Jurassic-Cretaceous Boundary Research Articles

The chromosome-level genome of Akebia trifoliata as an important resource to study plant evolution and environmental adaptation in the Cretaceous.

The environmental adaptation of eudicots is the most reasonable explanation for why they compose the largest clade of modern plants (>70% of angiosperms), which indicates that the basal eudicots would be valuable and helpful to study their survival and ability to thrive throughout evolutionary processes. Here, we detected two whole-genome duplication (WGD) events in the high-quality assembled Akebia trifoliata genome (652.73Mb) with 24 138 protein-coding genes based on the evidence of intragenomic and intergenomic collinearity, synonymous substitution rate (KS ) values and polyploidization and diploidization traces; these events putatively occurred at 85.15 and 146.43million years ago (Mya). The integrated analysis of 16 species consisting of eight basal and eight core eudicots further revealed that there was a putative ancient WGD at the early stage of eudicots (temporarily designated θ) at 142.72Mya, similar to the older WGD of Akebia trifoliata, and a putative core eudicot-specific WGD (temporarily designated ω). Functional enrichment analysis of retained duplicate genes following the θ event is suggestive of adaptation to the extreme environment change in both the carbon dioxide concentration and desiccation around the Jurassic-Cretaceous boundary, while the retained duplicate genes following the ω event is suggestive of adaptation to the extreme droughts, possibly leading to the rapid spread of eudicots in the mid-Cretaceous. Collectively, the A. trifoliata genome experienced two WGD events, and the older event may have occurred at the early stage of eudicots, which likely increased plant environmental adaptability and helped them survive in ancient extreme environments.

Age of the Tuchengzi Formation in Western Liaoning Province and the Jurassic–Cretaceous Boundary from the Continuous Core Records of Well YD1, Jinyang Basin

The Tuchengzi Formation is widely distributed in western Liaoning Province with a clear top and bottom. It is the focal area for the delineation of the terrestrial Jurassic–Cretaceous boundary in China. Based on continuous core samples taken from well YD1, detailed lithostratigraphic sequences and zircon uranium–lead (U-Pb) dating WERE used to investigate the Tuchengzi Formation. The zircon U-Pb ages of the tuff samples taken from the First and Third Members of the Tuchengzi Formation ranged from 153.8 to 137.16 Ma, indicating that they were formed in the late Middle Jurassic–Early Cretaceous. Dating results from the bottom of the Second Member of the Tuchengzi Formation indicate that the sedimentary time of the stratum is no later than 145.7 ± 2.1 Ma. We concluded that the Jurassic–Cretaceous boundary of the Jinyang Basin in western Liaoning province may be located at the interface at a depth of 464 m in well YD1. This conclusion is consistent with the Jurassic–Cretaceous boundary that has been presumed by other researchers based on paleontological assemblage features found in recent years, and can provide useful geological marker beds for the future study of the terrestrial Jurassic–Cretaceous boundary. In addition, the authors also systematically sorted the potential development areas and layers of the terrestrial Jurassic–Cretaceous boundary line, which may also provide useful geological marker beds for the future study of the terrestrial Jurassic–Cretaceous boundary.

The Jurassic – Cretaceous transition in the West Cameros Basin (Tera Group, Burgos, Spain): Sedimentological and palynostratigraphical insights

The Jurassic–Cretaceous boundary transition is widely distributed within the West Cameros Basin. This horizon is constituted by the Tera Group, consisting of two depositional sequences (DS1 and DS2) comprising four geological formations spanning the Tithonian to the Berriasian. Here, a combined sedimentological and palynostratigraphic study was made covering the Tera Group. The sedimentological results for the DS 1 differentiate three facies associations (FA 1-1, 1-2 and 1-3) corresponding to floodplains and braided channels of a distributive fluvial system shifting to shallow lacustrine facies in a semi-arid climate. For the DS 2, three FA were also differentiated (FA 2-1, 2-2, and 2-3), corresponding to tidal-influenced meandering channels and floodplains in a distal fluvial to a delta-plain system that shifts to shallow lacustrine facies. The palynological results were only productive in samples from the Jaramillo Formation. The palynological record shows an assemblage dominated by spores including Concavissimisporites, Leptolepidites and Taurocusporites, as well as Classopollis pollen. According to the stratigraphic ranges of some selected taxa, the suggested chronostratigraphic assignment is lower Berriasian for the Jaramillo Fm. A comparison of the microfloras from the Tithonian–Berriasian recorded in the Iberian Peninsula was carried out. The following observations can be concluded: 1) the change in the plant communities, already observed since the Late Jurassic onwards, became more pronounced between the Tithonian–Berriasian boundary and the late Berriasian; and 2) a noteworthy occurrence of some biostratigraphically important taxa, specifically for the Tithonian–Berriasian interval.

