Late Albian Time Research Articles

Mosirites, a New Cretaceous Heteromorph Ammonoid Genus from Hokkaido, Japan

A new genus Mosirites is herein erected for a Cretaceous heteromorph ammonoid that ranges from the upper Albian to the lower Cenomanian of Hokkaido. The new genus differs from previously described genera of the family Anisoceratidae by its large, loosely coiled, early helical whorls followed by loosely coiled elliptical planispiral whorls. Two new species, M. serpentiformis sp. nov. and M. mirabilis sp. nov., are described from the upper Albian and the upper Albian to lower Cenomanian interval, respectively. Mosirites was probably derived from Anisoceras during late Albian time and flourished during late Albian to early Cenomanian time in the North Pacific region.

Late Albian ammonites from northwest Algeria: Biostratigraphic, paleobiogeographic and paleoenvironmental inferences

Albian rocks of northwest Algeria are commonly clastic deposits with poor dating markers. This study records two late Albian ammonites from northwest Algeria for the first time. They are Elobiceras (Elobiceras) sp. and Pervinquieria (Pervinquieria) pricei (Spath). These ammonites are collected from the Mcharref Formation interpreted here to be deposited south of Tiaret city under a lagoonal carbonate ramp setting based on facies analysis. The recognition of the well-known early late Albian ammonite Pervinquieria (Pervinquieria) pricei biozone from the study area highly improves the assigned age for the studied sections. The unconformity surface separates the lower and upper members of the Mcharref Formation coincides with the upper boundary of this biozone. The correlation of this boundary with the global sequence boundaries in the frame of ammonite biozones reveals that it equivalent to the SB KAl 6 (∼103.8 my ago) that marks the end of the identified biozone everywhere. Therefore, the recorded biozone shows an excellent correlation between the study area and that of Tethyan, European, American, and Asian biozones. Paleobiogeography of the recognized biozone indicates a cosmopolitan distribution that is interpreted here as a response to the widespread distribution of the Tethyan shallow water that reached to form a connection with the northern and southern Atlantic oceans during the late Albian time.

Revised planktic foraminiferal biostratigraphy of the early late Albian of northern Tunisia (southern Tethys)

The upper Albian exposed near the Imbrication Zone of northern Tunisia is composed of a pelagic sequence that includes organic-rich beds. High-resolution biostratigraphy based on planktic foraminiferal bioevents across marker beds (i.e., organic-rich beds), allows revision of the lithostratigraphic subdivision and zonal scheme in comparison with late Albian time equivalents recorded in the northern Tethyan margins. In the present study, we proposed a new subdivision of the upper Albian lithostratigraphic section that now includes the Mellegue horizon as an equivalent of theAmadeus Segment rhythmic marl/limestone interval (central Italy). Three studied sections are subdivided into five biozones with the abundance and morphological trends of planktic foraminifera and inferred paleoenvironments: 1) The Biticinella breggiensis Total Range Zone is characterized by the appearance of ticinellids with pinched chambers (i,e., Ticinella praeticinensis) and is recorded a few meters above the first occurrence of Biticinella breggiensis. Agradual increase of trochospiral thick-walled (B. breggiensis, T. roberti, T. raynaudi) and Globigerinelloides taxa, reflecting mesotrophic conditions. 2) The Pseudothalmanninella subticinensis Interval Zone includes the first appearance of keeled forms (Ps.subticinensis), associated with abundant ticinellids with flattened chambers (B.breggiensis, T. roberti) below the base of the organic-rich Mellegue horizon, indicating an oligotrophic environment. 3) The Pseudothalmanninella ticinensis Interval Zone. The continuous occurrence of T. primula and the scarcity of Globigerinelloides throughout the lower part of this zone indicates an oligotrophic to weakly mesotrophic environment. The occurrences of radiolarian-rich horizons and abundant forms with elongated chambers (T. raynaudi) in the lower part of this zone indicate an abrupt shift to enhanced eutrophic and dysaerobic conditions. 4) The Thalmanninella appenninica Zone, which shows a decrease in abundance preceding the extinction of ticinellids, and a gradually increasing number of specialized keeled forms (i.e., Th. appenninica), indicates oligotrophic conditions. 5) The Planomalina buxtorfi Total Range Zone is characterized in its lower part by an increasing abundance of clavate forms with radially elongated chambers (i.e., Clavihedbergella) above the organic-rich bed of the Mouelha Member, indicating low-oxygen, oligotrophic to mesotrophic conditions.

