M. McGlue, Geoffrey S. Ellis, M. Brannon, J. Latimer, J. Stone, S. Ivory, Neema E. Mganza, M. Soreghan, C. Scholz
Lake Tanganyika ranks among the most valuable modern analogs for understanding depositional processes of carbonaceous sediments in ancient tropical rifts. Prior research on Lake Tanganyika has emphasized the importance of bottom-water anoxia, depositional processes (hemipelagic settling versus gravity flows), and large-scale (100s of meters) lake level change on the quality of sedimentary organic matter content. Here, facies analysis and numerous organic geochemical tools (elemental, carbon isotope, and programmed pyrolysis) were applied to a radiocarbon-dated core from southern Lake Tanganyika to investigate the accumulation of carbonaceous sediments in a deepwater slope environment influenced by high-frequency climatic fluctuations accompanied by only minor (10s of meters) lake level changes. Considerable variability in lithofacies and geochemistry characterizes the ∼ 1030-year-long core record, chiefly driven by climate-mediated changes to the lake's upwelling system. Laminated diatom oozes and sapropels with mean total organic carbon (TOC) concentrations and hydrogen indices of 6.9 wt.% and 385 mg hydrocarbon/g TOC, respectively, characterize sediments deposited during periods of strong upwelling and variable water levels. Silty sediments deposited via gravity-flow processes were likewise rich in organic matter, likely due to preservation-enhancing bottom-water anoxia. Dilution by reworked tephra was the chief constraint on organic enrichment at the study site. Data from this study reveal that oscillations in atmospheric and limnological processes in the absence of major shoreline movements can result in geochemically diverse deepwater slope sediments, which have implications for improving depositional models of petroliferous continental rift basins.
{"title":"Sedimentary geochemistry of deepwater slope deposits in southern Lake Tanganyika (East Africa): Effects of upwelling and minor lake level oscillations","authors":"M. McGlue, Geoffrey S. Ellis, M. Brannon, J. Latimer, J. Stone, S. Ivory, Neema E. Mganza, M. Soreghan, C. Scholz","doi":"10.2110/jsr.2021.104","DOIUrl":"https://doi.org/10.2110/jsr.2021.104","url":null,"abstract":"\u0000 Lake Tanganyika ranks among the most valuable modern analogs for understanding depositional processes of carbonaceous sediments in ancient tropical rifts. Prior research on Lake Tanganyika has emphasized the importance of bottom-water anoxia, depositional processes (hemipelagic settling versus gravity flows), and large-scale (100s of meters) lake level change on the quality of sedimentary organic matter content. Here, facies analysis and numerous organic geochemical tools (elemental, carbon isotope, and programmed pyrolysis) were applied to a radiocarbon-dated core from southern Lake Tanganyika to investigate the accumulation of carbonaceous sediments in a deepwater slope environment influenced by high-frequency climatic fluctuations accompanied by only minor (10s of meters) lake level changes. Considerable variability in lithofacies and geochemistry characterizes the ∼ 1030-year-long core record, chiefly driven by climate-mediated changes to the lake's upwelling system. Laminated diatom oozes and sapropels with mean total organic carbon (TOC) concentrations and hydrogen indices of 6.9 wt.% and 385 mg hydrocarbon/g TOC, respectively, characterize sediments deposited during periods of strong upwelling and variable water levels. Silty sediments deposited via gravity-flow processes were likewise rich in organic matter, likely due to preservation-enhancing bottom-water anoxia. Dilution by reworked tephra was the chief constraint on organic enrichment at the study site. Data from this study reveal that oscillations in atmospheric and limnological processes in the absence of major shoreline movements can result in geochemically diverse deepwater slope sediments, which have implications for improving depositional models of petroliferous continental rift basins.","PeriodicalId":17044,"journal":{"name":"Journal of Sedimentary Research","volume":" ","pages":""},"PeriodicalIF":2.0,"publicationDate":"2022-08-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43152082","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Detrital-zircon U–Pb geochronology has revolutionized sediment provenance studies over the last two decades, and zircon has been successfully analyzed from nearly all sedimentary lithologies, depositional environments, and sediment grain sizes. However, despite the ubiquity of this method and the far-reaching interpretations supported by detrital-zircon data, few studies have investigated the potential role of zircon grain size on age spectra and provenance interpretation. In this study, we investigate the connections between sample grain size, zircon grain size, U–Pb age spectra, and interpreted provenance using 18 detrital-zircon samples (4999 individual grains) collected from Pennsylvanian–Permian strata in central and southern Arizona, USA. In these samples, there is no clear correlation between sample grain size and zircon grain size and no clear correlation between sample grain size and age spectra. However, when all grains are grouped by zircon minimum long-axis dimension, the abundance of some age groups is correlated to zircon grain size. In Pennsylvanian samples, < 400 Ma grains and 2500–3000 Ma zircons are more abundant in the