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K–Ar dating and geological significance of clastic sediments of the Paleocene Sepultura Formation, Baja California, México
Journal of South American Earth Sciences 15 (2002) 725–730 www.elsevier.com/locate/jsames
K– Ar dating and geological significance of clastic sediments of the Paleocene Sepultura Formation, Baja California, Me´xico Miguel Agustı´n Te´llez Duarte*, Margarita Lo´pez Martı´nez Departamento de Geologı´a, Centro de Investigacio´n Cientı´fica y de Educacio´n Superior de Ensenada, Carretera Tijuana-Ensenada Km. 107, 22860 Ensenada, BC, Mexico Received 1 May 2001; accepted 1 June 2002
Abstract At its type locality, the Paleocene Sepultura Formation consists of two members: a lower glauconitic-rich clastic section and an upper calcareous section. Three samples of authigenic glauconite pellets from two localities with good clastic sediment exposures were dated using K– Ar. At the type locality of Mesa La Sepultura, pellets from the middle of the clastic section yield a date of 60 ^ 1 Ma (weighted average of four experiments), and pellets from the top of the clastic unit give a date of 60 ^ 1 Ma (weighted average of two experiments). Pellets from the base of the section at La Mesa, 60 km distant, give a date of 59 ^ 1 Ma (one experiment). Dates obtained are in good agreement with those reported by biostratigraphy and confirm a Late Danian age. Our results differ from those reported elsewhere, in which low potassium content glauconites yield younger K –Ar ages than expected. An explanation for our observed agreement in ages could be related to the tectonic setting of forearc basins, where the low geothermal gradient prevents argon loss and reliable dates can be obtained. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: K– Ar dating; Glauconite; Paleocene
Resumen En su localidad tipo la Formacio´n Sepultura del Paleoceno consiste de dos miembros: uno inferior cla´stico rico en glauconita y uno superior calca´reo. Se fecharon por K –Ar tres muestra de pellets de glauconita autige´nica provenientes de dos localidades donde los sedimentos cla´sticos se encuentran bien expuestos. En la localidad tipo de Mesa La Sepultura, la parte media de la seccio´n cla´stica proporciono´ una edad 60 ^ 1 Ma (promedio ponderado de cuatro experimentos), y el techo de la misma unidad 60 ^ 1 Ma (promedio ponderado de dos experimentos). En La Mesa, a 60 km de distancia, la base de la seccio´n rindio´ una edad de 59 ^ 1 Ma (un experimento). Las fechas obtenidas se encuentran en concordancia con las reportadas por bioestratigrafı´a, y confirman una edad Daniano tardı´o. Los resultados difieren de otros reportados en otras localidades en glauconitas con un bajo contenido de potasio, las cuales proporcionan edades mas bajas que las esperadas. La explicacio´n puede venir del escenario tecto´nico de las cuencas frente de arco, donde el gradiente geotermal bajo evita la pe´rdidad de argo´n y es posible obtener edades confiables. q 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction Although K –Ar dating with glauconite often produces ages younger than stratigraphically expected, glauconite is virtually the only mineral suitable for direct dating of sedimentary rocks because its authigenic origin and potassium content makes it useful for K – Ar dating (Dalrymple and Lanphere, 1970; Morton and Long, 1980; Amorosi and Centineo, 1997). K –Ar dating of glauconites * Corresponding author. Address: Facultad de Ciencias Marinas, Universidad Auto´noma de Baja California, Apdo. Postal 453, 22800 Ensenada, BC, Me´xico. E-mail address: [email protected] (M.A. Te´llez Duarte).
has frequently been used as a geochronometer for absolute dating and to address problems such as the timing, sedimentation rate, origin, and environmental settings of sedimentary rocks (Polevaya, 1961; Odin and Matter, 1981; Taylor and Curtis, 1995; Mitchum and Van Wagoner, 1991; Amorosi, 1995). Odin and Matter (1981) suggest that at least 103 –104 years of exposure are required to begin the glauconitization process. Thus, absolute dates from glauconites are not as accurate as those of minerals from igneous rocks. The Sepultura Formation of Baja California, Me´xico, includes a clastic unit interbedded with layers rich in glauconite. Biostratigraphically, the formation is dated as
0895-9811/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 8 9 5 - 9 8 1 1 ( 0 2 ) 0 0 0 8 5 - 8
M.A. Te´llez Duarte, M. Lo´pez Martı´nez / Journal of South American Earth Sciences 15 (2002) 725–730
726
Fig. 1. Map of the study area showing the sampling localities on the Sepultura Formation in north Baja California, Me´xico.
Paleocene (Abbott et al., 1993). To determine if there is agreement between the biostratigraphic and isotopic dates, we sampled glauconites from two localities: Mesa La Sepultura, the type locality, and La Mesa, 60 km to the southwest (Fig. 1). At Mesa La Sepultura, one sample came from the middle of the section (LS-12) in a layer rich in glauconite pellets (Fig. 2). The second sample (LS-17) came from the top of the section (Fig. 2). At La Mesa, one sample was collected near the base of the section (LM-3) (Fig. 2).
