Ángela Alonso

Inversion of an extensional-ramp basin by a newly formed thrust: the Cameros basin (N. Spain)

Abstract The Cameros basin (Iberian Chain) was developed under a very subsident extensional regime during the latest Jurassic-Early Cretaceous. Its sedimentary fill constitutes a megasequence of more than 5000 m of vertical thickness which contains six depositional sequences. Most of the sediments are continental (alluvial and lacustrine) with only minor marine intercalations. A lateral accretionary geometry at the basin scale shows a SSW-NNE migration of the depocentre and an onlap to the north of the sedimentary sequences on the Mesozoic substratum of the northern boundary of the basin. The overall Mesozoic structure of the basin, as seen from field studies and seismic profile interpretation, is a gentle WNW-ESE synclinorium 30–70 km wide and 150 km long. Extension in the pre-basin Mesozoic rocks is very small. The basin boundaries are defined by an unconformity of the basin filling rocks on the former Mesozoic substratum; it is bounded only locally by major faults at surface. The basin is interpreted as an extensional-ramp basin produced over an S-dipping ramp in a blind, flat-lying extensional fault some kilometres deep in the basement. The extensional displacement on the fault produced a synclinal basin which progressively widened with time. The depocentres of the successive depositional sequences were always located above the ramp and migrated to the north, inside the basin, as a result of the hangingwall displacement to the south. From a computer-modelled cross-section, we estimate the total displacement on the extensional fault to be about 33 km. This extension, in the hangingwall, would have taken place in the Pyrenean basin, north of the Cameros basin. Tertiary compression (Palaeogene to Lower-Middle Miocene) inverted the basin by means of a newly formed E-W north-directed thrust. Overthrusting in the Tertiary Ebro basin fill, this new fault formed in the Keuper beds, in a weakness zone with uniform dip within the extensional hangingwall (about 30 km in cross-section). During the deformation it expanded to the north and south until it branched to the Iberian sole thrust, which might coincide with the Mesozoic extensional sole fault. Although this thrust produced a slight (2–3 km) inversion of the basement-Mesozoic cover unconformity, its maximum displacement was of about 28 km. Pre-basin Mesozoic rocks overlie the thrust surface and are overlain by the basin-filling rocks. A south-directed imbricate-fan thrust system developed in the southern margin. The total shortening was about 38 km, leading to the complete inversion of the basin.

Intraplate deformation in the NW Iberian Chain: Mesozoic extension and Tertiary contractional inversion

The Iberian Chain developed within the Iberian plate during the Palaeogene and Early Miocene as a result of convergence between the African and Eurasian plates. It is a fold-and-thrust belt, which involves the Hercynian basement and the Mesozoic and Cenozoic cover. A generalized cross-section of the NW part of the Chain, of 195 km length, is presented. Mesozoic basins, developed on the Hercynian basement, display thickness variations across normal faults that bounded them and that can be recognized in the field. The Tertiary contraction deformed and inverted the Mesozoic basins, and it is inferred to have produced a thrust sheet about 5 km thick in its frontal (northeastern) part, which thickens to the SW. The total Tertiary shortening in the section is 66.6 km (26%). The structure and the crustal shortening and thickening of the Iberian Chain are explained by a major upper-crustal thrust system with simple flat-and-ramp geometry, which may branch to the Pyrenees or the Betics. This is combined with internal deformation of the thrust sheet. The contribution of the Iberian Chain shortening to the convergence of the African and Eurasian plates during the Tertiary is about one-half of that of the Pyrenees, and should be taken into account in any reconstruction of the kinematics of these plates.

Sandstone Petrography of Continental Depositional Sequences of an Intraplate Rift Basin: Western Cameros Basin (North Spain)

