Page 161 - Carbonate Sedimentology and Sequence Stratigraphy

152 WOLFGANG SCHLAGER

sea level sequence stratigraphy, the following characteristics of the M
factory are particularly important.
WB
➤ The primary material ranges from clay-size lime mud to
outer middle
shelf shelf pebble-size peloids or oncoids and also includes hard
framework that may extend over thousands of cubic
meters. Some of this framework may be reworked
to boulder-size clasts in the depositional environment.
Overall, the size spectrum (not the grain types) resem-
bles that of the T factory.
sea level
➤ Depth window of production extends from the shore-
WB outer middle line to bathyal, perhaps abyssal depths. However, com-
shelf shelf petition by the T factory may block the shallow part of
the window in the Phanerozoic.
➤ Lithification in the depositional environment is as rapid
as in the T factory and probably even more pervasive.
➤ Where the M factory builds platforms, it develops rims
at the shelf edge. In most instances, these structures
sea level
seem to be less robust than those of the T factory. How-
WB
ever, some Precambrian rims have been truly elevated
outer middle such that the platform developed an empty bucket
shelf shelf
(Adams et al., 2004).
Sequence anatomy and bounding surfaces
Where the M factory builds shoalwater platforms, the fa-
cies belts of these M platforms fit Wilson’s standard model
sea level (see chapter 4). The crucial element is a platform-margin
belt consisting of a rigid framework of automicrite and ma-
WB outer middle rine cement (Keim and Schlager, 2001; Stephens and Sum-
shelf shelf
ner, 2003). This margin may crest in tens of meters of wa-
lowstand deposits dominated by ter or build to sea level. Shallow rims shed much excess
sponge spicules and micrite sediment onto the platform and may develop debris aprons
transgressive and highstand analogous to the backreef aprons of coral reefs (e.g. Stephens
deposits dominated by tunicate and Sumner, 2003, p. 1285).
spicules, brown bioclasts, as well
Platform-interior deposits generally are flat-bedded, often
as bryozoan and corallinacean
debris showing shoaling cycles bounded by flooding or exposure
surfaces, just as in T carbonates. Small stromatolite mounds
genetic sequence boundary
(e.g. Chow and George, 2004) can be viewed as microbial
analogues of patch reefs on modern tropical platforms.
Fig. 8.6.— Model of systems tracts in Quaternary glacial- The slopes seaward of the rim are shaped by the interplay
interglacial cycles based on ODP drill holes and seismic data, Eu- of gravity-driven downslope transport and in-situ produc-
cla shelf, southern Australia. After Saxena and Betzler (2003), tion of automicrite and cement. Where in-situ production
modified. Lowstand tract (unshaded) forms a lens on the upper ◦
slope. Transgressive and highstand tracts extend with fairly uni- dominates, slope angles may exceed 50 and debris aprons
form thickness across upper slope and outer shelf but the part on at the toe-of-slope are small (Lees and Miller, 1995, p. 199).
the outer shelf is eroded during lowstands. The result is a pro- Where gravity-flows dominate, slope angles peak at 35 – 40
grading sediment wedge on the upper slope and a zone of reduced ◦ - the angles of repose of poorly sorted sand and rubble,
sedimentation or erosion on the shelf. and debris aprons are larger (Keim and Schlager, 2001).
The wide depth window of production allows the M fac-
MFACTORY tory to develop autonomous production centers in deep wa-
ter - the mud mounds. In the Phanerozoic, M platforms are
Overview
rare and mud mounds are the most characteristic products
The characteristic accumulations of the M factory are ei- of the M factory. They form groups and roughly contour-
ther swarms of mounds that may coalesce to a network of parallel belts of mounds where the individual structures
ridges, or rimmed platforms that geometrically resemble T may be tens to hundreds of meters thick and thus recogniz-
platforms. able in seismic data.
Sediment anatomy of the M factory is directly related to Mound flanks are often steep but debris aprons are small
the production process and its environmental controls, sim- or absent. The scarcity of debris and the fact that flank de-
ilar to the relationship observed in the T and C factories. For clivity commonly exceeds the angle of repose of sand and