New Mexico Geological Society Annual Spring Meeting — Abstracts

The Papalote diapir is one of several evaporite diapirs exposed in the La Popa basin, located approximately 100 km northwest of Monterey in northeastern Mexico. The diapir is elliptical in shape and is exposed over an area of approximately six square kilometers. The evaporites that form the diapir are of the Callovian Minas Viejas Formation which was deposited during marine transgressions in the Rio Grande Embayment during the Middle-Late Jurassic opening of the Gulf of Mexico. Surrounding the diapir are vertical limestone lentils which formed around the sides and top of the diapir during the Upper Cretaceous. Contained within the diapir are large blocks of igneous (monzonite, biotite-diorite, andesite), greenschist facies meta-diabase and carbonate rock. Previous work has been done in the La Popa basin (McBride, 1974 and Laudon, 1981); however, no age relationships between the blocks and the surrounding evaporite have been established. Several hypothesis concerning the origin and transport of the allochthogenic blocks have been developed; 1) The carbonate blocks represent the overlying ceiling that collapsed into the diapir, 2) The carbonate blocks represent pre-Upper Cretaceous or Jurassic section that was entrained during diapirism, 3) The igneous and meta-diabase blocks represent basement that was thrust over the salt and subsequently entrained, 4) The igneous rocks were intruded into the salt or the overlying sediments during diapirism, 5) The igneous blocks were intercalated with the salt during deposition, 6) The igneous and meta-volcanic blocks were entrained at the salt-basement interface and transported during diapirism.

Based on trace element analyses, the samples fall into several groups; high Nb, intermediate Nb, low Nb, plutonic rocks and outliers. Sample groups form correlative trends for less incompatible, immobile elements (P, Zr, Ti, Y). Incompatible, mobile elements (Rb, Ba, K) show large variation between samples due to alteration. REE data and ³⁹Ar/⁴⁰Ar dates will help constrain the age and tectonic environment of the blocks. If the basalt samples are rift related, they may represent intrusions associated with opening of the Gulf of Mexico during the Jurassic.

Petrographic comparison of the carbonate blocks with the limestone lentils shows that they are not related to the limestone lentils; therefore, they probably represent the Upper Cretaceous or late Jurassic section in the subsurface, and were entrained during diapirism. Igneous block textures indicate both plutonic and volcanic sources, which are not consistent with intrusion into or intercalation with the evaporite. There is no indication of a major thrust fault in the basin that places basement over evaporite during diapirism (Lawton, Shannon, pers. comm.). Numerical models show that it is possible to entrain blocks in the diapir from the saltJbasement interface. We propose that the basalt was extruded onto basement in the Rio Grande embayment prior to deposition of the evaporite. The allochthogenic blocks were then entrained during Upper Cretaceous diapirism.