Mallorca Exercises 2

Exercise 2 - sequence stratigraphic interpretation - Offset in sigmoids and Reef Crest Trajectory

 

Objectives
Two and then three measured sections are correlated using a combination of erosion surfaces (SB), (or their correlative conformities), their depositional facies patterns and the trajectory taken by the shelf margin crests and reef core that form a "sigmoid". The exercise shows how the trajectory of the reef crest can change and how this change in geometric position, or offset, of the sigmoid, can be used to establish how much of the section has accumulated or has been eroded, so enhancing ones ability to correlate high frequency carbonate cycles.

Data Set
Two cross sections, one involving the closely spaced measured sections at Cap Blanc and Sa Torre and the other cross section through measured sections at Sa Torre, Cala Carril, and Punta Negra are used in this exercise. These have been previously described by Pomar and Ward, (1999). 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 on this site. The cross-sections can be viewed on electronic media including PC, Notebook, Tablet, or Pad and interpreted with a variety of different visualization software (including but not exclusive of Adobe Acrobat, Adobe Illustrator, Freehand, Canvas, Photoshop and/or Power Point) or printed on paper and interpreted by hand. The exercises can be accessed by clicking on the appropriate thumbnail to the right or from the page listing the literature. As before you should also view the attached movie and read the earlier sections that introduce the geologic setting of the Late Miocene Llucmajor platform complex and its evolution.  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 downlap surface.

Click on the thumbnail below to view the movie that will enable you to recognize how the reef margin sigmoids change their position in response to changing sea level! 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!

 Movie

 

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 will build a depositional model and a sequence stratigraphic interpretation of the measured sections.
We reiterate once more that as you progress through the various exercises for the Late Miocene carbonate shelf margins of Mallorca you will see erosion surfaces, or their correlative surfaces, that 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 carbonate cycles that form the basic accretional unit of the section, particularly the building blocks of the different orders of sigmoid (Pomar, 1991) (sigmoid-sets, sigmoid-cosets and megasets). As Pomar indicated these erosion surfaces separate the younger strata above from older strata below. The hierarchy of these surfaces bounding the accretional units was established by determining which erosion surface truncates other erosion surfaces. These surfaces often also show evidence of subaerial erosion over which an abrupt marine transgression and increase in water depth was accompanied by minor submarine erosion and/or nondeposition, minor hiatuses are often indicated.

As indicated earlier one of the key characteristics of this Late Miocene Reef Complex is that it composed of high-frequency carbonates sequences, not parasequences. While the parasequence is the common building block of many carbonate depositional sequences, this is not the case for this Late Miocene Reef Complex. Here, the basic building block is the simple depositional sequence or high frequency carbonate cycle. Sets of sigmoids, cosets of sigmoids and megasets also form truncated depositional sequences.

There are at least two reasons for this:
One is the differing capacity of diverse depositional settings to record high frequency cycles of sea level. The modern to Miocene coral reefs, unlike reefs older than the Miocene, have the potential to construct a rigid framework up to the highest wave-energy level (sea level). Consequently, Holocene to Miocene carbonate platform settings have the capacity to build cemented wave resistant deposits to sea level. Any fall in sea level (even a minimum of a few meters) causes this rock to be truncated, and a visible and clearly identifiable erosion surface results. Associated with this sea-level fall is a basinward shift in the locus of "reefal-rock" accumulation. In other carbonate depositional systems, particularly those dominated by grains, the sediments can only accumulate up to a certain energy level (the base of wave action). A fall of sea level and its concomitant lowering of base level will increase sediment mobilization and basinward transference (shedding), forcing the sedimentary body to prograde. Although this fall will create an erosion surface, it tends not to be as prominent as when truncating a "rock". The amount of removal during fall cannot be ascertained and the basinward shift of sedimentation locus is not so clearly recognized. Moreover, this "regressive surface of erosion" will be often modified during subsequent transgression (ravinement surface). This eclectic array of causes will make it almost impossible (or at least very difficult) to recognize the effects falls in sea level have in modifying the upper boundary of a prograding sequence so that consequently, a parasequence will be interpreted.

Secondly in most "grain-dominated" depositional systems (and particularly those related to siliciclastic systems with high grain-density and clay content), the "pure" eustatic fall of sea level is commonly obliterated by the effect of compaction + subsidence. In this case, the fall in sea level (eustatic fall = compaction + subsidence) results in a "relative sea-level stillstand" and, a eustatic, high-frequency cycle of sea level becomes a paracycle of "relative sea level". In this situation, a parasequence is formed in relation to high-frequency eustatic cycles of sea level. In the Upper Miocene reefs of the Balearic Islands, however, compaction was negligible and subsidence was minimum.


