Carbonates Outcrop

Interpretation of high frequency carbonate cycles from outcrop:

Late Miocene of Mallorca

Introduction
The exercises in this section are designed to introduce geologists to interpreting the depositional setting of shallow water carbonate sediments exposed in outcrop and their relationship to varying sea level position. The most critical objective of the exercises is to understand how erosion plays a part during the development of carbonate margins. You will find that the ability to predict facies trends improves with an understanding of how much of carbonate geometries are eroded by the fall of sea level when coupled with knowledge of how much carbonate accumulation generated the geometries formed during aggradation, progradation and retrogradation during a sea level rise. This is further improved if it is understood how Louis Pomar used an inferred sinusoidal sea level curve to interpret the carbonate geometries of Mallorca.

While all geologic outcrops of carbonate and clastic sediments have a "unique" character imposed by the specifics of the depositional setting for that location and time, they also carry many common signals. The attached group of exercises for this section should help you identify these common and different signals. They are based on measured sections and field observations of the Late Miocene Llucmajor carbonate shelf and reef margin complex of Mallorca, one of the Ballearic Islands, Spain. These among the best exposed and most thoroughly studied Cenozoic platforms of the world. These carbonates accumulated in the Eastern Mediterranean during the Late Miocene, responding to changing sea level and productivity.

Most of the materials and text used on this site are based on the publications and the compendium of data assembled by Luis Pomar of Departament de Ciencies de la Terra, Universitat de les Illes Balears, Mallorca, Spain and colleagues he has worked with over the years. Many of the maps and diagrams presented in the attached gallery are from the publications linked to the page and were drafted by Luis Pomar. Also included in the gallery are a suite of digital photographs taken by Christopher Kendall during a field trip with Luis Pomar, when Luis kindly showed him the rocks and exchanged his ideas with him!! The exercises (accessed from the"Select Item" pull down menu above) are focused on examing the facies relationships close to the margin, particularly their stacking patterns and the path of the trajectories of the reef margin. Another gallery of photographs to visit are those taken by Steve Schulz during a field trip with Luis Pomar. For those planning a trip to view the Geology or need a more comprehensive guide to points of touristic interest etc click on Mallorca.

Background Geology
Reef-rimmed progradational platforms were widespread in the western Mediterranean during the late Miocene (Esteban, 1979, in press). Following the major overthrusting of the middle Miocene Alpine orogeny, several carbonate ramps and platforms developed and overlie the deformed Early and Middle Miocene, Paleogene and Mesozoic rocks on Mallorca, and the other Balearic Islands, (Pomar et al., 1983a; Alvaro et al., 1984; Simó and Ramón, 1986; Obrador and Pomar 1983; Obrador et al., 1983a, b). Late Tortonian to early Messinian reef-rimmed carbonate platforms were common (Bizon et al., 1973; Colom, 1980, 1985; Pomar, 1991; Pomar, 1993 ; Pomar, 2001; and Pomar et al 2004).

The Upper Miocene rocks of the Balearic archipelago (Mallorca, Menorca, Ibiza, Formentera and smaller islands) are commonly flat-lying limestones and dolostones,with only slight tilting and flexure, probably caused by normal and strikeslip faulting in the Late Neogene to Middle Pleistocene. They are composed of three third-order depositional sequences (Pomar et al., 1996):

 

  • The lower sequence, attributed to the Early Tortonian, corresponds to a carbonate ramp with extensive rhodalgal lithofacies and no coral reefs.
  • The middle sequence, attributed to the Late Tortonian-Early Messinian, corresponds to well-developed progradational reefal platforms, including the Llucmajor Platform reef complex.
  • The upper sequence, assigned to the Messinian, consists of a variety of lithologies including oolites and stromatolites.

 

On this web site only the lower and middle depositional sequences are considered and the exercises presented here are focused on the Llucmajor Platform reef complex. This is some 150 m thick and in southwestern Mallorca has prograded laterally over more than 20 km. Between Vallgornera and Cap Blanc in southwest Mallorca it is exceptionally well exposed along both depositional strike and dip directions for some 6 km in vertical sea cliffs and is only slightly deformed by Pliocene–Pleistocene uplift, faulting, and gentle flexure. Discussed below are the facies and stratal geometries of the Llucmajor Platform reef complex outcropping around Cap Blanc. These have been described in detail since 1991 by Pomar, in papers linked to this site.

