Abstract: Porosity Controls in Subsea Cemented Rocks from Deep-Flank Environment of Little Bahama Bank

R. Jude Wilber, A. Conrad Neumann

Products of deep-sea lithification exhibit a spectrum of porosity controls related to both the environment of deposition and early diagenesis. Samples of hardgrounds and lithoherms from the deep-flank environment of Little Bahama Bank range from dense micrites and biomicrites with little porosity to friable grain-supported biomicrites and biomicrudites, the porosity of which may be 50% or more. Biogenic structures due to burrowing and boring are particularly important. Soft-sediment burrowing by infauna results in a system of cavernous macropores (Thalassinoides-type) and sinuous micropores (Planolites-type) which are preserved during lithification. Stromatactoid structures can originate from the physical and/or biologic excavation and later infill of soft, intercrust are s. Boring sponges produce a variety of intricate porosities ranging from centimeter-sized chambers and pits to fine micropores a few microns in diameter. Microboring porosity due to the activity of deep-water fungi is present in all samples and is particularly abundant on exposed surfaces. Microbores destroy the thin septa within deep-water coral skeletons producing tubelike pores even within well-cemented biomicrudites. Selective dissolution of sponge spicules represents another from of microporosity control. Both well and poorly cemented rocks have a significant amount of primary intragranular porosity in the form of hollow or partly filled foram and pteropod tests. Intergranular porosity also varies widely within and between samples.

The study of porosity controls in ancient carbonate rocks commonly is concentrated on deciphering late-diagenetic sequences of cementation, solution, and/or fracture. The samples examined in this study show that some deep-water carbonate rocks form in situ as porous lithologies due to a combination of synsedimentary processes and early diagenetic influences. Not all of these porosities will be preserved but many will serve as pathways for later diagenetic controls of secondary porosities.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC