Origin of a sedimentary sequence on the north island of Todos Santos, B.C., Mexico

Abstract

The Todos Santos Islands are located within Todos Santos Bay, Baja California. A small sedimentary sequence of about 160 m length and 18 m height is located on the north island. The fine sediments that constitute this unit are associated with conglomerates and interbedded sedimentary breccias. Structurally, this sequence is affected by normal faults with a NE-SW dominant strike and a slip to the East. Sedimentary structures in this sequence are found represented by mud cracks and raindrops, which are present in the lower part of the sequence. The sedimentary lithology basically consists of arkoses, clays, muds, tuffs and conglomerates, and the volcanics are basalts and andesites. Due to the lack of fossils, the age has been established through correlation with other sedimentary sequences known in the area such as: the Rosarito Beach Formation (Las Glorias Member), the basalts and tuffs of La Misión and El Descanso and the Coronado Islands. Therefore, we propose the age for this sedimentary sequence as Miocene. The source of the volcanic material was located to the west of the actual coastline and the sedimentary sequence originated to the east. Based on sedimentary structures and textures, this study determined that the depositional environment of the sedimentary sequence was subaerial, that the nature of the sediment is typical of an alluvial fan and that this sequence belongs to the proximal part of this.