El Cretácico del Sistema Central representa las sucesiones sedimentarias de margen costero de las plataformas carbonatadas, que desarrolladas a partir de la cuenca Ibérica (cordillera Ibérica después de la orogénesis alpina), se extendieron sobre el margen oriental del macizo Ibérico a favor del gran ascenso eustático y de la subsidencia térmica regional. Tras la estructuración alpina del macizo Ibérico, la mayor parte de estos apilamientos sedimentarios han quedado ocultos bajo el relleno cenozoico de las cuencas del Duero y del Tajo; otra buena parte de ellos han desaparecido por la erosión subsecuente al levantamiento de la cordillera Cantábrica, el Sistema Central y los Montes de Toledo. Así, hoy en día afloran esporádicamente en el borde meridional de la Cantábrica occidental, borde septentrional de los Montes de Toledo y en depresiones internas del Sistema Central, y de manera más profusa, en los márgenes N y S del Sistema Central a lo largo de sendas bandas casi continuas. Estos afloramientos constituyen una de las escasas oportunidades que existen para conocer en detalle cómo son estas sucesiones de margen costero, reconstruyendo su arquitectura estratigráfica y su patrón de apilamiento deposicional. Ello ha permitido: (i) reconocer las relaciones laterales entre las facies carbonatadas de plataforma de áreas más internas de la cuenca y las facies siliciclásticas de margen costero, dentro del marco conceptual que impone la estratigrafía secuencial; (ii) demostrar y determinar el diacronismo de las unidades litoestratigráficas formales, cuyos límites superan, en muchos casos, los de las secuencias deposicionales; (iii) relacionar secuencialmente materiales afines de otros márgenes de la cuenca del Duero; (iv) identificar eventos de discontinuidad mayor a partir del reconocimiento de discordancias erosivas costeras, que puedan correlacionarse con sucesiones de áreas más centrales de la cuenca, y con ello, reconocer las paraconformidades presentes en el registro sedimentario de aquellas áreas; y (v) identificar cuatro unidades genéticas o mesosecuencias, asimilables a ciclos de 2º orden por su amplitud temporal y origen tecto-eustático, que permiten reconocer el marco evolutivo de este margen costero durante el Cretácico Superior.Cretaceous rocks at the Sistema Central (central Spain) represent the coastal margin sediments of the carbonate platforms developed in the Iberian basin during the large Upper Cretaceous eustatic sea-level rise. They consist of a lower succession of mainly terrigenous sediments (sands, sandstones, clays, marls and minor dolostones), an intermediate carbonate succession (dolostones, limestones and marls) and an upper terrigenous and evaporitic succession (sands, clays and gypsums), which have been traditionally considered uppermost Cretaceous - Paleocene in age but, as suggested by recent data, they should be considered older (Campanian-Maastrichtian).
Most of Cretaceous sediments at the western coastal margin of the Iberian basin were eroded after the tectonic inversion of the Iberian massif (Iberian microplate) along the Alpine orogeny, or are presently covered by cainozoic deposits of the cainozoic Duero and Tajo basins. Exceptionally, these sediments crop out on the southwest edge of the cordillera Cantábrica, on northern margin of Montes de Toledo and more widely, on both sides of the Sistema Central.
Then, the Cretaceous outcrops of the Sistema Central are one of the scarce areas we have to know in detail these sediments, reconstructing both their depositional stacking pattern, composed of different-order superimposed depositional sequences (Figure 5) and their wedge-shaped stratigraphic architecture as well; which, in general, shows that the subsequent sequences were progressively more extensive, onlapping onto the coastal margin, until the Santonian-Campanian, and from then they were continuously recessive prograding towards the Cantabrian margin (NW) until the end of the Cretaceous.
More specifically, the stacking pattern and the stratigraphic architecture allow to: (i) recognize the lateral relationships between platform carbonate facies deposited in central areas of the basin and siliciclastic facies at the coastal margin within the conceptual framework of sequence stratigraphy; (ii) demonstrate the diachronism of formal lithostratigraphic units, whose boundaries exceed sequence boundaries in many cases; (iii) correlate with these sequences, units from other margins of the Duero basin; (iv) identify major regional discontinuities from recognition of coastal erosive unconformities, which can be traced to their correlative sediments in more internal areas of the basin, recognizing thus paraconformable strata in the record of these areas; and (v) identify four mesosequences, tecto-eustatic origin, which subsequently allow to reconstruct the evolutionary framework of this coastal margin along the Upper Cretaceous.
Mesosequence I (Cenomanian Middle Turonian) is bounded by two major eustatic discontinuities, being represented on both sides of the Sistema Central by siliciclastic facies to the SW, mixed facies in the central part and carbonate facies at the NE. It is composed of three 3rd-order sequences with an overall transgressive-regressive trend (Figure 6), which is clearly recognizable by geometrical relationships of the internal sequences and by facies belt displacements.
Mesosequences II and III (Upper Turonian – Lower Santonian and Middle Santonian – Lower Campanian, respectively), consist of two and three 3rd-order sequences respectively. Both constitute the vast carbonate masses of the entire Cretaceous (intermediate carbonate succession) in this area. Southwestwards the thinning of this huge carbonate succession and the lateral transition to terrigenous facies of the coastal margin can be observed; some of these terrigenous deposits could be recognized on the west margin of the Duero basin (provinces of Zamora and Salamanca). The boundary between both mesosequences is a major tectonic discontinuity at microplate scale, and it might represent the onset of the first tectonic events of the alpine cycle in the studied area.
Mesosequence IV (Middle Campanian-Maastrichtian) is represented by the upper terrigenous-evaporitic succession. Its lower boundary is a major, eustatic discontinuity; although, it has been locally mapped as a cartographic unconformity (Valdeprados, Segovia), representing a major break in the general sedimentary trend, with the end of open marine sedimentation characteristic of the underlying mesosequences. In fact, until the recent finds of cretaceous vertebrate faunas at the north of the Sistema Central, the sediments of this mesosequence had been assigned mostly to the Paleocene, being correlated with other tertiary deposits of the Duero and Tajo basins. The depositional stacking pattern of their internal 3rd-order sequences has not been determined yet.Depto. de Geodinámica, Estratigrafía y PaleontologíaFac. de Ciencias GeológicasTRUEEspaña. Ministerio de Ciencia e Innovación. Dirección General de Investigaciónpu
