Sequence Stratigraphy(Part-II)
b) For a Muddy Siliciclastic
A parasequence developed on a muddy siliciclastic shoreline would have a different suite of facies, but they would also be arrayed vertically in a shallowing upward order and facies relationships would obey Walther's Law. A typical muddy shoreline parasequence would start with cross-bedded subtidal sands, continue with interbedded bioturbated mudstones and rippled sands of the intertidal, and pass upwards into entirely bioturbated and possibly coaly mudstones of the supratidal.
Parasequence with respect to Flooding Surface
The flooding surfaces that define the top and base of a parasequence display abrupt contacts of relatively deeper-water facies lying directly on top of relatively shallow-water facies. Rocks lying above and below a flooding surface commonly represent non-adjacent facies, such as offshore shales directly overlying foreshore sands or basinal shales directly overlying mid-fan turbidites. Thus, Walther's Law cannot be applied across flooding surfaces. Given that many parasequences are meters to tens of meters thick; this radically reduces the scale at which Walther's Law can be applied. Cases where Walther's Law has been applied to sections hundreds to thousands of meters thick are nearly always incorrect.
Flooding Surface Interpretations
Flooding surfaces may exhibit ;
Ø Small scale erosion, usually of a meter or less.
Ø Flooding surfaces may be mantled by a transgressive lag composed of shells, shale intraclasts, calcareous nodules, or siliciclastic gravel; such lags are usually thin, less than a meter thick.
Ø Flooding surfaces may display evidence of firmgrounds, such as Glossifungites ichnofacies, or hardgrounds that may be bored, encrusted, and possibly mineralized.
Lateral and Vertical Relationships within a Parasequence
One of the most powerful aspects to recognizing parasequences is understanding and applying the predictable vertical and lateral facies relationships within parasequences. Facies reflect increasingly shallower environments upwards within a parasequence. Although a complete vertical succession of facies can be compiled from a suite of parasequences, most parasequences will display only a portion of the entire shallowing-upward succession of facies.
Shallow and Deeper Water Facies
Because shallow water facies within a parasequence will pinch out laterally in a downdip direction and deeper water facies within a parasequence will pinch out in an updip direction, the facies composition of a single parasequence changes predictably updip and downdip. Thus, a single parasequence will not be composed of the same facies everywhere, but will be composed of deeper water facies downdip and shallower water facies updip, as would be expected. Because parasequence boundaries represent a primary depositional surface, that is, topography at the time of deposition, flooding surfaces will tend to be relatively flat but dip slightly seaward at angles typical of continental shelves. Finally, parasequence boundaries may become obscure in coastal plain settings and in deep marine settings because of a lack of facies contrast necessary to make flooding surfaces visible.
Parasequence Sets and Stacking Patterns
In most cases, there will not be simply one parasequence by itself, but there will be a series of parasequences. Sets of successive parasequences may display consistent trends in thickness and facies composition and these sets may be progradational, aggradational, or retrogradational.
a- Progradational Stacking
In a progradational set of parasequences, each parasequence builds out or advances somewhat farther seaward than the parasequence before. Because of this, each parasequence contains a somewhat shallower set of facies than the parasequence before. This produces an overall shallowing-upward trend within the entire parasequence set and the set is referred to as a progradational parasequence set or is said to display progradational stacking. The overall rate of deposition is greater than the rate of accommodation in progradational stacking pattern.
Recognition in a Single Outcrop
In a single outcrop, a progradational parasequence set can be recognized by the progressive appearance of shallower-water facies upward in the parasequence set as well as the progressive loss of deeper-water facies upward in the parasequence set. For example, in a set of progradationally stacked parasequences, perhaps all of the parasequences contain shoreface and foreshore facies, but only the uppermost parasequences may contain the coastal plain coal, and only the lowermost parasequences may contain offshore and transition zone facies.
