Laura González-Acebrón

Sandstone petrofacies in the northwestern sector of the Iberian Basin

During the most active rifting stages in the northwestern sector of the Iberian Basin (Cameros Basin and Aragonese Branch of the Iberian Range), thick sequences of continental clastic deposits were generated. Sandstone records from Rift cycle 1 (Permo-Triassic) and Rift cycle 2 (Late Jurassic-Early Cretaceous) show similarities in composition. Based on the most recent data, this paper describes sandstone petrofacies developed during both rifting periods. Six petrofacies can be distinguished: two associated with Rift cycle 1 (PT-1 and PT-2) and four with Rift cycle 2 (JC-1 to JC-4). All six petrofacies can be classifi ed as sedimentoclastic or plutoniclastic. Sedimentoclastic petrofacies developed during early rifting stages either through the recycling of pre-rift sediments or signifi cant palaeogeographical changes. These facies comprise a thin succession (<100 m) of clastic deposits with mature quartzose and quartzolithic sandstones containing sedimentary and metasedimentary rock fragments. Carbonate diagenesis is more common than clay mineral diagenesis. Sedimentoclastic petrofacies have been identifi ed in Rift cycle 1 (Saxonian facies, PT-1) and Rift cycle 2 (JC-1 and JC-3; Tithonian and Valanginian, respectively). In the absence of the pre-rift sedimentary cover, metasedimentoclastic petrofacies sometimes develop as a product of the erosion of the low- to medium-grade metamorphic substratum (Petrofacies JC-2, Tithonian-Berriasian). Plutoniclastic petrofacies were generated during periods of high tectonic activity and accompanied by substantial denudation and the erosion of plutonites. Forming thick stratigraphic successions (1000 to 4000 m), these feldspar-rich petrofacies show a rigid framework and clay mineral diagenesis. In Rift cycle 1, plutoniclastic petrofacies (PT-2) are associated with the Buntsandstein. This type of petrofacies also developed in Rift cycle 2 in the Cameros Basin (JC-4) from DS-5 to DS-8 (Hauterivian-Early Albian), and represents the main basin fi ll interval. Sedimentoclastic and plutoniclastic petrofacies can be grouped into three pairs of basic petrofacies. Each pair represents a ‘provenance cycle’ that records a complete clastic cycle within a rifting period. Petrofacies PT-1 and PT-2 represent the ‘provenance cycle’ during Rift-1. In the Cameros Basin, two provenance cycles may be discerned during Rift cycle 2, related both to the Tithonian-Berriasian and the Valanginian-Early Albian megasequences. Tectonics is the main factor controlling petrofacies. Other factors (e.g., maturation during transport, local supply) may modulate the compositional signatures of the petrofacies yet their main character persists and even outlines he hierarchy of the main bounding surfaces between depositional sequences in the intracontinental Iberian Rift Basin.

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.

Sand provenance and implications for paleodrainage in a rifted basin: the Tera Group (N. Spain)

Fluvial-fan and fluvial siliciclastic strata, developed during the rifting that generated the Cameros Basin (North Spain), record important provenance changes that reveal source areas compositions and locations, paleodrainage evolution and rift patterns. The Tera Group represents the first rifting stage in the Cameros Basin, containing fluvial-fan sediments at the lower part of the sedimentary fill that evolve to fluvial and lacustrine systems in the upper part of the record. Our quantitative sandstone petrographic analysis evidences the presence of three main petrofacies related closely to the rift basin evolution. At the base of the sedimentary succession, Petrofacies 1 (quartzolithic) indicates that the fluvial-fans source areas included Juras¬sic marine carbonates and older siliciclastic Mesozoic units, as well as metamorphic supplies from the West Asturian Leonese Zone (WALZ). Variscan basement sources of this metamorphic area (WALZ) were more abundant in the upper fluvial record (Petrofacies 2, quartzofeldspathic). Further, the influence of plutonic source areas with a mixed potassic and calcium-sodium composition is also recorded, probably related to the Central Iberian Zone (CIZ). In addition, a local sedimentary input was active during the fluvial and lacustrine stages (Petrofacies 2 and 3, both quartzofeldspathic), as a function of the palaeogeographical position of the Jurassic marine rocks and the level of erosion reached. Plutonic rock fragments have not been observed in the Tera Group sandstones of the western part of the basin. Thus, deeper erosion of the basement in the eastern Cameros Basin is suggested. The provenance evolution from quartzolithic to quartzofeldspathic petrofacies registered in Tera Group siliciclastic deposits is due to the higher influence of transversal supplies during the fluvial-fan stage (quartzolithic) to more important axial inputs during the fluvial stage (quartzofeldspathic). This provenance change represents the evolution from an undissected rift shoulder stage to more advanced stages of rifting (dissected rift shoulder) and during the beginning of a provenance cycle in a rifted basin.

