Cenomanian Santonian Research Articles

Paleoclimate changes across the Cenomanian–Santonian transition at the southern Neo-Tethys margin (SW Iran): An integrated approach

The Cenomanian–Santonian (C–S) carbonate successions include valuable petroleum reservoirs in the Zagros Basin. They also provide important information on the paleogeographic setting, paleoclimatic conditions, and tectonic evolution of the southern Neo-Tethys margin. This study focuses on the paleoclimatic changes across the C–S transition using the petrographical, petrophysical, paleontological, major and trace element and isotope geochemical data from three subsurface sections selected from the southern Dezful Embayment and northwestern part of the Persian Gulf. Remarkable differences in allochemical (skeletal and non-skeletal) elements indicate dissimilar paleoclimatic conditions during the Cenomanian–Turonian and Santonian. Paleoexposure events are related to diagenetic processes such as extensive meteoric dissolution (karstification) and the development of bauxitic–lateritic paleosols with high Fe, Al, and kaolinite concentrations indicating warm and humid (tropical) conditions. Increased values of Fe, Al, Rb, and 87Sr/86Sr isotopic ratios and depletion of Ca, Sr, δ18O, and δ13C considered together indicate meteoric diagenetic effects under this paleoclimatic condition. Spectral gamma measurements are used to investigate paleoclimatic conditions. Low content of Uranium (U) throughout the transgressive systems tract (TST) and high concentration in the regressive systems tract (RST) along with low contents of kaolinite within the Santonian sequence denote the predominance of arid conditions. The major fall in U concentrations at the Cenomanian–Turonian interface (CT-Exposure Surface) could be regarded as an outcome of the fall in the relative sea-level and a proxy of oxidizing conditions (Th/U > 7) that could be the consequence of a change in paleoclimate from humid/semi-humid to humid. Evidence for a 3–4 °C decrease in paleo-temperature and increase in aridity are recorded during the Cenomanian–Santonian transition in the Zagros Basin, which is in close agreement with global trends. This cooling pattern is attributed to a decline in atmospheric CO2 levels and alterations in oceanic circulation, specifically an increase in meridional circulation.

A new armored dinosaur with double cheek horns from the early Late Cretaceous of southeastern China

Ankylosaurines are the iconic armoured dinosaurs that characterize terrestrial vertebrate faunas in the Late Cretaceous of Asia and Laramidia (western North America). The earliest members of this clade are known from the early Late Cretaceous (Cenomanian–Santonian) times of Asia, but little consensus has emerged as to how they are related to the anatomically derived and chronologically younger forms. In southeastern China, the Cretaceous red sand beds crop out across basins from Zhejiang to Guandong provinces. However, the horizons corresponding to the early Late Cretaceous stages remain poorly sampled. Here, we report the first definitive vertebrate skeleton ¾ let alone that of an armoured dinosaur ¾ from the Coniacian/Turonian Ganzhou Formation, Datai yinliangis gen. et sp. nov. Despite the immature ontogenetic status of the type materials, D. yingliangis can be diagnosed with autapomorphic traits in the cranial caputegulae (such as double horns on the quadratojugal) and extensive gular osteoderms. Morphologically, it is intermediate between the chronologically older ankylosaurids from Asia (e.g., Crichtonpelta and Jinyunpelta) and derived post-Cenomanian ankylosaurines (e.g., Pinacosaurus). Phylogenetic analyses broadly corroborate this assessment. The new taxon either falls in the grade of Asian ankylosaurines proximal to the lineages of derived forms or forms a sister lineage to Pinacosaurus. Based on these insights, Datai makes a significant addition to the early Late Cretaceous vertebrate fauna from southeastern China and highlights the future potential in this region for improved understanding of the origin and early evolution of ankylosaurines.

