DOLOMITE COMPLEXITIES IN THE LOWER JURASSIC NAVAJO SANDSTONE, COLORADO PLATEAU: A PROGRESS REPORT

Carbonate constitutes 1-2% of the eolian Navajo Sandstone. The carbonate was deposited in freshwater spring mounds and lakes, a few m to >1.6 km wide. In the eastern and southern parts of the basin (east-central Utah, northern Arizona), the carbonate is ≥95% calcite. To the west, the carbonate is ≥95% dolostone, consistent with relatively deep burial as indicated by tectonic, stratigraphic, and clumped-isotopic data. In between, the carbonate is mixed dolomite and calcite on stratigraphic, geographic, and microscopic scales. Late diagenetic dolomitization with burial can be heterogeneous, but the mixture of dolomite and calcite on different scales suggests the possibility of different mechanisms of dolomitization. Stratigraphic scale: Near Tenmile Canyon, central Utah, the middle bed of three successive carbonate beds is mostly dolostone, whereas the other two are limestone. Dolomitization of the middle bed is explained by postdepositional disruption of the bed by soft-sediment deformation that permitted differential permeation of dolomitizing fluids. Geographic scale: The dolostone in the western part of the basin is mostly recrystallized, typical of late diagenetic dolomitization by saline groundwater after burial. In the region dominated by limestone, two localities are entirely dolostone, and the original petrographic fabric has been preserved at one of those sites. Nothing about these two sites suggests a similar mechanism of differential subsurface flow of dolomitizing fluids. The only known mechanism for fabric-preserving dolomitization is reflux dolomitization, a marine mechanism that cannot be extended to these systems. Although reflux from evaporating freshwater lakes is possible, the lack of Mg in this system is problematic for that mechanism. Microscopic scale: A few samples of the limestone in the eastern part of the basin contain rhombs or clusters of rhombs of dolomite. Although attribution of these to early diagenetic, microbially mediated processes is tempting, such processes do not produce rhombs. Cathodoluminescence to try to determine if the rhombs are overgrowths on the type of dolomite crystals that form through early diagenetic processes has been inconclusive. Examination of fluid inclusions to determine if the diagenetic processes were actually heterogeneous is planned.