Evaluating the segmented post-rift stratigraphic architecture of the Guyanas continental margin

Segmentation of the Guyanas continental margin of South America is inherited from the dual-phase Mesozoic rifting history controlling the first-order post-rift sedimentary architecture. The margin is divided into two segments by a transform marginal plateau (TMP), the Demerara Rise, into the Central and Equatorial Atlantic domains. This paper investigates the heterogeneities in the post-rift sedimentary systems at a mega-regional scale (>1000 km). Re-sampling seven key exploration wells and scientific boreholes provides new data (189 analysed samples) that have been used to build a high-resolution stratigraphic framework using multiple biostratigraphic techniques integrated with organic geochemistry to refine the timing of 10 key stratigraphic surfaces and three megasequences. The results have been used to calibrate the interpretation of a margin-scale two-dimensional seismic reflection dataset, and to build megasequence isochore maps, structural restorations and gross depositional environment maps at key time intervals of the margin evolution. Our findings revise the dating of the basal succession drilled by the A2-1 well, indicating that the oldest post-rift sequence penetrated along the margin is late Tithonian age (previously Callovian). Early Central Atlantic carbonate platform sediments passively infilled subcircular-shaped basement topography controlled by the underlying basement structure of thinned continental crust. Barremian–Aptian rifting in the Equatorial Atlantic, caused folding and thrusting of the Demerara Rise, resulting in major uplift, gravitational margin collapse, transpressional structures and peneplanation of up to 1 km of sediment capped by the regional angular Base Albian Unconformity. Equatorial Atlantic rifting led to margin segmentation and the formation of the TMP, where two major unconformities developed during the intra Late Albian and base Cenomanian. These two unconformities are time synchronous with oceanic crust accretion offshore French Guiana and in the Demerara–Guinea transform, respectively. A marine connection between the Central and Equatorial Atlantic is demonstrated by middle Late Albian times, coinciding with deposition of the organic-rich source rock of the Canje Formation (average total organic carbon 4.21%). The succession is variably truncated by the Middle Campanian Unconformity. Refining the stratigraphic framework within the context of the structural evolution and segmentation of the Guyanas margin impacts the understanding of key petroleum system elements. Supplementary material : Photographs of sandstone petrography thin sections (Fig. S1); calcareous nannofossil plates (Fig. S2); palynology reports for A2-1 and FG2-1 (Fig. S3); taxonomy description of new species; sample table and organic geochemistry results (Table S1); and nannofossil distribution charts (Table S2) are available at https://doi.org/10.6084/m9.figshare.c.5280490

Benthic foraminifera from the Albian shallow-marine limestones in the Geyik Dağı area (Central Taurides), southern Turkey

AbstractCretaceous carbonates in the Geyik Dağı area (Central Taurides, southern Turkey) are represented by two successions with different paleoenvironmental settings: open shelf to slope succession of Cenomanian to Danian age and inner platform succession of Albian to Maastrichtian age, which is interrupted by a post-Cenomanian disconformity. Outcropped lowermost part of the platform-type one is composed of rudistid limestones corresponding to the Urgonian-type carbonates and belongs to the Geyik Dağı Unit (=Anamas-Akseki Carbonate Platform). It contains a rich assemblage of larger benthic foraminifera including orbitolinid, chrysalidinid, cuneolinid, nezzazatid, and miliolid taxa, which has been illustrated and documented here for the first time from the upper Albian of the Tauride Carbonate Platform. The occurrence of such a diversified foraminiferal fauna indicates a prominent high diversity that took place in the Tauride Carbonate Platform during the late Albian time, which corresponds to a major emersion period in some parts of the platform.

The mid-Cretaceous transition from basement to cover within sedimentary rocks in eastern New Zealand: evidence from detrital zircon age patterns

AbstractDetrital zircon U-Pb ages for 30 Late Jurassic and Cretaceous sandstones from the Eastern Province of eastern New Zealand, combined with previously-published geochronological and palaeontological data, constrain the time of deposition in the Pahau and Waioeka terranes of the Cretaceous accretionary margin of Zealandia, and their adjacent cover strata. The zircon age patterns also constrain possible sediment source areas and mid-Cretaceous geodynamic models of the transition from basement accretionary wedge to passive-margin cover successions. Pahau Terrane deposition was mainly Barremian to Aptian but continued locally through to late Albian time, with major source areas in the adjacent Kaweka and Waipapa terranes and minor inputs from the inboard Median Batholith. Waioeka Terrane deposition was mainly Albian, with distinctive and exclusive sediment sources, principally from the Median Batholith but with minor inputs from the Western Province. Alternative tectonic models to deliver such exclusive Median Batholith and Western Province-derived sediment to the mid-Cretaceous Zealandia continental margin are: (1) the creation of a rift depression across Zealandia or (2) sinistral displacement of South Zealandia with respect to North Zealandia, to expose Western Province rocks directly at the Zealandia margin. Detrital zircon age patterns of Cretaceous cover successions of the Eastern Province of eastern New Zealand demonstrate purely local sources in the adjacent Kaweka and Waipapa terranes. Cretaceous zircon components show a decline in successions of late Early Cretaceous age and disappear by late Late Cretaceous time, suggesting the abandonment or loss of access to both the Median Batholith and Western Province as sediment sources.

