Saturday, December 31, 2016

Early Triassic Red Beds

Red beds between the lowest ammonoid bearing limestones and the basal conglomerates.  Commonly referred to the Black Dragon Formation (previously a member of the Moenkopi Fm.) or the Woodside Shale.  Exposed all over Utah, SE Idaho and SW Wyoming.

In the San Rafael Swell the lowest (and only) Triassic Limestone contains the Anasibirites Fauna, so the red beds below are older than Late Smithian age.

Lone Rock (right) above ridge of Sinbad Limestone with yellow and red Black Dragon Fm. below.  Foreground is Permian Black Box Dolomite.  On the road to the Black Box, northern San Rafael Swell.
In Weber Canyon the red beds overlie the Dinwoody Fm. and are below beds containing Meekoceras so these beds are either Dienerian or Early Smithian,

Woodside Shale with "Meekoceras Limestone" to the right.  In the canyon of the Weber River, just east of Morgan.

In the Confusion Range of western Utah.  A bed of limestone with large chert nodules and a chert pebble conglomerate separate the two sets of red beds shown (see this old post).  Kashmirites is found just above the calcarenites here and the brachiopod Xestotrema is found in limestones below the Permian red beds so these are probably Dienerian, or earliest Smithian.
Gerster Fm. redbeds (right), Thaynes red beds (center) and calcarenite (left).  Just north of Cowboy Pass in the northern Confusion Range.

Down south near Minersville the red beds are below limestones that contain the Early Smithian ammonoid Vercherites, so the beds here are probably Dienerian.

Moenkopi red beds below microbial limestones near Minersville.
  Farther south, around St. George, the limestones rest directly on the Permian and redbeds of this age are missing.