The influence of the provenance of arenite on its diagenesis in the Cameros Rift Basin (Spain)

Abstract The intraplate Cameros Rift Basin in northern Spain, which has sediments some 6500 m thick, developed between the Late Jurassic and Early Albian. Its facies and their distribution in the sedimentary record suggest the basin may contain hydrocarbon systems. The arenite composition of the basin reveals two main petrofacies: (1) a quartzolithic petrofacies, the provenance of which is related to recycling processes that took place in the pre-rift sedimentary cover; and (2) a quartzofeldspathic petrofacies mainly related to the erosion of a plutonic and metamorphic source of arenite. The succession of these petrofacies reflects two main cycles representing the progressive erosion of their sources, one of 10 Ma, the other of 30 Ma. Such succession is typical of a non-volcanic rift basin. The quartzolithic petrofacies shows early carbonate cements that inhibited compaction and later quartz, feldspar and clay mineral diagenetic phases. The quartzofeldspathic petrofacies has a rigid framework that maintained the original pores of the arenite during burial diagenesis. Quartz and K-feldspar overgrowths are common, with secondary porosity occurring as a product of feldspar dissolution. The quartzofeldspathic petrofacies has a greater potential to act as a hydrocarbon reservoir. This study corroborates the close relationship between the provenance of arenite and its reservoir potential in continental rift basins.

Easily altered minerals and reequilibrated fluid inclusions provide extensive records of fluid and thermal history: gypsum pseudomorphs of the Tera Group, Tithonian-Berriasian, Cameros Basin

This study reports a complex fluid and thermal history using petrography, electron microprobe, isotopic analysis and fluid inclusions in replacement minerals within gypsum pseudomorphs in Tithonian-Berriasian lacustrine deposits in Northern Spain. Limestones and dolostones, formed in the alkaline lakes, contain lenticularly shaped gypsum pseudomorphs, considered to form in an evaporative lake. The gypsum was replaced by quartz and non-ferroan calcite (Ca-2), which partially replaces the quartz. Quartz contains solid inclusions of a preexisting non-ferroan calcite (Ca-1), anhydrite and celestine.High homogenization temperatures (Th) values and inconsistent thermometric behaviour within secondary fluid inclusion assemblages in quartz (147–351°C) and calcite (108–352°C) indicate high temperatures after precipitation and entrapment of lower temperature FIAs. Th are in the same range as other reequilibrated fluid inclusions from quartz veins in the same area that are related to Cretaceous hydrothermalism.Gypsum was replaced by anhydrite, likely during early burial. Later, anhydrite was partially replaced by Ca-1 associated with intermediate burial temperatures. Afterward, both anhydrite and Ca-1 were partially replaced by quartz and this by Ca-2. All were affected during higher temperature hydrothermalism and a CO2-H2O fluid. Progressive heating and hydrothermal pulses, involving a CO2-H2O fluid, produce the reequilibration of the FIAs, which was followed by uplift and cooling.

