Diagenetic Pathways in Heterozoan Carbonates

Abstract

Understanding of controls on reservoir quality in marine carbonates is strongly influenced by classic diagenetic studies of Quaternary deposits of the Caribbean region. While these studies have provided a wealth of valuable information, deposits of the tropical realm cannot be considered as ideal analogs for understanding diagenesis of heterozoan (cool-water) carbonates. This investigation integrates sedimentological, petrographic, and geochemical observations from several modern and ancient heterozoan carbonate sequences that formed at mid to high latitudes with the aim of evaluating diagenetic pathways characteristic of these systems. Heterozoan carbonate factories operate under conditions distinct from those of their better-known photozoan counterparts. These conditions play a strong role in predetermining diagenesis and how deposits translate into the rock record. In these environments, weakly carbonate-saturated waters play a key role in determining the dominant mineralogy of benthos as well as the preservation potential of skeletal debris. Carbonate production is almost entirely biogenic, assemblages are dominated by calcitic taxa, and there are no significant calcareous phototrophs. Carbonate communities are typically not capable of building rigid frameworks, and thus their deposits accumulate as sands and gravels, and are prone to winnowing and reworking by waves and bottom currents. During long residence times on the seafloor, skeletal debris is infested by endolithic borers, who create intraparticle porosity, weaken grains, and increase susceptibility to fragmentation and dissolution. Thus, deposits of heterozoan carbonate systems tend to enter the geologic record as micrite-poor, unconsolidated sediments, reflecting accumulation of calcitic debris with low diagenetic potential in corrosive bottom waters. During progressive burial, physical and, ultimately, chemical compaction produce limestones with tightly packed, condensed and grain-supported fabrics, often with grains in sutured contact. Significant cementation is associated largely with the deep burial realm. Results reveal a significantly different diagenetic pathway than is typical for the more reactive deposits of tropical photozoan systems, in which alteration and cementation begins at the sea floor and during the early stages of burial. As such, this study highlights issues of importance relevant to assessing reservoir quality in the deposits of heterozoan carbonate systems.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016