The Neoproterozoic Cryogenian ('Marinoan') Ghaub Formation of northwestern Namibia represents an important founding pillar of the Snowball Earth hypothesis and its derivative, the Panglacial Earth hypothesis. These hypotheses assume oceans and continents covered by thick ice, even in the tropics, which caused a very distinct drop in eustatic sea-level. Over time, strongly increased CO2 contents of the atmosphere led to sudden ice melting, very substantial sea-level rise, and strong weathering on the continents associated with the deposition of cap carbonates in the newly ice-free oceans. The ongoing controversy about Snowball-type glaciations in Namibia and elsewhere is reviewed, and other hypotheses (Slushball Earth, Waterbelt Earth, Jormungand state of the Earth, Thin Ice state of the Earth, Zipper-Rift Earth, High-Obliquity Earth) are discussed. We prefer the term 'Waterbelt Earth' instead of the originally proposed `Waterbelt state' because of the clearer contrast with 'Snowball Earth'. Because a great deal of information related to Cryogenian glaciations comes from the Ghaub Formation of northwestern Namibia, these hypotheses should be tested independently based on a time-equivalent depositional system. This analogue was found in the carbonate-dominated successions of the Otavi Mountainland (OML), northeastern Namibia, and is highly comparable with the successions in the well-investigated northwest of the country. An extreme eustatic sea-level drop caused by a global glaciation of oceans and continents and imposed on a carbonate platform or ramp such as the one in the OML would have led either to glacial cover or widespread subaerial exposure and extensive erosion, including deeply incised valleys. The presence of such features would strongly support the Snowball Earth hypotheses if tectonic effects did not play a major role. During the post glacial transgression, distinct reworking of the carbonate platform/ramp surface would have occurred, leaving behind lag deposits, as well as infills of incised valleys with fluvial, reworked glacial, and marine deposits. The main objective of our research was to weigh and investigate the strengths and weaknesses of the proposed Snowball Earth model of glacially induced large-amplitude sea-level changes during Ghaub time, and to compare different models to obtain a rough estimate of the amount of glaciation. The study area in the OML includes two different, age-equivalent facies realms: platform sedimentation in the Southern area without diamictites, and slope deposits, including Ghaub diamictites, in the Northern area. The southern, continuously shallow-marine area shows a shallowing-upward succession from the pre-glacial lower Auros Formation, often varve-like laminated shales formed below wave base, to metre-high columnar stromatolites and microbial mat-related carbonates with intervals of vertical tubes (degassing features) of the upper Auros Formation, overlain by cap carbonates of the Maieberg Formation. The columnar stromatolites and the microbial tubestone lithotypes were clearly deposited in the euphotic zone. Indications for tidal conditions or subaerial exposure were not recorded in this platform succession without unconformities. Neither dropstones, nor incised channels, nor transgressive lag deposits were observed. The facies changes from below storm wave base to the photic zone and finally a shallow subtidal zone is explained by a prolonged, modest sea-level fall, partly counterbalanced by subsidence, followed by a slow transgression. In contrast, coarse-grained sedimentary rocks (e.g., oolites, debrites) characterise the time-equivalent successions in the Northern area. Starting with laminated shales at the base, similar to the Southern area, the overlying redeposited oolites and breccias of the Auros Formation show distinct lateral and vertical in homogeneities and thickness changes, which indicate long-lasting synsedimentary tectonism. The same phenomenon is observed in the overlying diamictites of the Ghaub Formation. Their variable clast content indicates erosion of a strongly uplifted local source area formerly covered by a thick carbonate succession, which was downstripped to the crystalline basement. The prograding diamictite succession with repeatedly intercalated silt stringers is interpreted as periglacial debris flows into a marine environment. Sparse striated clasts in the diamictites and very rare dropstones (much less common than in northwestern Namibia) are indicators of glaciations somewhere in the area. However, compared with other glacial sequences, e.g. Quaternary periglacial sediments at the forefront of continental ice, dropstones and striated clasts would be expected to be much more common and more uniformly distributed if the entire area was covered by melting continental ice, as proposed in the Snowball/Panglacial Earth scenario. In the Southern area, dropstones would be expected to occur on the flooded platforms/ramps as well, even when diamictites are absent. Both the relatively moderate sea-level change and the less common, irregular distribution of locally concentrated glacial rainouts provide strong evidence against the presence of a thick, laterally continuous ice cover over oceans and continents extending to equatorial areas. The oceans possibly corresponded to the scenario of a Waterbelt Earth or High-Obliquity Earth; evidence of open oceanic water exists, which would have enabled the continued evolution of biota. Glacial ice was present on tropical continents, but its occurrences may have been regional in patches, sourced from mountainous areas, and ice streams would have reached the oceans only locally, unrelated to a thick continental ice cover.

