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Interpretation of High Frequency Carbonate Sequences from Outcrop: Movie of the Progradation of the Llucmajor Reef Complex
Llucmajor movie Two movies model the evolution of the Llucmajor platform. One is annotated to guide you through the events and the other excludes the annotation so you can concentrate on the animation. Remember that the best way to view the Quicktime movies is to control their motion using the arrow keys on your keyboard.
If you use this scheme to view the film you will be able to step through it slowly while examining the products of each sea level position while gaining a better understanding of the principles explained below. The description below explaining the evolution of the Llucmajor platfom paraphrases some of the papers by Pomar and Ward (1994 and 1999) [see pdfs of these papers]. These papers complement the movie that captures the evolving character of the Llucmajor platform. This sequence of carbonate rocks exemplifies how sea level change determined not only the relative hierarchy of the accretional units of this complex but also how the relative positions of the facies belts developed. Up to four bundles (or systems tracts), which are related to specific parts of the sea level cycle, can be defined from characteristic changes in the hierarchical stacking patterns among these accretional units.
Carbonate Production
Fringing reefs with little or no lagoons predominated in all areas during falls of sea level when coral reefs shifted downward and basinward as a result of the decrease in accommodation. Steepness of the fore-reef slope was dependent mainly on the paleobathymetry and depositional profile of the basin floor, rather than on changes in accommodation. Volume of the fore-reef-slope sediments, however, was directly related to the extent of the existing lagoons. Maximum progradation rates occurred during the lowering of sea level across the shallowest gently inclined areas not only because accommodation was less, but also because carbonate production was greater in shallower water. In conclusion, in the absence of significant compaction and subsidence, the progradational geometry, facies and stratal architecture of this platform were controlled by the high-frequency changes both accommodation and carbonate production. Carbonate production was not an independent factor with respect to changes in accommodation; production varied according to changes in relative sea level and to the depositional profile of the basin floor. Thus during sea-level lowstands, carbonate production in shallow-basin settings was significant over a large area. The shallow open shelf was subjected to wave action which winnowed finer material and transported it to the deeper open platform so only coarse sediment rich in red algae remained. During the lowest stands of sea level, scattered corals grew on the open platform. The red-algal biostromes of this setting interfingered with the fore-reef slope of the LST on the shallower shelf. The red-algal deposits were not produced when and where the basin floor was too deep to allow sufficient light penetration. When sea level rose, most of the production of the carbonate sediment shifted to the shallower shelf, where extensive lagoons developed behind reefs. Pomar and Ward (1999) inferred from the stratigraphic architecture that during a rise of sea level, deposition on the open shelf was primarily the result of shedding of fine sediment from the shallow-shelf lagoons, although they were unable to specifically identify lagoonal sediment in the open-platform deposits. This interpretation matches that of other workers who considered that carbonate-sediment shedding is volumetrically important when platform tops are flooded (Driscoll et al., 1991; Glaser and Droxler, 1991; Grammer, 1991; Boschler, 1992; Grammer and Ginsburg, 1992; Schlager, 1992; Schlager et al., 1994). Proximal and distal open-shelf settings differed mainly in the volume of material shed from the shallower shelf. The thicker proximal deposits thinned out basinward, where amalgamated rhodalgal biostromes accumulated over time. Downslope shedding decreased during sea-level highstands and virtually ceased during falls and lowstands. On the shallower shelf, the changes in accommodation were reflected in the up-and-down shifts of the reef lithofacies and truncation of the upper AST/HST reef and lagoon lithofacies. In the lagoons, buildup was related to rising sea level and erosion to falling sea level. In summary, heterogeneity in the lithofacies architecture in this type of prograding platform reflects changes in carbonate production and depended on changes in accommodation and on basin-floor morphology. Exercises
The linked exercises below have been designed to help you better understand the controls on the evolution of the Llucmajor platform and help you apply the same principles to other shallow water carbonate systems. As indicated earlier high frequency carbonate sequences of this Late Miocene carbonate platform are bounded by erosion surfaces or their correlative conformities down dip. Updip in the shelf lagoon and within the reef crest these erosion surfaces are the products of a sea level fall and subaerial erosion, often complemented by kharstification. Though
parasequences are the products of changing base level, rates of
sediment accumulation and erosion, cycles formed by carbonates may
often be different in character from those formed by clastics. However,
as with the clastics parasequence, high frequency carbonate cycles
can be used in conjunction with Steno's
principles and Walther's
Law to provide control on the relative timing of deposition
of the studied section. As with clastic parasequences these high
frequency carbonate cycles often contain the fundamental units (reservoir
rock, seal and source rock) of a hydrocarbon reservoir or an aquifer.
Just as
with clastics, carbonate high frequency sequences are defined by
their bounding surfaces. In the case the Late Miocene carbonates
of Mallorca these bounding envelopes are marked by surfaces of erosion
that enclose layered wedges of sediment or "sigmoids".
The sediment contents of the sigmoids can be considered to have
been deposited synchronously but vary in character in response to
depositional setting. Using the exercises that follow you will be
to identify high frequency cycles in the carbonates of Mallorca
and see how the geometry of their carbonate facies change vertically
and laterally in response to changes in sea level.
In the
exercises it will be seen that the high frequency "cycle"
or "simple sequence" is the smallest set of genetically
related facies deposited during a single base level cycle.The cycle
boundaries (in this case erosion surfaces) mark the turnaround from
base level fall to base level rise (a period of time during which
sea level descends from a highstand position, through a lowstand,
and back to a highstand position). Cycles can be mapped across multiple
facies tracts and include multiple vertical facies successions (VFS)
and can therefore be considered chronostratigraphic units (Kerans
& Tinker, 1997 and Mitchum & Van Wagoner, 1991). One of
the commonest "quoted" manifestations of the carbonate
high frequency cycles or sequences is the shoaling upward cycle,
with finer deeper water facies at their base and coarser better
sorted facies towards their top. In the Late Miocene carbonate platform
of Mallorca this is not always the case and instead the commonest
signal of the sea level change is an erosion surface.
Exercise 1 Exercise 2 Exercise 3 |
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