Sequence stratigraphical and palaeoenvironmental implications of Cenomanian–Santonian dinocyst assemblages from the Trans‐Sahara epicontinental seaway: a multivariate statistical approach

AbstractThe Cretaceous was punctuated by episodic flooding of continental margins forming epicontinental seas. The Trans‐Sahara Seaway was one of these epicontinental seas, connecting the Gulf of Guinea with the Tethys Ocean. In this study, data including microplankton abundances, stable carbon isotopes of organic material and elemental geochemistry were integrated with traditional sedimentological analyses from the Trans‐Sahara Seaway. The carbon isotopic data provide the first evidence that oceanic anoxic event 2 was present in the Trans‐Sahara Seaway, and palynology shows it was associated with an increase in peridinioid dinocyst abundance. A combined study of microplankton assemblages and sedimentology reveals palaeoenvironmental trends linked to sea‐level change. Lowstand system tracts were characterised by increased siliciclastic grain size, low microplankton diversity, and were dominated by Chlorophyceae. Transgressive system tracts were associated with diversity increases during rising sea level, with open marine gonyaulacoid dinocysts dominating the assemblages. Maximum flooding surfaces were recognised by the highest increase in biological diversity in argillaceous deposits. As sea level started to fall, the peridinioid dinocysts became dominant, with decreased microplankton diversity during highstand systems tracts. This combination of sedimentology and interpretation of dinocyst assemblages allows the identification of shallow to deeper marine depositional sequences of Cenomanian–Santonian strata within the Yola Sub‐basin. This approach could be used to delineate marine depositional sequences where using conventional sedimentological methods alone is very challenging.

Discrete fracture modeling by integrating image logs, seismic attributes, and production data: a case study from Ilam and Sarvak Formations, Danan Oilfield, southwest of Iran

Understanding the fracture patterns of hydrocarbon reservoirs is vital in the Zagros area of southwest of Iran as they are strongly affected by the collision of the Arabian and Iranian plates. It is essential to evaluate both primary and secondary (fracture) porosity and permeability to understand the fluid dynamics of the reservoirs. In this study, we adopted an integrated workflow to assess the influence of various fracture sets on the heterogeneous carbonate reservoir rocks of the Cenomanian–Santonian Bangestan group, including Ilam and upper Sarvak Formations. For this purpose, a combination of field data was used including seismic data, core data, open-hole well-logs, petrophysical interpretations, and reservoir dynamic data. FMI interpretation revealed that a substantial amount of secondary porosity exists in the Ilam and Sarvak Formations. The upper interval of Sarvak 1-2 (3491 m to 3510 m), Sarvak 1-3 (3530 m to 3550 m), and the base of Sarvak 2-1 are the most fractured intervals in the formation. The dominant stress regime in the study area is a combination of compressional and strike-slip system featuring reverse faults with a NW–SE orientation. From the depositional setting point of view, mid-ramp and inner-ramp show a higher concentration of fractures compared to open marine environment. Fracture permeability was modeled iteratively to establish a realistic match with production log data. The results indicate that secondary permeability has a significant influence on the productivity of wells in the study area.

Geochemical expression of sequence stratigraphic surfaces: A case from Upper Cretaceous shallow-water carbonates of southeastern Neo-Tethys margin, SW Iran

Providing a chronostratigraphic framework often is problematic in shallow-marine carbonates with poor biostratigraphic resolution. In such cases, an integration of geochemical, sedimentological, and paleontological data can be used to construct a reliable sequence stratigraphic framework. This study focuses on elemental concentrations and isotopic ratios of Cenomanian–Santonian (C–S) neritic carbonates in SW Iran used to construct a sequence stratigraphic framework, asses sequence ages and durations of hiatus. The diagenetic and geochemical expression of two paleoexposure surfaces, representing type-I sequence boundaries (SBs), are discussed. Increase in Fe and Rb concentrations and decrease in Ca content are recorded at and/or below distinct erosional surfaces. Mg content shows a facies-dependent response with former LMC (low magnesium calcite) or aragonitic facies showing increase while HMC (high magnesium calcite) indicates decrease in response to SBs. Mn shows considerable changes apart from sequence surfaces that are strongly facies dependent. Sr concentrations show an increase in HMC and decrease in LMC in samples which experienced semi-closed meteoric diagenesis, far below the SBs. However, in samples characterized by open system diagenesis, directly underlying the SBs, Sr tends to decrease in all components. The Sr/Rb ratio is a reliable tool in detecting erosional SBs, shown as conspicuous negative excursions. C (carbon) and O (oxygen) isotopes show distinct negative excursions at erosional SBs and a slight increase at or around the MFSs. The 87Sr/86Sr ratio shows clear positive excursions at SBs probably due to 87Rb decay in incorporated aluminosilicate detritus, which makes it as the most reliable proxy for the detection of SBs in the studied sections. Two main erosional SBs are marked by 87Sr/86Sr ratios including the CT-ES (Cenomanian–Turonian boundary exposure surface) and mT-ES (middle Turonian exposure surface) with hiatus durations of 0.53 and 2.7 Myr, respectively. These paleoexposure surfaces are correlated with other neritic sections of C–S sequences in SW Iran.