The architecture of Middle to early Late Triassic carbonate platforms in the eastern Lombardian Alps is analysed following the accommodation and supply concept. This allows proposing a new sequence stratigraphic scheme. The selected study area can be seen as an earth sciences lab where in a relatively small area distinct synsedimentary tectonics in combination with global sea-level fluctuations caused pronounced differences in basin evolution, leading to repercussions in the carbonate platform development from the Anisian to the Carnian. A base-level fall in the Pelsonian favoured in the area of a long-lasting topographic high and shortly after the Permian-Triassic crisis of carbonate platforms the growth of the first in a cycle of carbonate build-ups, the Camorelli Bank. Slow subsidence combined with high carbonate production caused strong progradation of this carbonate bank. The Calcare di Camorelli is followed later but in the same area by the Pora Platform succession. This underlines that palaeotopography and related subsidence was an important mechanism for platform growth in the area. The well-dated base-level rise at the Pelsonian/Illyrian boundary caused platform flooding and the change to the marly Prezzo Fm (shallow basinal facies) and nodular-cherty Buchenstein Fm (deep basinal facies). In the early Longobardian, base-level fell gradually and the slow subsequent base-level rise triggered the growth of four carbonate platforms in the study area attributed to the Esino Fm: the Pora, Concarena, Pizzo Camino and Presolana platforms. In this paper, the results from detailed sequence stratigraphic studies from the Pora and Concarena area are presented. The cycle of carbonate platforms attributed to the evolution of the Esino Fm led to formation of a much thicker platform and slope succession compared to the preceding Camorelli platform cycle. This development can be best studied at the Concarena area. Initially, an aggradational to weak progradational pattern can be observed, indicating strong subsidence combined with high carbonate production. The lagoonal upper part of the Concarena platform shows much reduced cycle-thicknesses compared with the cycles of the preceding platform stage, combined with strong progradation of the platform slope. At its top, only the Pora Platform shows distinct emersion features, due to a base-level fall and underpinning the position on a local structural high, in contrast to the Concarena, which had somewhat higher subsidence rates and do not show signs of emersion.

The Upper Rhine Graben (URG) in SW-Germany, a classical hydrocarbon province, is part of the European Cenozoic Rift System. Rift graben development has led to a complex basin fill of terrestrial and marine deposits providing several reservoir and source rock units. Source rocks are restricted to the pelitic units of marine transgressive intervals. Although TOC is similar in the different source rock units, kerogen composition and therefore hydrocarbon potential, differs significantly. In general two types of source rocks are identified. Transgressive marine intervals during high rift tectonic activity with high subsidence and therefore high terrestrial input from the graben shoulders led to terrestrial dominated, mainly gas-prone source rocks. Even in the maximum transgressive interval (Rupel clay) high rift tectonic activity led to mainly gas-prone kerogen instead of mainly oil-prone kerogen as proposed before. In contrast, marine transgressions in times of low rift tectonic activity and therefore low subsidence and terrigenous sediment input led to the deposition of mainly marine-brackish derived, oil-prone kerogen, as expected for marine intervals. Kerogen composition also shows minor differences due to different structural positions. Thus organic matter accumulation and kerogen composition is strongly influenced by rift-related tectonic activity and less by sea-level variations, which differs clearly from previous models. For a better understanding and modelling of the petroleum system in the northern URG the palaeothermal history was studied by integrated maturation analysis of several wells across the study area. Most wells show almost vertical maturation trends, untypical for burial controlled maturation but clearly indicating secondary thermal overprint. This is related to long-lasting, very hot hydrothermal fluid systems, well known from the URG, which were mainly concentrated along reactivated fault zones. Therefore, maturation is mainly influenced by tectonically controlled distribution of hydrothermal systems and not by basin subsidence. This is confirmed by the uniform maximum maturation in the early oil-window in all wells, despite the different burial depths. Thus minor hydrocarbon generation can be expected only from oil-prone source rock units. Hence, the development of the depositional setting, kerogen composition, thermal maturation and hydrocarbon potential is directly linked and mainly controlled by the dynamics of the rift system. AAPG Datapages/Search and Discovery Article #90310 ©2017 AAPG/SEG International Conference and Exhibition, London, England, October 15-18, 2017

