Mudstone Member Research Articles

The influence of salt tectonics on the distribution of the Triassic Skagerrak Formation in the Ula Field, Norwegian North Sea

Distribution of the Triassic succession in the North Sea is poorly understood because of structural complexities associated with halokinesis and limited stratigraphic control. This study uses a seismic and well-based dataset to improve understanding of development of the Triassic succession in the Ula Field Area, in the Norwegian North Sea. Core interpretation revealed a fluvial-dominated depositional environment in the Ula Field Area. Palynological studies allowed dating of cored intervals, revealing Ladinian and Carnian sections, time-equivalent to the Julius and Joanne members of the Skagerrak Formation. Well-log interpretation provided insight into the intra-Triassic stratigraphy of the Ula Field Area. A section considered to be equivalent to and extending from the Smith Bank Formation to the Jonathan Member of the Skagerrak Formation was interpreted and correlated across the area. In the proposed correlation, the Julius Member thins towards the Ula Field Area and is replaced by a time equivalent sandstone unit. The Jonathan Member displays a sandier composition compared to the equivalent section in the UK sector. Seismic facies-based interpretation of Triassic stratigraphy within salt minibasins allowed recognition and mapping of intra-Triassic units and showed that mudstone members thin towards the NE. Interpreted internal geometries within minibasins allowed determination of the timing of halokinesis. Integration of different datasets allowed palaeogeographic reconstructions for the Anisian, early Ladinian, Carnian and Norian to be constructed. To conclude, the distribution of stacked fluvial channel deposits indicates that they occur both within minibasins and across salt highs such that ongoing halokinesis had no topographic expression and that channels were free to migrate across the area.

Litho- and biostratigraphy of a late Oligocene–Early Miocene succession in the Weber area, southern Hawke’s Bay, and implications for early Hikurangi subduction-margin evolution

ABSTRACT A deep-marine sedimentary succession of Whaingaroan to Altonian age (Early Oligocene–Early Miocene), cropping out near Weber, southern Hawke’s Bay, records abrupt changes in depositional paleoenvironments and sedimentary lithofacies. Highly calcareous early Waitakian (latest Oligocene) Weber Formation (Mangatu Group/Waka Supergroup) is unconformably overlain by terrigenous-dominated late Waitakian–Otaian (Early Miocene) Coast Road or Whakataki formations (Tolaga Group/Māui Supergroup). Most notably, in some localities, lowermost parts of the Coast Road Formation (Mangapuku Mudstone Member) contain prominent matrix-supported extra-formational olistoliths of centimetre to decametre scale derived from the underlying Weber Formation. Deposition of these blocks is attributed to deep-water collapse of parts of rapidly growing, over-steepened, reverse-fault-controlled thrust ridge(s). The onset of Coast Road and Whakataki formation deposition and associated abrupt changes in sedimentary lithofacies correlate to prominent Early Miocene lithological, mineralogical, and paleoenvironmental changes elsewhere along the Hikurangi Margin. We attribute these bio- and lithostratigraphic changes as evidence for significant shortening and upper-plate reverse faulting marking the onset of subduction beneath eastern North Island during the mid-Waitakian (c. 23 Ma). Proposed lithostratigraphic revisions presented herein help clarify depositional events and correlative sedimentary packages within the Miocene Hikurangi margin.

The Geochemical Study of Oil-Oil and Oil-Source Rock Correlations in the Wushi Sag of the Beibu Gulf Basin, South China Sea