 
 
 
 
Facies Model
 
The facies model presented on this site was established by Luis Pomar, in his description of the sea cliff outcrops, their rock textures, and the relative position of stratal geometries and boundaries (Pomar, 1991; Pomar and Ward, 1994, 1995, 1999, and Pomar et al. 1996). The four sub-facies have been established as the "reef-core", the landward coeval "lagoonal rocks", and the down slope "reef-slope" and "open-shelf deposits".

Reef Facies

As indicated carbonate geometries are eroded by a fall of sea level which modifies the carbonate geometries formed during aggradation, progradation and retrogradation of a sea level rise. Louis Pomar was able to interpret the amount of erosion using an inferred sinusoidal sea level curve to interpret the carbonate geometries of Mallorca. He describes and defines the basic accretional units of the Llucmajor reef complex as "sigmoidal" packages of coral framestones that are bounded by submarine and subaerial erosion surfaces and their correlative basinward conformities (Pomar, 1991). The reef core facies have a characteristic sigmoidal Bedding and are composed of skeletal grainstone /packstone/wackestone and rudstone within the coral framework composed of massive coral reefs that overlie and interfinger basinward with the fore-reef slope lithofacies and landward with the lagoonal lithofacies (Pomar, 1991; Pomar and Ward, 1994, 1995, 1999, and Pomar et al. 1996.

The reef framework and reefal rudstone are mainly composed of two coral genera: Tarbellastraea and Porites, although Siderastraea also occurs. Nevertheless in the sea cliffs most of the reefs are basically monospecific, with Porites being conspicous. Secondary framework components include encrustations of red algae, foraminifera, bryozoans, worm tubes and vermetid gastropods, as well as microcrystalline rinds and crusts. Red-algae genera include Spongites, Lithothamnion, Lithophyllum and Lithoporella (Perrin et al., 1995)..

The reefs show a vertical zonation that is dependent on the depth-controlled growth morphology of the corals. Deeper-water coral colonies are platy with finger-like vertical projections, intermediate colonies are predominantly branching, and shallow-water colonies are hemispheroidal to columnar or even domal in the reef crest position. Paleobathymetry of these coral morphology zones is estimated as 30-20 m, 20- 10 m, and less than 10 m, respectively. Inter-coral spaces may be filled with coarse skeletal grainstone- packstone and/or wackestone, but primary framework porosity may be locally significant.

The vertical shifts of the coral-morphology zones across and within the different orders of accretional units have been interpreted as an expression of high-frequency relative sea-level fluctuations (Pomar, 1991; Pomar and Ward, 1994). A reef crest curve (Pomar, 1991) has been defined by the successive positions of the reef crest, measured or inferred from the coral-morphology zonation, and reflects the amplitude of sea-level fluctuations in relation to progradation. This progradation has responded to the amount of sediment that has accumulated through time, and has lead to aggradation during a sea level rise, and shallowing or deepening upward trends dependant on the interplay between sea-level rise and carbonate production/sedimentation rates (accommodation versus production) (Bosence et al, 1994; Pomar and Ward, 1995; and Pomar, 2001).

An upper erosional surface truncates everything, particularly the patch reef corals and the grainstone sediments of the outer lagoon facies and is interpreted to follow a fall in sea level. In fact the upper erosional surface truncates the branching corals of the reef-core facies and correlates basinward with the conformity. In some sigmoids, coral morphology is characterized by a shallowing-upward zonation. On the more distal reef slope and open shelf intense bioturbation can destroy the internal arrangement of the lithofacies and obscure the conformable nature of this boundaformformformry. The bounding erosional surfaces are thought to be the products of falls in sea level, because in the reef core facies it matches a degree of shift in the coral morphology zones from relatively deeper to t relatively shallower facies across the boundaries. This is interpreted to capture the amount of the sea level fall. Even though there may be a lack of subaerial exposure features at most of the sigmoid boundaries the character of the biota has lead to the interpretation that they are the products of sea level fall which lowered wave base on a platform that remained submerged or are the product of submarine erosion during the subsequent flooding event.

