Middle Bajocian Research Articles

Tracking a drowning unconformity up to the peritidal zone: Proximal expression of the early Bajocian carbonate crisis in Morocco

The early Bajocian environmental crisis is marked by floral and faunal turnovers, and a positive carbon excursion recognized in both marine and continental archives. In the basinal setting of the Central High Atlas Basin, it is concomitant to a drastic drop of carbonate content, interpreted as a result of a severe carbonate demise event in neritic settings. In order to provide a carbonate platform perspective of this demise event, we have investigated a proximal–distal transect consisting of five sections of the Errachidia Platform, which represents upper neritic settings on the southern margin of the basin. New nannofossils and brachiopod findings, supported by organic matter carbon isotope chemostratigraphy, allow for the establishment of a high-resolution chronostratigraphic framework of the Errachidia Platform and a detailed reconstruction of carbonate factory evolution during the late Aalenian to middle Bajocian interval. The early Bajocian (Propinquans Zone) environmental crisis triggered a severe demise event which restricted carbonate production to supra-to intertidal settings, where microbial and peloidal limestones are the only trace of carbonate production. As a consequence, most of the platform is dominated by extended marl deposits that are usually interpreted as hemipelagic deposits. This observation demonstrates that caution must be exerted when interpreting the depositional environments of marl-dominated successions during time of neritic carbonate factory demise.

IL ROSSO AMMONITICO INFERIORE (BAIOCIANO - CALLOVIANO) NEI MONTI LESSINI SETTENTRIONALI (VERONA)

The aim of this paper is the description, from both the biostratigraphical and sedimen• tological point of view, Of the lower member of the Rosso Ammonitico Veronese, one of the most known red, nodular, pelagic limestones of the mediterranean Jurassic.
 The R.A.V. is a condensed sequence Of red, mostly nodular, limestones, Often rich in internal molds Of ammonites, which is areally restricted to the "Trento Ridge", a structural high isolated in the early Jurassic by the rifting Of a Triassic, large epicontinental carbonate—platform. It spans in age from late Bajocian to Tithonian and is separated from the underlying shallow—water carbonates Of Liassic and Aalenian age by a sharp break in sedimentation, underlined by a mineralized hard ground, which lasted for all early and middle Bajocian.
 Two members may bc recognized in the R.A.V.: a lower (Rosso Ammonitico Inferiore) and an upper one (Rosso Ammonitico Superiore) separated by a stratigraphic gap. The lower member, ranging in age from late Bajocian to middle Callovian, is well developed only in the southern part of the 'Trento Ridge" (i. e. in the Lessini Veronesi area) and has a variable thickness which reaches a maximum of 8 meters.

Recent geodynamics, active faults and earthquake focal mechanisms of the zone of pseudosubduction interaction between the Northern and Southern Caucasus microplates in the southern slope of the Greater Caucasus (Azerbaijan)

Our study was focused on the active tectonics of the southern slope of the Greater Caucasus within Azerbaijan. The study area is the zone of under-thrusting (pseudosubduction) interaction between Southern and Northern Caucasus continental microplates, which caused the tectonic stratification of the Alpine formations into various allochthonous and parauthochthonous thrust slices of southern vergency between the Middle Bajocian and Quaternary periods. These slices are grouped into the nappe complexes that form the modern structure of the trough in the study area. The large linearly stretched tectonic units (megazones) correspond to the axis of the Alpine marginal sea basin, the consolidated crust of which is subjected to destruction and thinning. The trough’s Alpine cover was compressed in the underthrust zone and pushed southwards. As a result, an accretionary prism formed allochthonously overlapping the northern side of the Southern Caucasus microplate by the system of gently dipping overthrusts. During the continental stage of Alpine tectogenesis (starting from the end of Miocene), intensive lateral compression process was caused by intrusion of the frontal wedge of the Arabian indenter into the buffer structures of the southern frame of Eurasia. This is evidenced by the GPS monitoring data on modern geodynamic activity, which demonstrates the Southern Caucasus block’s intensive (up to 29 mm/year) intrusion in the northern rhumbs as compared to the relative stability of the Northern Caucasus microplate (0–6 mm/year). This, in turn, is a reflection of the ongoing pseudosubduction regime (continental subduction or S-subduction) at the band of collision junction of these microplates. It is suggested that this process caused historically observed seismic activity in the study area, wherein the earthquakes occurred mainly in the southern slope’s accretionary prism area and the adjacent strip of the Southern Caucasus microplate. In this article, we analyze and correlate the whole range of seismic events that occurred in the study area until 2017 and the focal mechanisms of the recently recorded earthquakes (2012–2016). It is established that earthquake foci are confined either to the intersection nodes of variously trending ruptures with the faults of different directions or to the planes of deep tectonic ruptures and lateral displacements along the unstable contacts between the material complexes with different competence. The focal mechanisms of seismic events reveal various, mostly near-vertical, planes of normal and strike-slip faults. However, the earthquake foci are generally confined to the intersection nodes between the Caucasus and anti-Caucasus-striking rupture dislocations. The results of our studies are interesting in terms of their real-time application for drawing a regional summary of causes for both geodynamic and seismic activity of the Greater Caucasus system and the adjacent areas of Alpine-Himalayan fold belt.

The Jurassic–Early Cretaceous basalt–chert association in the ophiolites of the Ankara Mélange, east of Ankara, Turkey: age and geochemistry

AbstractThis study is focused on slide blocks including oceanic lavas associated with pelagic sediments within the eastern part of the Ankara Mélange. A detailed petrological characterization of the volcanic rocks and a detailed biochronological investigation of the associated radiolarian cherts in eight sections (east of Ankara) was carried out. The volcanic rocks are largely represented by basalts and minor ferrobasalts and trachytes. They show different geochemical affinities and overlapping ages including: (a) Late Jurassic – Early Cretaceous garnet-influenced MORB (middle late Oxfordian to late Kimmeridgian–early Tithonian and early–early late Tithonian; late Valanginian–early Barremian); (b) Early Cretaceous enriched-MORB (middle late Barremian–early early Aptian; Valanginian to middle Aptian–early Albian); (c) Middle Jurassic plume-type MORB (early–middle Bajocian to late Bathonian–early Callovian); (d) Late Jurassic – Early Cretaceous alkaline basalts (middle–late Oxfordian to late Kimmeridgian–early Tithonian; late Valanginian to late Hauterivian). All rock types show a clear garnet signature, as testified to by their high MREE/HREE (middle rare earth element/heavy rare earth element) ratios. The coexistence of chemically different rock types from Middle Jurassic to Early Cretaceous times suggests that they were formed in a mid-ocean ridge setting from partial melting of a highly heterogeneous mantle characterized by the extensive occurrence of OIB-metasomatized portions, which were likely inherited from Triassic mantle plume activity associated with the continental rift and opening of the Neotethys branch.

Radiolarian evidence for the age of chert blocks from the Upper Cretaceous ophiolitic mélange unit of the Erakh area, Armenia

Abstract Two new and relatively well-preserved radiolarian faunas were extracted from blocks included in the Cretaceous ophiolitic mélange unit that crops out in the Erakh area south of Yerevan, Armenia. One of these radiolarian assemblages was extracted from cherts that are in stratigraphic contact with variolitic lavas. This radiolarian assemblage suggests a late Aalenian to middle Bajocian age (Unitary Association Zones 2–3) based on the co-occurrence of the species Parasaturnalis diplocyclis (Yao) and Transhsuum hisuikyoense (Isozaki & Matsuda). A second radiolarian assemblage was obtained from a block, several metres thick, of alternating pelagic limestones and cherts. It was characterized by the co-occurrence of the species Hemicryptocapsa capita Tan and Dicerosaturnalis dicranacanthos (Squinabol). The presence of these two species within the same assemblage correlates with Unitary Association Zone 17 and these samples are therefore assigned to the late Valanginian. These results suggest that submarine volcanic activity took place during the late Aalenian–middle Bajocian interval and that mixed carbonate and siliceous ooze accumulated during the late Valanginian. Previous biochronological data on Radiolaria from the Erakh mélange and the Vedi ophiolite are reviewed and their implications for the geodynamic and palaeoenvironmental evolution of the Tethyan ocean in Armenia are discussed.

