Lower Cambrian bioconstructions in Northwestern Mexico (Sonora). Depositional setting, paleoecology and systematics of Archaeocyaths

Abstract

Investigations on Lower Cambrian carbonates of Caborca (NW Mexico) indicate that in the Puerto Blanco Formation the following two types of archaeocyathan-algal buildups occur: o I -Framework reefs occurring at the base of the carbonate interval of the Puerto Blanco Formation. These consist of an archaeocyathan framework with encrusting renalcids and abundant fibrous cements. Cavities are filled with mudstone and contain rare sponge spicules. Cements are well developed upon internal sediments. The archaeocyathan fauna is dominated by colonial Paranacyathus along with Batenevia and Claruscoscinus with taeniae and curved tabulae. A few regular archaeocyaths are present, including Palmericyatbus and Aulocricus. Lateral to the reef facies the lime mud sediment contains abundant skeletal fragments of trilobites, echinoderms, Chancelloria, hyoliths, and archaeocyaths. These buildups are interpreted to have formed as wave-resistant, framework reefs in a tidally dominated, shelf-margin setting seaward of a carbonate barrier. II - Girvanella-dominated buildups located in the upper part of the carbonate interval of the Puerto Blanco Formation and associated with oolites. These are built by stacked Girvanella crusts, with an important contribution of Renalcis and a less important presence of Epiphyton. Shelter cavities are filled by mudstone often rich in siltsize quartz. Cements are poorly developed and confined to the intraskeletal cavities. Archaeocyaths are scattered in the Girvanella frame either as isolated individuals or as clusters of solitary or colonial forms. Regular archaeocyaths are here abundant, represented by several taxa with an ethmophylloid inner wall, including Palmericyatbus and Cordilleracyathus. The dominant genus is Aulocricus. Colonial species of Arcbaeocyathus and Arrythmocricus are well developed. These latter two genera are now recorded from Mexico to northern Canada. The internal and lateral sediment contains a rich