Paleobiogeography of the pectinid bivalve Neithea, and its pattern of step-wise demise in the Albian Northwest Pacific

The pectinid bivalve genus Neithea is one of the most important indicators for understanding the biogeographic relationships between the Tethyan Realm and North Pacific Province during the Cretaceous Period. Changes in temporal species diversity, endemic/widespread species composition, and origination and demise ratios of Neithea at each Cretaceous stage boundary in the Northwest Pacific were analyzed from a biogeographic perspective. Neithea is continuously present in the Northwest Pacific during the Berriasian to late Albian time interval. Species diversity reached its maximum in the late Aptian, being correlated with the global warming phase. Step-wise demise of Neithea in the Northwest Pacific during the Albian is subdivided into three stages: the late Aptian/early Albian, early Albian/middle-late Albian, and late Albian/early Cenomanian. Thereafter, Neithea disappeared in the Northwest Pacific and never reappeared. This pattern is the reverse of the Albian diversification of Neithea in the Mediterranean, and also contrary to the Mid-Cretaceous global warming trend. Demise of Neithea in the Northwest Pacific occurred simultaneously with the step-wise demise of Mesogean taxa (e.g., rudists) which strongly supports the idea that the Northwest Pacific gradually became independent from the Tethyan Realm during the Albian. It also suggests a long-term deterioration of the faunal interchange between the North Pacific Province and Tethyan Realm throughout the Late Cretaceous. This biogeographic change was possibly caused by Albian “cooling” and changes in oceanic flow/heat transport in the Northwest Pacific.

Key elements to clarify the 110 million year hiatus in the Mesozoic of eastern Syria

ABSTRACT Log correlations, biostratigraphical results and seismic data were combined to show that from Late Triassic Norian to Early Cretaceous Aptian times, the Euphrates area (Eastern Syria) was part of a huge saddle-like northeast-trending ridge (the Hamad Uplift) characterized by a prolonged stratigraphic hiatus. This uplift, developed in the Late Triassic, was multiply reactivated during the Mesozoic, particularly in the Early Cretaceous Aptian-Albian times, during a major reorganization phase of the Neo-Tethyan rift system. This uplift marked the separation between two regions with distinctive tectono-sedimentary evolutions: an eastern isolated and starved region (Euphrates Graben) and a western region that was mainly influenced by the sedimentary dynamics of the westerly Bishri Trough, linked to the Palmyrides Basin. The Hamad Uplift broke-up into a N140°E-oriented graben system in late Albian times. This early NE-trending extensional stage was accompanied by volcanic activity and introduced the main phase of horst-and-graben development within the Euphrates Graben in the Late Cretaceous Senonian times.

THE AMMONOID DESMOCERAS IN THE UPPER ALBIAN (LOWER CRETACEOUS) OF JAPAN

The Upper Albian Desmoceras species of Japan are described, based on material from the Yezo Supergroup in Hokkaido, north Japan, and from the Fujikawa Formation in Shikoku, southwest Japan. Based on comparative analysis of morphologic variation and shell growth of relevant specimens, we assign the present material to two species: Desmoceras (Desmoceras) latidorsatum (Michelin, 1838) and Desmoceras (Pseudouhligella) poronaicum Yabe, 1904. Japanese examples of the latter species have been previously referred to “Desmoceras (Pseudouhligella) dawsoni,” a Canadian species present on Queen Charlotte Islands, British Columbia, but not represented in the Japanese Islands. Desmoceras (Pseudouhligella) poronaicum is also known from British Columbia.The subgenus Pseudouhligella presumably originated in the eastern Pacific during the Late Albian and subsequently migrated to the western Pacific during latest Albian to earliest Cenomanian time. Desmoceras (D.) latidorsatum is a Tethyan form; its presence in the Upper Albian of the North Pacific probably reflects a northward excursion of warm-water currents from the tropical region during Late Albian time.

