Displacive Calcite Syntaxial Overgrowths in Open Marine Limestones

Abstract

Some calcite syntaxial overgrowths in the Upper Greensand (Cretaceous) of southern England and the Onondaga Limestone (Devonian) of New York State had a displacive mode of formation. Precipitation of the overgrowths in both of these open shelf marine limestones occurred in a phreatic environment. Two displacive crystal growth patterns are recognized by cathodoluminescence: (1) unimpeded displacive precipitation, in which obstructions, such as adjacent grains and matrix, offer little resistance to crystal growth, and (2) impeded displacive precipitation, in which obstructions significantly decrease the calcite precipitation rate at overgrowth-obstruction contacts. Impeded displacive precipitation can be recognized by an irregular luminescence zonation, reflecting precipitation perpendicular to overgrowth-obstruction contacts rather than along rational crystal faces.