GEOLOGY OF THE LOWER MIOCENE MONTEREY FORMATION IN POINT DUME, CALIFORNIA

Exposures of the Miocene Monterey Formation, consisting of uplifted marine sediments, can be found in the coast ranges and sea cliffs along the southern California coast. The largest accessible and continuously exposed section near Cal Lutheran is located in Malibu at Point Dume state beach. The purpose of this study was to use clues within the formation to determine the ancient sedimentary processes that deposited the rocks and the subsequent deformation that produced the exposures found today. I investigated the lowermost section of the formation along the sea cliffs between Point Dume and Paradise Cove by measuring strike and dip, collecting samples and recording GPS coordinates of all field data sites and recording both sedimentary and deformation structures. Evaluation of the area centered on a large fault, not noted on previous geologic maps, separating two areas of different structural and sedimentological composition. Evidence of the fault displacement is exposed during low tide. While the rocks on one side of the fault dip an average of forty-two degrees the rocks on the other side dip a shallow nineteen degrees. West of the fault, the deeply dipping side, consists of a shale member interbedded with diatomite and chert with small scale brittle deformation. East of the fault, the shallow dipping side is a porcelanite member interbedded with mudstone that underwent both brittle and ductile deformation (folding). This eastern member contains many low angle thrust faults and folds that could be tied to drag-folding caused by slip on the larger fault. This study is important because much of the previous research in this area on the Monterey Formation has concerned its economic importance, i.e. oil and diatomite found in the formation, rather than structural analysis.