2. Geologic setting The Sepultura Formation was named by Santilla´n and Barrera (1930) from exposures between 30 and 328 latitude in the northern Baja California peninsula. They assigned it an Eocene age through correlation with the Martinez Formation of California. Subsequent work has determined that both units of the Sepultura Formation are Paleocene (Zinsmeister and Paredes-Mejı´a, 1988). The geology of
the Sepultura Formation at its type locality was described by Abbott et al. (1993). The type section is located in the northern part of Baja California, approximately 27 km south of the town of El Rosario (Fig. 1). The Formation is composed of two units: a lower clastic unit of fine sandstone resting unconformably, by a basal conglomerate, over Cretaceous sediments, and an upper unit of plane-bedded limestone. Abbott et al. (1993) estimate a low sedimentation rate in a deepening, upward clastic sequence followed by carbonate sedimentation. They use microfossils to assign an Early Maastrichtian age to the underlying Rosario Formation and a Late Paleocene age to the upper limestone unit. The lower clastic unit was dated as Early Late Paleocene on the basis of the presence of the gastropod Turritella peninsularis. According to the biostratigraphic evidence, the K-T boundary does not exist at this locality. Although with some differences, at the La Mesa section, the two stratigraphic units recognized at Mesa La Sepultura are present. The clastic lower unit of fine sandstones rests over the underlying Cretaceous Rosario Formation without
M.A. Te´llez Duarte, M. Lo´pez Martı´nez / Journal of South American Earth Sciences 15 (2002) 725–730
727
Fig. 2. Generalized stratigraphic sections of Mesa La Sepultura and La Mesa with sample locations shown.
a basal conglomerate. Lithologic characteristics are very similar in both Formations, but the most obvious difference in the inferred contact is the first occurrence of T. peninsularis, which indicates the presence of Early Late Paleocene sediments. Therefore, the K-T boundary does not exist. Carbonates at La Mesa consist of reworked and deeper water transported limestones, apparently derived from the top carbonate unit of Mesa La Sepultura. Channel fills of algal limestones with interbedded rip-ups of the underlying clastic unit characterize the gravity flow deposits. At the top of the carbonate layers, clastic sedimentation continues, but no more T. peninsularis is found. Because glauconite is abundant in the lower clastic unit in both localities and they are correlative, the purpose of this work is to determine if glauconite can provide K –Ar dates comparable with those obtained by biostratigraphy and to discuss its geological implications.
3. Methodology Two stratigraphic sections were measured in the best exposures of the Sepultura Formation sediments: at the type locality Mesa La Sepultura and La Mesa (Fig. 2). These are approximately 60 km apart. Glauconite is present in variable abundance in both sections, but mineral pellets are highly concentrated in a few dark layers, which were selected for the purposes of this study. We obtained two samples at the type locality, one from the middle (Sample LS-12) and another from the top (Sample LS-17), and a third sample (Sample LM-3) was collected close to the base of the La Mesa section. Most of the glauconite grains occur as polished ovoid pellets (Fig. 3a) and have few surface impurities. Some pellets are vermiform (Fig. 3b), and probably originated from the fecal pellets of decapods (Hugget and Gale, 1997).
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the Geochronology Laboratory at CICESE. We used the MS-10 mass spectrometer in static mode. The argon extraction line consists of a Modifications Ltd. Low-Ar blank Ta-furnace connected on-line with the mass spectrometer. The argon was extracted at 1500 8C. The liberated gases were purified in two stages. The first stage was accomplished with a LN2 cold trap and Ti getter at 700 8C. The second stage was performed with SAESq and SorbAcq getters. Argon quantification was achieved by adding a 38 Ar-enriched tracer during fusion. The argon isotopes were corrected for blank contribution and mass discrimination by running blanks before each experiment and performing a daily analysis of atmospheric argon samples. The blanks were generally less than 1 £ 1028 cc STP for 40Ar, which represents less than 2% of the total 40Ar measured. All errors reported are 1s. For details about this methodology, see Moreno Rivera and Lo´pez Martı´nez (1996).
4. Results
Fig. 3. Typical glauconite sand grains of the clastic portion of the Sepultura Formation. (a) Ovoid and (b) vermiform.