Chemometrics has been widely used to cope with the problems of oil-oil and oil-source correlations because of its unique advantages in the comprehensive consideration of multiple parameters and the classification of samples or variables. In this paper, three chemometric methods, especially multidimensional scaling, were used to revisit the genetic oil family and the relationship between the crude oil and the source rock, because the oil source in the Wushi Sag, a significant petroliferous sag in the Beibu Gulf Basin of South China Sea, is still controversial. Two genetic families of crude oils, namely group A and group B, have been identified based on chemometric results. Group A oils are characterized by relatively higher Pr/Ph ratios and a high abundance of C27 ααα 20R steranes and C30-methylsteranes than those of group B oils, suggesting that this group of oils was deposited under a more oxic condition with more contribution of algae organic matter. Group A oils have been interpreted to be a mixture derived from the member 2 and member 3 of the Liushagang Formation (LS-2 and LS-3), whereas group B oils can be ascribed to the LS-2 member. The contribution of LS-3 mudstone member to the Wushi oils in previous studies may have been underestimated to some extent, which was inferred from the chemometric oil-source correlation results. The results of oil-source rock correlation may be used to guide future petroleum exploration activities with the incorporation of geological evidence. The spatial distribution of oil and gas reservoirs varies with burial depth. Taking into consideration other geological evidence, we may infer that the reservoir in eastern Wushi Sag was mainly distributed in the deep of Liushagang Formation, whereas the reservoir in southeast Wushi Sag was likely located in the shallow Liushagang Formation.

Palaeoenvironmental reconstruction of Triassic floras from the Central North Sea

The Triassic sediments of the Central North Sea (CNS) are considered to have been deposited in a continental environment under a semi-arid climate. The Skagerrak Formation in particular comprises an alternation of sandstone and mudstone members, the development of which is considered to be climatically driven. However, conflicting models exist as to how climate influences member deposition. Here we analyse the climatic signal using a multivariate statistical approach in which detrended correspondence analysis (DCA) is applied to palynological observations to quantify environmental reconstruction. Using DCA it has been possible to define palaeoecological groups and construct a relative hydrological state trend showing hydrological conditions within the centre of the CNS basin during the Triassic. The resultant trends reveal that the relationship between hydrological conditions in the basin and the development of individual sandstone and mudstone members is perhaps not a simple as indicated by existing models. In particular, our data suggest that, although influenced by broader climate trends, in the basin centre there is no simple relationship between climate change and sandstone or mudstone development. The data also indicate that the Julius and Jonathan mudstone members were deposited under differing hydrological conditions. The DCA trends shown here also suggest that the Carnian Pluvial Episode documented from the South Permian Basin and Tethys is not expressed in the CNS.

LITHOSTRATIGRAPHY OF THE NEWLY PROPOSED MIDDLE CRETACEOUS “BIBAI GROUP”, WESTERN SULAIMAN FOLD-THRUST BELT, PAKISTAN

The newly proposed Middle Cretaceous “Bibai Group”, named after the Bibai peak, is exposed in Kach-Ziarat, Spera Ragha-Chingun areas of the Western Sulaiman Fold-Thrust Belt, Pakistan. It comprises thick succession of the mafic volcanic rocks, volcanic conglomerate, mudstone and sandstone. The stratigraphic nomenclature proposed by previous workers was not clear enough, as they used different names for the succession, such as “Kahan Conglomerate Member” of the Mughal Kot Formation, “Parh-related volcanics” by considering it as part of the “Parh Group, “Bibai Formation” and “Bela Volcanic Group”, which were confusing and misleading. Also previous workers did not realize that the succession may be further classified into distinct mappable lithostratigraphic units and deserved the status of a “Group”. Therefore, we carefully examined and mapped the area and hereby propose the name “Bibai Group” for the overall volcanic and volcaniclastic succession of the Middle Cretaceous age. Based on distinct lithostratigraphic characters we further subdivided the “Group” into two lithostratigraphic units of formation rank, for which we propose the names “Chinjun Volcanics” and “Bibai Formation”. Also based on distinct lithostratigraphic characters we further propose to subdivide our “Babai Formation” into three lithostratigraphic units of member rank, which we named as the “Kahan Conglomerate Member”, “Ahmadun Member” and “Kach Mudstone Member”. In this paper we have defined and briefly described the Bibai Group, its constituent formations and their members. Also we examined and discussed the validity and status of the proposed subdivisions; e.g. formations and members, of the Bibai Group, and are fully satisfied that the proposed subdivisions are appropriate and comply with the Article 24 and 25 of the North American Stratigraphic Codes (2005) and that the previous nomenclatures are inconsistent, confusing and do not comply with the International Stratigraphic Codes.

