MICROFACIES AND PALEONTOLOGICAL DATA OF THE JURASSIC – CRETACEOUS TRANSITION AT SIERRA DE LOS ÓRGANOS, PINAR DEL RÍO PROVINCE, WESTERN CUBA

Upper Jurassic/Lower Cretaceous marine carbonate deposits of the North America Mesozoic paleomargin in Sierra de Los Organos, western Cuba consist of a sequence of partially dolomitized limestone facies represented by the Guasasa Formation. These rocks often lack macrofauna, but different microfossil groups provided data that allowed us to further constrain their stratigraphic position.

We measured a section of an outcrop at Rancho San Vicente, Pinar del Río Province, which at its base shows facies of the uppermost San Vicente Member, the lowest unit of the Guasasa Formation. Detailed microfacies analyses of this unit reveal essentially peloidal, oncoidal, and oolithic grainstones and packstones with common intraclasts and bioclasts, typical of a carbonate bank where sedimentation occurred in a protected, shallow marine lagoon-like environment.

By contrast, the base of the suprajacent El Americano Member displays fossiliferous wackestones with debris from the shallow environments, but also contains scarce pelagic bioclasts such as ammonitellas and hyaline calpionellids typical of the late Tithonian. These deeper-water facies also yielded rare macroscopic fragmented molds of himalayitid ammonites that allowed further biocronostratigraphic correlation. Series of bioclastic wackestones with abundant calpionellids, radiolaria, ammonitellas, ostracods, and calcisphers that compose the El Americano Member indicate that the environment became increasingly pelagic through the latest Tithonian. Bioclastic wackestones at the top of this Member also showed abundant radiolaria and calpionellids characteristic of the Berriasian. Thus, in this section the Jurassic-Cretaceous boundary is recorded within the deeper facies of the uppermost part of the El Americano Member.

Based on the age of the basal strata of the El Americano Member we interpret the top of the underlying San Vicente Member to represent the first stage of Tithonian sedimentation in the area, followed by gradual inundation of the bank and relative sea-level rise within the middle-late Tithonian and early Berriasian.