The giant Aptian reservoirs and associated lacustrine rocks of the South Atlantic Pre-Salt section present a series of unique characteristics, which make the systems created for marine carbonate rocks quite inadequate for their classification. Based on our experience with the characterization of thousands of samples of the Pre-Salt reservoirs and associated deposits from the Brazilian basins, and on a wide literature evaluation, we propose an objective and operational system for the classification of the unusual, yet extremely important Pre-Salt lacustrine rocks. The system allows the coherent record of structure, fabric, primary texture and composition, and main diagenetic modifications of the in situ and resedimented rock types. The in situ rocks are directly classified according to the original proportion among calcite spherulites, fascicular shrubs and mud matrix. The resedimented rocks are classified according to the original volume of > 2mm particles and the proportion between sand and mud, using calcirudite, calcarenite and calcilutite as unbiased names. Such approach can also be applied for the classification of other particulate carbonate rocks, avoiding the conceptual problems and dubious interpretation of depositional environment and reservoir quality of currently used classification systems. The direct, objective and instinctive proposed classification system shall contribute to the understanding, exploration and production of the extraordinary South Atlantic Pre-Salt petroleum province.
{"title":"An operational classification system for the South Atlantic pre-salt rocks","authors":"L. F. De Ros, Daniel M. Oliveira","doi":"10.2110/jsr.2022.103","DOIUrl":"https://doi.org/10.2110/jsr.2022.103","url":null,"abstract":"The giant Aptian reservoirs and associated lacustrine rocks of the South Atlantic Pre-Salt section present a series of unique characteristics, which make the systems created for marine carbonate rocks quite inadequate for their classification. Based on our experience with the characterization of thousands of samples of the Pre-Salt reservoirs and associated deposits from the Brazilian basins, and on a wide literature evaluation, we propose an objective and operational system for the classification of the unusual, yet extremely important Pre-Salt lacustrine rocks. The system allows the coherent record of structure, fabric, primary texture and composition, and main diagenetic modifications of the in situ and resedimented rock types. The in situ rocks are directly classified according to the original proportion among calcite spherulites, fascicular shrubs and mud matrix. The resedimented rocks are classified according to the original volume of > 2mm particles and the proportion between sand and mud, using calcirudite, calcarenite and calcilutite as unbiased names. Such approach can also be applied for the classification of other particulate carbonate rocks, avoiding the conceptual problems and dubious interpretation of depositional environment and reservoir quality of currently used classification systems. The direct, objective and instinctive proposed classification system shall contribute to the understanding, exploration and production of the extraordinary South Atlantic Pre-Salt petroleum province.","PeriodicalId":17044,"journal":{"name":"Journal of Sedimentary Research","volume":" ","pages":""},"PeriodicalIF":2.0,"publicationDate":"2023-07-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44365295","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}
M. Gibling, R. Jia, R. Gastaldo, J. Neveling, H. Rochín-Bañaga
The Triassic Katberg Formation has played a central role in interpreting the end-Permian ecosystem crisis, as part of a hypothesis of aridification, vegetation loss, and sediment release in continental settings. We use drone images of an inaccessible cliff near Bethulie to investigate the Swartberg member, a 45 m thick braided-fluvial body, describing remote outcrop facies to identify geomorphic units and using spatial analysis to estimate their proportions in 2D sections. Here the Swartberg member comprises three channel belts within shallow valleys, the lowermost of which is ~500 m wide and incised into lacustrine deposits. The component channel bodies consist mainly of trough cross-bedded sand sheets (48%) and channel-scour fills (28%). Recognizable bars (15%) comprise unit bars with high-angle slipfaces and mounded bar cores (components of mid-channel compound bars), bars built around vegetation, and bank-attached bars in discrete, probably low-sinuosity conduits. Abandoned channels constitute 8% and 16% of flow-parallel and -transverse sections, respectively. When corrected for compaction, the average thalweg depth of the larger channels is 3.9 m, with an average bankfull width of 84 m, scaling broadly with the relief of the bars and comparable in scale to the Platte and South Saskatchewan rivers of North America. The fluvial style implies perennial but seasonably variable flow in a vegetated landscape with a humid paleoclimate. The northward paleoflow accords with regional paleoflow patterns and deposition on a megafan sourced in the Cape Fold Belt, where the Swartberg member represents the avulsion of a major transverse-flowing river. U-Pb dating of in situ and reworked pedogenic carbonate nodules from below the base of the Swartberg member yielded Anisian to Ladinian ages (Middle Triassic), younger than the previously assumed Early Triassic age and implying that considerable gaps in time exist within the succession. An assessment of the interval spanning the lower to mid Katberg Formation is needed to reevaluate the inferred unidirectional trend in fluvial style, aridification, and fossil distributions in this condensed, disjunct succession.