(1) Università di Milano-Bicocca, Department of Earth and Environmental Sciences, Milano, Italy (m.limonta1@campus.unimib.it), (2) Dipartimento di Scienze della Terra, dell’Ambiente e delle Risorse, Universita‘ Federico II di Napoli, Via Mezzocannone 8, 80134 Napoli, Italy, (3) GeoResources STC, Im Neuenheimer Feld 234, 69120 Heidelberg, Germany / Institute of Geological Sciences, Jagiellonian University, ul. Oleandry 2a, 30-063, Krakow, Poland

The main source of siliciclastic sediment in the Barbados accretionary prism is off-scraped quartzose to feldspatho-litho-quartzose metasedimentaclastic turbidites, ultimately supplied from South America chiefly via the Orinoco fluvio-deltaic system. Modern sand on Barbados island is either quartzose with depleted heavy-mineral suites recycled from Cenozoic turbidites and including epidote, zircon, tourmaline, andalusite, garnet, staurolite and chloritoid, or calcareous and derived from Pleistocene coral reefs. The ubiquitous occurrence of clinopyroxene and hypersthene, associated with green-brown kaersutitic hornblende in the north or olivine in the south, points to reworking of ash-fall tephra erupted from andesitic (St Lucia) and basaltic (St Vincent) volcanic centres in the Lesser Antilles arc. Modern sediments on Barbados island and those shed by larger accretionary prisms such as the Indo-Burman Ranges and Andaman-Nicobar Ridge define the distinctive mineralogical signature of Subduction Complex Provenance, which is invariably composite. Detritus recycled from accreted turbidites and oceanic mudrocks is mixed in various proportions with detritus from the adjacent volcanic arc or carbonate reefs widely developed at tropical latitudes. Ophiolitic detritus, locally prominent on the Andaman Islands, is absent on Barbados, where the prism formed above a westward subduction zone with a shallow décollement plane. The four-dimensional complexities inherent with multicyclic sediment dispersal along and across convergent plate boundaries require quantitative provenance analysis as a basic tool in paleogeographic reconstructions. Such analysis provides the link between faraway factories of detritus and depositional sinks, as well as clues on subduction geometry and the nature of associated growing orogenic belts, and even information on climate, atmospheric circulation and weathering intensity in source regions.

Burial hydrothermal dolomitization is a common diagenetic modification in sedimentary basins with implications for oil and gas reservoir performance. Outcrop analogues represent an easily accessible source of data to refine the genetic models and assess risk in hydrocarbon exploration and production. The Palaeozoic succession of northern Spain contains numerous excellent exposures of epigenetically dolomitized limestones, particularly in the Carboniferous and Cambrian. The epigenetic dolomites in the Cambrian carbonates of the Láncara Formation are volumetrically small, but have a large aerial distribution across different tectonic units of the Variscan fold and thrust belt. Coarse crystals, abundant saddle dolomite cement, negative δ18O and fluid inclusion homogenization temperatures between 80°C and 120°C characterize these dolomites, which are petrographically and geochemically similar to the tens of kilometre‐sized hydrothermal dolomites replacing the Upper Carboniferous succession in the same area. In both cases, the dolomitizing fluids are derived from highly evaporated sea water, modified to a limited degree through fluid‐rock interaction. The dolomitization events affecting both Cambrian and Carboniferous strata are probably related to the same post‐orogenic hydrothermal fluid flow. The formation of the post‐collisional (latest Carboniferous) Cantabrian arc fostered dolomitization: the extension related to bending of the arc generated deep‐reaching faults and strike‐slip movements, which favoured the circulation of hot dolomitizing fluids in the outer parts of this orocline. A similar dolomitization process affected other areas of Europe after the main stages of the Variscan orogeny. Dolomitization was a continuous, uninterrupted, isochemical process. Limestone replacement resulted in a major porosity redistribution and focused the fluid flow into the newly created porous zones. Replacement was followed immediately by partial to complete cementation of the pores (including zebra fabrics and vugs) with saddle dolomite. The amount of porosity left depends on the volume of cement and therefore on the volume of fluids available.

