The early sequence model of emphasized the accumulation of fluvial deposits during the late highstand phase of the sea-level cycle. The model suggested that the longitudinal profile of rivers that are graded to sea level would shift seaward during a fall in base level, and that this would generate accommodation for the accumulation of nonmarine sediments. This idea was examined critically and was described scenarios where this may and may not occur. The response of fluvial systems to changes in base level was examined from a geomorphic perspective in greater detail, and the sequence stratigraphy of nonmarine deposits was critically reviewed. In general, the importance of base-level change diminishes upstream. In large rivers, such as the Mississippi, the evidence from the Quaternary record indicates that sea-level changes affect aggradation and degradation as far upstream as the region of Natchez, Mississippi, about 220 km upstream from the present mouth. Farther upstream than this, source-area effects, including changes in discharge and sediment supply, resulting from tectonism and climate change, are much more important. In the Colorado River of Texas base-level influence extends about 90 km upstream, beyond which the river has been affected primarily by the climate changes of Neogene glaciations. At some point upstream rivers become completely independent of higher order relative changes in base level, and are responding to a tectonically controlled long-term average base level of erosion. The response of river systems to climate change is complex. Cycles of aggradation and degradation in inland areas may be driven by changes in discharge and sediment load, which are in part climate dependent. These cycles may be completely out of phase with those driven primarily by base-level change. The sequence boundary is commonly an incised valley eroded during the falling stage of the base-level cycle. This valley is filled with fluvial or estuarine deposits during the lowstand to transgressive part of the cycle, with the thickness and facies composition of these beds determined by the balance between the rates of subsidence, base-level change and sediment supply. Away from the incised valley, on interfluve areas, the sequence boundary may be marked by well-developed paleosol horizons. It is a matter of debate whether the fluvial fill of an incised valley should be assigned to the lowstand or the transgressive systems tract. The shape of the sea-level curve and the timing of these deposits relative to this curve are usually not knowable, and so this is a somewhat hypothetical argument.
Transgression is commonly indicated by the appearance of abundant tidal influence in the fluvial succession. Sigmoidal cross bedding, tidal bedding (wavy, flaser and lenticular bedding), oyster beds and brackish to marine trace fossils are all typical indicators of tidal-marine environments. The transition from fluvial to tidal is typically diachronous, and the filling of the incised valley changes from aggradational to retrogradational. Inland from tidal influence, the change from the lowstand to the transgressive phase may be marked by a change in fluvial style or by the development of coal beds. Coal commonly occurs during an initial increase in accommodation, before this is balanced by an increase in clastic supply. Within the valleys of major rivers, the increase in accommodation can result in more loose stacking of channel sand bodies and greater preservation of overbank fines. Changes in fluvial style are also common, with braided rivers typifying lowstand systems and anastomosed or meandering rivers common during times of high rate of generation of accommodation, as during the transgressive phase of the base-level cycle.
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| Meandering river |
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| Anastamous |
A highstand systems tract develops when base-level rise slows, and the rate of generation of accommodation space decreases to a minimum. There are two possible depositional scenarios for this phase of sequence development. Retrogradation of the river systems during transgression will have led to reduced slopes, and a low-energy landscape undergoing slow accumulation of floodplain deposits, limited channel aggradation, and closely-spaced, well-developed soil profiles. Given no change in source-area conditions, however, the sediment supply into the basin will continue, and vigorous channel systems will eventually be re-established. Under these conditions, channel bodies will form that show reduced vertical separation relative to the TST, leading to lateral amalgamation of sandstone units and high net-to-gross sandstone ratios. Basinward progradation of coastal depositional systems leads to downlap of deltaic and barrier-strandplain deposits onto the maximum flooding surface. It seems likely that the HST will be poorly represented in most nonmarine basins, because the highstand is usually followed by the next cycle of falling base level, which may result in the removal by subaerial erosion of much or all of the just-formed HST deposits. A minor increase in the sand-shale ratio immediately below the sequence boundary may be the only indication of the highstand phase, as in the Castlegate Sandstone of Utah.
