Neotectonics and taphonomy; Pleistocene molluscan shell accumulations in the northern Gulf of California

• Published in: Palaios Vol. 7; no. 2; pp. 187 - 197
• Main Authors: Meldahl, Keith H, Cutler, Alan Hughes
• Format: Journal Article
• Language: English
• Published: Tulsa, OK Society of Economic Paleontologists and Mineralogists 01-04-1992; SEPM
• Subjects: Beaches; Berms; Cenozoic; Coasts; Earth sciences; Earth, ocean, space; East Pacific; Exact sciences and technology; Fauna; Geologic unconformities; Geology; Gulf of California; Gulfs; Invertebrata; invertebrate; Invertebrate paleontology; Marine and continental quaternary; marine sedimentation; Mollusca; neotectonics; North Pacific; Northeast Pacific; Pacific Ocean; paleoecology; Paleontology; Pleistocene; Quaternary; Research Letters; sedimentation; shore features; Splashing; structural geology; Surficial geology; Taphonomy; Tectonics; Tectonics. Structural geology. Plate tectonics; tidal channels; unconformities; Gulf of California; North American Pacific; Pacific Ocean; mollusks; taphonomy; Quaternary; shells; structural controls; neotectonics; upper Pleistocene; transpression; strike-slip faults; marine environment; middle Pleistocene; uplifts; Baja California; shallow-water environment; Sonora
• DOI: 10.2307/3514929
• EISSN: 1938-5323
• ISSN: 0883-1351

Late Middle to early Late Pleistocene shallow marine deposits west of Bahia Adair, to the northern Gulf of California, have been exposed by tectonic uplift along the coast. Represented sedimentary environments include shallow subtidal, tidal flat, tidal estuary, tidal channel and shoreface. The deposits contain a diverse molluscan fauna, often concentrated in shell beds. Based on taphonomic data, stratigraphic geometry and faunal composition, the shell beds are identified as beach berm accumulations, tidal channel lags, subtidal community beds, or unconformity beds. While the first three types of shell beds form on many continental margins, the unconformity beds reflect the active tectonic setting of the northern Gulf of California. These beds are bioclast-supported tabular or sheet-like accumulations 5-15 cm thick, persist for more than 1000 m along out-crop, and cap angular unconformities truncating gently folded strata. Reworking in the swash zone is indicated by heavily abraded and fragmented shells exhibiting abraded endolith borings. The beds are encased in heavily bioturbated shallow subtidal or lower intertidal deposits containing abundant Ophiomorpha and Thalassinoides burrows, many of which either penetrate the beds or are cut by the beds. Unconformity beds are erosional lags created by beveling of structural bulges caused by uplift and deformation along the Pleistocene coast. At least two episodes of uplift and deformation are recognized in the field area, possibly caused by transpressional forces associated with the Cerro Prieto Fault, a major dextral strike-slip fault joining the Gulf of California to the San Andreas Fault system. In these heavily bioturbated strata, the unconformity shell beds may be the only means of recognizing deformation caused by neotectonic activity.