Publication: El Sinemuriense y el Pliensbachiense
en la sección de Alfara,
cordillera costero-catalana (Tarragona)
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1998
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Universidad Complutense de Madrid. Departamento de Estratigrafía
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Abstract
Los materiales del Jurásico Inferior de la región de Alfara, en la Cordillera Costero-Catalana, han sido motivo de numerosos estudios bioestratigráficos desde los primeros años del siglo xx, aunque son pocos los afloramientos donde pueden observarse en continuidad. La reciente construcción de la carretera que une Alfara de Carlés con Tortosa permite reconocer, sobre la Formación Imón del Triásico Superior, la totalidad de las unidades del Jurásico Inferior. En este trabajo se estudian la Formación Cuevas Labradas, «Unidad de Almonacid de la Cuba» y Formación Barahona, todas ellas con características similares a las reconocidas en la Cordillera Ibérica (Goy et al., 1976; Comas-Rengifo, 1982). Los materiales que las constituyen se organizan en secuencias de somerízación que pueden terminar con costras ferruginosas y a veces con hardgrounds. Los fósiles de invertebrados son escasos hasta los niveles superiores de la Fm. Cuevas Labradas, por lo que el límite Sinemuriense/Pliensbachiense no puede ser marcado con precisión. No obstante, existen asociaciones de braquiópodos y algunos ammonites que son típicos del Fliensbachiense inferior. Por otra parte, la Formación Calizas y calizas margosas de Sant Blai es notablemente más fosilífera e incluye en su parte inferior asociaciones de braquiópodos y ammonites característicos del Toarciense inferior.
Lower Jurassic deposits at the Alfara region, in the so-called Catatan coastal Chain (NE Spain) have been the subject of numerous biostratigraphic studies from the beginning of the 20ih century, even though favourable outcrops in this area are generally rare and sparse. Recent works on the road connecting ihe localities of Alfara de Carlés and Tortosa have lcd to tlíe appearance of fairly good exposures of the Lower Jurassic units which overlie the Upper Triassic Imón Fm. Some of the lithologic units distinguished in this area can easily be compared to, and correlated with, those defined for the Iberian Basin. Aboye the so-called Cortes de Tajuña Fm., the following units are recorded and studied in detail: Cuevas Labradas Fm. (mainly Sinemurian), Almonacid dc la Cuba Unit (Lower Pliensbachian) and Barahona Fm. (Upper Pliensbachian to Lower Toarcian). Sediments in these studied units are organized in shallowing upwards sedimentary sequences, generally bounded on top by ferruginous crusts and sometimes by hard-ground surfaces. Invertebrate fossils are extremely scarce until the upper part of the Cuevas Labradas Fm., so the Sinemurían- Pliensbachian boundary cannot be precisely delineated. Yet, some brachiopod and ammonite assemblages characterizing the Lower Pliensbachian are found in the upper levels of this unit.Unlike the mentioned units, the so-called Sant Blai Fm. is clearly more fossiliferous. lts lower part has yielded rich brachiopod and amrnonite assemblages characterizing Lower Toarcian.
Lower Jurassic deposits at the Alfara region, in the so-called Catatan coastal Chain (NE Spain) have been the subject of numerous biostratigraphic studies from the beginning of the 20ih century, even though favourable outcrops in this area are generally rare and sparse. Recent works on the road connecting ihe localities of Alfara de Carlés and Tortosa have lcd to tlíe appearance of fairly good exposures of the Lower Jurassic units which overlie the Upper Triassic Imón Fm. Some of the lithologic units distinguished in this area can easily be compared to, and correlated with, those defined for the Iberian Basin. Aboye the so-called Cortes de Tajuña Fm., the following units are recorded and studied in detail: Cuevas Labradas Fm. (mainly Sinemurian), Almonacid dc la Cuba Unit (Lower Pliensbachian) and Barahona Fm. (Upper Pliensbachian to Lower Toarcian). Sediments in these studied units are organized in shallowing upwards sedimentary sequences, generally bounded on top by ferruginous crusts and sometimes by hard-ground surfaces. Invertebrate fossils are extremely scarce until the upper part of the Cuevas Labradas Fm., so the Sinemurían- Pliensbachian boundary cannot be precisely delineated. Yet, some brachiopod and ammonite assemblages characterizing the Lower Pliensbachian are found in the upper levels of this unit.Unlike the mentioned units, the so-called Sant Blai Fm. is clearly more fossiliferous. lts lower part has yielded rich brachiopod and amrnonite assemblages characterizing Lower Toarcian.