Abstract

Five types of carbonate breccia are recognized in the Chiantla quadrangle of northwestern Guatemala. Depositional breccias and conglomerates (lithoclastic limestones) are widespread in the Cretaceous Ixcoy limestone and attain thicknesses up to 500 m. The lithoclasts were eroded from adjacent emergent areas, which probably resulted from faulting. Depositional breccias are recognized by the presence of primary sedimentary structures, original lime-mud matrix, and polymictic clasts.

A thick sequence (250 m) of evaporite-solution-collapse breccias is present in the lower part of the Ixcoy. These breccias are characterized by dolomite clasts in a matrix of sparry calcite and granulated dolomite. This breccia type is recognized primarily by its close relationship to thick evaporites in the subsurface and neighboring surface exposures. Secondary criteria for their recognition include dedolomite and dedolomitization textures and clasts composed of lithologies compatible with evaporite deposits.

Tectonic breccias are formed by extensive fracturing of carbonates during periods of deformation. This breccia type is identified by its proximity to faults; its matrix of either coarse-crystalline, sparry calcite or granulated carbonate material; and oligomictic clasts with matching edges.

Caliche breccias have formed by caliche cementation of carbonate rubble at the base of slopes, and by the development of caliche veins transecting dolomitic rocks.

Pseudobreccias, which form by selective grain growth, occur locally in limestones of the Ixcoy. They are distinguished by similarity in lithology between “clasts” and “matrix,” highly irregular “clast”-shapes, and by a complete graduation in grain size from micrite in the “clasts” to sparry calcite in the “matrix.”

These breccias, though similar in the field, may be distinguished by a combination of stratigraphic and petrographic criteria.

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