Stratigraphic Trends Research Articles

Sequence stratigraphy of deep-water systems

Abstract Stratigraphic cyclicity in the deep-water setting reflects the interplay of accommodation and sedimentation on the shelf, which controls shoreline trajectories, sediment supply to the shelf edge, and the timing of all elements of the sequence stratigraphic framework. Stratigraphic trends defined by changes in the types, volume, and composition of gravity flows during the shoreline transit cycles on the shelf provide the diagnostic criteria for the identification of deep-water systems tracts and bounding surfaces. Non-diagnostic variability in the sedimentological makeup of systems tracts reflects the unique tectonic and depositional settings of each sedimentary basin, and needs to be rationalized on a case-by-case basis. Contour currents may further modify the sedimentological makeup of deep-water sequences, but do not provide diagnostic elements for the definition of systems tracts and bounding surfaces. The application of sequence stratigraphy to the deep-water setting relies on the construction of composite profiles that illustrate the relative chronology of the different types of gravity-driven processes at regional scales. The cyclicity relevant to the definition of sequences is described by the composite rather than local profiles. The place of accumulation of depositional elements depends on the location of sediment entry points along the shelf edge, the types of gravity-driven processes, and the seafloor morphology. The allocyclic and/or autocyclic lateral shifts of deep-water depositional elements further enhance the offset between local trends and the regional composite profile in terms of timing and frequency of cycles, timing of coarsening- and fining-upward trends, and timing of the coarsest sediment. The sedimentological cycles defined by local trends must not be confused with the stratigraphic cycles defined by regional composite profiles.

Seismic stratigraphic analysis for hydrocarbon exploration in the Beta Field, Coastal Swamp Depobelt, Niger Delta

Hydrocarbon exploration analysis from a seismic stratigraphic approach was carried out within the Beta Field, Coastal Swamp Depobelt, Niger Delta, to identify system tracts and sequence boundaries that could be associated with potential hydrocarbon accumulation. 3D seismic volume and data from four wells were analyzed. Depositional sequences, system tracts, sequence boundaries, and candidate maximum flooding surfaces were picked from logs, while reflection patterns and terminations were interpreted from seismic sections. Log shapes from gamma rays in combination with seismic facies analysis were interpreted in order to delineate lithology, depositional environments and depositional sequences. Seismic attributes were extracted and were draped on gridded surfaces from the interpreted seismic horizons. These were integrated with structure maps to obtain structural and stratigraphic trends, and the possible presence of reservoir sand. Five depositional sequences and nine seismic facies were identified within the field. The depositional sequences were designated Sequences S1 to S5 based on the depth of the occurrences and stacking patterns. This study reveals a progression from fluvial depositional settings to the shelf. The main reservoirs identified are the sand units of the highstand and lowstand within three depositional sequences (S1, S4 and S5) although interbedded sands within the TST of S4 and S5 are also suspected of being potential reservoirs. The channel sand deposits within the study area are suspected to be hydrocarbon bearing as they occurred within the complex fault trapping system popular in the Niger Delta. The application of seismic stratigraphy, as shown in this study, serves to encourage exploration in the Niger Delta where it could be effectively employed for reducing risk in hydrocarbon exploration.

Palynological record of the Cretaceous ZKD14 core (Songliao Basin, Northeast China) and its paleoclimatic significance

AbstractTo better understand the vegetation and possible paleoclimate change, we recovered a palynological record obtained from the ZKD14 core covering from the Sifangtai and Mingshui Formations in the northern Songliao Basin, northeastern China. Based on the abundance and variations in determined palynomorphs, two palynological assemblages can be distinguished, including Schizaeoisporites–bisaccate–Tricolporopollenites and Taxodiaceaepollenites–bisaccate–Tricolporopollenites. These palynological assemblages are assigned to the late Campanian–Maastrichtian stage based on the geological range of some important elements and the correlation with relevant assemblages of other areas. In this paper, we applied a sporopollen–climate transforming methodology, which uses the percentage of xerophytic and thermophilic taxa to indicate stratigraphic trends of humidity and temperature, respectively. Then two vegetation and paleoclimate phases were reconstructed based on the ecological habits of the parent plant, as well as the abundance variation in xerophytic and thermophilic taxa: (i) The vegetation type was interpreted to reflect a mixed conifer forest with varying pteridophytes landscape, presenting a hot and arid subtropic climate during the Sifangtai to lower part of member 2 of the Mingshui Formations. (ii) The vegetation type was characterized by a conifer‐dominant landscape. Accordingly, the climate turned more humid and colder during the upper part of member 2 of the Mingshui Formation. The global cooling event (Campanian–Maastrichtian Boundary Event: CMBE) which occurred between 72.0–70.0 Ma appears to have an important influence on the climatic condition of Songliao Basin.

