Mallorca Exercises 3

Exercise 3 - Sequence stratigraphic interpretation - Sigmoids & Reef Crest trajectory

Low Sea Levels Position Favors Progradation

Objectives
Fifteen wells are correlated using a combination of erosional surfaces, depositional facies and the trajectory of the offsets in the "sigmoids" (reef core and crest). As in the previous exercise the reef crest trajectory is tracked using the geometries of the sigmoid and their offsets, while establishing how much of the section has been eroded following high stands in relative sea level, and how the greatest progradation is favored by lows.
 
Data Set
A fifteen kilometer long cross-section from water wells penetrating the Llucmajor platform in south east Mallorca extending from the town of Llucmajor and terminating at Cap Blanc is used in this exercise. This was previously described by Pomar and Ward, (1955 and 1999). As in the previous exercise the secret to this interpretation is to examine the cross section of the prograding reef margin crest, the associated linked diagrams and photographs assembled by Pomar and Ward, (1999), and in the photo galleries on this site. The cross-sections can be viewed on electronic media that include PC, Notebook, Tablet, or Pad. Using Power Point drawing tools, in particular the "curve",is an effective and easy way to complete the objectives of the exercise and provides a means to collectively view of the results in class. Click on red box for more details in a separate tab. 
The exercise images can be accessed by clicking on the appropriate thumbnails to the below or on the page listing the literature. As before you should also view the movie below and read the earlier sections of this site that introduce the geologic setting of the Late Miocene Llucmajor platform complex and its. Note that the initials on some of the cross-sections include "sb" which indicates a sequence boundary, "ci" that indicates a condensed interval and "dls" indicates a downlaps surface.
Image Gallery
 Movie 

Click on the thumbnail above to view the movie so you can see how the reef margin sigmoids change their position in response to changing sea level. Notice how much of the reef crest is eroded during the sea level falls and how much the platform margin progrades when the sea level is low for any length of time. As before don't forget to use the left and right keyboard arrows to control the forward and backward motion of the movie so you can review this as you view it.
Further click on the four thumbnails below which capture the evolving positions of the sigmoids and how, as the elevation of relative sea level moves up and down they change their elevation, and trajectory, and with drops in sea level can be eroded.
 
Interpretation Strategy and Techniques
Using a combination of your understanding of the regional geology with your understanding of the changing trajectory of the vertical, lateral facies relationships in this near shore carbonate settings (eg. the depositional setting of these Rocks including lagoon, reef crest, downslope reef, distal slope and offshore shelf) and Walther's Law, you should be able to build a depositional model and a sequence stratigraphic interpretation of the measured sections.
Once again we reiterate that as you have progressed through the various exercises for the Late Miocene carbonate shelf margins of Mallorca remember that erosion surfaces, or their correlative surfaces, represent the best means to separate this particular vertical association of facies types into packages of relatively conformable successions of genetically related beds or bedsets.  These surfaces envelope the high frequency cycles of the section that forming the building blocks of the sigmoids of Pomar (1991). Though these surfaces are there, it is not easy to identify them because they are not marked by changes in facies stacking patterns. This is because changes in carbonate production and accommodation are interdependent of each other. In siliciclastics, the mfs lies at the position of the turnaround the depocenter and mark the boundary between the landward migrating transgressive facies and the facies of the following basinward regression. Before the turnaround the facies deepened and after the turn around they form a shallowing upward succession. In the modern reefal system, there is not a landward migration of the depocenter during transgression since the reefal system keeps-up with sea level rise and, consequently, there is not a shallowing-upward trend in the vertically-stacked facies. As Pomar (1991) indicated these erosion surfaces separate the younger strata above from older strata below. The surfaces also often show evidence of subaerial erosion over which an abrupt marine transgression and increase in water depth is accompanied by minor submarine erosion and/or nondeposition, minor hiatus often being indicated.
Thus for this exercise, and others associated with Mallorca, each of these particular basic building block units are asymmetric shoaling upward cycles bounded by an erosion surface. Transgressive surfaces (TS) and maximum flooding surfaces (mfs) do occur in the Llucmajor platform complex, but their occurrence is the exception rather than the rule. Successive accretional units can be offset from one another forming the sigmoids of Pomar (1991). stacking patterns of of high frequency carbonate cycle sets are used in conjunction with boundaries and their position within a sequence to define the trajectories of the platform margin studied in this exercise and the next. The upper boundary is the top of the erosion surface at top of previous accretional unit. You can determine how much from the trajectory of the reef crest. Down stepping sigmoids that up dip are capped by erosion surfaces suggest build ups that have been removed. At the same time you should recognize that these carbonate platforms were produced by different biotic associations that had different capacities to record the high-frequency sea-level cyclicity and to construct internal architecture heterogeneities. In building a rigid framework up to sea level, this reefal system had a great potential to accurately record sea-level fluctuations. The different orders in the reef-crest curve or trajectory based on outcrops from the Llucmajor Platform of Mallorca are the key to correlating the parasequences. Click on the adjacent image of the simulation to see an interpreted reef crest trajectory and sketch this in on the cross sections to determine how you should correlate the sections. Remember that the sections record the following depositional settings.