A new genus of Early Jurassic earwigs from England (Dermaptera)

A new genus of archidermapteran earwigs is described for Brevicula teres Tihelka (Dermapteridae) from Sinemurian deposits of the Black Ven Mudstone Member, Charmouth Mudstone Formation, Dorset, United Kingdom. Dacryoderma Engel, new genus, is distinct from Brevicula Whalley in the broadly rounded anterior tegminal border lacking a submedial hump and overall teardrop shape, with the anterior margin tapering gradually in a comparatively straight line to the more acutely rounded apex. The following new combination is established: Dacryoderma teres (Tihelka).

Late middle Miocene Konan flora from northern Hokkaido, Japan

Plant macrofossils from the upper middle Miocene Konan Tuffaceous Sandstone and Mudstone Member of the Bifuka Formation, known as the Konan flora, northwest of Shibetsu City, Hokkaido, Japan, were taxonomically revised. A total of 31 taxa were recognized, which were assigned to 14 families and 19 genera, including a new fossil species, Salix palaeofutura sp. nov. The Konan flora includes three taxa of evergreen conifers, one perennial monocot herb and 27 deciduous dicots. The most abundant and common species were Fagus palaeojaponica, Acer subcarpinifolium, Acer protojaponicum, Picea sp. A and Cercidiphyllum crenatum, in addition to a number of species of the Betulaceae and Salicaceae. From the absence of evergreen angiosperms as well as the common occurrence of Fagus palaeojaponica, Picea, Acer and Betulaceae species, this flora was comparable to that seen in the modern Mixed Northern Hardwood Forest of East Asia, which is distributed in northernmost Honshu and extends toward lowland Hokkaido. On the basis of floral features, mode of occurrence, and the lithology of plant-bearing beds, the Konan flora was deemed to represent mountain to riverside vegetation with humid and cool temperate climatic conditions. In contrast to the early to late Miocene floras in Japan, the Konan flora belongs to the late Miocene–Pliocene Mitoku-type flora, with a few relict species from the early Miocene. The Konan flora represents one of the earliest occurrences of this type of flora, suggesting that floral modernization was initiated much earlier in areas with humid and cool temperate climate than previously thought.

Triassic mudstones of the Central North Sea: cross-border characterization, correlation and their palaeoclimatic significance

Abstract The Triassic of the Central North Sea is a continental succession that contains prolific hydrocarbon-bearing fluvial sandstone reservoirs stratigraphically partitioned by mudstones. Within the Skagerrak Formation of the UK sector, hydrocarbon accumulations in the Judy, Joanne and Josephine Sandstone members are top sealed by the Julius, Jonathan and Joshua Mudstone members, respectively. However, UK and Norwegian stratigraphic correlations have been problematical for decades, largely due to biostratigraphic challenges but also due to the non-uniqueness of the lithotypes and because the cross-border stratigraphic nomenclature differs and has yet to be rationalized. This study focuses on mudstones rather than sandstones to unify cross-border correlation efforts at a regional scale. The mudstone members have been characterized by integrating sedimentological, petrophysical and geophysical data. The facies are indicative of playa lakes that frequently desiccated and preserved minor anhydrite. These conditions alternated with periods of marshy, palustrine conditions favourable for the formation of dolostones. Regional correlations have detected lateral facies changes in the mudstones which are important for their seismically mappable extents, resulting palaeogeographies and, ultimately, their competency as intraformational top seals. Significant diachroneity is associated with the lithological transitions at sandstone–mudstone member boundaries and although lithostratigraphic surfaces can be used as timelines over short distances (e.g. within a field), they should not be assumed to represent timelines over longer correlation lengths. Palaeoclimatic trends are interpreted and compared to those of adjacent regions to test the extent and impact of climate change as a predictive allogenic forcing factor on sedimentation. Mudstone member deposition occurred as a result of the retreat of large-scale terminal fluvial systems during a return to more arid ‘background’ climatic conditions. The cause of the member-scale climatic cyclicity observed within the Skagerrak Formation may be related to volcanic activity in large igneous provinces which triggered the episodic progradation of fluvial systems.