Several lines of evidence support an authigenic origin for the glauconites. First, glauconite was not detected in the underlying Cretaceous rocks, and none is reported in the literature. Second, most grains are well preserved and lack traces of reworking, and some benthic foraminifera show glauconitization. For glauconite grain separation, we eliminated clays by wet sieving in a 4q mesh sieve (62.5 mm). The samples were then dried at 40 8C and sieved again in 1q intervals. Glauconite was concentrated in the 2q fraction (250 mm) at Mesa La Sepultura and in the 3q fraction (125 mm) at La Mesa. Micas were separated by water column settling in an Emery tube. The remaining grains were dried and individually separated under the microscope to ensure that only glauconite grains remained in the sample. Glauconite grains were analyzed by X-ray diffraction in the Centro de Fı´sica de la Materia Condensada de la UNAM to confirm that no other clay with K was present in the sample. Potassium content determinations were carried out by flame photometry at the Geochemistry Laboratory of CICESE. Four dating experiments were carried out on sample LS-12, two on LS-17, and one on LM-3 in
The clastic section at Mesa La Sepultura and La Mesa contains correlative glauconitic fine sandstone sediments with fossiliferous interbedded layers (Fig. 2). The late Danian gastropod T. peninsularis is the most abundant fossil and dates the sediments as between approximately 60 and 62 Ma (Saul, 1983). Both sedimentologic and paleontologic evidence suggest that clastic sedimentation could be a coeval sedimentary event, and a closely related date could be expected. Carbonates at Mesa La Sepultura and La Mesa are of similar composition, but the deposition mechanisms are different. Mesa La Sepultura is characterized by wellsorted algal limestones beds. At La Mesa, they are deposited as interbedded layers of reworked, channeled carbonates oriented from east to west. This evidence suggests mass transport from an eastward source such as Mesa La Sepultura and, therefore, diachronic sedimentation. Because clastic sediments continue after the carbonates at La Mesa and are missing at the top of the carbonate unit of Mesa La Sepultura, only the lower clastic unit is be expected to be correlative in age. Potassium contents were relatively low in the three samples (Table 1), with averages ranging from 4.13 ^ 0.05 to 5.15 ^ 0.04 wt%. The potassium values and grain features, such as rare inclusions, ovoid and vermiform shapes, and fractures, correspond with the descriptions of lightly evolved to evolved glauconite given by Odin and Fullangar (1988). Ages obtained are shown in Table 1. The weighted average age obtained from four experiments was 60 ^ 1 Ma for Mesa La Sepultura sample LS-12, that nearest to the bottom of the section. The weighted average age from two experiments was 60 ^ 1 Ma for sample LS-17 (top of the section). Only one experiment was performed on the La Mesa sample LM-3 that yielded an age of 59 ^ 1 Ma.
M.A. Te´llez Duarte, M. Lo´pez Martı´nez / Journal of South American Earth Sciences 15 (2002) 725–730
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Table 1 Summary of K–Ar age data obtained from three samples of glauconite grains from Mesa La Sepultura (LS12, LS17) and La Mesa (LM-3) Sample localities shown on Fig. 1 Arp £ 1026 (cc STP/g)
Sample
Ka (%)
K2Oa (%)
40
LS-17
4.57 ^ 0.01 4.57 ^ 0.01 4.78 ^ 0.01 K ¼ 4:64 ^ 0:12
5.50 ^ 0.01 5.50 ^ 0.01 5.76 ^ 0.01
8.204 8.576
5.07 ^ 0.02 5.14 ^ 0.03 5.17 ^ 0.03 5.17 ^ 0.03 5.17 ^ 0.01 5.14 ^ 0.01 5.17 ^ 0.01 K ¼ 5:15 ^ 0:04
6.11 ^ 0.02 6.19 ^ 0.03 6.23 ^ 0.03 6.23 ^ 0.03 6.23 ^ 0.01 6.19 ^ 0.01 6.23 ^ 0.01
4.15 ^ 0.02 4.05 ^ 0.02 4.15 ^ 0.01 4.17 ^ 0.01 K ¼ 4:13 ^ 0:05
5.00 ^ 0.02 4.88 ^ 0.02 5.00 ^ 0.01 5.02 ^ 0.01
LS-12
LM-3
a
40
Aratm (%)
23.9 23.1
Agea (Ma) 59 ^ 2 61 ^ 1 t ¼ 60 ^ 1
9.267 9.275 9.771 9.324
24.4 25.6 21.5 22.9
60 ^ 1 61 ^ 2 61 ^ 1 59 ^ 1 t ¼ 60 ^ 1
6.937
28.3
t ¼ 59 ^ 1
All errors reported are 1s.
