Sedimentary and Tectonic Models for Tightly Folded Neogene Marine Sandstone In Split Mountain, Eastern San Diego County, California A Thesis presented to the Faculty of the Institute for Creation Research In Partial Fulfillment of the Requirements for the Degree Master of Science in Geology by Paul Kerby Coski Santee, California December 1995 Abstract Spectacular tight folds in the Neogene Lycium member of the Imperial Formation are exposed in Split Mountain located in Anza Borrego Desert State Park of California, near the eastern-border of San Diego county. A tight anticlinal fold is exposed at the wash level on the western wall of Split Mountain gorge and adjacent to Split Mountain fault. This fold (informally designated the Split Mountain tight fold) along with four other folds, were examined to determine the origin of these structures. The Split Mountain tight fold was used as the "type fold" for detailed analysis of the mode of folding in the Lycium member because it is well exposed, relatively accessible, and is the most controversial in regards to it's origin. The two primary kinematic models proposed for folding Neogene marine sandstone are as follow: (1) a tectonic model in which fault movement on the Split Mountain fault allowed breccia blocks to down-drop and fold the Lycium member sandstone bedding, and (2) a rock avalanche-submarine slide emplacement model that resulted in significant disruption of the Lycium member sandstone during catastrophic emplacement of an approximately 50-90 m thick megabreccia unit. The local structure and stratigraphy of Split Mountain were examined along with the literature outlining the western Salton Trough regional geology, to help evaluate and determine the best fit kinematic model for fold development in the Lycium member. Four additional sandstone folds in the Lycium member, were examined along with the Split Mountain tight fold to determine if the structures developed at the same time and by the same mechanism. An evaluation of several tectonic models revealed that a tectonic interpretation was inadequate in explaining the origin of these structures. Penecontemporaneous folding during landslide emplacement best fits available data. At least three catastrophic landslide events occurred during the depositional history of the Fish Creek-Vallecito section. Orientations and structural configuration of "S" shaped folds, "C" shaped folds, sheath folds, fan folds, and striations on landslide slip surfaces helped to narrow possible transport directions of the UBB landslide events and further strengthen suspected source areas. These slides were most likely triggered by seismic ground shaking, movement on detachment faults, tsunamis wave run-up and/or sudden large-scale vertical offset and oversteepening along strike-slip fault scarps bounding the Fish Creek-Vallecito basin. Sediments deposited on the western Salton Trough margin document the Salton Trough-Gulf region developmental history. In a global catastrophe model for the majority of Phanerozoic sediments, the FCV section is interpreted to have been deposited during post-catastrophe geologic adjustments following rapid displacement of continental plates.