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.

Monday, February 22, 2016

Early Triassic Thaynes Group Isotopes

C. Thomazo, E. Vennin, A. Brayard, I. Bour, O. Mathieu, S. Elmeknassi, N. Olivier, G. Escarguel, K. G. Bylund, J. Jenks, D. A. Stephen, E. Fara
In the aftermath of the end-Permian mass extinction, Early Triassic sediments record some of the largest Phanerozoic carbon isotopic excursions. Among them, a global Smithian-negative carbonate carbon isotope excursion has been identified, followed by an abrupt increase across the Smithian–Spathian boundary (SSB; ~250.8 Myr ago). This chemostratigraphic evolution is associated with palaeontological evidence that indicate a major collapse of terrestrial and marine ecosystems during the Late Smithian. It is commonly assumed that Smithian and Spathian isotopic variations are intimately linked to major perturbations in the exogenic carbon reservoir. We present paired carbon isotopes measurements from the Thaynes Group (Utah, USA) to evaluate the extent to which the Early Triassic isotopic perturbations reflect changes in the exogenic carbon cycle. The δ13Ccarb variations obtained here reproduce the known Smithian δ13Ccarb-negative excursion. However, the δ13C signal of the bulk organic matter is invariant across the SSB and variations in the δ34S signal of sedimentary sulphides are interpreted here to reflect the intensity of sediment remobilization. We argue that Middle to Late Smithian δ13Ccarb signal in the shallow marine environments of the Thaynes Group does not reflect secular evolution of the exogenic carbon cycle but rather physicochemical conditions at the sediment–water interface leading to authigenic carbonate formation during early diagenetic processes.