5. Discussion The similar dates found at the top of the clastic section at Mesa La Sepultura (60 ^ 1 Ma) and at La Mesa (59 ^ 1 Ma) suggest that the horizons are temporally correlative and that clastic sedimentation in both localities was coeval. The replicate analyses conducted in different splits of the sample and the good agreement among the three dates is evidence that glauconite is pure and homogeneous. Acceptable analytical error allows that modifications in the glauconite structure during diagenesis could result in an estimated 10 –20%, and in some cases 40%, reduction in ages relative to the actual sedimentation age (Hurley et al., 1960). If our samples are 10– 20% too low because of diagenesis, the true date for sample LS-12 is between 66.6 and 72.6 Ma, 1.6 –7.6 Ma older than the K-T boundary. Because sedimentological and paleontological evidence clearly indicate a post-Cretaceous age, the error range must be less than 10%. This translates to a maximum difference of 6 Ma between the measured age and the true age. Taking into account the analytical error, these isotopic dates agree with those obtained through biostratigraphy by Abbott et al. (1993) and place the clastic unit in the late early Paleocene, as was determined on the basis of the presence of T. peninsularis. Thus, the clastic section was deposited at the end of the Danian in the early Paleocene. Hurley et al. (1960) argue that low K – Ar ages are commonly obtained from glauconites due to Ar loss by diffusion. However, the agreement among the three ages obtained in this study suggests that this is not the case for our samples. Because burial at depths with temperatures as low as 150 8C can cause Ar loss (Evernden et al., 1960), our results also suggest that the sedimentary column was not buried very
deeply over a long period, especially considering that the depositional setting was a forearc basin (Atwater, 1970) adjacent to a subduction zone where the geothermal gradient could be between 5 and 10 8C/km. A maximum of 10 – 20 km of sedimentary column could be expected (Raymond, 1995). However, the thickness was probably less than 10 km, because there is no evidence of sediment metamorphism or load structures by lithostatic pressure deformation. However, the excellent agreement between the radiometric and biostratigraphic dates eliminates the possibility of post-diagenetic potassium gain as a reason for low K – Ar ages (Hower, 1961; Hower et al., 1963). Odin (1982) suggests that reliable isotopic dates can be obtained only from evolved glauconites in excess of 6 wt% K2O. However, our glauconites varied between 4.88 and 6.23 wt% K2O. The agreement among the three glauconite ages, the reproducibility in replicate analyses on different splits of the sample, and the agreement with the biostratigraphic age is strong evidence that no significant Ar was inherited from the original glauconized substrate, in which case anomalous old ages would have been obtained. Our results also indicate that even poor potassium glauconites can yield useful results when the samples are pure and homogeneous and the tectonic setting prevents Ar loss due to a low geothermal gradient.
6. Conclusions Paleocene sedimentary rocks from Baja California were successfully dated by the K – Ar method using glauconite. The ages obtained for Mesa La Sepultura of 60 ^ 1 Ma
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close to the bottom of the section and 60 ^ 1 Ma for the top are similar to the 59 ^ 1 Ma obtained from La Mesa, approximately 60 km away. The K – Ar ages are in good agreement with those reported by biostratigraphy and confirm a late Danian age for the lower clastic unit of the Sepultura Formation. The samples analyzed show that reliable K – Ar ages can be obtained even with low potassium content glauconite, particularly in forearc basins where the geothermal gradient is low, which prevents argon loss by diffusion. The apparent lack of argon loss suggests that the Sepultura Formation sediments were not buried too deeply.
Acknowledgments We thank Vı´ctor Moreno and Susana Rosas for technical support with argon extraction, mass spectrometry, and K analyses by flame photometry; Eloisa Aparicio for the X-ray diffraction analysis on glauconite; Rolando Petterson for field assistance; and Universidad Auto´noma de Baja California for financial support throughout the project: No. 1358-6, Paleontologı´a, Tafonomı´a y relaciones estratigra´ficas de la Formacio´n Sepultura (Paleoceno) en el norte de Baja California. We acknowledge Gilles R. Odin and Andrew Hanson for their critical comments and helpful suggestions, which greatly improved the accuracy and readability of our manuscript, and J. Kellogg for his comments.
References Abbott, P.L., Hanson, A.D., Thompson, C.N., Logue, D.L., Bradshaw, K.D., Pollard, W.J., Seeliger, T.E., 1993. Geology of the Paleocene Sepultura Formation, Mesa de la Sepultura, Baja California. Ciencias Marinas 19, 75–93. Amorosi, A., 1995. Glaucony and sequence stratigraphy: a conceptual framework of distribution of siliciclastic sequences. Journal of Sedimentary Research B65, 419–425. Amorosi, A., Centineo, M.C., 1997. Glaucony from the Eocene of the Isle of Wight (southern UK). Journal of the Geological Society, London, 887– 896. Atwater, T., 1970. Implications of plate tectonics for the Cenozoic tectonic evolution of western North America. Geological Society of America Bulletin 81, 3513–3536.
