Sequence stratigraphic concepts and methods provide novel tools for performing stratigraphic analysis,allowing us to improve our understanding of depositional models and basin evolution.Main controls and depositional ...Sequence stratigraphic concepts and methods provide novel tools for performing stratigraphic analysis,allowing us to improve our understanding of depositional models and basin evolution.Main controls and depositional elements(e.g.,surfaces,systems tracts,parasequences,etc.)recognized in conventional sequence stratigraphy are designed for marine-related systems.In contrast,the sequence stratigraphy of lacustrine successions is much more complex and poorly understood,because it is not driven by sea-level changes,but by a complex interaction between tectonics and high-frequency climatic cycles.The comprehensive analysis of the water balance of lacustrine systems allows the recognition of three types of lake conditions:Underfilled,balanced-fill and overfilled lakes.Understanding the lake conditions in ancient successions is fundamental for unraveling lacustrine sequence stratigraphy,since these conditions effectively control water salinity,internal stacking pattern and the characteristics of systems tracts.Underfilled lakes are hydrologically closed lakes,and consequently,the lake-level can highly fluctuate,driven by high-frequency wet-dry climatic cycles.During wet periods,rivers supply water and sediments,resulting in fining-and thinning-upward elementary depositional sequences(EDS's)accumulated during the transgressive systems tract(TST).In contrast,dry periods are characterized by a relative lake-level fall with the subaerial exposure of lake margin areas during the regressive systems tract(RST).Lake water salinity can fluctuate from brackish to hypersaline.Balanced-fill lakes are partially closed lakes,and consequently,they have characteristics of both underfilled and overfilled lakes.During the TST,the lake is in underfilled condition,and consequently,the introduction of water and sediment will accumulate a fining-upward interval until reaching the spill point during the maximum flooding.The RST is accumulated under an overfilled lake condition,with coarseningupward progradational littoral deltas and related subaqueous delta deposits.Lake water salinity fluctuates from brackish to freshwater.Overfilled lakes are hydrologically open lakes.Most deposits accumulate during the RST,forming coarsening-upward progradational littoral deposits,with associated subaqueous deltas.All overfilled lakes are freshwater lakes.Subsidence is crucial for allowing the long-term preservation of lacustrine deposits.Lakes can temporarily store water and sediments in areas that lack subsidence,but these deposits will not be preserved in the stratigraphic record.Consequently,two types of lakes are recognized:1)subsiding lakes,which have permanent accommodation space and 2)hanging lakes,having temporary accommodation space.Although they cannot permanently store sediments,hanging lakes are fundamental for lacustrine sequence stratigraphy,since they can flood subsiding lakes with the near-instantaneous release of a substantial volume of water,creating favorable conditions for the accumulation of organic-rich shales.This rapid flooding from hanging lakes induced a forced transgression(FT),which is a large-scale rapid transgression(xenoconformity)not related to the normal sediment and water supply from local source areas.展开更多
Deltas constitute complex depositional systems formed when a land-derived gravity-flow(carrying water and sediments) discharges into a marine or lacustrine standing body of water. However, the complexity of deltaic se...Deltas constitute complex depositional systems formed when a land-derived gravity-flow(carrying water and sediments) discharges into a marine or lacustrine standing body of water. However, the complexity of deltaic sedimentary environments has been oversimplified by geoscientists over the years, considering just littoral deltas as the unique possible type of delta in natural systems. Nevertheless, a rational analysis suggests that deltas can be much more complex. In fact, the characteristics of deltaic deposits will depend on a complex interplay between the bulk density of the incoming flow and the salinity of the receiving water body. This paper explores the natural conditions of deltaic sedimentation according to different density contrasts. The rational analysis of deltaic systems allows to recognize three main fields for deltaic sedimentation, corresponding to(1) hypopycnal(2) homopycnal and(3) hyperpycnal delta settings. The hypopycnal delta field represents the situation when the bulk density of the incoming flow is lower than the density of the water in the basin. According to the salinity of the receiving water body, three different types of hypopycnal littoral deltas are recognized: hypersaline littoral deltas(HSLD), marine littoral deltas(MLD), and brackish littoral deltas(BLD). The basin salinity will determine the capacity of the delta for producing effective buoyant plumes, and consequently the characteristics and extension of prodelta deposits.Homopycnal littoral deltas(HOLD) form when the density of the incoming flow is roughly similar to the density of the water in the receiving basin. This situation is typical of clean bedload-dominated rivers entering freshwater lakes. Delta front deposits are dominated by sediment avalanches. Typical fallout prodelta deposits are absent or poorly developed since no buoyant plumes are generated. Hyperpycnal deltas form when the bulk density of the incoming flow is higher than the density of the water in the receiving basin. The interaction between flow type,flow density(due to the concentration of suspended sediments) and basin salinity defines three types of deltas,corresponding to hyperpycnal littoral deltas(HLD), hyperpycnal subaqueous deltas(HSD), and hyperpycnal fan deltas(HFD). Hyperpycnal littoral deltas are low-gradient shallow-water deltas formed when dirty rivers enter into brackish or normal-salinity marine basins, typically in wave or tide-dominated epicontinental seas or brackish lakes.Hyperpycnal subaqueous deltas represent the most common type of hyperpycnal delta, with channels and lobes generated in marine and lacustrine settings during long-lasting sediment-laden river-flood discharges. Finally,hyperpycnal fan deltas are subaqueous delta systems generated on high-gradient lacustrine or marine settings by episodic high-density fluvial discharges.展开更多
