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A section measured from the upper part of the Pimienta Formation and the lowest part of the Tamaulipas Formation located in the central-eastern part of the state of Hidalgo, NE Mexico was studied. The lower part of the succession consists of an argillaceous and laminated dark gray limestone, black shale, black calcareous shale, siltstone and bentonite; the Pimienta–Lower Tamaulipas transition is marked by the sporadic appearance of light-colored limestone in the upper part of the Pimienta and the shift to a succession dominated by thicker beds of yellowish and bioturbated limestone. The microfossil assemblage consists of calpionellids, radiolarians, and planktonic and benthic foraminifers. The biostratigraphic study of the calpionellids enabled three zones to be identifi ed: Calpionella, Calpionellopsis and Calpionellites as well as fi ve subzones, Elliptica, Oblonga, Murgeanui, Darderi and Major, ranging from early late Berriasian to early Valanginian. This allows a chronostratigraphic correlation to be drawn between the Tethys and the Southeastern Pacifi c provinces. In addition, we report the occurrence of abundant radiolarians and planktonic and benthic foraminifers. The paleoenvironmental inferences in the studied section are documented from the predominant wackestone texture and microfossil association, indicating a pelagic basin environment. The abundant occurrence of radiolarians mostly in the Elliptica and Oblonga subzones could suggest an increase in the input of nutrients. [ABSTRACT FROM AUTHOR]

The Upper Cretaceous stratigraphic succession in the western part of the Valles–San Luis Potosí Platform (central to eastern Mexico) reflect the interaction between various paleo-oceanographic factors including sea–level change, tectonic factors, and the type and supply of sediments. The El Abra Formation is a shallow–water carbonate deposit that is overlain by the hemipelagic–pelagic Soyatal Formation, which represents the transition to a deeper, eutrophic, open marine environment in the latest Cenomanian–earliest Turonian (Whiteinella archaeocretacea Partial Range Zone). This unit is linked to a sea-level rise that occurred on a global scale and flooded the platform, coinciding with the Oceanic Anoxic Event 2 (OAE 2). This change is well–marked by faunal assemblages with a low diversity, and environmental stress–resistant species as well as values of δ13C excursion, total organic carbon (TOC) and trace element signatures. These data suggest that in this time the ocean was oxygen deficient. During the early Turonian the platform was completely drowned, which led to re-establishment of the oligotrophic environment. These conditions favored for development of the planktonic foraminifera of the Helvetoglobotruncana helvetica Total Range Zone with the occurrence of keeled forms (k/strategists) even continuing as far as the late Santonian thus in the Soyatal interval two biostratigraphic zones are also recognized: the Dicarinella concavata Interval Zone (late Turonian–late Coniacian) and Dicarinella asymetrica Total Range Zone (early–late Santonian). This indicates that the studied part of the Valles–San Luis Potosi Platform remained flooded during this time, in contrast with Mediterranean Tethys regions where recovery of the shallow-water carbonate platforms occurred in the latest Turonian to mid–Coniacian. The planktonic foraminiferal assemblages of this interval are diverse, composed of large and complex keeled morphotypes, which indicate an open deep–sea environment. [ABSTRACT FROM AUTHOR]

