Bol. Soc. Geol. Mexicana XXXVII(1): 1-10, Junio. 1976


Las Rocas Metamórficas del Arroyo de la Pimienta, Zacatecas, Zac.

Richard V. McGehee*

Instituto de Geología, U.N.A.M.


A section of metasedimentary rocks of great lithologic and structural variety is found in the Arroyo of the Pimienta (or the Bote), about 3 km WNW of the city of Zacalecas. In order of abundance these rocks consist of (1) pelitic rocks (phyllite, schist, slate, "pencil rock"). (2) an alternation of meta-sandstone with phyllite, in thin beds, (3) massive meta-sandstone, (4) meta-conglomerate, (5) meta-limestone, with interlayered phyllite and schist, and (6) meta-chert(?). The total thickness of the section is around 1000 m. Original structures preserved include bedding fossils. flow casts and marks, original clastic grains in conglomerate and sandstone, graded bedding and possibly cross-bedding. Several blocks of massive sandstones situated within schist and phyllite may represent subaqueous sliding at the time of deposition, or possibly tectonic emplacement. The grade of metamorphism is greenschist facies: Bedding is folded into open to isoclinal forms, with dimensions from a few millimeters to several meters. Foliation is parallel with bedding in some of the rocks but angular relationships are also common. Where bedding is tightly folded cleavage is often seen parallel to the axial planes. In some places tectonic transport along foliation planes produced transposition of bedding Some phillites show two cleavage directions, which produce splinter or pencil-like forms. Where the foliation is folded, slip cleavage is produced. In one area of phyllite and meta-sandstone in thin beds, the latest effect of deformation is the formation of kink bands. which cut across all earlier structures. The metamorphic section is surrounded by greenstone. Post-metamorphic faults displace parts of the section.