexico Aeobgy, New Mexico Bureau of Mines & specimensinitially measured at New Mexico were then applied to the three specimens Mincral Resources. Socorro. NM 87801 Institute of MiningandTechnology were made from each sample to obtain sample mean di- Published os public domoin, therefore reproducible without at the PaleomagneticsLaboratory at the Uni- rections. Because declinations were arbi- perm ision. Sour@ credi I reques led. Cirulation: l,4N versity of Arizona. An ScT two-axis cry- trary, the declination for each specimen was Prirrer.
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