finer fractions, and 1400–1900 Ma zircons are more abundant in coarser fractions of both Pennsylvanian and Permian samples. In Permian samples, 500–800 Ma zircons are most abundant in the finer fractions, and 2500–3000 Ma grains are concentrated in the coarser fractions. Based on changes in abundance and grain-size distribution of 500–800 Ma grains, we interpret a change in zircon provenance across the Pennsylvanian–Permian boundary that reflects regional climate and paleogeographic changes driven in part by the northward drift of Laurentia across the equator. Specifically, we interpret the concentration of 500–800 Ma zircons in Permian samples in central and southern Arizona to indicate that these grains were: 1) sourced from Gondwana, 2) deposited in, and subsequently eroded (recycled) from, Mississippian–Pennsylvanian strata in the Arkoma, Anadarko, and Fort Worth basins at the margins of Laurentia, and 3) finally transported into the Arizona study area as loess by easterly trade winds. This study serves as a case study in the value and interpretive power of basic grain-size characterization of detrital-geochronology datasets.
{"title":"Mixed eolian–longshore sediment transport in the late Paleozoic Arizona shelf and Pedregosa basin, U.S.A.: A case study in grain-size analysis of detrital-zircon datasets","authors":"R. Leary, M. Smith, P. Umhoefer","doi":"10.2110/jsr.2021.101","DOIUrl":"https://doi.org/10.2110/jsr.2021.101","url":null,"abstract":"\u0000 Detrital-zircon U–Pb geochronology has revolutionized sediment provenance studies over the last two decades, and zircon has been successfully analyzed from nearly all sedimentary lithologies, depositional environments, and sediment grain sizes. However, despite the ubiquity of this method and the far-reaching interpretations supported by detrital-zircon data, few studies have investigated the potential role of zircon grain size on age spectra and provenance interpretation. In this study, we investigate the connections between sample grain size, zircon grain size, U–Pb age spectra, and interpreted provenance using 18 detrital-zircon samples (4999 individual grains) collected from Pennsylvanian–Permian strata in central and southern Arizona, USA. In these samples, there is no clear correlation between sample grain size and zircon grain size and no clear correlation between sample grain size and age spectra. However, when all grains are grouped by zircon minimum long-axis dimension, the abundance of some age groups is correlated to zircon grain size. In Pennsylvanian samples, < 400 Ma grains and 2500–3000 Ma zircons are more abundant in the finer fractions, and 1400–1900 Ma zircons are more abundant in coarser fractions of both Pennsylvanian and Permian samples. In Permian samples, 500–800 Ma zircons are most abundant in the finer fractions, and 2500–3000 Ma grains are concentrated in the coarser fractions. Based on changes in abundance and grain-size distribution of 500–800 Ma grains, we interpret a change in zircon provenance across the Pennsylvanian–Permian boundary that reflects regional climate and paleogeographic changes driven in part by the northward drift of Laurentia across the equator. Specifically, we interpret the concentration of 500–800 Ma zircons in Permian samples in central and southern Arizona to indicate that these grains were: 1) sourced from Gondwana, 2) deposited in, and subsequently eroded (recycled) from, Mississippian–Pennsylvanian strata in the Arkoma, Anadarko, and Fort Worth basins at the margins of Laurentia, and 3) finally transported into the Arizona study area as loess by easterly trade winds. This study serves as a case study in the value and interpretive power of basic grain-size characterization of detrital-geochronology datasets.","PeriodicalId":17044,"journal":{"name":"Journal of Sedimentary Research","volume":" ","pages":""},"PeriodicalIF":2.0,"publicationDate":"2022-08-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47051452","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Lateral migration is a key process shaping sinuous rivers and controlling sediment exchange with floodplains. The rate at which channels migrate is affected by bend curvature, bank erodibility, and sediment supply. The relationship between migration rate and sediment supply is poorly understood in dryland regions, where direct measurements are scarce. Here we propose a simple mass-balance model to estimate the sediment flux of ephemeral streams in North America's Great Basin and establish a comparison with timelapse photogrammetric data of lateral migration. The model takes into consideration variables such as long-term hillslope erosion, transient sediment storage in intra-catchment lowlands, and sediment bypass to depocenters. Our results point to first-order similarities in how sediment supply drives channel migration across diverse hydro-climatic regimes. However, we find that, for a given sediment supply and channel width, and despite their ephemeral discharge, dryland streams with minimal bank vegetation migrate about three times faster than humid-climate, vegetated ones. This difference in migration pace likely results from the compound effect of bank erodibility and bend geometry. Our model sheds new light on the driving mechanisms of channel mobility in dryland streams and may find application in assessing the sediment budgets of ungauged streams, reservoir trapping, and morphodynamic adjustments in stressed watersheds.
