Csoma, Anita E.¹, Robert H. Goldstein²
(1) ConocoPhillips Company, Houston, TX
(2) University of Kansas, Lawrence, KS

ABSTRACT: Porosity Reduction in Meteoric-Marine Mixing Zones: Case Studies Illustrate some of the Controls on Calcite and Aragonite Precipitation in Mixing-Zones

This study integrates stratigraphic, petrographic, and geochemical data from Cretaceous carbonates (Monte Camposauro, Italy), Plio-Pleistocene carbonates (Hope Gate Formation, Jamaica), and Pleistocene speleothems (Sa Bassa Blanca cave, Mallorca, Spain). It illustrates that calcite and aragonite precipitation is common in meteoric-marine mixing zones, documents the characteristics of mixing-zone cements, and evaluates some variables that lead to cementation.

Identification of mixing-zone cements is accomplished on the basis of fluid inclusion Tm ice. Mixing-zone cements include bladed, radial-fibrous, and overgrowth low-Mg calcite, bladed and dendritic high-Mg calcite, microporous calcite with variable Mg-content, and acicular aragonite. Many of the morphologies and mineralogies are similar to those of marine and meteoric cements. Mixing-zone ¹⁸O and ¹³C data show ranges and patterns that are similar to those of partially recrystallized marine cements and physical mixtures of meteoric and marine cements. Thus, caution is recommended in using morphology and mineralogy to distinguish among origins for low-temperature cements.

Tm ice data show precipitation from a wide range of salinities, suggesting that mixing ratio does not determine if there is dissolution or precipitation. ¹⁸O, ¹³C, and the distribution of speleothems of Sa Bassa Blanca cave demonstrate the importance of CO₂ degassing in causing cementation in the mixing-zone. Petrography, Sr, and Mg data from the Hope Gate Formation suggest that aragonite dissolution may have been a drive for mixing-zone cementation. Secular variation in seawater chemistry is recorded in the mineralogy of mixing-zone precipitates, low-Mg calcite cement characterizing the Cretaceous and mostly high-Mg calcite and aragonite in the Pleistocene.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.