{"title":"Braided-river architecture of the Triassic Swartberg Member, Katberg Formation, South Africa: assessing age, fluvial style, and paleoclimate after the End-Permian Extinction","authors":"M. Gibling, R. Jia, R. Gastaldo, J. Neveling, H. Rochín-Bañaga","doi":"10.2110/jsr.018","DOIUrl":"https://doi.org/10.2110/jsr.018","url":null,"abstract":"The Triassic Katberg Formation has played a central role in interpreting the end-Permian ecosystem crisis, as part of a hypothesis of aridification, vegetation loss, and sediment release in continental settings. We use drone images of an inaccessible cliff near Bethulie to investigate the Swartberg member, a 45 m thick braided-fluvial body, describing remote outcrop facies to identify geomorphic units and using spatial analysis to estimate their proportions in 2D sections. Here the Swartberg member comprises three channel belts within shallow valleys, the lowermost of which is ~500 m wide and incised into lacustrine deposits. The component channel bodies consist mainly of trough cross-bedded sand sheets (48%) and channel-scour fills (28%). Recognizable bars (15%) comprise unit bars with high-angle slipfaces and mounded bar cores (components of mid-channel compound bars), bars built around vegetation, and bank-attached bars in discrete, probably low-sinuosity conduits. Abandoned channels constitute 8% and 16% of flow-parallel and -transverse sections, respectively. When corrected for compaction, the average thalweg depth of the larger channels is 3.9 m, with an average bankfull width of 84 m, scaling broadly with the relief of the bars and comparable in scale to the Platte and South Saskatchewan rivers of North America. The fluvial style implies perennial but seasonably variable flow in a vegetated landscape with a humid paleoclimate. The northward paleoflow accords with regional paleoflow patterns and deposition on a megafan sourced in the Cape Fold Belt, where the Swartberg member represents the avulsion of a major transverse-flowing river. U-Pb dating of in situ and reworked pedogenic carbonate nodules from below the base of the Swartberg member yielded Anisian to Ladinian ages (Middle Triassic), younger than the previously assumed Early Triassic age and implying that considerable gaps in time exist within the succession. An assessment of the interval spanning the lower to mid Katberg Formation is needed to reevaluate the inferred unidirectional trend in fluvial style, aridification, and fossil distributions in this condensed, disjunct succession.","PeriodicalId":17044,"journal":{"name":"Journal of Sedimentary Research","volume":" ","pages":""},"PeriodicalIF":2.0,"publicationDate":"2023-07-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43641793","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}
D. Larue, Kimberly Mendez Mendez, José L. Corchado Albelo, Lauryn N. Martinez, S. Hughes, T. Hudgins, H. Santos, Alan L. Smith, C. Osterberg
Siliciclastic sandstone composition on the island of Puerto Rico, part of the Greater Antilles, was influenced by both tectonic setting and climate. Cretaceous through Eocene volcanic and plutonic rocks on Puerto Rico formed in an oceanic-arc setting. Sandstones deposited during arc volcanism are quartz-poor lithic and feldspatholithic arenites, whose geochemistry largely matches that of the oceanic-arc volcanic and plutonic rocks on Puerto Rico. After cessation of volcanism on Puerto Rico during collision of the Greater Antilles Island arc with the Bahamas Bank, an Oligocene through Holocene overlap assemblage was deposited unconformably above the arc deposits in the North Coast and South Coast basins of Puerto Rico. The overlap assemblage consists of some siliciclastic material, but largely shallow-water carbonate deposits. Siliciclastic deposition in the overlap assemblage reflects the volcanic-arc source, plus quartz derived from plutons exposed during uplift. In the Pleistocene and Holocene (and possibly earlier), poorly consolidated quartz-arenitic sands with SiOR2R values from 92 to 98% were deposited in the overlap assemblage. On geochemical tectonic-affinity diagrams, the lithic sandstones plot, as expected, in or near the fields for oceanic-arc provenance. However, the quartz-arenitic sands plot incorrectly in the passive-margin field when considering major elements, and correctly in or near the oceanic-arc field when considering trace elements. Trace elements in the quartz-arenitic sands are largely found in refractory minerals, whereas major elements in feldspars and lithic fragments are effectively removed by intense tropical weathering. Hence, future use of tectonic-affinity diagrams should rely on trace-element geochemistry. Sandstones derived from the Sierra Nevada continental arc in California have QFL quartz fractions below 60%, even though the source magmatic arc has significant modal quartz. This observation suggests a “climate cap,” which does not “allow” formation of more quartzose sediments. In contrast, tropical weathering on Puerto Rico removes this climate cap allowing the creation of quartz-arenitic sands from a source rock containing limited quartz. It is remarkable that quartz-arenitic sands occur on Puerto Rico, sourced from a provenance area containing extremely limited quantities of modal quartz (estimated at less than 5%) in an oceanic-arc environment bounded by two active subduction zones. Quartz-arenitic sands and sandstones are not uniquely continental nor of cratonal origin; chemical weathering is fundamentally important for the origin of first-cycle quartz-arenitic sands.
{"title":"Climato-tectonic evolution of siliciclastic sandstones on Puerto Rico: from lithic arenites to quartz-arenitic sands in an oceanic island-arc setting","authors":"D. Larue, Kimberly Mendez Mendez, José L. Corchado Albelo, Lauryn N. Martinez, S. Hughes, T. Hudgins, H. Santos, Alan L. Smith, C. Osterberg","doi":"10.2110/jsr.2023.016","DOIUrl":"https://doi.org/10.2110/jsr.2023.016","url":null,"abstract":"Siliciclastic sandstone composition on the island of Puerto Rico, part of the Greater Antilles, was influenced by both tectonic setting and climate. Cretaceous through Eocene volcanic and plutonic rocks on Puerto Rico formed in an oceanic-arc setting. Sandstones deposited during arc volcanism are quartz-poor lithic and feldspatholithic arenites, whose geochemistry largely matches that of the oceanic-arc volcanic and plutonic rocks on Puerto Rico. After cessation of volcanism on Puerto Rico during collision of the Greater Antilles Island arc with the Bahamas Bank, an Oligocene through Holocene overlap assemblage was deposited unconformably above the arc deposits in the North Coast and South Coast basins of Puerto Rico. The overlap assemblage consists of some siliciclastic material, but largely shallow-water carbonate deposits. Siliciclastic deposition in the overlap assemblage reflects the volcanic-arc source, plus quartz derived from plutons exposed during uplift. In the Pleistocene and Holocene (and possibly earlier), poorly consolidated quartz-arenitic sands with SiOR2R values from 92 to 98% were deposited in the overlap assemblage. On geochemical tectonic-affinity diagrams, the lithic sandstones plot, as expected, in or near the fields for oceanic-arc provenance. However, the quartz-arenitic sands plot incorrectly in the passive-margin field when considering major elements, and correctly in or near the oceanic-arc field when considering trace elements. Trace elements in the quartz-arenitic sands are largely found in refractory minerals, whereas major elements in feldspars and lithic fragments are effectively removed by intense tropical weathering. Hence, future use of tectonic-affinity diagrams should rely on trace-element geochemistry. Sandstones derived from the Sierra Nevada continental arc in California have QFL quartz fractions below 60%, even though the source magmatic arc has significant modal quartz. This observation suggests a “climate cap,” which does not “allow” formation of more quartzose sediments. In contrast, tropical weathering on Puerto Rico removes this climate cap allowing the creation of quartz-arenitic sands from a source rock containing limited quartz. It is remarkable that quartz-arenitic sands occur on Puerto Rico, sourced from a provenance area containing extremely limited quantities of modal quartz (estimated at less than 5%) in an oceanic-arc environment bounded by two active subduction zones. Quartz-arenitic sands and sandstones are not uniquely continental nor of cratonal origin; chemical weathering is fundamentally important for the origin of first-cycle quartz-arenitic sands.","PeriodicalId":17044,"journal":{"name":"Journal of Sedimentary Research","volume":" ","pages":""},"PeriodicalIF":2.0,"publicationDate":"2023-07-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44237939","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}
