Epeiric Shelves
There are no examples of epeiric shelves in modern day so all of the following information refers to ancient epeirc shelves. They existed during the highstand of Sloss Supersequences, when continents were deeply submerged.
Characteristics of epeiric shelves
include:
1. Equable, relatively warm climates
2. Vast, shallow (<10m), well-lit, clear seas
3. Little exposed land
a. Limited terrigenous influx
b. Limited freshwater runoff
c. Possible high evaporation rates (depends
on latitude and geography)
d. Sluggish circulation with deep ocean (frictional
drag retards wind and tidal driving
of currents)
4. Good organic and carbonate productivity
5. Vast areas of carbonate sedimentation
There are a couple of models used to explain the conditions and the processes associated with ancient epeiric shelves. The following model assumes that tidal currents were not maintained across the platform because of the shallow water has a dampening effect. Circulation is rather restricted, only influenced by wind driven waves. Storm events play a major role in shaping the environment.
The Clear Water Model
A. Zone X: offshore, subtidal shelf
1.
Open-ocean facing shelf with sediments deposited in sites ranging from inner
to outer neritic
2.
Several hundreds of km wide
3.
Facies distributions follow depth-energy patterns similar to those of clastic
facies
4.
Storm and fair weather waves dominate in low subsidence settings; micrites
better represented in
rapidly subsiding settings
5.
Fauna and sediments are a mixture of locally derived (autochthonous) and transported
(allochthonous) material
6.
Fabrics of in situ sediments reflect currents of removal
B. Zone Y: Banks and shoals
1.
Contrary to expectation from clastic shelves, highest energy facies (zone
of
shoaling and breaking waves) is located tens to hundreds of km ocean-ward
of
the strand line
2.
Narrow zone some tens of km wide with carbonate sand (calcarenite) shoals
as
dominant facies
3.
Sediment fabrics mimic those of clastic sands (bioclastic) reflecting dominance
of
local transport
4.
Micrite removed by currents
C. Zone Z: inner shelf.
1.
Protected, quiet water zone between shore and barrier shoal zone (Y).
2.
Tens to hundreds of km wide.
3.
Complex facies relations ranging from subtidal "lagoon" facies to
intertidal mud flats.
4.
Salinity and fauna may range form normal marine to highly restricted (hypersaline)
in cases of
high evaporation or brackish in cases of high coastal runoff.
5.
Syngenetic dolostone & evaporites; diagenetic dolostone.
a. Dolomite forms from high Mg++ brines.
b. Syngenetic (chalky) dolostone assoc. with anhydrite/gypsum in hypersaline
mud flats.
c. Diagenetic (sucrosic) dolostone assoc. with unconformity and overlying
evaporites:
secondary brine alteration
Examples of ancient eperic shelf
successions include the late PreCambrian to Ordovician of China, the Cambro-Ordivician
of North America, the Triassic to Jurassic of Europe, and the Tertiary of
the Middle East.
Proceed on to the carbonate platform question set
: