Marc Aurell

Upper Jurassic thrombolite reservoir play, northeastern Gulf of Mexico

In the northeastern Gulf of Mexico, Upper Jurassic Smackover inner ramp, shallow-water thrombolite buildups developed on paleotopographic features in the eastern part of the Mississippi Interior Salt basin and in the Manila and Conecuh subbasins. These thrombolites attained a thickness of 58 m (190 ft) and were present in an area of as much as 6.2 km2 (2.4 mi2). Although these buildups have been exploration targets for some 30 yr, new field discoveries continue to be made in this region. Thrombolites were best developed on a hard substrate during a rise in sea level under initial zero to low background sedimentation rates in low-energy and eurytopic paleoenvironments. Extensive microbial growth occurred in response to available accommodation space. The demise of the thrombolites corresponded to changes in the paleoenvironmental conditions associated with an overall regression of the sea. The keys to drilling successful wildcat wells in the thrombolite reservoir play are to (1) use three-dimensional seismic reflection technology to find paleohighs and to determine whether potential thrombolite reservoir facies occur on the crest and/or flanks of these features and are above the oil-water contact; (2) use the characteristics of thrombolite bioherms and reefs as observed in outcrop to develop a three-dimensional geologic model to reconstruct the growth of thrombolite buildups on paleohighs for improved targeting of the preferred dendroidal and chaotic thrombolite reservoir facies; and (3) use the evaporative pumping mechanism instead of the seepage reflux or mixing zone models as a means for assessing potential dolomitization of the thrombolite boundstone.

The Middle-Upper Jurassic ooliticironstone level in the Iberian range (Spain). Eustatic implications

Abstract The oolitic ironstone level occurring at the Callovian-Oxfordian boundary across wide areas in the central IberianRange (Eastern Spain) is interpreted here as formed on an extended, very shallow to temporarity emerged, uniform carbonate platform. Main evidence to support this interpretation, comes from both sedimentological analysis of facies and taphonomic analysis of the ammonites. The sedimentological analysis gives support to the idea of iron oolites being formed on, or in the surroundings of, emerged areas. On the other hand, the taphonomic analysis shows that some inner moulds of ammonites from this level display evidence of taphonomic reworking, such as the presence of ellipsoidal abrasion facets on the final part of the last preserved whorl, or annular abrasion furrows carved on the external region. These features would have developed by the action of directional currents under extremely shallow conditions. Bathymetric implications are relevant for the interpretation of the sea level fluctuations at this stratigraphic interval: A relative lowstand of sea level is proposed for the Upper Callovian-Lower Oxfordian interval in the studied area.

Identification of systems tracts in low-angle carbonate ramps: examples from the Upper Jurassic of the Iberian Chain (Spain)

Abstract The Upper Jurassic of the Iberian Chain in northeast Spain was selected for the study and identification of sequence boundaries and systems tracts in low-relief carbonate ramp systems. The study area provides a special opportunity for sequence stratigraphic analysis because the outcropping Upper Jurassic carbonate ramps are relatively undeformed, well-exposed from proximal to distal areas and have an established ammonite zonation. Two sequences are recognized. Sequence A, uppermost Callovian to uppermost Oxfordian in age (up to 60 m thick in the proximal areas and between 15 and 5 m thick in the middle and distal areas), and Sequence B, Kimmeridgian to lowermost Tithonian (from 80 to 160 m in thickness across the basin). The evolution of the first sequence is related to regional relative sea-level changes, i.e., tectono-eustatic changes superimposed on the sedimentary basin. On the other hand, the larger accommodation created during the second sequence is regarded as controlled mainly by steady subsidence. Sequence boundaries indicative of subaerial exposure show as unconformities with stratigraphic hiatuses in the proximal areas of the basin. The lowstand systems tract in Sequence A is represented by a decimetre-thick reworked level, consisting of several ammonite biozones. In Sequence B, this systems tract consists of a thick, wedge-shaped, marly unit. In both sequences, the transgressive surface overlaying these lowstand deposits is generally planar and marks a sharp boundary between the restricted and open marine fossils. However, in Sequence B, in the middle part of the basin, the transgressive surface can be absent; instead a transitional change to open marine facies exists. The transgressive systems tracts show sedimentary condensation at least in the distal areas of the basin with mostly ammonites and benthic fossil wackestones. The transgressive unit is generally overlain by a hard ground surface that corresponds to a maximum flooding surface. Finally, in the upper part of both sequences, highstand systems tracts are represented by progradational siliciclastic and shallow carbonate complexes in the proximal areas of the basin. In Sequence A, sedimentation is condensed in the distal areas with glauconite marl levels present. The descriptions of sedimentary features of the systems tracts of these two sequences, developed in a low-angle carbonate ramp, are suggested to be used as tools for the recognition of systems tracts patterns within sequences in comparable depositional settings.

Stratigraphie sequentielle de l'Aptien du sous-bassin de Galve (Province de Teruel, NE de l'Espagne)

A correlation is established in a north-south transect based on continuous outcrops. Considering the different reference surfaces and the geometry, three major depositional sequences can be distinguished which can be subdivided into a complex arrangement of parasequences. These third-order sequences are composed of a lower retrogradational and an upper progradational trends. The first sequence contains orbitolinid bioaccumulations in the retrogradational trend and oolitic-bioclastic shoals in the progradational trend. The second sequence exhibits, from bottom to top, a transgressive, a regressive and a forced-regressive trends. Ammonite-rich marls characterise the transgressive trend, whereas bioconstructions rich in coral-chaetetids-microbialites are abundant in both regressive and forced-regressive trends. The maximum flooding of this sequence is widely distributed across the whole Iberian platform. Finally, the third sequence shows the installation of homogeneous rudistid bioaccumulations in a retrogradational and a progradational trends. Each major sequence boundary marks a community replacement, whose respective fossil associations are dominated by (1) orbitolinids, (2) corals-microbialites, (3) corals-chaetetids-microbialites, and (4) rudists.