The fluvial-tidal transition(FTT)is a complex depositional zone,where fluvial flow is modified by tides as rivers approach a receiving marine basin.Variations in the relative importance of tidal versus fluvial process...The fluvial-tidal transition(FTT)is a complex depositional zone,where fluvial flow is modified by tides as rivers approach a receiving marine basin.Variations in the relative importance of tidal versus fluvial processes lead to a distinctive distribution of sediments that accumulate on channel bars.The FTT generally consists of three broad zones:(1)a freshwater-tidal zone;(2)a tidally influenced freshwater to brackish-water transition:and(3)a zone of relatively sustained brackish-water conditions with stronger tides.A very common type of deposit through the fluvial-tidal transition,especially on the margins of migrating channels,is inclined heterolithic stratification(IHS).At present,a detailed account of changes in the character of IHS across the FIT of a paleo-channel system has not been reported,although a number of modern examples have been documented.To fill this gap,we quantitatively assess the sedimentology and ichnology of IHS from seven cored intervals in three geographic areas situated within the youngest paleovalley("A"Valley)in the Lower Cretaceous McMurray Formation of Alberta.Canada.We compare the data to trends defined along the FTT in the present-day Fraser River in British Columbia.Canada to interpret paleo-depositional position in the ancient fluvial-tidal channels.Analysis determined that the mean mudstone thickness is 8.2 cm in the southern study area(SA).Mean thickness increases to 11 cm in the central study area(CA),and decreases again to 4.4 cm in the northern study area(NA).The proportion of mudstone is 31%in SA,44%in CA,and 27%in NA.Thicknessweighted mean bioturbation intensity in sands varied from 0.29 in SA and CA.to 0.28 in NA.On the other hand,thickness-weighted mean bioturbation intensity(Bl)in mudstone increases from 1.46 in SA.to 1.77 in CA.and is 1.94 in NA.The ichnological diversity also increased from south to north.Sedimentological results show sinilar trends to those of the Fraser River,enabling the identification of a freshwater to brackish-water transition zone with tidal influence.The interpreted position of the transition is underpinned by the bioturbation intensity and trace-fossil diversity trends,indicating periodic brackish-water conditions throughout SA in the McMurray Formation during low river flow conditions.Together,these data suggest that a broad FTT existed in the"A"Valley,with fluvial-dominated channels to the south that experienced seasonal brackish-water inundation during base flow,and channels experiencing increasing brackish-water influence lying further north towards a turbidity maximun zone.The FIT zone appears to have extended for several hundred kilometers fron south to north.Based on the sedimentological and ichnological data,as well as estimations of lateral accretion rates,we refute the colmonly applied Mississippi River depositional analogue for McMurray Formation channels.Rather,we show that while not a perfect fit,the tidally influenced Fraser River shows much greater agreement with the depositional character recorded in McMurray Formation IHS.Future work on the McMurray system should focus on characterizing tide-dominatecl deltaic and estuarine systems,such as the Ganges-Brahmaputra,and on forward-modeling the evolution of tide-dominated and tideinfluenced river systems.展开更多
Detailed fieldwork carried out in the southern part of Bida Basin, Nigeria, allowed the documentation of soft sediment deformation structures (SSDS) in the Maastrichtian Patti Formation. The aim of this study is to ex...Detailed fieldwork carried out in the southern part of Bida Basin, Nigeria, allowed the documentation of soft sediment deformation structures (SSDS) in the Maastrichtian Patti Formation. The aim of this study is to examine the sedimentary successions, describe and analyse these deformation features, discuss their deformation mechanisms and potential triggers. The Maastrichtian Patti Formation is composed of lithofacies interpreted to have been deposited in tidal and fluvial sedimentary environments. Soft sediment deformation structures recognised in the tidal sediments were clastic dykes, load cast, isolated sand balls, dish-and-pillar structures, convolute lamination, diapiric structures and recumbent folds. Severely deformed cross beds, ring structures, associated sand balls, normal folds and recumbent folds were identified in the fluvial sediments. SSDS recognised were interpreted to have been caused by effects of liquefaction and fluidization. Field observations, facies analysis and morphology of the SSDS indicate that there are relationship between the depositional environments and SSDS. Endogenic processes are considered as the trigger agents and they are represented by rapid sedimentation and overloading, impact of breaking waves, pressure fluctuations caused by turbulent water flow, cyclic stress and current generated by storm waves and changes in water table. The present study did not identify exogenic processes as trigger agent. The occurrence of SSDS in southern Bida Basin strongly favoured a non-tectonic origin but a clear relationship high energy processes in tidal and fluvial depositional environments.展开更多
