Abstract The Taraises and lower Tamaulipas formations of the Early Cretaceous (Berriasian–Valanginian) are basinal sedimentary sequences in northeastern and eastern Mexico, respectively. The first unit was penetrated in the San Javier 1 and 2 wells at depths between 1800 and 3200 m (5905 and 10,499 ft), and the second was drilled in the Bejuco 6 and the La Laja 8 wells between 2000 and 2500 m (6562 and 8202 ft). In the Bejuco 6 well, the lower Tamaulipas Formation represents a condensed section. Geophysical logs and ammonites allow the characterization and age dating of these units. Two important ammonite groups (Ancyloceratoidea and Perisphinctoidea) are described from cores in the four wells and from surface occurrences. Four new genera are proposed, Misantlites, Wiedmannites, Parvaites , and Parrasites ; new species are M. reyesi, Aegocrioceras eguiluzi, Capeloites lajense, C. peyroullesense, Thurmanniceras kleini, Neohoploceras bartolinii, Parvaites aguirrei, P. leanzai ; and one new subfamily is proposed, Capeloitinae.

Abstract The configuration of the top Jurassic is based on structural maps derived from oil-well data on the subsurface from the Poza Rica and Tampico regions, eastern Mexico. The maps are based on depths to the contact of the Pimienta and Lower Tamaulipas Formations at the Jurassic-Cretaceous boundary. Lithostratigraphic and chronostratigraphic data are from gamma-ray logs and ammonites, respectively. In the Poza Rica district, the La Mesa syncline was surveyed in the northwest, and the Sultepec homocline in the southeast. The depth to the top Jurassic varies from 2000 to 3700 m in the east and southeast of this region. An elongated area remained emergent during deposition of the Tithonian Pimienta Formation, along the present Gulf of Mexico coast east of Poza Rica, continuing to the southeast of the Tampico area. The Pimienta Formation was eroded in the southeast of Poza Rica along the initial cut of the San Andrés paleocanyon. In the Tampico district, the top of the Pimienta Formation is found from 1000 to 3000 m in depth. Two structures, the Tranquitas anticlinorium and the Tanquian anticlinorium, are observed in the northwest and central part of the Tampico district, respectively. Various areas remained emergent during deposition of this formation to the east and southeast of Tampico. They were part of an ancient continent composed of metamorphic and intrusive rocks, and upper Paleozoic continental origin. In the Bejuco region east of the Tampico district, two structures, the Piedra de Cal anticline and the Jabonera syncline, are observed; depth to the top Jurassic varies from 1400 to 3000 m, respectively. In this same region, two areas, Llano de Bustos and La Aguada, remained emergent during the upper Tithonian.

Abstract The Berriasian-Valanginian Taraises Formation in the subsurface of northeastern Mexico consists of an intercalation of argillaceous limestone and shale. This formation is divided into two members, based on lithologic characteristics defined from electric and radioactivity logs from oil wells in northeastern Mexico. In addition, two small ammonite groups of the subfamily Olcostephaninae (upper Valanginian) collected at Potrero de García, Nuevo León, northeastern Mexico, are described: (1) Capeloites neoleonense n. sp. and (2) Garcites potrerensis n. gen. and sp. and G. cavernensis n. gen. and sp. The presence of Dichotomites (Dichotomites ) sp. in the same beds supports an late Valanginian age for these strata. One specimen of Capeloites obtained from a core of the La Laja-8 well, south of Tampico City, eastern Mexico, designates an late Valanginian age for the interval drilled (2556-2562 m) in the so-called Lower Tamaulipas Formation in this area. Global geographic distribution of Capeloites is restricted to France, Spain, and Peru; its presence in Mexico supports a biogeographic link between Europe and America.

Abstract This study presents a regional structural analysis and a stratigraphic study in Hidalgo and Querétaro states, east-central Mexico, where strata of the Sierra Madre Oriental foreland fold-and-thrust belt are structurally juxtaposed against Middle (?) and Upper Jurassic back-arc sequences. Rocks of the San Juan de la Rosa Formation were apparently affected by major compressive deformation during the Early Cretaceous (Nevadian?) and Laramide orogenies. Toward the east, the style of deformation is that of a typical foreland fold-and-thrust belt, whose major thrust faults are located at the Zimapán Basin margins flanked by the El Doctor and Valles-San Luis Potosí carbonate platforms. Below the El Doctor thrust, at least seven thrust faults are exposed that crosscut the middle Cretaceous rocks and the enveloping Upper Jurassic-Lower Cretaceous and Upper Cretaceous marly shaly beds in the form of a duplex. The style of deformation between the two carbonate platforms is probably thin-skinned. Speculative, semibalanced cross sections indicate an average shortening between the Zimapán Basin margins of 39.4% of its initial width. Depth to the sole thrust is estimated to be about 2000 m below the present sea level.