Aalenian–Early Bathonian (Middle Jurassic) radiolarian assemblages in the Tavas nappe within Lycian nappes in the western Taurides (SW Turkey): The first dating of carbonate platform drowning

Stratigraphic sections (Kizilca and Kizilca North) in the Tavas nappe, western Taurides, Turkey records the event of carbonate platform drowning in the western Tethyan realm. Our detailed biostratigraphic study of radiolarians and other microfossils places the important temporal constraints on this event and indicates that drowning occured in the geochronological interval from Early Jurassic (Hettangian–Sinemurian) through to Middle Jurassic (early Bathonian). The basal part of the Kizilca section is represented by platform carbonates with abundant benthic foraminifera and algae of Hettangian–Sinemurian to Pliensbachian age. Toward the upper part of the section, the occurrence of brecciated limestones with some ammonites and belemnites reveals that the drowning event began in the Pliensbachian. The first radiolarian assemblages of early–middle to late Aalenian age were obtained from the overlying units characterized by pelagic limestones and chert alternation. Just after the subsidence of the platform, ribbon chert sedimentation began in the late Aalenian, as indicated by the radiolarians. Successively, early–late Bajocian and latest Bajocian–early Bathonian radiolarian assemblages were obtained from overlying chert with mudstone–silicified mudstones. Similar to that observed in the Kizilca section, the Kizilca North section include successively pelagic limestones, pelagic limestone–chert alternation and chert–mudstone alternation of late Aalenian to early–middle Bajocian age according to radiolarian biostratigraphy. All of these observations indicate that a continuous stratigraphy recorded the drowning event without a large gap from Hettangian–Sinemurian to early Bathonian in the Tavas nappe. This drowning event has close similarities with coeval events in other Tethyan realms in terms of mechanism, facies characteristics and fossil assemblages. Therefore, this study can aid correlation studies of the Tavas nappe with the Jurassic paleogeography worldwide.

Plate tectonic reconstructions and paleogeographic maps of the central and North Atlantic oceans1This article is one of a series of papers published in this CJES Special Issue on the theme ofMesozoic–Cenozoic geology of the Scotian Basin.2Earth Sciences Sector Contribution 20120172.

We have established a new plate kinematic model of the central and North Atlantic oceans between North America, Africa, Meseta, Iberia, Flemish Cap, and Galicia Bank from Late Triassic to Late Cretaceous to better understand the nature and timing of rifting of Nova Scotia and Morocco conjugate continental margins since Late Triassic. The maps of salt distributions at the Sinemurian–Pliensbachian limit (190 Ma; after salt deposition) and in middle Bajocian (170 Ma) show that an area of the Nova Scotia margin is devoid of allochthonous salt and that an area of similar size located oceanward of the West African Coast Magnetic Anomaly shows salt deposits, suggesting that a portion of the Nova Scotia margin with its overlying salt deposits could have been transferred onto the Moroccan side right after the formation of the conjugate East Coast Magnetic Anomaly and West African Coast Magnetic Anomaly. Seven paleogeographic maps, from Late Triassic to Late Cretaceous, are presented with structural elements and magnetic lineations. They show that the connection between the Central Atlantic and the Tethys, with an aborted rift between Iberia and North America ending in the north against the Flemish Cap – Galicia Bank dam, started to deepen at the end of the first rifting phase (190 Ma ago) after the rupture of the thinned continental crust. It is only during the Early Cretaceous, after the rupture of the Flemish Cap – Galicia Bank dam, that the deep connection around Iberia was finally established between the Central and North Atlantic, the Tethys, and the Bay of Biscay.

