31 The western equatorial Pangea: Geological, geophysical, geochronological, and paleoecological insights on the youngest supercontinent assembly and breakup on Earth (Part 1)

This session welcomes studies in Mexico, southwestern North America, and northwestern South America investigating the Pangea’s supercontinent cycle from tectonic, stratigraphic, paleoecologic, geophysical, geochronologic, and paleogeographic perspectives to offer insights into its assembly and breakup.

Friday, 24 April 2026
8:00 AM - 12:00 PM
LMH, Isla Carmen

Precambrian Metasedimentary Risco Prieto Unit of the Novillo Complex, Tamaulipas: Paleogeographic and Genetic Insights

Paper number: 31-2
Program: Technical Sessions
Time: 8:25 AM – 8:45 AM

Authors

Abstract

The Neoproterozoic Novillo Complex (NC) represents the oldest lithodeme of the Ciudad Victoria Block, to which several Paleozoic units are tectonically attached. Collectively, these units constitute the exposed multi-orogenic basement of the Huizachal–Peregrina Anticlinorium in the Sierra Madre Oriental. The NC is regarded as the northernmost extension of the Oaxaquia microcontinent. It comprises three distinct units: (a) Los Alamos, interpreted as a magmatic arc; (b) Risco Prieto, a metasedimentary suite; and (c) San Marcos, containing an AMCG suite. The boundaries between these units are tectonic, precluding any stratigraphic control. The Risco Prieto unit crops out mainly in the Peregrina and Caballeros canyons, where marble has been sporadically exploited for chlorite and talc.

The Risco Prieto (RP) suite is characterized by three principal lithologies: Mg-rich marbles, graphitic gneisses, and banded quartz-feldspathic and mafic gneisses. Marbles exhibit a modal composition of calcite + dolomite + phlogopite + chlorite + diopside ± gypsum ± talc ± scapolite. Graphitic gneisses consist of quartz + feldspar + graphite ± titanomagnetite. Banded gneisses display alternating light bands of plagioclase + quartz and dark bands of hornblende + plagioclase + quartz ± garnet.

The RP protoliths are interpreted as dolomitized limestones with evaporitic horizons, organic-rich shales, and arkosic–mafic tuff intercalations. Stratigraphic relationships among these lithologies remain uncertain due to the absence of contact exposures. The RP correlates with the El Marquez unit of the Oaxacan Complex, which includes metasedimentary rocks with calcareous (marble and calc-silicate rocks) and pelitic (paragneiss) protoliths.

Preliminary evidence suggests that the depositional basin of the RP sedimentary protoliths developed along a passive margin adjacent to Oaxaquia, with little to no contribution from external cratonic terranes. Subsequent closure of this basin occurred during the collision of Amazonia–Oaxaquia with Baltica or Laurentia. In this geodynamic context, RP is interpreted as a shallow crustal sector prior to the collision between the Oaxaquia arc and the cratonic continental crust, which together were metamorphosed into granulite facies during the Grenvillian event.

Geological Society of America Abstracts with Programs. Vol. 58, No. 3

doi: 10.1130/abs/2026CD-410861

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