Coastal wetlands as markers of transgression in proximal extensional systems (Berriasian, W Cameros Basin, Spain)

The early stages of intraplate extensional systems commonly are recorded by deposition of continental sediments. In this context, given appropriate tectonics and eustasy, transgressions can be well recorded in the areas of the basins located close to the sea, but they may be difficult to recognize in the innermost landwards areas of the system. This situation occurs in the innermost Upper Jurassic-Early Cretaceous Cameros Basin, part of the Iberian Extensional System (N. Spain), where a Berriasian transgression is recorded. The Berriasian succession in this area consists of siliciclastic deposits (sandstone and mudstone) of the Salcedal Formation and of carbonate and mixed carbonate-fine siliciclastic deposits (limestone and marl) of the San Marcos Formation. The sedimentological analysis of this depositional succession indicates that a Berriasian carbonate coastal wetland system occupied that sector of the Cameros Basin during deposition of the San Marcos Formation. This carbonate coastal wetland system consisted of shallow and quiet water bodies including some with marine influence others with no to very little marine influence, and palustrine areas. A semiarid climate characterized by the seasonal alternation of short wet and long dry periods caused water bodies of the system to undergo episodic desiccation and subaerial exposure. Moreover, this complex mosaic of sub-environments was connected laterally with a distal zone of a distributive fluvial system that was rimmed by siliciclastic tidal flats during phases of greater marine influence. The paleogeographic arrangement of this coastal wetland depositional system indicates that the marine influence came from the Basque-Cantabrian Basin to the north. During the period of Berriasian maximum marine influence, accommodation linked to the eustatic rise added to accommodation generated by tectonic subsidence from the extensional reactivation of late Variscan strike-slip faults. All these factors favored marine incursion into the west Cameros Basin from the Basque-Cantabrian Basin to the north. The example of the Berriasian transgression recorded in the W Cameros Basin by establishment of coastal wetland systems matches the interpretations of previous studies in neighboring areas. In those areas, complex coastal systems record transgressions in the innermost parts of the intraplate extensional basins of the Iberian Plate. This observation suggests that this paleogeographic and sedimentological arrangement may be common in the innermost parts of intraplate extensional basins during transgressive episodes throughout the geological record.ResumenEl registro de las primeras etapas de sedimentación en sistemas extensionales intraplaca comúnmente está formado por depósitos continentales. En este contexto, y si se dan las condiciones tectónicas y eustáticas adecuadas, las transgresiones pueden quedar registradas en las áreas que se encuentran más cerca del mar, aunque son difíciles de reconocer en las áreas más internas de los sistemas extensionales. Esta situación tuvo lugar en el sector occidental de la Cuenca de Cameros (N de España) durante el Jurásico Superior - Cretácico Inferior, donde quedó registrada una transgresión de edad Berriasiense. El registro berriasiense en esta área está constituido por los depósitos siliciclásticos (areniscas y lutitas) de la Formación Salcedal y por los depósitos carbonáticos y mixtos siliciclásticos - carbonáticos (calizas y margas) de la Formación San Marcos. Específicamente, el análisis sedimentológico de la Fm. San Marcos indica que su sedimentación se produjo en un sistema de humedales costeros carbonáticos que ocupó el sector occidental de la Cuenca de Cameros durante el Berriasiense. Este sistema de humedales costeros estaba formado por cuerpos de agua dulce o con influencia marina, poco profundos, con desecación episódica, y rodeados de zonas palustres. Este sistema probablemente se desarrolló bajo un clima estacional y semiárido en el que alternaban periodos húmedos y cortos y periodos secos prolongados. Este complejo mosaico de subambientes costeros conectaba lateralmente con un sistema fluvial distributivo que, durante las fases de mayor influencia marina, estuvo bordeado por llanuras mareales siliciclásticas. La disposición paleogeográfica de este sistema de humedales costeros indica que la influencia marina procedía de la Cuenca Vasco-Cantábrica situada al norte. Durante la etapa de máxima influencia marina, la acomodación vinculada al ascenso eustático se sumó a la acomodación generada por la subsidencia tectónica ligada a la reactivación extensional de fallas variscas. Todos estos factores favorecieron las incursiones marinas desde la Cuenca Vasco-Cantábrica en la Cuenca de Cameros occidental. El ejemplo de la transgresión berriasiense registrada en la Cuenca Cameros occidental coincide con las interpretaciones previas realizadas en áreas adyacentes, también situadas en una zona interna de la Placa Ibérica, y donde también se desarrollaron sistemas costeros complejos afectados por episodios transgresivos.