Dalrymple, G.B., Lanphere, M.A., 1970. Potassium – Argon Dating, Freeman, London, p. 258. Evernden, J.F., Curtis, G.H., Kistler, R.W., Obradovich, J., 1960. Argon diffusion in glauconite, microcline, sanidine, leucite and phlogopite. American Journal of Science 258, 583–604. Hower, J., 1961. Some factors concerning the nature and origin of glauconite. American Mineralogy 46, 313–334. Hower, J., Hurley, P.M., Pinson, W.H., 1963. The dependence of K– Ar age on the mineralogy of various particle size ranges in a shale. Geochimica Cosmochimica Acta 27, 405–411. Hugget, J.M., Gale, A.S., 1997. Petrology and paleoenvironmental significance of glaucony in the Eocene succession at Whitecliff Bay, Hampshire Basin, UK. Journal of the Geological Society, London, 897 –912. Hurley, P.M., Cornier, R.F., Hower, J., Fairbairn, H.W., Pinson, W.H., 1960. Reliability of glauconite for age measurement by K–Ar and Rb – Sr methods. American Association of Petroleum Geologists Bulletin 44, 1793–1808. Mitchum, R.M., Van Wagoner, J.C., 1991. High frequency sequences and their stacking patterns: sequence stratigraphic evidence of highfrequency eustatic cycles. Sedimentary Geology 70, 131–160. Moreno Rivera, V.M., Lo´pez Martı´nez, M., 1996. Manual del Laboratorio de Geocronologı´a, Comunicaciones Acade´micas, Serie Geologı´a CICESE,. Morton, J.P., Long, L.E., 1980. Rb –Sr dating of Paleozoic glauconite from the Llano region, central Texas. Geochimica Acta, 663–672. Odin, G.S., 1982. How to measure glaucony. In: Odin, G.S., (Ed.), Numerical Dating in Stratigraphy, Wiley, New York, pp. 387– 403. Odin, G.S., Fullangar, P.D., 1988. Geological significance of the glaucony facies. In: Odin, G.S., (Ed.), Green Marine Clays: Developments in Sedimentology, Elsevier, Amsterdam, pp. 295–332. Odin, G.S., Matter, A., 1981. De glauconarium origine. Sedimentology 28, 611 –641. Polevaya, N.I., 1961. The absolute geochronological time-scale based on glauconites. Trudy Laboratoriia Geologii Dokembriia 12, 123– 132. Raymond, L.A., 1995. Petrology: the Study of Igneous, Metamorphic and Sedimentary Rocks. W.C. Brown. Santilla´n, M., Barrera, T., 1930. Las posibilidades petrolı´feras en la costa occidental de la Baja California entre los Paralelos 308 y 328 latitud norte. Anales del Instituto Geolo´gico de Me´xico, 1 –37. Saul, L.R., 1983. Turritella Zonation Across the Cretaceous-Tertiary Boundary, Publications in Geological Sciences, vol. 125. University of California, California, p. 164. Taylor, K.G., Curtis, C.D., 1995. Stability and facies association of early diagenetic mineral assemblages: an example from a Jurassic ironstone– mudstone succession. UK Journal of Sedimentary Research A65, 358 –368. Zinsmeister, W.J., Paredes-Mejı´a, L.M., 1988. Paleocene biogeography of the west coast of North America: a look at the Molluscan fauna from Sepultura Formation, Mesa San Carlos, Baja California Norte. In: Filewicz, M.V., Squires, R.L. (Eds.), Paleogene Stratigraphy, West Coast of North America, Pacific Section, S.E.P.M., West Coast Paleogene Symposium, vol. 58., pp. 9–22.
K– Ar dating and geological significance of clastic sediments of the Paleocene Sepultura Formation, Baja California, Me´xico Miguel Agustı´n Te´llez Duarte*, Margarita Lo´pez Martı´nez Departamento de Geologı´a, Centro de Investigacio´n Cientı´fica y de Educacio´n Superior de Ensenada, Carretera Tijuana-Ensenada Km. 107, 22860 Ensenada, BC, Mexico Received 1 May 2001; accepted 1 June 2002
Abstract At its type locality, the Paleocene Sepultura Formation consists of two members: a lower glauconitic-rich clastic section and an upper calcareous section. Three samples of authigenic glauconite pellets from two localities with good clastic sediment exposures were dated using K– Ar. At the type locality of Mesa La Sepultura, pellets from the middle of the clastic section yield a date of 60 ^ 1 Ma (weighted average of four experiments), and pellets from the top of the clastic unit give a date of 60 ^ 1 Ma (weighted average of two experiments). Pellets from the base of the section at La Mesa, 60 km distant, give a date of 59 ^ 1 Ma (one experiment). Dates obtained are in good agreement with those reported by biostratigraphy and confirm a Late Danian age. Our results differ from those reported elsewhere, in which low potassium content glauconites yield younger K –Ar ages than expected. An explanation for our observed agreement in ages could be related to the tectonic setting of forearc basins, where the low geothermal gradient prevents argon loss and reliable dates can be obtained. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: K– Ar dating; Glauconite; Paleocene
Resumen En su localidad tipo la Formacio´n Sepultura del Paleoceno consiste de dos miembros: uno inferior cla´stico rico en glauconita y uno superior calca´reo. Se fecharon por K –Ar tres muestra de pellets de glauconita autige´nica provenientes de dos localidades donde los sedimentos cla´sticos se encuentran bien expuestos. En la localidad tipo de Mesa La Sepultura, la parte media de la seccio´n cla´stica proporciono´ una edad 60 ^ 1 Ma (promedio ponderado de cuatro experimentos), y el techo de la misma unidad 60 ^ 1 Ma (promedio ponderado de dos experimentos). En La Mesa, a 60 km de distancia, la base de la seccio´n rindio´ una edad de 59 ^ 1 Ma (un experimento). Las fechas obtenidas se encuentran en concordancia con las reportadas por bioestratigrafı´a, y confirman una edad Daniano tardı´o. Los resultados difieren de otros reportados en otras localidades en glauconitas con un bajo contenido de potasio, las cuales proporcionan edades mas bajas que las esperadas. La explicacio´n puede venir del escenario tecto´nico de las cuencas frente de arco, donde el gradiente geotermal bajo evita la pe´rdidad de argo´n y es posible obtener edades confiables. q 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction Although K –Ar dating with glauconite often produces ages younger than stratigraphically expected, glauconite is virtually the only mineral suitable for direct dating of sedimentary rocks because its authigenic origin and potassium content makes it useful for K – Ar dating (Dalrymple and Lanphere, 1970; Morton and Long, 1980; Amorosi and Centineo, 1997). K –Ar dating of glauconites * Corresponding author. Address: Facultad de Ciencias Marinas, Universidad Auto´noma de Baja California, Apdo. Postal 453, 22800 Ensenada, BC, Me´xico. E-mail address: [email protected] (M.A. Te´llez Duarte).
