Abstract

The Sierra Ladrones Formation is the uppermost syn-rift sedimentary unit of the Rio Grande Rift in central New Mexico. In the vicinity of the study site it was deposited in a variety of fluvial subenvironments. Portions of the Formation are strongly cemented by calcite, forming resistant ledges in otherwise poorly consolidated sediment. The calcite exhibits a wide range of macroscopic and microscopic textures and structures, which formed in different hydrologic environments, including vadose nonpedogenic, vadose pedogenic, and phreatic. Phreatic cements are characterized by sparite with uniform, disseminated, isopachous, poikilotopic, and drusy textures; vadose-nonpedogenic by micrite with meniscus textures; and vadose-pedogenic by textures similar to those in the vadose-nonpedogenic environment, plus macroscopic features such as root structures, destruction of primary sedimentary textures, and a red hue. Many of the cements are of mixed vadose/phreatic origin, with vadose micrite formation preceding phreatic sparite.

The calcite is near end-member in composition, with only minor substitution of trace elements (Sr, Mg, Mn, and Fe) for Ca in the calcite structure. Vadose micrite cements generally have a higher trace-element content than phreatic sparite, perhaps as a result of more rapid precipitation in the vadose zone. Carbon isotope values range from −4.7 to −3.5‰ PDB and δ18O values range from −10.3 to −7.2‰ PDB. Cements that are dominantly vadose have a wider range of δ13C values, and are more 13C-depleted overall than phreatic cements. Oxygen values are similar for both vadose and phreatic cements.

At large scales (i.e., several meters to decameters vertical dimension), calcite is present preferentially in coarse-grained sediments that had high pre-cementation saturated permeabilities relative to uncemented finer-grained strata. At smaller scales (i.e., several meters or less vertical dimension), however, there is no clear relationship between degree of cementation and pre-cementation permeability. At these scales, calcite cement is present preferentially in areas that contain early vadose calcite. This suggests that at large scales cementation was controlled by pre-cementation permeability, perhaps due to a greater flux of Ca2+ and/or HCO3 in permeable horizons, whereas at smaller scales, within high permeability units, the distribution of cementation was controlled by other factors, such as the presence of nucleating material.

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