Crocodyliform remains from the Upper Cretaceous of Central Asia – evidence for one of the oldest Crocodylia?

The Late Cretaceous is a key time interval for the understanding of origins and diversification of the crown-group Crocodylia and its extant lineages – Alligatoroidea, Crocodyloidea, and Gavialoidea. Yet, the fossil record of crocodylians in the beginning of the Late Cretaceous is scarce, obscuring the early evolutionary history of the clade. Here I review all known material of long-snouted crocodyliforms from the Upper Cretaceous (Cenomanian–Santonian) of Central Asia. Based on the extensive sampling in the Dzharakuduk locality (Bissekty Formation) and the distinctive anatomy of individual elements, they are referred to the enigmatic taxon Zholsuchus procerus Nessov et Cherepanov, 1989. Comparable specimens from other localities are tentatively attributed to Zholsuchus sp. or cf. Zholsuchus sp. Zholsuchus is herein confirmed as a valid taxon for the first time since its establishment. Based on extensive morphological comparison, I hypothesize that it is potentially a crown-group crocodylian that may be related to Gavialoidea. However, additional material is required to test taxonomic and phylogenetic hypotheses proposed in this study. Zholsuchus is allegedly a medium-sized crocodylian, attaining minimal total body lengths of 2 m and perhaps ranging up to 2.5–3 m. Remains associated with this taxon are rare but consistently recovered in the lower Cenomanian–Santonian deposits of Uzbekistan and Kazakhstan. Zholsuchus, along with Portugalosuchus from Cenomanian of Portugal, may be the oldest presently known crocodylians, which expands the fossil record of the crown-group back into the beginning of the Late Cretaceous and suggests the much earlier origins of the clade than it has been usually acknowledged.

Age, duration, and geochemical signatures of paleo-exposure events in Cenomanian–Santonian sequences (Sarvak and Ilam formations) in SW Iran: Insights from carbon and strontium isotopes chemostratigraphy

This study addresses the timing implications of chemical stratigraphy and chronostratigraphic correlation of unconformity-related shallow water (neritic) carbonate rock units. Cenomanian–Santonian sequences (i.e., Sarvak and Ilam formations) host considerable volumes of hydrocarbon in the Zagros region of SW Iran, which represent good examples of unconformity-related reservoirs worldwide. Geochemical traits including carbon, oxygen, and strontium isotopes of these sequences are measured in three wells from the Abadan Plain and one well from the Dezful Embayment. The altered intervals below the exposed erosional surfaces are characterized by remarkable variations in both elemental and isotopic compositions. These variations are marked by increasing in Mg, Al, Fe, Mn, and Rb elemental contents as well as decreasing in Sr and Ca elemental contents. The altered intervals show a positive shift in 87Sr/86Sr isotopic values and a negative trend in δ13Ccarb and δ18Ocarb isotopic values. The paleontological age that is available from previous studies provide a valuable basis for numerical dating of the studied intervals using the rubidium-corrected 87Sr/86Sr isotopic values. The duration of the stratigraphic hiatuses at the Cenomanian–Turonian and mid-Turonian paleo-exposures are calculated as 0.53 Myr and 2.7 Myr, respectively. The calculated durations corroborate the results of previous studies of these paleo-exposures within the Zagros region of western Iran. The concluded (carbon and Sr-isotope) chemostratigraphic correlation of the Cenomanian–Santonian sequences in the Abadan Plain and other parts of the Zagros region can provide a more reliable chronostratigraphic framework of larger scale. The latter can be used in regional correlation and interpretation of the geological-structural evolution of the Zagros region during Cenomanian–Santonian, particularly in western Iran.

The first benthic insects (Ephemeroptera and Coleoptera) from the Upper Cretaceous of Colombia

The Upper Cretaceous fossil record of insects in northern South America is almost unknown. Here we describe two fossil insects from the Chipaque Formation of (Cenomanian–Santonian), from central Colombia, representing the first report of fossil arthropods for the Upper Cretaceous of Colombia, and the first fossil record of a mayfly and beetle for northern South America. Although the specimens lack key anatomical features for a high-resolution taxonomic identification, we were able to attribute one of the specimens to Ephemeroptera and a second specimen to Coleoptera. This fossil finding opens the possibility for future discoveries of additional insect fossils in northern South American rock sequences that could fill the extensive gap that exists in this paleontological field.

