LOWER PALEOZOIC OF NORTHWEST GONDWANA—TERMINAL CAMBRIAN AND LOWEST ORDOVICIAN TIÑU FORMATION IN SOUTHERN MEXICO

The Tiñu Formation, the oldest fossiliferous unit in southern Mexico, occurs in several inliers along a N–S belt in central Oaxaca State. The thin (ca. 70 m) Tiñu Formation nonconformably overlies middle Proterozoic (ca. 1 Ga), high-grade metamorphics and intrusives, and is unconformably overlain by the Cretaceous or Tertiary or is in tectonic contact with the lower Carboniferous. Although traditionally referred to the lowest Ordovician (Tremadocian), restudy of the Tiñu now proves that it includes a very condensed, uppermost Cambrian lower member (to 13 m) and a lower, but not lowermost, Ordovician upper member (to 60 m). The limestone-rich lower member represents a wave-dominated shelf with echinoderm hash-trilobite fragment-intraclast pebble beds and dunes in the south (Rio Salinas). This facies changes into an upper slope facies with carbonate-clast debris flows and distal tempestites in the north (Santiago Ixtaltepec). The shell hash-dominated limestones are a non-Laurentian, non-tropical, cold-water lithofacies with abundant but very low-diversity conodonts that record two distinct eustatic onlaps in the lower and uppermost Cordylodus proavus Chron (terminal Cambrian). After Cambrian–Ordovician boundary offlap (recorded by a thin conglomerate or a phosphate horizon), strong middle Tremadocian eustatic onlap brought dendroid graptolite- and olenid trilobite-bearing, dysoxic mudstone across the lower–upper member unconformity. Shoaling into the high-stand facies again led to deposition of cold-water shell-hash limestones with terminal Tremadocian (Hunnebergian Stage) conodonts and macrofaunas. High-energy, wave-dominated sandstones with orthid brachiopods are the highest units of the Tiñu. Although a “Grenvillian” basement and the facies and biotas of Carboniferous and younger units of Oaxaquia are “North American,” the Lower Paleozoic of this terrane is non-Laurentian, non-Avalonian, and best considered deposited on the southern margin of the Rheic Ocean.