The Middle Jurassic basinal deposits of the Surmeh Formation in the Central Zagros Mountains, southwest Iran: facies, sequence stratigraphy, and controls

The lower part of the Lower to Upper Jurassic Surmeh Formation consists of a succession of shallow marine carbonates (Toarcian–Aalenian) overlain by a deep marine basinal succession (Aalenian–Bajocian) that grades upward to Middle to Upper Jurassic platform carbonates. The termination of shallow marine carbonate deposition of the lower part of the Surmeh Formation and the establishment of deep marine sedimentation indicate a change in the style of sedimentation in the Neotethys passive margin of southwest Iran during the Middle Jurassic. To evaluate the reasons for this change and to assess the basin configuration during the Middle Jurassic, this study focuses on facies analysis and sequence stratigraphy of the basinal deposits (pelagic and calciturbidite facies) of the Surmeh Formation, referred here as ‘lower shaley unit’ in the Central Zagros region. The upper Aalenian–Bajocian ‘lower shaley unit’ overlies, with an abrupt contact, the Toarcian–lower Aalenian platform carbonates. It consists of pelagic (calcareous shale and limestone) and calciturbidite facies grading to upper Bajocian–Bathonian platform carbonates. Calciturbidite deposits in the ‘lower shaley unit’ consist of various graded grainstone to lime mudstone facies containing mixed deep marine fauna and platform-derived material. These facies include quartz-bearing lithoclast/intraclast grainstone to lime mudstone, bioclast/ooid/peloid intraclast grainstone, ooid grainstone to packstone, and lime wackestone to mudstone. The calciturbidite layers are erosive-based and commonly exhibit graded bedding, incomplete Bouma turbidite sequence, flute casts, and load casts. They consist chiefly of platform-derived materials including ooids, intraclasts/lithoclasts, peloids, echinoderms, brachiopods, bivalves, and open-ocean biota, such as planktonic bivalves, crinoids, coccoliths, foraminifers, and sponge spicules. The ‘lower shaley unit’ constitutes the late transgressive and the main part of the highstand systems tract of a depositional sequence and grades upward to platform margin and platform interior facies as a result of late highstand basinward progradation. The sedimentary record of the ‘lower shaley unit’ in the Central Zagros region reveals the existence of a northwest–southeast trending platform margin during the Middle Jurassic that faced a deep basin, the ‘Pars intrashelf basin’ in the northeast. The thinning of calciturbidite layers towards the northeast and the widespread Middle Jurassic platform carbonates in the southern Persian Gulf states and in the Persian Gulf area support the existence of a southwest platform margin and platform interior source area. The platform margin was formed as a result of tectonic activity along the preexisting Mountain Front fault associated with Cimmerian continental rifting in northeast Gondwana. Flooding of the southwest platform margin during early to middle Bajocian resulted in the reestablishment of the carbonate sediment factory and overproduction of shallow marine carbonates associated with sea-level highstand, which led to vertical and lateral expansion of the platform and gradual infilling of the Pars intrashelf basin by late Bajocian time.

Faunal associations, paleoecology and paleoenvironment of marine Jurassic rocks in the Mae Sot, Phop Phra, and Umphang areas, western Thailand

