Jurassic Carbonate Research Articles

Cretaceous to Oligocene magmatic and tectonic evolution of the western Alaska Range: Insights from U-Pb and 40Ar/39Ar geochronology

AbstractNew U-Pb and 40Ar/39Ar ages integrated with geologic mapping and observations across the western Alaska Range constrain the distribution and tectonic setting of Cretaceous to Oligocene magmatism along an evolving accretionary plate margin in south-central Alaska. These rocks were emplaced across basement domains that include Neoproterozoic to Jurassic carbonate and siliciclastic strata of the Farewell terrane, Triassic and Jurassic plutonic and volcanic rocks of the Peninsular terrane, and Jurassic and Cretaceous siliciclastic strata of the Kahiltna assemblage. Plutonic rocks of different ages also host economic mineralization including intrusion-related Au, porphyry Cu-Mo-Au, polymetallic veins and skarns, and peralkaline intrusion-related rare-earth elements. The oldest intrusive suites were emplaced ca. 104–80 Ma into the Peninsular terrane only prior to final accretion. Deformation of the northern Kahiltna succession and underlying Farewell terrane occurred at ca. 97 Ma, and more widespread deformation ca. 80 Ma involved south-vergent folding and thrusting of the Kahiltna assemblage that records collisional accretion of the Peninsular-Wrangellia terrane and juxtaposition of sediment wedges formed on the inboard and outboard terranes. More widespread magmatism ca. 75–55 Ma occurred in two general pulses, each having distinct styles of localized deformation. Circa 75–65 Ma plutons were emplaced in a transpressional setting and stitch the accreted Peninsular and Wrangellia terranes to the Farewell terrane. Circa 65–55 Ma magmatism occurred across the entire range and extends for more than 200 km inboard from the inferred position of the continental margin. The Paleocene plutonic suite generally reflects shallower emplacement depths relative to older suites and is associated with more abundant andesitic to rhyolitic volcanic rocks. Deformation ca. 58–56 Ma was concentrated along two high-strain zones, the most prominent of which is 1 km wide, strikes east-northeast, and accommodated dextral oblique motion. Emplacement of widespread intermediate to mafic dikes ca. 59–51 Ma occurred before a notable magmatic lull from ca. 51–44 Ma reflecting a late Paleocene to early Eocene slab window. Magmatism resumed ca. 44 Ma, recording the transition from slab window to renewed subduction that formed the Aleutian-Meshik arc to the southwest. In the western Alaska Range, Eocene magmatism included emplacement of the elongate north-south Merrill Pass pluton and large volumes of ca. 44–37 Ma andesitic flows, tuffs, and lahar deposits. Finally, a latest Eocene to Oligocene magmatic pulse involved emplacement of a compositionally variable but spatially concentrated suite of magmas ranging from gabbro to peralkaline granite ca. 35–26 Ma, followed by waning magmatism that coincided with initiation of Yakutat shallow-slab subduction. Cretaceous to Oligocene magmatism throughout the western Alaska Range collectively records terrane accretion, translation, and integration together with evolving subduction dynamics that have shaped the southern Alaska margin since the middle Mesozoic.

Constraints of applying strontium isotope stratigraphy in coastal and shallow marine environments: insights from Lower Cretaceous carbonates deposited in an active tectonic setting (N Iberian Basin, Spain)

The Lower Cretaceous Leza Formation is an essentially carbonate unit deposited at the northernmost active margin of the Cameros Basin (N Spain) under an extensional tectonic regime. This unit is composed of freshwater, marine-influenced, marginal-marine and hypersaline marine carbonate facies, interbedded with variable amounts of alluvial deposits, mainly derived from the erosion of the Jurassic substrate. 87Sr/86Sr, δ18O and δ13C analyses were obtained from carbonate facies of the Eastern and Western sectors of the basin. δ18O values follow the expected trend in both sectors: they are more negative (down to − 7.9‰) in freshwater carbonates and more positive (up to + 2.8‰) in marginal-marine to hypersaline facies. However, independently of the seawater or freshwater influence, in the Western Sector the 87Sr/86Sr values (0.707373–0.707801) are significantly lower and closer to the published Lower Cretaceous seawater 87Sr/86Sr ratios, than those of the Eastern Sector (0.707988–0.709033), where the overall marine influence was relatively high and the alluvial input low. These data strongly suggest that 87Sr/86Sr ratios were mainly controlled by those of the riverine freshwater arriving to the coastal and marine areas after the weathering and erosion of the Jurassic carbonates or siliciclastic rocks, in the Western and Eastern sectors, respectively. Thus, data indicate that, in coastal and shallow marine carbonates, the influence of the riverine water on the 87Sr/86Sr ratios should be systematically evaluated. This is particularly necessary in active tectonic settings, where the uplifted areas are significantly prone to weathering and erosion and where alluvial fan systems commonly developed, eventually discharging into coastal and shallow marine areas.

