“A la recherche du temps perdu”: On geological condensation, with examples from the Jurassic Subbetic Plateau in Southeastern Spain

Abstract

A condensed deposit is a marine rock residue that accumulated autochthonously over a large time-span; it should have a recognizable and chronologlc biostratigraphic record. Condensation is a continuous process working during sedimentation by mechanical concentration (filteringlsieving, screening, by-passing, winnowing, or scouring of mud) or shortly after sedimentation (blOerosion on hard grounds in combination with mechanical erosion and exhumation of underlying strata). In the Subbetic Plateau (formerly "external Subbetic") of the eastern Betic Cordillera, the threshold for the development of a condensed situation is controlled by a tectonic process: emergence, subsidence or drowning of adjacent carbonate mud supplying platforms regulate retention or releasing of mud. As a consequence, the Plateau experienced four episodes of strongly reduced sedimentation: Carixlan -Early Domerian (condensation episode 1), Late Toarcian -Early Bajocian (condensation episode 2), Early Bathoman -Early Oxfordian (condensation episode 3), and Earliest Kimmeridgian (condensation episode 4). Hardgrounds in condensed successions produced during these condensation episode. are often covered by goethite crusts and goethite oncOlds. The most abundant types of goethite crusts show extremely thin (20 -50 JLm) laminae with clotted, globular or pseudofilamentous fabric which pOint to an origin as microbial crusts or consist of structureless, •sterile" goethite ore. From an estimation of growth rate, we conclude that most of the time which is buried In these crusts must be concentrated in some millimetres of massive, "sterile" goethite crust or lost within hardgrounds or erosional discontinuities. Condensed I This paper is an extended version of a talk given under the title "Tethyan JuraSSlc Unconformities: Sea-Level Dipsticks, Deformation Memories, and Recorders of Complex Sealing Processes• at a meeting of the Geological Society of London on "Diagenesis at unconformities" in Burlington House, London, May 8-9, 1991. We dedicate it to the memory of Manfred Gwinner. 0077-7749/93/0\89-0013 $ 4.75 e 1993 I-Schwe"lzerbart'sche Verla~\buchhandlun~. [)-70176 ~tutt.:.art 14A. Fels and H. Seyfried successions often contain a hierarchy of sequences ranging from metre to millimetrescale. Independently of their hierarchical level, these sequences commonly start with reduced sedimentation, pass through an omission stage and end with an overgrowth by goethitic crusts. The process controlling this trend most probably is a eustatic signal. Bundles of condensed deposits containing goethite crusts are therefore interpreted as pclagic parasequences.Zusammenfassung: Eine kondensierte Ablagerung ist eine marine Ruckstandsbildung, die sich autochthon und kontinuierlich uber einen langeren Zeitraum hinweg gebildet hat. Kondensation kann synsedimentar durch Abschirmung, Ablenkung oder Abtragung von Schlamm erfolgen; diagenetisch lauft Kondensation vor allem uber Bioerosion auf Hartboden in Verbindung mit unterschiedlich tiefgreifender Erosion ab. Im Jura des subbe...