has frequently been used as a geochronometer for absolute dating and to address problems such as the timing, sedimentation rate, origin, and environmental settings of sedimentary rocks (Polevaya, 1961; Odin and Matter, 1981; Taylor and Curtis, 1995; Mitchum and Van Wagoner, 1991; Amorosi, 1995). Odin and Matter (1981) suggest that at least 103 –104 years of exposure are required to begin the glauconitization process. Thus, absolute dates from glauconites are not as accurate as those of minerals from igneous rocks. The Sepultura Formation of Baja California, Me´xico, includes a clastic unit interbedded with layers rich in glauconite. Biostratigraphically, the formation is dated as
0895-9811/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 8 9 5 - 9 8 1 1 ( 0 2 ) 0 0 0 8 5 - 8
M.A. Te´llez Duarte, M. Lo´pez Martı´nez / Journal of South American Earth Sciences 15 (2002) 725–730
726
Fig. 1. Map of the study area showing the sampling localities on the Sepultura Formation in north Baja California, Me´xico.
Paleocene (Abbott et al., 1993). To determine if there is agreement between the biostratigraphic and isotopic dates, we sampled glauconites from two localities: Mesa La Sepultura, the type locality, and La Mesa, 60 km to the southwest (Fig. 1). At Mesa La Sepultura, one sample came from the middle of the section (LS-12) in a layer rich in glauconite pellets (Fig. 2). The second sample (LS-17) came from the top of the section (Fig. 2). At La Mesa, one sample was collected near the base of the section (LM-3) (Fig. 2).
2. Geologic setting The Sepultura Formation was named by Santilla´n and Barrera (1930) from exposures between 30 and 328 latitude in the northern Baja California peninsula. They assigned it an Eocene age through correlation with the Martinez Formation of California. Subsequent work has determined that both units of the Sepultura Formation are Paleocene (Zinsmeister and Paredes-Mejı´a, 1988). The geology of
the Sepultura Formation at its type locality was described by Abbott et al. (1993). The type section is located in the northern part of Baja California, approximately 27 km south of the town of El Rosario (Fig. 1). The Formation is composed of two units: a lower clastic unit of fine sandstone resting unconformably, by a basal conglomerate, over Cretaceous sediments, and an upper unit of plane-bedded limestone. Abbott et al. (1993) estimate a low sedimentation rate in a deepening, upward clastic sequence followed by carbonate sedimentation. They use microfossils to assign an Early Maastrichtian age to the underlying Rosario Formation and a Late Paleocene age to the upper limestone unit. The lower clastic unit was dated as Early Late Paleocene on the basis of the presence of the gastropod Turritella peninsularis. According to the biostratigraphic evidence, the K-T boundary does not exist at this locality. Although with some differences, at the La Mesa section, the two stratigraphic units recognized at Mesa La Sepultura are present. The clastic lower unit of fine sandstones rests over the underlying Cretaceous Rosario Formation without
M.A. Te´llez Duarte, M. Lo´pez Martı´nez / Journal of South American Earth Sciences 15 (2002) 725–730
727
Fig. 2. Generalized stratigraphic sections of Mesa La Sepultura and La Mesa with sample locations shown.
a basal conglomerate. Lithologic characteristics are very similar in both Formations, but the most obvious difference in the inferred contact is the first occurrence of T. peninsularis, which indicates the presence of Early Late Paleocene sediments. Therefore, the K-T boundary does not exist. Carbonates at La Mesa consist of reworked and deeper water transported limestones, apparently derived from the top carbonate unit of Mesa La Sepultura. Channel fills of algal limestones with interbedded rip-ups of the underlying clastic unit characterize the gravity flow deposits. At the top of the carbonate layers, clastic sedimentation continues, but no more T. peninsularis is found. Because glauconite is abundant in the lower clastic unit in both localities and they are correlative, the purpose of this work is to determine if glauconite can provide K –Ar dates comparable with those obtained by biostratigraphy and to discuss its geological implications.
3. Methodology Two stratigraphic sections were measured in the best exposures of the Sepultura Formation sediments: at the type locality Mesa La Sepultura and La Mesa (Fig. 2). These are approximately 60 km apart. Glauconite is present in variable abundance in both sections, but mineral pellets are highly concentrated in a few dark layers, which were selected for the purposes of this study. We obtained two samples at the type locality, one from the middle (Sample LS-12) and another from the top (Sample LS-17), and a third sample (Sample LM-3) was collected close to the base of the La Mesa section. Most of the glauconite grains occur as polished ovoid pellets (Fig. 3a) and have few surface impurities. Some pellets are vermiform (Fig. 3b), and probably originated from the fecal pellets of decapods (Hugget and Gale, 1997).
