Bathonian Time Research Articles

Comment on “Sedimentary and Tectonic Evolution of the Southern Qiangtang Basin: Implications for the Lhasa‐Qiangtang Collision Timing” by A. Ma et al.

AbstractMa et al. (2017, https://doi.org/10.1002/2017JB014211) proposed that the Lhasa‐Qiangtang collision commenced in late Bathonian time (~166 Ma), based on the observation of an angular unconformity that truncated the Bathonian Buqu Formation from the late Bathonian‐early Callovian Biluoco Formation and the recognition of siliciclastic provenance switch from arc igneous source to uplifted orogenic source in the Biluoco‐Qixiangco area of the southern Qiangtang basin. Unfortunately, however, extensive exposure of Late Jurassic‐Middle Cretaceous ophiolite fragments and marine sedimentary rocks along the Bangong suture zone indicates that the Bangong Meso‐Tethys Ocean did not close until the Late Cretaceous, which is consistent with a Late Jurassic‐Middle Cretaceous magmatic arc in the southern Qiangtang terrane as well as paleomagnetic data. The late Bathonian orogenic event in the southern Qiantang area observed by Ma et al. (2017, https://doi.org/10.1002/2017JB014211) can be explained by subduction of oceanic highs on the Bangong Meso‐Tethyan Ocean. Clearly, our submission is a significant clearance to Ma et al.'s work and the Tibetan Plateau geology and of timely significance in correctly understanding the evolution history of the Tibetan Plateau. Therefore, our paper possibly is of broad interest to our international colleagues.

Astrochronology of the Middle Jurassic Buqu Formation (Tibet, China) and its implications for the Bathonian time scale

The Buqu Formation in Qiangtang Basin in China was formed during the Bathonian Stage of the Middle Jurassic. The subject of this study is the formation at Well QZ11 at a depth of from 34 to 591m, located in the Southern Depression of Qiangtang Basin. The optimal accumulation rate from the result of ASM analysis was used as the “target” to tune the GR (gamma ray) and DLR (double lateral resistivity) series in QZ11. Power spectra analysis in tuned GR and DLR series demonstrate the existence of a series of prominent astronomical periods within the Buqu Formation in the Qiangtang Basin. Then ~100kyr bandpass filtered curve was used to establish a floating astronomical time scale for the Buqu Formation, and the maximum and minimum sedimentation rates without considering the factor of formation compaction were 150m/Myr and 395m/Myr, respectively. Using this scale, the lower limit for the duration of the formation is estimated to be approximately 2.09Myr. This duration can serve as a reference for temporal and chronological calibrations of the Bathonian Stage.

Sedimentary and tectonic evolution of the southern Qiangtang basin: Implications for the Lhasa‐Qiangtang collision timing

AbstractThe Mesozoic stratigraphic record of the southern Qiangtang basin in central Tibet records the evolution and closure of the Bangong‐Nujiang ocean to the south. The Jurassic succession includes Toarcian‐Aalenian shallow‐marine limestones (Quse Formation), Aalenian‐Bajocian feldspatho‐litho‐quartzose to feldspatho‐quartzo‐lithic sandstones (shallow‐marine Sewa Formation and deep‐sea Gaaco Formation), and Bathonian outer platform to shoal limestones (Buqu Formation). This succession is truncated by an angular unconformity, overlain by upper Bathonian to lower Callovian fan‐delta conglomerates and litho‐quartzose to quartzo‐lithic sandstones (Biluoco Formation) and Callovian shoal to outer platform limestones (Suowa Formation). Sandstone petrography coupled with detrital‐zircon U‐Pb and Hf isotope analysis indicate that the Sewa and Gaaco formations contain intermediate to felsic volcanic detritus and youngest detrital zircons (183–170 Ma) with εHf(t) ranging widely from +13 to −25, pointing to continental‐arc provenance from igneous rocks with mixed mantle and continental‐crust contributions. An arc‐trench system thus developed toward the end of the Early Jurassic, with the southern Qiangtang basin representing the fore‐arc basin. Above the angular unconformity, the Biluoco Formation documents a change to dominant sedimentary detritus including old detrital zircons (mainly >500 Ma ages in the lower part of the unit) with age spectra similar to those from Paleozoic strata in the central Qiangtang area. A major tectonic event with intense folding and thrusting thus took place in late Bathonian time (166 ± 1 Ma), when the Qiangtang block collided with another microcontinental block possibly the Lhasa block.