Groundwater dating with dissolved organic radiocarbon: A promising approach in carbonate aquifers

A complete hydrogeological understanding of the deep Upper Jurassic carbonate aquifer in the South German Molasse Basin is essential for the future development of this important drinking water resource and geothermally used system.Water chemistry data, δ13CDIC, 14C of the dissolved inorganic carbon (14CDIC) and stable water isotope (δ18O and δD) measurements have been used to evaluate a promising groundwater dating approach with 14C of dissolved organic carbon (14CDOC). The pre-concentration of dissolved organic matter (DOM) was performed by the easy applicable solid phase extraction (SPE) with a styrene-divinylbenzene copolymer sorbent (PPL).Based on the sampling campaign of seven groundwater wells conducted between 2017 and 2019, it was shown that the groundwater is mainly of Ca–HCO3 type with some evidence of ion exchange between Ca2+ and Na+ at two of the investigated wells. The δD values ranged from −89.4‰ to −70.9‰ while δ18O values varied between −12.5‰ and −9.8‰. The obtained stable water isotope signatures indicated that the groundwater is of meteoric origin and was recharged during warm climate (Holocene), intermediate climate and cold climate (Pleistocene) infiltration conditions.The measured 14CDOC activities varied from 5.7 pmC to 51.1 pmC and the calculated piston-flow water ages (ORAs) ranged from 4200 years to 25,248 years using an initial 14C0DOC of 85 pmC. The calculated ORAs showed a very good correlation to the infiltration temperature-sensitive δ18O values which were affirmed with noble gas infiltration temperatures for two wells after Weise et al. (1991) and were also in good accordance with the atmospheric temperature record of the northern hemisphere from Dokken et al. (2015).The results reflect a consistent hydrogeological picture of the carbonate aquifer, which also supports the applicability of the SPE-PPL method for 14CDOC dating in groundwater with a low DOC content (<1 mg/l). In contrast, 14CDIC activities of 1.4 pmC to 21.3 pmC led to geochemically corrected piston-flow ages between 8057 years and >30,000 years and generally to an overestimation of the apparent water ages.This study gives insights into the promising approach of 14CDOC groundwater dating in carbonate aquifers with low DOC contents and allows future sustainable groundwater resource management of the investigated aquifer system.

Multiphase, decoupled faulting in the southern German Molasse Basin – evidence from 3-D seismic data

Abstract. We use three-dimensional seismic reflection data from the southern German Molasse Basin to investigate the structural style and evolution of a geometrically decoupled fault network in close proximity to the Alpine deformation front. We recognise two fault arrays that are vertically separated by a clay-rich layer – lower normal faults and upper normal and reverse faults. A frontal thrust fault partially overprints the upper fault array. Analysis of seismic stratigraphy, syn-kinematic strata, throw distribution, and spatial relationships between faults suggest a multiphase fault evolution: (1) initiation of the lower normal faults in the Upper Jurassic carbonate platform during the early Oligocene, (2) development of the upper normal faults in the Cenozoic sediments during the late Oligocene, and (3) reverse reactivation of the upper normal faults and thrusting during the mid-Miocene. These distinct phases document the evolution of the stress field as the Alpine orogen propagated across the foreland. We postulate that interplay between the horizontal compression and vertical stresses due to the syn-sedimentary loading resulted in the intermittent normal faulting. The vertical stress gradients within the flexed foredeep defined the independent development of the upper faults above the lower faults, whereas mechanical behaviour of the clay-rich layer precluded the subsequent linkage of the fault arrays. The thrust fault must have been facilitated by the reverse reactivation of the upper normal faults, as its maximum displacement and extent correlate with the occurrence of these faults. We conclude that the evolving tectonic stresses were the primary mechanism of fault activation, whereas the mechanical stratigraphy and pre-existing structures locally governed the structural style.

