Bol. Soc. Geol. Mexicana, XLVII, No. 1, 1986

Estratigrafía de los alrededores de la mina de fierro de El Encino, Jalisco

http://dx.doi.org/10.18268/BSGM1986v47n1a1

Jerjes Pantoja Alor* Samuel Estrada Barraza**

*Instituto de Geología, Universidad Nacional Autónoma de México.
**Las Encinas S.A. (HYLSA). Monterrey, N.L.

 Abstract

The Tecalitlan Formation is the oldest unit that crops out in the vicinity of El Encino mine, in the Municipality of Pihuamo, State of Jalisco. Consists of a thick sequence of continental volcanic rocks, of acid to intermediate composition and of Neocomian age. It is overlied with marked angular unconformity by marine volcaniclastic deposits, making up the Encino Formarion, which has been subdivided into two members: a lower member of later Aptian age and an upper member of early Albian age.

An angular unconformity separates the Encino Formation from the overlying Vallecitos Formation, whose upper part consists of tuffs, lava flows and conglomerates with abundant celadonite, all deposited in a marine environment. Its upper part, a calcareous member, concordantly overlies the lower ruffaceous member and consists of dolomitized reef limestone with abundant rudists and bivalves, in which Coalcomana ramosa (Boehm) and Chondrodonta sp. are important for they indicate an early Albian age. However, these rocks may extend into the middle Albian.

Next ro rhe east, but outside of the studied area, the calcareous member is overlied concordantly by a thick sequence of marine and continental volcanics which may extend into the Cenomanian.

The Lower Cretaceous sequence is intruded by plutonic body and associated hypabissal rocks of quartzmonzonitic composition, with granodioritic to dioritic phases; because of its complex and composite character, its time of emplacement could correspond to two magmatic events: one during the early Albian, while the second towards the end of the Late Cretaceous or beginning of the Early Tertiary.

The magmatic activity which originated the volcanic an intrusive rocks of the region, was active within a calcalkaline magmatic island arc, which started probably toward the end of the Late Jurassic.