The Genesis of Condensed Sequences in the Tethyan Jurassic

Abstract

Pelagic limestones of the west Sicilian Middle Jurassic are used as type examples in this examination of Tethyan condensed sequences. Two main processes were involved in the genesis of this type of deposit: stratigraphic condensation (minimal sediment input) and reworking. The lack of nannoplankton during most of the Jurassic caused stratigraphic condensation, and the particular environment in which these beds were deposited – that of topographic high – was considerably influenced by currents; thus reworking led to a further reduction in sediment thickness. The presence of algal stromatolites in certain condensed sequences and the traces of boring algae in associated ferromanganese nodules suggest deposition within the photic zone. A maximum depth of 200 metres is inferred. Many features of condensed sequences are comparable with those on Recent seamounts; these include presence of ferromanganese nodules and crusts, mixed faunas, calcarenite lenses, algal biscuits, and evidence of submarine solution and lithification. Jurassic Tethyan condensed sequences can thus be interpreted as ancient seamount deposits.

Condensation processes in shallow platforms

“…The value of the mentioned rate is often expressed in centimetres per million years or in analogous units. The terms rate of sedimentation (Schindel, 1982;Tipper, 1983;Bates and Jackson, 1987;Ricken, 1991), net sediment accumulation rate (Jenkyns, 1971;Tipper, 1986;Sadler and Strauss, 1990), accumulation rate (Dott, 1983;Tipper, 1986), and rate of sediment accumulation (Schindel, 1980(Schindel, , 1982 have been used for the same condition.…”

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“…El término condensación estratigráfica ha sido empleado por numerosos autores para hacer referencia a la disminución de espesor de cualquier sucesión estratigráfica como consecuencia de una tasa de sedimentación más escasa o nula (Heim, 1934(Heim, , 1946(Heim, , 1958Went, 1970Went, , 1973Fürsich, 1971Fürsich, , 1978Fürsich, , 1979Jenkyns, 1971, Geyer & Hinkelbein, 1974Elmi, 1981;Seyfried, 1981). Como resultado del proceso de condensación estratigráfica se han formado capas, bancos, horizontes, niveles estratigráficos, secuencias o sucesiones estratigráficas de menor espesor que otra sucesión estratigráfica contemporánea, o de intervalo temporal equivalente, utilizada de referencia.…”

“…Una de las interpretaciones más usuales asocia los niveles y secuencias condensadas, que se encuentran en materiales pelágicos mesozoicos, con la presencia de altos topográficos submarinos o umbrales localizados en ambientes marinos abiertos y oceánicos (Wendt, 1970;Jenkyns, 1971;Vera, 1989). Además, varios autores han insistido en la idea de que en las plataformas epicontinentales afectadas por tempestades, los eventos de condensación y los «horizontes de condensación tempestítica» adquirieron mayor desarrollo durante las fases regresivas (Seilacher, 1982;Hagdorn, 1982;Gebhard, 1982) yen el término superior de las secuencias de somerización (Bayer et al, 1985).…”

High-Resolution Sequence Stratigraphy, Condensed Sections, and Flooding Events off the Great Barrier Reef: 0-1.5 Ma

“…Since then, researchers have increasingly recognized that intervals of sequence condensation also frequently were marked by the occurrence and, in some instances, a high (even minable) abundance of authigenic minerals (e.g., Glenn, 1990). Of continuing debate, however, is how much sediment reworking and winnowing contributes to the condensation process (e.g., Rod, 1946;Jenkyns, 1971;Kennedy and Garrison, 1975;Krajewski, 1984;Kidwell, 1991;Burnett et al, 1988;Glenn and Arthur, 1990;Föllmi, 1990;Snyder et al, 1990).…”

“…7). For example, this may be reflected in outcrop studies that link condensed sections to a variety of shallow-and deep-water settings, to starvations that coincide with regressions characterized by sediment bypassing, and to transgression (e.g., Rod, 1946;Jenkyns, 1971;Einsele and Seilacher, 1982;Bayer and Seilacher, 1985;Kidwell, 1991). On the singular basis of temporal variations in glauconite abundance, our data suggest that sequence condensation is predominantly limited to marine flooding and maximum flooding surfaces in this mixed depositional setting.…”