A pithonellid bloom in the Cenomanian-Turonian boundary interval from Cerritos in the western Valles–San Luis Potosí platform, Mexico: Paleoenvironmental significance
A calcisphere (Pithonellid) acme is recorded from Cerritos, western Valles–San Luis Potosí platform, Mexico. The abundance of these microfossils appears to constitute a global event in the Cenomanian-Turonian boundary. Their large quantity is interpreted as an indicator of changes in primary productivity during transgressive episodes. The pithonellids Bonetocardiella conoidea (Bonet, 1956), Pithonella sphaerica (Kaufmann, 1865), and P. ovalis (Kaufmann, 1865) are associated with the r and r-k strategists planktic foraminifera Muricohedbergella delrioensis (Carsey, 1926), M. planispira (Tappan, 1940), Heterohelix moremani (Cushman, 1938), Heterohelix reussi (Cushman, 1938), Macroglobigerinelloides caseyi (Bolli, Loeblich and Tappan, 1957), Whiteinella archaeocretacea Pessagno, W. cf. W. aprica (Loeblich and Tappan, 1961), W. brittonensis (Loeblich and Tappan, 1961), W. baltica (Douglas and Rankin, 1969) and W. paradubia (Sigal, 1952), which are also considered to be indicators of high nutrient content in unstable paleoenvironments. The abundance of pithonellids occurred at the base of the Whiteinella archaeocretacea Partial Range zone. This great temporal abundance in the material could be related to the environmental changes caused by the drowning of the Valles–San Luis Potosí platform, as nutrient supply increased in the latest Cenomanian, which is linked to a sea-level transgression that occurred on a global scale. In this interval, the occurrence of benthic foraminifera Gavelinella spp., Lingulogavelinella sp., Dorothia sp. and roveacrinids is also recorded.
This sequence overlies an extinction level of the platform benthic foraminifera dated as late Cenomanian.
The calcispheres Bonetocardiella conoidea (Bonet, 1956), Pithonella sphaerica (Kaufmann, 1865) and P. ovalis (Kaufmann, 1865) show a pithonelloid wall type as well as an inner space (pericoel) of the cyst, in-filled with sparry cements.