Recognition in a Cross Section
In a cross-section, a progradational parasequence set can be recognized by the seaward movement of a particular facies contact at an equivalent position in a parasequence. For example, the contact between the shoreline sands and the coastal plain facies at the top of each parasequence will appear to move farther basinward in each successive parasequence. Likewise, the same contact at the base of each parasequence will appear to move farther basinward in each successive parasequence.
b- Aggradational Stacking
In an aggradational set of parsequences, successively younger parasequences are deposited above one another with no significant lateral sifts. The rate of accommodation approximates the rate of deposition. Thus, each parasequence contains essentially the same suite of facies as the parasequences above and below. This lack of overall facies change results in no net vertical trend in water depth. Such a set is called an aggradational parasequence set or is said to display aggradational stacking.
Recognition in a Single Outcrop
In a single outcrop, an aggradational parasequence set can be recognized by the similarity of facies composition in each successive parasequence. No new deeper or shallower water facies will tend to appear near the top or base of the parasequence set.
Recognition in a Cross Section
In a cross-section, an aggradational parasequence set can be recognized by the relative stability of any particular facies contact at an equivalent position in a parasequence. For example, the contact between the shoreline sands and the coastal plain facies at the top of each parasequence will appear to stay at essentially the same position in each successive parasequence. Facies contacts rarely remain at exactly the same position, so aggradational parasequence sets are commonly characterized by relatively minor facies shifts that display no clear long-term trend.
c- Retrogradational Stacking
In a retrogradational set of parasequences, each parasequence progrades less than the preceding parasequence. The result is that each parasequence contains a deeper set of facies than the parasequence below. This net facies shift produces an overall deepening upward trend within the entire parasequence set and the set is referred to as retrogradational parasequence set or is said to display retrogradational stacking. Retrogradational stacking is also commonly called backstepping.
Recognition in a Single Outcrop
In a single outcrop, a retrogradational parasequence set can be recognized by the progressive appearance of deeper water facies upwards within the parasequence set as well as the progressive loss of shallower water facies upwards in the parasequence set. For example, in a set of retrogradationally stacked parasequences, offshore facies might be present in only the uppermost parsequences, and coastal plain coals might be present in only the lowermost parasequence.
Recognition in a Cross Section
In a cross-section, a retrogradational parasequence set can be recognized by the landward movement of a particular facies contact at an equivalent position in a parasequence. For example, the contact between the shoreline sands and the coastal plain facies at the top of each parasequence will appear to move farther landward in each successive parasequence.
DEPOSITIONAL SEQUENCES
“A relatively conformable succession of genetically related strata bounded by unconformities or their correlative conformities”.
Explanation
What the definition does emphasize is that every sequence is bounded above and below by unconformities, or by correlative conformities (surfaces that correlate updip to an unconformity). Every depositional sequence is the record of one cycle of relative sea level. Because of this, depositional sequences have a predictable internal structure consisting of major stratal surfaces and systems tracts, which are suites of coexisting depositional systems, such as coastal plains, continental shelves, and submarine fans.
Compositional Setting
In vertical succession, all depositional sequences are composed of the following elements in this order: sequence boundary, lowstand systems tract, transgressive surface, transgressive systems tract, maximum flooding surface, highstand systems tract, and the following sequence boundary. So, it can be divided into two classes as follows,
1- System Tracts
a- Lowstand Systems Tract (LST)
b- Falling Stage Systems Tract (FSST)
c- Transgressive Systems Tract (TST)
d- Highstand Systems Tract (HST)
2- Surfaces
a- Sequence Boundaries
b- Transgressive Surface
c- Maximum Flooding Surface
1- SYSTEMS TRACT
Genetically associated stratigraphic units that were deposited during specific phases of the relative sea-level cycle form a systems tract. It is a linkage of contemporaneous depositional systems. Each system is defined by stratal geometries at bounding surfaces, position within the sequence, and internal parasequence stacking patterns. Different systems tracts represent different phases of eustatic changes.