Sedimentological attributes of the Middle Jurassic peloids-dominated carbonates of eastern Tethys, lesser Himalayas, Pakistan

The carbonate factories of the tropical to sub-tropical regions characterize a number of facies, attributable to biological and oceanographic variables. For this purpose, the Middle Jurassic carbonate rock unit (Samana Suk Formation) of the Upper Indus Basin of Pakistan was studied to understand the various Tethyan platform attributes. Four sections from Hazara area were chosen which were also compared with a section from the Nizampur Basin. The resultant microfacies were placed in their paleoenvironmental settings to reveal the facies distribution and geometry of the platform. The platform represents a gently dipping ramp, dominated by peloidal–bioclastic–ooidal grainstones and peloidal lime mudstones. The allochems and fauna show some contrasting signatures regarding paleoclimate. These microfacies infer about existence of a relatively extended tropical climatic belt in the paleo Tethys during the Middle Jurassic. The notable presence of planktonic lime mudstone facies corresponds with global marine transgression, occurring in the middle of rock unit. The nature of ooids and types of fauna of sub-tropics to tropics advocate an extended warm climatic zone in comparison with today. The dominance of gently dipping inner-ramp shoals accompanied by lagoons to steep dipping outer-ramp planktonic lime mudstones restricted the development of least encountered middle-ramp facies. The ramp platform was struck by waves and tides of different strengths including low-middle amplitude storms and the overall absence of evaporites and primary dolostones designate a humid region as manifested by euryhaline conditions. The study has drawn paleoenvironmental and palaeoceanographic similarities with sections in Tanzania, western India, NW Iran, NW Afghanistan and Western Australia. The oceanic parameters compare with present day Trucial Coast of Persian Gulf, reflecting an almost similar nature of paleoceanography in the paleo Tethys.

Response of Upper Jurassic (Malm) Limestone to Temperature Change: Experimental Results on Rock Deformation and Permeability

The present study aimed to quantify the thermally induced deformation and the associated evolution of permeability in Blaubeuren limestone, an outcrop analogue of the Upper Jurassic (Malm) carbonate formation, providing references for hydro-thermo-mechanical responses of the reservoir rock to temperature changes within future enhanced geothermal systems as located in the Southern German Molasse Basin. Experiments deriving the changes in the primary (water-accessible) pore volume and the bulk volume of three water-saturated rock samples were conducted via loading the samples to defined stress states ( $${\sigma}_{3}$$ of 15 MPa; $${\sigma}_{1}$$ of up to 75 MPa ), respectively, and then cycling sample temperature between 30 °C and defined levels up to 120 °C at a sustained pore pressure of 0.5 MPa. Permeability was measured under isothermal conditions at each attained temperature. The primary voids dilated upon heating and partly contracted via cooling in each applied temperature cycle yielding thereby residual dilation. However, the concomitant bulk sample deformation manifested residual compaction. The permeability increased with increasing temperature and showed residual decreases by the end of the temperature cycling tests. Flow of pore fluid from the primary voids into the secondary voids at increased temperatures due to the decreased fluid viscosity mimicked irreversible dilation in the primary voids. The interplay between pressure solution-driven compaction and thermal expansion in rock solids was considered to account for the sample deformation and the development of permeability. It is presumable that the pressure solution-driven compaction and the contraction in rock solids would influence the long-term hydro-thermo-mechanical behavior of the Upper Jurassic carbonate reservoir rocks in the cooling process related to fluid injection at the geothermal systems located therein.

Investigating the impact of diagenesis on reservoir quality of the Jurassic shallow shelfal carbonate deposits: Kala Chitta Range, North Pakistan

The Middle Jurassic carbonate succession (Samana Suk Formation) in Kala Chitta Range, North Pakistan was studied in detail to investigate the impact of diagenesis on reservoir potential of the rock unit. The current study integrates detailed geological field, petrographic, scanning electron microscopic, and advance geochemical studies to reconstruct the diagenetic history of Jurassic carbonates and to evaluate its role in re‐shaping the reservoir potential. Based on field observations, five lithological units along with diagenetic features are recorded. Furthermore, various microfacies are defined within each lithological unit. Based on petrographic studies, it is observed that various diagenetic features including micritization, neomorphism, dissolution, compaction (mechanical and chemical), dolomitization, and cementation have played a crucial role in the porosity modification of various lithological units. The paragenetic sequence established in this study reveals modification of the carbonates in various diagenetic phases, for example, marine, meteoric, and burial (shallow and deep) diagenesis, followed by a phase of uplifting. Based on different types of microscopic porosities generated during various diagenetic phases, mudstone to wackestone carries good reservoir potential while the grainstone has moderate to poor potential. The plug porosity and permeability data further corroborate these findings. At outcrop, various large‐scale dissolution activities and dense network of inter‐connected fractures in all lithological units may have further enhanced the reservoir potential of the studied strata.

