Middle Miocene peralkaline ignimbrites in the Hermosillo region (Sonora, Mexico): Geodynamic implications

Scattered ignimbritic mesas crop out in the Hermosillo region (Sonora, Mexico). These rocks that have been dated at 12.5 Ma (Middle Miocene) have the petrography and chemical characteristics of comendites. Such a flare-up of peralkaline acidic volcanism, after a long period of subduction-related arc volcanism, emphasises an important change in the source of volcanism. It corresponds to the latest stage of continental extension prior to the marine invasion and the development of spreading centres in the Gulf of California. To cite this article: J. Vidal Solano et al., C. R. Geoscience 337 (2005).     

Further constraints for Permo-Carboniferous magnetostratigraphy: case study of the sedimentary sequence from San Salvador–Patlanoaya (Mexico)De nouvelles contraintes pour la magnétostratigraphie du Permo-Carbonifères : cas d'étude de la séquence sédimentaire de San Salvador–Patlanoaya (Mexique)

We report rock-magnetic and magnetostratigraphic results from one of the best exposed Permo-Carboniferous outcrops in Mexico, in order to determine some decisive magnetostratigraphy constraints. Some spinels, most probably titanomagnetites, seem to be responsible for aimantation, although greigite may also exist judging from thermal desaimantation of isothermal remanence. Remanence analyses indicate that only one remanent component could be recognized with minor secondary overprint, which were easily removed applying 100–180 °C. Six normal and four reverse magnetozones were recognized from bottom to top in Patlanoya section between 340 and 280 Myr. Both normal and reverse polarity rocks were found in Carboniferous time around 340 Myr, in agreement with previous paleomagnetic works. Our record revealed two normal subchrons within PCRS (Kiaman), the dominantly reverse superchron, at 305 and 280 Myr, respectively. These, which may be speculatively correlated to normal aimantation, occasionally occurred during PCRS interval. More detailed studies are needed in order to establish a more precise magnetostratigraphy for Permo-Carboniferous time. To cite this article: L.M. Alva-Valdivia et al., C. R. Geoscience 334 (2002) 811–817.     

Raman spectroscopy of natural silica in Chicxulub impactite,MexicoSpectroscopie Raman de silices naturelles dans l'impactite de Chicxulub,Mexique

A series of natural silica impactite samples from Chicxulub (Mexico) was investigated by Raman microprobe (RMP) analysis. The data yield evidence for high-pressure shock metamorphism in the rock. The impactite contains three polymorphs of silica: the original α-quartz, and two high-pressure varieties – coesite and disordered quartz representing various degrees of crystallinity. We found systematic changes in frequencies and half-widths of the Raman bands, caused by increasing irregularities of bond-lengths and bond-angles and a general breaking-up of the structure as a result of impact events. Therefore, RMP is an adequate tool for measuring the crystallinity of disordered quartz. The half-width Γ and the frequency ω of the symmetric SiOSi stretching vibrational band (A₁ mode) of the SiO₄ tetrahedra are the most amenable parameters for estimating the degree of crystallinity. In well-crystallized quartz, Γ=5 cm^?1 and ω=464 cm^?1, while in highly disordered quartz this line shifts up to ω=455 cm^?1 and broadens up to Γ=30 cm^?1. The Raman lineshapes appear to depend strongly on the degree of lattice disorder subsequent to impact events. To cite this article: M. Ostroumov et al., C. R. Geoscience 334 (2002) 21–26     

Impact ejecta and carbonate sequence in the eastern sector of the Chicxulub crater

The Chicxulub 200 km diameter crater located in the Yucatan platform of the Gulf of Mexico formed 65 Myr ago and has since been covered by Tertiary post-impact carbonates. The sediment cover and absence of significant volcanic and tectonic activity in the carbonate platform have protected the crater from erosion and deformation, making Chicxulub the only large multi-ring crater in which ejecta is well preserved. Ejecta deposits have been studied by drilling/coring in the southern crater sector and at outcrops in Belize, Quintana Roo and Campeche; little information is available from other sectors. Here, we report on the drilling/coring of a section of 34 m of carbonate breccias at 250 m depth in the Valladolid area (120 km away from crater center), which are interpreted as Chicxulub proximal ejecta deposits. The Valladolid breccias correlate with the carbonate breccias cored in the Peto and Tekax boreholes to the south and at similar radial distance. This constitutes the first report of breccias in the eastern sector close to the crater rim. Thickness of the Valladolid breccias is less than that at the other sites, which may indicate erosion of the ejecta deposits before reestablishment of carbonate deposition. The region east of the crater rim appears different from regions to the south and west, characterized by high density and scattered distribution of sinkholes.