Oxfordian deposits in northern Colorado and Wyoming, USA preserve proximal, intertidal, clastic, coastal deposits and distal, offshore to nearshore, subtidal bioclastic facies that accumulated during a forced regression of the Jurassic epeiric Sea. This contrasts with the common association between tidal deposits, carbonate facies, and rising sea level common to many depositional models of mixed clastic/carbonate systems. Ichnology, sedimentology, and architecture of these deposits were documented along a 460 km outcrop transect to test previous depositional interpretations and decipher the mechanisms driving the change from proximal, siliciclastic, intertidal flats to distal, bioclastic, subtidal macroforms. In southeastern Wyoming, the Windy Hill Sandstone (WH) is composed of very fine, SiO2-dominated, intertidal facies that truncate offshore to lower shoreface, storm-dominated deposits of the Redwater Shale Member (RS) of the Sundance Fm. Eolian and small fluvial systems delivered sand to the coastline after subaerial erosion reworked it from older, uplifted Jurassic strata onshore. The regional and time-transgressive J-5 unconformity separating the WH from the RS is readily identifiable using ichnological and sedimentological criteria. In the Wind River and Bighorn basins to the north, the Upper Sundance Fm (USF) is time-equivalent to the WH but is composed of glauconitic, silt-prone sandstone and meter to decimeter-scale, bioclastic, cross-bedded sandstone bodies. The abundance of molluscan shell material and limited volume of siliciclastic sediment in the geographically widespread outcrops suggests that nearby, marine shoals were the source of the coarse-grained material. Bioclastic, cross-stratified sandstone bodies represent two architectural elements: 1) coarsening-upward bodies with seaward-dipping foresets arranged into complexes bound by seaward-directed bounding surfaces interpreted to represent subtidal compound dunes and 2) landward- and laterally accreting tidal inlet fills composed of meter-scale, landward-accreting bodies with some landward-directed current ripples. The presence of transgressive tidal inlets supports previous interpretations that the WH and USF record high frequency transgressions superimposed on a tectonically-driven forced regression.
{"title":"Subtidal to intertidal deposits in a mixed clastic-carbonate epicontinental seaway, the Windy Hill Sandstone and Upper Sundance Formation (Oxfordian), Wyoming, U.S.A.","authors":"A. Wroblewski, J. Schueth","doi":"10.2110/jsr.2022.061","DOIUrl":"https://doi.org/10.2110/jsr.2022.061","url":null,"abstract":"Oxfordian deposits in northern Colorado and Wyoming, USA preserve proximal, intertidal, clastic, coastal deposits and distal, offshore to nearshore, subtidal bioclastic facies that accumulated during a forced regression of the Jurassic epeiric Sea. This contrasts with the common association between tidal deposits, carbonate facies, and rising sea level common to many depositional models of mixed clastic/carbonate systems. Ichnology, sedimentology, and architecture of these deposits were documented along a 460 km outcrop transect to test previous depositional interpretations and decipher the mechanisms driving the change from proximal, siliciclastic, intertidal flats to distal, bioclastic, subtidal macroforms. In southeastern Wyoming, the Windy Hill Sandstone (WH) is composed of very fine, SiO2-dominated, intertidal facies that truncate offshore to lower shoreface, storm-dominated deposits of the Redwater Shale Member (RS) of the Sundance Fm. Eolian and small fluvial systems delivered sand to the coastline after subaerial erosion reworked it from older, uplifted Jurassic strata onshore. The regional and time-transgressive J-5 unconformity separating the WH from the RS is readily identifiable using ichnological and sedimentological criteria. In the Wind River and Bighorn basins to the north, the Upper Sundance Fm (USF) is time-equivalent to the WH but is composed of glauconitic, silt-prone sandstone and meter to decimeter-scale, bioclastic, cross-bedded sandstone bodies. The abundance of molluscan shell material and limited volume of siliciclastic sediment in the geographically widespread outcrops suggests that nearby, marine shoals were the source of the coarse-grained material. Bioclastic, cross-stratified sandstone bodies represent two architectural elements: 1) coarsening-upward bodies with seaward-dipping foresets arranged into complexes bound by seaward-directed bounding surfaces interpreted to represent subtidal compound dunes and 2) landward- and laterally accreting tidal inlet fills composed of meter-scale, landward-accreting bodies with some landward-directed current ripples. The presence of transgressive tidal inlets supports previous interpretations that the WH and USF record high frequency transgressions superimposed on a tectonically-driven forced regression.","PeriodicalId":17044,"journal":{"name":"Journal of Sedimentary Research","volume":" ","pages":""},"PeriodicalIF":2.0,"publicationDate":"2023-07-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43760270","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}
Arnau Obradors-Latre, P. Haughton, Colm S. Pierce, P. Shannon, A. Lacchia, Simon P. Barker, O. Martinsen
Highly efficient sediment gravity flows can bypass mid fan channels and lobes and deposit significant volumes of sand, mud and particulate organic matter in outer fan and basin plain settings. The Serpukhovian to Bashkirian fill to the Shannon Basin, western Ireland, includes deep-water fan deposits (Ross Sandstone Fm) that gradationally overlie basin floor shales (Clare Shale Fm). As part of a broader progradational succession, the upward transition from muddy basin floor to sandy fan preserves the stacked deposits of settings present prior to and outboard of mid-fan channels and lobes. Three fully cored boreholes and associated wireline data constrain the facies tracts in an 18 km long panel orientated oblique to original depositional dip. Two distal successions dominated by hybrid event beds (HEBs) are recognised, separated by a prominent condensed section. The lower Cosheen system includes m-thick, tabular HEBs with prominent linked debrites that pass down dip into much thinner sandstones overlain by sand-speckled mudstone caps that thicken distally before thinning. The latter are interpreted as secondary mudflows released following reconstitution of more thoroughly mixed sections of the up-dip linked debrites. Significant bypass and accumulation of mud by this mechanism helped heal local topography and maintain a relatively flat sea floor promoting an overall tabular geometry for the deposits of larger volume hybrid flows reaching the distal sector of the basin. The overlying distal Ross system fringe is characterised by very fine to fine-grained sandstones and is lateral to compensationally-stacked lobes further to the west. It has a progradational (at least initially) stacking pattern, facies transitions developed over shorter length scales, and includes outsized event beds but these are thinner than those in the Cosheen system. Common banding and evidence for turbulence suppression by dispersed clay rather than entrained mud clasts indicate these were transitional flows. In this case, event beds are inferred to taper distally, with significant mud emplaced by plug flow retained as caps to sandy event beds rather than bypassing down-dip. Different flow transformation mechanisms thus impacted how mud was partitioned across the fringe of the two systems and this influenced bed geometries, larger scale bed stacking patterns and stratigraphy. Whereas the flow efficiency concept stresses the ability of flows to carry sand in a basinward direction, it is also imperative to consider the variable efficiency of mud transport given the operation of clay-induced flow transformations. These can either promote bypass or trigger premature fallout of mud with implications for how systems fill accommodation, bed -scale facies transitions and the burial and preservation of particulate organic carbon fractionated along with the clay in deep-water system fringes.
