Lourdes Omaña

Antillocaprina trilobata nov. sp. andUpper Cretaceous associated fauna from Chiapas, Southeast Mexico

Abstract Around the city of Tuxtla Gutierrez, the capital of Chiapas, which is the southern most state of Mexico,there are several fossiliferous localities with Upper Cretaceous rudists from which derived the specimens studied in this paper. It is described and proposed as a new species Antillocaprina trilobata, characterized by three prominent longitudinal ridges in the ventral region. It is also described Coralliochama gbohemi Bose ; this is the first record of the two genera in Chiapas. The fossils were found in the upper part of the Angostura Formation of late Campanian-Maastrichtian age. The Maastritchian age of the described species is established by the associated rudists Titonosarcolites giganteus ( Whitfield ) and Macgillavryia nicholasi ( Whitfield ), and this age is confirmed by the occurrence of the large foraminifers Chubbina jamaicensis Robinson , Kathina jamaicensis ( Cushman & Jarvis ), Smoutina cruysi Drooger , Sulcoperculina vermunti ( Thiadens ), S. dikersoni ( Palmer ), S. globosa De Cizancourt and Rhapydionina sp.

MAASTRICHTIAN INOCERAMID BIVALVES FROM CENTRAL CHIAPAS, SOUTHEASTERN MÉXICO

Abstract In central Chiapas (southeastern México) the last occurrence of inoceramid bivalves was recorded in strata of the predominantly terrigenous Ocozocoautla Formation. The inoceramids are associated with foraminifers of the Gansserina 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 tricostatus 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 México, which is a good index fossil for the Early Maastrichtian and has a cosmopolitan distribution.

Evidence for two Cretaceous superposed orogenic belts in central Mexico based on paleontologic and K-Ar geochronologic data from the Sierra de los Cuarzos

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.