Paleo-redox context of the Mid-Devonian Appalachian Basin and its relevance to biocrises

The Devonian Period witnessed the expansion of vascular land plants and an atmospheric oxygenation event associated with enhanced organic mass burial. The deposition of organic-rich shales (e.g. black shales of the Marcellus subgroup) and several biotic crises in the marine realm have been linked to Devonian ocean anoxia. However, it is not clear how redox conditions evolved in different parts of the water column in such a context of dynamic changes in the atmosphere-ocean system. To address this problem, we use the bulk carbonate I/Ca proxy on core samples from Yates County, NY, in order to reconstruct the water column redox history through the Onondaga Limestone into the lower Marcellus shale. On the secular scale, the range of I/Ca values support the notion of a Devonian rise in atmospheric oxygen, relative to time intervals earlier in the Paleozoic. In terms of Eifelian Stage stratigraphic trends, I/Ca ratios are generally stable and high in the Onondaga Formation but show large fluctuations in lower Marcellus strata. Low I/Ca ratios are found near the onset of organic-rich shale deposition indicating relatively reducing subsurface waters. The pattern of redox changes resembles that of contemporaneous sea-level changes. Finally, the reconstructed oxygenation changes are correlated to three biotic transitions in Devonian marine systems.

Spatiotemporal and stratigraphic trends in salt-water disposal practices of the Permian Basin, Texas and New Mexico, United States

Subsurface disposal of salt water coproduced with oil and gas has become a critical issue in the United States because of linkages with induced seismicity, as seen in Oklahoma and northcentral Texas. Here, we assess the spatiotemporal and stratigraphic variations of salt-water disposal (SWD) volumes in the Permian Basin. The results of this analysis provide critical input into integrated assessments needed for handling of produced water and for emerging concerns, such as induced seismicity. Wellbore architecture, permits, and disposal volumes were compiled, interpreted for disposal intervals and geologic targets, and summarized at formation, subregion, a 100-mi2 (260-km2) area, and monthly volumes for the years 1978–2016. Geologic targets were interpreted by intersecting the disposal intervals with gridded stratigraphic horizons and by reviewing well logs where available. A total of 30 billion bbl (∼5 trillion L) were disposed into 73 geologic units within 6 subregions via 8201 active SWD wells for 39 yr. Most disposal occurred in the Midland Basin and Central Basin Platform (CBP) over the first 34 yr but shifted from the CBP to the Delaware Basin over the last 5 yr (2011–2016) with the expansion of unconventional oil and gas production. Approximately half of the salt water is disposed above the major unconventional reservoirs into Guadalupian-aged formations, raising concerns of overpressuring and interference with production. Operators are exploring deeper SWD targets; however, proximity to crystalline basement poses concerns for high drilling costs and the potential for induced seismicity by reactivation of deep-seated faults.

Does fluvial channel-belt clustering predict net sand to gross rock volume? Architectural metrics and point-pattern analysis of a digital outcrop model

ABSTRACTSpatial point-pattern analyses (PPAs) are used to quantify clustering, randomness, and uniformity of the distribution of channel belts in fluvial strata. Point patterns may reflect end-member fluvial architecture, e.g., uniform compensational stacking and avulsion-generated clustering, which may change laterally, especially at greater scales. To investigate spatial and temporal changes in fluvial systems, we performed PPA and architectural analyses on extensive outcrops of the Cretaceous John Henry Member of the Straight Cliffs Formation in southern Utah, USA. Digital outcrop models (DOMs) produced using unmanned aircraft system-based stereophotogrammetry form the basis of detailed interpretations of a 250-m-thick fluvial succession over a total outcrop length of 4.5 km. The outcrops are oriented roughly perpendicular to fluvial transport direction. This transverse cross-sectional exposure of the fluvial system allows a study of the system's variation along depositional strike. We developed a workflow that examines spatial point patterns using the quadrat method, and architectural metrics such as net sand to gross rock volume (NTG), amalgamation index, and channel-belt width and thickness within moving windows. Quadrat cell sizes that are ∼ 50% of the average channel-belt width-to-thickness ratio (16:1 aspect ratio) provide an optimized scale to investigate laterally elongate distributions of fluvial-channel-belt centroids. Large-scale quadrat point patterns were recognized using an array of four quadrat cells, each with 237× greater area than the median channel belt. Large-scale point patterns and NTG correlate negatively, which is a result of using centroid-based PPA on a dataset with disparately sized channel belts. Small-scale quadrat point patterns were recognized using an array of 16 quadrat cells, each with 21× greater area than the median channel belt. Small-scale point patterns and NTG correlate positively, and match previously observed stratigraphic trends in the fluvial John Henry Member, suggesting that these are regional trends. There are deviations from these trends in architectural statistics over small distances (hundreds of meters) which are interpreted to reflect autogenic avulsion processes. Small-scale autogenic processes result in architecture that is difficult to correlate between 1D datasets, for example when characterizing a reservoir using well logs. We show that 1D NTG provides the most accurate prediction for surrounding 2D architecture.