{"title":"Flow transformations, mud partitioning and the variable stratigraphic architecture of basin-floor fan fringes","authors":"Arnau Obradors-Latre, P. Haughton, Colm S. Pierce, P. Shannon, A. Lacchia, Simon P. Barker, O. Martinsen","doi":"10.2110/jsr.2022.114","DOIUrl":"https://doi.org/10.2110/jsr.2022.114","url":null,"abstract":"Highly efficient sediment gravity flows can bypass mid fan channels and lobes and deposit significant volumes of sand, mud and particulate organic matter in outer fan and basin plain settings. The Serpukhovian to Bashkirian fill to the Shannon Basin, western Ireland, includes deep-water fan deposits (Ross Sandstone Fm) that gradationally overlie basin floor shales (Clare Shale Fm). As part of a broader progradational succession, the upward transition from muddy basin floor to sandy fan preserves the stacked deposits of settings present prior to and outboard of mid-fan channels and lobes. Three fully cored boreholes and associated wireline data constrain the facies tracts in an 18 km long panel orientated oblique to original depositional dip. Two distal successions dominated by hybrid event beds (HEBs) are recognised, separated by a prominent condensed section. The lower Cosheen system includes m-thick, tabular HEBs with prominent linked debrites that pass down dip into much thinner sandstones overlain by sand-speckled mudstone caps that thicken distally before thinning. The latter are interpreted as secondary mudflows released following reconstitution of more thoroughly mixed sections of the up-dip linked debrites. Significant bypass and accumulation of mud by this mechanism helped heal local topography and maintain a relatively flat sea floor promoting an overall tabular geometry for the deposits of larger volume hybrid flows reaching the distal sector of the basin. The overlying distal Ross system fringe is characterised by very fine to fine-grained sandstones and is lateral to compensationally-stacked lobes further to the west. It has a progradational (at least initially) stacking pattern, facies transitions developed over shorter length scales, and includes outsized event beds but these are thinner than those in the Cosheen system. Common banding and evidence for turbulence suppression by dispersed clay rather than entrained mud clasts indicate these were transitional flows. In this case, event beds are inferred to taper distally, with significant mud emplaced by plug flow retained as caps to sandy event beds rather than bypassing down-dip. Different flow transformation mechanisms thus impacted how mud was partitioned across the fringe of the two systems and this influenced bed geometries, larger scale bed stacking patterns and stratigraphy. Whereas the flow efficiency concept stresses the ability of flows to carry sand in a basinward direction, it is also imperative to consider the variable efficiency of mud transport given the operation of clay-induced flow transformations. These can either promote bypass or trigger premature fallout of mud with implications for how systems fill accommodation, bed -scale facies transitions and the burial and preservation of particulate organic carbon fractionated along with the clay in deep-water system fringes.","PeriodicalId":17044,"journal":{"name":"Journal of Sedimentary Research","volume":" ","pages":""},"PeriodicalIF":2.0,"publicationDate":"2023-07-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48430486","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}
Recent developments in fluvial geomorphology and sedimentology suggest that fluvial fans (also known as distributive fluvial systems) could be responsible for the accumulation of great volumes of clastic successions in continental basins. A general depositional model based on sedimentological and architectural trends has been formulated for these fluvial systems, however, their recognition in the stratigraphic record often relies on partially preserved, discontinuous successions. This study provides a sedimentological and architectural characterization of Paleogene alluvial strata of the Wasatch and Colton formations in the southwestern Uinta Basin, central Utah (U.S.A.), following an ∼ 120-km-long outcrop belt which records deposition from an alluvial wedge that prograded to the north and northwest, from the basin margin to distal lake-dominated environments, preceding the onset of ancient Lake Uinta. Lateral and vertical distribution of facies associations are presented from a dataset of field observations (thirteen logged stratigraphic sections, for a total of ∼ 2400 m) and virtual outcrop models along the proximal-to-distal extent of the Wasatch–Colton alluvial system. Four sectors are defined (proximal, medial, distal, and terminal) to mark the longitudinal heterogeneity of alluvial stratigraphy. Noteworthy trends comprise a downstream decrease in the overall thickness of the alluvial stratigraphic column, a reduction in the relative volume, architectural complexity, and amalgamation of fluvial-channel bodies away from the apex, a weak downstream-fining trend in channel sandstones, and a down-system increase in preserved overbank and floodplain deposits accompanied by increasing volume and facies complexity of preserved lacustrine and palustrine facies associations. A proximal-to-distal change in fluvial-channel architecture is noted, with proximal sections characterized by vertically and laterally amalgamated sheet-like channel fills, transitioning to a lesser degree of amalgamation towards the medial sector, whereas distal and terminal sections are dominated by floodplain fines enveloping a subordinate volume of isolated, ribbon-shaped channel-sandstone bodies. The temporal development of the stratigraphic succession is observed in its entirety throughout the field area and, albeit localized, channel-scale erosion and potential depositional hiatuses punctuate the stratigraphy. Two major system-scale trends have been described in the Wasatch–Colton System (WCS). A first-order, long-term progradational trend, especially evident in proximal and medial sections, encompasses the large-scale vertical patterns in facies and architecture vertically through most of the stratigraphy of the WCS. In contrast, the uppermost part of the stratigraphic succession is characterized by a reverse, retrogradational trend, possibly associated with the early transgression of Lake Uinta's southern margin, marking the base of the overlying Green River Formation.