Palaeobiogeography of the Bajocian–Oxfordian macrofauna of Gebel Maghara (North Sinai, Egypt): Implications for eustacy and basin topography

Based on newly collected material from the Bajocian–Oxfordian rocks of Gebel Maghara (North Sinai, Egypt) and available published information, the palaeobiogeographic pattern and the dispersal potential of the macrofauna including bivalves, ammonites, corals, and brachiopods are investigated. Both clustering methods and ordination techniques were applied and evaluated. During Bajocian times, G. Maghara occupied an intracratonic setting and was isolated from the main ocean by islands and shallows. These barriers may have limited the dispersal potential of the macrofauna and impeded the faunal exchange with even nearby areas. Although these barriers had disappeared by rejuvenation of faults during an active rift stage in Bathonian times, the same biogeographic patterns continued, which may be related to the global sea level lowstand. By Callovian times, a time of global sea level highstand, in contrast, the fauna of the study area became very similar to that of northeastern Africa. Similarly, diversity and extinction rates increased from the middle Bathonian onward, which may reflect intrusion of cosmopolitan taxa due to the newly established open marine setting and the global sea level highstand. Towards the Oxfordian, lowering of temperature may have limited the dispersal ranges of the macrofauna within the Ethiopian Province. As a result, a southeastern subprovince (Indo–Malagasy) including Tanzania, Madagascar, and India became established. Although the geographic patterns of the different faunal groups exhibit some similarities resulting from abiotic factors such as sea level, climate, and basin topography, biotic characteristics such as life habits and larval development played the dominant role in shaping the distribution pattern. The dispersal potential was highest for ammonites, followed by that of corals and then bivalves. Brachiopods had the lowest dispersal potential.

Four New Equisetites Species (Sphenophyta) from the Hojedk Formation, Middle Jurassic (Bajocian-Bathonian), the North of Kerman, Iran

Middle Jurassic (Bajocian-Bathonian) Sphenophyta fossils of the silicified level peats of the Pabdana Coal Mine (Kerman) recovered. Four new species of Sphenophyta described for the first time. They include Equisetites pabdanaii sp. nov., Equisetites sparseii sp. nov., Equisetites hojedkii sp. nov. and Equisetites irregularii sp. nov.,. Descriptions of the above mentioned new species has been done based on their exclusive length of internodes, rib shapes, grooves, jointed stems, ribs density, number of grooves and form of nodes location. Recovered specimens include huge stems, which their length and width ranges are 260-310mm and 140-150mm respectively. Recognized species are associated with some Bajocian –Bathonian genera including: Nilssonia, Podozamites and Phlebopteris. Comparison of the recognized species with equisetums to some parts of Gondwana indicated that the Kermanian equisetums are similar to those of Australia, China, Afghanistan, central and south of Europe. Therefore, the Kerman Basin was a part of Gondwana during the Bajocian – Bathonian time interval.

Ammonites and biostratigraphy of the Cuyo Group in the Sierra de Reyes (central Neuquén Basin, Argentina) and their sequential significance

This paper deals with the stratigraphy, the systematics of the ammonite faunas and the biostratigraphy of the Cuyo Group in the Sierra de Reyes (southern Mendoza Province, Argentina). The 230 m-thick marine siliciclastic succession of the Cuyo Group is composed, from base to top, of the Bardas Blancas Formation (mudstones, sandy mudstones and fine-grained sandstones), the Los Molles Formation (black shales) and the Lajas Formation (coarse- to fine-grained sandstones). Four ammonite levels have been recognised in the Cuyo Group. The first two levels (I and II) are located in the lower section of the Bardas Blancas Formation. The monospecific level I yielded Dumortieria cf. pusilla and is regarded as upper Toarcian-lower Aalenian, whereas level II is composed of an association of Podagrosiceras cf. athleticum, Planammatoceras sp., Phylloceras sp. and Eudmetoceras? sp. which can be assigned to the upper Aalenian-lower Bajocian Malarguense Standard Zone. The third level appears in the middle part of the Bardas Blancas Formation and contains Morphoceras sp., Phylloceras sp. and Lytoceras cf. eudesianum, and is assigned to the lower Bathonian. The younger level (IV) was identified in the lower section of the Lajas Formation and yielded Iniskinites crassus, Stehnocephalites gerthi and Lilloettia cf. steinmanni. This association corresponds to the upper Bathonian Steinmanni Standard Zone. The ammonite record of the Cuyo Group in the Sierra de Reyes allowed dating the transgressive succession represented by the Bardas Blancas and Los Molles formations as late Toarcian-early Bathonian. The ammonite fauna identified in the lower section of the Lajas Formation (level IV) indicates that the change from transgressive to regressive conditions recorded at the upper part of the Cuyo Group occurred around early-late Bathonian times. The uppermost strata of the Cuyo Group (upper section of the Lajas Formation), having a well defined regressive signature, can be dated as lower Callovian. This relatively well constrained biostratigraphic framework of the Cuyo Group in the Sierra de Reyes and its comparison with neighbouring areas allowed for a better understanding of the sequential evolution in central Neuquén Basin during the late Lower to Middle Jurassic.

