Mudstones as Exploration Guides to Tabular Sandstone-Type Uranium Deposits in Salt Wash Member, Morrison Formation (Upper Jurassic), Henry Basin, Utah: ABSTRACT

Abstract

Sedimentologic and palynologic studies in the Henry basin indicate that a particular type of mudstone can be used as an exploration guide to uranium deposits in nearby sandstone beds. Most Salt Wash mudstones bear no relation to the ore deposits and consist of non-organic red or green mudstones deposited in overbank environments on a flood plain. Other unfavorable but organic-rich gray mudstones contain carbonized wood fragments up to about 1-cm long, palynomorphs including the alga Botryococcus, powdery carbonaceous material, and structureless blebs of organic matter. These mudstones lack swelling clays, are moderately to highly calcareous, and are associated with thin limestones. Most of these mudstones were deposited in shallow and relatively large lakes at least sever l kilometers wide and several tens of kilometers long. Favorable organic-rich gray mudstones that lie near uraniferous sandstones contain minute carbonized wood particles less than about 0.5 mm long, a palynomorph suite lacking Botryococcus, cutinous and epidermal tissue, pyrite, and swelling clays. These mudstones are non-calcareous to slightly calcareous, and rarely contain limestone. Favorable mudstones occur just above or below uranium-bearing sandstones, or interfinger with sandstones that contain uranium anomalies within several hundred meters of the mudstone. These mudstones are associated with distal braided stream sandstones and were deposited in small shallow lakes or ponds up to several square kilometers in areal extent lying between stream courses or in abandoned stream channels. The mudstones lie in structural lows that were ctive during deposition, and they tend to occur in the part of the low where fluvial sedimentation was least active. Prospecting strategies based on these observations should search for favorable mudstones in tectonic lows in strata deposited by distal braided streams. End_of_Article - Last_Page 763------------