ABSTRACT The early to middle Pleistocene Colorado River Delta deposits exposed in the upper Gulf of California, Sonora, México, are host to a diverse paleo-fauna and paleo-flora (El Golfo local paleobiota) of Irvingtonian Land Mammal Age (Calabrian Stage). The fossiliferous exposures are found in badlands developed in fluvio-deltaic sediments that have been mildly deformed during late Pleistocene doming along the Cerro Prieto fault. The El Golfo Project is part of the resource inventory for the Upper Gulf of California and Colorado River Delta Biosphere Reserve. Through joint efforts by Arizona Western College, the La Brea Tar Pits and Museum, and the Centro de Geociencias, Universidad Nacional Autónoma de México, about 50% of the region has been prospected and mapped. To date, more than 13,500 mapped vertebrate fossil localities are documented, including important microvertebrate sites. New mammalian, avian, fish, and flora taxa have been recently added to a growing paleobiota list that now numbers more than 145 taxa. The preserved paleobiota suggests the existence of three ecologic communities: freshwater aquatic, shrub and brush woodland, and savannah-like grassland. Some of the fauna are presently endemic to geographic areas farther south, suggesting a more tropical to subtropical climate on the Colorado River Delta during this part of the Irvingtonian.

Abstract: The active Corinth rift records hanging-wall migration of faulting and slip-rate acceleration. The rift initiated at approximately 5–4 Ma, and older parts are well exposed in the northern Peloponnese. A new correlation of chrono- and lithostratigraphy and structure across the onland central to westernmost rift with offshore data reveals westward rift propagation, as well as northward fault migration. Northward fault migration ended first in the east, with the stabilization of major north-dipping faults that now bound the Gulf. The basin then propagated to the WNW in two stages, each involving the initiation of a new fault that propagated east to SE to link to the stable fault system. Extension rates accelerated in distinct steps as the rift opened to the west. The youngest faults in the westernmost rift are associated with high seismicity and highest geodetic extension due to rapid fault growth and linkage at depth. The early synrift succession infilled substantial inherited palaeo-relief. Antecedent rivers established vigorous sediment-routing systems that controlled facies distribution throughout rifting, albeit with drainage reorganization during fault-migration events. Multiple deepening events recorded in the stratigraphy can be due to lateral rift propagation. The transition from rift initiation to rift climax is, therefore, diachronous along the rift axis.