Page 33 - Carbonate Sedimentology and Sequence Stratigraphy

24 WOLFGANG SCHLAGER

The T factory is restricted to the warm, sunlit waters of or hard upon formation. A number of detailed case stud-
the ocean that are high in oxygen because of constant equili- ies suggests that precipitation of this fine-grained carbon-
bration with the atmosphere and low in nutrients because of ate was caused by a complex interplay of biotic and abi-
intensive competition. In modern oceans, the characteristic otic reactions with microbes and decaying organic tissue
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settings are the surface waters between 30 N and S of the playing a pivotal role (Reitner et al., 1995b; Monty, 1995;
equator. The northern and southern limit of the T factory Neuweiler et al., 1999; Neuweiler et al., 2000; Reitner et al.,
closely follows the line where the mean temperature of the 2000). The term “automicrite” for micrite precipitated in-
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coldest month is about 20 C (Fig. 2.9). The T factory may situ (Wolf, 1965; Reitner et al., 1995a) is very useful in in-
also pass into the cool-water factory downward in the wa- stances where the microbial origin is uncertain and the term
ter column, for instance at the boundary between the warm “microbialite” not justified, for instance if precipitation is
surface layer of the ocean and the thermocline. Furthermore, driven by non-living organic matter as in the concept of or-
transitions from T to C factory occur in shallow tropical up- ganomineralization (Trichet and Defargue, 1995; Neuweiler
welling areas where cool, nutrient-rich water comes to the et al., 2003). Abiotic marine cement is the second most im-
surface (Lees and Buller, 1972; Pope and Read, 1997, p. 423; portant product of this factory. It forms typically in vugs
Brandley and Krause, 1997, p. 365; James, 1997). (such as Stromatactis ) within the rigid framework of au-
tomicrite (Fig. 2.17.) Biotically controlled (skeletal) carbon-
ate may occur but is not characteristic.
Cfactory
The typical setting of the M factory in the Phanerozoic
are dysphotic or aphotic, nutrient-rich waters that are low
The letter C is derived from cool-water and controlled pre- in oxygen but not anoxic (Leinfelder et al., 1993; Neuweiler
cipitation. The products are almost exclusively biotically- et al., 1999; Stanton et al., 2000; Boulvain, 2001; Neuweiler et
controlled precipitates. Heterotrophic organisms dominate. al., 2001). These conditions often prevail in the thermocline,
The contribution of photo-autotrophic organisms in the i.e. at intermediate depths below the mixed layer of the sea
form of red algae and symbiotic larger foraminifers is some- (Fig. 2.18). However, in the Proterozoic and after severe ex-
times significant (Lees and Buller, 1972; Nelson, 1988; Hen- tinctions in the Phanerozoic, a carbonate production system
rich et al., 1997; James, 1997). The sediment typically con- dominated by biotically induced micrite and abiotic marine
sists of skeletal hash of sand-to-granule size. Cool-water car- cements also extended into the shallow environments nor-
bonates lack shoal-water reefs and oolites; carbonate mud mally occupied by the T factory. The products are suffciently
and abiotic marine cements are scarce (Figs 2.10, 2.11). similar to the classical M factory to include them here. Tex-
The cool-water factory extends poleward from the limit of tures and structures of the carbonate products do not in-
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the tropical factory (at about 30 ) to polar latitudes (Fig. 2.9). dicate that the involvement of phototrophic microbes – as
The transition to the T factory is very gradual (Betzler et al., opposed to aphotic ones – fundamentally changes the pre-
1997). The C factory also occurs at low latitudes in the ther- cipitation process of the biotically induced carbonates. At
mocline below the warm surface waters and in upwelling present it is very difficult to distinguish between photically
areas. and aphotically formed automicrite unless associated sessile
The oceanic environments of the C factory are photic or skeletal organisms provide the necessary information.
aphotic waters that are cool enough to exclude competition
by the T factory and sufficiently winnowed to prevent burial SEDIMENTATION RATES AND GROWTH
by terrigenous fines. Nutrient levels are generally higher POTENTIAL OF THE THREE FACTORIES
than in the tropical factory. These constraints set a wide
depth window for the cool-water factory from upper ner- Siliciclastic systems depend on outside sediment supply.
itic to bathyal and even abyssal depths. The most common For carbonate factories, the ability to grow upward and pro-
setting is the outer neritic, current-swept part of continen- duce sediment is an intrinsic property of the system. Con-
tal shelves. The transition to the T factory normally extends ceptually, one can distinguish between the ability to build
over more than thousand kilometers (Fig. 2.10; Schlanger, up vertically and track sea level, the aggradation potential,
1981; Collins et al., 1997). and the ability to produce and export sediment, the produc-
tion potential. In most instances, however, it is only possible
to quantify the lower limit of the growth potential by deter-
Mfactory mining aggradation rates.
Fig. 2.19 presents rates calculated from thickness and
The letter M alludes to mud-mound, micrite and mi- stratigraphic ages of ancient deposits. The upper limit of
crobes. Intensive work in the past 15 years established the observed rates was interpreted as a crude estimate of the
the significance of this carbonate factory in the Phanerozoic growth potential. Rates of all three factories were found to
(Lees and Miller, 1985; James and Bourque, 1992; Monty, decrease as the length of the time interval increases. Differ-
1995; Lees and Miller, 1995; Pratt, 1995; Reitner et al., 1995a; ent tests have shown that this trend has real, physical mean-
Webb, 1996, 2001). The characteristic component is fine- ing and is not just a consequence of the fact that geologists
grained carbonate that precipitated in situ and was firm calculate sedimentation rates by dividing thickness by time