We here report a paleoecological analysis and depositional history of the marine Jurassic (Toarcian-early Bajocian) strata cropping out in the western part of Thailand, based on bivalve assemblages with additional data from ammonites, brachiopods, and microfossils. Generally, the benthic bivalve facies in most outcrops is rich in infaunal, semi-infaunal and epifaunal suspension-feeders. Of these, infaunal forms dominate. The diversity of this benthic assemblage was influenced by energy level, substrate, sedimentation rate, and salinity. Low to intermediate energy levels and rather soft fine-grained siliciclastic substrate are proposed as factors governing faunal distribution and explaining the greater abundance and diversity of infaunal than epifaunal suspension-feeders. There were paleoenvironmental changes both in space and time, i.e., from south to north (Umphang to Mae Sot) and from Early Bajocian to Toarcian. In the Toarcian, most outcrops in Umphang are dominated by benthic bivalve facies (infaunal, semi-infaunal, and epifaunal associations). This implies warm, shallow water (inner neritic, 50-100 m) and oxygenated conditions except for the Mae Sot area where a deeper setting (outer neritic to possibly upper continental slope, 50-200 m) with restricted basinal anoxic conditions is favored as indicated by the presence of Bositra. After higher energy conditions in the Toarcian, lower energy conditions with low sediment supplies prevail in the Alenian, and the Mae Sot area was still a restricted basin. As a result of higher sea levels, the oxygen content in the basin is increased, resulting in the presence of the ammonites. By the end of the Alenian-early Bajocian, an ammonite-bivalve association (mixed facies A) and the presence of corals and microfauna (mixed facies B) are dominant but pass upwards to near-shore higher energy conditions in most areas except for restricted basin in Mae Sot. By the middle Bajocian the environment in all areas had changed from marine to nonmarine.

An overview of the stratigraphy and facies development of the Jurassic System on the Tabas Block, east-central Iran

Abstract The Tabas Block of east-central Iran shows very thick and well-exposed Upper Triassic–Jurassic sequences, which are crucial for the understanding of the Mesozoic evolution of the Iran Plate. The succession is subdivided into major tectonostratigraphic units based on widespread unconformities related to the Cimmerian tectonic events. As elsewhere in Iran, there is a dramatic change from Middle Triassic platform carbonates (Shotori Formation) to the siliciclastic rocks of the Shemshak Group (Norian–Bajocian), reflecting the onset of Eo-Cimmerian deformation in northern Iran. Following the marine sedimentation of the Norian–Rhaetian Nayband Formation, the change to non-marine, coal-bearing siliciclastic rocks (Ab-e-Haji Formation) around the Triassic–Jurassic boundary is related to the main uplift phase of the Cimmerian orogeny. Condensed limestones of the Toarcian–Aalenian Badamu Formation indicate widespread transgression, followed by rapid lateral facies and thickness variations in the succeeding Lower Bajocian Hojedk Formation. This tectonic instability culminated in the middle Bajocian compressional–extensional Mid-Cimmerian event. The resulting Mid-Cimmerian unconformity separates the Shemshak Group from the Upper Bajocian–Upper Jurassic Magu (or Bidou) Group. The succeeding Late Bajocian–Bathonian onlap of the Parvadeh and Baghamshah formations (Baghamshah Subgroup) was caused by increased subsidence of the Tabas Block rather than a eustatic sea-level rise, followed by the development of a large-scale platform–basin carbonate system (Callovian–Kimmeridgian Esfandiar Subgroup). Block faulting starting in the Kimmeridgian (Late Cimmerian event) resulted in the destruction of the carbonate system, which was covered by Kimmeridgian–Tithonian limestone conglomerates, red beds and evaporites (Garedu Subgroup or Ravar Formation). Virtually the same pattern of relative sea-level change, facies development and succession of geodynamic events is recorded from the Late Triassic–Jurassic of northern Iran (Alborz Mountains), suggesting that the Iran Plate behaved as a single structural unit at that time.

Siliceous-clayey rocks of the Jurassic accretionary prism, Khekhtsir Range, Sikhote Alin: Stratigraphy and genesis

Lithologic-stratigraphic aspects of siliceous-clayey rocks forming the Khabarovsk terrane of the Jurassic accretionary prism were studied in western spurs of the Bol’shoi Khekhtsir Range on the left side of the Ussuri River (Ussuri-Khekhtsir section). Two defined types of the examined section differ in the composition, age, and origin of their constituting rocks. The northern segment of the section is composed of middle Bajocian red-brown siliceous-tuffaceous silty and olive-gray silty mudstones that accumulated in the hemipelagic domain under the influence of continental provenance. Its southern segment is represented by lower Bathonian olive-gray siliceous mudstones, mudstones barren of any admixtures, and yellowish brown tuffaceous mudstones deposited far away from the continent in waters with abundant radiolarians. It is shown that these rocks are elements of two tectono-stratigraphic complexes that reflect different stages in the accretionary prism formation.

