Juan J. Gómez

Condensation processes in shallow platforms

Abstract Sequence stratigraphy allows elucidation of the time-space relationships of deposits in sedimentary basins. However, the relative distribution of unconformities and condensed sections can neither be used as diagnostic criteria for particular palaeogeographic settings, nor as indicators of maximum regional transgressions. Condensation processes also occur in the shallowest portions of platforms and during maximum regression episodes. Three separate processes of condensation are here distinguished: stratigraphic, sedimentary and taphonomic. These three categories of condensation can be used to analyze patterns of sediment distribution, and also to estimate lateral and vertical variations in sedimentation rates. Contrary to the usual interpretation, the maximum values of rate of sedimentation and degree of accommodation of sediments, as well as the highest relative sea level, occur in the lowest levels of the shallowing-upward sequences. The distinction between these condensation processes allows one to predict that the degree of sedimentary and stratigraphic condensation will be higher towards the distal portions of the platforms, whereas the stratigraphic condensation processes without sedimentary condensation will show the maximum intensity and frequency in the shallowest portions of the platforms.

A complementary section for the proposed Toarcian (Lower Jurassic) global stratotype: The Almonacid de la Cuba section (Spain)

We present the biostratigraphy (ammonites, brachiopods, foraminifers, and ostracodes), lithostratigraphy, sedimentology, sequence stratigraphy, magnetostratigraphy, and isotope stratigraphy of the Almonacid de la Cuba section located in the Iberian Range, central-eastern Spain. This section, which contains a continuous and expanded record of the Pliensbachian-Toarcian boundary (Early Jurassic), has been proposed as a complementary section for the Toarcian GSSP. An excellent ammonite-based biozonation has been obtained. Four ammonite assemblages characterized by the presence of Pleuroceras, Canavaria, Dactylioceras (Eodactylites), and Dactylioceras (Orthodactylites) have been distinguished. The base of the Toarcian is located at level CU35.2, based on the first occurrence of Dactylioceras. The occurrence of taxa from the NW European and the Mediterranean provinces is useful to improve the correlation between both provinces. Foraminiferal and ostracode assemblages are rich and diversified and no significant biostratigraphic events take place at the Pliensbachian-Toarcian boundary. The magnetostratigraphic data presented here are the most complete record of reversals of the earth magnetic field for the Pliensbachian-Toarcian boundary. A good record of the onset of the positive δ13C excursion reported in the Lower Toarcian of many European sections has been obtained. Average paleotemperatures measured at the latest Pliensbachian Spinatum Biochron of about 12.5°C, recorded a marked increase of the seawater temperature which started during the Toarcian, reaching average temperatures of 16.7°C at the Tenuicostatum Biochron. The obtained 87Sr/86Sr values fully agree with the LOWESS calibration curve.

THE MIDDLE JURASSIC EASTERN MARGIN OF THE IBERIAN PLATFORM SYSTEM (EASTERN SPAIN). PALAEOGEOGRAPHY AND BIODISPERSAL ROUTES OF AMMONOIDS

From a sedimentological and palaeogeographical point of view, in the eastern margin of the Iberian platform system, Middle Jurassic deposits are represented by a thick pile of carbonates with minor marls interbedded. Different facies, within the Tortosa, Aragonese and Castilian platforms, pertain to two categories: internal platform and external platform facies. A system of extensional faults was the major factor determining the differentiation of several shallow, carbonate, epicontinental platforms. The so-called Iberian Basin was a shallow intracratonic platform system, in which a relatively deep extensional basin (the Catalan Basin) started to be differentiated during the late Bajocian. In the eastern part of this faulted platform system, the occurrence of a listric fault, dipping towards the east, conditioned the development of a late Bajocian differentiated subsident area named the Tortosa Platform. From a palaeobiogeographical point of view, the ammonite fossil assemblages of the Iberian platforms are composed of Sub-Mediterranean taxa. At the eastern margin of the Iberian platform system, the appearance and distribution of shells of ammonoids were taphonomically and ecologically controlled by regional changes of relative sea level. A Sub-Mediterranean zonation can be recognized, although most of the ammonites shells correspond to adult individuals arrived by necroplanktic drift from more open-marine or oceanic areas. The following criteria are relevant in recognizing the maximum deepening episodes in the Iberian platform system: colonization (including ontogenic development) of open-marine taxa, immigration of oceanic taxa and development of endemic species. In terms of Sequence Stratigraphy, the development of expanded, external platform sections in the Tortosa Platform during the Garantiana Biochron (late Bajocian) is noteworthy. During this episode, the Iberian platform system reached the maximum bathymetric values and acted as a biogeographical dispersal area for some taxonomic groups of ammonoids between the Western Tethys and the Proto-Atlantic.

CONSERVATION MANAGEMENT OF THE AALENIAN STAGE GSSP IN FUENTELSAZ (CASTILLA-LA MANCHA, SPAIN)

The Global Boundary Stratotype Section and Point of the Toarcian-Aalenian Boundary (Lower-Middle Jurassic) in Fuentelsaz Section is the first GSSP defined in Spain. Among the requirements of the ICS for the choice of the best boundary level are accessibility to the type-section and free access for research, and guarantees from the respective authority concerning to the free access and the permanent protection of the site. In our point of view, the most suitable protection figure to safeguard the fulfilment of the ICS’s requirements according to the Spanish Conservation Policy and Legislation is the Natural Monument. This paper deals with the conservation management and classification of the Fuentelsaz area as a Natural Monument, being the Comunidad de Castilla-La Mancha Authorities the ones that have to take the necessary steps to protect the site.