The ammonoidDesmocerasin the Upper Albian (Lower Cretaceous) of Japan

The Upper AlbianDesmocerasspecies of Japan are described, based on material from the Yezo Supergroup in Hokkaido, north Japan, and from the Fujikawa Formation in Shikoku, southwest Japan. Based on comparative analysis of morphologic variation and shell growth of relevant specimens, we assign the present material to two species:Desmoceras (Desmoceras) latidorsatum(Michelin, 1838) andDesmoceras (Pseudouhligella) poronaicumYabe, 1904. Japanese examples of the latter species have been previously referred to“Desmoceras (Pseudouhligella) dawsoni,”a Canadian species present on Queen Charlotte Islands, British Columbia, but not represented in the Japanese Islands.Desmoceras (Pseudouhligella) poronaicumis also known from British Columbia.The subgenusPseudouhligellapresumably originated in the eastern Pacific during the Late Albian and subsequently migrated to the western Pacific during latest Albian to earliest Cenomanian time.Desmoceras (D.) latidorsatumis a Tethyan form; its presence in the Upper Albian of the North Pacific probably reflects a northward excursion of warm-water currents from the tropical region during Late Albian time.

Regional 2D seismic interpretation and exploration potential of offshore deepwater Sierra Leone and Liberia, West Africa

The offshore region of Sierra Leone and Liberia has been underexplored on the continental shelf and unexplored in deepwater. However, a reconnaissance interpretation of new 2D seismic data, in conjunction with a review of the reservoir and geochemical information available to date, is positive about the hydrocarbon potential of the region. Three distinct prospective hydrocarbon basins are present—Sierra Leone Basin, Liberia Basin, and Harper Basin (Figure 1). Figure 1. Location of Sierra Leone, Liberia, and Harper basins. Fault lines (blue) indicate major strike-slip fault systems. Nine exploratory wells have been drilled and abandoned on the shelf, at the limits of the study area, considerably shoreward of the potential deepwater basins. In terms of exploration maturity, considering a well density of one well per 6700 km2, the area should be classified as “frontier.” However, the regional potential is far from unknown. The nine wells provide an abundance of key data, which contribute to the identification of several hydrocarbon systems. The primary petroleum system contains at least three oil-prone, marine, and lacustrine source rock zones deposited during Lower Cretaceous time (Early Albian to Early Ceno-manian). A secondary petroleum system may have a Late Cenomanian to Turonian shale source rock. Trap types are numerous and relatively widespread, in particular those associated with Cretaceous syn-rift and transform related faulting and unconformities. In addition, traps on the slope and in the basin are ideally located for access to oil migration. The basins developed in two phases, a syn-rift phase and a passive margin phase, both significantly overprinted by wrenching associated with Atlantic transform fault systems. The main rift phase, accompanied by continental to marginally marine sedimentation, took place from Lower Cretaceous Aptian to Middle Albian time. The passive margin and wrenching phase, initiated with seafloor spreading, began in Late Albian time and continues to the …

Idanoceras, a new heteromorph ammonite genus from the Late Albian of eastern Australia

The late Albian marine fossil record from eastern Australia derives from the sedimentary succession of the Great Artesian Basin deposited in a vast epicontinental sea which then covered much of the continent (see Frakes et al., 1987). Ammonites of this age are common but their generic diversity is low. Heteromorph assemblages almost exclusively comprise the taxa Myloceras, Labeceras sensu stricto and Labeceras (Appurdiceras) of the Family Labeceratidae that were widely distributed in higher latitudes of the Southern Hemisphere during Late Albian time (see Aguirre Urreta and Riccardi, 1988; Klinger, 1989). Some 19 endemic species of these genera are recorded from the Great Artesian Basin in the present literature (Etheridge, 1892; Whitehouse, 1926; Reyment, 1964) and there are additional undescribed species (Henderson and McKenzie, unpublished data). The Australian Late Albian epicontinental sea was clearly a site of significant speciation for Labeceras and Myloceras and it has been argued that the Great Artesian Basin represents the evolutionary center for these genera (Henderson, 1990).