DOCUMENTATION AND SIGNIFICANCE OF TECTONIC EVENTS IN THE NORTHERN TABAS BLOCK (EAST-CENTRAL IRAN) DURING THE MIDDLE AND LATE JURASSIC

Apart from global sea-level fluctuations, the sedimentary pattern of the northern Tabas Block during the Middle and Late Jurassic was mainly governed by tectonic events of varying intensity and areal extent. These events took place during the Middle Bajocian (mid-Cimmerian tectonic phase), Early Bathonian, Late Bathonian, Early Callovian, Late Oxfordian, and Late Kimmeridgian. The importance and extent of each event and its influence on the facies pattern of the northern Tabas Block is briefly discussed and demostrated by some examples.

Carbon‐Cycle Perturbation in the Middle Jurassic and Accompanying Changes in the Terrestrial Paleoenvironment

Abstract Carbon‐isotope analyses of fossil wood from the Middle Jurassic Ravenscar Group, Yorkshire, NE England, reveal a significant excursion toward light isotopic values (δ13C change of −3 to −4‰) at about the Aalenian‐Bajocian boundary (∼174 Ma). A positive carbon isotopic excursion is also shown for the middle Bajocian (∼170 Ma) but is less clearly defined. These isotopic patterns are very similar to the few published marine carbonate records available for this time, in particular one based on belemnites from the Hebrides basin, NW Scotland, and others from pelagic limestones in Italy. The similarity of the terrestrial and marine isotope curves is an indication that the observed isotopic signal is a global phenomenon. Through parts of the Ravenscar Group (the Scarborough Formation), supplementary data from bulk organic carbon and palynofacies analysis confirm that isotopic curves based on bulk analyses may be strongly influenced by the balance of terrestrial versus marine organic matter present in th...

Patterns of biotic change in Middle Jurassic to Early Cretaceous Tethyan radiolaria

The rate of taxic turnover of nearly 400 radiolarian species/subspecies is analyzed in order to document long term biotic change of plankton during the Middle Jurassic to Early Cretaceous (Aalenian to Aptian). The pattern and dynamic of diversity change is described using four indices: rate of species first and last occurrence, rate of diversification and rate of turnover. Plots of cumulative sampling effort suggest that the analyzed data represent an adequate sample of total standing diversity for most examined stages. Rates of species first occurrence exceed rates of last occurrence for most of the Middle Jurassic, except for the middle Bajocian. In contrast, the Late Jurassic was a time of decreasing radiolarian diversity and the Kimmeridgian records the lowest rate of diversification. It is followed by a dramatic increase in first occurrences near the Jurassic–Cretaceous boundary with as a result the highest rate of diversification recorded in the late Tithonian. Regional radiolarian diversity was stable throughout most of the Early Cretaceous. A stratigraphic permutation test was performed to assess the influence of uneven sampling on the observed pattern of taxic turnover and identified the intervals for which randomly obtained patterns are significantly different from the observed pattern. The Kimmeridgian and late Tithonian events coincide with substantial climate-derived perturbations in water cycling, nutrient supply and oceanic productivity. They point to a negative relationship between radiolarian macroevolution and changes in the state of nutrient availability, although further work is needed to refine the temporal resolution of this relationship and to explore ecological aspects of its causal link with respect to radiolarian evolution.