a. Lowstand Systems Tract (LST)
The Lowstand Systems Tract is bounded by the Falling Stage Systems Tract (FSST) at the base, and the Transgressive Systems Tract (TST) at the top. It means that this systems tract lies directly on the upper surface of the Falling Stage Systems Tract (FSST) and is capped by the transgressive surface formed when the sediments onlap onto the shelf margin. It is overlain by the transgressive surface (TS) of the overlying TST. These Lowstand Systems Tract sediments form a lowstand wedge and often fill incised valleys that cut down into the Highstand Systems Tract. Because sediments are transported from land to sea, discrete depositional packages, called parasequences, are developed. Beach parasequences typically coarsen upward and in the progradational direction change facies from coastal plain (coal and clay) through marginal marine (sandstone) to offshore.
Traditionally the sediments of this systems tract include the deposits that accumulated after the onset of relative sea-level fall. In contrast to the new Lowstand Systems Tract, the traditional Lowstand Systems Tract lies directly on the sequence boundary over the Highstand Systems Tract. The Lowstand Systems Tract can be divided into Early and Late Phases.
Early Phase Lowstand Systems Tract is associated with:
a. Falling stage of relative sea level induced by eustasy falling rapidly and/or tectonic uplift outpacing the rate of change in sea level position
b. Fluvial incision up dip with formation of an unconformity or sequence boundary and the focus of sediment input at the shoreline
c. Forced regressions induced by the lack of accommodation producing stacking patterns of downward stepping prograding clinoforms over the condensed section formed during the previous transgressive and highstand systems tracts
d. Basin floor fans formed from sediment transported from the shelf margin when this fails under the weight of the rapid sediment accumulation associated with the forced regression
e. Onlap of sediments onto the prograding clinoforms below the shelf break
The lower bounding surfaces of the Early Phase Lowstand System Tract are the updip unconformity and the top of the downdip condensed section. These surfaces form by different mechanisms and have different time significance
The top of the downdip condensed section immediately underlies the downlapping prograding clinoforms of the forced regression
The top of the Early Phase Lowstand System Tract in theory is marked by an initial onlap onto the often eroded surface of the prograding clinoforms of the forced regression
Late Phase Lowstand Systems Tract is associated with:
a. A slow relative sea level rise is induced when eustasy begins to rise slowly and/or tectonic uplift slows
b. Sediment is now outpaced by an increase in accommodation and in response the sediment begins to onlap onto the basin margin
c. River profiles stabilize
d. Valleys backfill
e. Prograding lowstand clinoforms form and are capped by topset layers that onlap, aggrade, become thicker upward and landward
Figure 3. Lowstand systems tract
The old Lowstand Systems Tract is divided into the Falling Stage Systems Tract (FSST) with its basin-floor fans, and slope fans while, the Lowstand Systems Tract sediments form lowstand wedges, often filling incised valleys that cut down into the Highstand Systems Tract. Thus a terminology has been used, which suggests that the sediments of this systems tract can be equated with a relative fall in sea level forming the Falling Stage Systems Tract, and we now refer to the Late Lowstand Systems Tract as the Lowstand Systems Tract because this systems tract is equated with only a small rise in sea level and is an essentially lowstand set of deposits.
b. Falling Stage Systems Tract (FSST)
It includes all the regressional deposits that accumulated after the onset of a relative sea-level fall and before the start of the next relative sea-level rise. The Falling Stage Systems Tract is the product of a forced regression. The FSST lies directly on the type-1 sequence boundary and is capped by the overlying Lowstand Systems Tract sediments. This systems tract has also been termed the Early Lowstand Systems Tract (ELST). On seismic data, the upper boundary is the first definable horizon that onlaps the FSST, but when well logs and outcrops are used this boundary is instead recognized as the first marine-flooding surface that overlies the FSST. Coincidently it is often marked by a time transgressive ravinement surface overlain by a sediment lag.
