Ana Bertha Villaseñor

Major lithostratigraphic units in land-outcrops of north-central Mexico and the subsurface along the northern rim of Gulf of Mexico Basin (Upper Jurassic - lowermost Cretaceous): a proposal for correlation of tectono-eustatic sequences

The stratigraphic and geodynamic interpretation of Upper Jurassic lithostratigraphic units is revised in north-central Mexico and the northern rim of the Gulf of Mexico Basin through updated ammonite and calpionellid biochronostratigraphy. Significant events in the geodynamic evolution in these areas are evaluated and interpreted in terms of tectono-eustatic sequences (TES) of third and second orders. 3rd-TES-I and 2nd-TES-II/III in Mexico and 3rd-TES-I, 3rd-TES-II, and 3rd-TES-III at the northern rim of the Gulf of Mexico show the main traits of the Upper Jurassic Supercycle in these regions, allowing the identification of a combined ‘Atlantic-Tethyan cachet’ in the course of structuring/configuration of the Gulf of Mexico Basin during the Late Jurassic. The easy identification of 3rd-TES-I in north-central Mexico and at the northern rim of the Gulf of Mexico Basin shows no significant difference in geodynamic history during the Oxfordian, which contrasts with the increasing difference from the Kimmeridgian to the Early-Middle Berriasian. Shared trends in stratigraphic architecture with the European margin of the North Atlantic Basin, as well as with epicontinental shelves surrounding Iberia and other Tethyan areas, are interpreted to show phases of the geodynamic evolution in the central North Atlantic Basin, traces of which are recognizable also in western Africa.

Ammonite Biostratigraphy and Correlations in the Upper Jurassic-Lowermost Cretaceous La Caja Formation of North-Central Mexico (Sierra De Catorce, San Luis Potosí)

Ammonite stratigraphy of the Upper Jurassic-lowermost Cretaceous La Caja Formation is analyzed in two sections near Rancho Los Alamitos in the Sierra de Catorce, San Luis Potosi, Mexico. Numerous ammonites and other megainvertebrates, mainly bivalves, were collected bed by bed and complemented with data on rare calpionellids. The data obtained on ammonites provide the most precise biostratigraphy known from the region. The El Pastor Member is dated as late Early Kimmeridgian to Middle Tithonian and at least part of the El Verde Member as (late Early to Middle) Berriasian. The dominant Kimmeridgian ammonites are Idoceras and haploceratids. As common in north-central Mexico, Tithonian ammonoids occur in stratigraphically discontinuous assemblages, some of these in condensed deposits related to transgressions. According to the record of ammonites and calpionellids, the Kimmeridgian, Tithonian, and Berriasian are incomplete in the sections studied. The record of Kossmatia in the upper Lower to Middle Tithonian is confirmed.

Geographic control on phenotype expression. The case of Hybonoticeras mundulum (Oppel) from the Mexican Altiplano

Hybonoticeras mundulum (Oppel) (m) s.s. is reported from the Mexican Altiplano on the basis of material collected bed-by-bed in sections in the States of Durango and Zacatecas. The known range of this species in the Mexican Altiplano is interpreted to be uppermost Kimmeridgian (upper to uppermost Beckeri Zone) to lowermost Tithonian (basal to lower Hybonotum Zone). The Mexican specimens studied are the most complete and valuable collection of H. mundulum (Oppel) s.s. known from a given area, and reveal that Mexican populations show phenotypic features different from European ones. Vicariant events accord with data available about the areal and biostratigraphic distribution of this species, as well as with the combination of allocyclic and autocyclic factors influencing Mexican seas, the breaking of populations rather than colonization events, and the impoverished ammonite assemblages showing endemic traits within the stratigraphic interval studied.

New microconchiate Hybonoticeras from Mexico

A diversified and previously unknown record of microconchiate Hybonoticeras is analysed on the basisof the material collected in sections sampled bed-by-bed in the Mexican Altiplano. Five new species are described and compared to species traditionally recorded from the uppermost Kimmeridgian and the lowermost Tithonian in Europe. A general trend to smoothing in Mexican and European species during the earliest Tithonian is outlined.

Towards a consistent Oxfordian/Kimmeridgian global boundary : current state of knowledge

New data are presented in relation to the worldwide definition of the Oxfordian/Kimmeridgian boundary, i.e. the base of the Kimmeridgian Stage. This data, mostly acquired in the past decade, supports the 2006 proposal to make the uniform boundary of the stages in the Flodigarry section at Staffin Bay on the Isle of Skye, northern Scotland. This boundary is based on the Subboreal-Boreal ammonite successions, and it is distinguished by the Pictonia flodigarriensis horizon at the base of the Subboreal Baylei Zone, and which corresponds precisely to the base of the Boreal Bauhini Zone. The boundary lies in the 0.16 m interval (1.24–1.08 m) below bed 36 in sections F6 at Flodigarry and it is thus proposed as the GSSP for the Oxfordian/Kimmeridgian boundary. This boundary is recognized also by other stratigraphical data – palaeontological, geochemical and palaeomagnetic (including its well documented position close to the boundary between magnetozones F3n, and F3r which is placed in the 0.20 m interval – 1.28 m to 1.48 m below bed 36 – the latter corresponding to marine magnetic anomaly M26r).The boundary is clearly recognizable also in other sections of the Subboreal and Boreal areas discussed in the study, including southern England, Pomerania and the Peri-Baltic Syneclise, Russian Platform, Northern Central Siberia, Franz-Josef Land, Barents Sea and Norwegian Sea. It can be recognized also in the Submediterranean-Mediterranean areas of Europe and Asia where it correlates with the boundary between the Hypselum and the Bimmamatum ammonite zones. The changes in ammonite faunas at the boundary of these ammonite zones – mostly of ammonites of the families Aspidoceratidae and Oppeliidae – also enables the recognition of the boundary in the Tethyan and Indo-Pacific areas – such as the central part of the Americas (Cuba, Mexico), southern America, and southern parts of Asia. The climatic and environmental changes near to the Oxfordian/Kimmeridgian boundary discussed in the study relate mostly to the European areas. They show that very unstable environments at the end of the Oxfordian were subsequently replaced by more stable conditions representing a generally warming trend during the earliest Kimmeridgian. The definition of the boundary between the Oxfordian and Kimmeridgian as given in this study results in its wide correlation potential and means that it can be recognized in the different marine successions of the World.

