Jurassic-Lower Cretaceous Research Articles

Organic Carbon versus Major Elements Relationship in Rocks of the Bazhenov Formation, Western Siberia

A close relation of the organic carbon (Corg) content with major has been established for rocks of the Upper Jurassic–Lower Cretaceous Bazhenov Formation. Applying the method of multiple linear regression, it has been demonstrated that the Corgcontent in rocks of the Bazhenov Formation is stringently controlled by its bulk chemical composition. This inference is consistent with the existing ideas regarding a close interrelation between the following main components of rocks: organic carbon and authigenic quartz formed on remains of Radiolaria; pyrite formed in a highly reducing medium of Corg-rich sediments; and terrigenous clayey material diluting the authigenic siliceous–carbonaceous–pyritic matrix. These components chiefly determine the spectrum of major elements in the Bazhenov Formation. The establishment of the close relation between the Corgcontent and the group of major elements refutes the suggestion of some authors that siliceous material was supplied to nonlithified sediments of the Bazhenov sea by hydrothermal solutions, because this mechanism would have inevitably upset geochemical relations between elements in the studied rocks.

Geology, Geochemistry, and Evolution of the Divrigi and Kuluncak Ophiolitic Melanges, with Reference to Serpentinites in East-Central Turkey

The Divrigi and Kuluncak ophiolitic mélanges are located in central Anatolia in the Tauride ophiolite belt. The stratigraphic sequence in the Divrigi ophiolitic mélange includes, from bottom to top, the Upper Jurassic-Lower Cretaceous Akdag limestone, Upper Cretaceous Çalti ultramafic rocks, and the Curek listwaenite. The Divrigi ophiolitic mélange is intruded by the Late Cretaceous-Eocene Murmano pluton. The above stratigraphic sequence is followed by the Eocene-Paleocene Ekinbasi metasomatite and the Quaternary Kilise Formation. The oldest sequence of rocks in Kuluncak ophiolitic mélange in the GuvenÇ area is the Karadere serpentine/ultramafic body overlain successively by the Kurtali gabbro, Gundegcikdere radiolarite, the GuvenÇ listwaenites, and the Buldudere Formation. All of these units are Late Cretaceous in age. The Karamagra siderite deposit in the Hekimhan area probably was formed in the Lower Cretaceous at the contact between Çalti ultramafic rocks and the Buldudere Formation. The Kuluncak ophiolitic mélange was intruded by a subvolcanic trachyte in the Late Cretaceous. The Eocene-Paleocene Konukdere metasomatite, the Miocene Yamadag volcanic rocks, and Quaternary slope deposits are late in the stratigraphic sequence in the GuvenÇ area. The Kuluncak ophiolitic mélange in the Karakuz area is similar to that at GuvenÇ; however, gabbro, radiolarite, and Miocene volcanic rocks are not present. The Miocene is represented by the Ciritbelen Formation at Karakuz and the Karakuz iron deposit is hosted by a Late Cretaceous subvolcanic trachyte. The rareearth and trace-element concentration of serpentinite in the Divrigi and Kuluncak ophiolitic mélanges indicate that all of the ultramafics and their alteration products were derived from a MORB, which was depleted in certain elements and oxides. The results expressed in this study support the idea that the Divrigi and Kuluncak ophiolitic mélanges within the Tauride ophiolite belt originated from Northern Tauride oceanic lithosphere (Poisson, 1986), instead of a northern branch of Neo-Tethys (Sengor and Yilmaz, 1981).

Dinoflagellate Cysts and Acritarchs from the Jurassic-Cretaceous Boundary, Northwest Anatolia, Turkey

Dinoflagellate Cysts and Acritarchs from the Jurassic-Cretaceous Boundary, Northwest Anatolia, Turkey

Fluid inclusion and vitrinite-reflectance geothermometry compared to heat-flow models of maximum paleotemperature next to dikes, western onshore Gippsland Basin, Australia

