The highstand systems tract (HST):
The highstand systems tract (HST) corresponds to a period when little new accommodation is being added to the depositional environment. Base-level rise is in the process of slowing down as it reaches its highest point, immediately prior to commencement of a slow fall. If the sediment supply remains more or less constant, then Rd>Rs at this point. The most characteristic feature of this systems tract is the lateral progradation of coastal sedimentary environments. Major coastal barrier-lagoon and deltaic complexes are the result. Normal regression is the term used to describe the seaward advance of the coastline as a result of the progressive addition of sediment to the front of the beach or the delta systems, developing a broad topset environment. This is in contrast to the condition of forced regression, which is described below. Where the terrigenous sediment supply is high, delta systems may largely dominate the resulting sedimentary succession, as shown in the accompanying example of the Dunvegan delta. The allomember boundaries in this diagram indicate times of relative low sea level, followed by flooding. Each allomember boundary is overlain by a mudstone representing the maximum flooding surface, over which delta complexes prograded. Sedimentary environments characteristically include coastal mangrove swamps, and may include significant peat swamps, the sites of future coal development. The numbered subdivisions of each allomember indicate individual deltaic shingles. Subsurface mapping may indicate that shingles of this type shifted laterally as a result of delta switching, in a manner similar to the Mississippi delta and the Yellow River delta. This points to potential confusion in terminology, because upward-shoaling successions. The thickness of highstand shelf deposits depends on the accommodation generated by marine transgression across the shelf, typically a few tens of metres, up to a maximum of about 200 m. Where the shelf is narrow or thesediment supply is large, deltas may prograde to the shelf-slope break, at which point deltaic sedimentation may extend down slope into the deep basin. High-amplitude clinoforms may result, including significant volumes of sediment-gravity-flow deposits.
The falling-stage systems tract (FSST).
A fall in base level from the highstand position exposes the coastal plain and then the continental shelf, to subaerial erosion. River mouths retreat seaward, and under most conditions, river valleys incise themselves as they continually grade downward to progressively lower sea levels. Deeply incised paleovalleys may result. In the rock record, many of these show evidence of multiple erosional events, indicating repeated responses to autogenic threshold triggers or perhaps to minor cycles of base level change. Significant volumes of sediment are eroded from the coastal plain and the shelf, and are fed through the coastal fluvial systems and onto the shelf.Eventually, these large sediment volumes may be tipped directly over the edge of the shelf onto the continental slope, triggering submarine landslides, debris flows, and turbidity currents. These have a powerful erosive effect, and may initiate development of submarine erosional valleys at the mouths of the major rivers or off shore from major delta distributaries. Many submarine canyons are initiated by this process, and remain as major routes for sediment dispersal through successive cycles of base-level change. The FSST is typically the major period of growth of submarine fans.
The falling base level causes basinward retreat of the shoreline, a process termed forced regression. The occurrence of forced regression, as distinct from normal regression, may be detected by careful mapping of coastal shoreline sandstone complexes. Fall of sea level causes water depths over the shelf to decrease, increasing the erosive power of waves and tides. This typically leads to the development of a surface called the regressive surface of marine erosion (RSME), which truncates shelf and distal coastal (e.g., deltaic) deposits that had been formed during the preceding highstand phase. The first such surface to form, at the commencement of a phase of sea-level fall, is termed the basal surface of forced regression.. Given an adequate sediment supply, especially if there are pauses during the fall of sea level, shoreface sand accumulates above the RSME, forming what have come to be informally termed sharp-based sandstone bodies. These are internally identical to other coastal, regressive sandstone bodies, except that they rest on an erosion surface instead of grading up from the fine-grained shelf sediments, as in the initial coastal sands (which are the product of normal regression). Repeated pulses of sea-level fall punctuated by still stand may develop several off lapping surfaces of marine erosion. Shelf-margin deltas may form where the mouths of major river systems regress to the shelf-slope break during forced regression. The falling-stage is typically the interval during the sea-level cycle when the sediment supply to the continental shelf and slope is at its greatest. Most sediment accumulation on submarine fans occurs during this and the next phase, the lowstand. Most of the early sequence models showed submarine fans resting on a basal sequence boundary, but this configuration now seems unlikely. On the continental shelf and coastal plain, the sequence boundary is an erosion surface representing the lowest point to which erosion cuts during the falling stage of the base-level cycle. As sea-level fall slows to its lowest point, sediment delivery from the newly exposed coastal plain and shelf will gradually diminish. Sedimentation on submarine fans will correspondingly slow down, and the deposits may show a gradual upward decrease in average grain size. Sedimentation there may virtually cease once the next phase of sea-level rise commences, and the rivers feeding sediment to the slope become flooded (transgressive systems tract). The sequence boundary, therefore, is likely to be contemporaneous with the middle to upper part of the submarine-fan succession, possibly with the top of it. However, there is unlikely to be an actual mappable break in sedimentation at this level, and it may be difficult to impossible to locate the position in the section corresponding to the turnaround from falling to rising sea level. This horizon is therefore called a correlative conformity, although his original application of the term was to the fine-grained sediments formed in deep water beyond the submarine-fan wedge, out in the deep basin where it was assumed sedimentation would be continuous throughout a sea-level cycle.
