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 Island, in the southern Gulf of California. These strata overlie Miocene volcanic rocks along a low-angle unconformity and have a total thickness of similar to 1100 m. The section contains the following units: (1) lower conglomerate (similar to 750 m thick) representing submarine debris flows and grain flows, (2) conglomerate and sandstone (similar to 90-160 m) deposited by gravelly grain flows and sandy turbidity currents; (3) marlstone and mudstone (similar to 120 m) with benthic forams that record bathyal water depths (400-500 m) and an age of similar to3.5-3.1 Ma; (4) dacite breccia and boulder beds (similar to 20 m) emplaced by submarine avalanches and debris flows and (5) stratified conglomerate and bioclastic limestone (greater than or equal to 80 m) deposited by traction currents in a shallow shelf environment. Conjugate syn-sedimentary faults, deformation bands, and growth structures 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 section, indicating that the eastern normal fault was active during deposition of at least the upper similar to 300 m of the dipping section. Considering the thickness of the two units below the marlstone (similar to 900 m) and assuming a range of possible sediment-accumulation rates (0.2-1.0 m/ka), we postulate that deposition of the lower conglomerate was initiated in late Miocene or early Pliocene time (similar to8-4 Ma), possibly coincident with initial opening of the Gulf of California. Deposition of the marlstone unit above 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 and transform faulting were initiated in the Gulf of California. These tentative correlations suggest that the Baja California peninsula was structurally and mechanically linked to the deep Gulf during late Miocene to middle Pliocene time. (C) 2001 Elsevier Science B.V. All rights reserved.