Stratigraphically controlled silicification in Danian chalk and its implications for reservoir properties, southern Danish Central Graben

Abstract The effect of silica on reservoir quality in chalk is subject to continuing debate. In particular, the stratigraphic variations in silica content within the chalk matrix have not been addressed sufficiently. In this study, a new approach is applied to constrain silica variability in reservoir chalk utilizing hand-held X-ray fluorescence (HH-XRF) on samples from four Maastrichtian–Danian cores from hydrocarbon-producing fields in the southern Danish Central Graben. The results indicate a significant lateral and stratigraphic variability in silica concentrations across the study area; data display a common stratigraphic signature in all wells, however, with maximum silica concentrations and minimum porosities occurring in the lower part of the Danian Ekofisk Formation. Correlation between HH-XRF quantified silica concentrations and biostratigraphic data indicates the synchronous onset of an at least semi-regional silica event during the mid-Danian, nannofossil zones NNTp2F–G to NNTp4A–D, whereas the upper boundary of the peak silica unit is poorly constrained. Generally, stratigraphic trends in silica concentrations appear to be negatively correlated with porosity trends. However, comparison between the HH-XRF quantified silica content and reservoir data (porosity, permeability, estimated pore radius and calculated specific surface area) shows poor correlation indicating that silica content within the Ekofisk reservoir is not the only factor controlling the reservoir quality. Unknown complex diagenetic interactions between calcite, silica and clay probably influence the reservoir properties by promoting calcite cementation in stratigraphically controlled zones, making the Ekofisk reservoir much less predictable than the underlying Maastrichtian Tor Formation.

Architecture and preservation in the fluvial to marine transition zone of a mixed‐process humid‐tropical delta: Middle Miocene Lambir Formation, Baram Delta Province, north‐west Borneo

AbstractThe interaction of river and marine processes in the fluvial to marine transition zone fundamentally impacts delta plain morphology and sedimentary dynamics. This study aims to improve existing models of the facies distribution, stratigraphic architecture and preservation in the fluvial to marine transition zone of mixed‐process deltas, using a comprehensive sedimentological and stratigraphic dataset from the Middle Miocene Lambir Formation, Baram Delta Province, north‐west Borneo. Eleven facies associations are identified and interpreted to preserve the interaction of fluvial and marine processes in a mixed‐energy delta, where fluvial, wave and tidal processes display spatially and temporally variable interactions. Stratigraphic successions in axial areas associated with active distributary channels are sandstone‐rich, comprising fluvial‐dominated and wave‐dominated units. Successions in lateral areas, which lack active distributary channels, are mudstone‐rich, comprising fluvial‐dominated, tide‐dominated and wave‐dominated units, including mangrove swamps. Widespread mudstone preservation in axial and lateral areas suggests well‐developed turbidity maximum zones, a consequence of high suspended‐sediment concentrations resulting from tropical weathering of a mudstone‐rich hinterland. Within the fluvial to marine transition zone of distributary channels, interpreted proximal–distal sedimentological and stratigraphic trends suggest: (i) a proximal fluvial‐dominated, tide‐influenced subzone; (ii) a distal fluvial‐dominated to wave‐dominated subzone; and (iii) a conspicuously absent tide‐dominated subzone. Lateral areas preserve a more diverse spectrum of facies and stratigraphic elements reflecting combined storm, tidal and subordinate river processes. During coupled storm and river floods, fluvial processes dominated the fluvial to marine transition zone along major and minor distributary channels and channel mouths, causing significant overprinting of preceding interflood deposits. Despite interpreted fluvial–tidal channel units and mangrove influence implying tidal processes, there is a paucity of unequivocal tidal indicators (for example, cyclical heterolithic layering). This suggests that process preservation in the fluvial to marine transition zone preserved in the Lambir Formation primarily records episodic (flashy) river discharge, river flood and storm overprinting of tidal processes, and possible backwater dynamics.

