ISSN: 0378-102X www.ucm.es \JIG Journal of Iberian Geology 31 (2004) 25-50 Events of the Cenomanian-Turonian Succession, Southern Mexico Eventos de una Sucesión del Cenomaniano-Turoniano del Sur de México Noemí Aguilera-Franco1 and Peter Allison2 1Instituto Mexicano del Petróleo, Gerencia de Geociencias, Edificio 6, Eje Central Norte Lázaro Cárdenas 152, C.P. 07730, México D.F. MEXICO. E-mail address:[email protected] 2T.H. Huxley School of Environment, Earth Science and Engineering, Imperial College of Science Technology and Medicine, Prince Consort Road, London SW7 2BP, UK. Received: 22/10/03 / Accepted: 16/06/04 Abstract The Cenomanian-Turonian succession of the Guerrero-Morelos basin contains a number of paloecommunities that can be cor- related. These palaeocommunities have been recognized and interpreted as the result of environmental disturbances. Some of these bioevents are probably local (platform-wide) and reflect successive stages of the platform drowning, whereas others have equiva- lents in other parts of the world and are probably linked to global paleoceanographic changes. Bioevents that can be used for cor- relation are: 1) the last appearance of Pseudorhapydionina dubia (94.4Ma); 2) the disappearance of most large benthic foraminifers and calcareous algae (94.2Ma); 3) the first appearance of hippuritid mollusks (93.5Ma); 4) the first appearance ofHelvetoglobotrun - cana helvetica (93.0Ma) accompanied by a diversification of keeled planktonic foraminifers. Increase in13 C and TOC values in the deeper-water facies covering the carbonate platform suggests a probable link between the drowning of the platform and the global Cenomanian-Turonian Anoxic Event. The deposition of organic-rich facies in the upper Whiteinella archaeocretacea and lower Helvetoglobotruncana helvetica Zones is associated with the establishment of oxygen poor and eutrophic conditions. Keywords: bioevents, stable isotopes, TOC, Cenomanian-Turonian, Southern Mexico Resumen Se identificaron una serie de paleocomunidades en la cuenca de Guerrero-Morelos del sur de México. Estas paleocomunidades pueden ser correlacionadas e interpretadas como resultado de cambios ambientales anómalos. Algunos de estos bioeventos son locales y reflejan los diferentes estadíos del hundimiento de la plataforma, mientras que otros tienen equivalentes en otras partes del mundo y posiblemente estén ligados a cambios paleoceanográficos globales. La secuencia de eventos que se identificaron y que tienen valor de correlación son: 1) la ultima aparición de Pseudorhapydionina dubia (B1.2=93.83Ma); 2) la desaparición de la ma- yoría de foraminíferos bentónicos y algas calcáreas (B1.3=93.5Ma); 3) la primera aparición de moluscos hipurítidos (B4=93.31Ma); 4) la primera aparición de Helvetoglobotruncana helvetica (B7=93.0Ma) acompañada por una diversificación de foraminíferos planctónicos quillados. El incremento en los valores del isótopo de C13 y de Carbón Orgánico Total en las facies profundas, sugiere un relación entre el hundimiento de la plataforma y el Evento Oceánico Anóxico del Cenomaniano-Turoniano. El depósito de se- dimentos con materia orgánica en la parte superior de la Zona de Whiteinella archaeocretacea y en la parte inferior de la Zona de Helvetoglobotruncana helvetica está probablemente asociado al establecimiento de condiciones eutróficas y pobres en oxígeno. Palabras Clave: bioeventos, isótopos estables, COT, Cenomaniano-Turoniano, Sur de México. 26 Aguilera-Franco & Allison / Journal of Iberian Geology 31 (2004) 25-50 Aguilera-Franco & Allison / Journal of Iberian Geology 31 (2004) 25-50 27 1. Introduction Drowning of the platform was apparently related to local tectonic and paleoceanographic changes, compounded Earth’s prevailing climatic conditions during the Creta- with global forcing factors associated with the Ceno- ceous was very different than present and past recent cli- manian-Turonian Anoxic Event documented worldwide mates, as the record indicates that greenhouse conditions (Arthur and Schlanger, 1985). peaked in the earliest Turonian (Jenkyns et al., 1994), The present work is a detailed study of seven measured with the highest sea level stand ever recorded at a maxi- sections in the upper part of the Morelos Formation, and mum of 300 meters above present level (Barnes et al., the lower part of the Cuautla and Mexcala Formations 1996). In addition to volcanic activity associated with sea that coincide with the Cenomanian-Turonian succession. floor spreading, it has been postulated that plume-related Our goal was to identify changes in the fossil assem- intraplate volcanism was also an important contributor blage, lithology and geochemistry within the Cenoma- to high rates of mantle degassing (Tarduno et al., 1991; nian-Turonian succession