{"title":"Linking sediment flux to river migration in arid landscapes through mass balance","authors":"A. Ielpi, M. Lapôtre","doi":"10.2110/jsr.2022.118","DOIUrl":"https://doi.org/10.2110/jsr.2022.118","url":null,"abstract":"\u0000 Lateral migration is a key process shaping sinuous rivers and controlling sediment exchange with floodplains. The rate at which channels migrate is affected by bend curvature, bank erodibility, and sediment supply. The relationship between migration rate and sediment supply is poorly understood in dryland regions, where direct measurements are scarce. Here we propose a simple mass-balance model to estimate the sediment flux of ephemeral streams in North America's Great Basin and establish a comparison with timelapse photogrammetric data of lateral migration. The model takes into consideration variables such as long-term hillslope erosion, transient sediment storage in intra-catchment lowlands, and sediment bypass to depocenters. Our results point to first-order similarities in how sediment supply drives channel migration across diverse hydro-climatic regimes. However, we find that, for a given sediment supply and channel width, and despite their ephemeral discharge, dryland streams with minimal bank vegetation migrate about three times faster than humid-climate, vegetated ones. This difference in migration pace likely results from the compound effect of bank erodibility and bend geometry. Our model sheds new light on the driving mechanisms of channel mobility in dryland streams and may find application in assessing the sediment budgets of ungauged streams, reservoir trapping, and morphodynamic adjustments in stressed watersheds.","PeriodicalId":17044,"journal":{"name":"Journal of Sedimentary Research","volume":" ","pages":""},"PeriodicalIF":2.0,"publicationDate":"2022-08-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46480233","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Hairong Wang, Chengqian Yu, Z. Huo, Hongfang Gao, Wen Jiang
The Dongsha Continental Margin (DCM) projects seaward and is situated in the path of bottom currents coming through the only deep-water exchange passage, the Luzon Strait between the South China Sea (SCS) and the western Pacific Ocean. This provides an opportunity to observe the different interaction between the two wings of the convex margin and the bottom currents, and help understand the corresponding implications for provenance, debris transportation, and sedimentation in such an environment. The convexity of the DCM causes its eastern flank to shrink against upcoming bottom currents and internal solitary waves (ISWs), producing a funneling effect and forming strong erosion grooves or strips, remnant seamounts, and large seafloor coarse debris dunes. The concavity of the western flank induces the expansion of bottom currents that flow around the plateau, resulting in a depositional zone with weak erosion that mainly interacts with bottom currents and gravity flow. The strong erosion on the DCM caused by the bottom current forms the primary provenance of the deep-water environment, while the nepheloid layer that entraps the fine debris of the gravity flow that derives from Taiwan and that is transported by the bottom current is the secondary provenance. The different coupling patterns between the bottom currents and the two flanks determine the different modes of debris transportation and deposition. Debris eroded by the currents is mainly transported by the gravity flow on the eastern flank while sweeping of the outer shelf and upper slope by eddy currents, progradation of the gravity flow, and reworking by the bottom current mainly occur on the western flank. Two types of morphological breaks, namely, continental slope break and bottom-current slope break, have developed on the DCM. They control the evolution of the flow regime of the multi-layer bottom currents and the gravity flow of the DCM as well as the effects of erosion and deposition. These two types of slope breaks are coupled and form an area in front of Dongsha Island with the highest deposition rate in the SCS.