Testing carbonate-associated sulfate (CAS) extraction methods for sulfur isotope stratigraphy: A case study of a Lower–Middle Ordovician carbonate succession, Shingle Pass, Nevada, USA

Sulfur isotopes measured from sedimentary rocks are used to reconstruct the global sulfur cycle and chemostratigraphic correlation. Relative and absolute changes in the sulfur isotopic composition of the ocean (δ34Sseawater) are essential for estimating changes in redox conditions in ancient environments, particularly for geochemical models that utilize isotope mass balance. Thus, accurate estimates of δ34Sseawater must be measured using a laboratory protocol optimized for the extraction of primary sulfate (preserved as carbonate-associated sulfate; CAS) by removing any contaminant secondary sulfate (e.g., from diagenetic pyrite oxidation).Here we use two CAS-extraction protocols on Lower–Middle Ordovician carbonate rocks to test the degree to which similar absolute values and stratigraphic trends in δ34SCAS are produced by different CAS extraction methods. One method processes carbonate powders once with a single rinse of a 10% NaCl solution to remove sulfate minerals or weakly bonded sulfate ions, followed by a rinse in 5–6% bleach to remove organically bound sulfur before dissolution in hydrochloric acid (HCl). The second method treats carbonate powders with three rinses of NaCl, without a bleach rinse, to more aggressively remove secondary sulfate before dissolution in HCl. Isotopic results show that samples treated in three NaCl rinses produce δ34SCAS values that are on average 7‰ more positive than samples treated with a single NaCl rinse, but a similar stratigraphic trend is preserved in samples processed with either method. We interpret the difference in δ34SCAS between methods to reflect the incomplete removal of secondary sulfate derived from the oxidation of 32S-enriched pyrite when employing only a single NaCl rinse. Surprisingly, rocks with low CAS concentrations (≤10 mg/kg) using both methods show no significant difference in δ34SCAS values, as well as no significant difference in pyrite concentrations and pyrite δ34S (δ34SPY) between methods. Samples with low CAS concentrations are likely altered and secondary sulfate sourced from pyrite oxidation has likely become incorporated into recrystallized carbonate minerals or incorporated into pore-filling calcite cement, all of which is captured as CAS. Results suggest that both CAS-extraction protocols can produce broadly similar δ34SCAS trends, but a single NaCl rinse may not completely remove contaminant sulfate sorbed onto carbonate grains and thus will not yield meaningful data for reconstructing δ34Sseawater. This has clear implications for modeling studies aimed at reconstructing paleo-redox conditions using δ34S data assumed to record global δ34Sseawater trends if these δ34S data are generated using a non-optimized CAS protocol.

Recognition and significance of Upper Devonian fluvial, estuarine, and mixed siliciclastic-carbonate nearshore marine facies in the San Juan Mountains (southwestern Colorado, USA): Multiple incised valleys backfilled by lowstand and transgressive systems tracts

Abstract The Upper Devonian Ignacio Formation (as stratigraphically revised) comprises a transgressive, tide-dominated estuarine depositional system in the San Juan Mountains (Colorado, USA). The unit backfills at least three bedrock paleovalleys (10–30 km wide and ≥42 m deep) with a consistent stratigraphy of tidally influenced fluvial, bayhead-delta, central estuarine-basin, mixed tidal-flat, and estuarine-mouth tidal sandbar deposits. Paleovalleys were oriented northwest while longshore transport was to the north. The deposits represent Upper Devonian lowstand and transgressive systems tracts. The overlying Upper Devonian Elbert Formation (upper member) consists of geographically extensive tidal-flat deposits and is interpreted as mixed siliciclastic-carbonate bay-fill facies that represents an early highstand systems tract. Stratigraphic revision of the Ignacio Formation includes reassigning the basal conglomerate to the East Lime Creek Conglomerate, recognizing an unconformity separating these two units, and incorporating strata previously mapped as the McCracken Sandstone Member (Elbert Formation) into the Ignacio Formation. The Ignacio Formation was previously interpreted as Cambrian, but evidence that it is Devonian includes reexamined fossil data and detrital zircon U-Pb geochronology. The Ignacio Formation has a stratigraphic trend of detrital zircon ages shifting from a single ca. 1.7 Ga age peak to bimodal ca. 1.4 Ga and ca. 1.7 Ga age peaks, which represents local source-area unroofing history. Specifically, the upper plate of a Proterozoic thrust system (ca. 1.7 Ga Twilight Gneiss) was eroded prior to exposure of the lower plate (ca. 1.4 Ga Uncompahgre Formation). These results are a significant alternative interpretation of the geologic history of the southern Rocky Mountains.

