Federico Ortí

Oxygen and sulphur isotope compositions as indicators of the origin of Mesozoic and Cenozoic evaporites from Spain

Abstract The general investigation of oxygen and sulphur isotope compositions of the main evaporitic formations of Mesozoic and Cenozoic ages from Spain, for primary purposes, had to differentiate between marine and continental sulphate evaporites. Furthermore, the isotope data would also make it possible to determine the source of sulphate of the continental evaporites. The δ-values measured for the marine evaporites of Spain of Triassic, Lias, Cretaceous and Eocene age, are in good agreement with the general oxygen and sulphur isotopic curves of the oceanic sulphate. The continental evaporites deposited in the Ebro, Tajo, Calatayud and Teruel basins, since Paleocene up to Miocene times, exhibit wide ranges of δ-values both through time and space. The main source of sulphate reworked by continental waters was Triassic evaporitic sulphate, although another one was locally Cretaceous evaporite. In addition, in continental formations the local conditions of drainage and of in situ bacterial activity acted to modify the isotope composition of the sulphate source, thus imprinting specificity of the isotopic signature of each basin.

Sodium sulphate deposits of Neogene age: the Kirmir Formation, Beypazari Basin, Turkey

The Evaporite Member of the Kirmir Formation was deposited in shallow lacustrine environments during the upper Miocene. The most soluble minerals of this member can be currently observed in the Cayirhan mine. The Evaporite Member, which is composed of secondary gypsum at outcrop, can be subdivided into a bedded lower unit and a massive upper unit. In the bedded lower unit, most of the gypsum throughout the basin can be identified as having been transformed from glauberite. In the glauberite layers of the Cayirhan mine, some glauberite textures (‘clear glauberite’) suggest a primary, subaqueous, free precipitation on a depositional floor. More common, however, are the glauberite textures indicating an interstitial growth within a clayey-magnesitic matrix. In the thenardite layers accompanying the glauberite in the Cayirhan mine, some disruption structures can be assigned to synsedimentary dissolution. These structures together with the textures of the thenardite suggest that the original sodium sulphate was mirabilite, thenardite being a secondary phase, which formed during early to moderate burial diagenesis. The massive upper unit, in which evidence of sodium-bearing minerals is absent, is characterized by laminated to banded gypsum and nodular gypsum in the marginal areas of the evaporitic basin, whereas thick, clast-supported gypsum breccias prevail in the northern, deeper part of the basin. The brecciation of these calcium sulphate layers occurred as a result of synsedimentary, gravitative slumping under tectonic control. Although the sulphur isotopic values (d 34 S) of the sulphates of the Kirmir Formation suggest a marine-derived brine supply, the oxygen isotopic values (d 18 O) and the strontium ratios ( 87 Sr/ 86 Sr) do not support such a supply. The origin of the mother brines, the glauberite genesis, the depositional model of the sodium sulphates, and the salinity evolution are discussed. D 2002 Elsevier Science B.V. All rights reserved.

The stratigraphical record and activity of evaporite dissolution subsidence in Spain