Records of palaeoearthquakes in sedimentary rocks are often debated due to the potential confusion in distinguishing seismic versus aseismic trigger mechanisms causing liquefaction.The present paper documents some uni...Records of palaeoearthquakes in sedimentary rocks are often debated due to the potential confusion in distinguishing seismic versus aseismic trigger mechanisms causing liquefaction.The present paper documents some unique soft-sediment deformation structures(SSDS),characterized by their extremely large size,complex morphology and preservation in coarse-grained pebbly sandstone.The SSDS are present in the Permian Barren Measures Formation,a syn-rift depositional unit within the Lower Gondwana succession in the Pranhita-Godavari Valley,eastern Peninsular India.The^210 m thick succession is represented by stacked fining-up retrogradational cycles,characterized by coarse-grained trough cross-stratified pebbly sandstones near the base and fine-grained heterolithic sandstones–mudstones at the top.Each cycle signifies a change from fluvial-to tidal-influenced depositional systems.Coarse-grained pebbly sandstone beds near the base of each cycle record most SSDS,including complexly deformed layers,pseudonodules,load and flame structures,various water-escape structures like vertical/inclined sediment columns(sedimentary dykes)and contorted beds,and syn-sedimentary faults.The deformed beds are underlain and overlain by the undeformed beds.Complexly deformed SSDS are often sharply truncated at the top by undeformed beds manifesting syn-sedimentary character,which signifies that deformation took place just after deposition of the affected beds,but before deposition of the overlying beds while sedimentation was continuous.Facies analysis reveals the absence of processes like storms/pounding waves,slumps,rapid dumping(massive beds),impact shaking,volcanisms,tsunami waves or sediment gravity flows in the study area,thus negating their possibility as triggering agents for the liquefaction.The complex nature and large size of the deformation structures imply extensive liquefaction near the sediment–water interface.In addition,the deformed beds comply with most of the criteria of typical seismites.In half-graben type Gondwana basins,such seismites can be linked to palaeoearthquakes,which signify the phases of syn-rift fault reactivation,basinal sagging and associated accommodation changes.展开更多
基金The Natural Sciences and Engineering Research Council of Canada and Carbon Management Canada funded the rock record portion of this study
文摘The fluvial-tidal transition(FTT)is a complex depositional zone,where fluvial flow is modified by tides as rivers approach a receiving marine basin.Variations in the relative importance of tidal versus fluvial processes lead to a distinctive distribution of sediments that accumulate on channel bars.The FTT generally consists of three broad zones:(1)a freshwater-tidal zone;(2)a tidally influenced freshwater to brackish-water transition:and(3)a zone of relatively sustained brackish-water conditions with stronger tides.A very common type of deposit through the fluvial-tidal transition,especially on the margins of migrating channels,is inclined heterolithic stratification(IHS).At present,a detailed account of changes in the character of IHS across the FIT of a paleo-channel system has not been reported,although a number of modern examples have been documented.To fill this gap,we quantitatively assess the sedimentology and ichnology of IHS from seven cored intervals in three geographic areas situated within the youngest paleovalley("A"Valley)in the Lower Cretaceous McMurray Formation of Alberta.Canada.We compare the data to trends defined along the FTT in the present-day Fraser River in British Columbia.Canada to interpret paleo-depositional position in the ancient fluvial-tidal channels.Analysis determined that the mean mudstone thickness is 8.2 cm in the southern study area(SA).Mean thickness increases to 11 cm in the central study area(CA),and decreases again to 4.4 cm in the northern study area(NA).The proportion of mudstone is 31%in SA,44%in CA,and 27%in NA.Thicknessweighted mean bioturbation intensity in sands varied from 0.29 in SA and CA.to 0.28 in NA.On the other hand,thickness-weighted mean bioturbation intensity(Bl)in mudstone increases from 1.46 in SA.to 1.77 in CA.and is 1.94 in NA.The ichnological diversity also increased from south to north.Sedimentological results show sinilar trends to those of the Fraser River,enabling the identification of a freshwater to brackish-water transition zone with tidal influence.The interpreted position of the transition is underpinned by the bioturbation intensity and trace-fossil diversity trends,indicating periodic brackish-water conditions throughout SA in the McMurray Formation during low river flow conditions.Together,these data suggest that a broad FTT existed in the"A"Valley,with fluvial-dominated channels to the south that experienced seasonal brackish-water inundation during base flow,and channels experiencing increasing brackish-water influence lying further north towards a turbidity maximun zone.The FIT zone appears to have extended for several hundred kilometers fron south to north.Based on the sedimentological and ichnological data,as well as estimations of lateral accretion rates,we refute the colmonly applied Mississippi River depositional analogue for McMurray Formation channels.Rather,we show that while not a perfect fit,the tidally influenced Fraser River shows much greater agreement with the depositional character recorded in McMurray Formation IHS.Future work on the McMurray system should focus on characterizing tide-dominatecl deltaic and estuarine systems,such as the Ganges-Brahmaputra,and on forward-modeling the evolution of tide-dominated and tideinfluenced river systems.