IDANOCERAS, A NEW HETEROMORPH AMMONITE GENUS FROM THE LATE ALBIAN OF EASTERN AUSTRALIA

THE LATE Albian marine fossil record from eastern Australia derives from the sedimentary succession of the Great Artesian Basin deposited in a vast epicontinental sea which then covered much of the continent (see Frakes et al., 1987). Ammonites of this age are common but their generic diversity is low. Heteromorph assemblages almost exclusively comprise the taxa Myloceras, Labeceras sensu stricto and Labeceras (Appurdiceras) of the Family Labeceratidae that were widely distributed in higher latitudes of the Southern Hemisphere during Late Albian time (see Aguirre Urreta and Riccardi, 1988; Klinger, 1989). Some 19 endemic species of these genera are recorded from the Great Artesian Basin in the present literature (Etheridge, 1892; Whitehouse, 1926; Reyment, 1964) and there are additional undescribed species (Henderson and McKenzie, unpublished data). The Australian Late Albian epicontinental sea was clearly a site of significant speciation for Labeceras and Myloceras and it has been argued that the Great Artesian Basin represents the evolutionary center for these genera (Henderson, 1990). Middle Albian assemblages from the Great Artesian Basin include the heteromorph genera and subgenera Anisoceras, Hamites (s.s.), Protanisoceras, and Rossalites (McNamara, 1980; McKenzie, 1998), which are all widely distributed forms. However, a quite different pattern prevails in the Late Albian. Although heteromorph genera of pandemic distribution, such as Scaphites, Anisoceras, Hamites, Idiohamites, and Mariella, are represented on the Australasian borderlands, they are virtually unknown in the Great Artesian Basin (see Henderson, 1990) even though planktic foraminifera (Haig, 1979) and nannoplankton (Shafik, 1985) indicate that unhindered connections to the open ocean prevailed for at least part of this time. Among some 1,500 heteromorph specimens held in formal collections acquired for more that a century that we have examined, including the acquisition of extensive new collections, records of pandemic genera amount to one specimen of Worthoceras, and a cluster of Ptychoceras associated with a solitary Worthoceras specimen on a single hand-sample recently collected by Robert Jones of the Australian Museum. Colonisation of the Great Artesian Basin by pandemic taxa, such as those represented nearby on the northern Australian and New Zealand continental margins, was clearly possible but did not occur in any substantial manner. It is suggested here that Late Albian diversification of the members of the Labeceratidae occupied the niches available to ammonites of heteromorph morphology in the Australian epeiric sea, to the exclusion of pandemics. Part of that evolutionary diversification is documented here in the form of a new monotypic genus, Idanoceras, a novel representative of the Labeceratidae affiliated to, and perhaps derived from, Labeceras (Labeceras). Idanoceras mimics the shell form of the cosmopolitan genus Hamites and is thought to have adopted a similar life-style as a product of convergent evolution. The Labeceratidae represent the final evolutionary pulse of the Superfamily Ancylocerataceae which were the dominant group of Valanginian–Aptian heteromorph ammonites.

Stratigraphy and evolution of the Cretaceous forearc Celica-Lancones basin of southwestern Ecuador

The “Celica–Lancones” forearc Basin of southern Ecuador and northern Peru is located between the Paleozoic Amotape–Tahuin Massif to the west and NW and the continental volcanic arc to the east and SE. The study of nine sections and exhaustive sampling of the poorly fossiliferous, mainly clastic Cretaceous deposits of this Basin allowed us to define five distinct series, which display two depositional periods. The first period corresponds to the development of an Early (?) and Middle Albian carbonate shelf, interrupted during Late Albian times by the creation of a tectonically generated trough filled by turbidites of Late Albian–Coniacian age. Geological mapping indicates that this “Celica–Lancones Basin s.s.” includes distinct tectonic units, characterized by distinct early Late Cretaceous stratigraphic series and separated by major faults. These units can be grouped into two main paleogeographic domains. The southeastern one comprises mainly volcaniclastic deposits, whereas the northwestern domain exhibits quartz-rich deposits. Between Early Coniacian and Middle Campanian times, the “Celica–Lancones Basin s.s.” forerarc trough was deformed and eroded as a result of the Late Cretaceous “Peruvian” tectonic phase. The second period corresponds to the latest Cretaceous, during which a new forearc basin was created (Paita–Yunguilla Basin), which is much wider and strikes obliquely with respect to the Celica–Lancones Basin. The sediments of the Paita–Yunguilla Basin exhibit a comparable succession of Campanian–Maastrichtian age throughout the area and conceal the tectonic juxtaposition of the early Late Cretaceous tectonic units. The occurrence of thick Early(?) Maastrichtian coarse-grained conglomerates and breccias express a new significant tectonic event.

Evolution of Cupido and Coahuila carbonate platforms, Early Cretaceous, northeastern Mexico