{"title":"Effects of the coupling between slope morphology and bottom currents on flow erosion and sedimentation at the Dongsha Continental Margin, South China Sea","authors":"Hairong Wang, Chengqian Yu, Z. Huo, Hongfang Gao, Wen Jiang","doi":"10.2110/jsr.2021.097","DOIUrl":"https://doi.org/10.2110/jsr.2021.097","url":null,"abstract":"\u0000 The Dongsha Continental Margin (DCM) projects seaward and is situated in the path of bottom currents coming through the only deep-water exchange passage, the Luzon Strait between the South China Sea (SCS) and the western Pacific Ocean. This provides an opportunity to observe the different interaction between the two wings of the convex margin and the bottom currents, and help understand the corresponding implications for provenance, debris transportation, and sedimentation in such an environment. The convexity of the DCM causes its eastern flank to shrink against upcoming bottom currents and internal solitary waves (ISWs), producing a funneling effect and forming strong erosion grooves or strips, remnant seamounts, and large seafloor coarse debris dunes. The concavity of the western flank induces the expansion of bottom currents that flow around the plateau, resulting in a depositional zone with weak erosion that mainly interacts with bottom currents and gravity flow. The strong erosion on the DCM caused by the bottom current forms the primary provenance of the deep-water environment, while the nepheloid layer that entraps the fine debris of the gravity flow that derives from Taiwan and that is transported by the bottom current is the secondary provenance. The different coupling patterns between the bottom currents and the two flanks determine the different modes of debris transportation and deposition. Debris eroded by the currents is mainly transported by the gravity flow on the eastern flank while sweeping of the outer shelf and upper slope by eddy currents, progradation of the gravity flow, and reworking by the bottom current mainly occur on the western flank. Two types of morphological breaks, namely, continental slope break and bottom-current slope break, have developed on the DCM. They control the evolution of the flow regime of the multi-layer bottom currents and the gravity flow of the DCM as well as the effects of erosion and deposition. These two types of slope breaks are coupled and form an area in front of Dongsha Island with the highest deposition rate in the SCS.","PeriodicalId":17044,"journal":{"name":"Journal of Sedimentary Research","volume":" ","pages":""},"PeriodicalIF":2.0,"publicationDate":"2022-08-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49193923","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Nick Ferry, D. Sturmer, D. Ward, W. Taylor, C. Brett
Landslide deposits are common in basin fill of tectonically active areas, including the Basin and Range province of western North America. However, interpreting and reconstructing ancient landsliding events from these deposits is challenging, but can be aided by detailed field analysis. Remnants of the Blue Diamond landslide breccia are exposed capping hills and ridges in the foothills of the eastern Spring Mountains near Blue Diamond, Nevada, USA. Uncertainties surrounding the origin and emplacement of the ancestral Blue Diamond landslide have emerged based on the disparate distribution of landslide outcrops. Therefore, in this study we used detailed sedimentological data and observations to interpret a two-phase emplacement history for the Blue Diamond landslide. Sedimentological observations are consistent with Blue Diamond landslide breccia emplacement as a rock avalanche. The presence of clastic dikes and flame structures and negligible incorporation of bedrock substrate material suggest that runout occurred over a saturated substrate. Flow transformation into a debris avalanche is ruled out because clast-count data show that debris entrainment was not sufficient to act as the sole mechanism behind the excessive mobility experienced by the Blue Diamond landslide. Instead, we propose that the excessive mobility was driven by flow entrainment of large Aztec Sandstone boulders and interaction with a saturated runout path substrate that caused a reduced basal frictional resistance, enabling initial emplacement onto Blue Diamond Hill. We therefore suggest that the Blue Diamond landslide was derived from a source area about 8.5 km northwest of the Blue Diamond townsite and flowed into the Blue Diamond Hill site where it was emplaced onto Moenkopi Formation atop the hill during the Miocene. Due to loading by this new overburden, incompetent gypsum horizons failed in the upper Kaibab Formation stratigraphically below the Moenkopi Formation. These failed gypsum horizons then served as a compound landslide rupture surface, transporting the overlying Moenkopi Formation and landslide breccia. This secondary emplacement likely ceased by late Miocene to Pliocene time.
