MESO-SCALE JURASSIC PALEOTOPOGRAPHY AT THE J-2 UNCONFORMITY NEAR MOAB, UT

The J-2 unconformity in the area of Moab, UT, separates the Lower Jurassic Navajo Sandstone (Ss) from the Middle Jurassic lower Carmel Formation (Fm). The boundary is recognized by the inclusion of chert clasts in the sandstone immediately above the J-2 unconformity. As part of a comprehensive study of the stratigraphy, paleoecology, and paleoclimate of the Navajo Ss, we examined limestone beds in the Navajo Ss that occur near or at the unconformity.

One of these limestone beds caps an area of striking paleorelief north of Sevenmile Canyon. The limestone is 40-120 cm thick and stromatolitic to massive; the massive beds contain sparse to abundant red chert similar to that found at the J-2 unconformity. Three smaller areas of limestone to the northeast, which also cap the Navajo Ss, are mainly stromatolitic and thinner. They are lateral equivalents of the stromatolitic facies in the large cap, indicating that the limestone caps were originally one lake system.

Using a laser range-finder, we quantified the paleorelief at this locality. The flanks of the paleotopographic high indicate asymmetric downcutting on either side. The highest point of the paleotopography is the limestone. To the southwest, the vertical downcutting was 6 m within a horizontal distance of 60 m (av. slope = 5°); the unconformity cuts through the limestone and the underlying sandstone to a paleo-exposure surface characterized by mottling, burrows, and rhizoliths. To the northeast, the limestone thins and nearly pinches out, but the pinchout is truncated by the unconformity, which dips steeply. The paleorelief on the top of the Navajo Ss on this side is much greater, with 7.5 m of downcutting within a horizontal distance of 33 m (av. slope = 13°).

Downcutting truncated the limestone on either side and reworked chert clasts from the limestone into deposits at the J-2 unconformity. Erosion was particularly vigorous on the northeast side, where it formed a steep-sided scour. These relationships show the significant variability and unusual preservation of relief at the J-2 unconformity. Limestone beds in the Navajo Ss are responsible for modern topographic inversions throughout this region.