Chemical Constraints on Carbonate Sedimentation and Fabric Development, Barra Velha Formation, Santos Basin, Brazil

Abstract

Despite their importance in modifying reservoir characteristics, the origins of shrubby and spherulitic crystal aggregates of the Barra Velha Formation are poorly understood. What is clear, however, is that dendritic and spherulitic growth structures arise from dynamic instabilities at the solid-solution interface. As supersaturation increases, dislocation-controlled crystal growth transitions to growth via 2D surface nucleation, yielding euhedral and skeletal morphologies, respectively. With further supersaturation, unstable growth dominates. Diffusive transport becomes rate-limiting as growth front nucleation rates increase, and dendrites are produced. At still higher supersaturation, spherulitic aggregates are formed. This is because, as diffusive fluxes and growth front nucleation rates continue to increase, the diffusive boundary layer approaches the interface, producing spherical growth structures.

This interplay between transport and crystal growth provides a valuable framework to understand the origins of Barra Velha carbonates. Specifically, the development of spherulites with Mg-silicates requires the maintenance of high supersaturation at a regional scale, most readily achieved by evaporative concentration. In contrast, shrubby carbonates with complex dendritic morphology require a diffusion-controlled physical regime and a strong gradient of increasing CaCO₃ saturation above the sediment-water interface. Water column stratification, perhaps triggered through freshwater influx to a dense, saline water body, satisfies both criteria for shrub growth. Diffusion calculations show that once stratification was established, the influx of CO₂(aq) to the monimolimnion could have sustained sharp supersaturation gradients across the diffusive interface. This model predicts that shrub growth where the diffusive gradient intersects the sediment-water interface, establishing a bathymetric control on carbonate accumulation. Because diffusive transport fractionates a number of stable isotope systems, micro-scale isotopic measurements of carbonate components provide a test of the importance of transport-controlled growth. Together, the dynamics of evaporation, freshwater influx, and stratification may have strongly controlled the morphological development of carbonate precipitates. The locus of the carbonate factory itself would have shifted in space and time in concert with heterogeneities in CO₂(aq) and inhibitors to nucleation and growth.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018