Diagenetic evolution and associated dolomitization events in the middle Jurassic Samana Suk Formation, Lesser Himalayan Hill Ranges, NW Pakistan

The Jurassic carbonates of the Samana Suk Formation are extensively exposed in the foreland areas of Himalayas and form major reservoir of the upper Indus basin. These carbonates are composed of oolitic, pelitic fossiliferous and micritic limestone units which have been extensively modified by diagenetic alterations, particularly dolomitization. Field observations show two distinct types of dolostone geobodies (i) bedding parallel stratiform, and (ii) patchy dolostone units respectively. Bedding parallel stratiform dolostones are present in the basal part of the formation, while patchy dolostones are present at the middle and upper parts of the Samana Suk Formation. The dolomitization intensity of both geobodies increases from NW to SE in the study area. Petrographic studies reveal six phases of dolomites and three phases of calcites based on texture, crystal size and morphology. These phases are: matrix replacive dolomites (MD-I to MD-III); cementing dolomites include, replacive cementing dolomite (RD), saddle cementing dolomite (SD) and late stage cementing dolomites (CD); and calcite phases include CC-I and CC-II. XRD analyses reveal that stratiform matrix dolomites (MD-I, MD-II) are stoichiometric (51.08–51.86 mol percent of CaCO3) and contain up to 95% of the mineral dolomite. The patchy dolomite cement is non-stoichiometric (33.39–55.08 mol% of CaCO3) and contains around 65% of the mineral dolomite, whereas saddle dolomites is also non-stoichiometric (51.57 to 53.50 mol% of CaCO3) in origin. Stable isotope studies reveal non-depleted δ18O and δ13C values of matrix dolomites (MD-I, MD-II) represents coeval sea-water signatures of Jurassic carbonates, hence may have been formed by evaporative process. Dolomite cements (RD, SD) shows depleted δ18O values which represent elevated temperature, related to hydrothermal fluid source for their formation. The fracture filling calcite (CC-II) exhibits less depleted values indicative of meteoric fluids affected by shallow to moderate burial. The dedolomites shows depleted δ13C values suggests their formation from the meteoric water. Field, petrographic and geochemical studies suggest that diagenetic evolution of the Samana Suk Formation is the multistage process. In the first phase, marine diagenetic processes including marine cementation, stratiform dolomitization may have formed due to surface processes of marine water in peritidal to intertidal settings, while the second phase of diagenesis is due to burial associated processes which includes hydrothermal dolomitization occurred due to movement of magnesium rich fluids along weak planes such as fractures, faults, bedding planes and stylolites. Last stage includes formation of hydrothermal fracture filling calcites, replacive pyrites and dedolomites due to the uplift related processes.

3D forward stratigraphic modelling of the Lower Jurassic carbonate systems of Kuwait

This paper describes the architectural and texture/facies evolution of a complex carbonate system through the construction of a regional scale 3D stratigraphic forward model (252 × 152 km2, 4 km × 4 km grid size) encompassing onshore and offshore Kuwait. The model mimics the development of a Jurassic carbonate ramp system at a 2nd to 3rd sequence stratigraphic order (1–80 Myr duration; Schlager, 2010) by simulating sedimentary processes and ecological parameters such as carbonate production versus depth, time, wave energy, as well as erosional and gravitational transport. Multi-scale 1D, 2D, and 3D geological and geophysical constraints have been used to calibrate the Lower Jurassic Marrat stratigraphic model, which resulted in the identification of (1) a dominantly transgressive lower unit composed of onlapping packages of mudstone to wackestone textures, with anhydritic intercalations; (2) a dominantly regressive middle unit composed of prograding inner ramp shoal and lagoonal facies with wackestone to grainstone textures, and finally (3) a transgressive onlapping upper unit composed of mudstones to wackestones textures in a mid-ramp system. The identification of potential reservoir facies like oncolitic shoals located onshore Kuwait, as well as possible porous reworked material transported away from the shoal loci in a relatively deeper mid ramp settings provides a better insight on further model improvement needed to de-risk stratigraphic trapping mechanisms. The identification of porous features in 3D provides crucial insights on the influence of ecological constraints on the complex Jurassic carbonate ramp systems of Arabia.

