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 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 10 3 –10 4 years of exposure are required to begin the glauconitization process. Thus, absolute dates from glau- conites 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: S0895-9811(02)00085-8 Journal of South American Earth Sciences 15 (2002) 725–730 www.elsevier.com/locate/jsames * Corresponding author. Address: Facultad de Ciencias Marinas, Universidad Auto ´noma de Baja California, Apdo. Postal 453, 22800 Ensenada, BC, Me ´xico. E-mail address: mtellez@uabc.mx (M.A. Te ´llez Duarte).