THE PEÑA DE BERNAL NATURAL MONUMENT OF QUERETARO, MEXICO

Peña de Bernal is a natural monument of Queretaro State in central Mexico located next to the Magic town of Bernal. By its height of 433 m on the highest side, Peña de Bernal is among the tallest monoliths of the world. Peña de Bernal was recently declared Intangible Cultural Heritage of Humanity Patrimony by UNESCO. In spite of being both a natural and cultural monument, little is known about its origin, physical characteristics and chemical composition. Peña de Bernal is located at the intersection of three major geologic provinces, the Miocene-Quaternary Mexican Volcanic Belt, the mid-Tertiary Sierra Madre Occidental ignimbrite-dominated sequence, and the Mesozoic Sierra Madre Oriental folded belt, which in this area corresponds to the Sierra Gorda sector. Peña de Bernal intrudes marine Mesozoic sedimentary rocks, including Las Trancas (Kimmeridgian-Barremian) and Soyatal (Turonian) formations. Due to its aspect of a leucocratic-igneous rock it has been misinterpreted as a plutonic rock of Eocene or older age. However, our studies show that Peña de Bernal is a dacitic dome with a SiO₂= 67 wt% and an age of 8.7 ± 0.2 Ma (average of 2 ages). The complete Peña de Bernal body includes 3 lobes that crop out in a ca. 3.5 x 1.5 km area with an elongated shape oriented N40°E. Texture of the rock is porphyritic, nearly hollocrystalline (80 vol.% crystals and 20 vol.% glass), with a complex mineral assemblage of pyroxene, hornblende, biotite, plagioclase, and quartz, plus accessory apatite and zircon. Peña de Bernal is intensively fractured along its margins, but without being brecciated. The country rock is sheared along the contact surface with the dome, and vertical slicken-sides were observed in some cases due to this shearing. These observations, together with the highly crystallized nature of the rock, indicate a forceful injection of the dome and suggest that it may have been practically solid (80% crystalline) and relatively cold when it was emplaced. Peña de Bernal dacite is a spine-type endogenous dome that was forcefully injected through the Mesozoic sequence practically as a solid plug.