Stratigraphy, facies and geodynamic settings of Jurassic formations in the Bükk Mountains, North Hungary: its relations with the other areas of the Neotethyan realm

AbstractJurassic mélange complexes related to the subduction of the Neotethys Ocean occur in the Bükk Mountains, North Hungary. This paper characterizes the sedimentary sequence of basin and slope facies that occur in the southwestern part of the Bükk Mountains, placing special emphasis on the redeposited sedimentary rocks (olistostromes, olistoliths: Mónosbél Group) in order to obtain information on the provenance of the clasts, and the mode and time of their redeposition. The series of formations studied shows a general coarsening-upwards trend. Based on radiolarians and foraminifera, the Mónosbél Group formed in Early to Late Bathonian time. The lower part of the complex is typified by a predominance of pelagic carbonates, shale and radiolarite with andesitic volcaniclastic intercalations. The higher part of the succession is characterized by polymictic olistostromes. Large olistoliths that are predominantly blocks of Bathonian shallow marine limestone (Bükkzsérc Limestone) appear in the upper part of the sequence. Based on the biostratigraphic and sedimentological data, results of analyses of the redeposited clasts and taking into consideration the concepts of the development of the western Neotethys domain, the evolutionary stages of the sedimentary basins were defined. The onset of the compressional stage led to initiation of nappe stacking that led to the formation of polymict olistostromes and then to the redeposition of large blocks derived from out-of-sequence nappes of the former platform foreland.

New Bathonian (Middle Jurassic) ammonite assemblages from Kutch, India

The Bathonian ammonite assemblages have been previously poorly recorded in Kutch. The present study has unearthed a rich array of ammonite taxa ranging from the Middle to Upper Bathonian. While Oxycerites Rollier (1909) is a new record from Kutch, the oldest occurrence of Choffatia Siemiradzki (1898) has been found from the Middle Bathonian horizon. Oxycerites cf. orbis (Giebel) is a zonal index of the Late Bathonian in other areas and thus facilitates interprovincial correlation. Besides, palaeobiogeographic and stratigraphic distribution of many species have been modified based on new information. For example, macrocephalitin species, i.e., Kamptokephalites cf. etheridgei Spath (1928), Macrocephalites bifurcatus transient intermedius Spath (1928), M. cf. mantataranus Boehm (1912) were previously known from West Pacific, Indonesia have been now found in Kutch. Gracilisphinctes Buckman (1920) has been previously known to occur during the Middle Bathonian time, the present work extends its stratigraphic distribution up to the definite Upper Bathonian horizon. Procerites hians (Waagen) an endemic species in Kutch previously known from the Upper Bathonian beds, its stratigraphic range has been extended down to the Middle Bathonian. Detail taxonomy of the newly obtained taxa has been done and in many cases sexual dimorphism has been recognized.

Palaeoclimate reconstructions of the Middle Jurassic of Kachchh (western India): an integrated approach based on palaeoecological, oxygen isotopic, and clay mineralogical data

The Middle Jurassic sedimentary succession of the Kachchh Basin shows significant changes in lithology, faunal content and diversity across the Bathonian–Callovian boundary. The change in lithology from predominantly carbonates in the Middle and Late Bathonian to siliciclastics in the Callovian is accompanied by a drop in species diversity from high to moderate. Characteristic faunal elements of the Bathonian (corals, sponges, the bivalves Eligmus and members of the Opinae) are absent or rare in the Callovian. In contrast, Callovian sediments contain comparatively high proportions of nuculid bivalves. The clay mineral assemblage reveals prominent variations in the smectite vs. kaolinite abundance, especially during the Bathonian to Callovian transition. From the Bajocian to the Middle Bathonian, smectite becomes the dominant clay mineral and is nearly exclusively present in the Late Bathonian. Towards the Middle Callovian, its content gradually decreases and kaolinite becomes the dominant clay mineral. Illite generally is only a minor component throughout the Middle Jurassic. Based on these results, the Bajocian to Middle Bathonian time interval is interpreted to represent subtropical climatic conditions with seasonal droughts and a moderate supply of terrigenous clastics to the basin. The Late Bathonian was a period with a semi-arid climate, hot seasonal droughts and a minor input of terrigenous clastics. The increase in kaolinite contents in the Callovian suggests a subtropical humid climate with less prominent seasonal droughts and a higher input of siliclastics into the basin accompanied by a higher nutrient influx. Oxygen isotope ratios measured on Bathonian to early Callovian brachiopod shells give palaeotemperatures of 19–24 °C. Callovian to Oxfordian paleotemperatures calculated from oxygen isotopes measured on belemnite rostra range from 11 to 21 °C. The lower palaeotemperatures estimated from the oxygen isotopic composition of belemnite calcite are explained by assuming that the belemnites spent most of their life span in colder regions of the Malagassy Gulf and migrated into the warmer subtropical waters of the Kachchh Basin. Combining these various lines of evidence, the changes in lithology and faunal composition taking place in the Kachchh Basin during the Middle Jurassic appear to reflect largely a change towards increasing humidity rather than a distinct decrease in water temperature.