The Cupido and Coahuila platforms of northeastern Mexico are part of the extensive carbonate platform system that rimmed the ancestral Gulf of Mexico during Barremian to Albian time. Exposures of Cupido and Coahuila lithofacies in several mountain ranges spanning an ~80 000 km 2 area reveal information about platform morphology and composition, paleoenvironmental relations, and the chronology of platform evolution. New biostratigraphic data, integrated with carbon and strontium isotope stratigraphy, significantly improve chronostratigraphic relations across the region. These data substantially change previous age assignments of several formations and force a revision of the longstanding stratigraphy in the region. The revised stratigraphy and enhanced time control, combined with regional facies associations, allow the construction of cross sections, isopach maps, and timeslice paleogeographic maps that collectively document platform morphology and evolution. The orientation of the Cupido (BarremianAptian) shelf margin was controlled by the emergent Coahuila basement block to the northwest. The south-facing margin is a highenergy grainstone shoal, whereas the margin facing the ancestral Gulf of Mexico to the east is a discontinuous rudist-coral reef. A broad shelf lagoon developed in the lee of the Cupido margin, where as much as 660 m of cyclic peritidal deposits accumulated. During middle to late Aptian time, a major phase of flooding forced a retrograde backstep of the Cupido platform, shifting the locus of shallow-marine sedimentation northwestward toward the Coahuila block. This diachronous flooding event records both the demise of the Cupido shelf and the consequent initiation of the Coahuila ramp. The backstepped Coahuila ramp (AptianAlbian) consisted of a shallow shoal margin separating an interior evaporitic lagoon from a low-energy, muddy deep ramp. More than 500 m of cyclic carbonates and evaporites accumulated in the evaporitic lagoon during early to middle Albian time. Restriction of the platform interior dissipated by middle to late Albian time with the deposition of peloidal, miliolid-rich packstones and grainstones of the Aurora Formation. The Coahuila platform was drowned during latest Albian to early Cenomanian time, and the deep-water laminites of the Cuesta del Cura Formation were deposited. This study fills in a substantial gap in the Cretaceous paleogeography of the eastern Gulf of Mexico coast, improving regional correlations with adjacent hydrocarbon-rich platforms. The enhanced temporal relations and chronology of events recorded in the Cupido and Coahuila platforms significantly improve global correlations with coeval, economically important platforms worldwide, perhaps con

Cretaceous karst guyots: New evidence for inheritance of atoll morphology from subaerial erosional terrain

Data from recent surveys and drilling suggest that carbonate platforms over a wide region of the oceanic western Pacific became emergent by amounts of 100–200 m during late Albian time, ca. 100 Ma. The resulting erosional landscapes, now modified by differential compaction over the buried volcanic basement hills, included central basins surrounded by perimeter rims, sinkhole-like depressions, and perimeter benches interpreted as wave-cut terraces. Resubmergence resulted in the sealing of atoll-like erosional topography by pelagic sediments. These Cretaceous proto-atolls, now guyots, provide evidence that ancient as well as modern atolls inherit their form mainly from subaerial landscapes.

Aptian— Albian tectonic pattern of the Basque— Cantabrian Basin (Northern Spain)

During the Aptian and Albian thick terrigenous and carbonate successions of sediments up to 5000 m thick, including shallow water rudist limestones (Urgonian facies), were deposited in the Basque– Cantabrian Basin of northern Spain as a result of an intense rift-related subsidence. Based on criteria from surface and subsurface data, syn-sedimentary faults, folds, angular unconformities, diapirs and sub-basins are distinguished within the Urgonian successions. Faults are grouped into N– S, E– W, NW– SE and NE– SW families and most are normal and strike-slip. Folds are gentle anticlines and synclines related to major faults. The angular unconformities have small hiatuses, poor lateral continuity and they are associated with either folds or tilted blocks. Diapirs are related to the intersection of major basement faults and in at least one instance the diapir was fossilized by Late Albian times. Strong differential subsidence controlled by basement faults determined the division of the basin into many subbasins of different sizes, which acted as depocentres (e.g. Bilbao). Despite the tectonic inversion which affected the basin during the Tertiary and created thrusts in their margins and centre, the present position of the syn-sedimentary tectonic structures gives approximate clues about the broad structural style and this reveals the original model of basin extension. Features characteristic of strike-slip identified in different parts of the basin are displaced geological lines, wrench corridors, drag effects, thickness shifts, paired uplifts and basins, vaulting of ‘slabs’, decreasing displacements, horsetail and fault splays, ‘chessboard’, oroflexural bending, pull-apart geometries, in-line horst slices, and restraining/releasing bends. Sinistral strike-slip movements along major NW– SE faults are supposedly responsible for transtension, which characterized the basin particularly during the Albian. In this scenario, the main wrench movements would have concentrated along the Oiz domain (Biscay Tertiary Synclinorium) and is a situation that has more in common with the strike-slip model proposed by some workers for the western Pyrenees, than with the simple extension models proposed for the northern margin of the Bay of Biscay.