Multiple negative molybdenum isotope excursions in the Doushantuo Formation (South China) fingerprint complex redox-related processes in the Ediacaran Nanhua Basin

Abstract The Ediacaran Doushantuo Formation offers one of the most complete and extensively studied records of end-Neoproterozoic biotic and environmental change. Here, we report multiple coeval negative molybdenum (Mo) isotope excursions (to as low as δ98MoNIST+0.25 = −2.24 ± 0.10‰; 2SD) in shales from four separate sites in South China (Rongxi, Taoying, Wuhe, and Yuanjia) that preserve the Doushantuo Formation. The negative δ98Mo excursions appear coincident with previously discovered and seemingly peculiar redox-sensitive element (RSE) patterns in the same sedimentary rocks. We propose that these geochemical trends can be explained by some combination of (a) enhanced local marine oxygenation in the sedimentary basin where the Doushantuo Formation was originally deposited (the Nanhua Basin) and (b) changes in the degree of connectivity between this paleo basin and the open ocean. Enhanced local marine oxygenation, by exposing more sediments in the Nanhua basin to H2S-poor conditions, could have hindered quantitative tetrathiomolybdate formation within these sediments. Local marine oxygenation could have also stimulated the operation of a Mn oxide shuttle. Today, both of these processes are shown to promote the retention of lighter-mass Mo isotopes in sediments and also govern RSE enrichment patterns. Alternatively, or in addition, the Nanhua Basin may not have maintained an uninterrupted connection with the open ocean during the entirety of the Ediacaran Period. The negative δ98Mo excursions occur coincident with sea level highstands that could have also exposed more sediments in the basin to H2S poor conditions and/or catalyzed the operation of a local Mn oxide shuttle. When trying to infer temporal changes in ancient global ocean redox, it is important to consider the influence of sea level changes and associated variations in local depositional conditions on stratigraphic trends in RSE enrichments and isotope compositions.

Sedimentology and fossil vertebrates of the Santa Cruz Formation (early Miocene) in Lago Posadas, southwestern Patagonia, Argentina

Lago Posadas is located at the foot of the Southern Patagonian Andes, in southwestern Argentina, where the early Miocene Santa Cruz Formation (SCF) shows thick and laterally continuous exposures. This region has been scarcely explored for fossil vertebrates since the first efforts by J.B. Hatcher in 1898-99. In this contribution, we performed sedimentologic and paleontological studies in order to reconstruct depositional environments and the associated fossil vertebrate fauna. Sedimentologic data suggest that the sedimentary record begins with restricted marine-estuarine deposits grading upward to fluvial floodplains and fluvial channels. Extensive floodplains, occasionally interrupted by low-sinuosity, sand-dominated channels, show dominant reddish coloration, moderate to low paleosol development, abundant crevasse splay sandstones and lack of vegetal remains, suggesting deposition in a low gradient, oxygenated setting under elevated sedimentation rates. Vertical stratigraphic trends are subtle, suggesting little paleoenvironmental changes during deposition of the whole SCF in this region. Paleocurrent directions, sandstone composition and paleogeographic reconstructions all indicate that deposition of the SCF was strongly associated to the contemporaneous uplift of the Andes. Fossil vertebrates analyzed are the result of our collecting effort and revision of museum collections. The faunal assemblage includes 31 taxa: 28 mammals and three birds. Mammals belong to the main groups recorded in other areas of the SCF (metatherians, xenarthrans, notoungulates, litopterns, astrapotheres and rodents). The assemblage allows a Santacrucian Age sensu lato assignment for the fauna at Lago Posadas. Taxonomic revisions of several taxa are necessary to further adjust the biostratigraphic significance of this association. The combined record of arboreal, browser and frugivores, on one side, and grazer mammals and rheas, on the other, suggest the presence of both trees and open environments. Frugivores, among primary consumers, and the secondary consumers guild are under-represented due to sample and fossil remain size biases. The sedimentologic and paleontological record of the SCF in Lago Posadas suggests that the uplift of the Southern Patagonian Andes acted as a primary control on basin subsidence and sediment supply, providing a special signature for sub-andean localities. However, previously registered climatic changes are poorly recorded in this study.

Iron isotopic variations of the Cryogenian banded iron formations: A new model

The Fe isotope composition of banded iron formation (BIF) is regarded as a powerful tracer in paleoceanography, and the Cryogenian banded iron formation associated with “Snowball Earth Events” provides a precious record of environmental change in the Neoproterozoic. However, Fe isotope studies on Cryogenian BIFs are rare and previous interpretations of Cryogenian Fe isotope data are problematic. Here we present a systematic investigation of the Fe isotope composition of the Cryogenian Xinyu BIF sections from four localities in the Yangtze region, South China. These BIF sections comprise banded magnetite quartzite, banded chlorite-magnetite quartzite, and magnetite-bearing chlorite phyllite, in stratigraphically ascending order. The δ56FeIRMM-014 values of the Xinyu BIF vary significantly and show an overall increase upsection, from ca. 0‰ to ca. 1.5‰. This stratigraphic trend in Fe isotope compositions is similar to those reported previously for Cryogenian BIFs in North America and Australia, and thus seems to be a common phenomenon. We interpret Fe isotope variation in Cryogenian BIF to be essentially controlled by varying degrees of Fe precipitation in seawater, rather than resulted from “a water column Fe isotope gradient” as proposed previously. The variation in the degree of Fe precipitation can be controlled by changes of Eh and/or pH conditions in seawater resulting from transgression or ocean acidification.

