Abstract
The Rosario Formation crops out along the west side of the northern state of Baja California. Stratigraphically, the Rosario Formation has been subdivided into a lower sandstone-conglomerate, middle shale, and upper sandstone-shale-conglomerate sequence, although each unit exhibits changing paleoenvironmental and sedimentary facies from east to west. In Ensenada Bay, located 100 km from the Mexico-US border, the lower member of the Upper Cretaceous Rosario Formation crops out, exhibiting a variety of syngenetic structures indicating the oceanographic and environmental conditions that prevailed at the time of deposition. Among such structures are hummocky cross-stratification, symmetric and asymmetric ripple marks, convolute and flame structures rip-up clasts, and rhythmic sedimentation with irregularities (pinch and swell). Syngenetic structures possibly formed in deep water, such as along the break of the continental platform forming a turbidite deposit. However, based on the cross-stratification and the hydraulic conditions associated with the described structures, the authors believe the lower Rosario member in Ensenada Bay represents a transitional environment controlled by ocean tides.
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