Non-Actualistic
Carbonate Deposystems: Revising the Carbonate Factory-Depth Paradigm
Della Porta, Giovanna1, Jeroen
A.M. Kenter2, Paul M. Harris3 (1) Cardiff University,
Cardiff, United Kingdom (2) Chevron Energy Technology Company, Voorburg,
Netherlands (3) Chevron Energy Technology Company, San Ramon,
Non-actualistic (e.g. deviating from
depositional models developed for recent deposystems) carbonate platforms are
not the exception in the geologic record because of the wide range of
biological and environmental factors controlling facies character and
architecture.
A fundamental concept is the exponential
decrease of carbonate production with increasing water depth from a maximum at
shallow depths to the base of the photic zone. However, when the dominant
factory is microbially induced, high rates of carbonate production extend on
the slope down to dys-/aphotic depths and new production-water depth profiles
must be developed.
Studies of Carboniferous (Asturias,
Pricaspian Basin) and Permian (Capitan Reef) high-relief carbonate platforms
have shown that the microbial-boundstone production extends to 300m water
depth: 1) the detrital lower slope consists mostly of matrix-free cemented
rudstone sourced by the slope boundstone with subordinated platform top derived
material, 2) carbonate production on the slope is controlled by environmental
parameters (temperature, nutrients, oxygenation) that as well depend from water
depth but the microbial boundstone response to relative sea-level changes
differs from modern reefs, 3) carbonate growth is not seriously reduced during
sea-level falls because it can continue downslope, 4) progradation can take
place at high rates despite the lack of platform top shedding (slope vs.
highstand shedding), 5) concepts of leeward progradational vs. windward
aggradational margins have to be revised. Paleozoic high-relief platforms with
microbial boundstone-dominated margins seem to have developed in mesotrophic, starved restricted basins with oxygen-depleted
bottom waters that would not be suitable settings for the recent coral-reef
rimmed platforms.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California