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
Recent advances in the understanding of deltaic deposits provide new tools for the study and analysis of deltaic deposits in shallow epicontinental seas.After the introduction of sequence stratigraphic concepts,meter-...Recent advances in the understanding of deltaic deposits provide new tools for the study and analysis of deltaic deposits in shallow epicontinental seas.After the introduction of sequence stratigraphic concepts,meter-scale coarsening and thickening upward successions have been considered as“parasequences”originated by high-frequency sea-level changes.Nevertheless,recent studies enhanced the importance of wave-aided low-dense hyperpycnal flows in transporting fine-grained sediments in shallow shelfal areas.These poorly-known(and at the same time very common)types of delta,known as hyperpycnal littoral deltas(HLD),develop very low gradient progradational units,controlled by changes in the sediment supply instead of sea level changes.These small-scale progradational units are very common in shallow epicontinental seas like the Lower Cretaceous Agrio Formation in the Neuquén Basin.This study provides a first detailed analysis of hyperpycnal littoral deltas from the Agua de la Mula Member(upper Hauterivian-lower Barremian)of the Agrio Formation.This unit has been studied in three locations near Bajada del Agrio locality in the central part of the Neuquén Basin,Argentina.Six sandy facies,three heterolithic facies,three muddy facies and four calcareous facies were recognized.From facies analysis,three facies associations could be determined,corresponding to offshore/prodelta,distal ramp delta and proximal ramp delta.The three stratigraphic sections discussed in this study are internally composed of several small-scale sequences showing a coarsening and thickening upward pattern,transitionally going from muddy to sandy wave-dominated facies,and ending with calcareous bioclastics levels on top.These small-scale sequences are interpreted as delta front deposits of wave-influenced hyperpycnal littoral deltas,punctuated by calcareous intervals accumulated during periods of low sediment supply.It is interpreted that the development of hyperpycnal littoral deltas could have been facilitated by a decrease in sea water salinity related to an increasing runoff.展开更多
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.
基金financed by CONICET PUE0047COANPCyT 0419 projects。
文摘Recent advances in the understanding of deltaic deposits provide new tools for the study and analysis of deltaic deposits in shallow epicontinental seas.After the introduction of sequence stratigraphic concepts,meter-scale coarsening and thickening upward successions have been considered as“parasequences”originated by high-frequency sea-level changes.Nevertheless,recent studies enhanced the importance of wave-aided low-dense hyperpycnal flows in transporting fine-grained sediments in shallow shelfal areas.These poorly-known(and at the same time very common)types of delta,known as hyperpycnal littoral deltas(HLD),develop very low gradient progradational units,controlled by changes in the sediment supply instead of sea level changes.These small-scale progradational units are very common in shallow epicontinental seas like the Lower Cretaceous Agrio Formation in the Neuquén Basin.This study provides a first detailed analysis of hyperpycnal littoral deltas from the Agua de la Mula Member(upper Hauterivian-lower Barremian)of the Agrio Formation.This unit has been studied in three locations near Bajada del Agrio locality in the central part of the Neuquén Basin,Argentina.Six sandy facies,three heterolithic facies,three muddy facies and four calcareous facies were recognized.From facies analysis,three facies associations could be determined,corresponding to offshore/prodelta,distal ramp delta and proximal ramp delta.The three stratigraphic sections discussed in this study are internally composed of several small-scale sequences showing a coarsening and thickening upward pattern,transitionally going from muddy to sandy wave-dominated facies,and ending with calcareous bioclastics levels on top.These small-scale sequences are interpreted as delta front deposits of wave-influenced hyperpycnal littoral deltas,punctuated by calcareous intervals accumulated during periods of low sediment supply.It is interpreted that the development of hyperpycnal littoral deltas could have been facilitated by a decrease in sea water salinity related to an increasing runoff.
