PETROGRAPHY AND ENVIRONMENTAL INTERPRETATION OF TUFA MOUNDS AND CARBONATE BEDS IN THE JURASSIC NAVAJO SANDSTONE OF SOUTHEASTERN UTAH, U.S.A.

Abstract

Carbonate deposits in the Jurassic Navajo Sandstone of Utah reflect deposition in interdune lakes and springs. Interdune-lake deposits consist of flat-lying carbonate units. Springs formed tufa mounds that are interpreted as subaerial, ambient-temperature, artesian-spring deposits. In the first systematic study of the petrography of the carbonate deposits in the Navajo Sandstone, eleven facies were identified in several flat-lying carbonate deposits and two tufa mounds. Fenestral mudstone and peloidal facies dominate the lacustrine deposits, whereas thrombolitic mudstone characterizes the mounds. The biota consists of ostracodes, charophytes, fish, mollusks, a possible freshwater sponge, trace fossils, and fragments of vascular plants. Features resulting from penecontemporaneous weathering provide evidence of episodic exposure of the lacustrine carbonate beds during deposition and formation of the tufa mounds under subaerial conditions. Although carbonate deposits are not rare in eolian systems, few have been studied in detail; comparisons between these and the Navajo Sandstone carbonate deposits reveal some characteristics that may be unique to the latter.