Laura Rosell

Lacustrine oil-shale basins in Tertiary grabens from NE Spain (Western European rift system)

Abstract In the NE Iberian Plate, the convergent motion and collision of the European, Iberian and African plates resulted in the development of Paleogene compressional features (strike-slip systems, thrust-fold belts) and late Oligocene to mainly Neogene extensional structures which are superimposed on the former. These extensional structures are represented by horsts, half grabens and tilted blocks, often developed in connection with preexisting, inherited faults. From the late Oligocene and during the Neogene both strike-slip and extensional regimes alternated and coexisted, giving rise to a number of fault-bounded basins. The deposition of organic-rich facies, and in particular oil-shales, took place in the lacustrine complexes developed in some of these basins: Campins Basin (late Oligocene); Ribesalbes and Rubielos de Mora basins (early-middle Miocene); Libros Basin (late Miocene) and Cerdanya Basin (late Miocene). Deep lacustrine sequences ranging from 100 up to 250 m thick were deposited in all the above mentioned basins. The organic-rich sequences are characterized by thin lamination, absence of bioturbation, rare benthonic fauna (if present, it is supplied from other parts of the basin) and excellent preservation of “exotic” fossils (plant leaves, insects, amphibians). The suitable depth conditions needed for permanent stratification in the water bodies and anoxia in the deeper parts of the lakes, were due essentially to increasing subsidence outstripping deposition. Intense tectonic activity is recorded by the occurrence of olisthostromes and slumps affecting the lacustrine deposits as well as by the syntectonic structures recorded in the basin-fill successions. The late Oligocene-late Miocene paleoclimatic regimes, ranging from warm tropical to subtropical conditions, were favourable for the development of permanently stratified lakes. Moreover chemical ectogenic meromixis may have contributed in some cases to the establishment of permanent stratification. The Eastern Iberian rift system, makes up the southernmost part of the larger Western European rift system. The occurrence of lacustrine sediments ranging from the late Eocene to late Miocene, is frequent in the fault-bounded basins of this complex rift system and lacustrine oil-shale deposits have been recorded in several of these basins. Thus the Western European rift system presents an interesting model of intracontinental rifting in a foreland platform setting, where favourable conditions for organic rich deposits often took place. The high potential of this kind of tectonic settings for lacustrine oil-shale exploration must be stressed.

Carbonate replacement of lacustrine gypsum deposits in two Neogene continental basins, eastern Spain

Abstract Bedded nonmarine gypsum deposits in the Miocene Teruel and Cabriel basins, eastern Spain, are partly replaced by carbonate. The Libros gypsum (Teruel Graben) is associated with fossiliferous carbonate wackestones and finely laminated, organic matter-rich mudstones which accumulated under anoxic conditions in a meromictic, permanent lake. The gypsum is locally pseudomorphed by aragonite or, less commonly, replaced by calcite. Low δ13C values indicate that sulphate replacement resulted from bacterial sulphate reduction processes that were favoured by anacrobic conditions and abundant labile organic matter in the sediments. Petrographic evidence and oxygen isotopic composition suggest that gypsum replacement by aragonite occurred soon after deposition. A subsequent return to oxidising conditions caused some aragonite to be replaced by diagenetic gypsum. Native sulphur is associated with some of these secondary gypsum occurrences. The Los Ruices sulphate deposits (Cabriel Basin) contain beds of clastic and selenitic gypsum which are associated with limestones and red beds indicating accumulation in a shallow lake. Calcite is the principal replacement mineral. Bacterial sulphate reduction was insignificant in this basin because of a scarcity of organic matter. Stable isotope composition of diagenetic carbonate indicates that gypsum replacement occurred at shallow burial depths due to contact with dilute groundwaters of meteoric origin. Depositional environment evidently has a major influence upon the diagenetic history of primary sulphate deposits. The quantity of preserved organic matter degradable by sulphate-reducing bacteria is of particular importance and, along with groundwater composition, is the main factor controlling the mechanism of gypsum replacement by carbonate.

Sulfate Platform-Basin Transition of the Lower Werra Anhydrite (Zechstein, Upper Permian), Western Poland: Facies and Petrography

ABSTRACT The peripheral evaporite platform of the Zechstein (Upper Permian) Werra Anhydrite of western Poland comprises a series of shoals (with thick sulfate deposits) and lows (with thin sulfate and thick halite deposits). Three cores were selected to examine facies variations from a sulfate platform, slope, and basin. The basal unit of the Lower Werra Anhydrite is represented by different varieties of nodular, bedded-nodular, fluidal-nodular, and brecciated facies. Some nodular fabrics probably originated within sabkha and/or gypsumpond environments, and the deposit was then mechanically redistributed. The absence of nodular structures in units other than the lower one in two of the wells suggests that the origin of these structures cannot be related to deep burial. Other units of the Lower Werra Anhydrite represent subaqueous deposits. The facies in the middle and upper units have no recent analog in coastal salina environments. Turbidity currents displaced sulfates from the platform to the adjacent basin. The origin of lenticular-bedded anhydrite may be related to deformation by compaction or by ravitational instability and slumping, which may have initiated crystallization or recrystallization of anhydrite. The vertical sequence of facies shows a distinct deepening-upward trend starting from the top of the basal nodular units. Deposition in shallow (salina) environments dominated the early history of the Lower Werra Anhydrite basin, although it was probably preceded by transgressive sabkha deposits. Relatively thick nodular anhydrites probably formed by syndepositional diagenesis of probable salina deposits and minor sabkha sediments. Gypsum was the original sedimentary mineral; it was later transformed into anhydrite, during either early or late (burial) diagenesis. We show that in deeply buried evaporites not only some macroscopic primary textures remain but also even microscopic details, which allow reconstruction of depositional environments and primary mineralogy of ancient anhydrites.