The evaporite formations (in outcrop and at shallow depth) cover an extensive area of the Spanish territory. These soluble sediments are found in diverse geological domains and record a wide time span from the Triassic up to the present day. Broadly, the Mesozoic and Paleogene formations (Alpine cycle) are affected by compressional structures, whereas the Neogene (post-orogenic) sediments remain undeformed.The subsidence caused by subsurface dissolution of the evaporites (subjacent karst) takes place in three main types of stratigraphical settings: a) Subsidence affecting evaporite-bearing mesozoic and Tertiary successions (interstratal karst); b) Subsidence in Quaternary alluvial deposits related to the exorheic evolution of the present-day fluvial systems (alluvial or mantled karst); c) Subsidence in exposed evaporites (uncovered karst). These types may be represented by paleosubsidence phenomena (synsedimentary and/or postsedimentary) recognizable in the stratigraphical record, or by equivalent currently active or modern examples with surface expression.The interstratal karstification of the Mesozoic marine evaporites and the consequent subsidence of the topstrata is revealed by stratiform collapse breccias and wedge-outs in the evaporites grading into unsoluble residues.In several Tertiary basins, the sediments overlying evaporites locally show synsedimentary and/or postsedimentary subsidence structures. The dissolution-induced subsidence coeval to sedimentation gives place to local thickenings in basin-like structures with convergent dips and cumulative wedge out systems. This sinking process controls the generation of depositional environments and lithofacies distribution. The postsedimentary subsidence produces a great variety of gravitational deformations in the Tertiary supra-evaporitic units including both ductile and brittle structures (flexures, synforms, fractures, collapse and brecciation).The Quaternary fluvial terrace deposits on evaporite sediments show anomalous thickenings (>150 m) caused by a dissolution-induced subsidence process in the alluvial plain which is balanced by alluvial aggradation. The complex space and time evolution pattern of the paleosubsidence gives place to intricate and anarchical structures in the alluvium which may be erroneously interpreted as pure tectonic deformations. The current subsidence and generation of sinkholes due to suballuvial karstification constitutes a geohazard which affects to large densely populated areas endangering human safety and posing limitations to the development. An outstanding example corresponds to Calatayud historical city, where subsidence severely damages highly valuable monuments. The subsidence resulting from the underground karstification of evaporites has determined or influenced the generation of some important modern lacustrine basins like Gallocanta, Fuente de Piedra and Banyoles lakes. The sudden formation of sinkholes due to the collapse of cave roofs is relatively frequent in some evaporite outcrops. Very harmful and spectacular subsidence activity is currently occurring in the Cardona salt diapir where subsidence has been dramatically exacerbated by mining practices.

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.

Coastal salina evaporites of the Triassic-Liassic boundary in the Iberian Peninsula: the Alacón borehole

The evaporite unit (the Lecera Formation), which was formed at the Triassic–Liassic boundary in the Aragonian Branch of the Iberian Chain, was studied at the 01 Alacon borehole (Alacon village, Teruel province), where it is mainly constituted by a thick (>300 m) succession of predominant sulphates (anhydrite, secondary gypsum and carbonate) overlain by brecciated carbonates. In the evaporite succession, a number of lithofacies were recognized, which can be grouped into an “ideal cycle”, from base to top: (C1) massive to banded carbonate mudstone, (C2) alternation of carbonate and anhydrite laminae, (A1) alternation of anhydrite and carbonate bands, (A2) clastic intercalations in the alternation of anhydrite and carbonate bands, (A3) laminated to banded anhydrite, (A4) bedded pseudomorphs, (A5) interstitial pseudomorphs, and (A6) massive to nodular anhydrite. Fine-grained gypsum (anhydrite laminae and bands), bedded selenitic gypsum (bedded pseudomorphs), interstitial selenitic gypsum (interstitial pseudomorphs), and graded-nodular anhydrite (a particular fabric of nodular anhydrite) were the most outstanding depositional lithofacies. The evaporite succession exhibits a marked cyclicity: in the lower part the individual cycles are more similar to the ideal cycle and reflect deeper water settings, whereas in the upper part they correspond to shallower water settings. The evaporite sedimentation mainly occurred in a subsiding coastal basin of the salina or lagoon type. In this setting, the subaqueous precipitation of the carbonate and gypsum lithofacies was followed, in each cycle, by the interstitial growth of anhydrite in exposed conditions. As a whole, the evaporite succession reflects an infilling process. The conversion into anhydrite of the selenitic gypsum -probably also of the rest of depositional gypsum lithofaciesstarted under synsedimentary conditions and followed during shallow to moderate burial diagenesis.

