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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 8. (August)

First Page: 1502

Last Page: 1502

Title: Westwater Canyon Member of Morrison Formation: Is it Really One Fan?: ABSTRACT

Author(s): C. E. Turner-Peterson

Article Type: Meeting abstract


Three distinct fluvial packages can be recognized within the Westwater Canyon Member along the west side of the San Juan basin in New Mexico. Two of the fluvial packages (lower and upper) persist to the east and can be recognized as far east as Laguna. The middle fluvial package is as thick as the lower and upper on the west side of the basin but thins rapidly to the east; fluvial sandstones cannot be recognized in this interval east of the Gallup area.

Cross-bedding studies show that streams of the lower package flowed east-northeasterly, whereas streams of the middle and upper packages flowed generally easterly and southeasterly. Volcanic pebbles are present in abundance only in the middle package. Isopach maps of each separate package, together with maximum pebble-size data, show that major fluvial axes shifted through time. Northeasterly flowing streams of the lower fluvial package had two distinct lobes. One lobe, centered near Thoreau, apparently traversed the area of the present-day Zuni uplift, and the other lobe was centered northwest of the Gallup sag, near Asaayi Lake. The easterly and southeasterly flowing streams of the middle and upper fluvial packages dropped their coarsest bed-load material just east of the Defiance u lift as they entered the basin from the west. Farther east the middle unit changes facies rapidly and loses its fluvial character. The upper package maintains its fluvial character, but contours for maximum pebble size for this interval widen considerably in an eastward direction, suggesting a more constant energy level as the streams traversed the basin.

Results of these studies are not compatible with the concept of one fan emanating from the southwest, as has been proposed by previous workers. Separating the Westwater Canyon into discrete packages and treating them separately eliminates the need to have streams enter from the southwest, "turn," and then flow southeasterly in the vicinity of the mineral belt. Both northeast and southeast paleocurrent directions occur in the Westwater Canyon, but in different parts of the member.

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Copyright 1997 American Association of Petroleum Geologists