Nine basalt dikes, ranging from 6 cm to 40 m thick, intruding the Upper Jurassic–Lower Cretaceous Strzelecki Group, western onshore Gippsland Basin, were used to study maximum temperatures ( T max) reached next to dikes. T max was estimated from fluid inclusion and vitrinite-reflectance geothermometry and compared to temperatures calculated using heat-flow models of contact metamorphism. Thermal history reconstruction suggests that at the time of dike intrusion the host rock was at a temperature of 100–135°C. Fracture-bound fluid inclusions in the host rocks next to thin dikes (<3.4 m thick) suggest T max systematically increases towards the dike margin to at least 500°C. The estimated T max next to the thickest dike (thickness ( D)=40 m) suggests an extended zone of elevated R v-r to at least a distance from the dike contact ( X) of 60 m or at X/ D>1.5, using a normalized distance ratio used for comparing measurements between dikes regardless of their thickness. In contrast, the pattern seen next to the thin dikes is a relatively narrow zone of elevated R v-r. Heat-flow modeling, along with whole rock elemental and isotopic data, suggests that the extended zone of elevated R v-r is caused by a convection cell with local recharge of the hydrothermal fluids. The narrow zone of elevated R v-r found next to thin dikes is attributed to the rise of the less dense, heated fluids at the dike contact causing a flow of cooler groundwater towards the dike and thereby limiting its heating effects. The lack of extended heating effects suggests that next to thin dikes an incipient convection system may form in which the heated fluid starts to travel upward along the dike but cooling occurs before a complete convection cell can form. Close to the dike contact at X/ D<0.3, R v-r often decreases even though fluid inclusion evidence indicates that T max is still increasing. Further, fluid inclusion evidence indicates that the evolution of water vapor or supercritical fluids in the rock pores corresponds to the zone where R v-r begins to decrease. The generation of the water vapor or supercritical fluids near the dike contact seems to change vitrinite evolution reactions. These metamorphic conditions, closer to the dike than X/ D=0.3 make vitrinite-reflectance unreliable as a geothermometer. The form of the R v-r profile, as it indicates T max, can be interpreted using temperature profiles estimated from various heat-flow models to infer whether the dike cooled by conduction, incipient convection, or a convection cell. A contact aureole that consists of decreasing R v-r or T max extending out to X/ D≥2 and that has a T contact≫( T magma+ T host)/2 appears to be a signature of simple conductive cooling. Incipient convection is indicated by a R v-r profile that decreases to background levels at X/ D<1. A convection cell is indicated by a wave-like form of the R v-r profile and consistently high R v-r that may not decrease to background levels until beyond distances of X/ D>1.5.

MIDDLE JURASSIC-LOWER CRETACEOUS BIOSTRATIGRAPHY IN THE CENTRAL PONTIDES (TURKEY): REMARKS ON PALEOGEOGRAPHY AND TECTONIC EVOLUTION

The deposition of Jurassic-Lower Cretaceous carbonates in the Pontides was controlled mainly by the evolution of an Atlantic-type continental margin in the Tethys. The study of several stratigraphic sections from allochthonous slices and blocks of the North Anatolian Ophiolitic Melange provided insight into the Middle Jurassic-Early Cretaceous paleogeographic evolution of the Central Pontide Belt. The Callovian-Aptian successions span the Globuligerina gr. oxfordiana, Clypeina jurassica (equivalent of the Tubiphytes morronensis zone), Protopeneroplis ultragranulata (with the Haplophragmoides joukowskyi subzone), Montsalevia salevensis , Hedbergella delrioensis - Hedbergella planispira - Leupoldina - Globigerinelloides and Globigerinelloides algerianus biozones. Two major stratigraphic gaps corresponding to the pre-Callovian and Hauterivian-Early Aptian ages are recognised within the successions. Lithostratigraphic and biostratigraphic studies indicate strong similarities in the evolution of the successions in the Amasya region (Central Pontides) and Biga-Bursa-Bilecik (BBB ) Platform (North-western Anatolia).