Stratigraphic palynology of the Middle–Late Triassic successions of the Central North Sea

Historically, the continental Triassic successions of the Central North Sea have proven difficult to correlate, in part due to the poor palynomorph recovery associated with these sedimentary rocks. The existing framework for correlation is lithostratigraphic and, whilst this has proven effective in United Kingdom Continental Shelf (UKCS) Quad 30 where the mudstone members are well defined, elsewhere in the basin it is more problematic with confident identification of stratigraphic units becoming more difficult. Samples from 32 wells within UKCS Quads 22, 29 and 30, and Norwegian Quads 7, 15 and 16 underwent palynological analysis in which a processing method was utilized that was designed to concentrate palynomorph recovery from Triassic strata. The results of this analysis allowed the proposal of a new zonal scheme consisting of eight biozones. These biozones can then be used to correlate the Triassic successions of the Central North Sea (CNS), helping to provide both clarity and age constraint on previously disputed stratigraphic units, particularly the J-members of the Skagerrak Formation. Within the correlation framework outlined here, the Julius Mudstone Member is shown to be a productive horizon for palynomorph recovery, representing a widespread swamp environment. Here, its lateral extent is defined which is an important consideration when correlating the Triassic stratigraphy of the CNS given that this member can compartmentalize potential reservoirs contained within these successions. The stratigraphic palynology outlined here also allows clarity on the J-member equivalence of some of the informal units previously described within Triassic successions of the CNS, including the Marnock Shale and Heron Shale.

Chronostratigraphic Revision of the Cloverly Formation (Lower Cretaceous, Western Interior, USA)

The Cloverly Formation is an important geologic unit for understanding the development of North American terrestrial landscapes and ecosystems, but the age of this unit is poorly constrained. We report U–Pb radiometric dates determined by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and chemical abrasion thermal ionization mass spectrometry (CA-TIMS) from euhedral zircons derived from fluvial sandstone and bentonitic claystone. We reanalyzed published biostratigraphic, paleomagnetic, and radiometric datasets, which have generally disregarded younger (late Albian–Cenomanian) ages for the formation. New data reported in this study suggest that deposition of the Cloverly Formation spanned the Valanginian–Cenomanian stages (ca. 140 Ma–98 Ma), a longer time interval than the commonly cited Aptian–Albian depositional timeframe. The lowest member of the Cloverly Formation, the Pryor Conglomerate, was deposited ca. 140–130 Ma in response to the onset of the Sevier Orogeny shedding sediment from the west. The overlying Little Sheep Mudstone Member was deposited ca. 124–109 Ma in a time of low sediment supply. In the mid–late Albian to early Cenomanian (ca. 109–98 Ma), sediment sourced from the east was deposited as the Himes Member and Greybull Sandstone. Following this, the Sykes Mountain Formation began nearshore deposition as the Western Interior Seaway transgressed from the north. Our revised chronostratigraphic framework for the Cloverly Formation is congruent with tectonic subsidence analysis showing a rapid increase in accommodation space in the mid-Albian. We hypothesize that more intensive sampling may yield multiple fossil assemblages within the formation, paralleling its correlates to the south. Furthermore, we hypothesize that some poorly represented taxa will be synonymized with taxa from those same units now that their temporal equivalence has been demonstrated.

Biotic, geochemical and environmental changes through the early Sheinwoodian (Wenlock, Silurian) carbon isotope excursion (ESCIE), Buttington Quarry, Wales