  • Lagoon
  • Reef crest
  • Reef slope
  • basin slope
  • basin shelf
When you apply the techniques you have learnt in this exercise to other carbonate successions you should realize that the sequence boundary surface also often marks the boundary between the prograding highstand systems tract and the overlying of the transgressive systems tract. In carbonates the latter surface is also often characterized by the presence of hardgrounds and burrows, matching the underlying trangressive transgressive surface formed during or just after the initial transgressive phase that immediately follow sea level lowstands. In some cases glossifungites burrows may occur within this surface and the surface may be cemented by carbonates. When these occur you can use these associations to subdivide the sediments of measured section into their depositional settings.

Exercise 3 - The tasks

Identify parasequences in the measured section sections
Your interpretation process should be divided into four steps:
  • Examine the cross section and the associated diagrams that show how the hierarchies of sigmoidal shapes offset and change with sea level. Check in the previous section that introduced you to the Late Miocene carbonates of Mallorca and the information that Pomar and Ward (1999) provided to explain the origins of the offset sigmoids and their response to change sea-level.
  • Identify boundaries in the sections provided for the Exercise and use these to separate the parasequences and enable them to be and correlated from section to section.
  • Avoid your primary impulse in correlating facies tops and bases, tracing lines top-to-top and base-to-base: this would be a "physical" correlation. Try to think in terms of processes and the heterogeneities created by these processes and, just then, trace your correlation lines. They will provide you the key to predict permeability baffle or barrier surfaces
  • Track the reef crest trajectory or path from well to well and complete the exercise.
Remember that unlike Mallorca other, but not all, shallow carbonate sections may contain maximum flooding surfaces (mfs) or transgressive surfaces (TS). Where these occur they are often used to identify and bound the parasequences since these surfaces are normally more extensive and so better correlation tools than the SB. Needless to say such surfaces are not the norm in the Late Miocene platform shelf of Mallorca, but will be obvious after you finish the exercise and you can identify them. The movie above highlights the sequence boundary (SB) so you can see why they are so important to this suite of Rocks. Thus in this exercise each basic accretional unit is identified by the sequence boundary (SB) which caps horizons and are equated with surfaces of erosion formed when sea level dropped below the section, and mark a sediment surface that was reworked when sea level rose following a sea level low.
Once again your task will be to identify the base and top of the carbonate cycle units and identify the depositional facies within the basic accretional high frequency cycle units of the wells!
From your examination of the cross-section to establish how the hierarchies of sigmoids behave and track the trajectory of the reef crest as it progrades to the south east. Use this "trajectory" path to establish how the reef core migrates through time and space and use this to confirm the solution that Pomar and Ward (1999) have provided.

 

Solution to Exercise 3

As you can see in the Reef Complex, stratigraphic heterogeneities derive from the hierarchical stacking of high frequency accretional units that represent high frequency depositional sequences and in this exercise, with the resolution of data provided, you can trace fifth-order and, subsequently 4th-order sequences, within a third-order depositional sequence. The basic accretional unit or building block used in this exercise is the `sigmoid' (Pomar, 1991; Pomar and Ward, 1994; 1995, 1999). As you can see sigmoids stack into progressively larger-scale accretional units, forming sets, cosets, and megasets of sigmoids reflecting hierarchical orders of sea-level cycles. Each of the orders of accretional units are composed of horizontal lagoonal beds passing basinward into reef-core lithofacies with sigmoidal bedding, then into fore-reef slope clinoform beds, and then into flat lying open shelf (or shallow basin) beds. As you saw in the first exercise the lagoonal and reef-core units, boundaries are erosional surfaces (submarine and subaerial) which pass basinward into correlative conformities. The overall platforms show the same vertical succession of lithofacies: open-shelf lithofacies, composed of coarse-grained red-algal grainstone and fine-grained packstone/wackestone are overlain by progradational fore reef-slope and reef-core and, locally, by back-reef lagoon lithofacies. This exercise confirms that patterns in the stacking of parasequence sets (in this case sigmoids) can be used in conjunction with boundaries and their position within a sequence to define how a carbonate platform progrades and how heterogeneous, though ordered, the facies patterns can be (Pomar and Ward, 1999). As seen on the map and the cross section, low stands in sea level favor progradation.
Wednesday, March 11, 2015

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