Thus for this exercise and others associated with Mallorca each of these particular basic accretional units is a shoaling upward cycle 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. Though these surfaces exist it is not easy to identify them as they are not marked by changes in facies stacking patterns. This is because the carbonate production and accommodation changes are interdependent of each other. In contrast for siliciclastics the mfs marks the turnaround of the depocenter when it migrates landward during transgression and basinward during regression. The mfs expresses a turnaround from a deepening to shallowing upward succession of parasequences. In contrast the modern reefal system shows no landward migration of the depocenter during transgression, since the carbonate accumulation keeps pace with sea level and there is no consequent shallowing-upward trend in the vertically-stacked facies but instead, though each of the carbonate cycles shoal up, the overall trend remains shallow for the complete low frequency cycle of fill unlike the clastics that trend from deep to shallow, though the overall character or each cycle is to shoal up. Though the lower surface of the sequences cycle is the base of the deeper lithofacies that overlies, the upper surface of the underlying shallowing upward cycle, the high frequency cycles can be offset from one another forming the sigmoids of Pomar (1991). Stacking patterns of of shoaling upward 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 a shallowing upward lithofacies layer, which in the Late Miocene of Mallorca was often eroded and then overlain by a deeper lithofacies layer. You can determine how much from the reef crest trajectory. In fact, the backreef lagoonal sediments are characterized by top-truncated, deepening-upward successions: very-shallow water (inner lagoon) lithofacies overly the erosion surface and is overlain by deeper-water (open marine) lagoonal lithofacies bounded at top by the erosion surface (sequence boundary). This facies & boundaries arrangement can be seen at the level of the basic accretional unit (sigmoid), and also at the level of the set- and coset of sigmoids.

This reason for this is in part related to the position of base level up to which the sediment to accumulates. As the reef constructed a barrier up to sea level, the reef crest acts as an "wave-energy dam" and the lagoon was able to fill most of the available space up to sea level. Consequently, the shallow-water (inner, restricted) sediments accumulated onto the erosion surface as soon as the carbonate factory started to produce just after the flooding. As sea level continued to rise, the lagoon became deeper and the factory evovled from open marine to outer lagoon. When sea level stopped rising, the sediment infilled the available space and the factory progressively built a shallower, restricted lagoon. Nevertheless, a minor sea-level drop would develope subaerial conditions on the platform top, removing the shallow-water lagoonal cap and creating the erosion surface (SB). Remember that these carbonate platforms were the products of a variety of 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 high frequency carbonate cycles. 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 transgressed 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 2 - The tasks
Identify basic accretional units in the measured section sections. Do not worry about identification of the right (absolute) order of the accretional units. It is relative. Remember that all orders of accretional units have similar characteristics and are alike. Consequently, the volume of the basic accretional units, which depends of the sediment production rate, is what is significant in terms of reservoir characterization, and it is partially independent of the periodicity. When the sedimentation rate is low, 3rd-order sequences will only be relevant in terms of reservoir heterogeneity, although with high rates of carbonate production the fifth-order sequences (coset of sigmoids) determine the most important heterogeneity at reservoir scale. You can examine the Llucmajor platform map and the two cross-sections to show that in the Palma basin. Here the platform has prograded less than 2 km and the 3rd-order sequence is the only relevant, whereas to the south, where the platform has prograded 20 km during the same time and the key accretional units controlling reservoir-scale heterogeneities (reservoir compartmentalization) are the coset of sigmoids (2 km of progradation each)
Your interpretation process should be divided into several steps:

  • Examine the cross sections and the associated diagrams that show how the hierarchies of sigmoidal shapes are offset from each other and change with sea level position. 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 high frequency carbonate cycles and enable them to be and correlated from section to section.
  • Avoid your first impulse to correlate 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 in the facies and their geometries 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 section to section 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 common in the Late Miocene platform shelf of Mallorca but after you finish the correlation exercise you can identify them. The movie above highlights the sequence boundaries (SB) so you can see why they are so important to this suite of rocks. Thus in this exercise each high frequency carbonate cycle 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 each of the high frequency cycles of the measured sections!
From your examination of the cross-sections establish how the hierarchies of sigmoids behave and track the trajectory of the reef crest as it prograded 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 2

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 (in this case up to fifth and sixth order) 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; 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 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 forereef-slope and reef-core and, locally, by back-reef lagoon lithofacies. This exercise confirms that patterns in the stacking of high frequency cycle 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).

 

 

Sunday, October 27, 2013
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