The South American and Antarctic Peninsula fossil record of Salviniales (water ferns): Its implication for understanding their evolution and past distribution

We compiled the fossil record of the heterosporous water ferns (Salviniales) including macro- and microfossils from South America and the Antarctic Peninsula. Both extant families, Marsileaceae and Salviniaceae, are well represented and several fossil spore genera that cannot be placed within the extant families are included as Incertae sedis. Marsileaceae is first recorded in the Middle to Late Jurassic. Incertae sedis genera are recorded for first time in the Early Cretaceous while Salviniaceae in the Late Cretaceous. Two diversity spikes are recognized: one spanned the Aptian–Albian (Early Cretaceous) and is linked to an increase in species diversity within Marsileaceae; and the other occurred during the Campanian–Maastrichtian (Late Cretaceous) and it is associated with the diversification at generic level of the entire Salviniales. Two decreases in diversity are recognized: one during the Cenomanian–Santonian and affected Marsileaceae at specific level but not its generic diversity, and the second is related to the Cretaceous–Paleogene mass extinction and affected all Salviniales at generic and specific levels. From the Paleocene onwards there is a steady decline in the fossil record of the group, with most remains belonging to the extant genera. This study suggests that Southern Hemisphere aquatic ferns underwent a sharp radiation during the Late Cretaceous and a deep decline in the Paleocene that parallels that occurred in the Northern Hemisphere. Probably, the temporal co-occurrence of geological and climate events and the availability of suitable lineages promoted the evolutionary changes of the water ferns in the studied area, including the high diversification and distribution in the Cretaceous and the subsequent post-Maastrichtian decline.

Petrophysical rock typing (PRT) and evaluation of Cenomanian–Santonian lithostratigraphic units in southwest of Iran

Southern Iran is one of the most significant parts of the world in terms of economic aggregation of hydrocarbons. In this paper, reservoir quality of the Cenomanian–Santonian deposits has been evaluated in southwestern Iran. According to the reservoir qualification, five particular rock types have been classified based on log responses through K-nearest neighbors (KNN) method. 1–5 are representing poor, weak, moderate, superior and non-reservoir types of rock, respectively. Rock types 3 and 4 are determined as the best types which can play a suitable reservoir role in hydrocarbon accumulation. The mentioned groups generally consist of porous limestones with low level of clay content. Rock types number 1 and 2 indicate the low porosity tight limestones and rock type 5 belongs to clays and shaly layers with high gamma radiations. Lithology of these intervals is specified using Rhomaa–Umaa and NPHI–RHOB crossplots which displayed calcite mineral is the main mineral formed both formations. SEM analysis and Th–K crossplot confirmed that Illite, Montmorillonite and Kaolinite are major clay minerals settled in these intervals that may have impact on reservoir characteristics. Interparticle and micro-porosities are considered as dominant kinds of porosities based on velocity deviation log (VDL) and it is proved by SEM results. Rock types 3 and 4 have been considered as the most productive zones which can be the best target for future perforating operation. According to petrophysical evaluation results, Ilam (Santonian) reservoir quality is enhanced toward northeast and southwest (Maximum abundance of rock type 4) of the studied area in comparison to central part. The Sarvak (Cenomanian–Turonian) Formation is in the highest level of reservoir quality in the southwest of the studied area (abundance of rock types 4 and 3).

Controls on gravity-driven normal fault geometry and growth in stacked deltaic settings: a case study from the Ceduna Sub-basin

