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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
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Outcrop measurements taken from trough cross-beds throughout the Westwater Canyon Member indicate that two separate fluvial complexes, each with its own distinct current direction, compose the member along the southern San Juan Basin. Lower Westwater streams flowed east-northeast at all points between Gallup and Laguna, whereas upper Westwater streams flowed consistently southeast over the same area. Measurements in the overlying Poison Canyon sandstone (of economic usage) suggest a return to northeast-flowing streams. Mine maps from the Ambrosia Lake area confirm these current directions in the subsurface.
These data preclude the existence of a single fan emanating from the southwest, as proposed by previous workers. The upper Westwater had a source to the northwest which not only changes the overall picture of Westwater deposition but also raises significant questions about uranium mineralization and basin geometry during Morrison time.
In many surface sections the lower and upper Westwater sandstones merge as a result of scouring by upper Westwater streams. In the subsurface these areas might appear as "sand thicks" on an isopach map. Elsewhere on the surface, the Westwater contains numerous "shale breaks." Regardless of the number of shale breaks or total sandstone thickness, the type of deposition (braided) and the current directions (northeast-lower, southeast-upper) remain the same. This suggests that use of isopachs in the subsurface to determine stream type and current direction is invalid. Areas of greatest sandstone thickness in the mineral belt most likely reflect thickening
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in synclines that were actively growing during deposition.
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