Sedimentología de la Formación Matzitzi (Paleozoico superior) y signifi cado de sus componentes volcánicos, región de Los Reyes Metzontla-San Luis Atolotitlán, Estado de Puebla

Abstract

The Matzitzi Formation (Permian), located in the southern part of Puebla State, constitutes one of the few exposures of Paleozoic rocks in southern Mexico. Rocks of this formation were recognized since 1896 by Aguilera, and ever since they are known for its paleobotanical signifi cance, because of their diverse fossil fl ora. This unit overlies or is in fault contact with the Oaxacan Complex and is unconformably overlain by Mesozoic conglomerates. Measured columns along the dirt road between Los Reyes Metzontla and Coatepec allowed us to characterize six facies association. Based on these facies associations, we recognized debris fl ow deposits, sandy channel-fi ll, overbank, and crevasse splays deposits. These facies are interpreted as related to an anastomosed river system. In addition, evidence of an extrusive origin for the felsic rocks that crop out in San Luis Atolotitlán town are also discussed in this paper. These igneous rocks were previously interpreted as a shallow stock, or a sill or a dike. However, fi eld observations corroborated a volcanic origin, those are: 1) the basal contact is conformable, it shows no evidence of contact metamorphism, and is formed by a minor erosional surface due to fl ow (surge ondulite); 2) the upper contact is also conformable, and mildly erosional as well, and sandstone and conglomerate that rest upon contain abundant clasts of the underlying felsic rock; 3) rocks show reomorphic structures and cooling hexagonal prisms. Thus, we interpret these rocks as part of a pyroclastic fl ow, named as the Atolotitlán Tuff. Geochemical data suggests a volcanic-arc affi nity for this tuff, since it has low Ce, P, Nb and Ti, enriched light rare earth element (LREE) patterns, fl at heavy rare earth element (HREE) patterns, and negative Eu anomaly. This last interpretation is valid only if there was no considerable amount of crustal melts involved in the magma generation.

The fact that the Matzitzi Formation contains volcanic rocks is important for the paleogeographic and tectonic reconstruction of southern Mexico, because it corroborates that the paleo Pacifi c continental margin was active during Permian time. Permian arc related volcanic rocks have been already recognized, but only to the north of the studied area. This is also the fi rst record of upper levels of Paleozoic igneous activity, since only granitoids of Permian age have been found southern Mexico.