The Moenkopi Formation

Moenkopi Formation with ancient riverbed marks at capital reef national park Ut

Moenkopi Formation slopes in Canyonlands national park Ut

Swimming dinosaur tracks Capital reef national park Ut. Compared to 50mm lens cap

The Moenkopi Formation is a thick sequence of Triassic aged sedimentary rocks that outcrops in much of southern Utah.  It typically outcrops as deep reddish-brown, ledgey slopes and cliffs.  It's one of the more prominant rocks of southern Utah and makes up the bottom step of the Grand Staircase.  It is so named because its geology was first described at Moenkopi Wash in Arizona.  It is a wide-spread formation and is covers an area from western Colorado to southeastern California and parts of both New Mexico and Arizona.

     The age of the Moenkopi Formation ranges from the early Triassic to the middle Triassic period. The Moenkopi Formation generally contains thinly bedded mudstones and sandstones with a wide variety of ripple marks and some trace fossils. Secondary gypsum veins cut through the formation. The Moenkopi formation can contain localized fossils, but most exposures are devoid of them. Actual bone is even more rare. During the initial deposition of the Moenkopi, the climate was hot and dry, but later became progressively wetter. The Moenkopi sedients were deposited in tidal flats, near-shore, shallow marine, and floodplain environments. 

     There are a number of subdivisions or "Members" within the Moenkopi Formation.  These are characterized by different types of sediments and represent changing depositional environments.   All of the sedimentary environments preserved within the Moenkopi are somehow associated with coastal or near-coastal  systems.

     Trace fossils of reptiles are occasionally found preserved in the sediments of the Moenkopi Formation.  Not so long ago, one could see the trackway left in a sandy Triassic river bottom by a swimming Chirotherium.  As it swam across the river, its toes scraped grooves into the sand.  It's my understanding that these tracks have since been robbed by vandals.

Throughout much of southern Utah, the Moenkpoi consists primarily of mudstones and sandstones.  Farther to the West, into the Las Vegas, Nevada area, it is dominated by fossil-bearing, shallow marine, limestones.  The only time in my life as a geologist that I ever struck petroleum, was core drilling in the Moenkopi near Las Vegas.  The bore hole certainly wasn't a "gusher" but, a few hours after drilling, the core looked wet and smelled of oil.