Late Triassic – Jurassic development of the Danish Basin and the Fennoscandian Border Zone, southern Scandinavia

The continental to marine Upper Triassic – Jurassic succession of the Danish Basin and the Fennoscandian Border Zone is interpreted within a sequence stratigraphic framework, and the evolution of the depositional basin is discussed. The intracratonic Permian–Cenozoic Danish Basin was formed by Late Carboniferous – Early Permian crustal extension followed by subsidence governed primarily by thermal cooling and local faulting. The basin is separated from the stable Precambrian Baltic Shield by the Fennoscandian Border Zone, and is bounded by basement blocks of the Ringkøbing–Fyn High towards the south. In Late Triassic – Jurassic times, the basin was part of the epeiric shallow sea that covered most of northern Europe. The Upper Triassic – Jurassic basin-fill is subdivided into two tectono-stratigraphic units by a basinwide intra-Aalenian unconformity. The Norian – Lower Aalenian succession was formed under relative tectonic tranquillity and shows an overall layer-cake geometry, except for areas with local faults and salt movements. Deposition was initiated by a Norian transgression that led to shallow marine deposition and was accompanied by a gradual climatic change to more humid conditions. Extensive sheets of shoreface sand and associated paralic sediments were deposited during short-lived forced regressions in Rhaetian time. A stepwise deepening and development of fully marine conditions followed in the Hettangian – Early Sinemurian. Thick uniform basinwide mud blankets were deposited on an open storm-influenced shelf, while sand was trapped at the basin margins. This depositional pattern continued until Late Toarcian – Early Aalenian times when the basin became restricted due to renewed uplift of the Ringkøbing–Fyn High. In Middle Aalenian – Bathonian times, the former basin area was subjected to deep erosion, and deposition became restricted to the fault-bounded Sorgenfrei–Tornquist Zone. Eventually the fault margins were overstepped, and paralic–marine deposition gradually resumed in most of the basin in Late Jurassic time. Thus, the facies architecture of the Norian – Lower Aalenian succession reflects eustatic or large-scale regional sea-level changes, whereas the Middle Aalenian – Volgian succession reflects a strong tectonic control that gradually gave way to more widespread and sea-level controlled sedimentation. The uplift of the Ringkøbing–Fyn High and most of the Danish Basin occurred concurrently with the uplift of the North Sea and a wide irregular uplifted area was formed, which differs significantly from the postulated domal pattern.

The Lower–Middle Jurassic of the Anholt borehole: implications for the geological evolution of the eastern margin of the Danish Basin

This study of Upper Pliensbachian – Bajocian/Bathonian deposits in a borehole drilled on the island of Anholt, Denmark incorporates sedimentology, biostratigraphy (palynomorphs and foraminifera), palaeomagnetism and coal petrology. The studied succession records a gradual change from marine inner shelf storm-influenced clays to mainly terrestrial sands, clays, and lignite containing a flora of mainly freshwater algae and pollen. The regression was initiated at the Pliensbachian–Toarcian boundary and marine influence ceased during Bajocian–Bathonian times; the regression thus took place earlier at Anholt than in the centre of the Danish Basin. The sediments in the Anholt borehole are referred to the Fjerritslev and Haldager Sand Formations. Although the Lower–Middle Jurassic boundary is commonly placed at the boundary between the two formations, our data indicate that at Anholt the upper Fjerritslev Formation (member F-IV) is of Aalenian age. The Lower–Middle Jurassic boundary occurs close to the boundary between members F-III and F-IV of the Fjerritslev Formation. In contrast to other Lower–Middle Jurassic successions in the North Sea region, smectites of inferred volcanic origin are preserved in the Anholt section, suggesting limited burial and hence less intense diagenetic illitisation or chloritisation of smectites. A down-hole increase in diagenetic influence is reflected by the increase down-section both in the thermal stability of kaolinite and in the vitrinite reflectance. Kaolinite of inferred authigenic origin forms a white powder in the quartz-dominated sands of the Haldager Sand Formation; this kaolinite is thermally very unstable and is interpreted to be of late diagenetic, post-uplift origin. The vitrinite reflectance data indicate that the Jurassic formations have been exposed to thermal maturation corresponding to burial to a depth of 1000–1200 m below their present depth. Post-maturation uplift of the order of 1 km probably occurred partly during Late Cretaceous – Paleocene inversion in the Kattegat area and partly during Oligocene–Recent regional uplift, the latter being the most important of the two uplift phases. Palaeomagnetic data indicate that the main carrier of magnetic remanence is fine-grained magnetite. The stable remanence shows a pronounced inclination shallowing, which is attributed to post-depositional compaction.

Le jurassique inférieur et moyen de la Tunisie septentrionale; corrélations avec l'axe Nord-Sud et paléogéographie

Ammonites recently collected in Northern Tunisia specify precisely or modify the age of the Lower and Middle Jurassic formations, and allow correlations with Central Tunisia. The palaeogeographical reconstruction leads to distinguish from South to North: 1) two pelagic shoals (N-S Axis and Tunisian Dorsale) with reduced and incomplete sedimentation separated by a subsiding basin (El Haouareb Graben); 2) a slope zone situated towards the North of the Dorsale (Bou Garnine de Hammam Lif area) with pelagic sediments including tectogenetic brecias; 3) a trough filled up with pelagic sediments including radiolaritic beds since late Bajocian-? Bathonian time (Tunisian trough). Middle Jurassic sedimentary record of the N-S Axis and the Dorsale is characterized by major unconformities with important hiatuses. These later are related both to global eustatic changes and local tectonic movements associated to the Tethyan rifting.