Care must be taken to evaluate all the evidence in interpreting such data as net-to-gross sandstone ratios. Changes in this parameter may not always be attributable to changes in the rate of generation of accommodation space. An increase in the proportion of channel sandstones in a section seems to have been related not to changes in the rate of generation of accommodation space, but to increased sediment runoff resulting from increased rainfall. In the case of sequences driven by orbital forcing mechanisms, where both base-level change and climate change may be involved, unravelling the complexity of causes and effects is likely to be a continuing challenge. Rising base level is the main control on the rate and style of channel stacking, the rate of generation of accommodation is small during transgression in inland areas while the coastline is still distant, and increases only once transgression has brought the coastline farther inland where the effect of baselevel rise on the lower reaches of the river produces a more rapid increase in accommodation. The rate of generation of accommodation is greater during the highstand than during transgression, and results in low net-to-gross sandstone ratios. However, this line of reasoning omits the influence of upstream factors, and must therefore not be followed dogmatically. One must also be cautious in using systems-tract terminology derived from marine processes for the labeling of nonmarine events. There may be a considerable lag in the transmission of a transgression upstream to inland positions by the process of slope reduction, aggradation and tidal invasion. The inland reaches of the river will not “know” that a transgression is occurring, and it is questionable, therefore, whether the deposits formed inland during the initial stages of the marine transgression should be included with the TST. An alternative terminology for the standard systems tract terms used for marine basins because, obviously, terms that include such words as transgressive, highstand, etc., are inappropriate for basins that are entirely nonmarine. For falling stage and lowstand deposits the term degradational systems tract, for transgressive deposits, transitional systems tract, and for highstand deposits, aggradational systems tract. These terms provide analogous ideas regarding changes in accommodation and sediment supply and their consequences for depositional style.
In marine basins this will be marine base level (sea level). In inland basins it will be the lip or edge of a basin through which the trunk river flows out of the basin. The buffer is the zone of space above and below the current graded profile which represents the range of reactions that the profile may exhibit given changes in upstream controls, such as tectonism or climate change, that govern the discharge and sediment load of the river. For example, tectonic uplift may increase the sediment load, causing the river to aggrade towards its upper buffer limit. A drop in the buttress, for example as a result of a fall in sea level, may result in incision of the river system, but if the continental shelf newly exposed by the fall in sea level has a similar slope to that of the river profile, there may be little change in the fluvial style of the river. In any of these cases, the response of the river system is to erode or aggrade towards a new dynamically maintained equilibrium profile that balances out the water and sediment flux and the rate of change in accommodation. The zone between the upper and lower limits is the buffer zone, and represents the available (potential) preservation space for the fluvial system. Nowhere within coastal fluvial systems is there a single erosion surface that can be related to lowstand erosion. Such surfaces are continually modified by channel scour, even during transgression, because episodes of channel incision may reflect climatically controlled times of low sediment load, which are not synchronous with changes in base level. This is particularly evident landward of the limit of base-level influence. Post-glacial terraces within inland river valleys reveal a history of alternating aggradation and channel incision reflecting climate changes, all of which occurred during the last post-glacial rise in sea level. A major episode of valley incision occurred in Texas not during the time of glacioeustatic sea-level lowstand, but at the beginning of the postglacial sea-level rise, which commenced at about 15 ka. The implications of this have yet to be resolved for inland basins where aggradation occurs (because of tectonic subsidence), rather than incision and terrace formation. However, it would seem to suggest that no simple relationship between major bounding surfaces and base level change should be expected. Fluvial processes in relationship to glacially controlled changes in climate and vegetation, based on Dutch work. These studies, and those in Texas, deal with periglacial regions, where climate change was pronounced, but the areas were not directly affected by glaciation. A major period of incision occurred during the transition from cold to warm phases because run off increased while sediment yield remained low. Vegetation was quickly able to stabilize river banks, reducing sediment delivery, while evapotranspiration remained low, so that the run off was high. Fluvial styles in aggrading valleys tend to change from braided during glacial phases to meandering during interglacials. With increasing warmth, and consequently increasing vegetation density, rivers of anastomosed or meandering style tend to develop, the former particularly in coastal areas where the rate of generation of new accommodation space is high during the period of rapidly rising base level. Valley incision tends to occur during the transition from warm to cold phases. Reduced evapotranspiration consequent upon the cooling temperatures occurs while the vegetation cover is still substantial. Therefore run off increases, while sediment yield remains low. With reduction in vegetation cover as the cold phase becomes established, sediment deliveries increase, and fluvial aggradation is reestablished.
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| Channel incision |
It is apparent that fluvial processes inland and those along the coast may be completely out of phase during the climatic and base-level changes accompanying glacial to interglacial cycles. Within a few tens of kilometres of the sea, valley incision occurs at times of base level lowstand, during cold phases, but the surface may be modified and deepened during the subsequent transgression until it is finally buried. Inland, major erosional bounding surfaces correlate to times of climatic transition, from cold to warm and from warm to cold, that is to say during times of rising and falling sea level, respectively. The Dakota Group of north-east New Mexico and southeast Colorado provides a good example of an internally architecturally complex fluvial unit generated by a combination of upstream tectonic controls and downstream sea-level cycles. At the coastline, progradation and retrogradation creating three sequences were caused by sea-level cycles on a 105-year time scale. Each of these sequences can be traced up dip towards the west,where they are composed of repeated cycles of aggradational valley-fill successions and mutually incised scour surfaces. These cycles reflect autogenic channel shifting within the limited preservation space available under conditions of modest, tectonically-generated accommodation. This space is defined by a lower buffer set by maximum local channel scour, and an upper buffer set by the ability of the river to aggrade under the prevailing conditions of discharge and sediment load.
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