Palaeontological, sedimentological and geochemical data (particularly from LA-ICPMS analyses of pyrite) from a 12.4 m section through the lower Sheinwoodian Trewern Brook Mudstone Formation of Buttington Quarry, Wales are combined to reveal the environmental changes taking place during the early Wenlock Epoch of the Silurian, during the early part of the ESCIE (early Sheinwoodian carbon isotope excursion). The uppermost Cyrtograptus murchisoni through to lower Monograptus riccartonensis biozones show increased nutrient levels and peaks in palynomorph and graptolite abundance. At this time graptolite diversity globally was declining dramatically, possibly associated with metal (particularly zinc) concentrations attaining toxic levels and probably with the base of the euphotic zone being higher in the water column (resulting from blooms of phytoplankton) causing those graptolites that specialized in feeding on phytoplankton that were outcompeted in this diminished volume of illuminated surface waters to decline in numbers and eventually become extinct. Correlation of the graptolite and conodont biozonations enables the levels of the main stratigraphically higher Ireviken Event Datum Points to be proposed for the Buttington section and this in turn allows events recognised elsewhere (intervals of malformed palynomorphs and re-organization of polychaete faunas, together with an oceanic anoxic event) to be correlated with the lower part of the studied Buttington section. The commencement of the OAE, identified on Gotland at Ireviken Event Datum Point 4, appears to correlate rather well with a rapid transition, marking the top of the Butterley Mudstone Member at Buttington, from heavily bioturbated mudstones to laminated graptolitic mudstones. Surprisingly, the most rapid change in δ13Ccarb values within the Buttington section (from 0.6‰ at 8.19–8.25 m to 2.09‰ at 8.50–8.55 m) is not associated with any major faunal, palynological abundance or geochemical changes – these are all coincident with the initial rise in δ13Ccarb values, up to about the 5 m level in the Buttington section. Another unexplained feature of the graptolite record is a sudden change from Monograptus-dominated to Pristiograptus-dominated assemblages about half way through the studied section.

Western Tethyan Epeiric Ramp Setting in the Early Triassic: An Example from the Central Dinarides (Croatia)

In the central part of the External Dinarides in Plavno, Croatia, near Knin, a remarkably thick (927.5 m) Early Triassic depositional sequence was investigated. The Plavno sequence starts in the EarlyGriesbachian and ends with a continuous transition into the Anisian strata. A complete 13C isotope curve has been achieved and combined with conodonts, bivalves and ammonoids to establish and correlate stage and substage boundaries. The δ13C curve is consistent with former studies. It displays a general increase from the Griesbachian to a prominent maximum beyond the +8‰ amplitude around the Dienerian-Smithian boundary (DSB), followed by a steep and continuous decline to low, negative values in the Smithian. Around the Smithian-Spathian boundary (SSB) a steep rise to a second maximum occurred. It is followed by a saw-tooth shaped decline in the Spathian and a similar increase to a peak at the Spathian-Anisian boundary (SAB). Sedimentologically, the Plavno sequence is interpreted as having been deposited on an extensive epeiric ramp under long-term transgressive conditions, sharing depositional characteristics of both the epeiric platform and the carbonate ramp. The entire Plavno sequence was deposited above the storm-wave base and was storm influenced. Three informal members are differentiated: 1) the dolostone member (Early Griesbachian); 2) the siliciclastic member (red-coloured shale, siltstone, sandstone with oolitic/bioclastic grainstone intercalations), which can be further divided into lower, middle and upper intervals (Late Griesbachian, Dienerian and Smithian); and 3) the mudstone member (grey lime mudstones, marls and calcisiltites with common ammonoids and gastropods-Spathian). The Plavno sequence is compared with other western Tethyan sections. Observed differences stem from local controls on deposition in the overall shallow marine environment.