{"title":"Origin and emplacement of the Blue Diamond landslide breccia, southern Nevada, U.S.A.","authors":"Nick Ferry, D. Sturmer, D. Ward, W. Taylor, C. Brett","doi":"10.2110/jsr.2021.021","DOIUrl":"https://doi.org/10.2110/jsr.2021.021","url":null,"abstract":"\u0000 Landslide deposits are common in basin fill of tectonically active areas, including the Basin and Range province of western North America. However, interpreting and reconstructing ancient landsliding events from these deposits is challenging, but can be aided by detailed field analysis. Remnants of the Blue Diamond landslide breccia are exposed capping hills and ridges in the foothills of the eastern Spring Mountains near Blue Diamond, Nevada, USA. Uncertainties surrounding the origin and emplacement of the ancestral Blue Diamond landslide have emerged based on the disparate distribution of landslide outcrops. Therefore, in this study we used detailed sedimentological data and observations to interpret a two-phase emplacement history for the Blue Diamond landslide.\u0000 Sedimentological observations are consistent with Blue Diamond landslide breccia emplacement as a rock avalanche. The presence of clastic dikes and flame structures and negligible incorporation of bedrock substrate material suggest that runout occurred over a saturated substrate. Flow transformation into a debris avalanche is ruled out because clast-count data show that debris entrainment was not sufficient to act as the sole mechanism behind the excessive mobility experienced by the Blue Diamond landslide. Instead, we propose that the excessive mobility was driven by flow entrainment of large Aztec Sandstone boulders and interaction with a saturated runout path substrate that caused a reduced basal frictional resistance, enabling initial emplacement onto Blue Diamond Hill. We therefore suggest that the Blue Diamond landslide was derived from a source area about 8.5 km northwest of the Blue Diamond townsite and flowed into the Blue Diamond Hill site where it was emplaced onto Moenkopi Formation atop the hill during the Miocene. Due to loading by this new overburden, incompetent gypsum horizons failed in the upper Kaibab Formation stratigraphically below the Moenkopi Formation. These failed gypsum horizons then served as a compound landslide rupture surface, transporting the overlying Moenkopi Formation and landslide breccia. This secondary emplacement likely ceased by late Miocene to Pliocene time.","PeriodicalId":17044,"journal":{"name":"Journal of Sedimentary Research","volume":" ","pages":""},"PeriodicalIF":2.0,"publicationDate":"2022-08-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45790645","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
In models of siliciclastic sequence stratigraphy, the sequence boundary in distal marine environments, where the strata are mudstone dominated, is usually considered a correlative conformity—the seaward extension of a subaerial unconformity. Despite its wide usage in the literature, objective recognition criteria of a correlative conformity remain lacking, largely due to the limited number of case studies directly examining the characteristics of sequence boundaries in offshore mudstone-dominated environments. This study focuses on the mudstone-dominated transitional interval between the Tununk Shale Member and the Ferron Sandstone Member of the Mancos Shale Formation exposed in south-central Utah to extend our understanding of the characteristics of a sequence boundary developed in the distal shelf environment of a ramp setting. An integrated sedimentologic, petrographic, and sequence stratigraphic analysis was conducted to characterize the sequence boundary that separates the Tununk from the Ferron depositional system (hereafter referred to as the T-F sequence boundary) and its lateral along-depositional-strike variability. Although manifest as a mudstone-on-mudstone contact, the T-F sequence boundary in all three measured sections is a subtle unconformity, characterized by erosional truncation below and onlap above, and marks a distinct basinward shift in facies association. The T-F sequence boundary also marks the change from the Tununk offshore mud-belt system to the Ferron Notom delta system, and therefore represents a surface that divides two genetically different depositional systems. Based on two distinct marker beds that bracket the T-F sequence boundary, the T-F sequence boundary can be traced across the study area with confidence. The lateral variability in the characteristics of the T-F sequence boundary along depositional strike indicates that it was produced by an allogenic base-level fall. Offshore shelfal mudstone strata may contain a significantly higher incidence of subtle unconformities analogous to the T-F sequence boundary than currently appreciated. Careful sedimentologic and petrographic analyses, combined with lateral correlations constrained by reliable chronostratigraphic marker beds, are essential for identifying subtle unconformities in shelf mudstone successions. The accurate recognition of subtle unconformities in mudstone strata is critical to apply the sequence stratigraphic approach appropriately to distal shelf environments, as well as to better constrain the timing and cause (allogenic vs. autogenic) of relative changes of sea level recorded in these rocks.