The Jurassic in the south-eastern part of Bulgaria (stratigraphy, geodynamics, facies and palaeogeographic evolution)

In south-east Bulgaria the presence of Hettangian (by indirect data) and Sinemurian, Pliensbachian, Toarcian. Aalenian, Bajocian and Bathonian (on the basis of direct biostratigraphic proofs) has been established. The Kostina Formation is represented by quartzites, and almost everywhere is related to the Hettangian. However, in a restricted area, in the vicinity of Stara Zagora Town, the Kostina Formation probably belongs to the Upper Sinemurian. The Ozirovo Formation (represented by shallow water, sandy, bioclastic, partly ferrugineous limestones) and the Bliznak Formation (mainly consisting of fine-grained quartz sandstones) are related to the Sinemurian. The Ozirovo Formation and the Bliznak Formation comprise also the Pliensbachian, the Toarcian, the Aalenian and the lowermost part of the Lower Bajocian (the lower part of Hyperlioceras discites ammonite Zone). The Lower Bajocian (without the lower part of Hyperlioceras discites ammonite Zone) belongs to the Zvezdets Formation, represented by shales, rich in organic matter, with siderite and phosphorite concretions, in some places being interbedded with sandstones, or the Boyanovo Formation (new formation), represented by an alternation of terrigenous-clastic and pelitic rocks and bioclastic limestones. In the south-eastern part of the Strandzha Mountains, sandstones are more abundant in the Zvezdets Formation, and in addition some olistostromes, consisting mainly of limestone breccias, are found – Kazanski Member of the Zvezdets Formation. Only the Zvezdets Formation and its Kazanski Member belong to the Upper Bajocian. The Brushlyan Formation (represented by shallow water bioclastic limestones in alternation with phyllites) and the Hranova Formation (shallow-water calcareous phyllites and a lesser amount of limestones with bioclasts) belong to the Bathonian. All autochthonous Lower and Middle Jurassic rocks in southeast Bulgaria occurred within the limits of the Tundzha Early-Middle Jurassic Fault-Angle Valley which gradually subsided during Early Jurassic and the beginning of Middle Jurassic times. As a result of these phenomena, the sea rapidly ingraded from south-east, from the Vardar Ocean side. In the valley, 7 blocks (steps) were formed and separated bathymetrically from each other: Yambol, Indzhe Voyvoda, Botevo, Kraynovo, Stara Zagora, Elhovo and Kondolovo Steps. Along the valley, a sea sound was formed and carried out the connection between the Vardar Ocean and the epicontinental basin of the Moesian Early-Middle Jurassic Platform since the Hettangian up to the Bathonian time. The valley passed two stages in its development: (a) transgressive one (Hettangian-Bajocian) and (b) regressive one (Bathonian). During the transgressive stage the sea gradually overflowed the Yambol Step (during the Hettangian), the Indzhe Voyvoda, the Botevo and the Kraynovo Steps (during the Early Sinemurian), the Stara Zagora Step (during the Late Sinemurian), the Elhovo and the Kondolovo Steps (during the Early Bajocian). During Hettangian, Sinemurian and Pliensbachian, everywhere in the sea sound, a shallow sublittoral biotop with calcareous or sandy bottom and agitated waters existed. The depths did not exceed 50 m. During Toarcian, Aalenian and Early Bajocian, some bathymetric differentiation took place. The shallow sublittoral biotop remained near the sound coasts, while in the parts distant from the coasts, bottom subsidence took place. An environment of moderate deep sublittoral biotop mainly with calcareous and partly sandy bottom appeared there. The depths were of the order of 50-100 m. During the Early Bajocian, in the south-eastern part of the sea sound the most intensive deepening took place. There, environments of a deep sublittoral biotop with a silty and partly sandy bottom and relatively calm water have set in. The depths were 150-200 m. In the central parts of the sound, the sea overflowed new considerable parts of the Tundzha Valley. There, a shallow sublittoral biotop existed, and the sedimentation regime often changed (shallow-water limy silts, pelites and terrigenous-clastic sediments were deposited). In the north-western part of the sound, probably deeper-water pelitic sediments were deposited, but in the interval Callovian-Early Cretaceous, under conditions of emergence, they were eroded. During the Bathonian, a rapid general shallowing arose there, and under conditions of a shallow sublittoral biotop with calcareous and muddy bottom, limy silts with bioclasts and pelites were deposited. The depths did not cxceed 50 m. In considerably large parts of the Tundzha Fault-Angle Valley, a shallow sublittoral biotop with strong bottom currents existed and caused a sedimentation gap. At the end of the Bathonian, the Tundzha Fault-Angle Valley was uplifted over the sea level and joined the dry land of the Thracian Massif. As a result of this, the sea-strait between the Vardar Ocean and the Moesian Early-Middle Jurassic Platform stopped existing.

