A tectonically undeformed portion of the lower part of La Laja Formation is exposed at Cerro El Molle near San Juan, Precordillera of western Argentina. It consists of shallow-water, variably argillaceous lime mudstone and sporadically interbedded bioclastic grainstone deposited in an inner shelf setting. The El Estero Member and the basal 0.2 m of the Soldano Member contain a trilobite fauna of olenelloids and ‘simple’ ptychoparioids indicative of the early Cambrian (series 2, stage 4; Dyeran stage of Laurentia). The succeeding 50 m of the lower Soldano Member yield trilobites characteristic of the early middle Cambrian (series 3, stage 5; Delamaran stage of Laurentia). In ascending order of occurrence,Amecephalus arrojosensis,Kochiella maxeyiandEokochaspis nodosa, along with several other taxa, includingPtychobaban. gen. (type speciesPtychoparella buttsi), belong to the traditional lowerPlagiura–PoliellaBiozone. However, while this fauna is similar to that of the Great Basin, the nominative species of theEokochaspsis nodosaand overlyingAmecephalus arrojosensisbiozones recognized in southern Nevada occur in reverse order in the Soldano Member. This suggests that the ranges of these species overlap, thereby reducing the temporal resolution in the Precordillera into a combinedAmecephalus arrojosensis–Eokochaspis nodosaBiozone. Argillaceous lime mudstones at the top yieldMexicella mexicana, indicative of theMexicella mexicanaBiozone recognized in the Great Basin, which is equivalent to the traditionalAlbertellaBiozone of Laurentia. Because corynexochids are almost absent, the low-diversity ‘kochaspid’-dominated biofacies appears to typify the platform interior. The fauna is entirely Laurentian in composition, reinforcing notions of a close proximity of Cuyania to Laurentia during the Cambrian that enabled faunal migration and interchange. The absence of a late early Cambrian to early middle Cambrian hiatus correlative with the Hawke Bay Event, however, suggests no close affinity to the Iapetus-facing margin of eastern Laurentia.
Physical stratigraphy and trilobite biostratigraphy of the Carrara Formation (Lower and Middle Cambrian) in the southern Great Basin