Middle and Late Jurassic radiolarian palaeoecology versus carbon-isotope stratigraphy

Jurassic radiolarians are sensitive to palaeoclimatic–palaeoceanographic changes, recorded by the stable carbon-isotope curve. Carbon-isotopic variations have been linked to changes in ocean structure, productivity and concentrations of greenhouse gases in the atmosphere. In the Middle–Upper Jurassic Terminilletto section of the Umbria–Marche–Sabina Apennines (central Italy), well preserved radiolarians are present in a sequence of cherty limestones interbedded with platform-derived carbonates. The δ 13C curve in the Aalenian–Kimmeridgian interval indicates three main positive δ 13C `events' in the lower Bajocian, late Bathonian–lower Callovian and middle Oxfordian. The positive δ 13C shifts are correlated to increases in visible chert in the outcrop and may represent periods of increased biological productivity of surface sea-water. Spumellaria/Nassellaria ratios and the fluctuations of relative abundance of selected genera tend to correlate with δ 13C and chert abundance values, and seem to be related to episodes of high nutrient mobilisation. From the base of the Bajocian, the radiolarian associations show a slight tendency towards diversification, coincident with increasing δ 13C values. In the upper part of the middle Bajocian, last appearances prevail over first appearances associated with the peak of the δ 13C event. This may indicate maximum eutrophic conditions. Coincident with the late Bathonian–early Callovian positive δ 13C shift, the radiolarian associations record a different trend with respect to the Bajocian event. When the δ 13C values are increasing to a maximum, radiolarian associations tend to be reduced in species diversity, followed by a small diversification trend when the δ 13C values are decreasing. We speculate that during the Callovian a more `stressful' palaeoenvironment, due to high level of eutrophication, is indicated by the combination of high productivity of biogenic silica and low carbonate input. Although varying preservational conditions may have influenced the observed trends in the radiolarian associations, the good correlation between changes of sedimentation patterns, radiolarian assemblages and δ 13C positive anomalies, open interesting avenues for further investigations concerning the palaeoecology of Mesozoic radiolarians.

Response of shallow-marine carbonate facies to third-order and high-frequency sea-level fluctuations: Hauptrogenstein Formation, northern Switzerland

Facies patterns in shallow-marine carbonates of the Hauptrogenstein Formation of northern Switzerland (middle Bajocian to early Bathonian) reflect small changes in sea level. Third-order sea-level fluctuations, calculated from large-scale facies patterns (alternations of marl- and carbonate-dominated sedimentary units on the level of members) and subsidence, show two phases of sea-level rise and fall in the late Bajocian and a relative sea-level rise in the early Bathonian. Most of the sedimentary rocks were produced during phases of relative rise and highstand of sea level. No periods of emergence during lowstands are preserved. Sea-level changes of a higher order have been analyzed by facies changes at the outcrop scale. Thin shallowing-up parasequences (0.5–6 m thick) have been correlated across platform, ramp and offshore sedimentary environments. In each of these environments carbonates react differently to sea-level fluctuations. Relative rises in sea level are represented by non-deposition and biogenic sediments, periods of stable or slightly falling sea level are dominated by oolitic sediments: (1) in the offshoal environment, marl layers at the base of thin shallowing-up units are covered by increasingly proximal tempestites; (2) in the ramp environment, oblique-stratified, oolitic and bioclastic grainstones are bounded by marine flooding surfaces; (3) on the platform, marine flooding surfaces are overlain by beds deposited under moderate-energy conditions (coral beds, oncoid beds, platform tempestites) which are in turn covered by high-energy, oblique-stratified oolitic grainstones. Such parasequences, deposited in response to small-scale, high-frequency sea-level changes can be correlated over distances of more than 100 km. A model for evolution and response of carbonate facies to sea-level changes is presented.

Middle Jurassic (Middle Bajocian-Middle Bathonian) ammonites from Gebel Maghara, Sinai, Egypt: In the collections of the Geological Survey of Israel

Middle Jurassic ammonoids from Gebel Maghara (Sinai, Egypt), kept in the collection of the Geological Survey of Israel, ranging from the Dorsetensia beds (Middle Bajocian) up to the Micromphalites beds (Middle Bathonian) are studied. Fifty- nine species included in 30 genera are discussed, of them 44 species and 12 genera which have not been reported from Gebel Maghara by previous authors, and 5 New species. Douvilleia n. gen. is proposed for irregularly coiled Stephanoceratids that are similar externally to Normannites or Cadomites. Pseudotorrensia n. gen. is proposed for a homoeomorph of torrensia Sturani (1971). The sequence of Middle Jurassic ammonites at Gebel Maghara is compared with the sequence in the corresponding section at Jebel Tuwaiq (Central Arabia).