{"title":"Anatomy of a fluvial paleo-fan: sedimentological and architectural trends of the Paleocene–Eocene Wasatch–Colton System (western Uinta Basin, Utah, U.S.A.)","authors":"Davide Carraro, D. Ventra, A. Moscariello","doi":"10.2110/jsr.2022.095","DOIUrl":"https://doi.org/10.2110/jsr.2022.095","url":null,"abstract":"\u0000 Recent developments in fluvial geomorphology and sedimentology suggest that fluvial fans (also known as distributive fluvial systems) could be responsible for the accumulation of great volumes of clastic successions in continental basins. A general depositional model based on sedimentological and architectural trends has been formulated for these fluvial systems, however, their recognition in the stratigraphic record often relies on partially preserved, discontinuous successions. This study provides a sedimentological and architectural characterization of Paleogene alluvial strata of the Wasatch and Colton formations in the southwestern Uinta Basin, central Utah (U.S.A.), following an ∼ 120-km-long outcrop belt which records deposition from an alluvial wedge that prograded to the north and northwest, from the basin margin to distal lake-dominated environments, preceding the onset of ancient Lake Uinta. Lateral and vertical distribution of facies associations are presented from a dataset of field observations (thirteen logged stratigraphic sections, for a total of ∼ 2400 m) and virtual outcrop models along the proximal-to-distal extent of the Wasatch–Colton alluvial system. Four sectors are defined (proximal, medial, distal, and terminal) to mark the longitudinal heterogeneity of alluvial stratigraphy. Noteworthy trends comprise a downstream decrease in the overall thickness of the alluvial stratigraphic column, a reduction in the relative volume, architectural complexity, and amalgamation of fluvial-channel bodies away from the apex, a weak downstream-fining trend in channel sandstones, and a down-system increase in preserved overbank and floodplain deposits accompanied by increasing volume and facies complexity of preserved lacustrine and palustrine facies associations. A proximal-to-distal change in fluvial-channel architecture is noted, with proximal sections characterized by vertically and laterally amalgamated sheet-like channel fills, transitioning to a lesser degree of amalgamation towards the medial sector, whereas distal and terminal sections are dominated by floodplain fines enveloping a subordinate volume of isolated, ribbon-shaped channel-sandstone bodies. The temporal development of the stratigraphic succession is observed in its entirety throughout the field area and, albeit localized, channel-scale erosion and potential depositional hiatuses punctuate the stratigraphy. Two major system-scale trends have been described in the Wasatch–Colton System (WCS). A first-order, long-term progradational trend, especially evident in proximal and medial sections, encompasses the large-scale vertical patterns in facies and architecture vertically through most of the stratigraphy of the WCS. In contrast, the uppermost part of the stratigraphic succession is characterized by a reverse, retrogradational trend, possibly associated with the early transgression of Lake Uinta's southern margin, marking the base of the overlying Green River Formation.","PeriodicalId":17044,"journal":{"name":"Journal of Sedimentary Research","volume":" ","pages":""},"PeriodicalIF":2.0,"publicationDate":"2023-06-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44781206","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}
Abo Arroyo is a tributary of the Rio Grande in central New Mexico. Its alluvial sequence is made up of four informal units, their age defined by 44 AMS radiocarbon dates from 12.8 ka to 0.85 ka. The earliest, unit 1, is terminal Pleistocene to early Holocene (12.8 ka to 9.0 ka), including the Scholle wet meadow (12.3 ka to 11.1 ka) related to the Younger Dryas. After an erosional gap in the record from 9 ka to 6 ka, coarse gravel (unit 2) was deposited in the channel (5.8 ka and 4.3 ka) during the middle Holocene period of aridity. Subsequently, the late Holocene wet period is characterized by fine-textured alluvium (unit 3) (3.5 ka to 0.945 ka) with shells of an aquatic snail, indicating persistent stream flow. An upper 2-m zone of dark-colored clayey silt beds with high amounts of organic carbon and carbonate accumulated from 1.7 ka to 0.945 ka. A record of C4 signatures, previously interpreted to indicate dry conditions 1.4 ka to 0.945 ka, is reinterpreted as an interval of exceptionally wet floodplain conditions with C4 grasses and sedges. Abo Arroyo and other studies indicate three major episodes of late-Quaternary channel entrenchment: 1) from the full-glacial to late glacial-Bølling/Allerød (6.1 ky), 2) during the middle Holocene (2.5 ky), and 3) during the late Holocene Medieval Warm Period (0.4 ky), each erosional event less severe and shorter duration than the preceding one, and all three represent a significant change from wet to dry climate. During the Medieval Warm Period (A.D. 900 to 1300), a shallow channel formed in unit 3 alluvium, bracketed by AMS dates A.D. 1005 in unit 3 alluvium and A. D. 1100 in unit 4 channel fill. The channel cutting occurred with the shift from wet to dry climate, although the downcutting event was preceded by 100 years of landscape adjustment to the long drought. The canyon fill was entrenched again, deep and wide, by arroyo cutting in the late nineteenth and early twentieth centuries. The regional 83-year second-century drought (A.D. 100 to A.D. 182), documented by tree rings, shows up in the stable-carbon-isotope record from Abo Arroyo alluvium. The drought was more severe than the 400-year warm period but had little effect on the preserved alluvial record. The less severe but longer-duration Medieval Warm Period resulted in channel cutting at Abo Arroyo and elsewhere in the broad region at that time, but the second-century drought did not. Finally, the similarity of Abo Arroyo and Rio Grande late Holocene alluvial records with parallel stratigraphy, sedimentology, and geochronology illustrates that tributaries and main valleys respond alike and in concert to climate and climate change.