Origin and genetic family of Huhehu oil in the Hailar Basin, northeast China

The Huhehu Sag is one of the most important oil and gas depressions in the Hailar Basin. However, the origin of Huhehu oil is still controversial. Previous studies on source rocks have mainly focused on the Nantun Formation (K1n); a few studies have investigated the Damoguaihe Formation (K1d). Based on the Rock–Eval pyrolysis parameters, 172 drill cutting samples from the Huhehu Sag were analyzed to evaluate their geochemical characteristics. Based on the Rock–Eval data, the mudstones from the first member of the Damoguaihe Formation (K1d1) and the second member of the Nantun Formation (K1n2) have moderate to high hydrocarbon generation potential, while mudstones from the first member of the Nantun Formation (K1n1) have poor to good hydrocarbon generation potential. Additionally, both the K1n1 and K1n2 coal members have poor to fair hydrocarbon generation potential, but the K1n2 coal member has a better generative potential. Fifteen Huhehu oils were collected for molecular geochemical analyses to classify the oils into genetic families and to identify the source rock for each oil using chemometric methods. The Huhehu oils were classified into three groups with different maturity levels using hierarchical cluster analysis and principal component analysis. Group A oils (high maturity) are characterized by relatively moderate ratios of Pr/Ph, Pr/n-C17, and Ph/n-C18, as well as an abundance of C29 steranes, mainly derived from the K1n2 and K1n1 mudstone members. In comparison, group B oils (moderate maturity) have relatively low Pr/Ph ratios, moderate Pr/n-C17 and Ph/n-C18 ratios, and low concentrations of C29 steranes. Group C oils (low maturity) show relatively high ratios of Pr/Ph, Pr/n-C17, and Ph/n-C18, as well as high concentrations of C29 steranes. Furthermore, group B oils derived from the K1d1 mudstone member and group C oils derived from the K1n2 coal member were also identified by principal component analysis score plots. Correlation studies suggest a major contribution from the K1n mudstone Formation and the K1d1 mudstone member to the oils of the Huhehu Sag. So, the Nantun Formation and relatively shallow strata of the Damoguaihe Formation (e.g., the K1d1 member) represent important targets for future oil-reservoir exploration in the Huhehu Sag.

Stratigraphical and Sedimentological Review of the Merawu Formation, Serayu Basin, Central Jawa, Indonesia

DOI: 10.17014/ijog.5.2.117-128Merawu Formation is widely distributed in the Serayu Basin, Central Jawa, Indonesia. The formation is dominated by fine-grained clastic sediments comprising interbedded mudstone, siltstone, and sandstone. In the field, these outcrops are well exposed and well bedded. Stratigraphically, the Merawu Formation is underlain by the Paleocene Worawari Formation and overlain by the Late Miocene Penyatan Formation. Contacts between the Merawu Formation and the Worawari Formation are always found as fault types. Twelve sections have been done during the field work. Two members were identified within the Merawu Formation (Sandstone and Mudstone Members) in the field, where they interfinger each other. Previously, researchers interpreted the Merawu Formation as a turbidite sequence of deep marine deposits and depicted Early - Middle Miocene in age. However, the interpretation from the present research shows that the Merawu Formation was deposited on a tidal flat environment and has Early Miocene - Pliocene age, based on foraminifers found in some sections.

Diverse Sedimentary Conditions During Deposition Of Coal And Oil Shale From The Meihe Basin (Eocene, Ne China)

The Meihe Basin in northeastern China is a normal-fault-controlled basin filled with Eocene nonmarine deposits, the sediments assigned to the Meihe Formation. Sediment grain-sizes, core study, well-log analysis and micropetrographic study combined with sedimentary facies analysis indicate that the main sedimentary facies of the Meihe Basin represent fan-delta, lake, and turbidite deposits. Thick coal seams that are mainly distributed in the eastern part of the studied area were deposited in the swamp facies of the Lower Coal-Bearing Member. Oil shale was mainly deposited in the deep lake represented by the Mudstone Member, and is mainly distributed in the southern part of the studied area. In the Lower Coal-Bearing Member, organic-rich sediments are characterized by TOC content between 0.6 wt.% and 43.1 wt.%and contain Type II and III kerogen. The thick coal seams are the result of high landplant productivity and good preservation conditions. Thin coal seams in the fan-delta plain resulted from the migration and erosion of the fan-delta channel. In the shallow lake and fan-delta front, limited productivity coupled with dilution by inorganic matter resulted in lower organic-matter content in theMudstoneMember. Oil shale and mudstone rich in organic matter were deposited in a deep lake with an average TOC content of 3.4 wt.%. The origin of the organic matter was mainly algae and aquatic plants mixed with land plants. The improved preservation conditions and high bioproductivity in this environment resulted in three oil-shale layers in the lower part of this sedimentary succession. Three coal-bearing and oil-shale-bearing fault basins lie adjacent to the Dunhua-Mishan Fault zone, among which the Fushun Basin is located in the southwest, the Meihe Basin is in themiddle, and theHuadian Basin is in the southeast. From the Fushun Basin to the Huadian Basin, the oil shale and coal seams have distinct sedimentation characters laterally. Vertically in each basin, the organic-matter sources all have the same variation trend from land plants to land plants mixed with algae to algae then to algae mixed with land plants, and the TOC content also shows a similar trend from high to medium-high then to low in each basin.