Tectonically Controlled Facies Changes in the Jurassic of the Northern Border of the Central High Atlas East of Midelt (Morocco): ABSTRACT

Jurassic carbonate sedimentation and facies changes at the northern border of the Central High Atlas mountain range east of Midelt are mainly controlled by synsedimentary tectonics which took place during the development of the Central High Atlas rift. There is no evidence of the influence of global eustatic sea level changes. During early Liassic time the carbonate facies documents an extended platform with numerous isolated mud mounds surrounded by biodetritic limestone fans. From middle Liassic to early Dogger (Aalenian) time, different kinds of mass-flow deposits coincide with the maximum extension of the Central High Atlas trough. Large-scale slide blocks, conglomerates, mud-flow and debris-flow sediments, etc, show the existence of a platform-basin border formed by a preexisting fault system acting as downthrusts during Jurassic time. A sudden diminution of the subsidence at the Aalenian-Bajocian border is indicated by nearshore carbonates (tidal flats, tidal channels). Already at middle Bajocian time red beds were deposited. From the Oligocene onward during a compressive phase most likely due to the Rif orogenesis, these faults were reactivated. As a result, the east-northeast-striking faults which oriented at right angles toward the direction of compression were transformed to inverse faults and the north-northeast-striking faults became sinistral strike-slip faults.more » At the fault junction, areas of higher compression were created, inducing a cleavage of the Mesozoic sediments. Whereas upfolds predominantly occur over preexisting fault zones, wide flat downfolds developed above the basement blocks.« less

SEDIMENTARY MODELS FOR THE NON-MARINE SCALBY FORMATION (MIDDLE JURASSIC) AND EVIDENCE FOR LATE BAJOCIAN/BATHONIAN UPLIFT OF THE YORKSHIRE BASIN

SUMMARY Alternative alluvial and deltaic depositional models for the topmost Scarborough and Scalby Formation are outlined. Biostratigraphical and sedimentological arguments are used to deduce that a lengthy time gap of around 9.0 m.y. existed between deposition of the shallow marine Scarborough Formation (middle Bajocian) and the fluvial Moor Grit Member of the Scalby Formation (? upper Bathonian). This rules out acceptance of the deltaic model for the successions and is taken to indicate that much of the Yorkshire Basin was uplifted or became positive during late Bajocian/Bathonian times. Uplift may have been linked to rift-related processes acting in the North Sea basin, especially on the Pennine/Mid-North Sea High which acted as the hinterland and sediment source area for the Scalby alluvium.

Some Lower and Middle Jurassic Beds in Puale Bay-Alinchak Bay Area, Alaska Peninsula: GEOLOGIC NOTES

Studies of Jurassic ammonites and Inoceramus from the northeast shore of Puale Bay and from the south side of Alinchak Bay demonstrate the presence of at least 1,500 ft (457 m) of beds of early to middle Bajocian (Middle Jurassic) age. These form the upper part of a sequence that is 1,900 ft (580 m) thick and that consists mostly of dark-gray siltstone except for one unit of coarse conglomerate in its upper part. The siltstone also contains middle to late Toarcian (Early Jurassic) ammonites about 30 ft (9 m) above its base and lies with sharp contact on massive tufaceous conglomeratic sandstone of early Sinemurian age. The upper contact with the Shelikof Formation of Callovian age is marked by an upward change from nonvolcanic gray siltstone that weathers gray to siltston that bears thin beds of white to yellow volcanic ash and consequently weathers brown. The entire siltstone sequence is assigned to the Kialagvik Formation because it is similar lithologically and occupies the same stratigraphic position below the Shelikof Formation.