New data on the stratigraphy and depositional environment of the Jurassic-Cretaceous boundary sediments of the northwestern part of the Yenisei-Khatanga trough

Research subject. Boundary Jurassic-Cretaceous sediments of the western part of the Yenisei-Khatanga Regional Trough. Materials and methods. The main correlation tools were seismostratigraphic, logging, and biostratigraphic methods. Litho- and biofacies methods were used to reconstruct the development conditions of the Yanovstan Formation and the NKh group of the Ryazanian Stage reservoirs. Results. The composition of macro- and microfauna complexes was defined, analogues of the ammonite and foraminiferal zones of the Upper Volgian substage and the Ryazanian stage of Siberia were established. The structure of the Upper Jurassic and Ryazanian complexes of the western part of the Yenisei-Khatanga Regional Trough was detailed. Conclusions were made about the formation conditions of the Volgian and Ryazanian deposits. Conclusions. The sediments were found to have been formed in a sea basin with stable salinity and low water energy, in relatively deep-water conditions. Mixed turbidite systems with multipoint supply sources were identified, in which the terrigenous material was supplied from southeast to northwest.

Biogeographical and diversification analyses of Indian pseudoscorpions reveal the Western Ghats as museums of ancient biodiversity

The tropical forests of the Western Ghats (WG) of India are considered ‘refugia’ harbouring highly diverse and endemic taxa but these refugia are under immense anthropogenic pressure. Most phylogenetic studies have explained diversity patterns across the WG using vertebrates, however, the processes impacting the highly endemic invertebrate fauna are still poorly understood. Here we investigate the evolutionary history of an ancient and widespread arachnid lineage (Pseudoscorpiones: Chthoniidae: Tyrannochthoniini) in the WG through a variety of biogeographical and diversification analyses, including sequence data from three markers for 45 terminals from the WG and 22 from Africa, East and Southeast Asia, Australasia and the Neotropics. Our results show that WG Tyrannochthoniini are rendered paraphyletic by African taxa, a result consistent with continental drift. WG Tyrannochthoniini are further split into two major clades, a southern WG clade and an African-central WG clade, due to vicariance along a major biogeographical barrier, the Palghat Gap. Central WG pseudoscorpions diverged from their African relatives at the Jurassic-Cretaceous boundary, coinciding with the separation of India from Africa. Despite ancient vicariance, six dispersal events occurred across the Palghat Gap, beginning in the Early Cretaceous as India was rafting towards Southeast Asia and ending by the early Miocene, implying that the Palghat Gap became impermeable to dispersal for these humid-adapted organisms during Miocene aridification. Although the ancestor of WG Tyrannochthoniini originated in lowland WG forests, three dispersal events into high montane Shola forests occurred most likely as a result of Late Cretaceous orogenesis and Neogene uplift, allowing lineages to occupy niches at higher elevations. An exponentially declining diversification rate, typical of older lineages, supports the ‘museum’ model of diversification for WG Tyrannochthoniini. Our study sheds light on the historical biogeography of relictual soil-arthropods in the WG and emphasises the role of WG forest refugia in preserving ancient invertebrate biodiversity.

Further observations on the bio- and magnetostratigraphy of the J/K boundary interval in southern Ukraine

This is an account of finds of stratigraphically useful calcareous nannofossils and the magnetostratigraphy of the Jurassic-Cretaceous boundary interval of the eastern Crimean peninsula (southern Ukraine). We compare these new complementary results with those presented by our team in earlier publications. A missing interval in the Crimean sequence is filled, and the position of the Tithonian-Berriasian (J/K) boundary is confirmed

Insect and Plant Diversity in Hot-Spring Ecosystems during the Jurassic-Cretaceous Boundary from Spain (Aguilar Fm., Palencia).

