Estratigrafia y sedimentologia de los materiales pliocuaternarios del baix ebre y sectores adjacentes

A. ARASA TULIESA

Abstract


The Baix Ebre graben constitutes a small portion of the western mediterranean graben system related to the westem european Rift which is overimposed on the main structural trend. Other small grabens such as El Burga, l'Ametlla, l'Ampolla, Tortosa, La Sénia, Sant Mateu and Vinaròs, can be distinguished by their sedimentary infilling and structural geometries in the Baix Ebre graben. The basal units: "Arram Breccia" and "Rossell-l limestones" corresponding to the Paleogene can be distinguished. The upper units of "Rossell Conglomerates" and "Rossell-II limestones", separated from the former units by an unconformity, represent alluvial and palustrine environments which accumulated during the upper paleocene-lower eocene time span. The overlying units are Neogene in age: the "Rantxero Conglomerates" unit, corresponding to the Miocene, represents the midle alluvial fan deposits. The "Siliceous Conglomerates of Anguera" unit corresponds to the Late Miocene (Turolian) and records fluvial braided sedimentation Pliocene sedimentation began with the "Gravels with Oysters" unit. This is a transgressive unit where it is possible to differentiate foreshore and shoreface deposits. The "Marls of Camp-Redó" unit corresponds to shallow marine deposits. The "Sant Onofre Limestones" unit corresponds to palustrine deposits. The uppermost "Roca Corba Conglomerates" unit corresponds to alluvial deposits and represents The sedimentary relationships of the different subsurface stratigraphic units infilling the neogene Baix Ebre graben basin are described by means of drill hole data which permit a new lithostratigraphic correlation. These units are: "Mitanplana Conglomerates", "Vinaxarop Siliceous Gravels", "Vinallop Marls"and "Sénia Lutites". These units represent midfan conglomerates, confined marine marls and distal fan facies. The quaternary units are: Pelós gravels and Sant Antoni gravels. The neogene units represent midfan conglomerates sedimentary infilling of a paleovalley topography, confined marine marls and distal alluvial fan facies. During the Quaternary these units represent alluvial fan facies and braided fluvial facies. Different lithostratigraphic units are distinguished in the Baix Ebre Quaternary materials. Fluvial facies with a tabular character: "Ford Gravels", "Colada Sandstones" and "Aldover Poligenic Gravels" all with siliceous clastics. Alluvial facies with calcareous clastics: the "Aldover Monogenic Gravels". These units correspond to the lower and middle Quatemary. Other units constituting fluvial tenaces, whith calcareous monogenic clastics are differenciated by their textural character: "Xerta Blocks", "Llop Gravels" and the "Assud Sands"; these units correspond to the Thyrrhenian. The fluvial river basin is filled with the "Ebro Muds" corresponding to the Holocene. The Quatemary units described by borehole data are: "Pelós Gravels" and "Sant Antoni Gravels. During the Quatemary these units represent alluvial fan facies and braided fluvial facies. All these materials were affected by different stages of distensive tectonic fragmentation. In Paleocene materials the parallel fragmentation style affecting the mesozoic materials with a general NE-SW trend is observed. The Neogene lithostratigraphic units are affected by local the Villafranchian.  Al1 these materials were affected by different stages of distensive tectonic fragmentation. In Paleocene materials the parallel fragmentation style affecting the mesozoic materials with a general NE-SW trend is observed. The Neogene lithostratigraphic units are affected by local normal faults and have a dominant distensive general E-W and NE-SW trend. The Quaternary materials were affected by an important subsidence and fractured by normal faults approximately parallel to the mesozoic front. This faultis affect the main alluvial fan of the Baix Ebre- Montsih, l´Ampolla, l'Ametlla and Burga areas, with a main ESE-WSW distensive trend

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