Description
Approximately 1,820 meters of Aptian-Albian Alisitos Formation is continuously exposed within a 100 square kilometer area east of El Rosario, Baja California. To the east this formation is intruded by a granodiorite pluton of the Peninsular Range batholith and to the west is unconformably overlain by non-marine strata of the Late Cretaceous Rosario Formation. Three diabase sills intrude the Alisitos section. Within the Alisitos Formation nine members have been recognized. The members from oldest to youngest are: Member A (275 meters), composed of interbedded dacite flows, tuffaceous agglomerates, volcanic breccias, and acidic welded tuffs; Member B (50-500 meters), composed of dacite flows; Member C (165 meters), composed of fossiliferous, interbedded volcanically derived sandstones, siltstones, conglomerates, with minor andesite flows, pyroclastic rocks and limestones ; Member D (60 meters) , composed of volcanically derived conglomerates and sandstones; Member E (165 meters), composed of thinly interbedded, volcanically derived shales, siltstones, and sandstones, with locally abundant conglomerates and limestones; Member F (210 meters), composed of agglomerates, volcanic breccias, and tuffs; Member G (70 meters), composed of interbedded dacite, andesite, basalt flows, with interbedded agglomerates; Member H (50 meters), composed of interbedded tuffs and agglomerates; and Member I (325 meters), composed of interbedded tuffs, agglomerates, volcanic breccias with minor andesite flows. Members C through E are fossiliferous. Significant marine fauna are Quadratotrigonia, Apiotrigonia, Eucymatoceras, and Ostrea. Based on restricted occurrences of Trigoniids in western North America Late Aptian is suggested as the age of the lower portion of the fossiliferous section. Mineralogical, textural, and fossil evidence indicates that a nearby volcanic source supplied sediments to a relatively shallow basin distant or sheltered from continental sediments during the deposition of Members C through E. Stratigraphic evidence indicates that the source area was to the west during the deposition of Member C. Structurally, the Alisitos Formation is a northwest- southeast trending homocline dipping gently to the south. Gravity faults of small displacement parallel this regional trend. Two joint planes are developed, one paralleling the regional trend and one perpendicular to it. Field evidence indicates that the Alisitos Formation was folded by northeast-southwest compression, initially faulted, intruded by diabase sills, and metamorphosed prior to the intrusion of the pluton. It is proposed that subsequent to this intrusion and deposition of the Rosario Formation, renewed compressional forces acting along a similar northeast-southwest trend resulted in renewed faulting.