{"title":"Late Quaternary fluvial environments at Abo Arroyo, New Mexico, U.S.A.: response to millennial-scale climate change","authors":"S. Hall","doi":"10.2110/jsr.2022.099","DOIUrl":"https://doi.org/10.2110/jsr.2022.099","url":null,"abstract":"\u0000 Abo Arroyo is a tributary of the Rio Grande in central New Mexico. Its alluvial sequence is made up of four informal units, their age defined by 44 AMS radiocarbon dates from 12.8 ka to 0.85 ka. The earliest, unit 1, is terminal Pleistocene to early Holocene (12.8 ka to 9.0 ka), including the Scholle wet meadow (12.3 ka to 11.1 ka) related to the Younger Dryas. After an erosional gap in the record from 9 ka to 6 ka, coarse gravel (unit 2) was deposited in the channel (5.8 ka and 4.3 ka) during the middle Holocene period of aridity. Subsequently, the late Holocene wet period is characterized by fine-textured alluvium (unit 3) (3.5 ka to 0.945 ka) with shells of an aquatic snail, indicating persistent stream flow. An upper 2-m zone of dark-colored clayey silt beds with high amounts of organic carbon and carbonate accumulated from 1.7 ka to 0.945 ka. A record of C4 signatures, previously interpreted to indicate dry conditions 1.4 ka to 0.945 ka, is reinterpreted as an interval of exceptionally wet floodplain conditions with C4 grasses and sedges. Abo Arroyo and other studies indicate three major episodes of late-Quaternary channel entrenchment: 1) from the full-glacial to late glacial-Bølling/Allerød (6.1 ky), 2) during the middle Holocene (2.5 ky), and 3) during the late Holocene Medieval Warm Period (0.4 ky), each erosional event less severe and shorter duration than the preceding one, and all three represent a significant change from wet to dry climate. During the Medieval Warm Period (A.D. 900 to 1300), a shallow channel formed in unit 3 alluvium, bracketed by AMS dates A.D. 1005 in unit 3 alluvium and A. D. 1100 in unit 4 channel fill. The channel cutting occurred with the shift from wet to dry climate, although the downcutting event was preceded by 100 years of landscape adjustment to the long drought. The canyon fill was entrenched again, deep and wide, by arroyo cutting in the late nineteenth and early twentieth centuries. The regional 83-year second-century drought (A.D. 100 to A.D. 182), documented by tree rings, shows up in the stable-carbon-isotope record from Abo Arroyo alluvium. The drought was more severe than the 400-year warm period but had little effect on the preserved alluvial record. The less severe but longer-duration Medieval Warm Period resulted in channel cutting at Abo Arroyo and elsewhere in the broad region at that time, but the second-century drought did not. Finally, the similarity of Abo Arroyo and Rio Grande late Holocene alluvial records with parallel stratigraphy, sedimentology, and geochronology illustrates that tributaries and main valleys respond alike and in concert to climate and climate change.","PeriodicalId":17044,"journal":{"name":"Journal of Sedimentary Research","volume":" ","pages":""},"PeriodicalIF":2.0,"publicationDate":"2023-06-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47593802","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}
Keith P. Minor, R. Steel, C. Olariu, A. Wroblewski
An evolving tectonic setting during the Late Cretaceous and Paleocene drove the dramatic transformation of southern Wyoming from a broad marine shelf setting to a series of lacustrine basins. Early dominant control of depositional patterns by flexural tectonics gave way to control by dynamic topography and Laramide-style deformation. The southern Wyoming region demonstrates how progressive partitioning by Laramide movements exerted increasingly dominant control on regional depositional patterns. It did this by creating a long-lived, Laramide uplift-bounded fluvial catchment and basin largely controlled by Laramide-driven subsidence. The Rock Springs and Rawlins uplifts underpinned the basin and exerted their influence in the subsurface. Episodes of Laramide-driven subsidence produced a series of embayments, including the large Rusty–Allen Ridge embayment and the very large deepwater Lewis embayment. The degree of eustatic control in the region would have been greatest at short time scales in the low coastal–shallow-marine settings, causing shoreline oscillations. Several long transgressive episodes followed relatively punctuated progradational periods of coastal-plain construction when supply was generally high from the thrust belt and surrounding uplifts. Laramide activity in the Paleocene led to enclosure of the fluvial basin and the development of a series of smaller marine embayments that mark the end of influence in the region by open and coastal marine processes.
