Part I. Geological reconnaissance of the Petlalcingo-Santa Cruz area, Municipality of Acatlán, State of Puebla

Abstract

The area studied is located about 200 km southeast of Mexico City, in the border region of the Altipaís Oaxaqueña and Cuenca del Balsas Mexcala subprovinces of the physiographic province of the Sierra Madre del Sur of Mexico.

The sedimentary sequence rests on a metamorphic plinth, composed of sericite and quartzite schist included in the Acatlán Schist, of Precambrian or Paleozoic age.The oldest sedimentary unit consists of 600m of conglomerate, sandstone and siltstone, of red color and continental origin, which constitutes the Tecomazúchil Formation, of Middle Jurassic age. Overlying this unit, with transitional contact, is a limestone with a thickness of approximately 100 m, which forms the Chimeco limestone, of late Jurassic age (Oxfordian), which in turn is followed by about 400 m of clayey limestone and calcareous siltstone that constitute the Mapache Formation, also of late Jurassic age (Kimeridgian-Portlandian). Discordantly over this formation lies the Morelos Formation, of Late Cretaceous (Albian) age consisting of about 400 m of limestone and dolomite. Overlying the folded and eroded Mesozoic stratigraphic units was deposited the Huajuapan Formation, of Tertiary (Eocene-Oligocene) age, consisting of about 200 m or more of limestone conglomerate, sandstone and siltstone, all reddish in color, and freshwater limestone.

The area comprises a portion of the eastern flank of a broad, open, N-S trending anticline. The strata of this flank show an average dip of 20° to the E. Foliation of the metamorphic rocks outcropping in the core of this fold is NNE-SSW. Numerous transverse faults cut the flank of this fold and are posterior to the folding of the Mesozoic rocks.