Marcos Aurell

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.

Proximal–distal facies relationships and sedimentary processes in a storm dominated carbonate ramp (Kimmeridgian, northwest of the Iberian Ranges, Spain)

Abstract Facies analysis of the upper Kimmeridgian rocks in the outcrops located near Ricla (Zaragoza province, northeast Spain) and the integration of the resultant data in a broader context (the northern part of the Iberian Basin), has produced two general models showing the facies distribution and the processes that controlled the sedimentation in the Kimmeridgian carbonate ramp. Using these two models the transition from shallow to relatively deep environments of the carbonate ramp is examined in detail. Model 1 corresponds to the development of a mixed carbonate-siliciclastic ramp during a slow rise and stillstand of sea level (Sequence 1-HST), whereas Model 2 represents the growth of a pure carbonate ramp during a rapid rise of sea level (Sequence 2-TST). Carbonate production was higher in the shallow ramp domains (coral reefs and oolitic shoals in Model 1 and reefs in Model 2) than in deeper domains, where there is no indication of significant pelagic or benthic production. The activity of unidirectional return flows induced by winter storms and hurricanes, played an important role in the redistribution of the sediment across the ramp, generating different coarse-grained deposits. In the inner and mid-ramp settings dunes, lower scale bedforms and tempestites occur in Model 1, and storm lobes, bars and tempestites in Model 2. Moreover, a significant bulk of the carbonate mud produced in shallow areas would eventually be resedimented in the outer ramp as suspended load in the density currents. Stillstand of sea level in Model 1 involved a rapid progradation of the inner and proximal mid-ramp carbonate and siliciclastic facies. The rapid relative sea level rise of Model 2 is determined by the dominance of the carbonate facies and by the presence of aggradational geometries in the transitional area between shallow and deep-ramp domains. The presence of relatively thick sections in the outer-ramp settings (instead of condensed sections, as observed in Model 1) during times of sea level rise (Model 2) can mainly be explained by the increase of the shallow production in the reef dominated areas.

Kimmeridgian palaeogeography and basin evolution of northeastern Iberia

Abstract Two sedimentary basins were developed during the Kimmeridgian in the northeastern part of Iberia: the Basque-Cantabrian Basin, to the northwest, and the Iberian Basin, to the east. These basins were mainly filled by carbonates deposited in a shallow epicontinental sea that was connected to the open marine realms of both the North Atlantic and the Western Tethys respectively. During the Late Jurassic, they were temporarily linked by means of the Soria Seaway. The uppermost Oxfordian to lowermost Tithonian of northeastern Iberia consists of two depositional sequences. Sequence J3.5 spans latest Oxfordian to early late Kimmeridgian times; Sequence J3.6 spans the late Kimmeridgian and early Tithonian. During Sequence J3.5 coralgal reefs were developed in the sedimentary domains of the Basque-Cantabrian Basin and the Soria Seaway, whereas the marginal areas of the Iberian Basin were dominated by oolitic shoals with variable siliciclastic influence. Outer ramp areas, located in the central and eastern part of the Iberian Basin, were dominated by carbonate muds and marls. During the late Kimmeridgian-earliest Tithonian shallow reefal and oolitic facies prograded over the micritic outer ramp facies. The sedimentation areas were reduced, and Sequence J3.6 was only deposited in the Iberian Basin. Most of the western margin of the Iberian Basin and the southern areas of the Soria Seaway were covered by reefal facies during this sequence. Major transgressive events in the Iberian Basin took place in the latest Oxfordian-early Kimmeridgian, and in the middle late Kimmeridgian, whereas eustatic falls in sea level are recognised at the onset of the late Kimmeridgian and at the Kimmeridgian–Tithonian transition. These events have also been described in other West European basins. Synsedimentary extensional tectonics is also evident, and has been related to the onset of the Late Jurassic-Early Cretaceous rifting stage. Synsedimentary normal faulting resulted in the presence of several troughs and highs within the basin. The tectonic uplift of the western basin margin explains the progressive basinwards shift of the shorelines, as observed in the Iberian Basin towards the end of the Jurassic.