Kinematics of gravity-driven normal faults exerts a critical control on petroleum systems in deltaic settings but to date has not been extensively examined. The Ceduna Sub-basin (CSB) is a passive margin basin containing the White Pointer (Albian-Cenomanian) and Hammerhead (Campanian-Maastrichtian) delta systems that detach on shale layers of Albian-Cenomanian and Turonian-Coniacian ages, respectively. Here we present evidence for spatially variable fault growth styles based on interpretation of the Ceduna 3D seismic survey and fault kinematic analyses using displacement–distance, displacement–depth and expansion index methods. We identified faults that continuously grew either between the Cenomanian–Santonian or Santonian and the Maastrichtian located throughout the study area and faults that exhibit growth between the Cenomanian–Maastrichtian that are geographically separated into three areas according to their evolution histories: (i) Northern CSB faults exhibit constant growth between the Cenomanian and Maastrichtian. (ii) Central CSB faults show two dip-linkage intervals between (a) Cenomanian and Coniacian–Late Santonian, (b) Coniacian–Late Santonian and Late Santonian–Maastrichtian segments, respectively. (iii) Central and southern CSB faults exhibit dip-linkage intervals between Cenomanian–early Santonian and Late Santonian–Maastrichtian segments. Our study demonstrates a relationship between the location of the Cenomanian–Maastrichtian faults and their evolution history suggesting constant growth evolution at north and dip linkage at the central and south areas.

Laramide Orogenesis Driven by Late Cretaceous Weakening of the North American Lithosphere

AbstractThis paper investigates the causes of the Late Cretaceous transition from “Sevier” to “Laramide” orogenesis and the spatial and temporal evolution of effective elastic thickness (EET) of the North American lithosphere. We use a Monte Carlo flexural model applied to 34 stratigraphic profiles in the Laramide province and five profiles from the Western Canadian Basin to estimate model parameters which produce flexural profiles that match observed sedimentary thicknesses. Sediment thicknesses come from basins from New Mexico to Canada of Cenomanian–Eocene age that are related to both Sevier and Laramide crustal loads. Flexural models reveal an east‐to‐west spatial decrease in EET in all time intervals analyzed. This spatial decrease in EET may have been associated with either bending stresses associated with the Sevier thrust belt, or increased proximity to attenuated continental crust at the paleocontinental margin. In the Laramide province (i.e., south of ~48°N) there was a coeval, regional decrease in EET between the Cenomanian–Santonian (~98–84 Ma) and the Campanian–Maastrichtian (~77–66 Ma), followed by a minor decrease between the Maastrichtian and Paleogene. However, there was no decrease in EET in the Western Canada Basin (north of ~48°N), which is consistent with a lack of Laramide‐style deformation or flat subduction. We conclude that the regional lithospheric weakening in the late Santonian–Campanian is best explained by hydration of the North American lithosphere thinned by bulldozing by a shallowly subducting Farallon plate. The weakening of the lithosphere facilitated Laramide contractional deformation by focusing end‐loading stresses associated with flat subduction. Laramide deformation in turn may have further reduced EET by weakening the upper crust. Finally, estimates of Campanian–Maastrichtian and Paleogene EET are comparable to current estimates indicating that the modern distribution of lithospheric strength was achieved by the Campanian in response to flat subduction.

A Cenomanian–Santonian rudist–bearing carbonate platform on the northern Arabian Plate, Turkey: facies and sequence stratigraphy

Recent studies have shown that Cenomanian–Santonian carbonate sedimentary rocks rich in rudists are widespread throughout southeastern Turkey. The Derdere and Karababa formations have been analysed in the Sabunsuyu section (Kilis Province). On a field scale, we can distinguish rudist–rich beds that rhythmically alternate with bioclastic levels composed of bivalves, gastropods and roveacrinids in these formations. Although pelagic faunal elements (predominantly planktic foraminifera and pithonellids) are documented in the lowermost part of the series, the rudist–rich facies are represented mainly in the Derdere and Karababa formations. Rudists, benthic and planktic foraminifera indicate middle–late Cenomanian and Turonian–Santonian ages for the Derdere and Karababa formations, respectively. Based on studied facies, five microfacies have been recognised and categorised in three facies groups: inner ramp, mid ramp and outer ramp. Evidence such as gradual changes in facies and absence of extensive uninterrupted barrier reefs indicate that the Derdere Formation was deposited in a ramp depositional system. The sedimentology and taphonomic signature of the rudist shell beds have been described in order to obtain a better understanding of the depositional environment and the physical processes that controlled Cenomanian–Santonian sedimentation. Monospecific tabular beds characterise mainly the upper part of the series (topmost part of Derdere Formation and Karababa Formation); more complex rudist concentrations, characterised by moderate species diversity, increase upsection. The rudist levels are associated with highstand systems tract deposits because of the suitability of trophic conditions in the rudist–dominated ramp.

