Jurassic to Cretaceous radiolarian biostratigraphy and sedimentary evolution of the Budva zone (Dinarides, Montenegro)

The Budva Zone is the northernmost part of a long belt of Mesozoic basinal deposits, which extend southward to the Krasta-Cukali Zone in Albania and Pindos-Olonos Zone in Greece. Lowermost Jurassic to middle Cretaceous formations are defined and described. Radiolarians from 105 samples collected in ten sections allowed us to date pelagic sequences and to constrain ages of intervening carbonate gravity­flow deposits. Systematics of about 200 recorded radiolarian species is discussed and supported by illustrations. For the Middle Jurassic to Turonian tiJne interval, a local radiolarian zonation is constructed by means of the Unitary Association Method (Guex, 1977, 1991 ). One hundred and thirty-nine taxa were used in the database. Forty-eight Unitary Associations are established and grouped into 15 distinct "zones". The calibration is based on the existing zonations. The Budva Zone formations are correlated to time-equivalent lithologies in the tectonically overthrusting High Karst Platform. The correlation reveals a close relationship between the sedimentary and tectonic activity of the High Karst Platform margin, and facies evolution in the adjacent Budva Basin. The Hettangian to Sinemurian lime-poor "Passée Jaspeuse" Formation coincides with a subsidence of the High Karst Platform margin. In the Pliensbachian to lower Toarcian the entire basin was characterized by resedimented carbonates (Lower Bar Limestone Member). The margin-ward propagation of radiolarite sedimentation (Lastva Radiolarite) and retreat of resedimented carbonates (Upper Bar Limestone Member) in the Middle Jurassic are related to a development of continuous oolitic bars on the platform. The maximum expansion of radiolarites was attained in the Oxfordian and Kimmeridgian, when the platform margin was fringed by a large reef complex. Most of the carbonate mud in the Jurassic basinal succession was probably of platform origin. Periods of reduced periplatform-ooze supply were characterised by lime-poor to lime-free basinal sedimentation. In the late Tithonian, distal sequences show a transition from siliceous to carbonate deposition (Praevalis Limestone). In the Hauterivian-Barremian, again, radiolarite sedimentation (Bijela Radiolarite) progressively replaced pelagic carbonates and persisted to the Turonian. These facies changes are correlative with synchronous shifts in the Southern Alps and Apennines. The Budva Basin, however, differs from other Tethyan basins by a lower proportion of carbonate in the Upper Jurassic and Cretaceous sequences. The composition and distributional pattern of resedimented carbonates changed significantly by Late Jurassic time. Prior to that time, in the Early and Middle Jurassic, carbonate gravity-flow deposits were composed of remobilized pelagic sediments and penecontemporaneous platform debris. Contrary to this, since the Tithonian the bulk of the resedimented carbonates was derived from the erosion of lithified shallow water limestones. Coarse grained calcareous turbidites became restricted to the northwestern depositional area. This facies change is believed to reflect the evolution from an extensional to a compressive regime in the internal domains of the Dinaric Tethys, which induced a dif­ferential uplift of the High Karst Platform

THE JURASSIC SEDIMENTARY EVOLUTION OF A CARBONATE PLATFORM INTO A DEEP-WATER BASIN, MT. MANGART (SLOVENIAN-ITALIAN BORDER)

A complete Jurassic succession, recording the evolution from platform margin to a deep-water basin, is exposed at Mt. Mangart in the Julian Alps. The succession is a part of the Julian Nappe, where the Southern Alps overlap with the Dinarides. In the Jurassic, the area comprised part of the south Tethyan passive continental margin. The section was studied sedimentologically in detail and dated with radiolarians. It is divided into five lithostratigraphic units: Unit 1: Lower Jurassic shallow-water peloidal and oncoidal limestones; Unit 2: Pliensbachian distal shelf limestones rich in juvenile ammonites and sponge spicules topped by an Fe-Mn hardground; Unit 3: lower to possibly middle Toarcian sequence of black shales with interbedded siliceous limestone; Unit 4: upper Bajocian/Bathonian to lower Tithonian cherts, cherty limestones, and carbonate gravity-flow deposits; Unit 5: upper Tithonian red nodular cherty limestones with abundant calpionellids and aptychi. A stratigraphic gap, comprising the late Toarcian to early Bajocian, separates Unit 4 from Unit 3. In general, the succession correlates well with known Tethyan transgressive/regressive facies cycles. In addition, two periods of accelerated subsidence were recognized, the first, in the Pliensbachian, drowned the platform, the second, prior to the late Bajocian, created accommodation space for resedimented carbonate deposits from the adjacent Friuli Carbonate Platform. The present day position of the succession is between the Belluno Basin to the west and the Slovenian Basin to the south. The hitherto described successions of these two basins were located more distally from the Friuli Carbonate Platform than the Mt. Mangart succession.

