SEQUENCE STRATIGRAPHY AND STABLE ISOTOPES (DELTA-C-13,DELTA-O-18) OF THE LATE CRETACEOUS CARBONATE RAMP OF THE WESTERN MARGIN OF THE IBERIAN CHAIN (SORIA, SPAIN)

Within the Upper Cretaceous sediments of the western margin of the nor thern Iberian Chain, several formations of Turonian to Campanian age f orm a carbonate platform succession (165-270 m thick) developed during a second-order cycle of relative sea-level rise and fall. This succes sion consists of two depositional sequences, deposited during third-or der cycles, formed by sediments deposited in a broad, epeiric, ramp-ty pe platform open to the north. The lower depositional sequence (Moral de Hornuez Formation) began during the Cenomanian-Turonian transgressi on and represents the transgressive systems tract and highstand system s tract; it was terminated by a late Turonian regression resulting in a type 2 sequence boundary. Two types of parasequences (deepening-upwa rd and shallowing-upward, respectively), 1 to 3 m thick, deposited wit hin fourth- and higher-order cycles, have been identified, with featur es corresponding to lagoonal deposits. The upper depositional sequence (comprising the Hortezuelos, Caballar, Hontoria del Pinar, Burgo de O sma and Santo Domingo de Silos Formations) began during the early Coni acian transgression and was terminated by a Campanian regression. This upper depositional sequence also represents the transgressive systems tract and highstand systems tract. Eight types of shallowing-upward p arasequence, 1 to 4 m thick (locally up to 10 m), deposited in subtida l high-energy (shoals and channels) and low-energy settings (lagoon, l ocally with rudist patch-reefs) and in tidal-flat environments have be en identified. Carbon-isotope data, showing sharp and correlatable pos itive anomalies in different sections, strongly suggest that Oceanic A noxic Events are recorded in these shallow platform deposits. The Ceno manian-Turonian Oceanic Anoxic Event results in a positive delta(13)C excursion of up to 4 parts per thousand that coincides with the presen ce of small-size, low-diversity benthic foraminifers, agglutinated and planktonic foraminifers and calcispheres, despite the paucity or comp lete absence of black shales. The Santonian-Campanian Oceanic Anoxic E vent is recorded but is not so evident. The relative delta(13)C minimu m between these two maxima apparently coincides with the Turonian-Coni acian boundary. It is concluded that short-term variations in delta(13 )C can be useful for correlation in shallow platform areas, where the fossil record has little biostratigraphical precision. Accordingly, th e ages previously assigned to the sediments studied have been partiall y modified. The oxygen-isotope results are more negative than expected for carbonates precipitated in equilibrium with normal marine waters, thus indicating diagenetic alteration by meteoric waters.