Idoia Rosales

Transgressive–regressive cycles and Jurassic palaeogeography of northeast Iberia

Abstract A correlation and sequence stratigraphy study of Jurassic successions has been carried out in the main sedimentary basins of northeast Iberia, i.e., Asturias, Basque-Cantabrian, and Iberian basins, based on the identification of transgressive–regressive cycles. The development and palaeogeographic evolution of the epicontinental carbonate platforms of northeast Iberia were largely controlled by major tectonic activity at three main intervals, at the beginning of the Jurassic, in the Lower–Middle Jurassic transition, and during the uppermost Jurassic, respectively. During Early Jurassic times, northeast Iberia was the site of a single and large carbonate ramp opened to the north. This carbonate ramp suffered a progressive drowning, evolving from an inner to hemipelagic ramp systems, with the local development of suboxic environments in the deepest areas located to the north. During the Middle Jurassic, different open carbonate platforms were formed including the development of swells in intermediate areas. During the Upper Jurassic, the outer ramp areas were progressively moving to the east Iberian Basin, and the ammonite faunas showing a markedly Tethyan affinity thereafter. Three first order T–R cycles bounded by major discontinuities associated with significant time gaps are identified. They extend respectively from the latest Rhaetian to the Early Aalenian, from the Middle Aalenian to the Early Oxfordian and from the Early Oxfordian to the Late Berriasian. Major transgressive peaks occurred at the Middle Toarcian (Bifrons Zone), at the Late Bajocian (upper Niortense and Garantiana zones) and at the mid-Kimmeridgian (Divisum Zone). Each of the first order cycles includes four second-order T–R cycles. Cycles 1.1–1.4 are identified in the northern basins. Cycles 2.1–2.4 display some differences in age of transgressive peaks from one basin to another. Cycles 3.1–3.4 are mainly identified in the Iberian Basin. The correlation with other separated Boreal and Thethysian basins demonstrates that the number and age of T–R cycles varies from one basin to another and are mainly controlled by the local or regional tectonic development. The transgressive peaks may reflect episodes of eustatic rise during the Jurassic. However, their different age from one basin to another is explained by different subsidence evolution.

The uppermost Albian–lower Cenomanian Bielba Formation of the type-area (Cantabria, northern Spain): facies, biostratigraphy, and benthic Foraminifera

The lithofacies, microfacies, depositional sequences, and benthic Foraminifera of the uppermost Albian–lower Cenomanian Bielba Formation in its type-area in northern Cantabria, Spain, consisting of a lower siliciclastic member and an upper Cóbreces Member, have been investigated. The investigated section constitutes a hypostratotype for the Bielba Formation, and the lower siliciclastic member is here formally defined as the Somocuevas Member. The obtained data are framed in the major global paleoenvironmental and paleoceanographic changes that occurred during the Albian–Cenomanian transition. Eleven lithofacies types have been identified from facies analysis and vertical log characterization. These lithofacies accumulated in various depositional settings such as storm, wave, and tidal influenced delta front and shoreface, shallow-water carbonate ramp, and offshore marine environments. Within an overall transgressive trend, the succession of the Bielba Formation comprises two depositional transgressive–regressive sequences corresponding to the Somocuevas and Cóbreces Members, bounded by a major relative sea-level fall at the end of the Albian. The sea-level fall was followed by a rapid marine transgression and shifting from siliciclastic-dominated deposition of the Somocuevas Member to carbonate-dominated deposition of the Cóbreces Member. The microfauna of the Cóbreces Member is characterized by omnipresent orbitolinids, trocholinids, and a variety of other mostly small-sized foraminifera that so far have not attracted much attention. Among them, taxa with biostratigraphic importance and taxonomic–phylogenetic importance can yet be found. The benthic Foraminifera are associated with calcareous green algae (dasycladaleans, udoteaceans) and occur in calcarenites of poorly diversified microfacies containing variable amounts of siliciclastics. The microfauna closely compares to that of the overlying late early to middle Cenomanian Altamira Formation.

An interesting orbitolinid assemblage from the early Albian of Cantabria (N Spain)