of the Guerrero-Morelos basin, Kerr, 1998), resulting in a considerably warmer Earth at and to evaluate their use as stratigraphic markers for lo- that time. The Cretaceous elevated temperature may have cal, regional and global correlation. been caused by high atmospheric CO2 levels that allowed the development and expansion of tropical climate (Val- 2. Study Area and Geologic Setting des et al., 1996). The Cretaceous also includes one of the most conspicuous and well-documented mass extinction The study area is located in the southeastern part of in Earth’s history identified at the Cenomanian-Turonian the Morelos basin, which is in northern Guerrero State boundary. During this interval several groups of ammo- of southern Mexico (Fig. 1). The stratigraphic sequence nites, mollusks, corals, echinoderms, calcareous nanno- is composed of a thick succession of more than 800m of fossils, planktonic and benthic foraminifers were strongly shallow-marine limestones of the Morelos and Cuautla affected (Kauffman and Hart, 1996; Barnes et al., 1996; Formations that grade upwards into pelagic limestones Peryt and Lamolda, 1996). This event also coincides with and siliciclastics of the Mexcala Formation that compris- an anomalously high concentration of organic matter, es the Turonian and the Coniacian (Fig. 2). These rocks and positive excursions of the δ13C in biogenic organic are unconformably overlain by Tertiary continental de- marine carbonate (Raup and Sepkoski, 1986a, 1986b; posits of the Balsas Group, as well as by some remnants Arthur et al., 1987; Schlanger et al., 1987; Jarvis et al., of Oligocene rhyolitic volcanism, and by Quaternary vol- 1988; Koutsoukos et al., 1990; Jenkyns, 1991; Gusic canic rocks of the Trans-Mexican Volcanic Belt (Fries, and Jelaska, 1995; Caus et al., 1993; Ulicny et al., 1993; 1960; Morán-Zenteno, 1994). Peryt and Wyrwicka, 1993; Kauffman and Hart, 1996; Kerr, 1998 among others). The mechanisms put forward 2.1 Stratigraphy to explain this extinction are diverse and include: oceanic anoxia (Schlanger et al., 1987; Arthur et al., 1987), rise The following is a brief description of the lithostrati- (Jenkyns et al., 1980; Jarvis et al., 1988) and fall in sea graphic units of the Morelos and the basal part of the level (Jeans et al., 1991), salinity stratification (Brass et Cuautla and Mexcala Formations that coincide with the al., 1982) and an increase in productivity (Jarvis et al., Cenomanian-Turonian succession (Fig. 2). 1988; Vogt, 1989; Hilbretch et al., 1992). 2.1.1. Morelos Formation (Fries, 1960; de Cserna et al., 1980) The Cenomanian-Turonian event in the stratigraphic succession of Mexico is so far poorly documented. In Lithology. The Morelos Formation includes a succes- some areas, the sediments of that time interval occur as sion of limestones and some dolomites with a reported hemipelagic and pelagic facies (Basañez-Loyola et al., thickness that varies between 600m and 1000m (Fries, 1993; Cantú-Chapa, 1993), although shallow-marine 1960, González-Pacheco, 1991). Bioturbation, planar carbonate rocks are the dominant lithology (Enos and lamination, fenestral structures and desiccation cracks Stephens, 1993; Wilson and Ward, 1993). are the most common sedimentary structures (González- The Cenomanian-Turonian succession in the Guer- Pacheco, 1991; Hernández-Romano, 1995; Aguilera- rero-Morelos basin in southern Mexico is composed of Franco, 1995; Aguilera-Franco et al., 1998b). shallow marine limestones indicative of a platform that Petrographically it consists of bioclastic, peloidal became well established since the Early Cretaceous but and intraclastic packstone-wackestones with abundant was drowned during the latest Cenomanian (Hernán- microfossils of assemblages that are representative of dez-Romano et al., 1997; Aguilera-Franco et al., 2001). the inner shelf environment (e.g. miliolids, lituolids, ro- 26 Aguilera-Franco & Allison / Journal of Iberian Geology 31 (2004) 25-50 Aguilera-Franco & Allison / Journal of Iberian Geology 31 (2004) 25-50 27 Fig. 1.- Location of the study area. Adapted from Aguilera-Franco et.al. (2001). Fig. 1.- Localización del área de estudio. Modificado de Aguilera-Franco et al. (2001). taliids, discorbiids, dasycladacean, scarce red and other over the pre-Cretaceous rocks of the Acatlán Complex calcareous algae such as Thaumatoporella parvovesicu- toward the easternmost part of the basin, (de Cserna et lifera). Mollusks (rudists and gastropods), ostracods, al., 1981). scarce echinoderms that are of more restricted environ- 2.1.2. Cuautla Formation (Fries, 1960) ments are also present, and may dominate the limestones (Fries, 1960; Aguilera-Franco, 1995, Aguilera-Franco et Lithology. The Cuautla Formation was formally de- al., 1998b; Hernández-Romano