Upper Jurassic-Lower Cretaceous Morrison and Cedar Mountain formations, NE Utah-NW Colorado; relationships between nonmarine deposition and early Cordilleran foreland-basin development

ABSTRACT Sedimentologic, stratigraphic, and petrologic analysis of the Upper Jurassic-Lower Cretaceous Morrison and Cedar Mountain formations of Utah and Colorado provides information on the timing and nature of early Cordilleran foreland-basin development. The Morrison Formation can be subdivided into three depositional facies assemblages: (1) lower progradational shallow marine, lacustrine, fluvial, and eolian facies deposited during Oxfordian-Kimmeridgian retreat of the Stump-Sundance sea; (2) a middle assemblage containing sandy-gravelly braided fluvial deposits, which are overlain by meandering fluvial channel and overbank facies; and (3) an upper assemblage of laterally stable, low-sinuosity, fluvial channel facies deposited during Tithonian-early Neocomian (?) time. The upper part of this assemblage shows evidence of alteration and early diagenesis related to development of an Early Cretaceous unconformity. The overlying Cedar Mountain Formation is subdivided into two facies assemblages: (1) the Neocomian Buckhorn Conglomerate was deposited by northeast-trending, sandy-gravelly braided rivers that were incised into the underlying Morrison Formation; (2) an upper assemblage containing laterally stable, low-sinuosity, fluvial channel facies deposited during late Neocomian-Albian time. The base of the unit contains a thick calcrete zone that formed during an unconformity following Buckhorn deposition. Morrison and Cedar Mountain formation sandstones contain three petrofacies: a feldspar-rich lower Morrison petrofacies (%QmFLt = 70, 19, 11), and chert-rich upper Morrison and Buckhorn/Cedar Mountain petrofacies (%QmFLt = 54, 5, 41 and 69, 4, 27, respectively). Sandstone composition and paleocurrent data indicate a Cordilleran source area composed of Proterozoic, Paleozoic, and Mesozoic sedimentary rocks. The Morrison Formation and Buckhorn Conglomerate were deposited in the back-bulge depozone of the Late Jurassic Cordilleran foreland-basin system and onlapped a flexural forebulge located in central Utah. Late Neocomian eastward migration of the forebulge uplifted areas in eastern Utah, producing an unconformity, while the foredeep in central Utah underwent flexural subsidence. The upper part of the Cedar Mountain Formation represents overfilling of the foredeep and deposition above the forebulge.

The Jurassic–Lower Cretaceous Byers Group, South Shetland Islands, Antarctica: revised stratigraphy and regional correlations

The Byers Group, exposed in the western South Shetland Islands, is a thick (at least 2.7 km) succession recording Late Jurassic–Early Cretaceous sedimentation and volcanism on the Pacific flank of the Antarctic Peninsula magnetic arc. At least 1.3 km of marine clastic rocks are overlain by 1.4 km of non-marine strata. Penecontemporaneous, mainly intrusive, igneous rocks are present in much of the succession. At the base of the Byers Group, radiolarian-rich, hemipelagic mudstones of the Kimmeridgian–Tithonian Anchorage Formation are succeeded by terrigenous, slope-apron mudstones and sandstones of the Berriasian President Beaches Formation. These deep-marine strata are overlain by shallower-marine conglomerates and mudstones of the Berriasian–Valanginian Chester Cone Formation. The upper part of the marine succession also includes the volcanic breccias of the Start Hill Formation, interpreted as part of a debris-apron flanking a substantial volcanic edifice. Finally, following a Valanginian–Aptian erosional hiatus, non-marine volcaniclastic rocks of the Cerro Negro Formation were deposited in early Aptian times. Comparison with time-equivalent successions elsewhere in the Antarctic Peninsula region suggests that within the overall regressive trend, the Byers Group records regionally significant pulses or arc uplift in Tithonian–earliest Berriasian, latest Berriasian, and late Valanginian–Earliest Aptian times.

Upper Jurassic-Lower Cretaceous Morrison and Cedar Mountain Formations, Ne Utah-Nw Colordao: Relationships between Nonmarine Deposition and Early Cordilleran Foreland-Basin Development

Upper Jurassic-Lower Cretaceous Morrison and Cedar Mountain Formations, Ne Utah-Nw Colordao: Relationships between Nonmarine Deposition and Early Cordilleran Foreland-Basin Development

