RAPID BURIAL AND EXHUMATION OF THE EOCENE MANIOBRA BASIN, SOUTHERN CALIFORNIA

The Maniobra Formation comprises early Eocene marine strata located northeast of the Orocopia Mountains, southern California. Despite a handful of studies over the last 70 years, the relationship of these strata to the regional tectonic framework remains poorly resolved. This study combines new sedimentologic interpretations from the 1300 m-thick section with detrital zircon (DZ) U-Pb analyses and new apatite fission track data to infer the depositional and tectonic history of the basin. The base of the Maniobra Formation consists of a 500 m-thick megabreccia containing 100 m-scale, shattered granite ‘clasts’ derived from the local bedrock. Interbedded and discontinuous boulder and cobble conglomerate, sandstone, and shale units surround these large clasts and suggest deposition of landslide blocks within a marine setting. Above the megabreccia, the section is composed of fossiliferous marine shale with thin (0.2-1.0 m-thick) sandstone beds, interpreted as turbidites. The top of the section is dominated by interbedded calcareous shale, sandstone and pebble conglomerate beds, and is disconformably overlain by the Miocene Diligencia Formation. DZ spectra are dominated by Paleoproterozoic- aged peaks (~ 1.7 – 1.79 Ga), consistent with derivation from local Mojave basement. Subordinate Late Jurassic (168 – 156 Ma) and Late Cretaceous (91 – 78 Ma) populations – ages that are also present in the local basement – are observed and increase in proportion up- section. The top of the section yields the most complex DZ spectrum, which also includes minor Triassic (222 Ma) and Mesoproterozoic (1.2 Ga) populations, indicating a change in provenance to mixed and more distal sources. Five of the six new apatite fission track analyses (AFT) yield overlapping, late Eocene dates ranging from 41 – 37 Ma. One sample collected from a basal megabreccia clast records a 25 Ma date. These dates are younger than the depositional ages of the strata (~ 55-50 Ma) and require a previously-unrecognized episode of post-depositional burial and thermal resetting, followed by subsequent cooling in late Eocene time. Together, these data are consistent with relatively rapid initiation, filling, burial, and exhumation of the Maniobra basin between ~ 55– 37 Ma, possibly above the Orocopia Mts detachment.