Drowned or Smothered: The Semantics and the Processes of Carbonate Demise

Abstract

Fossil shallow-water carbonate build-ups are frequently said to have ‘drowned’ as they ceased to produce biogenic carbonate sediments. The generic term of ‘drowning’ is used variably in geological literature to imply a range of processes including: (i) subaquatic submergence of carbonate factories into depths too great for sustainable carbonate production, (ii) destruction of carbonate-producing ecosystems by excessive turbidity, temperature, salinity or nutrient supply, as well as (iii) burial of active carbonate factories under clastic sediments deposited at or above sea level. These processes result in diametrically different relationships between the now demised carbonate build-ups and their overlying sedimentary cover. Carbonate build-ups subject to subaquatic submergence into abysses too deep for sustainable carbonate production are by definition likely to be buried under deep-water sediments, which will attain a drape-like geometry in pelagic settings and downlap geometry in pro-delta settings. The overburden may be significantly younger than the build-ups, and hence free of shallow-water carbonate and likely fine-grained. On the other side of the spectrum, carbonate build-ups ‘smothered’ alive by regressive coastal siliciclastics are characterised by onlap relationships between both lithologies. The overburden may be coarse-grained and punctuated by shallow-water carbonate-clastic intercalations. These nuances are of fundamental importance in petroleum exploration, where overburden lithology controls the ability of fossil carbonate build-ups to contain hydrocarbons. The mode of demise also tends to be associated with different reservoir properties within the upper levels of the carbonate reservoirs. Build-ups subject to subaquatic submergence tend to be capped by tight argillaceous carbonate deposited during a gradual deepening of the depositional environment, and by ‘empty-bucket’ topography of elevated rims and back-stepped platforms. In contrast, the youngest carbonate strata of build-ups ‘smothered’ by prograding clastics at or near the sea level tend to consist of shallow-water facies and are frequently overprinted by subaerial exposure. A correct description of the mode of carbonate demise thus facilitates understanding of a variety of properties of the carbonate reservoirs and their sedimentary covers. These concepts are here illustrated on examples from the Miocene-Present Central Luconia carbonate province, offshore NW Borneo.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015