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the Geochronology Laboratory at CICESE. We used the MS-10 mass spectrometer in static mode. The argon extraction line consists of a Modifications Ltd. Low-Ar blank Ta-furnace connected on-line with the mass spectrometer. The argon was extracted at 1500 8C. The liberated gases were purified in two stages. The first stage was accomplished with a LN2 cold trap and Ti getter at 700 8C. The second stage was performed with SAESq and SorbAcq getters. Argon quantification was achieved by adding a 38 Ar-enriched tracer during fusion. The argon isotopes were corrected for blank contribution and mass discrimination by running blanks before each experiment and performing a daily analysis of atmospheric argon samples. The blanks were generally less than 1 £ 1028 cc STP for 40Ar, which represents less than 2% of the total 40Ar measured. All errors reported are 1s. For details about this methodology, see Moreno Rivera and Lo´pez Martı´nez (1996).
4. Results
Fig. 3. Typical glauconite sand grains of the clastic portion of the Sepultura Formation. (a) Ovoid and (b) vermiform.
Several lines of evidence support an authigenic origin for the glauconites. First, glauconite was not detected in the underlying Cretaceous rocks, and none is reported in the literature. Second, most grains are well preserved and lack traces of reworking, and some benthic foraminifera show glauconitization. For glauconite grain separation, we eliminated clays by wet sieving in a 4q mesh sieve (62.5 mm). The samples were then dried at 40 8C and sieved again in 1q intervals. Glauconite was concentrated in the 2q fraction (250 mm) at Mesa La Sepultura and in the 3q fraction (125 mm) at La Mesa. Micas were separated by water column settling in an Emery tube. The remaining grains were dried and individually separated under the microscope to ensure that only glauconite grains remained in the sample. Glauconite grains were analyzed by X-ray diffraction in the Centro de Fı´sica de la Materia Condensada de la UNAM to confirm that no other clay with K was present in the sample. Potassium content determinations were carried out by flame photometry at the Geochemistry Laboratory of CICESE. Four dating experiments were carried out on sample LS-12, two on LS-17, and one on LM-3 in
The clastic section at Mesa La Sepultura and La Mesa contains correlative glauconitic fine sandstone sediments with fossiliferous interbedded layers (Fig. 2). The late Danian gastropod T. peninsularis is the most abundant fossil and dates the sediments as between approximately 60 and 62 Ma (Saul, 1983). Both sedimentologic and paleontologic evidence suggest that clastic sedimentation could be a coeval sedimentary event, and a closely related date could be expected. Carbonates at Mesa La Sepultura and La Mesa are of similar composition, but the deposition mechanisms are different. Mesa La Sepultura is characterized by wellsorted algal limestones beds. At La Mesa, they are deposited as interbedded layers of reworked, channeled carbonates oriented from east to west. This evidence suggests mass transport from an eastward source such as Mesa La Sepultura and, therefore, diachronic sedimentation. Because clastic sediments continue after the carbonates at La Mesa and are missing at the top of the carbonate unit of Mesa La Sepultura, only the lower clastic unit is be expected to be correlative in age. Potassium contents were relatively low in the three samples (Table 1), with averages ranging from 4.13 ^ 0.05 to 5.15 ^ 0.04 wt%. The potassium values and grain features, such as rare inclusions, ovoid and vermiform shapes, and fractures, correspond with the descriptions of lightly evolved to evolved glauconite given by Odin and Fullangar (1988). Ages obtained are shown in Table 1. The weighted average age obtained from four experiments was 60 ^ 1 Ma for Mesa La Sepultura sample LS-12, that nearest to the bottom of the section. The weighted average age from two experiments was 60 ^ 1 Ma for sample LS-17 (top of the section). Only one experiment was performed on the La Mesa sample LM-3 that yielded an age of 59 ^ 1 Ma.
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Table 1 Summary of K–Ar age data obtained from three samples of glauconite grains from Mesa La Sepultura (LS12, LS17) and La Mesa (LM-3) Sample localities shown on Fig. 1 Arp £ 1026 (cc STP/g)
Sample
Ka (%)
K2Oa (%)
40
LS-17
4.57 ^ 0.01 4.57 ^ 0.01 4.78 ^ 0.01 K ¼ 4:64 ^ 0:12
5.50 ^ 0.01 5.50 ^ 0.01 5.76 ^ 0.01
8.204 8.576
5.07 ^ 0.02 5.14 ^ 0.03 5.17 ^ 0.03 5.17 ^ 0.03 5.17 ^ 0.01 5.14 ^ 0.01 5.17 ^ 0.01 K ¼ 5:15 ^ 0:04
6.11 ^ 0.02 6.19 ^ 0.03 6.23 ^ 0.03 6.23 ^ 0.03 6.23 ^ 0.01 6.19 ^ 0.01 6.23 ^ 0.01
4.15 ^ 0.02 4.05 ^ 0.02 4.15 ^ 0.01 4.17 ^ 0.01 K ¼ 4:13 ^ 0:05
5.00 ^ 0.02 4.88 ^ 0.02 5.00 ^ 0.01 5.02 ^ 0.01
LS-12
LM-3
a
40
Aratm (%)
23.9 23.1
Agea (Ma) 59 ^ 2 61 ^ 1 t ¼ 60 ^ 1
9.267 9.275 9.771 9.324
24.4 25.6 21.5 22.9
60 ^ 1 61 ^ 2 61 ^ 1 59 ^ 1 t ¼ 60 ^ 1
6.937
28.3
t ¼ 59 ^ 1
All errors reported are 1s.
