NEW UPPER TITHONIAN (JURASSIC) AMMONITES FROM THE CHINAMECA FORMATION IN SOUTHERN VERACRUZ, EASTERN MEXICO

ABSTRACT-

Upper Tithonian (Jurassic) ammonites occur in the Chinameca Formation at Chinameca, southern Veracruz, eastern Mexico. The fauna includes Eopaquiericeras peraltai n. gen. and sp. and E. pazi new genus and species of the family Platylenticeratidae, Chinamecaceras maldonadoi n. gen. and sp. of the subfamily Aspidoceratinae, Veracruciceras ruizi n. gen. and sp. and V. myczynskii n. gen. and sp. of the subfamily Simoceratinae. Durangites zigzagcostatus n. sp., and representatives of Haploceras, Salinites, Kossmatia, and Andiceras. Kossmatia is the most abundant and biostratigraphically significant ammonite in the Chinameca fauna and its age significance and distribution in Mexico is discussed. In addition, a comparison of the suture lines of Kossmatia and the related older genus Lemencia indicates that they both belong to the Tithonian subfamily Richterellinae rather than the Upper Oxfordian-Kimmeridgian families Ataxioceratidae and Lithacoceratidae. The Chinameca ammonites have affinities with taxa of the same age from central and northern Mexico, Cuba, and the southern United States.

INTRODUCTION

ONLY TWO outcrops with Upper Tithonian ammonites are known from the Gulf of Mexico coastal plain. The Chinameca Formation consists of limestone and chert in its type locality in southern Mexico. This small outcrop contains ammonites of the families Protancyloceratidae, Haploceratidae, and Perisphinctidae of Kimmeridgian to Lower Cretaceous age (Burckhardt, 1930) (Fig. 1). The second known outcrop is at Cruillas in northeastern Mexico. At this locality, argillaceous limestone and shale of the La Casita Formation contain the Upper Tithonian genera Protancyloceras Spath, 1924 and Kossmatia Uhlig, 1907 (Kellum, 1937). Both outcrops were discussed by Imlay (1980) and Cantú-Chapa (1982).

Subsurface.-In addition, there have been several subsurface stratigraphic studies of the Tithonian of the Gulf of Mexico coastal plain based on ammonites and well logs. Tithonian formations intersected by bore holes include the La Casita Formation in northeastern Mexico, the Pimienta Formation in eastern Mexico, and the Edzna Formation in southeastern Mexico. These formations lie at depths of between 400 and 4,800 m below sea level in the Gulf region (Cantú-Chapa, 1982, 1989; Cantú-Chapa and Ortuño, 2004).

Other localities.-An ammonite from the Malone Mountains in southwestern Texas which was referred to the Lower Cretaceous Neocomites cf. indicus Uhlig, 1910 by Albritton (1937) was later determined to be the Upper Tithonian species Kossmatia varicostata Imlay, 1943 by Cantú-Chapa (1976b) and Imlay (1980). The Artemisa Formation in the Pinar del Rio area, western Cuba, also contains a number of the same Tithonian ammonites that occur in Mexico (Imlay, 1942; Judoley and Furrazola-Bermudez, 1968; Myczynski, 1994a, 1994b, 1999a, 1999b).

The obvious affinity exhibited by Upper Tithonian ammonites from Mexico, the southern United States, and western Cuba suggests that they represent the same paleogeographic province, with similar faunal and sedimentological conditions (Imlay, 1980; Cantú-Chapa, 1989, 2001a). In addition, Tithonian ammonites are also known from east-central (Mazatepec), northeastern (Galeana), central (Mazapil, Real de Catorce, Symón), and northern (Chihuahua) Mexico (Burckhardt, 1906, 1912, 1919-1921; Imlay, 1939, 1943; Cantú-Chapa, 1967, 1968, 1976a, 1976b, 1993, 1998, 1999; Verma and Westermann, 1973; Oloriz et al., 1999).

Geological setting and location.-Ammonites described in this paper were recovered from the Chinameca Formation in the El Piedral quarry located about 5 km northeast of the village of Chinameca, in Veracruz, southern Mexico (Fig. 1). The Chinameca Formation consists of light brown, laminated, and fossiliferous limestone in beds 20-40 cm in thickness. It was originally defined by Benavides (1950) for exposures in the flanks of the Cerro Pelón Anticline, at Minatitlán, Veracruz, where it attains a thickness of 300 m. Unfortunately, the El Piedral section is in an active quarry constantly used to extract rock. The ammonite collection described herein was recovered from an area of only approximately 100 m^sup 2^. The exposure is surrounded by the Upper Oligocene Depósito Formation.

Paleontological, systematic, and biostratigraphic problems.-Recent studies of Upper Jurassic and Lower Cretaceous ammonites have emphasized classifications based on presumed sexual dimorphism, with attempts to explain their origins and centers of dispersion. Genera, subgenera, and species are divided arbitrarily into macroconchs and microconchs, depending on the relative size of the shell and the presence of lappets in complete specimens (Verma and Westermann, 1973; Donovan et al., 1981; Enay and Cecca, 1986). Such classifications are inconclusive, however, for incomplete ammonites (Cecca and Enay, 1991).

These studies have proposed ammonite classifications which employ a twofold nomenclature. For example, a form considered to be the microconch of one genus may also be considered to be the macroconch of another, such as the Upper Tithonian Durangites-Protacanthodiscus (Tavera-Benítez, 1985; Enay et al., 1998). Such classifications are highly subjective and arbitrary; moreover, taxonomic difficulties and inconsistencies such as those cited above have been recognized by the same paleontologists who proposed them (Verma and Westermann, 1973; Donovan et al., 1981). Consequently, they have been rejected by Ziegler (1974) and Cantú-Chapa (2001b).