Depositional environments of the Towsley Formation in the Tujunga Valley, San Gabriel Mountains, Los Angeles County, California

A portion of the upper Miocene to lower Pliocene Towsley Formation crops out in the eastern Ventura Basin on the southwestern slope of the San Gabriel Mountains, near Sylmar, Los Angeles County, California. The studied portion of the forn1ation, approximately 300 to 500 m thick, extends from the Lopez Canyon area southeastward to Big Tujunga Canyon area decreases in thickness toward the southeast. Stratigraphic analysis revealed ten lithosomes in the Towsley Fonnation and two lithosomes in the overlying Saugus Formation. They are light-yellowish-brown conglomerate (A); bimodal conglomerate (B); brown, very fine-grained, laminated sandstone (C); light-gray, low-angle cross-bedded sandstone (D); light-gray conglomerate (E); bioturbated, very fine-grained sandstone (F); light-yellowish-brown, very finegrained sandstone (G); light-gray, friable, medium- to coarse-grained sandstone (H); bioturbated, fine- to medium-grained sandstone (I); light-greenish-gray, fine- to mediumgrained sandstone (J); very light-gray, structureless, friable sandstone (K) (Saugus Formation); and very light-gray, well-rounded conglomerate (L) (Saugus Formation). Lithosomes A and B represent debris-flow deposits in an inner fan or submarine canyon environment; lithosomes C and F represent shelf deposits; lithosomes D, E, G, H, I, and J are shoreface deposits; and lithosomes K and L are fluvial. The rocks were derived mostly from felsic plutonic rocks that occur in the eastern San Gabriel Mountains north of the San Gabriel fault. The San Gabriel fault was moving right laterally, while coarse debris was being transported out of the San Gabriel Mountains continuously by the San Gabriel River across the San Gabriel fault. This debris was deposited into a deep-marine, inner fan environment that existed along the edge of a steeply sloped basin with a narrow shelf. After movement on the San Gabriel fault carried the coarse-elastic source away from the site of deposition, shelf and shoreface deposits formed as the shoreline regressed.