Integrated Triassic stratigraphy of the greater Culzean area, UK Central North Sea

The Culzean Field, located in UK CNS Block 22/25a between the Lomond Deep and the Heron Cluster, was discovered in 2008 by the 22/25a-9Z well and subsequently appraised by wells 22/25a-10, 22/25a-10Z, 22/25a-10Y and 22/25a-11. The primary reservoir is the Triassic Skagerrak Formation, with a secondary reservoir in the Middle Jurassic Pentland Formation. Total recoverable reserves are estimated to be 250 – 300 MMBOE. One principal challenge concerning the Skagerrak reservoir in Culzean is the stratigraphy, and especially how it relates to adjacent wells in Quadrant 22. Historically, biostratigraphic age determinations of the Skagerrak Formation in Quadrant 22 have been unsuccessful, unlike in Quadrant 30 where biostratigraphic recovery is good and ages are well established. However, good recovery in the Culzean discovery well 22/25a-9Z enabled identification of the Jonathan Mudstone Member, the Joanne Sandstone Member, the Julius Mudstone Member, the Judy Sandstone Member and Marnock Mudstone. The Joanne Sandstone Member and the Jonathan Mudstone Member are therefore more widely distributed in Quadrant 22 than has been hitherto recognized. The Skagerrak Formation in Culzean also has a well-defined heavy mineral stratigraphy, enabling correlation into the adjacent Marnock, Skua, Egret and Heron fields. This correlation shows that the reservoir succession in Egret and Heron is largely equivalent to the Joanne Sandstone Member, whereas Marnock and Skua contain the Judy Sandstone Member overlain by truncated Joanne Sandstone Member and Julius Mudstone Member intervals.

SHRIMP U–Pb zircon geochronology establishes that banded iron formations are not chronostratigraphic markers across Archean greenstone belts of the Pilbara Craton

Banded iron formation (BIF) of the c. 3020Ma Cleaverville Formation is correlated across the Pilbara Craton and is considered to be the first stratigraphic unit common to the West Pilbara Superterrane and the East Pilbara Terrane. New field mapping and geochronology from the Shay Gap and Goldsworthy belts on the northeastern margin of the East Pilbara Terrane test that correlation. Along that margin, the Farrel Quartzite at the base of the Gorge Creek Group nonconformably overlies granite and unconformably overlies metavolcanic rocks. The Farrel Quartzite is conformably overlain by what is interpreted to be Cleaverville Formation, which is divided into a lower BIF member, a middle mudstone member and an upper BIF member. We have dated a tuff from the middle mudstone member by the Sensitive High-Resolution Ion Microprobe (SHRIMP) U-Pb zircon method at 3104±16Ma. The underlying Farrel Quartzite has a maximum depositional age of 3295±5Ma and no zircon population at c. 3105Ma. Therefore, the SHRIMP date for the tuff does not represent reworking of underlying sedimentary rocks and is interpreted to be a depositional age. This new date refutes correlation of the Cleaverville Formation across the craton, and it implies that BIFs should not be used as chronostratigraphic markers across Archean greenstone terrains. We suggest reinstating the name Nimingarra Iron Formation for BIFs that crop out along the northeastern margin of the Pilbara Craton, to distinguish them from the Cleaverville Formation. If the Cleaverville Formation, as defined in the West Pilbara Superterrane, is not the oldest stratigraphic unit common to the craton, then our understanding of the Mesoarchean evolution of the Pilbara Craton needs to be revised.

