The Antimonio formation in Sonora, Mexico, and the Triassic-Jurassic boundary

The Antimonio Formation furnishes a record of sedimentation across the Triassic-Jurassic system boundary and is one of a few stratigraphic sections globally that preserves latest Triassic to Hettangian ammonoids in stratigraphic succession. The boundary falls near the middle of the formation, within a 155 m thick stratigraphic section, which is divided into five distinct sedimentary packages. The laminated shales and siltstones in the middle of package 4 represent deposition in an anoxic or disaerobic setting. Although shales of package 4 themselves are poorly fossiliferous, they are bounded below and above by Triassic and Jurassic biotas, respectively. The Triassic-Jurassic system boundary should fall within or stratigraphically close to the laminated beds. The transgressive-regressive signature from the Antimonio Formation corresponds closely to that of the Gabbs and Sunrise formations in Nevada and jointly shows eustatic regressive events at or near the beginning of the latest Triassic Crickmayi Zone and another near the top of the Hettangian. The beds from package 4 indicate a transgression closely associated with the Triassic-Jurassic system boundary.