Sequence stratigraphy and bedding rhythms of an outer ramp limestone succession (Late Kimmeridgian, Northeast Spain)

Facies, stratal and spectral analyses of an outer ramp lime mudstone succession (Aguilon, north Iberian Ranges, Spain) are presented in this work. The studied succession is Late Kimmeridgian (eudoxus and beckeri zones) in age and comprises the transgressive and highstand deposits of a third-order depositional sequence. A number of higher-order sequences (bundles and sets of bundles) have been identified based on the comparative analysis of the bedding planes. The bundles and sets of bundles show a well-defined stratal pattern. Spectral analysis has provided further independent confirmation of the cyclical nature of the bundles and sets of bundles defined from field analysis. The bundles have variable thickness (from 1 to 2 m) and are formed by up to 10 micritic beds. They have been related to sea- level changes controlled by the orbital precession cycle, affecting the shallow productivity area. A significant amount of the lime mudstones accumulated in outer ramp settings were derived from resedimentation of the shallow carbonate production areas. Many of the bundles show a lower interval with a thinning and fining-up trend, indicating a progressive decrease of the carbonate production (and carbonate export) during periods of high-frequency sea-level rise. The late transgressive and highstand deposits show sets of bundles (groups of five bundles, from 5 to 8 m) probably related to sea-level changes controlled by the short eccentricity cycle. The overall thickness and the stacking pattern observed in the sets of bundles are controlled by the long-term sea-level variation. The sets of bundles located in the late transgressive deposits show thinner micritic beds in their lower or middle part. The sets of bundles found in the highstand deposits are thinner and show a thickening-up and thinning-up trend. On the studied carbonate ramp, during periods of long-term sea-level rise, the overall carbonate production (and carbonate export) is high, although the superposition of the high-frequency sea-level rises may result in episodic flooding and drowning of the shallow ramp areas. During periods of long-term, early highstand of sea level, the overall carbonate production (and carbonate export) is more reduced, but it has maximum peaks during the transgressive (and early highstand) intervals of the high-frequency sea-level cycles. Sedimentation during the long-term sea- level fall (late highstand) was scarce and discontinuous in the outer ramp area, and resulted in the overall thickness reduction of the sets of bundles. D 2003 Elsevier Science B.V. All rights reserved.

New information about the stratigraphic position and age of the sauropod Aragosaurus ischiaticus from the Early Cretaceous of the Iberian Peninsula

The sauropod Aragosaurus ischiaticus Sanz, Buscalioni, Casanovas & Santafe, 1987 was the first dinosaur to be described in Spain. The holotype was recovered from the site of Las Zabacheras (Galve, Teruel province). This site has traditionally been situated in the El Castellar Formation (in the lower part of the Wealden facies). Recently, it has been proposed that the remains of Aragosaurus stem from the Villar del Arzobispo Formation (late Tithonian–upper part of the early Berriasian), which would mean that the sauropod was almost 15 million years older than previously thought. Detailed field work has been carried out, making it possible to pinpoint the position of the low-angle unconformity between the Villar del Arzobispo Formation and the El Castellar Formation. This unconformity originated as a result of block tilting that occurred during the early stages of the Early Cretaceous rifting episode. The upper levels of the Jurassic sequence (i.e. the Villar del Arzobispo Formation) were exposed to erosion and karstification, leading to the formation of a discontinuous conglomeratic level. This level has been locally preserved at the bottom of the Wealden syn-rift sequence (i.e. the El Castellar Formation). The results of our detailed mapping demonstrate that the Aragosaurus holotype was found in the lower part of the El Castellar Formation. Moreover, our revision of the existing datings suggests that the El Castellar Formation as a whole is Valanginian?–early Barremian in age. Given that Aragosaurus was located in its lower part, it is probably Valanginian?–Hauterivian in age.

Las cerradicas tracksite (Berriasian, Galve, Spain): Growing evidence for quadrupedal ornithopods

Dinosaur footprints occur in shallow marine sedimentary units of the Berriasian Villar del Arzobispo Formation in the Aliaga basin, NE Spain. Las Cerradicas is a small outcrop (25 m2), near Galve, with four dinosaur trackways. Three tridactyl trackways indicate bipedal animals, and have parallel orientation. A fourth or‐nithopod trackway indicates a quadrupedal animal and is the smallest among any reported in the literature (L = 23 cm, W = 23 cm of pes tracks). It has manus prints that are oval‐shaped impressions. These and other recent discoveries shed much light on the abundance of quadrupedal ornithopods during the early Cretaceous.