The sequence boundary (SB) marks the lowest point reached by erosion during the falling stage of the sea level cycle. On land this is represented by a subaerial erosion surface, which may extend far onto the continental shelf, depending on how far sea level falls. The sequence boundary cuts into the deposits of the highstand systems tract and is overlain by the deposits of the lowstand or transgressive systems tracts. It is therefore typically a surface where a marked facies change takes place, usually from a relatively lowerenergy deposit below to a high-energy deposit above. Mapping of such a surface in outcrop or in the subsurface, using well logs, is facilitated by this facies change, except where the boundary juxtaposes fluvial on fluvial facies. In such cases, distinguishing the sequence boundary from other large-scale channel scours may be a difficult undertaking.
The lowstand systems tract (LST):
This systems tract represents the interval of time when sea-level has bottomed out, and depositional trends undergo a shift from seaward-directed (e.g., progradational) to landward-directed (e.g.,retrogradational). Within most depositional systems there is little that may be confidently assigned to the lowstand systems tract. The initial basal fill of incised river valleys, and some of the fill of submarine canyons are deposited during this phase. Volumetrically they are usually of minor importance, but they may be of a coarser grain size than succeeding transgressive deposits. In parts of the incised valley of the Mississippi River, for example (the valley formed during Pleistocene glacioeustatic sea-level lowstands), the basal fill formed during the initial post-glacial transgression is a coarse braided stream deposit, in contrast to the sandy meandering river deposits that form the bulk of the Mississippi river sediments. The episode of active submarine-fan sedimentation on the continental slope and deep basin may persist through the lowstand phase. There may be a phase of normal regressive sedimentation at the lowstand coastline. On coastal plains, the lowstand is a time of still stand, when little erosion or sedimentation takes place. Between the major rivers, on the inter fluve uplands, this may therefore be a place where long-established plant growth and soil development takes place. Peat is unlikely to accumulate because of the lack of accommodation, but soils, corresponding in time to the sequence boundary, may be extensive, and the resulting paleosols may therefore be employed for mapping purposes.