Sedimentary Facies Analysis, Reservoir Characteristics and Paleogeography Significance of the Early Jurassic to Eocene Carbonates in Epirus (Ionian Zone, Western Greece)

Sedimentological, micropalaeontological, and marine geological results from the Early Jurassic to Eocene carbonate formations of the Ionian zone, from six localities of Epirus, provide new insights into the basin palaeogeographic evolution and better correlation with coeval analogous tectono-stratigraphic successions along the southern margin of the Neo-Tethys Ocean. Facies analysis allowed the recognition of several microfacies types and their depositional characteristics. During the Early Jurassic, autochthonous carbonates (Pantokrator Limestones) were deposited in shallow-water environment. The overlying (hemi)pelagic Siniais or their lateral equivalent Louros Limestones were deposited to the basin borders and mark the general deepening of the Ionian domain. During Toarcian to Tithonian, the Ionian Basin was characterized by an internal differentiation in small sub-basins with half-graben geometry presenting abrupt thickness and facies changes. The deeper parts were characterized by continuous sedimentation, while the elevated parts were marked by unconformities. The Early Cretaceous marks the homogenization of sedimentation by the deposition of the pelagic Vigla Limestones all over the Ionian zone. The transition from the Early to Late Cretaceous records a significant carbonate diversification in terms of biota assemblages, and related mineralogy due to intense tectonic activity in the region. From Late Cretaceous to Paleogene, allochthonous carbonates were transported to the outer shelf by turbidity currents (calciturbidites) and/or debris flows (limestones with breccia) formed by the gravitational collapse of the platform margin. Additional porosity and bulk density measurements showed that petrophysical behavior of these carbonates are controlled by the depositional environment and further influenced by diagenetic processes. The partly dolomitized neritic Jurassic carbonates, but mainly the Senonian calciturbidites and the microbrecciated Paleocene/Eocene limestones display the higher average porosity values, and therefore present enhanced carbonate reservoir quality.

De-risking hydrocarbon exploration in lower Jurassic carbonate systems of Kuwait through forward stratigraphic models

This paper discusses the results of an integrated case study using forward stratigraphic modelling of Lower Jurassic complex carbonate reservoirs along the Eastern Arabian Plate. Traditional stochastic or deterministic modelling techniques do not sufficiently capture carbonate reservoir heterogeneities in sectors with restricted datasets. The methodology applied uses a process-based approach that aims to define carbonate architectural and textural/facies heterogeneity and provides a framework to develop a more consistent conceptual geological model for exploration and field development.Through this communication we test the sensitivity of Lower Jurassic reference case model of complex carbonate systems (Al Wazzan et al., 2020) to various framework and sedimentological/ecological parameters as subsidence, eustacy, carbonate production rates, wave energies and evaporation versus precipitation rates. Results of this work help achieve a more refined and data calibrated model that tackles mainly the 3rd and 4th order sequence stratigraphic evolution of the Marrat Formation. Simulation results point to the main impact of differential subsidence and eustatic sea level changes on the architectural and textural variability along Kuwait with dominantly (1) transgressive onlapping packages of intercalated mudstone/wackestone and anhydritic textures in the Lower Marrat, (2) prograding to aggrading packages in the Middle Marrat localized around the southern, northern, northwestern and westernmost part of onshore Kuwait, and (3) an onlapping transgressive system composed of wackestone and lime mudstone with very restricted shoal patches.This work also highlights (1) the good fit with overall tendencies of the recently published Haq (2018) Jurassic global relative sea level curves, (2) the need to reduce the latter amplitudes of high frequency relative sea level drops to fit the G&G dataset, (3) the good anhydritic prediction using evaporation and precipitation predictions of the General Circulation Models (GCMs) (Sellwood and Valdes, 2006) and finally (4) the need for about 20–30% of additional differential subsidence in zones of thinner continental crust. Finally, a more geologically plausible prediction of the spatial and temporal extent of petroleum systems elements unraveled a new under-explored hydrocarbon potential in West and Central Kuwait.

Geochemical and Dynamic Model of Repeated Hydrothermal Injections in Two Mesozoic Successions, Provençal Domain, Maritime Alps, SE-France

A field, petrographic and geochemical study of two Triassic–Jurassic carbonate successions from the Maritime Alps, SE France, indicates that dolomitization is related to episodic fracturing and the flow of hydrothermal fluids. The mechanism governing hydrothermal fluids has been documented with the best possible spatio-temporal resolutions specifying the migration and trapping of hydrothermal fluids as a function of depth. This is rarely reported in the literature, as it requires a very wide range of disciplines from facies analysis (petrography) to very diverse and advanced chemical methods (elemental analysis, isotope geochemistry, microthermometry). In most cases, our different recognized diagenetic phases were mechanically separated on a centimetric scale and analyzed separately. The wide range of the δ18OVPDB and 87Sr/86Sr values of diagenetic carbonates reflect three main diagenetic realms, including: (1) the formation of replacive dolomites (Type I) in the eogenetic realm, (2) formation of coarse to very coarse crystalline saddle dolomites (Types II and Type III) in the shallow to deep burial mesogenetic realm, respectively, and (3) telogenetic formation of a late calcite cement (C1) in the telogenetic realm due to the uplift incursion of meteoric waters. The Triassic dolomites show a lower 87Sr/86Sr ratio (mean = 0.709125) compared to the Jurassic dolomites (mean = 0.710065). The Jurassic calcite (C1J) shows lower Sr isotopic ratios than the Triassic C1T calcite. These are probably linked to the pulses of the seafloor’s hydrothermal activity and to an increase in the continental riverine input during Late Cretaceous and Early Cenozoic times. This study adds a new insight into the burial diagenetic conditions during multiple hydrothermal flow events.