c. Transgressive Systems Tract (TST)
A transgressive systems comprises the deposits that accumulated from the onset of coastal transgression until the time of maximum transgression of the coast, just prior to the renewed regression of the HST. The TST lies directly on the transgressive surface (TS) formed when the sediments onlap the underlying LST and is overlain by the maximum flooding surface (mfs) formed when marine sediments reach their most landward position. Stacking patterns exhibit backstepping onlapping retrogradational clinoforms that thicken landward. In cases where there is high sediment supply the parasequences may be aggradational.
d- Highstand Systems Tract (HST)
These are the progradational deposits that form when sediment accumulation rates exceed the rate of increase in accommodation space. The HST lies directly on the maximum flooding surface (mfs) formed when marine sediments reached their most landward position. This systems tract is capped by a sequence boundary. The base of HST is formed by the maximum flooding surface (mfs) over which the Highstand Systems Tract sediments prograde and aggrade. The top of this systems tract is formed by the eroded unconformity surface that develops when a sea level fall initiates erosion of the now subaerial Highstand system sediment surface and the start of the Falling Stage Systems Tract.
A highstand systems tract is associated with:
1. Slow rise of relative sea level followed by a slow fall; essentially a still stand of base level when the slower rate eustatic change balances that of tectonic motion
2. Sediment outpacing loss of accommodation
3. River Profiles stabilize
4. River valleys are dispersed laterally in a position landward of the shelf margin.
Figure Highstand systems tract
This systems tract is commonly widespread on the shelf and may be characterized by one or more aggradation to progradational parasequence sets with prograding clinoform geometry. They onlap the sequence boundary in a landward direction and downlap the top of the Transgressive and/or Lowstand Systems Tracts in a basinward direction.
TYPE 1 and TYPE 2 Sequences
Not all relative falls in sea level occur at rates fast enough to expose the continental shelf. For example, during a eustatic fall, a rapidly subsiding margin may still experience a relative rise in sea level, provided the rate of eustatic fall is less than the rate of subsidence. Early seismic studies recognized two types of sequences reflecting the case of sea-level fall below the shelf-slope break (type 1) and the case where sea level does not fall below this break (type 2). Although there has been much subsequent confusion about the application of these two types to outcrop studies, their definitions have been modified such that a type 1 sequence now refers to one in which there is a relative fall in sea level below the position of the present shoreline and a type 2 sequence refers to a sequence in which the relative fall in sea level does not force a shift in the position of the shoreline.
Figure adapted from Van Wagoner et al. (1990)
Type 2 sequences (shown below) are similar to type 1 sequences (shown above) in nearly all regards except for the extent of the sequence-bounding unconformity and its expression in the marine realm. In addition, the two sequences differ in the name of the systems tract lying above the sequence boundary but below the transgressive surface.
Figure adapted from Van Wagoner et al. (1990)
In a type 2 sequence, the extent of the sequence-bounding unconformity can reach seaward only to the position of the previous shoreline, but no further. In other words, none of the marine areas of the previous highstand are subaerially exposed during a type 2 sequence boundary. Updip of these areas, the sequence bounding unconformity is expressed as for a type 1 sequence, but no incised valley forms as sea level does not fall far enough for incision. In the marine realm, no basinward shift of facies occurs as in a type 1 sequence, and the type 2 sequence boundary is characterized only by a slight change in stacking patterns from increasingly progradational in the underlying highstand to decreasingly progradational (possibly aggradational) above the sequence boundary. Detecting this subtle transition in marine sections may be difficult to impossible and many type 2 sequence boundaries probably go undetected.
The shelf margin systems tract in a type 2 sequence is equivalent in stratigraphic position to the lowstand systems tract of a type 1 sequence. As stated above, the shelf margin systems tract is characterized by aggradational stacking. Like the lowstand systems tract, the shelf margin systems tract is capped by the transgressive surface.
In general, far more type 1 sequences have been reported than type 2 sequences, possibly in part reflecting their comparative difficulty or ease of detection. Some workers have gone so far as to question the existence of any type 2 sequences.
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