Description
Stratigraphically enclosed gravity slide blocks (megabreccias) and superposed gravity slides are a key to the Cenozoic tectonic history of an area in the transition zone between the Colorado Plateau and the Basin and Range Province. The study area lies in the northeastern part of Yuma County and in the southeastern part of Mojave County. In Artillery Basin, Eocene monolithologic alluvial fan deposits, volcanic (41.5 and 43.3 m.y.), and sedimentary rocks are overlain by a dozen large blocks of crystalline rock (Slide I) which came from the ancestral Rawhide Mountains, five miles (8 km) to the southwest. The Eocene sequence and Slide I are overlain by Miocene volcanic (15.9 and 17.9 m.y.) and sedimentary rock units which contain a second megabreccia (Slide II), composed of an exotic suite of plutonic and gneissic rock. Stratigraphic relationships show that Slide II was sequentially emplaced from the northeast. In traversing the former topographic high (source of Slide I), Slide II incorporated additional material of Slide I lithology. Minimum distance traveled by some blocks in Slide II was 17 miles (27 km). A Miocene and/or Pliocene conglomerate, which is found in discontinuous outcrops over 300 square miles (777 square km), is characterized by the presence of well-rounded boulders and cobbles of granite. This fluvial deposit unconformably overlies Slide II and is composed of the same exotic suite of rocks, demonstrating erosion from the same source. Slide I resulted from northeastward regional tilting that probably occurred at the very end of the Laramide orogenesis. The importing of the exotic rocks (Slide II and the conglomerate) from the northeast in Miocene and Pliocene time marks the inception of uplift of the ancestral Colorado Plateau. This gradual uplift and resulting tension was broken by northwest trending normal faults which developed progressively from east to west across the area and formed modern plateaus and ranges. Two groups of Late Tertiary and/or Quaternary gravity slides (Slide A and Slide B), composed of an assemblage of older allochthons, blocks of Slides I and II, and consolidated Cenozoic sedimentary rocks, record uplift on the western (younger) faults. These younger blocks were shed radially from the newly uplifted Rawhide and Buckskin Mountains, and traveled distances on the order of hundreds to thousands of feet (30 m - l.5 km). Movement of Slides I, II, A, and B occurred on a near planar surface cut on crystalline basement rock. This surface may be equivalent to the erosional surface which developed on Older Precambrian rocks during Precambrian (post-Mazatzal Revolution) time.