Microfacies analysis, diagenetic overprints, geochemistry, and reservoir quality of the Jurassic Samanasuk Formation at the Kahi Section, Nizampur Basin, NW Himalayas, Pakistan

In Pakistan, carbonate rocks of Jurassic age are present entirely throughout the Indus Basin. The Jurassic carbonate rocks present in the Attock-Cherat requires a more detailed investigation as of recent discovery of gas in Jurassic carbonate rocks in the lower Indus basin. The evaluation of these Jurassic carbonate units in the Nizampur Basin of Attock-Cherat ranges should integrate the aspects of the depositional sediments, diagenetic history, and geochemistry to understand the reservoir behavior. In this study, the Samanasuk Formation of 90 m was sampled and measured in detail, and samples were collected with ~ 0.3 m interval and some samples were collected where reckoned necessary. Petrographic and microfacies analysis were conducted on about 100 samples; moreover, XRD analysis were performed. The present study aims to determine the depositional facies, diagenetic processes, and geochemical elements of this carbonate succession of the in an effort to explore their effect on reservoir quality. The Samanasuk Formation comprises eight microfacies assigning three facies belts including peritidal, lagoon, and shoal of a carbonate ramp. The recorded diagenetic processes include dolomitization, compaction, micritization, neomorphism, dissolution, and cementation in which dolomitization played an important role in enhancing the reservoir quality. In relation of their impacts on reservoir properties, the grainstone facies associated with peritidal facies and dolomudstone facie associated with carbonate sand and shoal shows the greatest reservoir quality, whereas lagoonal facies has the lowest reservoir quality. This study represents an approach to use the depositional facies, diagenetic alterations, and geochemical framework of carbonate succession in the reservoir characterization.

Volcanic rocks in Lower Jurassic marine carbonate successions in the southeastern Iberian Range (Spain): biochronostratigraphic dating

The Ligurian Tethys began to open in a roughly NE–SW direction in Pliensbachian times. West of the Ligurian oceanic crust, a carbonate-platform system constituted the eastern Iberian paleomargin. Lower and Middle Jurassic marine sediments deposited in these shallow platforms are well exposed today in the southeastern Iberian Range and host multiple volcanogenic deposits, mainly volcaniclastic in nature. Regional mapping reveals that volcanic outcrops show NW and NE (or NNE) oriented lineaments following tracks of cortical weakness. Volcanic ages were estimated by using biochronostratigraphic methods and sequence-stratigraphic correlations. As a result, ten Lower Jurassic volcanic episodes have been identified. The chronostratigraphic positions of the volcanic deposits span from the early Pliensbachian (Jamesoni Zone or later) to the late Toarcian (Thouarsense Zone). The extension-dominated regime of the eastern Iberian paleomargin also affected the southern Iberian paleomargin (Betic Cordillera) as a result of the eastward prolongation of the central Pangea fragmentation. The break-up of wide platforms and the first volcanic emissions seem to be roughly coeval in the Betic and Iberian domains and synchronous with the onset of rifting and spreading events of the Ligurian Tethys Ocean.

Demise of the Middle Jurassic carbonate platform: Responses to sea‐level fall in the Qiangtang Basin, Eastern Tethys