Lower Aptian AmmonitinaPseudohaploceras liptoviense (Zeuschner) in the Cumburindio Formation (Southwestern Mexico)

Abstract The presence of Pseudohaploceras liptoviense ( Zeuschner ) associated to Palorbitolina lenticularis ( Blumenbach )permits to interpret an Early Aptian age for the middle part of the lower member of the Cumburindio Formation nearby Turitzio (Michoacan). It is proposed that Boses species “Desmoceras” aguilerae, “D.” alzatei, “D.” burckhardti, “D.” durangense, “D.” tenuicostatum and “D.” wielandi are younger synonymous of Pseudohaploceras liptoviense ( Zeuschner ). An alternative interpretation as Mexican colonizers very close to Pseudohaploceras liptoviense ( Zeuschner ) cannot be conclusively disregarded.

INNER WHORLS OF GREGORYCERAS (AMMONITINA, PELTOCERATINAE) AS THE FIRST OCCURRENCE OF THE GENUS IN MEXICO

Two nuclei of Gregoryceras are described as the first record of the genus in Mexico. The occurrence in Mexico of the ubiquitous Tethyan genus Gregoryceras is interpreted as related to a major flooding episode that affected neritic shelves in the southern North-American plate.

Lower Tithonian Microconchiate Simoceratins from Eastern Mexico: Taxonomy, Biostratigraphy, and Palaeobiogeography

The precise record of simoceratins sampled bed-by-bed is first reported from Mexico (Mazatepec area in Puebla, central-eastern Mexico), as well as the existence of lappeted peristomes in these ammonites. Both Pseudovolanoceras aesinense and the subspecies Pseudovolanoceras aesinense chignahuapense are shown to occur among Mexican simoceratins. The European species and the Mexican subspecies share the same stratigraphic range in the studied sections, yet they differ in ephebic sculpture. Ecological adaptation to neritic seas corresponding to eastern Mexico areas is interpreted, forcing phenotypic deviation with geographic significance, i.e., subspeciation. The new subspecies would indicate stratigraphic horizons within the Semiformiceras semiforme/Haploceras verruciferum Chronozone in the Mediterranean Tethys. A revision of contemporaneous simoceratins in the Americas is founded on a comparative analysis with respect to the European species P. aesinense.

Palaeoecological significance of Barremian ammonite assemblages and facies variations from Southwest Mexico

Analyses of ammonite shell forms of two Barremian stratigraphic sections from Southwest Mexico consist of two well-defined morphotypes: (1) Small uncoiled, mostly leptoceratoid ancyloconic shells of the families Ancyloceratidae and Hamulinidae, and (2) middle-sized involute to moderately evolute oxycone to discocone shells of the family Pulchelliidae. Index taxa allow the recognition of standard ammonite biozones for the Barremian, which permit the relative dating of different processes that occurred through the water column in the environment of deposition. The vertical distribution of ammonite morphotypes and facies suggests changes of the palaeoceanographic and sedimentological conditions that prevailed in the area during Barremian time. Petrologic data, analyses of the organic carbon and carbonate contents of the rocks support the idea that oxygen-deficient bottom waters existed within a shallow marine, tectonically active area with little carbonate deposition during the early early Barremian (upper part of the Taveraidiscus hugii Zone through the base of the Nicklesia pulchella Zone). These conditions in the basin caused a proliferation of middle-water depth ammonites of Morphotype 1 but prevented the abundance of nektobenthic forms of Morphotype 2. Oxic conditions on a more calcareous and open normal marine environment seem to have been reestablished progressively during a transgressive episode from late early-early late Barremian (upper part of the Nicklesia pulchella Zone through the Gerhardtia sartousiana Zone). This environmental setting supported more facies dependent nektobenthic ammonites of Morphotype 2 to flourish within the basin.

First record of the genus Simocosmoceras Spath, 1925, Ammonitina, in Mexico. Biostratigraphic and palaeobiogeographic interpretation

Abstract The occurrence of Simocosmoceras in Mexico is first reported from the Mazatepec area (Puebla) in East-central Mexico. Simocosmoceras pszczolkowskii apulcoensis n. subsp. shows closest morphological resemblance with Simocosmoceras pszczolkowskii Myczynski registered in western Cuba (Myczynski 1989). Ecological adaptation to comparatively epeiric seas in eastern Mexico is interpreted to cause phenotypic deviation with respect to Simocosmoceras pszczolkowskii Myczynski reported from western Cuba. The lack of transient phenotypes between Mexican and Cuban Simocosmoceras is assumed to support the interpretation of Simocosmoceras pszczolkowskii apulcoensis n. subsp. as a geographic subspecies. The new subspecies is interpreted to indicate stratigraphic horizons within the Semiforme/Verruciferm Zone in the Mediterranean Tethys.