{"title":"The southern Wyoming Laramide basin: implications for long-term tectonic control on uplift and subsidence","authors":"Keith P. Minor, R. Steel, C. Olariu, A. Wroblewski","doi":"10.2110/jsr.2022.077","DOIUrl":"https://doi.org/10.2110/jsr.2022.077","url":null,"abstract":"\u0000 An evolving tectonic setting during the Late Cretaceous and Paleocene drove the dramatic transformation of southern Wyoming from a broad marine shelf setting to a series of lacustrine basins. Early dominant control of depositional patterns by flexural tectonics gave way to control by dynamic topography and Laramide-style deformation. The southern Wyoming region demonstrates how progressive partitioning by Laramide movements exerted increasingly dominant control on regional depositional patterns. It did this by creating a long-lived, Laramide uplift-bounded fluvial catchment and basin largely controlled by Laramide-driven subsidence. The Rock Springs and Rawlins uplifts underpinned the basin and exerted their influence in the subsurface. Episodes of Laramide-driven subsidence produced a series of embayments, including the large Rusty–Allen Ridge embayment and the very large deepwater Lewis embayment. The degree of eustatic control in the region would have been greatest at short time scales in the low coastal–shallow-marine settings, causing shoreline oscillations. Several long transgressive episodes followed relatively punctuated progradational periods of coastal-plain construction when supply was generally high from the thrust belt and surrounding uplifts. Laramide activity in the Paleocene led to enclosure of the fluvial basin and the development of a series of smaller marine embayments that mark the end of influence in the region by open and coastal marine processes.","PeriodicalId":17044,"journal":{"name":"Journal of Sedimentary Research","volume":" ","pages":""},"PeriodicalIF":2.0,"publicationDate":"2023-06-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44210939","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}
The Upper Cretaceous (Turonian?) Middendorf Formation is a sand-rich stratigraphic unit of fluvial origin that forms a large aquifer in the U.S. Atlantic Coastal Plain. In Chesterfield County (South Carolina), which is the site of the type locality, the formation ranges in thickness from 66.5 to > 119.7 meters. The base of the formation is an unconformity above Paleozoic metasiltstone, and the upper contact is an unconformity above which lies sand of the Quaternary Pinehurst Formation. Outcrops display the following five facies assemblages: 1) sandstone to conglomeratic sandstone (fluvial bar and channel deposits), 2) beds of alternating laminae of sandstone and mudstone (fluvial overbank or floodplain deposits), 3) ≥ 1 m-thick beds of clay (swamp deposits, floodplain deposits, and/or sediment that accumulated in abandoned fluvial channels), 4) 0.2–0.5 m-thick planar to slightly undulatory beds of framework-supported sandstone with a mud matrix (traction-dominated current deposits at the top of fluvial bars, upper-flow-regime bedform deposits in subsidiary fluvial channels, or coarse-grained overbank deposits), and 5) sandstone to conglomeratic sandstone cemented by iron (interpreted as fluvial bar and channel deposits, with the iron cement being a diagenetic “groundwater ferricrete” that formed via the circulation of shallow groundwater and the oxidation of iron-bearing minerals). Kaolinite in various forms is pervasive throughout the formation and is interpreted as an early diagenetic phenomenon that formed by prolonged postdepositional weathering and flushing by meteoric water under a warm and humid paleoclimate. The fluvial system that formed the Middendorf Formation prograded into the area from the west or northwest from uplifted margins of Mesozoic rift basins and/or the Appalachian Mountains. This progradation was a response to a base-level fall and the sediment accumulated during base-level lowstand and subsequent early transgression. In Chesterfield County, the Middendorf Formation can be subdivided into three fining-upward sequences. Each sequence consists predominantly of medium to coarse sand with a greater abundance of gravel in the lower part of the sequence and a greater abundance of clay and silt beds in the upper part. Each sequence is interpreted as either a response to autogenic processes or a response to allogenic sea-level changes, specifically a higher-order (higher-frequency) progression from relative lowstand conditions to early transgression whereby coarse sand and gravel (e.g., fluvial bar and channel deposits) were preserved during initial lowstand conditions and a greater proportion of mud and finer-grained sand (floodplain deposits) were preserved during subsequent early transgression. The Middendorf Formation is correlative with several other kaolinite-rich fluvial sandstones in North America including the Raritan Formation in New Jersey, the Tuscaloosa Formation of the eastern Gulf of Mexico (Alabama, Mississippi
上白垩纪(Turonian?)米登多夫组是一个富含砂的地层单元，起源于河流，在美国大西洋沿岸平原形成了一个大型含水层。在南卡罗莱纳切斯特菲尔德县(Chesterfield County, South Carolina)，即该类型位置的所在地，地层厚度为66.5 ~ bb0 ~ 119.7 m。地层底部为古生代变质泥质上的不整合面，上部接触面为不整合面，其上为第四纪松木组砂体。露头显示出以下5种相组合:1)砂岩-砾岩砂岩(河流坝和河道沉积)，2)砂岩-泥岩交替纹层(河流上滩或河漫滩沉积)，3)厚度≥1 m的粘土层(沼泽沉积、河漫滩沉积和/或废弃河道中堆积的沉积物)，4)厚度为0.2-0.5 m的平面至略带起伏的泥质框架支撑砂岩层(河流坝顶部以拖曳为主的水流沉积)，5)由铁胶结的砂岩到砾岩砂岩(解释为河流坝和河道沉积，其中铁胶结物是通过浅层地下水循环和含铁矿物氧化形成的成岩“地下水铁铁矿”)。不同形态的高岭石在整个地层中普遍存在，被解释为在温暖湿润的古气候下，经过长期的沉积后风化和大气水冲刷而形成的早期成岩现象。形成米登多夫组的河流系统从中生代裂谷盆地和(或)阿巴拉契亚山脉的隆起边缘向西或向西北推进。这种进积是对基准面下降和基准面低洼和随后的早期海侵期间沉积的反应。在切斯特菲尔德县，米登多夫组可以细分为三个向上细化的层序。每个层序主要由中至粗砂组成，层序下部含较多砾石，上部含较多粘土和粉砂层。每个层序都被解释为对自生过程的响应或对异体海平面变化的响应，特别是从相对低水位条件到早期海侵的高阶(高频率)进展，在初始低水位条件下保存了粗砂和砾石(例如河流坝和河道沉积)，在随后的早期海侵中保存了更大比例的泥和细粒砂(洪泛平原沉积)。米登多夫组与北美其他几个富含高岭石的河流砂岩相关，包括新泽西州的拉坦组、墨西哥湾东部(阿拉巴马州、密西西比州、路易斯安那州)的塔斯卡卢萨组、墨西哥湾中部(德克萨斯州)的伍德拜恩组和怀俄明州的边境组。这些地层的聚集和保存是对Turonian期海平面上升下降和随后的海侵的响应，这些地层中的早成岩高岭石归因于类似的温暖湿润的古气候条件。