Geochemistry of host rocks in the Howards Pass district, Yukon-Northwest Territories, Canada: implications for sedimentary environments of Zn-Pb and phosphate mineralization

Detailed lithogeochemical data are reported here on early Paleozoic sedimentary rocks that host the large Howards Pass stratiform Zn-Pb deposits in Yukon-Northwest Territories. Redox-sensitive trace elements (Mo, Re, V, U) and Ce anomalies in members of the Duo Lake Formation record significant environmental changes. During the deposition of lower footwall units (Pyritic siliceous and Calcareous mudstone members), bottom waters were anoxic and sulphidic, respectively; these members formed in a marginal basin that may have become increasingly restricted with time. Relative to lower members, a major environmental change is proposed for deposition of the overlying Lower cherty mudstone member, which contains phosphorite beds up to ∼0.8 m thick in the upper part, near the base of the Zn-Pb deposits. The presence of these beds, together with models for modern phosphorite formation, suggests P input from an upwelling system and phosphorite deposition in an upper slope or outer shelf setting. The overlying Active mudstone member contains stratabound to stratiform Zn-Pb deposits within black mudstone and gray calcareous mudstone. Data for unmineralized black mudstone in this member indicate deposition under diverse redox conditions from suboxic to sulphidic. Especially distinctive in this member are uniformly low ratios of light to heavy rare earth elements that are unique within the Duo Lake Formation, attributed here to the dissolution of sedimentary apatite by downward-percolating acidic metalliferous brines. Strata that overlie the Active member (Upper siliceous mudstone member) consist mainly of black mudstone with thin (0.5–1.5 cm) laminae of fine-grained apatite, recording continued deposition on an upper slope or outer shelf under predominantly suboxic bottom waters. Results of this study suggest that exploration for similar stratiform sediment-hosted Zn-Pb deposits should include the outer parts of ancient continental margins, especially at and near stratigraphic transitions from marginal basin facies to overlying slope or shelf facies.

Geochemistry of organic matter of the Neoproterozoic strata within the Berezovka depression (Seralakh Formation, Siberian Platform)

The results of study of geochemistry of the Neoproterozoic and Lower Cambrian deposits within the Berezovka depression are presented. A mudstone member of the Seralakh Formation is assumed to be an oil source bed. We compiled a series of geochemical maps for this formation and estimated the hydrocarbon generation potential and the scales of oil and gas formation in its mudstone member, using the volumetric-genetic method. The migrated liquid hydrocarbons amount to about 8 billion tons.

Palaeogene sequences with sedimentary characteristics controlling the lacustrine oil shale of the Meihe Basin

As a case study, the Meihe Basin, a typical Cenozoic faulted basin, was divided into and identified as five three-order sequences by utilizing core, well logging, and seismic data, as well as palaeontological and geochemical data. Field measurements of sections, core observations, and a comprehensive analysis revealed that the basin is mainly composed of deposits of alluvial fan, fan delta, lacustrine facies, and gravity flows, and oil shale is mainly developed in semi-deep and deep lacustrine environments. The comprehensive study of the sediment-sequence stratigraphy indicates that Sequence I was formed in the initial rifting stage of the basin, dominated by coarse clastic sediments of alluvial fan and fan delta. Sequence II was formed in the rifting expansion stage of the basin, with more developed sediments of fan delta and lacustrine. Sequence III was formed in the largest expansion stage of the basin, dominated by mudstone of deep lacustrine facies and gravity flow deposits. Sequence IV was formed in the shrinking stage of the basin, dominated by sediments of delta and lacustrine fan. Oil shale are mainly developed in the transgressive system tract (TST) and highstand system tract (HST) of Sequence III (Mudstone Member of lacustrine facies). The lake flooding effect of TST can reduce the decomposition amount of organic matter, increase in organic matter production, and reduce the amount of dilution, thus forming oil shale with a thin consistency but high quality. In the period of the HST, the larger accommodation space and excellent organic matter preservation conditions are conducive to developing stable oil shale with a greater consistency. During the high water level period, however, due to the oxygen brought in by turbidites, the decomposition of organic matter is often increased, resulting in the formation of low-quality oil shale.