{"title":"Correlative conformity or subtle unconformity? The distal expression of a sequence boundary in the Upper Cretaceous Mancos Shale, Henry Mountains Region, Utah, U.S.A.","authors":"Zhiyang Li, J. Schieber","doi":"10.2110/jsr.2021.103","DOIUrl":"https://doi.org/10.2110/jsr.2021.103","url":null,"abstract":"\u0000 In models of siliciclastic sequence stratigraphy, the sequence boundary in distal marine environments, where the strata are mudstone dominated, is usually considered a correlative conformity—the seaward extension of a subaerial unconformity. Despite its wide usage in the literature, objective recognition criteria of a correlative conformity remain lacking, largely due to the limited number of case studies directly examining the characteristics of sequence boundaries in offshore mudstone-dominated environments. This study focuses on the mudstone-dominated transitional interval between the Tununk Shale Member and the Ferron Sandstone Member of the Mancos Shale Formation exposed in south-central Utah to extend our understanding of the characteristics of a sequence boundary developed in the distal shelf environment of a ramp setting. An integrated sedimentologic, petrographic, and sequence stratigraphic analysis was conducted to characterize the sequence boundary that separates the Tununk from the Ferron depositional system (hereafter referred to as the T-F sequence boundary) and its lateral along-depositional-strike variability.\u0000 Although manifest as a mudstone-on-mudstone contact, the T-F sequence boundary in all three measured sections is a subtle unconformity, characterized by erosional truncation below and onlap above, and marks a distinct basinward shift in facies association. The T-F sequence boundary also marks the change from the Tununk offshore mud-belt system to the Ferron Notom delta system, and therefore represents a surface that divides two genetically different depositional systems. Based on two distinct marker beds that bracket the T-F sequence boundary, the T-F sequence boundary can be traced across the study area with confidence. The lateral variability in the characteristics of the T-F sequence boundary along depositional strike indicates that it was produced by an allogenic base-level fall.\u0000 Offshore shelfal mudstone strata may contain a significantly higher incidence of subtle unconformities analogous to the T-F sequence boundary than currently appreciated. Careful sedimentologic and petrographic analyses, combined with lateral correlations constrained by reliable chronostratigraphic marker beds, are essential for identifying subtle unconformities in shelf mudstone successions. The accurate recognition of subtle unconformities in mudstone strata is critical to apply the sequence stratigraphic approach appropriately to distal shelf environments, as well as to better constrain the timing and cause (allogenic vs. autogenic) of relative changes of sea level recorded in these rocks.","PeriodicalId":17044,"journal":{"name":"Journal of Sedimentary Research","volume":" ","pages":""},"PeriodicalIF":2.0,"publicationDate":"2022-07-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48777293","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Saline currents (SCs) have commonly been used to model muddy turbidity currents (TCs) in a laboratory. However, little is known about the limitations of this proxying, in particular when concerning processes and products related to their sedimentologic and stratigraphic imprints. The present study is aimed at investigating experimental hydraulic and sedimentological conditions and processes involved in the generation and development of bedforms by both SCs and TCs, when similar input conditions are applied (discharge, slope, densimetric Froude number). In all performed runs reported herein, only ripples were observed to form, and were identified and classified using known criteria such as their dimensions, near-bed shear stresses (), shear velocities (), and grain Reynolds values (Re*). Turbidity currents were observed to deposit sediments carried by the flow predominantly in the upstream section of the flume, increasing bed slope and thus increasing , , and near-bed concentrations (cb). This resulted in longer-wavelength bedforms compared to those generated by SCs under similar input conditions in those sections of the flume. On the other hand, along the downstream sections of the flume, bed slopes were observed to remain similar for all experiments, and both types of currents showed similar vertical distribution of velocities, concentrations, and stable stratification. Measured bedform wavelengths and heights were slightly higher when generated by SCs, due to the coarser bed material observed in SCs (which influenced the increase in near-bed turbulent intensities). Moreover, TCs presented a slight decrease in turbulence intensities due to their observed high near-bed suspended-sediment concentration. Spatial and temporal changes in several hydraulic parameters in both SCs and TCs highlight the role of sediment suspension in modifying turbulent processes and vertical stratification of these flows, depending on their concentrations. Both hydraulic and sedimentologic observations of this study support in principle the assumption that SCs can be used experimentally as a surrogate for diluted TCs to reproduce bedforms classified as ripples, as long as both type of currents reach similar hydraulic and sediment-transport conditions, in particular for depth-averaged concentration Cvol < 1% and near-bed concentration cb < 2%.
