Lithostratigraphy of Cretaceous strata, Cedros Island, Baja California, México: Vargas and Pinos formations defined

Recent field work involving mapping, sedimentology and biostratigraphic study of Cretaceous strata on Cedros Island suggest that Albian through Coniacian strata are divisible into two formations, each comprising several informal units. The chiefly fine-grained Vargas Formation forms a sedimentary wedge that thickens from 0m to over 1700 m in about 15 km lateral distance, and contains three laterally extensive units. The Pinos Formation is a relatively coarse-grained deposit overlying the Vargas on a chiefly conformable contact. It has an estimated minimum thickness of 1500 m. Although we describe several lithologic units within the Pinos Formation, much of the formation remains undivided owing to problems of mapping strata with rapid lateral facies variation, large scale slump deposits, and pervasive normal faults, some of which were syndepositionally active. Paleomagnetic results, described elsewhere, suggest that the Cedros Island-Vizcaino Peninsula region (Vizcaino terranc) has moved about 15° northward with respect to North America since Cretaceous time; however, we cannot discriminate whether or not such motion has occurred based on lithologic or biostratigraphic (i.e., non-paleomagnetic) data.