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.