(1) Department of Earth and Environmental Sciences, Università di Milano-Bicocca, 20126 Milano, Italy (m.limonta1@campus.unimib.it), (2) Geological Survey of India, Northern Region, Aliganj, Lucknow 226 024, India., (3) Department of Environmental Science, Lancaster University, LA1 4YQ Lancaster, UK., (4) Dipartimento di Scienze della Terra, Università di Napoli, 80134 Napoli, Italy, (5) Geological Institute (Petroleum Geology), Jagiellonian University, Kraków, Poland.

Paleozoic basins of southern Algeria comprise several HC systems: i) infra-Cambrian (?) source rock, Upper Ordovician reservoirs; ii) Silurian source rock, Lower and Middle Devonian reservoirs ; iii) Upper Devonian source rocks, Lower Carboniferous reservoirs. Since 2006 commercially productive gas reservoirs have been proven by several wells in the Reggane and Ahnet Basins. Previous basin evaluation has proposed HC generation predominantly during Mesozoic reburial, with pre-Hercynian generation having been largely ineffective (Purdy & MacGregor 2003, Geol. Soc. Spec. Publ. London, v. 207). Thermochronlogical data (Logan & Duddy 1998, Geol. Soc. Spec. Publ. London, v. 132) have indicated two-phased HC generation: i) simple burial heating to oil window before peak Hercynian orogeny (pre-Early Carboniferous); ii) Late Triassic heating to wet and dry gas window in the Latest Triassic, related to the development of the Central Atlantic Magmatic Province (CAMP) and doleritic dykes/sills (Reggane Basin); iii) subordinate heating of less mature basin margins until recently. In order to better assess the basin and HC development including the recently proven gas plays, an integrated study has been performed including: i) seismic and sequence stratigraphy at basin and reservoir resolution; ii) numerical basin modeling (subsidence/uplift, sediment flux); iii) palynostratigraphic (miospores, acritarchs) and organofacies analysis; iv) paleotemperature analysis including organic maturation, apatite and zircon fission track and (U-Th)/He dating. Focus is on the Reggane Basin, where ample 2D seismic coverage and calibration wells (logs, samples) have been available. Results include: i) trans- and regressive trends within the basin fill; ii) intra- and inter-basin correlation iii) lateral continuation and vertical connectivity of reservoir sandstones; iii) accommodation and sediment flux history; iv) paleotemperature development related to pre- and post-Hercynian burial and exhumation. The new data show, that the existing models of HC development for the Upper Silurian-Lower/Middle Devonian and Upper Devonian-Lower Carboniferous systems have to revised.

Key objectives include sequence stratigraphy of the Devonian basin fill, numerical modeling and basin development during the Paleozoic to Mesozoic. Total thicknesses of the Paleozoic basin fill range between 4500-5500 m. Twelve stratigraphic intervals have been analysed from 2D seismic surveys and well data. Two major pan-African structural domains in the subsurface of the Reggane Basin significantly influenced the Paleozoic basin development: the West African Craton (WAC) and the Hoggar Block (HB). Moderate to high subsidence rates persisted during the Ordovician to Silurian. Subsidence rates slow down until Eifelian and accelerated again until the Devonian-Carboniferous boundary. Initial moderate to high subsidence rates in the early Carboniferous are followed by low subsidence in the late Carboniferous. Basin inversion started at approx. 200 Ma. Maximum paleotemperatures were reached at 250-240 Ma with approx. 200-220°C. Sediment pathways and dispersion widths outline a low-gradient transition from proximal upper shelf areas in the SE to outer shelf areas in the NW. Sediment flux varies strongly in time. The burial and exhumation model indicates, that the paleotemperature development was primarily controlled by regional burial, rather than by a Late Triassic heat flow event (related to Central Atlantic Magmatic Province).