{"title":"Comparative hydraulic and sedimentologic study of ripple formation using experimental turbidity currents and saline currents","authors":"D. Koller, Rafael Manica, J. Fedele","doi":"10.2110/jsr.2021.076","DOIUrl":"https://doi.org/10.2110/jsr.2021.076","url":null,"abstract":"\u0000 Saline currents (SCs) have commonly been used to model muddy turbidity currents (TCs) in a laboratory. However, little is known about the limitations of this proxying, in particular when concerning processes and products related to their sedimentologic and stratigraphic imprints. The present study is aimed at investigating experimental hydraulic and sedimentological conditions and processes involved in the generation and development of bedforms by both SCs and TCs, when similar input conditions are applied (discharge, slope, densimetric Froude number). In all performed runs reported herein, only ripples were observed to form, and were identified and classified using known criteria such as their dimensions, near-bed shear stresses (), shear velocities (), and grain Reynolds values (Re*). Turbidity currents were observed to deposit sediments carried by the flow predominantly in the upstream section of the flume, increasing bed slope and thus increasing , , and near-bed concentrations (cb). This resulted in longer-wavelength bedforms compared to those generated by SCs under similar input conditions in those sections of the flume. On the other hand, along the downstream sections of the flume, bed slopes were observed to remain similar for all experiments, and both types of currents showed similar vertical distribution of velocities, concentrations, and stable stratification. Measured bedform wavelengths and heights were slightly higher when generated by SCs, due to the coarser bed material observed in SCs (which influenced the increase in near-bed turbulent intensities). Moreover, TCs presented a slight decrease in turbulence intensities due to their observed high near-bed suspended-sediment concentration. Spatial and temporal changes in several hydraulic parameters in both SCs and TCs highlight the role of sediment suspension in modifying turbulent processes and vertical stratification of these flows, depending on their concentrations. Both hydraulic and sedimentologic observations of this study support in principle the assumption that SCs can be used experimentally as a surrogate for diluted TCs to reproduce bedforms classified as ripples, as long as both type of currents reach similar hydraulic and sediment-transport conditions, in particular for depth-averaged concentration Cvol < 1% and near-bed concentration cb < 2%.","PeriodicalId":17044,"journal":{"name":"Journal of Sedimentary Research","volume":" ","pages":""},"PeriodicalIF":2.0,"publicationDate":"2022-07-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48041562","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
A. Cooper, Alan Smith, G. Rishworth, Carla Dodd, M. Forbes, H. Cawthra, C. Anderson
Contemporary microbialite formation has been documented on rock coasts in a variety of geomorphic, oceanographic, and climatic settings. Based on a synthesis of these diverse occurrences plus new observations, a generalized model is presented. At each locality microbialite development is associated with discharge of mineralized freshwater in the coastal zone. Microbialite formation in the high intertidal and supratidal zones of rock coasts occurs in a variety of sub-environments (cliff face, shore platform surface, platform surface pools, boulder beach, and sand beach) and forms a variety of laminated rock encrustations and oncoids. Allochthonous microbialites occur on the backshore as breccias of reworked microbialite clasts, oncoids transported from rock pools, and partly encrusted boulders. The microbialite-influenced rock coast is a distinct type of siliciclastic environment that offers potential comparison for ancient microbialite occurrences. It has preservation potential in both transgressive and regressive settings. Potential ancient examples are suggested.
{"title":"Microbialites of modern siliciclastic rock coasts","authors":"A. Cooper, Alan Smith, G. Rishworth, Carla Dodd, M. Forbes, H. Cawthra, C. Anderson","doi":"10.2110/jsr.2021.071","DOIUrl":"https://doi.org/10.2110/jsr.2021.071","url":null,"abstract":"Contemporary microbialite formation has been documented on rock coasts in a variety of geomorphic, oceanographic, and climatic settings. Based on a synthesis of these diverse occurrences plus new observations, a generalized model is presented. At each locality microbialite development is associated with discharge of mineralized freshwater in the coastal zone. Microbialite formation in the high intertidal and supratidal zones of rock coasts occurs in a variety of sub-environments (cliff face, shore platform surface, platform surface pools, boulder beach, and sand beach) and forms a variety of laminated rock encrustations and oncoids. Allochthonous microbialites occur on the backshore as breccias of reworked microbialite clasts, oncoids transported from rock pools, and partly encrusted boulders. The microbialite-influenced rock coast is a distinct type of siliciclastic environment that offers potential comparison for ancient microbialite occurrences. It has preservation potential in both transgressive and regressive settings. Potential ancient examples are suggested.","PeriodicalId":17044,"journal":{"name":"Journal of Sedimentary Research","volume":" ","pages":""},"PeriodicalIF":2.0,"publicationDate":"2022-07-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45076774","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Caio Bittencourt Guedes, M. Arena, H. N. dos Santos, B. Valle, Jeferson de Andrade Santos, J. Favoreto, L. Borghi
The presence of microbialites in the hypersaline lagoons of Rio de Janeiro is especially important in the study of recent analogs of carbonate rocks with microbial origins, mostly after the discovery of giant petroleum reservoirs in the Brazilian pre-salt section and their similarities with stromatolites from Lagoa Salgada (Rio de Janeiro State). Many studies have been conducted to analyze the biology, geochemistry, mineralogy, and geomicrobiology of these microbialites. This paper, however, focuses on the petrography, sedimentology, and geochemistry of recent and superficial microbial mats from Lagoa Vermelha to understand the interaction of carbonate and siliciclastic grains with an organic matrix and discuss their similarities and differences with pre-salt rocks. A sedimentologic description was performed to understand the sediment dynamics in microbial mats. A petrographic description involved the characterization of components and textures in microscale. Furthermore, geochemical analyses were performed using scanning electron microscopy and X-ray diffraction for detailed mineralogical characterization. This multitechnique study showed the lamellar and cracked texture of the matrix being displaced by biologically induced carbonate growth and siliciclastic grains. In addition, chemical analysis showed the concentration of magnesium and silica in the matrix, with the absence of Mg-clay minerals. Even though the studied microbial mats present relevant similarities with some pre-salt facies, a microbially dominated genesis for the pre-salt limestones cannot be supported by the studied data.
