Abstract

WEBER, Bodo; VALENCIA, Victor A.; SCHAAF, Peter  and  ORTEGA-GUTIERREZ, Fernando. Detrital zircon ages from the Lower Santa Rosa Formation, Chiapas: implications on regional Paleozoic stratigraphy. Rev. mex. cienc. geol [online]. 2009, vol.26, n.1, pp.260-276. ISSN 2007-2902.

Samples from the Rio Aguacate sequence that defines the Lower Santa Rosa Formation (SRF) in Chiapas were collected in the Jaltenango river valley that is currently partly flooded by the La Angostura Lake. U-Pb geochronologic analyses by Laser-Ablation Multicollector ICPMS have been conducted on 209 zircons from two quartz-rich samples of a monotonous slate and phyllite section cropping out above the lake. The maximum depositional age of the upper sample is defined by an overlapping group of three zircons having ages from 325 to 315 Ma. The youngest cluster of ages from the the second sample, which comes from an outcrop ~3000 meters below the first, range from 341 to 331 Ma. The results indicate that most of the Santa Rosa Formation (SRF) accumulated in Mississippian time. Most detrital zircons have ages that correspond to the Pan-African-Brasiliano orogenic cycle (~700-500 Ma). Secondary peaks in the age spectra are Mesoproterozoic (~1600-900 Ma), Paleoproterozoic (~2150 Ma), Archaean (~2.6-3.3 Ga), and also Silurian-Lower Devonian (~420-360 Ma). Mesoproterozoic zircons with a major peak at 1237 Ma and a second at 1025 Ma are present only in the stratigraphically lower sample. Otherwise, the provenance ages of the Lower SRF are similar to those from the Upper SRF, indicating that the SRF probably comprises a continuous sequence. The results further indicate that early Paleozoic sedimentary rocks of the Maya block are not part of the SRF . Similarities in detrital age distributions suggest a potential correlation with the Cosoltepec Formation of the Acatlán Complex, but a correlation with Paleozoic strata (Ixtaltepec and Santiago Formations) covering the Oaxacan complex is not indicated.

Keywords : provenance age; zircon; Santa Rosa Formation; Paleozoic; SE Mexico.

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