Evaporite-Carbonate Rocks of the Jurassic Lovelock Formation, West Humboldt Range, Nevada

Abstract

The Lovelock Formation is an assembly of late Early Jurassic or Middle Jurassic carbonate rocks and gypsum that constitute the youngest preserved sediments and, probably, the last marine deposits in successions of Jurassic rocks in the Carson Sink region of western Nevada. The formation records a set of transitional environments between open marine basinal conditions and an emergent tectonically active terrain. The current extent of the formation is probably a small fraction of its original distribution in the Carson region and, perhaps, in areas of Nevada farther west. The Lovelock Formation occurs only in a pair of nappes in the nappe pile of the northern West Humboldt Range. Geometric relations between intranappe and internappe folds suggest that nappe emplacement and folding were coupled deformations in major Middle Jurassic tectonism. Sites of deposition of the Lovelock Formation were probably not more than a few kilometers east or west of current exposures of the formation. The formation is comprised of three members in a maximum thickness of about 200 m. The lowest is a micrite which underwent emergence and fresh-water diagenesis before deposition of fragmental limestones of the middle member. Conglomerate of the middle member consists exclusively of intraclastic micrite pebbles from the lower member. Finer grained sedimentary breccia above the conglomerate is an accumulation of intraclasts and allochthonous particles from a variety of subenvironments on a carbonate bank. The upper member comprises gypsum and calcarenite, deposited at least in part in standing water of a barred basin. The sequence of motions of the sediment surface relative to sea level recorded in the deposits of the Lovelock Formation is conceivably due to eustatic variations, but the general temporal proximity of deposition of the formation to major tectonism suggests that such motions were probably tectonic. Postdepositional hydrothermal recrystallization occurs sporadically in carbonate rocks of the Lovelock Formation. Such metamorphism was perhaps synchronous with local generation of solution breccias in the upper member and local calcitization of sulfate.