The Campos, Santos and Pelotas basins (offshore southern Brazil) have been investigated in terms of 2D seismo-stratigraphy and numerical basin analysis. The processes controlling accommodation space evolution from the shelf top to the continental rise (i.e. eustacy, subsidence and sediment input) are discussed, and the evolution of the different basins is compared. Main results include: (i) classification of depositional seismic sequences from the syn-rift Barremian to the drift Holocene basin fill; (ii) numerical modeling of the subsidence/uplift history; (iii) forward stratigraphic simulation and quantification of erosion, transport and deposition rates regarding the basin-specific hydrocarbon potential

The Late Eocene Sobrarbe Delta System developed at the southern margin of the Ainsa basin as part of the Pyrenees foreland basin, northeastern Spain. It shows exceptional lateral and vertical exposure at seismic scale. Continuous outcrops between alluvial plain an basin margin in both time and space allow for high-resolution measuring/sampling of vertical sections and physical/optical tracing of sediment surfaces. This study includes preliminary data from three outcrop transects of 12-25 km each in the direction of long-term progradation and parallel to paleo-coastlines. The ongoing project will further develop and analyze a high-resolution 3D data set of reservoir architecture in the Sobrarbe Delta System with special focus on lateral continuity and vertical connectivity of reservoirs. Improved reservoir models are essential for forward numerical models of sedimentary systems and fluid-flow during subsequent burial. Key objective is the improved understanding of heterogeneities in reservoir facies and petrophysical parameters in foreland basin delta systems by high-resolution outcrop analysis. Outcrop-based data sets are processed with GIS (ArcGIS ®, ESRI 2009). 3D models will be performed and analyzed with Petrel ® (Schlumberger) and compared to producing subsurface delta systems in foreland basins.

Miocene Intrusives and Lower Cretaceous siliciclastic sedimentary rocks from the Basal Complex in western-Fuerteventura were analyzed with low-temperature thermochronometric methods such as fission-track, and (U–Th–Sm)/He dating, in order to reveal the evolution of the island’s exhumation history. The obtained thermochronometric data yields a very slow rate of cooling in the order of 1.5–3°C/Myr from ~50 to 20 Ma for the Early Cretaceous siliciclastic rocks. These sedimentary units have never been heated significantly above 240°C after deposition and still record the submarine onset of the island’s formation in the Eocene. Intrusive bodies associated with the early Miocene magmatic activity of the central volcanic complex of the island show rapid initial cooling rates of 50–70°C/Myr from ~20 to 14 Ma. Contemporaneous with the intrusions the cooling rate of the Cretaceous sedimentary units increased to 25–35°C/Myr and it is inferred that this increase is associated with enhanced uplift and erosion of the Central Volcanic Complex. After ~14 Ma rates slowed down to 3–6°C/Myr. Palaeosols overlying the sedimentary units are themselves covered by Pliocene basalt flows and reveal that the sedimentary rocks reached the surface before ~5 Ma. The thermochronometric data obtained in this study for central Fuerteventura is difficult to reconcile with the cooling history derived from previously obtained fission-track and K–Ar data from the north-western part of the island. This inconsistency is likely to indicate that the exhumation history of Fuerteventura is more complex and regionally subdivided than previously believed.

