Late miocene to pliocene stratigraphic evolution of northeast Carmen island, Gulf of California: implications for oblique-rifting tectonics

This paper describes a thick section of upper Miocene(?) to middle Pliocene marine sedimentary rocks exposed at Punta Perico, northeastern Carmen Isla nd, in the southern Gulf of California. These strata overlie Miocene volcan ic rocks along a low-angle unconformity and have a total thickness of simil ar to 1100 m. The section contains the following units: (1) lower conglomer ate (similar to 750 m thick) representing submarine debris flows and grain flows, (2) conglomerate and sandstone (similar to 90-160 m) deposited by gr avelly grain flows and sandy turbidity currents; (3) marlstone and mudstone (similar to 120 m) with benthic forams that record bathyal water depths (4 00-500 m) and an age of similar to3.5-3.1 Ma; (4) dacite breccia and boulde r beds (similar to 20 m) emplaced by submarine avalanches and debris flows and (5) stratified conglomerate and bioclastic limestone (greater than or e qual to 80 m) deposited by traction currents in a shallow shelf environment . Conjugate syn-sedimentary faults, deformation bands, and growth structure s are recognized within a 250-m-wide belt adjacent to a large N-striking, W -dipping normal fault that bounds the eastern margin of the Pliocene sectio n, indicating that the eastern normal fault was active during deposition of at least the upper similar to 300 m of the dipping section. Considering th e thickness of the two units below the marlstone (similar to 900 m) and ass uming a range of possible sediment-accumulation rates (0.2-1.0 m/ka), we po stulate that deposition of the lower conglomerate was initiated in late Mio cene or early Pliocene time (similar to8-4 Ma), possibly coincident with in itial opening of the Gulf of California. Deposition of the marlstone unit a bove a probable unconformity represents a major change in basinal tectonics at similar to3.5-3.1 Ma. which is about the time when seafloor spreading a nd transform faulting were initiated in the Gulf of California. These tenta tive correlations suggest that the Baja California peninsula was structural ly and mechanically linked to the deep Gulf during late Miocene to middle P liocene time. (C) 2001 Elsevier Science B.V. All rights reserved.