THE PERSEUS FIELD, NORTH WEST SHELF, AUSTRALIA: A GIANT GAS ACCUMULATION IN A COMPLEX STRUCTURAL/STRATIGRAPHIC TRAP

The Perseus Field contains reported expectation (proven plus probable) dry gas and condensate reserves of 6.48 TCF (183 × 109 m3) and 165.7 MMBBL (26.3 × 106 m3), respectively, in a complex structural/stratigraphic trap. Gas is predominantly reservoired in Bathonian shallow marine sandstones of the Legendre Formation which subcrop the Upper Jurassic-Lower Cretaceous 'Main Unconformity' in a graben separating the Goodwyn and North Rankin horsts. The Lower Cretaceous Muderong Shale forms the main regional top seal. The Lower-Middle Jurassic Athol Formation forms the seat seal for the Perseus trap and subcrops the Main Unconformity northwest of North Rankin. The maximum gross gas column for the field is approximately 360 m. The Athol Formation and underlying Murat Siltstone also form the topseal for the small Searipple Field, which underlies Perseus.The Perseus trap is dip-closed to the northwest. A steep fault scarp forms the southern margin of the trap. Trap integrity is dependent upon fault seal along the western flank of the field, where the reservoir section is juxtaposed against Athol Formation claystones in a long narrow fault block downthrown from the Goodwyn block. At the southwestern corner of the field this fault block is absent and the Perseus reservoir is juxtaposed against Triassic reservoir section in the Goodwyn block.The NRA22 well, drilled from the North Rankin A platform, has been producing gas from the Perseus Field since mid-1991. Reservoir pressure measurements and production history data have been of immense value in the exploration and appraisal of this field, both in driving further drilling and in understanding the results. Integration of geoscientific and engineering data and expertise within a multi-disciplinary team was essential for the efficient appraisal and evaluation of the field.

Sequence stratigraphy of nonmarine Jurassic–Cretaceous rocks, central Cordilleran foreland-basin system

An analysis of the Upper Jurassic–Lower Cretaceous Morrison and Cedar Mountain Formations of Utah and Colorado has resulted in a general sequence-stratigraphic model for nonmarine rocks. In this model, nonmarine deposition is governed by changes in basin accommodation development and corresponding shifts in depositional base level. These fluctuations result in deposition of systematically varying facies and architectural elements that allow nonmarine depositional sequences to be recognized. Internally, nonmarine depositional sequences comprise three systems tracts—degradational, transitional, and aggradational—which are analogous to the lowstand, transgressive, and highstand systems tracts of marine depositional sequences. Degradational systems tracts overlie sequence-bounding unconformities and consist of relatively coarse-grained, low-sinuosity fluvial deposits that are either contained within incised valleys or deposited as widespread, thin sheets above shallow erosion surfaces. Transitional systems tracts represent an increase in basin accommodation development following degradational systems-tract deposition. They are marked by the transition from laterally continuous, low-sinuosity fluvial channel sandstones and conglomerates to lenticular and ribbon-like, meandering and anastomosing channel sandstones. Aggradational systems tracts are characterized by meandering-anastomosing channel sandstones and abundant fine-grained overbank and lacustrine deposits. The Upper Jurassic–Lower Cretaceous nonmarine rocks of the study area contain three depositional sequences. The first of these, the UJ-1 sequence, consists primarily of an aggradational systems tract overlain by a sequence-bounding unconformity. However, the lower parts of this sequence are transitional with older marine rocks and can be considered the late stages of a marine highstand systems tract. The upper Morrison sequence (UJ-2) consists of degradational, transitional, and aggradational systems tracts. Above the UJ-2 sequence are a sequence-bounding unconformity and degradational and transitional systems tracts of the LK-1 sequence represented by the Buckhorn Conglomerate. The Buckhorn is overlain by a sequence-bounding unconformity and transitional-aggradational systems tracts of the LK-2 sequence that is composed of the upper part of the Cedar Mountain Formation. The Upper Jurassic–Lower Cretaceous sequences in Utah and Colorado can be traced regionally and correlated with nonmarine depositional sequences in central Utah and sequences that contain nonmarine, transitional, and marine rocks in central Wyoming. These sequences were deposited in the back-bulge, forebulge, and distal foredeep depozones of the Late Jurassic–Early Cretaceous foreland-basin system.

