Accretionary Wedge: Geo-Images ..Calcite Cements

Entry for the Accretionary Wedge carnival hosted by Chris and Anne at Highly Allochthonous.

When I viewed these crystals under a microscope for the first time... I felt more relief than elation. I finally had a story to tell my PhD committee!!

These are photomicrographs of pendant calcite crystals which precipitated within meteoric aquifers that developed during late Ordovician sea-level falls....location... Appalachians...northern Georgia.

On left is a view of the crystal stained with potassium ferricyanide. Non ferroan early cements are not stained. Burial Fe rich calcite stains blue. On right is a view of the same crystal in cathodoluminescence (crystals are bombarded with cathode rays in a vacuum chamber).

Cathodoluminescence helps understand pore fluid fluctuations between oxidizing and reducing conditions. In oxidizing pore-fluids, neither Mn+4 or Fe+3 is incorporated into growing calcite crystals, and thus cements are black (non-luminescent). In pore fluids with progressively lower Eh , reduction of Mn first and then Fe leads to their incorporation into the growing cements, giving the crystals a bright to dull luminescence. The image on left shows the changing geochemistry of pore fluids from oxidizing to mildly reducing to reducing as the sequence got buried.
 
All this geochemistry makes more sense when placed in a stratigraphic context. The image below shows a cyclic late Ordovician sequence and the position of calcite cements within it. Cathodoluminescent signatures help constraint the lower limits of the fresh water aquifer that developed during successive sea-level drops.

During the first sea-level drop both vadose and phreatic meteoric conditions are recognized and groundwater fluctuated between oxic to mildly reducing. During the next sea-level drop (Ordovician-Silurian unconformity) the meteoric aquifer was reducing and only bright luminescent phreatic cements precipitated. The vadose zone is not preserved in the younger sequence.

This kind of cement stratigraphy helps understand the geochemistry and the lateral and vertical extent of groundwater systems and fluid flow patterns that develop as sea-level drops and basins are exposed to fresh water infiltration. Fluid - sediment interaction create and destroy porosity and permeability,  properties which in turn influence the hydrocarbon and mineral potential of the sedimentary sequence.