Radiogenic isotope chemostratigraphy reveals marine and nonmarine depositional environments in the late Mesoproterozoic Borden Basin, Arctic Canada

Abstract The ca. 1050 Ma Bylot Supergroup in Arctic Canada is one of the best-preserved archives of late Mesoproterozoic geochemistry and biology and offers evidence that this period of Earth history may have been more biogeochemically dynamic than previously appreciated. The Bylot Supergroup was deposited in the Borden Basin and is the most thoroughly studied stratigraphic succession from a series of broadly contemporaneous late Mesoproterozoic intracratonic basins known as the Bylot basins. This ∼6-km-thick mixed carbonate-siliciclastic succession has undergone minimal postdepositional deformation and is now exposed on Baffin and Bylot Islands, Nunavut, Canada. Deep-water and tidally influenced carbonate facies, traditionally interpreted as marine, have yielded important insights into the evolution of Proterozoic seawater chemistry; however, more recent studies indicate that the Borden Basin was restricted marine or lacustrine for portions of its depositional history. Here, we present new multiproxy radiogenic isotope chemostratigraphic data spanning the Bylot Supergroup. A comparison of stratigraphic trends in radiogenic isotope data from hydrogenous (black shale 187Os/188Os and limestone 87Sr/86Sr) and detrital (whole-rock mudstone 143Nd/144Nd) sedimentary phases elucidates the complex hydrologic history of the Borden Basin and reconciles these disparate interpretations. Episodic coupling and decoupling between the composition of basin waters (from Os and Sr isotopes) and the local weathering input to the basin (from Nd isotopes) indicate that depositional environments within the Borden Basin fluctuated between marine and nonmarine (i.e., lacustrine). Variations in basin hydrology controlled secular sedimentation patterns through changes in basin water chemistry. These interpretations help to characterize the environment in which the early red algal fossil Bangiomorpha pubescens evolved. Episodically restricted epeiric seaways, such as within the Borden Basin, were relatively widespread within Rodinia and may have exerted unique selective pressures on eukaryotic evolution in the Mesoproterozoic Era. Hydrogenous and detrital radiogenic isotope chemostratigraphy, as implemented in this study, may provide a useful paleoenvironmental framework for future paleontological studies aimed at testing the role of freshwater environments in eukaryotic evolution. In addition, 87Sr/86Sr compositions from 81 new middle Bylot Supergroup marine limestone samples, calibrated by recent Re-Os geochronology, contribute to the terminal Mesoproterozoic marine 87Sr/86Sr curve. These data display a rise from ∼0.705 to 0.706 that reflects weathering of the active Grenville orogenic belt and demonstrates a global increase in chemical weathering during the amalgamation of Rodinia.

The distinctive character of lumpy limestones (Early Jurassic, Lusitanian Basin, W Portugal)

Ephemeral carbonate facies presenting restricted geographical and temporal occurrence hold the key to complete the knowledge of past environmental conditions. Far from representing environments widespread throughout the geological history, ephemeral lithofacies embody equally relevant but more unusual scenarios. This is the case of the lumpy limestones, a puzzling facies encased on a worldwide‐relevant stratigraphic section of the Portuguese Jurassic archive but still lacking a deeper understanding of the processes leading to their development. An extensive revision and update of the singularities of the lumpy limestones is presented, based on a systematic description of the lithic elements occurring in three sections. These represent different positions along the Pliensbachian (Early Jurassic) paleoplatform of the Lusitanian Basin (Western Portugal). The lumps were analysed regarding their abundance, distribution, appearance, and degree of sorting, based on field observation, macro‐ and microfacies inspection. Apart from shared stratigraphic trends, which highlight prevailing paleoenvironmental conditions during the Early Jurassic times, the main differences in spatial distribution across the Lusitanian Basin are also discussed, taking into account the specific paleogeographic setting of each studied locality. The presence of lumps but also absence is considered relevant, as for each case a specific paleoenvironmental context and/or syn‐ to post‐depositional processes can be assigned. A new lump classification scheme is brought forward, accounting for the high variety identified in lump architecture and aspect. A depositional model offering alternative paleoenvironmental scenarios is presented for the development of the variety of lump types identified, combining physical (mechanical) and biological processes involved in the occurrence and distribution of this facies. Depending on the presence/absence of lumps, lump type, abundance, and distribution, these scenarios include considerable fluctuations in oxygen availability and bottom current intensity, which affect deposition/accumulation, bioturbation, and related syn‐sedimentary conditions. Physical and mechanically dominated processes driven by sedimentary instability and/or heterogeneity are also introduced. A dynamic depositional framework is presented, clarifying the depositional to late‐diagenetic evolution accounting for the singularity of the lumpy limestone facies in the context of the Lower Jurassic in the western Iberia margin.