Many studies have been focused on the development of carbonate platforms, however, the controlling factors of their evolution are still unclear. The QTKT1 well, drilled in 2016 by the Chengdu Center of the China Geological Survey at the Northern Qiangtang Depression, has allowed further studies of Jurassic environments and resources in the Eastern Tethys. In this study, we investigate variations in spectral gamma‐ray logs and CaCO3 content and perform a cyclostratigraphic analysis in the Xiali mudstones and Buqu limestones and Xiali mudstone to analyse causes for the demise of the Middle Jurassic carbonate platform in the Qiangtang Basin. The concentrations of Th, K, U, and Th/U and Th/K ratios increase suddenly from the Buqu Formation to the Xiali Formation, suggesting increased terrigenous input and/or decreased carbonate production rates. Cyclostratigraphic analysis detects the existence of astronomical cycles and an abrupt change in sediment accumulation rates within the target intervals. Combined with sedimentary facies analysis, our research suggests that the demise of the Middle Jurassic carbonate platform may have been caused by terrigenous detrital inputs into the Qiangtang Basin. High‐frequency sea‐level fluctuations increased terrestrial inputs and carbonate production rates at the orbital scale. The regression happened before the facies change and controlled terrestrial inputs at a larger scale, which resulted in the demise of the carbonate platforms. Contrary to the Western Tethys, a regression took place from the Bathonian to Callovian ages in the Qiangtang Basin, Eastern Tethys, which increased the exposure levels. The large exposure led to significant erosion, accelerated detrital input from the land to the Northern Qiangtang Depression, reduced carbonate production as well as resulted in the demise of the Buqu carbonate platform. The relative sea‐level fall is mainly the result of the regional tectonism during this period.

Interacción entre vulcanismo, tectónica y sedimentación en una plataforma carbonatada somera: Ejemplo en el Tethys occidental (Jurásico Medio, sureste de la Cordillera Ibérica)

En el sureste de la Cordillera Ibérica (España) existen una serie de afloramientos de rocas volcánicas origi­nadas por erupciones submarinas, principalmente de tipo volcanoclástico, interestratificadas en las sucesiones carbonatadas del Jurásico Inferior y Medio. La cartografía geológica de los afloramientos de rocas volcánicas revela que se encuentran alineados a lo largo de tres elementos de debilidad cortical bien definidos (zonas de falla de Teruel, de Caudiel y de Alcublas), originados probablemente por fracturas relacionadas con la apertura y expansión del Tethys Ligur y del Tethys Alpino. En este trabajo se presentan los resultados del estudio de la evolución tectonosedimentaria de un ambiente submarino somero afectado por la presencia de una acumulación volcánica que tuvo lugar en el Jurásico Medio (límite Aaleniense‒Bajociense). Se trata del afloramiento volcánico de Caudiel, que es el más suroriental de todos los afloramientos localizados en uno de los tres elementos de debilidad estructural mencionados: la Zona de Falla de Caudiel. La presencia de un relieve positivo en el fondo de la plataforma, ya de por sí bastante somera, condiciona la sedimentación carbonatada, muy sensible a la pro­fundidad, hasta el punto de llegar a inducir cambios significativos de facies con respecto a otras áreas cercanas o de impedir la sedimentación. Los movimientos episódicos del substrato generaron una topografía irregular y cambiante a lo largo del tiempo, con tasas variables de hundimiento y levantamiento en diferentes áreas. El aflo­ramiento de Caudiel constituye un buen ejemplo fósil de interacción entre varios factores (vulcanismo, tectónica y sedimentación) en plataformas carbonatadas someras, en las que los cambios de acomodación y las tasas de sedimentación parecen estar subordinadas a perturbaciones tectónicas de carácter local.

Danian-lower Selandian Microcodium-rich calcarenites of the Subbetic Zone (SE Spain): Record of Nereites ichnofacies in a deep-sea, base-of-slope system

The Olivares Formation, a lower Paleogene unit of the Subbetic Zone in Southern Spain, is mostly composed of calcarenites comprising 60–90% of resedimented Microcodium remains, which are intercalated in hemipelagic marls rich in planktonic foraminifera. Microcodium is formed on the roots of terrestrial plants adapted to live on carbonate-rich substrates, its occurrence in the Olivares Formation entailing a massive resedimentation from a subaerially exposed carbonate source area. New field observations demonstrate that the unit was deposited, at the base of a faulted slope, within a graben flanked by massifs of mostly Jurassic carbonates, while an ichnological analysis shows an abundant and moderately diverse trace fossil assemblage consisting of 30 ichnospecies belonging to 21 ichnogenera. The nongraphoglyptid assemblage (24 ichnospecies, 16 ichnogenera) is dominated by Chondrites, Ophiomorpha, Planolites, and Scolicia. Zoophycos isp. is common, while Cladichnus, Gordia, Helminthopsis, Multina, Nereites, Parahaentzscheliana, Polykampton, Spirophycus, Strobilorhaphe and Thalassinoides isp. are scarce or rare. In the graphoglyptid assemblage (6 ichnospecies, 5 ichnogenera), Helminthorhaphe is common/scarce and Belorhaphe, Desmograpton, Paleodictyon and Urohelmintoida are scarce/rare. This trace fossil assemblage, with abundant Scolicia and Nereites and frequent graphoglyptids, can be assigned to the deep-seaNereites ichnofacies. Within the Nereites ichnofacies, the Ophiomorpha rudis and Paleodictyon subichnofacies are readily differentiated, and the Nereites subichnofacies probably also occurs, pointing to vertical changes in the turbiditic system most likely related to a coeval sea-level rise that eventually resulted in the flooding of the emergent Microcodium-producing carbonate massifs.