Architecture and controls on Bathonian–Kimmeridgian shallow‐marine synrift wedges of the Oseberg–Brage area, northern North Sea

The Oseberg Fault‐Block, situated along the eastern flank of the northern Viking Graben in the North Sea, was affected by Middle–Late Jurassic rifting initiated in Bajocian–Bathonian times. Temporal variations in stretching rates exerted the major control on the depositional infill patterns of the Bathonian–Kimmeridgian Heather Formation and its intercalated Middle Callovian to Early Oxfordian Fensfjord and Late Oxfordian to Kimmeridgian Sognefjord Formations. Three shallow‐marine, regressive–transgressive synrift wedges are recognized, and are interpreted in terms of discrete rift phases. The lower, regressive segments of the synrift wedges were deposited during periods of relatively low tectonic activity, whereas the upper, overall transgressive segments correspond to extensional pulses or stages during which significant fault‐related subsidence and fault‐block rotation occurred. These rotational tilt stages are further subdivided into an early, a climax and a late synrotational substage.The lower, regressive segments consist of stacked, shallowing‐upward units, which reflect the advance of wide shallow‐marine, rift‐marginal shorelines during the tectonically quiescent periods. During the intervening rotational tilt stages renewed basin floor tilting and increased basinal subsidence led to retreat of the rift‐marginal depositional systems, renewal of the half‐graben topography, formation of intrabasinal sediment sources (footwall islands) and the re‐establishment of localized footwall, hangingwall and axial depositional systems. These localized depositional systems generally have an overall forestepping‐to‐backstepping character superimposed on the larger‐scale transgressive trend. There was an associated shift from a wave‐ and storm‐dominated environment during deposition of the lower, regressive segment to a more protected, partly current‐(?tidally) influenced environment in the upper, transgressive segment. This reflects a shift from a broad open basin in tectonically quiescent periods to smaller subbasins (embayments or estuaries) during periods with increased rates of rifting.The footwall highs which formed intrabasinal sediment sources were of limited size compared with the volume of the adjacent depositional sinks. As a consequence, complete infilling of individual half‐grabens were not achieved during the synrotational stages, leaving the subbasins underfilled at the end of each successive rift phase. Mudstone drapes represent periods with deprivation of clastic material and basinal condensation during the latest synrotational to early tectonic quiescence substages, when footwall islands were small or completely submerged and there was a large distance to the (then progradational) rift‐marginal shoreline.

Evolution of a Middle Jurassic volcanic arc: stratigraphic, isotopic, and geochemical characteristics of the Harrison Lake Formation, southwestern British Columbia

The Harrison Lake Formation is an Early to Middle Jurassic volcanic-arc assemblage unconformably overlying Triassic oceanic basement in the eastern Coast Belt of southwestern British Columbia. The formation is subdivided into four members including, in ascending order, the Celia Cove Member (conglomerate), the Francis Lake Member (fine-grained strata), the Weaver Lake Member (flows and breccias), and the Echo Island Member (pyroclastic and epiclastic strata). New biostratigraphic constraints pinpoint the initiation of volcanism to late early Toarcian. U–Pb geochronology demonstrates the arc was active until at least late Bajocian–early Bathonian time (166.0 ± 0.4 Ma), and that the timing of arc volcanism strongly overlaps emplacement of both hypabyssal intrusions (Hemlock Valley stock) and deep-seated plutons (Mount Jasper pluton) within and adjacent to the arc. Geochemical data indicate the arc is of medium- to high-K calc-alkaline affinity, and is strongly light rare earth element enriched (LaN/YbN = 1.5 – 2.5). Nd and Sr isotopic data from primary volcanic rocks demonstrate the juvenile nature of the magmatic system, but isotopic data from associated fine-grained sedimentary rocks suggest temporally controlled variations in isotopic composition interpreted to represent two-component mixing between juvenile volcanic detritus and a more evolved detrital component. The succession of facies in the Harrison Lake Formation records initial basin subsidence in the Early Jurassic, initiation of explosive volcanism in the late early Toarcian, a change to effusive volcanism in the early Aalenian, and late-stage explosive volcanism in the late Bajocian. The Harrison Lake Formation contains mesoscopic folds and overturned bedding that are absent in the overlying Callovian Mysterious Creek Formation, strongly suggesting the existence of a regional Bathonian deformational event in the southern Coast Belt.

