Stratigraphy of the Cambrian (Miaolingian to Furongian) and Ordovician (Tremadocian) west of Arivechi, eastern Sonora, Mexico: Implications on the Cambrian-Ordovician boundary

The Cambrian-Ordovician transition in Mexico has been poorly documented. According to the literature published until now, the most complete middle-upper Cambrian (Miaolingian to Furongian)-Lower Ordovician (Tremadocian) (509-477 Ma) succession is exposed near the town of Arivechi, in northeastern Sonora. This Cambrian-Ordovician sedimentary succession is approximately 900 m thick and is divided into seven lithostratigraphic units. The Cambrian portion of the section consists of El Gavil & PRIME;an, El Mogall & PRIME;on, Milpillas, and La Huerta formations (-600 m) and range from the upper Wuliuan to Paiban stages (509-494 Ma). El Gavil & PRIME;an Formation consists of limestone and shale with chancelloriids and trilobites of the Altiocculus Subzone of the upper Ehmaniella Zone (Miaolingian, Wuliuan). El Mogall & PRIME;on formation (informal name) is characterized by shale, siltstone, and limestone with abundant poriferans and agnostids of the Ptychagnostus atavus/Bolaspidella Zone (Miaolingian, Drumian). Milpillas formation (informal name) consists of limestone and sandy limestone with trilobites of the Crepicephalus Zone (Miaolingian, Guzhangian). La Huerta formation (informal name) is a limestone and shale intercalations with abundant brachiopods, including Angulotreta triangularis, indicative from the Aphelaspis to the lower Dunderbergia Zones (Furongian, Paibian). The Lower Ordovician (Tremadocian) portion of the section consists of El Santisimo formation herein informaly divided into three members: El Ranchito, El Encinal and La Palma (-300 m). El Ranchito member consists of limestone, shale, and siltstone with dendroid graptolites of Anisograptus matanensis Zone (Early Ordovician, Tremadocian). El Encinal member consists of siltstone and sandstone with cross-stratification and graptolites of Anisograptus matanensis Zone (Early Ordovician, Tremadocian). La Palma member contains in the lower non-fossiliferous part, a limestone breccia and siltstone, and the upper part contains limestone with conodonts from the Rossodus manitouensis to the Scolopodus subrex Zone (Early Ordovician, Tremadocian). The Cambrian deposits of eastern Arivechi represent open marine platform facies with an abundant fauna of poriferans, brachiopods, trilobites, and echinoderms, typical of Laurentia. In contrast, the Ordovician deposits represent open ocean deposits dominated by graptolites and conodonts that indicate a shallow marine to a shelf environment.