Description
Cenomanian boulder-cobble conglomerate comprises a volumetrically important part of the Valle Group in the northern Vizcaino Peninsula of Baja California Sur. Provenance study of conglomerate clasts yields information about the nature of highland source regions that supplied the detritus. The conglomerates are part of a deep marine sequence of turbiditic deposits that prograded across a basin and plain mudstone-sandstone sequence. The conglomerate clast population is dominated by volcanic and plutonic rocks. Quartzite clasts are a distinctive minor component that make up less than 1% of the clast population. A polarizing microscope was used to characterize eight different quartzite clasts collected from three widely spaced localities. Five of the eight samples have well-preserved sedimentary textures characterized by very wellrounded, moderately to well-sorted, silica cemented quartz grains that comprise nearly 100% of the framework. Quartz exhibits variable degrees of undulose extinction. Very well-rounded detrital zircon and tourmaline are minor components. The remaining three samples are characterized by strongly recrystallized granoblastic quartz fabrics, but very well-rounded detrital zircon and tourmaline indicate a sedimentary origin. Detrital zircon and tourmaline from one sample were studied with a scanning electron microscope to examine surface morphology. The documented presence of mature quartzarenite sandstone clasts in the Valle Group Cenomanian conglomerates requires a continental provenance. Potential sources are prebatholithic wall rocks of the Peninsular Ranges batholith or cratonal sedimentary sequences farther to the east.
