Notes on the geology of the area between Santa Cruz and Ayuquila, Puebla and Oaxaca States
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Abstract
The area of this paper is within the Mixteca Baja region of southern Puebla and northern Oaxaca, and is underlain by Paleozoic metomorphics, Mesozoic marine and continental sediments, and Cenozoic continental clastic and volcaniclastic deposits, The Paleozoic rocks were included in the Acatlan Formation (Fries and Rincón-Orta, 1965), whose lower part consists of clastic metasediments, and mylonitized greenstones and granitic rocks, while its upper part is formed by phyllites, meta-sandstones and meta-limestones (Tecomate Formation of Rodríguez-Torres, 1970).
The Mesozoic rocks overlie unconformably the Acatlan Formation and were mapped as four lithostratigraphic units. The oldest of these, the Tecomazúchil Formation (Middle Jurassic), that contains fossil flora, is made up of marine and continental clastics. The overlying Chimeco Limestone (Oxfordian) has a lower transitional contact with the Tecomazúchil Formation, and is covered concordantly by argillacious limestones and calcareous siltstones of the Mapache Formation (Kimmeridgian-Portlandian). The Morelos Formation (Albian-lower Cenomanian) unconformably overlies the Jurassic sequence and consists of fossiliferous limestones and dolomitic limestones.
The Tertiary continental rocks were included in the Huajuapan Formation and these rest unconformably on the older lithostratigraphic units. The lower part of this formation consists of red conglomerates, whereas its upper part of clastic and vocaniclastic deposits.
Several intrusive bodies, of andesitic to dacitic composition, cut the formations mentioned; in these the most prominent, for its topographic expression, is the Peña de Ayuquila.
The metamorphic rocks have a steep N-S predominant foliation. The Jurassic rocks form a broad, gently north-plunging anticline, while the Cretaceous and Tertiary rocks constitute the east flank of a NW-SE elongated uplift, which is cut by a normal fault that enabled the juxtaposition of the Paleozoic metamorphics on the east.
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References
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