Provenance signatures recorded in transgressive sandstones of the Upper Cretaceous Iberian seaway

Abstract

Maximum Mesozoic transgressions flooded many continental areas worldwide, forming a wide and shallow seaway on the Iberian Microplate during the Late Cretaceous. Several siliciclastic and mixed siliciclastic–carbonate units developed along the Iberian Massif (cratonic area) while, at the same time, epeiric carbonate platforms spanned Iberia. These siliciclastic deposits unconformably cover the Variscan basement, as well as the Permian and Triassic deposits. This study focuses on sandstone provenance in central Spain, in a transgressive, relatively quiet tectonic setting with a wide rock variety in the potential source areas. Results shown in this paper can be applied in analogue systems; transgressive sandstones often form important hydrocarbon reservoirs due to their high primary porosity. The Cretaceous siliciclastic deposits are included in four formations ranging in age from the Cenomanian to the early Coniacian. The deposits were initially of fluvial origin, followed by a coastal-tidal setting. Quantitative petrographic analysis of sandstones from these formations reveals compositional variations related to provenance and weathering degree. The oldest formation (Utrillas Fm.) was deposited on a paleorelief surface. Sandstone composition changes from arkosic sandstones in the depressed area (Petrofacies P1a) to younger quartzarenites when sediments were deposited on top of the paleotopographic high (Petrofacies P1b). Quartz grains in both sub-petrofacies are angular to subangular, and the framework is poorly sorted. Intense weathering conditions are deduced for Petrofacies P1b, manifested by grain infiltration, clay coatings, and the absence of plagioclase grains and alteration features on K-feldspars. All these observations have been related to wetter conditions during P1b deposition. Main source areas of the Utrillas Fm were Ordovician metasediments and orthogneisses, Permian volcanic rocks, and Triassic Buntsandstein facies. Petrofacies 2 encompasses the three coastal-tidal formations, constituted mainly by subarkoses generally showing a subangular and poorly sorted framework. The deduced source areas are mainly associated with Variscan granitoids, pre-Ordovician schists and paragneisses, and Ordovician metasediments. Glauconite and carbonate grains support intrabasinal inputs. The low degree of sorting of this petrofacies related to a low-energy sedimentary environment produced lower values of original porosity compared to transgressive sandstone hydrocarbon reservoirs. Also, intrabasinal inputs promoted carbonate cementation and contribute to almost occlude the porosity.