Journal of Conference Abstracts

Origin of Early to Middle Jurassic Hardgrounds from Condensed Carbonate Sequences in se Spain

Tomasz Ciborowski (tciborow@twarda.pan.pl)¹, Michal Gruszczynski (beerbear@twarda.pan.pl)¹, Max Coleman (m.l.coleman@reading.ac.uk)² & Michael Isaacs²

¹ ING PAN ul.Twarda 51/55 00-818 Warsaw, Poland

² PRIS, University of Reading, Reading RG6 6AB

Examined hardgrounds located in SE Spain, about 1 km east from the village La Romana, in the vicinity of Cerro the la Cruz (Sierra de Reclot), 35 km west from Alicante. The whole region is situated within eastern part of the External Subbetic which is a part of the External Zones of the Betic Cordillera. The Jurassic sedimentary sequence includes the following formation: Gavilan Fm., Zegri Fm. and Upper Ammonitico Rosso Fm. Sediments are mainly limestones with admixture of marl, occasionally forming nodular limestones. The investigated hardgrounds occur in the top of Gavilan Fm. dated as late Pliensbachian and in the top of Zegri Fm. extended diachronously from Middle Toarcian up to Aalenian. Combined isotopic and elemental analyses have been used in studies of the both hardgrounds. The oxygen and carbon isotope results show very consistent values in the range from -1 to -2 per mil, and around +2 per mil respectively throught the whole sequence except the hardground surfaces. These surfaces are characterised by depleted isotope values. Interestingly, the carbon isotope values are more depleted than oxygen ones. This all suggestes that the reason of calcium carbonate non deposition could not be explained by the influx of fresh waters. Rather volcanic and hydrothermal activity, which might provide the seawater carbonate system with excess of carbon dioxide, could stop carbonate precipitation and created the hardground. The intensitivity of the hydrothermal and volcanic processes could not be very high because the isotopic pertubations is observed only at the very hardground surfaces. The possibility of submarine volcanic activity has been suggested elsewhere (Jimenes-Espinosa et al., 1997). Additionally, the detailed isotopic examination of originally preserved belemnite rostra show very consistent values throught. Calculating the temperature from the oxygen isotope values, the average of 17 degrees Celcius for the mean seawater has been obtained.

Jimenez-Espinosa R, Jimenez-Millan J & Nieto L, Sedim. Geol., 114, 97-107

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