5. Discussion The similar dates found at the top of the clastic section at Mesa La Sepultura (60 ^ 1 Ma) and at La Mesa (59 ^ 1 Ma) suggest that the horizons are temporally correlative and that clastic sedimentation in both localities was coeval. The replicate analyses conducted in different splits of the sample and the good agreement among the three dates is evidence that glauconite is pure and homogeneous. Acceptable analytical error allows that modifications in the glauconite structure during diagenesis could result in an estimated 10 –20%, and in some cases 40%, reduction in ages relative to the actual sedimentation age (Hurley et al., 1960). If our samples are 10– 20% too low because of diagenesis, the true date for sample LS-12 is between 66.6 and 72.6 Ma, 1.6 –7.6 Ma older than the K-T boundary. Because sedimentological and paleontological evidence clearly indicate a post-Cretaceous age, the error range must be less than 10%. This translates to a maximum difference of 6 Ma between the measured age and the true age. Taking into account the analytical error, these isotopic dates agree with those obtained through biostratigraphy by Abbott et al. (1993) and place the clastic unit in the late early Paleocene, as was determined on the basis of the presence of T. peninsularis. Thus, the clastic section was deposited at the end of the Danian in the early Paleocene. Hurley et al. (1960) argue that low K – Ar ages are commonly obtained from glauconites due to Ar loss by diffusion. However, the agreement among the three ages obtained in this study suggests that this is not the case for our samples. Because burial at depths with temperatures as low as 150 8C can cause Ar loss (Evernden et al., 1960), our results also suggest that the sedimentary column was not buried very
deeply over a long period, especially considering that the depositional setting was a forearc basin (Atwater, 1970) adjacent to a subduction zone where the geothermal gradient could be between 5 and 10 8C/km. A maximum of 10 – 20 km of sedimentary column could be expected (Raymond, 1995). However, the thickness was probably less than 10 km, because there is no evidence of sediment metamorphism or load structures by lithostatic pressure deformation. However, the excellent agreement between the radiometric and biostratigraphic dates eliminates the possibility of post-diagenetic potassium gain as a reason for low K – Ar ages (Hower, 1961; Hower et al., 1963). Odin (1982) suggests that reliable isotopic dates can be obtained only from evolved glauconites in excess of 6 wt% K2O. However, our glauconites varied between 4.88 and 6.23 wt% K2O. The agreement among the three glauconite ages, the reproducibility in replicate analyses on different splits of the sample, and the agreement with the biostratigraphic age is strong evidence that no significant Ar was inherited from the original glauconized substrate, in which case anomalous old ages would have been obtained. Our results also indicate that even poor potassium glauconites can yield useful results when the samples are pure and homogeneous and the tectonic setting prevents Ar loss due to a low geothermal gradient.
6. Conclusions Paleocene sedimentary rocks from Baja California were successfully dated by the K – Ar method using glauconite. The ages obtained for Mesa La Sepultura of 60 ^ 1 Ma
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close to the bottom of the section and 60 ^ 1 Ma for the top are similar to the 59 ^ 1 Ma obtained from La Mesa, approximately 60 km away. The K – Ar ages are in good agreement with those reported by biostratigraphy and confirm a late Danian age for the lower clastic unit of the Sepultura Formation. The samples analyzed show that reliable K – Ar ages can be obtained even with low potassium content glauconite, particularly in forearc basins where the geothermal gradient is low, which prevents argon loss by diffusion. The apparent lack of argon loss suggests that the Sepultura Formation sediments were not buried too deeply.
Acknowledgments We thank Vı´ctor Moreno and Susana Rosas for technical support with argon extraction, mass spectrometry, and K analyses by flame photometry; Eloisa Aparicio for the X-ray diffraction analysis on glauconite; Rolando Petterson for field assistance; and Universidad Auto´noma de Baja California for financial support throughout the project: No. 1358-6, Paleontologı´a, Tafonomı´a y relaciones estratigra´ficas de la Formacio´n Sepultura (Paleoceno) en el norte de Baja California. We acknowledge Gilles R. Odin and Andrew Hanson for their critical comments and helpful suggestions, which greatly improved the accuracy and readability of our manuscript, and J. Kellogg for his comments.
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