The Cameros Basin in Central Spain is an intraplate rift nbasin that developed from Late Jurassic to Middle Albian time along nNW–SE trending troughs. The sedimentary basin fill was deposited npredominantly in continental environments and comprises several depositional nsequences. These sequences consist of fluvial sandstones that ncommonly pass upward into lacustrine deposits at the top, producing nconsiderable repetition of facies. This study focused on the western nsector of the basin, where a total of seven depositional sequences (DS- n1 to DS-7) have been identified. nThe composition of sandstones permits the characterization of each nsequence in terms of both clastic constituents and provenance. In addition, nfour main petrofacies are identified. Petrofacies A is quartzosedimentolithic n(mean of Qm85F2Lt13) and records erosion of marine nJurassic pre-rift cover during deposition of fluvial deposits of DS-1 n(Brezales Formation). Petrofacies B is quartzofeldspathic (mean of nQm81F14Lt5) with P/F > 1 at the base. This petrofacies was derived nfrom the erosion of low- to medium-grade metamorphic terranes of nthe West Asturian–Leonese Zone of the Hesperian Massif during deposition nof DS-2 (Jaramillo Formation) and DS-3 (Salcedal Formation). nQuartzose sandstones characterize the top of DS-3 (mean of nQm92F4Lt4). Petrofacies C is quartzarenitic (mean of Qm95F3Lt2) with nP/F > 1 and was produced by recycling of sedimentary cover (Triassic narkoses and carbonate rocks) in the SW part of the basin (DS-4, Pen˜ - nacoba Formation). Finally, depositional sequences 5, 6, and 7 (Pinilla nde los Moros–Hortiguela, Pantano, and Abejar–Castrillo de la Reina nformations, respectively) contain petrofacies D. This petrofacies is nquartzofeldspathic with P/F near zero and a very low concentration of nmetamorphic rock fragments (from Qm85F11Lt4 in Pantano Formation nto Qm73F26Lt1 in Castrillo de la Reina Formation). Petrofacies D was ngenerated by erosion of coarse crystalline plutonics located in the Central nIberian Zone of the Hesperian Massif. In addition to sandstone npetrography, these provenance interpretations are supported by clay nmineralogy of interbedded shales. Thus, shales related to petrofacies nA and C have a variegated composition (illite, kaolinite, and randomly ninterlayered illite–smectite mixed-layer clays); the presence of chlorite ncharacterizes interbedded shales from petrofacies B; and Illite and kaolinite nare the dominant clays associated with petrofacies D. nThese petrofacies are consistent with the depositional sequences and ntheir hierarchy. An early megacycle, consisting of petrofacies A and B n(DS-1 to DS-3) was deposited during the initial stage of rifting, when ntroughs developed in the West Asturian–Leonese Zone. A second stage nof rifting resulted in propagation of trough-bounding faults to the SW, ninvolving the Central Iberian Zone as a source terrane and producing na second megacycle consisting of petrofacies C and D (DS-4, DS-5, DS-6, and DS-7). Sandstone composition has proven to be a powerful tool nin basin analysis and related tectonic inferences on intraplate rift basins nbecause of the close correlation that exists between depositional nsequences and petrofacies.

Holocene transgression recorded by sand composition in the mesotidal Galician coastline (NW Spain)

This study confirms several inferences regarding Holocene coastal dynamics and climate through a petrographic modal analysis of 60 Holocene sand samples recovered in seven sites along the NW coast of the Iberian Peninsula. Fluvial sand can be discriminated from more mature intertidal and aeolian sand according to texture and composition. Fluvial sand contains soil products and coastal sand has significant bioclasts. Quartzofeldspathic sand appears in the western area (produced by the erosion of granite and granitoid), and quartzolithic sand occurs in the eastern area (produced by the erosion of metasediment). Changes in sand composition during Holocene deposition are manifested by an increase in modern carbonate clasts (MC) correlated with the Holocene transgression. Episodes of faster sea-level rise and subsequent erosion of surrounding cliffs are indicated by the preservation of high proportions of feldspar in intertidal sand. In contrast, fluvial sand is characterized by greater quartz enrichment. These inferences were confirmed by petrographic indices (carbonate clasts/total clasts, MC/T; total feldspars/monocrystalline quartz, F/Qm; and plagioclase/total feldspars, P/F). The different maturity of intertidal and aeolian sands is revealed by their variable quartz contents, despite similar proportions of plagioclase and K-feldspar. This suggests mechanical abrasion as the main factor controlling maturity. In contrast, fluvial sand shows depleted plagioclase contents as the result of inland weathering processes. Intertidal, beach and aeolian sands are essentially the products of the erosion of coastal cliffs and head deposits, with only the scarce contribution of fluvial drainages. The long-distance transport of Galician coastal sands is discarded based on the close relationship between their composition and that of local sand sources. Our findings indicate that short-distance transport of sediments from the west closed off coastal wetlands and occluded estuarine mouths during the Holocene transgression by deposition on sediment-trap zones along the irregularly shaped Galician coast.