Upper Tithonian-Valanginian (Upper Jurassic-Lower Cretaceous) dinoflagellate cyst stratigraphy of the western Mediterranean

This study focuses on the qualitative dinoflagellate cyst distribution in the Upper Tithonian-Valanginian rocks of the western Mediterranean. Results are presented of research on the ammonite controlled pelagic Rı́o Argos succession near Caravaca (SE Spain) which are integrated with information from calibrated sections in southeast France and Switzerland. The ammonite framework permitted determination of contemporaneous first and last occurrences of dinoflagellate cyst taxa in the three areas. Mainly based on these biostratigraphic events, four successive dinoflagellate cyst zones are defined and identified in the Upper Tithonian-Valanginian succession of the western Mediterranean. The established zonation has potential for world-wide application. In addition, a high-resolution zonation, described as a suite of subzones, is established based on seven successive shifts in the dinoflagellate cyst development of the Upper Berriasian-Valanginian in the Rı́o Argos succession exclusively. These local palaeoecological changes are considered to reflect palaeoenvironmental and/or climatic changes of a regional, interregional or global nature.

A classification of rhizogenic (root-formed) calcretes, with examples from the Upper Jurassic-Lower Cretaceous of Spain and Upper Cretaceous of southern France — reply

Abstract Studies of fluvial sequences from the Upper Jurassic and Lower Cretaceous of the Cameros Basin (Spain) and the Upper Cretaceous of the Aquitaine Basin (France) have revealed large numbers of calcrete profiles which have formed wholly or largely through the action of plant root calcification. Those from Spain have formed by the calcification of root mats, whereas those from France resulted from the accumulation of calcified root cells ( Microcodium ). These root-related (rhizogenic) calcretes developed in stages allowing developmental time-series (chronosequences) to be identified, contrasting with existing models for calcrete profile growth. They provide unique examples where vegetation has been the dominant control on profile development and probably indicate specific vegetation types and palaeoecological settings. The role of root activity in forming some Quaternary calcretes is also discussed.

A classification of rhizogenic (root-formed) calcretes, with examples from the Upper Jurassic-Lower Cretaceous of Spain and Upper Cretaceous of southern France — Discussion

A classification of rhizogenic (root-formed) calcretes, with examples from the Upper Jurassic-Lower Cretaceous of Spain and Upper Cretaceous of southern France — Discussion

Shallow Geology of the Northern Barents Sea: Implications for Petroleum Potential

The shallow geology of the northern Barents Sea has been studied through analyses of geophysical data and geological samples from gravity cores. Mesozoic rocks subcrop in the entire study area, with Triassic-Middle Jurassic rocks dominating the shallowest and western part of the area, and Upper Jurassic-Lower Cretaceous rocks dominating the central areas. A palynologically investigated sample, representing in-situ bed rock, gives an Aptian-early Albian age, indicating that Lower Cretaceous units dominate the eastern part of the study area. These beds were deposited during shallow-marine conditions and show a petrographic composition comparable to the Carolinefjellet Formation of Svalbard. The thickness of the Lower Cretaceous section is almost 1000 m. Organic geochemical analyses of the Aptian-lower Albian rocks suggest a post-Early Cretaceous erosion of a maximum of 2000 m. An important tectonic event in the region was a compressive phase near the Cretaceous-Tertiary boundary. Occurrences of porous, well-sorted Lower Cretaceous sandstones succeeding organic-rich, mature Upper Jurassic and Lower Cretaceous shales, sealed by Lower Cretaceous shales within antiform settings, represent interesting play concepts in the study area. The petroleum potential has been limited by the post-Early Cretaceous uplift and erosion, resulting in the termination of hydrocarbon generation followed by gas expansion.

Depositional environment and burial history of a Lower Cretaceous carbonaceous claystone, Bornholm, Denmark