Multidisciplinary Study of Subsidence and Sinkhole Occurrences in the Acque Albule Basin (Roma, Italy)

AbstractWe present the results of a combined analysis of remote sensing and geophysical‐geotechnical data carried out in the Acque Albule Basin, a sinkhole prone area located close to the city of Roma, where a wide travertine wedge is present. We carried out geophysical measurements and borehole drillings over two test areas to image the subsoil where paroxysmal surficial dynamics occur. One site is marked by subsidence occurring at least since the early 2000s, whereas the other site hosts the “La Regina” and “Colonnelle” sinkhole lakes, which discharge sulfur‐carbonated waters. The stability of these two sites threatens highway, railway, and airport facilities, and this study helps to assess the geological hazard. For example, InSAR and LiDAR data helped define wide scale subsidence over the last 20 years and previously undetected small‐scale morphologies. Geophysical measurements of the latter revealed shallow and deep dissolution affecting the travertine and driving surficial paroxysmal events. Both study sites were found to lie inside a large depression located at the junction between Jurassic carbonate and Plio‐Pleistocene units in association with paleo karst morphologies in the travertine deposits and affected by the present‐past spillage of sulfurous waters. Given these elements, multidisciplinary geophysical observations are crucial for assessing and mitigating the geological risk and guiding land use planning and management.

Upper Jurassic carbonate buildups in the Miechów Trough, southern Poland – insights from seismic data interpretations

Abstract. The geometry and internal architecture of the Upper Jurassic carbonate depositional system in the epicontinental basin of central and western Europe and within the northern margin of the Tethyan shelf are hitherto only partly recognized, especially in areas with thick Cretaceous and younger cover such as the Miechów Trough. In such areas, seismic data are indispensable for the analysis of a carbonate depositional system, in particular for the identification of the carbonate buildups and the enveloping strata. The study area is located in the central part of the Miechów Trough that in the Late Jurassic was situated within the transition zone between the Polish part of the central and western European epicontinental basin and the Tethys Ocean. This paper presents the results of the interpretation of 2D seismic data calibrated by deep wells that document the presence of large Upper Jurassic carbonate buildups. The lateral extent of particular structures is in the range of 400–1000 m, and their heights are in the range of 150–250 m. The interpretation of seismic data revealed that the depositional architecture of the subsurface Upper Jurassic succession in the Miechów Trough is characterized by the presence of large carbonate buildups surrounded by basinal (bedded) limestone and marly-limestone deposits. These observations are compatible with depositional characteristics of well-recognized Upper Jurassic carbonate sediments that crop out in the adjacent Kraków–Częstochowa Upland. The presented study provides new information about carbonate open-shelf sedimentation within the transition zone in the Late Jurassic, which proves the existence of a much more extensive system of organic buildups which flourished in this part of the basin. The results obtained, due to the high quality of available seismic data, also provide an excellent generic reference point for seismic studies of carbonate buildups in other basins and of different ages.

Seismic Characteristics and Development of the Upper Jurassic Carbonate Buildups from the Miechów Trough (Southern Poland)

The presented study is devoted to the subsurface Upper Jurassic carbonate buildups and surrounding stratified inter-buildup deposits in the hitherto less recognized area, in comparison with other parts of the northern Tethyan shelf in Poland and Europe. The study area is located within the present-day Miechów Trough, almost entirely covered by thick Cretaceous and younger deposits. This paper shows results of the interpretation of 2D seismic data, calibrated by data from deep wells. Investigation of various elements of the Upper Jurassic carbonate depositional system in the Miechów Trough is supported by seismic facies and attribute analysis. The four distinctive seismic facies—(A) bedded, (B) mound-shaped, (C) contorted-chaotic, and (D) chaotic—were assigned to the main Upper Jurassic sedimentary facies, represented by (1) bedded facies, (2) massive facies (carbonate buildups) and (3) deposits of gravity mass-flows. The results of this study were used to construct a depositional model for the Upper Jurassic succession, that focuses on the initiation, growth and demise of the large carbonate buildups in this part of the basin. This paper also presents the more extensive distribution of the Upper Jurassic carbonate buildups than was previously proposed for the Miechów Trough.