DOLOMITIZATION PATTERNS IN FRESHWATER CARBONATE AS EVIDENCE OF DIFFERENTIAL BURIAL DEPTH OF THE JURASSIC NAVAJO SANDSTONE, UTAH, USA

The Jurassic Navajo Sandstone (Ss) of the Colorado Plateau thins dramatically west to east and is generally assumed to be thinner to the east owing to truncation, as the upper contact is an unconformity. We have argued that the thinning may be partially depositional based on the distribution of freshwater carbonate lake and spring deposits, and that groundwater surfaced more frequently and abundantly in the east than farther west. Recent dating of some of these carbonate rocks in eastern Utah (UT) indicates that the Navajo Ss is older in the eastern part of the basin than in southwest UT, raising the possibility of a different hydrology in the smaller, older erg. However, this does not address the question of thinning; indeed, a fully preserved section of Navajo Ss might have been thicker in the east, given the erg was established there first.

Although lake and spring deposits are most abundant in the eastern part of the basin, they occur as far west as Kanab, UT, and as far south as the Navajo Nation in northeast Arizona. In the eastern part of the basin, carbonate deposits are largely calcite with small patches of dolomitic micrite. Carbonate deposits in more central locations are mixed dolomite-calcite, and the mixing can be on a microscopic, macroscopic, or stratigraphic (i.e., interbedded limestone and dolostone) scale. For example, at Trough Canyon, a prominent, irregular brown horizon consists of both calcitic and dolomitic domains, whereas the surrounding rock is limestone. The dolomite in this horizon is, in places, sucrosic, with prominent rhombs. To the west and south, carbonate deposits examined are almost entirely dolomite. Generally, where dolomite predominates, the carbonate is recrystallized and most original texture is obscured.

Clumped isotopes (Δ₄₇) also show diagenetic temperatures (~65°C to 86°C) increasing from east to west; both trends, therefore, indicate increasing burial from east to west. Thus, the hypothesis that thinning of the Navajo Ss to the east is at least partially stratigraphic, rather than solely truncation, is supported. However, the complex nature of the pattern of the dolomitization, including the dolomite horizon at Trough Canyon and the southern sites in the Moab area in the eastern part of the basin, suggest further work is needed to understand dolomitization in the region.