Aptian— Albian tectonic pattern of the Basque— Cantabrian Basin (Northern Spain)

During the Aptian and Albian thick terrigenous and carbonate successions of sediments up to 5000 m thick, including shallow water rudist limestones (Urgonian facies), were deposited in the Basque– Cantabrian Basin of northern Spain as a result of an intense rift-related subsidence. Based on criteria from surface and subsurface data, syn-sedimentary faults, folds, angular unconformities, diapirs and sub-basins are distinguished within the Urgonian successions. Faults are grouped into N– S, E– W, NW– SE and NE– SW families and most are normal and strike-slip. Folds are gentle anticlines and synclines related to major faults. The angular unconformities have small hiatuses, poor lateral continuity and they are associated with either folds or tilted blocks. Diapirs are related to the intersection of major basement faults and in at least one instance the diapir was fossilized by Late Albian times. Strong differential subsidence controlled by basement faults determined the division of the basin into many subbasins of different sizes, which acted as depocentres (e.g. Bilbao). Despite the tectonic inversion which affected the basin during the Tertiary and created thrusts in their margins and centre, the present position of the syn-sedimentary tectonic structures gives approximate clues about the broad structural style and this reveals the original model of basin extension. Features characteristic of strike-slip identified in different parts of the basin are displaced geological lines, wrench corridors, drag effects, thickness shifts, paired uplifts and basins, vaulting of ‘slabs’, decreasing displacements, horsetail and fault splays, ‘chessboard’, oroflexural bending, pull-apart geometries, in-line horst slices, and restraining/releasing bends. Sinistral strike-slip movements along major NW– SE faults are supposedly responsible for transtension, which characterized the basin particularly during the Albian. In this scenario, the main wrench movements would have concentrated along the Oiz domain (Biscay Tertiary Synclinorium) and is a situation that has more in common with the strike-slip model proposed by some workers for the western Pyrenees, than with the simple extension models proposed for the northern margin of the Bay of Biscay.

The Guinea continental margin: an example of a structurally complex transform margin

The Guinean continental margin corresponds to a wide hinge zone which developed between the Jurassic Central Atlantic and the Cretaceous Equatorial Atlantic. The Guinea Marginal Plateau that characterizes the southern border of this margin is a wide triangular morphological feature bounded to the west by a typical rifted margin slope segment, while its southern edge corresponds to a narrow and morphologically complex continental slope probably resulting from intracontinental transcurrent motion during the Early Cretaceous. On the basis of seismic profiles recorded in two selected areas located along the southern Guinea margin, a seismic stratigraphy is established. At depth, the plateau is underlain by a thick Mesozoic and Cenozoic sedimentary wedge made of two main ensembles. A lower ensemble (or sequence 1) consists of Early Cretaceous clastics. An upper ensemble includes a series of four distinct sequences attributed to Albian to Tertiary deposits. Eastward, the sedimentary cover of the southern plateau edge is locally pierced by volcanic bodies of Paleocene age. Beneath the southern margin of the Guinea Plateau, the sedimentary sequences appear deformed by faults and folds resulting from, at least, two main tectonic episodes. A first event only affected sequence 1, which is cut by a set of extensional faults. A second tectonic phase including folding, reverse faulting and transcurrent faulting is responsible for the deformation of sequences 1 and 2. During this tectonic episode, former normal faults have been reactivated as reverse faults resulting in a structural inversion. A third tectonic phase was responsible for structures transverse to the general E-W trend of the southern Guinea margin and consisting of N-S-trending normal faults and associated volcanoes. The spatial distribution of the tectonic trends clearly substantiates a polyphase tectonic activity that can be related to plate motion changes during early stages of the continental separation. A first stage is characterized by a divergent rifting leading to N-S-trending synsedimentary normal faults. During the Cretaceous, the tectonic activity increased as a general transform motion occurred between the Guinea Plateau and its American conjugate margin (the Demerara Plateau). Local tensional stresses were responsible for a splay fault system active near the tip of the Guinea Plateau. In late Albian times, a slight adjustment of the Africa plate, with respect to the South American one, resulted in local squeezing of the whole sedimentary wedge. After this short compressive event, final continental separation occurred in an extensional regime characterized by a general collapse along two major scarps and volcanic activity.

