Abstract

Marine sediments constituting the Virgin limestone member of the Moenkopi formation in the St. George, Utah, area represent an easily mappable lithologic unit that unconformably overlies the continental strata of the lower red member of the Moenkopi formation and the alpha member of the Kaibab formation. It is conformably overlain by the continental middle red member of the Moenkopi.

The Virgin limestone “member” is raised to formational rank, and, consequently, the Moenkopi “formation” is made a group.

The limestone content of the Virgin formation averages about 30 per cent of the sediments; consequently the lithologic term “limestone” has been dropped from the formational name.

The Virgin formation was deposited under a near-shore, shallow-water, marine environment. Cyclic sedimentation characterizes the lower part of the formation, the cycles consisting of basal limestones overlain by siltstones. The upper part of the formation in the southeastern area was deposited in restricted lagoons under an arid environment, while at the same time marine deposition persisted in the northern part.

Ammonites, identified by Prof. S. W. Muller as Tirolites spinosus Mojsisovics s.l., ?Cordillerites, and ?Hungarites, were found near the middle of the formation. These definitely establish the stratigraphic position of the Virgin formation as near the top of the Lower Triassic sequence and indicate a Middle Triassic age for post-Virgin Moenkopi strata.

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