Simple SummaryIn this study, we show and identify new plant and insect remains found in travertine deposits from the Jurassic-Cretaceous boundary of the Aguilar Formation (Palencia, North Spain). From this hot-spring palaeoenvironment, we have identified the presence of dragonflies of the families Cymatophlebiidae and Aktassiidae, representing the first report of these families for the Iberian Peninsula. In addition, we find a flora dominated by Bennettitales and the presence of ferns that differ from other floras of the same age and geographical area. The unusual environmental and palaeoecological conditions of this hot-spring environment are also discussed, suggesting that this niche was an “ecological oasis” for some plants and insects.Hydrothermal palaeoenvironments are very uncommon in Upper Jurassic and Lower Cretaceous deposits worldwide. We present new plant and insect remains from travertines formed during the Jurassic-Cretaceous boundary in northern Spain (Aguilar Fm., Palencia province). A total of 136 plant specimens and three insect wings were collected and studied. This entomofauna consists of dragonfly (Odonata) wings including Cymatophlebiidae and an undetermined new genus and species of Aktassiidae, representing the first report of these families for the Iberian Peninsula. The fossil flora shows different morphotypes of plants, which have been tentatively assigned to three different genera. The taphocoenosis of the flora was dominated by Bennettitales (98.5%) including cf. Pterophyllum sp., Ptilophyllum cf. acutifolium, Ptilophyllum cf. pecten, Ptilophyllum cf. pectiniformis and cf. Ptilophyllum sp., and the occasional presence of ferns (1.5%) represented by the taxon Cladophlebis cf. denticulata. The presence of the Anisoptera Cymatophlebia cf. longialata suggests a higher affinity for a Tithonian age of the studied site, and the anatomy and palaeogeographical distribution of this species suggest capacity to migrate for rather long distances. The floristic composition of the site differs remarkably from other Tithonian-Berriasian floras of the Iberian Peninsula. The presence of Odonata and the distinctive flora in (semi)arid conditions could be due to the hot-spring providing an environmental niche with constant conditions of warmth and humidity forming an ‘ecological oasis’.

Environmental changes across the Jurassic–Cretaceous boundary in the western proto-Gulf of Mexico — Chemo- and biostratigraphic correlation of NE Mexican sections

Late Jurassic–earliest Cretaceous (Kimmeridgian–earliest Berriasian) marine shelf sediments in northeastern Mexico reveal repeated temperature-related oceanic and biotic turnovers in the northwestern proto-Gulf of Mexico. Our detailed geochemical analysis of four sections crossing the Jurassic-Cretaceous (J/K) boundary is based on chemostratigraphic correlation and repeated mollusk assemblage changes reflecting significant environmental fluctuations, including sea level and marine redox conditions. The present study confirms tectonic tilting and rapid drowning of shallow-water environments in the early Kimmeridgian, associated with the near-synchronous spread of bottom-water anoxia in shelf areas bordering the proto-Gulf of Mexico. Boreal cold-water faunal elements are associated with a sea-level drop in the late Kimmeridgian, while a transient transgressive-regressive phase is interpreted for the shelf transect in the earliest Tithonian. A general transgressional trend is reported for the Tithonian and across the J/K boundary, while the migration into the area of boreal and austral high-latitude faunal elements marks specific intervals. We hypothesize that oscillations in the oceanic current system were responsible for the intermittent establishment of cold-water boreal and austral faunal assemblages in the northwestern proto-Gulf of Mexico. • Integrated correlation of Jurassic–Cretaceous marine successions in northeastern Mexico. • Significant environmental changes including sea-level and marine redox conditions. • Repeated temperature-related oceanic and biotic turnovers. • Evidence for cold-water indicative belemnites occupying shallow-water habitats. • Diachronous spread of bottom-water anoxia during Early Kimmeridgian transgression.

Jurassic–Cretaceous boundary interval in the Ropice section from the perspective of biostratigraphy, stable isotopes and palaeomagnetism.

Jurassic–Cretaceous boundary interval in the Ropice section from the perspective of biostratigraphy, stable isotopes and palaeomagnetism.

