Page 369 - Carbonate Facies in Geologic History

356 Summary

are common in argillaceous limestones. Bedding surfaces ordinarily show dia-
stems and lag concentrates of fossils. Mud mounds and pinnacle reefs occur.
e) Terrigenous clastics: Quartz silt, siltstone, and shale commonly interbedded
with limestones in clearly segregated layers.
D Biota: Very diverse shelly fauna indicating normal marine salinity; preser-
vation of both infauna and epifauna. Fauna may not be very abundant in places
but is generally present. Notable presence of stenohaline forms such as brachio-
pods, corals, cephalopods, and echinoderms.

Belt 3. Basin Margin or Deep Shelf Margin Facies (Clinothem)

This facies is formed at the toe of slope of a carbonate-producing shelf. Carbonate
sediments consist of contribution from pelagic organisms plus fine detritus
moved ofT from adjacent shallow shelves. The water is at least the same depth as
Facies Belt 2 and perhaps 200-300 m deep. The basin is situated generally below
wave base and barely at oxygen level. The strata are chiefly thin, well-segregated
carbonate beds with minor interbeds or mere partings of clayey and siliceous
material, much of the fine terrigenous matter having drifted or blown farther out
into the basin. These rocks may resemble basinal sediments but are less argilla-
ceous and somewhat thicker. Some of these rhythmic, thin-bedded limestones are
hundreds of meters thick.
a) Prevailing rock type: Fine-grained limestone, in some places cherty.
b) Color: Dark to light.
c) Grain types and depositional texture: Mostly lime mudstone with some
calcisiltite and including some microbreccia beds, and coarser bioclastic-litho-
clastic packstones.
d) Bedding and sedimentary structures: Some beds are laminated lime mud-
stones; in even rhythmic (flysch-like) beds. Other thicker-bedded units are of
massive unlaminated lime mudstone. Some beds are graded. Megaslump features
within evenly bedded limestone cause major discontinuities in the bedding. It has
been suggested that large scale but shallow channeling due to density currents
causes the discontinuities in these even-bedded lime mudstones. Occasionally
micrite bioherms occur.
Some beds are laminated lying above micrograded lithoclastic and bioclastic
debris from upslope, i.e., beds of allodapic limestone. Sporadic turbidites, exotic
blocks, and debris flows are found in sequences relatively close to shelves in
unstable areas.
e) Terrigenous clastics: Rarely present except as fine shale partings. Chert is
common.
D Biota: The bioclastic detritus derived principally from upslope. The fauna is
open-shelf and normal marine but may be (I, mixture of older forms derived from
the shelf, benthonic organisms living on the slope and some pelagic forms.

Belt 4. Foreslope Facies of Carbonate Platform (Marine Talus, Clinothem)

The slope is generally located above the lower limit of oxygenated water and
stretches from above the wave base to below it. The material is debris deposited