Publication: Microcodium y trazas fósiles de invertebrados en facies continentales (Paleoceno de la Cuenca de Áger, Lérida)
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Publication Date
1997
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Sociedad Geológica de España
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Abstract
La sucesión thanetiense (Paleoceno superior) en el valle de Áger (Prepirineo de Lérida) está formada por depósitos continentales y marinos someros y representa en conjunto una secuencia transgresiva formada a su vez por 8 secuencias de orden menor. Las dos secuencias basales están formadas por facies fluviales que pasan hacia techo a depósitos de lagos costeros someros carbonáticos. Las microcoditas son un caso extremo de reemplazamiento de facies fluviales intercanal por Microcodium, y su aparición en la parte superior de las secuencias representa la retracción del sistema fluvial asociada con una subida relativa del nivel del mar. Microcodium se formó en paleosuelos hidromorfos de tipo pseudogley, apareciendo como masas globulares alineadas verticalmente a lo largo de trazas de raíces. Se describen nueve tipos de tubos de excavación de invertebrados. Cinco
presentan rellenos activos en menisco y han sido clasificados como Taenidium sp. Los organismos responsables fueron probablemente gusanos oligochaeta acuáticos y terrestres, escarabajos y larvas de insectos. En facies fluviales, los tipos 1 y 2 caracterizan depósitos de crevasse splay y sheetflood; los tipos 3 y 4 caracterizan paleosuelos hidromorfos de tipo pseudogley en la llanura de inundación y son coetáneos con la colonización por Microcodium y con la marmorización; el tipo 5 fue producido
por un invertebrado que se alimentaba de Microcodium; el tipo 6 caracteriza facies de canal. En los depósitos de lagos costeros los tipos 7 y 8 están asociados con margas y calizas lacustres someras y el tipo 9 fue originado en condiciones palustres
Thanetian (upper Paleocene) deposits in the Áger valley (South Central Pyrenees) include shallow marine and non marine sediments and form an overall transgressive sequence subdivided into eight minor sequences. In situ Microcodiu.m and trace fossils are found copiously in the two lower sequences (1 and 2). These sequences consist mainly of fluvial deposits, with lacustrine limes tones in the upper part (shallow coastallakes). Fluvial systems show an upward and distal increase in overbank deposits, calcitic paleosols and in situ Microcodium content in interchannel facies. Fluvial sandstones are intrabasinal in origino Extrabasinal supply occurs mainly as dissolved calcium carbonate derived from karstification of Cretaceous and lower Paleocene limestones. Microcodium-stones are an extreme stage of Microcodium replacement after fluviatile interchannel deposits, and its abundance in the upper part of sequences reflects fluvial retraction associated to a relative sea level rise. Microcodium was generated in flood-plain paleosols (pseudogleys); sulphate-rich pore water prevented its development. Replacement is by means of globular bodies with concentric bands of cylinqrical colonies. Globular bodies are vertically alligned around root traces and colonize their outer surfaces. Nine forms of invertebrate bUITOWS with paleoenvironmental significance are described. Five of them present active menuscus fills and are identified as Thaenidium sp. The traces are of probable Oligochaeta, beetle and insect larvae origino In fluviatile deposits, type 1 and 2 are associated to crevasse splay and sheet flood deposits; type 3 and 4 occur in flood-plain hydromorphic paleosols and are coeval with Microcodium and mottling structures; type 5 was generated by a Microcodium-feeder invertebrate; type 6 is closely associated with channel sandstones. In lacustrine-palustrine deposits (shallow coastal lakes and marshes) type 7 and 8 occur in lacustrine marls and limestones, and type 9 is associated to the palustrine stage and was generated by a Microcodium-feeder
Thanetian (upper Paleocene) deposits in the Áger valley (South Central Pyrenees) include shallow marine and non marine sediments and form an overall transgressive sequence subdivided into eight minor sequences. In situ Microcodiu.m and trace fossils are found copiously in the two lower sequences (1 and 2). These sequences consist mainly of fluvial deposits, with lacustrine limes tones in the upper part (shallow coastallakes). Fluvial systems show an upward and distal increase in overbank deposits, calcitic paleosols and in situ Microcodium content in interchannel facies. Fluvial sandstones are intrabasinal in origino Extrabasinal supply occurs mainly as dissolved calcium carbonate derived from karstification of Cretaceous and lower Paleocene limestones. Microcodium-stones are an extreme stage of Microcodium replacement after fluviatile interchannel deposits, and its abundance in the upper part of sequences reflects fluvial retraction associated to a relative sea level rise. Microcodium was generated in flood-plain paleosols (pseudogleys); sulphate-rich pore water prevented its development. Replacement is by means of globular bodies with concentric bands of cylinqrical colonies. Globular bodies are vertically alligned around root traces and colonize their outer surfaces. Nine forms of invertebrate bUITOWS with paleoenvironmental significance are described. Five of them present active menuscus fills and are identified as Thaenidium sp. The traces are of probable Oligochaeta, beetle and insect larvae origino In fluviatile deposits, type 1 and 2 are associated to crevasse splay and sheet flood deposits; type 3 and 4 occur in flood-plain hydromorphic paleosols and are coeval with Microcodium and mottling structures; type 5 was generated by a Microcodium-feeder invertebrate; type 6 is closely associated with channel sandstones. In lacustrine-palustrine deposits (shallow coastal lakes and marshes) type 7 and 8 occur in lacustrine marls and limestones, and type 9 is associated to the palustrine stage and was generated by a Microcodium-feeder