Reinterpretación estratigráfica y sedimentológica de las Formaciones Escucha y Utrillas de la Cordillera Ibérica

Resumen

The development of a Cretaceous sandy desert system (erg) has been recognized from stratigraphic, sedimentological, paleoclimate, palaeogeographic and synsedimentary tectonics studies in the Escucha and Utrillas Formations. These studies show that, in their reference area and surounding areas in the Iberian Range (Teruel, Soria, and Zaragoza Provinces) the thick sandy intervals with mudstones and siltstones of the upper part of the Escucha Formation and the whole Utrillas Formation constitute the sedimentary record of the first Cretaceous erg reported from Europe. The desert system developed over the Regional Unconformity Surface (SDR) which is included in the sedimentary record of the Escucha Formation and marks the regional disappearance of the coalbearing deposits of the Comarca de las Cuencas Mineras (Teruel). This regional surface is associated to subaerial exposure, erosion and synsedimentary tectonics and has been recognized in outcrop and in the subsurface over more than 230 km. This regional unconformity surface (SDR) separates two sedimentary successions: a Lower Sedimentary Succession (SSI) largely formed by mudstones and siltstones with coal-bearing deposits, sandstones and carbonates and an Upper Sedimentary Succession (SSS) formed by sandstones and a minor proportion of muddy and grey siltstones. The Lower Sedimentary Succession (SSI) comprises part of the lower and middle parts of the Escucha Formation of Pardo (1979) and contains the coal-bearing deposits that have been mined in the Comarca de las Cuencas Mineras de Teruel since long. This sedimentary succession is located between the urgonian carbonate platfoms (Oliete, Villarroya de los Pinares Fms) and the Benassal Fm below, and the Regional Unconformity Surface (SDR) above. It is late Aptian-early Albian in age, based on nannofossils, planktonic foraminifera, dinoflagellates and pollen biostratigraphy. This sedimentary succession comprises 1) a Lower Interval with Carbonates, developed in a carbonate platform with broad lagoons; 2) a Middle Interval with Coal-bearing deposits, developed in a dominated siliciclastic coastal system with barrier islands and marshes; 3) an Upper Muddy Interval characterized by mustones with pedogenetic features developed in a low-energy coastal setting. The Upper Sedimentary Succession (SSS) includes sandstones, siltstones and mudstones of the upper part of the Escucha Formation and sandstones, siltstones and mudstones of the Utrillas Formation. This Upper Sedimentary Succession is located between the Regional Unconformity Surface (SDR) below, and the Upper Cretaceous Carbonate Platforms above. It is early Albian to early Cenomanian in age. This Upper Sedimentary Succession constitutes a sandy desert depositional system (erg) that developed over the regional unconformity surface, over an area of more than 16,000 km2 in the provinces of Soria, Zaragoza and Teruel. The desert system displays a characteristic spatial zonation as previously described for other ancient desert depositional systems (e.g. Porter, 1986): (a) back-erg; the proximal area of the desert system close to the – then high – Variscan Iberian Massif (Soria, Zaragoza and Teruel Provinces) with the interaction of ephemeral fluvial systems and aeolian processes; (b) central-erg; central part of the desert system (Teruel Province) which is one of the main areas of aeolian sediments accumulation and draa (complex dunes) development; (c) fore-erg; distal area of the desert system (Teruel Province) where the erg and the coeval marine environment interacted. During its evolution, the erg-margin was transgressed by the Tethys, and aeolian sediments were reworked by the sea in restricted marine environments. After regional transgression an arid coast formed where marshes and mangroves developed forming thin coally mudstone layers. The development of this desert system shows periods of greater and less aeolian activity, associated to variations in aeolian input that conditioned the deflation and preservation of aeolian sediments. Such variations of allogenic processes led to a variety of regional bounding surfaces that divide the aeolian sedimentary record in different stages.