Origin and significance of rhodolith-rich strata in the Punta El Bajo section, southeastern Pliocene Loreto basinглава из книги
Аннотация: Pliocene sedimentary rocks at Punta el Bajo accumulated in the southeastern Loreto basin between about 2.3 and 2.0 Ma, during a period of relatively slow subsidence along the Loreto fault. Sediments in this section unconformably overlie Miocene volcanic rocks and consist of interbedded cobble to cobble-boulder conglomerate and rhodolith-bearing massive calcarenite. Based on the presence of boulder-size clasts, shelly calcarenite matrix, sharp erosional bases, and one lithofagid-bored unconformity, conglomerate beds are interpreted to have been deposited along a late Pliocene rocky shoreline. Interbedding of conglomerate and calcarenite beds preserves a record of four episodes of deposition of coarse shoreline gravels followed by submergence of the shoreline (transgression) and deposition of shoreface and shallow offshore calcarenites. The section displays a distinctive upsection decrease in bedding dips accommodated by nonerosional stratal wedging and one internal erosional unconformity. Repeated episodes of erosion and submergence of this rocky shoreline (as interpreted from vertical facies trends) and the well-expressed fanning-dip geometry are both attributed to discrete episodes of fault slip and related tilting along active intrabasinal faults. Calcarenite deposits in this section are rich in rhodoliths (coralline red algae) with accessory molluscan shell fragments and siliciclastic sand, thus comprising a rhodalgal lithofacies. Rhodoliths are present in three forms: (1) quasi-spherical, concentrically laminated calcareous nodules 1 to 3 mm in diameter; (2) cylindrical fragments 3 to 5 mm long, some displaying relict branches; and (3) broken to nearly complete ballshaped plants 0.5 to 4 cm in diameter. Forms 1 and 2 are dominant, and form 3 is found only high in the section. The highly fragmented nature of the rhodoliths and their close association with rocky-shore conglomerate beds indicate that they were deposited in a moderate- to high-energy shoreface setting. Rare whole rhodoliths high in the section suggest a slightly deeper, more offshore setting, possibly in a currentinfluenced tidal channel. Globally, rhodalgal lithofacies are typical of climate belts transitional between tropical and temperate zones. The Loreto basin currently is situated between a coral reef‐dominated carbonate shelf to the south and a siliciclastic shelf with echinoids, bivalves, bryozoans, and barnacles to the north. It is likely that, in spite of some modifications due to post‐2.0 Ma rifting, late Pliocene regional paleogeography of the Gulf of California resembled that of today. Thus, the occurrence of rhodolith-rich facies in the Punta el Bajo section appears consistent with facies models for carbonate deposition on climatically zoned continental shelves.
Год издания: 1997
Авторы: Rebecca J. Dorsey
Издательство: Geological Society of America
Источник: Geological Society of America eBooks
Ключевые слова: Geological and Geophysical Studies, Geological formations and processes, Geology and Paleoclimatology Research
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