Transgressive systems tract (TST):
A rise in base level is typically accompanied by flooding of incised valleys and transgression across the continental shelf. Base-level rise exceeds sediment supply, leading to retrogradation of depositional systems (Rd<Rs), except that at the mouths of the largest rivers sediment supply may be sufficiently large that deltas may continue to aggrade or prograde. Flooded river valleys are estuaries; they typically provide ample accommodation for sedimentary accumulation. In estuarine successions, the upward transition from lowstand to transgressive systems tract in estuaries and other coastal river systems is commonly marked by the development of wave- or tide influenced fluvial facies, such as tidal sand bars containing sigmoidal crossbedding or flaser bedding. The sedimentology of this environment has received much attention, because of the potential for the development of stratigraphic sandstone traps, in the form of valley-fill ribbon sands. Studies of ancient paleovalley fills have shown that many are complex, indicating repeated cycles of base level change and/or autogenic changes in sediment dispersal. On the continental shelf the most distinctive feature of most transgressive systems tracts is the development of a widespread transgressive surface (TS),a flooding surface covered with an equally widespread marine mudstone. A transgressive conglomeratic or sandy lag may blanket the flooding surface. Offshore, rapid transgression may cut the deep-water environment off from its sediment source, leading to slow sedimentation, and the formation of a condensed section. This is commonly a distinctive facies, consisting of concentrated shell or fish fragments, amalgamated biozones, and a “hot” (high gamma-ray) response on well-logs, reflecting a concentration of radioactive clays. Significant volumes of clastic sediment deposited on the shelf may be reworked during transgression. Numerous complexes of shelf sand ridges constituting parts of shelf transgressive systems tracts that were formed by vigorous wave and tide action. Offshore, limestones may be deposited, such as the several Jurassic and Cretaceous limestones and chalks in the Western Interior Seaway (Greenhorn Limestone, Austin Chalk). In the nearshore setting, wave erosion during transgression is usually the cause of ravinement, with the development of a diachronous ravinement surface. The juxtaposition of marine shelf sediments, above, over coastal shoreline or lagoonal sediments below, creates a prominent surface which should not be confused with a sequence boundary. A ravinement surface marks an upward deepening, the opposite of the facies relationships at most sequence boundaries. In some cases, ravinement erosion may cut down through lowstand deposits and into the underlying highstand systems tract, and in such cases the ravinement surface becomes the sequence boundary. Peat may be deposited on the coastal plain and in deltaic settings at any time during a cycle of base-level change. However, the thickest and most widespread coals are now known to be those formed from peat accumulated during transgression, because of the accommodation provided by rising base level, during a time when clastic influx into the coastal plain is “held back” by the landward-advancing shoreline.
The maximum flooding surface (MFS):
The maximum flooding surface (MFS) marks the end of the phase during which the difference between the rate of sea-level rise and the rate of sediment supply is at its greatest. Sea-level rise continues beyond this point, but as the rate of rise slows, sediment input begins to re-establish progradation at the shoreline, and this defines the transition into the highstand systems tract. The offshore shale formed around the time of the MFS is an excellent mapping marker, because of its widespread nature and distinctive facies. In areas distant from the shoreline, where clastic sediment supply is at a minimum, the MFS is commonly marked by calcareous shale, marl or limestone. In some studies, sequence mapping is accomplished using this surface in preference to the sequence boundary, because of its more predictable facies and its consistent horizontality. The preceding paragraphs constitute a set of useful generalizations. However, there are many exceptions and special cases. For example, consider the ultimate fate of the clastic sediment flux on continental margins during cycles of sea level change. In the traditional model on which this section is largely based, coastal plain complexes, including deltas, typically accumulate during highstand phases, following a period of coastal plain transgression and flooding, and basin slope and plain deposits, including submarine fans, accumulate during the sea-level falling stage and lowstand. However, these generalizations do not necessarily apply to all continental margins. The development of submarine fans on the California border land at times of sea-level high stand. The connection of canyon and fan dispersal systems to the littoral sediment supply is the key control on the timing of deposition in this setting. In addition to the physiographic variations noted here, which complicate the relationship between the base level cycle and systems-tract architecture and development, it is quite possible for episodic changes in systems-tract development at continental margins to have nothing to do with sea-level change at all. To cite two examples, in the case of the modern Amazon fan, the marked facies variations mapped by the ODP bear no relation to Neogene sea-level changes, but reflect autogenic avulsion processes on the upper fan. In the North Sea basin, peaks in submarine fan sedimentation occurred at times of regional uplift of the crust underlying the British Isles, as a consequence of episodes of magma under plating, resulting in increased sediment delivery to the marine realm. There is a possible confusion between the terminology of systems tracts (highstand, falling stage, etc.) and the actual state of the sea-level cycle which they represent.
ليست هناك تعليقات:
Write التعليقات