Palaeogeographical evolution of the Arabian platform during the obduction of the Samail ophiolite

International audienceObduction of a dense oceanic lithosphere on top of a light continental lithosphere remains one of the oddest phenomenons in plate tectonics. In northern Oman, the emplacement of the Samail (or Oman) ophiolite took place during Upper Cretaceous. The Samail ophiolite is probably the best studied ophiolite in the world because of its large size and because it has been relatively well preserved from deformation after its emplacement. If many studies have been carried out on the ophiolite itself in order to better understand the structure of an oceanic crust, little is known on the processes leading to its emplacement on the top of the Arabian platform. The sedimentary series deposited on the Arabian platform during the emplacement of the ophiolite could help us to understand the processes involved during the obduction by providing tectono-stratigraphic constraints on the evolution of the platform affected by the obduction. Moreover the reconstitution of the palaeoenvironments and the identification of source areas would allow discussing the evolution of reliefs formed in such a unique context. However, these series, being located just below the allochthonous units, have been affected by deformation. They are grouped in one formation, the Muti Formation, consisting of various lithologies of which the age is uncertain. In consequence, sedimentological analyses require careful mapping of the sedimentary series and biostratigraphical determinations remain difficult. The first results of our sedimentological and stratigraphical investigations show (i) a diachronism of the onset of syn-tectonic sedimentation (chemostratigraphic and biostratigraphic data) and (ii) a high variation of the source areas within the basin. These data are used in order to present a new model of evolution of the basin formed during the emplacement of the Samail ophiolite on top of the Arabian Platform. Implications for the processes involved during the obduction are also discussed

CALLOVIAN RADIOLARIANS FROM THE LOWERMOST CALCARE SELCIFERO DI FONZASO AT PONTE SERRA (TRENTO PLATEAU, SOUTHERN ALPS, ITALY)

This paper deals with radiolarian biostratigraphy of the Calcare Selcifero di Fonzaso at the type-section of Ponte Serra (Southern Alps, Italy). The Ponte Serra section is located on the easternmost part of the Trento Plateau and represents an early-drowned sector that marks the transition to the Belluno Trough.The Calcare Selcifero di Fonzaso consists of pelagic siliceous limestones of Middle to Late Jurassic age for a total thickness of about 70 m. A 24 metres thick package of resedimented limestone is interstratified. This research is focused on the lowermost part, 9 metres thick, that mainly consists of red and green siliceous micrites with interbedded clay beds and abundant chert lenses and nodules. The studied interval lies between the lower member of the Rosso Ammonitico Veronese Formation below and the resedimented limestone above. The recovered radiolarian assemblages indicate a Callovian age for the base of the Calcare Selcifero di Fonzaso. Correlation with other sections of the Trento Plateau reveals a significant diachronism in siliceous sedimentation that we relate to highly irregular sea-bottom paleotopography. The base of siliceous deposits can be even older (Bathonian) on the eastern margin of the Trento Plateau, but younger (Oxfordian) on its western margin. The Callovian succession of the Belluno Trough consists of radiolarian-bearing micrite alternating with oolitic limestone. The oolitic sands that filled in the Belluno Trough during the Middle Jurassic prograded over the eastern margin of the Trento Plateau in the early Oxfordian.

