New Mexico Geological Society Annual Spring Meeting — Abstracts

Heterogeneous calcite cements occur in the alluvial Sierra Ladrones Formation (Upper Santa Fe Group). The cement preferentially occurs in coarser-grained sediments of high- and low-energy channels and sandy paleosols (?). The spatial distribution of cementation within these units range from homogeneous to highly heterogeneous (in a variety of styles). Cementation in overbank fines and clay-rich paleosols (?) is less developed and usually occurs as layers at the tops of the units and as disseminated micritic concretions (more rarely as tubular and branching concretions).

The cements formed in three diagenetic environments: 1) phreatic, characterized by sparite with uniform, disseminated, isopachous; poikilotopic, and drusy textures; 2) vadose non-pedogenic, characterized by micrite with meniscus and pendent textures; and 3) vadose-pedogenic, which has cement characteristics similar to vadose plus macroscopic features sllch as tubular and branching concretions and a red hue. Many of the cements are of a mixed vadose/phreatic origin, with vadose cements occurring next to grains and phreatic cements filling some or all of the remaining pore space.

At the scale of the architectural elements (i.e., 100s of meters aerially and usually less than 2 vertically), cementation is largely controlled by pre-cementation permeability- elements with high pre-cementation permeability contain the most phreatic cement However, at smaller scales within architectural elements, the relationship between cementation and pre-cementation permeability is weaker. This weak relationship can be attributed to the presence of early vadose cements that control the distribution of later phreatic cements. Cement distribution is also controlled at the facies scale by contrasts in permeabilities between the facies and possibly other factors (e.g. sediment mineralogy and water/rock interaction).

Geochemical analyses of all cement types in the study area have similar values. Stable isotope analyses have the following mean and standard deviation values respectively: (1) δ¹³C pdb (-4.1, 0.33); and (2) δ¹⁸O pdb (-8.5, 0.63). Microprobe analyses have the following mean and standard deviation percentages respectively for end member compositions: (1) CaCO₃ (98.7, 0.74); (2) MgCO₃ (0.87, 0.47); (3) FeCO₃ (0.21, 0.25); (4) SrCO₃ (0.13,0.19); and (5) MnCO₃ (0.1 0.15). These results indicate that the stable isotope ratios and elemental compositions for all the cement in the study area are determined by a factor, or factors, that dominate(s) both temporally and spatially across all environments formation. However, small differences in Ca, Mg, Fe, Sr, and Mn amounts can be seen between micrite and sparite cements in all architectural elements.