Additional skulls of Talarurus plicatospineus (Dinosauria: Ankylosauridae) and implications for paleobiogeography and paleoecology of armored dinosaurs

Three new additional skull specimens of Talarurus plicatospineus have been recovered from the Upper Cretaceous (Cenomanian–Santonian) Bayanshiree Formation, of Bayan Shiree cliffs, eastern Gobi Desert, Mongolia. The skulls feature unique characters such as an anteriorly protruded single internarial caputegulum, around 20 flat or concave nasal-area caputegulae surrounded by a wide sulcus, a vertically oriented elongate loreal caputegulum with a pitted surface, an elongate lacrimal caputegulum positioned above the posterodorsal border of the maxilla, two longitudinally arranged large frontoparietal caputegulae surrounded by smaller rhomboid caputegulae, small but elongate medial supraorbital caputegulae, a posterior supraorbital caputegulum that is four times larger than the anterior one, up to three transverse parallel grooves on the dorsal surface of the posterior supraorbital caputegulum, postocular caputegulae along the ventral to posterior rim of the orbit that extend almost to the anteroventral margin of the squamosal horn, a longitudinal furrow tapering towards the apex of the squamosal horn, a lateral nuchal caputegulum four to five times larger than other nuchal caputegulae, and a pterygovomerine keel with a ventral margin that is dorsally positioned to the alveolar ridge. The phylogenetic analysis result showed that Talarurus is sister to the clade that includes the derived Asian ankylosaurines (Saichania chulsanensis, Tarchia kielanae, and Zaraapelta nomadis). It also shows that there was dispersal of ankylosaurines from Asia into western North America before the Cenomanian. Moreover, the rostral differences between Talarurus and Tsagantegia, another ankylosaur from the same formation, suggest possible niche partitioning between these taxa.

Classifying chalk microtextures: Sedimentary versus diagenetic origin (Cenomanian–Santonian, Paris Basin, France)

AbstractMicrotexture describes the type of particles and their arrangement in matrix samples at scanning electron microscopy scale. Although a microtexture classification exists for micritic limestone, it cannot be directly applied to chalk. This study therefore proposes a classification of chalk microtextures and discusses the origin of microtexture variability. Chalk was sampled at thirteen spatio‐temporal locations along the coastline of northern France (Cenomanian–Santonian). Four criteria are defined to describe, characterize and determine chalk matrix microtexture: (i) mineralogical content; (ii) biogenic fraction; (iii) micritic fraction; and (iv) cement fraction. From these criteria, two major groups are defined: Pure Chalk Microtexture Group, with seven classes, and Impure Chalk Microtexture Group, divided into two subgroups: Argillaceous Microtexture with four classes and Siliceous Microtexture with two classes. Microtexture variability is related both to initial sedimentation and to diagenesis. Sedimentological conditions (for example, climate and distance from shore) affect chalk composition (carbonate content and type of insoluble particles), thus influencing microtexture. Changes in Pure Chalk Microtexture are the result of increasing diagenetic intensity. This classification can also be used to characterize the microtexture of subsurface chalk reservoirs. Reservoir quality depends on the petrophysical and mechanical properties of reservoir rocks, which can be better understood by exploring their sedimentary and diagenetic history, revealed by the study of chalk microtexture variability.

Oil-source correlation in the West Esh El Mellaha, southwestern margin of the Gulf of Suez rift, Egypt

There are several source rock units in the Gulf of Suez graben, but the Upper Cretaceous–Miocene sequence has probably produced most of the commercial hydrocarbons in this petroleum province. One hundred and nine shale, marl and marly shale samples collected from five wells in the West Esh El Mellaha Concession and surrounding area were studied by petrographic and organic geochemical methods to determine organic facies, depositional paleoenvironment, and thermal maturity. We also investigated the molecular and stable carbon isotopic compositions of nine oil samples to establish genetic relationships with the source rock samples.Pyrolysis data for samples from the Cenomanian–Santonian Matulla, Paleocene Esna, Eocene Thebes, Oligo-Miocene Nukhul and Lower–Middle Miocene Rudeis formations contain hydrogen-poor terrigenous Type-III to II/III kerogen. However, the Campanian–Maastrichtian Brown Limestone samples contain Type-II and Type-II/III kerogens indicating very good to excellent generation potential. The acyclic isoprenoids and normal alkane distributions and biomarkers in the source rock extracts indicate derivation from bacteria and algae, with varying contributions from land plants, deposited under suboxic to anoxic conditions in normal marine to higher salinity waters.The oil samples have high sulfur content and low to moderately high API gravity, typical of mature oils from pre-rift marine carbonate or calcareous shale lithofacies. Hierarchical cluster analysis (HCA) of 18 source-related biomarker ratios revealed four genetic families of oil samples. The two South Malak-1 oils (Family-4) are substantially different from the Rabeh E−8 and Rabeh E−10 (Family-1), Rabeh-1 and Hurghada-1 (Family-2) and Abu Marwa-1 and Wadi El Sahl-1 (Family-3) oil families, which are more closely related. Principal component analysis (PCA) and HCA based on the 18 ratios for the total data from extracts and oils indicate three groups. The South Malak-1 oils seem to be most closely related to the two Brown Limestone extracts. Group-1 oils appear to represent mixtures with one end member consisting of the South Malak-1 oils and the Brown Limestone source rock, but the other end member is not in the dataset.