PurposeAn outcrop of early Cretaceous shallow water carbonates, partly rich in corals, is exposed in the Sorrozuela site, along the coast line of the Ajo Cape peninsula, Cantabria. The purpose of this work is to study the orbitolinid assemblage of these platform carbonates, which have been attributed to the Ramales Formation.MethodsFor that, rock samples were collected and thin-section were analyzed from the Albian limestones of the Ajo Cape area (Sorrozuela site).ResultsThin-section analysis yielded an interesting assemblage of orbitolinids including (in decreasing abundances Simplorbitolina manasi Ciry and Rat, Orbitolinopsis ajoensis n. sp., Dictyoconus? tunesianus (Peybernès), Simplorbitolina conulus Schroeder, and Palaeodictyoconus sp., indicating a late early Albian age. D.? tunesianus is recorded for the first time from Cantabria, representing also the youngest occurrence known so far. Orbitolinopsis ajoensis n. sp. is characterized by its medium-conical test, a subdivided marginal zone, and rather small, close-set cupules in the central zone. The new species represents one of the youngest lower Cretaceous records of the genus.ConclusionsThe new data provide further details on the biostratigraphic use of orbitolinid foraminifera in the Cantabrian Urgonian (Aptian-Albian) system.ResumenObjetivoSe ha estudiado un afloramiento de carbonatos de plataforma somera, parcialmente ricos en corales, que afloran en la localidad de Sorrozuela a lo largo de la costa del Cabo de Ajo, Cantabria. El objetivo de este estudio es analizar la asociación de orbitolínidos de estas calizas de plataforma atribuidas a la Formación Ramales.MétodosPara ello, se han recogido muestras y se han estudiado láminas delgadas de las calizas albienses del área de Cabo de Ajo (localidad de Sorrozuela).ResultadosEl análisis de láminas delgadas ha arrojado una asociación interesante de orbitolínidos que incluye (en orden decreciente de abundancia) Simplorbitolina manasi Ciry and Rat, Orbitoliniopsis ajoensis n. sp., Dictyoconus? tunesianus (Peybernès), Simplorbitolina conulus Schroeder y Palaeodictyoconus sp., indicando una edad de parte alta del Albiense inferior. Se registra D.? tunesianus por primera vez en Cantabria, representando además la aparición más moderna conocida hasta ahora. Orbitolinopsis ajoensis n. sp. está caracterizada por su caparazón cónico mediano, una zona marginal subdividida y cúpulas muy juntas y bastante pequeñas en las zona central. Esta nueva especie representa uno de los registros más modernos del género para el Cretácico inferior.ConclusionesEstos nuevos datos aportan detalles adicionales en el uso bioestratigráfico de los orbitolínidos para el estudio del sistema Urgoniano (Aptiense-Albiense) cantábrico.

Palaeoenvironmental Changes and C-Isotope Stratigraphy in the Alarcón Formation Stratotype (Upper Cenomanian-Lower Coniacian), Iberian Ranges, Spain

This study analyses the sedimentary facies, palaeosols, and δ13C stratigraphy of the Alarcon Formation in its stratotype in the Iberian Ranges (Spain). The aim is to characterize the palaeoenvironmental changes that occurred in the shallow-marine carbonate environments of central Iberia during the late Cenomanian to early Coniacian. These changes, recorded in seven transgressive–regressive cycles, are interpreted in the framework of regional sea-level variations, local subsidence, and climate. The possible influence of supraregional changes, such as those related to Oceanic Anoxic Event 2 (OAE2), on the sedimentary succession of the Alarcon Formation is also analysed and discussed.

Review of the late Aptian ammonoids of the Reocín area (Basque-Cantabrian Basin, Cantabria, Spain), stratigraphic implications and correlation to the third-order Ap 4 sequence

The present work intends to review the ammonoids collected a century ago by Louis Mengaud in the Reocín area, currently housed in the Paul Sabatier University (Toulouse, France). The ammonoids are studied from a taxonomic point of view, by means of thorough morphological description and comparison with other species defined in the literature. Three different species have been recognized: Ammonitoceras aff. ucetiae Dumas, Epicheloniceras gracile Casey and Epicheloniceras sp., allowing to date the assemblage as late Aptian. The biostratigraphic analysis of the studied ammonoids permits us to attribute the upper part of the Rodezas Formation of the Basque-Cantabrian Basin to the lowermost upper Aptian, Martini Zone, Gracile Subzone. This age-calibration makes it possible to compare the age of a transgressive/regressive third-order local sequence of the Basque-Cantabrian Basin to the global sequence Ap 4.ResumenEl presente trabajo pretende revisar los ammonoideos recolectados hace ya un siglo por Louis Mengaud en la zona de Reocín, que actualmente se encuentran depositados en la Universidad Paul Sabatier (Toulouse, Francia). Los ammonoideos se han estudiado desde un punto de vista taxonómico, por medio de la detallada descripción morfológica y comparación con otras especies definidas en la literatura. Se han reconocido tres especies diferentes: Ammonitoceras aff. ucetiae Dumas, Epicheloniceras gracile Casey y Epicheloniceras sp., permitiendo datar la asociación como Aptiense superior. El análisis bioestratigráfico de los ammonoideos estudiados permite asignar la parte superior de la Formación Rodezas de la Cuenca Vasco-Cantábrica a la parte baja del Aptiense superior, Zona Martini, Subzona Gracile. Esta calibración de la edad permite comparar la edad de una secuencia local transgresiva/regresiva de tercer orden de la Cuenca Vasco-Cantábrica con la secuencia global Ap 4.