THE JURASSIC HUASTECA SERIES IN THE SUBSURFACE OF POZA RICA, EASTERN MEXICO

The Author has studied 120 cores containing ammonites and lamellibranchs of Middle and Late Jurassic ages selected from the 230 wells drilled in the Poze Rica district, eastern Mexico. The wells are spread over an area of 8,000 sq. km.The Huasteca Series is a marine sedimentary sequence that ranges in age from Bathonian to late Tithonian, and is about 1,000‐m thick. This unit is subdivided into ammonite zones and substages which have previously been established by the Author. These zones are: Wagnericeras, Kepplerites, Reineckeia, Discosphinctes, Ataxioceras, Idoceras, Virgatosphinctes, Mazapilites, Suarites, and Parodontoceras‐Salinites.The restricted vertical distribution of these fossils and the general availability of gamma‐ray logs from the wells has made it possible for the first time to delimit the lateral facies changes that took place, especially in the late Bathonian—middle Callovian (calcarenite‐shale‐silstone) and in the early Tithonian (calcarenite‐argillaceous limestone). Structural and isopach maps for each interval show the presence in the northern half of the study area of the NW‐SE striking La Mesa syncline, and, in the southern half of the area, the east‐dipping Sultepec homocline.The transgressive phase of Huasteca Series sedimentation began during Bathonian time in the NW part of this region. Evaporites and calcarenites were deposited above rocks of continental origin. This transgression advanced progressively during Callovian to Kimmeridgian time towards the central and eastern parts of the area, where the initial marine deposits overlie igneous, metamorphic, and continental sedimentary rocks. The last phase of this transgression took place during early Tithonian time in the SE part of the Poza Rica district. It is represented here by the San Andrés was deposited on the same types of rocks—igneous, metamorphic, and continental‐sedimentary. Remnants of this late transgressive phase, characterized by calcarenites and deposited on continental sediments, are preserved in very limited areas NE of the Poza Rica district.Beginning in Kimmeridgian time, two well‐defined depocentres developed. The Huizotate Basin, with argillaceous limestones and shales of the Tamán Formation, formed in the central part of the district, while the Papantla Platform with calcarenite units of the San Andrés Member (Tamán Formation) formed around the northern, eastern, and SE margins. These depocentres are named here for the first timeAn important result of this study is that it explains the presence in structural traps of hydrocarbons of Middle and Late Jurassic ages

Tectonics of the Verkhoyansk-Kolyma Mesozoides in the context of plate tectonics

The Verkhoyansk-Kolyma Mesozoides, located to the northeast of the ancient Siberian platform, form a part of a Mesozoic orogenic belt which lies between the Siberian and North American platforms, the Pacific Ocean and Arctic Ocean. The outer part of the Verkhoyansk-Kolyma Mesozoides is adjacent to the Siberian platform and comprises the Verkhoyansk foldbelt, composed primarily of Carboniferous-Middle Jurassic clastic sequences and lower-middle Paleozoic and Riphean terrigenous-carbonate series deposited on the Siberian passive continental margin. Northeastward, the passivemargin shelf deposits grade into Upper Permian to Lower Jurassic black-shale sequences of the Kular Nera slate belt which formed at a continental rise and in a marginal sea. Thick (up to 8–10 km), largely carbonate Ordovician-Devonian deposits of the Cherskiy, Selennyakh and Ulakhan-Tas ranges lie allochthonously over the Mesozoic and represent formations of the Omulevka carbonate platform. Ophiolites of uncertain age and a serpentinite melange have been tectonically juxtaposed against the latter. The time of emplacement for the Ophiolites is constrained by their occurrence in a Bathonian-Callovian olistostrome. The northeast-trending Alazeya volcanic arc, of Carboniferous to early Jurassic age, is located in the central part of what was once considered as the Kolyma massif. An accretionary wedge associated with the arc is buried beneath Late-Cenozoic deposits and can be traced by a large magnetic anomaly which extends to the south and east. The Omolon and Okhotsk massifs have an Archean granulite basement and are independent microcontinents. The available paleomagnetic data do not allow of the quantitative assessment of the amount ofseparation of the various allochthonous terranes, either from each other or from the Siberian continent. An affinity between Paleozoic and Mesozoic faunas of the Mesozoides and those ofthe Siberian platform indicates that their relative separation did not exceed a few thousand kilometers. Remnants of basins with oceanic crust found in the inner areas of the Verkhoyansk-Kolyma Mesozoides indicate that they are more comparable to present-day marginal seas of eastern Asia than to the open Pacific Ocean. Early folding occurred as a result of the pre-mid-Jurassic attachment of the Prikolyma and Omolon terranes to the Alazeya arc, which led to the formation of the large, heterogeneous, Kolyma-Omolon microcontinent. Later folding was due to a collision of the Kolyma-Omolon microcontinent with the Omulevka platform and the Verkhoyansk passive margin. This deformation propagated from the inner (Cherskiy) areas, in Bathonian time, towards the Siberian platform, where it culminated in frontal thrusts in the latest Cretaceous. Transverse and longitudinal belts of Upper-Mesozoic granitoids are closely related to collision processes. Their long history (about 90 m.y.) reflects the duration of the collision.

