Abstract
A new mappable rock unit, the Cerro Rajón Formation, is proposed for the Cambrian succession of the Caborca region, Sonora, México. Formerly Unit 1 of the Puerto
Blanco Formation, the Cerro Rajón Formation is interpreted as a volcano-sedimentary succession deposited along the coast of a passive margin that was impacted by
rift-related volcanism. At its proposed type locality, in Cerro Rajón, the Cerro Rajón Formation consists of 270–285m of tuffaceous conglomerate, metabasalt, mafic
tuff, mafic lapillistone, mafic agglomerate, and quartzite with minor siltstone, limestone, and dolostone- and quartzite-dominated conglomerate. The unit contains a
major disconformity near its base, where m-to dm-thick conglomerate locally replaces the fine-grained clastics that make up the base of the Cerro Rajón Formation.
δ13C chemostratigraphy and biostratigraphy of the Rajón and its bounding strata limits Rajón deposition to the Fortunian Stage of the Terreneuvian Series. Volcanic
rocks in the Cerro Rajón Formation are represented by mafic to ultramafic flows, including picrobasalts and metabasalts with hydrothermal alteration characteristics,
evidenced by replacement of clinopyroxenes by chlorite, actinolite, and epidote. The mineral paragenesis of these volcanic rocks suggests the succession experienced
greenschist grade metamorphism. Basalt geochemistry is consistent with low silica (34.32–48.21%) magmatism with high TiO2 concentrations (3.63–7.52%), related
to continental rift volcanism with OIB characteristics. This volcanism could represent the last southern evidence of the rifting process that occurred along the western
margin of Laurentia or could be related to volcanic rift deposits further afield.
Blanco Formation, the Cerro Rajón Formation is interpreted as a volcano-sedimentary succession deposited along the coast of a passive margin that was impacted by
rift-related volcanism. At its proposed type locality, in Cerro Rajón, the Cerro Rajón Formation consists of 270–285m of tuffaceous conglomerate, metabasalt, mafic
tuff, mafic lapillistone, mafic agglomerate, and quartzite with minor siltstone, limestone, and dolostone- and quartzite-dominated conglomerate. The unit contains a
major disconformity near its base, where m-to dm-thick conglomerate locally replaces the fine-grained clastics that make up the base of the Cerro Rajón Formation.
δ13C chemostratigraphy and biostratigraphy of the Rajón and its bounding strata limits Rajón deposition to the Fortunian Stage of the Terreneuvian Series. Volcanic
rocks in the Cerro Rajón Formation are represented by mafic to ultramafic flows, including picrobasalts and metabasalts with hydrothermal alteration characteristics,
evidenced by replacement of clinopyroxenes by chlorite, actinolite, and epidote. The mineral paragenesis of these volcanic rocks suggests the succession experienced
greenschist grade metamorphism. Basalt geochemistry is consistent with low silica (34.32–48.21%) magmatism with high TiO2 concentrations (3.63–7.52%), related
to continental rift volcanism with OIB characteristics. This volcanism could represent the last southern evidence of the rifting process that occurred along the western
margin of Laurentia or could be related to volcanic rift deposits further afield.
| Original language | American English |
|---|---|
| Pages (from-to) | 197 |
| Number of pages | 210 |
| Journal | Journal of South American Earth Sciences |
| Volume | 89 |
| Issue number | 1 |
| State | Published - 10 Nov 2018 |