Foraminiferal Stratigraphy and Paleoenvironments of Late Jurassic to Early Cretaceous Deposits in Thakkhola, Nepal

This is the first detailed account of Late Jurassic, deeper marine, southern hemisphere, agglutinated benthic faunas, and their close taxonomic affinity to coeval assemblages from northern high latitudes. Over 1100m of dark shale, siltstone and sandstone in Thakkhola (Nepal) are dated by means of ammonites, dinoflagellate cysts and foraminifera as Oxfordian through latest Albian. The succession was deposited along the northern Gondwana margin, bordering Tethys, while Thakkhola lay at mid-latitudes (30-41?S). A highly diversified agglutinated faunal record in the Oxfordian lower Nupra Formation, with 44 taxa, is segmented (in ascending order) into the Eomarssonella paraconica assemblage, Vemeuilinoides graciosus assemblage and Spiroplectammina suprajurassica assemblage. The assemblages are of restricted deeper shelf nature, in agreement with geochemical, sedimentary and ammonite-based depositional interpretation. The Tithonian upper Nupra Formation is attributed to a prodelta depositional setting, and contains a low-diversity, mainly agglutinated assemblage named after Trochammina annae. The Early Cretaceous Tangbe Formation shows a change to coarser terrigenous clastics deposited under shallow shelf conditions characterized by the Trochammina aff. schaimica assemblage of agglutinated taxa with extremely low diversity. The Late Jurassic assemblages of Nepal have a majority of taxa in common with those of Western Siberia, Svalbard and the Canadian Arctic Archipelago, which suggests that these contain many cosmopolitan taxa, suitable for global stratigraphic correlations. INTRODUCTION During 1988 and 1991, international field excursions were carried out to the Mesozoic strata of the Himalayan mountain range (Gradstein et al. 1989, 1992). These passive continental margin sediments are preserved immediately north of the central Himalayan range in the Thakkhola region of northern Nepal. The present report describes new paleontologic findings, with emphasis on biostratigraphic, paleoecologic and paleogeographic interpretations from studies of foraminifera. Field observations and sediment samples originate from the above-mentioned excursions. The field area is delineated by the villages Jomosom-TangbeChhukgaon-Muktinath, at approximately 28?N and 84?E. Here, relatively easily accessible Late Jurassic and Early Cretaceous strata, over 1100m thick, are exposed at altitudes of 2800 to 5000m. The sediments were deformed to varying degrees by folding and faulting during Himalayan collision in Cenozoic time. The geological literature refers to this area as Thakkhola and we follow this usage. The location of the field sections studied is shown in text-figure 1. Samples are designated according to field section, e.g. J02, followed by the sample number. The Late Jurassic through Early Cretaceous formations of Thakkhola contain exclusively or predominantly agglutinated assemblages. An exception is the topmost (Late Albian) part of the succession, which contains a calcareous assemblage mainly of planktonic species. Preliminary accounts of these assemblages were published previously in Gradstein et al. (1989, 1992). The Late Jurassic agglutinated assemblages are new to southern Asia, and show unexpected affinities to the so-called boreal faunas of the northern hemisphere, which may be more cosmopolitan than known so far. PALEOGEOGRAPHIC SETTING Paleomagnetic reconstructions indicate that Australia, Antarctica and Greater India, which included northern Nepal, formed part of the southern landmass of Gondwana in Jurassic time (Sengor 1985; Powell et al. 1988). The Thakkhola area lay on the continental margin of northern Gondwana, bordering the Tethys Ocean to the south, until at least the Early Cretaceous (text-fig. 2). Consequ ntly, the Jurassic and Early Cretaceous succession of Thakkhola shows marked similarities with the sequences drilled at ODP sites off northwestern Australia (Gradstein and von Rad 1991; Gibling et al. 1994). Northerly paleoflow directions measured in Early Cretaceous fluvio-deltaic deposits of Thakkhola are in accordance with deposition on the subsiding northern Gondwana continental margin. Paleomagnetic data confirm that the Thakkhola area lay at mid latitudes, 30-41?S, in Late Jurassic to Early Cretaceous times (Ogg in Gradstein et al. 1992). Early Jurassic carbonates (Jomosom and Bagung formations) reflect Thakkhola's position in the subtropical zone, and correspond to the most northerly paleolatitude recorded. In the Late Jurassic to Early Cretaceous the area drifted to higher latitudes, reflected by deposition of terrigenous clastics composing the Nupra, Chukh and Tangbe formations. A return to hemipelagic carbonate sedimentation in Late Albian time may reflect in part a reversal of drift (with movement northward), and in part a global rise in sea level, which drowned the shelves and restricted influx of terrigenous clastic sediments. STRATIGRAPHIC AND DEPOSITIONAL FRAMEWORK The Late Jurassic to Early Cretaceous succession of Thakkhola comprises deltaic, marine shelf and upper slope sediments. The micropaleontology, vol. 41, no. 2, pp. 143-170, text-figures 1-12, plates 1-5, tables 1-4, 1995 143 This content downloaded from 157.55.39.163 on Fri, 18 Nov 2016 04:19:11 UTC All use subject to http://about.jstor.org/terms Jeno Nagy et al: Foraminiferal stratigraphy and paleoenvironments of Late Jurassic to Early Cretaceous deposits in Thakkhola, Nepal