Late Cretaceous paleosols as paleoclimate proxies of high-latitude Southern Hemisphere: Mata Amarilla Formation, Patagonia, Argentina

Although there is general consensus that a global greenhouse climate characterized the mid-Cretaceous, details of the climate state of the mid-Cretaceous Southern Hemisphere are less clearly understood. In particular, continental paleoclimate reconstructions are scarce and exclusively derived from paleontological records. Using paleosol-derived climofunction studies of the mid- to Upper Cretaceous Mata Amarilla Formation, southern Patagonia, Argentina, we present a reconstruction of the mid-Cretaceous climate of southern South America. Our results indicate that at ~60° south paleolatitude during the Cenomanian–Santonian stages, the climate was subtropical temperate–warm (12°C±2.1°C) and humid (1404±108mm/yr) with marked rainfall seasonality. These results are consistent with both previous estimations from the fossil floras of the Mata Amarilla Formation and other units of the Southern Hemisphere, and with the previous observations of the displacement of tropical and subtropical floras towards the poles in both hemispheres. The data presented here show a more marked seasonality and slightly lower mean annual precipitation and mean annual temperature values than those recorded at the same paleolatitudes in the Northern Hemisphere.

Elemental geochemistry and strontium-isotope stratigraphy of Cenomanian to Santonian neritic carbonates in the Zagros Basin, Iran

A Neo-Tethyan upper Cenomanian–Santonian neritic carbonate ramp succession (Sarvak and Ilam formations), drilled in the Zagros Basin in southwest Iran, was investigated via detailed sedimentology, microfacies analysis, elemental geochemistry and Sr-isotope stratigraphy (SIS). The succession contains two exposure surfaces, which are known as the CT-ES and mT-ES (Cenomanian–Turonian and middle Turonian, respectively), and associated prominent negative carbon-isotope excursions that represent important regional stratigraphic marker horizons. Precise knowledge about the onset of platform exposure and the duration of the exposure-related hiatus, however, is currently lacking due to a rather low-resolved shallow-water biostratigraphic framework and a bulk carbonate carbon-isotope pattern that clearly differs from global Late Cretaceous reference curves. Therefore, the existing bio-chemostratigraphic framework was complemented by bulk carbonate strontium-isotope stratigraphy (SIS). As bulk carbonate material is in particular prone to diagenetic alteration, a careful selection of least altered samples has been carried out by means of elemental geochemistry and petrography. In contrast to what could be expected, the meteoric alteration of limestones beneath both exposure surfaces is not clearly expressed by increasing iron and manganese and coeval decreasing strontium contents. On the contrary, the impact of meteoric diagenesis is well illustrated via pronounced increases in Rb concentrations and concomitant prominent positive shifts to radiogenic strontium-isotope values, an observation that clearly reflects the decay of continentally derived 87Rb into 87Sr. Rubidium corrected strontium-isotope values place the CT-ES around the Cenomanian–Turonian boundary and point to an exposure duration of less than 0.4Myr. This rather short-term CT-ES related hiatus is supported by petrographic evidence, which indicates a youth karstification stage of strata beneath the CT-ES. Following SIS, the Ilam–Sarvak transition at the top of Nezzazatinella–Dicyclina interval zone coincides with the mT-ES. Carbonates placing above this transition (Ilam Formation) are ascribed to the earliest to latest early Santonian, while carbonates immediately beneath the mT-ES (Sarvak Formation) are dated as late Turonian. SIS thus indicates a long-lasting hiatus of 4.5Myr associated with the mT-ES. Emergence represented by the CT-ES is here proposed as harbinger of the mT-ES in the Zagros Basin, which most likely resulted from stepwise peripheral bulging due to ophiolite obduction in combination with a small-scale global sea level fall around 94Ma.