{"title":"Sedimentological and geochemical characterization of microbial mats from Lagoa Vermelha (Rio de Janeiro, Brazil)","authors":"Caio Bittencourt Guedes, M. Arena, H. N. dos Santos, B. Valle, Jeferson de Andrade Santos, J. Favoreto, L. Borghi","doi":"10.2110/jsr.2021.072","DOIUrl":"https://doi.org/10.2110/jsr.2021.072","url":null,"abstract":"\u0000 The presence of microbialites in the hypersaline lagoons of Rio de Janeiro is especially important in the study of recent analogs of carbonate rocks with microbial origins, mostly after the discovery of giant petroleum reservoirs in the Brazilian pre-salt section and their similarities with stromatolites from Lagoa Salgada (Rio de Janeiro State). Many studies have been conducted to analyze the biology, geochemistry, mineralogy, and geomicrobiology of these microbialites. This paper, however, focuses on the petrography, sedimentology, and geochemistry of recent and superficial microbial mats from Lagoa Vermelha to understand the interaction of carbonate and siliciclastic grains with an organic matrix and discuss their similarities and differences with pre-salt rocks. A sedimentologic description was performed to understand the sediment dynamics in microbial mats. A petrographic description involved the characterization of components and textures in microscale. Furthermore, geochemical analyses were performed using scanning electron microscopy and X-ray diffraction for detailed mineralogical characterization. This multitechnique study showed the lamellar and cracked texture of the matrix being displaced by biologically induced carbonate growth and siliciclastic grains. In addition, chemical analysis showed the concentration of magnesium and silica in the matrix, with the absence of Mg-clay minerals. Even though the studied microbial mats present relevant similarities with some pre-salt facies, a microbially dominated genesis for the pre-salt limestones cannot be supported by the studied data.","PeriodicalId":17044,"journal":{"name":"Journal of Sedimentary Research","volume":" ","pages":""},"PeriodicalIF":2.0,"publicationDate":"2022-06-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46922105","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Y. Gan, Flavio N. De Almeida, V. Rossi, R. Steel, C. Olariu
The processes that transport sediment from the coastline to the shelf edge are key components of the sedimentary source-to-sink system, determining basin-margin building, deepwater deposition, organic-material accumulation, and the long-term carbon cycle. Research on shelf sediment transport has been aided recently by advances in modeling and marine technology. In this study we provide a much needed review of up-to-date findings on how sediment moves from the outer shelf onto the upper slope, and we summarize four dominant shelf-to-slope drivers: 1) river currents, 2) reworking storm waves and longshore currents, 3) strong tidal currents supplementing river outflow, and 4) small-scale to very large-scale gravity collapse of the shelf-edge area.
{"title":"Sediment transfer from shelf to deepwater slope: How does it happen?","authors":"Y. Gan, Flavio N. De Almeida, V. Rossi, R. Steel, C. Olariu","doi":"10.2110/jsr.2021.013","DOIUrl":"https://doi.org/10.2110/jsr.2021.013","url":null,"abstract":"\u0000 The processes that transport sediment from the coastline to the shelf edge are key components of the sedimentary source-to-sink system, determining basin-margin building, deepwater deposition, organic-material accumulation, and the long-term carbon cycle. Research on shelf sediment transport has been aided recently by advances in modeling and marine technology. In this study we provide a much needed review of up-to-date findings on how sediment moves from the outer shelf onto the upper slope, and we summarize four dominant shelf-to-slope drivers: 1) river currents, 2) reworking storm waves and longshore currents, 3) strong tidal currents supplementing river outflow, and 4) small-scale to very large-scale gravity collapse of the shelf-edge area.","PeriodicalId":17044,"journal":{"name":"Journal of Sedimentary Research","volume":" ","pages":""},"PeriodicalIF":2.0,"publicationDate":"2022-06-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42921676","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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