in Jurassic and Cretaceous Stratigraphy

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Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321 286 Evaluating Palaeoecological Response in Condensed Aalenian-Bajocian Deposits from western Tethys (Iberian Range, Spain) Alejandra García-Frank 1 , Soledad Ureta 1 , Ramón Mas 2 1. Departamento and UEI de Paleontología, Facultad C.C. Geológicas e Instituto de Geología Económica, (CSIC-UCM), José Antonio Novais 2. 28040, Madrid. Spain (E-mail: agfrank@geo.ucm.es; solureta@geo.ucm.es) 2. Departamento and UEI de Estratigrafía, Facultad C.C. Geológicas e Instituto de Geología Económica, (CSIC-UCM), José Antonio Novais 2. 28040, Madrid. Spain (E-mail: ramonmas@geo.ucm.es) The relationship of fossil assemblages, lithofacies and stratigraphic sequences within the uppermost Toarcian-lowermost Bajocian deposits of the North- western Iberian Range (Spain) has been studied in order to distinguish ecozones and ecostratigraphic sequences. The condensed carbonate deposits display specific features related with the depositional architecture of the studied area. A detailed study and comprehension of the timing of the sedimentation in this area, is reflected in the establishment of type sequences, which show the lithofacies changes and the temporal distribution of fossils based in accurate biostratigraphic data. The combined analysis of stratigraphic sequences and palaeontological data (macro- and microfacies analyses of the fossil content, including ichnofacies) allow to detect changes in the record of fossil assemblages, and to establish ecozones and ecostratigraphic sequences (Fig.1). Fig.1 Jurassic outcrops in the Iberian Peninsula and (enlarged, lower right) in the NW Iberian Range, with the stratigraphical and palaeostratigraphical remarks within the uppermost Toarcian-lowermost Bajocian deposits for the different studied areas The condensed deposits located in the San Leo- nardo, Madero-Moncayo and Demanda areas exhibit five discontinuity bounded sequences (García-Frank, 2007; García-Frank et al., 2006, 2008) in which five ecozones are observed. These ecozones are organized into three ecostratigraphic sequences. However, in the more expanded deposits of the Cameros area a re- markable distinctive style of sedimentation is recorded
Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321 (García-Frank et al., 2006, 2008), which is consistent with a different ecostratigraphical development. Despite the slow sedimentary rate that characterise these deposits, changes in the state of substrate have been detected, showing progressive lithification from loose to firm, and even, to hardground formation. The ichnites related with (stiff)/firm sediment are found within the Glossifungites ichnofacies, whereas the ichnites associated with lithified sediments and development of hardgrounds, correspond to the Trypanites ichnofacies. Some ichnotaxa such as Chondrites and Zoophycos are recorded in the Zoophycos ichnofacies. The changes or breaks in colonization or ichnofacies development are revealed at key stratal surfaces, where they are extremely important in order to detect environmental shifts, which would be underestimated or missed in condensed deposits. The substrate-controlled ichnofacies are linked with key stratal surfaces such non-depositional discontinuity surfaces: the Glossifungites ichnofacies is related with omission surfaces and the Trypanites ichnofacies related with hardgrounds. Specific ecostratigraphic events are detected within the ecostratigraphic sequences. Productivity (a notable increase of the abundance and diversity of fossil populations) and colonization events occurred at the lower (I) and intermedium (II) ecostratigraphic sequences respectively; some other colonization events (by pioneer organisms) took place above some barren or poorly fossiliferous beds in the upper ecostratigraphic sequence (III). The first colonization event was developed within the Madero-Moncayo area (M-M); whereas the second one occurred westwards, in the San Leonardo area (S-L). Acknowledgements: This work was funded by projects CGL2008-03112, CGL2005-04574 and Grupo 910431 UCM-BSCH. Key words: Palaeoecology; Condensed deposits; Early-Middle Jurassic; Iberian Basin; Spain References: García-Frank A. Evolución Biosedimentaria y Secuen- cial del Jurásico Medio Inferior en la Cuenca Ibérica (Sector NO). (Recurso electrónico) Servicio de Publicaciones, Universidad Complu- tense de Madrid, Madrid, 2007: 529 (in Spanish). García-Frank A., Ureta S., Mas R. Tectonically active Aalenian in the northwestern Iberian Basin (Spain). Volumina Jurassica, 2006, IV: 42. García-Frank A., Ureta S., Mas R. Aalenian pulses of tectonic activity in the Iberian Basin, Spain. Sedimentary Geology, 2008, 209(1-4): 15-35. 287
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