Abstract Marine straits and seaways are known to host a wide range of sedimentary processes and products, but the role of marine connections in the development of large river systems remains little studied. This study explores a hypothesis that shallow-marine waters flooded the lower Colorado River valley at c. 5 Ma along a fault-controlled former tidal strait, soon after the river was first integrated into the northern Gulf of California. The upper bioclastic member of the southern Bouse Formation provides a critical test of this hypothesis. The upper bioclastic member contains wave ripple-laminated bioclastic grainstone with minor red mudstone, pebbly grainstone with hummocky cross-stratification (HCS)-like stratification and symmetrical gravelly ripples, and calcareous-matrix conglomerate. Fossils include upward-branching segmented coralline-like red algae with no known modern relatives but confirmed as marine calcareous algae, echinoid spines, barnacles, shallow-marine foraminifers, clams, and serpulid worm tubes. These results provide evidence for deposition in a shallow-marine bay or estuary seaward of the transgressive backstepping Colorado River delta. Tsunamis generated by seismic and meteorological sources likely produced the HCS-like and wave-ripple cross-bedding in poorly-sorted gravelly grainstone. Marine waters inundated a former tidal strait within a fault-bounded tectonic lowland that connected the lower Colorado River to the Gulf of California. Delta backstepping and transgression resulted from a decrease in sediment output due to sediment trapping in upstream basins and relative sea-level rise produced by regional tectonic subsidence.

Abstract The approximately 350 m-thick stratigraphic succession of the Zagra Strait records an important oceanographic phase of basin interconnection between the Atlantic Ocean (Guadalquivir Basin) and the Mediterranean Sea through the Betic Cordillera (southern Spain) during the early Tortonian. The Zagra Strait developed as a narrow structurally controlled marine corridor. The sedimentary dynamics of the Zagra Strait were interpreted from the sedimentological features observed in six sections at well-exposed outcrops. Large-scale (>10 m high) compound dunes and compound–dune complexes moved parallel to the strait margins under strong tidal currents generated by tidal amplification at the strait entrance and exit. Dune distribution can be divided into three sectors with different palaeocurrent migration, lithological and topographical characteristics. The northern and central sectors were separated by a deep depression (>75 m water depth) where tidal currents were weaker and dunes were not generated. The southern sector records a relative decrease in current strength compared with the northern and central sectors, and a significant increase in the bioclastic content in the sediment. Terrigenous content generally increases towards the strait margins, and reciprocally, carbonates towards its axis. The closure of the Zagra Strait resulted from tectonic uplift of that part of the Betic Cordillera before the late Tortonian.

Abstract: The western Mediterranean region contains abundant examples of the different types of Lower, Middle and Upper Miocene reefs (hermatypic coral reefs, ahermatypic mounds, rhodalgal biostromes and stromatolitic reefs). Those corresponding to the Upper Tortonian-Messinian rock units are the ones that have attracted the most attention because of the extraordinary quality of the outcrops and their relation to the polemic Messinian events in the Mediterranean. This section is a general introduction to the region, with a review of the Lower-Middle Miocene rhodalgal biostromes and coral reefs of the Gulf of Valencia-Provençal Basin and the Middle-Upper Miocene reefs of southeastern Spain and northern Morocco. The emphasis in this paper will be on the complex Miocene stratigraphy and paleogeography of southeastern Spain (Betics) and northern Morocco (Rif). This part of the western Mediterranean is important in understanding the paleogeographic evolution of the entire Mediterranean and its connection with the Atlantic Ocean.