{"title":"Stratigraphic architecture and fluvial interpretations of the Upper Cretaceous (Turonian?) Middendorf Formation, Chesterfield County, South Carolina, U.S.A.","authors":"C. Swezey, Bradley A. Fitzwater, G. Whittecar","doi":"10.2110/jsr.2022.034","DOIUrl":"https://doi.org/10.2110/jsr.2022.034","url":null,"abstract":"\u0000 The Upper Cretaceous (Turonian?) Middendorf Formation is a sand-rich stratigraphic unit of fluvial origin that forms a large aquifer in the U.S. Atlantic Coastal Plain. In Chesterfield County (South Carolina), which is the site of the type locality, the formation ranges in thickness from 66.5 to > 119.7 meters. The base of the formation is an unconformity above Paleozoic metasiltstone, and the upper contact is an unconformity above which lies sand of the Quaternary Pinehurst Formation. Outcrops display the following five facies assemblages: 1) sandstone to conglomeratic sandstone (fluvial bar and channel deposits), 2) beds of alternating laminae of sandstone and mudstone (fluvial overbank or floodplain deposits), 3) ≥ 1 m-thick beds of clay (swamp deposits, floodplain deposits, and/or sediment that accumulated in abandoned fluvial channels), 4) 0.2–0.5 m-thick planar to slightly undulatory beds of framework-supported sandstone with a mud matrix (traction-dominated current deposits at the top of fluvial bars, upper-flow-regime bedform deposits in subsidiary fluvial channels, or coarse-grained overbank deposits), and 5) sandstone to conglomeratic sandstone cemented by iron (interpreted as fluvial bar and channel deposits, with the iron cement being a diagenetic “groundwater ferricrete” that formed via the circulation of shallow groundwater and the oxidation of iron-bearing minerals). Kaolinite in various forms is pervasive throughout the formation and is interpreted as an early diagenetic phenomenon that formed by prolonged postdepositional weathering and flushing by meteoric water under a warm and humid paleoclimate.\u0000 The fluvial system that formed the Middendorf Formation prograded into the area from the west or northwest from uplifted margins of Mesozoic rift basins and/or the Appalachian Mountains. This progradation was a response to a base-level fall and the sediment accumulated during base-level lowstand and subsequent early transgression. In Chesterfield County, the Middendorf Formation can be subdivided into three fining-upward sequences. Each sequence consists predominantly of medium to coarse sand with a greater abundance of gravel in the lower part of the sequence and a greater abundance of clay and silt beds in the upper part. Each sequence is interpreted as either a response to autogenic processes or a response to allogenic sea-level changes, specifically a higher-order (higher-frequency) progression from relative lowstand conditions to early transgression whereby coarse sand and gravel (e.g., fluvial bar and channel deposits) were preserved during initial lowstand conditions and a greater proportion of mud and finer-grained sand (floodplain deposits) were preserved during subsequent early transgression. The Middendorf Formation is correlative with several other kaolinite-rich fluvial sandstones in North America including the Raritan Formation in New Jersey, the Tuscaloosa Formation of the eastern Gulf of Mexico (Alabama, Mississippi","PeriodicalId":17044,"journal":{"name":"Journal of Sedimentary Research","volume":"1 1","pages":""},"PeriodicalIF":2.0,"publicationDate":"2023-06-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41432082","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}
Theresa J. Orr, E. Roberts, M. Bird, C. Mtelela, P. O’Connor, N. Stevens
The East African Rift System records a key interval in the evolution of modern African ecosystems, documenting significant floral changes and faunal dispersals in the context of environmental shifts. To date, Miocene-to-Pliocene data from eastern Africa have been derived primarily from richly fossiliferous rift basins along the far north of the Eastern Branch of the rift, with more limited windows emerging from the Malawi Rift and more recently, coastal Mozambique. Here, we present the first quantitative paleoclimate data for the Miocene–Pliocene transition from the Western Branch of the East African Rift System, based on analyses of paleosols from the Rukwa Rift Basin. Paleosols derived from the fossiliferous late Miocene–early Pliocene lower Lake Beds succession in southwestern Tanzania preserve a shallow lacustrine setting grading into a system of alluvial fans and braided rivers with abundant floodplain deposits. Paleoclimate reconstructions using bulk geochemistry and clay mineralogy reveal a highly seasonal, semiarid, mesic climate during the late Miocene, with increased moisture availability in the early Pliocene resulting in a shift to subhumid conditions. Stable-carbon-isotope composition of pedogenic carbonates document a woodland/bushland/shrubland paleoenvironment across the Miocene–Pliocene transition. Results support the presence of Pliocene subhumid to humid habitats, dominated by woody vegetation offering shade, food, and water for faunal dispersal along an inland corridor connecting northern segments of the East African Rift System with southern Africa.
{"title":"Paleosol-derived paleoclimate and paleoenvironment reconstruction of the Rukwa Rift Basin, Tanzania: implications for faunal dispersal in the Miocene–Pliocene","authors":"Theresa J. Orr, E. Roberts, M. Bird, C. Mtelela, P. O’Connor, N. Stevens","doi":"10.2110/jsr.2022.015","DOIUrl":"https://doi.org/10.2110/jsr.2022.015","url":null,"abstract":"\u0000 The East African Rift System records a key interval in the evolution of modern African ecosystems, documenting significant floral changes and faunal dispersals in the context of environmental shifts. To date, Miocene-to-Pliocene data from eastern Africa have been derived primarily from richly fossiliferous rift basins along the far north of the Eastern Branch of the rift, with more limited windows emerging from the Malawi Rift and more recently, coastal Mozambique. Here, we present the first quantitative paleoclimate data for the Miocene–Pliocene transition from the Western Branch of the East African Rift System, based on analyses of paleosols from the Rukwa Rift Basin. Paleosols derived from the fossiliferous late Miocene–early Pliocene lower Lake Beds succession in southwestern Tanzania preserve a shallow lacustrine setting grading into a system of alluvial fans and braided rivers with abundant floodplain deposits. Paleoclimate reconstructions using bulk geochemistry and clay mineralogy reveal a highly seasonal, semiarid, mesic climate during the late Miocene, with increased moisture availability in the early Pliocene resulting in a shift to subhumid conditions. Stable-carbon-isotope composition of pedogenic carbonates document a woodland/bushland/shrubland paleoenvironment across the Miocene–Pliocene transition. Results support the presence of Pliocene subhumid to humid habitats, dominated by woody vegetation offering shade, food, and water for faunal dispersal along an inland corridor connecting northern segments of the East African Rift System with southern Africa.","PeriodicalId":17044,"journal":{"name":"Journal of Sedimentary Research","volume":" ","pages":""},"PeriodicalIF":2.0,"publicationDate":"2023-05-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42212571","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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