A c. 1 m thick carbonaceous claystone from the type locality of the Lower Cretaceous Skyttegård Member (Rabekke Formation), Bornholm, has been investigated by organic pétrographie and organic geochemical methods in order to assess the depositional environment of the claystone and the thermal maturity of the organic matter. The claystone was deposited in a low-energy, anoxic lake which occasionally was marine influenced. The organic matter is terrestrial and can be classified as kerogen type III and lib. Detrital organic matter and cutinite are characteristic components. The organic matter is allochthonous but the transport distance was short, and the plant material was probably mainly derived from plants growing at the edge of, or nearto, the lake. Source-specific biomarkers such as norisopimarane suggest that the plant litter was derived from a gymnospermous, low-diversity vegetation. Evidence for early angiospermous plants cannot be demonstrated with any certainty. A huminite reflectance value of 0.24%Rm and several geochemical parameters indicate that the organic matter is highly immature. It has only experienced coalification corresponding to the peat stage. Estimates show that, prior to uplift, the claystone was buried to a maximum of approximately 260 m. Reflectance data further suggest that a maximum c. 550 m thick sediment package was removed by erosion prior to deposition of the ?uppermost Jurassic-Lower Cretaceous sediments on Lower Jurassic strata.

Geochemistry of the Upper Jurassic-Lower Cretaceous Bazhenov Formation, West Siberia

The Bazhenov Formation (Tithonian to Berriasian) occupies about 1 million km 2 in the central part of the West Siberian plate and is buried by 2,000 to 3,000 m of younger sediments. It was formed in a marine basin connected to the world oceans and is composed primarily of siliceous argillites rich in planktonic organic matter (about 8% of total organic carbon on average). Organic-rich sediments have been accumulating for about 5 to 6 million years; they supposedly formed periodically under anoxic conditions. The aggregate mass of organic matter in the Bazhenov Formation is as high as 18 trillion tons. It is known as one of the largest oil sources in the world. About 500 samples from 39 oil wells throughout the formation were analyzed; it was shown that Bazhenov rocks contain more U, Mo, V, Cu, Zn, and Ni than average black shales; moreover, the concentrations of these elements increase toward the center of the paleobasin. Based on the original data on concentrations of several metals in 500 samples, a representative set of 30 samples was selected and analyzed for 43 chemical elements (using atomic absorption, neutron activation, and X-ray fluorescence analyses) as well as for total organic carbon. Using correlation analysis, the elements determined can be divided into three groups: those associated with organic carbon, with aluminosilicate material, or with carbonates. There is almost complete coincidence between these groups in the Bazhenov rocks and in sapropel from the Black Sea: in both instances total organic cargon is accompanied by Mo, Br, Sb, Cd, V, Cu, S, Ni, As, Ag, U, and Se; for rocks from the Bazhenov Formation, this list also includes Zn. It has been suggested that some of the elements extracted primarily from the seawater by dead planktonic material were later released and that these accumulated in sulfides. By including an enormous amount of elements from the world oceans, the Bazhenov Formation became an important factor in the global geochemical balance of the Jurassic period.

Occurrence of Siderite in the Point Mcintyre Field North Slope Alaska: An Indicator of Paleo Aquifers: ABSTRACT

Iron, sourced from weathering of shales of the Jurassic Kingak and Lower Cretaceous Miluveach Formations, was transported by fluvial waters and coastal aquifers and fixed in nearshore fresh water-marine mixing zones as glauconite pellets and/or as siderite cement. The Neocomian section spent millions of years within 1000 feet of the surface, allowing for complex and extensive interaction between surface fluids and sediments. Siderite cement is most abundant in proximal facies that were exposed to meteoric water, and less abundant in fine-grained, well sorted more distal marine facies, which are isolated from meteoric input. Siderite cement associated with coarse-grained proximal facies is characterized by coarse-grained spar morphology, isotopically light oxygen, and Fe-rich composition. Siderite cement associated with distal marine facies is characterized by fine grain scattered rhombic crystals, isotopically heavy oxygen, and Fe-Mg rich composition. Carbon isotopic composition of siderite reflects degree of openness of the sediment to oxygenated water. Fine-grained, poorly sorted sediment associated with pyrite contains siderite with positive carbon isotopic values (PDB scale), whereas well sorted, porous-permeable sediment has siderite with negative carbon isotopic values. Siderite with isotopically light oxygen is associated with oxidized and dissolved glauconite grains and kaolinite cement. Siderite formed from the sediment-water interface tomore » burial temperatures of about 30[degrees]C, prior to the deposition of most of the overlying, laterally extensive, HRZ shale. In all cases, siderite predates the formation of quartz overgrowths, which precipitated at minimum temperatures of about 70[degrees]C.« less