Discovery of chitinoidellids and calpionellids from the Blue Nile Basin and the Jurassic-Cretaceous boundary in Ethiopia

The deposition of Mesozoic carbonates in the Horn of Africa is pervasive and forms an important marker for inter- and intra-basinal correlation. In central Ethiopia, this time interval is represented by transgressive deposits of the Antalo Limestone Formation in the Blue Nile and the Mekelle basins, and the Hamanlei Limestone Formation in the Ogaden Basin. These carbonate formations have traditionally been assigned to range from ?Lower to Upper Jurassic. Recently, based on calcareous nannofossils from the Blue Nile Basin, the Antalo Limestone Formation was constrained to between the lower Callovian and upper Tithonian with a suggestion of the presence of Cretaceous strata in the Blue Nile Basin, based on the record of Uniplanarius sissinghii in the Mugher section. Here, based on the discovery of chitinoidellids and calpionellids from two sections of the Blue Nile Basin, we confirm the presence of Cretaceous deposits and conclusively extend the age of the Antalo Limestone Formation to late Berriasian. Three calpionellid zones are recorded from the Blue Nile Basin: Bonetinae and Chitinoidellids/Primitive Calpionellids subzones, both assigned to the upper Tithonian Microcanthum ammonite Zone and the Calpionellopsis Zone (?Simplex‒Oblonga/Oblonga-Praecalpionellites Subzone) corresponding to the upper Berriasian ammonites Boissieri Zone. Two stratigraphic hiatuses: Kimmeridgian‒lower Tithonian and uppermost Tithonian‒lowermost upper Berriasian are noted in both sections. This discovery of calpionellids has a profound effect not only on the biostratigraphy of Ethiopia but also has allowed the integration of the study region of eastern Africa, for the first time, within the wide calpionellid domain of the South Tethysian margin.

Magnetostratigraphy and magnetic properties of the Jurassic to Lower Cretaceous Girón Group (northern Andes, Colombia)

AbstractWe report paleomagnetic results from the Jurassic to Lower Cretaceous continental sedimentary succession exposed in the eastern limb of the Los Yariguíes anticlinorium, Eastern Cordillera, Colombia. About 820 m of a stratigraphic section of the upper part of the Girón Group (Angostura del Río Lebrija and Los Santos Formations) was sampled to construct a magnetic polarity stratigraphy. A total of 199 independent samples that yield interpretable and acceptable data have a characteristic remanent magnetization component (ChRM) isolated between 400 °C and 680 °C in progressive thermal demagnetization. Demagnetization behavior and rock magnetic properties are interpreted to indicate that hematite is the principal magnetization carrier with a possible contribution by magnetite in some parts of the section. After tilt correction, 123 samples are of normal polarity (declination [D] = 44.9°, inclination [I] = +9.7°, R = 110.64, k = 9.87, and α95 = 4.3°), and the other 76 accepted samples are of reverse polarity (D = 216.4°, I = −6.1°, R = 68.29, k = 9.72, and α95 = 5.5°). The statistical reversal test conducted on virtual geomagnetic poles is positive (class B). Based on paleontologic age estimates for the Cumbre and Rosablanca Formations, we assume a Berriasian age for the Los Santos Formation. The magnetostratigraphic data from the Girón Group strata are interpreted to suggest an age for the sampled part of the section between early Kimmeridgian and early Valanginian (ca. 157–139 Ma). The age of the Angostura del Río Lebrija Formation is estimated as between early Kimmeridgian and early Tithonian (ca. 157–146.5 Ma). The age of the Los Santos Formation is estimated between early Tithonian and early Valanginian (146.5–139.3 Ma). With our proposed, but nonunique, correlation with the Geomagnetic Polarity Time Scale, the Jurassic-Cretaceous boundary is interpreted to be located within the Los Santos Formation. The Girón Group is characterized by two periods of high (>8 cm/k.y.) and two periods of low (< 2 cm/k.y.) sedimentation rates. An inferred clockwise rotation of ∼44°, based on paleomagnetic declination data from the Girón Group, is similar to rotation estimates reported in some previous studies in the general area, and this facet of deformation could be related to local and regional response to displacement along regional-scale strike-slip faults.

Discussion on the characteristics of the Jurassic-Cretaceous boundary correlation in the Songliao Basin and adjacent areas

Discussion on the characteristics of the Jurassic-Cretaceous boundary correlation in the Songliao Basin and adjacent areas

High latitude stratigraphical palynology of the Jurassic–Cretaceous boundary interval, Sverdrup Basin, Arctic Canada

