Abstract

Prediction of dolostone patterns requires knowledge of the hydrology of dolomitizing fluids. Reflux seepage of evaporated seawater is one the mechanisms invoked to explain dolomitization in restricted carbonate settings. Although this conceptual model has been broadly accepted over the decades, the hydraulics are nonetheless poorly understood. We hypothesize that the source of dolomitizing fluids was located in coastal carbonate depositional environments (tidal flats) and that a hydrologic model can be constructed on the basis of tidal-flat-related paleotopography and permeability distributions based on facies and rock types.

Because the Permian carbonate / evaporite succession of the San Andres Formation is often presented as an example of reflux dolomitization, we have chosen this formation to study dolomitization by reflux seepage. We present a comparison of outcrop and well log data for this formation in order to explain the geometry of the ramp and the distribution of dolostone, limestone, and evaporites.

We are interested in the carbonate / evaporite part of the ramp located seaward of the halite-rich depositional environments. The San Andres Formation is underlain by the Glorieta sands and peritidal succession of the Yeso Formation. In the inner ramp, limestone is overlain by interbedded anhydrite and dolostone. The limestone wedge decreases and abruptly disappears toward the middle ramp. Dolostone is the predominant lithology in the middle to outer ramp. Anhydrite beds thin and get progressively scarcer toward the outer ramp. The anhydrite-rich portion of the San Andres Formation has been eroded from the outcrop. Siliciclastics appear in the upper San Andres in the middle ramp and thicken toward the ramp margin, where they make up the upper half of the section. Limestone reappears in the lower portion of the ramp margin. Finally, the carbonate ramp gives way to basinal lowstand siliciclastics of the Delaware Group.

This geometry suggests that the dolomitizing fluids sink a short distance under the intertidal zone then travel horizontally for long distances following the bedding. The low permeability of the muddy facies dominating the limestone wedge or higher permeabilities of grainier facies in the middle to outer ramp may be the cause of such dolomite distribution. Further field studies and numerical modeling of the hydrologic system are under way in order for this hypothesis to be tested.