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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

2008

Vol. 6, no. 1

11-21

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.

Response of rotifers to hydrochemical and biotic factors

Żurek R.

Oceanological and Hydrobiological Studies

2006

Vol. 35, No.2

121-139

The zooplankton community in an open cast sulphur mine impoundment was investigated over two years. Complicated physical and chemical relationships in this pit ecosystem resulted in an atypical planktonic assemblage. Changes in the rotifer populations were examined over time and space in the pit ecosystem. Only 20 species of rotifers were identified in the impoundment. In winter, Polyarthra dolichoptera coexisted with P. bicerca; in spring, one of the dominant species was Keratella quadrata, while, in summer, Hexarthra fennica, Keratella cochlearis and Synchaeta tremula were all present. In autumn, Filinia longiseta dominated the population. On the border of the oxycline and beneath the thermocline lived Keratella testudo. Thermal and chemical stratification generated five variants of diurnal vertical migrations of planktonic animals including two typical, well-known patterns. Some species were able to penetrate the oxygen-free layer in the hypolimnion. These species were also resistant to low concentrations of hydrogen sulfide. Multidimensional scaling and correspondence analysis identified the following factors as important for rotifers: mineralization and the presence of sulfide with hydrogen sulfide, as well as biotic (predators) or the life supporting parameters: temperature, oxygen. Toxic compounds present in the water, especially sulfides or its derivates, were probably responsible for deformation of the rotifers’ spines in 0.1% of the population.