The Rollrock Section on northern Ellesmere Island (Arctic Canada) exposes a continuous succession of Upper Jurassic to Lower Cretaceous strata, of which 537 m were logged and sampled at 1.5 m intervals for palynological analysis. Consequently, these strata form one of the most comprehensive archives of Arctic biostratigraphy of the Jurassic–Cretaceous boundary interval. As such, their occurrence is valuable to our understanding of high latitude climate change and floral turnover over this poorly defined boundary. Previous macrofossil studies indicate a shallow, distal depositional environment for the succession. Palynomorph assemblages from the Rollrock Section display a sequence of palynostratigraphic events stretching from the Oxfordian to the early Valanginian. The dinocyst taxa reported in this study enable the division of the section into seven distinct biozones, as defined by first occurrences of Paragonyaulacysta capillosa, Prolixosphaeridiopsis spissa, Trichodinium erinaceoides, Oligosphaeridium complex, and Muderongia simplex, and the last occurrences of Gonyaulacysta jurassica adecta and Rhynchodiniopsis cladophora. The palynostratigraphic events displayed have enabled modifications to existing biostratigraphic schemes. Provincialism and the lack of a defined Jurassic–Cretaceous boundary complicate attempts at global correlation. The base of the Valanginian marks the first palynological event in the Cretaceous, which can be correlated across the Boreal Realm, with potential correlation to biological events in the Tethyan Realm.

Theropod tracks from the Jurassic–Cretaceous boundary, Tuchengzi Formation, Chengde, China: Review and new observations

Previously known theropod dinosaur footprints preserved as natural casts in the Tuchengzi Formation, on a rock wall beside the railway in Nanshuangmiao Village, Shangbancheng Town, Chengde City, were originally assigned to ichnogenus Anchisauripus and tentatively attributed to oviraptosaurs. The assemblage was restudied in more detail by examining the entire assemblage of 55 tracks associated with two horizons. The size range of the 27 measured tracks suggests a more diverse grallatorid–eubrontid assemblage and potentially greater diversity of theropod trackmakers. The label Anchisauripus, which has fallen into disuse in some recent literature, implies trackmakers of medium shape and size in the grallatorid–eubrontid morphological spectrum. However, given the presence of other theropod ichnotaxa in the Jurassic to Early Cretaceous strata of the Tuchengzi Formation and time equivalent units we suggest that explicit reference to the Grallator-Anchisauripus-Eubrontes (GAE) plexus, or simply the term Grallator-Eubrontes plexus be confined to Lower Jurassic assemblages as originally defined and intended. Further study centered on the 16 known Tuchengzi assemblages and older theropod ichnfaunas is necessary to confirm or refute the degree to which grallatorid–eubrontid assemblages from these different epochs are similar or convergent. Even if the tracks are morphologically very similar inferences regarding trackmaker identity are problematic because the same theropodan trackmaker species, genera or even families were not present in both epochs.

Late Jurassic syn-extensional sedimentary deposition and Cenozoic basin inversion as recorded in The Girón Formation, northern Andes of Colombia

The Yariguíes Anticlinorium, a regional structure located at the western flank of the Eastern Cordillera of Colombia, includes the thickest record of continental sedimentary rocks accumulated near to the Jurassic-Cretaceous boundary. The sedimentary rocks are lithoarenites and feldspathic arenites, grouped in the Girón Formation, and deposited in a Late Jurassic extensional basin interpreted in this work as a rift basin. We analysed the sedimentologic and compositional characteristics of two sections that accumulated in a complex rift system. We identified important thickness variations, from 3,350 m in the type section to at least 525 m in a reference section in the Zapatoca area, as well as petrographic and lithofacies changes. This led us to confirm that the Girón Formation encompasses all the continental facies, whose source rock correspond mainly to the exhumed blocks of the Santander Massif during the Late Jurassic. The synrift successions were segmented by transverse structures and regional longitudinal faults of the rift-shoulder, as the Suárez Fault. The tectonic frame of the study area shows the relevance of the W-E compressional regimes, explaining the local kinematics as a heritage of the former configuration and tectonic inversion of the basins. However, clockwise rotation of the stress field was detected from the stress tensor analysis. The latest orientation of the stress tensors and shear joints are related to the effect of the transpressional Bucaramanga and Lebrija faults along the study area.

