Abstract

Sedimentological characteristics suggest that the Precambrian quartz-pebble Vallecito Conglomerate of the Needle Mountains originated as an alluvial fan complex built by high-gradient, short duration peak discharge braided streams and occasional debris flows. Scott, Donjek and South Saskatchewan-type stream deposits dominate proximal, medial and distal fan sequences, respectively. Four fining-upward megacycles, averaging 127 m thick, suggest reactivation of basin-margin faults followed by scarp retreat and lowering of relief during deposition. Thinner cycles within these megacycles, ranging from a few meters to a few tens of meters in thickness, are related to second order extrinsic controls such as flood events. Although the sedimentological setting and gross lithological composition of the Vallecito Conglomerate are similar to Precambrian uranium-bearing fossil placers, the unit is too young to contain stable primary detrital uranium minerals and pyrite.

Field and petrographic evidence indicate that the unit rests unconformably on metavolcanic and metasedimentary rocks of the Irving Formation. Compositional data and limited paleocurrent data suggest that the Vallecito Conglomerate was derived from an uplifted subduction zone complex north of the present outcrop belt and deposited in a tectonically active continental-margin basin. This margin was the southern edge of the North American Plate during mid to late Proterozoic time. This setting, coupled with its age, are in general agreement with the plate-tectonic model proposed by Condie (1982) for Precambrian rocks of the southwestern United States. The depositional-tectonic model presented here represents a first attempt to develop such a model for Precambrian metasedimentary rocks of the southern Rocky Mountains.