Stratigraphy and structure of the Alisitos Formation near El Rosario, Baja California, Mexico

Thesis (M.S.) Geological Sciences. \ud Plate 1; Geologic map of an area near El Rosario, Baja California, Mexico. Plate 2; Structural cross-sections, in back pocket. \ud Original geological mapping performed on aerial photographs (scale 1:15,000). Base map (1:15,000) constructed from the aerial photographs using the hand-templet method with geology plotted on the base map.Approximately 1,820 meters of Aptian-Albian\ud Alisitos Formation is continuously exposed within a 100\ud square kilometer area east of El Rosario, Baja California.\ud To the east this formation is intruded by a granodiorite\ud pluton of the Peninsular Range batholith and to the west\ud is unconformably overlain by non-marine strata of the Late\ud Cretaceous Rosario Formation. Three diabase sills intrude\ud the Alisitos section.\ud Within the Alisitos Formation nine members have\ud been recognized. The members from oldest to youngest are:\ud Member A (275 meters), composed of interbedded dacite\ud flows, tuffaceous agglomerates, volcanic breccias, and\ud acidic welded tuffs; Member B (50-500 meters), composed\ud of dacite flows; Member C (165 meters), composed of fossiliferous,\ud interbedded volcanically derived sandstones,\ud siltstones, conglomerates, with minor andesite flows,\ud pyroclastic rocks and limestones ; Member D (60 meters) ,\ud composed of volcanically derived conglomerates and sandstones;\ud Member E (165 meters), composed of thinly interbedded,\ud volcanically derived shales, siltstones, and\ud sandstones, with locally abundant conglomerates and\ud limestones; Member F (210 meters), composed of agglomerates,\ud volcanic breccias, and tuffs; Member G (70 meters),\ud composed of interbedded dacite, andesite, basalt flows,\ud with interbedded agglomerates; Member H (50 meters),\ud composed of interbedded tuffs and agglomerates; and\ud Member I (325 meters), composed of interbedded tuffs,\ud agglomerates, volcanic breccias with minor andesite flows.\ud Members C through E are fossiliferous. Significant\ud marine fauna are Quadratotrigonia, Apiotrigonia,\ud Eucymatoceras, and Ostrea. Based on restricted occurrences\ud of Trigoniids in western North America Late Aptian\ud is suggested as the age of the lower portion of the\ud fossiliferous section.\ud Mineralogical, textural, and fossil evidence indicates that\ud a nearby volcanic source supplied sediments to\ud a relatively shallow basin distant or sheltered from\ud continental sediments during the deposition of Members C\ud through E. Stratigraphic evidence indicates that the\ud source area was to the west during the deposition of\ud Member C.\ud Structurally, the Alisitos Formation is a northwest-\ud southeast trending homocline dipping gently to the\ud south. Gravity faults of small displacement parallel this\ud regional trend. Two joint planes are developed, one\ud paralleling the regional trend and one perpendicular to it.\ud Field evidence indicates that the Alisitos Formation\ud was folded by northeast-southwest compression,\ud initially faulted, intruded by diabase sills, and metamorphosed\ud prior to the intrusion of the pluton. It is\ud proposed that subsequent to this intrusion and deposition\ud of the Rosario Formation, renewed compressional forces\ud acting along a similar northeast-southwest trend resulted\ud in renewed faulting