Facies analysis and stratigraphy across the Jurassic-Cretaceous boundary in a new basinal Tithonian–Berriasian section of the Vaca Muerta Formation, Las Tapaderas, Southern Mendoza Andes, Argentina

This paper presents a multidisciplinary study of a new basinal section of Tithonian-Berriasian the Vaca Muerta Formation at Las Tapaderas area, including detailed, biostratigraphic, sedimentologic, sequence stratigraphic and cyclostratigraphic analysis. Biostratigraphy based on ammonite, calpionellids and calcareous dinoflagellate cysts indicate that Las Tapaderas section spans through the Lower Tithonian - lowermost Upper Berriasian, however, its upper part is covered through an erosive unconformity by Pleistocene volcaniclastic deposits, and therefore Las Tapaderas section could reach the Lower Valanginian. Two facies associations were identified, corresponding to basinal and distal outer ramp subenvironments. Recognition of flooding surfaces allowed the identification of three composite depositional sequences and eight high-frequency depositional sequences, which can be correlated with other sections throughout the basin. Cyclostratigraphic analysis based on the recognition of marlstone/limestone couples (elementary cycles) allowed to build a time series based on bed thickness. Fourier analysis indicates the characteristic mid latitude precession-eccentricity syndrome, with 220 precessional cycles (~20.4 and ~23 kyr), 53 low frequency eccentricity cycles (~79, ~90 and ~140 kyr) and 11 high frequency eccentricity cycles (~400 kyr). Spectral analysis also allowed to recognize the presence of the obliquity cycle (38.5 kyr), which has been erratically recorded in the Vaca Muerta Formation. Our data allowed the construction of an orbital scale, calibrated by cosmopolitan markers (calpionellids and calcareous dinoflagellate cysts), for this section. The precise bio- and cyclostratigraphic location of the Jurassic-Cretaceous boundary was established for this section. The sedimentation rate was studied at the scale of the precession cycle, showing values between 0.6 and 3 cm/kyr, while at the low-frequency eccentricity scale it shows values between 1 and 2 cm/kyr.

UPPER KIMMERIDGIAN AND TITHONIAN AMMONITES FROM THE TUNISIAN "DORSALE" (NE TUNISIA): UPDATED BIOSTRATIGRAPHY FROM THE JEBEL OUST

In the context of updating biostratigraphic analysis in the Upper Kimmeridgian-lowermost Berriasian from the whole of the Tunisian Dorsale (NE Tunisia), new results obtained from the Jebel Oust reference section are presented. The bed-by-bed-sampling of a rather rich ammonite fauna gathered in the two most favourable profiles in the area allowed to propose, for the first time, a biozonation for Upper Kimmeridgian to Tithonian limestones. Calpionellids relayed or completed ammonite assemblages, and were also useful for biostratigraphy in beds close to the Jurassic-Cretaceous boundary. The composition of ammonite assemblages, as well as the known ranges for genera and species, is in accordance with the available information from epioceanic areas in West-Mediterranean Tethys. The recognized biozones fit the standard proposal given by Groupe Francais D'Etude du Jurassique in 1997. Age-interpretation for turbiditic horizons in the Jebel Oust is made for the first time. In spite of some limitations, the biostratigraphic results obtained in Jebel Oust sections provide a valuable tool for potential correlations with poorly known sections in northern Tunisia, as well as with other Tethyan areas.

Volcanic Tuffs and Tuffites in Jurassic–Cretaceous (Volgian–Ryazanian) Boundary Rocks of Western Siberia

Altered ash tuff and tuffite layers in the Jurassic–Cretaceous boundary rocks (Bazhenovo Formation) were studied in detail. These layers represent an interval ranging from the upper part of the middle Volgian substage to the uppermost Ryazanian and cover a huge area (more than 400 000 km2). Despite small thickness (10–3–10–2 m), these layers are prominent in borehole cores and correlated confidently in interborehole spaces. Therefore, they can be used as marker horizons and isochronous stratigraphic levels in the Bazhenovo Formation. The composition of ash layers is quite consistent and marked by lateral zoning, probably, related to fractionation of the primary pyroclastic material during the eolian and hydrodynamic transport and deposition. The petrographic, mineralogical, and geochemical data suggest that the original ash corresponds to basaltic andesite in composition and is related to arc (suprasubduction) volcanism beyond the West Siberian Plate. The probable source of ash material was located most likely southwest of the study region in the Transcaucasus. The studied tuff layers are very important for the regional stratigraphy and global correlation, since they can potentially be traced in both Boreal and Tethyan successions, and zircons found in tuffs can be used for the U–Pb dating.