 
 Movie
 
 
 
 
 
 
Lagoon Facies

Pomar, (1991, 1993) Pomar and Ward (1994, 1995, 1999), Bosence et al, (1994), Pomar et al. (1996) and later Pomar (2001) explain how the lagoon facies landward of the reef crest are horizontal beds bounded by erosion surfaces. These lagoonal sediments are formed by skeletal grainstones and packstones with lenses of coral breccia and patch reefs. Outer lagoonal areas in back-reef position are formed by beds of bioturbated skeletal grainstone- packstone with coral patch reefs with an abundaformformformnt contents of red-algae fragments, echinoids, mollusks, benthic foraminifera and coral fragments, as well as minor amounts of Halimeda, planktonic foraminifera, bryozoans, peloids and intraclasts. Coral morphologies in the lagoon facies commonly vary spatially, with larger, hemispherical and columnar forms seaward and smaller hemispherical forms landward. Inner lagoonal areas have fewer and smaller coral colonies and patch reefs, and bioclastic sediments predominate. The inner-lagoonal lithofacies are thin- to medium- Bedded grainstones, packstones and wackestones- mudstones. Skeletal components are miliolids, thin bivalves, peloids and cerithid gastropods. Some layers also contain abundaformformformnt benthic foraminifera or ooids. Ostracods, dasyclad algae, echinoids, Favreina pellets, calcispheres and red algae are less common component. Miliolid grainstones and packstones with vertical root casts are interpreted as mangrove deposits. Stromatolites and muddy sediments with ostracods, Chara, and oncolites characterize the innermost lagoonal facies. Beach deposits with upper shoreface and foreshore facies also are present. In the lagoonal facies thin laminites or gastropod rich wackestone ( restricted facies) rest on the erosional surface and are overlain by packstone, wackestone and grainstone with red algae, echinoids, mollusks, and benthic foraminifera (open lagoon) In the outer lagoonal facies, the basal laminites are overlain by both coral parch reefs and coarse skeletal grainstone (interpatch sediments). As at the reef crest, the upper erosional surface truncates everything, particularly the patch reef corals and the grainstone sediments of the outer lagoon facies. In the lagoonal facies, the basal laminites record the flooding of the platform top and the overlying coral parches record the submergence of the platform to the optimum production conditions. In the lagoonal facies the erosional truncation of the coral patches is interpreted to follow a fall in sea level. The upper shallowing up part of the cycle is missing, and the physical correlation of the erosional surface from the lagoon to the reef core supports this interpretation.

 
 
 

Reef-slope .

The proximal forereef slope facies are characterized by seaward-dipping clinoformformformBeds ');">Beds composed of skeletal rudstones/floatstones to grainstones and packstones with coral fragments, rhodoliths, mollusk fragments, etc. Proximal reef slopes show a gradual, lateral increase of grain size from the distal (deeper) slope facies upward to the reef. There is a general trend for a coarsening and thickening upward sequence that formed as the platform prograded. The upper levels include abundaformformformnt coral breccia and Halimeda

Distal reef-slope deposits are gently inclined layers of white, chalky calcisiltites and calcarenites (coralline algal, molluskan packstones and grainstones) with bioturbated beds (for details see Pomar et al., 1996).

Open-Shelf

The distal slope facies pass basinward to bioturbated, fine-grained wackestones. These fine-grained deposits accumulated as the result of carbonate shedding from the shallow-water platform, during periods of sea-level rise, when carbonate production was enhanced in the lagoonal areas. During lowstands of sea level however, carbonate production occurred in off-reefs areas where the light reached the sea floor. In this setting, a sediment dominated by red-algae was produced. This is composed of red-algal grainstone to rhodolithic rudstone or to biostromal layers of laminar or branching red algae. Oysters and pectinids are common. Larger foraminifera (Heterostegina) locally occur, and planktonic foraminifera are present. Coral colonies of Tarbellastraea and/or Porites occur at the top of some Beds. Meter-scale cyclic textural changes in composition are interpreted to record high-frequency cyclicity in this lithofacies. Bioturbated sediments are characteristic although laminations are locally visible. Branching red-algal biostromes also occur within the slope deposits. Based on the coarse-grained texture and abundaformformformnce of red algae it is interpreted that deposition of most of this lithofacies took place in the oligophotic or in the mesophotic zone, with sufficient water energy (currents and storm waves) to winnow away fines (Pomar, 2001). Nevertheless, the platform architecture indicates that this lithofacies was deposited in off-reef, shallow-basin settings, during lowstands of fourth-order relative sea-level cycles. Corals and other shallow-water biota at the top of the internal cycles of this lithofacies are interpreted to represent the lowest positions of relative sea level, within the high-frequency cyclicity. Sufficient light reached the sea floor to allow some corals to grow during these lowstands of sea level (Pomar et al., 1996; Pomar and Ward, 1999; Pomar, 2001).