La préservation exceptionnelle du système turbiditique carbonaté au Jurassique moyen

National audienc

Comment on the species and age determination published by Li et al. "Discovery of Radiolaria from Upper Cretaceous Oceanic Red Beds in Daba, Kangmar and its paleogeographic implication" [Palaeogeography, Palaeoclimatology, Palaeoecology 312 (2011): 127-13

Li et al. (2011a, Discovery of Radiolaria from Upper Cretaceous Oceanic Red Beds in Daba, Kangmar and its paleogeographic implication. Palaeogeography, Palaeoclimatology, Palaeoecology 312, 127-137) reported three radiolarian assemblages from Cretaceous Oceanic Red Beds (CORBs) in Chuangde Formation, which they assigned to the Late Cretaceous. With only very few exceptions all species have been erroneously determined and, consequently, the age of the samples was wrongly interpreted. None of these radiolarians suggest a Late Cretaceous age, late Santonian to early Maastrichtian, as proposed in the publication. On the contrary, all radiolarians are indicators of an Early Cretaceous age, early Barremian to earliest Albian

La préservation exceptionnelle du système turbiditique carbonaté au Jurassique moyen

National audienc

Mesozoic deep-water carbonate deposits from the southern Tethyan passive margin in Iran (Pichakun nappes, Neyriz area): biostratigraphy, facies sedimentology and sequence stratigraphy

International audienceThe objective of this work is to study the Mesozoic turbiditic sediments from the southern Tethys margin in Iran. These sediments are exposed as nappes in the Pichakun Mountains (i.e. the Zagros Mountains in the Neyriz area), which inverted during latest Cretaceous time. Radiolarians are used to both define and date four main lithostratigraphic formations: (1) the Bar Er Formation (undated, probably Late Triassic to Early Jurassic); (2) the Darreh Juve Formation (Aalenian­early Bajocian to middle Callovian­early Oxfordian); (3) the Imamzadeh Formation (middle Callovian­early Oxfordian to Aptian); (4) the Neghareh Khaneh Formation (late Aptian to Turonian­Coniacian). Most of the sediments are deep-sea gravity-flow lobe deposits. Channel deposits occurred during the Bajocian (i.e. the Darreh Juve Fm) and deeply incised channels (canyons?) occurred during the Albian (i.e. the Neghareh Khaneh Fm). Twenty-seven facies, grouped into eight facies associations, are defined. Based on a sequence stratigraphic study (i.e. the stacking pattern), five second-order cycles (10­30 Ma duration), defined between two successive distal facies time-intervals, are characterized: (1) the J2 (Toarcian?­middle Oxfordian, unconformity: Late Toarcian­Aalenian); (2) the J3 (middle Oxfordian­Berriasian, unconformity: middle? Tithonian); (3) the K1.1 (Berriasian­undated top); (4) the K1.2 (undated base­early Aptian, unconformity: late Hauterivian); (5) the K1.3 (early Aptian­at least Turonian­Coniacian, unconformity: Aptian­Albian boundary). The most important tectonic event recorded occurred at the Aptian­Albian boundary (a deposition of olistoliths, from a few metres to 100 m thick, in debris flows; related to Austrian deformations). The Arabian-scale late Toarcian and early Tithonian deformations have been recorded as unconformities. It is expected that another tectonic event occurred during the late Hauterivian. The unconformity of cycle K1.1 could be a late Valanginian eustatic fall of climatic origin

Review of, Mesozoic Radiolarian Biochronology - Current Status and Future Directions

Mesozoic radiolarian biochronologic scales have been developed since the 1970s and most of them reached their present day status in the 1990s. The degree of temporal resolution, on average, corresponds to substage level and is sufficient to provide a meaningful framework for general geological studies. The great majority of zonal schemes were elaborated in low-latitude sections but are applicable in high latitudes as well because an adequate number of species occur worldwide. This paper presents a short historical review and a synthesis of currently used zonations developed in North America, Europe and Asia. The advantages and the shortcomings of the existing zonations are discussed. As a general rule, the zonations including a high number of taxa in each zone have a much greater applicability for global correlations than those defined exclusively with marker taxa. In the forthcoming years, particular studies will focus on zonal division of under-explored time intervals and on improved calibration to chronostratigraphy. Two joint objectives for future research are briefly introduced. The first objective, achievable in a relatively short time, is to compile a composite Mesozoic zonation that would provide a single reference standard for radiolarian dating at a global scale. The second objective is to refine the radiolarian zonal schemes, which, in certain intervals, have already attained the resolution comparable to that of the standard ammonite zones. To increase the degree of precision and accuracy to this level through the entire Mesozoic is a long-term goal that requires additional high-resolution sampling and emphasis on detailed documentation of evolutionary first and last appearances in different phylogenetic lineages