A summary of the Jurassic System in North and East-Central Iran

The rocks of the Jurassic System in North and Central Iran are represented by very thick sedimentary successions reflecting the deposition during two tectono-stratigraphic megacycles, bounded by three important unconformities. The older (Early to early Middle Jurassic) cycle starts, after the Late Triassic closure of the Palaeotethys and the collision of the Iran Plate with the southern margin of Eurasia (Turan Plate), with an abrupt uplift of the Cimmerian Mountains around the Triassic–Jurassic boundary (Main-Cimmerian Event) and is terminated by the Mid-Cimmerian Tectonic Event in the mid-Bajocian. The predominantly siliciclastic sediments of this tectono-stratigraphic cycle are accommodated in the Ab-e-Haji Subgroup of the (upper) Shemshak Group. The Ab-e-Haji Subgroup consists of thick and widespread, partly coal-bearing siliciclastic strata and also records some important marine ingressions, especially in the Toarcian and Aalenian. The younger tectono-stratigraphic cycle starts with an extensive marine transgression in the Late Bajocian and ends around the Jurassic-Cretaceous boundary with the Late Cimmerian Tectonic Event. In the aftermath of the Mid-Cimmerian Tectonic Event, the Iran Plate was dissected into several structural units, the geological histories of which differed and so did their facies patterns. Thus, in North Iran two main sedimentary areas developed, namely the Alborz and Koppeh Dagh basins, which show moderate diffe-rences in lithology and thicknesses. However, the differentiation is much more pronounced on the Central-East Iranian Microcontinent, which comprises three N–S-oriented and independent structural units, i.e., from east to west the Lut, Tabas and Yazd blocks. During the Jurassic Period, the sea mostly covered the Tabas and Lut blocks, whereas the Yazd Block remained largely emergent. The lithologically diverse and numerous formations of the area are combined in the Magu and Bidou groups. Early to Early Middle Jurassic ammonite fau-nas of North and Central Iran are palaeobiogeographically closely related to Northwest European (Subboreal) ammonite faunas, allowing a similar biozonation. Concurrent with the Late Bajocian transgression, there is an abrupt change in faunal composition and the Middle and Late Jurassic ammonite faunas of North and Central Iran are much more similar to those of epicontinental seas bordering the nort-hern margin of the western Tethys and occupy an intermediate position between the Mediterranean and the Submediterranean Province. The results of the studies on ammonoid palaeobiogeography are in line with Early Jurassic palaeogeographic reconstructions that place the Iran Plate at fairly high palaeo-latitudes of about 45°N, followed by a rapid southward shift throughout the Middle and Late Jurassic to a position of about 30°N.

Comments on the Liassic age ascribed to the Bou Adel oceanic gabbro (Central Mesorif, Morocco) by Michard et al. (2018), and stratigraphic and palaeogeographic arguments for a latest Jurassic-earliest Cretaceous age

In the median part of the Rif Belt (Mesorif Zone), Upper Jurassic-Lower Cretaceous deposits contain numerous relics of volcanic materials (basalt pebbles and interstratified lava flows), and several large gabbro massifs, especially in the Central Mesorif. The most important of these massifs, Bou Adel, was regarded as an intrusive gabbro of varying age (according to successive authors: Barremian, Late Jurassic to Senonian, and Bathonian respectively), and recently as an oceanic gabbro of latest Jurassic-earliest Cretaceous. Based on a radiometric (190±2 Ma) date from zircon grains, Michard et al. (2018) ascribed an Early Jurassic (Liassic) age to this gabbro. Consequently these authors proposed a new interpretation of the oceanic opening of the External Rif Domain related to the eastwards expansion of the Central Atlantic oceanic rift. They also hypothesized that the Upper Jurassic-Lower Cretaceous volcanic lavas and the ophiolite complexes of the Mesorif Zone are heterochronous and correspond to two, distinct and separated in time, magmatic events. This new age ascribed to the Bou Adel gabbro and the new interpretation of the oceanic opening of the External Rif disagree with the stratigraphic data and the geodynamic framework of the External Rif domain during the Mesozoic. On the base of numerous stratigraphic data and the palaeogeographic evolution, we evidence herein that the volcanic lavas and the oceanic gabbros of the Mesorif Zone all originated from the same geodynamic event related to the westwards expansion of the Tethys Ocean at the Jurassic-Cretaceous boundary, and that these oceanic gabbros are ophiolite complexes of the Senhadja and Bou Haddoud nappes, thrusted over the External Mesorif Zone during the Alpine Miocene orogeny.