| Exercise
1
Carbonate depositional setting interpretation from outcrops - Measured
sections at Cap Blanc and Sa Pedrera Blanca, Mallorca
Objectives
This section
is an introduction on how to identify high frequency carbonate cycles
or the simple carbonate sequence using their bounding surfaces, in
this case erosion
surfaces (SB), and their lithologies in carbonate outcrops. The
high frequency carbonate cycle can be considered to be the simplest
of carbonate sequences, forming the basic carbonate accretional unit
in much the same way that the parasequence
is often the basic unit of clastic sedimentary sections. The intent
of this exercise is to learn how to identify vertical sets of high
frequency carbonate cycles within a measured section of shallow
water carbonates and to interpret their depositional setting.
The models considered in the exercises that follow are compatible
with the conventional carbonate sequence stratigraphic models of Handford
and Lucks, (1993), and Hunt and Tucker, (1993) but not, in the strictest
sense, the clastic models of Van Wagoner et al, (1999). This is because,
unlike the parasequence, the high frequency carbonate cycle (simple
sequence) and the allied basic reefal accretional unit used in the
exercises, the sigmoid,
are bounded by clear erosion surfaces (the product of sea level lowering
and erosion with a matching correlative surface downdip) but no obvious
marine
flooding surfaces. However these units and their associated sigmoid
sets, and sigmoid cosets, are similar to the parasequence in that
they are composed of a relatively conformable succession of genetically
related beds or bedsets, and that they lend themselves to process/product
oriented types of analyses. Thus the geometric patterns exhibited
by stacked sigmoids and high frequency carbonate cycles can be used
like the patterns of stacked parasequence
sets, along with their position within a sequence, to define systems
tracts.
Despite this one could argue, and in many cases can demonstrate, that
erosion surfaces bounding the sigmoid or high frequency carbonate
cycle also involve the immediately following flooding event, and for
this reason, though not strictly true, the parasequence can be considered
to be equivalent to the sigmoid or cycle. We believe that this over
simplification often leads to confusion and we argue against its use
in this and the exercises that follow!
Data
Set
The section used in this exercise was previously described
by Luis Pomar (1991) for San Pedrera Blanca on the south eastern
coast of Mallorca. The secret to its interpretation is to
use the block diagram of the reef margin crest created by
Pomar and Ward, (1999, 1994), along with tthe associated linked
diagrams and photographs of the vertical association of sedimentary
structures, depositional systems. 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 sequence stratigraphy.
Click on the thumbnail
below to view the movie that will enable you to recognize
how important the erosion is that occurs when the shelf is
subaerially exposed! 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
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Interpretation
Strategy and Techniques
Using a combination of an understanding of the regional geology,
and the vertical and lateral facies relationships in this near
shore carbonate settings (e.g. the depositional setting of these
rocks including lagoon, reef crest, downslope reef, distal slope
and offshore shelf) and Walther's
Law, you should build a depositional model of the measured
section. A sequence stratigraphic interpretation of the area
will be made later in Exercises 2 and 3
where geometric relationships play a greater role. |
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| As you progress
through the various exercises for the Late Miocene carbonate
shelf margins of Mallorca you will see 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 building blocks of the section including
the high frequency carbonate cycles, sigmoid-sets and sigmoid-cosets
of Pomar (1991). As Pomar 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 has
been accompanied by minor submarine erosion and/or nondeposition,
indicating minor hiatae. |
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Thus for this
exercise and others associated with Mallorca, each of these
genetic units represents a shoaling upward cycle, or simple
sequence, bounded by an erosion surface. Transgressive
surfaces (TS) and maximum
flooding surfaces (mfs) occur in the Llucmajor platform
complex, but their occurrence is not easy to interpret, even
though the carbonate production and accommodation changes
were interdependent. This is because the cycles are often
not marked by changes in facies stacking patterns, though
locally offsets are characteristic in the sigmoids described
by Pomar (1991). This contrasts with interpreting siliciclastic
sections, in which the mfs
that lies between the landward transgression and basinward
regression, forms the boundary between a deepening- to shallowing
upward succession. Thus, as with the modern-reefal system,
the Late Miocene of Mallorcan carbonate accumulation often
matched sea level change so that there is no landward migration
of the depocenter during transgression and consequently the
vertically-stacked facies had a continuously shoaling-upward
character. Despite this, the trajectories
of the platform margin studied in the later exercises
can be determined from a combination of stacking
patterns of sets of shoaling upwards cycles in conjunction
with bounding surfaces and their position within a sequence;
particularly with respect to the truncation of reef facies
overlain by outer or inner lagoonal lithofacies. These later
often formed in water that was not deeper, but equal or even
shallower than the underlying section. Thus the sequence
boundary on a coset of sigmoids is often overlain by inner-lagoonal
beds that accumulated in water shallower than that of the
underlying lagoonal or reefal facies
In the exercise
we suggest that you draw a triangle immediately beside each
cycle you identify, with its base and top coinciding with
the base and top of the cycle.. The triangle appex should
point in the direction of the shallowing upward cycle. Alternatively
you can place a curved arrow beside the cycle to capture its
shoaling upward character, and/or grain size variation. An
arrow inclined to the left indicates that the water became
shallower and/or the grain size is coarser. Note the key to
making progress with this exercise is to use the triangle
to track variations in the depth and/or grain size for each
cycle you identify. Having done this you should match sediment
types and sedimentary structures to one of the following depositional
settings: |
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Lagoon
-
Reef
crest
-
Reef
slope
- Basin slope
-
Basin
shelf
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| When you apply the
techniques you have learnt in this exercise to other carbonate
successions you may find differences. For instance you should
realize that the sequence boundary surface can also often marks
the boundary between the prograding Highstand
System Tract and the overlying of the Transgressive
System Tract. In carbonates the latter surface is also often
characterized by the presence of hardgrounds and burrows, matching
the underlying trangressive 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 features occur you can use
their associations to subdivide the sediments of measured section
into their depositional settings. |
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Exercise
- The Tasks
Identify high frequency carbonate cycles or simple sequences
in the measured section.
Your interpretation process should be divided into three
steps:
-
Examine the block diagram and the photographs that match
hierarchies of sedimentary structures and depositional
setting. Check in the previous section that introduced
you to the Late Miocene carbonates of Mallorca for the
examples that Pomar and Ward (1990) provided of the fining
upward parasequences of the lagoon; the crestal reef,
reef slope, shelf slope and shelf basin.
-
Identify
erosion surfaces in the section provided for the Exercise
and use these to separate the high frequency carbonate
cycles from each other.
-
Use
a combination of this subdivision, and abrupt changes
in grain size with sedimentary structures to interpet
the depositional setting of the cycles.
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Most sedimentary
stratigraphers interpreting sections of carbonate rock are confronted
with the porblem of the difference in the sequence stratigraphy,
facies architecture and bounding surfaces of various Phanerozoic
carbonate platforms. This is a reflection of diverse biotic
systems producing differing products for the same processes.
This can be seen when the distally steepened ramp of Menorca
is compared to the reefal platform of Mallorca. Although both
these carbonate platforms were deposited during similar high-frequency
sea-level fluctuations both have completely different architectures.
For this reason, unlike the Late Miocene reefs of Mallorca (which
match modern reef systems), often many shallow carbonate sections
may contain maximum
flooding surfaces (mfs) or Transgressive
surfaces (TS). Where these surfaces occur they may be 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. 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
cycle or simple sequence is identified by the sequence boundary
(SB) which caps horizons and is 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.
Your task will be to identify the base and top of the units
of the carbonate cycle and identify the sequence boundaries
(SB) at the top of the each cycle and identify these within
the reef- or lagoonal section. On the slope and open shelf facies,
sequence boundaries are conformities and can be identified as
condensed intervals. For example, scattered corals on photic
(red algae dominated) open shelf/slope, represent the lowest
position of the sea-level allowing good light to reach the sea
floor and, consequently they mark the sequence boundary. On
proximal slope facies, rhodolithic-rich intervals represent
"in-situ" carbonate production (by slowly growing
red algae) in low-sedimentation rate area; low sedimentation
rates on fore-reef slope areas only occur when the lagoonal
areas do not shed sediment and this happens when the lagoon
did not exist during fall of sea level and the platform-top
was emergent. |
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| Use your examination
of the block diagram to establish how the hierarchies of sedimentary
structures match depositional setting. Use these associations
to subdivide the sediments of the measured sections into their
depositional settings. Now use a combination of this subdivision,
erosion surfaces and condensed intervals to identify the sequence
boundaries (surfaces of erosion) in the sections. |
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Solution
to Exercise 1
Be
sure to identify each of the high frequency carbonate cycle
or simple carbonate sequence either with triangles that narrow
in the direction of shallow water or arrows, so an arrow that
moves to the left indicates that the water is becoming shallower.
The high frequency cycles, or simple sequences, of the sections
are represented by shoaling upward cycles bounded by an erosion
surface followed by a reworked marine flooding surface. Thus
the lower surface of each of the cycles is the base of the
deeper lithofacies layer that overlies the top of a shallowing
upward cycle. The upper boundary is the top of a shallower
lithofacies layer that is overlain by a deeper lithofacies
layer. Patterns of the stacking of the high frequency cycle
sets or simple sequences are used in conjunction with bounding
surfaces and their position within a sequence like the parasequences
of Van Wagoner et al., (1988) to define system tracts. |
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