Gypsum-Hydroboracite Association in the Sijes Formation (Miocene, NW Argentina): Implications for the Genesis of Mg-Bearing Borates

ABSTRACT This paper deals with sedimentologic and diagenetic aspects of the evaporitic facies of the Sijes Formation (Miocene, central Andes, NW Argentina), which contains the largest known hydroboracite reserves in the world. In outcrop, the sulfate minerals are secondary gypsum and minor anhydrite, and the borate minerals are hydroboracite with subordinate inyoite and colemanite, and some ulexite. In the Monte Amarillo Member of the Sijes Formation it is possible to distinguish two coeval, shallow lacustrine subbasins, in which the gypsum accumulated in the margins and the hydroboracite in the centers, the intermediate zones being characterized by mixed gypsum-hydroboracite layers. In the depositional sequence, primary gypsum (gypsarenite) and syndepositional anhydrite, in association with limited amounts of calcium borates (colemanite, inyoite) precipitated first, followed by hydroboracite (calcium/magnesium borate). Alternations of gypsum and hydroboracite layers also formed. Hydroboracite is mainly a primary mineral, although it replaced some gypsum under synsedimentary conditions. The formation of colemanite, which occurred during early diagenesis, is linked to the precipitation of calcium sulfates (gypsum and anhydrite), whereas inyoite coexists with both calcium sulfates and magnesium-bearing borates. Transformations among the various borate minerals during burial diagenesis were not detected. Primary gypsum was transformed into anhydrite from early diagenesis to moderate burial diagenesis. The boron source of these deposits seems to be related to the volcanic/hydrothermal activity in the central Andes during the Miocene.

Paleosubsidence and active subsidence due to evaporite dissolution in Spain

Evaporite formations crop out or are at shallow depth present in an extensive area of Spain. These soluble sediments occur in diverse geological domains and were deposited over a long time span, from the Triassic up to the present day. Broadly, the Mesozoic and Paleogene formations (Alpine cycle) are affected by compressional structures, wheras the Neogene (post-orogenic) sediments remain underformed. Subsidence caused by subsurface dissolution of evaporites (subjacent karst) takes place in three main types of stratigraphic settings: a) subsidence affecting evaporite-bearing Mesozoic and Tertiary successions (interstratal karst); b) subsidence in Quaternary alluvial deposits related to the exorheic evolution of present-day fluvial systems (alluvial or mantled karst); and c) subsidence in exposed evaporites (uncovered karst). These types may be represented by paleosubsidence phenomena (synsedimentary and/or postsedimentary) recognizable in the stratigraphic record, or by equivalent, currently active or modern examples which have a surface expression. Interstratal karstification of Mesozoic marine evaporites, and the consequent subsidence of overlying strata, is revealed by stratiform collapse breccias and wedge outs of the evaporites grading into unsoluble residues. In several Tertiary basins, the sediments overlying evaporites locally show synsedimentary and/or postsedimentary subsidence structures. Dissolution-induced subsidence coeval with sedimentation is accompanied by local thicknening of strata in basin-like structures with convergent dips and cumulative wedge-out systems. This sinking process controls the generation of depositional environments and lithofacies distribution. Postsedimentary subsidence produces a great variety of gravitational deformations in Tertiary supra-evaporitic units, including both ductile and brittle structures (flexures, synforms, fractures, collapse, and brecciation). Quaternary fluvial terrace deposits overlying evaporites show anomalous thickenings (>150m) caused by a dissolution-induced subsidence process in the alluvial plain, which is balanced by alluvial aggradation. The complex evolution (in time and space) of paleosubsidence leads to intricate and chaotic structures in the alluvium, which may be erroneously interpreted as pure tectonic deformations. The current subsidence and generation of sinkholes due to suballuvial karstification constitutes a geohazard which affects large, densely populated areas, and thus endangers human safety and poses limitations on development. An outstanding example can be seen in Calatayud, an important historical city where subsidence has severely damaged highly valuable monuments. Subsidence resulting from the underground karstification of evaporites has caused or influenced the generation of some important modern lacustrine basins, such as Gallocanta, Fuente de Piedra, and Banyoles Lakes. The sudden formation of sinkholes due to collapse of cave roofs is fairly frequent in some evaporite outcrops. Very harmful and spectacular subsidence activity is currently occurring in the Cardona salt diapir, where subsidence has been dramatically exacerbated by mining practices.