Abstract

Late Leonardian to Early Guadalupian marine carbonates exposed in the Sacramento Mountains relate a marked change from equatorial tidal flat rocks of the Middle Leonardian Yeso Formation, which were deposited during a worldwide sea level lowstand, to submerged shelf limestones of the San Andres Formation following a Late Leonardian transgression of the Northwest Shelf. The Yeso-San Andres contact, considered to be gradational in the Sacramento Mountains, is here interpreted as a flooding surface resulting from a eustatic sea level rise after the lowstand.

Inundation of the Northwest Shelf led to deposition of the thick San Andres Formation marine limestone sequence within a shallow shelf or lagoon setting. Depositional environments are predominantly sub-tidal and intertidal. Microfacies include packstones of comminuted bioclasts of normal saline affinities which were redistributed by circulation on the shelf. This facies periodically shoaled upwards to dasycladacean algal grainstones which are interpreted as tidal bars prograding across the lagoon or shelf. More restricted wackestones and laminated mudstones at the base of the section record the transition from the restricted evaporitic shelf of the Yeso.

Aggradation of sediment into the intertidal zone cyclicly lowered relative sea level during San Andres time and interrupted shelf circulation, resulting in the deposition of restricted facies grainstones. Major cycles of aggradation (50 to 90m) resulted in evaporitic conditions which were recorded by horizontally laminated dolostones and a fine-grained, arenitic tongue of the Glorieta Sandstone. These tidal flat rocks were previously attributed to intertonguing of the Yeso with the San Andres, but are considered here to represent the lowering of relative sea level by aggradation coupled with eustatic sea level fall within the San Andres stratigraphic sequence. Two such major cycles occur within the partial San Andres section (135m) exposed in the southern Sacramento Mountains.