Blanca Estela Buitrón

Biostratigraphie par fusulines des calcaires carbonifères et permiens de San Salvador Patlanoaya (Puebla, Mexique)

Resume Les calcaires de la Formation San Salvador Patlanoaya (Mexique) livrent six biozones a fusulines. Les principaux genres permettant de dater sont des Triticites et des Kansanella du Missourien et du Virgilien inferieur (Carbonifere superieur), des Pseudofusulina et des Rugosochusenella du Wolfcampien (Permien inferieur) et des Paraskinnerella et des Skinnerella du Leonardien moyen. Les differentes especes de chacun des genres sont decrites et illustrees: Triticites sp. 1, Triticites burgessae, Triticites milleri, Triticites piloncillosensis, Triticites acutuloides, Triticites oryziformis (=T. homecreekensis), Triticites moorensis, Triticites primarius, Triticites aff. confertoides, Triticites aff. lepidus, Kansanella neglecta, Rugosochusenella emend. (=Pseudochusenella), Rugosochusenella gregaria, Skinnerella emend., Skinnerella imlayi (=S. robusta), Paraskinnerella skinneri (=P. leonardensis) .

Sur une nouvelle localitéa Fusulines du Wordien (Permien supérieur) du Mexique; conséquences paléogéographiques

Resume Les calcaires tectonises de San Juan Ihualtepec (Oaxaca, Mexique) contiennent fusuline schwagerinoides Parafusulina deliciasensis (= P. maleyi ). Ils permettent de dater les couches du Wordien inferieur ou moyen ( zones PG2 ou PG3 de Wilde, 1990 ). Grâce a ce microfossile une paleogeographie du Wordien peut etre dessinee en Californie, au Texas et dans les Etats mexicains de Coahuila, Sonora, Guerrero et Oaxaca.

Lower Ordovician microfacies and microfossils from Cerro San Pedro (San Pedro de la Cueva, Sonora, Mexico), as a westernmost outcrop of the newly defined Nuia Province

The lowermost carbonate beds of the Cerro San Pedro (San Pedro de la Cueva, Sonora State, Mexico) previously assigned to the Mississippian, belong in fact to the lower Ordovician. The limestone of this succession is often dolomitized and chertified, and displays gastropods, pelmatozoans, sponges, and trilobites as major bioclastic components. The different microfacies show that high-energy grainstones, proximal tempestites, and distal tempestites dominated the sedimentation. The paleoenvironments of deposition correspond to an inner ramp, a mid-ramp, and perhaps the upper part of an outer ramp. The strata are characterized by the incertae sedis cyanobacteria Nuia sibirica. A taxonomic revision and discussion of these cyanobacteria, often confused with ooidic grains, is emphasized. Some data are presented on other microfossils, such as primitive, monothalamous foraminifers: Rauserina sp., Vicinesphaera sp., and Neoarchaesphaera sp., leperditicopida and their endolithic microperforations, and primitive chaetetids. Compared to the contemporaneous deposits of the USA, some paleobiological components, e.g., the lithistid siliceous sponges Archaeoscyphia, stromatoporoids? Pulchrilamina, and receptaculacean algae Calathium, are quite rare in the studied section of Sonora, but the predominance of tempestites in the carbonate succession shows that boundstones formed before were systematically eroded and resedimented. The paleogeographic implications are the following: (1) a lower Ordovician intertropical Nuia Province is newly defined in the western part of the lower Ordovician intertropical belt; (2) San Pedro de la Cueva constitutes one of the westernmost outcrops of this new Nuia Province; (3) from Sonora, Nuia extends eastward as far as South China; (4) due to its westernmost paleoposition, San Pedro de la Cueva was frequently affected by tropical storms; and (5) northern lower Ordovician terranes of Mexico (Sonora, Chihuahua, and Baja California) are entirely distinct from the southern ones (Oaxaca). The paleopositions and paleogeographic connections of these northern Mexican terranes with Laurentia, Avalonia, and peri-Gondwanan parts of South America, through the Iapetus and Rheic oceans, still remain disputable or unknown.