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Interpretation
of High Frequency
Carbonate Cycles from Outcrop:
Late
Miocene of Mallorca
Introduction
The exercises in this
section are designed to introduce geologists to building sequence
stratigraphic models from sections of shallow water carbonate sediments
exposed in outcrop. 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, Spain (Sequence
Stratigraphy of Ballearic Islands - Papers), one of the best
exposed and most thoroughly studied Cenozoic platforms of the world.
These carbonates accumulated during the Late Miocene in the Eastern
Mediterranean, 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 information 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 |
Click
to View |
| 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). |
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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. |
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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". |
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Reef
Facies
The basic accretional
units of the Llucmajor reef complex have been described
and defined 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 boundary. 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.
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Movie
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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 abundant 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 abundant 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.
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Reef-slope
The proximal forereef slope facies are characterized
by seaward-dipping clinobeds 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 abundant 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 abundance 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). |
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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). |
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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, bounding
surfaces, 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 clinobeds, 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. |
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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.
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