Palaeogeographical significance of the Triassic-Jurassic unconformity in the north Iberian basin (Sierra del Moncayo, Spain)

Abstract Field work carried out on the Lower Jurassic (i.e., Hettangian-lower Pliensbachian) of the north Iberian basin (Sierra del Moncayo, Spain), including the mapping of unconformities and sedimentary units as well as the facies analysis, has allowed us to recognize two episodes in the sedimentary evolution of the basin. A lower one, Hettangian in age, includes sediments deposited during the period of extensional tectonic activity, which involved the formation of half-graben basins by normal listric faults. Sedimentary breccias and rudites, located mainly at the edges of the half-grabens are derived from the erosion of blocks uplifted during the Triassic. In the depocenter of the half-grabens which show most subsidence these breccias pass into thick dolomitic and evaporitic successions, deposited in sabkha environments, under negligible marine influence. The later episode, Sinemurian and lower Pliensbachian in age, consists of shallow marine carbonate units deposited in a low-angle ramp setting. Evaporite accumulation took place during phases of relative lowstand of sea level (i.e., lowstand systems tract), when the basin was cut ooff from open marine conditions; whereas the carbonate sediments mainly represent transgressive and highstand systems tracts. In the Iberian basin, the lowstand episode during the Hettangian was mainly induced by the breakup of the Upper Triassic ramp, which favoured the isolation from open marine conditions of several subsident sub-basins. In addition, it should be noted that the Hettangian is considered as a lowstand sea level episode in some of the proposed global eustatic cycle charts, so that a eustatic component may also be postulated for the isolation of the basin at this time. The infilling of the Hettangian half-grabens left an even topography which was flooded during the Sinemurian sea level rise, leaving shallow carbonate deposits. The results presented here give new information on the paleogeography of northeast Iberia at the Triassic-Jurassic boundary in the context of the opening of the western margin of the Tethys.

Análisis de facies de la Capa de Oolitos ferruginosos de Arroyofrío en la Sierra de Arcos (Jurásico, Cordillera Ibérica septentrional)

In this work we have studied the Arroyofrio Bed. This is a classic and largely studied ironstone unit in the Iberian Range. The age of this bed ranges from the Lower Callovian to Middle Oxfordian. This is a complex lithologic unit formed by irregular sedimentary events and several discontinuities associated with stratigraphic gaps of variable extent. In this work five facies containing ferruginous ooids are described and a preliminary study of the carbonate cements and Fe oolite facies is presented. This ironstone bed was formed near to an emergent high (Ejulve high). The oolites formed in a shallow marine subtidal environment. The facies and cement analysis shows that there were no long episodes of subaereal exposure at the Callovian-Oxfordian boundary. These data also demonstrate that these sediments were not deposited in deep marine conditions. The most probable hypothesis is that Fe ooids were formed in shallow subtidal and perhaps intertidal conditions and that they were eventually transported into more distal and relatively deep basinal areas.

Cratered cobbles in Triassic Buntsandstein conglomerates in northeastern Spain: An indicator of shock deformation in the vicinity of large impacts: Comment and Reply COMMENT

[Ernstson et al. (2001)][1] interpreted pitted and fractured quartzite cobbles of Lower Triassic conglomerates as new evidence in support of the existence of two large impact structures in the Iberian chain: Azuara and Rubielos de la Cerida ([Fig. 1][2]). In this discussion we disagree with their

Cool episode and platform demise in the Early Aptian: New insights on the links between climate and carbonate production

The Early Aptian encountered several crises in neritic and pelagic carbonate production, major perturbations in the carbon cycle, and an oceanic anoxic event (OAE1a). Yet the causal links between these perturbations and climate changes remain poorly understood, partly because temperature records spanning the Early Aptian interval are still scant. We present new δ18O data from well-preserved bivalves from a carbonate platform of the Galve subbasin (Spain) that document a major cooling event postdating most of OAE1a. Our data show that cooling postdates the global platform demise and cannot have triggered this event that occurred during the warmest interval. The warmest temperatures coincide with the time equivalent of OAE1a and with platform biotic assemblages dominated by microbialites at Aliaga as well as on other Tethyan platforms. Coral-dominated assemblages then replace microbialites during the subsequent cooling. Nannoconids are absent during most of the time equivalent of the OAE1a, probably related to the well-known crisis affecting this group. Yet they present a transient recovery in the upper part of this interval with an increase in both size and abundance during the cool interval portion that postdates OAE1a. An evolution toward cooler and drier climatic conditions may have induced the regional change from microbial to coral assemblages as well as nannoconids size and abundance increase by limiting continent-derived input of nutrients.