Porosity and Cementation in Upper Cretaceous Mooreville and Demopolis Chalks, Central Alabama

Ira Holston, David T. King, Jr., Enid Bittner

Cyclically arranged chalky marl, marl, limestone, and sand facies comprise the Upper Cretaceous Mooreville and Demopolis Chalks, the lower two formations in the Selma Group, inner Coastal Plain of Alabama. In the central Alabama study area (Dallas, Lowndes, and Montgomery Counties), the Mooreville-Demopolis section is 305 m thick and the two main facies are chalky marl and marl. Chalky marl consists of 50-70% carbonate (nannofossil component plus isopachous cement) and is relatively impermeable (average permeability is 0.9 md). The marl has 30-50% carbonate (mainly nannofossil component) and has an average permeability of 0.13 md. Helium-calibrated porosity values range from 31 to 35% in chalky marl and 36 to 41% in the marl. Scanning-electron microscopy (SEM) of chalky m rl and marl shows a relatively poor alignment of phyllosilicate grains in both facies. Under SEM, the calcareous nannofossil component, mainly coccoliths and rhabdoliths, shows pristine to cement-coated exterior surfaces. The cement coating is most common in the chalky marl. Sampling throughout the Mooreville-Demopolis section shows no apparent vertical (stratigraphic) trends in facies-specific petrologic characteristics such as permeability, helium-calibrated porosity, phyllosilicate grain alignment, nannofossil content, and extent of cementation. The most parsimonious explanation of the development and evolution of porosity and cementation in the study area is as follows. First, simple mechanical compaction from burial under a few hundred meters of sediment can readily explain the redu tion in porosity to less than 40% in most samples from the Mooreville-Demopolis section. Secondary (facies-selective) cementation is a further cause for porosity reduction in chalky marl. Cementation is likely the result of calcareous nannofossil dissolution during compaction and carbonate solution-transfer via groundwater movement.

AAPG Search and Discovery Article #91029©1989 AAPG GCAGS and GC Section of SEPM Meeting, October 25-27, 1989, Corpus Christi, Texas.