Page 146 - Carbonate Sedimentology and Sequence Stratigraphy

CHAPTER 7: SEQUENCE STRATIGRAPHY OF THE T FACTORY 137

Epoch My SITE 627 SITE 628 long to the sediment fill of intra-montane molasse basins of
Setting. The Miocene limestones, dolomites and marls be-
seismic secequences
Pleis- A the Betic Cordillera, an orogenic belt of the Alpine chains
tocene A of southern Europe. Unlike the Bahamas, the Betic molasse
Plio-
cene 5 basins experienced long periods of post-depositional uplift
B B and terrestrial erosion. The carbonates on Mallorca are Late
Miocene (Tortonian-Messinian) in age; Pomar et al. (1996,
10 C p. 197) estimate that the phase of deposition lasted about 2
Miocene 15 C million years. The facies clearly show the characteristic at-
tributes of the T factory: abundant hermatypic corals and

20 green algae, mud-rich lagoons and fast-growing coral reefs
at the platform margin. The platforms grew on a shelf of
less than 100 m water depth that subsided slowly during the
D
25 D interval in question. Eustatic sea level fluctuated, but the
Oligocene 30 Thus, the rate of accommodation creation was rather low.
long-term trend was flat or falling (e.g. Abreu et al., 1998).
The rate of carbonate growth, on the other hand, was persis-
35 D tently high. Consequently, the studied platform prograded
20 km during the examined interval.
The combination of excellent outcrops and careful obser-
40 vation yielded a number of important insights.
Eocene 45 E ➤ Depositional rhythms appear in a wide range of scales.
Individual depositional units, called “sigmoids” (Po-

E mar, 1991; Pomar et al., 1996; Pomar and Ward, 1999),
50 are bundled into sigmoid sets, sets into cosets, and
cosets into megasets. The entire formation is consid-
ered equivalent to the major portion of a third-order se-
55
Paleocene 60 E (not drilled) pattern in this hierarchy of cycles remains the same. Po-
quence in the standard model. The basic depositional
mar and Ward (1995, p. 96) observe that the sigmoid “is

65 not a parasequence ... but ... a small depositional se-
quence” and “sets, cosets and megasets of sigmoids ...
also show characteristics of depositional sequences ...”.
Fig. 7.39.— Hiatuses in the Cenozoic sediments of the Blake ➤ The platform is characterized by strong progradation
Plateau. Boreholes are only 10 km apart, yet the history of sedi- and very minor aggradation (Fig. 7.40, 7.41). Backstep-
mentation and erosion differs considerably. Both sites are located ping of the margin or deepening-upward intervals have
on the rise seaward of the slope of Little Bahama Bank. Sediments not been observed. The succession is an extreme ex-
reflect the interplay pelagic settling, input from the platform by distal ample of a supply-dominated system, one where the
sediment gravity flows, and reworking by bottom currents. Gravity rate of carbonate production nearly always exceeded
flows are at their distal end, thus weak and hardly erosive. The the rate of accommodation creation by a fair margin
hiatuses, therefore, are probably caused by contour currents. After
Austin, Schlager et al. (1986), modified. (see Fig. 7.4). The sequence-stratigraphic characteristics
listed below are a direct consequence of this fundamen-
tal setup.
Late Miocene of Mallorca ➤ Bounding surfaces are erosional with frequent evidence
of terrestrial exposure. Flooding events are extremely
Late Miocene carbonates of the Mediterranean island
subtle; they normally consist of open-shelf limestones
of Mallorca, documented in exemplary fashion by Pomar
onlapping tongues of reef debris in the clinoforms.
(1991, 1993), Pomar et al. (1996) and Pomar and Ward (1999)
Tongues of open-shelf sediment extending to the crest
complement the Bahama case study in several important as-
of the margin reef are exceedingly rare.
pects. Rather than being buried under a modern carbon-
➤ Transgressive systems tracts as defined in Figs 7.3 and
ate platform, the rocks on Mallorca are exposed in many
7.4 are not developed. The record consists of highstand
kilometer-long sea cliffs and have been drilled in hundreds
tracts and lowstand tracts. However, there are two pat-
of water wells, thus offering a truly exceptional opportunity
terns that indicate that the rate of accommodation cre-
to examine sequence anatomy at the scale of meters to tens
ation by relative sea-level rise sometimes exceeded the
of meters. Detail at this scale has been described from the rate of carbonate production: (1) The reef rim at the
Bahamian wells but was not resolved seismically or in well- platform margin occasionally rose above the lagoon
to-well correlations.