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Pedogenic and lacustrine features of the Brushy Basin Member of the Upper Jurassic Morrison Formation in western Colorado: Reassessing the paleoclimatic interpretations

Tanner L., Galli K., Lucas S.

Volumina Jurassica

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2014

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Vol. 12, no. 2

115--130

EN

Study of the pedogenic features of the Upper Jurassic Morrison Formation in western Colorado, USA, shows a clear difference in the types of paleosols between the strata of the lower and upper Brushy Basin Member. Lower Brushy Basin paleosols are mostly calcareous Aridisols with Stage I through Stage III calcrete Bk horizons, abundant root traces, occasional vertic features, but only rarely with ochric epipedons. Upper Brushy Basin paleosols are mainly thicker and commonly display ochric epipedons and well-developed Bt and Bw horizons. We assign these paleosols to the order Inceptisol. Limestones occur in the Brushy Basin Member and include both uniformly micritic limestones and limestones with strongly brecciated textures. The former contain sparse body fossils and charophyte debris, while the latter are characterized by clotted-peloidal fabrics with circumgranular cracking and silica replacement. We interpret these limestones as the deposits of carbonate in small water bodies on a low-gradient flood plain, with the textures resulting from pedogenic reworking of the carbonate sediment. We find no evidence for the presence of extensive lacustrine or wetlands (Lake T’oo’dichi’) deposits in the study area. The paleoclimate suggested by all of these features is strongly seasonal, but subject to variations on orbital (precessional and higher) timescales causing intervals of semi-aridity during weaker monsoons, to alternate with sub-humid periods during stronger monsoons. The apparent long-term change in climate during Brushy Basin deposition potentially resulted from northward drift of North America.

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Magnetostratigraphy and paleopoles of the Kayenta Formation and the Tenney Canyon Tongue

Steiner M., Tanner L.

Volumina Jurassica

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2014

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Vol. 12, no. 2

31--38

EN

The Kayenta Formation is the third in a series of stratigraphic units making up the Glen Canyon Group that were sampled along US Hwy 89 in southern Utah. The Kayenta is dominantly reversed polarity with a number of very short normal polarity intervals. Above the Kayenta and interbedded in the Navajo Sandstone is the Tenney Canyon Tongue of the Kayenta Formation. The lower half of the Tenney Canyon Tongue was also sampled and is dominantly normal polarity with three short reversed polarity intervals. The dominantly reversed magnetostratigraphy of the Kayenta appears to match that of Early Pliensbachian polarity interval “e-Pli R.” The dominance of normal polarity of the Tenney Canyon Tongue suggests that the Tenney Canyon may have been deposited in the upper half of the Pliensbachian polarity interval “ePli-N.” The suggested polarity matches indicate that the Kayenta and Tenney Canyon Tongue strata are 187–190 Ma in age. The paleopoles of the two units are statistically identical. The combined data of the Kayenta-Springdale-Whitmore Point show that the J-1 cusp terminated before the deposition of the Kayenta Formation. The North American continent/pole returned to its Late Triassic position during/after Springdale time, apparently along the same path used to reach the apex of the J-1 cusp.

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The Whitmore Point Member of the Moenave Formation: Early Jurassic Dryland Lakes on the Colorado Plateau, Southwestern USA

Tanner L., Lucas S.

Volumina Jurassica

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2008

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Vol. 6, no. 1

11-21

EN

The Lower Jurassic Whitmore Point Member of the Moenave Formation in Arizona-Utah, USA, comprises fish- and coprolite-bearing shales, siltstones, sandstones, and minor limestones. These facies were deposited in ephemeral and perennial lakes subject to episodic desiccation and incursions of coarse clastics during floods. Meromictic conditions developed during perennial episodes, probably due to salinity stratification, which enhanced preservation of organic matter in gray to black shales. These lakes formed on the floodout of a north-northwest oriented (relative to modern geography) system of mainly ephemeral streams on a broad and open floodplain. The Whitmore Point Member both overlies and interfingers laterally with alluvial red-bed facies of the Dinosaur Canyon Member of the Moenave Formation. The vertical transition from alluvial to lacustrine sedimentation recorded by the Dinosaur Canyon and Whitmore Point members of the Moenave Formation most probably resulted from a eustatically-controlled rise in base level during the Early Jurassic (Hettangian). The Dinosaur Canyon Member also interfingers laterally with eolian dune deposits of the Wingate Sandstone, which was deposited by winds that reworked coastal plain sediments to the north of the study area. Thus, on this part of the Colorado Plateau, fluvial, lacustrine and eolian sedimentary facies were deposited contemporaneously in laterally adjacent paleoenvironments.

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Tetrapod trace fossils from lowermost Jurassic strata of the Moenave Formation, northern Arizona, USA

Tanner L., Lucas S.

