RAPID SUBSIDENCE AND COLLAPSE OF THE EOCENE CORDILLERAN MARGIN AND DEPOSITION OF THE MANIOBRA FORMATION, CA

Eocene strata are poorly preserved in the southwestern Cordillera and are most-commonly related to forearc deposition west of the Sierra Nevada-Peninsular Ranges arc. The Maniobra Fm., however, comprises at least 1000 m of Eocene strata that cannot be obviously related to the forearc at that time. This study combines new geologic observations and mapping with detrital zircon (DZ) analysis of 4 samples collected throughout the stratigraphic section. The base is composed of a 400-m thick megabreccia unit nonconformably above the Late Jurassic Hayfield Granite. Megabreccia clasts are 10¹-10³ m-diameter, heavily fractured, and are composed of Hayfield granite. Clasts are surrounded by silt and shale with interbedded 0.5m-thick sandstone beds interpreted as turbidites. We interpret the megabreccia to have formed from landslide blocks into a rapidly subsiding marine slope or within a steep-walled submarine canyon. Above the megabreccia the stratigraphy comprises 350 m of marine shale with mollusk fossils and interbedded thin sandstones interpreted as turbidites. Two DZ samples (DZ 1901, 1903) at 420m and 500m stratigraphically above the base contain prominent peaks at 1790 Ma, 168-156 Ma, and 91-85 Ma. The source of the megabreccia clasts and the DZ peak at ~160 Ma are likely Hayfield Granite (U/Pb age 156±2Ma; this study), although other sources are likely based on the DZ peaks. At 750 m (strat. height) the stratigraphy changes to a calcareous sandstone with some mollusk fossils and interbedded gypsum. One DZ sample from 800 m contains similar peaks to the lower samples but also has additional prominent peaks at 1200 Ma and 222 Ma peaks, implying a changing provenance over time. The sandstone and conglomerate, increasing gypsum, and increasing grain size implies shallowing of the basin. Altogether, our data imply a rapidly subsiding basin formed during the Eocene, which quickly filled with strata initially from the Hayfield Mts but shifted to another source over time. The juxtaposition of the Maniobra Fm. against the Orocopia and Clemens Well detachment systems to the southwest suggest that these strata may be related to rapid subsidence during detachment faulting during the Eocene, which occurred as the result of subduction of a large igneous province beneath the Mojave region.