Mesozoic to recent geological history of southern Crimea and the Eastern Black Sea region

Abstract We provide a synthesis of stratigraphic data to unravel the history of the geological evolution of South Crimea in the Mesozoic and Cenozoic. The South Crimea Orogen consists of three major mega-sequences: (1) the Triassic–Early Jurassic; (2) the Aalenian–Bathonian; and (3) the Callovian–Eocene. The Late Triassic–Early Jurassic deposits formed in the environment of a forearc basin and a remnant basin. The Aalenian–Bathonian deposits formed above subduction extension and a volcanic belt. Three main Callovian–Eocene tectonic units can be identified in South Crimea: (1) the South Crimean Shelf Basin; (2) the Sudak Deepwater Trough; and (3) the Alchak–Kaya Shelf Basin at the northern margin of the Shatsky Ridge. The Oligocene–Quaternary deposits are considered to be syn-orogenic. A description of the anticipated stratigraphic units on the Shatsky Ridge is suggested for the Middle Jurassic, Callovian–Late Jurassic, Neocomian, Aptian–Albian, Late Cretaceous–Paleocene, Eocene and Maykopian. We propose a model for the geological history of the Eastern Black Sea Basin. Graben formed during the Late Barremian–Albian at the location of the future Eastern Black Sea Basin and a phase of volcanism occurred in the Albian. The main phase of rifting and spreading of oceanic crust took place during Cenomanian–Santonian time. Supplementary material: A Google Earth kmz file of the location of the outcrops and sections is available at http://www.geolsoc.org.uk/SUP18850

Late Cretaceous radiolarian biochronology of the Pedro Brand section, Tireo Group, eastern Central Cordillera, Dominican Republic: A contribution to the stratigraphy of the Caribbean Large Igneous Province

The Tireo Group in the eastern Central Cordillera (Dominican Republic) is part of the Jarabacoa Block, composed of a Pacific-type Jurassic ocean floor (Loma La Monja, overlain by the El Aguacate ribbon Chert), intruded and overlain by an early CLIP (Carribean Large Igneous Province)-type plateau, the Duarte Complex, which is in turn unconformably overlain by arc-type rocks of the Tireo Group. This group exhibits a 3-km thick sequence of arc-related volcanic and volcano-sedimentary rocks, including tuffaceous chert and mudstone studied for radiolarians in this paper. The Siete Cabezas Formation, considered to be the last Campanian–Maastrichtian CLIP-type volcano-sedimentary sequence, overlies the Tireo Group. A controversy about earlier radiolarian dating of the Tireo Group, considered until now as part of the Siete Cabezas Formation, encouraged us to study a well-exposed section located 3km NE of Pedro Brand village. Seven samples of laminated siliceous mudstones and cherts yielded around 40 common and well-preserved radiolarian taxa. Based on maximum ranges of taxa published in several regional zonations and on a comparison with a Turonian–Coniacian sample calibrated by planktonic foraminifera (the Deva Beds from Romania), we determine a Turonian–Coniacian age for the Pedro Brand section. A 40Ar–39Ar whole rock age of 75.1±1.1Ma, obtained in a basalt dyke crosscutting the radiolarian bearing rocks, provides a Late Campanian consistent minimum age for the pelagic–hemipelagic Pedro Brand section. Including the re-interpretation of earlier radiolarian work, we conclude that the studied rocks of the Tireo Group are older than the Maastrichtian 40Ar–39Ar ages on plagioclase of the Siete Cabezas Formation. The studied dyke in the Pedro Brand section geochemically resembles the overlying Siete Cabezas and Pico Duarte basalts and could be a feeder dyke of those. However, a tectonic superposition of the Siete Cabezas cannot be excluded, since earlier 40K–40Ar basalt ages of this unit are Aptian–Albian and Cenomanian–Turonian. The Jarabacoa Block is considered as the most complete outcrop section of Pacific ocean crust overlain by a first (Aptian–Albian) phase of CLIP-type activity, followed by the development of a Cenomanian–Santonian intraoceanic arc, which is in turn overlain by a late Campanian–Maastrichtian CLIP-like phase.