Middle–Upper Ordovician (Darriwilian–Sandbian) paired carbon and sulfur isotope stratigraphy from the Appalachian Basin, USA: Implications for dynamic redox conditions spanning the peak of the Great Ordovician Biodiversification Event

The Evans Ferry section from the Appalachian Basin of eastern North America has been analyzed for chemostratigraphic trends to elucidate possible causal mechanisms facilitating the Great Ordovician Biodiversification Event (GOBE). Paired stable isotope (δ13C and δ34S) analyses were used in this carbonate-dominated locality from the Appalachian Basin to reconstruct the marine redox states during this key period of rapid biodiversification. This succession is one of the most expanded Sandbian Stage (Upper Ordovician) deposits known from North America, and it allows new high-resolution reconstructions of long-term, global carbon and sulfur cycle fluctuations. The integrated geochemical and sequence-stratigraphic investigation presented here from the Laurentian epeiric seaway allows for possible identification of the contraction and expansion of reducing water masses during the Middle–Late Ordovician that may have been linked to global-marine paleoredox dynamics. Utilizing new conodont-based 87Sr/86Sr isotope stratigraphy along with previous conodont biostratigraphy, we can confidently correlate our stable isotope profiles to other carbonate successions globally. Carbonate facies indicate that this area experienced partial restriction from open-marine conditions during certain intervals in the Sandbian. These semi-restricted environments record carbon (δ13Ccarb) isotope trends that are secular in nature, as they can be correlated to other successions across Laurentia and Baltica. We identify several intervals when δ13Ccarb and δ34SCAS (carbonate-associated sulfate) trends are decoupled. These inverse stratigraphic trends are, at times, followed by parallel positive shifts in δ13Ccarb and δ34SCAS. Causal mechanisms for the observed decoupled δ13C and δ34S trends may include a wide variety of factors such as more closed-system, local biogeochemical processes associated in part with diagenesis, or they could instead reflect variations in the global fluxes of organic matter and pyrite burial linked to changing marine paleoredox conditions. The positive covariation in trends presented here likely represents transient increases in organic carbon and pyrite burial in response to expansion of reducing marine environments.

Using Gamma Emissions to Identify Cycles in the Lower Cambrian Monkton Formation (NW Vermont): Implications for Identifying Sea Level Variation on the Iapetus Margin

The Monkton Formation is described as a Lower Cambrian regressive sandstone unit containing shallowing-up cycles, called parasequences, which record tidal flat progradation. Spatial variation and limited outcrops of continuous stratigraphy have made it difficult to characterize how cycles change in architecture and thickness through the entirety of the Monkton Formation. This study seeks to identify stratigraphic trends in the parasequence architecture and thickness from the successful recognition of facies in the subsurface, information that will clarify how the sea level changes and impacts accommodation space. New geophysical data allow this research project to explore the stratigraphy of the Monkton at higher levels of resolution than previously achieved. Initial statistical study of gamma ray data from a well through the Monkton suggests that meter-scale parasequences are identifiable in geophysical logs. Using continuous gamma log data, this work identified clastic carbonate ratios from a 1034’ deep geothermal well drilled at Champlain College in Burlington, VT. Although cycles were identified in the gamma log, the spatial variation, complexity and variety of parasequence types within the Monkton made it difficult to correlate the sequences with specific environments or changes in sea level. To see if specific architectural elements of parasequences and lithologies could be characterized by gamma values, outcrops of the Monkton were surveyed containing both parasequences and lithologies that were useful paleobathymetric indicators. The survey determined gamma ray patterns and relative values of a common Monkton parasequence representing tidal flat progradation and a carbonate lithology indicating an abrupt sea level rise. Gamma data from a second well on the University of Vermont campus, along with accompanying borehole camera video, confirmed that the outcrop survey results could be useful in subsurface interpretations of the Monkton. Intervals representing the parasequences and carbonate lithologies were identified in the Champlain College Well and were combined with the UVM Fleming Well findings to create a composite stratigraphic section of the Monkton. The parasequences and lithologic trends identified in the composite section indicated that, over the entirety of the Monkton, accommodation space decreased. The decrease in accommodation space is interpreted to represent a change in the rate of sea level rise. The Monkton Formation deposition occurred in a transgressive systems tract with varying rates of sea level rise, which implied that the overlying Winooski Formation would represent highstand systems tract deposition.