Abstract The geological paradox of at least two Neoproterozoic glacial intervals at tropical latitudes intercalated within carbonates remains an unsolved puzzle. Several conceptual models have been proposed to explain these apparent rapid swings between climatic extremes and the associated isotopic changes in sea-water chemistry. In Oman, post-glacial transgressive sedimentary successions represent important hydrocarbon source rocks. Source rock characteristics of Neoproterozoic post-glacial successions in other parts of the world (even if not directly correlatable) are, therefore, of special economic interest. This paper concentrates on the Ghaub Formation diamictite interval in northern Namibia and the major environmental change in the aftermath of the assumed glaciation. The relationship of the post-glacial sediments with the underlying different types of cap carbonate and diamictite successions is discussed, and a model of the succession of events is presented. The palaeotopography, caused mostly by ongoing tectonic activity including uplift on the scale of thousands of metres, strongly influenced the petroleum system created and played an important role for the hydrocarbon prospectivity of this post-glacial succession. Tectonic activity on the shelf of the southern margin of the Congo Craton was repeated, and different sub-basins were created before, during and after the Ghaub glaciation. The newly formed relief was flooded, and the different sub-basins were affected by restricted circulation for quite some time. This general scenario bears many similarities to the late Ordovician–early Silurian petroleum system, also formed during post-glacial sea-level rise.

A combination of thermal history, numerical basin-reverse and sequence-stratigraphic forward modelling is applied to the Mesozoic outcrop analogue of the Rosengarten carbonate platform area in the Dolomites of northern Italy. This integrated multidisciplinary approach of numerical simulation quantifies the thermal, subsidence, geometrical and subsequent facies evolution of the area. Calibration data during modelling were vitrinite reflectance (VR) and apatite fission-track (FT) analyses as well as detailed outcrop studies. Vitrinite reflectance values in strata underlying the carbonate platform vary between 0.5 and 0.8% VRr; apatites from these formations reveal cooling ages of around 165.6 Ma and track lengths of approximately 9.8 mu m. This low thermal maturity combined with the FT data in apatites indicates a relatively cool (< 110 degrees C), protracted (between 250 and 30 Ma) and shallow burial (thickness of eroded strata overlying present-day topography is < 1100 m), as well as a fast exhumation from the Middle Miocene onward. Maximum temperatures are reached during the Middle/Late Triassic, when the basal heat flow was elevated owing to regional volcanic and hydrothermal activity. Local anomalies in vitrinite reflectance of up to 1.1% VRr in the immediate surroundings of the Predazzo/Monzoni volcanic centre show that its thermal influence decreased rapidly with increasing distance. The geometrical evolution of the Middle Triassic (Anisian/Ladinian) Rosengarten platform is twofold: the first stage reveals aggradation, the second progradation of the platform margin. Basin-reverse modelling results indicate that these two intervals originate from a temporal change in tectonic subsidence. Spatial variations in flexural and tectonic subsidence along the 6 km transect are insignificant due to the rigidity of the basement (up to 2500 m of Late Permian ignimbrites). During the first stage of platform evolution, high pulse-like total subsidence rates of up to 820 m Myr(-1) led to aggradation, whereas the subsequent drop to 100 m Myr(-1) initiated platform progradation. The short-spanned subsidence peak was linked to block movements in a strike-slip tectonic setting (Cima Bocche Anticline-Stava Line approximately 10 km southeast of the study area). Stratigraphic forward modelling quantifies the sediment volumes involved in the geometrical evolution of the platform. In order to replicate platform architecture, constant carbonate accumulation rates between 900 and 1000 m Myr(-1) - increasing from periplatform environments to the slope - have to be assumed throughout the existence (approximately 5.8 Myr) of the Rosengarten. As the carbonate factory successfully keeps up with the modelled accommodation rates, it must have completely recovered from the Permian-Triassic biotic crisis during the onset of platform growth in latest Anisian times despite the low biotic diversity of the platform succession seen elsewhere in the Dolomites. Our forward modelling confirms that the main carbonate factory was situated on the slope at water depths from shallow subtidal to 300 m ('slope-shedding') and that it therefore switched on during all possible stages of accommodation change.

