Blackening of Eolianite and Caliche Adjacent to Saline Lakes, Isla Mujeres, Quintana Roo, Mexico

Abstract

ABSTRACT A partly blackened limestone and caliche rubble has been produced in situ at the edge of hypersaline lakes on the island of Mujeres off the northeast coast of the Yucatan peninsula. The angular gravel along the quiet lake shores is formed by dissolution and desiccation-fracturing of dark and lighter-colored country rock and caliche crusts. Dark-colored constituents of this rubble gravel are of two types: 1) blackened Pleistocene (?) eolianite country rock and 2) black porous laminated caliche-algal micrite crusts. Pale to dark yellowbrown constituents, which make up 50 to 75% of the rubble, are fragments of caliche and eolianite. Chemical analyses of the dark and light-colored fragments of this deposit indicate that concentrations of Fe, Mn, and S in the black rocks are equal to or less than that in the brown rocks. HCl-insoluble organic C, however, is slightly higher in the darker rocks. The blackened calcarenite is infiltrated with finely-disseminated organic matter of algal origin. The black color is attributed to preservation of the organic matter. A secondary source of the dark color may be minor amounts of iron sulfide, produced in the presence of H2S derived from bacterial reduction of sulfates within the reducing environment of an algal mat. Apparently dark-colored caliche-like crusts may be laid down in localities where caliche deposition is simultaneous with production of black algal matter. These conditions probably existed at several northeast Yucatan localities where black caliche-algal micrite fragments are found near hypersaline water. The environment necessary for the blackening of caliche or limestone is probably one in which the pH is high enough and the Eh low enough that the preservation of finely-disseminated organic material produced by algae can be accompanied by deposition of secondary calcite which includes and preserves the organic matter. In such a reducing environment associated with sulfate deposition, bacterial action may produce H2S. The black rocks of the northeast Yucatan hypersaline reas contain H2S. Blackened limestone in ancient carbonate sections may call attention to stratigraphic breaks. A few workers have recognized an association between blackened limestones and unconformities in certain Mesozoic and Paleozoic sections. The presence of layers of fragmented, dark-colored limestone may indicate subaerial exposure adjacent to hypersaline water. Black fragments in limestone strata, then, may be an aid in locating coastal areas in ancient carbonate provinces and the landward edge of evaporite deposits.