 
 
 
 

Fine grained Reef-Slope to Open-Shelf Facies

In this off reef position the sediments are highly burrowed, subhorizontal to flat-lying fine-grained skeletal packstone/ wackestones that may overlie and/or underlie the coarse-grained red-algal grainstone/ packstone lithofacies (Pomar, 1991, 2001; Pomar and Ward, 1994, 1995, 1999, and Pomar et al. 1996). Locally, extensive networks of vertical to horizontal burrows (Ophiomorpha, and Thalassinoides, are present. Geometric constraints and the relationship of carbonate production to photic conditions suggest open-shelf water depths more than 100 m, well below the photic zone. Common components include planktonic foraminifera, ostracods, and fine-grained detritus of oysters, other bivalves, echinoids, and red algae. Common megafossils are relatively deep-water oysters, irregular echinoids, pectinids and, locally, some rhodoliths and branching red algae (Pomar, 2001). High-frequency cycles are defined by alternation of meter-scale megafossil-rich- and megafossil-poor layers or burrow-rich/burrow-poor layers. Sediment texture, fossil content and architectural position within the platform indicate that this lithofacies represents an accumulation of fine-grained sediment shed-off from the shallow-shelf lagoons during periods of rise of the fourth and fifth-order relative sea-level cycles. When sea level rose, open-shelf settings were placed below the photic limit and most of the production of sediment shifted to the shallower shelf, where extensive lagoons developed behind reefs (Pomar and Ward, 1994, 1999; Pomar, 2001).
 
 
 

Response of Platform Architecture to Eustasy

Maps of the Late Miocene reef-slope through reef-core to lagoonal sediments of the Llucmajor reef complex indicate that it has prograded south-southwest some 20 km (Pomar and Ward, 1995, 1999).The arcuate coastline of Mallorca in the vicinity of Cap Blanc is demarked by the vertical sea cliffs that are up to 90 m high and reveal the three-dimensional features of the internal architecture and facies distribution of the Llucmajor reef complex (Pomar, 1991; and Pomar and Ward, 1994, 1995, 1999).

The outcropping facies and stratigraphic geometries form the "sigmoids" of Pomar (1991). These capture several orders of cyclic change in the position of the reefal facies, and stack into progressively larger-scale accretional units of sets, cosets, and megasets. They are interpreted to represent a response to hierarchical orders of cyclic fluctuations in Late Miocene glacioeustatic movement (Pomar, 1991; and Pomar and Ward, 1994, 1995, 1999). The estimated amplitudes of these cycles are respectively less than 15 m, 20-30 m, 60-70 m, and about 100 m. The hierarchy of the cyclicity is interpreted as four orders below that of the 3rd order global cycles of Haq et al. (1987) and has been used to construct the relative sea level curve of Bosence et al (1994) which is used in the simulation movies of the site.

The accretional units of the high-frequency depositional sequences (seventh to fourth order) have similar stratal geometries, boundaries, and facies architectures (see the adjacent figures). Each of these units are composed of horizontal lagoonal beds that pass basinward into reef-core lithofacies with sigmoidal Bedding, through forereef-slope clinoformformformBeds, and then into flat-lying openshelf (or shallow-basin) Beds. The bounding envelopes to the lagoonal and reef-core units are erosion surfaces (submarine and subaerial), which pass basinward into correlative conformities. Many of the basic accretional units are wedge shaped as a result of non-deposition or erosional truncation of the upper part of this accretional unit (lagoon and upper portion of ref-core lithofacies), or both.

Movie
 
 
 
 

Movie

The character of the Llucmajor platform exemplifies how sealevel change has determined not only the relative hierarchy of the accretional units but also the relative positions of the facies belts developed within them. Before taking the exercises we advise you to visit next page and run the movie to see how this reef complex evolved and read the accompanying explanation for this.

Click here to go to the movie page and links that take you to the exercises that examine the outcrops of the Late Miocene carbonates of Mallorca.

Sunday, March 08, 2015
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