Geology of the Agua Dulce Canyon area, Los Angeles County, California

Abstract

The Agua Dulce Canyon area is in the northwest quarter of Los Angeles County, California. Within it are exposed 4000 feet of the Oligocene (?) Vasquez series and 1400 feet of the Miocene Tick Canyon formation. These are clastic sedimentary rocks distinguished by their unusual coarseness. The Vasquez series is composed of sandstones and conglomerates, with some siltstone at the top of the section. These were deposited in a narrow continental basin that drained to the west, and were deprived from neighboring highlands to north, south, and east. Most appear to have been laid down in the form of alluvial fans or piedmont alluvial plains. A 3000 foot section of anorthosite-rich conglomerates, here referred doubtfully to the Vasquez series, is exposed in a fault block in the south part of the area. The Tick Canyon formation lies unconformably above the Vasquez series. Only its basal parts are exposed within the Agua Dulce Canyon area. Those consist wholly of conglomerate beds, the lowest of which contain numerous cobbles derived from lavas of the Vasquez series. Quaternary gravels form a maximum of thirty feet of terrace capping, and also fill Pleistocene and Recent stream channels. Two groups of faults cut the Tertiary sediments. Those are located within the area as follows: the Soledad fault is near the southern border, the Burke fault and Little Escondido fault are in the southeast corner, the Bee Canyon fault and Escondido fault are in the center, and the Green Ranch fault is in the northwest corner. The Soledad fault and Bee Canyon fault are older than the others, which are on echelon faults. Vertical movement has been predominant. A gentle syncline trending northeast-southwest folds Vasquez and Tick Canyon rocks in the northern half of the area. A subdued erosion surface existed in Quaternary time. Remnants can now be seen near the northern border of the area. Well developed stream terraces in Agua Dulce Canyon were contemporaneous with this surface. It is now being dissected by rejuvenated streams.