Geology Reference
In-Depth Information
15.7 Platform-Slope-Basin
Transects
and Florida (Mullins and Neumann 1979; Mullins et
al. 1980, 1984, 1986; Mullins and Cook 1986) and off
Jamaica (Land and Moore 1977), Belize (James and
Ginsburg 1979) and Grand Cayman (Rigby and Rob-
erts 1976). The accretionary Bahamian slopes are dif-
ferentiated into an upper gullied slopes and lower non-
gullied slopes or a basin-margin rise that passes later-
ally to the flat ocean floor. The sediment of the upper
slope consists of periplatform oozes composed of plat-
form-derived material and pelagic debris. Cementation
and hardground formation takes place in the shallow
part of the upper slope. The oozes are strongly bio-
turbated in the deeper part. Differences in the compo-
sition and the dominant mineralogy of periplatform
ooze layers are explained by changes in the submer-
gence and exposure of the platform, and by differences
in the dissolution of aragonitic grains. Lower slope
deposits are gravity deposits, interbedded with peri-
platform ooze. Mud- and grain-supported debris flows
and turbidites contribute to the formation of aprons near
the base of the slopes. Particular elements of the lower
slope are deep-water reefs (lithoherms) with aherm-
atypic corals (see Sect. 2.4.4.3).
Deep-marine carbonates comprise two major sedimen-
tary groups differing in depositional criteria and diage-
netic development:
•
Pelagic carbonates
characterized by fine-grained
sediments formed by pelagic planktonic and nektonic
organisms (Sect. 2.4.5.3), and
•
Resedimented allochthonous carbonates
(Sect.
2.4.5.4), whose constituents are derived from platform
and slope settings and exported to slopes and into ba-
sins.
Microfacies criteria of the first group are discussed
in Sect. 15.8. This section deals with the second group,
specifically with carbonates produced by gravity mass
transport and consisting of resediments. The first part
gives a summary of the origin, the composition and
the diagnostic criteria of carbonate slopes. The second
part describes case studies and demonstrates the po-
tential of microfacies analyses for understanding slope
history and slope geometries.
Significance:
Recent research on fossil and recent
submarine slope systems has provided new insights into
the relationships between sediment composition and
slope morphology. Declivity and stabilization stages
are reflected in the depositional texture, grain types,
and biota, and revealed by microfacies features, includ-
ing compositional data, diagenetic criteria and paleon-
tological proxies. Sea-level fluctuations leading to dif-
ferent kinds of carbonate shedding are reflected by grain
compositional logs that can be successfully used in cor-
relating platform-, slope- and basin sediments.
Carbonate slopes pass seawards from shallow-wa-
ter to deeper-water environments. Their development
is controlled by extrinsic and intrinsic factors. Extrin-
sic factors are sea level, tectonics, oceanography, si-
liciclastic input and climate. Intrinsic factors are bio-
logical productivity, sediment transport and erosion,
biological stabilization and cementation, the growth of
mounds and reefs, and bioerosion and dissolution. A
concise synthesis of these controls and an overview of
the large-scale patterns of carbonate slopes was given
by Coniglio and Dix (1992).
15.7.1 Types and Composition of Carbonate
Slopes
Slope types and facies models
of slopes
have been
summarized by (1) Cook et al. (1972), with regard to
the sediment wedge underlying the slopes and forming
aprons adjacent to the shallow-water platform; (2) James
and Mountjoy (1983) and McIlreath and James (1984),
distinguishing bypass and depositional slopes based on
the nature of the platform margin and the relief between
platform and basin; (3) Mullins and Cook (1986), em-
phasizing similarities and differences between silici-
clastic fans and carbonate aprons, and (4) Coniglio and
Dix (1992). Commonly used classifications are those
by McIlreath and James, and Coniglio and Dix. The
models described by the latter are based primarily upon
the geometry of shelf deposits and the adjacent plat-
form margin, but also consider the essential factors
listed below:
• The intensity and location of the carbonate produc-
tion on the platform.
• The type of the carbonate production and the mor-
phology of the platform margin.
• The feeding areas of the exported carbonates (line
source and a multitude of feeding channels).
• The line source of platform-derived sediments origi-
nates from numerous channels dissecting oolitic and
bioclastic shoals, reefs and islands along margins of
Modern slopes:
Carbonate slopes are deep-water
depositional environments. They exhibit a variety of
depositional angles and sediment types that are differ-
ent for erosional slopes, bypass slopes, and accretion-
ary or depositional slopes. Modern carbonate slope
models have been intensively studied in the Bahamas