Palaeoenvironmental Reconstruction of the Puerto Madryn Formation (Middle to Late Miocene), Northeast of Patagonia: Palynology, Nannofossils and Stratigraphy

A 20 m thick interval of the Miocene Puerto Madryn Formation, composed of nearshore to shelf mudstones and sandstones was the subject of a detailed, multiproxy study, including sedimentological and stratigraphical observations as well as marine-derived amorphous organic matter (AOM), phytoclast, terrestrial and marine palynomorph and calcareous nannofossil assessments. Three palynofacies were defined based on qualitative and quantitative analysis of the palynological organic matter. Based on their sedimentology and microfossils, the section was divided into four intervals: Interval 1 accumulated in relatively stable, warm water conditions, without salinity fluctuations, in an inner shelf; Intervals 2 and 3 show a marked upward fining trend and a progressive change towards a phytoclast-dominated (continental) palynofacies; Interval 4 represents a new reinstauration of normal marine conditions with a regressive trend. The accumulation of the finest-grained deposits on the shelf corresponds to the most continental signature of the succession which seems to contradict the classic grain-size stratigraphic trends for shallow marine sedimentary successions. We interpret this as the result of the progressive increase of fluvial, mud-laden freshwater input into the depositional system, interpreted as produced by switching of deltaic channels processes. The dinoflagellate cyst assemblages reflect overall warm-water conditions. Nevertheless, the presence of cool-water indicators species in some levels suggests the influence of Antarctic cold–waters in the area. Based on diagnostic dinoflagellate cyst and nannofossil taxa, the age of the studied interval was estimated between the Serravalian and the Tortonian.

Regional migration and trapping frameworks in the frontier ceduna sub-basin: new insights from stratigraphic forward modelling and ‘triangle juxtaposition’ diagrams.

Despite a recent renewal in exploration, the Ceduna Sub-basin is underexplored with high uncertainty regarding lithofacies distribution. This results in limited understanding of reservoir and top seal coupling, trends for fluid migration and structural trapping for the Late Cretaceous marine and deltaic interval. In order to address these uncertainties a stratigraphic forward model was developed to recreates the development and preservation of stratigraphic successions. Over an area of interest in the centre of the sub-basin, an early investigation tool consists in a rapid screening of the stratigraphic forward model to forecast vertical and lateral stratigraphic trends which outline structural trapping opportunities.The model shows an overall north-west to south-east net-to-gross distribution trend and stacked reservoir-seal successions predicted in both the marine and deltaic intervals. Structural trapping potential was assessed with Shale Gouge Ratio calculation and triangle diagrams. The marine-dominated Tiger and lower Hammerhead Supersequences are predicted with the higher potential for structural traps with probability of juxtaposition and membrane fault seals. A predicted sand-rich Early Campanian nearshore marine sequence is likely to act as a migration fairways.

Coupled isotopic evidence for elevated pCO2 and nitrogen limitation across the Santonian-Campanian transition

The Santonian–Campanian (S–C) transition notably coincides with the termination of the Cretaceous Normal Superchron, a positive carbon isotope anomaly, and the enhanced preservation of organic matter associated with an oceanic anoxic event at ca. 84 Ma. The driving factors for the OAEs should include global warming, widely believed to be from volcanic sources, the delivery of nutrients to surface oceans from the continents, anoxic sediments and atmosphere, and variations in the configuration of ocean basins. To further investigate this phenomenon and evaluate whether there was a teleconnection between the marine and continental realm, we have studied a lacustrine succession in northeastern China (the Songliao Basin, which is ~3× the size of Lake Superior) that preserves two thick horizons of oil shales and a positive carbon isotope anomaly recorded in ostracode carbonate near the S–C boundary. This freshwater basin is one of the many that formed as a consequence of Mesozoic extensional tectonics in eastern Asia. The stratigraphic trends illustrate broad excursions in the isotopic compositions of carbonate, total organic carbon (including black carbon), and total nitrogen. The carbon isotope data suggest that maximal fractionation (εTOC) at the S–C boundary is caused by enhanced burial of organic carbon and inputs from anoxygenic photoautotrophs and chemoautotrophs, or from the buildup of CO2 in a lake environment. On the other hand, the nitrogen isotope data suggest that the lake became limited in nitrogen immediately before the deposition of the younger oil shale, resulting in the addition of new nitrogen to the system, likely as the result of diazotrophy or ammonium assimilation. To some extent, all of the features we described from the Songliao Basin have been recognized in Jurassic and Cretaceous OAEs. Insofar as climatically-sensitive lakes are typically supersaturated with CO2 relative to the atmosphere, we speculate that nutrient-driven primary productivity in massive lake systems may have released enough greenhouse gas (CO2, CH4, and N2O) into the atmosphere and therefore played a role in enhancing global temperatures and driving widespread anoxia in the Mesozoic oceans.