The investigated section of the Lower Devonian La Vid Group (Cantabrian Zone, northern Spain) was deposited in a rifted-continental-margin setting, which in the Carboniferous evolved into a foreland basin, affected by Variscan thinskinned folding and thrusting. Soon thereafter the originally linear orogenic belt underwent secondary curvature, forming the Variscan Ibero-Armorican Arc. This bending caused extension, crustal thinning, and strike-slip movements in the outer part of the orocline, to which the area of our investigation belongs. Subsequently the Variscan orogen subsided in Mesozoic and Cenozoic fimes, was covered by sediments of unknown thickness, and was affected by Alpidic tectonics. The Paleozoic Cantabrian Basin is an outcrop analogue for kilometer-scale differences in type and degree of porosity and cementation that can be expected in foreland basins of other areas as well.Origin of cements and related fluids in the La Vid carbonates can be described by three distinct fluid-evolution mechanisms: local in-situ fluid generation, mixing of external with internally generated fluids, and incursion of exotic fluids. The oldest recorded fluid belongs to the basin stage in pre-Variscan times. Upward migration of an in-situ generated fluid from underlying organic-rich shales through iron-rich sandstones into the carbonates of the La Vid Group was accompanied by the precipitation of various Fe-carbonates in various stratigraphic positions of this succession. Among the cements are ferroan saddle dolomites with inclusions of solid bitumen in the lower part and mature petroleum in the upper part of the succession. Assuming a common origin of petroleum and aqueous fluid inclusions in the ferroan saddle dolomite, trapping conditions of 114 to 130 degrees C and a pressure of about 38.5 to 40.5 MPa were determined; this corresponds to a burial depth of about 3850 to 4050 m.The second important fluid event exhibits mixing of a low-salinity internally generated local fluid with an exotic high-salinity fluid of widespread occurrence. This fluid is attributed to the time of Variscan orogenesis, probably the formation of the Cantabrian orocline, based on several tectonic features occurring in the cements. Clear saddle dolomite precipitated during this fluid event in the dolostone unit of the La Vid Group only. These dolomites exhibit a distribution of zoned fluid inclusions from low salinity in the cores to high salinity in the outer parts of the crystals. Minimum trapping temperatures are 160 to 170 degrees C at the core and temperatures of 110 to 120 degrees C at the rim. Higher temperatures in the rim (up to 210 degrees C) appear to have resulted from post-entrapment reequilibration (stretching) of the inclusions during Variscan deformation.The last cementation phase was caused by a cool and oxidizing fluid, related to tectonic activity, most probably of Alpidic age. This fluid precipitated calcite, celestite, and kaolinite and recrystallized former Fe-carbonate cements to calcite and to Feoxvhvdroxides. Fluid inclusions in celestite have low salinities, ruling out basinal brines and pointing to a meteoric or marine origin. Occurrences of these cements in various locations all over the Cantabrian Zone imply a large-scale fluid event.

Large-scale, massive burial dolomitization affected the Carboniferous and Cambrian carbonate succession of the Cantabrian Zone (Asturia, Cantabria and Leon Provinces, NW Spain). These dolomites are excellently exposed and represent an important outcrop analogue for hydrocarbon reservoirs elsewhere. The dolomitization is discontinuous and irregular but has a remarkable spatial distribution in different tectonic units of the area. Dolomitization has no relation to local early diagenetic dolomitization. Replacive and void-filling dolomites formed and were often post-dated by calcite cementation. Each dolomite type is characterized by an ample range in ?O, on a regional scale, and by more constant ?C, buffered by the carbon signature of the precursor limestone. The Sr/Sr ratios are slightly but constantly more radiogenic than Cambrian or Carboniferous seawater values. The minor element composition (Sr, Na, Fe, Mn) is consistent with a burial origin of the dolomite. Fluid inclusion studies give more insight into the composition of the dolomitizing fluids. The low melting temperature of ice in fluid inclusions points to highly saline brines characterized by the presence of different cation species. Low temperature Raman spectroscopy confirmed the presence of hydrohalite and al least one other salt-hydrate phase. The halogen ratios are consistent with basinal brines derived from evaporated seawater, which has been modified through water/rock interaction. A regional, longlasting fluid flow can be hypothesized, affecting the Cantabrian Zone. The dolomites probably formed in Early Permian time during post-thrusting orocline formation in an extensional setting. Lithospheric delamination enabled increased heat flow and thermal convection of the fluids.