文摘Sequence stratigraphic concepts and methods provide novel tools for performing stratigraphic analysis,allowing us to improve our understanding of depositional models and basin evolution.Main controls and depositional elements(e.g.,surfaces,systems tracts,parasequences,etc.)recognized in conventional sequence stratigraphy are designed for marine-related systems.In contrast,the sequence stratigraphy of lacustrine successions is much more complex and poorly understood,because it is not driven by sea-level changes,but by a complex interaction between tectonics and high-frequency climatic cycles.The comprehensive analysis of the water balance of lacustrine systems allows the recognition of three types of lake conditions:Underfilled,balanced-fill and overfilled lakes.Understanding the lake conditions in ancient successions is fundamental for unraveling lacustrine sequence stratigraphy,since these conditions effectively control water salinity,internal stacking pattern and the characteristics of systems tracts.Underfilled lakes are hydrologically closed lakes,and consequently,the lake-level can highly fluctuate,driven by high-frequency wet-dry climatic cycles.During wet periods,rivers supply water and sediments,resulting in fining-and thinning-upward elementary depositional sequences(EDS's)accumulated during the transgressive systems tract(TST).In contrast,dry periods are characterized by a relative lake-level fall with the subaerial exposure of lake margin areas during the regressive systems tract(RST).Lake water salinity can fluctuate from brackish to hypersaline.Balanced-fill lakes are partially closed lakes,and consequently,they have characteristics of both underfilled and overfilled lakes.During the TST,the lake is in underfilled condition,and consequently,the introduction of water and sediment will accumulate a fining-upward interval until reaching the spill point during the maximum flooding.The RST is accumulated under an overfilled lake condition,with coarseningupward progradational littoral deltas and related subaqueous delta deposits.Lake water salinity fluctuates from brackish to freshwater.Overfilled lakes are hydrologically open lakes.Most deposits accumulate during the RST,forming coarsening-upward progradational littoral deposits,with associated subaqueous deltas.All overfilled lakes are freshwater lakes.Subsidence is crucial for allowing the long-term preservation of lacustrine deposits.Lakes can temporarily store water and sediments in areas that lack subsidence,but these deposits will not be preserved in the stratigraphic record.Consequently,two types of lakes are recognized:1)subsiding lakes,which have permanent accommodation space and 2)hanging lakes,having temporary accommodation space.Although they cannot permanently store sediments,hanging lakes are fundamental for lacustrine sequence stratigraphy,since they can flood subsiding lakes with the near-instantaneous release of a substantial volume of water,creating favorable conditions for the accumulation of organic-rich shales.This rapid flooding from hanging lakes induced a forced transgression(FT),which is a large-scale rapid transgression(xenoconformity)not related to the normal sediment and water supply from local source areas.
基金the continuous support provided by the Departamento de Geologia de la Universidad Nacional del Sur and the CONICET (National Research Council from Argentina)。
文摘Deltas constitute complex depositional systems formed when a land-derived gravity-flow(carrying water and sediments) discharges into a marine or lacustrine standing body of water. However, the complexity of deltaic sedimentary environments has been oversimplified by geoscientists over the years, considering just littoral deltas as the unique possible type of delta in natural systems. Nevertheless, a rational analysis suggests that deltas can be much more complex. In fact, the characteristics of deltaic deposits will depend on a complex interplay between the bulk density of the incoming flow and the salinity of the receiving water body. This paper explores the natural conditions of deltaic sedimentation according to different density contrasts. The rational analysis of deltaic systems allows to recognize three main fields for deltaic sedimentation, corresponding to(1) hypopycnal(2) homopycnal and(3) hyperpycnal delta settings. The hypopycnal delta field represents the situation when the bulk density of the incoming flow is lower than the density of the water in the basin. According to the salinity of the receiving water body, three different types of hypopycnal littoral deltas are recognized: hypersaline littoral deltas(HSLD), marine littoral deltas(MLD), and brackish littoral deltas(BLD). The basin salinity will determine the capacity of the delta for producing effective buoyant plumes, and consequently the characteristics and extension of prodelta deposits.Homopycnal littoral deltas(HOLD) form when the density of the incoming flow is roughly similar to the density of the water in the receiving basin. This situation is typical of clean bedload-dominated rivers entering freshwater lakes. Delta front deposits are dominated by sediment avalanches. Typical fallout prodelta deposits are absent or poorly developed since no buoyant plumes are generated. Hyperpycnal deltas form when the bulk density of the incoming flow is higher than the density of the water in the receiving basin. The interaction between flow type,flow density(due to the concentration of suspended sediments) and basin salinity defines three types of deltas,corresponding to hyperpycnal littoral deltas(HLD), hyperpycnal subaqueous deltas(HSD), and hyperpycnal fan deltas(HFD). Hyperpycnal littoral deltas are low-gradient shallow-water deltas formed when dirty rivers enter into brackish or normal-salinity marine basins, typically in wave or tide-dominated epicontinental seas or brackish lakes.Hyperpycnal subaqueous deltas represent the most common type of hyperpycnal delta, with channels and lobes generated in marine and lacustrine settings during long-lasting sediment-laden river-flood discharges. Finally,hyperpycnal fan deltas are subaqueous delta systems generated on high-gradient lacustrine or marine settings by episodic high-density fluvial discharges.