文摘Detailed fieldwork carried out in the southern part of Bida Basin, Nigeria, allowed the documentation of soft sediment deformation structures (SSDS) in the Maastrichtian Patti Formation. The aim of this study is to examine the sedimentary successions, describe and analyse these deformation features, discuss their deformation mechanisms and potential triggers. The Maastrichtian Patti Formation is composed of lithofacies interpreted to have been deposited in tidal and fluvial sedimentary environments. Soft sediment deformation structures recognised in the tidal sediments were clastic dykes, load cast, isolated sand balls, dish-and-pillar structures, convolute lamination, diapiric structures and recumbent folds. Severely deformed cross beds, ring structures, associated sand balls, normal folds and recumbent folds were identified in the fluvial sediments. SSDS recognised were interpreted to have been caused by effects of liquefaction and fluidization. Field observations, facies analysis and morphology of the SSDS indicate that there are relationship between the depositional environments and SSDS. Endogenic processes are considered as the trigger agents and they are represented by rapid sedimentation and overloading, impact of breaking waves, pressure fluctuations caused by turbulent water flow, cyclic stress and current generated by storm waves and changes in water table. The present study did not identify exogenic processes as trigger agent. The occurrence of SSDS in southern Bida Basin strongly favoured a non-tectonic origin but a clear relationship high energy processes in tidal and fluvial depositional environments.
基金supported by the Institute (MHRD) Fellowship provided to AS by the Indian Institute of Technology,Roorkee,India。
文摘Records of palaeoearthquakes in sedimentary rocks are often debated due to the potential confusion in distinguishing seismic versus aseismic trigger mechanisms causing liquefaction.The present paper documents some unique soft-sediment deformation structures(SSDS),characterized by their extremely large size,complex morphology and preservation in coarse-grained pebbly sandstone.The SSDS are present in the Permian Barren Measures Formation,a syn-rift depositional unit within the Lower Gondwana succession in the Pranhita-Godavari Valley,eastern Peninsular India.The^210 m thick succession is represented by stacked fining-up retrogradational cycles,characterized by coarse-grained trough cross-stratified pebbly sandstones near the base and fine-grained heterolithic sandstones–mudstones at the top.Each cycle signifies a change from fluvial-to tidal-influenced depositional systems.Coarse-grained pebbly sandstone beds near the base of each cycle record most SSDS,including complexly deformed layers,pseudonodules,load and flame structures,various water-escape structures like vertical/inclined sediment columns(sedimentary dykes)and contorted beds,and syn-sedimentary faults.The deformed beds are underlain and overlain by the undeformed beds.Complexly deformed SSDS are often sharply truncated at the top by undeformed beds manifesting syn-sedimentary character,which signifies that deformation took place just after deposition of the affected beds,but before deposition of the overlying beds while sedimentation was continuous.Facies analysis reveals the absence of processes like storms/pounding waves,slumps,rapid dumping(massive beds),impact shaking,volcanisms,tsunami waves or sediment gravity flows in the study area,thus negating their possibility as triggering agents for the liquefaction.The complex nature and large size of the deformation structures imply extensive liquefaction near the sediment–water interface.In addition,the deformed beds comply with most of the criteria of typical seismites.In half-graben type Gondwana basins,such seismites can be linked to palaeoearthquakes,which signify the phases of syn-rift fault reactivation,basinal sagging and associated accommodation changes.