Volumina Jurassica

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2008

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Vol. 6, no. 1

133-141

EN

At Moenkopi Wash along the Ward Terrace escarpment of northern Arizona strata of the upper Dinosaur Canyon Member of the Moenave Formation contain sedimentary structures we interpret as casts of tetrapod burrows. Sandstone casts and in situ burrows occur concentrated in two horizons that extend several hundred meters along the Ward Terrace escarpment. The structures, hosted in beds of eolian sandstone, form interconnecting networks of burrows that branch at right angles. Individual burrow casts have sub-circular cross sections and consist of nearvertical tunnels and horizontal to low-angle galleries that connect to larger chambers. Most burrow casts measure 5 to 15 cm in diameter, are filled by sandstone of similar grain size as the host rock, and have walls that are unlined and lack external ornamentation. Bedding plane exposure of the lower horizon reveals that the density of burrows exceeds 30 vertical tunnels per square meter. One exposure in the upper horizon reveals burrows concentrated in a mound-like structure with 1 m of relief. Rhizoliths, distinguished from burrows by their typical smaller diameters, calcareous infilling, and downward branching, co-occur with these burrows in the upper horizon. The fossil burrows in the Moenave Formation appear to have been constructed by a fossorial tetrapod with social behavior similar to the modern Mediterranean blind mole-rat. Although no skeletal remains are associated with the burrows, the fossil record suggests that the most likely producers of the Moenave burrows were tritylodontid cynodonts.

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Onset of erg sedimentation during the Early Jurassic on the Colorado Plateau, southwestern U.S.A.: climatic, eustatic and tectonic controls

Tanner L., Lucas S.

Volumina Jurassica

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2006

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Vol. 4, no. 1

106-106

EN

Deposition of the Moenave and Wingate formations took place during the latest Triassic to Early Jurassic in a mosaic of terrestrial subenvironments including fluvial, lacustrine, and eolian. Fluvial-lacustrine processes dominated Moenave deposition, which included channelized flow in ephemeral to perennial streams, unconfined flow (sheetwash) in interchannel areas, ephemeral lakes (playas), and perennial lakes that were subject to episodic desiccation. The Moenave terminal floodplain, which was dotted by broad, shallow lakes, interfingered with the Wingate erg, where eolian processes dominated. The Moenave-Wingate outcrop belt exposes a north-south lithofacies gradient from distal (erg margin) to proximal (erg interior) as dominantly fluvial-lacustrine lithofacies in the north are replaced by mainly eolian dune and interdune deposits to the south, recording encroachment of the Wingate erg. The prevalence of ephemeral stream and eolian environments during deposition of these strata indicates a seasonally arid climate during the latest Triassic to earliest Jurassic. We see no sedimentologic evidence for significant climate change at the Triassic/Jurassic boundary, or at any time encompassed by this sedimentary succession. The growth and incursion of the Wingate erg into the Moenave fluvial system may have been driven by the availability of sediment in the up-wind source area, the coastal plain and coastline to which the Moenave streams delivered sediment. We interpret a eustatic signal as responsible for formation of this erg as long-term regression during the Rhaetian and continued Hettangian low-stand exposed a broader area of shallow marine sediments to eolian reworking. Preservation of the erg deposits may have been enhanced by tectonically controlled accommodation space as continental shortening led to crustal flexure during the Early Jurassic.

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Criterion for definition of the Triassic/Jurassic boundary

Lucas S., Guex J., Tanner L.

Volumina Jurassica

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2006

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Vol. 4, no. 1

291-291

EN

The criterion for definition of the Triassic/Jurassic boundary (TJB) should be a marker event of optimal global correlateability. Only an ammonite event meets this criterion; other potential marker events for definition of the TJB have less correlation potential. Since the 1960s, the LO of the ammonite Psiloceras (usually the species P. planorbis) has provided the working definition of the TJB. However, other criteria for boundary definition have been advocated, including a change in the bivalve fauna (LO of Agerchlamys), a sudden negative excursion of carbon isotopes and the LO of Psiloceras tilmanni, which precedes the LO of P. planorbis. Other criteria that can be advocated include the supposed TJB mass extinction, the HO (highest occurrence) of conodonts or a significant evolutionary turnover of radiolarians. Distinction of the Triassic and Jurassic systems in marine biostratigraphy has a long tradition rooted in ammonite biostratigraphy. This is because the ceratitedominated ammonite faunas of the Triassic virtually disappeared across the system boundary and were totally replaced by the smooth-shelled psiloceratids of the Early Jurassic. Because of the long history of study of this ammonite turnover, its details are extremely well documented on a global scale, especially in western North America, South America and Western Europe. This ammonite turnover thus provides wide-ranging correlations that are intensively studied, extensively published and documented. No other bio-event associated with the TJB can claim such investigation, and no bio-event is comparable to the ammonite turnover to provide a globally correlateable criterion for boundary definition. Using the LO of Psiloceras tilmanni as to define the TJB thus has these advantages: 1. it maintains longstanding tradition of placing the boundary so that all smooth-shelled psiloceratids are Jurassic; 2. it is a boundary above all bio-events traditionally considered Triassic (Fig. 1); 3. it provides an ammonite-based definition of broad correlation potential (P. tilmanni has a distribution from Nevada to Chile); 4. it places the boundary close to (just above) other marker events that can be used to identify the TJB in sections that lack ammonites (Fig. 1). The LO of P. tilmanni thus defines a TJB of optimal correlation potential.