Lithologic and foraminiferal analysis from the western part of the Valles-San Luis Potosí platform reflect a sea-level rise suggesting that the platform was drowned at the Cenomanian-Turonian boundary interval. Benthic and planktic foraminiferal content of the succession was identified to establish the transition from the shallow-water carbonate sedimentation with abundant larger benthic foraminifera, algae, rudists and other molluscs, corals, echinoderms, worms changing to hemipelagic-pelagic sediments containing calcispheres and opportunistic foraminifers (r-strategists), such as whitenellids, hedbergellids, heterohelicids and elongated chambered forms, as well as radiolarians. The El Abra Limestone is dated as the middle-late Cenomanian age (Pseudolituonella reicheliAssemblage Zone) and was deposited on a warm shallow-marine rimmed platform. Peloidal-bioclastic packstone-grainstone is the dominant texture of these rocks. Lithological variations determined by the high detrital influx were followed by a major transgression when wackestone and marly limestone of the lower part of the Soyatal Formation were deposited. The transition to deeper open marine environments is dated as latest Cenomanian/earliest Turonian represented by the Whiteinella archaeocretacea Partial Taxon Range Zone and coincides with a global sea-level rise and the Oceanic Anoxic Event 2 (OAE 2). Deeper overlying deposits of the Helvetoglobotruncana helvetica Total Range Zone (early/middle Turonian) are characterized in the study area by the occurrence of the keeled forms (k strategists). The Cenomanian/Turonian boundary is placed within the Whiteinella archaeocretacea Partial Range Zone in absence of ammonite markers. When theValles-San Luis Potosí platformwas flooded, the carbonate factorywas stressed due to the trophic resource modifications. Production of a larger amount of nutrients led to the extinction of the k-strategist benthic foraminifera; only small benthic foraminifera survived for a short period of time in the newly established mesotrophic conditions. High productivity continued and in the eutrophic episode the calcispheres, r-opportunists, intermediate taxa and the planktic foraminifera with elongate chambers proliferated. Later, complete drowning enabled the recovery of oligotrophic conditions with the occurrence of k-strategist keeled planktic taxa. [ABSTRACT FROM AUTHOR]

In central Chiapas (southeastern Mexico) the last occurrence of inoceramid bivalves was recorded in strata of the predominantly terrigenous Ocozocoautla Formation. The inoceramids are associated with foraminifers of the Gansserinc, gansseri Zone (Late Campanian to Early Maastrichtian). The described species are: Cataceramus? cf. barabini (Morton, 1834), Cataceramus? cf. subcircularis (Meek, 1876), Trochoceramus aff. costaecus (Khalafova, 1966), Trochoceramus nahorianensis (Kociubynskij, 1968), and Trochoceramus tricostarus n. sp. Based on the inoceramid and foraminifer species present, the assemblage is assigned to the Early Maastrichtian. This age is important because at this level most of the typical inoceramids globally disappeared. This is the first report of the genus Trochoceramus in Mexico, which is a good index fossil for the Early Maastrichtian and has a cosmopolitan distribution. [ABSTRACT FROM AUTHOR]

The continental interior of Mexico is characterized by a Late Cretaceous- Eocene fold-thrust belt named the Mexican Fold-Thrust Belt, which shows characteristics of an eastward-tapering orogenic wedge. The juxtaposition of the Guerrero terrane to the west of the Mexican Fold-Thrust Belt has motivated many to propose the accretion of this terrane as the cause for the regional shortening in the Mexican continental interior. The Sierra de los Cuarzos is located in the westernmost Mexican Fold- Thrust Belt, directly to the east of the boundary with the Guerrero terrane. Based on its position, the Sierra de los Cuarzos is key in reconstructing the possible propagation of shortening deformation from the Guerrero terrane suture belt into the Mexican continental interior. Our new paleontologic and K-Ar geochronologic data from the Sierra de los Cuarzos show that shortening in the westernmost Mexican Fold-Thrust Belt started ca. 83 Ma, which is ~30 m.y. later than the late Aptian age of the Guerrero terrane suture boundary. Therefore, the integration of our new paleontologic and geochronologic determinations with previous data suggests that the Guerrero terrane was already accreted and amalgamated with the Mexican continental interior at the time of the formation of the Mexican Fold-Thrust Belt. In light of this, the shortening structures in the western Mexican Fold-Thrust Belt do not show any in-sequence relationship with the adjacent Guerrero terrane suture belt. Therefore, we favor the idea that the Guerrero terrane suture boundary and the Mexican Fold-Thrust Belt are two distinct orogens that mark two distinct stages of tectonic evolution of the North American Pacific margin. [ABSTRACT FROM AUTHOR]