The concretions of the Bearreraig Sandstone Formation: geometry and geochemistry

ABSTRACTThe sandbodies of the Bearreraig Sandstone Formation (Inner Hebrides, UK) are cemented by two generations of calcite. The first generation, an inhomogeneous ferroan calcite (0.05−3.28 mol% FeCo3) formed during sulphate reduction (δ13C =−24 to −32%o PDB) in marine porewaters (δ18O of cement from −1 to −4%o PDB) at very shallow burial depths (a few centimetres). These cements are rare but form millimetre‐scale clusters of crystals which acted as nuclei to the later, concretionary cements.The second generation of cements are more homogeneous ferroan calcites (mean 1−58% mol% FeCo3) which evolve to progressively higher Fe/Mg ratios. They are sourced by shell dissolution (δ13C of cement from +1 to −3%o PDB) into meteoric (δ18O of cement from −6 to −10%o PDB) or mixed marine meteoric waters (δ18O of cement from −4 to −6%o SMOW). These were introduced into the formation either during Bathonian times as a freshwater lens, or, subsequent to partial inversion, by confined aquifer flow. Corroded feldspars within the concretions suggest that an interval of at least 8 Ma separated the deposition of the sediments from the onset of concretion growth.Abundant concretions are preferentially developed at certain horizons within the sandbodies, where the early generation of ferroan calcite cements provided nuclei. The latter formed close to the sediment‐water interface, the concentration of cement within the sediment being related to sedimentation rate. The relatively high concentrations of the first generation of cement, upon which the concretionary horizons are nucleated, formed during periods of minimal sedimentation.

Origin of late cements in oolitic reservoir facies: a fluid inclusion and isotopic study (Mid-Jurassic, southern England)

During Bathonian times a carbonate ramp was constructed over southern England by the progradation of oolitic, skeletal and oncoidal grainstones and packstones. The grainstones provide a significant hydrocarbon reservoir facies. Although these carbonates suffered early diagenesis under marine and, more significantly, meteoric conditions, later diagenetic cements such as saddle dolomite and, more particularly, mildly ferroan calcite are volumetrically more important in causing the occlusion of porosity on a regional scale. We have analysed systematically these late cements employing a number of techniques: standard petrography (including CL and UVL); C and O isotopes; Sr isotopes and fluid inclusion studies. Integration of the results of these approaches constrains both the pore-fluid composition and the temperature regime at the time of cement precipitation. Saddle dolomite was emplaced before the mildly ferroan calcite at temperatures between 75 and 85°C and from highly saline brines (18–19.5 wt.% NaCl equivalents). Mildly ferroan calcite precipitated over a generally lower temperature range and from less saline brines (6.5–15 wt.% NaCl equivalents). Five types of fluid inclusion have been recognised: (1) monophase aqueous (liquid); (2) monophase oil (liquid); (3) two-phase aqueous (liquid+vapour); (4) two-phase oil (liquid hydrocarbon +vapour); (5) three-phase aqueous-oil (liquid hydrocarbon+aqueous liquid+vapour). During burial warm (> 75°C) brines passed through the limestones and precipitated saddle dolomite. Later, slightly cooler oil-rich brines precipitated mildly ferroan calcite wherever hydrocarbon accumulation had not already occurred. Our Sr isotopic studies suggest that both these brines were derived from underlying, and more deeply, buried Lias shales and that these brines were generated from Late Jurassic times onwards. Brine movements are likely to have been largely complete by the Late Cretaceous.

Stratigraphy and Depositional History of Thousand Pockets Tongue of Page Sandstone and Crystal Creek Member of Carmel Formation (Middle Jurassic), Southwestern Utah: ABSTRACT

Between the East Kaibab monocline and Zion National Park in southwestern Utah, gray, cliff-forming, cross-bedded quartz sandstone of the Thousand Pockets Tongue of the Page Sandstone grades northwestward into red, slope-forming, flat-bedded quartz sandstone of the Crystal Creek Member of the Carmel Formation. On the basis of stratigraphic position, both members are considered Bathonian in age. Coastal deposits of the Thousand Pockets Tongue prograded northwestward and interfingered with contemporaneous submarine deposits of the Crystal Creek Member along a northeast-southwest trending shoreline of the Middle Jurassic seaway. The Thousand Pockets Tongue is divided into three parts characterized by distinct sedimentary features and paleoenvironments. The gray lower part contains cross-bedded eolian sandstones (cross-bedded sandstone facies) which grade into evenly laminated and cross-bedded beach sandstones (Round Valley Draw facies). The red middle part is characterized by flat laminations, echinodern fragments, and a distribution which suggest a lagoon/tidal-flat environment. In the gray upper part, cross-bedded sandstones of eolian origin (Paria Canyon facies) grade into evenly laminated and cross-bedded beach sandstones (yellow sandstone ledge facies) and locally enclose red, flat-bedded units deposited in a washover channel-wind-tidal flat complex (red lenticular sandstone facies). The Crystal Creek Me ber is typified by evenly laminated, massive, and cross-bedded units and a lateral facies relation which suggest a lower beach and subtidal environment where storm and tidal currents dominated. Cross-stratification measurements from eolian facies in lower and upper Thousand Pockets Tongue indicate north-northeasterly winds during Bathonian time. The marginal-marine and submarine interpretation of the red beds in the Thousand Pockets Tongue and Crystal Creek Member, respectively, lends support to a diagenetic origin for the red color of these beds. End_of_Article - Last_Page 556------------