Thermochronological and Petroleum Generation Studies next to Dikes in the Upper Jurassic-Lower Cretaceous Strzelecki Group, Onshore Gippsland Basin, Australia: ABSTRACT

Fluid inclusion, apatite fission track analysis (AFTA[reg sign]) annealing, mean random vitrinite reflectance (Rv-r) data and heat flow modeling studies of nine basalt dikes ranging from 0.06 m to 40 in thick in the Strzelecki Group were used to constrain the conditions of contact metamorphism of coaly sedimentary organic matter (SOM) to investigate it as an extended-duration natural analog to hydrous pyrolysis experiments. Thermal history reconstruction suggests that at the time of dike intrusion the host rock was at a paleotemperature of 100 to 120[degrees]C. Reequilibrated fluid inclusions in the host rocks next to thin dikes (< 3.4 m thick) suggest maximum temperature (T[sub peak]) systematically increases towards the dike margin to around 550[degrees]C. These T[sub peak] data are in fair agreement with those predicted by a conductive heat flow model and measured Rv-r. AFTA indicates the time of uplift and cooling as mid to Late Cretaceous and that paleotemperatures were too high for heating duration information to be recorded by annealing of tracks in apatite near the dikes. The estimated T[sub peak] next to some larger dikes (20 to 40 m thick) as indicated by an extended zone of elevated Rv-r did not fit the pattern seen next tomore » the thin dikes. Heat flow modeling along with whole rock elemental and isotopic data suggest that the extended zone of increased Rv-r is caused by convection of heated fluids. This is in contrast to the apparent advection effects found adjacent to thin dikes. Laboratory hydrous pyrolysis compares well with contact metamorphism next to thin dikes with respect to the composition of the free hydrocarbons except in samples very close to the dike. Close to the dike, fluid inclusion evidence indicates a water vapor or super-critical phase in the rock making the organic geochemistry results diverge from.« less

A classification of rhizogenic (root-formed) calcretes, with examples from the Upper Jurassic-Lower Cretaceous of Spain and Upper Cretaceous of southern France

Studies of fluvial sequences from the Upper Jurassic and Lower Cretaceous of the Cameros Basin (Spain) and the Upper Cretaceous of the Aquitaine Basin (France) have revealed large numbers of calcrete profiles which have formed wholly or largely through the action of plant root calcification. Those from Spain have formed by the calcification of root mats, whereas those from France resulted from the accumulation of calcified root cells ( Microcodium). These root-related (rhizogenic) calcretes developed in stages allowing developmental time-series (chronosequences) to be identified, contrasting with existing models for calcrete profile growth. They provide unique examples where vegetation has been the dominant control on profile development and probably indicate specific vegetation types and palaeoecological settings. The role of root activity in forming some Quaternary calcretes is also discussed.

The Upper Jurassic-Lower Cretaceous carbonate-terrigenous succession and the development of a carbonate mound in western Shikoku, Japan

The stratigraphy and sedimentology of the Upper Jurassic-Lower Cretaceous Torinosu Group were studied in western Shikoku (western Kochi Prefecture). The Torinosu Group is interpreted as a transgressive succession deposited in a foreland basin. The group can be divided into a lower coarse siliciclastic unit (the Tsukadani Formation) and an upper carbonate and fine siliciclastic unit (the Yatsuji Formation). The Tsukadani Formation is characterized by conglomerates with chert gravel and probably represents submarine fan deposits. The gravels were derived from an exposed accretionary wedge containing pelagic siliceous rocks. The Yatsuji Formation includes a limestone unit up to 150 m in thickness and a few kilometres in lateral extension, which comformably rests on siliciclastic rocks and marlstone. The limestone unit is interpreted as a carbonate mound and laterally shows considerable variation in thickness. The conglomerate with limestone gravels which infills the channel and flanks of the mounds supports a topographic relief associated with the carbonate build-ups. The facies of the limestone unit changes vertically from marlstone to stromatoporoid facies. Change in fossil components associated with a decrease of terrigenous components indicates that the initial community was replaced by the reef-forming community which is interpreted as a response to the decrease of terrigenous influx. Periodical input of terrigenous sediment originating from movement of thrust mass interrupted the carbonate deposition.